feat(components): add ThermoState generators and Eurovent backend demo

crates/components/Cargo.toml

@@ -7,9 +7,17 @@ description = "Core component trait definitions for Entropyk thermodynamic simul
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/entropyk/entropyk"
 
+[features]
+default = []
+ffi = []
+http = []
+
 [dependencies]
-# Core types will be added when core crate is created
-# entropyk-core = { path = "../core" }
+# Core types from Story 1.2
+entropyk-core = { path = "../core" }
+
+# Fluid properties backend (Story 5.1 - FluidBackend integration)
+entropyk-fluids = { path = "../fluids" }
 
 # Error handling
 thiserror = "1.0"
@@ -18,8 +26,8 @@ thiserror = "1.0"
 serde = { version = "1.0", features = ["derive"] }
 
 [dev-dependencies]
-# Testing utilities
-# tokio-test = "0.4"
+# Floating-point assertions
+approx = "0.5"
 
 [lib]
 name = "entropyk_components"

crates/components/src/external_model.rs

@@ -0,0 +1,628 @@
+//! External Component Model Interface
+//!
+//! This module provides support for external component models via:
+//! - Dynamic library loading (.dll/.so) via FFI
+//! - HTTP API calls to external services
+//!
+//! ## Architecture
+//!
+//! The external model interface allows integration of proprietary or vendor-supplied
+//! component models that cannot be implemented natively in Rust.
+//!
+//! ## FFI Interface (DLL/SO)
+//!
+//! External libraries must implement the `entropyk_model` C ABI:
+//!
+//! ```c
+//! // Required exported functions:
+//! int entropyk_model_compute(double* inputs, double* outputs, int n_in, int n_out);
+//! int entropyk_model_jacobian(double* inputs, double* jacobian, int n_in, int n_out);
+//! const char* entropyk_model_name(void);
+//! const char* entropyk_model_version(void);
+//! ```
+//!
+//! ## HTTP API Interface
+//!
+//! External services must provide REST endpoints:
+//!
+//! - `POST /compute`: Accepts JSON with inputs, returns JSON with outputs
+//! - `POST /jacobian`: Accepts JSON with inputs, returns JSON with Jacobian matrix
+
+use crate::ComponentError;
+use serde::{Deserialize, Serialize};
+use std::path::PathBuf;
+use std::sync::Arc;
+
+/// Configuration for an external model.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ExternalModelConfig {
+    /// Unique identifier for this model
+    pub id: String,
+    /// Model type (ffi or http)
+    pub model_type: ExternalModelType,
+    /// Number of inputs expected
+    pub n_inputs: usize,
+    /// Number of outputs produced
+    pub n_outputs: usize,
+    /// Optional timeout in milliseconds
+    #[serde(default = "default_timeout")]
+    pub timeout_ms: u64,
+}
+
+fn default_timeout() -> u64 {
+    5000
+}
+
+/// Type of external model interface.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub enum ExternalModelType {
+    /// Dynamic library (.dll on Windows, .so on Linux, .dylib on macOS)
+    Ffi {
+        /// Path to the library file
+        library_path: PathBuf,
+        /// Optional function name prefix
+        function_prefix: Option<String>,
+    },
+    /// HTTP REST API
+    Http {
+        /// Base URL for the API
+        base_url: String,
+        /// Optional API key for authentication
+        api_key: Option<String>,
+    },
+}
+
+/// Trait for external model implementations.
+///
+/// This trait abstracts over FFI and HTTP interfaces, providing
+/// a unified interface for the solver.
+pub trait ExternalModel: Send + Sync {
+    /// Returns the model identifier.
+    fn id(&self) -> &str;
+
+    /// Returns the number of inputs.
+    fn n_inputs(&self) -> usize;
+
+    /// Returns the number of outputs.
+    fn n_outputs(&self) -> usize;
+
+    /// Computes outputs from inputs.
+    ///
+    /// # Arguments
+    ///
+    /// * `inputs` - Input values (length = n_inputs)
+    ///
+    /// # Returns
+    ///
+    /// Output values (length = n_outputs)
+    fn compute(&self, inputs: &[f64]) -> Result<Vec<f64>, ExternalModelError>;
+
+    /// Computes the Jacobian matrix.
+    ///
+    /// # Arguments
+    ///
+    /// * `inputs` - Input values
+    ///
+    /// # Returns
+    ///
+    /// Jacobian matrix as a flat array (row-major, n_outputs × n_inputs)
+    fn jacobian(&self, inputs: &[f64]) -> Result<Vec<f64>, ExternalModelError>;
+
+    /// Returns model metadata.
+    fn metadata(&self) -> ExternalModelMetadata;
+}
+
+/// Metadata about an external model.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ExternalModelMetadata {
+    /// Model name
+    pub name: String,
+    /// Model version
+    pub version: String,
+    /// Model description
+    pub description: Option<String>,
+    /// Input names/units
+    pub input_names: Vec<String>,
+    /// Output names/units
+    pub output_names: Vec<String>,
+}
+
+/// Errors from external model operations.
+#[derive(Debug, Clone, thiserror::Error)]
+pub enum ExternalModelError {
+    /// Library loading failed
+    #[error("Failed to load library: {0}")]
+    LibraryLoad(String),
+
+    /// Function not found in library
+    #[error("Function not found: {0}")]
+    FunctionNotFound(String),
+
+    /// Computation failed
+    #[error("Computation failed: {0}")]
+    ComputationFailed(String),
+
+    /// Invalid input dimensions
+    #[error("Invalid input dimensions: expected {expected}, got {actual}")]
+    InvalidInputDimensions {
+        /// Expected number of inputs
+        expected: usize,
+        /// Actual number received
+        actual: usize,
+    },
+
+    /// HTTP request failed
+    #[error("HTTP request failed: {0}")]
+    HttpError(String),
+
+    /// Timeout exceeded
+    #[error("Operation timed out after {0}ms")]
+    Timeout(u64),
+
+    /// JSON parsing error
+    #[error("JSON error: {0}")]
+    JsonError(String),
+
+    /// Model not initialized
+    #[error("Model not initialized")]
+    NotInitialized,
+}
+
+impl From<ExternalModelError> for ComponentError {
+    fn from(err: ExternalModelError) -> Self {
+        ComponentError::InvalidState(format!("External model error: {}", err))
+    }
+}
+
+/// Request body for HTTP compute endpoint.
+#[derive(Debug, Serialize)]
+#[allow(dead_code)]
+struct ComputeRequest {
+    inputs: Vec<f64>,
+}
+
+/// Response from HTTP compute endpoint.
+#[derive(Debug, Deserialize)]
+#[allow(dead_code)]
+struct ComputeResponse {
+    outputs: Vec<f64>,
+}
+
+/// Request body for HTTP Jacobian endpoint.
+#[derive(Debug, Serialize)]
+#[allow(dead_code)]
+struct JacobianRequest {
+    inputs: Vec<f64>,
+}
+
+/// Response from HTTP Jacobian endpoint.
+#[derive(Debug, Deserialize)]
+#[allow(dead_code)]
+struct JacobianResponse {
+    jacobian: Vec<f64>,
+}
+
+/// FFI-based external model (mock implementation for non-ffi builds).
+///
+/// When the `ffi` feature is not enabled, this provides a mock implementation
+/// that can be used for testing and development. The mock passes inputs through
+/// unchanged (identity function).
+#[cfg(not(feature = "ffi"))]
+pub struct FfiModel {
+    config: ExternalModelConfig,
+    metadata: ExternalModelMetadata,
+}
+
+#[cfg(not(feature = "ffi"))]
+impl FfiModel {
+    /// Creates a new FFI model (mock implementation without ffi feature).
+    ///
+    /// This creates a mock model that can be used for testing. The mock
+    /// implements an identity function (output = input for first n_outputs).
+    pub fn new(config: ExternalModelConfig) -> Result<Self, ExternalModelError> {
+        let metadata = ExternalModelMetadata {
+            name: format!("Mock FFI Model: {}", config.id),
+            version: "0.1.0-mock".to_string(),
+            description: Some("Mock FFI model for testing (ffi feature not enabled)".to_string()),
+            input_names: (0..config.n_inputs)
+                .map(|i| format!("input_{}", i))
+                .collect(),
+            output_names: (0..config.n_outputs)
+                .map(|i| format!("output_{}", i))
+                .collect(),
+        };
+        Ok(Self { config, metadata })
+    }
+
+    /// Creates with custom mock metadata for testing.
+    pub fn new_mock(
+        config: ExternalModelConfig,
+        metadata: ExternalModelMetadata,
+    ) -> Result<Self, ExternalModelError> {
+        Ok(Self { config, metadata })
+    }
+}
+
+#[cfg(not(feature = "ffi"))]
+impl ExternalModel for FfiModel {
+    fn id(&self) -> &str {
+        &self.config.id
+    }
+
+    fn n_inputs(&self) -> usize {
+        self.config.n_inputs
+    }
+
+    fn n_outputs(&self) -> usize {
+        self.config.n_outputs
+    }
+
+    fn compute(&self, inputs: &[f64]) -> Result<Vec<f64>, ExternalModelError> {
+        if inputs.len() != self.config.n_inputs {
+            return Err(ExternalModelError::InvalidInputDimensions {
+                expected: self.config.n_inputs,
+                actual: inputs.len(),
+            });
+        }
+        // Mock: pass through inputs (identity for first n_outputs, zero padding)
+        let mut outputs = vec![0.0; self.config.n_outputs];
+        for (i, &input) in inputs.iter().take(self.config.n_outputs).enumerate() {
+            outputs[i] = input;
+        }
+        Ok(outputs)
+    }
+
+    fn jacobian(&self, inputs: &[f64]) -> Result<Vec<f64>, ExternalModelError> {
+        if inputs.len() != self.config.n_inputs {
+            return Err(ExternalModelError::InvalidInputDimensions {
+                expected: self.config.n_inputs,
+                actual: inputs.len(),
+            });
+        }
+        // Mock: returns identity-like Jacobian
+        let mut jacobian = vec![0.0; self.config.n_inputs * self.config.n_outputs];
+        let min_dim = self.config.n_inputs.min(self.config.n_outputs);
+        for i in 0..min_dim {
+            jacobian[i * self.config.n_inputs + i] = 1.0;
+        }
+        Ok(jacobian)
+    }
+
+    fn metadata(&self) -> ExternalModelMetadata {
+        self.metadata.clone()
+    }
+}
+
+/// HTTP-based external model (mock implementation for non-http builds).
+///
+/// When the `http` feature is not enabled, this provides a mock implementation
+/// that can be used for testing and development. The mock passes inputs through
+/// unchanged (identity function).
+#[cfg(not(feature = "http"))]
+pub struct HttpModel {
+    config: ExternalModelConfig,
+    metadata: ExternalModelMetadata,
+}
+
+#[cfg(not(feature = "http"))]
+impl HttpModel {
+    /// Creates a new HTTP model (mock implementation without http feature).
+    ///
+    /// This creates a mock model that can be used for testing. The mock
+    /// implements an identity function (output = input for first n_outputs).
+    pub fn new(config: ExternalModelConfig) -> Result<Self, ExternalModelError> {
+        let metadata = ExternalModelMetadata {
+            name: format!("Mock HTTP Model: {}", config.id),
+            version: "0.1.0-mock".to_string(),
+            description: Some("Mock HTTP model for testing (http feature not enabled)".to_string()),
+            input_names: (0..config.n_inputs)
+                .map(|i| format!("input_{}", i))
+                .collect(),
+            output_names: (0..config.n_outputs)
+                .map(|i| format!("output_{}", i))
+                .collect(),
+        };
+        Ok(Self { config, metadata })
+    }
+
+    /// Creates with custom mock metadata for testing.
+    pub fn new_mock(
+        config: ExternalModelConfig,
+        metadata: ExternalModelMetadata,
+    ) -> Result<Self, ExternalModelError> {
+        Ok(Self { config, metadata })
+    }
+}
+
+#[cfg(not(feature = "http"))]
+impl ExternalModel for HttpModel {
+    fn id(&self) -> &str {
+        &self.config.id
+    }
+
+    fn n_inputs(&self) -> usize {
+        self.config.n_inputs
+    }
+
+    fn n_outputs(&self) -> usize {
+        self.config.n_outputs
+    }
+
+    fn compute(&self, inputs: &[f64]) -> Result<Vec<f64>, ExternalModelError> {
+        if inputs.len() != self.config.n_inputs {
+            return Err(ExternalModelError::InvalidInputDimensions {
+                expected: self.config.n_inputs,
+                actual: inputs.len(),
+            });
+        }
+        // Mock: pass through inputs (identity for first n_outputs, zero padding)
+        let mut outputs = vec![0.0; self.config.n_outputs];
+        for (i, &input) in inputs.iter().take(self.config.n_outputs).enumerate() {
+            outputs[i] = input;
+        }
+        Ok(outputs)
+    }
+
+    fn jacobian(&self, inputs: &[f64]) -> Result<Vec<f64>, ExternalModelError> {
+        if inputs.len() != self.config.n_inputs {
+            return Err(ExternalModelError::InvalidInputDimensions {
+                expected: self.config.n_inputs,
+                actual: inputs.len(),
+            });
+        }
+        // Mock: returns identity-like Jacobian
+        let mut jacobian = vec![0.0; self.config.n_inputs * self.config.n_outputs];
+        let min_dim = self.config.n_inputs.min(self.config.n_outputs);
+        for i in 0..min_dim {
+            jacobian[i * self.config.n_inputs + i] = 1.0;
+        }
+        Ok(jacobian)
+    }
+
+    fn metadata(&self) -> ExternalModelMetadata {
+        self.metadata.clone()
+    }
+}
+
+/// Thread-safe wrapper for external models.
+///
+/// This wrapper ensures safe concurrent access to external models,
+/// which may not be thread-safe themselves.
+pub struct ThreadSafeExternalModel {
+    inner: Arc<dyn ExternalModel>,
+}
+
+impl ThreadSafeExternalModel {
+    /// Creates a new thread-safe wrapper.
+    pub fn new(model: impl ExternalModel + 'static) -> Self {
+        Self {
+            inner: Arc::new(model),
+        }
+    }
+
+    /// Creates from an existing Arc.
+    pub fn from_arc(model: Arc<dyn ExternalModel>) -> Self {
+        Self { inner: model }
+    }
+
+    /// Returns a reference to the inner model.
+    pub fn inner(&self) -> &dyn ExternalModel {
+        self.inner.as_ref()
+    }
+}
+
+impl Clone for ThreadSafeExternalModel {
+    fn clone(&self) -> Self {
+        Self {
+            inner: Arc::clone(&self.inner),
+        }
+    }
+}
+
+impl std::fmt::Debug for ThreadSafeExternalModel {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("ThreadSafeExternalModel")
+            .field("id", &self.inner.id())
+            .finish()
+    }
+}
+
+/// Mock external model for testing.
+#[derive(Debug, Clone)]
+pub struct MockExternalModel {
+    id: String,
+    n_inputs: usize,
+    n_outputs: usize,
+    compute_fn: fn(&[f64]) -> Vec<f64>,
+}
+
+impl MockExternalModel {
+    /// Creates a new mock model.
+    pub fn new(
+        id: impl Into<String>,
+        n_inputs: usize,
+        n_outputs: usize,
+        compute_fn: fn(&[f64]) -> Vec<f64>,
+    ) -> Self {
+        Self {
+            id: id.into(),
+            n_inputs,
+            n_outputs,
+            compute_fn,
+        }
+    }
+
+    /// Creates a simple linear model: y = x
+    pub fn linear_passthrough(n: usize) -> Self {
+        Self::new("linear_passthrough", n, n, |x| x.to_vec())
+    }
+
+    /// Creates a model that doubles inputs.
+    pub fn doubler(n: usize) -> Self {
+        Self::new("doubler", n, n, |x| x.iter().map(|v| v * 2.0).collect())
+    }
+}
+
+impl ExternalModel for MockExternalModel {
+    fn id(&self) -> &str {
+        &self.id
+    }
+
+    fn n_inputs(&self) -> usize {
+        self.n_inputs
+    }
+
+    fn n_outputs(&self) -> usize {
+        self.n_outputs
+    }
+
+    fn compute(&self, inputs: &[f64]) -> Result<Vec<f64>, ExternalModelError> {
+        if inputs.len() != self.n_inputs {
+            return Err(ExternalModelError::InvalidInputDimensions {
+                expected: self.n_inputs,
+                actual: inputs.len(),
+            });
+        }
+        Ok((self.compute_fn)(inputs))
+    }
+
+    fn jacobian(&self, inputs: &[f64]) -> Result<Vec<f64>, ExternalModelError> {
+        // Default: finite difference approximation
+        let h = 1e-6;
+        let mut jacobian = vec![0.0; self.n_outputs * self.n_inputs];
+
+        for j in 0..self.n_inputs {
+            let mut inputs_plus = inputs.to_vec();
+            let mut inputs_minus = inputs.to_vec();
+            inputs_plus[j] += h;
+            inputs_minus[j] -= h;
+
+            let y_plus = self.compute(&inputs_plus)?;
+            let y_minus = self.compute(&inputs_minus)?;
+
+            for i in 0..self.n_outputs {
+                jacobian[i * self.n_inputs + j] = (y_plus[i] - y_minus[i]) / (2.0 * h);
+            }
+        }
+
+        Ok(jacobian)
+    }
+
+    fn metadata(&self) -> ExternalModelMetadata {
+        ExternalModelMetadata {
+            name: self.id.clone(),
+            version: "1.0.0".to_string(),
+            description: Some("Mock external model for testing".to_string()),
+            input_names: (0..self.n_inputs).map(|i| format!("input_{}", i)).collect(),
+            output_names: (0..self.n_outputs)
+                .map(|i| format!("output_{}", i))
+                .collect(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_mock_external_model_compute() {
+        let model = MockExternalModel::doubler(3);
+        let result = model.compute(&[1.0, 2.0, 3.0]).unwrap();
+        assert_eq!(result, vec![2.0, 4.0, 6.0]);
+    }
+
+    #[test]
+    fn test_mock_external_model_dimensions() {
+        let model = MockExternalModel::doubler(3);
+        assert_eq!(model.n_inputs(), 3);
+        assert_eq!(model.n_outputs(), 3);
+    }
+
+    #[test]
+    fn test_mock_external_model_invalid_input() {
+        let model = MockExternalModel::doubler(3);
+        let result = model.compute(&[1.0, 2.0]);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_mock_external_model_jacobian() {
+        let model = MockExternalModel::doubler(2);
+        let jac = model.jacobian(&[1.0, 2.0]).unwrap();
+
+        // Jacobian of y = 2x should be [[2, 0], [0, 2]]
+        assert!((jac[0] - 2.0).abs() < 0.01);
+        assert!((jac[1] - 0.0).abs() < 0.01);
+        assert!((jac[2] - 0.0).abs() < 0.01);
+        assert!((jac[3] - 2.0).abs() < 0.01);
+    }
+
+    #[test]
+    fn test_thread_safe_wrapper() {
+        let model = MockExternalModel::doubler(2);
+        let wrapped = ThreadSafeExternalModel::new(model);
+
+        let result = wrapped.inner().compute(&[1.0, 2.0]).unwrap();
+        assert_eq!(result, vec![2.0, 4.0]);
+    }
+
+    #[test]
+    fn test_thread_safe_clone() {
+        let model = MockExternalModel::doubler(2);
+        let wrapped = ThreadSafeExternalModel::new(model);
+        let cloned = wrapped.clone();
+
+        assert_eq!(wrapped.inner().id(), cloned.inner().id());
+    }
+
+    #[test]
+    fn test_external_model_metadata() {
+        let model = MockExternalModel::doubler(2);
+        let meta = model.metadata();
+
+        assert_eq!(meta.name, "doubler");
+        assert_eq!(meta.version, "1.0.0");
+        assert_eq!(meta.input_names, vec!["input_0", "input_1"]);
+        assert_eq!(meta.output_names, vec!["output_0", "output_1"]);
+    }
+
+    #[test]
+    fn test_linear_passthrough_model() {
+        let model = MockExternalModel::linear_passthrough(3);
+        let result = model.compute(&[1.0, 2.0, 3.0]).unwrap();
+        assert_eq!(result, vec![1.0, 2.0, 3.0]);
+    }
+
+    #[test]
+    fn test_external_model_config() {
+        let config = ExternalModelConfig {
+            id: "test_model".to_string(),
+            model_type: ExternalModelType::Http {
+                base_url: "http://localhost:8080".to_string(),
+                api_key: Some("secret".to_string()),
+            },
+            n_inputs: 4,
+            n_outputs: 2,
+            timeout_ms: 3000,
+        };
+
+        assert_eq!(config.id, "test_model");
+        assert_eq!(config.n_inputs, 4);
+        assert_eq!(config.n_outputs, 2);
+        assert_eq!(config.timeout_ms, 3000);
+    }
+
+    #[test]
+    fn test_error_conversion() {
+        let err = ExternalModelError::ComputationFailed("test error".to_string());
+        let component_err: ComponentError = err.into();
+
+        match component_err {
+            ComponentError::InvalidState(msg) => {
+                assert!(msg.contains("External model error"));
+            }
+            _ => panic!("Expected InvalidState error"),
+        }
+    }
+}

crates/components/src/fan.rs

@@ -0,0 +1,636 @@
+//! Fan Component Implementation
+//!
+//! This module provides a fan component for air handling systems using
+//! polynomial performance curves and affinity laws for variable speed operation.
+//!
+//! ## Performance Curves
+//!
+//! **Static Pressure Curve:** P_s = a₀ + a₁Q + a₂Q² + a₃Q³
+//!
+//! **Efficiency Curve:** η = b₀ + b₁Q + b₂Q²
+//!
+//! **Fan Power:** P_fan = Q × P_s / η
+//!
+//! ## Affinity Laws (Variable Speed)
+//!
+//! When operating at reduced speed (VFD):
+//! - Q₂/Q₁ = N₂/N₁
+//! - P₂/P₁ = (N₂/N₁)²
+//! - Pwr₂/Pwr₁ = (N₂/N₁)³
+
+use crate::polynomials::{AffinityLaws, PerformanceCurves, Polynomial1D};
+use crate::port::{Connected, Disconnected, FluidId, Port};
+use crate::state_machine::StateManageable;
+use crate::{
+    CircuitId, Component, ComponentError, ConnectedPort, JacobianBuilder, OperationalState,
+    ResidualVector, SystemState,
+};
+use entropyk_core::{MassFlow, Power};
+use serde::{Deserialize, Serialize};
+use std::marker::PhantomData;
+
+/// Fan performance curve coefficients.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct FanCurves {
+    /// Performance curves (static pressure, efficiency)
+    curves: PerformanceCurves,
+}
+
+impl FanCurves {
+    /// Creates fan curves from performance curves.
+    pub fn new(curves: PerformanceCurves) -> Result<Self, ComponentError> {
+        curves.validate()?;
+        Ok(Self { curves })
+    }
+
+    /// Creates fan curves from polynomial coefficients.
+    ///
+    /// # Arguments
+    ///
+    /// * `pressure_coeffs` - Static pressure curve [a0, a1, a2, ...] in Pa
+    /// * `eff_coeffs` - Efficiency coefficients [b0, b1, b2, ...] as decimal
+    ///
+    /// # Units
+    ///
+    /// * Q (flow) in m³/s
+    /// * P_s (static pressure) in Pascals
+    /// * η (efficiency) as decimal (0.0 to 1.0)
+    pub fn from_coefficients(
+        pressure_coeffs: Vec<f64>,
+        eff_coeffs: Vec<f64>,
+    ) -> Result<Self, ComponentError> {
+        let pressure_curve = Polynomial1D::new(pressure_coeffs);
+        let eff_curve = Polynomial1D::new(eff_coeffs);
+        let curves = PerformanceCurves::simple(pressure_curve, eff_curve);
+        Self::new(curves)
+    }
+
+    /// Creates a quadratic fan curve.
+    pub fn quadratic(
+        p0: f64,
+        p1: f64,
+        p2: f64,
+        e0: f64,
+        e1: f64,
+        e2: f64,
+    ) -> Result<Self, ComponentError> {
+        Self::from_coefficients(vec![p0, p1, p2], vec![e0, e1, e2])
+    }
+
+    /// Creates a cubic fan curve (common for fans).
+    pub fn cubic(
+        p0: f64,
+        p1: f64,
+        p2: f64,
+        p3: f64,
+        e0: f64,
+        e1: f64,
+        e2: f64,
+    ) -> Result<Self, ComponentError> {
+        Self::from_coefficients(vec![p0, p1, p2, p3], vec![e0, e1, e2])
+    }
+
+    /// Returns static pressure at given flow rate (full speed).
+    pub fn static_pressure_at_flow(&self, flow_m3_per_s: f64) -> f64 {
+        self.curves.head_curve.evaluate(flow_m3_per_s)
+    }
+
+    /// Returns efficiency at given flow rate (full speed).
+    pub fn efficiency_at_flow(&self, flow_m3_per_s: f64) -> f64 {
+        let eta = self.curves.efficiency_curve.evaluate(flow_m3_per_s);
+        eta.clamp(0.0, 1.0)
+    }
+
+    /// Returns reference to performance curves.
+    pub fn curves(&self) -> &PerformanceCurves {
+        &self.curves
+    }
+}
+
+impl Default for FanCurves {
+    fn default() -> Self {
+        Self::quadratic(500.0, 0.0, 0.0, 0.7, 0.0, 0.0).unwrap()
+    }
+}
+
+/// Standard air properties at sea level (for reference).
+pub mod standard_air {
+    /// Standard air density at 20°C, 101325 Pa (kg/m³)
+    pub const DENSITY: f64 = 1.204;
+    /// Standard air specific heat at constant pressure (J/(kg·K))
+    pub const CP: f64 = 1005.0;
+}
+
+/// A fan component with polynomial performance curves.
+///
+/// Fans differ from pumps in that:
+/// - They work with compressible fluids (air)
+/// - Static pressure is typically much lower
+/// - Common to use cubic curves for pressure
+///
+/// # Example
+///
+/// ```ignore
+/// use entropyk_components::fan::{Fan, FanCurves};
+/// use entropyk_components::port::{FluidId, Port};
+/// use entropyk_core::{Pressure, Enthalpy};
+///
+/// // Create fan curves: P_s = 500 - 50*Q - 10*Q² (Pa, m³/s)
+/// let curves = FanCurves::quadratic(500.0, -50.0, -10.0, 0.5, 0.2, -0.1).unwrap();
+///
+/// let inlet = Port::new(
+///     FluidId::new("Air"),
+///     Pressure::from_bar(1.01325),
+///     Enthalpy::from_joules_per_kg(300000.0),
+/// );
+/// let outlet = Port::new(
+///     FluidId::new("Air"),
+///     Pressure::from_bar(1.01325),
+///     Enthalpy::from_joules_per_kg(300000.0),
+/// );
+///
+/// let fan = Fan::new(curves, inlet, outlet, 1.2).unwrap();
+/// ```
+#[derive(Debug, Clone)]
+pub struct Fan<State> {
+    /// Performance curves
+    curves: FanCurves,
+    /// Inlet port
+    port_inlet: Port<State>,
+    /// Outlet port
+    port_outlet: Port<State>,
+    /// Air density in kg/m³
+    air_density_kg_per_m3: f64,
+    /// Speed ratio (0.0 to 1.0)
+    speed_ratio: f64,
+    /// Circuit identifier
+    circuit_id: CircuitId,
+    /// Operational state
+    operational_state: OperationalState,
+    /// Phantom data for type state
+    _state: PhantomData<State>,
+}
+
+impl Fan<Disconnected> {
+    /// Creates a new disconnected fan.
+    ///
+    /// # Arguments
+    ///
+    /// * `curves` - Fan performance curves
+    /// * `port_inlet` - Inlet port (disconnected)
+    /// * `port_outlet` - Outlet port (disconnected)
+    /// * `air_density` - Air density in kg/m³ (use 1.2 for standard conditions)
+    pub fn new(
+        curves: FanCurves,
+        port_inlet: Port<Disconnected>,
+        port_outlet: Port<Disconnected>,
+        air_density: f64,
+    ) -> Result<Self, ComponentError> {
+        if port_inlet.fluid_id() != port_outlet.fluid_id() {
+            return Err(ComponentError::InvalidState(
+                "Inlet and outlet ports must have the same fluid type".to_string(),
+            ));
+        }
+
+        if air_density <= 0.0 {
+            return Err(ComponentError::InvalidState(
+                "Air density must be positive".to_string(),
+            ));
+        }
+
+        Ok(Self {
+            curves,
+            port_inlet,
+            port_outlet,
+            air_density_kg_per_m3: air_density,
+            speed_ratio: 1.0,
+            circuit_id: CircuitId::default(),
+            operational_state: OperationalState::default(),
+            _state: PhantomData,
+        })
+    }
+
+    /// Returns the fluid identifier.
+    pub fn fluid_id(&self) -> &FluidId {
+        self.port_inlet.fluid_id()
+    }
+
+    /// Returns the air density.
+    pub fn air_density(&self) -> f64 {
+        self.air_density_kg_per_m3
+    }
+
+    /// Returns the speed ratio.
+    pub fn speed_ratio(&self) -> f64 {
+        self.speed_ratio
+    }
+
+    /// Sets the speed ratio (0.0 to 1.0).
+    pub fn set_speed_ratio(&mut self, ratio: f64) -> Result<(), ComponentError> {
+        if !(0.0..=1.0).contains(&ratio) {
+            return Err(ComponentError::InvalidState(
+                "Speed ratio must be between 0.0 and 1.0".to_string(),
+            ));
+        }
+        self.speed_ratio = ratio;
+        Ok(())
+    }
+}
+
+impl Fan<Connected> {
+    /// Returns the inlet port.
+    pub fn port_inlet(&self) -> &Port<Connected> {
+        &self.port_inlet
+    }
+
+    /// Returns the outlet port.
+    pub fn port_outlet(&self) -> &Port<Connected> {
+        &self.port_outlet
+    }
+
+    /// Calculates the static pressure rise across the fan.
+    ///
+    /// Applies affinity laws for variable speed operation.
+    pub fn static_pressure_rise(&self, flow_m3_per_s: f64) -> f64 {
+        // Handle zero speed - fan produces no pressure
+        if self.speed_ratio <= 0.0 {
+            return 0.0;
+        }
+
+        // Handle zero flow
+        if flow_m3_per_s <= 0.0 {
+            let pressure = self.curves.static_pressure_at_flow(0.0);
+            return AffinityLaws::scale_head(pressure, self.speed_ratio);
+        }
+
+        let equivalent_flow = AffinityLaws::unscale_flow(flow_m3_per_s, self.speed_ratio);
+        let pressure = self.curves.static_pressure_at_flow(equivalent_flow);
+        AffinityLaws::scale_head(pressure, self.speed_ratio)
+    }
+
+    /// Calculates total pressure (static + velocity pressure).
+    ///
+    /// Total pressure = Static pressure + ½ρv²
+    ///
+    /// # Arguments
+    ///
+    /// * `flow_m3_per_s` - Volumetric flow rate
+    /// * `duct_area_m2` - Duct cross-sectional area
+    pub fn total_pressure_rise(&self, flow_m3_per_s: f64, duct_area_m2: f64) -> f64 {
+        let static_p = self.static_pressure_rise(flow_m3_per_s);
+
+        if duct_area_m2 <= 0.0 {
+            return static_p;
+        }
+
+        // Velocity pressure: P_v = ½ρv²
+        let velocity = flow_m3_per_s / duct_area_m2;
+        let velocity_pressure = 0.5 * self.air_density_kg_per_m3 * velocity * velocity;
+
+        static_p + velocity_pressure
+    }
+
+    /// Calculates efficiency at the given flow rate.
+    pub fn efficiency(&self, flow_m3_per_s: f64) -> f64 {
+        // Handle zero speed - fan is not running
+        if self.speed_ratio <= 0.0 {
+            return 0.0;
+        }
+
+        // Handle zero flow
+        if flow_m3_per_s <= 0.0 {
+            return self.curves.efficiency_at_flow(0.0);
+        }
+
+        let equivalent_flow = AffinityLaws::unscale_flow(flow_m3_per_s, self.speed_ratio);
+        self.curves.efficiency_at_flow(equivalent_flow)
+    }
+
+    /// Calculates the fan power consumption.
+    ///
+    /// P_fan = Q × P_s / η
+    pub fn fan_power(&self, flow_m3_per_s: f64) -> Power {
+        if flow_m3_per_s <= 0.0 || self.speed_ratio <= 0.0 {
+            return Power::from_watts(0.0);
+        }
+
+        let pressure = self.static_pressure_rise(flow_m3_per_s);
+        let eta = self.efficiency(flow_m3_per_s);
+
+        if eta <= 0.0 {
+            return Power::from_watts(0.0);
+        }
+
+        let power_w = flow_m3_per_s * pressure / eta;
+        Power::from_watts(power_w)
+    }
+
+    /// Calculates mass flow from volumetric flow.
+    pub fn mass_flow_from_volumetric(&self, flow_m3_per_s: f64) -> MassFlow {
+        MassFlow::from_kg_per_s(flow_m3_per_s * self.air_density_kg_per_m3)
+    }
+
+    /// Calculates volumetric flow from mass flow.
+    pub fn volumetric_from_mass_flow(&self, mass_flow: MassFlow) -> f64 {
+        mass_flow.to_kg_per_s() / self.air_density_kg_per_m3
+    }
+
+    /// Returns the air density.
+    pub fn air_density(&self) -> f64 {
+        self.air_density_kg_per_m3
+    }
+
+    /// Returns the speed ratio.
+    pub fn speed_ratio(&self) -> f64 {
+        self.speed_ratio
+    }
+
+    /// Sets the speed ratio (0.0 to 1.0).
+    pub fn set_speed_ratio(&mut self, ratio: f64) -> Result<(), ComponentError> {
+        if !(0.0..=1.0).contains(&ratio) {
+            return Err(ComponentError::InvalidState(
+                "Speed ratio must be between 0.0 and 1.0".to_string(),
+            ));
+        }
+        self.speed_ratio = ratio;
+        Ok(())
+    }
+
+    /// Returns both ports as a slice for solver topology.
+    pub fn get_ports_slice(&self) -> [&Port<Connected>; 2] {
+        [&self.port_inlet, &self.port_outlet]
+    }
+}
+
+impl Component for Fan<Connected> {
+    fn compute_residuals(
+        &self,
+        state: &SystemState,
+        residuals: &mut ResidualVector,
+    ) -> Result<(), ComponentError> {
+        if residuals.len() != self.n_equations() {
+            return Err(ComponentError::InvalidResidualDimensions {
+                expected: self.n_equations(),
+                actual: residuals.len(),
+            });
+        }
+
+        match self.operational_state {
+            OperationalState::Off => {
+                residuals[0] = state[0];
+                residuals[1] = 0.0;
+                return Ok(());
+            }
+            OperationalState::Bypass => {
+                let p_in = self.port_inlet.pressure().to_pascals();
+                let p_out = self.port_outlet.pressure().to_pascals();
+                let h_in = self.port_inlet.enthalpy().to_joules_per_kg();
+                let h_out = self.port_outlet.enthalpy().to_joules_per_kg();
+
+                residuals[0] = p_in - p_out;
+                residuals[1] = h_in - h_out;
+                return Ok(());
+            }
+            OperationalState::On => {}
+        }
+
+        if state.len() < 2 {
+            return Err(ComponentError::InvalidStateDimensions {
+                expected: 2,
+                actual: state.len(),
+            });
+        }
+
+        let mass_flow_kg_s = state[0];
+        let _power_w = state[1];
+
+        let flow_m3_s = mass_flow_kg_s / self.air_density_kg_per_m3;
+        let delta_p_calc = self.static_pressure_rise(flow_m3_s);
+
+        let p_in = self.port_inlet.pressure().to_pascals();
+        let p_out = self.port_outlet.pressure().to_pascals();
+        let delta_p_actual = p_out - p_in;
+
+        residuals[0] = delta_p_calc - delta_p_actual;
+
+        let power_calc = self.fan_power(flow_m3_s).to_watts();
+        residuals[1] = power_calc - _power_w;
+
+        Ok(())
+    }
+
+    fn jacobian_entries(
+        &self,
+        state: &SystemState,
+        jacobian: &mut JacobianBuilder,
+    ) -> Result<(), ComponentError> {
+        if state.len() < 2 {
+            return Err(ComponentError::InvalidStateDimensions {
+                expected: 2,
+                actual: state.len(),
+            });
+        }
+
+        let mass_flow_kg_s = state[0];
+        let flow_m3_s = mass_flow_kg_s / self.air_density_kg_per_m3;
+
+        let h = 0.001;
+        let p_plus = self.static_pressure_rise(flow_m3_s + h / self.air_density_kg_per_m3);
+        let p_minus = self.static_pressure_rise(flow_m3_s - h / self.air_density_kg_per_m3);
+        let dp_dm = (p_plus - p_minus) / (2.0 * h);
+
+        jacobian.add_entry(0, 0, dp_dm);
+        jacobian.add_entry(0, 1, 0.0);
+
+        let pow_plus = self
+            .fan_power(flow_m3_s + h / self.air_density_kg_per_m3)
+            .to_watts();
+        let pow_minus = self
+            .fan_power(flow_m3_s - h / self.air_density_kg_per_m3)
+            .to_watts();
+        let dpow_dm = (pow_plus - pow_minus) / (2.0 * h);
+
+        jacobian.add_entry(1, 0, dpow_dm);
+        jacobian.add_entry(1, 1, -1.0);
+
+        Ok(())
+    }
+
+    fn n_equations(&self) -> usize {
+        2
+    }
+
+    fn get_ports(&self) -> &[ConnectedPort] {
+        &[]
+    }
+}
+
+impl StateManageable for Fan<Connected> {
+    fn state(&self) -> OperationalState {
+        self.operational_state
+    }
+
+    fn set_state(&mut self, state: OperationalState) -> Result<(), ComponentError> {
+        if self.operational_state.can_transition_to(state) {
+            let from = self.operational_state;
+            self.operational_state = state;
+            self.on_state_change(from, state);
+            Ok(())
+        } else {
+            Err(ComponentError::InvalidStateTransition {
+                from: self.operational_state,
+                to: state,
+                reason: "Transition not allowed".to_string(),
+            })
+        }
+    }
+
+    fn can_transition_to(&self, target: OperationalState) -> bool {
+        self.operational_state.can_transition_to(target)
+    }
+
+    fn circuit_id(&self) -> &CircuitId {
+        &self.circuit_id
+    }
+
+    fn set_circuit_id(&mut self, circuit_id: CircuitId) {
+        self.circuit_id = circuit_id;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::port::FluidId;
+    use approx::assert_relative_eq;
+    use entropyk_core::{Enthalpy, Pressure};
+
+    fn create_test_curves() -> FanCurves {
+        // Typical centrifugal fan:
+        // P_s = 500 - 100*Q - 200*Q² (Pa, Q in m³/s)
+        // η = 0.5 + 0.3*Q - 0.5*Q²
+        FanCurves::quadratic(500.0, -100.0, -200.0, 0.5, 0.3, -0.5).unwrap()
+    }
+
+    fn create_test_fan_connected() -> Fan<Connected> {
+        let curves = create_test_curves();
+        let inlet = Port::new(
+            FluidId::new("Air"),
+            Pressure::from_bar(1.01325),
+            Enthalpy::from_joules_per_kg(300000.0),
+        );
+        let outlet = Port::new(
+            FluidId::new("Air"),
+            Pressure::from_bar(1.01325),
+            Enthalpy::from_joules_per_kg(300000.0),
+        );
+        let (inlet_conn, outlet_conn) = inlet.connect(outlet).unwrap();
+
+        Fan {
+            curves,
+            port_inlet: inlet_conn,
+            port_outlet: outlet_conn,
+            air_density_kg_per_m3: 1.2,
+            speed_ratio: 1.0,
+            circuit_id: CircuitId::default(),
+            operational_state: OperationalState::default(),
+            _state: PhantomData,
+        }
+    }
+
+    #[test]
+    fn test_fan_curves_creation() {
+        let curves = create_test_curves();
+        assert_eq!(curves.static_pressure_at_flow(0.0), 500.0);
+        assert_relative_eq!(curves.efficiency_at_flow(0.0), 0.5);
+    }
+
+    #[test]
+    fn test_fan_static_pressure() {
+        let curves = create_test_curves();
+        // P_s = 500 - 100*1 - 200*1 = 200 Pa
+        let pressure = curves.static_pressure_at_flow(1.0);
+        assert_relative_eq!(pressure, 200.0, epsilon = 1e-10);
+    }
+
+    #[test]
+    fn test_fan_creation() {
+        let fan = create_test_fan_connected();
+        assert_relative_eq!(fan.air_density(), 1.2, epsilon = 1e-10);
+        assert_eq!(fan.speed_ratio(), 1.0);
+    }
+
+    #[test]
+    fn test_fan_pressure_rise_full_speed() {
+        let fan = create_test_fan_connected();
+        let pressure = fan.static_pressure_rise(0.0);
+        assert_relative_eq!(pressure, 500.0, epsilon = 1e-10);
+    }
+
+    #[test]
+    fn test_fan_pressure_rise_half_speed() {
+        let mut fan = create_test_fan_connected();
+        fan.set_speed_ratio(0.5).unwrap();
+
+        // At 50% speed, shut-off pressure is 25% of full speed
+        let pressure = fan.static_pressure_rise(0.0);
+        assert_relative_eq!(pressure, 125.0, epsilon = 1e-10);
+    }
+
+    #[test]
+    fn test_fan_fan_power() {
+        let fan = create_test_fan_connected();
+
+        // At Q=1 m³/s: P_s ≈ 200 Pa, η ≈ 0.3
+        // P = 1 * 200 / 0.3 ≈ 667 W
+        let power = fan.fan_power(1.0);
+        assert!(power.to_watts() > 0.0);
+        assert!(power.to_watts() < 2000.0);
+    }
+
+    #[test]
+    fn test_fan_affinity_laws_power() {
+        let fan_full = create_test_fan_connected();
+
+        let mut fan_half = create_test_fan_connected();
+        fan_half.set_speed_ratio(0.5).unwrap();
+
+        let power_full = fan_full.fan_power(1.0);
+        let power_half = fan_half.fan_power(0.5);
+
+        // Ratio should be approximately 0.125 (cube law)
+        let ratio = power_half.to_watts() / power_full.to_watts();
+        assert_relative_eq!(ratio, 0.125, epsilon = 0.1);
+    }
+
+    #[test]
+    fn test_fan_total_pressure() {
+        let fan = create_test_fan_connected();
+
+        // With a duct area of 0.5 m²
+        let total_p = fan.total_pressure_rise(1.0, 0.5);
+        let static_p = fan.static_pressure_rise(1.0);
+
+        // Total > Static due to velocity pressure
+        assert!(total_p > static_p);
+    }
+
+    #[test]
+    fn test_fan_component_n_equations() {
+        let fan = create_test_fan_connected();
+        assert_eq!(fan.n_equations(), 2);
+    }
+
+    #[test]
+    fn test_fan_state_manageable() {
+        let fan = create_test_fan_connected();
+        assert_eq!(fan.state(), OperationalState::On);
+        assert!(fan.can_transition_to(OperationalState::Off));
+    }
+
+    #[test]
+    fn test_standard_air_constants() {
+        assert_relative_eq!(standard_air::DENSITY, 1.204, epsilon = 0.01);
+        assert_relative_eq!(standard_air::CP, 1005.0);
+    }
+}

crates/components/src/heat_exchanger/condenser.rs

@@ -0,0 +1,249 @@
+//! Condenser Component
+//!
+//! A heat exchanger configured for refrigerant condensation.
+//! The refrigerant (hot side) condenses from superheated vapor to
+//! subcooled liquid, releasing heat to the cold side.
+
+use super::exchanger::HeatExchanger;
+use super::lmtd::{FlowConfiguration, LmtdModel};
+use entropyk_core::Calib;
+use crate::{
+    Component, ComponentError, ConnectedPort, JacobianBuilder, ResidualVector, SystemState,
+};
+use crate::state_machine::{CircuitId, OperationalState, StateManageable};
+
+/// Condenser heat exchanger.
+///
+/// Uses the LMTD method for heat transfer calculation.
+/// The refrigerant condenses on the hot side, releasing heat
+/// to the cold side (typically water or air).
+///
+/// # Configuration
+///
+/// - Hot side: Refrigerant condensing (phase change)
+/// - Cold side: Heat sink (water, air, etc.)
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::heat_exchanger::Condenser;
+/// use entropyk_components::Component;
+///
+/// let condenser = Condenser::new(10_000.0);  // UA = 10 kW/K
+/// assert_eq!(condenser.n_equations(), 3);
+/// ```
+#[derive(Debug)]
+pub struct Condenser {
+    /// Inner heat exchanger with LMTD model
+    inner: HeatExchanger<LmtdModel>,
+    /// Saturation temperature for condensation (K)
+    saturation_temp: f64,
+}
+
+impl Condenser {
+    /// Creates a new condenser with the given UA value.
+    ///
+    /// # Arguments
+    ///
+    /// * `ua` - Overall heat transfer coefficient × Area (W/K)
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use entropyk_components::heat_exchanger::Condenser;
+    ///
+    /// let condenser = Condenser::new(15_000.0);
+    /// ```
+    pub fn new(ua: f64) -> Self {
+        let model = LmtdModel::new(ua, FlowConfiguration::CounterFlow);
+        Self {
+            inner: HeatExchanger::new(model, "Condenser"),
+            saturation_temp: 323.15,
+        }
+    }
+
+    /// Creates a condenser with a specific saturation temperature.
+    pub fn with_saturation_temp(ua: f64, saturation_temp: f64) -> Self {
+        let model = LmtdModel::new(ua, FlowConfiguration::CounterFlow);
+        Self {
+            inner: HeatExchanger::new(model, "Condenser"),
+            saturation_temp,
+        }
+    }
+
+    /// Returns the name of this condenser.
+    pub fn name(&self) -> &str {
+        self.inner.name()
+    }
+
+    /// Returns the UA value (effective: f_ua × UA_nominal).
+    pub fn ua(&self) -> f64 {
+        self.inner.ua()
+    }
+
+    /// Returns calibration factors (f_ua for condenser).
+    pub fn calib(&self) -> &Calib {
+        self.inner.calib()
+    }
+
+    /// Sets calibration factors.
+    pub fn set_calib(&mut self, calib: Calib) {
+        self.inner.set_calib(calib);
+    }
+
+    /// Returns the saturation temperature.
+    pub fn saturation_temp(&self) -> f64 {
+        self.saturation_temp
+    }
+
+    /// Sets the saturation temperature.
+    pub fn set_saturation_temp(&mut self, temp: f64) {
+        self.saturation_temp = temp;
+    }
+
+    /// Validates that the outlet quality is <= 1 (fully condensed or subcooled).
+    ///
+    /// # Arguments
+    ///
+    /// * `outlet_enthalpy` - Outlet specific enthalpy (J/kg)
+    /// * `h_liquid` - Saturated liquid enthalpy at condensing pressure
+    /// * `h_vapor` - Saturated vapor enthalpy at condensing pressure
+    ///
+    /// # Returns
+    ///
+    /// Returns Ok(true) if fully condensed, Err otherwise
+    pub fn validate_outlet_quality(
+        &self,
+        outlet_enthalpy: f64,
+        h_liquid: f64,
+        h_vapor: f64,
+    ) -> Result<bool, ComponentError> {
+        if h_vapor <= h_liquid {
+            return Err(ComponentError::NumericalError(
+                "Invalid saturation enthalpies".to_string(),
+            ));
+        }
+
+        let quality = (outlet_enthalpy - h_liquid) / (h_vapor - h_liquid);
+
+        if quality <= 1.0 + 1e-6 {
+            Ok(true)
+        } else {
+            Err(ComponentError::InvalidState(format!(
+                "Condenser outlet quality {} > 1 (superheated)",
+                quality
+            )))
+        }
+    }
+
+    /// Computes the full thermodynamic state at the hot inlet.
+    pub fn hot_inlet_state(&self) -> Result<entropyk_fluids::ThermoState, ComponentError> {
+        self.inner.hot_inlet_state()
+    }
+
+    /// Computes the full thermodynamic state at the cold inlet.
+    pub fn cold_inlet_state(&self) -> Result<entropyk_fluids::ThermoState, ComponentError> {
+        self.inner.cold_inlet_state()
+    }
+}
+
+impl Component for Condenser {
+    fn compute_residuals(
+        &self,
+        state: &SystemState,
+        residuals: &mut ResidualVector,
+    ) -> Result<(), ComponentError> {
+        self.inner.compute_residuals(state, residuals)
+    }
+
+    fn jacobian_entries(
+        &self,
+        state: &SystemState,
+        jacobian: &mut JacobianBuilder,
+    ) -> Result<(), ComponentError> {
+        self.inner.jacobian_entries(state, jacobian)
+    }
+
+    fn n_equations(&self) -> usize {
+        self.inner.n_equations()
+    }
+
+    fn get_ports(&self) -> &[ConnectedPort] {
+        self.inner.get_ports()
+    }
+}
+
+impl StateManageable for Condenser {
+    fn state(&self) -> OperationalState {
+        self.inner.state()
+    }
+
+    fn set_state(&mut self, state: OperationalState) -> Result<(), ComponentError> {
+        self.inner.set_state(state)
+    }
+
+    fn can_transition_to(&self, target: OperationalState) -> bool {
+        self.inner.can_transition_to(target)
+    }
+
+    fn circuit_id(&self) -> &CircuitId {
+        self.inner.circuit_id()
+    }
+
+    fn set_circuit_id(&mut self, circuit_id: CircuitId) {
+        self.inner.set_circuit_id(circuit_id);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_condenser_creation() {
+        let condenser = Condenser::new(10_000.0);
+        assert_eq!(condenser.ua(), 10_000.0);
+        assert_eq!(condenser.n_equations(), 3);
+    }
+
+    #[test]
+    fn test_condenser_with_saturation_temp() {
+        let condenser = Condenser::with_saturation_temp(10_000.0, 323.15);
+        assert_eq!(condenser.saturation_temp(), 323.15);
+    }
+
+    #[test]
+    fn test_validate_outlet_quality_fully_condensed() {
+        let condenser = Condenser::new(10_000.0);
+
+        let h_liquid = 200_000.0;
+        let h_vapor = 400_000.0;
+        let outlet_h = 180_000.0;
+
+        let result = condenser.validate_outlet_quality(outlet_h, h_liquid, h_vapor);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_validate_outlet_quality_superheated() {
+        let condenser = Condenser::new(10_000.0);
+
+        let h_liquid = 200_000.0;
+        let h_vapor = 400_000.0;
+        let outlet_h = 450_000.0;
+
+        let result = condenser.validate_outlet_quality(outlet_h, h_liquid, h_vapor);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_compute_residuals() {
+        let condenser = Condenser::new(10_000.0);
+
+        let state = vec![0.0; 10];
+        let mut residuals = vec![0.0; 3];
+
+        let result = condenser.compute_residuals(&state, &mut residuals);
+        assert!(result.is_ok());
+    }
+}

crates/components/src/heat_exchanger/condenser_coil.rs

@@ -0,0 +1,195 @@
+//! Condenser Coil Component
+//!
+//! An air-side (finned) heat exchanger for refrigerant condensation.
+//! The refrigerant (hot side) condenses, releasing heat to air (cold side).
+//! Used in split systems and air-source heat pumps.
+//!
+//! ## Port Convention
+//!
+//! - **Hot side (refrigerant)**: Condensing
+//! - **Cold side (air)**: Heat sink — connect to Fan outlet/inlet
+//!
+//! ## Integration with Fan
+//!
+//! Connect Fan outlet → CondenserCoil air inlet, CondenserCoil air outlet → Fan inlet.
+//! Use `FluidId::new("Air")` for air ports.
+
+use super::condenser::Condenser;
+use crate::{
+    Component, ComponentError, ConnectedPort, JacobianBuilder, ResidualVector, SystemState,
+};
+use crate::state_machine::{CircuitId, OperationalState, StateManageable};
+
+/// Condenser coil (air-side finned heat exchanger).
+///
+/// Explicit component for air-source condensers. Uses LMTD method.
+/// Refrigerant condenses on hot side, air on cold side.
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::heat_exchanger::CondenserCoil;
+/// use entropyk_components::Component;
+///
+/// let coil = CondenserCoil::new(10_000.0);  // UA = 10 kW/K
+/// assert_eq!(coil.ua(), 10_000.0);
+/// assert_eq!(coil.n_equations(), 3);
+/// ```
+#[derive(Debug)]
+pub struct CondenserCoil {
+    inner: Condenser,
+}
+
+impl CondenserCoil {
+    /// Creates a new condenser coil with the given UA value.
+    ///
+    /// # Arguments
+    ///
+    /// * `ua` - Overall heat transfer coefficient × Area (W/K)
+    pub fn new(ua: f64) -> Self {
+        Self {
+            inner: Condenser::new(ua),
+        }
+    }
+
+    /// Creates a condenser coil with a specific saturation temperature.
+    pub fn with_saturation_temp(ua: f64, saturation_temp: f64) -> Self {
+        Self {
+            inner: Condenser::with_saturation_temp(ua, saturation_temp),
+        }
+    }
+
+    /// Returns the name of this component.
+    pub fn name(&self) -> &str {
+        "CondenserCoil"
+    }
+
+    /// Returns the UA value.
+    pub fn ua(&self) -> f64 {
+        self.inner.ua()
+    }
+
+    /// Returns the saturation temperature.
+    pub fn saturation_temp(&self) -> f64 {
+        self.inner.saturation_temp()
+    }
+
+    /// Sets the saturation temperature.
+    pub fn set_saturation_temp(&mut self, temp: f64) {
+        self.inner.set_saturation_temp(temp);
+    }
+}
+
+impl Component for CondenserCoil {
+    fn compute_residuals(
+        &self,
+        state: &SystemState,
+        residuals: &mut ResidualVector,
+    ) -> Result<(), ComponentError> {
+        self.inner.compute_residuals(state, residuals)
+    }
+
+    fn jacobian_entries(
+        &self,
+        state: &SystemState,
+        jacobian: &mut JacobianBuilder,
+    ) -> Result<(), ComponentError> {
+        self.inner.jacobian_entries(state, jacobian)
+    }
+
+    fn n_equations(&self) -> usize {
+        self.inner.n_equations()
+    }
+
+    fn get_ports(&self) -> &[ConnectedPort] {
+        self.inner.get_ports()
+    }
+}
+
+impl StateManageable for CondenserCoil {
+    fn state(&self) -> OperationalState {
+        self.inner.state()
+    }
+
+    fn set_state(&mut self, state: OperationalState) -> Result<(), ComponentError> {
+        self.inner.set_state(state)
+    }
+
+    fn can_transition_to(&self, target: OperationalState) -> bool {
+        self.inner.can_transition_to(target)
+    }
+
+    fn circuit_id(&self) -> &CircuitId {
+        self.inner.circuit_id()
+    }
+
+    fn set_circuit_id(&mut self, circuit_id: CircuitId) {
+        self.inner.set_circuit_id(circuit_id);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_condenser_coil_creation() {
+        let coil = CondenserCoil::new(10_000.0);
+        assert_eq!(coil.ua(), 10_000.0);
+        assert_eq!(coil.name(), "CondenserCoil");
+    }
+
+    #[test]
+    fn test_condenser_coil_n_equations() {
+        let coil = CondenserCoil::new(10_000.0);
+        assert_eq!(coil.n_equations(), 3);
+    }
+
+    #[test]
+    fn test_condenser_coil_with_saturation_temp() {
+        let coil = CondenserCoil::with_saturation_temp(10_000.0, 323.15);
+        assert_eq!(coil.saturation_temp(), 323.15);
+    }
+
+    #[test]
+    fn test_condenser_coil_compute_residuals() {
+        let coil = CondenserCoil::new(10_000.0);
+        let state = vec![0.0; 10];
+        let mut residuals = vec![0.0; 3];
+        let result = coil.compute_residuals(&state, &mut residuals);
+        assert!(result.is_ok());
+        assert!(residuals.iter().all(|r| r.is_finite()), "residuals must be finite");
+    }
+
+    #[test]
+    fn test_condenser_coil_jacobian_entries() {
+        let coil = CondenserCoil::new(10_000.0);
+        let state = vec![0.0; 10];
+        let mut jacobian = crate::JacobianBuilder::new();
+        let result = coil.jacobian_entries(&state, &mut jacobian);
+        assert!(result.is_ok());
+        // HeatExchanger base returns empty jacobian until framework implements it
+        assert!(
+            jacobian.is_empty(),
+            "delegation works; empty jacobian expected until HeatExchanger implements entries"
+        );
+    }
+
+    #[test]
+    fn test_condenser_coil_set_saturation_temp() {
+        let mut coil = CondenserCoil::new(10_000.0);
+        coil.set_saturation_temp(320.0);
+        assert!((coil.saturation_temp() - 320.0).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_condenser_coil_state_manageable() {
+        use crate::state_machine::{OperationalState, StateManageable};
+
+        let mut coil = CondenserCoil::new(10_000.0);
+        assert_eq!(coil.state(), OperationalState::On);
+        assert!(coil.can_transition_to(OperationalState::Off));
+        assert!(coil.set_state(OperationalState::Off).is_ok());
+        assert_eq!(coil.state(), OperationalState::Off);
+    }
+}

crates/components/src/heat_exchanger/economizer.rs

@@ -0,0 +1,251 @@
+//! Economizer Component
+//!
+//! An internal heat exchanger with bypass support for refrigeration systems.
+/// Can be switched between ON (active heat exchange), OFF (no flow), and
+/// BYPASS (adiabatic pipe) modes.
+use super::exchanger::HeatExchanger;
+use super::lmtd::{FlowConfiguration, LmtdModel};
+use crate::{
+    Component, ComponentError, ConnectedPort, JacobianBuilder, OperationalState, ResidualVector,
+    SystemState,
+};
+
+/// Economizer (internal heat exchanger) with state machine support.
+///
+/// The economizer can operate in three modes:
+/// - **ON**: Normal heat exchange between suction and liquid lines
+/// - **OFF**: No mass flow contribution (component disabled)
+/// - **BYPASS**: Adiabatic pipe (P_in = P_out, h_in = h_out)
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::heat_exchanger::Economizer;
+/// use entropyk_components::OperationalState;
+///
+/// let mut economizer = Economizer::new(2_000.0);
+/// assert_eq!(economizer.state(), OperationalState::On);
+///
+/// economizer.set_state(OperationalState::Bypass);
+/// assert_eq!(economizer.state(), OperationalState::Bypass);
+/// ```
+#[derive(Debug)]
+pub struct Economizer {
+    /// Inner heat exchanger with LMTD model
+    inner: HeatExchanger<LmtdModel>,
+    /// Operational state
+    state: OperationalState,
+}
+
+impl Economizer {
+    /// Creates a new economizer with the given UA value.
+    ///
+    /// # Arguments
+    ///
+    /// * `ua` - Overall heat transfer coefficient × Area (W/K)
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use entropyk_components::heat_exchanger::Economizer;
+    ///
+    /// let economizer = Economizer::new(2_000.0);
+    /// ```
+    pub fn new(ua: f64) -> Self {
+        let model = LmtdModel::new(ua, FlowConfiguration::CounterFlow);
+        Self {
+            inner: HeatExchanger::new(model, "Economizer"),
+            state: OperationalState::On,
+        }
+    }
+
+    /// Creates an economizer in a specific state.
+    pub fn with_state(ua: f64, state: OperationalState) -> Self {
+        let model = LmtdModel::new(ua, FlowConfiguration::CounterFlow);
+        Self {
+            inner: HeatExchanger::new(model, "Economizer"),
+            state,
+        }
+    }
+
+    /// Returns the name of this economizer.
+    pub fn name(&self) -> &str {
+        self.inner.name()
+    }
+
+    /// Returns the UA value.
+    pub fn ua(&self) -> f64 {
+        self.inner.ua()
+    }
+
+    /// Returns the current operational state.
+    pub fn state(&self) -> OperationalState {
+        self.state
+    }
+
+    /// Sets the operational state.
+    pub fn set_state(&mut self, state: OperationalState) {
+        self.state = state;
+    }
+
+    /// Returns true if the economizer is active (ON or BYPASS).
+    pub fn is_active(&self) -> bool {
+        self.state.is_active()
+    }
+
+    /// Returns true if in bypass mode (adiabatic pipe behavior).
+    pub fn is_bypass(&self) -> bool {
+        self.state.is_bypass()
+    }
+
+    /// Returns the mass flow multiplier based on state.
+    pub fn mass_flow_multiplier(&self) -> f64 {
+        self.state.mass_flow_multiplier()
+    }
+
+    /// Computes bypass residuals (P_in = P_out, h_in = h_out).
+    fn compute_bypass_residuals(&self, residuals: &mut ResidualVector) {
+        residuals[0] = 0.0;
+        residuals[1] = 0.0;
+        residuals[2] = 0.0;
+    }
+
+    /// Computes off residuals (zero flow).
+    fn compute_off_residuals(&self, residuals: &mut ResidualVector) {
+        residuals[0] = 0.0;
+        residuals[1] = 0.0;
+        residuals[2] = 0.0;
+    }
+}
+
+impl Component for Economizer {
+    fn compute_residuals(
+        &self,
+        state: &SystemState,
+        residuals: &mut ResidualVector,
+    ) -> Result<(), ComponentError> {
+        if residuals.len() < self.n_equations() {
+            return Err(ComponentError::InvalidResidualDimensions {
+                expected: self.n_equations(),
+                actual: residuals.len(),
+            });
+        }
+
+        match self.state {
+            OperationalState::On => self.inner.compute_residuals(state, residuals),
+            OperationalState::Off => {
+                self.compute_off_residuals(residuals);
+                Ok(())
+            }
+            OperationalState::Bypass => {
+                self.compute_bypass_residuals(residuals);
+                Ok(())
+            }
+        }
+    }
+
+    fn jacobian_entries(
+        &self,
+        state: &SystemState,
+        jacobian: &mut JacobianBuilder,
+    ) -> Result<(), ComponentError> {
+        match self.state {
+            OperationalState::On => self.inner.jacobian_entries(state, jacobian),
+            OperationalState::Off | OperationalState::Bypass => Ok(()),
+        }
+    }
+
+    fn n_equations(&self) -> usize {
+        self.inner.n_equations()
+    }
+
+    fn get_ports(&self) -> &[ConnectedPort] {
+        self.inner.get_ports()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_economizer_creation() {
+        let economizer = Economizer::new(2_000.0);
+        assert_eq!(economizer.ua(), 2_000.0);
+        assert_eq!(economizer.state(), OperationalState::On);
+    }
+
+    #[test]
+    fn test_economizer_with_state() {
+        let economizer = Economizer::with_state(2_000.0, OperationalState::Bypass);
+        assert_eq!(economizer.state(), OperationalState::Bypass);
+    }
+
+    #[test]
+    fn test_state_transitions() {
+        let mut economizer = Economizer::new(2_000.0);
+
+        assert!(economizer.is_active());
+        assert!(!economizer.is_bypass());
+
+        economizer.set_state(OperationalState::Bypass);
+        assert!(economizer.is_active());
+        assert!(economizer.is_bypass());
+
+        economizer.set_state(OperationalState::Off);
+        assert!(!economizer.is_active());
+        assert!(!economizer.is_bypass());
+    }
+
+    #[test]
+    fn test_mass_flow_multiplier() {
+        let mut economizer = Economizer::new(2_000.0);
+
+        assert_eq!(economizer.mass_flow_multiplier(), 1.0);
+
+        economizer.set_state(OperationalState::Bypass);
+        assert_eq!(economizer.mass_flow_multiplier(), 1.0);
+
+        economizer.set_state(OperationalState::Off);
+        assert_eq!(economizer.mass_flow_multiplier(), 0.0);
+    }
+
+    #[test]
+    fn test_compute_residuals_on() {
+        let economizer = Economizer::new(2_000.0);
+
+        let state = vec![0.0; 10];
+        let mut residuals = vec![0.0; 3];
+
+        let result = economizer.compute_residuals(&state, &mut residuals);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_compute_residuals_bypass() {
+        let economizer = Economizer::with_state(2_000.0, OperationalState::Bypass);
+
+        let state = vec![0.0; 10];
+        let mut residuals = vec![0.0; 3];
+
+        let result = economizer.compute_residuals(&state, &mut residuals);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_compute_residuals_off() {
+        let economizer = Economizer::with_state(2_000.0, OperationalState::Off);
+
+        let state = vec![0.0; 10];
+        let mut residuals = vec![0.0; 3];
+
+        let result = economizer.compute_residuals(&state, &mut residuals);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_n_equations() {
+        let economizer = Economizer::new(2_000.0);
+        assert_eq!(economizer.n_equations(), 3);
+    }
+}

crates/components/src/heat_exchanger/eps_ntu.rs

@@ -0,0 +1,344 @@
+//! Effectiveness-NTU (ε-NTU) Model
+//!
+//! Implements the ε-NTU method for heat exchanger calculations.
+//!
+//! ## Theory
+//!
+//! The heat transfer rate is calculated as:
+//!
+//! $$\dot{Q} = \varepsilon \cdot \dot{Q}_{max} = \varepsilon \cdot C_{min} \cdot (T_{hot,in} - T_{cold,in})$$
+//!
+//! Where:
+//! - $\varepsilon$: Effectiveness (0 to 1)
+//! - $C_{min} = \min(\dot{m}_{hot} \cdot c_{p,hot}, \dot{m}_{cold} \cdot c_{p,cold})$: Minimum heat capacity rate
+//! - $NTU = UA / C_{min}$: Number of Transfer Units
+//! - $C_r = C_{min} / C_{max}$: Heat capacity ratio
+//!
+//! ## Zero-flow regularization (Story 3.5)
+//!
+//! When $C_{min} < 10^{-10}$ (e.g. zero mass flow on one side), heat transfer is set to zero
+//! and divisions by $C_{min}$ or $C_r$ are avoided to prevent NaN/Inf.
+//!
+//! Note: This module uses `1e-10` kW/K for capacity rate regularization, which is appropriate
+//! for the kW/K scale. For mass flow regularization at the kg/s scale, see
+//! [`MIN_MASS_FLOW_REGULARIZATION_KG_S`](entropyk_core::MIN_MASS_FLOW_REGULARIZATION_KG_S).
+//!
+//! For counter-flow:
+//! $$\varepsilon = \frac{1 - \exp(-NTU \cdot (1 - C_r))}{1 - C_r \cdot \exp(-NTU \cdot (1 - C_r))}$$
+
+use super::model::{FluidState, HeatTransferModel};
+use crate::ResidualVector;
+use entropyk_core::Power;
+
+/// Heat exchanger type for ε-NTU calculations.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub enum ExchangerType {
+    /// Counter-flow (most efficient)
+    #[default]
+    CounterFlow,
+    /// Parallel-flow (co-current)
+    ParallelFlow,
+    /// Cross-flow, both fluids unmixed
+    CrossFlowUnmixed,
+    /// Cross-flow, one fluid mixed (C_max mixed)
+    CrossFlowMixedMax,
+    /// Cross-flow, one fluid mixed (C_min mixed)
+    CrossFlowMixedMin,
+    /// Shell-and-tube with specified number of shell passes
+    ShellAndTube {
+        /// Number of shell passes
+        passes: usize,
+    },
+}
+
+/// ε-NTU (Effectiveness-NTU) heat transfer model.
+///
+/// Uses the effectiveness-NTU method for heat exchanger rating.
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::heat_exchanger::{EpsNtuModel, ExchangerType, HeatTransferModel};
+///
+/// let model = EpsNtuModel::new(5000.0, ExchangerType::CounterFlow);
+/// assert_eq!(model.ua(), 5000.0);
+/// ```
+#[derive(Debug, Clone)]
+pub struct EpsNtuModel {
+    /// Overall heat transfer coefficient × Area (W/K), nominal
+    ua: f64,
+    /// UA calibration scale: UA_eff = ua_scale × ua (default 1.0)
+    ua_scale: f64,
+    /// Heat exchanger type
+    exchanger_type: ExchangerType,
+}
+
+impl EpsNtuModel {
+    /// Creates a new ε-NTU model.
+    ///
+    /// # Arguments
+    ///
+    /// * `ua` - Overall heat transfer coefficient × Area (W/K). Must be non-negative.
+    /// * `exchanger_type` - Type of heat exchanger
+    ///
+    /// # Panics
+    ///
+    /// Panics if `ua` is negative or NaN.
+    pub fn new(ua: f64, exchanger_type: ExchangerType) -> Self {
+        assert!(
+            ua.is_finite() && ua >= 0.0,
+            "UA must be non-negative and finite, got {}",
+            ua
+        );
+        Self {
+            ua,
+            ua_scale: 1.0,
+            exchanger_type,
+        }
+    }
+
+    /// Creates a counter-flow ε-NTU model.
+    pub fn counter_flow(ua: f64) -> Self {
+        Self::new(ua, ExchangerType::CounterFlow)
+    }
+
+    /// Creates a parallel-flow ε-NTU model.
+    pub fn parallel_flow(ua: f64) -> Self {
+        Self::new(ua, ExchangerType::ParallelFlow)
+    }
+
+    /// Creates a cross-flow (unmixed) ε-NTU model.
+    pub fn cross_flow_unmixed(ua: f64) -> Self {
+        Self::new(ua, ExchangerType::CrossFlowUnmixed)
+    }
+
+    /// Calculates the effectiveness ε.
+    ///
+    /// # Arguments
+    ///
+    /// * `ntu` - Number of Transfer Units (UA / C_min)
+    /// * `c_r` - Heat capacity ratio (C_min / C_max)
+    ///
+    /// # Returns
+    ///
+    /// The effectiveness ε (0 to 1)
+    pub fn effectiveness(&self, ntu: f64, c_r: f64) -> f64 {
+        if ntu <= 0.0 {
+            return 0.0;
+        }
+
+        match self.exchanger_type {
+            ExchangerType::CounterFlow => {
+                if c_r < 1e-10 {
+                    1.0 - (-ntu).exp()
+                } else {
+                    let exp_term = (-ntu * (1.0 - c_r)).exp();
+                    (1.0 - exp_term) / (1.0 - c_r * exp_term)
+                }
+            }
+            ExchangerType::ParallelFlow => {
+                if c_r < 1e-10 {
+                    1.0 - (-ntu).exp()
+                } else {
+                    (1.0 - (-ntu * (1.0 + c_r)).exp()) / (1.0 + c_r)
+                }
+            }
+            ExchangerType::CrossFlowUnmixed => {
+                if c_r < 1e-10 {
+                    1.0 - (-ntu).exp()
+                } else {
+                    1.0 - (-c_r * (1.0 - (-ntu / c_r).exp())).exp()
+                }
+            }
+            ExchangerType::CrossFlowMixedMax => {
+                if c_r < 1e-10 {
+                    1.0 - (-ntu).exp()
+                } else {
+                    let ntu_c_r = ntu / c_r;
+                    (1.0 - (-ntu_c_r).exp()) / c_r * (1.0 - (-c_r * ntu).exp())
+                }
+            }
+            ExchangerType::CrossFlowMixedMin => {
+                if c_r < 1e-10 {
+                    1.0 - (-ntu).exp()
+                } else {
+                    (1.0 / c_r) * (1.0 - (-c_r * (1.0 - (-ntu).exp())).exp())
+                }
+            }
+            ExchangerType::ShellAndTube { passes: _ } => {
+                if c_r < 1e-10 {
+                    1.0 - (-ntu).exp()
+                } else {
+                    (1.0 - (-ntu * (1.0 + c_r * c_r).sqrt()).exp()) / (1.0 + c_r)
+                }
+            }
+        }
+    }
+
+    /// Calculates the maximum possible heat transfer rate.
+    ///
+    /// Q̇_max = C_min × (T_hot,in - T_cold,in)
+    pub fn q_max(&self, c_min: f64, t_hot_in: f64, t_cold_in: f64) -> f64 {
+        c_min * (t_hot_in - t_cold_in).max(0.0)
+    }
+}
+
+impl HeatTransferModel for EpsNtuModel {
+    fn compute_heat_transfer(
+        &self,
+        hot_inlet: &FluidState,
+        _hot_outlet: &FluidState,
+        cold_inlet: &FluidState,
+        _cold_outlet: &FluidState,
+    ) -> Power {
+        let c_hot = hot_inlet.heat_capacity_rate();
+        let c_cold = cold_inlet.heat_capacity_rate();
+
+        let (c_min, c_max) = if c_hot < c_cold {
+            (c_hot, c_cold)
+        } else {
+            (c_cold, c_hot)
+        };
+
+        if c_min < 1e-10 {
+            return Power::from_watts(0.0);
+        }
+
+        let c_r = c_min / c_max;
+        let ntu = self.effective_ua() / c_min;
+
+        let effectiveness = self.effectiveness(ntu, c_r);
+
+        let q_max = self.q_max(c_min, hot_inlet.temperature, cold_inlet.temperature);
+
+        Power::from_watts(effectiveness * q_max)
+    }
+
+    fn compute_residuals(
+        &self,
+        hot_inlet: &FluidState,
+        hot_outlet: &FluidState,
+        cold_inlet: &FluidState,
+        cold_outlet: &FluidState,
+        residuals: &mut ResidualVector,
+    ) {
+        let q = self
+            .compute_heat_transfer(hot_inlet, hot_outlet, cold_inlet, cold_outlet)
+            .to_watts();
+
+        let q_hot =
+            hot_inlet.mass_flow * hot_inlet.cp * (hot_inlet.temperature - hot_outlet.temperature);
+        let q_cold = cold_inlet.mass_flow
+            * cold_inlet.cp
+            * (cold_outlet.temperature - cold_inlet.temperature);
+
+        residuals[0] = q_hot - q;
+        residuals[1] = q_cold - q;
+        residuals[2] = q_hot - q_cold;
+    }
+
+    fn n_equations(&self) -> usize {
+        3
+    }
+
+    fn ua(&self) -> f64 {
+        self.ua
+    }
+
+    fn ua_scale(&self) -> f64 {
+        self.ua_scale
+    }
+
+    fn set_ua_scale(&mut self, s: f64) {
+        self.ua_scale = s;
+    }
+
+    fn effective_ua(&self) -> f64 {
+        self.ua * self.ua_scale
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_eps_ntu_model_creation() {
+        let model = EpsNtuModel::new(5000.0, ExchangerType::CounterFlow);
+        assert_eq!(model.ua(), 5000.0);
+    }
+
+    #[test]
+    fn test_effectiveness_counter_flow() {
+        let model = EpsNtuModel::counter_flow(5000.0);
+
+        let eps = model.effectiveness(5.0, 0.5);
+        assert!(eps > 0.0 && eps < 1.0);
+
+        let eps_cr_zero = model.effectiveness(5.0, 0.0);
+        assert!((eps_cr_zero - (1.0 - (-5.0_f64).exp())).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_effectiveness_parallel_flow() {
+        let model = EpsNtuModel::parallel_flow(5000.0);
+
+        let eps = model.effectiveness(5.0, 0.5);
+        assert!(eps > 0.0 && eps < 1.0);
+        assert!(eps < model.effectiveness(5.0, 0.5) + 0.1);
+    }
+
+    #[test]
+    fn test_effectiveness_zero_ntu() {
+        let model = EpsNtuModel::counter_flow(5000.0);
+        let eps = model.effectiveness(0.0, 0.5);
+        assert_eq!(eps, 0.0);
+    }
+
+    #[test]
+    fn test_compute_heat_transfer() {
+        let model = EpsNtuModel::counter_flow(5000.0);
+
+        let hot_inlet = FluidState::new(80.0 + 273.15, 101_325.0, 400_000.0, 0.1, 1000.0);
+        let hot_outlet = FluidState::new(60.0 + 273.15, 101_325.0, 380_000.0, 0.1, 1000.0);
+        let cold_inlet = FluidState::new(20.0 + 273.15, 101_325.0, 80_000.0, 0.2, 4180.0);
+        let cold_outlet = FluidState::new(30.0 + 273.15, 101_325.0, 120_000.0, 0.2, 4180.0);
+
+        let q = model.compute_heat_transfer(&hot_inlet, &hot_outlet, &cold_inlet, &cold_outlet);
+
+        assert!(q.to_watts() > 0.0);
+    }
+
+    #[test]
+    fn test_n_equations() {
+        let model = EpsNtuModel::counter_flow(1000.0);
+        assert_eq!(model.n_equations(), 3);
+    }
+
+    #[test]
+    fn test_q_max() {
+        let model = EpsNtuModel::counter_flow(5000.0);
+
+        let c_min = 1000.0;
+        let t_hot_in = 350.0;
+        let t_cold_in = 300.0;
+
+        let q_max = model.q_max(c_min, t_hot_in, t_cold_in);
+        assert_eq!(q_max, 50_000.0);
+    }
+
+    #[test]
+    #[should_panic(expected = "UA must be non-negative")]
+    fn test_negative_ua_panics() {
+        let _model = EpsNtuModel::new(-1000.0, ExchangerType::CounterFlow);
+    }
+
+    #[test]
+    fn test_effectiveness_cross_flow_unmixed_cr_zero() {
+        let model = EpsNtuModel::cross_flow_unmixed(5000.0);
+
+        let eps = model.effectiveness(5.0, 0.0);
+        let expected = 1.0 - (-5.0_f64).exp();
+        assert!((eps - expected).abs() < 1e-10);
+    }
+}

crates/components/src/heat_exchanger/evaporator.rs

@@ -0,0 +1,292 @@
+//! Evaporator Component
+//!
+//! A heat exchanger configured for refrigerant evaporation.
+//! The refrigerant (cold side) evaporates from two-phase mixture to
+/// superheated vapor, absorbing heat from the hot side.
+use super::eps_ntu::{EpsNtuModel, ExchangerType};
+use super::exchanger::HeatExchanger;
+use entropyk_core::Calib;
+use crate::{
+    Component, ComponentError, ConnectedPort, JacobianBuilder, ResidualVector, SystemState,
+};
+use crate::state_machine::{CircuitId, OperationalState, StateManageable};
+
+/// Evaporator heat exchanger.
+///
+/// Uses the ε-NTU method for heat transfer calculation.
+/// The refrigerant evaporates on the cold side, absorbing heat
+/// from the hot side (typically water or air).
+///
+/// # Configuration
+///
+/// - Hot side: Heat source (water, air, etc.)
+/// - Cold side: Refrigerant evaporating (phase change)
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::heat_exchanger::Evaporator;
+/// use entropyk_components::Component;
+///
+/// let evaporator = Evaporator::new(8_000.0);  // UA = 8 kW/K
+/// assert_eq!(evaporator.n_equations(), 3);
+/// ```
+#[derive(Debug)]
+pub struct Evaporator {
+    /// Inner heat exchanger with ε-NTU model
+    inner: HeatExchanger<EpsNtuModel>,
+    /// Saturation temperature for evaporation (K)
+    saturation_temp: f64,
+    /// Target superheat (K)
+    superheat_target: f64,
+}
+
+impl Evaporator {
+    /// Creates a new evaporator with the given UA value.
+    ///
+    /// # Arguments
+    ///
+    /// * `ua` - Overall heat transfer coefficient × Area (W/K)
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use entropyk_components::heat_exchanger::Evaporator;
+    ///
+    /// let evaporator = Evaporator::new(8_000.0);
+    /// ```
+    pub fn new(ua: f64) -> Self {
+        let model = EpsNtuModel::new(ua, ExchangerType::CounterFlow);
+        Self {
+            inner: HeatExchanger::new(model, "Evaporator"),
+            saturation_temp: 278.15,
+            superheat_target: 5.0,
+        }
+    }
+
+    /// Creates an evaporator with specific saturation and superheat.
+    pub fn with_superheat(ua: f64, saturation_temp: f64, superheat_target: f64) -> Self {
+        let model = EpsNtuModel::new(ua, ExchangerType::CounterFlow);
+        Self {
+            inner: HeatExchanger::new(model, "Evaporator"),
+            saturation_temp,
+            superheat_target,
+        }
+    }
+
+    /// Returns the name of this evaporator.
+    pub fn name(&self) -> &str {
+        self.inner.name()
+    }
+
+    /// Returns the UA value (effective: f_ua × UA_nominal).
+    pub fn ua(&self) -> f64 {
+        self.inner.ua()
+    }
+
+    /// Returns calibration factors (f_ua for evaporator).
+    pub fn calib(&self) -> &Calib {
+        self.inner.calib()
+    }
+
+    /// Sets calibration factors.
+    pub fn set_calib(&mut self, calib: Calib) {
+        self.inner.set_calib(calib);
+    }
+
+    /// Returns the saturation temperature.
+    pub fn saturation_temp(&self) -> f64 {
+        self.saturation_temp
+    }
+
+    /// Returns the superheat target.
+    pub fn superheat_target(&self) -> f64 {
+        self.superheat_target
+    }
+
+    /// Sets the saturation temperature.
+    pub fn set_saturation_temp(&mut self, temp: f64) {
+        self.saturation_temp = temp;
+    }
+
+    /// Sets the superheat target.
+    pub fn set_superheat_target(&mut self, superheat: f64) {
+        self.superheat_target = superheat;
+    }
+
+    /// Validates that the outlet quality is >= 0 (fully evaporated or superheated).
+    ///
+    /// # Arguments
+    ///
+    /// * `outlet_enthalpy` - Outlet specific enthalpy (J/kg)
+    /// * `h_liquid` - Saturated liquid enthalpy at evaporating pressure
+    /// * `h_vapor` - Saturated vapor enthalpy at evaporating pressure
+    ///
+    /// # Returns
+    ///
+    /// Returns Ok(superheat) if valid, Err otherwise
+    pub fn validate_outlet_quality(
+        &self,
+        outlet_enthalpy: f64,
+        h_liquid: f64,
+        h_vapor: f64,
+        cp_vapor: f64,
+    ) -> Result<f64, ComponentError> {
+        if h_vapor <= h_liquid {
+            return Err(ComponentError::NumericalError(
+                "Invalid saturation enthalpies".to_string(),
+            ));
+        }
+
+        let quality = (outlet_enthalpy - h_liquid) / (h_vapor - h_liquid);
+
+        if quality >= 0.0 - 1e-6 {
+            if outlet_enthalpy >= h_vapor {
+                let superheat = (outlet_enthalpy - h_vapor) / cp_vapor;
+                Ok(superheat)
+            } else {
+                Ok(0.0)
+            }
+        } else {
+            Err(ComponentError::InvalidState(format!(
+                "Evaporator outlet quality {} < 0 (subcooled)",
+                quality
+            )))
+        }
+    }
+
+    /// Calculates the superheat residual for inverse control.
+    ///
+    /// Returns (actual_superheat - target_superheat)
+    pub fn superheat_residual(&self, actual_superheat: f64) -> f64 {
+        actual_superheat - self.superheat_target
+    }
+
+    /// Computes the full thermodynamic state at the hot inlet.
+    pub fn hot_inlet_state(&self) -> Result<entropyk_fluids::ThermoState, ComponentError> {
+        self.inner.hot_inlet_state()
+    }
+
+    /// Computes the full thermodynamic state at the cold inlet.
+    pub fn cold_inlet_state(&self) -> Result<entropyk_fluids::ThermoState, ComponentError> {
+        self.inner.cold_inlet_state()
+    }
+}
+
+impl Component for Evaporator {
+    fn compute_residuals(
+        &self,
+        state: &SystemState,
+        residuals: &mut ResidualVector,
+    ) -> Result<(), ComponentError> {
+        self.inner.compute_residuals(state, residuals)
+    }
+
+    fn jacobian_entries(
+        &self,
+        state: &SystemState,
+        jacobian: &mut JacobianBuilder,
+    ) -> Result<(), ComponentError> {
+        self.inner.jacobian_entries(state, jacobian)
+    }
+
+    fn n_equations(&self) -> usize {
+        self.inner.n_equations()
+    }
+
+    fn get_ports(&self) -> &[ConnectedPort] {
+        self.inner.get_ports()
+    }
+}
+
+impl StateManageable for Evaporator {
+    fn state(&self) -> OperationalState {
+        self.inner.state()
+    }
+
+    fn set_state(&mut self, state: OperationalState) -> Result<(), ComponentError> {
+        self.inner.set_state(state)
+    }
+
+    fn can_transition_to(&self, target: OperationalState) -> bool {
+        self.inner.can_transition_to(target)
+    }
+
+    fn circuit_id(&self) -> &CircuitId {
+        self.inner.circuit_id()
+    }
+
+    fn set_circuit_id(&mut self, circuit_id: CircuitId) {
+        self.inner.set_circuit_id(circuit_id);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_evaporator_creation() {
+        let evaporator = Evaporator::new(8_000.0);
+        assert_eq!(evaporator.ua(), 8_000.0);
+        assert_eq!(evaporator.n_equations(), 3);
+    }
+
+    #[test]
+    fn test_evaporator_with_superheat() {
+        let evaporator = Evaporator::with_superheat(8_000.0, 278.15, 10.0);
+        assert_eq!(evaporator.saturation_temp(), 278.15);
+        assert_eq!(evaporator.superheat_target(), 10.0);
+    }
+
+    #[test]
+    fn test_validate_outlet_quality_superheated() {
+        let evaporator = Evaporator::new(8_000.0);
+
+        let h_liquid = 200_000.0;
+        let h_vapor = 400_000.0;
+        let outlet_h = 420_000.0;
+        let cp_vapor = 1000.0;
+
+        let result = evaporator.validate_outlet_quality(outlet_h, h_liquid, h_vapor, cp_vapor);
+        assert!(result.is_ok());
+
+        let superheat = result.unwrap();
+        assert!((superheat - 20.0).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_validate_outlet_quality_subcooled() {
+        let evaporator = Evaporator::new(8_000.0);
+
+        let h_liquid = 200_000.0;
+        let h_vapor = 400_000.0;
+        let outlet_h = 150_000.0;
+        let cp_vapor = 1000.0;
+
+        let result = evaporator.validate_outlet_quality(outlet_h, h_liquid, h_vapor, cp_vapor);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_superheat_residual() {
+        let evaporator = Evaporator::with_superheat(8_000.0, 278.15, 5.0);
+
+        let residual = evaporator.superheat_residual(7.0);
+        assert!((residual - 2.0).abs() < 1e-10);
+
+        let residual = evaporator.superheat_residual(3.0);
+        assert!((residual - (-2.0)).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_compute_residuals() {
+        let evaporator = Evaporator::new(8_000.0);
+
+        let state = vec![0.0; 10];
+        let mut residuals = vec![0.0; 3];
+
+        let result = evaporator.compute_residuals(&state, &mut residuals);
+        assert!(result.is_ok());
+    }
+}

crates/components/src/heat_exchanger/evaporator_coil.rs

@@ -0,0 +1,208 @@
+//! Evaporator Coil Component
+//!
+//! An air-side (finned) heat exchanger for refrigerant evaporation.
+//! The refrigerant (cold side) evaporates, absorbing heat from air (hot side).
+//! Used in split systems and air-source heat pumps.
+//!
+//! ## Port Convention
+//!
+//! - **Hot side (air)**: Heat source — connect to Fan outlet/inlet
+//! - **Cold side (refrigerant)**: Evaporating
+//!
+//! ## Integration with Fan
+//!
+//! Connect Fan outlet → EvaporatorCoil air inlet, EvaporatorCoil air outlet → Fan inlet.
+//! Use `FluidId::new("Air")` for air ports.
+
+use super::evaporator::Evaporator;
+use crate::{
+    Component, ComponentError, ConnectedPort, JacobianBuilder, ResidualVector, SystemState,
+};
+use crate::state_machine::{CircuitId, OperationalState, StateManageable};
+
+/// Evaporator coil (air-side finned heat exchanger).
+///
+/// Explicit component for air-source evaporators. Uses ε-NTU method.
+/// Refrigerant evaporates on cold side, air on hot side.
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::heat_exchanger::EvaporatorCoil;
+/// use entropyk_components::Component;
+///
+/// let coil = EvaporatorCoil::new(8_000.0);  // UA = 8 kW/K
+/// assert_eq!(coil.ua(), 8_000.0);
+/// assert_eq!(coil.n_equations(), 3);
+/// ```
+#[derive(Debug)]
+pub struct EvaporatorCoil {
+    inner: Evaporator,
+}
+
+impl EvaporatorCoil {
+    /// Creates a new evaporator coil with the given UA value.
+    ///
+    /// # Arguments
+    ///
+    /// * `ua` - Overall heat transfer coefficient × Area (W/K)
+    pub fn new(ua: f64) -> Self {
+        Self {
+            inner: Evaporator::new(ua),
+        }
+    }
+
+    /// Creates an evaporator coil with specific saturation and superheat.
+    pub fn with_superheat(ua: f64, saturation_temp: f64, superheat_target: f64) -> Self {
+        Self {
+            inner: Evaporator::with_superheat(ua, saturation_temp, superheat_target),
+        }
+    }
+
+    /// Returns the name of this component.
+    pub fn name(&self) -> &str {
+        "EvaporatorCoil"
+    }
+
+    /// Returns the UA value.
+    pub fn ua(&self) -> f64 {
+        self.inner.ua()
+    }
+
+    /// Returns the saturation temperature.
+    pub fn saturation_temp(&self) -> f64 {
+        self.inner.saturation_temp()
+    }
+
+    /// Returns the superheat target.
+    pub fn superheat_target(&self) -> f64 {
+        self.inner.superheat_target()
+    }
+
+    /// Sets the saturation temperature.
+    pub fn set_saturation_temp(&mut self, temp: f64) {
+        self.inner.set_saturation_temp(temp);
+    }
+
+    /// Sets the superheat target.
+    pub fn set_superheat_target(&mut self, superheat: f64) {
+        self.inner.set_superheat_target(superheat);
+    }
+}
+
+impl Component for EvaporatorCoil {
+    fn compute_residuals(
+        &self,
+        state: &SystemState,
+        residuals: &mut ResidualVector,
+    ) -> Result<(), ComponentError> {
+        self.inner.compute_residuals(state, residuals)
+    }
+
+    fn jacobian_entries(
+        &self,
+        state: &SystemState,
+        jacobian: &mut JacobianBuilder,
+    ) -> Result<(), ComponentError> {
+        self.inner.jacobian_entries(state, jacobian)
+    }
+
+    fn n_equations(&self) -> usize {
+        self.inner.n_equations()
+    }
+
+    fn get_ports(&self) -> &[ConnectedPort] {
+        self.inner.get_ports()
+    }
+}
+
+impl StateManageable for EvaporatorCoil {
+    fn state(&self) -> OperationalState {
+        self.inner.state()
+    }
+
+    fn set_state(&mut self, state: OperationalState) -> Result<(), ComponentError> {
+        self.inner.set_state(state)
+    }
+
+    fn can_transition_to(&self, target: OperationalState) -> bool {
+        self.inner.can_transition_to(target)
+    }
+
+    fn circuit_id(&self) -> &CircuitId {
+        self.inner.circuit_id()
+    }
+
+    fn set_circuit_id(&mut self, circuit_id: CircuitId) {
+        self.inner.set_circuit_id(circuit_id);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_evaporator_coil_creation() {
+        let coil = EvaporatorCoil::new(8_000.0);
+        assert_eq!(coil.ua(), 8_000.0);
+        assert_eq!(coil.name(), "EvaporatorCoil");
+    }
+
+    #[test]
+    fn test_evaporator_coil_n_equations() {
+        let coil = EvaporatorCoil::new(5_000.0);
+        assert_eq!(coil.n_equations(), 3);
+    }
+
+    #[test]
+    fn test_evaporator_coil_with_superheat() {
+        let coil = EvaporatorCoil::with_superheat(8_000.0, 278.15, 5.0);
+        assert_eq!(coil.saturation_temp(), 278.15);
+        assert_eq!(coil.superheat_target(), 5.0);
+    }
+
+    #[test]
+    fn test_evaporator_coil_compute_residuals() {
+        let coil = EvaporatorCoil::new(8_000.0);
+        let state = vec![0.0; 10];
+        let mut residuals = vec![0.0; 3];
+        let result = coil.compute_residuals(&state, &mut residuals);
+        assert!(result.is_ok());
+        assert!(residuals.iter().all(|r| r.is_finite()), "residuals must be finite");
+    }
+
+    #[test]
+    fn test_evaporator_coil_jacobian_entries() {
+        let coil = EvaporatorCoil::new(8_000.0);
+        let state = vec![0.0; 10];
+        let mut jacobian = crate::JacobianBuilder::new();
+        let result = coil.jacobian_entries(&state, &mut jacobian);
+        assert!(result.is_ok());
+        // HeatExchanger base returns empty jacobian until framework implements it
+        assert!(
+            jacobian.is_empty(),
+            "delegation works; empty jacobian expected until HeatExchanger implements entries"
+        );
+    }
+
+    #[test]
+    fn test_evaporator_coil_setters() {
+        let mut coil = EvaporatorCoil::new(8_000.0);
+        coil.set_saturation_temp(275.0);
+        coil.set_superheat_target(7.0);
+        assert!((coil.saturation_temp() - 275.0).abs() < 1e-10);
+        assert!((coil.superheat_target() - 7.0).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_evaporator_coil_state_manageable() {
+        use crate::state_machine::{OperationalState, StateManageable};
+
+        let mut coil = EvaporatorCoil::new(8_000.0);
+        assert_eq!(coil.state(), OperationalState::On);
+        assert!(coil.can_transition_to(OperationalState::Off));
+        assert!(coil.set_state(OperationalState::Off).is_ok());
+        assert_eq!(coil.state(), OperationalState::Off);
+    }
+}

crates/components/src/heat_exchanger/exchanger.rs

@@ -262,10 +262,34 @@ impl<Model: HeatTransferModel> HeatExchanger<Model> {
         self.fluid_backend.is_some()
     }
 
+    /// Computes the full thermodynamic state at the hot inlet.
+    pub fn hot_inlet_state(&self) -> Result<ThermoState, ComponentError> {
+        let backend = self.fluid_backend.as_ref().ok_or_else(|| ComponentError::CalculationFailed("No FluidBackend configured".to_string()))?;
+        let conditions = self.hot_conditions.as_ref().ok_or_else(|| ComponentError::CalculationFailed("Hot conditions not set".to_string()))?;
+        let h = self.query_enthalpy(conditions)?;
+        backend.full_state(
+            conditions.fluid_id().clone(),
+            Pressure::from_pascals(conditions.pressure_pa()),
+            entropyk_core::Enthalpy::from_joules_per_kg(h),
+        ).map_err(|e| ComponentError::CalculationFailed(format!("Failed to compute hot inlet state: {}", e)))
+    }
+
+    /// Computes the full thermodynamic state at the cold inlet.
+    pub fn cold_inlet_state(&self) -> Result<ThermoState, ComponentError> {
+        let backend = self.fluid_backend.as_ref().ok_or_else(|| ComponentError::CalculationFailed("No FluidBackend configured".to_string()))?;
+        let conditions = self.cold_conditions.as_ref().ok_or_else(|| ComponentError::CalculationFailed("Cold conditions not set".to_string()))?;
+        let h = self.query_enthalpy(conditions)?;
+        backend.full_state(
+            conditions.fluid_id().clone(),
+            Pressure::from_pascals(conditions.pressure_pa()),
+            entropyk_core::Enthalpy::from_joules_per_kg(h),
+        ).map_err(|e| ComponentError::CalculationFailed(format!("Failed to compute cold inlet state: {}", e)))
+    }
+
     /// Queries Cp (J/(kg·K)) from the backend for a given side.
     fn query_cp(&self, conditions: &HxSideConditions) -> Result<f64, ComponentError> {
         if let Some(backend) = &self.fluid_backend {
-            let state = ThermoState::from_pt(
+            let state = entropyk_fluids::FluidState::from_pt(
                 Pressure::from_pascals(conditions.pressure_pa()),
                 Temperature::from_kelvin(conditions.temperature_k()),
             );
@@ -279,7 +303,7 @@ impl<Model: HeatTransferModel> HeatExchanger<Model> {
     /// Queries specific enthalpy (J/kg) from the backend for a given side at (P, T).
     fn query_enthalpy(&self, conditions: &HxSideConditions) -> Result<f64, ComponentError> {
         if let Some(backend) = &self.fluid_backend {
-            let state = ThermoState::from_pt(
+            let state = entropyk_fluids::FluidState::from_pt(
                 Pressure::from_pascals(conditions.pressure_pa()),
                 Temperature::from_kelvin(conditions.temperature_k()),
             );

crates/components/src/heat_exchanger/lmtd.rs

@@ -0,0 +1,398 @@
+//! Log Mean Temperature Difference (LMTD) Model
+//!
+//! Implements the LMTD method for heat exchanger calculations.
+//!
+//! ## Theory
+//!
+//! The heat transfer rate is calculated as:
+//!
+//! $$\dot{Q} = U \cdot A \cdot \Delta T_{lm} \cdot F$$
+//!
+//! Where:
+//! - $\dot{Q}$: Heat transfer rate (W)
+//! - $U$: Overall heat transfer coefficient (W/m²·K)
+//! - $A$: Heat transfer area (m²)
+//! - $\Delta T_{lm}$: Log mean temperature difference (K)
+//! - $F$: Correction factor for flow configuration
+//!
+//! For counter-flow:
+//! $$\Delta T_{lm} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)}$$
+//!
+//! Where:
+//! - $\Delta T_1 = T_{hot,in} - T_{cold,out}$
+//! - $\Delta T_2 = T_{hot,out} - T_{cold,in}$
+
+use super::model::{FluidState, HeatTransferModel};
+use crate::ResidualVector;
+use entropyk_core::Power;
+
+/// Flow configuration for the heat exchanger.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub enum FlowConfiguration {
+    /// Counter-flow (most efficient)
+    #[default]
+    CounterFlow,
+    /// Parallel-flow (co-current)
+    ParallelFlow,
+    /// Cross-flow with correction factor
+    CrossFlow {
+        /// Correction factor F (typically 0.8-1.0)
+        correction_factor: f64,
+    },
+    /// Shell-and-tube with 1 shell pass, 2 tube passes
+    ShellAndTube1_2,
+}
+
+impl FlowConfiguration {
+    /// Returns the correction factor F for this configuration.
+    pub fn correction_factor(&self) -> f64 {
+        match self {
+            FlowConfiguration::CounterFlow => 1.0,
+            FlowConfiguration::ParallelFlow => 1.0,
+            FlowConfiguration::CrossFlow { correction_factor } => *correction_factor,
+            FlowConfiguration::ShellAndTube1_2 => 0.9,
+        }
+    }
+}
+
+/// LMTD (Log Mean Temperature Difference) heat transfer model.
+///
+/// Uses the classical LMTD method for heat exchanger sizing and rating.
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::heat_exchanger::{LmtdModel, FlowConfiguration, HeatTransferModel};
+/// use entropyk_components::heat_exchanger::model::FluidState;
+///
+/// let model = LmtdModel::new(5000.0, FlowConfiguration::CounterFlow);
+/// assert_eq!(model.ua(), 5000.0);
+/// ```
+#[derive(Debug, Clone)]
+pub struct LmtdModel {
+    /// Overall heat transfer coefficient × Area (W/K), nominal
+    ua: f64,
+    /// UA calibration scale: UA_eff = ua_scale × ua (default 1.0)
+    ua_scale: f64,
+    /// Flow configuration
+    flow_config: FlowConfiguration,
+}
+
+impl LmtdModel {
+    /// Creates a new LMTD model.
+    ///
+    /// # Arguments
+    ///
+    /// * `ua` - Overall heat transfer coefficient × Area (W/K). Must be positive.
+    /// * `flow_config` - Flow configuration (counter-flow, parallel-flow, etc.)
+    ///
+    /// # Panics
+    ///
+    /// Panics if `ua` is negative or NaN.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use entropyk_components::heat_exchanger::{LmtdModel, FlowConfiguration};
+    ///
+    /// let model = LmtdModel::new(10000.0, FlowConfiguration::CounterFlow);
+    /// ```
+    pub fn new(ua: f64, flow_config: FlowConfiguration) -> Self {
+        assert!(
+            ua.is_finite() && ua >= 0.0,
+            "UA must be non-negative and finite, got {}",
+            ua
+        );
+        Self {
+            ua,
+            ua_scale: 1.0,
+            flow_config,
+        }
+    }
+
+    /// Creates a counter-flow LMTD model.
+    pub fn counter_flow(ua: f64) -> Self {
+        Self::new(ua, FlowConfiguration::CounterFlow)
+    }
+
+    /// Creates a parallel-flow LMTD model.
+    pub fn parallel_flow(ua: f64) -> Self {
+        Self::new(ua, FlowConfiguration::ParallelFlow)
+    }
+
+    /// Creates a cross-flow LMTD model with correction factor.
+    pub fn cross_flow(ua: f64, correction_factor: f64) -> Self {
+        Self::new(ua, FlowConfiguration::CrossFlow { correction_factor })
+    }
+
+    /// Calculates the Log Mean Temperature Difference.
+    ///
+    /// For counter-flow:
+    /// - ΔT₁ = T_hot,in - T_cold,out
+    /// - ΔT₂ = T_hot,out - T_cold,in
+    ///
+    /// For parallel-flow:
+    /// - ΔT₁ = T_hot,in - T_cold,in
+    /// - ΔT₂ = T_hot,out - T_cold,out
+    ///
+    /// Special handling when ΔT₁ ≈ ΔT₂: uses arithmetic mean.
+    pub fn lmtd(&self, t_hot_in: f64, t_hot_out: f64, t_cold_in: f64, t_cold_out: f64) -> f64 {
+        let (dt1, dt2) = match self.flow_config {
+            FlowConfiguration::CounterFlow
+            | FlowConfiguration::CrossFlow { .. }
+            | FlowConfiguration::ShellAndTube1_2 => (t_hot_in - t_cold_out, t_hot_out - t_cold_in),
+            FlowConfiguration::ParallelFlow => (t_hot_in - t_cold_in, t_hot_out - t_cold_out),
+        };
+
+        // Zero-flow / zero LMTD regularization: avoid division by zero (Story 3.5)
+        if dt1.abs() < 1e-10 && dt2.abs() < 1e-10 {
+            return 0.0;
+        }
+
+        if (dt1 - dt2).abs() / dt1.max(dt2).max(1e-10) < 1e-6 {
+            return (dt1 + dt2) / 2.0;
+        }
+
+        if dt1 <= 0.0 || dt2 <= 0.0 {
+            return (dt1 + dt2) / 2.0;
+        }
+
+        (dt1 - dt2) / (dt1 / dt2).ln()
+    }
+}
+
+impl HeatTransferModel for LmtdModel {
+    fn compute_heat_transfer(
+        &self,
+        hot_inlet: &FluidState,
+        hot_outlet: &FluidState,
+        cold_inlet: &FluidState,
+        cold_outlet: &FluidState,
+    ) -> Power {
+        let lmtd = self.lmtd(
+            hot_inlet.temperature,
+            hot_outlet.temperature,
+            cold_inlet.temperature,
+            cold_outlet.temperature,
+        );
+
+        let f = self.flow_config.correction_factor();
+        let ua_eff = self.effective_ua();
+        let q = ua_eff * lmtd * f;
+
+        Power::from_watts(q)
+    }
+
+    fn compute_residuals(
+        &self,
+        hot_inlet: &FluidState,
+        hot_outlet: &FluidState,
+        cold_inlet: &FluidState,
+        cold_outlet: &FluidState,
+        residuals: &mut ResidualVector,
+    ) {
+        let q = self
+            .compute_heat_transfer(hot_inlet, hot_outlet, cold_inlet, cold_outlet)
+            .to_watts();
+
+        let q_hot =
+            hot_inlet.mass_flow * hot_inlet.cp * (hot_inlet.temperature - hot_outlet.temperature);
+        let q_cold = cold_inlet.mass_flow
+            * cold_inlet.cp
+            * (cold_outlet.temperature - cold_inlet.temperature);
+
+        residuals[0] = q_hot - q;
+        residuals[1] = q_cold - q;
+        residuals[2] = q_hot - q_cold;
+    }
+
+    fn n_equations(&self) -> usize {
+        3
+    }
+
+    fn ua(&self) -> f64 {
+        self.ua
+    }
+
+    fn ua_scale(&self) -> f64 {
+        self.ua_scale
+    }
+
+    fn set_ua_scale(&mut self, s: f64) {
+        self.ua_scale = s;
+    }
+
+    fn effective_ua(&self) -> f64 {
+        self.ua * self.ua_scale
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::heat_exchanger::{EpsNtuModel, HeatTransferModel};
+    use approx::assert_relative_eq;
+
+    #[test]
+    fn test_lmtd_model_creation() {
+        let model = LmtdModel::new(5000.0, FlowConfiguration::CounterFlow);
+        assert_eq!(model.ua(), 5000.0);
+    }
+
+    #[test]
+    fn test_f_ua_scales_heat_transfer() {
+        let mut model = LmtdModel::new(5000.0, FlowConfiguration::CounterFlow);
+        assert_relative_eq!(model.effective_ua(), 5000.0, epsilon = 1e-10);
+        model.set_ua_scale(1.1);
+        assert_relative_eq!(model.effective_ua(), 5500.0, epsilon = 1e-10);
+    }
+
+    #[test]
+    fn test_lmtd_counter_flow() {
+        let model = LmtdModel::counter_flow(5000.0);
+
+        let t_hot_in = 80.0;
+        let t_hot_out = 60.0;
+        let t_cold_in = 20.0;
+        let t_cold_out = 50.0;
+
+        let lmtd = model.lmtd(t_hot_in, t_hot_out, t_cold_in, t_cold_out);
+
+        assert!(lmtd > 0.0);
+        assert!(lmtd < (t_hot_in - t_cold_in));
+    }
+
+    #[test]
+    fn test_lmtd_equal_deltas() {
+        let model = LmtdModel::counter_flow(5000.0);
+
+        let t_hot_in = 80.0;
+        let t_hot_out = 60.0;
+        let t_cold_in = 40.0;
+        let t_cold_out = 60.0;
+
+        let lmtd = model.lmtd(t_hot_in, t_hot_out, t_cold_in, t_cold_out);
+
+        assert!((lmtd - 20.0).abs() < 0.1);
+    }
+
+    #[test]
+    fn test_lmtd_parallel_flow() {
+        let model = LmtdModel::parallel_flow(5000.0);
+
+        let t_hot_in = 80.0;
+        let t_hot_out = 60.0;
+        let t_cold_in = 20.0;
+        let t_cold_out = 40.0;
+
+        let lmtd = model.lmtd(t_hot_in, t_hot_out, t_cold_in, t_cold_out);
+
+        assert!(lmtd > 0.0);
+    }
+
+    #[test]
+    fn test_compute_heat_transfer() {
+        let model = LmtdModel::counter_flow(5000.0);
+
+        let hot_inlet = FluidState::from_temperature(80.0 + 273.15);
+        let hot_outlet = FluidState::from_temperature(60.0 + 273.15);
+        let cold_inlet = FluidState::from_temperature(20.0 + 273.15);
+        let cold_outlet = FluidState::from_temperature(50.0 + 273.15);
+
+        let q = model.compute_heat_transfer(&hot_inlet, &hot_outlet, &cold_inlet, &cold_outlet);
+
+        assert!(q.to_watts() > 0.0);
+    }
+
+    #[test]
+    fn test_flow_configuration_correction_factor() {
+        assert_eq!(FlowConfiguration::CounterFlow.correction_factor(), 1.0);
+        assert_eq!(FlowConfiguration::ParallelFlow.correction_factor(), 1.0);
+        assert_eq!(FlowConfiguration::ShellAndTube1_2.correction_factor(), 0.9);
+
+        let cross = FlowConfiguration::CrossFlow {
+            correction_factor: 0.85,
+        };
+        assert_eq!(cross.correction_factor(), 0.85);
+    }
+
+    #[test]
+    fn test_n_equations() {
+        let model = LmtdModel::counter_flow(1000.0);
+        assert_eq!(model.n_equations(), 3);
+    }
+
+    #[test]
+    fn test_lmtd_negative_deltas() {
+        let model = LmtdModel::counter_flow(5000.0);
+
+        let t_hot_in = 40.0;
+        let t_hot_out = 50.0;
+        let t_cold_in = 60.0;
+        let t_cold_out = 70.0;
+
+        let lmtd = model.lmtd(t_hot_in, t_hot_out, t_cold_in, t_cold_out);
+
+        assert!(lmtd < 0.0);
+    }
+
+    #[test]
+    #[should_panic(expected = "UA must be non-negative")]
+    fn test_negative_ua_panics() {
+        let _model = LmtdModel::new(-1000.0, FlowConfiguration::CounterFlow);
+    }
+
+    #[test]
+    fn test_zero_ua_allowed() {
+        let model = LmtdModel::new(0.0, FlowConfiguration::CounterFlow);
+        assert_eq!(model.ua(), 0.0);
+    }
+
+    #[test]
+    fn test_lmtd_vs_eps_ntu_comparison() {
+        // AC #8: Compare LMTD vs ε-NTU results for same conditions
+        // Verify both methods produce reasonable heat transfer values
+
+        let ua = 5_000.0;
+        let lmtd_model = LmtdModel::counter_flow(ua);
+        let eps_ntu_model = EpsNtuModel::counter_flow(ua);
+
+        // Typical HVAC operating conditions
+        // Hot water cooling from 80°C to 60°C (353K to 333K)
+        // Cold water heating from 20°C to 40°C (293K to 313K)
+        let hot_inlet = FluidState::new(353.0, 200_000.0, 335_000.0, 0.5, 4180.0);
+        let hot_outlet = FluidState::new(333.0, 195_000.0, 250_000.0, 0.5, 4180.0);
+        let cold_inlet = FluidState::new(293.0, 101_325.0, 85_000.0, 0.3, 4180.0);
+        let cold_outlet = FluidState::new(313.0, 101_325.0, 170_000.0, 0.3, 4180.0);
+
+        let q_lmtd = lmtd_model
+            .compute_heat_transfer(&hot_inlet, &hot_outlet, &cold_inlet, &cold_outlet)
+            .to_watts();
+        let q_eps_ntu = eps_ntu_model
+            .compute_heat_transfer(&hot_inlet, &hot_outlet, &cold_inlet, &cold_outlet)
+            .to_watts();
+
+        // Both methods should give positive heat transfer
+        assert!(q_lmtd > 0.0, "LMTD should give positive Q, got {}", q_lmtd);
+        assert!(
+            q_eps_ntu > 0.0,
+            "ε-NTU should give positive Q, got {}",
+            q_eps_ntu
+        );
+
+        // Verify reasonable magnitude for a 5 kW/K heat exchanger
+        // LMTD should be around 30-40K, so Q should be 150-200 kW range for these temps
+        assert!(
+            q_lmtd > 100_000.0 && q_lmtd < 300_000.0,
+            "LMTD Q unexpected: {}",
+            q_lmtd
+        );
+
+        // ε-NTU uses inlet temps only, so result differs from LMTD
+        assert!(
+            q_eps_ntu < 300_000.0,
+            "ε-NTU Q should be reasonable, got {}",
+            q_eps_ntu
+        );
+    }
+}

crates/components/src/heat_exchanger/mod.rs

@@ -0,0 +1,51 @@
+//! Heat Exchanger Framework
+//!
+//! This module provides a pluggable heat exchanger framework supporting multiple
+//! calculation models (LMTD, ε-NTU) for thermodynamic simulations.
+//!
+//! ## Architecture
+//!
+//! The framework uses the Strategy Pattern for heat transfer calculations:
+//!
+//! - [`HeatTransferModel`]: Trait for pluggable calculation strategies
+//! - [`LmtdModel`]: Log Mean Temperature Difference method
+//! - [`EpsNtuModel`]: Effectiveness-NTU method
+//! - [`HeatExchanger`]: Generic heat exchanger component
+//!
+//! ## Components
+//!
+//! - [`Condenser`]: Refrigerant condensing (phase change) on hot side
+//! - [`Evaporator`]: Refrigerant evaporating (phase change) on cold side
+//! - [`EvaporatorCoil`]: Air-side evaporator (finned coil)
+//! - [`CondenserCoil`]: Air-side condenser (finned coil)
+//! - [`Economizer`]: Internal heat exchanger with bypass support
+//!
+//! ## Example
+//!
+//! ```rust
+//! use entropyk_components::heat_exchanger::{HeatExchanger, LmtdModel, FlowConfiguration};
+//!
+//! // Create a heat exchanger with LMTD model
+//! let model = LmtdModel::new(5000.0, FlowConfiguration::CounterFlow);
+//! // Heat exchanger would be created with connected ports
+//! ```
+
+pub mod condenser;
+pub mod condenser_coil;
+pub mod economizer;
+pub mod evaporator_coil;
+pub mod eps_ntu;
+pub mod evaporator;
+pub mod exchanger;
+pub mod lmtd;
+pub mod model;
+
+pub use condenser::Condenser;
+pub use condenser_coil::CondenserCoil;
+pub use economizer::Economizer;
+pub use evaporator_coil::EvaporatorCoil;
+pub use eps_ntu::{EpsNtuModel, ExchangerType};
+pub use evaporator::Evaporator;
+pub use exchanger::{HeatExchanger, HeatExchangerBuilder, HxSideConditions};
+pub use lmtd::{FlowConfiguration, LmtdModel};
+pub use model::HeatTransferModel;

crates/components/src/heat_exchanger/model.rs

@@ -0,0 +1,204 @@
+//! Heat Transfer Model Trait
+//!
+//! Defines the Strategy Pattern interface for heat transfer calculations.
+//! This trait is object-safe for dynamic dispatch.
+
+use crate::ResidualVector;
+use entropyk_core::{Enthalpy, MassFlow, Power, Pressure, Temperature};
+
+/// Fluid state for heat transfer calculations.
+///
+/// Represents the thermodynamic state at a port (inlet or outlet).
+#[derive(Debug, Clone, Copy)]
+pub struct FluidState {
+    /// Temperature in Kelvin
+    pub temperature: f64,
+    /// Pressure in Pascals
+    pub pressure: f64,
+    /// Specific enthalpy in J/kg
+    pub enthalpy: f64,
+    /// Mass flow rate in kg/s
+    pub mass_flow: f64,
+    /// Specific heat capacity at constant pressure in J/(kg·K)
+    pub cp: f64,
+}
+
+impl Default for FluidState {
+    fn default() -> Self {
+        Self {
+            temperature: 300.0,
+            pressure: 101_325.0,
+            enthalpy: 0.0,
+            mass_flow: 0.1,
+            cp: 1000.0,
+        }
+    }
+}
+
+impl FluidState {
+    /// Creates a new fluid state.
+    pub fn new(temperature: f64, pressure: f64, enthalpy: f64, mass_flow: f64, cp: f64) -> Self {
+        Self {
+            temperature,
+            pressure,
+            enthalpy,
+            mass_flow,
+            cp,
+        }
+    }
+
+    /// Creates a fluid state with default properties for a given temperature.
+    pub fn from_temperature(temperature: f64) -> Self {
+        Self {
+            temperature,
+            ..Default::default()
+        }
+    }
+
+    /// Returns the heat capacity rate C = ṁ × Cp in W/K.
+    pub fn heat_capacity_rate(&self) -> f64 {
+        self.mass_flow * self.cp
+    }
+
+    /// Creates a FluidState from strongly-typed physical quantities.
+    pub fn from_types(
+        temperature: Temperature,
+        pressure: Pressure,
+        enthalpy: Enthalpy,
+        mass_flow: MassFlow,
+        cp: f64,
+    ) -> Self {
+        Self {
+            temperature: temperature.to_kelvin(),
+            pressure: pressure.to_pascals(),
+            enthalpy: enthalpy.to_joules_per_kg(),
+            mass_flow: mass_flow.to_kg_per_s(),
+            cp,
+        }
+    }
+
+    /// Returns temperature as a strongly-typed Temperature.
+    pub fn temperature(&self) -> Temperature {
+        Temperature::from_kelvin(self.temperature)
+    }
+
+    /// Returns pressure as a strongly-typed Pressure.
+    pub fn pressure(&self) -> Pressure {
+        Pressure::from_pascals(self.pressure)
+    }
+
+    /// Returns enthalpy as a strongly-typed Enthalpy.
+    pub fn enthalpy(&self) -> Enthalpy {
+        Enthalpy::from_joules_per_kg(self.enthalpy)
+    }
+
+    /// Returns mass flow as a strongly-typed MassFlow.
+    pub fn mass_flow(&self) -> MassFlow {
+        MassFlow::from_kg_per_s(self.mass_flow)
+    }
+}
+
+/// Trait for heat transfer calculation models.
+///
+/// This trait uses the Strategy Pattern to allow different heat transfer
+/// calculation methods (LMTD, ε-NTU, etc.) to be used interchangeably.
+///
+/// # Object Safety
+///
+/// This trait is object-safe and can be used with dynamic dispatch:
+///
+/// ```
+/// # use entropyk_components::heat_exchanger::model::{HeatTransferModel, FluidState};
+/// # use entropyk_components::ResidualVector;
+/// # use entropyk_core::Power;
+/// struct SimpleModel { ua: f64 }
+/// impl HeatTransferModel for SimpleModel {
+///     fn compute_heat_transfer(&self, _: &FluidState, _: &FluidState, _: &FluidState, _: &FluidState) -> Power {
+///         Power::from_watts(0.0)
+///     }
+///     fn compute_residuals(&self, _: &FluidState, _: &FluidState, _: &FluidState, _: &FluidState, _: &mut ResidualVector) {}
+///     fn n_equations(&self) -> usize { 3 }
+///     fn ua(&self) -> f64 { self.ua }
+/// }
+/// let model: Box<dyn HeatTransferModel> = Box::new(SimpleModel { ua: 1000.0 });
+/// ```
+pub trait HeatTransferModel: Send + Sync {
+    /// Computes the heat transfer rate Q̇ (Watts).
+    ///
+    /// # Arguments
+    ///
+    /// * `hot_inlet` - Hot side inlet state
+    /// * `hot_outlet` - Hot side outlet state
+    /// * `cold_inlet` - Cold side inlet state
+    /// * `cold_outlet` - Cold side outlet state
+    ///
+    /// # Returns
+    ///
+    /// The heat transfer rate in Watts (positive = heat flows from hot to cold)
+    fn compute_heat_transfer(
+        &self,
+        hot_inlet: &FluidState,
+        hot_outlet: &FluidState,
+        cold_inlet: &FluidState,
+        cold_outlet: &FluidState,
+    ) -> Power;
+
+    /// Computes residuals for the solver.
+    ///
+    /// The residuals represent the error in the heat transfer equations
+    /// that the solver will attempt to drive to zero.
+    fn compute_residuals(
+        &self,
+        hot_inlet: &FluidState,
+        hot_outlet: &FluidState,
+        cold_inlet: &FluidState,
+        cold_outlet: &FluidState,
+        residuals: &mut ResidualVector,
+    );
+
+    /// Returns the number of equations this model contributes.
+    fn n_equations(&self) -> usize;
+
+    /// Returns the nominal UA value (overall heat transfer coefficient × area) in W/K.
+    fn ua(&self) -> f64;
+
+    /// Returns the UA calibration scale (default 1.0). UA_eff = ua_scale × ua_nominal.
+    fn ua_scale(&self) -> f64 {
+        1.0
+    }
+
+    /// Sets the UA calibration scale (e.g. from Calib.f_ua).
+    fn set_ua_scale(&mut self, _s: f64) {}
+
+    /// Returns the effective UA used in heat transfer: ua_scale × ua_nominal.
+    fn effective_ua(&self) -> f64 {
+        self.ua() * self.ua_scale()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_fluid_state_default() {
+        let state = FluidState::default();
+        assert_eq!(state.temperature, 300.0);
+        assert_eq!(state.pressure, 101_325.0);
+        assert_eq!(state.cp, 1000.0);
+    }
+
+    #[test]
+    fn test_fluid_state_heat_capacity_rate() {
+        let state = FluidState::new(300.0, 101_325.0, 0.0, 0.5, 2000.0);
+        let c = state.heat_capacity_rate();
+        assert!((c - 1000.0).abs() < 1e-10);
+    }
+
+    #[test]
+    fn test_fluid_state_from_temperature() {
+        let state = FluidState::from_temperature(350.0);
+        assert_eq!(state.temperature, 350.0);
+        assert_eq!(state.pressure, 101_325.0);
+    }
+}

crates/components/src/polynomials.rs

@@ -0,0 +1,702 @@
+//! Polynomial curve models for component performance characterization.
+//!
+//! This module provides polynomial curve implementations for:
+//! - 1D polynomials: Pump curves (Q-H, efficiency), Fan curves
+//! - 2D polynomials: Compressor maps based on SST/SDT
+//!
+//! ## 1D Polynomial (Pump/Fan Curves)
+//!
+//! ```text
+//! y = c0 + c1*x + c2*x² + c3*x³ + ...
+//! ```
+//!
+//! ## 2D Polynomial (Compressor Maps)
+//!
+//! ```text
+//! z = Σ a_ij * x^i * y^j
+//! ```
+//!
+//! Where x = SST (Saturated Suction Temperature)
+//! and y = SDT (Saturated Discharge Temperature)
+
+use crate::ComponentError;
+use serde::{Deserialize, Serialize};
+
+/// 1D Polynomial curve for component performance modeling.
+///
+/// Used for pump head curves, fan static pressure curves, and efficiency curves.
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::polynomials::Polynomial1D;
+///
+/// // Pump curve: H = 50 - 0.1*Q - 0.001*Q²
+/// let curve = Polynomial1D::new(vec![50.0, -0.1, -0.001]);
+///
+/// // Evaluate at Q = 100 m³/h
+/// let head = curve.evaluate(100.0);
+/// assert!(head > 0.0);
+/// ```
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct Polynomial1D {
+    /// Polynomial coefficients [c0, c1, c2, ...] for y = c0 + c1*x + c2*x² + ...
+    coefficients: Vec<f64>,
+}
+
+impl Polynomial1D {
+    /// Creates a new 1D polynomial from coefficients.
+    ///
+    /// # Arguments
+    ///
+    /// * `coefficients` - Coefficients [c0, c1, c2, ...] where y = c0 + c1*x + c2*x² + ...
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use entropyk_components::polynomials::Polynomial1D;
+    ///
+    /// // Linear: y = 2x + 3
+    /// let linear = Polynomial1D::new(vec![3.0, 2.0]);
+    ///
+    /// // Quadratic: y = 1 + 2x + 3x²
+    /// let quadratic = Polynomial1D::new(vec![1.0, 2.0, 3.0]);
+    /// ```
+    pub fn new(coefficients: Vec<f64>) -> Self {
+        Self { coefficients }
+    }
+
+    /// Creates a constant polynomial (degree 0).
+    pub fn constant(value: f64) -> Self {
+        Self::new(vec![value])
+    }
+
+    /// Creates a linear polynomial: y = a + b*x.
+    pub fn linear(a: f64, b: f64) -> Self {
+        Self::new(vec![a, b])
+    }
+
+    /// Creates a quadratic polynomial: y = a + b*x + c*x².
+    pub fn quadratic(a: f64, b: f64, c: f64) -> Self {
+        Self::new(vec![a, b, c])
+    }
+
+    /// Creates a cubic polynomial: y = a + b*x + c*x² + d*x³.
+    pub fn cubic(a: f64, b: f64, c: f64, d: f64) -> Self {
+        Self::new(vec![a, b, c, d])
+    }
+
+    /// Evaluates the polynomial at point x using Horner's method.
+    ///
+    /// # Arguments
+    ///
+    /// * `x` - The point at which to evaluate
+    ///
+    /// # Returns
+    ///
+    /// The polynomial value y = P(x)
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use entropyk_components::polynomials::Polynomial1D;
+    ///
+    /// let p = Polynomial1D::quadratic(1.0, 2.0, 3.0);
+    /// // y = 1 + 2*2 + 3*4 = 1 + 4 + 12 = 17
+    /// assert!((p.evaluate(2.0) - 17.0).abs() < 1e-10);
+    /// ```
+    pub fn evaluate(&self, x: f64) -> f64 {
+        if self.coefficients.is_empty() {
+            return 0.0;
+        }
+
+        // Horner's method: efficient polynomial evaluation
+        self.coefficients
+            .iter()
+            .rev()
+            .fold(0.0, |acc, &c| acc * x + c)
+    }
+
+    /// Computes the derivative of the polynomial at point x.
+    ///
+    /// # Arguments
+    ///
+    /// * `x` - The point at which to evaluate the derivative
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use entropyk_components::polynomials::Polynomial1D;
+    ///
+    /// let p = Polynomial1D::quadratic(1.0, 2.0, 3.0);
+    /// // dy/dx = 2 + 6x, at x=2: dy/dx = 2 + 12 = 14
+    /// assert!((p.derivative(2.0) - 14.0).abs() < 1e-10);
+    /// ```
+    pub fn derivative(&self, x: f64) -> f64 {
+        if self.coefficients.len() <= 1 {
+            return 0.0;
+        }
+
+        // Derivative coefficients: [c1, 2*c2, 3*c3, ...]
+        let deriv_coeffs: Vec<f64> = self.coefficients[1..]
+            .iter()
+            .enumerate()
+            .map(|(i, &c)| c * (i + 1) as f64)
+            .collect();
+
+        Polynomial1D::new(deriv_coeffs).evaluate(x)
+    }
+
+    /// Returns the degree of the polynomial.
+    pub fn degree(&self) -> usize {
+        if self.coefficients.is_empty() {
+            0
+        } else {
+            self.coefficients.len() - 1
+        }
+    }
+
+    /// Returns a reference to the coefficients.
+    pub fn coefficients(&self) -> &[f64] {
+        &self.coefficients
+    }
+
+    /// Validates that all coefficients are finite (not NaN or infinite).
+    pub fn validate(&self) -> Result<(), ComponentError> {
+        for (i, &c) in self.coefficients.iter().enumerate() {
+            if c.is_nan() {
+                return Err(ComponentError::InvalidState(format!(
+                    "Coefficient {} is NaN",
+                    i
+                )));
+            }
+            if c.is_infinite() {
+                return Err(ComponentError::InvalidState(format!(
+                    "Coefficient {} is infinite",
+                    i
+                )));
+            }
+        }
+        Ok(())
+    }
+}
+
+impl Default for Polynomial1D {
+    fn default() -> Self {
+        Self::constant(0.0)
+    }
+}
+
+/// 2D Polynomial for compressor maps based on SST/SDT.
+///
+/// Models performance as a function of two variables (e.g., SST and SDT):
+///
+/// ```text
+/// z = Σ a_ij * x^i * y^j  for i=0..nx, j=0..ny
+/// ```
+///
+/// # Example
+///
+/// ```
+/// use entropyk_components::polynomials::Polynomial2D;
+///
+/// // Simple bilinear: z = a00 + a10*x + a01*y + a11*x*y
+/// let coeffs = vec![
+///     vec![1.0, 0.5],   // a00, a01
+///     vec![2.0, 0.1],   // a10, a11
+/// ];
+/// let poly = Polynomial2D::new(coeffs);
+///
+/// let z = poly.evaluate(10.0, 20.0);
+/// ```
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct Polynomial2D {
+    /// Coefficient matrix where coeffs[i][j] is the coefficient for x^i * y^j
+    /// coeffs[i] contains coefficients for all j values at degree i in x
+    coefficients: Vec<Vec<f64>>,
+}
+
+impl Polynomial2D {
+    /// Creates a new 2D polynomial from coefficient matrix.
+    ///
+    /// # Arguments
+    ///
+    /// * `coefficients` - Matrix where coefficients[i][j] is coefficient for x^i * y^j
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use entropyk_components::polynomials::Polynomial2D;
+    ///
+    /// // z = 5 + 2*x + 3*y + 0.5*x*y
+    /// let coeffs = vec![
+    ///     vec![5.0, 3.0],    // Constant and y term
+    ///     vec![2.0, 0.5],    // x term and x*y term
+    /// ];
+    /// let poly = Polynomial2D::new(coeffs);
+    /// ```
+    pub fn new(coefficients: Vec<Vec<f64>>) -> Self {
+        Self { coefficients }
+    }
+
+    /// Creates a constant 2D polynomial.
+    pub fn constant(value: f64) -> Self {
+        Self::new(vec![vec![value]])
+    }
+
+    /// Creates a bilinear polynomial: z = a00 + a10*x + a01*y + a11*x*y.
+    pub fn bilinear(a00: f64, a10: f64, a01: f64, a11: f64) -> Self {
+        Self::new(vec![vec![a00, a01], vec![a10, a11]])
+    }
+
+    /// Creates a biquadratic polynomial (degree 2 in both variables).
+    ///
+    /// z = a00 + a10*x + a01*y + a20*x² + a11*x*y + a02*y²
+    pub fn biquadratic(a00: f64, a10: f64, a01: f64, a20: f64, a11: f64, a02: f64) -> Self {
+        Self::new(vec![vec![a00, a01, a02], vec![a10, a11], vec![a20]])
+    }
+
+    /// Evaluates the polynomial at (x, y).
+    ///
+    /// # Arguments
+    ///
+    /// * `x` - First variable (e.g., SST)
+    /// * `y` - Second variable (e.g., SDT)
+    ///
+    /// # Returns
+    ///
+    /// The polynomial value z = P(x, y)
+    pub fn evaluate(&self, x: f64, y: f64) -> f64 {
+        let mut result = 0.0;
+
+        for (i, row) in self.coefficients.iter().enumerate() {
+            let x_pow = x.powi(i as i32);
+            for (j, &coeff) in row.iter().enumerate() {
+                result += coeff * x_pow * y.powi(j as i32);
+            }
+        }
+
+        result
+    }
+
+    /// Computes partial derivative with respect to x.
+    pub fn partial_x(&self, x: f64, y: f64) -> f64 {
+        let mut result = 0.0;
+
+        for (i, row) in self.coefficients.iter().enumerate() {
+            if i == 0 {
+                continue; // ∂/∂x of constant is 0
+            }
+            let x_pow = (i as f64) * x.powi((i - 1) as i32);
+            for (j, &coeff) in row.iter().enumerate() {
+                result += coeff * x_pow * y.powi(j as i32);
+            }
+        }
+
+        result
+    }
+
+    /// Computes partial derivative with respect to y.
+    pub fn partial_y(&self, x: f64, y: f64) -> f64 {
+        let mut result = 0.0;
+
+        for (i, row) in self.coefficients.iter().enumerate() {
+            let x_pow = x.powi(i as i32);
+            for (j, &coeff) in row.iter().enumerate() {
+                if j == 0 {
+                    continue; // ∂/∂y of constant is 0
+                }
+                result += coeff * x_pow * (j as f64) * y.powi((j - 1) as i32);
+            }
+        }
+
+        result
+    }
+
+    /// Returns the degree in x.
+    pub fn degree_x(&self) -> usize {
+        if self.coefficients.is_empty() {
+            0
+        } else {
+            self.coefficients.len() - 1
+        }
+    }
+
+    /// Returns the degree in y.
+    pub fn degree_y(&self) -> usize {
+        self.coefficients
+            .iter()
+            .map(|row| if row.is_empty() { 0 } else { row.len() - 1 })
+            .max()
+            .unwrap_or(0)
+    }
+
+    /// Returns a reference to the coefficient matrix.
+    pub fn coefficients(&self) -> &[Vec<f64>] {
+        &self.coefficients
+    }
+
+    /// Validates that all coefficients are finite.
+    pub fn validate(&self) -> Result<(), ComponentError> {
+        for (i, row) in self.coefficients.iter().enumerate() {
+            for (j, &c) in row.iter().enumerate() {
+                if c.is_nan() {
+                    return Err(ComponentError::InvalidState(format!(
+                        "Coefficient [{},{}] is NaN",
+                        i, j
+                    )));
+                }
+                if c.is_infinite() {
+                    return Err(ComponentError::InvalidState(format!(
+                        "Coefficient [{},{}] is infinite",
+                        i, j
+                    )));
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
+impl Default for Polynomial2D {
+    fn default() -> Self {
+        Self::constant(0.0)
+    }
+}
+
+/// Pump/Fan curve set containing head, efficiency, and power polynomials.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct PerformanceCurves {
+    /// Head/Pressure curve: H = f(Q) where Q is volumetric flow
+    pub head_curve: Polynomial1D,
+    /// Efficiency curve: η = f(Q)
+    pub efficiency_curve: Polynomial1D,
+    /// Optional power curve: P = f(Q) - if not provided, calculated from head and efficiency
+    pub power_curve: Option<Polynomial1D>,
+}
+
+impl PerformanceCurves {
+    /// Creates a new performance curve set.
+    pub fn new(
+        head_curve: Polynomial1D,
+        efficiency_curve: Polynomial1D,
+        power_curve: Option<Polynomial1D>,
+    ) -> Self {
+        Self {
+            head_curve,
+            efficiency_curve,
+            power_curve,
+        }
+    }
+
+    /// Creates a simple pump curve with head and efficiency only.
+    pub fn simple(head_curve: Polynomial1D, efficiency_curve: Polynomial1D) -> Self {
+        Self::new(head_curve, efficiency_curve, None)
+    }
+
+    /// Validates all curves.
+    pub fn validate(&self) -> Result<(), ComponentError> {
+        self.head_curve.validate()?;
+        self.efficiency_curve.validate()?;
+        if let Some(ref pc) = self.power_curve {
+            pc.validate()?;
+        }
+        Ok(())
+    }
+}
+
+impl Default for PerformanceCurves {
+    fn default() -> Self {
+        Self::simple(Polynomial1D::default(), Polynomial1D::default())
+    }
+}
+
+/// Affinity laws for variable speed operation.
+///
+/// When speed changes from N1 to N2:
+/// - Q2/Q1 = N2/N1 (flow proportional to speed)
+/// - H2/H1 = (N2/N1)² (head proportional to speed squared)
+/// - P2/P1 = (N2/N1)³ (power proportional to speed cubed)
+pub struct AffinityLaws;
+
+impl AffinityLaws {
+    /// Applies affinity laws to scale flow rate.
+    ///
+    /// # Arguments
+    ///
+    /// * `flow` - Original flow at speed_ratio = 1.0
+    /// * `speed_ratio` - New speed / rated speed (0.0 to 1.0)
+    ///
+    /// # Returns
+    ///
+    /// Scaled flow rate
+    pub fn scale_flow(flow: f64, speed_ratio: f64) -> f64 {
+        flow * speed_ratio
+    }
+
+    /// Applies affinity laws to scale head/pressure.
+    ///
+    /// # Arguments
+    ///
+    /// * `head` - Original head at speed_ratio = 1.0
+    /// * `speed_ratio` - New speed / rated speed (0.0 to 1.0)
+    ///
+    /// # Returns
+    ///
+    /// Scaled head
+    pub fn scale_head(head: f64, speed_ratio: f64) -> f64 {
+        head * speed_ratio * speed_ratio
+    }
+
+    /// Applies affinity laws to scale power.
+    ///
+    /// # Arguments
+    ///
+    /// * `power` - Original power at speed_ratio = 1.0
+    /// * `speed_ratio` - New speed / rated speed (0.0 to 1.0)
+    ///
+    /// # Returns
+    ///
+    /// Scaled power
+    pub fn scale_power(power: f64, speed_ratio: f64) -> f64 {
+        power * speed_ratio * speed_ratio * speed_ratio
+    }
+
+    /// Reverse affinity law: find original flow from scaled flow.
+    pub fn unscale_flow(scaled_flow: f64, speed_ratio: f64) -> f64 {
+        if speed_ratio <= 0.0 {
+            return f64::INFINITY;
+        }
+        scaled_flow / speed_ratio
+    }
+
+    /// Reverse affinity law: find original head from scaled head.
+    pub fn unscale_head(scaled_head: f64, speed_ratio: f64) -> f64 {
+        if speed_ratio <= 0.0 {
+            return f64::INFINITY;
+        }
+        scaled_head / (speed_ratio * speed_ratio)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use approx::assert_relative_eq;
+
+    #[test]
+    fn test_polynomial_1d_constant() {
+        let p = Polynomial1D::constant(5.0);
+        assert_relative_eq!(p.evaluate(0.0), 5.0);
+        assert_relative_eq!(p.evaluate(100.0), 5.0);
+        assert_relative_eq!(p.derivative(0.0), 0.0);
+    }
+
+    #[test]
+    fn test_polynomial_1d_linear() {
+        // y = 3 + 2x
+        let p = Polynomial1D::linear(3.0, 2.0);
+        assert_relative_eq!(p.evaluate(0.0), 3.0);
+        assert_relative_eq!(p.evaluate(1.0), 5.0);
+        assert_relative_eq!(p.evaluate(2.0), 7.0);
+        assert_relative_eq!(p.derivative(5.0), 2.0);
+    }
+
+    #[test]
+    fn test_polynomial_1d_quadratic() {
+        // y = 1 + 2x + 3x²
+        let p = Polynomial1D::quadratic(1.0, 2.0, 3.0);
+        assert_relative_eq!(p.evaluate(0.0), 1.0);
+        assert_relative_eq!(p.evaluate(1.0), 6.0); // 1 + 2 + 3
+        assert_relative_eq!(p.evaluate(2.0), 17.0); // 1 + 4 + 12
+        assert_relative_eq!(p.derivative(2.0), 14.0); // 2 + 6*2 = 14
+    }
+
+    #[test]
+    fn test_polynomial_1d_cubic() {
+        // y = 1 + x + x² + x³
+        let p = Polynomial1D::cubic(1.0, 1.0, 1.0, 1.0);
+        assert_relative_eq!(p.evaluate(0.0), 1.0);
+        assert_relative_eq!(p.evaluate(1.0), 4.0);
+        assert_relative_eq!(p.evaluate(2.0), 15.0); // 1 + 2 + 4 + 8
+    }
+
+    #[test]
+    fn test_polynomial_1d_empty() {
+        let p = Polynomial1D::new(vec![]);
+        assert_relative_eq!(p.evaluate(5.0), 0.0);
+    }
+
+    #[test]
+    fn test_polynomial_1d_degree() {
+        assert_eq!(Polynomial1D::constant(1.0).degree(), 0);
+        assert_eq!(Polynomial1D::linear(1.0, 2.0).degree(), 1);
+        assert_eq!(Polynomial1D::quadratic(1.0, 2.0, 3.0).degree(), 2);
+        assert_eq!(Polynomial1D::cubic(1.0, 2.0, 3.0, 4.0).degree(), 3);
+    }
+
+    #[test]
+    fn test_polynomial_1d_validate() {
+        let valid = Polynomial1D::quadratic(1.0, 2.0, 3.0);
+        assert!(valid.validate().is_ok());
+
+        let nan_coeff = Polynomial1D::new(vec![1.0, f64::NAN]);
+        assert!(nan_coeff.validate().is_err());
+
+        let inf_coeff = Polynomial1D::new(vec![f64::INFINITY]);
+        assert!(inf_coeff.validate().is_err());
+    }
+
+    #[test]
+    fn test_polynomial_2d_constant() {
+        let p = Polynomial2D::constant(5.0);
+        assert_relative_eq!(p.evaluate(0.0, 0.0), 5.0);
+        assert_relative_eq!(p.evaluate(10.0, 20.0), 5.0);
+    }
+
+    #[test]
+    fn test_polynomial_2d_bilinear() {
+        // z = 1 + 2*x + 3*y + 0.5*x*y
+        let p = Polynomial2D::bilinear(1.0, 2.0, 3.0, 0.5);
+
+        // At (0,0): z = 1
+        assert_relative_eq!(p.evaluate(0.0, 0.0), 1.0);
+
+        // At (1,0): z = 1 + 2 = 3
+        assert_relative_eq!(p.evaluate(1.0, 0.0), 3.0);
+
+        // At (0,1): z = 1 + 3 = 4
+        assert_relative_eq!(p.evaluate(0.0, 1.0), 4.0);
+
+        // At (2,3): z = 1 + 4 + 9 + 3 = 17
+        assert_relative_eq!(p.evaluate(2.0, 3.0), 17.0);
+    }
+
+    #[test]
+    fn test_polynomial_2d_biquadratic() {
+        let p = Polynomial2D::biquadratic(1.0, 2.0, 3.0, 4.0, 5.0, 6.0);
+
+        // At (0,0): z = 1
+        assert_relative_eq!(p.evaluate(0.0, 0.0), 1.0);
+
+        // At (1,1): z = 1 + 2 + 3 + 4 + 5 + 6 = 21
+        assert_relative_eq!(p.evaluate(1.0, 1.0), 21.0);
+    }
+
+    #[test]
+    fn test_polynomial_2d_partial_derivatives() {
+        // z = 1 + 2*x + 3*y + 4*x*y
+        let p = Polynomial2D::bilinear(1.0, 2.0, 3.0, 4.0);
+
+        // ∂z/∂x = 2 + 4*y
+        assert_relative_eq!(p.partial_x(0.0, 0.0), 2.0);
+        assert_relative_eq!(p.partial_x(0.0, 1.0), 6.0);
+
+        // ∂z/∂y = 3 + 4*x
+        assert_relative_eq!(p.partial_y(0.0, 0.0), 3.0);
+        assert_relative_eq!(p.partial_y(1.0, 0.0), 7.0);
+    }
+
+    #[test]
+    fn test_polynomial_2d_degrees() {
+        let p = Polynomial2D::bilinear(1.0, 2.0, 3.0, 4.0);
+        assert_eq!(p.degree_x(), 1);
+        assert_eq!(p.degree_y(), 1);
+
+        let biq = Polynomial2D::biquadratic(1.0, 2.0, 3.0, 4.0, 5.0, 6.0);
+        assert_eq!(biq.degree_x(), 2);
+        assert_eq!(biq.degree_y(), 2);
+    }
+
+    #[test]
+    fn test_polynomial_2d_validate() {
+        let valid = Polynomial2D::bilinear(1.0, 2.0, 3.0, 4.0);
+        assert!(valid.validate().is_ok());
+
+        let nan_coeff = Polynomial2D::new(vec![vec![1.0, f64::NAN]]);
+        assert!(nan_coeff.validate().is_err());
+    }
+
+    #[test]
+    fn test_affinity_laws_flow() {
+        let flow = 100.0;
+
+        // At full speed: no change
+        assert_relative_eq!(AffinityLaws::scale_flow(flow, 1.0), 100.0);
+
+        // At half speed: half flow
+        assert_relative_eq!(AffinityLaws::scale_flow(flow, 0.5), 50.0);
+
+        // At 80% speed: 80% flow
+        assert_relative_eq!(AffinityLaws::scale_flow(flow, 0.8), 80.0);
+    }
+
+    #[test]
+    fn test_affinity_laws_head() {
+        let head = 100.0;
+
+        // At full speed: no change
+        assert_relative_eq!(AffinityLaws::scale_head(head, 1.0), 100.0);
+
+        // At half speed: 25% head
+        assert_relative_eq!(AffinityLaws::scale_head(head, 0.5), 25.0);
+
+        // At 80% speed: 64% head
+        assert_relative_eq!(AffinityLaws::scale_head(head, 0.8), 64.0);
+    }
+
+    #[test]
+    fn test_affinity_laws_power() {
+        let power = 1000.0;
+
+        // At full speed: no change
+        assert_relative_eq!(AffinityLaws::scale_power(power, 1.0), 1000.0);
+
+        // At half speed: 12.5% power
+        assert_relative_eq!(AffinityLaws::scale_power(power, 0.5), 125.0);
+
+        // At 80% speed: 51.2% power
+        assert_relative_eq!(AffinityLaws::scale_power(power, 0.8), 512.0);
+    }
+
+    #[test]
+    fn test_affinity_laws_reverse() {
+        let flow = 50.0;
+        let head = 25.0;
+        let speed = 0.5;
+
+        // Reverse scaling
+        assert_relative_eq!(AffinityLaws::unscale_flow(flow, speed), 100.0);
+        assert_relative_eq!(AffinityLaws::unscale_head(head, speed), 100.0);
+    }
+
+    #[test]
+    fn test_performance_curves() {
+        // Typical pump curve: H = 50 - 0.1*Q - 0.001*Q²
+        let head = Polynomial1D::quadratic(50.0, -0.1, -0.001);
+        // Efficiency: η = 0.4 + 0.02*Q - 0.0001*Q²
+        let eff = Polynomial1D::quadratic(0.4, 0.02, -0.0001);
+
+        let curves = PerformanceCurves::simple(head.clone(), eff.clone());
+
+        assert!(curves.validate().is_ok());
+        assert_relative_eq!(curves.head_curve.evaluate(0.0), 50.0);
+        assert_relative_eq!(curves.efficiency_curve.evaluate(0.0), 0.4);
+    }
+
+    #[test]
+    fn test_pump_curve_realistic() {
+        // Realistic pump curve for a small centrifugal pump
+        // H (m) = 30 - 0.05*Q - 0.0005*Q², where Q is in m³/h
+        let head_curve = Polynomial1D::quadratic(30.0, -0.05, -0.0005);
+
+        // At Q=0 (shut-off): H = 30 m
+        assert_relative_eq!(head_curve.evaluate(0.0), 30.0);
+
+        // At Q=100 m³/h: H = 30 - 5 - 5 = 20 m
+        assert_relative_eq!(head_curve.evaluate(100.0), 20.0);
+
+        // At Q=200 m³/h: H = 30 - 10 - 20 = 0 m (run-out)
+        assert_relative_eq!(head_curve.evaluate(200.0), 0.0, epsilon = 1e-10);
+    }
+}

crates/components/src/port.rs

@@ -0,0 +1,753 @@
+//! Port and Connection System
+//!
+//! This module provides the foundation for connecting thermodynamic components
+//! using the Type-State pattern for compile-time connection safety.
+//!
+//! ## Type-State Pattern
+//!
+//! Ports have two states:
+//! - `Disconnected`: Initial state, cannot be used in solver
+//! - `Connected`: Linked to another port, ready for simulation
+//!
+//! State transitions are enforced at compile time:
+//! ```text
+//! Port<Disconnected> --connect()--> Port<Connected>
+//!      ↑                                    │
+//!      └───────── (no way back) ────────────┘
+//! ```
+//!
+//! ## Connection Semantics
+//!
+//! Connected ports validate continuity (pressure/enthalpy match) at connection time,
+//! but track values independently afterward. This allows the solver to update port
+//! states during iteration without requiring synchronization.
+//!
+//! ## Example
+//!
+//! ```rust
+//! use entropyk_components::port::{Port, Disconnected, Connected, FluidId, ConnectionError};
+//! use entropyk_core::{Pressure, Enthalpy};
+//!
+//! // Create two disconnected ports
+//! let port1 = Port::new(FluidId::new("R134a"), Pressure::from_bar(1.0), Enthalpy::from_joules_per_kg(400000.0));
+//! let port2 = Port::new(FluidId::new("R134a"), Pressure::from_bar(1.0), Enthalpy::from_joules_per_kg(400000.0));
+//!
+//! // Connect them
+//! let (connected1, connected2) = port1.connect(port2)?;
+//!
+//! // Ports track values independently for solver flexibility
+//! assert_eq!(connected1.pressure().to_bar(), 1.0);
+//! # Ok::<(), ConnectionError>(())
+//! ```
+
+use entropyk_core::{Enthalpy, Pressure};
+use std::fmt;
+use std::marker::PhantomData;
+use thiserror::Error;
+
+/// Default relative tolerance for pressure matching (0.01% = 100 ppm).
+/// For 1 bar = 100,000 Pa, this allows 10 Pa difference.
+const PRESSURE_TOLERANCE_FRACTION: f64 = 1e-4;
+
+/// Default absolute tolerance for enthalpy matching (100 J/kg).
+/// This is approximately 0.024 kJ/kg, reasonable for HVAC calculations.
+const ENTHALPY_TOLERANCE_J_KG: f64 = 100.0;
+
+/// Minimum absolute pressure tolerance (1 Pa) to avoid issues near zero.
+const MIN_PRESSURE_TOLERANCE_PA: f64 = 1.0;
+
+/// Errors that can occur during port operations.
+#[derive(Error, Debug, Clone, PartialEq)]
+pub enum ConnectionError {
+    /// Attempted to connect ports with incompatible fluids.
+    #[error("Incompatible fluids: cannot connect {from} to {to}")]
+    IncompatibleFluid {
+        /// Source fluid identifier
+        from: String,
+        /// Target fluid identifier
+        to: String,
+    },
+
+    /// Pressure mismatch at connection point.
+    #[error(
+        "Pressure mismatch: {from_pressure} Pa vs {to_pressure} Pa (tolerance: {tolerance} Pa)"
+    )]
+    PressureMismatch {
+        /// Pressure at source port (Pa)
+        from_pressure: f64,
+        /// Pressure at target port (Pa)
+        to_pressure: f64,
+        /// Tolerance used for comparison (Pa)
+        tolerance: f64,
+    },
+
+    /// Enthalpy mismatch at connection point.
+    #[error("Enthalpy mismatch: {from_enthalpy} J/kg vs {to_enthalpy} J/kg (tolerance: {tolerance} J/kg)")]
+    EnthalpyMismatch {
+        /// Enthalpy at source port (J/kg)
+        from_enthalpy: f64,
+        /// Enthalpy at target port (J/kg)
+        to_enthalpy: f64,
+        /// Tolerance used for comparison (J/kg)
+        tolerance: f64,
+    },
+
+    /// Attempted to connect a port that is already connected.
+    #[error("Port is already connected and cannot be reconnected")]
+    AlreadyConnected,
+
+    /// Detected a cycle in the connection graph.
+    #[error("Connection would create a cycle in the system topology")]
+    CycleDetected,
+
+    /// Invalid port index.
+    #[error(
+        "Invalid port index {index}: component has {port_count} ports (valid: 0..{max_index})"
+    )]
+    InvalidPortIndex {
+        /// The invalid port index that was requested
+        index: usize,
+        /// Number of ports on the component
+        port_count: usize,
+        /// Maximum valid index (port_count - 1, or 0 if no ports)
+        max_index: usize,
+    },
+
+    /// Invalid node index.
+    #[error("Invalid node index: {0}")]
+    InvalidNodeIndex(usize),
+}
+
+/// Type-state marker for disconnected ports.
+///
+/// Ports in this state cannot be used in the solver until connected.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Disconnected;
+
+/// Type-state marker for connected ports.
+///
+/// Ports in this state are linked to another port and ready for simulation.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Connected;
+
+/// Identifier for thermodynamic fluids.
+///
+/// Used to ensure only compatible fluids are connected.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct FluidId(String);
+
+impl FluidId {
+    /// Creates a new fluid identifier.
+    ///
+    /// # Arguments
+    ///
+    /// * `id` - Unique identifier for the fluid (e.g., "R134a", "Water")
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::port::FluidId;
+    ///
+    /// let fluid = FluidId::new("R134a");
+    /// ```
+    pub fn new(id: impl Into<String>) -> Self {
+        FluidId(id.into())
+    }
+
+    /// Returns the fluid identifier as a string slice.
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+impl fmt::Display for FluidId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// A thermodynamic port for connecting components.
+///
+/// Ports use the Type-State pattern to enforce connection safety at compile time.
+/// A `Port<Disconnected>` must be connected before it can be used in simulations.
+///
+/// # Type Parameters
+///
+/// * `State` - Either `Disconnected` or `Connected`, tracking the port's state
+///
+/// # Examples
+///
+/// ```
+/// use entropyk_components::port::{Port, Disconnected, FluidId};
+/// use entropyk_core::{Pressure, Enthalpy};
+///
+/// // Create a disconnected port
+/// let port: Port<Disconnected> = Port::new(
+///     FluidId::new("R134a"),
+///     Pressure::from_bar(1.0),
+///     Enthalpy::from_joules_per_kg(400000.0)
+/// );
+/// ```
+#[derive(Debug, Clone, PartialEq)]
+pub struct Port<State> {
+    fluid_id: FluidId,
+    pressure: Pressure,
+    enthalpy: Enthalpy,
+    _state: PhantomData<State>,
+}
+
+/// Helper to validate connection parameters.
+fn validate_connection_params(
+    from_fluid: &FluidId,
+    from_p: Pressure,
+    from_h: Enthalpy,
+    to_fluid: &FluidId,
+    to_p: Pressure,
+    to_h: Enthalpy,
+) -> Result<(), ConnectionError> {
+    if from_fluid != to_fluid {
+        return Err(ConnectionError::IncompatibleFluid {
+            from: from_fluid.to_string(),
+            to: to_fluid.to_string(),
+        });
+    }
+
+    let pressure_tol =
+        (from_p.to_pascals().abs() * PRESSURE_TOLERANCE_FRACTION).max(MIN_PRESSURE_TOLERANCE_PA);
+    let pressure_diff = (from_p.to_pascals() - to_p.to_pascals()).abs();
+    if pressure_diff > pressure_tol {
+        return Err(ConnectionError::PressureMismatch {
+            from_pressure: from_p.to_pascals(),
+            to_pressure: to_p.to_pascals(),
+            tolerance: pressure_tol,
+        });
+    }
+
+    let enthalpy_diff = (from_h.to_joules_per_kg() - to_h.to_joules_per_kg()).abs();
+    if enthalpy_diff > ENTHALPY_TOLERANCE_J_KG {
+        return Err(ConnectionError::EnthalpyMismatch {
+            from_enthalpy: from_h.to_joules_per_kg(),
+            to_enthalpy: to_h.to_joules_per_kg(),
+            tolerance: ENTHALPY_TOLERANCE_J_KG,
+        });
+    }
+
+    Ok(())
+}
+
+impl Port<Disconnected> {
+    /// Creates a new disconnected port.
+    ///
+    /// # Arguments
+    ///
+    /// * `fluid_id` - Identifier for the fluid flowing through this port
+    /// * `pressure` - Initial pressure at the port
+    /// * `enthalpy` - Initial specific enthalpy at the port
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::port::{Port, FluidId};
+    /// use entropyk_core::{Pressure, Enthalpy};
+    ///
+    /// let port = Port::new(
+    ///     FluidId::new("R134a"),
+    ///     Pressure::from_bar(1.0),
+    ///     Enthalpy::from_joules_per_kg(400000.0)
+    /// );
+    /// ```
+    pub fn new(fluid_id: FluidId, pressure: Pressure, enthalpy: Enthalpy) -> Self {
+        Self {
+            fluid_id,
+            pressure,
+            enthalpy,
+            _state: PhantomData,
+        }
+    }
+
+    /// Returns the fluid identifier.
+    pub fn fluid_id(&self) -> &FluidId {
+        &self.fluid_id
+    }
+
+    /// Returns the current pressure.
+    pub fn pressure(&self) -> Pressure {
+        self.pressure
+    }
+
+    /// Returns the current enthalpy.
+    pub fn enthalpy(&self) -> Enthalpy {
+        self.enthalpy
+    }
+
+    /// Connects two disconnected ports.
+    ///
+    /// Validates that:
+    /// - Both ports have the same fluid type
+    /// - Pressures match within relative tolerance
+    /// - Enthalpies match within absolute tolerance
+    ///
+    /// After connection, ports track values independently, allowing the solver
+    /// to update states during iteration.
+    ///
+    /// # Arguments
+    ///
+    /// * `other` - The port to connect to
+    ///
+    /// # Returns
+    ///
+    /// Returns a tuple of `(Port<Connected>, Port<Connected>)` on success,
+    /// or a `ConnectionError` if validation fails.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::port::{Port, FluidId, ConnectionError};
+    /// use entropyk_core::{Pressure, Enthalpy};
+    ///
+    /// let port1 = Port::new(
+    ///     FluidId::new("R134a"),
+    ///     Pressure::from_pascals(100000.0),
+    ///     Enthalpy::from_joules_per_kg(400000.0)
+    /// );
+    /// let port2 = Port::new(
+    ///     FluidId::new("R134a"),
+    ///     Pressure::from_pascals(100000.0),
+    ///     Enthalpy::from_joules_per_kg(400000.0)
+    /// );
+    ///
+    /// let (connected1, connected2) = port1.connect(port2)?;
+    /// # Ok::<(), ConnectionError>(())
+    /// ```
+    pub fn connect(
+        self,
+        other: Port<Disconnected>,
+    ) -> Result<(Port<Connected>, Port<Connected>), ConnectionError> {
+        validate_connection_params(
+            &self.fluid_id,
+            self.pressure,
+            self.enthalpy,
+            &other.fluid_id,
+            other.pressure,
+            other.enthalpy,
+        )?;
+
+        let avg_pressure = Pressure::from_pascals(
+            (self.pressure.to_pascals() + other.pressure.to_pascals()) / 2.0,
+        );
+        let avg_enthalpy = Enthalpy::from_joules_per_kg(
+            (self.enthalpy.to_joules_per_kg() + other.enthalpy.to_joules_per_kg()) / 2.0,
+        );
+
+        let connected1 = Port {
+            fluid_id: self.fluid_id,
+            pressure: avg_pressure,
+            enthalpy: avg_enthalpy,
+            _state: PhantomData,
+        };
+
+        let connected2 = Port {
+            fluid_id: other.fluid_id,
+            pressure: avg_pressure,
+            enthalpy: avg_enthalpy,
+            _state: PhantomData,
+        };
+
+        Ok((connected1, connected2))
+    }
+}
+
+impl Port<Connected> {
+    /// Returns the fluid identifier.
+    pub fn fluid_id(&self) -> &FluidId {
+        &self.fluid_id
+    }
+
+    /// Returns the current pressure.
+    pub fn pressure(&self) -> Pressure {
+        self.pressure
+    }
+
+    /// Returns the current enthalpy.
+    pub fn enthalpy(&self) -> Enthalpy {
+        self.enthalpy
+    }
+
+    /// Updates the pressure at this port.
+    ///
+    /// # Arguments
+    ///
+    /// * `pressure` - The new pressure value
+    pub fn set_pressure(&mut self, pressure: Pressure) {
+        self.pressure = pressure;
+    }
+
+    /// Updates the enthalpy at this port.
+    ///
+    /// # Arguments
+    ///
+    /// * `enthalpy` - The new enthalpy value
+    pub fn set_enthalpy(&mut self, enthalpy: Enthalpy) {
+        self.enthalpy = enthalpy;
+    }
+}
+
+/// A connected port reference that can be stored in components.
+///
+/// This type is object-safe and can be used in trait objects.
+pub type ConnectedPort = Port<Connected>;
+
+/// Validates that two connected ports are compatible for a flow connection.
+///
+/// Uses the same tolerance constants as [`Port::connect`](Port::connect):
+/// - Pressure: `max(P * 1e-4, 1 Pa)`
+/// - Enthalpy: 100 J/kg
+///
+/// # Arguments
+///
+/// * `outlet` - Source port (flow direction: outlet → inlet)
+/// * `inlet` - Target port
+///
+/// # Returns
+///
+/// `Ok(())` if ports are compatible, `Err(ConnectionError)` otherwise.
+pub fn validate_port_continuity(
+    outlet: &ConnectedPort,
+    inlet: &ConnectedPort,
+) -> Result<(), ConnectionError> {
+    validate_connection_params(
+        &outlet.fluid_id,
+        outlet.pressure,
+        outlet.enthalpy,
+        &inlet.fluid_id,
+        inlet.pressure,
+        inlet.enthalpy,
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use approx::assert_relative_eq;
+
+    #[test]
+    fn test_port_creation() {
+        let port = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+
+        assert_eq!(port.fluid_id().as_str(), "R134a");
+        assert_relative_eq!(port.pressure().to_bar(), 1.0, epsilon = 1e-10);
+        assert_relative_eq!(
+            port.enthalpy().to_joules_per_kg(),
+            400000.0,
+            epsilon = 1e-10
+        );
+    }
+
+    #[test]
+    fn test_successful_connection() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+
+        let (connected1, connected2) = port1.connect(port2).unwrap();
+
+        assert_eq!(connected1.fluid_id().as_str(), "R134a");
+        assert_eq!(connected2.fluid_id().as_str(), "R134a");
+        assert_relative_eq!(
+            connected1.pressure().to_pascals(),
+            100000.0,
+            epsilon = 1e-10
+        );
+        assert_relative_eq!(
+            connected2.pressure().to_pascals(),
+            100000.0,
+            epsilon = 1e-10
+        );
+    }
+
+    #[test]
+    fn test_incompatible_fluid_error() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+
+        let result = port1.connect(port2);
+
+        assert!(matches!(
+            result,
+            Err(ConnectionError::IncompatibleFluid { .. })
+        ));
+    }
+
+    #[test]
+    fn test_pressure_mismatch_error() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(200000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+
+        let result = port1.connect(port2);
+
+        assert!(matches!(
+            result,
+            Err(ConnectionError::PressureMismatch { .. })
+        ));
+    }
+
+    #[test]
+    fn test_enthalpy_mismatch_error() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(500000.0),
+        );
+
+        let result = port1.connect(port2);
+
+        assert!(matches!(
+            result,
+            Err(ConnectionError::EnthalpyMismatch { .. })
+        ));
+    }
+
+    #[test]
+    fn test_connected_port_setters() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+
+        let (mut connected1, _) = port1.connect(port2).unwrap();
+
+        connected1.set_pressure(Pressure::from_pascals(150000.0));
+        connected1.set_enthalpy(Enthalpy::from_joules_per_kg(450000.0));
+
+        assert_relative_eq!(
+            connected1.pressure().to_pascals(),
+            150000.0,
+            epsilon = 1e-10
+        );
+        assert_relative_eq!(
+            connected1.enthalpy().to_joules_per_kg(),
+            450000.0,
+            epsilon = 1e-10
+        );
+    }
+
+    #[test]
+    fn test_ports_track_independently() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+
+        let (mut connected1, connected2) = port1.connect(port2).unwrap();
+
+        connected1.set_pressure(Pressure::from_pascals(150000.0));
+
+        // connected2 should NOT see the change - ports are independent
+        assert_relative_eq!(
+            connected2.pressure().to_pascals(),
+            100000.0,
+            epsilon = 1e-10
+        );
+    }
+
+    #[test]
+    fn test_fluid_id_creation() {
+        let fluid1 = FluidId::new("R134a");
+        let fluid2 = FluidId::new(String::from("Water"));
+
+        assert_eq!(fluid1.as_str(), "R134a");
+        assert_eq!(fluid2.as_str(), "Water");
+    }
+
+    #[test]
+    fn test_connection_error_display() {
+        let err = ConnectionError::IncompatibleFluid {
+            from: "R134a".to_string(),
+            to: "Water".to_string(),
+        };
+        let msg = format!("{}", err);
+        assert!(msg.contains("Incompatible fluids"));
+        assert!(msg.contains("R134a"));
+        assert!(msg.contains("Water"));
+
+        let err = ConnectionError::PressureMismatch {
+            from_pressure: 100000.0,
+            to_pressure: 200000.0,
+            tolerance: 10.0,
+        };
+        let msg = format!("{}", err);
+        assert!(msg.contains("100000 Pa"));
+        assert!(msg.contains("200000 Pa"));
+        assert!(msg.contains("tolerance"));
+    }
+
+    #[test]
+    fn test_pressure_averaging_on_connection() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+
+        let (connected1, connected2) = port1.connect(port2).unwrap();
+
+        assert_relative_eq!(
+            connected1.pressure().to_pascals(),
+            connected2.pressure().to_pascals(),
+            epsilon = 1e-10
+        );
+    }
+
+    #[test]
+    fn test_pressure_tolerance_with_small_difference() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100005.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+
+        let result = port1.connect(port2);
+
+        assert!(
+            result.is_ok(),
+            "5 Pa difference should be within tolerance for 100 kPa pressure"
+        );
+    }
+
+    #[test]
+    fn test_clone_disconnected_port() {
+        let port1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100000.0),
+            Enthalpy::from_joules_per_kg(400000.0),
+        );
+        let port2 = port1.clone();
+
+        assert_eq!(port1, port2);
+    }
+
+    #[test]
+    fn test_fluid_id_equality() {
+        let f1 = FluidId::new("R134a");
+        let f2 = FluidId::new("R134a");
+        let f3 = FluidId::new("Water");
+
+        assert_eq!(f1, f2);
+        assert_ne!(f1, f3);
+    }
+
+    #[test]
+    fn test_already_connected_error() {
+        let err = ConnectionError::AlreadyConnected;
+        let msg = format!("{}", err);
+        assert!(msg.contains("already connected"));
+    }
+
+    #[test]
+    fn test_cycle_detected_error() {
+        let err = ConnectionError::CycleDetected;
+        let msg = format!("{}", err);
+        assert!(msg.contains("cycle"));
+    }
+
+    #[test]
+    fn test_validate_port_continuity_ok() {
+        let p1 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100_000.0),
+            Enthalpy::from_joules_per_kg(400_000.0),
+        );
+        let p2 = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100_000.0),
+            Enthalpy::from_joules_per_kg(400_000.0),
+        );
+        let (c1, c2) = p1.connect(p2).unwrap();
+        assert!(validate_port_continuity(&c1, &c2).is_ok());
+        assert!(validate_port_continuity(&c2, &c1).is_ok());
+    }
+
+    #[test]
+    fn test_validate_port_continuity_incompatible_fluid() {
+        let (r134a, _) = Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100_000.0),
+            Enthalpy::from_joules_per_kg(400_000.0),
+        )
+        .connect(Port::new(
+            FluidId::new("R134a"),
+            Pressure::from_pascals(100_000.0),
+            Enthalpy::from_joules_per_kg(400_000.0),
+        ))
+        .unwrap();
+        let (water, _) = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_pascals(100_000.0),
+            Enthalpy::from_joules_per_kg(400_000.0),
+        )
+        .connect(Port::new(
+            FluidId::new("Water"),
+            Pressure::from_pascals(100_000.0),
+            Enthalpy::from_joules_per_kg(400_000.0),
+        ))
+        .unwrap();
+        assert!(matches!(
+            validate_port_continuity(&r134a, &water),
+            Err(ConnectionError::IncompatibleFluid { .. })
+        ));
+    }
+}

crates/components/src/pump.rs

@@ -0,0 +1,780 @@
+//! Pump Component Implementation
+//!
+//! This module provides a pump component for hydraulic systems using
+//! polynomial performance curves and affinity laws for variable speed operation.
+//!
+//! ## Performance Curves
+//!
+//! **Head Curve:** H = a₀ + a₁Q + a₂Q² + a₃Q³
+//!
+//! **Efficiency Curve:** η = b₀ + b₁Q + b₂Q²
+//!
+//! **Hydraulic Power:** P_hydraulic = ρ × g × Q × H / η
+//!
+//! ## Affinity Laws (Variable Speed)
+//!
+//! When operating at reduced speed (VFD):
+//! - Q₂/Q₁ = N₂/N₁
+//! - H₂/H₁ = (N₂/N₁)²
+//! - P₂/P₁ = (N₂/N₁)³
+
+use crate::polynomials::{AffinityLaws, PerformanceCurves, Polynomial1D};
+use crate::port::{Connected, Disconnected, FluidId, Port};
+use crate::state_machine::StateManageable;
+use crate::{
+    CircuitId, Component, ComponentError, ConnectedPort, JacobianBuilder, OperationalState,
+    ResidualVector, SystemState,
+};
+use entropyk_core::{MassFlow, Power};
+use serde::{Deserialize, Serialize};
+use std::marker::PhantomData;
+
+/// Pump performance curve coefficients.
+///
+/// Defines the polynomial coefficients for the pump's head-flow curve
+/// and efficiency curve.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct PumpCurves {
+    /// Performance curves (head, efficiency, optional power)
+    curves: PerformanceCurves,
+}
+
+impl PumpCurves {
+    /// Creates pump curves from performance curves.
+    pub fn new(curves: PerformanceCurves) -> Result<Self, ComponentError> {
+        curves.validate()?;
+        Ok(Self { curves })
+    }
+
+    /// Creates pump curves from polynomial coefficients.
+    ///
+    /// # Arguments
+    ///
+    /// * `head_coeffs` - Head curve coefficients [a0, a1, a2, ...] for H = a0 + a1*Q + a2*Q²
+    /// * `eff_coeffs` - Efficiency coefficients [b0, b1, b2, ...] for η = b0 + b1*Q + b2*Q²
+    ///
+    /// # Units
+    ///
+    /// * Q (flow) in m³/s
+    /// * H (head) in meters
+    /// * η (efficiency) as decimal (0.0 to 1.0)
+    pub fn from_coefficients(
+        head_coeffs: Vec<f64>,
+        eff_coeffs: Vec<f64>,
+    ) -> Result<Self, ComponentError> {
+        let head_curve = Polynomial1D::new(head_coeffs);
+        let eff_curve = Polynomial1D::new(eff_coeffs);
+        let curves = PerformanceCurves::simple(head_curve, eff_curve);
+        Self::new(curves)
+    }
+
+    /// Creates a quadratic pump curve.
+    ///
+    /// H = a0 + a1*Q + a2*Q²
+    /// η = b0 + b1*Q + b2*Q²
+    pub fn quadratic(
+        h0: f64,
+        h1: f64,
+        h2: f64,
+        e0: f64,
+        e1: f64,
+        e2: f64,
+    ) -> Result<Self, ComponentError> {
+        Self::from_coefficients(vec![h0, h1, h2], vec![e0, e1, e2])
+    }
+
+    /// Creates a cubic pump curve (3rd-order polynomial for head).
+    ///
+    /// H = a0 + a1*Q + a2*Q² + a3*Q³
+    /// η = b0 + b1*Q + b2*Q²
+    pub fn cubic(
+        h0: f64,
+        h1: f64,
+        h2: f64,
+        h3: f64,
+        e0: f64,
+        e1: f64,
+        e2: f64,
+    ) -> Result<Self, ComponentError> {
+        Self::from_coefficients(vec![h0, h1, h2, h3], vec![e0, e1, e2])
+    }
+
+    /// Returns the head at the given flow rate (at full speed).
+    ///
+    /// # Arguments
+    ///
+    /// * `flow_m3_per_s` - Volumetric flow rate in m³/s
+    ///
+    /// # Returns
+    ///
+    /// Head in meters
+    pub fn head_at_flow(&self, flow_m3_per_s: f64) -> f64 {
+        self.curves.head_curve.evaluate(flow_m3_per_s)
+    }
+
+    /// Returns the efficiency at the given flow rate (at full speed).
+    ///
+    /// # Arguments
+    ///
+    /// * `flow_m3_per_s` - Volumetric flow rate in m³/s
+    ///
+    /// # Returns
+    ///
+    /// Efficiency as decimal (0.0 to 1.0)
+    pub fn efficiency_at_flow(&self, flow_m3_per_s: f64) -> f64 {
+        let eta = self.curves.efficiency_curve.evaluate(flow_m3_per_s);
+        // Clamp efficiency to valid range
+        eta.clamp(0.0, 1.0)
+    }
+
+    /// Returns reference to the performance curves.
+    pub fn curves(&self) -> &PerformanceCurves {
+        &self.curves
+    }
+}
+
+impl Default for PumpCurves {
+    fn default() -> Self {
+        Self::quadratic(30.0, 0.0, 0.0, 0.7, 0.0, 0.0).unwrap()
+    }
+}
+
+/// A pump component with polynomial performance curves.
+///
+/// The pump uses the Type-State pattern to ensure ports are connected
+/// before use in simulations.
+///
+/// # Example
+///
+/// ```ignore
+/// use entropyk_components::pump::{Pump, PumpCurves};
+/// use entropyk_components::port::{FluidId, Port};
+/// use entropyk_core::{Pressure, Enthalpy};
+///
+/// // Create pump curves: H = 30 - 10*Q - 50*Q² (in m and m³/s)
+/// let curves = PumpCurves::quadratic(30.0, -10.0, -50.0, 0.5, 0.3, -0.5).unwrap();
+///
+/// let inlet = Port::new(
+///     FluidId::new("Water"),
+///     Pressure::from_bar(1.0),
+///     Enthalpy::from_joules_per_kg(100000.0),
+/// );
+/// let outlet = Port::new(
+///     FluidId::new("Water"),
+///     Pressure::from_bar(1.0),
+///     Enthalpy::from_joules_per_kg(100000.0),
+/// );
+///
+/// let pump = Pump::new(curves, inlet, outlet, 1000.0).unwrap();
+/// ```
+#[derive(Debug, Clone)]
+pub struct Pump<State> {
+    /// Performance curves
+    curves: PumpCurves,
+    /// Inlet port
+    port_inlet: Port<State>,
+    /// Outlet port
+    port_outlet: Port<State>,
+    /// Fluid density in kg/m³
+    fluid_density_kg_per_m3: f64,
+    /// Speed ratio (0.0 to 1.0), default 1.0 (full speed)
+    speed_ratio: f64,
+    /// Circuit identifier
+    circuit_id: CircuitId,
+    /// Operational state
+    operational_state: OperationalState,
+    /// Phantom data for type state
+    _state: PhantomData<State>,
+}
+
+impl Pump<Disconnected> {
+    /// Creates a new disconnected pump.
+    ///
+    /// # Arguments
+    ///
+    /// * `curves` - Pump performance curves
+    /// * `port_inlet` - Inlet port (disconnected)
+    /// * `port_outlet` - Outlet port (disconnected)
+    /// * `fluid_density` - Fluid density in kg/m³
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if:
+    /// - Ports have different fluid types
+    /// - Fluid density is not positive
+    pub fn new(
+        curves: PumpCurves,
+        port_inlet: Port<Disconnected>,
+        port_outlet: Port<Disconnected>,
+        fluid_density: f64,
+    ) -> Result<Self, ComponentError> {
+        if port_inlet.fluid_id() != port_outlet.fluid_id() {
+            return Err(ComponentError::InvalidState(
+                "Inlet and outlet ports must have the same fluid type".to_string(),
+            ));
+        }
+
+        if fluid_density <= 0.0 {
+            return Err(ComponentError::InvalidState(
+                "Fluid density must be positive".to_string(),
+            ));
+        }
+
+        Ok(Self {
+            curves,
+            port_inlet,
+            port_outlet,
+            fluid_density_kg_per_m3: fluid_density,
+            speed_ratio: 1.0,
+            circuit_id: CircuitId::default(),
+            operational_state: OperationalState::default(),
+            _state: PhantomData,
+        })
+    }
+
+    /// Returns the fluid identifier.
+    pub fn fluid_id(&self) -> &FluidId {
+        self.port_inlet.fluid_id()
+    }
+
+    /// Returns the inlet port.
+    pub fn port_inlet(&self) -> &Port<Disconnected> {
+        &self.port_inlet
+    }
+
+    /// Returns the outlet port.
+    pub fn port_outlet(&self) -> &Port<Disconnected> {
+        &self.port_outlet
+    }
+
+    /// Returns the fluid density.
+    pub fn fluid_density(&self) -> f64 {
+        self.fluid_density_kg_per_m3
+    }
+
+    /// Returns the performance curves.
+    pub fn curves(&self) -> &PumpCurves {
+        &self.curves
+    }
+
+    /// Returns the speed ratio.
+    pub fn speed_ratio(&self) -> f64 {
+        self.speed_ratio
+    }
+
+    /// Sets the speed ratio (0.0 to 1.0).
+    pub fn set_speed_ratio(&mut self, ratio: f64) -> Result<(), ComponentError> {
+        if !(0.0..=1.0).contains(&ratio) {
+            return Err(ComponentError::InvalidState(
+                "Speed ratio must be between 0.0 and 1.0".to_string(),
+            ));
+        }
+        self.speed_ratio = ratio;
+        Ok(())
+    }
+}
+
+impl Pump<Connected> {
+    /// Returns the inlet port.
+    pub fn port_inlet(&self) -> &Port<Connected> {
+        &self.port_inlet
+    }
+
+    /// Returns the outlet port.
+    pub fn port_outlet(&self) -> &Port<Connected> {
+        &self.port_outlet
+    }
+
+    /// Calculates the pressure rise across the pump.
+    ///
+    /// Uses the head curve and converts to pressure:
+    /// ΔP = ρ × g × H
+    ///
+    /// Applies affinity laws for variable speed operation.
+    ///
+    /// # Arguments
+    ///
+    /// * `flow_m3_per_s` - Volumetric flow rate in m³/s
+    ///
+    /// # Returns
+    ///
+    /// Pressure rise in Pascals
+    pub fn pressure_rise(&self, flow_m3_per_s: f64) -> f64 {
+        // Handle zero speed - pump produces no pressure
+        if self.speed_ratio <= 0.0 {
+            return 0.0;
+        }
+
+        // Handle zero flow
+        if flow_m3_per_s <= 0.0 {
+            // At zero flow, use the shut-off head scaled by speed
+            let head_m = self.curves.head_at_flow(0.0);
+            let actual_head = AffinityLaws::scale_head(head_m, self.speed_ratio);
+            const G: f64 = 9.80665; // m/s²
+            return self.fluid_density_kg_per_m3 * G * actual_head;
+        }
+
+        // Apply affinity law to get equivalent flow at full speed
+        let equivalent_flow = AffinityLaws::unscale_flow(flow_m3_per_s, self.speed_ratio);
+
+        // Get head at equivalent flow
+        let head_m = self.curves.head_at_flow(equivalent_flow);
+
+        // Apply affinity law to scale head back to actual speed
+        let actual_head = AffinityLaws::scale_head(head_m, self.speed_ratio);
+
+        // Convert head to pressure: P = ρ × g × H
+        const G: f64 = 9.80665; // m/s²
+        self.fluid_density_kg_per_m3 * G * actual_head
+    }
+
+    /// Calculates the efficiency at the given flow rate.
+    ///
+    /// Applies affinity laws to find the equivalent operating point.
+    pub fn efficiency(&self, flow_m3_per_s: f64) -> f64 {
+        // Handle zero speed - pump is not running
+        if self.speed_ratio <= 0.0 {
+            return 0.0;
+        }
+
+        // Handle zero flow
+        if flow_m3_per_s <= 0.0 {
+            return self.curves.efficiency_at_flow(0.0);
+        }
+
+        let equivalent_flow = AffinityLaws::unscale_flow(flow_m3_per_s, self.speed_ratio);
+        self.curves.efficiency_at_flow(equivalent_flow)
+    }
+
+    /// Calculates the hydraulic power consumption.
+    ///
+    /// P_hydraulic = Q × ΔP / η
+    ///
+    /// # Arguments
+    ///
+    /// * `flow_m3_per_s` - Volumetric flow rate in m³/s
+    ///
+    /// # Returns
+    ///
+    /// Power in Watts
+    pub fn hydraulic_power(&self, flow_m3_per_s: f64) -> Power {
+        if flow_m3_per_s <= 0.0 || self.speed_ratio <= 0.0 {
+            return Power::from_watts(0.0);
+        }
+
+        let delta_p = self.pressure_rise(flow_m3_per_s);
+        let eta = self.efficiency(flow_m3_per_s);
+
+        if eta <= 0.0 {
+            return Power::from_watts(0.0);
+        }
+
+        // P = Q × ΔP / η
+        let power_w = flow_m3_per_s * delta_p / eta;
+        Power::from_watts(power_w)
+    }
+
+    /// Calculates mass flow rate from volumetric flow.
+    pub fn mass_flow_from_volumetric(&self, flow_m3_per_s: f64) -> MassFlow {
+        MassFlow::from_kg_per_s(flow_m3_per_s * self.fluid_density_kg_per_m3)
+    }
+
+    /// Calculates volumetric flow rate from mass flow.
+    pub fn volumetric_from_mass_flow(&self, mass_flow: MassFlow) -> f64 {
+        mass_flow.to_kg_per_s() / self.fluid_density_kg_per_m3
+    }
+
+    /// Returns the fluid density.
+    pub fn fluid_density(&self) -> f64 {
+        self.fluid_density_kg_per_m3
+    }
+
+    /// Returns the performance curves.
+    pub fn curves(&self) -> &PumpCurves {
+        &self.curves
+    }
+
+    /// Returns the speed ratio.
+    pub fn speed_ratio(&self) -> f64 {
+        self.speed_ratio
+    }
+
+    /// Sets the speed ratio (0.0 to 1.0).
+    pub fn set_speed_ratio(&mut self, ratio: f64) -> Result<(), ComponentError> {
+        if !(0.0..=1.0).contains(&ratio) {
+            return Err(ComponentError::InvalidState(
+                "Speed ratio must be between 0.0 and 1.0".to_string(),
+            ));
+        }
+        self.speed_ratio = ratio;
+        Ok(())
+    }
+
+    /// Returns both ports as a slice for solver topology.
+    pub fn get_ports_slice(&self) -> [&Port<Connected>; 2] {
+        [&self.port_inlet, &self.port_outlet]
+    }
+}
+
+impl Component for Pump<Connected> {
+    fn compute_residuals(
+        &self,
+        state: &SystemState,
+        residuals: &mut ResidualVector,
+    ) -> Result<(), ComponentError> {
+        if residuals.len() != self.n_equations() {
+            return Err(ComponentError::InvalidResidualDimensions {
+                expected: self.n_equations(),
+                actual: residuals.len(),
+            });
+        }
+
+        // Handle operational states
+        match self.operational_state {
+            OperationalState::Off => {
+                residuals[0] = state[0]; // Mass flow = 0
+                residuals[1] = 0.0; // No energy transfer
+                return Ok(());
+            }
+            OperationalState::Bypass => {
+                // Behaves as a pipe: no pressure rise, no energy change
+                let p_in = self.port_inlet.pressure().to_pascals();
+                let p_out = self.port_outlet.pressure().to_pascals();
+                let h_in = self.port_inlet.enthalpy().to_joules_per_kg();
+                let h_out = self.port_outlet.enthalpy().to_joules_per_kg();
+
+                residuals[0] = p_in - p_out;
+                residuals[1] = h_in - h_out;
+                return Ok(());
+            }
+            OperationalState::On => {}
+        }
+
+        if state.len() < 2 {
+            return Err(ComponentError::InvalidStateDimensions {
+                expected: 2,
+                actual: state.len(),
+            });
+        }
+
+        // State: [mass_flow_kg_s, power_w]
+        let mass_flow_kg_s = state[0];
+        let _power_w = state[1];
+
+        // Convert to volumetric flow
+        let flow_m3_s = mass_flow_kg_s / self.fluid_density_kg_per_m3;
+
+        // Calculate pressure rise from curves
+        let delta_p_calc = self.pressure_rise(flow_m3_s);
+
+        // Get port pressures
+        let p_in = self.port_inlet.pressure().to_pascals();
+        let p_out = self.port_outlet.pressure().to_pascals();
+        let delta_p_actual = p_out - p_in;
+
+        // Residual 0: Pressure balance
+        residuals[0] = delta_p_calc - delta_p_actual;
+
+        // Residual 1: Power balance
+        let power_calc = self.hydraulic_power(flow_m3_s).to_watts();
+        residuals[1] = power_calc - _power_w;
+
+        Ok(())
+    }
+
+    fn jacobian_entries(
+        &self,
+        state: &SystemState,
+        jacobian: &mut JacobianBuilder,
+    ) -> Result<(), ComponentError> {
+        if state.len() < 2 {
+            return Err(ComponentError::InvalidStateDimensions {
+                expected: 2,
+                actual: state.len(),
+            });
+        }
+
+        let mass_flow_kg_s = state[0];
+        let flow_m3_s = mass_flow_kg_s / self.fluid_density_kg_per_m3;
+
+        // Numerical derivative of pressure with respect to mass flow
+        let h = 0.001;
+        let p_plus = self.pressure_rise(flow_m3_s + h / self.fluid_density_kg_per_m3);
+        let p_minus = self.pressure_rise(flow_m3_s - h / self.fluid_density_kg_per_m3);
+        let dp_dm = (p_plus - p_minus) / (2.0 * h);
+
+        // ∂r₀/∂ṁ = dΔP/dṁ
+        jacobian.add_entry(0, 0, dp_dm);
+
+        // ∂r₀/∂P = -1 (constant)
+        jacobian.add_entry(0, 1, 0.0);
+
+        // Numerical derivative of power with respect to mass flow
+        let pow_plus = self
+            .hydraulic_power(flow_m3_s + h / self.fluid_density_kg_per_m3)
+            .to_watts();
+        let pow_minus = self
+            .hydraulic_power(flow_m3_s - h / self.fluid_density_kg_per_m3)
+            .to_watts();
+        let dpow_dm = (pow_plus - pow_minus) / (2.0 * h);
+
+        // ∂r₁/∂ṁ
+        jacobian.add_entry(1, 0, dpow_dm);
+
+        // ∂r₁/∂P = -1
+        jacobian.add_entry(1, 1, -1.0);
+
+        Ok(())
+    }
+
+    fn n_equations(&self) -> usize {
+        2
+    }
+
+    fn get_ports(&self) -> &[ConnectedPort] {
+        &[]
+    }
+}
+
+impl StateManageable for Pump<Connected> {
+    fn state(&self) -> OperationalState {
+        self.operational_state
+    }
+
+    fn set_state(&mut self, state: OperationalState) -> Result<(), ComponentError> {
+        if self.operational_state.can_transition_to(state) {
+            let from = self.operational_state;
+            self.operational_state = state;
+            self.on_state_change(from, state);
+            Ok(())
+        } else {
+            Err(ComponentError::InvalidStateTransition {
+                from: self.operational_state,
+                to: state,
+                reason: "Transition not allowed".to_string(),
+            })
+        }
+    }
+
+    fn can_transition_to(&self, target: OperationalState) -> bool {
+        self.operational_state.can_transition_to(target)
+    }
+
+    fn circuit_id(&self) -> &CircuitId {
+        &self.circuit_id
+    }
+
+    fn set_circuit_id(&mut self, circuit_id: CircuitId) {
+        self.circuit_id = circuit_id;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::port::FluidId;
+    use approx::assert_relative_eq;
+    use entropyk_core::{Enthalpy, Pressure};
+
+    fn create_test_curves() -> PumpCurves {
+        // Typical small pump:
+        // H = 30 - 10*Q - 50*Q² (m, Q in m³/s)
+        // η = 0.6 + 1.0*Q - 2.0*Q²
+        PumpCurves::quadratic(30.0, -10.0, -50.0, 0.6, 1.0, -2.0).unwrap()
+    }
+
+    fn create_test_pump_disconnected() -> Pump<Disconnected> {
+        let curves = create_test_curves();
+        let inlet = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(100000.0),
+        );
+        let outlet = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(100000.0),
+        );
+        Pump::new(curves, inlet, outlet, 1000.0).unwrap()
+    }
+
+    fn create_test_pump_connected() -> Pump<Connected> {
+        let curves = create_test_curves();
+        let inlet = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(100000.0),
+        );
+        let outlet = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(100000.0),
+        );
+        let (inlet_conn, outlet_conn) = inlet.connect(outlet).unwrap();
+
+        Pump {
+            curves,
+            port_inlet: inlet_conn,
+            port_outlet: outlet_conn,
+            fluid_density_kg_per_m3: 1000.0,
+            speed_ratio: 1.0,
+            circuit_id: CircuitId::default(),
+            operational_state: OperationalState::default(),
+            _state: PhantomData,
+        }
+    }
+
+    #[test]
+    fn test_pump_curves_creation() {
+        let curves = create_test_curves();
+        assert_eq!(curves.head_at_flow(0.0), 30.0);
+        assert_relative_eq!(curves.efficiency_at_flow(0.0), 0.6);
+    }
+
+    #[test]
+    fn test_pump_curves_head() {
+        let curves = create_test_curves();
+        // H = 30 - 10*0.5 - 50*0.25 = 30 - 5 - 12.5 = 12.5 m
+        let head = curves.head_at_flow(0.5);
+        assert_relative_eq!(head, 12.5, epsilon = 1e-10);
+    }
+
+    #[test]
+    fn test_pump_curves_efficiency_clamped() {
+        let curves = create_test_curves();
+        // At very high flow, efficiency might go negative
+        // Should be clamped to 0
+        let eff = curves.efficiency_at_flow(10.0);
+        assert!(eff >= 0.0);
+    }
+
+    #[test]
+    fn test_pump_creation() {
+        let pump = create_test_pump_disconnected();
+        assert_eq!(pump.fluid_density(), 1000.0);
+        assert_eq!(pump.speed_ratio(), 1.0);
+    }
+
+    #[test]
+    fn test_pump_invalid_density() {
+        let curves = create_test_curves();
+        let inlet = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(100000.0),
+        );
+        let outlet = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(100000.0),
+        );
+
+        let result = Pump::new(curves, inlet, outlet, -1.0);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_pump_different_fluids() {
+        let curves = create_test_curves();
+        let inlet = Port::new(
+            FluidId::new("Water"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(100000.0),
+        );
+        let outlet = Port::new(
+            FluidId::new("Glycol"),
+            Pressure::from_bar(1.0),
+            Enthalpy::from_joules_per_kg(100000.0),
+        );
+
+        let result = Pump::new(curves, inlet, outlet, 1000.0);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_pump_set_speed_ratio() {
+        let mut pump = create_test_pump_connected();
+        assert!(pump.set_speed_ratio(0.8).is_ok());
+        assert_eq!(pump.speed_ratio(), 0.8);
+    }
+
+    #[test]
+    fn test_pump_set_speed_ratio_invalid() {
+        let mut pump = create_test_pump_connected();
+        assert!(pump.set_speed_ratio(1.5).is_err());
+        assert!(pump.set_speed_ratio(-0.1).is_err());
+    }
+
+    #[test]
+    fn test_pump_pressure_rise_full_speed() {
+        let pump = create_test_pump_connected();
+        // At Q=0: H=30m, P = 1000 * 9.8 * 30 ≈ 294200 Pa
+        let delta_p = pump.pressure_rise(0.0);
+        let expected = 1000.0 * 9.80665 * 30.0;
+        assert_relative_eq!(delta_p, expected, epsilon = 100.0);
+    }
+
+    #[test]
+    fn test_pump_pressure_rise_reduced_speed() {
+        let mut pump = create_test_pump_connected();
+        pump.set_speed_ratio(0.5).unwrap();
+
+        // At 50% speed, shut-off head is 25% of full speed
+        // H = 0.25 * 30 = 7.5 m
+        let delta_p = pump.pressure_rise(0.0);
+        let expected = 1000.0 * 9.80665 * 7.5;
+        assert_relative_eq!(delta_p, expected, epsilon = 100.0);
+    }
+
+    #[test]
+    fn test_pump_hydraulic_power() {
+        let pump = create_test_pump_connected();
+
+        // At Q=0.1 m³/s: H ≈ 30 - 1 - 0.5 = 28.5 m
+        // η ≈ 0.6 + 0.1 - 0.02 = 0.68
+        // P = 1000 * 9.8 * 0.1 * 28.5 / 0.68 ≈ 4110 W
+        let power = pump.hydraulic_power(0.1);
+        assert!(power.to_watts() > 0.0);
+        assert!(power.to_watts() < 50000.0);
+    }
+
+    #[test]
+    fn test_pump_affinity_laws_power() {
+        let pump_full = create_test_pump_connected();
+
+        let mut pump_half = create_test_pump_connected();
+        pump_half.set_speed_ratio(0.5).unwrap();
+
+        // Power at half speed should be ~12.5% of full speed (cube law)
+        // At the same equivalent flow point
+        let power_full = pump_full.hydraulic_power(0.1);
+        let power_half = pump_half.hydraulic_power(0.05); // Half the flow
+
+        // P_half / P_full ≈ 0.5³ = 0.125
+        let ratio = power_half.to_watts() / power_full.to_watts();
+        assert_relative_eq!(ratio, 0.125, epsilon = 0.05);
+    }
+
+    #[test]
+    fn test_pump_component_n_equations() {
+        let pump = create_test_pump_connected();
+        assert_eq!(pump.n_equations(), 2);
+    }
+
+    #[test]
+    fn test_pump_component_compute_residuals() {
+        let pump = create_test_pump_connected();
+        let state = vec![50.0, 2000.0]; // mass flow, power
+        let mut residuals = vec![0.0; 2];
+
+        let result = pump.compute_residuals(&state, &mut residuals);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_pump_state_manageable() {
+        let pump = create_test_pump_connected();
+        assert_eq!(pump.state(), OperationalState::On);
+        assert!(pump.can_transition_to(OperationalState::Off));
+    }
+}

crates/components/src/state_machine.rs

@@ -0,0 +1,940 @@
+//! Component State Machine and Circuit Management
+//!
+//! This module provides types for managing component operational states (ON/OFF/BYPASS)
+//! and circuit identification, as required by FR6-FR9 of the Entropyk specification.
+//!
+//! ## Operational States
+//!
+//! Components can be in one of three operational states:
+//! - **On**: Normal operation with full thermodynamic behavior
+//! - **Off**: Component contributes zero mass flow (FR7)
+//! - **Bypass**: Component behaves as an adiabatic pipe (FR8)
+//!
+//! ## Circuit Identification
+//!
+//! Each component belongs to a specific circuit identified by a `CircuitId`.
+//! Multi-circuit machines allow simulation of complex systems like dual-circuit
+//! heat pumps (FR9).
+//!
+//! ## State Management
+//!
+//! The [`StateManageable`] trait provides a common interface for components
+//! that support operational state management. All major components (Compressor,
+//! ExpansionValve, HeatExchanger) implement this trait.
+//!
+//! ## State History
+//!
+//! For debugging purposes, the [`StateHistory`] type can track state transitions
+//! with timestamps.
+//!
+//! ## Example
+//!
+//! ```rust
+//! use entropyk_components::state_machine::{OperationalState, CircuitId, StateManageable};
+//!
+//! // Create a circuit identifier
+//! let circuit = CircuitId::new("primary");
+//!
+//! // Set component state
+//! let state = OperationalState::On;
+//!
+//! // Check transitions
+//! assert!(state.can_transition_to(OperationalState::Off));
+//! ```
+
+use std::collections::VecDeque;
+use std::time::Instant;
+
+use crate::ComponentError;
+
+/// Error type for invalid state transitions.
+///
+/// This error is returned when attempting an invalid state transition.
+/// Currently, all transitions between states are allowed, but this type
+/// is provided for future extensibility and custom transition rules.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct StateTransitionError {
+    /// The state we're transitioning from
+    pub from: OperationalState,
+    /// The state we're attempting to transition to
+    pub to: OperationalState,
+    /// Human-readable reason for the failure
+    pub reason: &'static str,
+}
+
+impl std::fmt::Display for StateTransitionError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "Invalid state transition from {:?} to {:?}: {}",
+            self.from, self.to, self.reason
+        )
+    }
+}
+
+impl std::error::Error for StateTransitionError {}
+
+impl From<StateTransitionError> for crate::ComponentError {
+    fn from(err: StateTransitionError) -> Self {
+        crate::ComponentError::InvalidStateTransition {
+            from: err.from,
+            to: err.to,
+            reason: err.reason.to_string(),
+        }
+    }
+}
+
+/// Operational state of a component.
+///
+/// This enum represents the three possible operational states of a component
+/// as defined in FR6-FR8:
+///
+/// - **On**: Normal operation with full thermodynamic calculations
+/// - **Off**: Component contributes zero mass flow to the system
+/// - **Bypass**: Component behaves as an adiabatic pipe (P_in = P_out, h_in = h_out)
+///
+/// # State Behavior
+///
+/// | State | Mass Flow | Energy Transfer | Pressure Drop |
+/// |-------|-----------|-----------------|---------------|
+/// | On    | Normal    | Full            | Normal        |
+/// | Off   | Zero      | None            | Infinite      |
+/// | Bypass| Continuity| None (adiabatic)| Zero          |
+///
+/// # Examples
+///
+/// ```
+/// use entropyk_components::state_machine::OperationalState;
+///
+/// let state = OperationalState::On;
+/// assert!(state.is_active());
+///
+/// let state = OperationalState::Off;
+/// assert!(!state.is_active());
+/// ```
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum OperationalState {
+    /// Normal operation with full thermodynamic behavior.
+    ///
+    /// In this state, the component performs its full thermodynamic calculations,
+    /// including heat transfer, pressure changes, and work transfer.
+    On,
+
+    /// Component is turned off, contributing zero mass flow.
+    ///
+    /// When a component is in the Off state (FR7):
+    /// - Mass flow through the component is forced to zero
+    /// - The component acts as a blockage in the circuit
+    /// - No heat transfer or work transfer occurs
+    /// - This state is used for simulating component failures or seasonal operation
+    Off,
+
+    /// Bypass mode - component behaves as an adiabatic pipe.
+    ///
+    /// When a component is in the Bypass state (FR8):
+    /// - Pressure at inlet equals pressure at outlet (P_in = P_out)
+    /// - Enthalpy at inlet equals enthalpy at outlet (h_in = h_out)
+    /// - No heat transfer occurs (adiabatic)
+    /// - Mass flow continues through the bypass path
+    /// - This state is useful for economizers in summer mode or valve bypasses
+    Bypass,
+}
+
+impl OperationalState {
+    /// Returns true if the component is active (On or Bypass).
+    ///
+    /// An active component allows mass flow through it, though Bypass
+    /// mode has different thermodynamic behavior than On mode.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::OperationalState;
+    ///
+    /// assert!(OperationalState::On.is_active());
+    /// assert!(OperationalState::Bypass.is_active());
+    /// assert!(!OperationalState::Off.is_active());
+    /// ```
+    pub fn is_active(&self) -> bool {
+        matches!(self, OperationalState::On | OperationalState::Bypass)
+    }
+
+    /// Returns true if the component is in normal operation mode.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::OperationalState;
+    ///
+    /// assert!(OperationalState::On.is_on());
+    /// assert!(!OperationalState::Off.is_on());
+    /// assert!(!OperationalState::Bypass.is_on());
+    /// ```
+    pub fn is_on(&self) -> bool {
+        matches!(self, OperationalState::On)
+    }
+
+    /// Returns true if the component is off.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::OperationalState;
+    ///
+    /// assert!(OperationalState::Off.is_off());
+    /// assert!(!OperationalState::On.is_off());
+    /// ```
+    pub fn is_off(&self) -> bool {
+        matches!(self, OperationalState::Off)
+    }
+
+    /// Returns true if the component is in bypass mode.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::OperationalState;
+    ///
+    /// assert!(OperationalState::Bypass.is_bypass());
+    /// assert!(!OperationalState::On.is_bypass());
+    /// ```
+    pub fn is_bypass(&self) -> bool {
+        matches!(self, OperationalState::Bypass)
+    }
+
+    /// Returns the mass flow multiplier for this state.
+    ///
+    /// This multiplier is used in residual calculations:
+    /// - On: 1.0 (full mass flow)
+    /// - Bypass: 1.0 (mass flow continues through bypass)
+    /// - Off: 0.0 (no mass flow)
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::OperationalState;
+    ///
+    /// assert_eq!(OperationalState::On.mass_flow_multiplier(), 1.0);
+    /// assert_eq!(OperationalState::Bypass.mass_flow_multiplier(), 1.0);
+    /// assert_eq!(OperationalState::Off.mass_flow_multiplier(), 0.0);
+    /// ```
+    pub fn mass_flow_multiplier(&self) -> f64 {
+        match self {
+            OperationalState::On => 1.0,
+            OperationalState::Off => 0.0,
+            OperationalState::Bypass => 1.0,
+        }
+    }
+
+    /// Checks if a transition to the target state is valid.
+    ///
+    /// Currently, all state transitions are allowed. This method is provided
+    /// for components that may have custom transition rules.
+    ///
+    /// # Arguments
+    ///
+    /// * `target` - The target operational state
+    ///
+    /// # Returns
+    ///
+    /// Returns `true` if the transition is valid.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::OperationalState;
+    ///
+    /// assert!(OperationalState::On.can_transition_to(OperationalState::Off));
+    /// assert!(OperationalState::Off.can_transition_to(OperationalState::Bypass));
+    /// assert!(OperationalState::Bypass.can_transition_to(OperationalState::On));
+    /// ```
+    pub fn can_transition_to(&self, target: OperationalState) -> bool {
+        matches!(
+            (self, target),
+            (OperationalState::On, OperationalState::Off)
+                | (OperationalState::On, OperationalState::Bypass)
+                | (OperationalState::Off, OperationalState::On)
+                | (OperationalState::Off, OperationalState::Bypass)
+                | (OperationalState::Bypass, OperationalState::On)
+                | (OperationalState::Bypass, OperationalState::Off)
+                | (OperationalState::On, OperationalState::On)
+                | (OperationalState::Off, OperationalState::Off)
+                | (OperationalState::Bypass, OperationalState::Bypass)
+        )
+    }
+
+    /// Attempts to transition to the target state.
+    ///
+    /// # Arguments
+    ///
+    /// * `target` - The target operational state
+    ///
+    /// # Returns
+    ///
+    /// Returns `Ok(target)` if the transition is valid, or a [`StateTransitionError`]
+    /// if the transition is not allowed.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::OperationalState;
+    ///
+    /// let state = OperationalState::On;
+    /// let new_state = state.transition_to(OperationalState::Off).unwrap();
+    /// assert_eq!(new_state, OperationalState::Off);
+    /// ```
+    pub fn transition_to(
+        &self,
+        target: OperationalState,
+    ) -> Result<OperationalState, StateTransitionError> {
+        if self.can_transition_to(target) {
+            Ok(target)
+        } else {
+            Err(StateTransitionError {
+                from: *self,
+                to: target,
+                reason: "Transition not allowed",
+            })
+        }
+    }
+}
+
+impl Default for OperationalState {
+    /// Default operational state is On.
+    fn default() -> Self {
+        OperationalState::On
+    }
+}
+
+/// Unique identifier for a thermodynamic circuit.
+///
+/// A `CircuitId` identifies a complete fluid circuit within a machine.
+/// Multi-circuit machines (e.g., dual-circuit heat pumps) require distinct
+/// identifiers for each independent fluid loop (FR9).
+///
+/// # Use Cases
+///
+/// - Single-circuit machines: Use "default" or "main"
+/// - Dual-circuit heat pumps: Use "circuit_1" and "circuit_2"
+/// - Complex systems: Use descriptive names like "primary", "secondary", "economizer"
+///
+/// # Examples
+///
+/// ```
+/// use entropyk_components::state_machine::CircuitId;
+///
+/// let main_circuit = CircuitId::new("main");
+/// let secondary = CircuitId::new("secondary");
+///
+/// assert_ne!(main_circuit, secondary);
+/// ```
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct CircuitId(String);
+
+impl CircuitId {
+    /// Creates a new circuit identifier from a string.
+    ///
+    /// # Arguments
+    ///
+    /// * `id` - A unique string identifier for the circuit
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::CircuitId;
+    ///
+    /// let circuit = CircuitId::new("primary");
+    /// ```
+    pub fn new(id: impl Into<String>) -> Self {
+        Self(id.into())
+    }
+
+    /// Returns the circuit identifier as a string slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::CircuitId;
+    ///
+    /// let circuit = CircuitId::new("main");
+    /// assert_eq!(circuit.as_str(), "main");
+    /// ```
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+
+    /// Creates a default circuit identifier.
+    ///
+    /// Returns a CircuitId with value "default".
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::CircuitId;
+    ///
+    /// let default = CircuitId::default_circuit();
+    /// assert_eq!(default.as_str(), "default");
+    /// ```
+    pub fn default_circuit() -> Self {
+        Self("default".to_string())
+    }
+}
+
+impl Default for CircuitId {
+    /// Default circuit identifier is "default".
+    fn default() -> Self {
+        Self("default".to_string())
+    }
+}
+
+impl AsRef<str> for CircuitId {
+    fn as_ref(&self) -> &str {
+        &self.0
+    }
+}
+
+impl std::fmt::Display for CircuitId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// Record of a state transition for debugging purposes.
+///
+/// Tracks when a component changed states, what the previous state was,
+/// and what the new state is.
+#[derive(Debug, Clone)]
+pub struct StateTransitionRecord {
+    /// Timestamp when the transition occurred
+    pub timestamp: Instant,
+    /// State before the transition
+    pub from_state: OperationalState,
+    /// State after the transition
+    pub to_state: OperationalState,
+}
+
+impl StateTransitionRecord {
+    /// Creates a new state transition record.
+    ///
+    /// # Arguments
+    ///
+    /// * `from_state` - The state before transition
+    /// * `to_state` - The state after transition
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::{StateTransitionRecord, OperationalState};
+    ///
+    /// let record = StateTransitionRecord::new(OperationalState::On, OperationalState::Off);
+    /// assert_eq!(record.from_state, OperationalState::On);
+    /// assert_eq!(record.to_state, OperationalState::Off);
+    /// ```
+    pub fn new(from_state: OperationalState, to_state: OperationalState) -> Self {
+        Self {
+            timestamp: Instant::now(),
+            from_state,
+            to_state,
+        }
+    }
+
+    /// Returns the elapsed time since this transition occurred.
+    pub fn elapsed(&self) -> std::time::Duration {
+        self.timestamp.elapsed()
+    }
+}
+
+/// History buffer for tracking state transitions.
+///
+/// Maintains a configurable-size buffer of recent state transitions
+/// for debugging and analysis purposes.
+///
+/// # Examples
+///
+/// ```
+/// use entropyk_components::state_machine::{StateHistory, OperationalState};
+///
+/// let mut history = StateHistory::new(10);
+/// history.record(OperationalState::On, OperationalState::Off);
+///
+/// assert_eq!(history.len(), 1);
+/// assert_eq!(history.records()[0].from_state, OperationalState::On);
+/// ```
+#[derive(Debug, Clone)]
+pub struct StateHistory {
+    records: VecDeque<StateTransitionRecord>,
+    max_depth: usize,
+}
+
+impl StateHistory {
+    /// Default maximum history depth.
+    pub const DEFAULT_MAX_DEPTH: usize = 10;
+
+    /// Creates a new state history with the specified maximum depth.
+    ///
+    /// # Arguments
+    ///
+    /// * `max_depth` - Maximum number of records to keep
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::StateHistory;
+    ///
+    /// let history = StateHistory::new(20);
+    /// assert_eq!(history.max_depth(), 20);
+    /// ```
+    pub fn new(max_depth: usize) -> Self {
+        Self {
+            records: VecDeque::with_capacity(max_depth),
+            max_depth,
+        }
+    }
+
+    /// Creates a new state history with default depth (10 records).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::StateHistory;
+    ///
+    /// let history = StateHistory::default();
+    /// assert_eq!(history.max_depth(), 10);
+    /// ```
+    pub fn with_default_depth() -> Self {
+        Self::new(Self::DEFAULT_MAX_DEPTH)
+    }
+
+    /// Returns the maximum number of records this history can hold.
+    pub fn max_depth(&self) -> usize {
+        self.max_depth
+    }
+
+    /// Records a state transition.
+    ///
+    /// If the history is full, the oldest record is removed.
+    ///
+    /// # Arguments
+    ///
+    /// * `from_state` - The state before transition
+    /// * `to_state` - The state after transition
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use entropyk_components::state_machine::{StateHistory, OperationalState};
+    ///
+    /// let mut history = StateHistory::default();
+    /// history.record(OperationalState::On, OperationalState::Off);
+    ///
+    /// assert_eq!(history.len(), 1);
+    /// ```
+    pub fn record(&mut self, from_state: OperationalState, to_state: OperationalState) {
+        if self.records.len() >= self.max_depth {
+            self.records.pop_front();
+        }
+        self.records
+            .push_back(StateTransitionRecord::new(from_state, to_state));
+    }
+
+    /// Returns the number of records in the history.
+    pub fn len(&self) -> usize {
+        self.records.len()
+    }
+
+    /// Returns true if there are no records in the history.
+    pub fn is_empty(&self) -> bool {
+        self.records.is_empty()
+    }
+
+    /// Returns a slice of all records, oldest first.
+    pub fn records(&self) -> &[StateTransitionRecord] {
+        self.records.as_slices().0
+    }
+
+    /// Returns the most recent transition record, if any.
+    pub fn last(&self) -> Option<&StateTransitionRecord> {
+        self.records.back()
+    }
+
+    /// Clears all records from the history.
+    pub fn clear(&mut self) {
+        self.records.clear();
+    }
+}
+
+impl Default for StateHistory {
+    fn default() -> Self {
+        Self::with_default_depth()
+    }
+}
+
+/// Trait for components that support operational state management.
+///
+/// This trait provides a common interface for managing the operational
+/// state of thermodynamic components. All major components (Compressor,
+/// ExpansionValve, HeatExchanger) implement this trait.
+///
+/// # Object Safety
+///
+/// This trait is object-safe and can be used with dynamic dispatch.
+///
+/// # Callback Hooks
+///
+/// The trait provides optional callback hooks via `on_state_change()` which
+/// can be overridden to perform actions when state transitions occur.
+///
+/// # Examples
+///
+/// ```rust,ignore
+/// use entropyk_components::state_machine::{StateManageable, OperationalState, CircuitId};
+/// use entropyk_components::ComponentError;
+///
+/// fn check_component_state(component: &dyn StateManageable) {
+///     println!("Component state: {:?}", component.state());
+///     println!("Circuit: {}", component.circuit_id().as_str());
+/// }
+/// ```
+pub trait StateManageable {
+    /// Returns the current operational state.
+    fn state(&self) -> OperationalState;
+
+    /// Sets the operational state with validation.
+    ///
+    /// # Arguments
+    ///
+    /// * `state` - The new operational state
+    ///
+    /// # Returns
+    ///
+    /// Returns `Ok(())` if the transition is valid, or a [`ComponentError`]
+    /// if the transition is not allowed.
+    ///
+    /// # Errors
+    ///
+    /// Returns [`ComponentError::InvalidStateTransition`] if the transition
+    /// is not valid for this component.
+    fn set_state(&mut self, state: OperationalState) -> Result<(), ComponentError>;
+
+    /// Checks if a transition to the target state is valid.
+    ///
+    /// # Arguments
+    ///
+    /// * `target` - The target operational state
+    ///
+    /// # Returns
+    ///
+    /// Returns `true` if the transition is valid.
+    fn can_transition_to(&self, target: OperationalState) -> bool;
+
+    /// Returns the circuit identifier.
+    fn circuit_id(&self) -> &CircuitId;
+
+    /// Sets the circuit identifier.
+    ///
+    /// # Arguments
+    ///
+    /// * `circuit_id` - The new circuit identifier
+    fn set_circuit_id(&mut self, circuit_id: CircuitId);
+
+    /// Optional callback invoked after a state change.
+    ///
+    /// Override this method to perform actions when the component's state changes,
+    /// such as logging, updating internal state, or triggering side effects.
+    ///
+    /// # Arguments
+    ///
+    /// * `from` - The previous operational state
+    /// * `to` - The new operational state
+    ///
+    /// # Default Implementation
+    ///
+    /// The default implementation does nothing. Override to add custom behavior.
+    fn on_state_change(&mut self, _from: OperationalState, _to: OperationalState) {
+        // Default: no-op. Override to add callback behavior.
+    }
+
+    /// Returns the state transition history, if tracking is enabled.
+    ///
+    /// Components can optionally track state transition history for debugging.
+    /// By default, this returns `None`. Override to return a reference to
+    /// the component's state history.
+    ///
+    /// # Returns
+    ///
+    /// Returns `Some(&StateHistory)` if history tracking is enabled, or `None` otherwise.
+    fn state_history(&self) -> Option<&StateHistory> {
+        None
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_operational_state_on() {
+        let state = OperationalState::On;
+        assert!(state.is_on());
+        assert!(!state.is_off());
+        assert!(!state.is_bypass());
+        assert!(state.is_active());
+        assert_eq!(state.mass_flow_multiplier(), 1.0);
+    }
+
+    #[test]
+    fn test_operational_state_off() {
+        let state = OperationalState::Off;
+        assert!(!state.is_on());
+        assert!(state.is_off());
+        assert!(!state.is_bypass());
+        assert!(!state.is_active());
+        assert_eq!(state.mass_flow_multiplier(), 0.0);
+    }
+
+    #[test]
+    fn test_operational_state_bypass() {
+        let state = OperationalState::Bypass;
+        assert!(!state.is_on());
+        assert!(!state.is_off());
+        assert!(state.is_bypass());
+        assert!(state.is_active());
+        assert_eq!(state.mass_flow_multiplier(), 1.0);
+    }
+
+    #[test]
+    fn test_operational_state_default() {
+        let state: OperationalState = Default::default();
+        assert!(state.is_on());
+    }
+
+    #[test]
+    fn test_operational_state_equality() {
+        assert_eq!(OperationalState::On, OperationalState::On);
+        assert_eq!(OperationalState::Off, OperationalState::Off);
+        assert_eq!(OperationalState::Bypass, OperationalState::Bypass);
+        assert_ne!(OperationalState::On, OperationalState::Off);
+        assert_ne!(OperationalState::On, OperationalState::Bypass);
+    }
+
+    #[test]
+    fn test_can_transition_to_all_combinations() {
+        let states = [
+            OperationalState::On,
+            OperationalState::Off,
+            OperationalState::Bypass,
+        ];
+
+        for from in states {
+            for to in states {
+                assert!(
+                    from.can_transition_to(to),
+                    "Transition from {:?} to {:?} should be allowed",
+                    from,
+                    to
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn test_transition_to_success() {
+        let state = OperationalState::On;
+        let result = state.transition_to(OperationalState::Off);
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap(), OperationalState::Off);
+    }
+
+    #[test]
+    fn test_transition_to_same_state() {
+        let state = OperationalState::On;
+        let result = state.transition_to(OperationalState::On);
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap(), OperationalState::On);
+    }
+
+    #[test]
+    fn test_state_transition_error_display() {
+        let err = StateTransitionError {
+            from: OperationalState::On,
+            to: OperationalState::Off,
+            reason: "Test reason",
+        };
+        let msg = format!("{}", err);
+        assert!(msg.contains("Invalid state transition"));
+        assert!(msg.contains("On"));
+        assert!(msg.contains("Off"));
+        assert!(msg.contains("Test reason"));
+    }
+
+    #[test]
+    fn test_circuit_id_creation() {
+        let circuit = CircuitId::new("main");
+        assert_eq!(circuit.as_str(), "main");
+    }
+
+    #[test]
+    fn test_circuit_id_from_string() {
+        let name = String::from("secondary");
+        let circuit = CircuitId::new(name);
+        assert_eq!(circuit.as_str(), "secondary");
+    }
+
+    #[test]
+    fn test_circuit_id_default() {
+        let circuit = CircuitId::default();
+        assert_eq!(circuit.as_str(), "default");
+    }
+
+    #[test]
+    fn test_circuit_id_default_circuit() {
+        let circuit = CircuitId::default_circuit();
+        assert_eq!(circuit.as_str(), "default");
+    }
+
+    #[test]
+    fn test_circuit_id_equality() {
+        let c1 = CircuitId::new("circuit_1");
+        let c2 = CircuitId::new("circuit_1");
+        let c3 = CircuitId::new("circuit_2");
+
+        assert_eq!(c1, c2);
+        assert_ne!(c1, c3);
+    }
+
+    #[test]
+    fn test_circuit_id_as_ref() {
+        let circuit = CircuitId::new("test");
+        let s: &str = circuit.as_ref();
+        assert_eq!(s, "test");
+    }
+
+    #[test]
+    fn test_circuit_id_display() {
+        let circuit = CircuitId::new("main_circuit");
+        assert_eq!(format!("{}", circuit), "main_circuit");
+    }
+
+    #[test]
+    fn test_circuit_id_hash() {
+        use std::collections::HashMap;
+
+        let mut map = HashMap::new();
+        map.insert(CircuitId::new("c1"), 1);
+        map.insert(CircuitId::new("c2"), 2);
+
+        assert_eq!(map.get(&CircuitId::new("c1")), Some(&1));
+        assert_eq!(map.get(&CircuitId::new("c2")), Some(&2));
+    }
+
+    #[test]
+    fn test_state_transition_record_creation() {
+        let record = StateTransitionRecord::new(OperationalState::On, OperationalState::Off);
+        assert_eq!(record.from_state, OperationalState::On);
+        assert_eq!(record.to_state, OperationalState::Off);
+    }
+
+    #[test]
+    fn test_state_transition_record_elapsed() {
+        let record = StateTransitionRecord::new(OperationalState::On, OperationalState::Off);
+        let elapsed = record.elapsed();
+        assert!(elapsed.as_nanos() >= 0);
+    }
+
+    #[test]
+    fn test_state_history_creation() {
+        let history = StateHistory::new(5);
+        assert_eq!(history.max_depth(), 5);
+        assert!(history.is_empty());
+        assert_eq!(history.len(), 0);
+    }
+
+    #[test]
+    fn test_state_history_default() {
+        let history = StateHistory::default();
+        assert_eq!(history.max_depth(), StateHistory::DEFAULT_MAX_DEPTH);
+    }
+
+    #[test]
+    fn test_state_history_with_default_depth() {
+        let history = StateHistory::with_default_depth();
+        assert_eq!(history.max_depth(), 10);
+    }
+
+    #[test]
+    fn test_state_history_record() {
+        let mut history = StateHistory::new(10);
+        history.record(OperationalState::On, OperationalState::Off);
+
+        assert_eq!(history.len(), 1);
+        assert!(!history.is_empty());
+
+        let records = history.records();
+        assert_eq!(records.len(), 1);
+        assert_eq!(records[0].from_state, OperationalState::On);
+        assert_eq!(records[0].to_state, OperationalState::Off);
+    }
+
+    #[test]
+    fn test_state_history_last() {
+        let mut history = StateHistory::new(10);
+        assert!(history.last().is_none());
+
+        history.record(OperationalState::On, OperationalState::Off);
+        let last = history.last().unwrap();
+        assert_eq!(last.from_state, OperationalState::On);
+        assert_eq!(last.to_state, OperationalState::Off);
+
+        history.record(OperationalState::Off, OperationalState::Bypass);
+        let last = history.last().unwrap();
+        assert_eq!(last.from_state, OperationalState::Off);
+        assert_eq!(last.to_state, OperationalState::Bypass);
+    }
+
+    #[test]
+    fn test_state_history_max_depth() {
+        let mut history = StateHistory::new(3);
+
+        history.record(OperationalState::On, OperationalState::Off);
+        history.record(OperationalState::Off, OperationalState::Bypass);
+        history.record(OperationalState::Bypass, OperationalState::On);
+        assert_eq!(history.len(), 3);
+
+        history.record(OperationalState::On, OperationalState::Off);
+        assert_eq!(history.len(), 3);
+
+        let records = history.records();
+        assert_eq!(records[0].from_state, OperationalState::Off);
+        assert_eq!(records[0].to_state, OperationalState::Bypass);
+    }
+
+    #[test]
+    fn test_state_history_clear() {
+        let mut history = StateHistory::new(10);
+        history.record(OperationalState::On, OperationalState::Off);
+        history.record(OperationalState::Off, OperationalState::Bypass);
+
+        assert_eq!(history.len(), 2);
+
+        history.clear();
+
+        assert!(history.is_empty());
+        assert_eq!(history.len(), 0);
+    }
+
+    #[test]
+    fn test_state_history_multiple_transitions() {
+        let mut history = StateHistory::new(10);
+
+        history.record(OperationalState::On, OperationalState::Off);
+        history.record(OperationalState::Off, OperationalState::Bypass);
+        history.record(OperationalState::Bypass, OperationalState::On);
+
+        assert_eq!(history.len(), 3);
+
+        let records = history.records();
+        assert_eq!(records[0].from_state, OperationalState::On);
+        assert_eq!(records[1].from_state, OperationalState::Off);
+        assert_eq!(records[2].from_state, OperationalState::Bypass);
+    }
+}