Story 1.10: Pipe Helpers for Water and Refrigerant

Status: done

Story

As a HVAC engineer modeling refrigerant and incompressible fluid circuits (water, seawater, glycol), I want convenient constructors Pipe::for_incompressible() and Pipe::for_refrigerant() with explicit ρ/μ from a fluid backend, So that I can create pipes without hardcoding fluid properties in the component.

Architecture: Fluid properties (ρ, μ) belong in the fluids crate (Story 2.7 IncompressibleBackend). Pipe must NOT hardcode water/glycol properties—user obtains them from FluidBackend.

Acceptance Criteria

Pipe::for_incompressible (AC: #1)
- Pipe::for_incompressible(geometry, port_inlet, port_outlet, density, viscosity) — explicit ρ, μ from backend
- Doc states: obtain ρ, μ from IncompressibleBackend (water, seawater, glycol)—do not hardcode
- Doc examples show water and glycol circuit usage
Pipe::for_refrigerant (AC: #2)
- Pipe::for_refrigerant(geometry, port_inlet, port_outlet, density, viscosity) — explicit ρ, μ at design point
- Doc states ρ, μ vary with P,T — design-point values from CoolProp/tabular
- Doc examples show refrigerant circuit usage
Documentation (AC: #3)
- Module-level doc: Pipe serves refrigerant and incompressible (water, seawater, glycol)
- "Fluid Support" section: refrigerant (ρ/μ from backend) vs incompressible (ρ/μ from IncompressibleBackend)
- No hardcoded fluid properties in components crate

Tasks / Subtasks

Add Pipe::for_incompressible (AC: #1)
- Constructor accepting (geometry, ports, density, viscosity)
- Doc: obtain from IncompressibleBackend, do not hardcode
Add Pipe::for_refrigerant (AC: #2)
- Constructor accepting (geometry, ports, density, viscosity)
- Doc: design-point values from CoolProp/tabular
Update documentation (AC: #3)
- pipe.rs module doc: Fluid Support section
- Pipe struct doc: dual refrigerant/incompressible usage
- Doc tests for both constructors
Tests
- test_pipe_for_incompressible_creation
- test_pipe_for_incompressible_glycol
- test_pipe_for_refrigerant_creation
- test_pipe_inlet_outlet_same_fluid

Dev Notes

Previous Story Intelligence

From Story 1.8 (Auxiliary & Transport):

Pipe uses Darcy-Weisbach, Haaland friction factor
PipeGeometry: length_m, diameter_m, roughness_m
Pipe::new(geometry, port_inlet, port_outlet, fluid_density, fluid_viscosity)
Already validates inlet/outlet same FluidId

Typical Values

Fluid	ρ (kg/m³)	μ (Pa·s)
Water 20°C	998	0.001
Water 40°C	992	0.00065
R134a liquid 40°C	~1140	~0.0002
R410A liquid 40°C	~1050	~0.00015

References

pipe.rs: Pipe::new, PipeGeometry
port.rs: FluidId, Port
Story 2.7: Incompressible fluids (water polynomial when done)

Dev Agent Record

Implementation Plan

Added Pipe::for_incompressible(geometry, port_inlet, port_outlet, density, viscosity) — no hardcoding
Added Pipe::for_refrigerant(geometry, port_inlet, port_outlet, density, viscosity)
Module doc: Fluid Support section (refrigerant vs incompressible)
Pipe struct doc: dual refrigerant/incompressible usage
Architecture: Properties (ρ, μ) obtained from FluidBackend (IncompressibleBackend for water/glycol)

Completion Notes

for_incompressible and for_refrigerant: explicit ρ, μ from user (who gets from backend)
No hardcoded water/glycol properties in components crate
Unit tests: test_pipe_for_incompressible_creation, test_pipe_for_incompressible_glycol, test_pipe_for_refrigerant_creation

Architecture Refactor (2026-02-15)

Removed for_water, for_water_at_temp — hardcoded water-only properties (violated FR40, Story 2.7)
Replaced with for_incompressible(density, viscosity) — user provides ρ, μ from IncompressibleBackend
Water, seawater, glycol have different properties — must not hardcode in components

Code Review Fixes (2026-02-21)

Fixed solver bug where jacobian_entries unconditional numerical gradient calculation applied to Off/Bypass states.
Fixed compute_residuals for OperationalState::Bypass to correctly output zero pressure drop (p_in - p_out).
Fixed Haaland friction factor clipping the regularized Reynolds number improperly to 1.0, breaking linear laminar pressure drop curve near 0 flow.
Removed dead and unused code (swamee_jain and simplified friction factors).
Refined numerical differentiation stepping h to avoid numerical instability for zero/tiny mass flows.

File List

crates/components/src/pipe.rs (modified)

Change Log

2026-02-15: Implemented Pipe::for_water, Pipe::for_water_at_temp, Pipe::for_refrigerant
2026-02-15: Code review fixes
2026-02-15: Architecture refactor — Removed hardcoded water properties; replaced with Pipe::for_incompressible(density, viscosity). Properties from FluidBackend (Story 2.7).
2026-02-21: Fixed logic and numerical stability issues found during adversarial code review.

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