Chilled Water Pipe Sizing: engineer's guide
Chilled water pipes are sized to keep pressure drop under a target value (typically 4 ft/100 ft) while keeping velocity in the 2 to 8 fps range to avoid erosion and noise.
Open the Chilled Water Pipe Sizing →
What this calculator does
The calculator applies the Darcy-Weisbach equation with steel pipe roughness and reports pipe diameter, velocity, and friction head loss.
Formulas used
- Flow rate:
GPM = Q (BTU/h) / (500 × ΔT) - Velocity:
V = GPM × 0.4085 / D² (fps for D in inches) - Head loss:
Δh ≈ f × L/D × V²/(2g)
Variables
- Q = cooling load, BTU/h
- ΔT = supply-to-return temperature difference (typically 10 to 14°F)
- D = pipe internal diameter
Worked example
Scenario: 200 TR primary chilled water at ΔT 12°F.
- GPM = 200 × 12,000 / (500 × 12) = 400 GPM
- Trial 4 in schedule 40: V = 400 × 0.4085 / 16 ≈ 10.2 fps — too high
- Upsize to 5 in: V ≈ 6.5 fps — acceptable
- Head loss ≈ 3 ft/100 ft — within target
Common mistakes to avoid
- Velocity above 8 fps for continuous operation (erosion)
- Sizing on nominal ID instead of actual (schedule 40 vs 80 differ)
- Ignoring fitting equivalent length in long runs
- Same ΔT for primary and secondary loops without a decoupler check
Frequently asked questions
Steel or copper for chilled water?
Steel for large diameters > 2 in, copper for small; use compatible corrosion inhibitor for mixed systems.
Insulate chilled water pipes?
Always — both for thermal efficiency and to prevent condensation.
Standards and references
ASHRAE Fundamentals Ch. 22, ASPE Data Book
Related tools on this site
Open the calculator that pairs with this guide: Chilled Water Pipe Sizing.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.