HVAC Duct Pressure Loss
Friction loss in round and rectangular HVAC ducts using Darcy-Weisbach with Swamee-Jain (Colebrook approximation) friction factor. ASHRAE Fundamentals 2021 ch. 21 method, equivalent diameter for rectangular ducts.
Defaults: 14 in round duct, 1500 cfm, 100 ft long, galvanized steel — typical small-commercial supply main. Standard air ρ = 0.075 lb/ft³ (1.20 kg/m³). For high-altitude or hot air, correct ρ.
Velocity targets in commercial design
- Supply main, low-velocity: 1000–1500 fpm (5–7.5 m/s). Quiet, low energy.
- Supply branch, low-velocity: 600–800 fpm.
- Supply main, high-velocity: 2000–3500 fpm. Louder, smaller ducts, higher fan power.
- Return: 800–1200 fpm (lower than supply for noise control).
- Industrial dust collection: 3500–4500 fpm minimum to prevent dust settling.
Higher velocity = smaller duct, lower first cost, but exponentially higher fan energy and noise. Standard practice is to size mains for ~1200 fpm and branches for < 800 fpm in offices and classrooms.
Friction rate targets
Most commercial systems are sized for 0.08–0.10 in. w.g. per 100 ft of friction loss. Energy-conscious design targets 0.05–0.08. Constant-friction-rate sizing simplifies design: pick a target ΔP/100, then size every duct segment to that rate. The fan must produce the longest path's total loss (= rate × longest equivalent length).
Equivalent length for fittings
This calculator handles straight-duct friction only. Fittings (elbows, transitions, takeoffs, terminals) add minor losses, often expressed as equivalent length:
- 90° smooth elbow: 1 × duct diameter
- 90° mitered elbow: 25 × duct diameter
- Branch takeoff with damper: 5–10 × main diameter
- Transition (gradual): 1–3 × diameter
For complete duct design, ASHRAE provides a comprehensive C-coefficient table (loss coefficient per fitting) — multiply by velocity pressure for fitting losses.
Roughness for HVAC materials
- Galvanized steel duct: ε = 0.0003 ft (0.09 mm)
- PVC duct: ε = 0.00005 ft (0.015 mm)
- Aluminum: ε = 0.0001 ft
- Spiral seam steel: ε = 0.0005 ft (0.15 mm)
- Flexible duct (extended): ε = 0.003 ft (0.91 mm) — use only for short runs (< 6 ft)
- Flexible duct (compressed 50%): up to 4× pressure loss penalty
Reference: ASHRAE Handbook Fundamentals (2021), ch. 21 Duct Design. SMACNA HVAC Systems Duct Design (2006), 4th ed. Colebrook, C.F. (1939). "Turbulent flow in pipes." JICE, 11(4).