Ductulator & Duct Friction Loss Calculator
Size ducts, convert between round and rectangular shapes, and calculate friction loss at standard or non-standard air conditions.
Preliminary Duct Design
ASHRAE Fundamentals Ch.21 basis
ASHRAE Fundamentals Ch.21 basis
Result
Awaiting input
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Formula & Method
ΔP/L = f · (ρ·V²) / (2·D) [Darcy–Weisbach]
f via Swamee–Jain explicit approximation of Colebrook–White:
f = 0.25 / [ log10( ε/(3.7D) + 5.74/Re^0.9 ) ]²
Re = ρ·V·D / μ
Round-equivalent (equal friction), Huebscher (ASHRAE):
De = 1.30·(a·b)^0.625 / (a+b)^0.25
| Var | Meaning | Basis |
|---|---|---|
| ΔP/L | Friction rate | Pa/m or in.wg/100ft |
| f | Darcy friction factor | dimensionless |
| ρ | Air density | kg/m³ or lb/ft³ |
| V | Mean velocity | m/s or fpm |
| D | Diameter (or De) | m or in |
| ε | Absolute roughness | m or ft |
| μ | Dynamic viscosity (Sutherland) | Pa·s |
| a, b | Rectangular sides | m or in |
Assumptions & Limits
- Standard air: 1.204 kg/m³ (0.075 lb/ft³) at 20°C, 101,325 Pa, dry.
- Non-standard conditions computed from altitude, dry-bulb temperature and RH via the ProExergy PsychroLib API; viscosity via Sutherland's law.
- Included: friction (major) losses along straight duct; optional lumped minor (dynamic) losses via ΣC coefficients.
- Excluded: duct leakage, insulation liner roughness build-up, acoustic/NC constraints, structural/seismic bracing, fire/smoke damper losses.
- Valid for fully-developed turbulent flow, Re > 4,000; aspect ratio ≤ 4:1 recommended for rectangular ducts (flagged if exceeded).
- Rectangular sizing uses ASHRAE equal-friction equivalent diameter (Huebscher); actual velocity is based on true cross-sectional area.
Engineering Notes
Design friction rates of 0.08–0.15 in.wg/100 ft (0.65–1.2 Pa/m) are typical for low-pressure supply/return trunks; higher rates suit high-pressure/VAV systems where space is constrained. Keep branch velocities below noise-generating thresholds (commonly 1,500–2,000 fpm for occupied-space branches; verify against project NC criteria).
Method reference: ASHRAE Fundamentals, Ch. 21 (Duct Design); Huebscher equal-friction equivalent diameter; Swamee–Jain explicit Colebrook approximation. Non-standard air density/viscosity sourced from ProExergy PsychroLib API.
Preliminary design tool. Results are for early-stage duct sizing and friction-loss estimation only. Verify final duct sizes, static pressure budgets, and acoustic performance against manufacturer data, project specifications, and applicable codes (SMACNA/ASHRAE) with a qualified engineer before construction.
