ℹ️ About This Calculator
Accurate duct pressure drop calculations ensure fans are correctly sized and air is distributed evenly throughout the HVAC system. The equal friction method sizes ducts to maintain a constant friction rate (typically 0.8–1.2 Pa/m) through all branches, balancing the system. This calculator covers both rectangular and circular ducts using the Darcy-Weisbach equation with Colebrook-White friction factors.
ASHRAE Fundamentals Handbook (Chapter 21) and ISHRAE guidelines govern duct sizing in India. Equal friction method is most common for comfort conditioning; static regain method for high-velocity systems; constant velocity for exhaust/return ducts. Maximum duct velocity limits: living areas < 5 m/s (noise); offices < 8 m/s; plant rooms < 12 m/s. Sheet metal ducts: Gauge 26 (0.5 mm) for up to 750 mm width; Gauge 24 (0.6 mm) for 750–1500 mm; Gauge 22 (0.7 mm) for over 1500 mm.
📐 Duct Pressure Drop (Darcy-Weisbach)
❓ Frequently Asked Questions
What friction rate should I use for duct sizing?
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For standard comfort cooling/heating in offices and commercial buildings: 0.8–1.0 Pa/m friction rate. For quiet applications (bedrooms, boardrooms): 0.5–0.8 Pa/m to keep velocities low and noise down. For industrial ventilation and large plant rooms: 1.0–1.5 Pa/m. Higher friction rates give smaller ducts (cheaper) but increase system pressure and fan energy consumption.
How do I convert between rectangular and circular duct sizes?
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Use the equal friction diameter (hydraulic equivalent diameter): D_eq = 1.30 × (a×b)^0.625 / (a+b)^0.25, where a and b are the rectangular dimensions (mm). This gives the circular duct diameter that has the same friction loss per unit length at the same air volume. The actual circular duct will be slightly different from the hydraulic diameter formula used for pressure loss.
What is the maximum velocity in air ducts to prevent noise?
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Velocity limits for acceptable noise (NC 35): main supply ducts – 7 m/s max; branch ducts – 5 m/s; duct to diffuser – 3 m/s; diffuser face velocity – 0.5–1.5 m/s. For bedrooms and quiet spaces (NC 25): reduce all velocities by 20–30%. Higher velocities generate duct noise (rushing sound) and diffuser noise. Fan speed must be reduced, not duct size increased, to fix a noisy system.
Do I need to consider static regain in duct design?
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Static regain analysis is important for long high-velocity systems (supply trunks over 50 m) or where the equal friction method results in very unbalanced system pressures. In the static regain method, velocity is progressively reduced at each branch, converting velocity pressure to static pressure to partially offset friction losses. Result: more uniform pressure at all outlets without balancing dampers. For short systems, equal friction with volume control dampers is simpler.
What duct leakage should I allow in the calculation?
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ASHRAE and SMACNA classify duct leakage: Class A (maximum 0.5% fan flow) for clean rooms and pressurised systems; Class B (2%) for typical commercial supply ducts; Class C (4%) for return ducts and low-pressure systems. Add the expected leakage to the fan flow calculation. For unsealed galvanised ducts in Indian buildings, assume 5–10% leakage if leakage testing is not specified.
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