Duct Sizing & Fan Static Pressure Lab
Lay out a supply duct tree, size every section by equal friction with velocity limits, convert to rectangular sizes that fit your ceiling void, and find the index run and fan static pressure. Then watch the system balance as the dampers are set.
How to use: edit the sections table (flows go on the terminal sections only), set your friction rate and void depth, and the sizes update. Tap i for plain explanations.
1. Design criteria
Typical values for offices and hotels are preset.
2. Duct layout
Width shows duct size, colour shows velocity. The index run is outlined in orange.
3. Checks
4. Sections
Edit names, parent, length, fitting loss coefficients and terminal flows. Upstream flows add up automatically.
5. How the sizing works
Equal friction
Each section gets the smallest standard round size whose friction loss is at or below the target, and whose velocity is within the limit for mains or branches. 0.8 to 1.0 Pa/m is a common target for low velocity systems in offices and hotels.
Friction
Pressure loss per metre is Darcy-Weisbach: Δp/L = f / D × ρV²/2, with the friction factor from the Altshul-Tsal form of Colebrook, which ASHRAE uses for duct design.
Rectangular ducts
A rectangular duct carries the same flow at the same friction as a round duct of equivalent diameter De = 1.30 (ab)0.625 / (a + b)0.25. The lab keeps the depth within your ceiling void and works out the width, flagging aspect ratios above 4 to 1.
Index run and fan static
Every path from the fan to a terminal adds friction, fitting losses (C × ρV²/2) and the terminal pressure. The highest is the index run. Fan static is the index run plus the AHU internal losses, the return path and a margin. Shorter runs have pressure to spare, which their volume control dampers must take up at commissioning.
Learning and checking tool. Final duct design should be checked in your design software against the manufacturer data for terminals, fittings and the AHU.