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Duct Sizing

Velocity, friction loss and fitting loss of each segment of a duct run and the total static pressure the fan must supply, by the Darcy equation with the Colebrook friction factor and the duct material's roughness; rectangular ducts through their hydraulic diameter for velocity and their Huebscher equivalent diameter for friction; fittings by loss coefficients summed per segment. For each segment the tool also gives the round diameter that would hold the entered design friction rate (the equal-friction method), so an undersized or oversized segment shows at once. A chart of the pressure along the run shows where the loss accumulates.

  • Segments of round or rectangular duct with length, size, airflow and the sum of fitting loss coefficients

  • Velocity, velocity pressure, Reynolds number, Colebrook friction factor, friction and fitting losses per segment and cumulative

  • Equal-friction sizing: the round diameter that gives the design friction rate for each segment's flow

  • Total pressure loss of the run, fan static pressure, and a chart of pressure against distance

Duct Sizing

Unit System: /

Air and Duct Material





Segments (fan outlet to the farthest terminal)
1
2
3
4

Terminal



Friction, Fittings and the Equal-Friction Method | Method page
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Duct Sizing

Friction, Fittings and the Equal-Friction Method

Friction loss

Air moving through a duct loses pressure to wall friction by the Darcy equation, Δp = f (L/D) ρV²/2: the friction factor f times the length in diameters times the velocity pressure. The friction factor depends on the Reynolds number and the relative roughness ε/D through Colebrook's implicit relation, which the tool solves by iteration; at the Reynolds numbers of HVAC ducts (10⁴ to 10⁶) f lies between 0.015 and 0.03 and is governed more by the roughness than by the velocity. The roughness classes are ASHRAE's: galvanized steel with normal seams about 0.0003 ft, smooth aluminum or plastic 0.0001 ft, lined or fully extended flexible duct 0.003 ft, and compressed flexible duct an order of magnitude rougher still, which is why a sagging flex run can double a branch's loss.

Rectangular ducts

The velocity comes from the actual area. For friction a rectangular duct of sides a and b behaves like a round duct of Huebscher's equivalent diameter De = 1.30 (ab)0.625/(a + b)0.25, which carries the same flow at the same friction rate; it is not the hydraulic diameter 4A/P, which overstates the capacity of a flat duct.

Fittings

Elbows, transitions, branches and dampers lose pressure in proportion to the velocity pressure, Δp = C ρV²/2, with C from the ASHRAE or SMACNA fitting tables (a smooth radius elbow 0.1 to 0.3, a mitered elbow without vanes about 1.2, an abrupt entry 0.5). The coefficients of a segment's fittings are entered as their sum, referred to the velocity in that segment.

Equal-friction sizing

The common design method sizes every segment so that the friction rate is the same, typically 0.08 to 0.10 in. w.g. per 100 ft (0.8 to 1.0 Pa/m) for low-pressure supply ducts, which keeps velocities moderate and balances the branches roughly. For each segment the tool solves for the round diameter at which the entered design rate is met at that segment's flow; a segment whose actual rate is far above the design rate is undersized, far below it is oversized. Velocities above about 2,500 ft/min (12.7 m/s) in occupied-space ducts are flagged for noise.

Fan pressure

The sum of the segment losses along the index run (fan to the farthest or highest-loss terminal) plus the terminal's own loss is the external static pressure the distribution needs; adding filters, coils and other equipment losses gives the fan static pressure to select the fan at the total airflow. Return ducts are sized the same way and their loss added.

Not included

Static regain and the velocity-reduction methods, system-effect factors at the fan inlet and outlet, leakage, thermal effects on density along the run, and acoustic calculations beyond the velocity flag.



References

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2021). ASHRAE Handbook: Fundamentals. ASHRAE, Peachtree Corners, GA.

Colebrook, C. F. (1939). "Turbulent Flow in Pipes, with Particular Reference to the Transition Region between the Smooth and Rough Pipe Laws." Journal of the Institution of Civil Engineers, Vol. 11, No. 4, pp. 133–156.

Huebscher, R. G. (1948). "Friction Equivalents for Round, Square and Rectangular Ducts." ASHVE Transactions, Vol. 54, pp. 101–118.

Sheet Metal and Air Conditioning Contractors' National Association (2006). HVAC Systems Duct Design, 4th ed. SMACNA, Chantilly, VA.

Worked Examples

Published examples this tool reproduces; each page lists the inputs to enter and the printed answers.

All worked examples

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