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Ducts and Airflow

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.

1 worked example reproduced, listed on the tool page.

Fitting Loss Coefficients

The pressure loss of the fittings in a duct run, each from a catalog of loss coefficients of the common elbows, transitions, entries, exits, branches, dampers and grilles, applied to the velocity pressure of the air through it: Δp = C ρV²/2. Pick each fitting, give its count and the airflow and duct size it sits in, and the tool lists the coefficient, the velocity pressure and the loss, sums the run, and reports the equivalent length of each fitting in its duct so the losses can be carried into the Duct Sizing tool as a ΣC or an added length. The coefficients are representative values from the ASHRAE and SMACNA tables; the ASHRAE Duct Fitting Database gives the exact coefficient of a specific geometry.

Duct Leakage

Leakage airflow of a duct system from its surface area, the static pressure in it and the SMACNA leakage class of its seal level: Q = C_L p^0.65 per unit of surface, the class being 3 to 48 cfm per 100 ft² at 1 in. w.g. depending on the duct shape and the seal class (A, B, C or unsealed). The tool gives the leakage flow, its share of the system airflow, the fan power and energy it costs over the operating hours, and compares the seal classes so the value of sealing can be weighed against its labor.

Static Regain Sizing

Sizing of a supply trunk by the static-regain method: after each takeoff the trunk velocity is reduced so that the regain of static pressure from the lower velocity pressure, less the regain efficiency, pays for the friction and fitting loss of the next section. The static pressure then stays nearly constant at every takeoff, which is what balances the branches of a long trunk. Enter the first section's size and flow and the flow and length of each following section; the tool solves each section's diameter, reports the velocities, pressures and the residual, and compares the result with equal-friction sizing.

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