The design of the ductworks in ventilation systems are often done by using the
Proper air flow velocities for the application considering the environment are selected. Sizes of ducts are then given by the continuity equation like:
A = q / v (1)
A = duct cross sectional area (m2)
q = air flow rate (m3/s)
v= air speed (m/s)
Alternatively in Imperial units
Ai = 144 qi / vi (1b)
A = duct cross sectional area (sq.in.)
q = air flow rate (cfm)
v= air speed (fpm)
A proper velocity will depend on the application and the environment. The table below indicate commonly used velocity limits:
|Type of Duct||Comfort Systems||Industrial Systems||High Speed Systems|
|Main ducts||4 - 7||780 - 1380||8 - 12||1575 - 2360||10 - 18||1670 - 3540|
|Main branch ducts||3 - 5||590 - 985||5 - 8||985 - 1575||6 - 12||1180 - 2360|
|Branch ducts||1 - 3||200 - 590||3 - 5||590 - 985||5 - 8||985 - 1575|
Be aware that high velocities close to outlets and inlets may generate unacceptable noise.
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A proper speed is selected in the main duct close to the fan. The pressure loss in the main duct are then used as a template for the rest of the system. The pressure (or friction) loss is kept at a constant level throughout the system. The method gives an automatic velocity reduction through the system. The method may add more duct cross sectional changes and can increase the number of components in the system compared to other methods.
With the static pressure recovery method the secondary and branch ducts are selected to achieve more or less the same static pressure in front of all outlets or inlets. The major advantage of the method are more common conditions for outlets and inlets. Unfortunate the method is complicated to use and therefore seldom used.