Sizing Ducts

Duct sizing in ventilation systems is often done with the Velocity, Constant Pressure Loss or the Static Pressure Recovery Method

The Velocity Method

With this method a proper speed for the application is selected. The duct size is then given by the continuity equation expressed as:

A = q / v (1)

where

A = duct cross sectional area (m²)

q = air flow rate (m³/s)

v= air speed (m/s)

Proper speed depends on the application. The table below can be used as an indication of common velocities:

Type of Duct	Comfort Systems	Industrial Systems	High Speed Systems
Main ducts	4 - 7 m/s	8 - 12 m/s	10 - 18 m/s
Main branch ducts	3 - 5 m/s	5 - 8 m/s	6 - 12 m/s
Branch ducts	1 - 3 m/s	3 - 5 m/s	5 - 8 m/s

High velocity close to the outlets and inlets generates noise.

The Constant Pressure Loss Method

A proper speed in the start of the system is selected and the pressure loss in rest of the system is kept at a constant level. This will give an automatic velocity reduction in the system. The method adds more cross sectional changes and complicates the system.

The Static Pressure Recovery Method

With the static pressure recovery method secondary and connection ducts are selected so that the static pressure in front of the outlets are more or less the same. The advantage of the system is similar conditions for outlets or inlets. Unfortunate is the method complicated and seldom used.

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