| With artificial ventilation, the exhaust ventilator determines the size of the air outlet aperture.
The aperture needed for an exhaust ventilator is normally much smaller than that required for natural ventilation.
The size of the air inlet aperture A in depends on the ventilator output  V and the maximum flow speed v S in the inlet aperture.
It is preferable to calculate with a flow speed of v S = 3 m/s. However, if structural considerations do not permit the size of aperture resulting from this calculation, it is also possible to use a flow speed of v S = 5 m/s.
The minimum size of the air inlet aperture is calculated with the aid of the following formula:
Ain = minimum area of air inlet aperture [m³]
 = Ventilator output [m³/h]
vS = maximum flow speed [m/s]
Note
It is to be remembered when choosing exhaust ventilators, that the flow of cooling-air is subject to the same laws of physics as the compressed air. Even when cooling-air flows through ducts and apertures, when flow speed increases the dynamic pressure Dp (pressure loss) rises. A ventilator can only overcome dynamic pressure that lies below its defined surface pressure. If the dynamic pressure is higher than the surface pressure of the ventilator, no volume flow can occur.
The maximum dynamic pressure is determined from the shape and size of the air inlet and outlet apertures together with the respective ducts (if fitted). The flow speed must also be taken into account.
A Dp = 100 Pa (10 mm WH) can be assumed for simple apertures without unfavourable diversion (ducting). |