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It depends on the volume flow rate of the system. Normally velocity of air in taken250 -350 FPM at all air outlets.
Q = AV
Q = flowrate
A = area
V = velocity
If you know two parameters then you can find the third one
Hope this is clear
During selection & calculation of Grill/Diffuser Air flow, free area factor for grill/diffusers must be considered , recommended between0.85 to0.9
While designing the grill size we keep400 to500 fpm as duct velocity. Hence by area and velocity we get the flow.
the velocity of air is critical issue for some applications specially Pharmaceutical facilities
i supose that velocity of400-600 fpm in air out lets is accepted
Determine the throw(m) and air jet velocity(m/s) you need from the outlet (diffuser or grille) first, then match that to the corresponding required flow rate (L/s or m3/h) in the manufacturers selection chart. Most diffuser manufacturers will give their selection chart if you request them. With these three factors (throw, airjet velocity and flow rate) corresponding to the desired noise level we can determine the outlet size. Or as rule of thumb take15 to30 percent extra area from the duct size required for same air flow.
