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Normally there are no calculations except to adjust velocity to avoid noise.
Hope this will work
Regards
Maximum acceptable air velocity at plenum box is210-250 fpm grater the velocity of air would carrying the condensation from cooling coil. Design velocity of air 150 fpm is acceptable
A plenum is a distribution box attached to the supply outlet and often return inlet of the air handling unit. And the purpose of plenum box is to distribute the cooled or heated air to several spaces. It is also used to balance the air and attenuate the sound caused by air movement in the duct.
To size a plenum box we need to look at the famous equation of Continuity
Q = V x A --------- (1)
Where;Q = flow rate (Cubic feet per minute) >> CFMV = Velocity (feet per minute) >> ft/min
A = Area of plenum box (Square feet) >> ft2 As per the standards velocity for Air handling unit (AHU) is 800 ft/min, and let say our calculated heat load tonnage is = 8 TR.As rule of thumb 1TR needs 400 CFM So, 8 TR needs = 8 x 400 = 3200 CFM As per equation 1:
