Supply air flow rate to a space is based only on the total space sensible heat load, thus
Q = 1.08 x [Hs / (TR – TS)]
Where
• Q = air flow in cubic feet per minute (CFM)
• 1.08 = conversion constant = 0.244 X (60/13.5); 0.244 = specific heat of moist air, Btu/lb of dry air and 13.5 = specific volume of moist air, cu-ft. per lb of dry air (@70° F, 50% RH) and 60 is conversion from hour to minute.
• HS = total room sensible heat gain, BTU per hr.
• TR = Room dry bulb temperature, °F usually 75ºF
• TS = Supply air or leaving air temperature from the cooling coil in ºF
The selection of temperature differential (TR – TS) is stated for simplicity above but actually it is little tricky as the real operating temperature differential is determined by the laws of “Psychrometrics” governing the performance of air system. As a rule of thumb following table provides an indicative relationship between the sensible heat ratio and leaving air temperature. Once the TD is known, the
supply airflow rate (CFM) can be calculated.
