The air volume for an HVAC system is calculated from the sensible heat in the conditioned space. For the typical heating and cooling unit, the calculation is for cooling because more air is needed to cool a space than to heat it. If the conditioned space can be properly cooled with a given amount of cfm, then the space can be heated correctly with the same amount of cfm.
The sensible cooling load in a conditioned space is the heat that needs to be removed from the space to maintain the desired room temperature (75 °F, for example). The cooling load is calculated from the sensible heat from the people (approximately, 225 Btuh), lights, computers and other equipment, and the heat transfer through the building envelope on the historically warmest day and time of the year (e.g., 90 °F, 3 pm, July 7).
Example: The sensible cooling load for conditioned space (Building A) is 113,400 Btuhs. Find required fan cfm. The heat transfer equations are:
Btuhs = cfm × 1.08 × TD
cfm = Btuhs ÷ (1.08 × TD)
TD = Btuhs ÷ (1.08 × cfm)
- Btuhs = Btu per hour sensible heat,
- cfm = volume of airflow
- 1.08 = constant, 60 min/hr × 0.075 lb/cf × 0.24 Btu/lb F
- TD = the difference between the supply air temperature (SAT) dry bulb and the room air temperature (RAT) dry bulb.
To find cfm required, use cfm = Btuhs ÷ (1.08 × TD)
cfm = Btuhs ÷ (1.08 × TD)
cfm = 113,400÷ (1.08 × 20 °F) (SAT is 55 °F and RAT is 75 °F)
cfm = 5250
