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
FREQUENTLY ASKED QUESTIONS
The air volume calculation is based on cooling load because more air is needed to cool a space than to heat it. This is because cooling requires the removal of heat from the space, which requires a higher air flow rate than heating, which involves adding heat to the space.
The sensible cooling load directly affects the air volume calculation, as it determines the amount of heat that needs to be removed from the conditioned space. A higher sensible cooling load requires a higher air volume to cool the space effectively.
In HVAC systems, air volume is typically measured in CFM (cubic feet per minute). The air volume calculation determines the required CFM to cool or heat a space effectively. A higher air volume requires a higher CFM, and vice versa.
Yes, 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. This is because the air volume calculation is based on the cooling load, which is typically higher than the heating load.
The size of the air handling unit (AHU) affects the air volume calculation, as it determines the maximum amount of air that can be circulated through the system. A larger AHU can handle a higher air volume, while a smaller AHU may require a lower air volume to operate efficiently.
Common mistakes to avoid when calculating air volume for an HVAC system include neglecting to consider the sensible cooling load, using incorrect CFM values, and failing to account for factors such as duct losses and fan efficiency. These mistakes can lead to inadequate cooling or heating, reduced system efficiency, and increased energy costs.
