How much airflow do we need to ventilate? Airflow is the volume of air moved by a fan per unit of time, usually expressed in cubic feet per minute (cfm) or meters cubed per second (m³/s). For general ventilation, airflow can be calculated by:
- Area method
- Air change method
- Occupancy method
- Heat removal method
Derives the ventilation rate from the area of the space (in square feet) to be ventilated multiplied by the ventilation rate per square foot.
Example: For residential bathrooms up to 100 sq. ft. in area, HVI recommends an exhaust rate of 1 cfm per square foot. A bathroom is 6′ to 10′ or 60 square feet.
Airflow = Q = 1 cfm/sq.ft. x 60 sq. ft. = 60 cfm
Air change method
Derives the ventilation rate from the volume of the space (in cubic feet) to be ventilated multiplied by the number of total air changes in one hour.
Example: For an auditorium, the suggested air change rate is 4 to 15 air changes per hour. An auditorium is 80′ x 90 ‘ with 20’ ceiling or 144,000 cu. ft. Use 10 air changes per hour.
Airflow = Q- 144,000 cu. ft. x 10 AC/hr/60 min/hr = 24,000 cfm
Derives the ventilation rate from the number of people that will occupy the space at any given time.
Example: For an office, the recommended ventilation rate is 20 cfm per person. The occupancy of a general office is one person per 80 to 150 sq. ft. An office is 40′ x 60′ or 2,4000 sq. ft. Occupancy = 2,400 sq. ft. / 150 sq. ft. per person = 16 people.
Airflow = Q = 16 people x 20 cfm per person = 320 cfm
Heat removal method
When the temperature of a space is higher than the ambient outdoor temperature, general ventilation can be used to provide “free” cooling. What is needed to calculate airflow is the amount of heat to be removed in BTU/hr, the desired indoor temperature and design outdoor dry bulb temperature.
Example: 200,000 BTU/hr to be removed, 70 degree desired indoor temperature and 50 outdoor dry bulb temperature.
Airflow = Q = 200,000 (BTU/hr) / (1.08×20) = 9,260 cfm
For a quick refresher on fan fundamentals, view our article on Fan Basics