How do I size my exhaust fans and inlet shutters?
Printer-friendly versionSizing fans and shutters without the presence of an evaporative cooling system is pretty straightforward.
Finding your CFMs:
- Length X Width X Height = CFM (As a rule, we typically use 12 for the height value in this equation.)
- Let's assume a 30'x48' structure:
30 X 48 X 12 = 17,280 CFMs
Using this you can size your fans and inlet shutters.
Sizing the Fans:
Exhaust fans pull air out of the building. This air is replaced by cool air from outside the building coming in through the inlet shutters.
A typical exhaust fan arrangement is two fans. This increases the efficiency of your airflow. So:
- 17,280 (your total CFMs) / 2 (number of fans) = 8,640 CFMs per fan
This means each of your two fans should have a CFM rating of over 8,640 to have standard air exchange rates.
Sizing the Inlet Shutters:
Without an evaporative cooler it's easy to size your inlet shutters. Divide by 2 (just like the fans) and size to that number.
What happens if the CFM rating per shutter (or fan) is more than the rating of the largest shutter?
If the CFM requirement per shutter (or fan) is larger than the maximum CFM rating of the largest shutters, you will need to add shutters until you have an arrangement that suits you.
- Assuming a 41.5'x60' structure:
(41.5 X 60 X 12 = 29,880 CFMs) / 2 (number of shutters) = 14,940
Our largest shutters have a 14,000 CFM rating. They are also 5' tall, which may or may not be acceptable depending on your specific arrangement. So you will need to add shutters until you can meet your airflow requirements and/or you have a shutter size that is acceptable.
- 29,880 / 3 (one additional shutter) = 9,960
- 29,880 / 4 (two additional shutters) = 7,470
Continue to add shutters until you have an airflow and shutter size to your liking.
