Each fan has a rating that tells you how many cubic feet of air per minute (CFM) it will move. When dealing with above average temperatures you will want your fan to exchange the grow room air 3-5 times in one minute, so for a room that is 40 cubic feet, a fan that is capable of moving 120-200 cfm (cubic feet per minute) is recommended. If you only want to replace depleted levels of co2 and are growing in a closet using fluorescents, one room change per five minutes (divide room size by 5) will be adequate.
To calculate your room size, multiply Width by Length by Height, this will give you the CFM rating required for one room change per minute.
Air that’s exhausted will be replaced by fresh air, which is drawn from
located opposite the exhaust at the floor of your grow room (Recommended for closet grows) or forced in using another blower (recommended for larger grow rooms).
For internal circulation oscillating fans are In my opinion the most efficient devices for circulating air in a room. The gentle back and forth sway of the fan is very beneficial for the developing plants. These fans tend to keep anaerobic moulds down by constantly freshening any potentially stagnant air. Home improvement centres carry a large array of various types and styles of air-moving fans, there are wall-mounted styles available and most are relatively inexpensive.
NOTE: To convert CFM to CMH use the folowing formula CFM X 0.03 X 60 = CMH