Calculating Air Exchange Rates For HEPA Filter Systems

When discussing the size or amount of CFM (cubic feet per minute) of air flow required from one of our HEPA systems to effectively remove or reduce the levels of airborne particulate it is good to know a few basic rules of thumb when determining the right size, quantity and location of the HEPA systems. The math is really simple.

You need to determine the area of the room in cubic feet, so you first need to  measure the length and width of the room. Let’s say it is 15 feet long and 20 feet wide. Multiply 15 x 20 and you get 300 square feet. Now multiply that number ( 300 ) times the height of the room. We will make in 1o feet high. So you multiply 10 x 300 and you get 3,000 cubic feet. This is all it takes to determine the area of the room.

Depending upon the type of application and particulate loading and size range of the airborne particulate you will require a certain number of air changes per hour to accomplish your goal of air contaminant removal. In the case of a clean room type requirement, those are usually designated by Class. Such as Class 100,000, 10,000, 100, 10 and 1. These are rated as .3 microns per cubic foot. As an example a class 10,000 clean room environment would need to meet the criteria of nothing over 10,000 particles per cubic foot at .3 microns or smaller.  Considering the room without filtration could easily register 400,000 particles plus per cubic foot at .3 microns, this reduction level requirement is fairly significant.

Since not all applications require a clean room environment, however, even for residential and light commercial applications, a large reduction of particulates in the air is still necessary. So understanding some simple rules of thumb regarding air exchange rates is necessary. So now that you know the area of the room you need to figure out how many air changes per hour ( how many time you want all the air in the room to be cleaned) and this will bring you to the size  requirement of the air filtration system.

Effective removal of airborne particles is dependent upon two factors. A room in stasis or an active room. When a room is unoccupied or no equipment is running in a room the levels of particulate stay fairly constant. As an example we will take our 3,000 cubic foot room and say it is a small health clinic. During its normal daily operation it will most likely be occupied all of the time. People, clothing, equipment all generate sub-micron sized particles. If the room was seldom occupied probably  2 to 3 air changes per hour would suffice and maintain a relatively clean environment. Three air changes per hour would require a system that produces 150 CFM  or 9,000 cubic feet per hour. For a medical clinic that is occupied a minimum of 10 air changes per hour would a filtration system that produces 30,000 cubic feet per hour or 500 CFM.

By using these simple math formulas you can determine the amount of HEPA filtered air required for any size room. Remember, the higher the exchange rate the cleaner the air. Medical and high-tech manufacturing requirements will require higher exchange rates. Residential and light commercial or institutional may be lower. For more information on how to determine air exchange rates for HEPA filtration systems please go to our website at: or visit us on Twitter @pureairsystems. You can also like us on Facebook and call us anytime on our toll-free number: 800-869-8025.

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