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Written By: Robert E. Sheriff, MS, CIH, CSP, President
March 26, 2018
There are two primary methods of measuring air movement in an industrial/commercial building. The most obvious is to actually measure all the supply points and exhaust points in the building. Compiling that information can lead to calculations of air changes per hour (ACH) which can be compared to recommended levels for similar building activities.
A good reference for recommended ACH’s for typical activities (but not the only one) is Engineering Tool Box found at www.engineeringtoolbox.com.
Another method for determining ACH’s is to use a tracer gas—such as Sulfur Hexafluoride by injecting the gas into the building’s intakes and measuring the degradation rate of the tracer gas and then calculating the ACH. The American Society for Testing and Materials (ASTM) has actually established a method using a tracer gas—ASTM E741-11.
Using the actual measurement of supply and exhaust point, the first thing is to determine if the building is under positive or negative pressure with respect to the outside—or adjacent areas. If it is difficult to open the exterior doors, it is likely under negative pressure since there is more exhaust then supply air. If there is not enough negative or positive pressure to test, use smoke tubes. If smoke flows in the door/windows, it means negative pressure. If smoke flows out, then it is positive pressure. If it is negative pressure, it is best to measure all the exhaust points. If positive pressure, it is best to measure supply points.
Of course, if you want to determine how negative or positive the air volume is, then you must measure both. This is necessary of the intent is to balance the supply and exhaust volumes.
A common consequence of too much exhaust (in addition to not being able to open the doors unless you’re a weight lifter) is that there is a great imbalance between the temperature in various parts of the building. Hot in one area and cold in another.
Another point to add when determining ACH’s is to be sure to measure the local exhausts, as well as the roof fans and wall fans.
When the building is so configured that accessing all the air supply or exhaust points are not possible, then tracer gas testing is a viable option.
Another important use of tracer gas is where buildings are constructed as a “Safe Haven” in the event of a hazardous chemical spill or leak. Such buildings, when closed up, are not supposed to have more than 1/10 ACH to be classified as a “Safe Haven” building. The test method for such situations is covered in ASTM E741-11.
I should mention that the American Conference of Governmental Industrial Hygienists (ACGIH) has recently come out with the 29th Edition of “Industrial Ventilation: A Manual of Recommended Practice-2 Volume Set.” VOLUME ONE is “Industrial Ventilation: A Manual of Recommended Practice for Design” and VOLUME TWO, “Industrial Ventilation: A Manual of Recommended Practice for Operation and Maintenance” (www.acgih.org) look under PUBLICATIONS. This is an excellent guide on Industrial Ventilation.
If you need additional information on measurement of ACH’s, you can send your inquiries to: email@example.com or call us at 1-800-344-4414.
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