Filter Cake Is Not A Dessert
Filter cake is a term that is not used much anymore in the air filtration industry but it is a very important part of the mechanical method by which air filters capture smaller particles. Removing airborne particles by the use of air filters has been around for over 70 years. The filter medias have changed but the method of removing particle has not.
Whether you use a simple fiberglass furnace filter, a polyester dual layer ring panel filter or a HEPA filter the filter cake is essential in the overall efficiency of the filter. When any mechanical filter ( non-mechanical would be electronic or ozone or electrostatic air purifiers) is used it is normally placed in a return air duct or return air grill and always on the dirty side of the air moving fan device.
Media air filters have different efficiencies and are normally rated using the MERV rating system. In essence the higher the MERV rating ( a rating of 11 or 12 is very good while a MERV of 4 or 5 is low) the better the initial effectiveness of the filter. The operative word here is “initial.” When a new filter is placed in a dirty air stream the tiny spaces between the filter fibers are all open. Initially some of the smaller dust particles will pass through the spaces until these small spaces fill up. As the filter media “loads up” or gets dirtier a cake like layer of dust forms on the air incoming side of the filter. This is normally called the filter cake. This filter cake performs a significant role in the overall dust collection efficiency of the filter.
Since many of the smaller, less than 10 micron particles are tiny enough to pass through many filter medias, the filter cake that forms on the outside layer of the filter itself has enough different dust size particles mashed together to form, in essence, a primary filter bed that actually will capture many of the smaller, tiny airborne particles. The filter cake, or layer of dust, is the primary filter while the filter media, in essence, becomes the secondary filtration media.
As the filter cake builds up so does the pressure drop or resistance of air through the filter. At some point the filter loads up so much that the air flow through the air handling device is actually reduced. This is usually the point at which the filter has to be changed out. Interestingly, the higher rated MERV filters will normally be changed out more often than a lower rated filter because the more efficient filter captures more smaller particles and loads up faster.
Pure Air Systems has been manufacturing HEPA and Carbon based air filtration systems since 1985. All of the HEPA filters used in the PAS fan powered air filtration systems utilize a polyester ring panel filter ahead of the HEPA to remove all the larger particles and reduce the heavy dust loading so that the HEPA filter only sees the fine, sub-micron particulate. For more information on our complete product line please go to our website at: www.pureairsystems. com