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


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How To Turn Three Negatives Into A Positive!

You always hear about the eternal optimist that says you can always turn a negative into a positive if you just rethink the problem. All too often we have something happen in our lives that creates a negative feeling or puts us in a negative mood. But these negative thoughts are a waste of energy. Instead of dwelling on what could go wrong or did go wrong, look at all the positive things in you life and don’t waste energy worrying about the negative aspects of the issue.

The title of this blog relates to turning two negatives into a positive and while your life  maybe filled with multiple negative thoughts the two negatives I am referring to relate to negative pressure of the physics kind.

Pure Air Systems has been making HEPA and Carbon based, fan powered, commercial grade air filtration systems since 1985. The use of certified, commercial grade HEPA filters creates a design issue in how you incorporate the motor/blower assembly and HEPA filter to obtain maximum performance with minimal noise and air turbulence.

HEPA filters, by their nature, have a relatively high resistance to air flow due to the extremely dense HEPA paper that is used in the fabrication of the filter. When moving air through a fan/motor assembly with a HEPA filter in line you have two options. You can put the fan in front of the HEPA and blow air through the HEPA filter keeping the fan/motor assembly in the dirty air stream or place the HEPA in front of the fan/motor assembly.

Since the resistance to moving air across the HEPA is so high ( over 1.0 inches as measured in water column) pushing the air through the HEPA filter creates an air buffeting affect and requires more horsepower to push the air through the filter. By placing the fan/motor assembly on the clean air side or “negative” side of the HEPA you get a very smooth flow of air, little or no buffeting and a reduction in horsepower requirements.

The HEPA systems manufactured by Pure Air Systems also incorporate a carbon media filter and a polyester, dual layer ring panel roughing filter. All of these filters are placed ahead of the fan/motor assembly. So they are all on the negative side of the blower and offer the same benefit of having a smooth, non-buffeting air flow across their surface areas and keep the blower and motor clean. So in essence, you are turning three negatives into a positive.

Many other companies that utilize HEPA filters in their systems place their blower/motor assembly ahead of the HEPA and this creates more noise fills up the HEPA filter faster as it tends to push the dirt into the HEPA paper rather than pull the dirt through, which is a more efficient way of using the HEPA filter.

For more information on this aspect of the HEPA systems made by Pure Air Systems please go to our website at:



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