Blog Tag: pressure drop.

Most people refer to air filtration devices as either air filters or air purifiers. In truth, these two methods are worlds apart in the way they collect or remove airborne contaminants. The term non-mechanical refers to any media type filter;  i.e. fiberglass, polyester, cotton, woven and non-woven, HEPA, etc. Mechanical or Electronic filters would include electronic air cleaners, ion generators, ozone generators, and any filtering device that uses electricity or some type of moving part incorporated into the device.

Before I get any further I do want to state that the term ” air purifier” is a misnomer. Nothing purifies the air. You can purify water or chemical solutions but not the air. However, this term has become a standard part of the filtration market so I will use it.

From a performance point of view, the purpose of any air filter air purifier should be to “remove” the offending airborne particles from the room, house, or area where you live or work. The operative word here is “remove.” The non-mechanical type of filter incorporates a media that has openings in the media itself that make it either more or less dense. The less dense medias like fiberglass filters allow a lot of air to pass through but collect only the larger dust particles and debris. The more dense medias like polyester and some non-wovens like the Filtrete filter from 3M, collect a much higher percentage of the smaller and finer particles.  The less dense media has a lower pressure drop across its surface and has little resistance to air flow. The more dense media is much more restrictive.

Air purifiers, for the most part, use a combination of a positive/negative process to capture airborne particulate. This usually takes the form of a surface or plate that is charged to collect particles that have the opposite electrical charge. It is like running a comb through your hair to build up a static charge and then placing the comb next to small pieces of paper and watch them stick to the comb. Same principle. Unlike media filters that use the collected particulate on their surface ( sometimes referred to as dust cake ) thus collecting more and smaller particles, the electronic air cleaners or purifiers have no depth collection process and once the plates or collection surface gets too dirty or the collected particles loose their charge, the particulate detach and are put back into the air stream.

There are trade-offs in using the two methods. The non-mechanical filters eventually have to be replaced. As they collect more particles the ability to pass air through them is reduced and the pressure drop becomes too high for the fan/motor assembly normally used to pull the air through the filters. The plates or collection surfaces of the electronic devices can be cleaned manually and then put back into service. They also have very little resistance to air flow. The non-mechanical filters, however, are much more effective at removing small, respirable sized particulate.

The type and design of filtration device you choose depends upon the purpose and type of particulate you need to remove. For what it is worth, non-mechanical filters are predominantly used in the following types of applications and markets. Hospital surgery rooms, medical clinics, pharmaceutical manufacturing, clean rooms, isolation rooms, micro chip manufacturing, optics manufacturing, the CDC, medical laboratories and any process or operation that requires the removal of harmful airborne allergens or pathogens.

For more information on the design and operation air filtering devices please go to our website a: www. pureairsystems.com.

It is always interesting to see the amount of new technology being developed every day. New computing devices, new 3-D TV’s, new ways to generate energy and yet we understand so little about how these systems actually work. We just assume that when it says to push the red button the system will turn on or off or clicking on this URL will get us to internet nirvana.

This lack of interest in actually knowing “how things work” can be to our detriment. If we don’t know the how or why of a product or system we will make a buying decision based solely on looks and price. For some items this is appropriate. But in many cases some basic research on the product or system is warranted.

Take for example the term “Delta P” or sometimes it is written out using the pyramid symbol with the P following it. This term means pressure differential, pressure drop, resistance in air flow as measured in inches of water and has a lot to do with some products you buy. Specifically air filtration systems and their relationship to their performance and performance of systems they are attached to.

Air filters or any type, style or media make up all create a certain amount of resistance when placed in an enclosed device like a furnace filter in a filter frame or a filter in a fan powered air filtering (air purifying) device.

When the filter is placed on the air entering side of the fan or air moving device it will create some resistance or increase the amount of pressure the fan has to see in order for the air to move through the filtering device. This pressure drop or Delta P is important as it affects both the air flow performance of the heating/cooling system or the filtering device itself.

When you have an air moving device like the fan in the heating/cooling system, the fan/motor assembly is designed to move a certain amount of air ( measured in CFM) at a specific pressure drop or resistance ( measured in inches of water). Let’s say the fan is designed to move 1200 CFM at .5″ of water. This is with a general duty, Walmart fiberglass furnace filter( this filter has a rated pressure drop clean of  .1″).

If you want to install a better, more effective filter in the system, say a MERV 9 or 10 filter, this media has an initial resistance or Delta P of .4″ . This means the furnace fan will have to work harder to pull the air through this filter. If the motor/blower system is only designed to move 1200 CFM at .5″ or .6″ then something has to give. What happens is the furnace fan actually backs down due to the increase in pressure drop and the air flow is reduced. Depending upon the type of fan and motor being used in the HVAC unit the air flow can be reduced by as much as 100 CFM per additional .1″. So if you add .3″ of static that the fan system is not designed for, you can reduce the air flow by 300 CFM.

This air flow reduction is significant in that it now requires the system to run 30% longer to either heat or cool the house due to the additional resistance across the filter. When using filter media such as HEPA filters the initial resistance across these filters can be as high as 1.0″. So it is important to understand the relationship between air flow and Delta P.

For more information on this subject matter and information on the complete line of HEPA filtration systems offered by Pure Air Systems please go to our website at: www.pureairsystems.com

It is always interesting to see how companies that manufacture air filters, especially those for the residential market, use images of dust mites, dirt, dander and other graphic photos to illustrate the significance of using their filter if they really want to rid their home of these nasty airborne creatures.

Usually these companies compare the efficiency of their filter against the standard air filter that you use in your heating/cooling system and show how theirs is much more effective in removing these harmful airborne allergens.

For the most part, all of the air filters available from the retail stores are made from similar filter media and have similar efficiencies. Some are pleated, some aren’t, some are washable and reusable and many are not. Almost all of them have a cardboard frame and come is a wide variety of sizes.  And, their prices vary from less than $1.00 each up to $10.oo or more per filter.

When shopping for these filters you will notice that many or most of them now come with a MERV rating label to indicate their overall efficiency.  These MERV ratings (which are an extension of the old ASHRAE ratings) indicate their filtration efficiency in relationship to how well they remove various sizes of airborne particulate.

In truth, what you really want is a filter that removes as much of the less than 5 micron in size particulate but doesn’t have a high resistance so it won’t affect the performance of your HVAC unit. Typically a MERV 7 or 8 rated filter offers the best combination of particulate removal (small particles) with minimal adverse affect on your heating/cooling air flow.

While it may seem that using a filter with a higher MERV rating than 8 would be better, the truth is that as a filter starts to get dirty and gets a layer of dust on the filter surface, the efficiency of the filter actually gets better as the filter loads up.  And, as the filter gets dirtier the pressure drop or resistance across the filter goes up as well.

Therefore, if you start out with a MERV 9 or higher rated filter you will have  a slightly higher efficiency at first but also a higher initial pressure drop as well. And as the filter loads up the pressure drop will increase rapidly and the air flow across the air handler or HVAC system will drop quickly.  This is why using a MERV 9, 10, 11 or higher rated filter usually requires you to change the filter often, usually once a month verses once every three months for a MERV 8 or lower rated filter. And, these higher rated filters are more expensive.

Pure Air Systems offers a line of MERV 8 rated BioPanel dual-layer polyester ring panel filters that are less expensive than the higher rated media filters, fit more tightly in the filter frame, have zero leakage around the filter and have a much higher dust holding capacity with a lower pressure drop or resistance over the the life of the filter.  These commercial grade filters are not available in the retail market but can be purchased directly from the Pure Air Systems website.

For more information on filter efficiency and the BioPanel filters available from PAS go to our website at: www.pureairsystems.com.