Operation of Air Filters

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Types of residential filters are discussed on the Filters page. This is about how and how well they work. There are two types, mechanical and electronic. Filter media thickness, air speed and particle size are the main factors for how well filters perform in a given situation.

Mechanical filters use three different processes for cleaning. Impaction or impingement, interception and straining. All methods exist in each filter but one method will be predominate.
Impaction is when larger particles can't make the turn around a filter fiber. They fall out of the air stream, impact and stick to a filter fiber.
Interception is a particle following the air stream but still coming in contact with a filter fiber and staying. Filter fiber diameter will effect the size of particle it can most easily capture and hold. Smaller fibers generally do better on smaller particles. Interception is a major factor in electrostatic filters. The generated charge increases the attraction and holding power of the filter media.
Straining is exactly that. The filter fibers are to close together for the particle to fit. Straining has the least effect in the filtering process.

Electronic filters require an outside power source. There are three steps in a good electronic filter installation. The pre filter, ionizing wires and collection plates.
Pre filters are used to filter out the larger particles that may be moving to fast for proper collection. They may also cause "arcing". This makes noise and can generate ozone (O3). Some manufacturers included washable metal pre filters. Disposable spun fiber or pleated filters may be used if the system pressures allow.
Ionizing has a electrified wire in the air stream to place a positive charge into passing particles. Air speed, wire spacing and voltage are the main variables. Voltage usually does not exceed 12kV because of ozone creation.
Collector plates are flat metal strips with a negative charge. The voltage is less the 6kV. Positive charged particles are drawn out of the air stream and stick to the plates. Smooth air flow is essential for small particle collection. Small particles may have a adequate charge but buffeting in the air stream can keep them away from the plates. Positive charged particles that miss the negative collector plates will stick to anything with a sufficient negative charge.

Residential Air Filter Types
The air filter in your home heating and air conditioning system is the only way to keep all the air in your home clean. As the unit runs, air is drawn through the filter where the dust and dirt is captured and held for disposal. There are several different types of filters available for home use.

Spun fiber, generally is a long strand polyester or fiber glass material. These filters have fairly low efficiency ratings. They will stop and hold only the larger airborne particles. They have the least amount of air restriction. The efficiency actually will improve as material builds up on the filter making the air paths through the filter smaller. However, with smaller air paths the air resistance and velocity will increase. More on this later. Some spun filters are treated with a "tacifier" (sticky stuff) to improve efficiency. Some spun fiber filters are washable but usually they are the throwaway type.

Pleated, a woven or pressed short fiber material formed into a pleated shape. With ratings of 15 to 40%, these filters are more efficient then the spun type. This is done by having much smaller air paths then the spun fiber filters. With smaller air paths the resistance and velocity for the same area has increased. This is why this material has a pleated face. By forming these filters into this shape we are able to increase the surface area. The increased surface area allows the same volume of air to move through at a lower overall resistance and velocity for a given area of filter material. Pleated filters are disposable.

Electrostatic, made of multiple layers of materials that rub against one another by the air motion. Efficiencies vary between 15 to 40% with some claiming higher numbers. These filters usually have less initial resistance then pleated, do to a more open design. The motion of the filter materials generate a static charge, like rubbing a balloon on your hair, to attract and hold dirt. Electrostatic filters are washable.

Electronic, somewhat like an electrostatic filter, but require an outside power source. There is a pre filter to trap the largest particles. Then the electric plates charge and hold the smaller materials. Efficiencies vary between 15 to 50% and in some cases even higher. Electronic filters are washable and some brands are dishwasher safe. With improper airflow and filter sizing they will make a crackling noise and generate ozone (O3). A drop in efficiency will also be experienced.

Additives, we are seeing tacifiers, antimicrobials and anti allergens added to some spun and pleated filters. The tacifiers do increase the capture and holding power of filters. The others only seem to be affective if the particle was actually captured and held. It seems that this is what a normal filter does also.