The Different Types Of Air Filters

You can see to the right that there are all kinds of air filters. The main categories are; pre-filters, final filters and HEPA filters. Then you have special filters that include carbon filters, electrostatic and gas phase filters.

The pre-filter is the most common and the least expensive. Pre-filters usually start with a media that will be able to capture particles from the 5-micron particle up to 100-micron. These are usually considered OK for home air filters and office air filters use and for pre-filtering a more expensive filter.

There are several types of pre-filters. First you have the fiberglass filters or poly media filters known as furnace filters or through away filters. These filters are so inefficient that they are not even tested. The next in line is the pleated filter. Pleated so you can expose the air to more media. The pleated filter is usually a 30% filter. There are some pleats now that have an electrostatic charge that makes them much more efficient until the charge dissipates (usually in several months). Next you have the type that is made from a 3-ply or 4-ply graduated density media. These are the best because they capture particles that are smaller than a pleat can capture. The 4-ply is better than the 3-ply, however, the 3-ply is efficient enough to give the average home a healthy environment.

The final filter (ASHRAE CELL) gets it's name because for ages it was the final filter in a situation where cleaner air is needed. These filters come in different configurations but all have a dust spot rating of 65%, 85% or 95%. One configuration is the ASHRAE CELL, made in the form of a box usually with 6" or 12" deep pleats. Another type would be what is call a pocket filter. This is a frame that holds 6 to 10 pockets. The pocket media is rated at the same efficiencies as the cell. A 90% in either style is usually able to remove near 100% on a 1-micron particle.

HEPA filters are designed to capture a .3-micron particle. A .23 to .3 micron particle is the hardest particle size to capture. So if you design a filter to capture this size you can capture a smaller particle at a higher efficiency. A .01-micron particle is easier to capture than the .23 size. The principle used to capture these small particles is call diffusion. HEPA filters are rated 99.97%, 99.99% or 99.999% on a .3-micron particle. This means that they will capture a .3-micron particle at these efficiencies.

Carbon filters are usually used to capture an odor or a vapor. They need to be pre-filtered as do all expensive filters.

A hospital operating room where bone work is being done would have, first a pre-filter protecting a ASHRAE cell and that protecting a HEPA filter. The HEPA can cost as much as $200.00 so you would protect it with a filter that costs about $60.00 and this filter would be protected with a pre-filter that costs $10.00 or less.

This is an overview of the types of filters. If you want to learn more go to the ASHRAE web site. (ashrae.org)  Back to Home