Air Conditioner Reviews, HVAC info and Buyers Guide

The Ins and Outs of Filtration


On October 8th, 2001, while the US was still reeling from the World Trade Center attacks, the President established the Office of Homeland Security. One of the tasks of the new agency was to develop ways citizens could protect themselves from terrorist attack.

What has that got to do with filtration? Well, the agency commissioned a report to address the worry that a terrorist group would introduce some toxic substance into a building’s air handling equipment. The report, Filtration and Air-Cleaning Systems to Protect Building Environments, ended up being published by the Centers for Disease Control and has become one of the best guides for air filtration available outside of the HVAC industry. Our tax dollars at work.

Some basic terms

A barrier that removes particles (dust, dander, and other solids) from the air is called an air filter, while something that removes gases or vapors is called an air cleaner.

Filter performance is a measure of a particular filter’s description of a filter’s collection efficiency, pressure drop, and dust-holding capacity over time.

  • Collection efficiency is the fraction of the number of particles collected that are retained by the filter. Since the size of the particle matters (smaller sizes mean more get through), this figure is sometimes given for different size ranges.
  • Pressure Drop is a measure of airflow resistance through a filter. The higher the pressure drop, the faster air will move through a filter.
  • Holding Capacity — the amount of particulates a filter will hold before it starts getting blocked.

How do all these relate? The best filter is a balance between how well it removes particles from the air with the lowest amount of work (pressure drop needed) and the longest lifespan (before it gets clogged).

Sounds easy, right? It isn’t. All filters trade off one or the other of these factors. How well they do so is directly related to the price you end up paying for the filter.

 

Types, benefits and costs

In this article, we aren’t delving into the more exotic, specialty filters. Some of these were covered in the Homeland Security guide, but most homeowners just want a simple filter to reduce dust or other junk (cigarette smoke, pollen) floating in the air.

The cheapest are simple fiber filters. These use a membrane made of paper, cotton, fiberglass or almost any fibrous medium. They are generally “in depth” filters, rather than surface filters. In depth just means they filter throughout their thickness (up to half an inch or more) rather than just in one layer. A coffee filter is an example of a surface type filter.

These have the advantage of not needing much of a positive pressure to push air through them. Low cost fans will do. The trade off is they aren’t very good at capturing fine particles and many airborne pollutants will make their way through them. The cheapest is simply a plastic grate type. These catch large bits of dust and keep them out of the air conditioner, but not much else. The advantage of these is are they are washable – just rinse them, pat dry and reinstall. Fiber filters and washable screens are sometimes installed together, with the plastic mesh catching larger stuff and the fiber batting some smaller items.

The relatively loose weave of simple fiber filters means they have a large holding capacity – air flow remains good and they last a long time between changes (depending on how dirty the environment is).

HEPA filters are also made of fibers, but these are compressed into a tightly woven mat. This mat is then crinkled up to give it more thickness. It offers much better filtration, but the price comes in reduced airflow. More pressure is needed to pull air through the smaller spaces. They also clog faster because they trap more particles.

For those who suffer allergies or are sensitive to dust, a HEPA filter may still be the better choice, even though it is more expensive.

Activated Carbon filters also gather particles, but their main benefit is in trapping vapors and chemicals too small for either a HEPA or fiber filter to get. Activated carbon has microscopic spaces on the carbon granules that chemicals can lodge in. It’s this reason why they are used as odor busters – they can grab the very small molecules we detect as foul scents. Because the chemicals are trapped on small grains of carbon, air can still flow between the grains easily, giving them good air flow.

The downside of activated carbon is the cost. They are almost always used as an additional filter, behind and after either a HEPA or fiber type filter. This keeps many larger particles from reaching the activated carbon filter and extends life.

Filter stacks

Since each of the common filter types above traps different particle sizes, a filter stack is sometimes used with fiber followed by HEPA, followed by carbon. This lets each filter remove those particles it is best suited for. There is a tradeoff with lower efficiency and air flow.

Check out acboy’s tips on buying the right air purifier here!

In air conditioners, very expensive or high-tech filters aren’t usually needed. Their primary purpose is to protect the air conditioner itself from the damage that can come when dust accumulates inside. If air purification is the desired result, it is almost always better to buy a dedicated unit to improve air quality.

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