Some things don’t change. One of them is how airborne particles are captured by media air filters. Understanding this will help explain how the oldest and most common air filtration technology can be effective in some ways, and why they are not in others.
Two fundamental factors play into the effectiveness of media, or mechanical, air filters. First is the likelihood that airborne particles that are in the ductwork’s airstream will be on a collision course with the media fibers, and second, that the captured particle will stay captured and not re-enter the airstream or the living space. If any particles that reach the in-duct filter material are not captured, or if they are captured and then later released back into the airstream, then the filter is not as effective as we might have thought.
There are four ways that media filters remove particles from the airstream:
Straining occurs when the smallest dimension of a dust particle is greater than the distance between adjoining media fibers. In other words, they are so big that they just get stuck.
The bigger particles, think of them as airborne bowling balls, carried in the ductwork’s airstream, with their larger size and mass, will simply smash into the media filter fibers and stick to them. The heavier and larger the particle, the more likely it will be captured. The deeper the filter, and the tighter the weave of the filter material, the more likely the particle will be removed from the airstream.
Smaller particles with less mass will tend to follow the airflow around the fibers. When the particle size is similar to the fiber size, the particle will tend to rub against the fiber and stick to it. Additionally, van der Waal’s forces, which are weak electrical forces caused by air friction around the fibers, can provide attraction between particle and fiber.
Very small particles are affected by Brownian Motion, a phenomenon where the smallest particles are constantly affected by electrical forces that exist in all matter. As these tiny particles pass through the filter, their erratic motions carry them into the filter’s fibers and stick there.
There is no doubt that media, or mechanical, filters can be effective at removing some particles from the air that gets to the filter. As you already know, filter materials can be constructed to remove particles of larger sizes, but there becomes a point where the static pressure issues will limit a media filter’s capability to remove smaller particles.
Here are Five Questions to ask about any IAQ device or strategy:
- Will it remove particles from the occupied space?
- Will it remove all contaminants, including the smallest particles, pathogens and VOCs?
- Will it kill pathogens and prevent their reproduction?
- Will it produce any byproducts that are harmful to the occupant’s health?
- Will it negatively impact the performance of the HVAC equipment or air distribution system?
All of these are good to consider, aren’t they? Most IAQ products, including media filters, can provide the desirable answers…but there are shortcomings to each.
Media Filter Limitations
- Media filters will remove larger particles only from the airstream that reaches the filter.
- Media filters will not remove VOC’s or the smallest particles.
- Media filters will not kill airborne pathogens nor pathogens in the occupied space.
- Media filters will not produce any byproducts that can be harmful to the occupant’s health.
- Media filters often have a static pressure rating that exceeds recommended levels when they are new and clean, and their static pressure continues to increase as it loads.
According to the EPA and ASHRAE, even with their limitations, mechanical filters are the only IAQ products that have been proven to improve the health of people in the living or occupied space.
EPA Air Cleaners and Air Filters in the Home, August 2018
“Using air cleaners has also been linked to reductions in some allergy and asthma symptoms and lowering indoor PM concentrations with air cleaners has been shown to beneficially impact some markers of cardiovascular effects associated with exposure to indoor PM of both indoor and outdoor origin.”
ASHRAE Position Document on Filtration and Air Cleaning, July 2018
“One key statement is that, at present, there is only significant evidence of health benefits
for porous media particle filtration systems.”
Next time, Ion Generators FAQ.
Until then, Good Selling.