©2024 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 68, No. 12, December 2024.
How Do Particle Filters Work?
By Kathleen Owen, Brent Stephens, Ph.D., William P. Bahnfleth, Ph.D., P.E., and Marwa Zaatari, Ph.D.
Kathleen Owen is a consulting engineer at Owen Air Filtration Consulting, LLC. Brent Stephens, Ph.D., is the Arthur W. Hill Endowed Professor in Sustainability and department chair in the Department of Civil, Architectural and Environmental Engineering at the Illinois Institute of Technology. William P. Bahnfleth, Ph.D., P.E., is professor of Architectural Engineering at Pennsylvania State University. Marwa Zaatari, Ph.D., is chief science officer of DZine Partners and head of product at Poppy.
Particle air filters—also called media filters or mechanical filters—are the most common air cleaning technology used today, but not everyone knows how they work or how they interact with systems to clean air. This column reviews filter basics including particle sizes, aerosol capture theory, resistance to airflow and practical issues associated with their application in buildings to improve indoor air quality (IAQ).
Filters are components made of fibrous media, which capture particles as air flows through them. They may be mounted in HVAC systems or in-room air cleaners (IRAC), hoods, small appliances and more. Common filter media are fiberglass or polymer fibers in mixtures of fiber sizes held in plastic, metal or cardboard frames. They can also be made from metal mesh, cotton fabric or other media that allow air to pass and capture particles.
For any given filter, particle filtration efficiency depends mainly on particle size and, to a lesser extent, the flow rate of air through the filter. The size of particles in the air vary from a few nanometers (nm) to 10s of micrometers (µm). At the larger end of this range, particles tend to fall out of the air quickly due to gravity, so most particles suspended in air, in most environments, will remain below 10 µm. Thus, removal of particles below 10 µm by filters is of the most interest in IAQ.
For a specific filter, lower airflow rates generally result in higher efficiency for smaller particles and lower efficiency for larger particles. Thus, it is important to know the airflow at which a filter is rated. Modern filter rating standards consider efficiency in several particle size ranges. ASHRAE Standard 52.2 defines filter minimum efficiency reporting values (MERV) based on 12 particle sizes in three size ranges (E1 covers 0.3 µm – 1.0 µm; E2, 1 µm –3 µm; and E3, 3 µm –10 µm).2
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