What You Need to Know About Filtration


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Every HVACR technician knows that air filters are designed to remove particulate contaminants from the air. Some of these particulates come from natural sources such as plants, mold, small insect parts, or dust from soil erosion. Manmade sources including fires, car exhaust, fossil fuel appliances, industrial processes, etc., also add particulate matter to the air. Smaller and lighter airborne particulates float in the air and can stay suspended longer. I believe everyone has seen light reflected off of dust floating in front of a window? Fortunately, for us, a majority of those airborne particles are harmless. However, some particulates – like microorganisms, droplet nuclei, and spores from fungi which can cause an allergic reaction and/or illness, are not a good thing to inhale. Worse yet, for those living near industrial areas, breathing in particulates like asbestos, coal dust, lead dust, and silica, can be a serious health risk and may cause permanent lung damage.

Most HVAC systems are designed to use basic filtration that protects coils and heat exchangers from relatively large dust particulates. Large particles in the air are trapped as the air passes through the filter. When selecting filters, most designers will try to either meet or exceed local codes. For example, some local codes have adopted ANSI/ASHRAE 62.1 Ventilation for Acceptable Indoor Air Quality. In the 2007 version, a MERV 6 or better rating was required for filtration of air brought in from the outside. That was upgraded in 2013 to a minimum MERV value of 8. Thus, a designer who has used a MERV 6 filter since 2007 might need to upgrade to a MERV 8 when the updated code starts being enforced. Chart 1 can be used as a basic application guideline based on ASHRAE 52.2 Method of Testing General Ventilation Air-Cleaning devices for Removal Efficiency by Particle Size. Note that for general filtration purposes, MERV’s 1-4 are currently considered the norm. A MERV 1-4 filter would remove less than 20% of the largest range particles – 3.0 to 10.0 micrometers – and almost none of the particles in the ranges 0.3 to 1.0 and 1.0 to 3.0 micrometers. For example, a MERV 1-4 filter would remove less than 20% of particles from fertilizer, plant spores, human hair, most pet dander, and some types of household dust.

If, due to environmental or health concerns, protection from smaller particulates is required, the filter will need to be more efficient. For filters that does not mean the filter is saves on operating expense, it means it is able to remove smaller particles. Removing smaller particles means smaller air openings and generally that means the pressure drop across the filter is higher, and it will cost more to pull air through it. Thus, in order to maintain the HVAC equipment’s required airflow, fan speed modifications may be required when a filter’s efficiency is increased.

Warning:  the MERV value of the HVAC system will only be as good as the total combined MERV value of the HVAC cabinet plus the filter rack and the filter’s MERV value.  It is important in high efficiency MERV applications that the filters are installed correctly.  Additionally, the filter rack needs to be designed to hold the filter in a way that meets or exceeds the MERV value of the filter.  Filter rack system designs are very robust in applications where HEPA filters are used.  This is due to their clean operating pressure drops in the range of 0.8 to 1.0 inches water gauge.

For further information on filters, and filtration design, ACCA members can access the Filtration Primer Technical Bulletin available for downloading in the File Downloads section of ACCA’s webpage: https://www.acca.org/members/downloads.

Don Prather

Posted In: Technical Tips

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