Air filtration is included in Engineered Systems’ editorial calendar this month, so I’m going to touch on some of the issues and concerns regarding the design, installation, commissioning, and operation of filters. You might want to stop reading this column and pick up your handy 2020 ASHRAE Handbook, “HVAC Systems and Equipment,” to refresh your knowledge of air filters by reading Chapter 29, “Air Cleaners for Particulate Contaminants,” as well as the 2021 Handbook, Chapter 11, “Air Contaminants.” Collectively, these two chapters represent 37 pages on this topic.
Here is a list of things to do when designing an air filtration system or air filters in series for your central air-handling system project application.
1. What is the bare-essential filter needed upstream of a fan to protect the components within a fan unit, e.g., heating coil and/or to recirculate this air within a space? Remember, filters are used to help keep HVAC components clean to allow these components to continue to operate efficiently as well as strive to capture the dust and other particles in the air space.
2. What is the optimum type of filter or combination of filters for the job? ASHRAE has drawn upon MERV rating as a quick reference chart for design engineers to use as select air filters based on each project’s design intent.
3. Coordinate with the other consultants, such as the design electrical engineer, especially if electrostatic filters are used for the job. More importantly, coordination with the building owner’s operation and maintenance (O&M) manager is needed whenever possible to mutually agree on the selection of filter(s) to be used so as to determine the optimum filter sizes and types to keep in stock. One trick of the trade I learned from a filter manufacturer representative years ago was to use standardized filter sizes to minimize the inventory of filters to keep in stock. My “filter mentor,” Max Share, told me of a high-rise building where his firm had the filter maintenance contract that included 156 different filter sizes, e.g., 12-by-12-inch, 15-by-15-inch, etc. From this “value-engineering information,” I was able to specify one standardized 24-by-24-inch flat pre-filter with a MERV-8 rating and one standardized 24-by-24-inch flat final filter with a MERV-14 rating to serve 12 central air-handling units delivering a total of 1 million cfm of supply air to 800,000 square feet of commercial building space. The maintenance cost and ability to keep a reasonable number of replacement filters proved to be very cost-effective. I’ve applied this standardized filter concept to every project since then.
4. Just because a filter looks dirty doesn’t mean it needs to be replaced. Filter manufacturers catalog the initial air resistance when a new filter is installed along with a recommended air resistance limit indicating when it is time to replace the filter. The designer, as well as the maintenance supervisor, should always have a magnehelic gage, sensing the air pressure upstream and downstream of the filter unit to visually see what the air resistance is so that filters are replaced based on manufacturer’s recommendation and not simply changed because they look dirty.
Next month’s editorial calendar includes refrigerants in transition, so I will examine issues related to refrigerants next month.