Air Filtration for Healthcare Facilities
There are few places where indoor air quality (IAQ) is of greater concern than hospitals and other healthcare facilities. Proper air filtration including filter selection, installation and maintenance plays an important role in this area. This article will focus on the requirements for central ventilation filter selection in healthcare facilities, show how proper installation is essential to obtain good performance and discuss some of the aspects of filter maintenance.

According to the ASHRAE HVAC Design Manual for Hospitals and Clinics rigid filters are preferred for healthcare facilities. Therefore, these high efficiency filters usually are Rigid Box Filters, Rigid Cell Filters or Mini-pleat Filters.

Rigid Box Filters

Rigid box filters are available in depths of 4”, 6” and 12”. They are made with pleated high efficiency media (synthetic or fiberglass) bonded to an expanded metal grid. The pleats are separated and supported by “fingers” made from plastic, cardboard or metal. The boxes are generally made from metal or plastic. They either come as a square box or have a “header” on the front of the filter that enables them to be installed in a 1” filter track/frame. Pleats in a rigid box filter are about an inch apart. A typical 24”X24”X12” rigid box filter will have about 60 square feet of media. Pressure drop for a MERV 14 rigid box filter at 500 fpm is approximately .50” w.g..

Rigid Cell Filters

Rigid cell filters are made with microglass media separated by corrugated aluminum. They are available in either 6” or 12” depths and are sealed into a box constructed from metal or wood. Rigid cell filters come with a single or double “header” or are made in a straight box configuration. A typical 24”X24”X12” MERV 14 rigid cell filter has a media area of about 110 square feet and a pressure drop of about .60” w.g. at 500 fpm.

Mini-pleat Filters

Mini-pleat filters are made from microglass or synthetic media with very tight pleat spacing. While a standard pleated filter might have one pleat per inch, a mini-pleat filter typically has 4-6 pleats per inch. Therefore, it has 4-6 times more media surface area for the same depth filter. Mini-pleat filters come in thicknesses of 2” or 4.” They are also made into 1” panels. These 1” thick panels are then configured into a 12” deep frame in a “V” shape to give the maximum surface area. There are anywhere from 2 to 4 “V’s” in a 24”X24”X12” filter. Several manufacturers state that their 24”X24”X12” V-cell mini-pleat filter incorporates up to 190 square feet of media surface area and a pressure drop of about .35-.40” w.g. at 500 fpm.

Which One?

There are a number of factors that influence the choice of the best high efficiency filter for a particular application including pressure drop, energy usage, projected filter life, ease of installation, speed of the air flow in the HVAC unit and more. When all of these factors are considered it just may be that the more expensive filter in initial cost will turn out to be the least expensive filter to operate in the long run. Filter test results vary significantly and it is always a good idea to insist that all filters being considered are supported by testing of an independent third-party laboratory. The best way to assess what filter to use is to work with someone with extensive knowledge of filtration such as an individual with a Certified Air Filter Specialist (CAFS) designation from the National Air Filtration Association (NAFA).

Recent advances in filter media technology have enabled filter manufacturers to build filters achieving a MERV 13 while using the same frames and pleat configurations as standard pleated filters. These new filters have relatively low resistance and high efficiency. While they are significantly more expensive than a standard pleated filter, they are less expensive than the more traditional filters mentioned above. It is anticipated that they will be widely used in hospitals and healthcare facilities in the future.

Proper Filter Installation

A healthcare facility can have the best filters, but still have poor results unless the filters are installed properly. Filter holding frames should be durable and should be sized properly to provide an airtight fit within the enclosing ductwork. Because high efficiency filters have higher resistance, they are more susceptible to the problems of air bypass. Small gaps between filters or between filters and the surrounding ductwork and unit doors and panels can create serious efficiency loss. For example, in a study conducted by Dr. Jeffrey Siegel at the University of Texas in Austin, it was found that just a 10 millimeter gap between filters could reduce filter efficiency from a MERV 15 to a MERV 8.

All joints between filter frame banks and the enclosing ductwork should be caulked or gasketed to provide a positive seal against air leakage. Gaskets should also be used in the space between the last filter in the row and the filter bank door and between high efficiency filters. Gasket material is made from neoprene or some other compressible rubber-like material that will provide an airtight seal. It usually has an adhesive on one side protected by a peel off backing.

Are Pre-filters Necessary?

Several of the filtration guidelines above do not call for pre-filtration of high efficiency filters. Generally, whenever there are filters of MERV 12 or above,it is a good idea to use a pre-filter to capture the larger particles. This extends the life of the high efficiency filters and preserves their integrity for capturing smaller particles. When you consider that the high efficiency MERV 12 and above filters cost anywhere from 10 times to 40 times the cost of a pleated filter, it is easy to see the savings involved. Changing pleated filters on a quarterly basis is more economical than replacing an $80 filter six months prematurely because it had been used to remove dirt, dust and debris.

High Efficiency Final Filters Should be Downstream from the Coils

It is important to understand that in all healthcare filtration when there are two filter beds, the first one is installed before the blower and coils. The second one is always downstream of the coils. This set-up ensures that the first filter is used to protect the equipment from dirt and other contaminants in the air stream. The second filter bank captures anything that makes it through the first filter or comes off of the HVAC equipment itself. For example, if the coils have mold on them, the high efficiency filter would capture the mold spores before they could enter the healthcare facility.

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