An air filter frame is just a bent up piece of sheet metal isn't it? For many manufacturers the answer is yes, but for the end user that wants to ensure a proper fit and minimal to no bypass of air then the answer is a resounding NO! There are a few components to an air filter frame that make sure it not only holds the filter but also eliminates the chances of bypass of air around the filter or through the frame system itself. Here are those components:
1) Correct & Consistent Sizing. This seems like a prerequisite and should not even need to be mentioned, but we've had more than one project ruined by allowing customers to go with a more "budget friendly" air filter frame supplier only to have the frames not be square, have tolerances that makes the frames vary in size, and to have the sealing surface too large which prevents the use of some filter types that are designed to pass through the frame and seal against it using a peripheral header (sometimes refered to as a "lip"). The sealing surface should not extend further than 3/4" into the center of the frame to allow for a variety of manufacturer's products to work in the frames. Demanding strict tolerances and that the manufacturing be done in an ISO 9001 and NQA-1 facility are a good protection against these problems.
2) Easily Replaceable & Duarable Gasket. One of the main reasons to use a built-up air filter frame system as opposed to the space saving side-access systems is to have the lowest possible leak rate/bypass of air around the filter and the gasket is critical to achieve this. Most common is a cheap neoprene gasket, but with advancements in technology and a narrowing gap in raw material costs there are frames available with a high-quality rubber. Rubber gaskets are more duarable as they tend not to compress and lose the ability to seal as quickly as neoprene gaskets deteriorate. Another advantage of the rubber gaskets is they are much easier to replace, traditional gaskets are held in palce by two-sided tape which means when they need to be replaced maintenance staff needs to spend hours scraping the old adhesive off the frames so the new gasket material will adhere to the frame - this acts as a deterrent and causes frame gasketing to not get replaced.
3) Installation guides/dimples. Even if you get the best frame in the world, if it is installed poorly the benefits are gone. Having installation guides, dimples, or pre-manufactured bolt holes as an integral part of the frame system will help to make sure that the installation is done properly.
4) Stout Material. Another easy was for a manufacturer to make things less expensive is to use a thin sheet metal or to not weld the frame together - opting for a "wrap around" style of manufacturing. 16-gauge steel should be the standard for HVAC applications in order for the frame to hold up to decades of abuse in an air handling unit. If the corners aren't welded it creates a product that is weaker diagonally - if you grab it in opposite corners and attempt to twist it, the frame should remain rigid and not buckle or bend.
5) Interferance-free Clips/Fasteners. Another one of those, "why do we even need to mention this" topics but the filter needs to be held in the frame somehow and commonly this is done with something called a "knock-on" or "P" clip. These should not be used as they interfere with the gasket and create an area that produces air by-pass reducing the effectiveness of the frame system. We have seen more high-quality frames get their value negated by using these clips than any of the other offences in air filter frame building. The clip/fastner system should be integral to the side of the frame itself either with a series of lances for a clip to be held by or a compression tab type system that does not "knock-on" over or under the gasket to the sealing surface of the frame.
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