Maximum Filter Velocity

Tonight I wanted to take a few moments to talk about air velocity through filters. Typical specifications limit velocity through filters to no more than 500 feet per minute. Likewise Manufacturers typically list filter data at 500 feet per minute. The reality is that filters will work at different velocities. What is uncertain is how well they will work. The pressure drop through the filter goes up. In most cases the efficiency goes down and the life cycle of the filter is reduced.
The reason the reduction in efficient is uncertain, is because of the way that modern air filters capture particles. Modern filters are not sieves. They are not a plate with holes smaller than the smallest particle size you are trying to remove from the air. Modern filters are a matrix of fibers, usually of 3 or 4 different diameters. The density of the weave and crisscross path that a particle would take through the filter increases the probability of a particle being captured in the surface of the filter media. A change in particle velocity might make the particle capture more or less likely as it tries to remain in the airstream without crashing into a fiber.
With all that said, what we have to remember is that all designs are a trade off. You can’t always get what you want without giving up something. Sometimes it’s dollars, sometimes it’s space. If you are going to push filters hard, don’t do it on a clean room project. If it’s a comfort cooling application, but you are out of space, overdriving the filter velocity isn’t going to be the end of the world. Just remember a few basics.
Plan for a higher than normal pressure drop through the filters. More fan horsepower will be required per cfm.
If you are looking for a certain efficiency of filter performance, oversize your selection at least one step. i.e. If a MERV 12 filter is required, use at least a MERV 14 filter.
Warn the end user through proper documentation that filters will need to be changed more often. Ideally consider active monitoring for dirty filter status, by measuring differential pressure across the filter bank.
The higher velocity puts you at risk for pulling the filters out of the frame. To mitigate this occurrence, consider adding some 1/4″ rod across the middle of the rack.
Know when to walk away. You might get away with a 50% overdrive, but I wouldn’t try to push it any harder.

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