Today 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 air stream 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, over driving 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.
