I'm working with the formula that CFM = FPM X Cross sectional Area and I'm trying to calculate an accurate FPM through my air filter in my air handler.
Originally, I was simply using the cross sectional area of the filter opening, 20×20 (19.5 x 19.5 actual) which is about 2.6 sq ft. My air handler is pushing 1098 CMF, and so I have:
1098 CFM / 2.6 sq ft = 422 FPM
Which I believe is accurate, if the opening were just a gaping hole instead of an actual filter. The trouble comes when I referenced this number 422 FPM and someone said I should actually be using the total square footage of the pleated media in the filter, through which the air is passing. In my 1" filter thats 5.55 sq ft, and in my 2" filter thats 12 sq ft. That would yield the following calculations:
1098 CFM / 5.55 sq ft = 197 FPM
1098 CFM / 12 sq ft = 91 FPM
What I'm ultimately trying to do is determine an accurate FPM calculation, so that I can compare that value to data sheets on various filters. The data sheets I have, have tables and graphs where the X axis is FPM and the Y axis is Pressure Drop. Thus, if I can get an accurate FPM calculation, I can see the estimated pressure drop at various sizes of filter, and at various CFM settings on my air handler.
Should I therefore be using total filter media as the sq ft variable, or just the cross sectional area of the opening, or something entirely different?
Best Answer
Your method of FPM X cross sectional area won’t come out right for a large trunk line, the problem is the airflow is faster in the middle and slower at the edges so usually a grid of measurements are taken and an average flow is created. I have a book that gives the recommended number of points based on the size of memory serves 3 points 2 on the sides and one in the middle averaged gets close with the big book. Measurements were not supposed to be taken the width of the opening from a turn, that’s silly because most openings are at a 90 to the duct within an inch or so. I just take the measurements wherever and it has always been close enough for residential.