filter sizing

[r2]

are there ANY guidelines what comes to filter sizing?
i have this one guy telling me that you can use a duculator to size filters, so for 4 ton rooftop 16" round return a 22x22x1 pleated filter is pleeeeeenty. he seems to think anything over 16" is extra.

the trane filter rack accommodates 2-16x25x2 filters itself.

i'm not looking to get too technical on this, perhaps just a good guideline.

[r2]

let me know if this would work;
since 1600cfm is about 1100fpm with a 16" round duct or 15x15 rectangular,,, and the filter box is little more than 20x20 can i ductulate that the fpm's are now about 600..,,,and from there conclude that if the filter was stretched out it would be something like 22x32 therefore fmp's against the filter would be 360fpm??
what are the recommendet values for pleated filters anyway?

[drk]

Need more filter area need bigger return

[r2]

well i figured that i'd need more filter area, but the return should be in the ballpark. can you be more specific how much more filter area and how did you figure it out.

[midhvac]

Originally posted by r2
well i figured that i'd need more filter area, but the return should be in the ballpark. can you be more specific how much more filter area and how did you figure it out.

CFM / 2.08 = square inches of filter surface area for 300 FPM velocity (which is what most disposable filters are designed for).

[midhvac]

You can't use the stretched out size of a pleated filter for the filter size. It doesn't work that way. The actual 2 dimensional physical width and height that the filter (stretched out or not) occupies in a cross section of duct determines the static pressure drop. Therefore, the size of the duct or filter rack determines the static pressure drop for a given type of filter media in a system. The thickness of the filter has no effect on this. All the pleats do is provide more area for the dirt to collect on, thereby increasing the time for the filter to plug up. So if you have a 20 x 20 pleated filter with an unfolded size of 20 x 40 placed in the shape of a sock in a 20 x 20 duct, it will have the same pressure drop as it did before, other than some minor aerodynamic differences.