Is a 24x30 filter grille large enough for a 3.5 ton system?
Yes & No...
RETURN AIR FILTER RACKS
A filter grille is not to exceed Manual D's 300-fpm maximum velocity.
Use the tonnage's CFM /divided by filter grille's free-air-area = FPM velocity.
You really can't have too much filter grille area; more is always better because they are always loading.
It all depends on the type of filter! Typical disposable 1-inch capacity return air filter is 2 cfm per square inch of gross filter area. Recommended actual filter velocity is 300-fpm or less, lower is better.
Filter Velocities higher than 500 fpm will decrease filter performance. Increase flow resistance, and possibly blow off collections of debris. Measure Velocity 1” from RA grille face.
Average Free Air area of most Return Air grilles about 75%.
75% of 720-sq.ins., is 540-sq.ins. of free air area.
So, what is the filter's free-air-area?
The filter mfg'ers ought to be required to list the free-air-area of their filters, & then state on the packaging or on the filter, the limit of CFM to stay within 300-fpm velocity through a clean filter.
To achieve 300-fpm through a clean filter, you would need 700-sq.ins of free-air-area / 144 = 4.886111-sq.ft., | 3.5-Ton @1400-cfm / 4.86111= 288-fpm velocity through a clean filter. (Edited)
Then how much free-air-area does that filter design have?
It depends on the type of filter(s) used.
Most 3.5 to 5-Ton systems require two filter racks exterior of the airhandler to achieve a low enough air velocity through those filters. Most 4 & 5-Ton systems nearly always have too much air velocity through initial clean filters, let alone when they begin loading. - Darrell
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udarrell
Last edited by udarrell; 10-08-2009 at 06:33 PM.
Reason: 300-fpm or less... depends on the type of filter(s) used
Linear Mode
