Just FYI - The Fujitsu standard accessory filters on the slim ducts, or the long life filters offered on other models probably have an initial resistance of .03-.05 depending on sizing.  If I was designing a system and using the slim duct with a Fujitsu filter I would subtract .03” -.05” from the rated external static pressure in the Fujitsu D&T manuals.  Depending on ducted unit size and fan speed (ex; the ARUL7 would be a .03” filter drop value).  Unless specifically designed for higher velocities, AHU filter systems are selected for a maximum 300 FPM – 500 FPM face velocity. 

As proof I offer the following:

High Static Unit:  ARUH96TLAV max rated CFM = 2,855,  our filter essentially covers the factory return air opening and is for the most part flat or possibly mildly pleated,  the RA opening on this unit = 6.32 Sqft.

2,855 CFM/6.32 sqft = 452 FPM < 500 FPM ( the max velocity across an evaporator to minimize condensate carry-over).

Slim Duct Unit:  ARUL7TLAV max rated CFM = 324 CFM, same filter installation across the RA opening,  the RA opening on this unit = 1.089 sqft.

324 CFM/1.089 sqft = 297 FPM < 300 FPM

As far as MERV rating goes, to my knowledge our filters have never been tested. However based on the thickness and material I would guess that they are a MERV 2 or 3.  A typical 1” fiberglass throw-away can be a MERV 3 or 4 (See the Purolator F312 PDF’s attached).

For your design question, it is not so much a calculation as a selection based on filter manufacturer data.  The Purolator Hi-E-40 2” Filter (see the PDF) is a MERV 8 and is what I used on RTU maintenance contracts for office type applications.  A filter must be selected for the occupancy application (see the MERV Rating chart PDF), Total designed CFM for the AHU, physical dimension constraints, and available external static pressure of the AHU. Applying a highly efficient filter that is excessive for the type of particulates associated with the application will cause problems because it will load up quickly and decrease AHU CFM output. 

Compare the F312 fiberglass criteria to the Hi-E-40 pleated data and you will notice significant differences.  All filter manufacturers give a final resistance value of the filter when it is completely full. The Purolator Hi-E-40 is 1”wg pressure drop.

On a design where little external static pressure is available for filtration, it will require possibly a low efficiency filter, possibly a  pleated filter to get more filter surface area and more CFM per filter, or possibly a thicker pleated filter to get a better efficiency at a lower initial pressure drop.  (See below)

Certainly the selection needs to allow for the filter to load up some because that will be its typical performance point during the AHU operation.  The filter won’t always be new and clean.

Every filter size is rated for maximum CFM per filter.  CFMtot Fan/Max CFM Filter = #of that filter needed for that AHU.

High Static Unit:  ARUH96TLAV max rated CFM = 2,855,   Look at the HI-E-40 2” thick cut sheet 14x25x2.   2,855 CFM/1220 CFM (14x25x2 Rated CFM) = 2.34 = 3 Filters or a combination of (2) 14x25x2 + (1) 14x20x2.  Initial resistance for this filter type is .28”W.G.  If an ARUH96TLAV designed for 1.1”W.G. external static pressure  1.1”-.28” = .82” of external static remaining for the duct system, electric heat coils, diffusers/grilles, etc.

If the field filter rack is properly fabricated, it is possible to use (2) 16x25x4” thick filters rated at 1,400 CFM each, total of 2,800.  These have an initial resistance of .20”W.G.  The problem here is someone will not use 4” and then the system will be thrown off because they used 2” thick out of convenience during maintenance.

The information listed above also applies to the new RGLX-D Medium Static Halcyon units.