Below is sample calculation for the pressure drop or static pressure drop to cater for fan and airflow calculation.For Standard 2 layers MERV6/7 and MERV14 filters. 

Prefilter/ Primary Filter

IF30 24x24x2” (MERV7)             55Pa@ 3400cmh (Rated)

       12x24x2”  1700cmh

       12x12x2”  850cmh

 For Airflow Volume 450cmh, can use 12x12x2” will do.

Initial Pressure Drop: 450/850 * 55Pa      = 29 Pa

Final Pressure Drop: min twice of initial >58Pa to 150Pa 

Secondary filter

C90 2in 24x24x2" (MERV14)  128pa@3400cmh (2.5m/s)

C90 2in 12x24x2"                  128Pa@1700cmh

C90 2in 12x12x2”                  128Pa@850cmh

For 450cmh, using C90 2in 12x12x2"

Intial Pressure Drop: 450/850 * 128Pa = 68Pa

Final Pressure Drop: min twice =136Pa or 250Pa

So Total Pressure Drop to cater for fan static = 58+136=194Pa. Typically can use to 250Pa to cater extra.

  Thus for 450cmh, if C90 2in 12x24x2" filter is used instead, the initial pressure drop = 450/1700 * 128Pa = 34Pa.


* For Quick Calculation for Ducted FCU, Fresh Air Fan for design engineers and consultants

For design running at 1.8m/s , Pressure drop =1.8/2.5 * 128Pa = <92Pa. Final presure drop can take as twice of initial min 184Pa

at 1.25m/s of 1700cmh for face area 24x24" Pressure Drop = 1.25/2.5 * 128Pa = 64Pa. Final presure drop can take as twice of initial, min 128Pa

By looking at the pressure drop curve is good  but if one can notice face velocity which is less than rated/maximum 2.5m/s (3400cmh) for 24x24" face area, the pressure drop is much lesser. Thus using proportional ratio will already cater the extra pressure drop or airflow required if there is unexpected shortfall and it is so much easy.

Secondary filter (option) example

IP90 8P 24x24x22" MERV14  89Pa@3400cmh

For Face velocity 1.85m/s, Air volume 2500cmh, 2500/3400 * 89Pa = 65Pa

Final Pressure Drop: min twice = 130Pa to 250Pa

For Rated velocity or max velocity 2.5m/s, pressure drop should cater for Pre 150Pa and Secondary 250Pa , this applies to AHU and Pau as well. However most AHU and Pau cater nowaday are 300Pa as running/design velocity is typically at <75% or 1.85m/s.


Using Larger Face Area, increase depth of filter, underrating and using lower Velocity 75%(typical airflow design) of rated velocity 2.5m/s (Rated means maximum) use will reduce the final pressure drop required in fan selection, increase lifespan, lower noise, cater for unexpected final built results.


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