Filter Pressure Loss

[Norm01]

I am just wondering what some you are doing when sizing your fans to incorporate the friction loss of a filter. For instance I have a situation where I need 2000 cfm and a MERV 13 filter. When I go to the manufacturers literature I find that for 2000 cfm the pressure drop through a 2" filter is 0.4" wg. The literature also states that the final resistance is 1.0" wg. So when sizing your fan do you using the clean filter pressure drop (0.4"wg), the dirty filter pressure drop (1.0"wg) or somewhere in between?
Thanks.

 

[EnergyProfessional]

I'd use final and the VFD reduces fan speed when the filter is clean. Obvioulsy fan shoudl not be in surge region whne filter is clean.

 

[ChasBean1]

The latter. Size for dirty filters.

 

[IRstuff]

I don't know if these filters will get progressively worse, but you might actually want to size for a somewhat worse resistance, on the assumption that most filters are rarely replaced on schedule.

TTFN

FAQ731-376

 

[Zesti]

I guess it depends on the application of the system.

For most applictions (office HVAC etc.) it is probably alright to size the fans for filters at halfway between new and dirty.

The difference between "half dirty" and "dirty" is probably only 75Pa and with a total fan-pressure of 1000Pa or so the difference in airflow is of little or no consequence in such an application.

But for a critical application where you need 100% flow under all circumstances you should size for dirty filters.

 

[willard3]

It is always a good idea to put a differential pressure sensor across the filter and attach it to a gauge that is easily visible in the mechanical room.

Put in a point for this information on the DDC.

Then, when complaints start about poor cooling/heating-stuffiness, you can find the problem right away.