Filter Tracking Controls
I am doing construction support on a project that has eleven new, mostly, air handling units, mostly standard rooftop gaspacks for a health care facility. All units have a 30% pre-filter in the unit, and ducted onto the discharge, a 90% final filter. Most units were selected for about 2000 cfm at 1.25"w.g. external static pressure.
My first question is, since I am told the pre-filter is considered part of the internal static pressure, how dirty can that pre-filter get before it starts deducting from the external static pressure? If a 30% filter can load to a published 0.9"w.g., I'm guessing maybe 0.4 is allowed as internal, and the other 0.5 of that deducts from the external static pressure. The pre-filter is 0.1"clean.
The 90% final filter is published as about 0.15"w.g. clean and 1.0"w.g. dirty.
Now running the unit with clean filters, we have 1.25 + 0.4 - 0.1 pre-filter - 0.15 final filter = 1.4"w.g. static pressure after the final filter, and this is when the Air Balancer came in and balanced the duct system.
Four months later, one pre-filter later, same final filter, we have 1.25 + 0.4 - 0.6 pre-filter - 0.6 final filter = 0.45" static pressure after the final filter. Do you think the duct system is performing anything like what it was balanced at?
In health care facilities, where maintaining the air balance is important, and the static pressure loss through two banks of filters may vary greatly between clean filters and dirty filters, you need a control system to maintain the final static pressure constant, regardless of the great change in pressure loss from the filters. Its a shame so many HVAC engineers don't address this problem, until it is too late.
A variable speed control on the fan would do the job nicely.
An old fashioned method is to install a "filter tracking control." Here you start with clean filters and a set of half-closed dampers. As the clean filters get dirty, the dampers gradually open. The net result is that the total static pressure drop stays constant.