The AHU pressure drop can be reduced by:
Lowering face velocities necessitates larger area coils and filter elements and bigger AHU housings than conventionally used. These larger items increase the AHU's cost, but a resulting decrease in face velocity from 500 fpm to 400 fpm usually makes up for the increased cost over the life of the AHU. Face velocity reductions from 500 fpm to 400 fpm can have a simple payback of about three years. A comparable decrease in face velocity at filters can have a simple payback of one year or less. [Brown, 1990]
Variable system resistance, primarily resulting from filter loading, can be efficiently corrected for with a VFD on the fan. AHU pre-filter pressure drop will typically vary from 0.5 in. H2O (125 Pa) when "clean" to 1.7 in H2O (423 Pa) when "dirty." The system design point is usually sized for the "dirty" condition. However, on average, about 90 percent of the design head and energy is actually required. The energy saved by a VFD will pay for the drive in two to five years, depending upon system capacity. [Brown, 1990]
Some AHU components are only utilized during a portion of the year; therefore, bypass dampers should be used to reduce parasitic energy losses when these components are not needed. [Brown, 1990]
Energy engineers must carefully evaluate inlet and outlet configurations of the AHU because physical restrictions imposed by architectural considerations often limit the space allocated to air-handling equipment. [Hunt et al., 1990]
