Naughton ("HVAC Systems… Part 1," 1990) discusses the choice of filter fans.
For many small FFUs with motor sizes less than 1 hp (0.75 kW) and in-line centrifugal forward-curved fans, the total efficiency drops to 50% or less. An 85% mechanically efficient, direct-drive vane-axial fan has a motor efficiency of 94% and total efficiency of (0.85 x 0.94) x 100% = 79.9%.
Compare this with an FFU operating at 1.5 in. WC [W.G.] (373.5 Pa) of static pressure, 55% mechanical efficiency, 70% motor efficiency, and total efficiency of (0.55 x 0.70) x 100% = 38.5%. The total efficiency ratio of 79.9% over 38.5%, or 2.07, means the large fan system must have 2.07 times higher static pressure, or 2.07 x 1.5 inc. - 3.11 inc. WC [W.G.] (775.1 Pa), in order to use the same amount of energy as the FFU system. Therefore, when one considers the higher maintenance costs and shorter equipment life of the FFU, the lower static pressure FFU may not be as attractive a choice as the large centralized vane-axial fan system. The energy trade-off in reduced efficiency can also be seen when comparing other fan choices.
