Book - School of Science and Technology

Power PressurePowerPressureFan types and performance 483F 1F 3F 2F 1F 2F 3F 1C 160 80 100VolumeC 160 80 100VolumeP 1P 1P 2F 2P 2P 3F 3F 2F 1P 3F 3VolumeFigure 17.9 Control of fan volume (axial flow) byvariable pitchVolumeFigure 17.10 Control of fan volume (centrifugal)by variable speedconstant system characteristic curve. The resultant dramatic savings in power are shownin (d). Throttling dampers would only be considered for use in very small systems usedintermittently.Where axial flow fans are used with systems demanding volume reduction, the facilityof change in pitch angle of the blades may be automated. This permits both volume andpower to be reduced as required. Figure 17.9 shows the effect of blade pitch angle onpower consumption at various duties. As with inlet guide vanes on centrifugal fans, asignificant power reduction results.The variable fan speed method is well suited to variable air volume systems in comfortair-conditioning applications. In these systems, the torque varies with the square of thespeed and the power with the cube of the speed. Usually a drive is supplied capable ofworking against a torque, with the reduced power and torque requirements at lowerspeeds taken into account in the design of the motor. Special attention should be givenwhere a constant pressure has to be maintained to all, or to a major part of, the systemdistribution since, to achieve the required performance over the range of volume variation,the motor characteristics would need to match the particular system requirements.Fan operating characteristics for centrifugal fans with variable speed control are shown inFigure 17.10.Multi-speed dual wound or pole-change motors may be used where the operatingrequirements are in clearly defined steps, such as winter/summer or day/nightoperation.