Book - School of Science and Technology

478 Fans and air treatment equipment30° to 90° dischargeDrivecasingImpellerImpellerGuide vanesFigure 17.4 Mixed flow fansproduce a given pressure, and are liable to be more noisy: this may be overcome, to someextent, by the use of an attenuator. Their limited pressure development may be overcomeby installing multiple fans to operate in series.Mixed flowSuch fans are designed for in-line flow or radial-discharge and comprise an impellerhaving a number of blades, on a conical shaped hub. The efficiency and pressure developedare generally higher than for the axial flow type, with lower noise levels. Pressuresup to 1750 Pa can be achieved with the range of fans currently available. Variable duty canbe obtained using either variable speed drives or inlet guide vanes. Either direct drive orbelt drive configurations are available. Figure 17.4 shows diagrammatically the operationof a mixed flow fan.The fan curves indicate a non-overloading power characteristic, making it suitable foruse with ducted systems. Efficiency is good at about 80% over a limited range of duty, butthe pressure characteristic, being similar to that of a forward curved centrifugal, makes itunsuitable for parallel operation.Centrifugal fan arrangements and drivesCentrifugal fans may be open or cased. When open they can only be used for exhausting,and the discharge is tangential from the perimeter of the impeller, as might be suitable in alarge roof turret.The usual arrangement is the cased type, and the suction is then either on one side, as inFigure 17.5(a), with a single inlet, or both sides, as Figure 17.5(b), with double inlet. Thedouble inlet double-width fan is useful in packaged air handling plant or where largevolumes are concerned as it gives double the capacity of the single inlet with the sameheight of casing.Fans are almost invariably driven by electric motor except for cases where there may bea requirement for independent drive from a petrol or diesel engine. Figure 17.5(c) shows atypical arrangement with the fan impeller mounted on a shaft extension of the motor.This is a compact arrangement, but generally used for small or medium-sized fans only.A motor direct-coupled to a fan with a flexible coupling is illustrated in Figure 17.5(d).The fan shaft runs in its own bearings independently of the motor. This is obviously to bepreferred for heavy duty and for large fans. The motor can be removed and replacedwithout affecting the fan.