Book - School of Science and Technology

476 Fans and air treatment equipment(a) Forward curved (b) Radial (c) Backward curvedFigure 17.2 Types of runner in centrifugal formThe three types of runner (a), (b) and (c), are shown in Figure 17.2.. Propeller type:(a) ordinary propeller or disc fan(b) axial flow.. Mixed flow type.Figure 17.3(a)±(f) gives typical curves for the main types. Paddle-wheel fans arenot given, as they are now little used in ventilating workon account of their noise,being confined chiefly to dust removal and industrial uses. Radial bladed fans havecharacteristics which are similar to those of the backward curved type but withoutthe advantage of power limitation. They are not commonly used in ventilation applications.The curves for static pressure, power input, and static efficiency are drawn from testsat constant speed, as already described. The base of the curve is percentage of fullopening of the orifice. The vertical scale is percentage of pressure, efficiency or powerinput.Forward curvedIt will be noted that the forward-bladed centrifugal fan most commonly used in ventilationreaches a maximum efficiency at about 50% opening, where at the same time thestatic pressure is fairly high. Fans are generally selected to worknear this point. It willalso be observed that the power curve rises continuously. Thus, if in a duct system thepressure loss is less than calculated, the air delivered will be more and the power absorbedmore, which may lead to overloading of the motor.Backward curvedThis type of fan runs at a higher speed to achieve the same output as a forward curved.The efficiency reaches a maximum at about the same point, and the power, after reachinga peak, begins to fall. This is called a self-limiting characteristic, and means that if themotor installed is large enough to cover this peakit cannot be overloaded. This is oftenuseful in cases where the pressure is variable or indeterminate. The pressure curve issmooth without the dip of the forward curved; for this reason this kind of fan is to bepreferred where two fans are working in parallel. The forward curved type under suchconditions is apt to hunt from one peakto the next, so that one fan may take more than itsshare of the load and the other much less. The backward-bladed fan is often made withaerofoil blades, so raising efficiency. It is much used in high velocity systems where highpressures are required.