Book - School of Science and Technology

480 Fans and air treatment equipmentCertain applications require fans to have an automatic standby capability. The beltdrive configuration enables duty and standby motors to be installed to drive shaft of asingle fan, although this increases the load on the running motor. The alternative is to usetwin fans, with the resultant increase in space and capital cost.CentrifugalIt will be noted that in the illustrations a variety of different positions of the dischargeopening in relation to the suction eye of the fan is given in each case. It is usually possibleto obtain a fan with its discharge at any angle, vertical, horizontal top, horizontal bottom,downwards, and intermediately at an angle of 45 .Inlet guide vanes, or variable speed drive using static inverters are usually adopted togive variable duty.Axial flow fan arrangementsThis type of fan can produce pressures up to a maximum of about 1000 Pa, as a single unitwithin the normal range of noise generation. To achieve such performance, guide vaneswould be fitted to improve the operating efficiency by reducing the swirl effect.These fans can be built in one, two or three stages, to obtain increased pressure, thevolume remaining the same. Alternatively they may have two sets of blades made counterrotating.Both direct drive and belt drive types are available.Such fans are illustrated in Figure 17.6, many are manufactured to permit adjustmentof the pitch angle of the blade. Since this angle, for a given fan diameter and speed,determines the volume delivered it follows that adjustment facilities permit output to bematched to the duty required with some precision. Figure 17.6(d) and (e) show how pitchmay be adjusted.(a) Single stage fan(b) Two stage fan(contra-rotating)(c) Belt driven fanPitch angleRotation(d) Blade sectionFigure 17.6 Arrangements for axial flow fans(e) Variable pitchBlade pitchadjustable