Book - School of Science and Technology

Axial flow fans of the bifurcated type, where the air stream is directed around the motorwhich is enclosed in a protective casing, are suitable for handling corrosive gases, fromfume cupboards and the like, and also high temperature gases/air such as experienced insmoke extract during fire conditions.Standby arrangements are also possible with axialflow fans. A common way is to installthe two fans in series with the increased pressure loss over the idling impeller to beconsidered. Alternatively it is also possible to mount two fans in a parallel arrangementwith automatic changeover dampers coupled to the fan controls. A standby motorarrangement is also possible where the motors are out of the airstream driving the shaftvia pulleys and belts.The range of fan types, speeds, pressures, and volumes is too great for any indication tobe given here of sizes, duties, power requirements, etc., or of the problem of motor typessuitable for fan drives.Where it is necessary, as in the case of a variable volume air-conditioning system, toexercise control over fan output this may be achieved in a number of ways, which may besummarised conveniently under two headings:Constant fan speed. throttling dampers. inlet guide vanes. variable pitch blade angle (axial flow fans).Variable fan speed. eddy current coupling. variable voltage (fixed frequency). variable frequency, variable voltage. thyristor converter (AC to variable voltage DC)*. fluid couplings. variable pulley drive. slip-ring motor. switched reluctance drive.Fan types and performance 481Figure 17.7(a) and (b) shows for constant and variable fan speed drives respectively, thepercentage power input to a fan assembly relative to volume flow.Where throttling dampers are introduced into the system, volume reduction will beachieved at the expense of efficiency as illustrated in parts (a) and (c) of Figure 17.8. In(a), the full volume operation of a system is represented by the intersection of the fancharacteristic F 1 with the system characteristic C 1 . The use of throttling dampers changesthe system characteristic to C 2 and C 3 at 80% and 60% volume respectively. The resultantsmall savings in power absorbed are shown in (c).The use of radial inlet vane dampers fitted to the eye of a centrifugal fan provides amuch more effective means of volume control since such devices act, by changing the airflow pattern at the fan inlet, to modify the performance of the fan impeller in much thesame way as would result from change in the speed of drive. This effect is illustrated inparts (b) and (d) of Figure 17.8. As before, F 1 represents full volume operation with thesystem characteristic C 1 : operation of the radial inlet vanes changes the fan characteristicto curves F 2 and F 3 , which represent 80% and 60% volume, respectively, against a* AC, alternating current (mains supply); and DC, direct current (supply to motor).