Document nmzL2RoRDyDKydw62Xdx0GE76

686: CHAPTER 32 1950 Guide. time. If A and B are the limiting characteristic curves of the systems, the fan performance will never be outside the points iX or Y. . FAN ARRANGEMENTS Centrifugal fan arrangements have been standardized by the National Association of Fan Manufacturers. Figs. 6, 7 and 8 show, the accepted designation as: to arrangement of drive, rotation, discharge and motor position, for belt drive. Axial flow fans are either belt driven or direct connected, in accordance, with individual manufacturer's arrangements. Usually a choice of anti-friction, or sleeve bearings is available. I FAN CONTROL In some heating and ventilating systems it. is desirable to vary the volume of air handled by the fan, and this may be accomplished by a Location of motor is determined by facing the drive side of fan or blower and designating the motor po sition by letters W,X,'Y or Z as the case may ' ... Fig. 8. Motob Position, Belt ob Chain Drive number of methods,. -Where the change,is:made infrequently, the pulley, or.sheave on the driving motor, or fan, may be changed to vary the speed of. the fan and alter the,air volume. .. Dampers may be placed in the duct, system to vary the volume. Variable speed pulleys or transmissions, such! as fan belt change boxes, or electric or hydraulic couplings, may be used to vary the fan speed. Variable speed motors, and variable fan inlet vanes may-also be'used to adjust the fan volume. All of these methods will give * Control. From a power consumption consideration, a reduction of fan- speed is most efficient. Inlet vanes save some power, and- dampers save* the least. From consideration of first cost, dampers usually are the lowest in'cost.' In some installations, adjustments of volume are desirable At' various times during the day, or continuously. In others, an increased; supply of air in summer,-over. that needed in winter,, is demanded. The demands of each case "will .dictate which ,type of. control is most desirable., Where noise is a factor, lowering the fan speed, ifpossible ispreferred as a- control means,'.because of.the resulting reduction in sound level. :j f ^lri,Addition to'the: above types'of control,-`pubeaxial and varieaxial fans Fans '- ' - 687* are sometimes made with adjustable blades tb permit balancing the fan against the system, or making seasonal adjustment. MOTIVE POWER . Heating, ventilating and air conditioning fans are usually driven by electric motors, although Other prime movers may be used. The small sizes of fans, and especially those operating in the higher speed range, are equipped. with direct-connected motors. For larger size fans, and those operating at lower speed, V-belt drives are generally used. In selecting the size of motor for operating a fan, it is advisable to select at least, the standard size next larger than the fan requirements., Direct-connected motors do not require so great a safety factor as belted units. Justification for liberal power provision. exists only in systems, where it is possible that, larger volumes of air may be required at intervals, and made available by use of by-pass dampers, thus greatly reducing the system resistance. If such a system includes a fan with forward-curvedblades, it would be necessary that the motor be sized for the maximum volume and duty. If such a system includes fans with backward-curved blades, the volume peak would not make.it necessary to provide addi tional motor , power. In selecting fans for such a system, sound ratings should be given careful consideration. Where a system is constant, and has no provision for volume change that would materially, reduce the resistance, and when the resistance calculations are reasonably accurate, there is no. necessity for; too liberal a motor allowance, even where fans with forward-curved blades Are used, if the fan has been properly selected. Fig. 5 shows that the system re sistance varies as the square of the volume and the fan static pressure varies approximately inversely as the volume, thus greatly offsetting the trend toward both increase in air delivery and motor load. Reference to Fig. 5 indicates that there is no justification for allowing large spare motor capacity. It is generally more economical to operate motors well loaded. Since the power consumption of fans varies as the cube of the speed, very little starting torque is required of the motor. Refer to Chapter 39-for. characteristics of various types of motors. FAN SELECTION The following information is required to select the proper type and size of fan: 1 1. Capacity in cubic feet per minute. 2. Static pressure or system resistance. 1 3. Air density if other than standard. .4. Type of application or-service... 5. Arrangement of system. 6. Prevailing sound level or use of space served. - 7. Nature of load. . . 8. Type of motive power available. " .. - , ' In order to facilitate, the choice of apparatus, the various fan manu facturers supply fan tables or curves which usually show the following factors for each size of fan operating against a wide range of static pres sures: (1) volume of air in cubic feet per minute (68 F, 50 per cent rela tive humidity, 0;075 lb per cubic foot), (2) outlet velocity, (3) revolutions per minute, (4) brake horsepower, (5) tip or peripheral-speed, and .(6) static pressure. ' The most efficient operating point is usually shown by either bold-face or italicized figures in the capacity tables. -;