Document 4w9zy9d3B2dM1VVY1mQyGLop

744 CHAPTER 33 1954 Guide time. If A and B are the limiting characteristic curves of the systems, the fan performance will never be outside the points X 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.- 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 be. Fig. 8. Motoh Position, Belt or 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, while 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 in each case will dictate which type of control is most desirable. Where noise is a factor, lowering the fan speed if possible is preferred as a control means, because of the resulting reduction in sound level. In addition to the above types of control, tubeaxial and vaneaxial fans Fans 745 are sometimes made with adjustable blades to 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 se lect at least the standard size next larger than the fan requirements. Di rect-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-curyed blades, 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 precise, 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 40 for characteristics of various types of motors, FAN SELECTION The following information is required to select the proper type and size of fan: 1- Capacity in cubic feet per minute. 2. Static pressure or system resistance. 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- ures: (1) volume of air in cubic feet per minute (68 F, 50 percent 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.