Document 7ObbQ72og5v370k2yQ6kOYvER

836 CHAPTER 32 1958 Guide 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 mays 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, oyer 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. 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. Motor Position, Belt or Chain Drive In addition to the above types of control, tubeaxial and vaneaxial fans 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 fa,n 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-curved 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 Fans 837 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 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. 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 sures: (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. Often the service determines the type of fan. When operation occurs with little or no resistance, and particularly without a duct system, the propeller fan is indicated for convenience and low cost. When resistance B lw the power required is low, and efficiency becomes a secondary im portance. When a duct system is involved the choice is usually made between a centrifugal fan and a tubeaxial or vaneaxial. At times the capacity-pressure-speed relationship (specific speed)9 dictates a choice. Usually, space, efficiency, sound, cost and serviceability must all be con sidered.10 In general, centrifugal and axial fans are comparable in effi ciency and sound, but the latter are lighter and require considerably less Pace, especially if arranged for straight-through operation. The compari son cannot be made on the cost of fans only, but the difference in cost of ductwork, mounting and servicing must be included. A vaneaxial is more efficient and quieter than a tubeaxial, but is more expensive, and frequently inquires more space. While requiring less space than the centrifugal, the axial flow fan is inherently less accessible for service. When high-temperatore air or air containing corrosive elements is being conveyed, motors and bearings should be located outside of the air stream. This requirement