Document 91KjGeJpooGmBN5KOjzVRKjBL

578 CHAPTER. 32 1________ ' _______1946 Guide clockwtse. (The driving side of a single inlet fan is considered to be the side opposite the inlet regardless of the actuaMocation of the drive.)1 ' This method of designation will apply to all centrifugal fans, single or double width, and single or double inlet. Do not use the word hand but specify clockwise or counterclockwise. The discharge of a fan will be determined by the direction of the line of air discharge and its relation to the fan shaft, as follows: Bottom horizontal: If the line of air discharge is horizontal and below the shaft. Top horizontal: If the line of air discharge is horizontal and above the shaft. Up blast: If the line of air discharge is vertically up. Down blast: If the line of air discharge is vertically down. All intermediate discharges will be indicated as angular discharge as follows: Either top or bottom angular up discharge or top or bottom angular down discharge, the smallest angle made by the line of air discharge with the horizontal being specified. Fig. 6 shows the names and definitions of types of fans recently dis tributed by the National Association of Fan Manufacturers *. 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 number of methods. Where the change is made infrequently, the pulley Propeller Fan A propeller fan consists of a propeller or disc type wheel within a mounting ring or plate and including driving mecha nism supports either for belt drive or direct connection. Tubeaxial Fan A tubeaxial fan consists of a propeller or disc type wheel within a cylinder and including driving mechanism supports either for belt drive or direct connection. Vaneaxiat Fan A vaneaxial fan consists of a disc type wheel within a cylinder, a set of air guide vanes located either before or after the wheel and including driving mechanism supports either for belt drive or direct connection. Centrifugal Fan A. centrifugal fan consists of a fan rotor or wheel within-a scroll type of housing and including driving mechanism sup ports either for belt drive or direct connection.... ' Fig. 6.- Names and Definitions of Types of Fans Fans 579 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, electric or hydraulic couplings, may bemsed 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 standpoint, a reduction of the fan speed is most efficient. Inlet vanes save some power and dampers save the least.. From the standpoint 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 if possible is preferred as a . control means, because of the resulting reduction in sound level. 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-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 blades, the volume peak would not make it necessary to provide addi- . tional motor power. In selecting ferns 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. 4 shows that the system resistance 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. 4 indicates that there is no justification for allowing large spare motor capacity, and it is generally more economical to operate motors well loaded. REFERENCES i-See Standard Test Code for Centrifugal and Axial Fans. 3rd Edition, 1938. Bulletin No. 103, National Association of Fan Manufacturers. a--Recommendations adopted by the National Association of Fan Manufacturers. v -Supplement No. B to Form X-12.'National Association of Fan Manufacturers.. . BIBLIOGRAPHY Mine Ventilation, by J. J. Walsh'(A.S.H.V.E. Transactions, Vol. 23, 1917, p. 659). Fan Blower Design, by H. F. Hagen (A.S.H.V.E. Transactions, Vol. 28,1922, p. 175). S' '