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American Society of Heating and Ventilating Engineers Guide, 1935 So! .Ca aos: ^ oEi d 3 i! 1 t -c Sj i!S~d &M to: 521 -/1.5 sk 9.y-pQ *kkb. k- -_pTQ5 o ,JZ <J K. 5 - <<*=. *fOcCS3'SC? Hi |g.s Zvaa - ,1 & 292 ' Hi s!" c l?-s <o ^ k k .ss 322* W 3 0.2 = i? -S."rtjOovl *S '& Sgtoc 'i Chapter 17--Fans and Motive Power The backward curved and double curved types with backward tip operate satisfactorily in double or in parallel operation. MOTIVE POWER It is no easy matter to predetermine the exact resistance to be encoun tered by a fan or, having determined this resistance, to insure: that no changes in construction or operation shall ensue which may increase air resistance, thus requiring more fan speed and power to deliver the required volume, or which may reduce air resistance, thus causing delivery of more air and a consequent increase of power even at constant speed. It is recommended, therefore, for centrifugal type fans that the. rated power to be supplied shall exceed the rated fan power by a liberal margin, when forward curved types are used. When backward or double curved blade types are used, motors with ratings very close to that of the fan horsepower demand can be employed. Justification for liberal power provision exists also in the possibility of varying demand due to changes in ventilation requirements, intensity of occupation, and weather conditions. The motive power of fans should be determined in accordance 'with the Standard Test Code for Disc and Propeller Fans, Centrifugal Fans and Blowers, as adopted by the American Society of Heating and Venti lating Engineers and the National Association of Fan Manufacturers. Fans may be driven by electric motors, steam engines {either horizon tal or vertical), gasoline or oil engines, and turbines, but as previously stated the drive commonly used is the electric motor. ELECTRIC POWER Each type of.electric motor and kind of electric current has its advan tages and disadvantages as applied to a fan. For motor specifications and standards, the Motor and Generator:Standards of the National Electrical Manufacturers Association should be consulted. Direct-connected electric motors usually are very efficient,' for fan driving because there is no slippage due to belts, and no wear or noise due to chains or gears. There is less maintenance and upkeep tb a d.irectconnected unit, and with an overhung fan wheel on the, motor shaft, the usual fan bearings are eliminated. The disadvantage of a 'slow-speed direct-connected motor is that it may be unduly large and heavy as well as Costly, but this1 may be offset by the compactness of the unit as a whole due to limited space for fan equipment. Should anything go wrong with a slow-speed direct-connected motor there may be a considerable delay in securing replacements, as these motors are not usually carried in stock, as is the Case, with moderately high-speed motors. If a change of speed is found necessary with' a direct-connected rriotor, it will mean a change of motor, which may necessitate a change in the motor foundation usually built with the fan in such cases. On the other hand, non-direct-connected motors have transmissions subject, to.: wear and slippage, and chains or gears may be noisy with this latter type. 293