Document zVBmmbBZG1M1XBrm1XaoDpZz

American Society of Heating and Ventilating Engineers Guide, 1936 Chapter 17--Fans increased supply of air in summer over that needed for winter is demanded. Experience is required in deciding whether speed-control or damper- control shall be used for specific cases. Where noise is a factor, it may be exceedingly desirable to reduce the speed at times, while on the other hand, any fan which has its normal speed reduced as much as 50 per cent without change in resistance will move only 50 per cent of the air. DESIGNATION OF FANS Facing the driving side of the fan, blower, or blast wheel, if the proper direction of rotation is clockwise, the fan, blower, or blast wheel will be designated as clockwise. If the proper direction of rotation is counter-clockwise, the designation will be counter clockwise. (The driving side of a single inlet fan is considered to be the side opposite the inlet regardless of the actual location of the drive.)* 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 "counter-clockwise." ^ 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. In order to prevent misunderstandings, which cause delays and losses, the arrangements of fan drives adopted by the National Association of Fan Manufacturers and indicated in Fig. 6 are suggested. If double width, double inlet fans are selected, care must be taken that both inlets have the same free area. If one inlet of a fan is obstructed more than the other, the fan will not operate properly, as one half of the wheel will deliver more air than the other half. 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, provided the fan has a limiting horsepower characteristic. `Recommendations adopted by the National Association of Fan Manufacturers. 321