Document Evx8ELedOQ84GD1NZYwDKybY0

American Society of Heating and Ventilating Engineers Guide, 1932 Chapter 34--Fans and Motive Power service is required than for the other type multiblade fans. The dimen sional bulk for a given duty often is 150 to 200 per cent that of a forward curve multiblade type fan. Typical fan performance curves indicating static pressure, static effi ciency and horsepower are shown in Fig. 8. SELECTION OF FANS Fans for Ventilation The following information is required to select the proper type of fan : 1. Cubic feet of air per minute to be moved. 2. Static pressure required to move the air through the system. 3. Type of motive power available. 4. Whether fans are to operate singly or in parallel on any one duct. 5. What degree of noise is permissible. 6. Nature of the load, such as variable air quantities or pressures. '> Knowing the requirements of the system, the main points to be con sidered for fan selection are: 1. Efficiency. 2. Reliability of operation. 3. Size and weight. 4. Speed. 5. Noise. 6. Cost. 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 pressures: 1. Volume of air in cubic feet per minute, (68 F, SO per cent relative humidity, 0.07488 lb per cubic foot). 2. Outlet velocity. ` 3. Revolutions per minute. 4. Brake horsepower. 5. Tip or peripheral speed. 6. Static pressure. The most efficient operating point of the fan is usually shown, by either bold or italicized figures in the capacity tables. Two important factors in selecting fans for ventilating systems are efficiency (which affects the cost of operation) and noise. First cost and space available are secondary. The fans should be selected to operate at maximum efficiency without noise. Noise in a ventilating system is irritating and a cause for complaint. Fans must be selected of proper size in order to reduce it to a minimum. Noise may be caused by other factors than the fan, namely, high velocity in the duct work, unsatisfac tory location of the fan room, improper construction of floors and walls and poor installation. Where noise is chargeable directly to the fan, it is caused either by excessive peripheral speeds, or the fan is of insufficient size. It should be remembered, however, that the tip speed required for 489