Document om146nV7ageOMkOyqD9N0OwVo

SELECTION OF FANS The following information is required to select the proper type of f^. 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) speed, (3) noise, (4) size and weight, and (5) 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, 50 per cent relative humidity, 0.07488 lb per cubic foot). 2. Outlet velocity. 3. Revolutions per minute. 4. Brake power. 5. Tip or peripheral speed. 6. Static pressure. 324 <fhe most efficient operating point of the fan is usually shown by either bold-face or italicized figures in the capacity tables. fans to Ventilating and Air Conditioning Systems 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. Because 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, unsatisfactory location of the fan room, improper construction of floors and walls, and poor installation. Where, noise is chargeable directly to ' " v 'c either by excessive peripheral speeds, or the fan is of 1 Table 1 Good Operating Velocities and Tip Multiblade Ventilating Static Pressure Ikches or Water K HH - 78 H% 1 iH m IK VA > 2H 3 Outlet Velocitt Feet peb Minute 1000-1100 1000-1100 1000-1200 1100^1300 1200-1400 1300-1600 1500-1800 1600-1900 1800-2100 1900-2200 2000-2400 2200-2600 2300-2600 2500-2800 Tip Speed Feet per Minim 1520-1700 1760-1900 1970-2150 2225-2450 2480-2700 2660-2910 2820-3120 3162-3450 3480-3810 3760-4205 4000-4500 4250-4740 4475-4970 4900-5365 required for a specified capacity and pressure varies with the type of blade, and that a tip speed which may be excessive for the forward curved type is not necessarily so for the backward or slightly backward type. A noisy fan usually is one which is operated at a point considerably beyond maximum efficiency. For a given static pressure there is a corresponding outlet velocity and peripheral speed wherein maximum efficiency is obtained. If a fan is selected to operate at this point, the cost of operation and the noise can be held within control. To aid in selecting, fans as near as possible to the point of maximum efficiency, there are listed in Tables 1 and 2 for each static pressure cor responding outlet velocities and tip speeds which will give satisfactory results. The proper tip speed for a given static pressure varies with the design of wheel and with the number of blades or vanes in the wheel. Lower outlet velocities than those listed in Table 1 may be employed, but care must be exercised to avoid selecting a fan for operation below its useful range. The useful range of the fans of Table 2 extends over the fulMength of the performance curve.