Document wgjGEeLE7w6xMVEr1naZwO7y6

American Society of Heating and Ventilating Engineers Guide, 1931 power curve indicates that a large reserve in horsepower of the motn required as with the forward curved blade type of fan. Fig. 5 shows characteristic curves of this type of fan. "e The outstanding characteristics of the full backward curve multihiaj type fan are the steep pressure curves, the non-overloading power cunf* and the high speed. (See Fig. 6). This fan operates at a peripheral speed Cf approximately 250 per cent of the forward curve multiblade type for liv results. The pressure curves begin to drop at very low capacity and con6 tinue to fall rapidly to full outlet opening. This makes the fan adaptable y' 1. i $f So 70 (4o -} GO ISO & 50 tod 5 40 80 4 30 &o 3 20 40 2 to SO t oo o fat? No. ~0 Oesrga - /? '"rye it7(Cf ybi it? ttr 1* hr te fb \V&i `Ar. tK \ V fa. Of? ?y > V\ A S0OO C.Sfdf. Fig. 7. Typical Fan Performance Curve to parallel operation for such application as stoker installations. The steep pressure curves tend to produce constant capacity under changing pres sures. Where wide fluctuations occur, the use of this type of fan is desirable to prevent overloading of motors. The maximum power require ment occurs at about the maximum efficiency. Consequently a motor selected to carry the. load at this point will be of sufficient capacity to drive the fan over its full range of capacities at a given speed. The high speed of this type makes it adaptable for direct connected electric motor drives. The high speed necessitates heavier construction, and more operating attention and service is required than for the other type multi blade fans. The dimensional bulk for a given duty is 150 to 200 per cent that of a forward curve multiblade type fan. 512 Chapter 32--Fans and Motive Power P?.: , r ,, nerformance curves indicating static pressure, static effi- Spg5 horsepower are shown in Fig. 7. SELECTION OF FANS Wfej(jFfchfeofroVlloewntinilagtiionnformation is required to select the proper type of fan : Ill Cubic feet of air Per minute to be moved. '`Static pressure required to move the air through the system. Hs&rype of motive power available. StyWhether fans are to operate singly or in parallel on any one duct. What degree of noise is permissible. Nature of the load, such as variable air quantities or pressures. lUifcnowing the requirements of the system, the main points to be con- Stfiered for fan selection are: Efficiency. IjSf2. Reliability of operation. |Sj^3. Size and weight, jp|^4. Speed. Noise. Jy 6. Cost, ity: ; 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 if.pre1s.sVuorelusm: e of air in cubic feet per minute, (68 F, 50 per . cent relative humidity, i|(0.p7488 lb per cubic foot). \p it;2. Outlet velocity. - '3. Revolutions per minute. iggC 4. Brake horsepower. 1*ty 5. Tip or peripheral speed. 6. Static pressure, .t\f'. The most efficient operating point of the fan is usually shown by either v? bold or italicized figures in the capacity tables. i ` Two important factors in selecting fans for ventilating systems are y 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 4; 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 *s caused either by excessive peripheral speeds, or the fan is of insufficient : size. It should be remembered, however, that the tip speed required for / a specified capacity and pressure varies with the type of blade, and a tip ; >: speed that is excessive for the forward curved type is not necessarily so 513