Document 6BnEGNBB7D6daGVgVmZJZMJ16

HEATING VENTILATINC AIR CONDITIONING GUIDE 1943 general class can be made to perform the same duty, although mechanical difficulties, noise or lack of efficiency may limit the use to one or another type. Until a few years ago, the most common field of service for fans of the propeller type was in moving air against moderate pressures, where no long ducts were involved, or no heavy frictional resistance had to be overcome. However, recent developments in the design of axial flow fans based on the application of aero-dynamic principles, and furthermore, the use of multi-stage fans, have greatly increased the range of pressures against which the modern propeller fan can be applied. Single stage axial flow fans of moderate diameter are now available to operate against static pressures of 3 and 4 in. of water, while maintaining moderate noise levels. These pressures are readily doubled by the simple device of double staging. In multi-stage units, intermediate guide vanes are employed to properly redirect the air discharged by the first stage into the second stage wheel. In the most common form of two-stage axial flow fan, the motor is provided with shaft extensions on each end, one carrying the first stage fan, and the other the second stage. The motor is supported radially from a: cylindrical.casing and the intermediate guide vanes are mounted in the annular space around the motor. FAN PERFORMANCE Fans of ali types follow certain laws-of performance which are useful in ' determining the effect of changes in the conditions of operation. These laws apply to installations comprising any type of fan, any given piping system and constant air density, and are as follows: 1. The air capacity varies directly as the fan speed. . 2. The pressure (static, velocity, and total) varies as the square of the fan speed; 3. The power demand varies as the cube of the fan speed. Example 1. A certain fan.delivers 12,000 cfm.at a static pressure of 1 in. of water .when operating at a speed of 400 rpm and requires an input of 4 hp. If in the same installation 15,000 cfm'are desired, what will be the speed, static pressure, and power? 14 mn Speed = 400 X = 500 rpm (500\2 . 400/ = >n- ' . Power = 4 X = 7.81 hp \. When the density of the air varies the following laws apply: 4. At constant speed and capacity the pressure and power vary.directly;as the density. Example S. A certain fan delivers 12,000 cfm at 70 F and normal barometric pressure (density 0.075 lb per cubic foot) at a static pressure of 1 in. of water when operating at 400 rpm, and requires 4 hp. If the air temperature is increased to 200 F (density 0.0602 . lb) and the speed of the fan remains the same, what will be the static pressure.and power? ' ' Static pressure = 1 X 0.0602-.=. 0n.o8n0 i.n. 0.07 5 ` ' ' 1. .. Power = 4 X = 3.20 hp. . 576 CHAPTER 30. FANS- 5. At constant pressure the speed, capacity and power vary inversely as the square root of the density. Example S. If the speed of the fan of Example 2 is increased so as to produce a static pressure of 1 in. of water at the 200 F temperature, what will be the speed, capacity, and power? Speed = 400 X^-1^ = 446 rpm Capacity = 12,000 X -J = 13,392 cfm (measured at 200 F) yi 0.0o02 Power = 4 X =4.46hp 6. For a constant weight of air: (a) The speed, capacity, and pressure vary inversely as the density. (b) The horsepower varies inversely as the square of the density. Example 4* If the speed of the fan of the previous examples is increased.so as to deliver the same weight of air at 200 F as at 70 F, what will be the speed, capacity, static pressure, and power? Speed = 400 X = 498 rpm Capacity = 12,000 X ~ 14,945 cfm (measured at 200 F) , w 0.075 ' . . Static pressure = 1 X n0.n0f6in0y2 = 1-25 in. Power ; FAN EFFICIENCY The efficiency of a fan may be defined as the ratio of the horsepower output to the horsepower input. . The horsepower output is expressed by the formula: - _A. .ir"Hrrorsepower*, = c--f-m-- :-X----t--o--t-a--l--p--ressure inin,.ches .of,--wa--ter' / (.1..) When the static pressure is used iff the computation in place of total pressure it is assumed that this represents the useful pressure and that the velocity pressure is lost in the piping system and iff the air which leaves the system. Since in most installations a higher velocity exists'at the fan outlet than at the point of delivery into the atmosphere, sorne of the velocity pressure at the fan outlet may be utilized by conversion to static pressure within the system, but, owing to the uncertainty of friction losses which occur at the- places where changes in velocity take place, the amount of velocity pressure which is actually utilized is seldom known, and the static pressure alone may best represent the useful pressure. In the standards for published capacity tables as adopted by the National Associatwhof Fan Manufacturers, the term static pressure-refers to the 'See Standard Test Code for Centrifugal and Axial FansVThird Edition of 1938. 577