104 misc. Publication 540, u. s. dept. of agriculture 



which, at any given speed and specific volume, prevent it from 

 absorbing more than a limited amount of power. This is of special 

 importance when the fan is used in dehydrating systems where the 

 static pressure in the tunnel drier may be suddenly decreased as a 

 result of opening of doors, or, in the case of a bin drier, where the 

 static load can be changed by removal of the product from the bin. 



Propeller, Axial-Flow, or Disk Fans 



Unless very special designs are used, the disk-type propeller fan is 

 generally suitable for use only where the static pressure is less than 1 

 inch of water pressure. There are two general classes of disk-type pro- 

 pellers : Those with thin, steel, curved blades, and those with air-foil 

 sections similar to those used on airplane propellers. The former type 

 is available commercially and the latter can be readily built of wood. 

 Irrespective of the type used, the entrance and exit cones should be 

 streamlined and the propeller shrouds should be built with minimum 

 clearances in order to secure the highest efficiencies. 



High-speed disk fans are likely to be very noisy, and this factor may 

 restrict their use. When properly applied, they are efficient and con- 

 stitute a relatively cheap means of obtaining a high volume of air 

 movement with a minimum of equipment. 



Although simple in appearance and operation, propellers are intri- 

 cate in design. An improperly designed propeller may be extremely 

 inefficient, wasteful, and sometimes very dangerous. Properly de- 

 signed, however, they can be readily constructed and are efficient and 

 safe. Those who contemplate their use should make use of publications 

 that contain adequate design information (5, 8, 17, 23, 25) . 



Application of Fan Laws in Choice of Fan 



The proper choice of fan size and the prediction of its performance 

 under specific conditions requires a knowledge of basic fan laws. These 

 laws are applicable to all geometrically similar centrifugal or disk- 

 type fans. It should be understood that two fans are geometrically 

 similar only if they are alike in all proportions and in all details re- 

 gardless of size. 



B= barometric pressure. 

 D = diameter of fan wheel. 

 hp. = fan power, horsepower. 

 Q= capacity or volume of air per unit of time, cubic feet per minute 

 (c. f. m.). 

 Ps= static pressure of fan, inches of water, 

 r. p. m. = fan speed, revolutions per minute. 



T= absolute temperature of air (°F. + 460) or (°C. + 273). 

 V— specific volume of air, cubic feet per pound (specific volume of air at 

 standard conditions for fan tables is approximately 13.3 cubic feet 

 per pound of air). 



1. For a given fan speed and a constant specific volume of air when the size of 

 fan varies: 



a. QccD 3 



b. PsozD 2 



c. hpccD 5 



2. For a given fan size and a constant specific volume of air when the speed of 

 the fan varies: 



a. Qoc(r. p. m.) 



b. Psoc(r. p. in.) 2 



c. hpcc(r. p. m.) 3 



