150 COMMERCIAL ELECTRICAL TESTING 



readings before the curves are plotted, but it does not 

 affect volume. 



The sixth column gives the air velocity and may be 

 obtained from the formula 



/if// 

 *_ 

 \ w 



The volume must be given in the seventh column. It 

 is obtained by multiplying the velocities given in the 

 sixth column by the effective area of the pipe; i.e., 0.91. 



The horse power in the air can be calculated from the 

 formulae : 



Air horse power = or or 



The horse power input to the fan is the horse power 

 output of the motor. 



Unless instructions are issued to the contrary, all 

 fan tests for government work should be plotted with 

 pounds per square foot, horse power input to fan, and 

 efficiency, as ordinates; and volume in cubic feet per 

 minute as abscissae. Both static and impact pressure 

 should be plotted. 



Cone Method of Test 



In the cone method of test an adapter is used, where 

 necessary, to change the fan outlet from rectangular 

 to circular, a cone being placed on the circular end. This 

 cone is made up of sections about one foot in length, the 

 sides of which slope about two inches to the foot. Read- 

 ings are taken by a single Pitot tube, the open end of 

 which is held flush with the opening in the cone and 

 pointed against the stream of air. Pressure is registered 

 as before by a manometer or U tube. The readings are 

 taken, one at the top, one at the bottom, and one at 

 each side of the cone, at a distance from the edge of the 

 pipe of about | of the diameter of the opening. A reading 

 is also taken in the center of the cone opening. The 

 average of these five readings represents the impact 

 pressure produced by the fan, and is taken as the velocity 

 head. The velocity may be obtained from the formula 

 given for the double tube test. 



