152 



COMMERCIAL ELECTRICAL TESTING 



with the inside of the box to avoid eddy currents. The 

 pressure shown by this pipe will be somewhat higher 

 than that registered at the end of the cone, and both 

 pressures should be corrected for standard air and 

 plotted on the final curve sheet. 



The volume must be calculated as in the cone test, 

 but the pressure obtained in the box is taken as the 

 static pressure produced by the fan, since the velocity 

 head is lost in the box. To obtain the impact pressure, 

 the volume obtained should be divided by the area of 

 the opening of the fan, and the corresponding velocity 

 head figured from the formula. This velocity head should 

 be added to the static pressure shown by the cone read- 

 ings. For transformer ventilation it is customary to 

 calculate the pressure in ounces, measured at the cone 

 opening. 



The following form should be used in tabulating the 

 calculations : 



Fan rating 



Motor rating 



Box test taken at r.p.m. 



Wet bulb deg. F. 



Dry bulb deg. F. 



Barometer in. 



Wt. of air Ib. 



Air horse power should be calculated from the static 

 pressure and the efficiency obtained will be the static 

 efficiency. 



Formulae for Blower Tests 



H = Head of air in feet. 



h = Head of water in inches shown by manometer. 

 h' = Static head; h" = impact head; h'" = h"-h f 

 velocity head. 



