110 PRINCIPLES OF ELECTRICAL DESIGN 



The watts dissipated by the cylindrical cooling surfaces may 

 be calculated by the formula 



/l,500+j;\ 

 4 \ 100,000 / 

 where W = the watts dissipated. 



T = the surface temperature rise in degrees Centigrade. 

 A = the cooling area in square inches. 

 v = the peripheral velocity in feet per minute. 



This formula is generally similar to one proposed some years 

 ago by DR. GISBERT KAPP. 



If w c is a cooling coefficient representing the watts that can 

 be dissipated per square inch of surface for 1C. difference of 

 temperature, we have 



1,500 + v ,_ 



100,000 

 and the temperature rise will be 



r- w 



w c A 



where W stands for the watts that have to be dissipated through 

 the cooling surface A. 



The watts dissipated by the air ducts and end surfaces may 

 be calculated by the formula 



w = TA mm ^ 



where W, T, and A have the same meaning as before, but v d 

 stands for the average velocity of the air through the ducts in 

 feet per minute. This velocity is very difficult to estimate in 

 the case of self-ventilating machines, but the constant in the 

 formula has been selected to give good average results if Vd is 

 taken as one-third of the peripheral velocity of the armature. 



If Wd is a cooling coefficient representing the watts that can 

 be dissipated per square inch of duct surface for each degree 

 Centigrade rise of temperature, we have 



(56) 



and the temperature rise of the vent duct surfaces will be 



where W stands for the watts that have to be dissipated through 

 the surface of area A. 



