182 THE DESIGN OF STATIC TRANSFORMERS 



surface of the case, not including in this expression the 

 increased surface provided by the ribs. 



The temperature rise is in all cases estimated from the total 

 watts loss in the transformer divided by the total external 

 smooth surface of the case, this surface of reference being the 

 cylindrical surface at the root of the ribs. The temperature 

 rise is taken as the rise above the temperature of the surround- 

 ing air when indicated by a thermometer placed in the oil at 

 the hottest part, which is usually near the top of the core of 

 the transformer. 



These constants (i.e., for cases A and B) relate to the 

 ultimate temperature rise for continuous running when the 

 transformer has attained a steady temperature. The time 

 taken for medium-sized oil-immersed transformers to attain a 

 steady temperature is generally at least 10 hours. After six 

 hours the temperature-rise in such transformers is often 

 of the order of seven-tenths of the ultimate rise. Hence, if 

 transformers are required to be designed for 40 rise after six 

 hours' run (and not 40 ultimate temperature rise), the value of 

 the watts per sq dm may be some 1,4 times the values given 

 above. Thus, for guarantees of 40 rise after six hours, 

 smooth-case transformers may be proportioned for some 6 to 6 

 watts per sq dm. But it is best to keep the value down, since 

 estimates of temperature rise are, at the best, only very rough. 

 An air-cooled transformer has generally only some two-thirds 

 of the heat-emitting capacity of the same transformer in the 

 same case, but immersed in oil. 



The extent to which the heat-emitting capacity is increased 

 by the addition of ribs on the exterior of the case is fairly repre- 

 sented by the statement B above. The total external cooling 

 surface may be increased to far more than twice the smooth 

 surface of the case by the addition of very deep and numerous 

 ribs. The cooling capacity is not, however, increased nearly in 

 proportion, for the ends of the ribs are so removed from the 

 surface of the case as to be much less effective. Consequently 



