I ol;( KD-COOLED TRANSFORMERS 161 



st it ute a three-phase transforming set, and one, two or all 

 three groups are used according to the amount of the load 

 being supplied. Tin- power necessary for circulating the 

 required amount of air varies considerably in different installa- 

 tions, according to the length and size of the supply duct and 



. lural arrangements adopted. The blower rarely requires 



i in >mall installations) to have a capacity of more than 

 1 p-r cent, of the :< tc rated capacity of transformers 



in-tailed, and it is more usually, especially for groups of very 

 large transformers, well down toward one-tenth of 1 per cent 

 It is usually safe to estimate on a motor whose capacity is one- 

 half of 1 per cent, of the aggregate capacity of the transformers 

 cooled It i- important that the duct or passage supply- 

 ing the t ran > formers shall be of large size, and that plenty of 

 -hall reaeh r\vn the most distant transformers of the group. 



vel.M-ity at whieh the air Hows along the duct should be of 



the order of 1 meter per second, and the air pressure at the 



transformers will need to be some 8 to 4 grams per sq cm 



(some three-quarter- of an ounce per sq in) or some 5 to 6 



er sq mi as delivered from the blower. The quantity 



! and the power ivquiivd for 500-kva and 1000-kva sizes 

 will be roughly as shown in Table 17. 



I.E 17. 



n disastrous consequences of a prolonged inter- 

 niptioii ( ,f the supply ..t' air. it is very desirable to duplicate 

 the air-cirrulatiiiL,' apparai n-. and in general every care must 

 be exercised in providing for contingencies. I recall a case 



8.T. M 



