TRANSFORMERS 453 



ognized as an absolute limit and must not be exceeded It should 

 be held only during an emergency period of short duration. 



The ingoing cooling water should never have a maximum tem- 

 perature of over 25 C. 



Nearly all cooling water will in time cause scale or sediment to 

 form in the cooling coils. The time required to clog up a coil 

 depends on the nature and amount of foreign matter in the water. 

 The clogging materially decreases the efficiency of the coil and is 

 indicated by a high oil temperature and a decreased flow of water, 

 load conditions and water pressure remaining the same. 



The most frequent cause of clogging of iron cooling coils is a 

 large quantity of air in the water, resulting in the formation of a 

 scaly oxide. 



Scale and sediment can be removed from cooling coils without 

 removing the coils from the tank. Both inlet and outlet pipes 

 should be disconnected from the water system and temporarily 

 piped to a point a number of feet away from the transformer, 

 where the coil can be filled and emptied safely. Especial care 

 must be taken to prevent any acid, dirt or water from getting 

 into the transformer. 



Blow or siphon all the water from the cooling coil and then fill 

 it with a solution of hydrochloric acid, specific gravity 1.10. 

 (Equal parts of concentrated hydrochloric acid and commercially 

 pure water will give this specific gravity.) After the solution has 

 stood in the coils about an hour, flush out thoroughly with clean 

 water. If all the scale is not removed the first time, repeat until 

 the coil is clean, using a new solution each time. The number of 

 times it is necessary to repeat the process will depend on the con- 

 dition of the coil, though ordinarily one or two fillings will be suf- 

 ficient. 



The chemical action which takes place is very noticeable 

 and often forces acid, sediment, etc., from both ends of the coils; 

 therefore, it is well to leave both ends open to prevent abnormal 

 pressure. 



When water-cooled transformers have operated for some time, 

 especially if the operating temperature? are high, the oil may 

 leave a deposit on the outside surface of the cooling coils. Any 

 deposit decreases the efficiency of the coils and should be re- 

 moved. This condition of the coils is indicated by higher oil 

 temperature, water flow and load conditions remaining the same 



