136 STEAM BOILERS 



filter, passing the water through layers of sand, gravel, hay, or 

 equivalent substances, or through layers of cloth. Hay and 

 cloth are of service especially where the feedwater contains oil 

 or grease, as is the case where a surface condenser is used and 

 the condensed steam is used over again. 



Purification by Chemicals. Chemical purification may 

 take place before or after the water enters the boiler, the former 

 method being somewhat more expensive. However, the 

 purification is better carried out before the water enters the 

 boiler for the reason that the amount of impurities enter- 

 ing the boiler will be greatly reduced. The chemical pro- 

 cess to be adopted depends on the substances present in the 

 water. 



Use of Quicklime. When the water contains only car- 

 bonate of lime, it may be treated with slaked quicklime, 

 using 28 gr. of lime for every 50 gr. of carbonate of lime present 

 in the water, the quicklime precipitating the carbonate of 

 lime and being transformed into carbonate of lime itself during 

 the process. 



Use of Caustic Soda. Water containing carbonate of lime 

 may be treated with caustic soda, which precipitates the car- 

 bonate of lime and leaves carbonate of soda, which is harmless. 

 For every 100 gr. of carbonate of lime 80 gr. of caustic soda 

 should be added. 



Use of Sal Ammoniac. Sal ammoniac is sometimes added 

 to water containing carbonate of lime and will cause the latter 

 to precipitate. Its use is not advisable, however, on account 

 of the danger of the formation of hydrochloric acid, which will 

 attack the boiler. The formation of this acid is due to the use 

 of an excessive quantity of sal ammoniac. 



Treatment of Sulphate of Lime. While slaked lime will 

 precipitate carbonate of lime, it will have no effect on sulphate 

 of lime, and water containing the latter, either alone or in con- 

 junction with carbonate of lime, must be treated with other 

 chemicals. The most available ones for water containing both 

 are carbonate of soda and caustic soda. These are often fed 

 into the boiler and will precipitate the carbonate of lime and 

 sulphate of lime there, requiring the sediment to be blown out 

 or otherwise removed periodically. 



