Steam Engines. 617 



649. Boiler Incrustation. The use of hard water for 

 making steam results in the precipitation of the carbonates 

 of lime and magnesia, and their sulphates also, when these 

 are present, on the flues and walls of the boiler in the form 

 of a more or less resistant scale which may be harmful in 

 several ways: (1) The incrustation on the boiler is not 

 a good conductor of heat and both the capacity and effi- 

 ciency of the boiler are decreased. (2) When a heavy 

 crust forms on the boiler which prevents perfect contact 

 of the water the boiler may become overheated and the 

 scale thus weaken it by allowing it to <f burn out." (3) It 

 is thought that even boiler explosions may sometimes orig- 

 inate from the thick scale suddenly flaking off when the 

 boiler underneath is overheated and thus letting the hot 

 water come suddenly in contact with the hot surface, which 

 results in the sudden evolution of a large volume of steam. 



To prevent the formation of scale on boilers and to re- 

 move it when formed many methods have been proposed. 

 A common one is to use the simple sodium carbonate or 

 sal soda of commerce, dissolving a quantity in water and 

 letting it be fed into the boiler with the water. Its action 

 is to cause the carbonates to be precipitated in a more or 

 less powdery form which does not adhere to the flues so 

 firmly. It is possible that the influence of the sodium 

 carbonate, besides taking up the excess of carbon dioxide 

 from the bicarbonates of lime and magnesia, is to floc- 

 culate the lime and magnesium carbonates and sul- 

 phates, causing them to fall in larger granules which 

 have not the power of adhering to the walls of the 

 boiler and flues as the molecules do. Sometimes ammoni- 

 um chloride is used and in this case the carbonates are 

 converted into chlorides, which are very soluble in the 

 water, while the ammonium carbonate is volatile and 

 passes off with the steam. Where the steam is not to be 

 used for any other purpose than driving the engine, kero- 

 sene is sometimes employed but its method of action is 



not clearly understood. 

 S3 



