CREAMERY MECHANICS 283 



forms into a hard impervious scale which usrally con- 

 sists largely of lime. 



Objection to Scale, i. The excessive formation of 

 boiler scale is the immediate cause of most boiler explo- 

 sions. The scale acts as a non-conductor of heat, so that 

 in cases where the capacity of the boiler is severely taxed, 

 the metal becomes overheated, thus materially weakening 

 it. The scale is, therefore, not only dangerous, but by 

 overheating the metal, also materially shortens the life 

 of the boiler. 2. Another most serious objection to scale 

 is its wastefulness of fuel. This becomes evident when 

 we note that the heat before reaching the water must first 

 be conducted through a non-conducting layer of incrusta- 

 tion. 



Prevention of Scale. Since nearly all water used for 

 boilers is more or less impure, it is evident that to prevent 

 scale, boilers must receive frequent cleaning. How often 

 this needs to be done is, of course, dependent upon the 

 amount and character of the impurity in the water. Boilers 

 are kept clean in three different ways : ( i ) by blowing off 

 at low pressure, (2) by cleaning through man hole, and 

 (3) by using boiler compounds. 



( i.) By blowing the boiler off at low pressure most of 

 the mud will be blown out. But care must be taken 

 that the pressure is not above ten pounds and that there is 

 no more fire in the fire box, otherwise the mud, instead 

 of flowing out with the water, will bake on and form 

 scale. 



(2.) A good way of removing mud is to allo\\T the 

 boiler to cool off and then run a rubber hose through the 

 man hole. By working the hose and forcing water 

 through it the sediment can be removed. 



(3.) Boiler compounds are used to keep boilers free 



