CHAPTER XVIII 



STEAM BOILERS 



438. Principle. A kettle over the fire filled with water 

 is a boiler of small proportions. When fuel is burned be- 

 neath the kettle heat is transferred to the metal of the 

 kettle and from the metal to the water at the bottom. 

 Thus the water in direct contact with the bottom is 

 heated, and, since warm water is lighter than cold, the 

 warmer water rises to the top and the cold settles in its 

 place. In physics this action of the water rising and 

 falling in the kettle, conveying the heat from one part to 

 another, is known as convection. In the steam boiler it 

 is known as circulation. When sufficient heat has been 

 transferred to the water to raise the temperature to 

 212 F. it will commence to boil and throw off steam. 



The reason why the water had to be heated to 212 

 before the particles of water would be thrown off as 

 steam was because the atmosphere, having a pressure of 

 14.7 pounds to each square inch, pressed upon it so hard 

 that the steam could not be thrown off until this tem- 

 perature had been reached. If the kettle were up on a 

 mountain where the atmospheric pressure is not nearly 

 as great, steam would have been thrown off at a lower 

 temperature. 



The same process which takes place in a steam boiler 

 also takes place in a kettle, only under less economical 

 conditions. A fire is maintained within the furnace of 

 the boiler and the heat is transferred to the metal of the 

 boiler shell and tubes, thence to the water, which is con- 



