380 AGRICULTURAL ENGINEERING 



70 lbs. pressure by the gauge from the feed water at a temper- 

 ature of 100° F. 



It is easy to see, however, that the capacity of any boiler 

 depends on its ability to burn fuel, or the area of the grate 

 surface, and on the heating surface which will absorb the heat 

 produced. Thus it is possible to estimate the capacity of the 

 steam boiler from the size of the grates, allowing from Y2 to 1^ 

 square foot for each horsepower. In like manner the horse- 

 power may be calculated by determining the entire heating 

 surface of the boiler, or the area of the plates and tubes which 

 have heated gases on one side and water on the other, and 

 allowing 14 square feet of heating surf ace for each horsepower. 



Quality of Steam. As steam leaves the boiler there is a 

 tendency for it to carry water with it in the form of spray. 

 It is the purpose of the steam dome to cause the water to 

 settle from the steam as fast as possible. Steam which con- 

 tains water in the form of spray is called wet steam, and the 

 proportion of water to steam is sometimes called the quality 

 of steam. Steam which does not contain any water is said 

 to be dry steam. When dry steam is passed through highly 

 heated tubes it is heated above the boiling point of water for 

 the pressure under which the steam is confined. When in 

 this condition the steam is said to be superheated. Some 

 boilers are provided with superheaters for raising the tem- 

 perature of the steam in this way. To prevent the loss of 

 heat it is customary to cover the pipes leading the steam from 

 the boiler to the engine with some non-conductive material in 

 the shape of pipe covering. 



Boiler Accessories. All boilers must be provided with 

 certain accessories, in order to permit of their successful 

 operation and management. 



Gauge Cocks. Boilers are usually provided with two or 

 three gauge cocks to enable the fireman to determine the 



