STEAM BOILERS 339 



or the rear end, and in fire-box boilers it is beneath the 

 fire box or in the water legs. 



461. Spark arrester. Where some method of forced 

 draft is used in a boiler there is danger of sparks being 

 carried out and causing fires. Traction engines guard 

 against this by means of a spark arrester. This may con- 

 sist of a screen which catches the sparks and allows them 

 to fall into the stack, or it may be accomplished by turn- 

 ing the smoke around a sharp corner and, as the sparks 

 are heavier than the smoke, they will be thrown out and 

 are caught in a receptacle for that purpose. The smoke 

 box or front end of the boiler may be long for the 

 purpose. 



BOILER CAPACITY 



462. The capacity of a boiler depends upon the amount 

 of heat generated and the proportion of that heat trans- 

 ferred to the water. The amount of heat generated de- 

 pends upon the quantity of coal, the draft, and area of 

 grate surface. The amount of heat transferred from the 

 coal to the water depends upon the amount and position 

 of the heating surface. 



There is no entirely satisfactory method of stating the 

 capacity of a boiler or its economy, but they are com- 

 monly stated as boiler horse power and the pounds of 

 steam evaporated per pound of coal. This method of 

 rating is on the assumption that the steam is all dry 

 saturated steam and that there is no priming or super- 

 heating. 



When water is carried along with steam from the boiler 

 it is called priming. Very seldom is a boiler designed 

 which does not prime at least 2 per cent, but if it primes 

 over 3 per cent it is improperly designed. When steam 

 passes over a hot surface after leaving the boiler it will 

 absorb additional heat and become superheated. That 



