340 FARM MOTORS 



part of the tubes which is above the water line in a 

 vertical boiler is superheating surface. In other styles 

 of boilers the steam in order to be superheated generally 

 passes through a coil of pipe within the fire box or a 

 furnace made purposely for it. 



463. Steam space. The surface for the disengagement 

 of steam and the steam space should be of sufficient size 

 so that there is no tendency for the water to pass off 

 with the steam. It has been found by experiment that if 

 the steam space has capacity to supply the engine with 

 steam for 20 seconds, there will be no trouble with prim- 

 ing. To determine whether the boiler has sufficient steam 

 space, find the volumes of the engine cylinder, less the 

 volume of the piston, and multiply this by twice the 

 number of revolutions that the engine makes in 20 sec- 

 onds. This should be about equal to the volume of the 

 steam space, which is the space above the water in the 

 boiler, plus that in the dome. 



464. Boiler horse power. There are two common 

 methods of approximately determining the horse power of 

 a boiler, and a third one which is sometimes resorted to. 

 One of the common methods is by test, and the other is by 

 heating surface, while the third method is by grate sur- 

 face. 



465. Horse power by test. A committee of the Ameri- 

 can Society of Mechanical Engineers has recommended 

 that one horse power be equivalent to evaporating 30 

 pounds of water at 100 F. under a pressure of 70 pounds 

 gauge. This is equivalent to 33,320 B.T.U. an hour. 



Example. H a 15 H.P. boiler evaporate 15 X 30 or 450 pounds of 

 water in one hour with feed water at 100 and under a gauge 

 pressure of 70 pounds, it would be doing its rated horse power. To 

 make the test, fill the boiler to its proper level and tie a string 

 around the glass at this point, then keep the water in the boiler at 

 this level. If the feed water is below 100, turn steam into it until 



