382 



AGRICULTURAL ENGINEERING 



be taken to see that the pressure valve is kept in working 

 order, and that it is not set too high for the strength of the 

 boiler. It should also have sufficient capacity to release the 

 steam as fast as it can be produced in 

 the boiler under any condition. 



The Fusible Plug. As an additional 

 safety device, a fusible plug, containing 

 a core made of some metal with a low 

 melting point, like tin, is placed at the 

 Fig. 243. A spring- highest point of the crown sheet which 



loaded safety valve. . , i i i -r-m 



will be first exposed by low water. When, 

 because of low water, the plate becomes heated, the soft 

 metal core of the plug melts away, causing the steam to blow 

 on the fire and put it out. 



The Boiler Feeder. In order to re- 

 ceive additional water the boiler must be 

 provided with some sort of feeder. One 

 such device is the crosshead pump, which 

 is attached directly to the crosshead of 

 the engine and can be operated only when 241< 

 the engine is running. The independent pump has a steam 

 cylinder of its own and may be operated by steam from the 

 boiler. This type of pump is practically a small steam engine. 



Another form of boiler feeder is 

 I (<*!'' Jbi ^ ne i n J ec t r > which takes steam 



f F^rrl from the boiler and, by allowing 



it to expand, converts its energy 

 into kinetic energy. As this steam 

 strikes a supply of cold water 

 within the injector it condenses. 

 Fig. 245. A standard type of but the impact drives the water 



injector. . 



into the boiler. 



The Feed Water Heater. Many boilers are provided witn 

 feed water heaters which use the exhaust steam from the engine 



pi ug f" which A placed 

 c a r t w 8 in ee pi" 



