FARM MOTORS 377 



In an open vessel water cannot be heated above the boil- 

 ing point of 212° F., but heat continues to be absorbed and is 

 used in the formation of vapor. Water under pressure boils 

 at a higher temperature. Thus if the pressure inside the con- 

 taining vessel were two pounds greater than atmospheric 

 pressure, the boiling point would be about 228° F. Changing 

 water into vapor increases its volume many fold. At atmos- 

 pheric pressure the volume of the vapor is about 1700 times 

 that of. the liquid. At 100 pounds pressure the volume of 

 the steam is about 240 times the volume of the liquid. Water 

 vapor, or steam, is a colorless gas which obeys all of the laws 

 of gases as far as expansion and change of temperature are 

 concerned. 



Functions of a Boiler. The functions of a boiler are to 

 absorb heat from the hot gases produced by the burning of 

 fuel and to transmit it to the water contained within, causing 

 it to vaporize into steam. The steam boilers used in agricul- 

 tural plants and in traction engine service include the firebox, 

 or furnace, which may be placed either directly underneath 

 the main part of the boiler or entirely within it. 



Location of the Furnace. Boilers with the fire box out- 

 side of the boiler proper are called externally-fired boilers. 

 This type can safely be used for stationary work and are 

 usually set in brick work, which forms a large part of the 

 furnace. Those which have the furnace within the main 

 body of the boiler, or shell, as it is called, are said to be inter- 

 nally-fired boilers. Most of the boilers used in agricultural 

 practice and all of the boilers used for traction engine service 

 are internally fired. 



The Vertical Boiler. The vertical boiler is used in small 

 units and where space is especially valuable. It consists of 

 a cylindrical shell containing a furnace in the lower end, over 

 which is placed a tube sheet or plate and a system of tubes. 



