BOII.KR 



792 



BOILER 



If steam is confined it endeavors to escape, and 

 in doing so exercises pressure. The higher the 

 temperature is raised the greater the pressure 

 exerted. 



Strength of 

 Boilers. The 

 first essential of a 

 boiler is strength 

 to withstand tin- 

 pressure of the 

 steam without 

 bursting. Boilers 

 are therefore 

 carefully con- 

 structed with a 

 strength sufficient 

 to resist a cer- 

 tain amount of 

 power or pressure 

 to the square 

 inch. Each boiler 

 is fitted with a 

 gauge which indi- 

 cates the pressure 

 of steam. There 

 is also an in- 

 genious contriv- 

 ance called a 

 safety valve 

 which, if the 

 pressure rises too 

 high, automatic- 

 ally opens and 

 allows sufficient 



VERTICAL FIRE-TUBE 

 BOILER 



The tubes are like chim- 

 steam to escape neys for the fire, and by 

 ,, passing through the water 



to bring the pres- give a large heating surface. 



Methods of Heating. The main object of all 

 boilers is the same, but that object is arrived 

 at by different methods. It is desirable quickly 

 to heat the water, which is supplied by pipes 

 leading into the boiler, and to maintain that 

 heat as economically as possible. Fire-tube 

 boilers are provided with flues, or passages by 

 which the heated gases generated by the fire 

 pass through the chamber containing water. 

 Water-tube boilers have the water in a series 

 of tubes instead of in one compartment and 

 are considered the safest and most effective 

 style of boiler. 



Upright and Horizontal Models. Boilers may 

 be divided into two classes, namely, upright 

 and horizontal. The principles of operation 

 are the same in both cases, the difference in 

 shape being simply a matter of convenience. 

 On board steamships upright boilers are obvi- 

 ously more convenient than the horizontal 

 boilers used in locomotives on land would be. 

 Small upright boilers are used on land for 

 industrial purposes, and in some cases for 

 heating large buildings. The cylindrical shape 

 has been universally adopted as being most 

 efficient and most economically maintained in 

 a state of cleanliness. 



Horse Power of Boilers. In stating that a 

 boiler is of a certain horse power it is not 

 intended to convey the idea that the power 

 exerted is equal to the power of the same num- 

 ber of horses. Horse power is a term indicating 

 a definite unit of force, established by the 

 American Society of Mechanical Engineers. 

 The horse-power unit as applied to steam 

 boilers is thirty pounds of water evaporated 



HORIZONTAL FIRE-TUBE BOILER 

 This is the type used on locomotives. 



sure within the safety limit. Powerful modern 

 boilers are constructed to stand a pressure of 

 as much as 200 to 225 pounds per square inch. 



per hour from a temperature of 100 F. under 

 a pressure of seventy pounds by gauge. See 

 STEAM ENGINE. C.H.H. 



