BOILERS, FUELS AND FLUES 203 



burning gases on their way to the chimney. 

 Direct fire surface is three times as effective 

 as indirect. It does not follow, however, 

 that boilers having the greatest amount of 

 direct fire surface are the most efficient, for 

 there must be sufficient length of fire travel 

 to consume the gases and enable them to give 

 up the greater part of the heat of combus- 

 tion to the water. 



To be most effective the fire surface is so 

 arranged that the heat will impinge at right 

 angles against it. This is accomplished with- 

 out serious interference with the draft, 

 and without making the course of the water 

 in the boiler so long and tortuous as to in- 

 terfere with its rapid circulation. The pro- 

 portion of fire surface to grate surface dif- 

 fers so widely in the different forms of boil- 

 er construction that no definite rule can be 

 given. It may vary from 15 to 35 square 

 feet to each square foot of grate area. 



Types of Boilers. Broadly speaking, 

 there are three types of boilers, when classi- 

 fied as to their form of construction : ( i ) Boil- 

 ers in which the water is spread out in thin 

 sheets between layers of iron or steel and 

 against which the heat strikes; (2) tubular 



