BOILERS, FUELS AND FLUES 223 



unions should be made with push or screw 

 nipples. 



8. Soft coal burners require a somewhat 

 different construction than do hard coal burn- 

 ers. The kind of fuel to be burned should 

 be clearly in mind when selecting a boiler. 



9. The ash pit should be deep and com- 

 modious. Shallow ash pits are likely to be- 

 come filled so that the draft is impaired 

 and the grate bars ruined. 



CHIMNEYS AND FLUES 

 A very essential part. of the heating equip- 

 ment is the chimney or flue. Its purpose is 

 twofold: First, to create a draft in order 

 to furnish air to promote combustion; and 

 second, to carry off smoke and gas. The 

 size and height of the chimney required de- 

 pends on the size of the grate surface. Mere 

 velocity does not necessarily indicate that 

 the draft is sufficient; the chimney must be 

 of sufficient size to carry the required 

 quantity. 



The velocity of the gas in the flue depends 

 on the height of the flue and upon the tem- 

 perature of the gas. The difference be- 

 tween the weight of the hot gases in the 

 chimney, and a column of cold air of equal 



