108 



STEAM BOILERS 



CHIMNEYS 



Production of Draft. It is well known that any volume of 

 gas is lighter when heated than the same volume of gas when 

 cool. When hot gases pass into the chimney, they have a tem- 

 perature of from 400 to 600 P., while the air outside the chim- 

 ney has a temperature of from 40 to 90 F. Roughly speak- 

 ing, the air weighs twice as much, bulk for bulk, as the hot 

 gases. Naturally, then, the pressure in the chimney is less 

 than the pressure of the outside air. The production of draft 

 and the satisfactory operation of a chimney 

 depend on this pressure difference. The pres- 

 sure of the draft depends on the temperature 

 of the furnace gases and the height of the 

 chimney. Chimney draft is affected by so 

 many varying conditions that no absolutely 

 reliable rules can be given for proportioning 

 chimneys to give a certain desired draft pres- 

 sure. The rules given for chimney proportions 

 are based on successful practice rather than on 

 pure theory. 



Measurement of Draft. The intensity 

 the draft may be measured by means of 

 water gauge such as is shown in the accom- 

 panying illustration. As will be seen, it is a 

 glass tube open at both ends, bent to the shape 

 of the letter U ; the left leg communicates with 

 the chimney. The difference in the two water 

 levels H and Z in the legs represents the intensity of the draft, 

 and is expressed in inches of water. 



The draft produced by a chimney may vary from J in. to 

 2 in. of water, depending on the temperature of the chimney 

 gases and on the height of the chimney. Generally speaking, 

 it is advantageous to use a high chimney and as low a chimney 

 temperature as possible. The draft pressure required depends 

 on the kind of fuel used. Wood requires but little draft, 

 say J in. of water or less; bituminous coal generally requires 

 less draft than anthracite. To burn anthracite, slack, or culm, 

 the draft pressure should be about H in. of water. 



