OTHER FUELS 



169 



passes up through the fire, producing combustion and absorbing 

 the heat produced, except that which escapes through the side of the 

 firebox. This heated air may then be utilized for heating in one 

 of several ways, as by letting it pass around water tubes or through 

 flues in a boiler, around air flues in hot air furnaces, around the oven 

 in a cooking range, or through a 

 heating drum and length of pipe 

 in the common heating stove. 

 Whatever heat is left in the gas 

 when it enters the chimney is use- 

 less except for increasing the draft. 

 It is therefore important to utilize 

 as large a proportion of heat as 

 possible before the gases reach 

 the chimney. This can be done 

 by (1) having the gases as hot as 

 possible, (2) allowing them to pass 

 out as slowly as practicable, and 

 (3) bringing them into the best 

 possible contact with the flues, 

 oven sides, or other heating sur- 

 faces. 



Good contact between the flue gases and the heating flues can 

 be obtained in any given stove or furnace only by keeping the flues 

 clean and free from soot and ashes. This is very much more im- 

 portant than most persons realize. 



The gases will be hotter and will also pass more slowly the less 

 their amount; therefore, no more air should be admitted to the stove 

 or furnace than is necessary. This applies particularly to air which 

 might be admitted over the fire by opening the fire door or a draft in 

 it, except under the following conditions: 



When combustion gases are produced by the heating of fuels 

 such as soft coal, or wood, it is sometimes necessary to admit air 

 over the fire so as to permit the gases to burn. Therefore, with 

 such fuels some air should be admitted through the fire door, or 

 in some other manner immediately over the fire, so long as a bright 

 flame is produced. 



FIG. 134. Electric oven. Why is 

 it necessary to have heavy non- 

 conducting walls on this oven? 



