12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



carbon dioxide never reaches 20 per cent in practice, the best average 

 being around 15 per cent. Leaky furnace settings and other condi- 

 tions may very materially affect the best practical figure for carbon 

 dioxide, and accordingly that which proves to be the best for one 

 furnace may not be the best for another. Each furnace operation, 

 therefore, becomes to a more or less degree a case unto itself. A 

 number of tests of furnaces in operation have shown an average yield 

 of between 5 and 7 per cent carbon dioxide, which means that as 

 nearly as generalities may be drawn, there is an undue volume of 

 flue gas and a corresponding loss of heat. Satisfactory results, how- 

 ever, are measured in terms of steam produced per pound of fuel 

 burned, so that for a given case the best practice is to secure that 

 percentage of carbon dioxide which will produce the maximum of 

 steam. Right here is to be seen the need of co-ordinating between a 

 carbon dioxide indicator and a steam flow gage. 



Air Control 



We have observed how the analysis of flue gases is an index of 

 the extent to which proper combustion is effected, but it must be 

 apparent also that these analyses do not give any indication as to the 

 conditions which produced them. For example, a furnace may be 

 operating with a good carbon dioxide yield and steam output when 

 suddenly the flue gas analyzer indicates a falling off of the former and 

 the steam output drops. It is decided that too much air is entering the 

 furnace and the damper is adjusted to cut down the draft, but the 

 steam output continues to fall. Again the damper is adjusted, this 

 time to increase the draft, but still no improvement is observed. The 

 facts in the matter are that one or more of a number of things may 

 have occurred such as the formation of clinkers, holes burned in the 

 baffles, holes in the fuel bed, or an opening in the setting, any one 

 of which may have caused the trouble but which damper manipula- 

 tions alone could not correct and which are not indicated by the gas 

 analyzer. 



There is need, therefore, for an indicator of furnace conditions 

 which is furnished by the draft gage. Its method of operation and 

 the interpretation of its readings, however, require a little explanation. 



We are all familiar, in a general way, with chimney drafts. The 

 pull of this draft creates a partial vacuum within the furnace so that 

 a pressure exists there which is less than that outside. To equalize 

 this pressure difference, air enters the furnace and is sucked up 

 through the fuel bed, passes through the combustion chamber and 



