HEAT CONSUMPTION AND HEAT UTILIZATION. 503 



The composition of the flue-dust is a matter of considerable indus- 

 trial importance. It is composed of the lighter and finer particles of 

 the cement mix and the ash, plus a certain amount of material deposited 

 from the stack-gases. This last factor includes in some cases a large 

 percentage of alkali salts, whose recovery has been suggested as a profit- 

 able by-product. (See p. 510.) 



Sources of Heat-supply. 



To counterbalance the heat utilized and the heat wasted, as above 

 noted, heat is always supplied to the kiln from two sources, and occa- 

 sionally from two other sources. The invariable sources of supply 

 are: 



(1) A large and well-known supply is derived from the combustion 

 of the fuel fed to the kiln. 



(2) A smaller and very poorly defined supply is obtained from 

 exothermic chemical combinations which take place in the kiln during 

 clinkering. 



Supplies from these two sources are necessarily received in every kiln. 

 In addition, however, heat may be supplied from 



(3) Regeneration of the clinker heat. 



(4) Utilization of the heat in the stack-gases. 



Heat supplied by combustion of fuel. The most important source 

 of the heat supplied to the kiln is, of course, the burning of the fuel 

 injected into it. This can be estimated accurately enough, for any 

 given kiln, if the composition of the coal and the amount of coal used 

 per barrel of cement are known. It must be borne in mind, however, 

 that any defects in the coal-feeding arrangements, or deficiencies in 

 the fineness of coal grinding, should not properly be charged against 

 the efficiency of the kiln, but against the efficiency of the superintendent. 

 In calculating the heat supplied to the kiln by combustion of fuel the 

 assumption is always made that the coal is ground as fine as is econom- 

 ically possible, and that the injecting apparatus gives perfect combus- 

 tion. Actually we know that neither of these assumptions is ever quite 

 justified an<J that in some mills both are very incorrect. 



If the best bituminous coal from western Pennsylvania or West 

 Virginia be used, a theoretical heating value of 14,000 B.T.U. per Ib. 

 may be assumed : but the coals used in practice often fall very far short 

 of this. Such a coal, used at the rate of 120 Ibs. per barrel of cement, 

 would give a heat supply of 1,680,000 B.T.U. per barrel. This is prob- 

 ably about equal to the average practice with 60-foot kilns on a dry 

 mixture of limestone and clay. With longer kilns, under specially 



