504 CEMENTS, LIMES, AND PLASTERS 



favorable circumstances, a fuel consumption of 90 Ibs. per barrel may 

 be expected, corresponding to a heat supply of 1,260,000 B.T.U. per 

 barrel. With the wet process a fuel consumption of 160 Ibs. per barrel 

 is rather better than the average. This corresponds to a heat supply 

 of 2,240,000 B.T.U. per barrel. These three estimates have therefore 

 be used in making up the summary table. 



Heat supplied by chemical combinations. It is undoubtedly true 

 that a considerable quantity of heat must be liberated when the lime 

 and magnesia combine, at the clinkering temperature, with the silica, 

 alumina and iron oxide, and that in this way considerable heat is added 

 to that derived from the fuel. Unfortunately, however, we have no 

 very definite knowledge as to the exact chemical combinations which 

 take place during clinkering, and lacking such knowledge any estimate 

 of the amount of heat thus liberated must be considered as merely a 

 wild guess. 



Both Helbig and Richards, in the papers previously cited, have 

 quoted Berthelot on this point as giving the following data for the heat 

 liberated during this combination. 



1 kilogram lime (CaO) liberates 530 calories 



1 " magnesia (MgO) liberates 827 " 



These figures, changed into English measures, are: 



1 pound lime (CaO) liberates . 954 B.T.U. 



1 " magnesia (MgO) liberates 1489 " 



For convenience these figures might be adopted in discussion, but both 

 the reader and experimenter must bear in mind that they represent 

 very doubtful assumptions, and are accepted merely because no better 

 data are obtainable. In the present discussion of the subject no esti- 

 mate of this type will be used. 



Heat derived from the clinker. A large part of the heat carried out 

 in the hot clinker may be used to heat the incoming air. In Carpenter's 

 experiments a little less than half of the clinker heat was thus utilized, 

 but other experimenters have claimed 80 to 90 per cent efficiency for 

 various types of clinker-heat regenerators. The amount of heat thus 

 returned to the kiln might therefore vary from 90,000 to 175,000 B.T.U. 

 per barrel of cement. 



Heat derived from the stack-gases. Heat may also be taken from 

 the stack-gases and used to heat either the raw material or the air- 

 supply. Usually, however, stack-gas heat when utilized is used in the 

 power department of the mill, rather than in the kiln. 



