252 ANIMAL PROTEINS 



The capacity and efficiency of an evaporator depends 

 upon a good many factors, some of which are worthy of 

 discussion at this point. 



The transference of heat and the amount of evaporation 

 are directly proportional to the mean temperature difference 

 between the heating steam and the liquor being evaporated. 

 These temperatures, however, both vary somewhat, the steam 

 losing part of its pressure and temperature as it passes along 

 the heating surface ; the liquid generally increases in tempe- 

 rature. The mean difference in temperature, moreover, 

 is not the arithmetic mean between the smallest and largest 

 temperature differences, but is given by the following 

 expressions, which yield results not wide apart : 



If a =temperature difference at commencement ; 



Oe end ; 



and m =mean temperature difference ; 

 then. 



'--' or -?-' 



This mean temperature difference is in practice usually 

 spoken of as the " temperature head " or " heat drop." 

 It will be clear that this temperature head is increased by 

 using steam at higher pressure (temperature), and by 

 evaporating under reduced pressure. Since most liquids 

 have their boiling points reduced about 40 C. by operating 

 in vacuo, the advantage of the vacuum is apparent. It 

 should be remembered that the temperature head has not 

 the same value in any part of the scale : it has more value 

 higher up the scale, because the steam is denser and more 

 heat units come in contact with a given area in a given time. 

 It must also be remembered that whilst the pressure gauge 

 is a most useful indicator of steam temperature, it is not 

 necessarily accurate. The pressure in the hot space is the 

 sum of the pressures of air and steam, and since the tempe- 

 rature (the important condition) of the hot space depends 

 upon the pressure of the steam, and not on the sum of the 



