EVAPORATION 251 



It will be understood that for high vacua, low tempe- 

 rature of cooling water (T$) is more important than copious 



supply frr It is advantageous, however, to choose a 



site yielding plenty of cold water, such as a river or canal 

 side. Otherwise it is often necessary to use cooling towers 

 or spray nozzles. The cooling is by evaporation (=60 to 80 

 per cent, of W), cold water replacing that evaporated, 



and yielding water 75 to 80 F. If T<=8o F. and =7O , 



W 



a vacuum of 28-34" is possible, but the 0-34" should be 

 allowed for the partial pressure of the air, determined exactly 

 by the air entering and by the displacement of the air-pump. 



Another feature of the modern evaporator is the " heater " 

 or " calorifier," by which the liquor to be evaporated is led 

 in a continuous rapid stream through heated tubes immediately 

 prior to its entry into the first effect. It is the aim of the 

 heater to raise the temperature of the liquor to the tempera- 

 ture of evaporation, and so to avoid this being necessary 

 in the first effect. The heater thus further avoids stewing, 

 ensures steady running, and effectively increases the capacity 

 of a machine. 



It is noteworthy that superheated steam is not desirable 

 for working an evaporator. The principle of evaporation 

 by steam is not merely that the temperature of the liquor is 

 raised to boiling point ; it is that in the condensation of the 

 heating steam its latent heat is yielded to the liquor being 

 evaporated. To evaporate quickly, therefore, the heating 

 steam must condense rapidly. Hence, as superheated steam 

 has a rate of condensation 20-30 times slower than saturated 

 steam, the latter is much to be preferred. A slight super- 

 heating, however, may be justifiable where the steam has 

 any distance to travel before use. It is the fact that it is 

 the latent heat of steam which is mainly utilized which gives 

 steam its great practical advantage over hot non-condensable 

 gases. Steam in condensing yields an enormously greater 

 number of heat units per Ib. than hot waste gases. Steam 

 has also the advantage of more constant temperature. 



