EVAPORATION 



357 



Use of Superheated Steam. Claassen, experimenting with coil vacuum 

 pans, condemned the use of superheated steam, and has advised that such 

 steam before entering the coils or calandria be reduced to a saturated one 

 by the introduction of " atomized " water, but later experiments with evapor- 

 ators by Saillard 19 , Jaks 20 , and Kerr 10 have shown that with a moderate 

 degree of superheat the coefficient of transmission is the same as with satur- 

 ated steam. In Kerr's experiments the superheat varied from 3-9 to 31-7 

 F., and a uniform coefficient of transmission was found. 



FIG. 210 



Condensers. The final reduction of pressure, or production of the 

 " vacuum " in the last cell of a multiple effect, is obtained by withdrawing 

 the air by a pump, and is maintained by the rapid condensation of the steam 

 with the removal of whatever air accompanies it. The apparatus in which 

 this condensation takes place are called condensers, and fall into two main 



FIG. 211 



classes surface and injection. In the former the steam and cooling water 

 are separated by a partition, through which the heat is transmitted. The 

 cells of the effects, juice heaters, etc., are all types of surface condensers, 

 which are very rarely used to obtain the final reduction in pressure. 



Two specialized forms of surface condensers which have found a very 

 limited use in the cane sugar industry may be mentioned. The first consists 



