THE EVAPORATION OF THE JUICE TO SYRUP. 



Size of Pumps. Although it is impossible to calculate on first 

 principles the necessary sizes of pumps to maintain a vacuum, certain principles 

 can be developed : the treatment here adopted is mainly after Hausbrand. 



FIG. 187. 



The pressure in a condenser is made up of two parts : a. the pressure due to 

 the water vapour which depends only on the temperature of the water ; b. 

 that due to the incondensible gases (referred to below as Air). Thus, with a 



pf 



FIG. 188. 



vacuum of 24 inches, the pressure is 2'92 Ibs. per square inch. Let the tem- 

 perature of the water in the condenser be 100 F. ; the pressure of water vapour 

 at this temperature is '95 Ibs. per square inch ; hence the gases are at a 



331 



