EVAPORATION 



347 



A rise in the pressure in calandria 4 results, which is reflected back to vapour 

 space 3, causing a rise in pressure therein. The result is a greater temperature 

 difference in cell 4 and a more rapid evaporation there and throughout the 

 whole apparatus. The steam which is short-circuited from vapour space 2, 

 however, now only works at triple effect and the economy falls. Similarly, 

 steam may be short-circuited from vapour space I to calandria 3, in which 

 case some steam will only operate at double effect. At times, when the 

 last cell is getting foul at the end of a run, the concentration of the syrup 

 and the capacity of the apparatus may be maintained by this means, and 

 generally it will be economical to keep at high density in the syrup and 

 eliminate as much single effect boiling in the vacuum pans as is possible. 



Evacuation of the Condensed Water. The condensed water in the first 

 cell being normally under pressure can flow out by gravity. This water is 

 always separated from the other condensed waters and is used for boiler 

 feed. The water may pass through a trap or inverted syphon to prevent 



FIG. 194 



FIG. 195 



simultaneous loss of steam. If the apparatus is on a sufficiently high level 

 the water can flow directly to the boiler feed tank, otherwise a pump is 

 necessary. 



In the latter cells, which operate at less than atmospheric pressure, 

 the following methods may be adopted : 



(a) The water may pass from cell to cell through inverted syphons 

 (Chapman's patent 1752, 2511, 1888). In Fig. 194, a and b represent two 

 cells, in which the vacua are respectively five and fifteen inches, or a difference 

 in pressure equivalent to a ten-foot head of water. If the syphon in this is 

 more than ten feet long a water seal will be formed at the lower part of the 

 U due to the balancing of the two columns of water, and the water will 

 continuously and automatically pass from cell to cell. This system is also 

 applicable to juice circulation. 



The " flashing " of the water into steam as it passes to the lower pressure 

 seems to disturb this system, and in actual operation it is found necessary 

 to make the syphon twice as long as the prevailing pressures indicate to 

 be necessary. 



(b) The water from each cell gravitates to a sealing tank by a fall pipe. 



