THE EVAPOEATION OF THE JUICE TO SYRUP. 



It was, and still is, a very general rule for makers to design evaporators 

 with heating surface increasing from vessel to vessel. Hausbrand recommends 

 that at the very most the heating surface in the last body should be not greater 

 than and perhaps a little smaller than in the first. 



The Efficiency of a Multiple Effect. A very important 

 factor in dealing with multiple effects is the evaporation per square foot of 

 heating surface, or, in other words, the capacity ; the chief factor is the 

 available fall in temperature from vessel to vessel. A numerical example, 

 which is founded on the methods used by Dessin 9 , will show how this fall 

 is controlled. 



Let the heating steam be at a pressure of 8 Ibs. per square inch, cor- 

 responding to a temperature of 234 F., and let the pressure in the last body 

 be 2*42 Ibs. per square inch, corresponding to a vacuum of 25 inches, and to a 

 temperature of 132F. 



Then the total fall in temperature is 234 - 132 102F., and the 

 average fall from vessel to vessel is 34 F. Let the juice under treatment 

 enter at a density of 1-0709 or 18 Brix, and leave at a density of 1-253 or 

 55 Brix. 



Now for convenience of calculation let there be equal evaporation* in 

 each vessel, then the total evaporation per cent, by weight on original juice is 



55 _ 10 



x 100 = 67/27 per cent. 

 oo 



of which each vessel evaporates one-third or 22-42 per cent, of the juice. 

 Then if B. is the degree Brix of juice in the first vessel 



whence B. = 23-2, corresponding to a density of 1 '093. Similarly the juice 

 in the second vessel is found to be of density 32'6 Brix or 1-136. 



These densities are correct at a temperature of 84 F., and the densities 

 are required at the temperatures prevailing in the vessels which, assuming the 

 fall in temperature equally divided, are 210 F., 180 F., 160 F. ; the 

 respective densities at these temperatures are 1-054, 1*106, 1-229. 



Now consider the case of a vertical submerged tube apparatus with tubes 

 4 feet long, and let the level of the liquid be 6 inches above the level of the 

 tube plate ; the vapour formed in the tubes must, so as to disengage itself, be 

 at a pressure equal to the pressure in the vapour space increased by the 

 pressure due to the weight of the column of liquid. This increase in pressure 

 varies with the place where the vapour is formed ; the maximum height is 

 4 feet, the minimum 6 inches, the mean is 27 inches. 



* The results obtained by Hausbrand show that this condition can never obtain ; this 

 section is introduced to explain certain important principles in the multiple evaporator, 

 and an ideal case is chosen for simplicity ; the application of the conclusions is in no wise 

 invalidated. 



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