CANE SUGAR. 



Then since the vapour at the moment of its formation is at a greater 

 pressure than that already formed, and since the temperature of a boiling 

 liquid is equal to that of the vapour which it gives off, it follows that the 

 mean temperature of the juice in ebullition is greater than that of the vapour 

 in the vapour space. 



Again, the juice is emulsioned with the vapour it gives off and this has 

 a tendency to diminish the density of the boiling juice. Now to obtain an 

 idea of the volume of the contained vapour, Dessin recalls the phenomenon 

 observed when an evaporator stops boiling, namely, that the level of the 

 liquid falls. This fall in level, which is 3 inches in extent, Dessin takes as 

 giving the volume of the contained vapour. 



Take the case of the third body of a triple, the vacuum being 25 inches, 

 the vessel 6 feet internal diameter, the tubes 4 feet long, 2 inches diameter, 

 and 630 in number. The total volume of the juice is the volume of the 

 tubes -f- the volume above the tube plate the volume occupied by the vapour. 



We have, then, at rest 



c.ft. 

 Yol. of the tubes = 630 TT ( T V x T V X 4) c. ft. = 55-0 



Vol. above tube plate = TT x 3 X 3 x c. ft. = 14-1 



69-1 

 During ebullition the total volume is that found above + that occupied 



by vapour = TT x 3 x 3 X i c. ft. = 7'0 c. ft. The total volume during 



ebullition is then 76-1 c. ft. 



The finally corrected densities of the juices then appear 



First vessel, 1-054 X-^~T= '957 

 76'1 



Second vessel, 1-106 x-= = 1*004 



76 - l 



69*1 



Third vessel, 1-229 x-r7r-r-= 1-116 

 7o*l 



The vapour then in the third vessel will have to raise a column of liquid of 

 mean height 27 inches, and of density 1-116 ; this is equivalent to 2'20 inches 

 of mercury or 1-09 Ibs. per square inch. The vapour itself was taken at a 

 pressure of 2-42 Ibs. per square inch, so that the pressure of the vapour at the 

 moment of its formation is 2'42 -f 1-09 = 3 51 Ibs. per square inch, corres- 

 ponding to a temperature of 147 F. ; that of the vapour was 132 F., a 

 difference of 15F. 



Similarly for the second body a difference of 7 F., and for the first of 

 4F. are found. 



The efficient fall in temperature from body to body is then 

 284 - 132 - (15 + 7 + 4) = ^^ 



3 



or 76-5 per cent, of what would be the case if the height of juice in ebullition 

 was zero. 



312 



