THE EVAPORATION OF THE JUICE TO SYRUP. 



To illustrate of what vital importance it is to have as great as possible a 

 vacuum in the last body, figures are given below for an identical apparatus 

 where the vacuum in the last body is 27 inches corresponding to a temperature 

 of 114 P., and a pressure of 1-43 Ibs. per square inch. Taking the vacua in 

 the other two bodies as 4 inches and 17 inches, the efficient fall in temperature 

 is found to be 30-3 P., that is to say the heating surface compared with the 

 apparatus where the vacuum in the last body is only 25 inches, is 30 per 

 cent, more valuable. 



In describing the Welner-Jelinek apparatus it was stated that the height 

 of the column of liquid was only 24 inches. A calculation as above for a final 

 25 inch vacuum gives the efficient fall in temperature as 28 F.- 29 P., 

 and proportionally higher for a 27 inch vacuum. It was with particular 

 reference to this point that the Welner-Jelinek was designed. 



In the film evaporators of the Lillie and Yaryan type there is no increase 

 in pressure due to the height of the boiling column of liquid, but whereas in 

 the latter the juice is contained in the tubes and the vapour formed can only 

 escape at the end of a unit, this vapour must, while in the tubes, be under 

 pressure and tend to increase the boiling point to the detriment of the efficiency 

 of the heating surface. In the Lillie design the vapour formed is free to escape 

 in all directions, and the juice falling in a rain or film there is no increase of 

 temperature in the body of the liquid for the reasons detailed above, and the 

 efficiency of the heating surface must approach the maximum value. 



In a quadruple effect submerged vertical tube apparatus the efficient fall 

 in temperature from vessel to vessel on the lines given above works out with 

 a 25 inch vacuum at about 14 P., and with 27 inch vacuum at about 18 P. ; 

 for a Welner-Jelinek the corresponding figures are about 16 P. and 20 P. 

 The maximum for the two cases, there being no increase due to head of liquid 

 under treatment, is 25 F. and 30 P. respectively. 



In many sugar houses the evaporation is done by triple effects, and it was 

 supposed once that the maximum working number of effects was three, and 

 that with four a sufficient fall in temperature could not be obtained to allow 

 heat to pass from condensed vapour to boiling juice. Formerly vertical 

 apparatus were constructed with tubes even 6 feet long, causing the efficiency 

 of the heating surface to become very low. By reducing the length of the 

 tubes to 3 feet or 3 feet 6 inches, and by attending to the necessity for as low 

 a vacuum as possible in the last body, quadruple effects using back pressure 

 steam only are working in many sugar houses, and do not require high 

 pressure steam at all. In some sugar houses, however, quadruples can be seen 

 where the juice in the first vessel boils under more than atmospheric pressure. 

 With film evaporators quadruple evaporation is general, and quintuple effects 

 have also been constructed. Assuming a minimum fall in temperature for 

 efficient working from vessel to vessel of 20 F., the maximum number of 



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