CANE SUGAR. 



Heating 1 Surface. Calculations of the heating surface requisite for 

 vacuum pans can be made on the same lines as have already been given when 

 dealing with the designs of multiple effect evaporators; coefficients of the 

 heat transmission for the conditions in the vacuum pan were given there ; these 

 calculations are not of much avail as the data are uncertain and of necessity 

 vary with the actual conditions of working. Further, while in the multiple 

 evaporator the whole heating surface is constantly active and the juice level 

 remains constant, in the vacuum pan during part of the process only is the 

 whole of the heating surface in use, and the level of the contents of the pan is 

 constantly changing. This is the more marked the taller the pan is, and 

 consequently the higher the pan is the less is the efficiency of the heating 

 surface ; the effect of a high level in the liquid under treatment in decreasing 

 the mean temperature difference between heating steam and heated liquid was 

 explained in Chapter XVI., and a similar cause is at work in the vacuum 

 pan ; hence pans that are shallow in proportion to height are found to be 

 ' quick boiling* pans. As an example of an actual case let the maximum 

 height of the massecuite in a pan be 10 feet, so that the average height for 

 the boiling is 5 feet ; the average density of the contents of the pan can be 

 only very approximately estimated; the syrup enters at a density of, say, 1*25, 

 and is discharged as massecuite at a density of about 1*50 ; allowing for the 

 decrease in density due to einulsioning with gases, as was done when dealing 

 with the multiple evaporator, the mean density of the boiling mass is found to 

 be about 1*21 ; the weight of a column of liquid 5 feet high and of density 

 1*21, and of 1-inch cross section, is equivalent to a column of mercury 5'34 

 inches high, or to a pressure of 2-62 Ibs. per square inch ; if the pressure of 

 the liberated vapour be taken as 2 '42 Ibs. per square inch corresponding to a 

 vacuum of 25 inches, the mean pressure of the vapour at the moment of its 

 formation is 2'62 + 2'42 =: 5*04 Ibs. per square inch, corresponding to a 

 temperature of 162F., which may be taken as the mean temperature of the 

 boiling mass. 



Now the mean pressure of the* steam in the coils may be taken as 20 Ibs. 

 per square inch, corresponding to a temperature of 259 F. The difference in 

 temperature between the heating steam and the boiling liquid is then 97 F. ; 

 for a triple effect dealt with in the previous chapter a mean temperature 

 difference of 24-3 F. was obtained. A modern triple effect evaporator will 

 evaporate 6 Ibs. of water per square foot per hour, so that on these grounds a 

 single effect pan should evaporate under the conditions detailed about 24 Ibs. 

 As a matter of fact, experience has shown that in the single effect vacuum pan 

 an evaporation of only 12 to 15 Ibs. can be reckoned on; the greater density 

 of the material under treatment implying greater viscosity and consequent 

 greater difficulty for the disentanglement of vapour, and at the same time 

 decreasing the coefficient of transmission of heat. In addition, whatever be 

 the actual evaporation during working, for a part of the operation the whole 



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