CANE SUGAR. 



effects with steam at 234 F., and a final vacuum of 27 inches, will be six, and 

 this result could only be obtained with film evaporators. 



It should be noticed that this increase in temperature due to height of 

 liquid is greatest when the pressure is least ; in the following table are given 

 figures showing how this varies for different pressures, the density of juice 

 being taken uniformly as unity : 



Prom the lines immediately above it may be thought that the lower levels 

 of juice in an evaporator will be of higher temperature than the upper ; such 

 indeed is not the case, as was shown by Jelinek, whose observations have been 

 extended to the vacuum pan by Curin. 10 



In a well-designed evaporator with a rapid circulation the transfer of heat 

 from the ' superheated ' particles at the lower levels is very rapid, so much so 

 as almost altogether to mask the effect of the hydrostatic pressure, so that the 

 objection to long tubes becomes of diminished importance ; in apparatus with 

 bad circulation the effect of hydrostatic pressure is very pronounced, and the 

 low duty of many of the evaporators dating back to the eighties may be 

 attributed to these causes. Extreme length of tubes, combined with a high 

 duty, is seen in the Stillman and Kestner apparatus, which utilize the principle 

 of ' ruissellement ' or * grimpage? best rendered in English as ' creeping' This 

 principle can be used in bulk evaporators of conventional design, but the type 

 of labour usually available in the tropics is not to be trusted to use it ; 

 hence the writer thinks that a bulk evaporator with tubes of a conservative 

 length with a well-arranged circulation is the best type to adopt. 



Conditions are rather different in vacuum pans, and here the effect of the 

 hydrostatic head is very noticeable; that is to say, it is a matter of clinical 

 observation that tall narrow pans are much less rapid boilers than are broader 

 shallower apparatus. 



Capacity of Multiples. A vertical submerged tube triple effect 

 may be considered as giving a high duty when it evaporates 8 Ibs. of water 

 per square foot per hour ; a quadruple is satisfactory when it evaporates 6 ; a 

 Lillie quadruple film evaporator will be satisfactory when it gives a duty of 

 10 Ibs., or a triple when it reaches 13 Ibs. ; a horizontal tube low level appar- 

 atus will have a duty lying between these figures, and a ' creeping* vertical tube 

 apparatus of the * Stillman ' type will approach or equal in duty the film type. 



314 



