CANE SUGAR. 



The writer has found, however, in evaporators of old design, with small 

 vapour spaces and pipes of restricted area, losses amounting to 2 per cent, of 

 the sugar entering the boiling house. 



Theory of Multiple Evaporation. The theory of multiple 

 evaporation in vacuo has been elaborated by Jelinek, Horsin-Deon, Dessin, 

 Claassen, Bolk, and Hausbrand; the most complete and elegant theory is that 

 due to Hausbrand 8 ; in what follows, the writer is indebted to all these 

 experts, and especially to the last mentioned. 



It may be stated at once that it is impossible to calculate OD first princi- 

 ples the pounds of water evaporated in each body of a multiple effect, unless 

 the different temperatures in each body are assumed. The temperatures, 

 however, depend on a number of factors the temperature of the heating 

 steam, the vacuum in the last body, the height of the liquid under treat- 

 ment, &c., all of which are variable or unknown. Very considerable informa- 

 tion can, however, be obtained on the lines below. 



In a simple unexact way it is understood that a pound of steam introduced 

 into the first body evaporates there a pound of water, the resulting steam 

 passing on to the second body again evaporates a second pound of water and 

 so on ; but this supposition is only approximate.* 



In the first instance, as the temperature at which evaporation occurs 

 decreases, the latent heat increases, so that for this reason the steam from an 

 earlier vessel evaporates less than its own weight of water in the succeeding 

 one, but on the other hand the liquid entering from an earlier vessel itself 

 suffers a fall in temperature and the heat given up can go only towards the 

 formation of steam. Water evaporated by reason of this last cause is termed 

 self-evaporation. As a numerical example the following data may be taken : 

 100 units of water enter the first vessel and from it are evaporated 25 units of 

 water, so that 75 units pass on to. the next vessel. Let the temperature in the 

 first vessel be 212 F. and in the second vessel be 194 P. ; the latent heats 

 of steam at the two temperatures are 966 and 978 B.T.U. respectively. The 



Q A f\ 



25 units of steam will then evaporate by their condensation 25 X =- = 24-7 



y / o 



units of water. But the water in cooling down from 212 F. to 194F. gives 

 up 75 (212 194) = 1350 units of heat ; this quantity will evaporate 



1350 



= 1-4 units of water. This latter is the self-evaporation and the total 

 y /o 



evaporation is 24-7 -f 1-4 = 26-1 units. Another cause tends to make the 



* It should be pointed out that the evaporation in a multiple evaporator is best expressed 

 as water evaporated per Ib. of steam received + steam used in driving the vacuum pump ; 

 actually most of the direct steam admitted to the cylinder of the pump passes out as exhaust, 

 the heat of which is utilized in evaporation, but the steam condensed in the cylinder + that 

 represented by the fall in total heat of the portion which passes out, should be added to the 

 steam condensed in the calandria of the first effect when estimating the evaporation per Ib. of 

 steam. Since a quadruple of equal capacity with a triple will require a smaller pump, the 

 economy of the former compared with the latter will tend to be greater than in the ratio 4 : 3. 



308 



