CHAPTER XVIII 



Evaporation 



After the processes of defecation and filtration have been completed, 

 there results a more or less clear juice, varying in quantity from So to 120 

 per cent, of the weight of the cane. This juice contains from 13 to 20 per 

 cent, solid matter, of which 70 to go per cent, is cane sugar. In order to 

 obtain the sugar as crystals the greater part of the water has to be removed. 

 Its removal is effected in two stages : the first is referred to as evaporation, 

 the second as boiling or graining. There is, however, no fundamental 

 reason why these stages should not be continuous ; their discontinuity is 

 due to the nature of the operations involved. 



In the first stage the concentration is carried on until the percentage of 

 solids has reached not less than 50 per cent., and it may reach 70 per cent. ; 

 the variation depends on the capacity of the evaporators, the caprice of the 

 superintendent, and the purity of the juice. The process is conducted 

 under a system of multiple effect evaporation, whereby one unit of steam may 

 evaporate n units of water, where n may be very great. The extreme limit 

 in practice is reached with an eight-fold evaporation ; the apparatus in 

 common use are of triple, quadruple, or quintuple effect. The evaporator 

 may be operated as an independent unit, or it may be worked in combination 

 with the juice heaters or the graining pans. The first-named combination 

 will be referred to as an isolated system, and the second as a connected 

 system. 



In this chapter an attempt is made to bring together an account of the 

 elementary principles involved, of the chief tj'pes of apparatus used (together 

 with their essentia] accessories), of the different sj^stems and combinations, 

 with their bearing on the general economy of the factory as a whole. 



Boiling Points. — All liquids continually give off to the surrounding 

 atmosphere a part of their substance in the form of vapour, which exerts 

 a definite pressure known as the vapour pressure. For every temperature 

 there is a corresponding vapour pressure, which increases with the tem- 

 perature. When this pressure becomes equal to the pressure of the surround- 

 ing atmosphere, vapour is given off freely from all points of the liquid, and 

 the latter is said to boil, the temperature at which this occurs being called 

 the boiling point at that particular pressure. When no qualification is applied 

 to the boiling point, the pressure is taken to be the normal atmospheric 

 pressure ; this is 14-707 lbs. per sq. in. ; 1-034 kilos per sq. cm., or that 

 exerted by a column of mercury 29-92 ins. or 76-0 cm. high. It is common 

 practice to refer to pressures less than atmospheric in terms of vacuum ; 

 thus an absolute vacuum would be referred to as 29-92 inches of vacuum, 

 and a liquid boiling under a vacuum of 25 inches boils under a pressure of 



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