THE SEPARATION OF THE CRYSTALS 423 



There will then on this basis be required : — 



Machines. 



77 purity 75 purity massecuite — = 4 -8 of 40-in. x 24-in. 



' 6800 



55 ,, ,, — — = 16-9 of 36-in. X i8-in. 



850 



„ 49672 , 



85 „ 75 „ „ 1 — '— = 7 -3 of 40-in. X 24-in, 



6800 



10296 , . 



55 ,, „ = 12 -o of 36-in. X i8-in. 



850 



As the installation must allow for the maximum at the different condi- 

 tions, the design would resolve itself into six 40-in. x 24-in. machines for 

 high grade and eighteen 36-in. x i8-in. machines for low grade, some mul- 

 tiple of three being taken in this case, since one unit of labour can handle 

 three machines. 



Alternatively, a design might be offered comprising six 40-in. x 24-in. 

 machines on high-grade and twelve 36-in. x i8-in. machines on low grade, 

 with six 36-in. X i8-in. machines connected to work on either, changes being 

 made dependent on the purity of the material being handled. 



It is not unusual to express centrifugal capacity as so many square feet 

 of screen area per ton-cane-hour. A 40-in. X 24-in. machine offers 21 -i 

 sq. ft., and a 36-in. X i8-in. machine 14-1 sq. ft. In this case then there 

 will be 423 sq. ft. in all, and if the 100 tons of juice are derived from 100 tons 

 of cane the proportion is 23 sq. ft. per ton-cane-hour, of which 40 per cent, 

 is used on high grade and 60 per cent, on low grade. Screen area is not, 

 however, an altogether satisfactory basis of comparison except as between 

 machines of the same size, for, whilst the screen area varies as the product 

 of the diameter and height of basket, the capacity varies as the net cubic 

 contents. 



In any case a flat rate does not form a good system of design, which 

 should be considered in detail for every case with a knowledge of the purities 

 and densities of the juice, as well as of the tonnage of cane to be handled. 



Handling of Low Sugars. — In the older processes of repeated boilings a 

 quantity of fine-grained molasses sugars of 88 test or thereabouts was ob- 

 tained. This material is of low comparative value, and its marketing is 

 attended with difficulty. The best way to dispose of it is to remelt it or 

 to take it into the pans as seed grain. In the two and three-massecuite 

 processes described in the previous chapter the low sugars are boiled on a 

 footing of high grade massecuite, so that they are of large grain, and if neces- 

 sary can be washed up to 96 test. It is, however, much more convenient 

 to double-cure these sugars. They are accordingly dropped wet from the 

 baskets, mixed with sufficient high-grade molasses to allow of pumping, 

 and mixed with the high-grade massecuite ; alternatively, they may be re- 

 dried separately in independent machines. This process of double purging 

 was first used by G. L. Spencer at Tinguaro, in Cuba, about igoo. 



When these low sugars are mixed with high-grade massecuite, a uniform 

 distribution should be obtained. This is best done by running a canal 

 parallel to and over the centrifugal supply tank. Part of the canal is cut 

 away precisely as is done with the " cush cush " distributors in use at the 

 mills. 



