SUGAR BOILING AND CRYSTALLIZATION-IN-MOTION 



597 



which crystals form is not instantaneous, but lags behind the rate at which 

 water is removed. Actualh^ it would not be advisable to count on more 

 than four full strikes per day of twenty-four hours. Each pan would then 

 have a working capacity of 759 cu. ft., and the total capacity would be 3,037 

 cu. ft. If, further, the 100 tons of juice be derived from 100 tons of cane, 

 this computation would give 30-4cu. ft. per ton-cane-hour. In the example 

 quoted the gravity solids accepted were low, and as a general rule 40 cu. ft. 

 per ton-cane-hour represents modern practice. 



The heating surface in the pans depends on the pressure of the steam 

 which is to be used. Coil pans of the t\^pe described on page 39S are usually 

 worked ^\•ith live steam, and as usually constructed have i sq. ft. heating 

 surface to i cu. ft. of working capacity. If, then, pans of this tj-pe are to 

 be installed there would also be required 40 sq. ft. heating surface per ton- 

 cane-hour. 



On the other hand, if a svstem of steam utilization be decided which 



Fig. 239 



employs low-pressure steam in the pans, calandria pans or pans with short 

 coils and distributing boxes will be used. These are made with as much as 

 two sq. ft. heating surface per cu. ft. of capacitj', and maybe taken as operating 

 equally as fast as a coil pan used with Live steam. With such a system the 

 heating surface would reach 80 sq. ft. per ton-cane-hour, and with a combina- 

 tion of the s\'stems would lie anywhere between these limits. 



Rate of Evaporation. — The rate of evaporation in a vacuum pan is very 

 variable. In a l}Te coil pan tested b}^ the %^Titer there were in all 2,555 sq- 

 ft. heating surface, of which 1,570 sq. ft. were in the body and 855 sq. ft. 

 in the saucer. The capacity of the pan was 2,700 cu. ft. The steam used was 

 40 lbs. gauge and the s^Tup boiled was of 60° Brix. On commencing 

 operations a charge of 640 cu. ft. was admitted, and steam was turned on to 

 1,540 sq. ft. Grain appeared in 35 minutes ; all the heating surface was in 

 operation in 59 minutes ; the last charge was taken in 219 minutes after 

 starting, and the strike was completed in 264 minutes. The rate of evapora- 

 tion was determined by collecting the water discharged from the coils in 

 tanks of 63-5 cu. ft. capacity. The times taken to fill a tank were : 810, 780, 



