274 



CHAPTER XIV 



Fig. 157 



gutter c. They are usually provided with the syphon float discharge 

 indicated at d in Fig. 157. French practice changed the straight tubes in 

 the Taylor system to a coil and adopted a circular vessel as shown in Fig. 158. 

 This type is usually found provided with draw-off cocks at different levels, 

 and is the form generally found in Cuba. In both designs, i sq. ft. heating 



surface is found per three 

 cub. ft. of capacity. In 

 place of either of these 

 designs the very efficient 

 Witcowitz heating device 

 may be used ; this, as ar- 

 ranged in an evaporator, is 

 shown in Fig. 205. 



When the heating is 

 done entirely in tubular 

 heaters, the tanks which 

 receive the hot juice serve 

 merely as settling and storage tanks. In the defecator a system of 

 flotation obtains. On appl5dng heat the emulsioned air attaches itself 

 to particles of the solid matter, and causes them to rise as a blanket 

 to the surface. At the same time the particles of greater specific gravity 

 fall to the bottom. Between these two layers lies the great bulk of 

 the juice in a state of clarity. The operation of heating, once known as 

 "cracking" requires to be carefully carried out, for, if the juice be allowed to 

 boil, the floating blanket is broken up. 



Java practice in raw sugar manufacture combines the French defecator 

 with discontinuous settling. Generally three defecators of the type shown 

 in Fig. 157 are used, the passage of juice (together with the separation of the 

 scum) through these being continuous ; afterwards the partially defecated 

 juice passes to settling tanks, where the separation of suspended matter is 

 completed. The literal translation of the term used for this operation is 

 " troubled defecation." 



The Design of Defecators. — If the defecator be considered as a setthngtank, 

 the fundamental factor in its design is the rate of settling. Based on the 

 experiments described above, a rate to the critical position of 7 cms. per 

 minute should be obtained, 

 the critical position being « 

 taken as 0-75^ where h is ^ 

 the height of the tank. After < 

 the critical point is reached ^ 

 twenty minutes should be ^ 

 sufficient to so reduce the '-^ 

 rate that further settling 

 is uneconomical. Under such 

 conditions the volume of 

 the mud should lie between 



Fig 158 



10 per cent, and 15 per cent, of the voliune of the juice. 



A second factor in design is concerned with the conservation of heat. 

 With circular tanks without a cover, the surface is a minimum for a stated 

 volume, when the height is half the diameter and with square tanks when 

 the height is half the length of one side. In addition, the exposed surface 

 for a given volume decreases as the number of tanks decreases. There is a 



