4O2 



CHAPTER XIX 



bottom is made sloping to allow of the discharge of the contents. They 

 appear but rarely, if at all, in the cane sugar industry. 



Mechanical Agitation. A patent (13286, 1850), taken out by Shears as 

 agent, claims the use of a vertical screw in a vacuum pan. Many years 

 later this same device appears in the Freytag pan, with a tubular heating 

 surface, and in the Grosse coil pan, Fig. 249, and in the Reboux pan, 

 Fig. 250. These pans, used in the- beet industry for the slow methodical 

 boiling of low-grade material, have not up to the present entered into use 

 in the cane industry. 



Certain patents have for their object agitation by means of moving 

 heating surfaces. Thus McNeil's patent (8814 of 1899) employs a device 

 similar to that of Bour's evaporator (q.v.)> while Czapiowski (15031 of 1902) 



FIG. 250 



employs a rotating coil similar to those once used in the Wetzel pan (q.v.). 

 These devices have never come into use. 



Technique of Crystalli2ation-in-Motion. In a previous section it was 

 stated that low-grade products when dropped from the pan should have a 

 coefficient of supersaturation of 1-5 to I- 6. The object of operating with so 

 high a coefficient is to push the work in the pan to the limit and to obtain 

 there as great a crystallization as is possible. On cooling such a massecuite 

 owing to the high viscosity crystallization will be very slow, and eventually 

 a supersaturated mother liquor may remain, when it is time to dry the 

 strike. There may be also much fine grain present, and the strike may 

 dry badly and require much water to remove the viscous mother liquor. 

 The supersaturation of such a material should be systematically reduced 

 in the crystallizers by the addition of water until a saturated molasses 



