338 SORGHUM. 



Evaporation in Open Pans and by Steam. 



Owing to the far greater ease with which the heat may be applied 

 and regulated, it is extensively employed, both in the operations of 

 defecation and evaporation. In open pans it is applied by means of 

 a coil of iron (or, preferably, copper) pipe, as in the defecator; and 

 since the heat may thus be applied over a greatly increased surface, 

 and at any temperature and pressure, the evaporation is proportionately 

 more rapid than from a pan which is heated directly over the fire. The 

 relative evaporating capacity of pans heated by direct heat and by 

 eteam, may be learned by the following comparisons. The average 

 evaporation from an open pan over a fire, the grate surface of which is 

 about one-fourth to one-third the size of the pan, is about one gallon 

 of water to each square foot of surface in the pan per hour. Pans of 

 large size evaporate a larger amount proportionately than small ones, 

 and with greater economy of fuel, and less relative waste. To illustrate: 

 pans of the following dimensions are claimed to evaporate as follows : 



Gallons per hour. 

 Size. Feet of surface. per foot surface. 



40 X 120 inches 33>3' 4 



40 X 192 inches 53)^ 7 



40 X 28S inches 80 1.3 



44 X 108 inches 35 8 



44 X 144 inches 44 11, 



44 X 180 inches 55 11 



Experiments Were made at the Department of Agriculture, by the 

 author, with two evaporators heated by copper coils with steam at the 

 pressure of 55 pounds. 



As an average of seven experiments, with two evaporators, it was 

 found that there were evaporated four gallons of water per hour from 

 each squai-e foot of surface of the juice in the evaporator. One evap- 

 orator was seven feet in diameter ; the other, four feet ten inches. 

 Both were circular. ' 



Practically, the same results are reported by others in their work 

 with similar evaporators. 



The importance of keeping up a brisk boiling, must not be forgotten; 

 for it has been found that the transmission of heat from steam pipes is two 

 and a half times as rapid when the water is boiling vigorously, as when 

 it is at rest as in the heating up of the juice in the act of defecation. 

 This great difference is obviously due to the rapid circulation and dis- 

 tribution of the heat when the boiling is going on, as compared with 

 the very slow movements in the mass of water when not boiling. 



It is found, also, that the heat given out by steam coils to boiling 

 juice, is about fifteen per cent less than that given out to boiling watefj 



