COMPARISON OF PROCESSES 65 



der an air pump where a vacuum is maintained, boils near 

 its freezing point. The heat required for it is taken by the 

 water from the surroundings namely, the air pump. But 

 to do this requires almost a perfect vacuum which we can 

 not produce in practice, but we can readily boil water in a 

 vacuum pan at 135 Fahr. If the steam employed for this 

 purpose is 230 Fahr. then the difference between 1 35 ° Fahr. 

 at which the water boils and the temperature of the steam 

 supplied is 95°and we can produce a very rapid evaporation 

 provided we have sufficient heating surface at our disposal. 

 Hence steam at any temperature will cause the evaporation 

 of a liquid, of which the boiling point is below the tempera- 

 ture of the steam. 



From the description given of the methods employed 

 in the manufacture of salt, it is evident that a perfect com- 

 parison in regard to the practical results is a most difficult 

 problem, since the quality of the brine and the fuel will 

 often vary from week to week in the same work. Taking 

 therefore the average results for a season or a year we find 

 that with proper attention, good condition of the works and 

 fuel, a kettle-block can produce 45 bushels (56 pounds each) 

 of salt with a brine of 67 Salometer per ton of anthracite 

 dust or, one pound of fuel evaporates 5.83 pounds of water, 

 while a pan under the same conditions but with a brine of 

 96 Salometer yields 73 bushels of salt, hence a pound of 

 fuel evaporates 6 pounds of water. The results with the 

 upper (direct) grainer together with that of the lower or 

 dividend grainer using a brine of 96 Salometer is 70 bush- 

 els, corresponding to an evaporation of 5.74 pounds of 

 water per pound of fuel. These results are very low indeed 

 as compared to those obtained in good steam boilers, but it 

 must be taken into consideration that the conditions in both 

 cases are very different. The expense for labor is greatest 

 in the kettle method, next comes the pan and finally the 

 grainer method. The wear and tear is very heavy in all salt 

 works but especially in kettle and pan block. Allowing a 

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