CANE SUGAK. 



If a series of calculations be made for different vacua in order to obtain 

 the optimum temperature of discharge on the supposition that the gases intro- 

 duced are proportional to the amount of water, it will be found that as the water 

 increases in temperature so does the quantity required. The calculation leads 

 to the following very rough approximation : With initial temperatures of 60, 

 70, 80, 90 F., the water admitted should be about 10, 25, 35, and 50 times 

 the amount of steam to be condensed. 



If calculations on the principles assumed above be made, it will be found 

 for vacua of 24, 25, 26, 27 inches that the volume of gases is very roughly 

 as 6, 9, 15, and 25; i.e., on this supposition a pump to produce a 27-inch 



vacuum must be -~- times as large as one to produce a 24 inch vacuum. If, 



however, a quantity of air, x, enters the condenser which is independent of the 

 quantity of water admitted, then the sizes of the pump capacity will be as 

 25 + x : 6 -f x, and as x is positive the ratio of pump capacity will be less 

 than calculated above ; if, however, x is small, as should be the case in a well- 

 built evaporator, a ratio similar to that calculated will be found. 



The dry air pump has only to remove the air in the water passing down 

 the barometric column. Since the air is last of all in contact with freshly 

 entering cooling water, it will approximate in temperature to the cooling water. 

 Also since a counter current system of cooling is used, less quantities of water 

 are required. If a series of calculations be made on the lines developed above, 

 comparing the volume of air for the most favourable conditions with wet and 

 dry condensation, it will be found that on an average the volume of air with 

 dry condensation is one-third that obtained with wet condensation. This 

 comparison shows how small need be a dry vacuum pump compared with a wet 

 vacuum pump ; in addition the efficiency and speed of an air pump is much 

 greater than that of a water and air pump. 



Empirical rules are very dangerous tools, and when the very variant 

 conditions described above are remembered, it will bo at once seen how foolish 

 it is to attempt to give any hard and fast rule connecting size of pump and 

 capacity of evaporator, since this will vary 



(a) With temperature of cooling water. 



(i) With the efficiency of the apparatus as regards air leaks. 



(c) With the amount of air in the heating steam which finds its way 



eventually to the condenser. 



(d) With the vacuum considered desirable. 



Of the empirical relations the following may be mentioned : 



In marine engineering practice the pump displacement is five times the 

 volume of water to be removed. This would be a very low capacity for sugar 

 house evaporators. 



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