EVAPORATION 369 



Temperature of Cubic feet 



the discharge of cooling Cu. ft. of Cu. ft. of 



water, F°. water. air. air and water. 



80 0-8192 0'ii29 o '9321 



90 0-5888 0-0825 0'67i3 



100 0-4112 0-0703 o 4815 



no 0-2996 0-0683 0-3649 



120 0-2736 0-0747 0-3483 



130 0-2554 0-1155 0-3709 



140 0-2049 1-7712 1-9761 



140-5 (X (X 



On examining these figures it will be seen that the volume of the air at 

 first decreases as the quantity of water decreases, reaches a minimum, and 

 then rapidly increases. Hence, if the air present in a condenser is proportional 

 to the amount of cooling water admitted, there is a definite temperature in the 

 ivaste-water at which the volume of the air and water is least. This temperature 

 in the waste-water is then the optimum for the particular condition, and the 

 admission of more water beyond this quantity instead of affording a better 

 vacuum has the reverse effect. Experimental data to calculate in advance 

 this condition are wanting : it exists, however, and can probably be found 

 by trial and error for each apparatus. 



If a series of calculations be made for different vacua, to obtain the 

 optimum temperature of discharge, under the supposition that the gases 

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

 the water increases in temperature so does the quantity required. 



The calciilations lead to the following very rough approximation : — 



With initial temperatures of 60, 70, 80, 90° F, the water admitted should 

 be 10, 25, 35 and 50 times the amount of steam to be condensed. 



If further calculation on the above lines be made, it will be seen that for 

 vacua of 24, 25, 26, 27 inches the volume of the air to be removed is roughly 

 as 6, 9, 15, 25 : that is to say, to maintain a 27-inch vacuum requires a pump 



25 



-~ times as large as for a 24-inch vacuum. If, however, a quantity of air, 



25 -f- X 

 X, enters which is independent of the water, the ratio will be ' — , and 



as X is positive the rate of pump capacity will not increase so fast. 



As regards relative pump capacity in wet-air pumps and dry-air pumps, 

 some idea may be obtained from calculations made on the above lines. 

 Under the same conditions it will be found that the volume of air from 

 the dry system is usually only one-third or thereabouts that from the wet 

 system, a condition which gives some idea of the relative pump capacity 

 as cu. ft. developed per sq. ft. of heating surface, etc. As dry-air pumps can 

 work at much higher speeds than can wet-air pumps, the actual size of the 

 dry-air pump decreases still more in comparison. 



Empirical rules are very dangerous tools unless the basis upon which 

 they are developed is known and appreciated. This is particularly true 

 of vacuum pumps, into the necessary capacity of which so many factors 

 enter. A collection of data of very many installations leads to the following 

 very rough rules referred to 6xy vacuum pumps : — 



2C 



