EVAPORATION 



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that a surface of 10 square feet will evaporate fully one pound of water per minute, or 

 725 x 10 = 7250 gr., the same as over a naked fire ; consequently the condensing sur- 

 face must be equally extensive. Suppose that the vessel is to receive of water 2,500 Ibs. 

 which corresponds to a boiler 5 feet long, 4 broad, and 2 deep, being 40 cubic feet by 

 measure, and let there be laid over the bottom of this vessel 8 connected tubes, each 

 4 inches in diameter and 5 feet long, possessing therefore a surface of 4'8 feet square. 

 If charged with steam, they will cause the evaporation of half a pound of water per 

 minute. The boiler to supply the steam for this purpose must expose a surface of 4'8 

 square feet to the fire. It has been proved experimentally that 10 square feet surface 

 of thin copper can condense 3 Ibs. of steam per minute, with a difference of tempera- 

 ture of 90 degrees Fahr. In the above example, 10 square feet evaporate 1 Ib. of 

 water per minute; the temperature of the evaporating fluid being 212 F., conse- 

 quently 3 : 1 : : 90 : ^. During this evaporation the difference of the temperature is 

 therefore = 30. Consequently the heat of the steam placed in connection with the 

 interior of the boiler, to produce the calculated evaporation, should be, 212 + 30 = 

 242, corresponding to an elastic force of 53 '6 inches of mercury. Were the tem- 

 perature of the steam only 224, the same boiler in the same time would produce a 

 diminished quantity of steam, in the proportion of 12 to 30 ; or to produce the same 



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quantity the boiler or tubular surface should be enlarged in the proportion of 30 to 

 12. In general, however, steam boilers employed for this mode of evaporation are of 

 such capacity as to give an unfailing supply of steam. 



We shall now illustrate, by some peculiar forms of apparatus, different systems of 

 evaporation. Fig. 838 explains the principles of evaporating in vacuo. A B repre- 

 sents a pan or kettle charged with the liquor to be evaporated. The somewhat wide 

 orifice c, secured with a screw-plug, serves to admit the hand for the purpose of 

 cleaning it thoroughly out when the operation is finished ; h is the pipe of communi- 

 cation with the steam boiler ; b is a tube prolonged and then bent down with its end 

 plunged into the liquor to be evaporated, contained in the charging back (not shown 

 in the figure). H is a glass, tube communicating with the vacuum pan at the top and 

 bottom, to show by the height of the column the quantity of liquid within. The 

 eduction evaporating pipe c is provided with a stop-cock to cut off the communication 

 when required, i is a tube for the discharge of the air and the water from the steam- 

 case or jacket ; the refrigerator E- is best formed of thin copper tubes about 1 inch in 

 diameter, arranged zigzag or spirally like the worm of a still in a cylinder. The 

 small air-tight condenser F, connected with the efflux-pipe / of the refrigerator, is 

 furnished below with a discharge-cock g, and surrounded by a cooling case, for the 

 collection of the water condensed by the refrigerator. In its upper part there is a 

 tube Je t also furnished with a cock, which communicates with the steam boiler, and 

 through which the pan A B is heated. 



