EVAPORATION 315 



is lower, and may more than counterbalance the effect of its higher tem- 

 perature. 



Conception of Multiple Effect Evaporation. Heat may be quantitatively 

 exchanged from one body to another, the heat always passing from that 

 body with the higher temperature to that with the lower. If one pound of 

 water at 80 F. be mixed with the same quantity at 60 F. there will result 

 two pounds at 70 F. ; similarly, if one pound of water at 80 F. be contained 

 in a vessel, separated by a partition from a second pound of water at 60 F., 

 eventually 10 B.T.U. will pass from the hotter water to the colder, and there 

 will again result two pounds at 70 F. If steam be conducted into water, 

 the former will condense until the temperature of the water has been raised 

 to the temperature at which water boils under the prevailing pressure, after 

 which nearly equal quantities of steam will enter and pass away. If, how- 

 ever, the steam be not conducted directly into the water, but be directed 

 against the outer wall of the vessel containing the water, it will condense 

 and transfer its latent heat to the water ; and, if the heating steam be at a 

 higher pressure than that prevailing on the surface of the water, the latter 

 will eventually boil. There will then be a system in which the water and the 

 vessel containing it act the part of a surface condenser, as opposed to an 

 injection condenser, where the steam is conducted directly into the water. 

 To give an arithmetical calculation let there be 10 Ibs. of water at 82 F. 

 contained in a vessel open to the atmosphere, and acting as a surface con- 

 denser to a current of steam at 227 F., which condenses, and is by some 

 device or other removed at this temperature. Referring to the table at 

 the end of this chapter, the latent heat of steam at 227 F. is 960-1 B.T.U. ; 

 to raise the 10 Ibs. of water from 82 to 212 requires 130 B.T.U., and hence 

 when (130 X 10) -4- 960-1 Ibs. = 1-354 ^ s - f steam have been condensed, 

 the water will begin to boil. The latent heat of water at 212 F. is 969-7 

 B.T.U., and after boiling has begun each pound of steam condensed will 

 cause the evaporation of 960 I ~ 969 -7=0- 991 Ib. of water as steam at 

 212 F. 



Now let the steam evaporated at atmospheric pressure be collected 

 and conducted to a second surface condenser, in which a pressure of less 

 than one atmosphere is maintained ; exactly the same process is repeated, 

 and the original pound of steam can in this way be conceived as causing 

 the evaporation of an infinite quantity of water. Multiple effect evaporation 

 is, then, a scheme for the alternate condensation and generation of steam 

 under continually decreasing pressure. 



It is to be observed that " vacuum " (or pressures less than atmospheric) 

 has as such nothing to do with the principle, which is applicable over any 

 range of temperature or pressure. The adoption of vacuum multiple effects 

 in the sugar industry is due to the destruction of sugar which occurs when 

 temperatures considerably above 212 F. are reached, and also to the pro- 

 duction in the engines of large quantities of low pressure steam, the multiple 

 utilization of which is possible only under reduced pressure. The method 

 of obtaining the successively decreasing- pressures will be understood by 

 reference to the diagram, Fig. 183, which represents a vertical submerged 

 tube triple effect. Fach body consists of a vertical cylinder divided into two 

 compartments by means of two transverse partitions, which are connected 

 by tubes open at both ends. The transverse partitions are known as 

 tube-plates, and between them and exterior to the tubes connecting them is 



