EVAPORATION 



345 



A system of circulation found in some designs is that known as progressive 

 evaporation, in which the Hquid is constrained to travel in a definite path. 

 This system is first found in British patent 3965, 18 16, taken out by Dihl 

 for an unnamed inventor. It appears in Chapman's patents (1752 and 2511 

 of 1888) and in Foster's (13284 of 1890), where the flow is defined by vertical 

 partitions alternately above the upper and below the lower tube plate of a 

 vertical submerged tube apparatus. 



Mechanical circulation as obtained by a screw propeller located below 

 the lower tube plate is found in Fletcher's patent (14164 of 1886), but this 

 means does not seem to have come into common use. 



Distribution of the hquid over the heating surface in thin layers as a spray, 

 or by other means, forms what is known as film evaporation. It appears 

 first in Cleland's patent (4696, 1822), and is also shown in RilUeux's first 



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w)ni/f/, 



wiiniik 



Fig. 190 



patent (U.S. 3237, 1843). Since then the principle appears in many designs, 

 particularly in those of Yarj-an, LiUie, and Kestner (q.v.), and more recently 

 in that of Meyer and Arbuckle (4212, 7078, 19962, of 1903), who employ a 

 perforated pipe rotating in a horizontal plane above the upper tube plate 

 of a vertical submerged tube apparatus. 



Circulation by a locahzed use of high pressure steam is found in Heck- 

 man's circulators, which consist of a supplementary calandria, as shown 

 in Fig. 190. In Rohrig and Koenig's design, the live steam is used in a 

 small annular tubular cluster arranged round the vertical axis of a cell. 



Incondensihle Gases. — In the process of evaporation a certain amount 

 of incondensihle gas is formed ; some air enters \vith the juice, and some 

 leaks into the apparatus. This accumulation of gas both retards the rate 

 of boiling and causes corrosion of the tubes, so that it is necessar}' to remove 

 it as fast as possible. In the first calandria there is generally a pressure, 

 so that the gas can be vented to the air ; as the heating steam contains but 

 little air, generally all that is necessary is to expel the air present in the 

 calandria when commencing work. In the second and subsequent calandrias 

 communication is made by means of pipes from the interior of the calandria 



