EVAPORATION 317 



As the steam in each calandria condenses to water it is removed from the 

 calandria by the drain pipes c, and evacuated against the atmospheric pressure 

 b}^ pimips, or other devices described elsewhere. At e is shown a washout 

 pipe. 



In actual working in sugar manufacture the material to be evaporated is 

 nearly always introduced to the first body only, and passes on to the last 

 body with continually increasing concentration, whence it is pumped out 

 against the atmospheric pressure. Similarly the steam is generally intro- 

 duced to the first cell ; there is no reason why the direction of flow of juice 

 or of steam either separately or simultaneously may not be reversed, and this 

 scheme forms a feature of one t3'pe of apparatus referred to elsewhere. It 

 is also employed for special purposes in other industries. 



Coefficient of Transmission. — The coefficient of transmission is that 

 quantit}' of heat which passes through a partition of unit area, in unit time, 

 under unit temperature difference. In British and American engineering 

 practice the units selected are the square foot, the hour and the degree 

 Fahrenheit, the quantity of heat being expressed in British Thermal Units. 

 In European practice the square metre, hour, degree Centigrade and calorie 

 are used, so that the British or American coefficient is 3-96 times as great 

 as the continental European value. 



No difficult}' attaches to expressing the coefficient of transmission in a 

 heater or single effect evaporator, but in a multiple effect it is to be noted 

 that the mean coefficient is not the average of the individual coefficients 

 even when the cells are all of the same area ; actually, if h be the total heat 

 transmitted, t be the total temperature difference, a be the total heating 

 surface, and n be the number of effects, then the value of the coefficient 



n h 



IS t or — r, and if A be the heat transmitted m one cell the coefficient 



a X~ at 

 n 



becomes — - 

 a t 



For example in a triple effect of 1,000 sq. ft. in each cell, with temperature 

 falls of 10° F., 30° F. and 60° F., and transmitting 6,000,000 B.T.U. per hour 

 per cell, the coefficients in the first, second, and third cells are, respectively, 

 600, 200 and 100. The total heat transmitted is 18,000,000 B.T.U. , and, since 

 the transmission occurs in three stages, the mean coefficient for the whole 



18,000,000 



apparatus is 100 = 180. 



3,000 X ~;^ 



Distribution of Heating Surface for Maximum Efficiency.* — In a double 

 effect let k■^^ and k.^ be the coefficients of transmission in the first and second 

 ceUs in which the heating surfaces are «i and a^, and the temperature differ- 

 ences t^ and t^- Let a^ + a^ = i, and also t-y -\- t^ = '^- Then evidently 

 h^ = h^ = ^1 «i ti = ^2 ^2 ^2 where /?j and h.^ are the quantities of heat 

 transmitted. 



* For this demonstration I am indebted to Mr. Louis Wachenberg. 



