312 CHAPTER XVIII 



RATE OF EVAPORATION AS DETERMINED BY TEMPERATURE DIFFERENCE. 



When, however, the temperature differences are very great, and are 

 located in different positions in the thermometric scale, a different law 

 obtains. Rankine 1 assumed the difference was in proportion to the square 

 of the temperature difference, as was later indicated by the experiments 

 of Blechynden. 2 



The passage of heat through a partition takes place in three stages : 

 i. The passage from the hot fluid to the partition. 2. The passage through 

 the partition. 3. The passage from the partition to the cold fluid. 



Peclet's classical equation 3 representing these conditions is : Let a, b, c 

 be the coefficients of heat transfer at entry, through the partition, and at 

 exit ; then if k be the quantity of heat transferred in unit time, through unit 



area with unit temperature difference, -r = -- f~T"H -- or ^ = - 



Now suppose b is very large compared with a and c ; then it follows that 

 will be very small, and the heat transferred will depend on the resistances 



at entry and at exit or to and . 



a c 



For evaporators this subject has been studied by Holborn and Ditten- 

 berger 4 and by Austin 5 ; using their results, Aulard 6 finds the following 

 values for a, b, c in beet sugar juices in multiple effects : 



Conductance at entry = a 



,, through partition 



at exit = c 



Cell I Cell II Cell III Cell IV 



0-133 0*125 o-in 0-067 



i i i i 



O -222 O -2OO O -069 O -042 



The above values for b refer to brass tubes : for the fourth cell the relative 



