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^ 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^ 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 



III, I 



k 



I I I I , 



area with unit temperature difference, -^= l~xH ^^^ = j 





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



—will be verv small, and the heat transferred will depend on the resistances 

 b -^ 



at entry and at exit or to — and - - 



•^ a c 



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

 berger^ and by Austin^ ; using their results, Aulard^ 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-iii 0-067 



I I I I 



O -222 O -200 O -069 O -042 



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



