EVAPORATION 



325 



Mean height of 



column ot water 



causing . increase 



in pressure. 



\'acuum in Inches of Mercurv. 



27-5 



This loss in the fall of temperature was first discussed by Jelinek, who also 

 observed that thermometers inserted at different levels in the cells of evapor- 

 ators did not indicate different temperatures. This is due to the rapid move- 

 ment of the boiling liquid, and the equally rapid transfer of heat from the 

 overheated to the cooler particles : it was again in consideration of this 

 effect that the Welner-Jelinek type of evaporator was designed, in which a 

 low level of juice is obtained. The further development of this idea leading 

 to elimination of the effect of hj^drostatic head is seen in the Lillie t3'pe of 

 film evaporator. The decrease in the rate of evaporation due to loss of tem- 

 perature difference in vertical submerged tube apparatus is, however, 

 masked by a vigorous circulation, so much so that the disadvantages of 

 decreasing the length of the tube will offset any increased efficiency to be 

 derived therefrom : a tube 5 ft. long, more or less, which most makers 

 have adopted, seems to be the economic length. 



The Rate of Evaporation as influenced by Change in the Temperature 

 Difference and by Change in the Position of the Temperature Difference in 

 the Absolute Scale of Temperature. — The temperature difference in an evapor- 

 ator may be increased by increasing the pressure of the steam entering the 

 first body (increase of Cj), or by decreasing the pressure under which ebulli- 

 tion occurs in the last body (increase of " vacuum " or decrease of FS4 

 or J^) : if also C^ and VS^ be altered simultaneously, C^ — VS^, which is the 

 temperature difference, ma}^ remain unchanged. 



Some experiments made to connect these changes with the rate of 

 evaporation are detailed below. 



The rate of evaporation may be determined experimentally in three 

 ways : — 



1. Measurement of the s\Tup discharged from the last body, combined 

 with observation of the solids in the incoming juice and outgoing 

 syrup. 



2. Measurement of the juice admitted, combined with the same observa- 

 tions as in I. Either of these methods demands that the contents of the 

 apparatus be the same at the end as at the beginning of an experi- 

 ment. 



3. Measurement of the water discharged from a cell. This will not give 

 the absolute evaporation in the apparatus unless the relative evaporation 

 in each cell is know'n, combined with a knowledge of the ratio between 

 steam condensed and water evaporated ; results as between different 

 experiments will, however, be comparative. The first cell is most amenable 



