254 ANIMAL PROTEINS 



viscosity of the liquor being evaporated ; the greater the 

 viscosity, the lower the coefficient of heat transmission. 

 Unfortunately for this process of evaporation, gelatine sols 

 are exceedingly viscous, and thus the difficulty in obtain- 

 ing a concentrated sol is thus greatly enhanced. 



The transference of heat is often greatly hindered by 

 incrustations of the tubes, which incrustations generally 

 conduct heat very badly. Thus the relative heat conduc- 

 tivities of copper and chalk are as 1000 : 5. 



The amount of heat transferred is of course determined 

 also by the area of the heating surface. The amount of 

 evaporation needed thus determines the number of tubes 

 (of standard size) in the evaporator, and thus the capacity 

 of the machine. An evaporator should have its heating 

 surface area chosen with a view to the duty required of it. 



In practice the working of an evaporator is often not a 

 very difficult matter, and large numbers of machines are 

 operated by unskilled labour. Troubles generally arise 

 from inconstant steam pressure, incrustation, leakages of 

 air, which reduce the vacuum, the temperature head, and 

 hinder heat transmission. For the evaporation of gelatine 

 liquors the Yaryan, the Kestner, and the Blair-Campbell 

 film evaporators are the most widely used. The velocity 

 of the liquor through some of these machines is so great 

 that occasionally no vacuum is used. The temperature 

 obtained is high (200 F.), but the time is very short, if 

 rapid cooling of the evaporated liquor is arranged. 



REFERENCES. 



"Evaporating, Condensing and Cooling Apparatus,'" by E. Haus- 

 brand. Scott, Greenwood & Son (1916 Ed.). 



" Evaporation," by E. Kappeschaar. Norman Rodger (1914). 



" Evaporation in the Chemical Industry," by J. A. Reavell, M.I.Mech.E., 

 /. S.C.I., 1918, April nth. 



" Glue and Glue Testing," S. Rideal, D.Sc., pp. 56-59. 



" Gelatine, Glue, and their Allied Products," T. Lambert, pp. 26-29. 



" Notes on Condensing Plant," J. M. Newton, B.Sc., /. Junior Inst. 

 Engineers, Aug., 1912. 



