EVAPORATION 249 



These disadvantages were still fatal to the production of 

 the highest-grade gelatine. There were also the practical 

 difficulties of entrainment ("blowing over"), in which 

 parts of the sol were carried away by the escaping vapour, 

 and also of " incrustation " which so rapidly reduces the 

 heating efficiency and evaporative capacity of the machine. 

 The vacuum pan, however, presented two decided advantages 

 evaporation at a low temperature, and, as a corollary, 

 the possibility of utilizing exhaust steam to attain this 

 temperature. 



Whilst the vacuum pan was a satisfactory machine for 

 many branches of chemical engineering, the problem of 

 evaporation was still unsolved for gelatine liquor because 

 of the " stewing " involved, until the advent of the " film 

 evaporator," which dealt with the liquor not in bulk, but 

 in a continuous stream. In this way the product was only 

 exposed to heat for a comparatively short time. Many 

 evaporators of this type came into being, and rapid improve- 

 ment was made in the constructional details. The film 

 evaporators retained usually the advantage of evaporation 

 in racuo, so that it was now possible to evaporate gelatine 

 sols by exposure for a short time to a comparatively low 

 temperature. Of this type of evaporator, the Lillie, Yaryan, 

 Schwager, Claassen, Greiner, Blair Campbell, and the 

 Kestner machines are well-known examples. 



A further advance in solving this problem was the 

 application of the principle of multiple -effect evaporation. 

 The vapour driven off during evaporation possesses of 

 course many heat units, and is of very considerable volume. 

 In multiple-effect evaporators this vapour is used to work 

 a similar evaporator, and the evaporated liquor passes im- 

 mediately into what is practically a second machine, and is 

 further evaporated by the heat from the vapour just driven 

 from it. Such an arrangement would be termed a double- 

 effect evaporator. The vapour from the second effect 

 may of course be similarly used to operate a third effect, 

 and the vapour from this to work a fourth effect, and so on. 

 Thus, we may have triple effect, quadruple effect, etc., even 



