METHODS OF EVAPORATION. 33^ 



and this result is obviously due to the comparatively sluggish juice, as 

 compared with the water. 



Surface or Film Evaporation. 



A very large number of evaporators have been devised (some of 

 which appear to have given excellent results in practice), based upon 

 the evaporation of the water from thin films of juice distributed over 

 heated surfaces, or exposed to hot and dry air. Among the many 

 forms o'f apraratus constructed upon this principle, may be mentioned 

 the following : 



1. A continuous sheet of cloth, passing over two sets of rollers, at 

 the top and bottom of a chamber through which, by means of a fan, 

 fresh hot and dry air is made to pass. The lower rolls are placed be- 

 neath the surface of the juice to be evaporated, so that, by the revolu- 

 tion of tlie cloth, a fresh portion of the juice is constantly exposed to 

 the hot air. 



2. A long, hollow cylinder, heated by the waste steam of the sugar 

 house, which passes through it, and which, revolving in the juice, in 

 which it is partly immersed, presents a fresh film constantly to the air. 



3. The substitution of hollow discs upon a common axle, the hollow 

 axle permitting the steam to enter and heat the discs, which are caused 

 to revolve partially immersed in the juice to be evaporated. 



4. A spiral hollow coil, through which steam passes, which also re- 

 volves partially in the juice. 



5. An upright hollow cylinder (15 or 20 inches in diameter, 10 to 12 

 feet long), heated within by steam, over which, from the upper end, the 

 juice is allowed to stream, the distribution of the juice over the surface 

 of the cylinder being eflfected by circular troughs about the cylinder, 

 at intervals, which, by their overflow, secure the distribution of the 

 juice in a film, which otherwise would form streams from the top to 

 the bottom of the cylinder. 



6. Still another form is a skeleton cylinder, consisting of two steam 

 tight circular drums, which are connected together by a series of tubes 

 which enter the periphery of each drum, and through which the steam 

 passes from one drum to the other. This cylinder revolves horizontally, 

 with its lower portion dipping in the juice, and thus presents a large 

 surface for evaporation. 



None of these surface evaporators permit the removal of any impuri- 

 ties by skimming; and they can not, therefore, be used except for the 

 concentration of semi-syrup of 22° to 25° Beaume, since the juice 

 requires more or less skimming during its evaporation, until it has 

 reached that density. 



