314 CHAPTER XVIII 



Accordingly, the diameter of the vapour pipes should increase from cell to 

 cell. In order to effect standardization and to reduce initial cost, some appar- 

 atus are found with equal-sized vapour pipes. If a uniform velocity of the 

 vapour is decided on, the areas will depend on what are the conditions 

 assumed as obtaining in the evaporator. Based on an evaporation of 7 lbs. 

 per sq. ft.-hour and velocities of 50 ft. and 150 ft. to the condenser, the 

 vapour pipes of a quadruple should be about : 1-2, 0-21 a* ; 2-3, 0-24 a* ; 

 3-4, 0-30 h^ ; 4-condenser, • 39 ai, where a is the heating surface in the 

 whole apparatus. 



Size of Tubes. — When evaporators of the vertical submerged tube type 

 were first built European makers adopted a 5 cm. tube, and British makers 

 followed with a 2-inch one. The length of tube was at first exaggerated, 

 and early examples may be found with a tube length of 8 feet. At the present 

 time 5 feet or thereabouts seems to be standard practice. At the same 

 time there is a tendency to decrease the diameter of the tube, European 

 makers often adopting a two or two and one-half centimetre tube, but British 

 and American firms seldom use one less than one and three-quarter inch. 

 The adoption of a smaller tube increases the heating surface, which can be 

 arranged in vessels of equal diameter, and may possibly increase the trans- 

 mission of heat since the mean distance of the juice from the partition 

 decreases. The increase in heating surface is indicated in the annexed table, 

 and it may be remembered that the volume of juice contained in vessels 

 of equal diameter is independent of the diameter of the tubes, provided the 

 pitch vary with the diameter. 



Heating Surface and Tube Diameter, 



No. of tubes 



Diameter, Pitch, per sq.ft. 



inches. inches. of tube plate. 



2 -oo 2 -50 27 



I -75 2 -25 33 



I -50 2 -oo 42 



I -25 I -75 55 



I -oo I -50 74 



Juice Distribution and Circulation. — In very many apparatus the juice 

 is introduced above the upper tube plate, the pipe ending flush with the side 

 of the shell. This method is, the writer believes, quite wrong. Conversely, 

 the juice should enter at the bottom and rise through the tubes ; this intro- 

 duction may be made by means of a perforated pipe or at a number of places 

 symmetrically located with reference to the axis of a cell. The former 

 method is specifically claimed in Smith's U.S. patent (881351, 1908). 



In order to obtain a circulation, apparatus are constructed with a large 

 tube or down- take, the route of the juice being up through the tubes and down 

 through the down-take. This tube is usually located centrally, but in some 

 designs is placed eccentricalty, and in others takes the form of a segment cut 

 out of the calandria. In other designs the circulating tube takes the form 

 of an annular space between the calandria and the shell, this type being 

 referred to as a drum calandria, as indicated in Fig. 189. 



Circulation may also be assisted by shaping or bellying the saucer, as 

 seen in Fletcher's patent (13857 of 1894), or by incUning the calandria, as 

 found in McNeil's patent (8763 of 1900). 



