324 CHAPTER XVIII 



1,280 sq. ft. bare ; water discharged separately from each cell ; gases as 

 in I ; Cj 235° F. ; VS^ 140° F. ; water evaporated /steam condensed 3-83 ; 

 efficiency 92-6 per cent. 



4. Vertical submerged tube, 12,521 sq. ft. h.s., evaporating at rate of 

 5-6 lbs. per sq. ft.-hr. ; 2,010 sq. ft. area protected with 2 ins. asbestos, 

 or wood and | inch air space ; 240 sq. ft. bare ; water and gases as in 3 ; 

 Ci 217° F. ; VS^ 146° F. ; water evaporated /steam condensed, 4-07 ; 

 efficiency 95-3 per cent. 



5. Vertical submerged tube, 14,146 sq. ft. h.s., evaporating at rate of 

 9-10 lbs. per sq. ft.-hr. ; 2,460 sq. ft. protected with 2 ins. asbestos, 290 sq. 

 ft. bare ; water and gases as in 3 ; C^ 229° F., VS^ 125° F. ; water evapor- 

 ated/steam condensed, 4-31 ; efficiency 99 9 per cent. 



Another series of tests made by Kerr^° gave efficiencies varying from 

 85-1 to 100 • 4, and are in good agreement with those quoted above. 



The Apparent and the Effective Fall in Temperature. — If C^ be the tem- 

 perature of the entering steam and J^ be the temperature of the juice boiling 

 in the last cell of a quadruple, the total fall in temperature over which heat 

 is transmitted is C^ — J^.* The effective fall in temperature is C^ — J^ -\- C^ 

 — J2 + etc. ; which is always less than C^ — J^. 



There are three factors which influence the effective fall. 



1. In the passage of steam from one cell to the next and in the passage 

 through the calandria some loss of temperature must occur ; the magnitude 

 of this loss will depend on the size of the connection pipes, on their insulation, 

 and on the pitch and arrangement of the tubes in the calandria. In an 

 apparatus in which the vapour pipes were covered with 2 ins. asbestos, 

 and were of diameter 23 ins., 27 ins. and 31 ins. in a quadruple effect of 14,146 

 sq. ft. h.s., the writer found the following losses : VS^ to Cg, 0-2° F. ; VS2 

 to C3, 0-7° F. ; VSs to C4, o-8° F. ; or in all, 1-7° F. The loss from VS^ 

 to the condenser varied from i ° F. to 3 ° F. in different apparatiis, depending 

 on the velocity of the gases in and the length of the vapour pipe. The total 

 loss in the calandrias has been estimated b}' Claassen^^ as of the order 0-5° F. 

 as in the second, third, and fourth vessels, and is hence very small. 



2. As the material under evaporation is not water but a solution, the 

 boiling point is raised above the nonnal boiling point of water corresponding 

 to the various pressures, but the vapour given off will be, or tends to be, 

 at the temperature corresponding to the vapour pressure ; actually, however, 

 the vapours are found to be superheated. 



3. The formation of steam at the bottom of the colrmin of boiling juice 

 takes place under the pressure due to the vapour plus that due to the weight 

 of the column of liquid ; this is usually called the hydrostatic head. For 

 example, suppose in the fourth body the pressure is 2-42 lbs. per sq. in. ; 

 the height of the column of liquid is 4 feet, and its density 1-25 ; then 

 the weight of a column of i sq. in. section is 48 X 1-25 X o • 036 =2-16 lbs., 

 and the total pressure is 4-58 lbs. per sq. in. ; under a pressure of 2-42 lbs. 

 per sq. in., the boiling point of water is 132° F., and at 4-58 lbs. per sq. in. 

 it is 159° F., so that the temperature difference appears to be 27° F. less at 

 the bottom than at the top. In the annexed table are given the results of 

 other calculations, whence it will appear that the effect of the hydrostatic 

 head increases as the pressure decreases. 



* Some writers take C, — VS^ as the total fall in temperature, and further the temperature in the condenser 

 might be substituted for J4. 



