EVAPORATION 339 



Taking the latent heat of water as 1,000 B.T.U., the evaporation in terms 

 of water evaporated per sq. ft. per hour will be 



Kr (C, - C.J ^ K, [C, - C3) ^ ^^^ _ 

 1,000 1,000 



An evaporation equal to that indicated above is very generall}' accepted 

 as indicative of actual working conditions. The question is, however, 

 much more complicated. The experiments quoted in the earlier portion 

 of this chapter show that the capacit}' or rate of evaporation is controlled 

 by the temperature difference and by the position of the temperature differ- 

 ence in the thermometric scale, and accordingly, when the capacity of an 

 evaporator is spoken of, the conditions under which it operates should be 

 specified. A second most important factor is the cleanliness of the tubes. 

 This factor is controlled by the facility with which the juices form scale, 

 a point often be3-ond the control of the operator, and bj'^ the care and 

 attention given to cleaning. An evaporator worked continuously for several 

 weeks \\-ill be found to fall in capacit}-, and its efficienc\' can only be main- 

 tained by periodic stops for cleaning either by boiling wdth soda and acid 

 or by brushing, or by a combination of the methods. The third factor 

 controlling the capacity lies in the design, included herein such points as 

 juice circulation, elimination of incondensible gases, and of condensed 

 water, steam circulation, height and diameter of tubes. 



On actual tests on well-designed quadruple vertical submerged tube 

 apparatus operated \nih. steam at 5 lbs. gauge pressure, and \\ith 26 ins. 

 vacuum in the last bod}', the \\Titer has found at the end of a week's continu- 

 ous operation a rate of evaporation of 9 lbs. per sq. ft. per hour, when the 

 juice was admitted at the temperature prevailing in the first body. This 

 figure is much higher than is usually accepted in design, but it has been, 

 and can be, obtained in apparatus which are regularly cleaned everj?- week, 

 and with juices which show no abnormal tendency to scale. If, however, 

 the steam pressure be allowed to fall below the stated figure, or if the vacuum 

 be not maintained, so high a duty is not obtained. The data recorded 

 earlier in this chapter indicate the extent to which the rate of evaporation 

 will fall off with variation from these standards. 



As regards the horizontal submerged tube apparatus, the \vriter has only 

 been able to experiment with one, and under such conditions that the 

 cleanliness of the tubes was not under control. The results found were lower 

 than those quoted above, but on the other hand Kerr has found, referred 

 to the same pressure and vacuum as selected by the writer as a standard, 

 a maximimi evaporation of 9-55 lbs. per sq. ft. per hour. 



The third common type of evaporator, the horizontal film, has a distinctly 

 higher rate of evaporation when it operates satisfactorily ; that is to say, 

 when the distributing system is not choked by scale, or when some minor 

 accident or defect does not develop in the pumps. Kerr found a maximum 

 value of 16-45 lbs. per sq. ft. per hour at 5 lbs. gauge pressure, and 26 ins. 

 of vacuum ; the writer obtained a value of rather over 9 lbs., with steam at 

 less than i lb. gauge pressure and 27 ins. of vacuum. These results can be 

 shown to be in reasonable agreement following on the observations given 



