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T H E J b U R N A L OF I N D U X T R I A L A N D ENGINEERING C H E M I S T R Y
Vol. 14, No. 9
Heat Transmission and Evaporation By C H E M I C A L ENGINEERING DEPARTMENT,
T
W. L. Badger U N I V E R S I T Y OF
MICHIGAN, ANN ARBOR,MICH.
HERE is Scarcely a process with which the chemical rem is being made in this field, he must first admit that so engineer is concerned that does not involve the trans- far as general engineering practice is concerned, the last few years have seen little change. Rising coal prices have fer of heat in some form. In a great variety of &cumstances he must pass heat through a separating wall with wme- thrown the weight of opinion toward triple or quadruple times liquids, sometimes vapors, and sometimes permanent effect evaporators, where before the war the double effect was most common. Mechanical engineers cases on either side. But if he wishes to have made progress in the design of suriesign such apparatus intelligently and face condensers and their auxiliaries. A accurately he finds the literature practifew new ideas have been advanced, nocally devoid of information that might tably that of recompression of vapors, but help him. this is not yet commercially feasible in this This has come about for several reasons. country. At the University of Michigan The fundamental principles of heat transwe are trying to make some headway on a fer (such as the fact that it is proportional study of evaporator theory. The Massato temperature drop) have long been chusetts Institute of Technology has pubfamiliar; but though more than a generalished some beginnings of a study of real tion ago the variables which affect heat film coefficients. Some of the larger cortransfer were thoroughly discussed, only a porations are making tests and developing couple years ago an engineer stated in new ideas, but this work is rarely made the proceedings of an important society available by publication. Perhaps the that since copper had seven times the most promising feature is the increasing thermal conductivity of iron, an evaporanumber of engineers who are really thinktor with copper tubes should do seven ing accurately about heat transmission times the work of one with iron tubes. problems, and who are coming to realize Actually, while a plate of copper’0.l in. the need for work in this field. thick and 1 ft. sq., with a difference of 1’ Our work at the University of Michigan F. between the two sides, will transmit is only beginning and is as yet largely about 25,000 B. t. u. per hr., a layer of water 0.01 in. thick b n each side will W. I ,. BADGBR empirical. The writer has literally dozens of problems in connection with standard reduce this to about 200 B. t. u. The resistance of the copper will then be only about 1,450 of evaporator design which have not yet been touched; and if the total, and the substitution of iron for copper would have one also considers the possibility in the way of new types, practically no effect. This film concept as applied to heat the field is correspondingly broadened. For instance, the transfer has been more or less recognized for about 30 yrs., average evaporator operates with a heat transfer coefficient and other factors, such as the effect of viscosity, circulation, of somewhere from 100 to 400 B. t. u. per sq. ft. per hr. per O F . elevation of boiling point, hydrostatic head, etc., were rec- Feedwater heaters may, under special conditions, reach 1000 ognized even further back; but the number of engineers to 1100 B. t. u. The writer has seen a special evaporator who fully appreciate the significance of these factors is dis- in operation for which 1500 t o 2000 B. t. u. are claimed. This was accomplished by accepting the film concept and appointingly small. A series of heaters were designed by a man well known in attempting to scrape off the film. The problems now facing us are varied. How can we prehis field of engineering and installed in a plant operated by graduate mechanical engineers. The heaters were not doing dict accurately the temperature distribution in a multiple what they should and the writer was called on to revise the effect evaporator? What are the exact effects of such facequipment, It was found that each heater had three hun- tors as viscosity, circulation, hydrostatic head, etc., on the dred and sixty 21/2-in. tubes, all in one pass. The amount film coefficient between boiling liquids and metal surfaces? of material being circulated was such that its average velocity What is the effect of superheat in steam on heat transmission? was about 0.1 ft. per sec. Inquiry developed that neither Why do salt crystals grow on the heating surfaces instead the designer nor the operating engineers had ever thought of in the solution? If we turn to artificial circulation, where shall we find increased heat transmission offset by cost of figuring liquid velocity. While we may expect to see in the near future a certain of stirring? What is the field for re-use of vapors by recomamount of information in fairly satisfactory shape on heat pression? Where are we to get the physicochemical data transfer when heating liquids or gases, we will have to wait (such as elevation of boiling point, solubility, specific grava long time for information on evaporation. When a liquid ity, specific heat, latent heat of evaporation, etc.) for the is actually boiling, conditions are so complex that as yet they strong solutions which we use in practice and which the have defied mathematical analysis. The evaporator, whether physical chemist has not considered because they fail to single or multiple effect, is one of the chemical engineer’s follow his laws for dilute solutions? Several of these probmost important tools, yet we really know very little about it. lems we have already attacked, and on one or two we have Hausbrand’s classic analysis leaves us just short of being made considerable progress. There is plenty of room, able to really design an evaporatbr, and constants are however, for more workers. Besides the vast amount of experimental work yet to be almost wholly lacking. Even the standard types still need done, there is much ahead of us in educating engineers to much study. If one attempts to list the particular points in which prog- think in terms of the heat balances of a plant as a whole.
