McADAMS, WOODS, BRYAN—VAPORIZATION IN SID E HORIZONTAL TUBES 551 the tube, “suggesting that the film became thinner near the top and. . .disappeared entirely, leaving part of the tube surface dry.” It is important to note that, although the last part of the tube surface may be badly vapor-bound, the average heat-transfer coefficient for the entire boiling section may still be quite high. Stroebe’s coefficients at low vaporization run higher than those obtained in the horizontal tubes. While this may in part be due to the comparison of film versus over-all coefficients, it is more probably due to the difference in vertical and horizontal tubes. When only small percentages of the feed had been vaporized in the horizontal-tube apparatus, observation of the glass return bends showed that the liquid phase was carried as a separate goes through a maximum and decreases sharply; in the latter range the effect of a return bend is beneficial in increasing the coefficient in the jacket immediately following. 3 W ith excessive temperature difference, equal to or somewhat greater than that required to produce the maximum coefficient, the surface becomes vapor-bound, and an increase in cumulative weight per cent vaporized is not beneficial and may be detrimental, Fig. 7. 4 In the boiling section, the fluid stream, immediately following the return bend, is at substantially the saturation temperature corresponding to the pressure. 5 In the superheating section, the coefficients are of the same order as those predicted from the accepted equation for heating gases. 6 In the preheating section the coefficients are considerably higher than those predicted for warming liquids; this may be due to local boiling followed by condensation in the bulk of the stream of liquid, and to the surging flow inside the tube. * FEED VAPORIZED F i o . 1 2 D a t a T a k e n a t t h e U n i v e r s i t y o f M i c h i g a n (5 ) W h e n B o i l i n g W a t e r a t F l u i d D i s c h a r g e P r e s s u r e O t h e r T h a n A t m o s p h e r i c (Vapor-binding at high cum ulative vaporization causes decrease in average coefficient at high total vaporization.) layer, moving at low velocity and filling only a fraction of the horizontal tube. Occasional surges resulted in an intimate mixture of liquid and vapor which completely filled the tube. This intimate mixture prevailed steadily at higher vaporization, eventually giving way to a spray of liquid drops suspended in a stream of vapor. This phenomenon is illustrated in Figs. 10 and 11. In the range A B of Fig. 9, it is quite possible that the beneficial effect of increasing the per cent vaporized in horizontal tubes is largely due to an increase in the fraction of the wall wetted, as an intimate mixture of liquid and vapor replaces the liquid layer which in earlier jackets filled only a fraction of the tube. The data of Stroebe (5) at pressures other than atmospheric, Fig. 12, also suggest the existence of vapor-binding at high cumulative vaporization, for no coefficient greater than 1500 was obtained when the total vaporization in the tube was greater than 70 per cent. Tube diameter apparently is not important when boiling liquids outside of horizontal pipes (9), but it may be very critical when boiling liquids inside of horizontal pipes because of its possible effect on the character of flow. Thus, with a low entering velocity, in a 4-in-diam horizontal pipe, stratification of liquid and vapor might persist to far greater cumulative vaporization, whereas, in V2-in-diam horizontal pipe, appreciable stratification might never occur. C o n c l u s i o n s For pure liquids entering at low velocities and boiling inside horizontal tubes the following conclusions are drawn: 1 Vapor-binding, accompanied by a decrease in the local overall coefficient of heat transfer, can be caused by excessive cumulative per cent vaporization when using moderate temperature difference (Figs. 2, 5, and 8) or by excessive temperature difference at moderate per cent vaporization, Fig. 4. 2 W ith moderate temperature difference, as the cumulative per cent vaporization increases, the coefficient at first rises, then A c k n o w l e d g m e n t The authors acknowledge their indebtedness to Prof. H. E. Edgerton for use of the photographs shown in Figs. 10 and 11, and to Mr. L. C . Heroman for his participation in runs B-1A to B-7A. BIBLIOGRAPHY 1 “Temperature Drops and Liquid-Film Heat-Transfer Coefficients in Vertical Tube,” by R. M. Boarts, W. L. Badger, and S. J. Meisenburg, Trans. American Institute of Chemical Engineers, vol. 33, 1937, pp. 363-389; same title, Industrial and Engineering Chemistry, vol. 29, 1937, pp. 912-918. 2 “Heat-Transfer Coefficients in the Boiling Section of a Long- Tube Natural-Circulation Evaporator,” by C. H. Brooks and W. L. Badger, Trans. American Institute of Chemical Engineers, vol. 33, 1937, pp. 392-413; same title, Industrial and Engineering Chemistry, vol. 29, 1937, pp. 918-923. 3 “Liquid Velocity and Coefficients of Heat Transfer in a Natural- Circulation Evaporator,” by A. S. Foust, E. M. Baker, and W. L. Badger, Trans. American Institute of Chemical Engineers, vol. 35, 1939, pp. 45-71; same title, Industrial and Engineering Chemistry, vol. 31, 1939, pp. 206-214. 4 “Film-Heat-Transfer Coefficients for SOs in a Vertical Evaporator,” by F. C. Stewart and F. G. Hechler, Refrigeration Engineering, vol. 31, Feb., 1936, pp. 107-111; same title, Ice and Cold Storage, vol. 39, 1936, pp. 126-127; same title, Ice and Refrigeration, vol. 90, Jan., 1936, p. 3. 5 “Boiling-Film Heat-Transfer Coefficients in a Long-Tube Vertical Evaporator,” by G. W. Stroebe, E. M. Baker, and W. L. Badger, Trans. American Institute of Chemical Engineers, vol. 35, 1939, pp. 17-41; same title, Industrial and Engineering Chemistry, vol. 31, 1939, pp. 200-206. 6 “Heat Transfer for Boiling Inside Horizontal Tubes,” by W. K. Woods, D.Sc. thesis, chemical engineering, Massachusetts Institute of Technology, Cambridge, Mass., 1940. 7 “Boiling: Heat Transfer in Natural Convection Evaporator,” by G. A. Akin and W. H. McAdams, Trans. American Institute of Chemical Engineers, vol. 35, 1939, pp. 137-155; same title, Industrial and Engineering Chemistry, vol. 31, 1939, pp. 487-491; “Heat Transfer to Boiling Liquids,” by E. T. Sauer, H. B. H. Cooper, G. A. Akin, and W. H. McAdams, Mechanical Engineering, vol. 60, 1938, pp. 669- 675. 8 “Principles of Chemical Engineering,” by W. H. Walker, W. K. Lewis, W. H. McAdams, and E. R. Gilliland, third edition, McGraw- Hill Book Company, Inc., New York, N. Y., 1937, pp. 111—113. 9 “Heat Transfer to Boiling Liquids Outside of Horizontal Tubes,” by G. A. Akin, D.Sc. thesis, chemical engineering, Massachusetts Institute of Technology, Cambridge, Mass., 1940. D iscussion V i c t o r J. S k o g l u n d . 7 The authors analyze pressure-drop data under the assumption that the pressure drop has only two 7 Project Engineer, Pratt & W hitney Aircraft Division, United Aircraft Corp., East Hartford, Conn. Jun. A.S.M .E.
552 TRANSACTIONS OF TH E A.S.M.E. AUGUST, 1941 components: (a) wall friction, and (6) change in kinetic energy. In the flow of media of more than one phase of different densities, the different phases will have different velocities, and therefore, there will be a transfer of momentum between phases. A transfer of momentum between particles, having different velocities, results in a loss of mechanical energy. This loss of energy adds another component to the pressure drop. The equation of motion of a liquid particle evaporating in a pipe is very complex. The equation should include the following terms: 1 force term due to the pressure gradient. 2 A force term due to the relative velocity between liquid and vapor phases. This is the component discussed by the writer. 3 A momentum term due to the changing velocity of the particle. 4 A momentum term due to the changing mass of the particle. A u t h o r s ’ C l o s u r e The analysis of the pressure drops is to appear in a paper to be presented at a subsequent meeting of the Society. The friction losses were calculated from the observed pressure drops in each of the last three passes, allowing for the changes in kinetic energy due to changes in the mass and velocity of both vapor and liquid. It was assumed that the friction loss could be correlated by an equation of the Fanning type, without allowance for the unknown slip between the vapor and liquid. In the range where the cumulative vapor generation exceeded 20 per cent by weight, the friction factors so obtained were intermediate between the usual friction factors for one-phase isothermal flow, corresponding to Reynolds’ numbers for all liquid, and all vapor, respectively. The reasonable values of these apparent friction factors suggest that friction, arising from the transfer of momentum between vapor and liquid phases, is of minor importance under the conditions of the experiments described. In the range where the percentage of feed vaporized was small, and where separation by gravity into two continuous phases sometimes occurred (as shown in Fig. 10 of the paper, at the end of the first pass), the pressure drops were too small to warrant any conclusions.
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