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V aporization Inside H orizontal Tubes This paper reports an investigation carried out to determ ine the changes in the coefficient of heat transfer for the evaporation of a liquid flowing inside a heated horizontal tube. For this research, a semiworks apparatus was constructed, consisting of 48 ft of standard 1-in. copper pipe, provided with 12 individual steam jackets, steam traps, and condensate lines. In the benzene runs, the velocities ranged from 0.26 to 1 fps at the inlet and 80 to 240 fps at the outlet; in the water runs, the corresponding values were 0.27 to 0.85 and 205 to 540 fps. With moderate temperature differences, as the fluid is progressively vaporized, the local over-all coefficient at first increases, goes through a maxim um , and then decreases sharply toward values typical of superheating dry vapor. Such “vapor-binding” is attributed to insufficient liquid to wet the wall, sm all droplets of liquid being carried down th e center of th e tube, as observed at the entrance to the glass return bend. With high temperature differences, the type of vapor-binding previously observed when boiling liquids outside submerged tubes, where (due to excessive temperature difference) a vapor film insulates the tube wall from the bulk of the liquid, was encountered. By W. H. McADAMS,1 W. K. WOODS,2 a n d R. L. BRYAN3 N om en cla tu r e The following nomenclature is used in the paper: h „ s = average film coefficient for entire boiling section, Btu per hr per sq ft of inside surface, divided by the length-mean temperature difference from inner wall to fluid inside of the tube p = cumulative weight per cent of feed vaporized, based on cumulative heat transferred and feed rate P gage pressure on steam header, psi q/A = local heat flux, Btu per hr transferred in an individual jacket, divided by 0.88 sq ft of inside surface of copper tube U = local over-all coefficient q/A divided by difference (deg F) between saturation temperature of steam and temperature of fluid. In the boiling section, temperature of fluid was taken as saturation temperature U „t = average value of U for boiling section, based on lengthmean temperature difference W = feed rate, lb per hr I n tro d u ctio n Vaporization of liquids inside tubes is of such industrial importance that considerable experimental research has been devoted to measuring heat-transfer coefficients under such conditions. The usual method of reporting the results of such investigations has been to base the heat-transfer coefficient on the “apparent” tem perature difference. For steam-heated apparatus, the apparent over-all temperature difference involves the condensing temperature of the steam and either the outlet temperature of the partially vaporized liquid or an average of the inlet and outlet temperatures. Where tube-wall temperatures have been measured by thermocouples, the length-mean wall temperature may be substituted for the condensing-steam temperature in order to obtain apparent “film temperature differences.” Although apparent over-all or film coefficients are of great value to the designer, who usually knows only the apparent temperature difference, they can safely be used only when design conditions are almost identical with those used in obtaining the data. Thus, the use of a longer or shorter tube might cause considerable variation in the effective temperature difference and capacity without affecting the apparent temperature difference. Some investigators (1,2,3,4, 5)4 have reported “true” temperature differences obtained by means of a traveling thermocouple which measures the temperature of the fluid at various distances along the inside of the tube; the true temperature difference being taken as the length-mean average of the local temperature differences between the inside wall and the fluid.5 Since the fluid velocity may vary by several hundredfold during passage through the tube, a large variation in the local heattransfer coefficient throughout the length of the tube would not be unexpected. Even if heat-transfer coefficients based upon true temperature differences and total heat flux in the boiling section are known, the designer still does not know whether these same coefficients would prevail with a different heated area. The principal object of the work to be described in this paper was to study the variation in local heat-transfer coefficients in a semiworks apparatus in which large percentages of the liquid feed were vaporized. The results obtained when boiling pure benzene and pure water are given. An analysis of the pressure drops, and the results obtained when boiling mixtures of benzene and lubricating oil will be published subsequently. A ppa r a tu s a n d E x p e r im e n t a l P ro c ed u r e The apparatus used in this investigation was a special semicommercial evaporator, Fig. 1, consisting of four horizontal 12-ft lengths of copper pipe, 1-in. standard pipe size, connected in series by glass return bends. Each copper pipe carried three separate steam jackets, 3 ft 2 in. long. Condensate was collected separately from each of the twelve steam jackets in order that changes in the rate of heat transfer along the pipe could be measured. Dry steam from a cyclone separator was supplied to the jackets, as shown in Fig. 1. The vapor-liquid mixture leaving the last pass was separated, the vapor condensed at atmospheric pressure, and the twe liquid streams continuously mixed and returned by a pump through an orifice to the first pass. An over-all heat balance could be obtained from the rate of condensation of steam 1 Professor, Chemical Engineering, Massachusetts Institute of and the rate of flow and temperature rise of the water in the con- Technology, Cambridge, Mass. 2 Technical Division, Engineering Department, Experimental Station, E. I. du Pont de Nemours & Co., Wilmington, Del. the paper. * Numbers in parentheses refer to the Bibliography at the end of * Technical Division, Rayon Department, E. I. du Pont de Nemours & Co., Seaford, Del. with a rough surface inside of a vertical steam-heated glass tube re * T. B. Drew has reported that insertion of a metal thermocouple Contributed by the Process Industries Division and presented at sulted in radical changes in the boiling action, minimizing superheat the Annual Meeting, New York, N. Y., December 2-6, 1940, of T h e and causing boiling to commence earlier in the tube than when the A m e r i c a n S o c i e t y or M e c h a n i c a l E n g i n e e r s . couple was absent. Such a phenomenon should not be encountered N o t e : Statements and opinions advanced in papers are to be in commercial tubes where the additional nuclei for bubble formation understood as individual expressions of their authors and not those offered by the thermocouple are small in number compared with the of the Society. numerous nuclei along the metal wall. 545
546 TRANSACTIONS OF TH E A.S.M.E. AUGUST, 1941 F i g . 1 D i a g r a m o f F o u r - P a s s E v a p o r a t o r (This diagram shows pressure taps P, thermometers T, tube-wall thermocouples tw, Pyrex-glas denser. The feed rate was determined by means of an orifice and checked by means of a heat balance in those runs where the liquid was completely vaporized. Fluid temperatures were measured at the entrance to each pass. Pressure taps were provided near the end of each pass, and pressure drops across each pass and the preceding bend were measured by mercury-water differential manometers. At the midpoint of each jacket, two thermocouples were embedded in the top and bottom of the outer wall of the copper pipe. Dropwise condensation of steam was promoted by the use of octylthiocyanate. To permit removal of noncondensable gases, a steam vent was provided at the top of each jacket, near the condensate outlet. A series of runs was made on commercial benzene, followed by a series of runs on benzene-oil mixtures and, finally, a series of runs on distilled water. High-speed photographs of the glass return bends were taken by stroboscopic light during some of the runs on water. During the initial runs on benzene, the thermocouples in the tube wall read so nearly like the condensing-steam temperature that the film heat-transfer coefficients were almost identical with the over-all heat-transfer coefficients. Consequently, the thermocouple readings were dispensed with during the remainder of the runs on benzene, in order to reduce the length of the run and the possibility of variation in steam pressure during the run. The runs on boiling water were started 3 months after the initial runs on boiling benzene; during the intervening time the thermocouples were impaired to such an extent that their operation during the runs on boiling water was unsatisfactory. One observer could record all temperatures, pressures, and orifice readings, while a second operator collected the steam condensates. The twelve condensate streams were collected almost simultaneously by starting the collection from each line at 5-sec intervals. After steady operating conditions had been maintained for 20 min, the actual test period lasted less than 10 min. The operation of the apparatus was characterized by fluctuating flow in the glass return bends, accompanied by fluctuations of all readings, except the rate of flow of water through the condenser. Reported readings represented a visual time average over the space of several seconds. The saturation temperature of the fluid, corresponding to the observed fluid pressure, varied over smaller limits than did the observed fluid temperature. Hence, all temperature differences in the boiling section were based upon return bends, and twelve steam jackets.) the saturation temperature of the liquid, although the observed temperatures were usually within 1 C of the saturation temperature. The observed temperatures fluctuated over a range of as much as 3 C or more. From the steam-condensate readings and the known temperature and pressure of the fluid, the cumulative weight per cent p of the feed vaporized, up to and including any specified jacket, was calculated. The local over-all coefficients U in Btu per hr per sq ft per deg F, were calculated by dividing the heat flux in each jacket by the difference between the saturation temperature of the steam and that of the liquid. R e s u l t s a n d D is c u s s io n o f R u n s o n B o i l i n g B e n z e n e The results of three representative runs on boiling benzene are shown in Figs. 2, 3, and 4. In Fig. 2, it is noted that the rate of heat transfer prior to boiling is high, due to the high temperature difference. In the boiling section, the heat-transfer coefficient starts at 290 in jacket No. 3, passes through a maximum of 710 in jacket No. 9 (in which the cumulative weight per cent of the feed vaporized p had increased from 55 to 70), and then decreases to about 60 in the last jacket, where p was 95 per cent. The initial increase in the heat-transfer coefficient might be ascribed (a) to the increasing temperature difference in the later jackets, due to pressure drop, or (6) to the increase in the cumulative weight per cent of the feed vaporized.6 The run illustrated in Fig. 3, was taken at a higher steam pressure than that of the run illustrated in Fig. 2, at approximately the same feed rate. Consequently, 67 per cent of the feed was vaporized in the first three jackets and the fluid leaving jacket No. 8 was superheated vapor. It is noted that jackets Nos. 4 and 7, which immediately follow a return bend, gave rates of heat transfer and heat-transfer coefficients considerably higher than obtained in the adjacent jackets. It is also noted that jackets Nos. 5 and 6 gave heat-transfer coefficients substantially the same as obtained when dry benzene vapor was being superheated in jackets Nos. 8 to 12, inclusive. 6 T h a t t h e in c r e a s e in t h e h e a t - t r a n s f e r c o e f f ic ie n t is n o t d u e s o le ly t o in c r e a s in g t e m p e r a t u r e d iffe r e n c e i s b e s t p r o v e d b y r e fe r e n c e t o d a t a (6 ) o n b o ilin g b e n z e n e - o il m ix t u r e s , in w h ic h a s im ila r in c r e a s e in t h e h e a t - t r a n s f e r c o e f f ic ie n t w it h in c r e a s e in t h e c u m u la t iv e p e r c e n t v a p o r iz a t io n o c c u r r e d d e s p it e a d e c r e a s in g te m p e r a t u r e d iffe r e n c e a s t h e b e n z e n e w a s b o ile d o u t o f t h e o il.
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Transactions of the Significance of
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Significance of Coal-A sh Fusing T
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BAILEY, ELY—COAL-ASH FUSING TEM P
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BAILEY, ELY—COALrASH FUSING TEMPE
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BAILEY, ELY— COAL-ASH FUSING TEM
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BAILEY, ELY—COAL-ASH FUSING TEMPE
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L ubrication of G eneral E lectric
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Page 75 and 76: Discussion M. F. Behar.4 This paper
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