Heat Transfer of Condensing
Organic Vapors ALBERT H. COOPER1, R. HALL MORRISONI, AND I%%RVEYE. HENDERSON Virginia Polytechnic Institute, Blacksburg, Va.
I
N THIS investigation an attempt has been made to determine and correlate the film heat transfer coefficients for individual and mixed vapors condensing on a vertical surface. Only limited data are available on specific condensing vapors other than with steam. Steam distillation and azeotropic mixtures have found important applications in the chemical industry, but thus far little has been done on coefficients of heat transfer for condensation of mixed vapors. Most of the correlations have been based on the theoretical equations developed by Nusselt (26). For horizontal tubes:
An attempt is made to determine and comelate the film heat transfer coefficients for vapors condensing on a vertical surface. This investigation included primarily a study of two series of organic liquids, alcohols and acetates. Five members of each of these series (methyl through amyl) and in addition a constant-boiling mixture of ethyl acetate-isopropyl alcohol and an immiscible liquid mixture of steam and butyl acetate were studied. Members of each of the organic series investigated fell in parallel curves, the film coefficients decreasing with in creasing molecular weights. The film coefficients of the condensing immiscible liquid mixture fell intermediate between the two pure components, while that of the azeotropic mixture appeared to be higher than either of the individual components.
dicted for the viscous region. PossiblJ' this affects the rate Of heat transfer. The derivation of the Nusselt equation for vapors condensing on a vertical surface was baaed on the following assumptions, which may account for this deviation from actual data: (a) Film condensate in viscous flow. (b) Condensation is true film type condensation. (c) The effect of vapor velocity upon the thickness of condensate film may be neglected. ( d ) Physical properties may be taken at the mean film temperature. (e) Film condensate is so thin that the temperature gradient through it is a straight line. Extensive work was done on the film aspect of heat transfer by McAdams and Frost who reported the results of their investigations in a series of articles in 1922 (20). Monrad and Badger (92)discussed the Nusselt derivation a t length, and pointed out the effects of superheat, vapor velocity, and impurities. They developed an equation involving turbulence and showed that turbulence accounts for most of the observed deviations from the Nusselt theory. Kirkbride (16) and Colburn (7)pointed out that at high rates of heat transfer on long vertical tubes the filmshould flow in turbulent, rather than streamline motion, over the lower part of the tube. Colburn (7) calculated the condensation with a portion of condensate layer in turbulent motion. Hebbard and Badger (11) in determining steam film coefficients found that deviation from the Nusselt theory was practically constant. McCormack (21) pointed out that a general correlation of observed steam film coefficients on a vertical surface by the Nusselt equation requires a more complex relation than that provided by a constant correction factor. Badger, Monrad, and Diamond (3) presented data for condensing diphenyl, whicli deviated considerably from those predicted by the Nusselt theory, the coefficient increasing with increasing temperature differences. Rhodes and Younger (28) determined the coefficients of condensing vapors of hydrocarbons and alcohols series, using the indirect method of over-all resistance at various cooling water velocities. Wallace and Davison (31) . , determined film coefficients for mixtures 'Ondensing compositions Of on a horizontal tube. Baker and Tsao (6) determined the
For vertical tubes:
wherer = latent heat of vaporization of saturated vapor,
Tb.
density B*t* o Condensate, 1bJft.a I C = thermal conductivity of condensed vapor, B. t. u./
P E
ft./hr./" F./ft. acceleration of gravity, ft./hr.a (4.18 X lo*) viscosity of condensate film, lb./(ft.) (hr.) c1D = outside pipe diameter, ft. At = te3Rerature differences between vapor and metal, 8'
-
P.
L = length of tube,
ft.8
Application of the Nusselt theory to data obtained on vapor condensation outside of horizontal tubes appears to be in fair agreement. Application of the theory to condensation of vapors on a vertical surface, particularly on long tubes, however, shows a considerable disagreement with experimental data. Linear velocity of a film on a vertical surface has been found to be substantially higher a t the vapor interface than would be predicted on the basis of laws of viscosity. I n this region the film moves with a wave motion. However, average film thickness and velocity correspond to values preaddress, Chemical Warfare Service, Edgewood Arsenal, Md. Present addreas, Tennessee Eastman Corporation. Kingnport. Tenn.
1 Temporary 8
79
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 1
In order to ensure sufficient vapor in the condenser a t all times, an excess supply of vapor was supplied. Uncondensed vapors from the main condenser were passed through a glass auxiliary condenser, where they were totally condensed and returned to the boiler. A vent was provided at the auxiliary condenser for the elimination of permanent gases from the system. Thermocouples for the measurement of condensing surface temperatures were made of constantan wire ( I ) . Grooves were cut at right angles to the axis of the tube, and the thermocouple junctions were made between the constantan wire and the copper tube and soldered into place, similar to the arrangement used by Hebbard and Badger (lb). The excess solder was polished off flush with the surface of the tube. The constantan wires were insulated with APPARATUS FOR CONDENSING ( FIGURE 1. HEATTRANSFER Bakelite resin, from the thermocouple junction to the point a t which the lead leaves the groove and tube surface in order to provide thermal concoefficients for condensing vapor films of water and nontact without electrical contact. This was done in order miscible organic liquids, using a method of measuring tube t o minimize the conduction of heat along the wire from the wall temperature described by Jeffrey (14) in which the copper vapor space to the thermocouple junction. After leaving tube itself acts as a resistance thermometer. Patterson (26) the tube surface, the wires were insulated with Bakelite reported investigations on mixtures of immiscible liquids conenamel, enclosed in a fine capillary glass tube, and led out densing on vertical tubes, with empirical equations for the through a packing gland a t the end of the condenser. Therfilm coefficients. Baker and Mueller (5) determined coeffimocouples were placed a t 6-inch intervals along the length of cients of condensing mixed vapors of immiscible liquids, and the condenser tubing and staggered around the circumference described a method of measuring the tube surface temperaof the tube. tures. Drew el al. (8, 25, 24), Fitzpatrick, Baum, and McAdams Experimental Procedure (IO),and Shea and Krase (29) reported on the conditions for For the purpose of standardizing the experimental techdropwise condensation of steam and the resulting increases nique and checking the data obtained from the equipment in steam film coefficients obtained for this mechanism of conagainst accepted data by other investigators, a series of rum densation. Emmons (9) discussed the mechanism of dropwas made upon steam. Approximately one hour was rewise condensation of vapors. quired for each run. On beginning a run, a tune-up period Apparatus of approximately 30 minutes was allowed during which the conditions of operation were maintained as constant as posA boiler was made of a flanged section of 10-inch standard sible in order t o ensure equilibrium conditions before taking cast iron pipe, and equipped with a coil of half-inch copper data. Preliminary readings were taken a t intervals until tubing for the heating unit. Vapor was conducted from the conditions appeared to be constant, which indicated that top of the boiler, through an entrainment separator, into the equilibrium had been attained. After equilibrium had been experimental condenser (Figure 1). The main condenser consisted of a 3-fOOt section of 18-gage '/*-inch copper condenser tubing, having a surface area of 0.6876 JOPO square foot and mounted-vertically in a 4-inch Pyrex glass pipe. Rubber stoppers were used as packing glands where the tube passed through the ends of the jacket. The boiler, condenser jacket, and all water and -3000 e-. vapor lines were lagged with 2 inches of magnesia covert ing. Cooling water from the city lines was fed t o a 8 I500 constant-head reservoir, from which it flowed by gravity to the condenser. The cooling water passed up0 ward through the copper condenser tubing and to a calic' 1000 brated weighing tank for determining the rate of flow. In order to minimize the amount of heat lost by con800 duction along the tube wall past the ends of the con700 5 6 8 IO IS 20 30 40 so 60 BO m denser section, a short section of rubber hose mas used d r, -6 to join the condenser to the outside cooling water system. FIGURE 2. HEATTRANSFER COEFFICIENTS FOR CONDENSING STEAM
$
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INDUSTRIAL
January, 1942
EMISTRY
COEFFICIENTS FOR TABLE I. HEATTRANSFER d o o l i n g WaterInlet Outlet Temp. Run temp., temp., rise,
No. 10 15 6 4 5 8 1 2 9 3 11 7 12
0
F.
51.0 52.0 50.0 60.3 50.0 62.0 50.0 60.0 52.0 51.5 52.0 52.0 51 0
0
F.
81.0 87.0 89.0 110.1 95.0 126.5 85.0 123.0 81.2 88.8 84.5 81.2 82 6
F.
30.0 35.0 39.0 49.8 45.0 64.5 35.0 63.0 29.2 37.3 32.5 29.2 31.6
Rate of Q/& Heat flow Transferred Ib./h;. B. T. U./Hr: 480 480 480 330 480 267 480 260 624 680 735 1016 1220
14,400 16,800 18.720 16,435 21,600 17,220 16,800 10,380 18.220 25,380 23,900 29,680 38,580
CONDENSINQ
STEAM FILMS
W Con- -Temperature. Av. ' DifferF.dinsate, Lb./Hr. Vapor surface ence, At hv 21.1 210.0 202.0 8.0 2620 198.5 11.5 2125 210.0 23 9 1756 194.5 15.5 25 1 210.0 22.1 28.9 23.4 22.5 22.0 20.5 33.9 32.6 40.4 52 0
211.1 210 0 212.0 210.0 210.5 213.3 210.5 210 0 212.7 211.0
194.0 191.5 191.5 189.5 189.5 191.5 184.5 179.5 172.8 156.0
17.1 18.5 20.5 20.5 21.0 21.8 26.0 30.5 39.9 55 0
1398 1697 1222 1192 1134 1215 1420 1140 1085 1020
81
temperature of the heating medium. The cold junctions were maintained a t 32' F. by an ice bath. The thermocouple readings were measured by a Leeds & Northrup potentiometer, Vapor, cooling water, and condensate temperatures were measured with a Brown resistance thermometer, using a multiple-point switch.
Results
This investigation included primarily a study of two series of organic liquids, alcohols and aceTAFJL 11.~ HEATTRANSFER COEFFICIENT8 FOR CONDENSINQ ALCOHOL FILMS tates. Five members of each of -Cooling Waterthese series (methyl through amyl), F.W ,Con- -Temperature, Inlet Outlet Temp. Rate of Q/e, Heat Av. Differflow Transferred densate rise a constant-boiling mixture of ethyl Run temp., temp Vapor surface ence, At hv Ib./h;. B. T . U./Hr: Lb./H
