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INDUSTRIAL AND ENGINEERING CHEMISTRY
The use of the proper filter aid, added in the proper amount, is then applied to production runs. The maximum pressure for satisfactory results has also been noted as the critical pressure and should not be exceeded on the production filters. The application of this information is then checked on production filtration and should result in the maximum efficiency. This system of testing has been applied many times to difficult problems in clarification filtration and has proved highly successful and flexible as tool for attacking annoying types of sedimentation. This description is presented with the purpose of providing a simple, practical means of ap-
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proach to filtration difficulties in the specific field of clarification.
Literature Cited Carman, p. c,, ENa. cHnaa., 31, 104, (1939). (2) Ruth, Ibid., 27, 708 (1935). (3) Ruth, Montillon, and Montonna, Ibid., 25, 153 (1933). (4) Sperry, Ibid., 20, 892 (1928). (5) Walker, Lewis, and McAdams, “Principles of Chemical Engineering”, 2nd ed., pp. 363-72 (1923). (6) Walker, Lewis, McAdams, and Gilliland, Ibid., 3rd ed., PP. 346. 358 (1937).
Heat Transfer Coefficients for the Condensation of Mixed Vapors of Turpentine and Water on a Single Horizontal Tube E. L. PATTON AND R. A. FEAGAN, JR. Naval Stores Station, Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture, Olustee, Fla. INCE the beginning of the naval stores industry in this
be calculated, the data taken in these experiments will permit the calculation of over-all coefficients of heat transfer and the country, turpentine gum has been subjected to a crude size of the condenser required for a given gum-processing form of steam distillation in a direct-fired still. The plant. This should encourage the use of the more efficient condenser used at these fire stills consists of a copper coil tubular condenser to replace the old-fashioned tub and worm. surrounded by water in a wooden tub. The present tubs are usually 10 feet in diameter and 10 to 12 feet high although the size varies throughout the industry. This type Apparatus of condenser has certain inherent advantages for use on small The apparatus used in these experiments was similar to stills in the woods in that it is cheap and will permit some that of Baker and Mueller (1) and of Baker and Tsao (2) use of the still over a considerable period without the addition with the exception of the method of tube-wall thermocouple of more cooling water. This is an important consideration in installation. localities where the water supply is limited during dry seasons. Condensation of mixed vapors of turpentine and water Moreover, within limits, this type of condenser as used in the took place on a measured length of extra-heavy copper pipe naval stores industry serves as a cooler as well as a condenser. enclosed in a wellThe present trend of t h e i n d u s t r y , 0 i n s u l a t e d comer however, is toward jacket. This jacket centralized processwas equipped with ing and hence more sight glasses to perefficient plants. m i t c o n s t a n t obThe purpose of this s e r v a t i o n of t h e formation of coninvestigation was to densate on the tube. determine the coefThe rate and temcient of heat transp e r a t u r e of t h e fer on the vapor water flowing side of a single-tube t h r o u g h t h e tube horizontal conwere carefully condenser handling trolled during each mixed v a p o r s of turpentine and experiment, a n d PARTIAL FINAL water. Since the equilibrium vapors CONDENSER CONDENSER of turpentine and coefficients for the cooling medium can w a t e r w e r e conFIG^ 1. TESTCONDE~NSER AND AUXILIARY EQUIPMENT
S
W I N MET
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INDUSTRIAL AND ENGINEERING CHEMISTRY
densed on the outside of the tube. The equilibrium vapors were produced by passing the mixed vapors from the turpentine still through a large partial condenser before entering the test condenser. The work of Baker and Mueller ( I ) and Langen (4) showed that the temperature variation around the perimeter of a tube on which condensation was taking place could cause serious errors in heat transfer coefficients determined by measuring the tube-wall temperature at one point on the perimeter of the tube. These authors showed that no single point could be depended on t o give a n average value of the tube-wall temperature. For this reason the jacket of the test condenser was fitted with packing glands a t each end t o receive the condenser tube and permit rotation of the tube during each experiment. The condenser tube had a measured outside diameter of 1.315 inches and an inside diameter of 0.951 inch, and was 12 feet long. A 6-foot section of the total length was enclosed by the insulated copper jacket which was 12 inches in diameter. Following the method of Baker and Mueller ( I ) , the area of the tube from which condensate was collected and weighed was limited by placing asbestos fins around the tube at points exactly 48 inches apart. A trough 48 inches long and 2 inches wide collected the material condensing on the measured length of the tube. The fins prevented condensate from running to or from the measured section and by limiting the condensing area to only a portion of the tube in the jacket, the possibility of end effects was eliminated. Details of the apparatus and equipment arrangements are shown in Figure 1.
Installation of Thermocouples Thermocouples were of the copper-constantan type, and the potentiometer for making measurements was a Leeds & Noithrup student-type, limiting the accuracy of the temperature measurements to ~ 0 . 2 ’F. The standard cell used with this Potentiometer mas an Eppley No. 94220 which mas calibrated by the National Bureau of Standards to a n accuracy of 0.01 per cent.
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POLAR COORDINATE PLOTOF FI~UR 3. E TYPICAL T U B E - W A L L TEMPERATURE US. ANGULAR POSITION AROUND THE TUBEWHILE COXDER’GINIJ MIXED VAPORSOF TURPENTINE AND WATER
copper pipe was milled and fitted with tube-wall thermocouples, as installed in the test condenser. This short section was then exposed to the vapors of boiling liquids and the measured electromotive forces fell (within the limit of error of the potentiometer) on the “38 calibration” of the Leeds & Northrup Company scale for copper-constantan thermocouples. The “38 calibration” was then used throughout the experiments. This short test section mas taken apart, and it was found that the insulation of the wires had not been injured by the heat from the soldering operation. Vapor thermocouples were installed in the jacket opposite each end of the 48-inch condensing section and extended to within 1 inch of the condenser tube. Experimental Procedure
ANGLE
FIGURE 2. TYPICAL CURVEOF TEMPERATURE VARIATIONAROUXD THE TUBE \vALI, IJ’H1I.E C O X D E N S I N G MIXEDVAPORS OF TURPENTINE AND WATER
Top of tube is designated as an angle of zero
degrees.
Four thermocouple junctions were installed in the tube wall by our method (6). The two end junctions were 2 inches from each end of the 48-inch test section, and the other two junctions were spaced equidistant from the two end couples and 14.67 inches apart. T o check this method of installation and t o calibrate the tube-wall thermocouples, a short section (about 4 inches) of
Before starting the series of runs, the surface of the tube was oxidized by passing mixed vapors through the jacket, since Baker and Mueller (I) had shown that this produces a surface having reproducible surface characteristics. To ensure the removal of noncondensable gases from the jacket, the test conrlenser was vented to a final condenser. All data were taken with a slight positive pressure (1 to 2 cm. of water) in the condenser shell to ensure proper venting, During each experiment, the condensate collected in the trough was run into an automatic gravity separator where the turpentine and water vere separated and diverted to individual weighing vessels where they were weighed simultaneously at intervals. At the start of a run, mixed equilibrium vapors were passed into the test condenser for approximately 30 minutes while the speed of the still, water rate, and temperature were adjusted, after which time the vapor and tube-wall temperatures were recorded, the turpentine and water vcssels were weighed, and the rate and temperature of the cooling water were checked. The condenser tube was then rotated through an angle of 30” and the temperature measuremrnts were repented. This procedure was continued until the tube had been rotated through 360” in 30” increments, when the individual run was considered complete. The time required for a complete experiment ranged from about 30 to 45 minutes.
October, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Since the rate of condensation of the turpentine and water was found to be substantially constant during a run, the rate of heat flow in B. t. u. per hour was found by multiplying the net weights of the components obtained during the run by their respective latent heats and dividing the sum by the length of the run in hours. Since the tube-wall temperature varied around the perimeter of the tube, an average value was obtained by plotting the thermocouple readings in millivolts against the angular position of the thermocouple, and integrating the resulting curve with a polar planimeter. This procedure gave a true average of the temperature around the tube, since the calibration curve is a straight line over small intervals of temperature. The average temperatures of the various points were then plotted against the position of the thermocouple along the length of the tube. This curve was integrated (by means of a polar planimeter) over the length of the condensing section to give a mean of the tube-wall temperature. From this value and the vapor temperature, the temperature drop ( A T ) through the film of condensate was obtained. No correction was made for the slight temperature drop through the thickness of copper between the thermocouple junctions and the condensate film, since the greatest possible error was of the order of 0.2 to 0.3 per cent of the corrected temperature drop through the h.
Results The turpentine used in these experiments was freshly distilled from gum obtained from the slash pine (P. curibaea) and had a specific gravity of 0.8675 (6Oo/6O0 Fa). Typical curves of temperature variation around the tube at two points along its length are shown in Figures 2 and 3. Figure 2 shows a typical curve of temperature variation around the tube at a point along the tube and is plotted on rectangular coordinate paper. Figure 3 is a curve which was obtained in another experiment and is plotted on polar coordinate paper. I n both cases the angular position on the tube (0" being the top) is plotted against the tube-wall temperature a t that point. The protractor used in measuring the angle was set in a horizontal position by eye, and the zero point was off about 15" in several of the experiments. However, this would not affect the integrated value of temperature. The curves are similar to those obtained by Baker and Mueller (1).
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When pure turpentine was condensed, film-type condensation was obtained. The coefficient of heat transfer through a condensate film oan be expressed by the equation: h = q/AAT
where h
film heat transfer coefficient, B. t. u./(hr.)/(sq. ft.)/(" F.) A = he&&nsfer area, sq. ft. q = heat transferred, B. t. u./hr. AT = temperature drop through condensate film, F. =
All terms except h are determined from the data, and h is calculated. The value for latent heat of steam was taken from Keenan (3)and that for the latent heat of turpentine from Perry (6). Figure 4 shows effect of temperature drop through the film on the film coefficients. The data were obtained using equilibrium vapors of turpentine and water with the exception of the one point, well above the general curve, which represents a nonequilibrium mixture, in that excessive water vapor was present. These data are given in Table I. The latter condition is generally present in a turpentine-water condenser, and since this point is higher than the curve, Figure 4 will furnish conservative values of h for the design of condensers for turpentine stills. Further work is contemplated using the vapor mixtures encountered in actual practice, but until this is done, the values determined from the curve of Figure 4 are recommended for predicting the film coefficient on the vapor side of a condenser handling the vapor of a turpentine still.
TABLB I. RESULTS OBTAINEDBY CONDENSING MIXBD VAPORS OF TURPENTINE AND WATERON A SINGLE HORIZONTAL TUBE" h, B. T. Turpentine Steam Av. TubeU./HrJ/ Run Condensed, Condensed, Wall Temp., Mean Vapor (Sq. Ft.)/ Lb./Hr. No. Lb./Hr. F. Temp., F. (" F.)
1) 2 3 4 5 6 a
b
44.7 31.7 27.6 18.8 19.2 23.8
38.4 25.0 22.1 15.0 15.8 19.4
119.8 139.8 152.2 179.7 177.6 169.4
204.1 203.G 204.0 203.9 203.6 203.7
374 326 354 514 502 488
Area of test section, 1.378 square feet. Nonequilibrium.
Literature Cited 600,
I
I
I
I
l
l
1
(1) Baker, E . M., and Mueller, A. C., IND.ENG.CHEIM., 29, 1067-72 500
. f ?*
400
. d
300
=
r
!
X
2001
20
I
30
I
40
NON EQUILIBRIUM
50
60
70
80 90
(1937). (2) Baker, E. M., and Tsao, U., Trans. A m . Inst. Chem. Engrs., 36, 617-39, 783 (1940). (3) Keenan, J. H.,"Steam Tables and Mollier Diagram", New York, Am. SOC.Mech. Engrs., 1930. (4) Langen, Forsch. Gebiete Ingenieurw., 2, 359 (1931). (5) Patton, E. L., and Feagan, R. A., Jr., IND.ENG.CHEM.,ANAL. ED., t o be published. (6) Perry, J. H., et al., Chemical Engineers' Handbook, New York, MoGraw-Hill Book Co., 1934.
A T . .F,
FIGURE4. FILMCOEFFICIENTS FOR CONDENSING MIXEDEQUILIBRICM VAPORS OF TURPENTINE AND WATERON A SINGLE HORIZONTAL TUBE
The condensation obtained while condensing mixed vapors of turpentine and water was the film-dropwise type described by Baker and Mueller (1). The turpentine apparently condensed as a film, and the steam condensed dropwise in and on this film.
Permeability and Absorption Properties of Bituminous Coatings-Correction On page 992 of the August, 1941, issue the sentence six lines from the bottom of the first column should read: "The data ( I ) have shown that the asphaltene portion of asphalt is much less permeable to water vapor than the oily viscous medium." A. P. ANDERSON AND K. A. WRIGHT
