BOILING Heat Transfer in Natural Convection Evaporators Film coefficients were determined from a single horizontal 0.75-inch nickel-plated copper tube to water and three alcohols boiling a t atmospheric pressure and under vacuum. Vapor binding was encountered with all four liquids. A t atmospheric pressure the maximum heat flux in B. t. u. per hour per square foot was 360,000 for distilled water, 100,000 for n-butanol, 110,000 for isobutanol, and 110,000 for isopropanol; the corresponding film temperature drops were 50", 65", 8 O 0 , and 88" F. The percentage reduction in h with decrease in boiling temperature was not nearly so large as reported by Cryder and Finalborgo. The data for n-butanol and for water boiling a t atmospheric pressure check reasonably well with those of Cryder and Finalborgo for a 1.5-inch brass tube. Data are included on the effect of wetting agents. A model evaporator containing sixty nearly horizontal chromium-plated 0.5-inch copper tubes was used to boil distilled water a t atmospheric pressure with over-all temperature differences ranging from 20" to 100" F. The distribution of heat flux in the various rows was determined. The maximum flux for the bundle when slightly scaled was 340,000 B. t. u. per hour per square foot, which corresponded to a n over-all temperature dserence of 100" F. The bundle gave approximately the same results as the single-tube apparatus. An increase in tube clearance from one to one and a half tube diameters reduced the heat transfer by less than 10 per cent.
G. A. AKIN, BuffaloStation of the M. I. T.School of Chemical Engineering Practice, Bethlehem Steel Company, Lackawanna, N. Y.
W. H. MCADAMS, Massachusetts Institute of Technology, Cambridge, Mass.
The following table summarizes the dimensions and nature of the heat transfer surfaces, and the shape and size of the boilers: Depth
An-
of
~~
paraCopper tus -Tube DimensionsTube No. 0. D. I. D. Length Inch Inch Inches I 0.75 0.63 8.5 Ni-plated I1 0 , 5 4 0 0.364 5 . 0 6 Ni-plated I11 0.625 0.500 7 2 . 0 Unplated
Boiler Shape Cubioal Cubical Vertical oylinder
Liqiid Boiler over Refer=am. Tube enoe Inches Inches 8.5 5.06
12.0
a ;1
I n apparatus I and I1 the tubes were straight; in 111,the tube was coiled in the form of a helix with a slight pitch as the steam flowed downward inside the helix. I n the first two cases the vapor was condensed in a reflux condenser, and the heat transfer rate was based upon the heat absorbed in the condenser, corrected for separately measured losses. I n the first two cases thermocouples were used to measure tube-wall temperatures, as described in detail later. The temperatures of the boiling liquids were measured by thermometers or thermocouples. The individual coefficients, h, from tube t o liquid were based on p, the B. t. u. transfer per hour, the outside area of the tube, A , and the temperature differenoe from tube to boiling liquid :
T
WO recent articles (6, 9) dealing with laboratory-scale natural convection boilers, heated by steam condensing inside short metal tubes, showed that nuclear boiling occurred a t the smaller temperature differences, and that the heat flux, q/A, went through a maximum value as the temperature difference was increased. Film boiling occurred at the large temperature differences beyond the hump in the curve of q/A vs. At. However, mainly over-all coefficients were reported, and the liquids were boiled only at atmospheric pressure. The object of this paper is to report the results of recent experiments with natural convection boilers. Several sizes and arrangements of heating surface were used, data are included for a number of liquids, and tube temperatures were measured in most cases. I n contrast to earlier work on film coefficients, a sufficient range of temperature difference was used to obtain both nuclear and film boiling. The description of apparatus will be divided into two parts-single-tube and multitubular.
= q/(Nt,
- td
The over-all coefficients were based on the temperature of the condensing steam less the boiling point of the liquid :
u
- 2L)
= q/(A)(t*
Where aging of the surface had an effect, this is noted in discussing the results. I n the first two cases benzyl mercaptan (8) was added to the steam line prior to each run, to promote dropwise condensation.
Results with Apparatus I I n some of the early experiments with small temperature differences, boiling occurred mainly a t the points where the thermocouple wires left the surface. If the tube temperatures were measured by the usual method of spot-welding or soldering the junctions to the tube, except for errors due to thermal conduction in the leads, the temperature would be obtained at the point where the wire left the surface; this temperature would not be typical of the surface as a whole, entirely
Single-Tube Apparatus This type is similar to those described previously (9). I n brief, the shells of the boilers consisted of either a cubical metal box with plane windows or a vertical glass cylinder. 487
INDUSTRIAL AND ENGINEERING CHEMISTRY
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VOL. 31, NO. 4
Consider a copper tube, 0.75-inch o.d., with a wall 0.06 inch thick; neglecting the effect of the tin: k l L = 220/0.005 = 44,000
When the flux has the high value of, say, 360,000 B. t. u. per hour per square foot of outer surface, the temperature drop through the entire wall is 360.000 44,000
FIGURE 1. DIAGRAM OF BURIED THERMOCOUPLE, USEDFOR APPARATUS
I AND 11
aside from any variations in temperature from top to bottom of the tube or from end to end. In order to avoid this difficulty, in apparatus I and I1 the thermocouples were located as shown in Figure 1,so that the junction was not a t the point where the wires left the surface. A drill with a diameter of 0.026 inch was used to make a hole in the tube wall, with the closed end, a, almost at the outer surface of the tube. A single 30-gage Ideal wire, sealed in Pyrex tubing except for the tip, ba, was then inserted and tin-soldered so that the thermojunction was a t point b.
q/A 400 000
-
I
x - RUN W - I Y .
--
0
= 8.9"
F.
An error of 0.01 inch in estimating the position of the thermojunction would introduce an error of 1.8' F. However, the temperature drop from the junction to the boiling liquid would be of the order of perhaps 45" F. or more; hence the error due to not knowing the exact location of the couple would not exceed 4 per cent a t the high flux and would be less for lower flux. However, for tubes with high thermal resistances, an error of 0.01 inch in estimating the location of the junction would become serious. I n apparatus I, temperatures on opposite sides of the tube were measured by three pairs of couples; one pair was located midway of the 8.5-inch length of tube, and the others were 2 inches distant from the mid-point. The average reading of these side couples was used in determining the temperature difference, At, from tube to boiling liquid. A number of runs were made, boiling distilled water, n-butanol, isobutanol, and isopropanol, both a t atmospheric pressure and under vacuum. With each liquid the atmospheric run, involving a number of temperature differences, was run first; then the vacuum runs were made, and later one or two readings were taken a t atmospheric pressure. Unless the later atmospheric runs showed reasonable agreement with the original atmospheric runs, indicating that the surface condition of the tube had remained substantially constant during the vacuum runs, the vacuum runs were rejected. As noted in Figure 2, with some liquids a new tube was used, and in other cases the tube had been lightly polished with medium-grade steel wool.
POLISHED Ni-PL. COPPER 213'F. RUN W - Y , 173'F. v RUN W-m, 131 'F. + LATER ATM. RUN
NEW Ni -PL. COPPER RUN 8-1, 241 'F. RUN 8-E, 207'F. 0 RUN 6-1, 173 'F. + LATER ATM. RUN x
A
(E)
-
--
I
X
- RUN W - I , 213 'F. RUN W - I I , 191'F. - RW W - E . 155°F.
0-
V
+ - LATER ATM. RUN
I
I
200 000 100 000 80 000 60 000
40 OOC 20 ooc I O 000
6000 600C
4000 2000
IO
20
40
60 60 100
u
10
200
OF.
TEMPERATURE DROP, TUBE
20
40
60 80100
TO LIQUID
FIGURE 2. DATAFOR DISTILLED WATERAND TZ-BUTANOLIN APPARATUSI WITH SIDETHERMOCOUPLES, COMPARED WITH LITERATURE DATAAT Low FLUX
APRIL, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
In the literature, results have been plotted as h us. Ai, h us. q / A , and q / A us. At. Since by dehition h equals q / ( A ) (At), the first method involves a plot of q / ( A ) (At) us. (At) and the second, q/(A) (At) us. q / A . The third method involves a direct plot of the resulting flux, q/A, us. the temperature difference employed; and since it cannot distort the basic results in comparing data of different observers, it is
489
used here. The eoaeients may be obtained readily from the xrauhs by simple division.' The results are dotted in FiguTes-2 and 3. Compsrison of the two atmospheric runs on distilled water (Figure 2) shows that polishing the nickel-plated copper tube did not appreciably change the maximum flux (360,000 to 380,oM)B. t. u. per hour per square foot) and moved the curve only slightly to the left, reducing the critical temperature difference (correspondinz to maximum flux) from 50" to 45" F. foot, a t temperature differences ranging &om 65"to 88' F. With a11 four liquids, a t temperature differences less than the critical values a t atmospheric pressure a reduction in boiling i Tsbles of dataere sivenin the completearticlstosppearinths April 25th number of the Tronsadiona o/rhe American Innaae of Chemical Enginears.
FIQUKE
At
AI
-
.e
48'
F.; d A
= 26.000;
-
u
-
-
PoLISEED
540
41.000; U 560 PHOTOMICROCR~.PHSOF ETHYL ACETATEB o l m o ON A
73' F.; p/A
3. ADDITIONALDATAFOR
NICKEL-PLATED COPPER TUBE(0.75-INCH 0.D.) WITH SIDE TEEEM~C~UPLES. APPARATUS I
At = 122'
F.:. .o/A
= 5700:
U
0 . 5 1 0. ~ D. ~ ~ALWMIKUM TUBE
-
247
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INDUSTRIAL AND ENGINEERING CHEMISTRY
490
point due to vacuum reduced the flux a t a given temperature difference; but the percentage reduction from the atmospheric value was not nearly so large as that found by Cryder and Finalborgo (3) who used both water and n-butanol. The discrepancy might be due to the accidental presence of ebullition promoters in apparatus I, where bubbles formed on the unheated bottom of the metal still; to the fact that Cryder and Finalborgo used spot-welded couples a t locations different from those in apparatus I and used 1.5-inch 0. d. brass tubes
flux was substantially the same as with distilled water, the corresponding temperature difference was increased from 45 O to 80" F. (Figure 5).
Results with Apparatus I11 The results given above, as well as those previously reported @), were obtained with tubes not over 8 inches long. In order to determine whether or not the surprisingly high flux of 250,000 to 400,000 B. t. u. per hour per square foot could be obtained with considerably longer tubes, apparatus I11 was constructed with a 0.625-inch 0.d. copper tube 74 inches long. The flux obtained upon boiling water a t atmospheric pressure is plotted against the over-all temperatures in Figure 6. The maximum flux with the 74-inch length of polished tube is in between those shown by the dotted curves for the results obtained previously (9) for 4-inch lengths of polished copper. The 74-inch tube was then given a heavy coat of oxide by exposing the outside to air while the wall was electrically heated to redness. The oxidized tube gave substantially the same results as when cleaned and polished with medium-grade steel wool. Effect of Wetting Agents
IO
20
40
60 80 100 SIDE At,'F.
IO
20
40
60 80 100
ZOO
A saturated solution of benzyl mercaptan in distilled water, tws- t, boiled a t atmospheric pressure by a single horizontal chroFIGURE4. DATAFOR DISTILLED WATERBOILING AT ATMOSPHERIC PRESSURE ON NEW NICKEL-PLATED COPPERTUBES mium-plated tube, raised the value of q / A approximately 30 per
cent over that for distilled water in the region to the left of the hump. An excess of mercaptan on a copper tube caused FIGURE5. DATAFOR CITYWATERBOILING AT ATMOSPHERIC deposition of a yellow scale, which lowered the flux a t the PRESSURE IN APPARATUS 11 (0.54-INCH 0. D.) ON A SCALE-INCRUSTED NICKEL-PLATED COPPER TUBE(RUNS5 AND 6) COM- various temperature differences. With a steel tube 0.2 per PARED WITH CURVEFROM FIGURE 4 (DISTILLED WATERON A cent of sodium oleate in distilled water raised the flux a t the Run W-1 made with side couples
NEWTUBE,RUNS17 AND 18)
instead of 0.75-inch 0. d. nickel-plated copper; or to the small flux employed by Cryder and Finalborgo. (Cryder and Finalborgo did not state how many couples were used or where they were located. I n the case of n-butanol, where they made duplicate runs, q/A differed substantially a t a given At, as shown in Figure 2.) I n view of these various differences, the data of the two sets of observers on water and n-butanol agree surprisingly well for atmospheric pressure, as shown in Figure 2. Further data are needed to clarify the effect of changing the boiling point when various temperature differences are used. The data of Cryder and Gilliland (4) are high compared with the other data in Figure 2.
FIGURE6. FLUXFOR WATER BOILINGAT ATM o s PHE R I c PRESSURE us. OVER-ALL At, FOR COPPER TUBES OF VARIOUS LENQTHS
Results with Apparatus I1 Top, side, and bottom thermocouples were installed as shown in Figure 1. Only atmospheric runs were made. Figure 4 gives the results for new nickel-plated copper tubes and distilled water for runs 17 and 18, based on the single side thermocouple in order to compare these data with those of apparatus I where only side couples were available. The bottom couple in runs 17 and 18 was apparently out of order, but the top and side couples were working and gave nearly the same readings; the top couple read slightly higher on the whole. Run 18 was merely a continuation of run 17, made without polishing the tube between runs; it lies a little to the right of run 17, perhaps owing to a slight deposition of scale which was not detected at the end of run 18. The dotted curve for the larger tube of run W-1 (taken from Figure 2 ) lies somewhat to the right of run 17, but the results do not differ widely. Runs 5 and 6 in apparatus I1 were made with city water boiling a t atmospheric pressure on a scale-incrusted nickelplated copper tube 0.54-inch 0. d.; and although the maximum
IL
20
,
:4
, 60,80IO0
2[
I TUBE, APP I , RUN W- I 60 TUBES,APP 1,RUNS IB- ID I TUBE,APP.IL, RUN 17
I----
400 000
FIGURE7. FLUXFOR WATER BOILING AT ATMOSPHERIC PRESSURE u s . OVER-ALL A t , USINGNICKEL-PLATED COPPERTUBES
*-
200 ooa 100 000 80 000 60 ooa 4 0 000
2 0 000 IO
FIG. 7
20
40 60 80 100 OVERALL A t , 'F.
200
APRIL, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
smaller values of At and lowered it a t values of At above 34" F. However, check runs were not made, and hence the data are not included.
Results with a Sixty-Tube Evaporator A model evaporator (1)containing sixty nearly horizontal chromium-plated copper tubes, 0.50-inch 0 . d., 0.438-inch i. d., and 10 inches long, was used to boil water a t atmospheric pressure; steam condensing under pressure was employed in the tubes. The shell had a rectangular cross section, and the water level was 5 inches above the top of the tubes. The temperature of the boiling water was measured by a thermometer suspended a t the side of the tube bank. The water evaporated was condensed in a reflux condenser and returned by gravity t,o the evaporator. Two different arrangements of tubes were used; in both cases they were ten rows deep and six rows abreast and were not staggered. I n the first, the tubes were placed on 1-inch centers, and in the second, on 1.25-inch centers; both horizontal and vertical minimum clearances of 0.50 and 0.75 inch, respectively, were thus provided. Thermocouples were not used. Prior to each run, benzyl mercaptan (8)was added to the steam to promote dropwise condensation, but the steam was not filtered. The over-all temperature differences were taken as the saturation temperature corresponding to the pressure in the condensate discharge line, less the boiling point of the water. The rate of heat flow was determined from measured condensate rates, and the outside surface was used in computing A . Condensate was collected separately from each of the five pairs of adjacent horizontal rows, containing twelve tubes each. The over-all At was determined separately for each of the five sets of twelve tubes; 212" F. was taken as the boiling point a t all levels. With arrangement 1 a preliminary run (1A) with small temperature differences gave 20-40 per cent higher flux a t a given At0 than did subsequent runs lB, lC,and lD, which checked quite satisfactorily. A thin brown scale was on the tubes a t the end of run 1A but was not removed. The results of series lB, IC, and 1D for arrangement 1, and of series 2A and 2B for arrangement 2, were plotted as separate curves for each pair of rows. The following values of p/1000A and At, were read from these curves a t temperature differences a t intervals of 20" F.:
491
the data for the sixty-tube apparatus are bracketed by data for single tubes. The sixty-tube bundle with the narrower tube clearance gave a maximum flux of 340,000 B. t. u. per hour per square foot a t At, of 100" F. as compared with 360,000 a t At0 of 70" F. for the single tube of apparatus I.
Conclusions 1. With distilled water boiling a t atmospheric pressure on horizontal nickel-plated or chromium-plated tubes, the maximum flux was approximately the same (350,000 B. t. u. per hour per square foot) whether a single-tube or a sixty-tube apparatus was used. City water boiling on a scaled tube gave approximately the same maximum flux a t a higher temperature difference. 2. Under similar conditions three paraffin alcohols (isopropyl, isobutyl, and n-butyl) gave only 30 per cent as much maximum flux as distilled water, and required higher temperature, differences. 3. At a given temperature difference, the effectof vacuum in reducing the flux is not so large in the range of high flux as that reported by others who used low flux. 4. Further work is needed to study the effects of a number of factors, such as pressure, nature of the surface, nature of the liquid, and variations in temperature with position on the tube.
Acknowledgment The authors wish to express their appreciation of the enthusiastic cooperation of M. D. Abbott, H. L. Christison, W. D. Comley, R. A. Dreselly, A. F. Kaulakis, and L. M. Sherman, upon whose data this article is based. Thanks are due to T. H. Chilton, T. B. Drew, and W. K. Woods for valuable suggestions, and to C. R. Bailey, L. D. Etherington, and M. D. Parekh for assistance in calculations. W. B. Tucker made the photomicrographs of ethyl acetate boiling on the aluminum tube, in the runs made by E. T. Sauer.
Nomenclature A = area of outer surface of tube, sq. ft. h = coefficient of heat transfer from tube to liquid, B. t. u./(hr.)(sq. ft. of outer surface)( O F.) ' k = thermal conductivity of heating surface, B. t. u./(hr.)(sq. ft.)( F. per ft.) L = thickness of tube wall, f t . P = heat transfer rate, B. t. u./hr. t L , t a , t, = temperature of boiling liq$d, condensing steam, and tube wall, respectively, F. U = over-all coefficient of heat transfer from condensing steam to boilingliquid, B. t. u./(hr.)(sq. ft. of outside surface)( O F.) At, At0 = temperature difference from tube to liquid,oand from condensing steam to liquid, respectively, F. O
For Arrangement 1. Narrow Spacing 20°F. 40OF. 60'F. 80' F. Over-all At, F. 29 103 210 290 Top 2 rows 26 90 188 2nd 2 rows 33 132 250 320 290 3rd 2 rows 4th 2 rows. 32 107 230 300 Av. top 8 row8 30 108 219 300 For Arrangement 2, Wide Spacing 28 103 220 300 Top 2 rows 25 90 182 242 2nd 2 rows 240 300 3rd 2 rows 32 119 210 275 4th 2 rows 20 94 190 270 Bottom 2 rows 18 80 25 97 210 278 Av. all rows 213 279 Av. top 8 rows 26 102 94 97 90 yo of arrangement 1 87 ~~
1OOOF. 320 350 350 330 338
For both tube bundles the third pair of horizontal rows gave results somewhat above the average for the entire bundle. The steam trap on the bottom pair of rows did not function properly in arrangement 1, and those data are not included. Comparing the averages for the top eight rows at a given At,, the wider spacing of tubes gave, on the average, 8 per cent less flux than the narrower spacing, a difference within the precision of the data. The smooth curve for the sixty-tube apparatus with the narrower tube clearance is compared in Figure 7 with data for single chromium-plated copper tubes in the small-scale apparatus. Below over-all temperature differences of 55" F.,
Literature Cited (1) Abbott, M. D., and Comley, W. D., S.M. thesis in chem. eng.. Mass. Inst. Tech., 1938. (2) Christison, H. L.,S.B.thesis in chem. eng., Mass. Inst. Tech., 1938. (3) Cryder, D. S., and Finalborgo, A. C., Trans. Am. Inst. Chem. Engrs., 33, 346-61 (1937). (4) Cryder, D. S.,and Gilliland, E. R., IND.ENG.CHEM., 24, 13827 (1932): Refrig. Enu.. 25,78-83 (1933). ( 5 ) Dreselly,-R. A., S.M.-thesis in chem. eng., Mass. Inst. Tech., 1938. (6) Drew, T. B.,and Mueller, A. C., Trans. Am. Inst. Chem. Engrs., 33, 449-71 (1937). (7) Kaulakis, A. F., and Sherman, L. M., S.B. thesis in chem. eng., Mass. Inst. Tech., 1938. (8) Nagle, W.M., U. S. Patent 1,995,361 (March 26, 1935). (9) Sauer. E.T., Cooper, H. B. H., Akin, G. A., and McAdams, W. H.. Mech. Eng., 60, 669-75 (Sept., 1938). PRESENTIOD before the meeting of the American Institute of Chemical Engineers, Philadelphia, Pa.
