Heat Transfer to Boiling Liquids F. H. RHODES
AND C. H. BRIDGES Cornel1 University, Ithaca, N. Y.
The critical temperature difference at which the transition from nuclear boiling to film boiling takes place depends to a very great extent on the wettability of the solid heating surface by the boiling liquid. The presence of a film of oil or oleic acid at the surface of a steel heating tube markedly lowers this critical temperature. The addition of sodium carbonate or sodium chloride to the water tends to displace the film of oil or fatty acid and thus promotes nuclear boiling. With water boiling at a chromium-plated surface, the transition t o film boiling occurs at a much lower temperature than with water boiling at a clean steel surface.
T
*
-
HAT the thermal conductance between a hot solid surface and a boiling liquid is determined, in part a t least, by the ease with which the liquid wets the solid is suggested by observations made by various investigators. Jakob and his co-workers (2,s)found that when water is boiled a t a meballic surface coated with a thin film of oil, larger bubbles are formed and lower rates of heat transfer are obtained than when the ebullition takes place a t a clean metal surface. They attributed the difference in the size of the bubbles and in the rate of heat transfer to the difference in the smoothness of the surface. Pridgeon and Badger (6) obtained a higher rate of heat transfer from a polished copper tube than from a clean iron tube. Here again the difference may be due, in part a t least, to a difference in wettability and not entirely to the difference in roughness or to the presence on the iron of a thin insulating film of rust. I n most of the earlier discussions of the rate of heat transfer to boiling liquids it was tacitly assumed that the manner of boiling remains essentially constant over a wide range of values for the difference between the temperature of the solid surface and that of the boiling liquid. Isolated observations indicated that a t certain critical values for the temperature differential a marked change in the manner of boiling occurs, but the significance of these observations was not generally appreciated. Pridgeon and Badger (6) observed that in an experimental evaporator the rate of heat transfer increased regularly with the temperature differential until the latter reached a value of about 25” C. and then decreased. They made the following comment: “Does this indicate a film so thin and a rate of heat transfer so high that the tube begins to be insulated by a film of steam?” They did not, however, follow up this suggestion or make any further investigation of this phenomenon. Lang (4), working with an evaporator for salt water, also found that at certain rather definite critical values for the temperature differential there is a distinct maximum in the over-all rate of heat transfer. He did not further investigate this phenomenon. Jakob and Linke (2, 3) observed that when water was boiled a t the surface of an electrically heated wire, the wire burned out when the difference in temperature
between the surface of the wire and the water reached about 15.5’ C . They did not further investigate the possibility of an abrupt change in the unit rate of heat transfer a t this point. Nukiyama (6) appears to have been the first to appreciate the possibility of the existence of a definite maximum, a t comparatively low values for the temperature differential, in the unit rate of heat transfer to a boiling liquid. Drew and Mueller (1) published the first really comprehensive discussion of the subject of the effect of temperature differential on the unit rate of transfer of heat from a solid surface to a boiling liquid. They found that boiling a t a solid surface may take place in either of two distinctly different ways. With relatively low values for the temperature differential, separate bubbles are formed a t the solid surface; these break free and rise separately through the solution. They designate this type of boiling as nucleate boiling. As the temperature difference is increased, no change occurs a t first in the manner of boiling; the effect of the increased temperature difference is to increase the rate a t which the separate bubbles are formed and evolved. This, in turn, results in an increase in the agitation of the solution and therefore in the unit rate of heat transfer. Finally, however, a point is reached a t which the vapor formed a t the surface of the heating element is not liberated as separate bubbles but is evolved as a continuous film covering the heating surface. This film is an effective insulating agent; as soon as “film boiling” begins, the unit rate of heat transfer drops rapidly to far below the maximum value obtainable with nucleate boiling. Some heat is still being transferred, even through the continuous layer of vapor; therefore the formation of vapor continues, although a t a much lower rate. The vapor that is formed disengages rather slowly from the continuous film. Drew and Mueller, working with a copper coil heated internally with steam and immersed in the boiling liquid, found that the maximum in the over-all rate of heat transferi. e., the point a t which the transition from nucleate boiling to film boiling takes place-occurs a t a temperature differential of 30” to 50” C., depending on the nature of the liquid. Their reported results deal entirely with organic liquids, although in the discussion of the paper it was brought out that 1401
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INDUSTRIAL AND ENGINEERING CHEMISTRY
they had observed a similar maximum in the rate of heat transfer to boiling water when the temperature differential was about 40" to 50" C. They commented, however, that with water the critical temperature difference "varies considerably with the nature of the heating surface and with the presence of small addition agents to the water." In the work of Drew and Mueller, as well as in most of the other work on this subject, the rates of heat transfer and the temperature difference that were measured were the over-all values. The present knowledge of the transfer of heat to boiling liquids has been well summarized in an article by Sauer, Cooper, Aiken, and McAdams (7). In these previously published observations on the rate of transfer of heat to a boiling liquid, it has frequently been observed that the transition from nucleate boiling to film boiling may be influenced markedly by changes in the condition of the surface or by the presence in the liquid of substances that affect the wettability of the solid. No specific information on this aspect of the problem has, however, been published. It appears probable that a decrease in the pressure under which the boiling occurs should affect the rate of heat transmission in the region of nucleate boiling and the temperature difference a t which the change to film boiling should occur. This point has, apparently, not been thoroughly investigated.
Experimental Procedure The commonly used method for the determination of the individual film resistance between a solid surface and a fluid involves the direct determination of the temperature of the surface by the use of thermocouples. In the determination of the film resistance between a heated solid and a boiling liquid, this method is not very satisfactory. It is difficult to insert a thermocouple into the solid without changing either the condition of the surface in the neighborhood of the junction or the flow of heat a t that point. Furthermore, the formation of a bubble of vapor immediately above the junction of the thermocouple temporarily interferes with the flow of heat and causes the temperature to' rise locally; the release of the bubble brings cool liquid into contact with the metal and results in a sharp local drop in temperature. The result is that the readings obtained from the thermocouple are so erratic that they have little or no quantitative significance. It has been shown that the rate of heat transfer between a liquid flowing through a cylindrical pipe and the inner wall of the pipe can be fairly accurately expressed by the Dittus-Boelter equation: hD/k = 0.0225(Dup/~)o~'(Cp/k)s~3
where h = film conductance, P. c. u./sq. ft./hr./" C. D = internal diameter of tube, ft. k = thermal conductivity of liquid, P. c. u./hr./sq. ft./O C./ft. u = velocity of liquid, ft./hr. p = density, lb./cu. f t . p = viscosity, lb., hr., ft. units C = specific heat of liquid
If we circulate a hot liquid of known physical properties a t a known rate of flow through a cylindrical tube of known material and dimensions immersed in a boiling liquid, we should be able to compute the magnitude of the inner film conductance. The conductance of the wall of the tube can be calculated. The over-all conductance can be computed from the measurement of the total amount of heat transferred in unit time and the known dimensions of the tube. From the over-all conductance and the conductances of
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the wall of the tube and of the inner film,we can find the resistance of the film between the outer surface of the tube and the boiling liquid. The value thus obtained is an average value for the surface of the tube. As a heating liquid mercury offers several advantages. It can be heated to considerably above the boiling point of water or of many organic liquids. Because of its high density and its thermal conductivity, it gives large values for the inside film conductance-i. e., low values for the resistance between the liquid and the inner wall of the tube. Thus the inside film resistance is only a relatively small fraction of the total over-all resistance; a small percentage error in measuring or computing this inner film resistance has only a slight relative effect on the calculated value for the resistance of the film between the outer wall of the tube and the boiling liquid. The apparatus used in these experiments consisted essentially
of a horizontal cylindrical boiling chamber provided with a
horizontal heating tube through which hot mercury was circulated, means for circulating and heating the mercury and measuring its rate of flow, means for measuring the temperature of the mercury entering and leaving the boiling chamber, and a condenser to collect and return to the boiling chamber the vapor evolved by the boiling liquid. The boiling chamber was made from a 6-inch section of Pyrex glass tubing,? about 31/2 inches in internal diameter, mounted between metal end plates. The heating tube was of drawn steel tubing, 0.25 inch in external diameter and 0.192 inch in internal diameter, insulated and held in place by asbestos-packed stuffing boxes in the end plates. The bottom of the heating tube was about 11/& inches above the bottom of the boiling tube. The length of heating tube exposed to the boiling liquid was 51a/10 inches. The heating tube was continued beyond the inlet end to provide an adequate calming section. One end plate of the boiling tube was provided with an opening through which vapor could be discharged to a reflux condenser; the condensate was returned throu h a trapped return line to the opposite end of the boiling &amber. Provision was made for connecting a vacuum pump to the discharge from the condenser so that the entire system could be maintained under controlled reduced pressure. The mercury discharged from the heating tube was passed through a water-cooled coil to bring it to about room temperature. (In some of the experiments, this coil was by-passed.) It then passed to the iron gear pump that provided circulation, and from there to a thin-plate orifice meter ( 3 / ~ i n c horifice in a 1/2-inch chamber), and to a heater section consisting of a length of pipe wound with a resistance wire. The rate of flow of mercury was controlled by a steel valve set before the gear pump; the temperature of the mercury entering the boiling chamber was controlled by a rheostat in circuit with the resistance heater. The entire line carrying hot mercury was thermally insulated. Calibrated iron-constantan thermocouples were used to measure the temperature of the mercury entering and leaving the heating chamber. I n computing the rate of flow of mercury from the readings on the differential manometer connected across the orifice meter, the density of the mercury at the temperature a t the meter was used. The rates of flow, as computed, are probably accurate to within a t least 3 per cent. The electromotive forces developed by the thermocouples were read to within 0.001 millivolt ; the temperatures are probably accurate to within about 0.2' C. I n no case were readings taken until constant conditions had been attained, as indicated by constant temperatures of the mercury entering and leaving the boiling chamber. At each rate of flow and temperature difference, duplicate readings were taken. Because of the relatively large amount of heat lost from the boiling chamber and the vapor lines leading to the condenser, no attempt was made to obtain a balance between the amount of heat supplied to the boiling tube and the amount removed in the condenser. The point a t which the transition to true film boiling occurs was always sharply defined. The continuous film of vapor a t the surface of the tube appeared a t the end a t which the hot
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INDUSTRIAL AND ENGINEERING CHEMISTRY
mercury entered; as soon as such film appeared it acted as an effective insulator and greatly decreased the drop in temperature of the mercury flowing through the tube. Almost immediately after the appearance of the continuous film a t any point, this film spread over the entire surface of the tube. In a few cases and under very special conditions it was possible to maintain, for a short time, film boiling a t the inlet end of the chamber and nucleate (or "intermediate") boiling a t the outlet end, but this condition was a very unstable one. When two types of boiling could be maintained simultaneously, we had the rather anomalous result of having a much lower over-all rate of heat transfer between the warmer end of the tube and the liquid than between the cooler end and the liquid. I n some instances it was observed that just prior to the establishing of true film boiling the bubbles formed a t the surface of the lower half of the heating tube appeared to agglomerate, above the upper half, to form a more or less continuous layer.
1403
surface of a steel tube. When distilled water was allowed to stand for some time in contact with the steel heating tube, there was a slight corrosion, which resulted in a change in the surface condition of the metal. This corrosion may be prevented by the addition of a trace of sodium chromate (100 parts per million) to the water. Experiments showed that the rate of heat transfer to a solution of this composition is identical with that to pure water under the same temperature differential.
Method of Computation From any one series of measurements made a t constant conditions, the total amount of heat transferred in unit time was computed from the weight of mercury flowing in unit time, the specific heat of the mercury, and the drop in temperature of the mercury flowing through the tube. From this, the known dimensions of the tube, and the average difference between the temperature of the mercury and that of the boiling liquid, the over-all rate of heat transfer was calculated. This was expressed in terms of P. c. u. per square foot of external surface of the heating tube per hour per O C. From the Dittus-Boelter equation, hD/k
0.0225(pD~/~)'.*(C~/k)'.'
values of inner film conductance, h, for various rates of flow of mercury and various average temperatures of the mercury were calculated. These were converted to the basis of P. c. u. per square foot of the external surface of the heating tube per hour per " C. The conductance of the wall of the tube was computed and expressed in the same units and on the same basis. From these values, graphs were drawn showing the variation of the total resistance of the inner film and the tube wall as a function of the average temperature of the mercury. The total resistances were expressed in terms of reciprocal P. c. u. per hour per " C. difference in temperature per square foot of external tube surface. One such graph was drawn for each of several rates of flow within the range used in the experimental work. From the overall thermal resistance, computed BS described above, and the combined resistance of the inner film and the tube wall, as read from the graphs, the resistance of the external film was calculated directly. From the over-all difference in temperature between the mercury and the liquid and the ratio of the outer film resistance to the total resistance, the drop in temperature between the outer surface of the tube and the liquid was computed. I n the following discussion, all temperature differences are expressed in terms of the difference in temperature through this film. I n computing the resistance of the film between the tube wall and the mercury, the values for the physical properties of mercury a t various temperatures were taken from International Critical Tables.
Experimental Results The first series of results was made to determine the effect of the pressure on the manner of boiling of water a t the
TEMP.
DIFF. "C.-
~
FIGURE 1. EFFECTOF PRESSURE ON FILM
CONDUCTANCE BETWEEN CLEANSTEEL TUBEAND BOILING WATER 1 AtmosDheric Dressure -. Absolite bre&ure, 20 inches mercury Absolute pressure, 10 inches mercury 4. Abaolute pressure, 5 inches mercury 5 . Absolute pressure. 2 inohes mercury
2.
3.
Figure 1shows, for various pressures, the change in the film conductance with change in the difference between the temperature of the surface of the tube and the temperature of the boiling water. At atmospheric pressure (curve 1) and at an absolute pressure of 20 inches of mercury (curve 2) the film conductance increased continuously throughout the entire range of temperature differentials used in the experiments. Even with a temperature differential of 60" C. there was no evidence of film boiling. At pressures of 10 and 5 inches of mercury (curves 3 and 4) there was a more or less definite maximum in the film conductance a t a temperature differential of about 60" to 65" C. I n neither of these cases was true film boiling established; a t the high temperature differences the evolution of vapor was so rapid that the surface of the tube was more or less blanketed by the bubbles, but the boiling was still definitely of the nucleate type. Even a t an absolute pressure of 2 inches of mercury (curve 5 ) no evidence of film boiling was obtained. I n general, even with nucleate boiling there seems to be a more or less consistent decrease in the outside film conductance with decrease in the absolute pressure. This can be attributed to tlie increase in the specific volume of the vapor, with resulting increase in blanketing action. It is also possible that the increase in the viscosity of the boiling liquid which occurs when the pressure, and therefore the temperature, is decreased may be a factor in determining the change in film conductance. I n one experiment a waxed packing material was used for sealing the heating tube in the stuffing boxes a t the end of the boiling chamber. I n the insertion of the heating tube through this packing, the surface of the tube became coated with a very thin film of the wax. With this contaminated tube were obtained the results shown on Figure 2. At a temperature differential of about 18" C. the boiling was definitely nucleate. When the temperature differential was increased to about 26" C., there was a definite change in
INDUSTRIAL AND ENGIlVEERING CHEMISTRY
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the manner of boiling, although true and typical film boiling was not yet fully established. Individual bubbles were formed as in nucleate boiling, but these separate bubbles tended to flow together a t the surface of the metal so that relatively large areas of the heating surface were temporarily blanketed. At a temperature differential of about 34" C. this intermediate type of boiling still persisted, although the tendency towards the agglomeration of the bubbles was even more p r o n o u n c e d . At a t e m p e r a t u r e g r a d i e n t of about 45" C. true film boiling was established; the tube was covered with a continuous layer of vapor and the film conductance had dropped to about one-tenth of the value obtained at a temperature gradient of 20" c. This series of experiments shows defin i t e l y t h e import a n c e of the wettabilits of the solid FIGURE 2. COEFFICIENT OF surface in determinHEAT TRANSFERTO BOILING ing the film conductWATERFROM A STEELTUBE COVERED WITH A FILMOF WAX ance between the solid and a boiling liquid. The presence on the surface of the metal of a thin film of material not readily wetted by the liquid tends to promote film boiling. In the region in which true nucleate boiling occurs, the presence of the thin film of wax appears not to have any marked effect on the rate of heat transmission; a t a temperature difference of 20" C. the contaminated tube showed a film conductance of 1390 P. c . u., while the clean tube at the
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same temperature difference showed a conductance of about 1310 P. c. u. I n another series of experiments about 0.05 gram of oleic acid was added to the 300 cc. of water in the boiling chamber. The effects of this addition are shown by Figure 3. Curve 1 was obtained with pure water; curve 2 shows the results obtained after the addition of the oleic acid. I n this case the tube was heated to a temperature high enough to establish film boiling; then the temperature of the mercury flowing through the tube was progressively decreased. True film boiling was obtained a t a temperature difference of 55" C. At temperature differences of 35", 25.45O, and 19.03" C. the boiling was of the intermediate or transition type. Only when the temperature gradient had dropped to about 15" C. was true nucleate boiling established. When typical nucleate boiling was finally obtained, the rate of heat transmission was comparable with that observed with clean water on a clean surface under the same temperature gradient.
B a a
I
ii = i u R'
FIQURE 4. EFFECT OF MINERAL OIL ON FILM CONDUCTANCE AND MANNEROF BOILING 1. 2. 3. 0 '.
Et' LL
s3 8
R
s'
B 5
3 i s
TEMP.
DIFF. 'C.
FIGURE3. EFFECTOF PRESENCEOF OLEICACIDON FILMCONDUCTANCE AND MANNER OF BOILING 1. 2. 3.
Clean steel tube in clean water After addition of oleic acid After addition of sodium carbonate
Clean steel tube with clean water Tube with film of mineral oil Tube with film of mineral oil after addition of sodium carbonate
After this series of experiments one gram of sodium carbonate was added to the solution in the boiling chamber, and the temperature of the circulating mercury was again raised and lowered. A series of readings was made during the period of decreasing temperature. The results are shown by curve 3, Figure 3. Even with a temperature difference higher than 65" C. true film boiling was not obtained. When the differential temperature was reduced to 26.8" C., nucleate boiling was established; a t and below this temperature gradient the results agreed closely with those obtained with clean water on a clean tube. That not only fatty acids but also petroleum oils may induce film boiling is shown by Figure 4. Curve 1 represents the results obtained with pure water on a clean tube. After these experiments were made, a drop of paraffin-base lubricating oil was added to the water in the boiling chamber. At first the added oil floated on top of the water and was not carried to the metal surface; there was no immediate effect on the rate or manner of boiling. Sufficient water was then
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INDUSTRIAL AND ENGINEERING CHEMISTRY
withdrawn to bring the level of the water to about the midpoint of the tube, and the liquid was boiled for some time to ensure that the surface of the metal was well coated with oils. Water was then added to restore the normal level, the temperature of the mercury was increased to induce iilm boiling, and a series of measurements was made with decreasing temperature of the circulating mercury. The results are shown by curve 2. True film boiling persisted until the temperature differential had dropped to about 40" C.; even a t a temperature differential as low as 20" C. true nucleate boiling was not established. One gram of sodium carbonate was then added, and the series of measurements was repeated. Even a t a temperature difference of 70" C. true film boiling was not obtained. When the temperature gradient had dropped to 23.4" C. nucleate boiling was resumed; below this point the results parallel those obtained with clean metal and clean liquid. The effect of sodium carbonate addition in aiding the restoration of nucleate boiling on a tube that has been coated with oleic acid is easily explained on the basis of the assumption that the sodium carbonate reacts with the organic acid with the formation of soap and thus tends either to remove the adsorbed film completely or to convert it into a material which, even if adsorbed, is less effective in promoting film boiling. The similar action of sodium carbonate in counteracting the effect of the addition of petroleum oil cannot be explained by any such simple hypothesis. It is probable that in this case the added salt simply displaces the adsorbed film of oil.
1405
chloride was introduced into the chamber. There was an immediate change in the appearance of the boiling zone. The rate of boiling increased to about double the value obtained before the addition of the salt, and the continuous film of vapor was disrupted, although typical nucleate boiling was not established. As the temperature of the circulating mercury was decreased, the film conductance increased progressively, as shown by curve 2, Figure 5. (Curve 1 represents the results obtained with the oiled tube without salt.) Although typical nucleate boiling was not obtained until relatively' low temperature differentials were reached, the film conductance was always considerably higher in the p r e s e n c e of t h e sodium chloride. After this series of experiments the chamber was drained u' and cleaned, the tube a was again coated aw with a thin film of oil, water was placed t' in the boiling chamd ber, and film boiling E was e s t a b l i s h e d . T h e n 2 g r a m s of sodium chloride were added. Immediately the rate of boiling increased enormously. a' As the temperature of t h e c i r c u l a t i n g mercury was lowered, FIGURE6. IRREVERSIBILITY OF DISPLACEMENT OF OIL FILM BY the film conductance SODIUM CHLORIDU increased rapidly to a 1. Decreasing temperature differentials maximum of about 2. Increasing t~tiiporaturedifferentials 1900 a t about 25" C. temperature gradient and then again decreased. With temperature differentials between about 25" and 65" C. the boiling was of the intermediate type. With salt solution of this concentration, the tube that had been coated with oil gave, in the lower ranges of temperature differentials, considerably higher film conductances than were observed with a clean tube within these same ranges. T o determine whether or not the displacement of the adsorbed film of oil by the salt is permanent and irreversible, the following series of measurements was made. A tube freshly coated with mineral oil was placed in the boiling chamber, 300 cc. of water were added, and through the tube was circulated mercury a t a high enough temperature to provide true film boiling. Then one gram of sodium chloride was added, and a series of measurements was made with decreasing temperatures of the mercury. Boiling of the intermediate type occurred; with decrease in the temperature differential there was a progressive increase in the film conductance until finally a value substantially identical with that shown by pure water was obtained. These results are illustrated by curve 1, Figure 6. When this point was reached, the temperature differential was again increased and a second series of measurements was made. With increase in the temperature difference, the film conductance increased progressively (curve 2, Figure 6). No film boiling was observed even with a temperature drop of 50" C. through the film. Curve 2, depicting the change in the film conductance with the difference in temperature through the film, is approximately parallel with the graph obtained with pure water on a clean tube but lies slightly below it. These results indicate that the displacement of the oil film by the salt solution is substantially irreversible.
2 E
I
TEMR
DIFF. OC.
FIGURE 5. EFFECT OF SODIUM CHLORIDE ADDITION ON FILM CONDUCTANCE AT STEEL SURFACE COATED WITH MINERAL OIL 1. 2. 3.
'
Oil-coated tube with water alone Same tube with 0.33 per cent solution of sodium chloride Same tube with 0.66 per cent solution of sodium chloride
Not only sodium carbonate but also sodium chloride may be effective in displacing an adsorbed film of mineral oil. This is shown by the following experiments: The boiling chamber was washed free from soluble salts and the surface of the heating tube was coated with a thin film of the petroleum oil by wiping it with a cloth wet with the oil. The chamber was then filled with water, and hot mercury was circulated through the heating tube until true and typical film boiling was established. Then one gram of sodium
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Several series of experiments were made with a chromiumplated heating tube. The tube used in these experiments wa8 of the same material and dimensions as that used in previous work but was electrolytically plated with chromium on the outer surface. It was not polished after plating. The results obtained with the clean tube in pure water are shown by curve 1, Figure 7. Up to a temperature differential of slightly above 20" C . nucleate boiling was obtained, and the unit rates of heat transmission were high. A slight further increase in the temperature gradient resulted in a transition to film boiling; true film boiling was attained when the gradient reached about 45" C.
400
I
200 IO
20
30 TEMP.
40 5 0 . 60 DIFF: C.
70
80
FIGURE7. RATESOF HEATTRANSFER FROM A CHROMIUM-PLATED TUBE 1. 2. 3. 4.
Chromium-plated tube with pure water Same tube i n 0.166 per cent sodium carbonate solution Same tube with pure water, after boiling with dilute sodium carbonate Same tube with 0.66 per cent sodium chloride solution
When film boiling was established, 0.5 gram of sodium carbonate was added to the water. Immediately there was an enormous increase in the rate of boiling and a transition from film boiling to true nucleate boiling. The film conductance increased from about 150 to above 2300, which is considerably higher than the value observed for a clean iron tube under the same temperature differential. As the temperature gradient was decreased, nucleate boiling persisted. The film conductance diminished regularly with the temperature differential; when the differential was reduced to about 15" C., the film conductance was about comparable with that observed with clean iron and clean water. To show that the effect of the sodium carbonate was not caused by any marked permanent change in the smoothness or other characteristics of the surface of the chromium, the boiling chamber was rinsed thoroughly with water and was then filled with distilled water. A series of measurements of film conductance was then made. The results are shown by curve 3, Figure 7. The results were comparable with those obtained with the original chromiumplated tube, although true film boiling was not obtained until temperature differentials had been established that were appreciably higher than those required to give film boiling
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a t the surface of the original chromium-plated tube. Furthermore, a t low temperature differentials the conductances were considerably higher than those obtained with the original tube. The marked tendency of the chromium-plated tube to cause film boiling may be due to a true lack of wettability of chromium metal by water, or to the presence on the surface of the metal of a very thin film o f chromium oxide that is not readily wetted. The action of the sodium carbonate may be attributed either to the effect of this salt in increasing the interfacial tension between the metal and the solution or to its effect in removing the film of oxide. The difference between the results obtained with the original tube and those shown by the tube that had been exposed to the action of the sodium carbonate may be due to a slight change in the roughness of the surface, to the presence on the tube of a very thin adsorbed film of some material that promotes wetting, or to the removal by the carbonate of a film of chromium oxide that was not immediately and completely regenerated by exposure to water alone. After this series of experiments the tube was heated until true film boiling was established, and 2 grams of sodium chloride were added. There was an immediate increase in the rate of boiling and a transition to what appeared to be typical nucleate boiling. The film conductance, however, was not as high as that obtained with the solution of sodium carbonate. As the temperature differential was decreased, there was a t first only a gradual decrease in the film conductance. At a temperature gradient of about 35" C., however, the rate of change in film conductance with change in gradient began to increase. At the lower values for the temperature gradient the results obtained with the chromium-plated tube in the presence of salt solution were lower than those shown by the same tube in a solution of sodium carbonate but were somewhat above those obtained with a clean iron tube in clean water.
Conclusions The results of these experiments indicate that the manner of boiling of a liquid a t the surface of a solid and the observed unit rate of heat transfer are determined largely by the ease with which the liquid wets the solid. Relatively small amounts of certain substances in solution or in suspension in the liquid may have a great effect in altering the manner of boiling and in changing the unit rate of heat transfer. Some of the large variations in the evaporative capacities of evaporators operating under apparently similar or comparable conditions may be due to the presence of small amounts of contaminating substances that change the wettability of the solid by the liquid. The observed variation in the effects of various salt solutions in the quenching of steel may be caused by variations in the ease of wetting of the metal by the solution.
Literature Cited (1) Drew and Mueller, Trans. Am. Inst. Chem. Engrs., 33, 449
(1937). (2) Jakob, Mech. Eng., 58, 643 (1936). (3) Jakob and Fritz, Forsch. Gebiete Ingenieurw., 2,435 (1931). (4) Lang, Trans. Inst. Engrs. Shipbuilders Scot., 32, 279 (1888). (5) Nukiyama, J. SOC.Mech. Engrs. (Japan), 37, 367,S 53 (1934). (6) Pridgeon and Badger, IND. ENQ.CHEM.,16, 474 (1924). (7) Sauer, Cooper, Aiken, and MoAdams, Mech. Eng., 60, 673 (Sept., 1938).
RECEIVED June 23, 1938. Presented before the meeting of the Amerioan Institute of Chemical Engineers, Philadelphia, Pa., November 9 to 11, 1938.
