CA1170512A - Heat transfer surface for efficient boiling of liquid r-11 and its equivalents - Google Patents
Heat transfer surface for efficient boiling of liquid r-11 and its equivalentsInfo
- Publication number
- CA1170512A CA1170512A CA000402739A CA402739A CA1170512A CA 1170512 A CA1170512 A CA 1170512A CA 000402739 A CA000402739 A CA 000402739A CA 402739 A CA402739 A CA 402739A CA 1170512 A CA1170512 A CA 1170512A
- Authority
- CA
- Canada
- Prior art keywords
- flame
- metallic particles
- metal substrate
- substrate
- coating
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
- F28F13/00—Arrangements for modifying heat-transfer, e.g. increasing, decreasing
- F28F13/18—Arrangements for modifying heat-transfer, e.g. increasing, decreasing by applying coatings, e.g. radiation-absorbing, radiation-reflecting; by surface treatment, e.g. polishing
- F28F13/185—Heat-exchange surfaces provided with microstructures or with porous coatings
- F28F13/187—Heat-exchange surfaces provided with microstructures or with porous coatings especially adapted for evaporator surfaces or condenser surfaces, e.g. with nucleation sites
Landscapes
- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Coating By Spraying Or Casting (AREA)
Abstract
Abstract A heat transfer surface for boiling liquid re-frigerant 11 and its equivalents and method for producing the surface. The surface comprises a porous, open-cell coating at least 15 mils thick, of oxidized metallic par-ticles which are flame-sprayed onto a metal substrate.
This surface includes a substantial number of nucleate boiling cavities having an equivalent radius in the range of 1.5 to 4 mils, which are the result of one or more of the following conditions in the flame-spraying process:
1) The flame-spraying nozzle is oriented so that the metallic particles impact the metal substrate at an angle in the range of 30° to 75°; 2) The metal substrate is pre-heated to a temperature which is at least 600°F, but below the melting point of the substrate; and 3) The flame-spraying apparatus is positioned such that the metallic particles travel from 3 to 6 inches before impacting the substrate. These conditions create a porous coating with substantially more nucleate boiling cavities of the re-quired size for boiling liquid R-11 than the flame-spraying process disclosed in the prior art.
This surface includes a substantial number of nucleate boiling cavities having an equivalent radius in the range of 1.5 to 4 mils, which are the result of one or more of the following conditions in the flame-spraying process:
1) The flame-spraying nozzle is oriented so that the metallic particles impact the metal substrate at an angle in the range of 30° to 75°; 2) The metal substrate is pre-heated to a temperature which is at least 600°F, but below the melting point of the substrate; and 3) The flame-spraying apparatus is positioned such that the metallic particles travel from 3 to 6 inches before impacting the substrate. These conditions create a porous coating with substantially more nucleate boiling cavities of the re-quired size for boiling liquid R-11 than the flame-spraying process disclosed in the prior art.
Description
Description Title HEAT TRANSFER SVRFACE FOR
E~FICIENT BOILING OF LIQUID R-ll AND ITS EQUIVALENTS
Technical ~ield The subject invention generally pertains to a heat transfer surface and the method by which it is pro-duced, and specifically to-a porous boiling surface for efficiently boiling liquid R-ll and its equivalents and the method for flame-spraying such a surface.
Backaround Art -It is well known that a porous boiling surface is much more efficient in transferring heat to a boiling liquid than is a smooth surface. The improvement in ef-ficiency is due to the interconnected nucleate boiling cavities provided in the porous surface, which act as sites for the liquid to vaporize and form bubbles. In a porous coating having the proper physical characteristics, liqui~ is drawn into the open-celled cavities by capillary action. Ideally, when a vapor bubble forms in a nucleate boiling cavity and breaks away, part of it is retained in the cavity to act as a nucleus or seed for the next bubble.
If the cavities are too large, the vapor bubble may escape completely, cr capillary force may not effectively draw liquid into the cavity. If the cavities are too small, vapor bubbles may not readily form without substantial superheat of the liquid surrounding the surface.
7~
The above theory is discussed at much greater length in U.S. Patents, Nos. 3,384,154 to Milton and 3,990,862 to Dahl et al. The patent to Milton discloses a method of producing a porous boiling surface by sin-tering metallic particles of from 1 to 50 micron size toa metal surface. The ~articles are applied as a slurry mixed with a plastic binder. When tl~e slurry i5 heated in a furnace, the binder is driven off and the Darticles are sintered to the metal base.
In the patent to Dahl et al, as in the present invention, metallic particles are flame-sprayed onto a metal substrate to form a porous, open-celled coating.
An excess of oxygen, beyond the stoichiometric requirement for complete combustion of the acetylene fuel gas, is pro-vided in the flame-spraying process. As the particles transit from the flame-spraying nozzle to the metal sur-face, an oxide film is thus formed on the metallic par-ticles due to the heat and excess oxygen in the flame.
These particles impact the surface and are adhesively fused to the surface and to each other by the oxide film, thereby forming a porous coating with interconnected open cells.
It has been experimentally determined that a flame-spraying porous boiling surface, produced as taught by the Dahl et al patent, is very effective in transferring heat to a variety of liquids, particularly refrigerants such as R-12 and R-22. However, that surface has also - been shown to be not as efficient for boiling R-11 or R-113, for reasons that are not obvious. The '862 patent to Dahl et al states that the average pore radius of the flame-sprayed surface (prepared as taught in the specifi-cation of that patent) is in the approximate range of 0.3 to 6.0 mils. This range should encompass the desired nucleate boiling cavity size required to effeciently boil R-ll, yet the surface so-produced lacks the very high ~ ~?
.
-heat transfer capability with this liquid that alterna-tive enhanced heat transfer surfaces provide.
When used in a refrigeration cycle, R-ll has one of the highest co-efficient of performance (COP) ratings 5; of any of the commonly used and commercially available ` ' refrigerants. It is thus the preferred refrigerant for use in many temperature conditioning systems. There is therefore, a substantial economic motivation to develop a low-cost, highly efficient enhanced boiling surface for use with R-ll. For use with R-12 and R-22, the porous boiling surface produced as taught by the Dahl et al patent provides one of the lowest cost, high efficiency heat transfer boiling surfaces available. Alternative sur-faces for boiling R-ll are generally higher in cost, but are more efficient for this purpose than the prior art surface.
For these reasons, it is an object of this in-vention to produce a flame-sprayed porous boilin~ surface that is as efficient at transferring heat for boiling R-ll, R-113, and their equivalents as are higher cost alterna-tives.
It is a further object of this invention to provide in a flame-sprayed porous surface a greater density of nucleate boiling cavities having an equivalent radius in the range l.S to 4 mils to more efficiently transfer heat to a li~uid having surface tension characteristics similar to R-ll and R-113.
These and other objects of the subject invention will become evident from the disclosure which follows and by reference to the attached drawings.
1 1 70$ 1-2 DISCLOSURE OF THE INVENTION
Our heat transfer surface is especially efficient in boiling liquid R-ll and its equivalents, and is produced using a flame-spraying apparatus. The surface is produced by a process which includes preheating a metal substrate to a temperature below its melting point while flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame. The flame-spraying apparatus is oriented such that the metallic particles impact the metal substrate at an angle substantially less than 90. One or more passes of the flame-spraying apparatus deposits on the substrate an open-celled porous coating at least 15 mils Lhick comprising the oxidized metallic particles, parts of which are fused to the substrate and to each other. A substantial quantity of nucleate boiling cavities are thus formed by the open cells of the coating, having an equivalent radius of from 1.5 to 4 mils.
According to one aspect of the present invention, there is provided a heat transfer surface, especially efficient in boiling liquid refrigerant-ll and its equivalents, produced by a process in which a flame-spraying apparatus is used, said process comprising the steps of flame spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45 to 60, depositing on the substrate an open-cell, porous coating at least 15 mils thick, said , .
~ 170512 coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
According to a further aspect of the present invention there is provided a method for producing a heat transfer surface, especially efficient in boiling liquid refrigerant-ll and its equivalents, using a flame-spraying apparatus, comprising the steps of flame spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45 to 60, the.e~v deposit:ina on tlle substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
Brief Description of the Drawinqs Figure 1 is a drawing depicting a cross-sectional photomicrograph view of the flame-sprayed surface produced accordinq~
to the teachings of the prior art.
Figure 2 is a drawing depicting a cross-sectional photomicrograph view of the flame-sprayed surface oE the subject invention, at the same magnification factor (35 times) as that in 1 1705~ 2 Figure 1.
Figure 3 illustrates -schematically the process by which the porous coating of the subject invention is applied to a metal substrate, specifically to a rotating tubular structure for use in a heat exchanger.
'~~` ll7nsl2 `
Figure 4 is a graph showing the relationshiP of the angle at which the metallic particles impact the metal substrate to the boiling performance of a resulting sur-face with liquid R-ll.
Figure 5 is a gra~h s~lowing the èffect of the thickness of the flame-sprayed coating relative to the - boiling performance of the resulting surface with li~uid R-ll.
Figure 6 is a graph showing the relationship between the tem~erature of the metal substrate at the time the metallic particles impact thereon and the boiling performance of the resulting surface in liquid R-ll.
Figure 7 is a graph showing the effect of s?ray distance, i.e., the distance which the metallic particles travel between the flame-s~raying apparatus and the point of impact on the metal substrate, upon the boiling per-formance of the resulting surface in liquid R-11.
Description of the Preferred Embodiments With reference to Figure 1, the flame-sprayed porous heat transfer surface 10, produced by the method taught in the prior art patent to Dahl et al illustrates the open-cell structure which forms nucleate boiling cavities 11. This surface 10 comprises oxidized metallic particles 12 which are cohesively bound together and to the metal substrate 13 in a rando~l pattern.
As taught in the '862 patent to Dahl et al, heat transfer surface 10 was produced using a flame-spraying apparatus supplied with acetylene gas as a fuel and oxygen gas as the oxidizer. Substantially pure alu-30 minum particles, ranging in size from -100 to +325 mesh were applied to a copper metal substrate 13 with a spray apparatus oriented at 90 thereto. The flow rates of the acetylene and oxygen gases were set at 18 and 45 cubic 11~0512 ..
feet per hour, respectively. Approximately 3.75 pounds per hour of oxidized metallic particles 12 were applied to the metal substrate 13 from a distance of 12 inches, to form the metal substrate 13. The coatin~ depth or thickness is approximately 12 to 15 mils. Prior to flame-spraying, metal substrate 13 was cleaned and roughened by a grit blasting process. The above conditions of the flame-spraying process used to produce prior art surface 10 are generally in accord with the teachings of the patent to Dahl et al.
For purposes of comparison, the heat transfer surface of the subject invention, senerally denoted by reference numeral 14, i5 illustrated in Figure 2. Both Figures 1 and 2 represent photomicrographs taken of a randomly selected cross section of the heat transfer surfaces, enlarged by the same magnification factor.
Likewise, heat transfer surface 14 comprises a copper metal substrate 15 which is flame-sprayed with oxidized aluminum metallic particles 16; however, the process by which these metallic particles 16 are applied, differs significantly from the process taught in the prior art.
In comparing the prior art heat transfer surface 10 and the heat transfer surface of the subject invention 14, it should be immediately evident that these two surfaces differ in thickness of the coating, and in the relative size of the nucleate boiling cavities formed therein.
Heat transfer surface 14 has a coating applied to a depth - of 20 to 28 mils and includes nucleate boiling cavities 11 formed in the prior art surface 10. Although the random structure of prior art surface 10 and the present heat transfer surface 14 are difficult to describe objectively, it may be said that heat transfer surface 10 has a more compact structure, whereas heat transfer surface 14 appears to have a more open structure, or to be "fluffier" in ap-pearance.
R~ 9 ~.
The process used to produce the heat transfersurface 14 differs from that used to produce the prior art surface 10 in the followin~ manner. The copper metal substrate 15 was ~reheated so that the temperature of its surface at the pointof impact of the oxidized metallic particles 16 reached approximately 730F. The aluminum particle feed rate was set for 5.5 lb/hr. Two passes of the flame-spraying apparatus were used to deposit the coating of the heat transfer surface 14. In the first pass, flame-spraying apparatus was oriented ~7ith the nozzle directed generally in the relative axial direction of tube travel so that the aluminum metallic particles 16 impacted the metal substrate 15 at a relative angle of 45 thereto. In addition, the flame-spraying nozzle was po-sitioned substantially closer to the metal substrate 15than was recommended in the prior art, so that the oxidized metallic particles 16 travelled only approximately 4 inches before impacting the metal substrate 15. In the second pass made with the nozzle directed generally opposite the relative axial direction of tube travel, the oxidized aluminum metallic particles 16 impacted the metal substrate 15 at an angle of approximately 135 relative to the sub-strate 15, i.e., at an angle of approximately 90 relative to their line of flight in the first pass. All other con-ditions of the process were substantially the same asthose used to produce the prior art heat transfer surface 10 .
Although the subject invention may be used in conjunction with both a flat and a curved metal substrate 15, its primary use will likely be in conjunction with heat transfer tubes used in evaporative heat exchangers for boiling a liquid refrigerant, such as R-ll or its equivalents. For such a use, a liquid (e.g., water) cir-culated through the heat exchanger tubes would be cooled by heat transferred through the metal substrate 15 to evaporate the refrigerant liquid exposed to the nucleate boiling cavities 17 on the external sur~ace of ~he tube.
With reference to Figure 3, a preferred process is shown by which the flame-sprayed porous coating comprising oxidized metallic particles 16 may be applied to produce a heat exchanger tube 18. In a preferred embodiment of the subject invention illustrated in Figure 3, the wall of the heat exchanger tube 18 comprises the metal sub-strate 15. In the Figure, tube 18 is shown moving from left to right, while rotating about its longitudinal axis.
In the preferred production process, tube 18 is cau~ed to rotate at approximately 600 rpm and to traverse below the flarne-spraying apparatus at approximately 66 inches per minute. The metal substrate 15 of tube 18 is preheated ahead of the flame-spraying a~paratus by burner 19 USing ~PP gas or acetylene, and oxygen as the fuel and oxidizer, respectively. In the first pass of the flame-spraying process, a flame-spraying nozzle 20 is oriented so that a line through the longitudinal axis of the nozzle forms an angle A e~ual to approximately ~5 relative to the surface of the metal substrate 15. Angle A therefore nominallv represents the angle at which the oxidized metallic par-ticles 16 impact the metal substrate 15. The metallic particles 16 travel approximately 4 inches after leaving the flame-spraving nozzle 20 before impacting upon the metal substrate 15. Burner 19 is adjusted in its position and firing rate so that the temperature of the surface of the metal substrate 15 at the point where metallic particles 16 impact is approximately 730F. It should be understood that the metal substrate attains this temperature as a result o' both the heat provided by burner 19 and the heat provided by flame-spraying nozzle 20. ~etallic particles 16 are heated and oxidized as they travel to the metal substra~e 15 in the flame from nozzle 20, which generates and transfers substantial heat to the sub.strate in addition to that provided by the gas flame from burner 19.
.. . i 117~512 A second flame-spraying nozzle 21 is oriented so that a line through its longitudinal axis forms an angle B equal to 135, relative to the surface of the metal substrate 15. As shown in Figure 3, an~les A and B are co-planar; however, it will be apparent that flame-spray nozzle 21 may be located at some other oosition around the longitudinal axis of heat exchange tube 1~
while still providing a flame-sprayed porous layer ac-cording to the present invention. If flame-spraying nozzles 20 and 21 are relatively close together, the tem-perature of the metal substrate 15 may exceed the soften-ing temperature of the material comprising the heat exchange tube 18, causing it to deform. For this reason, a cooling blower 22 may be provided to direct a stream of cooling air onto the porous surface deposited by flame-spraying nozzle 20, prior to the deposition of the porous surface deposited by flame-spraying nozzle 21. Blower 22 cools the metal substrate 1~ and the first layer of the surface 14 such that the added heat from flame spray nozzle 21 does not overheat the heat exchange tube 18.
If flame-spray nozzles 20 and 21 are spaced sufficiently far apart either in time and/or distance, blower 22 is not required since the tube 18 will cool between flame-spray nozzles 20 and 21, thereby avoiding this overheating effect. Cooling blower 22 is therefore considered an optional requirement depending u~on the relative proximity of flame-spray nozzles 20 and 21 to each other in time and position.
As an alternative to the process shown in Figure 3, the porous coating 1~ may be applied in two separate flame-spraying operations, using onlv one nozzle.
Tube 18 would be reversed between passes or else the nozzle would be reversed. For a sufficiently short heat exchanger tube 18, preheating by burner 19 may not be re-quired prior to the second pass by the flame-spraying nozzle, depending upon the elapsed time between passes.
~.~
;' 1~70512 /~
~ urthermore, although the heat exchanger tube 18 is shown as moving past the flame-spraying nozzles 20 and 21, the necessary relative motion may be provided by traversing the burner 19 (and blower 22, if required) and flame-spray nozzles 20 and 21 along an axially sta-- tionary, rotating tube 18. Variations such as these will of course be apparent to those skilled in the art.
It has also been determined that the heat trans~
fer surface 14 may be produced by using a single angled application of metallic particles to the metal substrate 15 by flame-spraying nozzle 20. In this case, the rate of traverse is reduced to approximaely 33 inches/minute while the metallic particle feed rate is maintained at 6.6 lbs./hr. Since the heat exchanger tube 18 is moving at one-half the speed that it is when coated with a double angle flame-spraying process, the POrOUs heat transfer surface 14 achieves approximately the same depth. However, as will be shown hereinbelow, the single angle flame-spraying process does not produce a heat transfer surface 14 having the same high efficiency for boiling refrigerant 11 as does the surface 1~ produced by the double angle process.
In the '862 ~atent to Dahl et al, it is sug-gested that spray distance and angle, and substrate surface temperature are variables affecting porosity of the flame-sprayed deposit. This patent also teaches that a distance of generally 12" is appropriate for flame-spraying alu-minum particles, to allow a time of flight for the particles to be heated and oxidized. Furthermore, the prior art suggests that the coating may be applied to a thickness of greater or less than 12-15 mils, and that there should be a good distribution of size among the nucleate boiling cavities so that the resulting surface might be usable for boiling a variety of liquids. Never-theless, the prior art does not specifically teach or 7~S12 A~
su~gest a method which may be used to produce a surfacesuitable for very efficient boiling of refrigerant R-ll and its equivalents. The present inven~ion was developed after substantial experimentation as will be apparent from the following discussion.
It is believed that the heat transfe~ surface 14 includes substantially more nucleate boiling cavities 17 having an equivalent radius in the range of 1.5 to 4 mils than the prior art heat transfer surface 10. A
larger nucleate boiling cavity is required for the for-mation of vapor bubles in liquid R-ll than in liquid R-12 or R-22, because of R-ll's greater sur~ace tension. An equivalent of R-ll would have a similar surface tension.
It is thus believed that the subject invention provides more efficient nucleate boiling in R-ll than the prior art surface 10 because it has a higher proportion of nucleate boiling cavities of the required larger size.
An excellent method of determining the efficiency of a heat transfer surface for boiling a particular liquid involves immersing a tube provided with that surface in the liauid and measuring the tem~erature difference at boiling, between the liquidsurrounding the surface and the surface of the tube, when heat is applied to the in-ternal surface of the tube by means of an electric heater.
Typically, several thermocouples are attached to the sur-face of the test heat transfer tube and their average temperature indication during boiling is compared against the indicated saturation temperature of the li~uid in which the tube is immersed. The difference in temperature represents the wall superheat required for a given heat flux. A boiling superheat number low in magnitude in-dicates that the heat transfer efficienc~ of the surface under test is relatively high. From an economic standpoint, a low cost, high efficiency heat transfer surface provides a competitive advantage since heat exchangers of a given rating may be built with relatively less heat transfer 1~
a.~ ~
surface, resulting in less material used and correspondingly lower cost. This is especially significant when a heat transfer surface includes an expensive material such as copper.
5- Turning now to Figures 4-7, the affect of varying several different parameters involved in the flame-s~raying process to produce a porous boiling surface are shown in terms of observed boiling superheat when the resulting specimens were tested in liquid R-ll. Specimens for which test results are shown in the same Figure were prepared under e~uivalent conditions except where noted. In all tests, a heat flux e~ual to 9,000 BTU/hr-Ft2 was provided by the electric heating element sealed in the center of the test specimen. Results are shown for both samples prepared with a single spray angle (dash lines) and samples prepared with two angles (solid lines) relative to the copper metal substrate.
With reference to Fiaure 4, none of the samples were preheated prior to the application of the porous boiling surface to the metal substrate. The single angle sample produced with a spray angle equal to 90 has a boiling superheat equal to 2.7F and is representative of the performance of the heat transfer surface lO produced according to the teachings of the prior art. By comparison, a single angle surface produced with the spray angle equal to 60 shows a substantial improvement, having a boiling superheat equal to 2.3F. A more significant improvement is obtained however, when two angles are used, the first equal to 45 and the second e~ual to 135 (90 to the first 30 application). It will be understood that two angles are shown on the abscissa of this graph representing nominal angles as shown for angles A and B in Figure 3. As Figure 4 shows, even with no preheat, a double angle sample made with spray angles equal to 45/135 produces a boiling 35 superheat of only 1.9F.
In Figure 5, results are shown for specimens produced by the sinqle angle process which were not pre-heated prior to flame-spraying the metallic particles;
these specimens were made at mixed traverse speed. The results for specimens produced using the double angle process include some made with preheat; all were made at the same speed. All specimens for which results are shown in Figure 5 were made using a flame-spraying angle equal to 45, and in the case of the double angle process, with the second angle equal to 135, as previously ex-plained. The results of this series of tests indicates that a coating thickness in the range of 20-30 mils pro-vides the lowest boiling superheat inliquid R-ll.
The effects of the surface temperature of the metal substrate at the point at which the metallic par-ticles impact is shown in Fisure 6. In this test, all the specimens were made with the spray nozzle at an angle of 45, and in the case of the double angle specimens, the second angle was 135. This graph shows that an op-timum surface temperature lies in the range of 700 to800F, for both the single angle and double angle processes.
Figure 7 shows the effect of the distance be-tween the flame-spray nozzle and the point of im~act of the metallic particles on the metal substrate. All 25 specimens for this test were made without preheat with a spray angle equal to 45, and in the case of the double angle specimens, with the second angle at 135. As ~shown, optimum performance occurs at a spray distance of 3 inches for the single angle, and of 4 inches for the 30 double angle specimens.
As Figures 4-7 illustrate, the optimum ef-ficiency of a flame-spraYed porous boiling surface for boiling R-ll is achieved by producing that surface using a double angle process, with the f]ame-spraying a~paratus oriented so that the metallic particles impact the ~etal substrate from a distance of 4 inches at angles of 45 and 135, respectively, thereby providing a porous coating approximately 21 mils thick, and preheating the metal sub-strate so the temperature of its surface at the point where the metallic particles impact reaches approximately 730F. Although these conditions were found optimum specifically for aluminum particles sprayed on a copper metal substrate, it is believed that an improvement in the heat transfer performance for boiling specific liquids such as R-ll might also result if the subject inven~ion were practiced using other materials. Besides copper, the metal substrate might comprise steel, aluminum, or titanium; likewise, copper, steel or nickel metallic par-ticles might be used.
The process for applying a flame-sprayed porous boiling surface to efficiently boil R-ll and its equiva-lents has been disclosed with detail directed to its use on the exterior surface of heat exchange tubing. Those skilled in the art will understand how this process may be easily adapted to flame-spraying a porous boiling sur-face which is equally efficient on other types of heatexchange surfaces, such as plates, or finned surfaces - having enhanced heat exchange area. It will be under-stood that modifications to the invention such as these will be apparent to those skilled in the art within the scope of the invention, as definéd in the claims which follow.
E~FICIENT BOILING OF LIQUID R-ll AND ITS EQUIVALENTS
Technical ~ield The subject invention generally pertains to a heat transfer surface and the method by which it is pro-duced, and specifically to-a porous boiling surface for efficiently boiling liquid R-ll and its equivalents and the method for flame-spraying such a surface.
Backaround Art -It is well known that a porous boiling surface is much more efficient in transferring heat to a boiling liquid than is a smooth surface. The improvement in ef-ficiency is due to the interconnected nucleate boiling cavities provided in the porous surface, which act as sites for the liquid to vaporize and form bubbles. In a porous coating having the proper physical characteristics, liqui~ is drawn into the open-celled cavities by capillary action. Ideally, when a vapor bubble forms in a nucleate boiling cavity and breaks away, part of it is retained in the cavity to act as a nucleus or seed for the next bubble.
If the cavities are too large, the vapor bubble may escape completely, cr capillary force may not effectively draw liquid into the cavity. If the cavities are too small, vapor bubbles may not readily form without substantial superheat of the liquid surrounding the surface.
7~
The above theory is discussed at much greater length in U.S. Patents, Nos. 3,384,154 to Milton and 3,990,862 to Dahl et al. The patent to Milton discloses a method of producing a porous boiling surface by sin-tering metallic particles of from 1 to 50 micron size toa metal surface. The ~articles are applied as a slurry mixed with a plastic binder. When tl~e slurry i5 heated in a furnace, the binder is driven off and the Darticles are sintered to the metal base.
In the patent to Dahl et al, as in the present invention, metallic particles are flame-sprayed onto a metal substrate to form a porous, open-celled coating.
An excess of oxygen, beyond the stoichiometric requirement for complete combustion of the acetylene fuel gas, is pro-vided in the flame-spraying process. As the particles transit from the flame-spraying nozzle to the metal sur-face, an oxide film is thus formed on the metallic par-ticles due to the heat and excess oxygen in the flame.
These particles impact the surface and are adhesively fused to the surface and to each other by the oxide film, thereby forming a porous coating with interconnected open cells.
It has been experimentally determined that a flame-spraying porous boiling surface, produced as taught by the Dahl et al patent, is very effective in transferring heat to a variety of liquids, particularly refrigerants such as R-12 and R-22. However, that surface has also - been shown to be not as efficient for boiling R-11 or R-113, for reasons that are not obvious. The '862 patent to Dahl et al states that the average pore radius of the flame-sprayed surface (prepared as taught in the specifi-cation of that patent) is in the approximate range of 0.3 to 6.0 mils. This range should encompass the desired nucleate boiling cavity size required to effeciently boil R-ll, yet the surface so-produced lacks the very high ~ ~?
.
-heat transfer capability with this liquid that alterna-tive enhanced heat transfer surfaces provide.
When used in a refrigeration cycle, R-ll has one of the highest co-efficient of performance (COP) ratings 5; of any of the commonly used and commercially available ` ' refrigerants. It is thus the preferred refrigerant for use in many temperature conditioning systems. There is therefore, a substantial economic motivation to develop a low-cost, highly efficient enhanced boiling surface for use with R-ll. For use with R-12 and R-22, the porous boiling surface produced as taught by the Dahl et al patent provides one of the lowest cost, high efficiency heat transfer boiling surfaces available. Alternative sur-faces for boiling R-ll are generally higher in cost, but are more efficient for this purpose than the prior art surface.
For these reasons, it is an object of this in-vention to produce a flame-sprayed porous boilin~ surface that is as efficient at transferring heat for boiling R-ll, R-113, and their equivalents as are higher cost alterna-tives.
It is a further object of this invention to provide in a flame-sprayed porous surface a greater density of nucleate boiling cavities having an equivalent radius in the range l.S to 4 mils to more efficiently transfer heat to a li~uid having surface tension characteristics similar to R-ll and R-113.
These and other objects of the subject invention will become evident from the disclosure which follows and by reference to the attached drawings.
1 1 70$ 1-2 DISCLOSURE OF THE INVENTION
Our heat transfer surface is especially efficient in boiling liquid R-ll and its equivalents, and is produced using a flame-spraying apparatus. The surface is produced by a process which includes preheating a metal substrate to a temperature below its melting point while flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame. The flame-spraying apparatus is oriented such that the metallic particles impact the metal substrate at an angle substantially less than 90. One or more passes of the flame-spraying apparatus deposits on the substrate an open-celled porous coating at least 15 mils Lhick comprising the oxidized metallic particles, parts of which are fused to the substrate and to each other. A substantial quantity of nucleate boiling cavities are thus formed by the open cells of the coating, having an equivalent radius of from 1.5 to 4 mils.
According to one aspect of the present invention, there is provided a heat transfer surface, especially efficient in boiling liquid refrigerant-ll and its equivalents, produced by a process in which a flame-spraying apparatus is used, said process comprising the steps of flame spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45 to 60, depositing on the substrate an open-cell, porous coating at least 15 mils thick, said , .
~ 170512 coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
According to a further aspect of the present invention there is provided a method for producing a heat transfer surface, especially efficient in boiling liquid refrigerant-ll and its equivalents, using a flame-spraying apparatus, comprising the steps of flame spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45 to 60, the.e~v deposit:ina on tlle substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
Brief Description of the Drawinqs Figure 1 is a drawing depicting a cross-sectional photomicrograph view of the flame-sprayed surface produced accordinq~
to the teachings of the prior art.
Figure 2 is a drawing depicting a cross-sectional photomicrograph view of the flame-sprayed surface oE the subject invention, at the same magnification factor (35 times) as that in 1 1705~ 2 Figure 1.
Figure 3 illustrates -schematically the process by which the porous coating of the subject invention is applied to a metal substrate, specifically to a rotating tubular structure for use in a heat exchanger.
'~~` ll7nsl2 `
Figure 4 is a graph showing the relationshiP of the angle at which the metallic particles impact the metal substrate to the boiling performance of a resulting sur-face with liquid R-ll.
Figure 5 is a gra~h s~lowing the èffect of the thickness of the flame-sprayed coating relative to the - boiling performance of the resulting surface with li~uid R-ll.
Figure 6 is a graph showing the relationship between the tem~erature of the metal substrate at the time the metallic particles impact thereon and the boiling performance of the resulting surface in liquid R-ll.
Figure 7 is a graph showing the effect of s?ray distance, i.e., the distance which the metallic particles travel between the flame-s~raying apparatus and the point of impact on the metal substrate, upon the boiling per-formance of the resulting surface in liquid R-11.
Description of the Preferred Embodiments With reference to Figure 1, the flame-sprayed porous heat transfer surface 10, produced by the method taught in the prior art patent to Dahl et al illustrates the open-cell structure which forms nucleate boiling cavities 11. This surface 10 comprises oxidized metallic particles 12 which are cohesively bound together and to the metal substrate 13 in a rando~l pattern.
As taught in the '862 patent to Dahl et al, heat transfer surface 10 was produced using a flame-spraying apparatus supplied with acetylene gas as a fuel and oxygen gas as the oxidizer. Substantially pure alu-30 minum particles, ranging in size from -100 to +325 mesh were applied to a copper metal substrate 13 with a spray apparatus oriented at 90 thereto. The flow rates of the acetylene and oxygen gases were set at 18 and 45 cubic 11~0512 ..
feet per hour, respectively. Approximately 3.75 pounds per hour of oxidized metallic particles 12 were applied to the metal substrate 13 from a distance of 12 inches, to form the metal substrate 13. The coatin~ depth or thickness is approximately 12 to 15 mils. Prior to flame-spraying, metal substrate 13 was cleaned and roughened by a grit blasting process. The above conditions of the flame-spraying process used to produce prior art surface 10 are generally in accord with the teachings of the patent to Dahl et al.
For purposes of comparison, the heat transfer surface of the subject invention, senerally denoted by reference numeral 14, i5 illustrated in Figure 2. Both Figures 1 and 2 represent photomicrographs taken of a randomly selected cross section of the heat transfer surfaces, enlarged by the same magnification factor.
Likewise, heat transfer surface 14 comprises a copper metal substrate 15 which is flame-sprayed with oxidized aluminum metallic particles 16; however, the process by which these metallic particles 16 are applied, differs significantly from the process taught in the prior art.
In comparing the prior art heat transfer surface 10 and the heat transfer surface of the subject invention 14, it should be immediately evident that these two surfaces differ in thickness of the coating, and in the relative size of the nucleate boiling cavities formed therein.
Heat transfer surface 14 has a coating applied to a depth - of 20 to 28 mils and includes nucleate boiling cavities 11 formed in the prior art surface 10. Although the random structure of prior art surface 10 and the present heat transfer surface 14 are difficult to describe objectively, it may be said that heat transfer surface 10 has a more compact structure, whereas heat transfer surface 14 appears to have a more open structure, or to be "fluffier" in ap-pearance.
R~ 9 ~.
The process used to produce the heat transfersurface 14 differs from that used to produce the prior art surface 10 in the followin~ manner. The copper metal substrate 15 was ~reheated so that the temperature of its surface at the pointof impact of the oxidized metallic particles 16 reached approximately 730F. The aluminum particle feed rate was set for 5.5 lb/hr. Two passes of the flame-spraying apparatus were used to deposit the coating of the heat transfer surface 14. In the first pass, flame-spraying apparatus was oriented ~7ith the nozzle directed generally in the relative axial direction of tube travel so that the aluminum metallic particles 16 impacted the metal substrate 15 at a relative angle of 45 thereto. In addition, the flame-spraying nozzle was po-sitioned substantially closer to the metal substrate 15than was recommended in the prior art, so that the oxidized metallic particles 16 travelled only approximately 4 inches before impacting the metal substrate 15. In the second pass made with the nozzle directed generally opposite the relative axial direction of tube travel, the oxidized aluminum metallic particles 16 impacted the metal substrate 15 at an angle of approximately 135 relative to the sub-strate 15, i.e., at an angle of approximately 90 relative to their line of flight in the first pass. All other con-ditions of the process were substantially the same asthose used to produce the prior art heat transfer surface 10 .
Although the subject invention may be used in conjunction with both a flat and a curved metal substrate 15, its primary use will likely be in conjunction with heat transfer tubes used in evaporative heat exchangers for boiling a liquid refrigerant, such as R-ll or its equivalents. For such a use, a liquid (e.g., water) cir-culated through the heat exchanger tubes would be cooled by heat transferred through the metal substrate 15 to evaporate the refrigerant liquid exposed to the nucleate boiling cavities 17 on the external sur~ace of ~he tube.
With reference to Figure 3, a preferred process is shown by which the flame-sprayed porous coating comprising oxidized metallic particles 16 may be applied to produce a heat exchanger tube 18. In a preferred embodiment of the subject invention illustrated in Figure 3, the wall of the heat exchanger tube 18 comprises the metal sub-strate 15. In the Figure, tube 18 is shown moving from left to right, while rotating about its longitudinal axis.
In the preferred production process, tube 18 is cau~ed to rotate at approximately 600 rpm and to traverse below the flarne-spraying apparatus at approximately 66 inches per minute. The metal substrate 15 of tube 18 is preheated ahead of the flame-spraying a~paratus by burner 19 USing ~PP gas or acetylene, and oxygen as the fuel and oxidizer, respectively. In the first pass of the flame-spraying process, a flame-spraying nozzle 20 is oriented so that a line through the longitudinal axis of the nozzle forms an angle A e~ual to approximately ~5 relative to the surface of the metal substrate 15. Angle A therefore nominallv represents the angle at which the oxidized metallic par-ticles 16 impact the metal substrate 15. The metallic particles 16 travel approximately 4 inches after leaving the flame-spraving nozzle 20 before impacting upon the metal substrate 15. Burner 19 is adjusted in its position and firing rate so that the temperature of the surface of the metal substrate 15 at the point where metallic particles 16 impact is approximately 730F. It should be understood that the metal substrate attains this temperature as a result o' both the heat provided by burner 19 and the heat provided by flame-spraying nozzle 20. ~etallic particles 16 are heated and oxidized as they travel to the metal substra~e 15 in the flame from nozzle 20, which generates and transfers substantial heat to the sub.strate in addition to that provided by the gas flame from burner 19.
.. . i 117~512 A second flame-spraying nozzle 21 is oriented so that a line through its longitudinal axis forms an angle B equal to 135, relative to the surface of the metal substrate 15. As shown in Figure 3, an~les A and B are co-planar; however, it will be apparent that flame-spray nozzle 21 may be located at some other oosition around the longitudinal axis of heat exchange tube 1~
while still providing a flame-sprayed porous layer ac-cording to the present invention. If flame-spraying nozzles 20 and 21 are relatively close together, the tem-perature of the metal substrate 15 may exceed the soften-ing temperature of the material comprising the heat exchange tube 18, causing it to deform. For this reason, a cooling blower 22 may be provided to direct a stream of cooling air onto the porous surface deposited by flame-spraying nozzle 20, prior to the deposition of the porous surface deposited by flame-spraying nozzle 21. Blower 22 cools the metal substrate 1~ and the first layer of the surface 14 such that the added heat from flame spray nozzle 21 does not overheat the heat exchange tube 18.
If flame-spray nozzles 20 and 21 are spaced sufficiently far apart either in time and/or distance, blower 22 is not required since the tube 18 will cool between flame-spray nozzles 20 and 21, thereby avoiding this overheating effect. Cooling blower 22 is therefore considered an optional requirement depending u~on the relative proximity of flame-spray nozzles 20 and 21 to each other in time and position.
As an alternative to the process shown in Figure 3, the porous coating 1~ may be applied in two separate flame-spraying operations, using onlv one nozzle.
Tube 18 would be reversed between passes or else the nozzle would be reversed. For a sufficiently short heat exchanger tube 18, preheating by burner 19 may not be re-quired prior to the second pass by the flame-spraying nozzle, depending upon the elapsed time between passes.
~.~
;' 1~70512 /~
~ urthermore, although the heat exchanger tube 18 is shown as moving past the flame-spraying nozzles 20 and 21, the necessary relative motion may be provided by traversing the burner 19 (and blower 22, if required) and flame-spray nozzles 20 and 21 along an axially sta-- tionary, rotating tube 18. Variations such as these will of course be apparent to those skilled in the art.
It has also been determined that the heat trans~
fer surface 14 may be produced by using a single angled application of metallic particles to the metal substrate 15 by flame-spraying nozzle 20. In this case, the rate of traverse is reduced to approximaely 33 inches/minute while the metallic particle feed rate is maintained at 6.6 lbs./hr. Since the heat exchanger tube 18 is moving at one-half the speed that it is when coated with a double angle flame-spraying process, the POrOUs heat transfer surface 14 achieves approximately the same depth. However, as will be shown hereinbelow, the single angle flame-spraying process does not produce a heat transfer surface 14 having the same high efficiency for boiling refrigerant 11 as does the surface 1~ produced by the double angle process.
In the '862 ~atent to Dahl et al, it is sug-gested that spray distance and angle, and substrate surface temperature are variables affecting porosity of the flame-sprayed deposit. This patent also teaches that a distance of generally 12" is appropriate for flame-spraying alu-minum particles, to allow a time of flight for the particles to be heated and oxidized. Furthermore, the prior art suggests that the coating may be applied to a thickness of greater or less than 12-15 mils, and that there should be a good distribution of size among the nucleate boiling cavities so that the resulting surface might be usable for boiling a variety of liquids. Never-theless, the prior art does not specifically teach or 7~S12 A~
su~gest a method which may be used to produce a surfacesuitable for very efficient boiling of refrigerant R-ll and its equivalents. The present inven~ion was developed after substantial experimentation as will be apparent from the following discussion.
It is believed that the heat transfe~ surface 14 includes substantially more nucleate boiling cavities 17 having an equivalent radius in the range of 1.5 to 4 mils than the prior art heat transfer surface 10. A
larger nucleate boiling cavity is required for the for-mation of vapor bubles in liquid R-ll than in liquid R-12 or R-22, because of R-ll's greater sur~ace tension. An equivalent of R-ll would have a similar surface tension.
It is thus believed that the subject invention provides more efficient nucleate boiling in R-ll than the prior art surface 10 because it has a higher proportion of nucleate boiling cavities of the required larger size.
An excellent method of determining the efficiency of a heat transfer surface for boiling a particular liquid involves immersing a tube provided with that surface in the liauid and measuring the tem~erature difference at boiling, between the liquidsurrounding the surface and the surface of the tube, when heat is applied to the in-ternal surface of the tube by means of an electric heater.
Typically, several thermocouples are attached to the sur-face of the test heat transfer tube and their average temperature indication during boiling is compared against the indicated saturation temperature of the li~uid in which the tube is immersed. The difference in temperature represents the wall superheat required for a given heat flux. A boiling superheat number low in magnitude in-dicates that the heat transfer efficienc~ of the surface under test is relatively high. From an economic standpoint, a low cost, high efficiency heat transfer surface provides a competitive advantage since heat exchangers of a given rating may be built with relatively less heat transfer 1~
a.~ ~
surface, resulting in less material used and correspondingly lower cost. This is especially significant when a heat transfer surface includes an expensive material such as copper.
5- Turning now to Figures 4-7, the affect of varying several different parameters involved in the flame-s~raying process to produce a porous boiling surface are shown in terms of observed boiling superheat when the resulting specimens were tested in liquid R-ll. Specimens for which test results are shown in the same Figure were prepared under e~uivalent conditions except where noted. In all tests, a heat flux e~ual to 9,000 BTU/hr-Ft2 was provided by the electric heating element sealed in the center of the test specimen. Results are shown for both samples prepared with a single spray angle (dash lines) and samples prepared with two angles (solid lines) relative to the copper metal substrate.
With reference to Fiaure 4, none of the samples were preheated prior to the application of the porous boiling surface to the metal substrate. The single angle sample produced with a spray angle equal to 90 has a boiling superheat equal to 2.7F and is representative of the performance of the heat transfer surface lO produced according to the teachings of the prior art. By comparison, a single angle surface produced with the spray angle equal to 60 shows a substantial improvement, having a boiling superheat equal to 2.3F. A more significant improvement is obtained however, when two angles are used, the first equal to 45 and the second e~ual to 135 (90 to the first 30 application). It will be understood that two angles are shown on the abscissa of this graph representing nominal angles as shown for angles A and B in Figure 3. As Figure 4 shows, even with no preheat, a double angle sample made with spray angles equal to 45/135 produces a boiling 35 superheat of only 1.9F.
In Figure 5, results are shown for specimens produced by the sinqle angle process which were not pre-heated prior to flame-spraying the metallic particles;
these specimens were made at mixed traverse speed. The results for specimens produced using the double angle process include some made with preheat; all were made at the same speed. All specimens for which results are shown in Figure 5 were made using a flame-spraying angle equal to 45, and in the case of the double angle process, with the second angle equal to 135, as previously ex-plained. The results of this series of tests indicates that a coating thickness in the range of 20-30 mils pro-vides the lowest boiling superheat inliquid R-ll.
The effects of the surface temperature of the metal substrate at the point at which the metallic par-ticles impact is shown in Fisure 6. In this test, all the specimens were made with the spray nozzle at an angle of 45, and in the case of the double angle specimens, the second angle was 135. This graph shows that an op-timum surface temperature lies in the range of 700 to800F, for both the single angle and double angle processes.
Figure 7 shows the effect of the distance be-tween the flame-spray nozzle and the point of im~act of the metallic particles on the metal substrate. All 25 specimens for this test were made without preheat with a spray angle equal to 45, and in the case of the double angle specimens, with the second angle at 135. As ~shown, optimum performance occurs at a spray distance of 3 inches for the single angle, and of 4 inches for the 30 double angle specimens.
As Figures 4-7 illustrate, the optimum ef-ficiency of a flame-spraYed porous boiling surface for boiling R-ll is achieved by producing that surface using a double angle process, with the f]ame-spraying a~paratus oriented so that the metallic particles impact the ~etal substrate from a distance of 4 inches at angles of 45 and 135, respectively, thereby providing a porous coating approximately 21 mils thick, and preheating the metal sub-strate so the temperature of its surface at the point where the metallic particles impact reaches approximately 730F. Although these conditions were found optimum specifically for aluminum particles sprayed on a copper metal substrate, it is believed that an improvement in the heat transfer performance for boiling specific liquids such as R-ll might also result if the subject inven~ion were practiced using other materials. Besides copper, the metal substrate might comprise steel, aluminum, or titanium; likewise, copper, steel or nickel metallic par-ticles might be used.
The process for applying a flame-sprayed porous boiling surface to efficiently boil R-ll and its equiva-lents has been disclosed with detail directed to its use on the exterior surface of heat exchange tubing. Those skilled in the art will understand how this process may be easily adapted to flame-spraying a porous boiling sur-face which is equally efficient on other types of heatexchange surfaces, such as plates, or finned surfaces - having enhanced heat exchange area. It will be under-stood that modifications to the invention such as these will be apparent to those skilled in the art within the scope of the invention, as definéd in the claims which follow.
Claims (21)
1. A heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, produced by a process in which a flame-spraying apparatus is used, said process comprising the steps of:
flame spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45° to 60°, depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
flame spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45° to 60°, depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
2. A heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, produced by a process in which a flame-spraying apparatus is used, said process comprising the steps of:
a) preheating a metal substrate to a temperature in the range 650° F. to 800° F.;
b) flame-spraying the preheated metal substrate with metallic particles which are oxidized by heat and excess oxygen provided in the flame; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
a) preheating a metal substrate to a temperature in the range 650° F. to 800° F.;
b) flame-spraying the preheated metal substrate with metallic particles which are oxidized by heat and excess oxygen provided in the flame; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
3. A heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, produced by a process in which a flame-spraying apparatus is used, said process comprising the steps of:
a) preheating a metal substrate to a temperature in excess of 650° F.;
b) flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45° to 60°; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
a) preheating a metal substrate to a temperature in excess of 650° F.;
b) flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45° to 60°; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
4. A heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, produced by a process in which a flame-spraying apparatus is used, said process comprising the steps of:
flame-spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 30° to 60° and in addition, at an angle in the range 120° to 150°; depositing on the substrate an open-cell, porous coating at least 15 mils thick; said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
flame-spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 30° to 60° and in addition, at an angle in the range 120° to 150°; depositing on the substrate an open-cell, porous coating at least 15 mils thick; said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
5. A heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, produced by a process in which a flame-spraying apparatus is used, said process comprising the steps of:
a) preheating a metal substrate to a temperature in excess of 650° F.;
b) flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic angle in the range of 30° to 60°
and in addition, at an angle in the range of 120° to 150°; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
a) preheating a metal substrate to a temperature in excess of 650° F.;
b) flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic angle in the range of 30° to 60°
and in addition, at an angle in the range of 120° to 150°; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
6. The heat transfer surface of claim 1 or 3 wherein the metallic particles impact the metal substrate at about a 45°
angle.
angle.
7. The heat transfer surface of claim 1, 2 or 3, wherein the steps of flame-spraying includes depositing the coating of metallic particles in two or more passes.
8. The heat transfer surface of claim 1, 2 or 3, wherein the flame-spraying apparatus is positioned during the process so that the distance the metallic particles travel from the flame-spraying apparatus to their point of impact on the metal substrate is within the range of 3 to 6 inches.
9. The heat transfer surface of claim 4 or 5, wherein the flame-spraying apparatus is positioned during the process so that the distance the metallic particles travel from the flame-spraying apparatus to their point of impact on the metal substrate is within the range of 3 to 6 inches.
10. The heat transfer surface of claim 2, 3 or 5 wherein the metal substrate is heated so that the temperature at the surface of the metal substrate is about 730°F. where the metallic particles impact thereon.
11. The heat transfer surface of claim 4 or 5 wherein the metallic particles impact the metal substrate at about a 45° angle in one or more passes, and in additional one or more passes, impact the metal substrate and the coating already deposited thereon at about a 135° angle, both angles being measured relative to the metal substrate.
12. The heat transfer surface of claim 1, 2 or 3 wherein the metal substrate is copper or an alloy of copper, and the metallic particles are aluminum or an alloy thereof.
13. The heat transfer surface of claim 4 or 5 wherein the metal substrate is copper or an alloy of copper, and the metallic particles are aluminum or an alloy thereof.
14. The heat transfer surface of claim 3 or 5 wherein the flame-spraying apparatus is oriented at an angle to the metal substrate and passes sufficiently slow in relation thereto that its flame preheats the metal substrate ahead of where the metallic particles impact.
15. A method for producing a heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, using a flame-spraying apparatus, comprising the steps of:
flame spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45° to 60°,thereby depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
flame spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45° to 60°,thereby depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
16. A method for producing a heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents,using a flame-spraying apparatus, comprising the steps of:
a) heating a metal substrate to a temperature in the range 650 F. to 800 F.;
b) flame-spraying the heated metal substrate with metallic particles which are oxidized by heat and metallic particles provided in the flame; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, comprising the oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
a) heating a metal substrate to a temperature in the range 650 F. to 800 F.;
b) flame-spraying the heated metal substrate with metallic particles which are oxidized by heat and metallic particles provided in the flame; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, comprising the oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
17. A method for producing a heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, using a flame-spraying apparatus, comprising the steps of:
a) preheating a metal substrate to a temperature in excess of 650 F.;
b) flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45° to 60°; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
a) preheating a metal substrate to a temperature in excess of 650 F.;
b) flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 45° to 60°; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
18. A method for producing a heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, using a flame-spraying apparatus, comprising the steps of:
flame-spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 30° to 60° and in addition, at an angle in the range 120° to 150°; depositing on the substrate an open-cell, porous coating at least 15 mils thick; said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
flame-spraying a metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic particles impact the metal substrate at an angle in the range of 30° to 60° and in addition, at an angle in the range 120° to 150°; depositing on the substrate an open-cell, porous coating at least 15 mils thick; said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of 1.5 to 4 mils.
19. A method for producing a heat transfer surface, especially efficient in boiling liquid refrigerant-11 and its equivalents, using a flame-spraying apparatus, comprising the steps of:
a) preheating a metal substrate to a temperature in excess of 650 F.;
b) flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic angle in the range of 30° to 60°
and in addition, at an angle in the range of 120° to 150°; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
a) preheating a metal substrate to a temperature in excess of 650 F.;
b) flame-spraying the metal substrate with metallic particles which are at least partly oxidized by heat and excess oxygen provided in the flame, the flame-spraying apparatus being oriented such that the metallic angle in the range of 30° to 60°
and in addition, at an angle in the range of 120° to 150°; and c) depositing on the substrate an open-cell, porous coating at least 15 mils thick, said coating comprising oxidized metallic particles, parts of which are fused to the substrate and to each other, the open cells of said coating forming a substantial quantity of nucleate boiling cavities having an equivalent radius of from 1.5 to 4 mils.
20. The method of claim 13, 14 or 15 including the step of positioning the flame-spraying apparatus so that the distance the metallic particles travel from the flame-spraying apparatus to their point of impact on the metal substrate is within the range of 3. to 6 inches.
21. The method of claim 16 or 17 including the step of positioning the flame-spraying apparatus so that the distance the metallic particles travel from the flame-spraying apparatus to their point of impact on the metal substrate is within the range of 3. to 6 inches.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000402739A CA1170512A (en) | 1982-05-11 | 1982-05-11 | Heat transfer surface for efficient boiling of liquid r-11 and its equivalents |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000402739A CA1170512A (en) | 1982-05-11 | 1982-05-11 | Heat transfer surface for efficient boiling of liquid r-11 and its equivalents |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1170512A true CA1170512A (en) | 1984-07-10 |
Family
ID=4122757
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000402739A Expired CA1170512A (en) | 1982-05-11 | 1982-05-11 | Heat transfer surface for efficient boiling of liquid r-11 and its equivalents |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1170512A (en) |
-
1982
- 1982-05-11 CA CA000402739A patent/CA1170512A/en not_active Expired
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4354550A (en) | Heat transfer surface for efficient boiling of liquid R-11 and its equivalents | |
| Chang et al. | Enhanced boiling heat transfer from microporous surfaces: effects of a coating composition and method | |
| AU602751B2 (en) | Heat exchanger tube for evaporation or condensation | |
| JPS645111B2 (en) | ||
| JPS61184395A (en) | Corrosion preventive process for aluminum heat exchanger | |
| US4101691A (en) | Enhanced heat transfer device manufacture | |
| US4303737A (en) | Coating material | |
| Kim et al. | Effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of horizontal tubes | |
| CA1170512A (en) | Heat transfer surface for efficient boiling of liquid r-11 and its equivalents | |
| Cieśliński | Flow and pool boiling on porous coated surfaces | |
| JPH03229667A (en) | Method for manufacture of highly efficient heat transfer surface and said surface manufactured by said method | |
| Koch et al. | Study on plasma enhanced CVD coated material to promote dropwise condensation of steam | |
| JPH05164496A (en) | Fin tube for open rack type carburetor | |
| JPH06300482A (en) | Heat exchanger | |
| EP1242650B1 (en) | Patterned hydrophilic-oleophilic metal oxide coating and method of forming | |
| JP4382162B2 (en) | Member coating and manufacturing method thereof | |
| IL103174A (en) | Hydrophilic and corrosion resistant fins for a heat exchanger | |
| JPS57200536A (en) | Preparation of corrosion resistant structural member | |
| JP2004360945A (en) | Heat exchanger tube for flow-down liquid film type heat exchanger | |
| JPH1096599A (en) | Outdoor heat exchanger unit and air conditioner using it | |
| JP3480514B2 (en) | Heat transfer tube for falling film evaporator | |
| US4780373A (en) | Heat-transfer material | |
| JPS58129191A (en) | Method of forming core material for heat pipe | |
| CN110468366A (en) | It is a kind of to disappear the laser secondary remelting amorphous composite coating and preparation method thereof of fair and clearization open system formula sorption type heat pump for flue gas | |
| JPH1143777A (en) | Aluminum or aluminum alloy material excellent in water repellency and frosting preventability and its production |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |