Heat transfer surface for efficient boiling of liquid R-11 and its equivalents

A heat transfer surface for boiling liquid refrigerant 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 particles 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.degree. to 75.degree.; (2) The metal substrate is preheated to a temperature which is at least 600.degree. 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 required size for boiling liquid R-11 than the flame-spraying process disclosed in the prior art.

DESCRIPTION 
1. Technical Field 
The subject invention generally pertains to a heat transfer surface and the 
method by which it is produced, and specifically to a porous boiling 
surface for efficiently boiling liquid R-11 and its equivalents and the 
method for flame-spraying such a surface. 
2. Background 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 efficiency 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, liquid 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, or 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. 
The above theory is discussed at much greater length in U.S. Pat. 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 sintering 
metallic particles of from 1 to 50 micron size to a metal surface. The 
particles are applied as a slurry mixed with a plastic binder. When the 
slurry is heated in a furnace, the binder is driven off and the particles 
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 provided 
in the flame-spraying process. As the particles transit from the 
flame-spraying nozzle to the metal surface, an oxide film is thus formed 
on the metallic particles 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 opon 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 specification 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-11, yet the surface so-produced lacks the very high heat transfer 
capability with this liquid that alternative enhanced heat transfer 
surfaces provide. 
When used in a refrigeration cycle, R-11 has one of the highest 
co-efficient of performance (COP) ratings 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-11. 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 surfaces for boiling R-11 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 invention to produce a 
flame-sprayed porous boiling surface that is as efficient at transferring 
heat for boiling R-11, R-113, and their equivalents as are higher cost 
alternatives. 
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 1.5 to 4 mils to more efficiently transfer 
heat to a liquid having surface tension characteristics similar to R-11 
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. 
DISCLOSURE OF THE INVENTION 
The subject invention is a heat transfer surface which is especially 
efficient in boiling liquid R-11 and its equivalents, and which 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.degree.. One or more passes of the 
flame-spraying apparatus deposits on the substrate an open-celled, porous 
coating at least 15 mils thick 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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 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 random 
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 aluminum 
particles, ranging in size from -100 to +325 mesh were applied to a copper 
metal substrate 13 with a spray apparatus oriented at 90.degree. thereto. 
The flow rates of the acetylene and oxygen gases were set at 18 and 45 
cubic 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 coating 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, generally denoted by reference numeral 14, is illustrated in 
FIG. 2. Both FIGS. 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 different 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 appearance. 
The process used to produce the heat transfer surface 14 differs from that 
used to produce the prior art surface 10 in the following manner. The 
copper metal substrate 15 was preheated so that the temperature of its 
surface at the point of impact of the oxidized metallic particles 16 
reached approximately 730.degree. F. 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 with 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.degree. thereto. In addition, the flame-spraying nozzle was 
positioned substantially closer to the metal substrate 15 than 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.degree. relative to the substrate 15, i.e., at an angle 
of approximately 90.degree. relative to their line of flight in the first 
pass. All other conditions of the process were substantially the same as 
those 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-11 or its equivalents. For 
such a use, a liquid (e.g., water) circulated 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 surface of the tube. With reference to FIG. 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 FIG. 3, the wall of the heat exchanger tube 18 comprises 
the metal substrate 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 caused to rotate at approximately 600 rpm 
and to traverse below the flame-spraying apparatus at approximately 66 
inches per minute. The metal substrate 15 of tube 18 is preheated ahead of 
the flame-spraying apparatus by burner 19 using MAPP 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 equal to 
approximately 45.degree. relative to the surface of the metal substrate 
15. Angle A therefore nominally represents the angle at which the oxidized 
metallic particles 16 impact the metal substrate 15. The metallic 
particles 16 travel approximately 4 inches after leaving the 
flame-spraying 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 730.degree. F. It should be 
understood that the metal substrate attains this temperature as a result 
of both the heat provided by burner 19 and the heat provided by 
flame-spraying nozzle 20. Metallic particles 16 are heated and oxidized as 
they travel to the metal substrate 15 in the flame from nozzle 20, which 
generates and transfers substantial heat to the substrate in addition to 
that provided by the gas flame from burner 19. 
A second flame-spraying nozzle 21 is oriented so that a line through its 
longitudinal axis forms an angle B equal to 135.degree., relative to the 
surface of the metal substrate 15. As shown in FIG. 3, angles A and B are 
co-planar; however, it will be apparent that flame-spray nozzle 21 may be 
located at some other position around the longitudinal axis of heat 
exchange tube 18 while still providing a flame-sprayed porous layer 
according to the present invention. If flame-spraying nozzles 20 and 21 
are relatively close together, the temperature of the metal substrate 15 
may exceed the softening 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 15 and the first layer of the surface 14 such that the 
added heat from the 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 upon 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 FIG. 3, the porous coating 14 may 
be applied in two separate flame-spraying operations, using only 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 required prior to the second pass by the 
flame-spraying nozzle, depending upon the elapsed time between passes. 
Furthermore, 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 stationary, 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 transfer 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 approximately 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 14 produced by the double angle 
process. 
In the '862 patent to Dahl et al, it is suggested 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 aluminum 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. 
Nevertheless, the prior art does not specifically teach or suggest a 
method which may be used to produce a surface suitable for very efficient 
boiling of refrigerant R-11 and its equivalents. The present invention was 
developed after substantial experimentation as will be apparent from the 
following discussion. 
It is believed that the heat transfer 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 formation of vapor bubbles in 
liquid R-11 than in liquid R-12 or R-22, because of R-11's greater surface 
tension. An equivalent of R-11 would have a similar surface tension. It is 
thus believed that the subject invention provides more efficient nucleate 
boiling in R-11 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 liquid and measuring the temperature difference 
at boiling, between the liquid surrounding the surface and the surface of 
the tube, when heat is applied to the internal surface of the tube by 
means of an electric heater. Typically, several thermocouples are attached 
to the surface of the test heat transfer tube and their average 
temperature indication during boiling is compared against the indicated 
saturation temperature of the liquid 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 indicates 
that the heat transfer efficiency 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 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. 
Turning now to FIGS. 4-7, the affect of varying several different 
parameters involved in the flame-spraying process to produce a porous 
boiling surface are shown in terms of observed boiling superheat when the 
resulting specimens were tested in liquid R-11. Specimens for which test 
results are shown in the same Figure were prepared under equivalent 
conditions except where noted. In all tests, a heat flux equal to 9,000 
BTU/hr-Ft.sup.2 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 FIG. 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.degree. has a 
boiling superheat equal to 2.7.degree. F. and is representative of the 
performance of the heat transfer surface 10 produced according to the 
teachings of the prior art. By comparison, a single angle surface produced 
with the spray angle equal to 60.degree. shows a substantial improvement, 
having a boiling superheat equal to 2.3.degree. F. A more significant 
improvement is obtained however, when two angles are used, the first equal 
to 45.degree. and the second equal to 135.degree. (90.degree. to the first 
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 FIG. 3. As FIG. 4 shows, even with no preheat, a double angle 
sample made with spray angles equal to 45.degree./135.degree. produces a 
boiling superheat of only 1.9.degree. F. 
In FIG. 5, results are shown for specimens produced by the single angle 
process which were not preheated 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 FIG. 5 were made using a flame-spraying angle equal 
to 45.degree., and in the case of the double angle process, with the 
second angle equal to 135.degree., as previously explained. The results of 
this series of tests indicates that a coating thickness in the range of 
20-30 mils provides the lowest boiling superheat in liquid R-11. 
The effects of the surface temperature of the metal substrate at the point 
at which the metallic particles impact is shown in FIG. 6. In this test, 
all the specimens were made with the spray nozzle at an angle of 
45.degree., and in the case of the double angle specimens, the second 
angle was 135.degree.. This graph shows that an optimum surface 
temperature lies in the range of 700.degree. to 800.degree. F., for both 
the single angle and double angle processes. 
FIG. 7 shows the effect of the distance between the flame-spray nozzle and 
the point of impact of the metallic particles on the metal substrate. All 
specimens for this test were made without preheat with a spray angle equal 
to 45.degree., and in the case of the double angle specimens, with the 
second angle at 135.degree.. As shown, optimum performance occurs at a 
spray distance of 3 inches for the single angle, and of 4 inches for the 
double angle specimens. 
As FIGS. 4-7 illustrate, the optimum efficiency of a flame-sprayed porous 
boiling surface for boiling R-11 is achieved by producing that surface 
using a double angle process, with the flame-spraying apparatus oriented 
so that the metallic particles impact the metal substrate from a distance 
of 4 inches at angles of 45.degree. and 135.degree., respectively, thereby 
providing a porous coating approximately 21 mils thick, and preheating the 
metal substrate so the temperature of its surface at the point where the 
metallic particles impact reaches approximately 730.degree. F. 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-11 
might also result if the subject invention were practiced using other 
materials. Besides copper, the metal substrate might comprise steel, 
aluminum, or titanium; likewise, copper, steel or nickel metallic 
particles might be used. 
The process for applying a flame-sprayed porous boiling surface to 
efficiently boil R-11 and its equivalents 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 surface which is equally efficient on 
other types of heat exchange surfaces, such as plates, or finned surfaces 
having enhanced heat exchange area. It will be understood that 
modifications to the invention such as these will be apparent to those 
skilled in the art within the scope of the invention, as defined in the 
claims which follow.