Method of manufacturing high efficiency heat exchange tube

A high thermal efficiency multi-wall heat exchange tube is sealed at one end and has the gap between the tube walls filled with a silicon liquid oil that is nontoxic, has a boiling point above that of water, and increases heat transfer efficiency between the inner and outer walls of the tube. The liquid oil is inserted into the gap between the tube walls by placing the tube into a fixture which receives ten tubes simultaneously, heating the tube and the oil, evacuating the gap and inserting oil into the evacuated gap. The fixture includes a manifold connector assembly which is sealingly positioned over the open end of the one or more tubes to provide a manifold connection between the gap of each tube being filled and a source of vacuum as well as a source of oil and a drain. A hydraulic ram facilitates connection and removal of the manifold connector assembly.

BACKGROUND OF THE INVENTION 
As energy costs rise it is becoming increasingly necessary for industry to 
minimize these costs in order to become and remain competitive. One way of 
reducing energy costs is to use waste energy which is a byproduct of one 
activity in another activity. For example a heat exchanger can be used to 
receive two fluids and transfer heat energy from one of the fluids to 
another. Thus, instead of wasting the heat energy in the first fluid and 
using more energy to heat the second fluid, the same heat energy is used 
two or more times. 
Such heat exchangers typically employ a plurality of heat exchange tubes 
which carry one fluid on the inside thereof and are exposed to the second 
fluid on the outside thereof. If the walls of the heat exchange tubes are 
made of a thermally conductive material such as copper or some other 
metal, heat readily passes through the walls of the tube from the warmer 
fluid to the cooler fluid. Typical examples of heat exchangers employing 
such tubes are shown in European patent application No. 66,425 A3 and 
European patent application No. 120,497. In each of these heat exchanger 
arrangements a plurality of heat exchange tubes are nested inside a 
container while first and second fluids are directed through the container 
to improve heat transfer efficiency. 
Where heat from a toxic heat exchange fluid such as a refrigerant is to be 
exchanged with a consumable fluid such as potable water, safety or code 
requirements frequently demand the use of a multi-walled heat exchange 
tube. To be effective or comply with code requirements a gap must be 
maintained between adjacent tube walls so that if a leak develops in one 
wall the leaking fluid will flow through the gap to the exterior of the 
tube where it can be detected. At the same time, the second wall maintains 
the leaking fluid separated from the other fluid. For certain applications 
it becomes necessary to further improve safety by using a triple wall 
tube. When a triple wall tube is used, two heat exchange fluids can become 
mixed only if all three walls develop a leak simultaneously. 
While the use of multi-walled heat exchange tubes significantly increases 
the safety of a heat exchanger, the efficiency of heat transfer between 
the two fluids is significantly impaired. The tubes themselves can be made 
of a relatively good heat conductive material. However, the gap between 
the tubes becomes a thermal insulator and significantly reduces heat 
transfer efficiency of a heat exchange tube. The heat transfer efficiency 
can be improved somewhat by swaging a helical groove into the outer tube 
over a substantial portion of the length of the tube. However, in order to 
maintain a sufficient gap between adjacent tube walls that provides 
communication with an open end of a tube so that a fluid leak can be 
detected, the groove must not be permitted to produce wall to wall contact 
between adjacent tubes over more than a small portion of the total surface 
area of a heat exchange tube. 
This limitation on adjacent wall direct contact means the heat transfer 
efficiency remains low when compared to a single wall tube. As a result, 
either less heat is transferred between the two fluids, thus increasing 
manufacturing costs, or a larger heat exchange tube surface area must be 
provided, thus increasing the size and cost of the heat exchanger. 
The present invention significantly increases the efficiency of 
multi-walled heat exchange tubes by inserting a liquid in the gap between 
adjacent tube walls. The liquid provides a heat transfer efficiency that 
is far superior to that of the gases that are normally found in the 
interwall gap. Since the liquid will be pushed out of the gap and detected 
in the event of a leak in any of the walls of the tube, the safety of the 
tube is not impaired. If one end of the tube is sealed, the liquid can be 
inserted by first evacuating the tube and then forcing the liquid into the 
gap. The seal at the closed end, coupled with the relatively small size of 
the gap, causes the liquid to be retained within the gap unless forced out 
by fluid leaking into the gap. It thus becomes possible to significantly 
reduce the cost of an industrial process by either reducing the heat 
exchange surface area or by increasing the energy transfer between two 
heat exchange fluids. 
SUMMARY OF THE INVENTION 
A high efficiency multi-walled heat exchange tube in accordance with the 
invention includes inner and outer walls that are sealed closed at one 
end, open at the opposite end and have a leakage gap between them. A 
helical groove extends over a substantial length of the tube to produce 
contact between adjacent walls in the vicinity of the groove while 
allowing communication through the gap between the open and closed ends of 
the tube. Heat transfer efficiency through the gap is further improved by 
filling the gap with a nontoxic silicon oil or other nontoxic liquid 
having a boiling point above the boiling point of the hottest fluid being 
used in the heat exchanger. 
Apparatus for inserting the liquid into the interwall gap of a heat 
exchange tube includes a heat insulating housing that receives and 
supports a plurality of tubes that are sealed at a closed end, a heater 
connected to direct a flow of hot air into the housing sufficient to heat 
the tubes to approximately 200 degrees F., a manifold connector assembly 
sealingly engaging the inner and outer tubes and providing a fluid flow 
path between the gaps of each of the tubes, a vacuum pump connected 
through a pump valve to the fluid path to selectively evacuate the fluid 
flow path and the tube interwall gaps, and a container of liquid oil 
selectively coupled to the fluid flow path through a source valve to fill 
the tube gaps with liquid after they have been evacuated. An immersion 
heater placed in the liquid before filling advantageously heats the liquid 
to approximately 200 degrees F. to reduce the liquid viscosity and allow 
the liquid to more completely fill the vacuumized gaps. 
Since one end of each tube is sealed and the gaps are rather small, 
atmospheric pressure retains the liquid within the gaps unless a leak 
allows additional fluid to enter the gap and force the liquid from the 
tube. The intergap liquid thus provides a significant increase in heat 
transfer efficiency without impairing the safety or durability of the heat 
exchanger in which the enhanced tubes are installed.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, a system or apparatus 10 for inserting liquid into 
an inter-wall gap of a multi-wall heat exchanger tube includes one or more 
multi-wall heat exchanger tubes 12, a thermally insulated housing 14 
enclosing the heat exchanger tubes 12, and a manifold connector assembly 
16 providing an interconnected fluid flow path to the inter-wall gaps of 
each of the heat exchange tubes. The manifold connector assembly 16 
sealingly engages the outer and inner walls of each heat exchange tube 12 
to facilitate first the evacuation of the inter-wall gaps and then the 
insertion of a liquid therein to improve the heat transfer efficiency for 
each of the multi-wall tubes 12. Although not explicitly shown in FIG. 1, 
each tube 12 is sealed by welding, by an expanded bushing or by other 
suitable means to close each inter-wall gap at a closed end opposite the 
open end at which the manifold connector assembly is connected. As is 
known in the art, helical grooves 18 are swaged into the outer surface of 
the outer tube wall 20 which penetrates into the inner wall of the inner 
tube to improve heat transfer efficiency by producing a direct contact in 
the vicinity of the groove 18 between adjacent walls in the multi-wall 
tube 12. 
A vacuum pump 30 is connected by a tube section 32 to a liquid separator 34 
which assures that any liquid is separated from air or other gases being 
evacuated from the one or more tubes 12 before they can flow through tube 
section 32 and reach vacuum pump 30. A drain tube 36 connects through a 
valve V5 and extends to a container 38 to permit any liquids collecting 
within liquid separator 34 to be selectively drained into container 38. 
Liquid separator 34 also connects through a tube section 40 which in turn 
connects to a tube section 42. Tube section 42 connects through an 
electrically controlled valve V3 which in turn connects through a tube 
segment 44 and a tube segment 46 to the manifold connector assembly 16. A 
vacuum gage 48 is connected to tube section 46 to monitor the pressure 
within tube section 46 and hence the connector assembly 16 and the 
inter-wall gaps of heat exchange tubes 12. 
Tube section 42 connects to a tube section 50 and through an electrically 
controlled valve V4 to a vent pipe 52. It will be apparent that when valve 
V3 is open and valve V4 is closed, a flow path is provided between vacuum 
pump 30 and the manifold connector assembly 16 so that vacuum pump 30 may 
operate to evacuate the inter-wall gaps in the heat exchange tubes 12. 
A sealed reservoir 60 is closed by a cap 62 and holds a supply of liquid 64 
for improving heat transfer efficiency of the heat exchange tubes 12. The 
liquid 64 must have a boiling point above the temperatures of any fluids 
to which the heat exchange tubes 12 will be subjected and must be nontoxic 
if one of the fluids is to be a human consumable. In an application where 
potable water is to be heated by a fluid carrying heat from an industrial 
process, a silicon oil having the commercial designation 200 FL 350 CS has 
been found to provide the required qualities. 
A submersible heater 66 has a submersion heating element 68 extending into 
the liquid 64 and is operated under thermostat control to maintain the 
liquid 64 at approximately 200 degrees F. 
A stand pipe 70 is connected through tube segment 50 to the top of 
container 60 and through a tube segment 72 to the bottom of the container 
60 and includes a transparent viewing window which enables observation of 
the fluid level of the liquid 64 within container 60. 
The tube segment 72 at the bottom of container 60 is also connected through 
a constriction 74 and a tube segment 76 to an electrically controlled 
valve V2. Valve V2 provides a selective coupling of tube segment 76 
through tube segment 46 to the manifold connector assembly 16. Hence, 
after the inter-wall gaps of tubes 12 have been evacuated, they can be 
filled with the liquid 64 by opening valve V2, closing valve V3, and 
opening valve V4 to provide communication between the top of container 60 
and the vent pipe 52. Atmospheric pressure on the liquid 64 as well as a 
pressure head of about two feet or more have been found sufficient to then 
force the liquid 64 into the evacuated inter-wall gaps of the heat 
exchange tubes 12. It has been found that when valve V2 is first opened, 
the rush of liquid through tube segment 76 is faster than the rate of 
supply through tube segment 72 to the bottom of container 60. As a result, 
the liquid level within stand pipe 70 is temporarily pulled down to the 
point that air passes through stand pipe 70 into tube segment 76 and the 
inter-wall gaps of heat exchange tubes 12. The constriction 74 is 
therefore placed in tube segment 76 to limit this initial surge of fluid 
and prevent air from entering the tube segment 76. The constriction 74 may 
be conveniently implemented as simply a small diameter section of tube 
segment 76. 
After the inter-wall gaps of tubes 12 have been filled, valve V2 is closed, 
valve V3 is open and valve V4 is opened to provide a venting of the tube 
section 46 and hence the manifold connector assembly 16 to atmosphere. The 
bottom of manifold connector assembly 16 is connected through a tube 
segment 78 and an electrically controlled valve V1 to the drain tank 38 so 
that when valve V1 is open excess fluid is permitted to drain from the 
manifold connector assembly through tube segment 78 to the container 38. 
The tubes 12 may then be removed and a new set of tubes installed for 
evacuation and insertion of a heat exchange enhancement liquid into the 
inter-wall gaps thereof. 
Prior to insertion of the liquid, a forced air heater 80 is operated to 
provide hot air through a duct 82 to the interior of housing 14 to preheat 
the heat exchange tubes 12. In one preferred example, the air has a 
temperature of approximately 220 degrees to enable the heat exchange tubes 
to be heated to a temperature of approximately 200 degrees Fahrenheit. The 
tubes and the oil 64 have been found to facilitate insertion of the oil 
into the inter-wall gaps by thermally expanding the tubes 12 and hence the 
gaps, by decreasing the density of the air within the gaps and by reducing 
the viscosity of the liquid oil 64. Heating the tubes 12 prevents the oil 
from cooling and increasing in viscosity as it fills the tubes 12. 
An electrical control system 88 is electrically connected through control 
signals C1-C7 to control the forced air heater 80, the energization of 
vacuum pump 30, and the opening and closing of the five valves V1-V5 with 
control signals C1-C5 respectively. The control system 88 is generally 
conventional in construction and includes a plurality of cams mounted on a 
shaft which is subjected to a single rotation for each operating cycle of 
the apparatus 10. Each cam is shaped to activate a cam follower switch 
which in turn energizes or de-energizes the signals C1-C7 to control the 
various components in the apparatus 10. Alternatively, manual switches are 
provided to manually control the operation of each of the control devices 
within the apparatus 10. 
A fixture 100 incorporating the housing 14 and manifold connector assembly 
16 is shown in FIG. 2 to which reference is now made. The full length of 
one of the heat exchange tubes 12 is shown in FIG. 2. It will be observed 
that the closed end is closed by a bushing 102 having a stepped axial bore 
with a large diameter section which receives the outer wall 20 and a small 
diameter section which receives an inner wall 104 therethrough. A tool is 
placed within the inner wall 104 to expand both the inner wall 104 and 
outer wall 20 against the bushing 102 and create an air tight seal. It 
will be appreciated that other sealing means such as epoxy bonding, 
welding or O-ring seals could be utilized if desired to seal the closed 
end 106 of tube 12. The housing 14 is conveniently mounted on four legs 
108 and has a bottom 110 in communication with the duct 82, two side walls 
112, 114, an end wall 116 and a lid 118 which is hinged to side wall 114 
to provide convenient access to the interior of housing 14. Each of the 
walls forming the housing 14 is conventionally insulated to reduce heat 
loss. An aperture (not shown) is formed in the bottom 110 adjacent the end 
wall 116 to provide an exit for hot air entering through the duct 82. The 
exit aperture may be advantageously connected to a vent duct if desired. A 
rack 120 is disposed transversely of the fixture of the housing 14 and 
provides ten axially extending grooves 122 in the top surface thereof for 
receiving and supporting the closed ends of ten heat exchange tubes 12. 
The front end of housing 14 is closed by a steel front or main plate 124 
which has ten axially extending bores therethrough for receiving the open 
ends of ten heat exchange tubes (only one being shown by way of example in 
FIG. 2). 
A pair of hydraulic ram cylinders 130, 132 are mounted on opposite sides of 
the housing 14 adjacent the main or front leg 124 and have ram rods 134, 
136 respectively which extend slideably through axial bores in opposite 
ends of a manifold plate 140 to be suitably fastened to opposite ends of 
an end plate 142. Each of the ram cylinders 130, 132 has an internal 
cylinder diameter of 1.75 inches and is connected by hydraulic lines 146 
at each end thereof to a conventional hydraulic pressure source which 
maintains a hydraulic pressure of 200 psi. After ten individual tubes are 
installed in the housing 14 with their open ends extending through the ten 
apertures 148 through main plate 124, the lid 118 is closed and large 
O-ring seals 150 are concentrically disposed over the outer wall 20 of 
each of the tubes 12 and positioned adjacent a front surface 152 of main 
plate 124. The manifold plate 140 is then slid on ram rods 134, 136 into 
contact with the large O-rings 150 with the 3/4 inch large walls 20 of 
each tube extending partway into apertures 154 in the manifold plate 140 
and the small walls 144 extending completely through the apertures 154 
(see also FIG. 5). A small O-ring 156 is then concentrically slipped over 
the end of each 5/8 inch inner wall 104 and the rams 130, 132 are 
energized to close the manifold connector assembly by forcing end plate 
142 toward the main plate 144 with the manifold plate 140 and O-ring seals 
150, 156 sandwiched therebetween. It should be apparent that other tube 
sizes could be used as well. For example, outer wall diameters of one inch 
or 1.25 inch are frequently used in heat exchangers. 
As best seen in FIGS. 3 and 5, the manifold plate 140 comprises a laterally 
extending center bar 158, and a laterally extending top bar 160 which is 
welded to the top surface of center bar 158 and a laterally extending 
bottom bar 162 which is welded to the bottom surface of center bar 158. A 
pair of bushings 166, 168 extend through the center bar 158 adjacent 
opposite sides thereof to receive the ram rods 134, 136 respectively. 
The ten tube receiving apertures 154 which extend through the manifold 
plate 140 each have a small diameter portion 170 adjacent the front side 
of manifold plate 140 and a large diameter portion 172 adjacent the back 
side of manifold plate 140. A top fluid flow channel 174 is cut partly 
into the top bar 160 and partly into the bottom bar 158 and extends across 
the top of all ten tube receiving bores 154. Channel 174 extends with a 
downward slope from a vacuum port 176 at the right side thereof to 
facilitate gravity feed of the liquid 64 to all of the tubes 12. A lower 
fluid flow channel 178 is cut partly into lower bar 62 and partly into 
upper bar 158 and extends with a downward slope from the vicinity of the 
right most bore 154 to the left most bore 154 and a drain port 180 
adjacent the left-hand end of the channel. The downward slope facilitates 
the drainage of excess liquid 64 from the manifold plate 140 through drain 
port 180 and tube segment 78 to the drain container 38. Vertically 
extending bores 182, 184 provide a fluid flow path between a transition 
region of the large bore 154 and the upper and lower fluid flow channels 
174, 178 respectively. The fluid flow channels 174, 178 and vertical bores 
182, 184 thus provide communication between each of the ports 176, 178 and 
the inter-wall gap of each of the ten heat exchange tubes 12 which are 
disposed with their open ends extending into the manifold plate 140. It 
will be apparent that the channels 174 and 178 as well as the bores 182 
and 184 may readily be formed before the bars 156, 158 and 160 are welded 
together. 
As best seen in FIG. 4, the end plate 142 has a cavity 190 disposed in 
opposed relationship to the inner wall 104 of each of the ten heat 
exchange tubes 12 which may be positioned in the fixture 14. The cavities 
130 are each circular in shape and have a 45 degree annular wall 192 which 
is sized and positioned to matingly engage the small O-ring seals 156 when 
end plate 142 is forced into a closed position. Making further reference 
to FIG. 5, as the annular wall 192 engages the small O-ring seal 156, the 
O-ring seal 156 is forced into sealing engagement with a front surface 194 
of manifold plate 140 and simultaneously into sealing engagement with the 
outer surface of inner tube wall 104. 
It will be observed in FIG. 5 that the manifold plate 140 is shown in 
combination with a triple wall tube having an outer wall 20, an inner wall 
104 and a middle wall 198. It will be further observed that the middle 
wall 198 does not quite extend to the end of outer wall 20 and that the 
end of outer wall 20 does not quite extend to the end of the large 
diameter bore section 154. A manifold region 200 thus exists adjacent the 
end of outer wall 20 to provide communication with the vertical bores 182, 
184. An inner gap 202 between inner wall 104 and middle 198 as well as an 
outer gap 204 between outer wall 20 and middle wall 198 thus remains in 
communication with the annular manifold 200 and hence the channels 174, 
178 and ports 176, 180. 
Referring now to FIG. 6, the front plate or main plate 124 has ten equally 
spaced bores 148 therethrough for receiving the open ends of the ten heat 
exchange tubes 12. For purposes of illustration, the bores 148 are shown 
in FIG. 6 as receiving a double walled tube l2A having an outer wall 20A, 
an inner wall 104A and a single inter-wall gap 208 therebetween. Because 
the outer diameter of outer walls 20 and 20A remain the same at 0.75 inch, 
the thickness of outer wall 20A can be somewhat greater than the thickness 
of triple-wall tube outer wall 20. Since the outer wall 198 of the 
double-wall tubes 12 does not extend beyond the outer wall 20, the loading 
of double and triple-wall tubes into the fixture 14 is identical. Main 
plate 124 has a cavity 210 formed in the front surface thereof concentric 
with each of the ten bores 148. Each of the cavities 210 has an annular 45 
degree side wall chamber 212 which matingly engages the larger O-ring 
seals 150 when the manifold plate 140 is forced into engagement with the 
main plate 124. Each circular cavity 210 forces a larger O-ring seal 150 
into sealing engagement with a front surface 212 of the manifold plate 140 
and simultaneously into sealing engagement with the outer surface of an 
outer tube wall 20 or 20A. The manifold plate 140 is thus sealing coupled 
to both the inner wall 104 and the outer wall 20 of each of the ten heat 
exchange tubes which are to be filled with liquid. 
Once the tubes 12 are installed in the fixture 100, the container 60 is 
filled with an adequate quantity of liquid 64, the heater 66 is turned on 
and the end plate is closed by the rams 130, 132, the control system 88 is 
activated to start a multi-step automatic process. During a startup step 
the forced air heater 80 is turned on to begin heating the heat exchange 
tubes 12 and valve V1 is opened to assure that any excess liquid 64 from a 
previous operation is drained from the manifold 140 and tube segment 78. 
The vacuum pump 30 remains off and all other valves V2-V5 are closed. The 
startup step lasts for about one minute. 
At the end of the startup step a vacuum step begins. At the beginning of 
the vacuum step the following events occur in sequence. The drain valve V1 
is closed, valve V3 is opened to open a path between manifold plate 140 
and the vacuum pump and vent valve V4 is opened so that vacuum pump 30 
does not have to start under load. Next, vacuum pump 30 is started and 
then vent valve V4 is closed. Vacuum pump 30 now begins to pull a vacuum 
on the container 60, the manifold plate 140 and the inter-wall gaps 
themselves. Valves V2 and V5 remain closed. The vacuum step continues for 
a sufficient period of time for the system pressure to be pulled down to 
approximately 0.0000003 in Hg. A time period of approximately 10 to 15 
minutes has been found sufficient to pull an adequate vacuum and heat the 
tubes 12. It appears that a vacuum of 0.49 to 1.49 or better is required 
to substantially fill the inter-wall gaps with the liquid 64 and optimize 
the heat transfer capacity of the multi-wall tubes 12. During the vacuum 
step container 64 is evacuated along with the tube gaps. This assures that 
any air is removed from the liquid oil 64 and cannot flow back into the 
inter-wall gaps with the oil 64. 
A fill tubes step is next executed at the end of the vacuum step. During 
the fill tubes step the following sequence of events occurs. First the 
valve V3 is closed to isolate the gaps from the vent 52. Second, fill 
valve V2 is opened to allow liquid 64 to begin running into the manifold 
140 and the inter-wall gaps in the heat exchange tubes 12. Third the vent 
valve V4 is opened to introduce atmospheric pressure to the liquid 
container 60. The resulting atmospheric pressure combines with a pressure 
head of about 2 feet produced by elevating the container above the tubes 
12 to force the liquid 64 into the inter-wall gaps of the heat exchange 
tubes 12. Finally, the vacuum pump is turned off and the forced air heater 
is turned off to allow the tubes 12 to begin to cool. The fill tubes step 
continues for approximately 12 minutes while the liquid 64 fills the 
inter-wall gaps. During this time the cooling of the tubes 12 tends to 
reduce the size of the inter-wall gaps, thus forcing out excess liquid 64 
and making drainage of further liquid from the tiny gaps more difficult. 
Finally, a 3 minute drain cycle is initiated. During the drain cycle fill 
valve V2 is closed and then drain valve V1 is opened to drain excess 
liquid 64 from the manifold plate 140 into drain container 38. Valve V3 is 
then opened while vent valve V4 remains open to connect the fill side of 
manifold plate 140 to atmosphere and assure that proper drainage can 
occur. Valve V5 is then opened to drain any liquid accumulated in liquid 
separator 34 into the drain container 38. The forced air heater 80 remains 
off. 
In practice it has proven convenient to simply pour any contents of the 
drain container 38 back into the source container 60 at the end of each 
gap fill cycle. However, the system 10 is designed to automatically 
transfer the drainage fluid back to container 60 using the vacuum pump 30. 
This is accomplished by opening valves V1 and V2 and starting vacuum pump 
30 while valves V3, V4 and V5 remain closed. This configuration pulls a 
vacuum on the container 60 and causes drain liquid to be drawn under 
atmospheric pressure from drain container 38 upward through valve V1, 
manifold plate 140 and valve V2 to the container 60. When the excess drain 
liquid has been returned to container 60 the vacuum pump 30 is turned off, 
fill valve V2 is closed to retain the liquid within container 60 and all 
valves V1, V3, V4 and V5 are opened to allow any liquid within the system 
to drain back into the container 38. 
While there have been shown and described herein an improved multi-wall 
heat exchanger tube and a method and apparatus for manufacturing the tube 
for the purpose of enabling a person of ordinary skill in the art to make 
and use the invention, it should be appreciated that the invention is not 
limited thereto. Accordingly, any modifications, variations or equivalent 
arrangements within the scope of the attached claims should be considered 
to be within the scope of the invention.