Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HEAT TRANSFER BOILING SURFACE
This invention relates to a heat transfer device,
and more particularly to a heat transfer tube having an
improved nucleate boiling surface.
One mode of heat transfer from a surface to a
fluid in contact with such surface is nucleate boiling.
This phenomenon is well known and consists in that, during
boiling, ma~y vapour bubbles are generated on the heat trans-
fer surface from active areas known as nucleation sites and
rise to the surface of the liquid. This creates agitation
and increases heat transfer. It is also known that these
vapour bubbles are more readily formed at surface irregular-
ities. Therefore, in order to obtain a large heat transfer
coefficient, it is generally recognized to roughen the sur-
face of heat transfer devices to create as many nucleation
sites as possible. Up to now, various methods of forming
nucleation sites have been proposed. U.S. Patent No.
3,326,283 teaches the idea of knurling an already finned
tube. U.S. Patent No. 3,454,081 teaches a method for in-
creasing t~e number of nucleation sites in which ridges
formed by scoring are deformed by a subsequent knurling
operation to create partially enclosed and connected sub-
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surface cavities for vapour entrapment so as to promote
nucleate boiling. U.S. Patent No. 3,683,656 teaches another
method of increasing the number of nucleation sites by
partially bending the fins of a finned tube to form
cavities. U.S. Patent No. 3,893,233 teaches the idea of
first knurling a smooth tube with a diamond pattern and then
subjecting the knurled tube to a finning operation to form
small splits of a controlled geometry and depth which become
efficient nucleation sites for boiling enhancement.
Applicant has found that an improved heat transfer
can be obtained by a method similar to the one disclosed in
U.S. Patent No. 3,326,283 mentioned above. In the prior
art patent, the nucleation sites were formed by knurling a
finned tube in such a way as to create a regular pattern of
14 to 33 teeth per circumferential inch in each fin. Thepatentee specifically points out that at less than 14 teeth
per circumferential inch a low increase in heat transfer is
obtained and that at a density higher that 33 teeth per cir-
cumferential inch, an irregular pattern is formed due to inter-
ference between the knurling tools and that the heat trans-
fer is reduced.
Applicant has surprisingly found that an increase
in heat transfer of 200 to 300% over that of a smooth tube
may be obtained by performing an improved knurling operatlon
on a finned tube.
The heat transfer device, in accordance with the
invention, comprises a base wall of heat conductive material,
a plurality of spaced apart fins formed integrally with the
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surface of the base wall at about 30 to 40 fins per
inch, and a plurality of indentations formed in the
peripheral edge of said fins by a diamond knurling
tool forming two series of parallel threads in the range
S of 40-80 threads per inch intersecting each other at an
angle of 10 to 80 degrees, preferably about 60 degrees.
The base wall is preferably a tube and the inden-
tations are formed as a knurled diamond pattern around
:-...... the outer periphery of the tube.
The height of the fin is preferably in the range
of .025 to .040 inch and the depth of the indentations
in the range of .012 to .020 inch.
The invention will now be disclosed, by way of
example, with reference to the accompanying drawings in
which:
Figure 1 illustrates a finned tube upon which has
been formed a finning operation as a first step in the
making of a heat transfer boiling surface, followed by
a diamond knurling operation;
Figure 2 is an enlarged fragmentary longitudinal
section through a portion of the tube of figu~e 1 upon
which both the finning and knurling operations in
accordance with the invention have been performed; and
Figure 3 is a graphical presentation of the heat
flux for the tubes tested over a range of Log Mean Tem-
perature Difference.
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Referring to Figure 1, there is shown a tube
10 having integrally formed external fins 12. The fins
are preferably arranged in configuration from 30 to 40
fins per inch (FPI) and have a height of about .032 inch.
.. . . .... ... . .
Such tube is subsequéntly subjected to ~ knurling operation
ation known as diamond knurling wherein two series of
parallel threads 14 and 16 in the range of 40-80 threads per
inch (TPI) intersecting each other at an angle of about
60 are formed on the fins at a depth of about .016 inch.
This operation forms a plurality of subsurface cavities
18 with restricted openings 20 to the outer surface of
the tube as illustrated in Figure 2 of the drawings.
Heat transfer tests were performed on five tubes
hereinafter designated C-0 to C-4. All tubes had internal
I5 smooth surfaces. Tube C-0 had an external smooth surface.
Tube C-l was finned at 30 FPI and knurled at 80 TPI.
Tube C-2 was finned at 40 FPI and knurled at 40 TPI.
Tube C-3 was finned at 40 FPI and knurled at 80 TPI.
Finally, tube C-4 was finned at 30 FPI and knurled at
30 TPI.
The apparatus used for making the tests is an
apparatus boiling refrigerant R-ll such as disclosed in
a paper by T. C. Carnavos entitled "An Experimental
Study: Condensing R-ll on Augmented Tubes" presented at
the joint ASME/AICHE National Heat Transfer Conference,
Orlando, Florida, July 27-30, 1980. The apparatus
consisted of an insulated rectangular shell having
within the shell a single condensing tube in the upper
portion and a single boiling tube in the lower portion
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-- 5 --
for vapour generation. The tested boiling tubes were
3/4" nominal and approximately 52" long. Hot water
flowed in a closed loop through a calibrated 250 mm
rotameter and the boiling tube, and returned to a
Variac controlled 9kw hot water heater for reheating.
Cold water flowed in a closed loop through a calibrated
600 mm rotameter and condensing tube, and returned to
a holding tank. A pump took water from the tank, put it
through a shell and tube heat exchanger then back to
the tank. City water was used to cool the test water in
the heat exchanger. Temperature measurements were made
with precision glass stem mercury thermometers having
0.056C minimum graduations and 76 mm immersion. All
thermometers were properly immersed and their positions
were switched in stream during data acquisition to min-
imize temperature difference inaccuracy for heat balance
determination. A mercury manometer was used to measure
shell pressure to determine shell temperature.
Data acquisition was conducted under steady
state conditions. Heat balances were made between the
waterside heat loads of the boiling and condensing tubes
and fell predominantly in the range of +10~. Average data
values were used in the analysis. The tubeside mass flux
was held constant at 1540 kg/sec m2 in order to make
direct comparisons of overall heat transfer capability
meaningful. The magnitude of 1540 kg/sec m2 of nominal
flow area represents the approximate lower end commonly
used in commercial practice. In addition,larger
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temperature differences resulted for closer heat
balances. The heat loads Qb (boiling) and Qc (condensing)
were calculated as follows:
Qb- Wb cp (Tbi ~ Tbo)
Qc~ Wc cp (Tco ~ Tci)
where
Wb (boiling tube) and Wc (condensing tube) = Flow rate -
kg/hr
Cp - Specific heat - k J/kgC
Tbi, Tbo (inlet, outlet boiling tube) = Temperature - C
TCi, Tco (inlet, outlet condensing tube) =Temperature -C
The heat flux Q was calculated by:
Q , Qb + Qc
An (L) 2 An (L)
where Qb and Qc are defined above and
An - Nominal Heat Transfer Area in m2 based on
nominal outside diameter of tube over
augmentation
L - Length of tube - m
The Log Mean Temperature Difference ~LMTD) was calculated
as follows:
LMTD - Tbi ~ Tbo
ln [(~ O b
where Tbo, Tbi are as defined above and Tb is the boiling pool
temperature in C.
Figure 3 provides the graphical presentation of
Heat Flux for all the tube tested over the Log Mean
Temperature Difference (LMTD). Tube C-3 having the
geometry 40 FPI/80TPI exhibited the highest overall heat
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flux, some 200 to 300% above smooth tube C-0, across a
broad LMTD range. The C-3 tube is especially a good
performer in the lower LMTD range, where operation is
most prevalent for these types of augmented boiling
tubes. Tubes C-l and C-2 having the geometry 30 FPI/
80 TPI and 40 FPI/40 TPI, respectively, exhibited
a heat flux slightly ~ower than C-3, more particularly
at the lower LMTD but their performance is still much
better than smooth tube C-0. Tube C-4 is a finned tube
which was knurled at 30 TPI and which contains about
the same number of nucleation sites per unit area as the
tubes disclosed in U.S. Patent No. 3,326,283. It will be
noted that the performance of the tube C-4 is much lower
than that of tubes C-l, C-2 and C-3 which are made in
accordance with the present invention, that is knurled
at 40-80 FPI. It will thus be seen from the above that
the performance gains obtained with the finned tubes
knurled at 40-80 FPI are very substantial, not only over
a smooth tube but also over the tubes disclosed in the
above U.S. Patent No. 3,326,283.
