Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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D E S C R I P T I 0 N
Title
HEAT EXCHANGER TUBE HAVING
MINUTE INTERNAL FINS
Technical Field
~ The subject invention generally pertains to heat
exchanger tubes, and more specifically to tubes having internal
fins.
Background of the Invention
Refrigeration systems, such as air conditioners and
heat pumps, typically include a compressor, one heat exchanger
functioning as an evaporator, an expansion device, and a second
heat exchanger functioning as a condenser, all of which are
connected in series to circulate a refrigerant in a closed loop.
The two heat exchangers each include at least one heat exshanger
tube for transferring heat either to or from the refrigerant.
The rate of heat transfer is enhanced by providing the heat
exchanger tubes with internal fins.
In designing tubes having internal fins, many
interrelated factors need to be considered such as fin height,
fin spacing, helix angle, heat flux, flow rate, and various
properties of the fluid being conveyed through the tube. Varying
each of these factors provides an infinite combination of
factors, making it difficult and costly to extensively study all
the possibilities. As a result, some apparently conflicting
conclusions have been drawn based on different lab tests.
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U.S. Patent 4,044,797 suggests using an internal fin
` height of .02 to .2 mm (.0008 to .0079 inches) and a helix angle
of 4- to 15-, while U.S. Patent 4,118,944 concludes that the most
effective helix angles are greater than 20-. ~.S. Patent
4,545,428 suggests a fin height of .1 to .6 mm (.0039 to .0236
inches) and a helix angle of 16- to 35-. It should be noted,
however, that none of the three patents recommends a helix angle
of less than 4-.
In view of the above patents alone, it is difficult, if
not impossible, to determine the optimum fin height and helix
angle. Therefore, in developing the present invention, lab tests
were performed to determine the best internal fin design. The
unexpected results of the tests showed that straight internal
fins, i.e., a zero degree helix angle, provide a surprisingly
effective internal heat transfer surface. The results were so
surprising that additional, more extensive tests were performed
to check the validity of the earlier tests. The more extensive
tests conflrmed the surprising results.
Since a heat exchanger tube having internal helical
fins is more difficult and more costly to manufacture than a
straight finned tube, it is an object of the invention to provide
a heat exchanger tube with substantially straight internal fins
that provides a greater heat transfer coefficient than that of a
similar tube with helical fins.
It has now been discovered that it is possible to
provide a heat exchanger tube with substantially straight
internal fins having an optimum fin height.
It has also been found that it is possible to provide a
tube with substantially straight internal fins that are
circumferentially spaced apart at optimum intervals to avoid
excessively thin, fragile fins and to avoid excess space between
the fins.
In addition, it is possible to provide an effective
heat exchanger tube for conveying refrigerant of varying quality.
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Furthermore, it is possible to provide a refrigerant
flow rate that takes full advantage of substantially straight
internal fins.
It is also possible to provide an internally finned
S heat exchanger tube that provides heat transfer when conveying a
fluorinated hydrocarbon refrigerant.
The invention will be more clearly understood with
reference to the attached drawings and the description of the
preferred embodiment which follows hereinbelow.
Summa~y of the Invention
. ' , .
According to one aspect of the present invention there
is provided a heat exchanger comprising a heat exchanger tube
conveying a réfrigerant said tube having a plurality of
longitudinally running internal fins disposed along an inner
surface of said tube at a helix angle of less than 4 degrees with
respect to a longitudinal axis of said tube, said fins having a
fin height of .007 inches to .030 inches and being
circumferentially spaced around said inner surface at intervals
of .010 to .040 inches.
Brief ~escriotion of the Drawinos
Figure 1 illustrates a half-section of a preferred
embodiment of the invention.
Figure 2 illustrate a half-section of a heat exchanger
tube having a helix angle that is greater than zero.
Figure 3 shows how a heat transfer multiplier varies as
30 a function of fin height and helix angle.
'i~
Figure 4 shows how a heat transfer multiplier varies as
a function of helix angle for a given fin height.
Figure 5 shows how a heat transfer multiplier varies as
a function of fin height for a given helix angle.
Figure 6 shows how a heat transfer multiplier varies as
a function of mass flow rate for a preferred embodiment of the
invention.
Descriptio of the Preferred Embodiment
Heat exchanger tube 10, shown in Figure 1, is one
embodiment of the subject invention. Tube 10 includes generally
straight longitudinal running internal fins 12, i.e., its helix
angle ~ is zero degrees with respect to a longitudinal axis 14 of
the tube. Fins 12 have a height "h" of .0155 inches and are
distributed circumferentially around the inner surface of tube 10
at .017 inch intervals 16. Interval 16 is defined as the
distance between a center point of one fin to the center point of
an adjacent fin.
Under equivalent test conditions, tube 10 proves to be
superior to a variety of other internally finned tubes having
various fin heights and helix angles, such as tube 18 shown in
Figure 2. The variety of other tubes that have been tested and
compared to tube 10 are generally suitable in refrigeration
systems. This means that the tubes have a nominal outside
diameter of approximately one inch or less and the internal
surface of the tubes have a high concentration (intervals of .010
to .040 inches) of relatively minute fins (fin height below .035
inches) to enhance heat transfer while minimizing flow
resistance, or pressure drop through the tube. Minimizing
pressure drop is especially important when conveying refrigerant,
because the refrigerant's temperature and vapor/liquid quality
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(ratio of vapor to liquid mass) changes significantly with
pressure which, in turn, affects the rate of heat transfer across
the tube. The fins are closely distributed around the tube's
inner circumference at .013 to .033 inch intervals 16 to maximize
fin surface area while avoiding the use of excessively thin,
fragile fins.
Each tube that was tested had a nominal outer diameter
of 3/8 inches and was tested by conveying a refrigerant through
the interior of the tube. The specific refrigerant that was used
in the tests was "FREON", which is a trademark for a fluorinated
hydrocarbon. More specifically, the refrigerant was "FREON R-22"
which is a trademark for difluoromonochloromethane. The
temperature of the external surface of the tube was controlled to
provide a constant heat flux of 5,000 Btu/hr-ft2. The inlet and
outlet temperature of the refrigerant was controllably changed to
test each tube as i~ conveyed refrigerant of different
vapor/liquid qualities. The quality was varied at five
incremental values ranging from .15 to .85, and the results that
are shown in Figures 3 through 6 are based on an average quality
of .6 with the tubes functioning as an evaporator. Figures 3, 4
and 5 are based on a refrigerant mass flow rate of 200 lbs/hr.
The tests provided the heat transfer coefficient
(Btu/hr-ft2-F) of various tubes having internal fins. The
coefficients were compared to those of smooth tubes having no
internal fins. From the comparison, a dimensionless improvement
factor, referred to hereinbelow as a heat transfer multiplier or
simply a multiplier, was determined by dividing the heat transfer
coefficient of the finned tube by the coefficient a comparable
smooth tube.
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The results of the tests are summarized in Figure 3.
Eight different internally finned tubes are represented by points
"A-H" which have been plotted according to fin height h and helix
angle e. As indicated at point B, each point A-H is accompanied
by its heat transfer multiplier 20 as determined by actual tests.
Below each multiplier 20, in parentheses, is a calculated
- multiplier "Z" based on an empirically derived equation 22 having
a 96% coorelation with the measured multipliers 20. Equation 22
defines multiplier Z as a function of fin height h and helix
angle e, with h being expressed in mils (1 mil = .001 inches) and
e expressed in degrees with respect to a longitudinal axis of the
tube.
Regions 24 and 26 of Figure 3 represent combinations of
fin height h and helix angle e that provide a multiplier greater
than two based on equation 22. In other words, the heat transfer
rate of a smooth tube could be expected to double if it were
modified to include internal fins having a combination height h
and helix angle e that lies within regions 24 or 26. It is worth
noting that U.S. Patents 4,545,428 and 4,118,944 have pointed out
the importance of regions 28 and 30 respectively which generally
coincides with region 26; however, the importance of the
relatively narrow region 24 has not been appreciated. Region 24
identifies a specific set of tubes that represent the preferred
embodiment of the invention.
Figure 4 illustrates how varying the helix angle
affects heat transfer for given a fin height of approximately
.008 inches (8 mils). Tubes represented by points C, D, G and H
have a fin height of .008, .0075, .0085, and .008 inches
respectively. A V-shape curve 32 represents multiplier Z as a
function of helix angle e based on equation 22 for a constant fin
height of .008 inches. Curve 32 illustrates how the multiplier
increases as the helix angle increases or decreases from a low
point 34 of 12 degrees.
i;~O;~39~;
The effect of fin height for a given helix angle is
illustrated in Figure 5. Curve 36 represents multiplier Z as a
function of fin height based on equation 22 with a constant helix
angle of 6. Internally finned tubes represented by points B, C,
D and E have a helix angle of 7, 7, 5 and 6 respectively.
Point 38 represents a comparable smooth tube having no internal
- fins. Figure 5 shows that for a helix angle of 6, optimum heat
transfer is obtained at a fin height of at least .007 inches.
However, its best to limit the height to less than .030 inches to
facilitate fin forming. This generally limits the fin height to
no more than a nominal thickness 40 (Figure 2) of readily
available tubing having a nominal pre-finned wall thickness
ranging from .012 to .033 inches. Moreover, when using 3/ô inch
O.D. tubing having a nominal wall thickness of .027 inches, an
internal fin having a fin height of .020 inches, for example,
will only project 7~ across the internal diameter of the tube to
provide minimal flow restriction.
The tests also show that the heat transfer multiplier Z
is highest at lower flow rates. This is illustrated by curve 42,
shown in Figure 6, which is based on actual data points 44 and 46
and empirically derived points 48. Curve 42 clearly shows that a
mass flow rate below 400 lbs/hr is the optimum flow rate for tube
10 which represents one embodiment of the invention. Line 43
represents a smooth tube which, by definition, has a multiplier
equal to one. Comparing curve 42 to line 43 shows that tube 10
is superior to a comparable smooth tube at flow rates below 650
lbs/hr. For a tube 10 having a nominal 3/8 inch ouside diameter
and a .321 inch inside diameter, 650 lbs/hr provides a mass flow
per cross-sectional area (mass flux) of 8,032 lbs/hr-in2.
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Tube 10 was also tested in a condensing mode. The
tests were similar to the tests performed in the evaporating
mode, except the refrigerant being conveyed through the tube was
cooled instead of heated. The tests showed that multiplier 20
of tube 10 only decreased from 2.12 in the evaporating mode to
2.02 in the condensing mode. The small 4.7% decrease indicates
that tube 10 is suitable to function as both an evaporator and a
condenser.
Although the invention is described wih respect to a
preferred embodiment, modifications thereto will be apparent to
those skilled in the art. Therefore, the scope of the invention
is to be determined by reference to the claims which follow.
I claim:
