U.S. patent application number 11/522365 was filed with the patent office on 2007-08-16 for heat transfer tube and heat exchanger using same.
This patent application is currently assigned to HITACHI CABLE, LTD.. Invention is credited to Mamoru Hofuku, Ken Horiguchi, Kenichi Kikuchi, Ryuichi Kobayashi, Kenji Kodama, Katsumi Nomura.
Application Number | 20070187067 11/522365 |
Document ID | / |
Family ID | 38367135 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070187067 |
Kind Code |
A1 |
Horiguchi; Ken ; et
al. |
August 16, 2007 |
Heat transfer tube and heat exchanger using same
Abstract
A heat transfer tube is formed with a corrugated water tube to
be used in a heat exchanger, and satisfying 0.04.ltoreq.Hc/OD,
where Hc is the corrugated groove depth of the corrugated tube and
OD is the corrugation outside diameter thereof.
Inventors: |
Horiguchi; Ken; (Tsuchiura,
JP) ; Kikuchi; Kenichi; (Tsuchiura, JP) ;
Kobayashi; Ryuichi; (Hitachinaka, JP) ; Hofuku;
Mamoru; (Inashiki-gun, JP) ; Kodama; Kenji;
(Tsuchiura, JP) ; Nomura; Katsumi; (Tsuchiura,
JP) |
Correspondence
Address: |
MCGINN INTELLECTUAL PROPERTY LAW GROUP, PLLC
8321 OLD COURTHOUSE ROAD, SUITE 200
VIENNA
VA
22182-3817
US
|
Assignee: |
HITACHI CABLE, LTD.
Tokyo
JP
|
Family ID: |
38367135 |
Appl. No.: |
11/522365 |
Filed: |
September 18, 2006 |
Current U.S.
Class: |
165/70 ; 165/154;
165/177 |
Current CPC
Class: |
F28F 1/003 20130101;
F28F 1/426 20130101; F28D 7/106 20130101; F28F 1/42 20130101; F28F
1/08 20130101 |
Class at
Publication: |
165/70 ; 165/154;
165/177 |
International
Class: |
F28F 11/00 20060101
F28F011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 15, 2006 |
JP |
2006-038531 |
Claims
1. A heat transfer tube, comprising: a corrugated water tube to be
used in a heat exchanger, and satisfying 0.04.ltoreq.Hc/OD, where
Hc is a corrugated groove depth of the corrugated tube and OD is a
corrugation outside diameter thereof.
2. The heat transfer tube according to claim 1, wherein:
0.04.ltoreq.Hc/OD.ltoreq.0.1.
3. The heat transfer tube according to claim 1, wherein: a twist
angle .beta.c defined between a corrugated groove of the corrugated
tube and a tube axis thereof is .beta.c.gtoreq.40.degree..
4. A heat exchanger, comprising: a heat transfer tube comprising a
corrugated water tube to be used in a heat exchanger, and
satisfying 0.04.ltoreq.Hc/OD, where Hc is a corrugated groove depth
of the corrugated tube and OD is a corrugation outside diameter
thereof.
5. The heat exchanger according to claim 4, further comprising: an
outer tube provided outside of the heat transfer tube that is used
as an inner tube, the heat exchanger formed so that a refrigerant
flows through an annular portion between the heat transfer tube and
the outer tube.
6. The heat exchanger according to claim 4, further comprising: a
plain tube sheathed on the heat transfer tube to form a leak
detection portion; and an outer tube arranged outside of the plain
tube, the heat exchanger formed so that a refrigerant flows through
an annular portion between the plain tube and the outer tube.
7. The heat exchanger according to claim 5, wherein: the outer tube
comprises a corrugated tube.
8. The heat exchanger according to claim 6, wherein: the outer tube
comprises a corrugated tube.
9. A heat exchanger, comprising: a heat transfer tube comprising a
corrugated water tube to be used in a heat exchanger, and
satisfying 0.04.ltoreq.Hc/OD, where Hc is a corrugated groove depth
of the corrugated tube and OD is a corrugation outside diameter
thereof; and a refrigerant-conducting heat transfer tube wound
around on the heat transfer tube.
Description
[0001] The present application is based on Japanese patent
application No. 2006-038531, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a heat transfer tube and a
heat exchanger using the heat transfer tube, and particularly, to a
heat transfer tube for a water-refrigerant heat exchanger used in a
natural refrigerant heat pump water heater (which may herein be
referred to as simply "heat pump water heater"), and a heat
exchanger using the heat transfer tube.
[0004] 2. Description of the Related Art
[0005] Conventionally well-known heat pump water heaters using a
natural refrigerant (e.g., carbon dioxide refrigerant) employ
generally two kinds of heat exchangers, i.e., a heat radiator and a
heat absorber. The two heat exchangers use, as a heat transfer
tube, refrigerant tubes for the heat radiator and for the heat
absorber, respectively.
[0006] In the heat pump water heaters, the heat radiator is also
called "water-refrigerant heat exchanger" and uses also another
heat transfer tube (which is called "water tube for the heat
radiator" or herein also called simply "water tube") for heat
exchange to the refrigerant other than the above two heat transfer
tubes. Fluid to flow inside the three individual heat transfer
tubes (i.e., the water tube for the heat radiator, the refrigerant
tube for the heat radiator and the refrigerant tube for the heat
absorber) is different in kind, pressure and flow rate. Therefore,
the technical specifications to be required respectively for the
heat transfer tubes are also different.
[0007] For example, as the water-refrigerant heat exchanger (herein
called simply "heat exchanger") used for the natural refrigerant
heat pump water heater, there is a double tube heat exchanger that
comprises two tubes of an outer tube through which water flows, and
an inner tube through which a refrigerant flows. In such a double
tube heat exchanger, in the inner tube through which a refrigerant
flows, hole formation may be caused by corrosion, which leads to a
mixing of the water and refrigerant. For this reason, a leak
detection portion (a leak detection tube with leak detection
grooves) is often provided that detects water or refrigerant leak
to stop the apparatus (providing the leak detection tube causes the
heat exchanger to actually have a triple tube structure).
[0008] On the other hand, the natural refrigerant heat pump water
heater is used for boiling water during night, and has a small
water flow rate which causes a laminar flow. For this reason, to
enhance heat exchanger performance, it is essential to enhance heat
transfer performance of the water tube that becomes a
bottleneck.
[0009] JP-A-2004-360974 discloses a heat exchanger for the purpose
of enhancement in heat transfer performance, which comprises a
first heat transfer tube and a plural-tube-helically-twisted second
heat transfer tube arranged in the first heat transfer tube.
According to JP-A-2004-360974, the heat exchanger disclosed therein
is small in water pressure loss and in dissolution of a
scale-forming constituent, and allows heat transfer promotion
without using another heat transfer promotion component.
[0010] Also, JP-A-2002-228370 discloses a heat exchanger that
comprises a water tube as its core tube and a refrigerant tube
wound therearound, where the core tube is constructed from a plain
tube, a corrugated tube or an inner-grooved tube, or by inserting a
torsion sheet in the core tube. According to JP-A-2002-228370, the
heat exchanger disclosed therein has the advantages of ease of
fabrication and conveyance, enhancement of heat exchange, reduction
of cost, etc.
[0011] In the heat exchanger disclosed in JP-A-2004-360974,
however, there are the problems that the
plural-tube-helically-twisting step itself is complicated and
costly (twisting a hollow tube that tends to deform (collapse,
break, etc.) is not so easy compared to twisting a solid wire), and
that the treatment (structure) of the heat exchanger ends, in which
the first heat transfer tube and the plural-tube second heat
transfer tube are separated from each other, is complicated. There
is also the problem that, when the above-mentioned leak detection
portion is provided, it is necessary to cause each of the second
heat transfer tube to comprise a double tube, which leads to an
even higher cost.
[0012] Also, in JP-A-2002-228370, simply forming the core tube in a
corrugated shape or inserting the torsion sheet in the core tube
may make no desired heat transfer performance, and an increase in
cost or pressure loss. Also, where an inner-grooved tube is used as
the core tube, a laminar flow region by a small flow-rate has no
effect caused by an increase of heat transfer area even though the
heat transfer area increases. Further, because of limitations on an
inner-grooved tube fabrication method, it is difficult to form a
shape change to cause a turbulence effect in a laminar flow region
by the small flow-rate.
SUMMARY OF THE INVENTION
[0013] Accordingly, it is an object of the present invention to
provide a heat transfer tube for a heat exchanger, capable of
enhancing heat transfer performance of the heat exchanger when used
for a small water-flow-rate in a natural refrigerant heat pump
water heater, and a heat exchanger using the heat transfer tube.
[0014] (1) According to a first aspect of the invention, a heat
transfer tube comprises:
[0015] a corrugated water tube to be used in a heat exchanger, and
satisfying 0.04.ltoreq.Hc/OD, where Hc is a corrugated groove depth
of the corrugated tube and OD is a corrugation outside diameter
thereof.
[0016] In the above invention (1), the following modifications and
changes can be made.
[0017] (i) 0.04.ltoreq.Hc/OD.ltoreq.0.1.
[0018] (ii) A twist angle .beta.c defined between a corrugated
groove of the corrugated tube and a tube axis thereof is
.beta.c.gtoreq.40.degree.. [0019] (2) According to a second aspect
of the invention, a heat exchanger comprises:
[0020] a heat transfer tube comprising a corrugated water tube to
be used in a heat exchanger, and satisfying 0.04.ltoreq.Hc/OD,
where Hc is a corrugated groove depth of the corrugated tube and OD
is a corrugation outside diameter thereof.
[0021] In the above invention (2), the following modifications and
changes can be made.
[0022] (iii) The heat exchanger further comprises an outer tube
provided outside of the heat transfer tube that is used as an inner
tube, the heat exchanger formed so that a refrigerant flows through
an annular portion between the heat transfer tube and the outer
tube.
[0023] (iv) The heat exchanger further comprises a plain tube
sheathed on the heat transfer tube to form a leak detection
portion, and an outer tube arranged outside of the plain tube, the
heat exchanger formed so that a refrigerant flows through an
annular portion between the plain tube and the outer tube.
[0024] (v) The outer tube comprises a corrugated tube. [0025] (3)
According to a third aspect of the invention, a heat exchanger
comprises:
[0026] a heat transfer tube comprising a corrugated water tube to
be used in a heat exchanger, and satisfying 0.04.ltoreq.Hc/OD,
where Hc is a corrugated groove depth of the corrugated tube and OD
is a corrugation outside diameter thereof; and
[0027] a refrigerant-conducting heat transfer tube wound around on
the heat transfer tube.
ADVANTAGES OF THE INVENTION
[0028] According to the present invention, it is possible to
provide a heat transfer tube for a heat exchanger that is capable
of enhancing heat transfer performance of the heat exchanger when
used for a small water-flow-rate in a natural refrigerant heat pump
water heater, and a heat exchanger using the heat transfer
tube.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The preferred embodiments according to the invention will be
explained below referring to the drawings, wherein:
[0030] FIG. 1A is an explanatory diagram of an entire view showing
structure of a heat transfer tube in a first preferred embodiment
according to the present invention;
[0031] FIG. 1B is an enlarged cross-sectional view in region A of
FIG. 1A;
[0032] FIG. 2 is an explanatory diagram showing structure of a heat
transfer tube in a second preferred embodiment according to the
present invention;
[0033] FIG. 3 is an explanatory diagram showing structure of a heat
exchanger in a third preferred embodiment according to the present
invention;
[0034] FIG. 4 is an explanatory diagram showing structure of a heat
exchanger in a fourth preferred embodiment according to the present
invention;
[0035] FIG. 5 is an explanatory diagram showing structure of a heat
exchanger in a fifth preferred embodiment according to the present
invention;
[0036] FIG. 6 is an explanatory diagram showing structure of a heat
exchanger in a sixth preferred embodiment according to the present
invention;
[0037] FIG. 7 is an explanatory diagram showing structure of a heat
exchanger in a seventh preferred embodiment according to the
present invention;
[0038] FIG. 8 is a diagram showing the comparison of heat transfer
performance of the corrugated heat transfer tube of the first
embodiment (example 1), of a plain tube (comparison example 1), and
of an inner-grooved tube (comparison example 2);
[0039] FIG. 9 is a diagram showing the relationship between Hc/OD
and heat transfer performance of the corrugated heat transfer tube,
i.e., heat transfer performance ratio relative to a plain tube for
Reynolds number Re=1000;
[0040] FIG. 10 is a diagram showing the relationship between twist
angle .beta.c and heat transfer performance of the corrugated heat
transfer tube, i.e., heat transfer performance ratio relative to a
plain tube for Reynolds number Re=1000; and
[0041] FIG. 11 is a diagram showing the relationship between Hc/OD
and friction coefficient of the corrugated heat transfer tube,
i.e., tube friction coefficient ratio relative to a plain tube for
Reynolds number Re=1000.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0042] FIG. 1A is an explanatory diagram of an entire view showing
structure of a heat transfer tube in a first preferred embodiment
according to the present invention, and FIG. 1B is an enlarged
cross-sectional view in region A of FIG. 1A.
[0043] A heat transfer tube (a corrugated heat transfer tube) 10 in
this embodiment is formed of a one-thread corrugated tube
("one-thread" means that a number of a corrugated groove is one),
and is used as a water tube that constitutes a heat exchanger
(e.g., a water-refrigerant heat exchanger for a heat pump water
heater), where a heat is exchanged between a water flowing inside
the heat transfer tube 10 and a refrigerant flowing outside the
heat transfer tube 10. The corrugated tube generally refers to a
tube with an undulating helical structure in its inner/outer
surface.
[0044] Let the corrugated groove depth and corrugation outside
diameter of the corrugated heat transfer tube 10 in this embodiment
be Hc and OD respectively, Hc/OD, which represents the
unevenness-to-outside-diameter ratio of the corrugated heat
transfer tube 10, is then large compared with the
unevenness-to-outside-diameter ratio (="groove depth/outside
diameter") of a typical inner-grooved tube. The corrugated heat
transfer tube 10 satisfies 0.04.ltoreq.Hc/OD, preferably
0.04.ltoreq.Hc/OD.ltoreq.0.1, more preferably
0.04.ltoreq.Hc/OD.ltoreq.0.07, and can thereby have good heat
transfer performance and low pressure loss.
[0045] Also, let the angle which corrugated grooves 1 of the
corrugated heat transfer tube 10 make with tube axis Ta thereof be
a twist angle .beta.c, .beta.c is then desirably 40.degree. or
higher, more desirably 40.degree..ltoreq..beta.c.ltoreq.82.degree..
This allows promoting fluid turbulence which has crossed the
unevenness. From the above definition of the corrugated tube, the
twist angle .beta.c ranges 0.degree.<.beta.c<90.degree..
[0046] Thickness Tw of an end region with a plain shape and
corrugation pitch Pc of the corrugated heat transfer tube 10 are
not particularly limited, but may be 0.4 mm.ltoreq.Tw.ltoreq.1.7 mm
and 3 mm.ltoreq.Pc.ltoreq.10 mm, respectively, for example. Also,
its material is not particularly limited, but may, from the point
of view of thermal conductivity and mechanical strength, be
preferably copper or copper alloy, aluminum or aluminum alloy, or
the like.
Second Embodiment
[0047] FIG. 2 is an explanatory diagram showing structure of a heat
transfer tube in a second preferred embodiment according to the
present invention.
[0048] While the heat transfer tube 10 in the first embodiment is
formed from the one-thread corrugated tube, a heat transfer tube 20
in this second embodiment is formed from a three-thread corrugated
tube ("three-thread" means that a number of a corrugated groove is
three), and is used as a water tube that constitutes a heat
exchanger. The more the number of threads, the higher the
fabrication rate, which therefore results in a large cost
merit.
[0049] Although the twist angle .beta.c in three thread fabrication
tends to be smaller than that of one thread fabrication, by
reducing the spacing between the adjacent corrugated grooves 1,
i.e., corrugation pitch Pc, a twist angle of 40.degree. or higher,
which is difficult to fabricate in the inner-grooved tube, can be
realized.
[0050] Next, there is explained a heat exchanger equipped with the
above corrugated heat transfer tube.
Third Embodiment
[0051] FIG. 3 is an explanatory diagram showing structure of a heat
exchanger in a third preferred embodiment according to the present
invention.
[0052] A heat exchanger (a double tube heat exchanger) 100 in this
embodiment includes an outer tube 11 provided outside of the heat
transfer tube (e.g., corrugated heat transfer tube 10) in the
above-described embodiments that is used as an inner tube, where
the heat exchanger is formed so that a refrigerant flows through an
annular path between the corrugated heat transfer tube 10 and the
outer tube 11.
Fourth Embodiment
[0053] FIG. 4 is an explanatory diagram showing structure of a heat
exchanger in a fourth preferred embodiment according to the present
invention.
[0054] A heat exchanger (a triple tube heat exchanger) 200 in this
embodiment includes a leak detection tube 12 comprising a plain
tube arranged in contact with the periphery of the heat transfer
tube (e.g., corrugated heat transfer tube 10) in the
above-described embodiments that is used as an inner tube, to form
therearound a leak detection portion (a leak detection grooves 13),
and an outer tube 11 arranged outside of the leak detection tube
12, where the heat exchanger is formed so that a refrigerant flows
through an annular path between the leak detection tube 12 and the
outer tube 11.
Five and Sixth Embodiments
[0055] FIG. 5 is an explanatory diagram showing structure of a heat
exchanger in a fifth preferred embodiment according to the present
invention. FIG. 6is also an explanatory diagram showing structure
of a heat exchanger in a sixth preferred embodiment according to
the present invention
[0056] Heat exchangers 300 and 400 shown in FIGS.5 and 6 use
corrugated outer tubes 21 in place of the outer tubes in the heat
exchangers of FIGS.3 and 4, respectively. This allows enhancement
in flexibility for bending of the heat exchanger.
Seventh Embodiment
[0057] FIG. 7 is an explanatory diagram showing structure of a heat
exchanger in a seventh preferred embodiment according to the
present invention.
[0058] A heat exchanger 500 in this embodiment is constructed by
winding a refrigerant-conducting helical heat transfer tube 31
along the corrugated grooves 1 of the heat transfer tube (e.g.,
corrugated heat transfer tube 10) in the above-described
embodiments. The corrugated grooves 1 and heat transfer tube 31 may
be securely brazed to each other, if desired.
[0059] In the heat exchanger 500, heat is exchanged between a water
flowing inside the heat transfer tube 10 and a refrigerant flowing
inside the helical heat transfer tube 31 in contact with the
periphery of the heat transfer tube 10. Because the heat transfer
tube 31 is wound along the corrugated grooves 1, it is possible to
increase effective contact area (effective heat transfer area)
between the heat transfer tube 10 and the heat transfer tube
31.
Other Embodiments
[0060] As the embodiments of the present invention, besides the
above-described embodiments, there are various embodiments. For
example, while the one-thread and three-thread corrugated heat
transfer tubes have been explained, the corrugated heat transfer
tube may comprise two, or four or more threads. The
one-to-three-thread corrugated heat transfer tube is desirable in
that it is easy to realize a high twist angle difficult to
fabricate in the inner-grooved tube.
Advantages of the Embodiments
[0061] The embodiments of the present invention have the following
advantages:
[0062] (1) In a prior-art heat transfer tube (a plain tube, an
inner-grooved tube, etc.), there is the problem of very low heat
transfer performance due to a very small water-flow-rate in a
water-refrigerant heat exchanger of a heat pump water heater,
leading to a laminar flow in the heat transfer tube. Also, a
prior-art heat transfer tube using a corrugated tube does not
define unevenness-to-outside-diameter ratio Hc/OD, and is
indefinite in heat transfer performance effect. In contrast to
these, according to the corrugated heat transfer tube in the
present embodiments, the unevenness-to-outside-diameter ratio Hc/OD
can be sufficiently large at low cost even compared to an
inner-grooved tube, and the heat transfer performance can be
substantially enhanced by the turbulence effect of fluid crossing
unevenness defined by this Hc/OD. Particularly, it is possible to
realize twice or more the performance compared to a plain tube, in
a low Reynolds number Re range (e.g., 1000-5000, particularly
1000-3000) difficult to enhance the performance in the prior-art
product.
[0063] (2) According to the corrugated heat transfer tube in the
present embodiments, because the twist angle .beta.c which the
corrugated grooves make with the tube axis can be 40.degree. or
higher, which is difficult to form in the inner-grooved tube, it is
possible to increase the frequency of fluid crossing unevenness,
and thereby promote fluid turbulence effect. Also, by adjusting the
relationship between the number of threads and the corrugation
pitch Pc, it is possible to make the twist angle .beta.c large at
low cost compared to the inner-grooved tube, etc.
[0064] (3) According to the corrugated heat transfer tube in the
above third to sixth embodiments, because it is possible to
maximize enhancement in the heat transfer performance of the water
tube and the heat transfer area of the water tube relative to the
refrigerant, the heat exchanger efficiency is enhanced. Further,
according to the above third and fifth embodiments, it is possible
to ensure enhancement in the heat transfer performance of the
refrigerant in addition to the heat transfer performance of the
water tube.
[0065] (4) It is possible to relatively easily provide a large leak
detection portion, in comparison to the inner-grooved tube.
Specifically, although leak detection groove formation typically
requires use of an inner-grooved tube with high fins as a leak
detection tube, because the corrugated heat transfer tube is used
as the inner tube, it is possible to make the leak detection
grooves large (at low cost), and thereby use a plain tube as the
leak detection tube 12.
[0066] (5) According to the above fifth and sixth embodiments, the
corrugated outer tube allows enhancement in flexibility for bending
of the heat exchanger.
[0067] (6) According to the above seventh embodiment, because the
outer tube through which a refrigerant flows is helically wound
along the corrugated grooves of the corrugated heat transfer tube,
it is possible to have flexibility for bending of the heat
exchanger, and increase effective contact area (effective heat
transfer area) between the outer tube and the water tube (the heat
transfer tube around which is wound by the outer tube), compared to
the case where the outer tube is wound around a plain tube or an
inner-grooved tube.
EXAMPLE 1
[0068] FIG. 8 is a diagram showing the comparison of heat transfer
performance, in the laminar flow regions (small Reynolds number
regions), of the corrugated heat transfer tube of the first
embodiment (example 1), of a plain tube (comparison example 1), and
of an inner-grooved tube (comparison example 2). Table 1 below
shows specifications of the tested corrugated heat transfer tube
and inner-grooved tube. The heat transfer tubes all comprise
phosphorus deoxidized copper, and have the outside diameter (OD) of
9.52 mm. Here, the heat transfer performance is defined by dividing
Nusselt number Nu by Prandtl Number Pr raised to the power of 0.4
(Nu/Pr.sup.0.4, the same applies to examples below), to cancel the
affects of fluid properties. Also, comparison is made for Reynolds
numbers Re=1000, 2000, and 3000 that correspond to water flow
amounts actually used in a heat pump water heater.
TABLE-US-00001 TABLE 1 Sample-tube specifications Example 1 Hc/OD
Twist angle .beta.c No. of threads Corrugated tube 0.1 81.degree. 1
Comparison Groove depth/ Twist angle No. of grooves example 2
outside diameter Inner-grooved tube 0.03 35.degree. 40
[0069] As shown in FIG. 8, it is revealed that, in evaluated
Reynolds-number regions, the inner-grooved tube (comparison example
2) and plain tube (comparison example 1) have substantially the
same heat transfer performance, whereas the corrugated heat
transfer tube 10 has the substantially enhanced heat transfer
performance of 3 times or more those of the inner-grooved tube and
the plain tube.
EXAMPLE 2
[0070] FIG. 9 is a diagram showing the relationship between Hc/OD
and heat transfer performance of the corrugated heat transfer tube,
i.e., heat transfer performance ratio relative to a plain tube for
Reynolds number Re=1000. Both of the twist angle .beta.c and the
number of threads of the corrugated heat transfer tube are the same
as in example 1 (Table 1). Also, as shown in FIG. 8, because the
heat transfer performance of the inner-grooved tube is on the same
order as that of the plain tube in this flow rate region, the heat
transfer performance of the corrugated heat transfer tube is
compared with that of the plain tube.
[0071] As shown in FIG. 9, it is revealed that, at less than 0.04
of Hc/OD, the heat transfer performance drops sharply. Thus, it is
desirable that 0.04.ltoreq.Hc/OD.
EXAMPLE 3
[0072] FIG. 10 is a diagram showing the relationship between the
twist angle .beta.c and the heat transfer performance of the
corrugated heat transfer tube, i.e., heat transfer performance
ratio relative to a plain tube for Reynolds number Re=1000. Both of
the Hc/OD and the number of threads of the corrugated heat transfer
tube are the same as in example 1 (Table 1). Also, as shown in FIG.
8, because the heat transfer performance of the inner-grooved tube
is on the same order as that of the plain tube in this flow rate
region, the heat transfer performance of the corrugated heat
transfer tube is compared with that of the plain tube.
[0073] As shown in FIG. 10, it is found that, for Hc/OD=0.1, the
heat transfer performance of the corrugated heat transfer tube is
higher by the order of 1.5 times that of the plain tube even in the
event of a small twist angle .beta.c (.beta.c=35.degree., for
example). In addition, it is clarified that, by making the twist
angle .beta.c equal to or higher than 40.degree., the heat transfer
performance of the corrugated heat transfer tube can be enhanced to
twice or more that of the plain tube.
EXAMPLE 4
[0074] FIG. 11 is a diagram showing the relationship between Hc/OD
and friction coefficient of the corrugated heat transfer tube,
i.e., tube friction coefficient ratio relative to a plain tube for
Reynolds number Re=1000. Here, the tube friction coefficient refers
to a dimensionless number .lamda. defined by the relation of
".DELTA.P=.lamda..times.L/de.times.(.rho.v.sup.2)/2", and can be
regarded as an indicator of pressure loss where the affects of flow
passage area, fluid flow rate, etc. are canceled. .DELTA.P is the
pressure loss of the heat transfer tube, L is the length of the
heat transfer tube, de is the equivalent diameter (4.times.flow
passage area/wetted perimeter) of the heat transfer tube, .rho. is
the fluid density, and v is the fluid flow rate. Both of the twist
angle .beta.c and the number of threads of the corrugated heat
transfer tube are the same as in example 1 (Table 1). Also, as
shown in FIG. 8, because the heat transfer performance of the
inner-grooved tube is on the same order as that of the plain tube
in this flow rate region, the tube friction coefficient of the
corrugated heat transfer tube is compared with that of the plain
tube.
[0075] As shown in FIG. 11, it is found that, at less than 0.04 of
Hc/OD, the tube friction coefficient ratio drops sharply as in the
heat transfer performance ratio, making turbulence promotion
impossible. On the other hand, for 0.04 or higher Hc/OD, the tube
friction coefficient ratio (i.e., pressure loss) continues to
increase. Further, it is found that, beyond 0.1 of Hc/OD
(0.1<Hc/OD), the tube friction coefficient ratio exceeds the
heat transfer performance ratio (see FIG. 9). For example, at
Hc/OD=1.1, the heat transfer performance ratio is 4.3, whereas the
tube friction coefficient ratio is 4.5. It is therefore desirable
that 0.04.ltoreq.Hc/OD.ltoreq.0.1, which makes it possible to
provide a high-performance corrugated heat transfer tube with low
pressure loss.
[0076] Although the invention has been described with respect to
the specific embodiments for complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art which fairly fall within the
basic teaching herein set forth.
* * * * *