US20070187067A1

US20070187067A1 - Heat transfer tube and heat exchanger using same

Info

Publication number: US20070187067A1
Application number: US11/522,365
Authority: US
Inventors: Ken Horiguchi; Kenichi Kikuchi; Ryuichi Kobayashi; Mamoru Hofuku; Kenji Kodama; Katsumi Nomura
Original assignee: Hitachi Cable Ltd
Current assignee: Hitachi Cable Ltd
Priority date: 2006-02-15
Filing date: 2006-09-18
Publication date: 2007-08-16
Also published as: JP2007218486A; CN101021393A

Abstract

A heat transfer tube is formed with a corrugated water tube to be used in a heat exchanger, and satisfying 0.04≦Hc/OD, where Hc is the corrugated groove depth of the corrugated tube and OD is the corrugation outside diameter thereof.

Description

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

1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
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).
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.
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.
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.
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.
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

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.

(1) According to a first aspect of the invention, a heat transfer tube comprises:

a corrugated water tube to be used in a heat exchanger, and satisfying 0.04≦Hc/OD, where Hc is a corrugated groove depth of the corrugated tube and OD is a corrugation outside diameter thereof.
In the above invention (1), the following modifications and changes can be made.
(i) 0.04≦Hc/OD≦0.1.
(ii) A twist angle βc defined between a corrugated groove of the corrugated tube and a tube axis thereof is βc≧40°.

(2) According to a second aspect of the invention, a heat exchanger comprises:

a heat transfer tube comprising a corrugated water tube to be used in a heat exchanger, and satisfying 0.04≦Hc/OD, where Hc is a corrugated groove depth of the corrugated tube and OD is a corrugation outside diameter thereof.
In the above invention (2), the following modifications and changes can be made.
(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.
(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.
(v) The outer tube comprises a corrugated tube.

(3) According to a third aspect of the invention, a heat exchanger comprises:

a heat transfer tube comprising a corrugated water tube to be used in a heat exchanger, and satisfying 0.04≦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.

ADVANTAGES OF THE INVENTION

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

The preferred embodiments according to the invention will be explained below referring to the drawings, wherein:

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;

FIG. 1B is an enlarged cross-sectional view in region A of FIG. 1A;

FIG. 2 is an explanatory diagram showing structure of a heat transfer tube in a second preferred embodiment according to the present invention;

FIG. 3 is an explanatory diagram showing structure of a heat exchanger in a third preferred embodiment according to the present invention;

FIG. 4 is an explanatory diagram showing structure of a heat exchanger in a fourth preferred embodiment according to the present invention;

FIG. 5 is an explanatory diagram showing structure of a heat exchanger in a fifth preferred embodiment according to the present invention;

FIG. 6 is an explanatory diagram showing structure of a heat exchanger in a sixth preferred embodiment according to the present invention;

FIG. 7 is an explanatory diagram showing structure of a heat exchanger in a seventh preferred embodiment according to the present invention;

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);

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;

FIG. 10 is a diagram showing the relationship between twist angle βc 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

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

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.
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.
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≦Hc/OD, preferably 0.04≦Hc/OD≦0.1, more preferably 0.04≦Hc/OD≦0.07, and can thereby have good heat transfer performance and low pressure loss.
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 βc, βc is then desirably 40° or higher, more desirably 40°≦βc≦82°. This allows promoting fluid turbulence which has crossed the unevenness. From the above definition of the corrugated tube, the twist angle βc ranges 0°<βc<90°.
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≦Tw≦1.7 mm and 3 mm≦Pc≦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

FIG. 2 is an explanatory diagram showing structure of a heat transfer tube in a second preferred embodiment according to the present invention.
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.
Although the twist angle βc 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° or higher, which is difficult to fabricate in the inner-grooved tube, can be realized.
Next, there is explained a heat exchanger equipped with the above corrugated heat transfer tube.

Third Embodiment

FIG. 3 is an explanatory diagram showing structure of a heat exchanger in a third preferred embodiment according to the present invention.
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

FIG. 4 is an explanatory diagram showing structure of a heat exchanger in a fourth preferred embodiment according to the present invention.
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

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
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

FIG. 7 is an explanatory diagram showing structure of a heat exchanger in a seventh preferred embodiment according to the present invention.
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.
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

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

The embodiments of the present invention have the following advantages:
(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.
(2) According to the corrugated heat transfer tube in the present embodiments, because the twist angle βc which the corrugated grooves make with the tube axis can be 40° 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 βc large at low cost compared to the inner-grooved tube, etc.
(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.
(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.
(5) According to the above fifth and sixth embodiments, the corrugated outer tube allows enhancement in flexibility for bending of the heat exchanger.
(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

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^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 1

Sample-tube specifications

Example 1	Hc/OD	Twist angle βc	No. of threads
Corrugated tube	0.1	81°	1
Comparison	Groove depth/	Twist angle	No. of grooves
example 2	outside diameter
Inner-grooved tube	0.03	35°	40

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

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 βc 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.
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≦Hc/OD.

EXAMPLE 3

FIG. 10 is a diagram showing the relationship between the twist angle βc 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.
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 βc (βc=35°, for example). In addition, it is clarified that, by making the twist angle βc equal to or higher than 40°, the heat transfer performance of the corrugated heat transfer tube can be enhanced to twice or more that of the plain tube.

EXAMPLE 4

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 λ defined by the relation of “ΔP=λ×L/de×(ρv²)/2”, and can be regarded as an indicator of pressure loss where the affects of flow passage area, fluid flow rate, etc. are canceled. ΔP is the pressure loss of the heat transfer tube, L is the length of the heat transfer tube, de is the equivalent diameter (4×flow passage area/wetted perimeter) of the heat transfer tube, ρ is the fluid density, and v is the fluid flow rate. Both of the twist angle βc 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.
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≦Hc/OD≦0.1, which makes it possible to provide a high-performance corrugated heat transfer tube with low pressure loss.
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.

Claims

1. A heat transfer tube, comprising:

a corrugated water tube to be used in a heat exchanger, and satisfying 0.04≦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≦Hc/OD≦0.1.

3. The heat transfer tube according to claim 1, wherein:

a twist angle βc defined between a corrugated groove of the corrugated tube and a tube axis thereof is βc≧40°.

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≦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≦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.