US20080066488A1

US20080066488A1 - Heat Exchanger, Intermediate Heat Exchanger, and Refrigeration Cycle

Info

Publication number: US20080066488A1
Application number: US11/571,893
Authority: US
Inventors: Koichiro Take; Shigeharu Ichiyanagi
Original assignee: Showa Denko KK
Current assignee: Resonac Holdings Corp
Priority date: 2004-08-06
Filing date: 2005-08-05
Publication date: 2008-03-20
Also published as: DE112005001885T5; JP2006071270A; CN1977139A; WO2006014032A1

Abstract

A heat exchanger includes an outer tube and an inner tube having a plurality of fins for ed on an external periphery of the inner tube and disposed in the outer tube. The heat exchanger is designed to exchange heat between first fluid passing through the inner tube and second fluid passing in between the outer tube and the inner tube. A gap is formed between an internal periphery of the outer tube and a tip end of each of the plurality of fins of the inner tube. This structure can improve the heat exchanging performance and provide a heat exchanger excellent in vending workability.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. §111(a) claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date of U.S. Provisional Application No. 60/600,357 filed on Aug. 11, 2004, pursuant to 35 U.S.C.§111(b).
This application claims priority to Japanese Patent Application No. 2004-230778 filed on Aug. 6, 2004 and U.S. Provisional Application Ser. No. 60/600,357 filed on Aug. 11, 2004, the entire disclosures of which are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to a refrigeration cycle to be employed, for example, in an automobile air-conditioning refrigeration cycle, and also relates to an intermediate heat exchanger and a heat exchanger for use in such a cycle.

BACKGROUND ART

In most conventional refrigeration cycles of vapor compression type, Freon series refrigerant has been used. In recent years, however, it is getting to draw the attention of a refrigeration cycle using natural refrigerant such as carbon dioxide (CO₂).
In such a refrigeration cycle, refrigerant circulates such that high pressure refrigerant from a compressor and a gas cooler (condenser) passes through a decompressor and an evaporator into low pressure refrigerant and returns to the compressor. To improve the refrigeration performance in such a refrigeration cycle, it has been proposed to exchange heat between high pressure refrigerant and low pressure refrigerant.
As an intermediate heat exchanger for use in a CO₂refrigeration cycle, tubular structure intermediate heat exchangers disclosed in, for example, the following patent documents have been known.
In a heat exchanger as disclosed in Japanese Unexamined Laid-open Patent Document No. 2001-56188 (hereinafter, “Patent Document 1”) and Japanese Unexamined Laid-open Patent Document No. 2002-181466 (hereinafter, “Patent Document 2”), it is constituted by a multi-bored tubular element with a central passage and a plurality of external passages arranged around the central passage. In such a heat exchanger, high pressure refrigerant passes through the central passage and low pressure refrigerant passes through the external passages to exchange the heat therebetween.
In a heat exchanger as disclosed in International Publication No. WO 03/085344 (hereinafter, “Patent Document 3”), it is constituted by a tubular element in which an inner tube provided with a plurality of fins on an external periphery of the inner tube is inserted in an outer tube. In this heat exchanger, high pressure refrigerant passes through the inner tube and low pressure refrigerant passes in between both tubes to exchange the heat therebetween.
In the case of the heat exchanger as disclosed by Patent Documents 1 and 2, although the heat exchanger is usually formed by an extrusion method, it is difficult to form a heat exchanging tube having a plurality of passages by extrusion, resulting in restricted diameter and/or configuration of the passage for example. This causes a small diameter passage, thereby increasing the flow resistance, and also causes an increased thickness of partitions (fins) between passages, thereby deteriorating heat transfer performance, which in turn results in deteriorated heat exchanging performance. Furthermore, the entire tube is formed into an integral formed article and the partition wall (fin) between passages is thick, and therefore it is difficult to perform the bending work. For example, in the case of employing such a heat exchanger in a refrigeration cycle for air-conditioners, it cannot be formed into a configuration corresponding to a limited mounting space configuration in an automobile, resulting in a deteriorated freedom of design.
Furthermore, in the case of the heat exchanger as disclosed by Patent Document 3, since the inner tube is restrained by the outer tube, external force applied to the outer tube can easily act on the inner tube, causing cracks in the bending external side of the fins of the inner tube, which in turn results in deteriorated pressure resistance and durability. Thus, this heat exchanger is also not suitable to be subjected to bending work.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.

DISCLOSURE OF INVENTION

The preferred embodiments of the present invention have been developed in view of the above-mentioned and/or other problems in the related art. The preferred embodiments of the present invention can significantly improve upon existing methods and/or apparatuses.
Among other potential advantages, some embodiments can provide a heat exchanger capable of improving heat exchanging performance and excellent in bending workability.
Among other potential advantages, some embodiments can provide an intermediate heat exchanger capable of improving heat exchanging performance and excellent in bending workability.
Among other potential advantages, some embodiments can provide a refrigeration cycle using the aforementioned heat exchanger or intermediate heat exchanger.
To attain the aforementioned objects, the heat exchanger according to the present invention has the following structure as recited in the following Items [1] to [20].
[1] A heat exchanger, comprising:
an outer tube;
an inner tube having a plurality of fins formed on an external periphery of the inner tube, the inner tube being disposed in the outer tube;
first fluid passing in the inner tube; and
second fluid passing in between the outer tube and the inner tube,
wherein the heat exchanger exchanges heat between the first fluid and the second fluid, and
wherein a gap is formed between an internal periphery of the outer tube and a tip end of each of the plurality of fins of the inner tube.
[2] The heat exchanger as recited in the aforementioned Item [1], wherein the plurality of fins extend along a longitudinal direction of the inner tube and are arranged in a circumferential direction thereof at certain intervals.
[3] The heat exchanger as recited in the aforementioned Item [1] or [2], wherein the gap is set to 0.2 to 1 mm.
[4] The heat exchanger as recited in any one of the aforementioned Items [1] to [3], wherein the plurality of fins are arranged along a circumferential direction of the inner tube and the number of fins is set to 13 to 18.
[5] The heat exchanger as recited in any one of the aforementioned Items [1] to [4], wherein a thickness of each of the plurality of fins is set to 0.3 to 1.3 mm.
[6] The heat exchanger as recited in any one of the aforementioned Items [1] to [5], wherein an opening angle between adjacent fins is set to 15 to 30°.
[7] The heat exchanger as recited in any one of the aforementioned Items [1] to [6], wherein the plurality of fins are integrally formed on the inner tube.
[8] The heat exchanger as recited in any one of the aforementioned Items [1] to [7], wherein both the outer tube and the inner tube are bent by bending work.
[9] The heat exchanger as recited in any one of the aforementioned Items [1] to [8], wherein the first fluid is high pressure heat medium and the second fluid is low pressure heat medium.
[10] The heat exchanger as recited in any one of the aforementioned Items [1] to [9], wherein the inner tube is provided with a plurality of inner fins on an internal periphery thereof.
[11] An intermediate heat exchanger for exchanging heat between high pressure refrigerant and low pressure refrigerant among refrigerant circulating a refrigeration cycle,
the intermediate heat exchanger, comprising:
an inner tube provided with a plurality of fins on an external periphery thereof; and
an outer tube in which the inner tube is disposed in a manner such that a gap is formed between an internal periphery of the outer tube and a tip end of each of the plurality of fins of the inner tube,
wherein one of the high pressure refrigerant and the low pressure refrigerant passes through a first heat exchanging passage formed in the inner tube, and
wherein the other of the high pressure refrigerant and the low pressure refrigerant passes through a second heat exchanging passage between the outer tube and the inner tube.
[12] The heat exchanger as recited in the aforementioned Item [11], wherein the high pressure refrigerant passes through the first heat exchanging passage, and wherein the low pressure refrigerant passes through the second heat exchanging passage.
[13] The heat exchanger as recited in the aforementioned Item [11] or [12], wherein the refrigerant is carbon dioxide refrigerant.
[14] An intermediate heat exchanger for exchanging heat between high pressure refrigerant and low pressure refrigerant among refrigerant circulating a refrigeration cycle,
the intermediate heat exchanger, comprising:
an inner tube provided with a plurality of fins on an external periphery thereof; and
an outer tube in which the inner tube is disposed in a manner such that a gap is formed between an internal periphery of the outer tube and the plurality of fins of the inner tube,
wherein one of the high pressure refrigerant and the low pressure refrigerant passes through a first heat exchanging passage formed in the inner tube, and
wherein the other of the high pressure refrigerant and the low pressure refrigerant passes through a second heat exchanging passage between the outer tube and the inner tube.
[15]A refrigerant cycle in which refrigerant circulates such that the refrigerant passes through a compressor, a condenser, a decompressor and an evaporator, and then returns to the compressor,
the refrigerant cycle, comprising:
an intermediate heat exchanger for exchanging heat between high pressure refrigerant passing through a high pressure circuit from the compressor to the decompressor and low pressure refrigerant passing through a low pressure circuit from the decompressor toward the compressor,
wherein the intermediate heat exchanger includes:
an inner tube provided with a plurality of fins on an external periphery thereof; and
an outer tube in which the inner tube is disposed in a manner such that a gap is formed between an internal periphery of the outer tube and a tip end of each of the plurality of fins of the inner tube,
wherein one of the high pressure refrigerant and the low pressure refrigerant passes through a first heat exchanging passage formed in the inner tube, and
wherein the other of the high pressure refrigerant and the low pressure refrigerant passes through a second heat exchanging passage between the outer tube and the inner tube.
[16] A refrigerant cycle in which refrigerant circulates such that the refrigerant passes through a compressor, a condenser, a decompressor and an evaporator, and then returns to the compressor,
the refrigerant cycle, comprising:
an intermediate heat exchanger for exchanging heat between high pressure refrigerant passing through a circuit located between the condenser and the decompressor and low pressure passing through a circuit located between the evaporator and the compressor,
wherein the intermediate heat exchanger includes:
an inner tube provided with a plurality of fins on an external periphery thereof; and
an outer tube in which the inner tube is disposed in a manner such that a gap is formed between an internal periphery of the outer tube and a tip end of each of the plurality of fins of the inner tube,
wherein one of the high pressure refrigerant and the low pressure refrigerant passes through a first heat exchanging passage formed in the inner tube, and
wherein the other of the high pressure refrigerant and the low pressure refrigerant passes through a second heat exchanging passage between the outer tube and the inner tube.
[17] The refrigerant cycle as recited in the aforementioned Item [16], wherein the high pressure refrigerant passes through the first heat exchanging passage, and wherein the low pressure refrigerant passes through the second heat exchanging passage.
[18] The refrigerant cycle as recited in the aforementioned Item [16]or [17], wherein the condenser is a gas cooler.
[19] The refrigerant cycle as recited in any one of the aforementioned Items [16]to [18], wherein the refrigerant is carbon dioxide refrigerant.
[20] The refrigerant cycle as recited in any one of the aforementioned Items [16]to [19], wherein both the outer tube and the inner tube are bent by bending work.

EFFECTS OF THE INVENTION

According to the heat exchanger as recited in the aforementioned Item [1], the heat exchanger can be formed by combining the inner tube having fins and the outer tube. Therefore, as compared with the case in which a heat exchanging multi-bored tube is formed by a single extrusion procedure, the fin and tube can be decreased in thickness, and minute structure thereof can be formed. Accordingly, the heat exchanging performance can be improved.
Furthermore, since a gap is formed between the tip end of the fin of the inner tube and the internal periphery of the outer tube, the inner tube will not be restrained excessively by the outer tube. Therefore, it is possible to prevent occurrence of defects such as damages of fins of the inner tube due to the stress caused at the time of the bending work. Thus, the heat exchanger can be bent easily and accurately into a desired configuration. Thus, the heat exchanger is excellent in bending performance.
In addition, since the gap is formed at the tip end of each fin, the second fluid can be mixed via the gaps, which can prevent deflection of the refrigerant temperature distribution. Thus, the heat exchanging efficiency can be further improved.
According to the heat exchanger as recited in the aforementioned Item [2], the heat exchanging efficiency can be improved.
According to the heat exchanger as recited in the aforementioned Item [3], excellent bending workability can be attained.
According to the heat exchanger as recited in the aforementioned Item [4] to [6], the heat exchanging efficiency can be further improved.
According to the heat exchanger as recited in the aforementioned Item [7] to [10], the aforementioned effects can be obtained more assuredly.
According to the heat exchanger as recited in the aforementioned Item [11] to [14], an intermediate heat exchanger having the effects mentioned above can be provided.
According to the heat exchanger as recited in the aforementioned Item [15] to [20], a refrigeration cycle having the effects mentioned above can be provided.

BRIEF DESCRIPTION OF DRAWINGS

The preferred embodiments of the present invention are shown by way of example, and not limitation, in the accompanying figures, in which:

FIG. 1 is a refrigeration circuit diagram of an automobile air-conditioning refrigeration system in which an intermediate heat exchanger according to an embodiment of this invention is employed; and

FIG. 2 is a cross-sectional view showing the intermediate heat exchanger of the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following paragraphs, some preferred embodiments of the invention will be described by way of example and not limitation. It should be understood based on this disclosure that various other modifications can be made by those in the art based on these illustrated embodiments.
FIG. 1 shows a refrigerant circuit diagram showing an automobile air-conditioning refrigeration system in which a heat exchanger according to an embodiment of this invention is employed. As shown in this figure, this refrigeration cycle uses carbon dioxide as refrigerant and includes a compressor 1, a gas cooler (condenser 2), a decompressor such as an expansion valve 3, an evaporator 4, and an intermediate heat exchanger 10 which will be detailed. In this refrigeration cycle, a refrigerant circulation circuit is formed. That is, the refrigerant compressed by the compressor 1 is cooled by the gas cooler 2, and then decompressed by the expansion valve 3. Thereafter, the refrigerant is evaporated by the evaporator 4 and then returns to the compressor 1. Furthermore, the high pressure refrigerant (forwarding refrigerant) flowing from the gas cooler 2 toward the expansion valve 3 passes through a high pressure refrigerant heat exchanging passage 25 in the intermediate heat exchanger 10, and the low pressure refrigerant (returning refrigerant) flowing from the evaporator 4 toward the compressor 1 passes through a low pressure refrigerant heat exchanger passage 35 to exchange the heat therebetween.
As shown in FIG. 2, the intermediate heat exchanger 10 has a double-tube structure including an inner tube 20 which is an aluminum (including its alloy) extruded member and an outer tube 30 which is an aluminum (including its alloy) extruded member.
The inner tube 20 is provided a plurality of fins 21 integrally formed on the external periphery of the inner tube. The fins 21 extend along the longitudinal direction of the tube and arranged on the external periphery at certain equal intervals in the circumferential direction. In the inner tube 20, a plurality of inner fins 22 extending along the longitudinal direction of the inner tube and arranged at certain equal intervals in the circumferential direction are integrally provided.
The outer tube 30 has a tube aperture having an internal diameter larger than the external diameter of the fins 21 of the inner tube 20, and the inner tube 20 is inserted in the tube aperture of the outer tube in a manner such that the axial center of the inner tube 20 coincides with that of the outer tube 30. The inside of the inner tube 20 constitutes a first heat exchanging passage 25 through which high pressure refrigerant (first fluid) passes, and the space between the inner tube 20 and the outer tube 30 constitutes a second heat exchanging passage 35 through which low pressure refrigerant (second fluid) passes.
In this embodiment, the inner tube 20 is disposed in the outer tube 30 so as to form a gap S between the tip end of the fin 21 and the internal periphery of the outer tube 30 so that the inner tube 20 is not restrained by the outer tube 30.
Concretely, it is preferable that the size Ls of the gap S is adjusted to 0.2 to 1 mm. In other words, it is preferable that the difference between the inner diameter of the outer tube 30 and the external diameter of the inner tube 20 including the fins 21 is adjusted to 0.4 to 2 mm. If the gap S is smaller than the lower limit, the inner tube 20 may be restrained by the outer tube 30, and therefore external force applied to the outer tube 30 greatly acts on the inner tube 20. Therefore, when the intermediate heat exchanger 10 constituted by both the tubes 20 and 30 is subjected to bending work, the bending stress will concentrate on the outside of the bending portion of the fins 21 of the inner tube 20, which may cause cracks in fins 21. On the other hand, if the gap S is larger than the upper limit, the size (height) of the fin 21 becomes small (low), which may cause deteriorated heat transfer property, resulting in deteriorated heat exchanging performance.
In this embodiment, it is preferable that the number of fins 21 is set to 13 to 18, more preferably 15 to 17. If the number of fins is smaller than the lower limit, the heat transfer property may deteriorate, which in turn may cause a deterioration of heat exchanging performance. On the other hand, if the number of fins exceeds the upper limit, the fin pitch becomes small, decreasing the width between adjacent fins, which results in deteriorated heat exchanging performance due to the increased flow resistance of the refrigerant passing threrethrough.
Furthermore, in this embodiment, it is preferable that the thickness T of the fin 21 is set to 0.3 to 1.3 mm, more preferably 0.5 to 1.1 mm. If the fin thickness T is smaller than the lower limit, it becomes difficult to secure sufficient strength. To the contrary, if the fin thickness T exceeds the upper limit, the heat transfer property deteriorates and the flow resistance increases, resulting in deteriorated heat exchanging performance.
The opening angle θ of the adjacent fins 21 and 21 is preferably set to 15 to 30°, more preferably 18 to 26°. If the opening angle θ is smaller than the lower limit, the width between the adjacent fins 21 and 21 becomes small, causing increased flow resistance of the refrigerant passing therethrough, which in turn results in deteriorated heat exchanging performance. To the contrary, if the opening angle θ exceeds the upper limit, the number of fins 21 decreases, causing deteriorated heat transfer performance, which in turn results in deteriorated heat exchanging performance.
As mentioned above, according to the intermediate heat exchanger of this embodiment, the inner tube 20 with fins is inserted and disposed in the outer tube 30 as mentioned above. Therefore, the heat exchanger can be manufactured by forming both the tubes 20 and 30 separately and then combining them. Accordingly, as compared with the case in which a heat exchanging multi-bored tube is formed by a single extrusion procedure, the fin and tube can be decreased in thickness, and minute structure thereof can be formed. Accordingly, desired heat transfer performance and heat exchanging performance can be attained more assuredly.
Furthermore, in this embodiment, since a gap S is formed between the tip end of the fin 21 of the inner tube 20 and the internal periphery of the outer tube 30, the inner tube 20 will not be restrained excessively by the outer tube 30. Therefore, it is possible to prevent the bending stress from being concentrated on the outer side of the bending portion of the fins 21 of the inner tube 20 when the intermediated heat exchanger 10 is subjected to bending work, which in turn can assuredly prevent defects such as the occurrence of cracks or damages. Thus, the heat exchanger can be bent easily and accurately into a desired configuration because of the excellent bending performance. Especially, when it is employed in a refrigeration cycle for automobile air-conditioners, the heat exchanger can be bent into a desired configuration in accordance with the limited available installing space in the automobile, which dramatically improves the degree of design freedom.
In addition, in this embodiment, since the gap S is formed at the tip end of each fin 21 in the second heat exchanging passage 35, the refrigerant in the heat exchanging passage 35 will be mixed via the gaps S. Therefore, deflection of the refrigerant temperature distribution can be effectively prevented, which further improves the heat exchanging efficiency.

EXAMPLE

Hereinafter, examples of this embodiment will be explained.

Example 1

In a refrigeration system for automobile air-conditioners shown in FIG. 1 employing the intermediate heat exchanger 10 in which an inner tube 20 having fins 21 on the external periphery is inserted in the outer tube 30, the heat exchanging amount of each of the intermediate heat exchangers 10 with different number of fins was obtained by computer simulation. The results are shown in Table 1 in which the heat exchanging amount is represented by e (100% when the number of fin is “0”).
The conditions were set as follows: the length of the intermediate heat exchanger (length of the outer tube) was set to 500 mm, the external diameter of the outer tube 30 was set to 21.0 mm, the internal diameter of the outer tube 30 was set to 15.0 mm, the external diameter of the inner tube 20 including the outer fins 21 was set to 14.0 mm, the external diameter of the inner tube 20 excluding the outer fins 21 was set to 7.0 mm, the internal diameter of the tubular portion of the inner tube 20 excluding the inner fins 22 was set to 4.0 mm, and the inner diameter of the inner tube 20 including the inner fins 22 was set to 3.5 mm.

Example 2

Under the same conditions as in Example 1, the flow resistance of the low pressure side refrigerant heat exchanging passage (passage between the inner tube and the outer tube) of each of the heat exchangers with respect to the number of fins was obtained by computer simulation. The results are shown in Table 1 in which the flow resistance is represented by % (100% when the number of fin is “0”).
As shown in Table 1, as the number of fins increases, the heat transfer performance increases and therefore the heat exchanging amount increases. On the other hand, as shown in Table 2, as the number of fins increases, the flow resistance increases and therefore the heat exchanging amount decreases. When it is judged in a comprehensive manner, when the number of fins is 13 to 18, appropriate heat exchanging performance can be attained while restraining the flow resistance to some extent. Especially, when the number of fins is 15 to 17, sufficient heat exchanging performance can be attained while sufficiently restraining the flow resistance.
Needless to say, in cases where the number of fins was extremely small, although the flow resistance decreased, it was difficult to attain sufficient heat exchanging amount. As a result, the overall heat exchanging performance deteriorated. To the contrary, in cases where the number of fins was extremely large, although the heat exchanging amount increased, the flow resistance also increased. As a result, the overall heat exchanging performance deteriorated.

INDUSTRIAL APPLICABILITY

The heat exchanger, intermediate heat exchanger and refrigeration cycle according to the present invention can be employed in a refrigeration system for use in, example, automobile air-conditioners.

Broad Scope of the Present Invention

While the present invention may be embodied in many different forms, a number of illustrative embodiments are described herein with the understanding that the present disclosure is to be considered as providing examples of the principles of the invention and such examples are not intended to limit the invention to preferred embodiments described herein and/or illustrated herein.
While illustrative embodiments of the invention have been described herein, the present invention is not limited to the various preferred embodiments described herein, but includes any and all embodiments having equivalent elements, modifications, omissions, combinations (e.g., of aspects across various embodiments), adaptations and/or alterations as would be appreciated by those in the art based on the present disclosure. The limitations in the claims are to be interpreted broadly based on the language employed in the claims and not limited to examples described in the present specification or during the prosecution of the application, which examples are to be construed as non-exclusive. For example, in the present disclosure, the term “preferably” is non-exclusive and means “preferably, but not limited to.” In this disclosure and during the prosecution of this application, means-plus-function or step-plus-function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) “means for” or “step for” is expressly recited; b) a corresponding function is expressly recited; and c) structure, material or acts that support that structure are not recited. In this disclosure and during the prosecution of this application, the terminology “present invention” or “invention” may be used as a reference to one or more aspect within the present disclosure. The language present invention or invention should not be improperly interpreted as an identification of criticality, should not be improperly interpreted as applying across all aspects or embodiments (i.e., it should be understood that the present invention has a number of aspects and embodiments), and should not be improperly interpreted as limiting the scope of the application or claims. In this disclosure and during the prosecution of this application, the terminology “embodiment” can be used to describe any aspect, feature, process or step, any combination thereof, and/or any portion thereof, etc. In some examples, various embodiments may include overlapping features. In this disclosure and during the prosecution of this case, the following abbreviated terminology may be employed: “e.g.” which means “for example;” and “NB” which means “note well.”

Claims

1. A heat exchanger, comprising:

an outer tube;

an inner tube having a plurality of fins formed on an external periphery of the inner tube, the inner tube being disposed in the outer tube;

first fluid passing in the inner tube; and

second fluid passing in between the outer tube and the inner tube.

wherein the heat exchanger exchanges heat between the first fluid and the second fluid, and

wherein a gap is formed between an internal periphery of the outer tube and a tip end of each of the plurality of fins of the inner tube.

2. The heat exchanger as recited in claim 1, wherein the plurality of fins extend along a longitudinal direction of the inner tube and are arranged in a circumferential direction thereof at certain intervals.

3. The heat exchanger as recited in claim 1, wherein the gap is set to 0.2 to 1 mm.

4. The heat exchanger as recited in claim 15 wherein the plurality of fins are arranged along a circumferential direction of the inner tube and the number of fins is set to 13 to 18

5. The heat exchanger as recited in claim 1, wherein a thickness of each of the plurality of fins is set to 0.3 to 1.3 mm.

6. The heat exchanger as recited in claim 1, wherein an opening angle between adjacent fins is set to 15 to 30°.

7. The heat exchanger as recited in claim 1, wherein the plurality of fins are integrally formed on the inner tube.

8. The heat exchanger as recited in claim 1, wherein both the outer tube and the inner tube are bent by bending work.

9. The heat exchanger as recited in claim 1, wherein the first fluid is high pressure heat medium and the second fluid is low pressure heat medium.

10. The heat exchanger as recited in claim 1, wherein the inner tube is provided with a plurality of inner fins on an internal periphery thereof,

11. An intermediate heat exchanger for exchanging heat between high pressure refrigerant and low pressure refrigerant among refrigerant circulating a refrigeration cycle,

the inter-mediate heat exchanger, comprising:

an inner tube provided with a plurality of fins on an external periphery thereof and

an outer tube in which the inner tube is disposed in a manner such that a gap is formed between an internal periphery of the outer tube and a tip end of each of the plurality of fins of the inner tube,

wherein one of the high pressure refrigerant and the low pressure refrigerant passes through a first heat exchanging passage formed in the inner tube, and

wherein the other of the high pressure refrigerant and the low pressure refrigerant passes through a second heat exchanging passage between the outer tube and the inner tube,

12. The heat exchanger as recited in claim 11, wherein the high pressure refrigerant passes through the first heat exchanging passage, and wherein the low pressure refrigerant passes through the second heat exchanging passage.

13. The heat exchanger as recited in claim 11, wherein the refrigerant is carbon dioxide refrigerant.

14. An intermediate heat exchanger for exchanging heat between high pressure refrigerant and low pressure refrigerant among refrigerant circulating a refrigeration cycle,

the intermediate heat exchanger, comprising:

an inner tube provided with a plurality of fins on an external periphery thereof; and

an outer tube in which the inner tube is disposed in a manner such that a gap is formed between an internal periphery of the outer tube and the plurality of fins of the inner tube,

wherein one of the high pressure refrigerant and the low pressure refrigerant passes through a first heat exchanging passage formed in the inner tube and

wherein the other of the high pressure refrigerant and the low pressure refrigerant passes through a second heat exchanging passage between the outer tube and the inner tube.

15. A refrigerant cycle in which refrigerant circulates such that the refrigerant passes through a compressor, a condenser, a decompressor and an evaporator, and then returns to the compressor.

the refrigerant cycle, comprising:

an intermediate heat exchanger for exchanging heat between high pressure refrigerant passing through a high pressure circuit from the compressor to the decompressor and low pressure refrigerant passing through a low pressure circuit from the decompressor toward the compressor,

wherein the intermediate heat exchanger includes:

16. A refrigerant cycle in which refrigerant circulates such that the refrigerant passes through a compressor, a condenser, a decompressor and an evaporator and then returns to the compressor,

the refrigerant cycles comprising:

an intermediate heat exchanger for exchanging heat between high pressure refrigerant passing through a circuit located between the condenser and the decompressor and low pressure refrigerant passing through a circuit located between the evaporator and the compressor,

wherein the intermediate heat exchanger includes:

an inner tube provided with a plurality of tins on an

external periphery thereof, and

an outer tube in which the inner tube is disposed in a manner such that a gap is for ed between an internal periphery of the outer tube and a tip end of each of the plurality of fins of the inner tube,

wherein one of the high pressure refrigerant and the low pressure refrigerant passes through a first heat exchanging passage for ed in the inner tube, and

17. The refrigerant cycle as recited in claim 6, w herein the high pressure refrigerant passes through the first heat exchanging passage, and wherein the low pressure refrigerant passes through the second heat exchanging passage.

18. The refrigerant cycle as recited in claim 16, wherein the condenser is a gas cooler.

19. The refrigerant cycle as recited in claim 12 , wherein the refrigerant is carbon dioxide refrigerant.

20. The refrigerant cycle as recited in claim 16, wherein both the outer tube and the inner tube are bent by bending work.