DOUBLE PIPE FOR HEAT EXCHANGER

Abstract

A double pipe for a heat exchanger includes an inner pipe having a plurality of spiral projections extending radially outward from an outer peripheral surface of the inner pipe. An outer pipe is disposed around the inner pipe. At least an inner peripheral surface of the outer pipe is at least substantially smooth and has an at least substantially circular cross section. The inner peripheral surface of the outer pipe contacts all of the projections of the inner pipe so as to define a plurality of peripherally-separated, outer flow paths between the inner pipe and the outer pipe. Fluid flowing within an inner flow path defined by the inner pipe exchanges heat with fluid flowing through the outer flow paths. Furthermore, an inscribed circle of the inner pipe has a first diameter (d1), an inscribed circle of the outer pipe has a second diameter (d2), and 0.6<d1/d2.

Description

CROSS-REFERENCE

This application claims priority to Japanese patent application no. 2012-019776 filed on Feb. 1, 2012, the contents of which are entirely incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to a double pipe suitable for use in a heat exchanger that may be utilized, e.g., in the heat exchange cycle system of a vehicle air conditioning system or the like.

BACKGROUND ART

The heat exchange cycle system (also referred to as the “refrigeration cycle system”) of a vehicle air conditioner or the like generally includes a condenser, an evaporator, a compressor and a thermal expansion valve. Known heat exchange cycle systems have been configured to circulate a refrigerant, such as chlorofluorocarbon (CFC) gas, hydrochlorofluorocarbon (HCFC) gas, hydrofluorocarbon (HFC) gas, CO₂ or ammonia gas, through a refrigerant circulation path that fluidly connects the above-mentioned components.

Previously-proposed arrangements for improving the heat exchange performance of such heat exchange cycle systems include disposing a double pipe within the circulation route in a counter-flow arrangement such that hot refrigerant exiting the condenser flows in an opposite direction of cold refrigerant exiting the evaporator through the respective spaces defined in two-layers by the double pipe (see the below-identified Patent Documents 1 and 2).

Meanwhile, new types of refrigerants having a relatively low global-warming potential or coefficient (GWP) have been considered and already used as refrigerants in such heat exchange cycle systems in order to minimize environmental problems resulting from the use of such refrigerants. However, there are concerns that such environmentally-friendly refrigerants exhibit relatively poor heat exchange performance (thermodynamic properties) as compared with currently-used refrigerants. Therefore, a heat exchange cycle system arrangement incorporating a double pipe structure has been considered to be an effective way to improve the heat exchange performance properties of the heat exchange cycle system and to minimize any reduction or loss of thermodynamic performance of the overall heat exchange cycle system caused by the use of a more environmentally-friendly refrigerant.

By incorporating a double pipe into the heat exchange cycle system designed to compress the gaseous refrigerant discharged from an evaporator by using a compressor, it is possible to overcome a problem that can arise when the refrigerant is not thoroughly gasified or vaporized (and hence is mixed with refrigerant that is in a liquid state) and flows into the compressor. That is, the refrigerant is heated in the double pipe before it flows into the compressor so that it can be thoroughly gasified or vaporized.

Many of the previously-proposed double pipes are comprised of a twisted inner pipe that is intended to improve the heat exchange (thermodynamic) performance (see e.g., the below-mentioned Patent Documents 1 and 2).

PATENT DOCUMENTS

• Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2002-318015
• Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2006-162241 and its family members, e.g., US 2006/0096744
• Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2008-232449

SUMMARY OF THE INVENTION

Patent Document 1 discloses the use of a double pipe structure for a heat exchange cycle system that has a spiral groove part formed on the outer peripheral surface of the inner pipe or on the inner peripheral surface of the outer pipe of the double pipe. However, no further details concerning the structure of the double pipe are disclosed. Furthermore, the double pipe disclosed in Patent Document 1 is based on the assumption that straight or linear pipe members are used and hence, is not applicable to pipe parts of vehicle air conditioners and the like that need to be bent in a machine bending process prior to installation in the vehicle air conditioning system.

Patent Document 2 discloses a double pipe structure that is a combination of an inner pipe, inside of which a spiral groove is formed, and a smooth outer pipe, wherein the inner diameter of the outer pipe is larger than the outer diameter of the inner pipe. In this previously-proposed structure, the inner pipe and the outer pipe generally do not contact each other. If they do come into contact, however, they contact each other only at a location within the straight part of the double pipe. Such a structure is problematic, because noise (rattling) will be generated as a result of vibrations that normally take place when the heat exchange system is in operation.

Patent Document 3 discloses a double pipe including an inner pipe implemented as a multi-coiled pipe having a plurality of relatively oblong projections similar to the shape of several leaves in a cross section. However, when a double pipe having such a structure is machine bent in order to utilize it in a vehicle air conditioner or the like, the profile of the inner pipe can be deformed to a relatively large extent in a disadvantageous manner. In particular, adjacently-disposed oblong projections can collide/contact with each other and consequently close one or more portions of the outer flow paths and/or reduce the cross sectional area of the outer flow paths, thereby obstructing the fluid flow in the outer flow paths. In this case, a large pressure loss in the pipe could disadvantageously result with a consequent reduction in the thermal efficiency of the flow path.

In view of the above-identified problems of the prior art, it is therefore an object of the present teachings to provide a double pipe suitable for use in a heat exchanger, e.g., that can be operated with less or no noise and/or is suitable to be machine bent at the time of installation without causing any loss or reduction in the heat exchange capabilities (thermodynamic properties) of the double pipe during operation.

According to one aspect of the present teachings, a double pipe for a heat exchanger has a double pipe structure formed by arranging an inner pipe within an outer pipe and such that fluid flowing through the inner pipe exchanges heat with fluid flowing through the outer pipe;

the inner pipe has a profile formed by partly deforming its original circular outer periphery in cross section so as to exhibit a plurality of longitudinally straight and narrow outward projections and subsequently twisting the inner pipe having the plurality of longitudinally straight and narrow outward projections into a plurality of spiral and narrow outward projections;

the outer pipe is formed from a smooth pipe and at least its inner peripheral surface has a circular cross section;

the inner peripheral surface of the outer pipe contacts the tops of the projections of the inner pipe so as to define a plurality of peripherally separated flow paths;

the diameter, d1, of the inscribed circle of the inner pipe and the diameter, d2, of the inscribed circle of the outer pipe satisfy the relationship 0.6<d1/d2.

Thus, a double pipe for a heat exchanger according to this aspect of the present teachings comprises an inner pipe, which is spirally twisted to provide a special outer profile, and an outer pipe formed from a smooth pipe, at least the inner peripheral surface of which has a circular cross section. The tops or apexes of the plurality of projections of the inner pipe contact the inner peripheral surface of the outer pipe. With this arrangement, the space between the inner pipe and the outer pipe is divided into a plurality of outer flow paths that are peripherally separated and each of the outer flow paths exhibits a spiral profile or helical path.

Additionally, the inside of the inner pipe is configured so as to provide or define an inner flow path having a spirally- or helically-twisted outer wall. Thus, as fluid (refrigerant) is caused to flow through the outer flow paths and also through the inner flow path of the double pipe, the length of the outer flow paths is greater than the length of a comparable simple linear flow path and consequently leads to improved heat exchange efficiency. The fluid (refrigerant) flowing through the inner flow path can also be expected to flow in a sufficiently turbulent manner, which also can contribute to improved heat exchange efficiency.

Furthermore, the inner pipe (fixedly) contacts the outer pipe so as to become integral with the latter in the double pipe. Thus, even if a heat exchange cycle system incorporating such a double pipe vibrates during operation, any noise, which would otherwise be produced when the inner pipe collides with (rattles against) the outer pipe in the double pipe, is prevented or at least substantially minimized.

Finally, the diameter, d1, of the inscribed circle of the inner pipe and the diameter, d2, of the inscribed circle of the outer pipe are set to satisfy the relationship 0.6<d1/d2. By limiting the difference between d1 and d2 to this range, the extent by which the projections formed on the inner pipe project can be limited to a certain degree. With this arrangement, it is possible to prevent the projections from being deformed during a bending operation, which might cause the (partial or complete) blockage or closure of one or more of the outer flow paths defined between adjacent projections.

Thus, this aspect of the present teachings provides a double pipe for a heat exchanger that can be operated with little or no noise and is suited to be bent using a bending machine at the time of installation, while also having a structure that performs exceedingly well in heat exchange operations.

Further objects, aspects, embodiments, advantages and designs of the present teachings will be explained in the following, or will become apparent to the skilled person, with the assistance of the exemplary embodiments and the appended Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic transverse, cross sectional view of a first embodiment (Example 1) of the double pipe according to the present teachings, showing only the outer pipe.

FIG. 2 is a schematic transverse, cross sectional view of the first embodiment (Example 1) of double pipe, showing only the inner pipe.

FIG. 3 is a schematic transverse, cross sectional view of the first embodiment (Example 1) of the double pipe.

FIG. 4 is a schematic illustration of the inner pipe and the outer pipe of the first embodiment (Example 1) of the double pipe, prior to forming the double pipe.

FIG. 5 is a schematic transverse, cross sectional view of a second first embodiment (Example 2) of the double pipe according to the present teachings.

FIG. 6 is a schematic transverse, cross sectional view of a third embodiment (Example 3) of the double pipe according to the present teachings.

FIG. 7 is a schematic transverse, cross sectional view of a fourth embodiment (Example 4) of the double pipe according to the present teachings.

FIG. 8 is a schematic transverse, cross sectional view of a double pipe prepared as a Comparative Example.

FIG. 9 is a schematic illustration of the radius of curvature R produced in a bending test in a Test Example.

FIG. 10 is a schematic illustration of the results of the visual observation of Test Sample T7.

FIG. 11 is a schematic illustration of the results of the visual observation of Test Sample T10.

FIG. 12 is an elevation view of a fifth embodiment (Example 5) of the double pipe according to the present teachings.

FIG. 13 is a schematic transverse, cross sectional view taken along line A-A of the fifth embodiment (Example 5) shown in FIG. 12.

FIG. 14 is a schematic illustration of the inner pipe of the fifth embodiment (Example 5).

FIG. 15 is an elevation view of a sixth embodiment (Example 6) of the double pipe according to the present teachings.

FIG. 16 is a schematic transverse, cross sectional view taken along line B-B of the sixth embodiment (Example 6) shown in FIG. 15.

FIG. 17 is a schematic illustration of the inner pipe of the sixth embodiment (Example 6).

FIG. 18 shows a table of results comparing Test Samples according to the present teachings with comparative samples.

DESCRIPTION OF THE EMBODIMENTS

In one aspect of a double pipe for a heat exchanger according to the present teachings, the diameter, d1, of the inscribed circle of the inner pipe and the diameter, d2, of the inscribed circle of the outer pipe preferably satisfy the relationship 0.6<d1/d2. If d1/d2 were to be less than 0.6, the extent by which the projections of the inner pipe radially project would be too large or great. In this case, there is a higher probability that the projections will be deformed (in an undesirable manner) during a pipe bending operation and one or more of the outer flow paths defined between adjacent projections could be closed (blocked) or narrowed.

As will be described hereinafter in the Examples, when the diameter, d1, of the inscribed circle of the inner pipe and the diameter, d2, of the inscribed circle of the outer pipe satisfy the relationship 0.6<d1/d2 in a double pipe according to the present teachings, it is possible to prevent or avoid the problem that the projections will be deformed and one or more of the outer flow paths defined between adjacent projections will be closed or narrowed when the double pipe is machine bent as long as the radius of curvature R (at the innermost peripheral side) that is produced when the double pipe is machine bent is relatively small; preferably, the radius of curvature R of the bent double pipe is not greater than 100 mm.

In the below-described Examples, 45 mm is the smallest-mentioned value for the radius of curvature R of the exemplary double pipes after being machine bent. However, a double pipe according to the present teachings may further be bent so to exhibit a smaller radius of curvature, e.g., about 35 mm. But, even in such an embodiment, the above-described problem can effectively be avoided or prevented by maintaining (satisfying) the relationship 0.6<d1/d2.

The upper limit value for the ratio of d1/d2 is determined as a function of the lower limit value of the thickness of the inner pipe and the necessary minimal extent by which the projections of the inner pipe radially project. It is less than one (unity) as a matter of course in terms of the structure of the pipe.

The number of the projections that are formed on the inner tube is not particularly limited according to the present teachings. The skilled person can appropriately set or determine the best number of projections for a particular application of the present teachings by taking into consideration the liquid (refrigerant) flow rate, the heat exchange performance of the heat exchange system, and/or the ease or difficulty of forming projections on the inner pipe.

In preferred embodiments of a double pipe for a heat exchanger according to the present teachings, the inner pipe may have, e.g., 2 to 12 projections. On the other hand, if the inner pipe were to have only a single projection, it is likely to be unstably supported when the projection is formed, which may result in a poor formability of the projection. To the contrary, if two or more projections are formed on the inner pipe, the inner pipe does not readily deform when the inner pipe is machine bent. In this case, the entire double pipe becomes highly resistant to undesired deformation, e.g., of the projections. Likewise, if the number of projections is not greater than twelve, projections can be formed on the inner pipe in a stable or durable manner even when the inner pipe has a relatively small diameter, which is typically the case for a double pipe of a heat exchanger to be used in a vehicle air conditioner. Furthermore, when the number of projections is between two and twelve, the fluid (refrigerant) flow rate will not fall excessively, thereby making it easy to maintain a high heat exchange performance.

The projections may be formed at any peripheral position, as viewed in cross section, although adjacent projections are preferably arranged at peripherally regular intervals (e.g., equidistantly in the circumferential direction of the inner pipe). In preferred embodiments, as viewed in cross section, two projections may be formed so as to be angularly separated from each other by 180° or three projections may be formed so as to be angularly separated from each other by 120°. Alternatively, as viewed in cross section, four projections may be formed and adjacent projections may be angularly separated from each other by 90° or six projections may be formed and adjacent projections may be angularly separated from each other by 60°. In still further embodiments of the present teachings, as viewed in cross section, eight projections may be formed and adjacent projections may be angularly separated from each other by 45° or twelve projections may be formed and adjacent projections may be angularly separated from each other by 30°. With any of these arrangements (embodiments), the fluid (refrigerant) flow rates in the outer flow paths are equal, or at least substantially equal, to each other as viewed in the peripheral direction. As a result, it is possible to easily prevent or minimize uneven heat exchange between the fluid (refrigerant) in the outer flow paths and the fluid (refrigerant) in the inner flow path.

In some aspects of the present teachings, the outer diameter of the outer pipe may be set so as to fall within the range of 10 mm to 30 mm. If the outer diameter of the outer pipe falls within the above-specified range, the outer pipe will be adaptable to heat exchangers of many different types.

The outer pipe is preferably formed from a smooth pipe, and at least the inner peripheral surface of the outer pipe has a circular cross section.

In addition, the cross section of the outer peripheral surface may be circular as shown in Examples 1-4 or it may be deformed (non-circular) to some degree in one or more portions along the periphery of the circle as shown in Examples 5 and 6 below.

In certain aspects of the present teachings, the wall thickness of the inner pipe may be set so as to fall within the range of 0.5 mm to 2.0 mm. If the wall thickness of the inner pipe falls within the above-specified range, the inner pipe can be easily machined so as to provide the projections having the above-described spiral or helical profile.

In certain aspects of the present teachings, the angle of the inner pipe spiral projections relative to the longitudinal axis of the inner pipe may fall within the range of 10° to 70°. If the angle were to be smaller or less than the above-identified range, there may be reductions in the effects of (i) producing a turbulent flow in the inner flow path, (ii) increasing the length of the outer flow paths and/or (iii) improving the heat exchange efficiency, even though the pressure loss of the fluid flow can be reduced. On the other hand, if the angle were to be greater or larger than the above-specified range, the pressure loss of the fluid flow may become too large, even though the heat exchange efficiency can be improved. Preferably, the outward projections having a spiral profile of the inner pipe make a complete turn within the longitudinal range of 20 mm to 150 mm (which will also be referred to as “spiral pitch” hereinafter). When the spiral pitch falls within the above-specified range, the projections of the inner pipe can be produced relatively easily by machining.

In certain aspects of the present teachings, one or both of the inner pipe and the outer pipe of a double pipe for a heat exchanger according to the present teachings may be made of an aluminum alloy or a copper alloy. The term “aluminum alloy” as used herein covers pure aluminum as well as any aluminum alloys containing aluminum as a principal ingredient. Similarly, the term “copper alloy” as used herein covers pure copper as well as any copper alloys containing copper as a principal ingredient. The above-described metal materials possess excellent heat conduction characteristics and enable the double pipe to achieve highly effective heat exchange operations. The use of an aluminum alloy is most preferable in case it is desired to realize a lightweight heat exchanger.

When an aluminum alloy is selected as the material for a double pipe for a heat exchanger according to the present teachings, the use of pure aluminum (e.g., A1050, A1100) or a specific aluminum alloy (e.g., A3003, A6063) is preferable. When copper is selected, the use of phosphorous-deoxidized copper, pure copper, or a copper alloy possessing a high thermal conductivity is preferable.

While a highly workable material is preferably selected from the above-specified materials, the present teachings are by no means limited to them, particularly when corrosion-resistance and/or high strength are required.

In certain aspects of the present teachings, the cross sectional area, S1, of the outer flow paths and the cross sectional area, S2, of the inner flow path within the inner pipe preferably satisfy the relationship S2/S1≧1.5. When the above-defined relationship is realized between the cross sectional areas of the outer flow paths and the inner flow path, it is possible to easily design the profile of the projections of the inner pipe so as to make the inner pipe suitable for bending and hence hardly crushable. In this case, a double pipe having such an inner pipe rarely leads to a partial (or complete) closure or narrowing of the flow paths between the inner and outer pipes.

Furthermore, a double pipe according to the present teachings is highly suitable for use in a heat exchanger when the above-defined ratio is realized between the cross sectional areas of the outer flow paths and the inner flow path. During operation, the state of the refrigerant can change within the heat exchange cycle system. That is, the refrigerant may be either liquid or gas at high temperatures under high pressure. For a refrigerant having a certain mass, its volume is greater in a gaseous state than in a liquid state. Therefore, the refrigerant can be made to flow smoothly by causing refrigerant in a gaseous state to flow through the inner flow path and by causing refrigerant in a liquid state to flow through the outer flow paths.

Preferably, the upper limit value of the ratio of S2/S1 is ten so as to avoid or prevent excessively large pressure losses in the outer flow paths.

In certain aspects of the present teachings, the width of the projections, as viewed in a direction orthogonal to the radial direction extending through the respective tops (apexes) of the projections, preferably gradually or continuously decreases towards the outside in the radial direction. With such a design of the projections, it is possible to prevent or at least substantially minimize the problem of the projections becoming undesirably deformed to a large extent when bent by machine bending, which would disadvantageously lead to the (partial or complete) closing or narrowing of one or more of the outer flow paths.

A double pipe for a heat exchanger according to the present teachings may be advantageously used as piping for circulating a refrigerant in a vehicle air conditioner. The space for installing a vehicle air conditioner is normally limited. Therefore, whether the double pipe can be bent without significant limitations on the radius of curvature is vitally important, because the degree of freedom of installation depends on it. Thus, a double pipe according to the present teachings is highly suitable for a vehicle air conditioner.

A representative method for manufacturing a double pipe according to the present teachings will be described in the following.

First, the inner pipe can be prepared by linearly drawing (applying a tensile force to) a smooth pipe having a circular cross section that is used as a starting pipe, while rotating a die having inner grooves with profiles that correspond to the desired profiles of the spiral projections. However, other techniques that do not require rotation of the die may also be used for preparing the inner pipe.

A smooth pipe having a circular cross section with an inner diameter that is greater than the outer diameter of the inner pipe is preferably used as the starting pipe for the outer pipe. After the drawing process, the inner pipe is inserted into the starting pipe of the outer pipe to produce a double pipe structure. Then, the starting pipe of the outer pipe of the double pipe structure is subjected to a drawing process that reduces the diameter of the outer pipe. As a result, the tops of the projections having an arch-like (rounded peak) profile of the inner pipe will firmly or fixedly come into contact with the inner surface of the outer pipe to produce a double pipe, in which the inner pipe and the outer pipe become integral, i.e. no relative motion will be permitted between the inner pipe and the outer pipe, thereby preventing, e.g., rattling.

The inner flow path of the inner pipe and the outer flow paths defined between the inner pipe and the outer pipe are connected to respective external pipes at the opposite ends of the double pipe. The connection structures of the double pipe and the external pipes are not particularly limited according to the present teachings. In other words, various connection structures and various connection techniques can be used for connecting the double pipe to the external pipes and include, but are not limited to, caulking bonding, brazing joining, adhesion joining and friction stir welding.

Two or more grooves, e.g., straight grooves, may be defined in the outer peripheral surface of the outer pipe and may extend longitudinally (axially) along two or more respective locations in the circumferential direction of the outer pipe. In such an embodiment of the present teachings, the grooves may be formed on the outer surface of the outer pipe while the outer pipe and the inner pipe are being subjected to the above-described drawing process that reduces the diameter of the outer pipe. In this embodiment as well, the tops of the projections extending radially outwardly from the inner pipe will firmly or fixedly come into contact with the outer pipe, whereby the inner pipe and the outer pipe become integral, i.e. no relative movement is permitted therebetween. Preferably, the grooves are separated or spaced at regular (equidistant) intervals around the circumferential direction of the outer pipe.

EXAMPLES

Example 1

A first representative, non-limiting embodiment of a double pipe for a heat exchanger according to the present teachings will be described below with reference to FIGS. 1 through 4.

As shown in FIG. 3, a double pipe 1 according to the present teachings has a double pipe structure, in which an inner pipe 2 is arranged or disposed within the interior of an outer pipe 10. This double pipe structure enables heat to be exchanged or transferred between a fluid flowing through the interior of the inner pipe 2 and a fluid flowing between the inner pipe 2 and the outer pipe 10.

As shown in FIGS. 2 and 4, the inner pipe 2 has a profile, in which two projections 21 have been formed by deforming the outer periphery of a smooth pipe having a circular cross section so as to produce the projections 21 that protrude outwardly in the radial direction. The projections 21 helically or spirally extend in the longitudinal direction, thereby providing a spiral profile on the outer surface of the inner pipe 2.

The outer pipe 10 is formed from a smooth pipe having a circular cross section. As shown in FIG. 3, the tops (apexes or vertices) 210 of the projections 21 of the inner pipe 2 firmly and fixedly contact the inner peripheral surface of the outer pipe 10. Therefore, two outer flow paths 31 are defined between the outer pipe 10 and the inner pipe 2 and are peripherally separated from each other. The (inner) diameter, d1, of the inscribed circle of the inner pipe 2 and the (inner) diameter, d2, of the inscribed circle of the outer pipe 10 satisfy the relationship 0.6<d1/d2.

The double pipe of this example will now be described in greater detail below.

The double pipe 1 of the present example was manufactured in the following manner.

First, two extruded smooth pipes made of aluminum alloy A3003 and having an outer diameter of 10 mmø, a wall thickness of 1.0 mm, a length of 500 mm and a circular cross section were utilized as starting materials (starting pipes).

The pipe that served as the starting material (pipe) of the inner pipe was first subjected to an end-diameter reduction process to reduce the diameter of each of its ends in the following manner. One end of the pipe was inserted into a die having an inner diameter of 8 mmø and then the pipe was drawn 100 mm. This process was repeated for the opposite end of the pipe, thus obtaining a pipe having two end portions, in which a length of 100 mm had a diameter of 8 mmø while the outer diameter of the middle portion of the pipe remained 10 mmø.

This end-diameter reduction process was conducted as a pre-treatment process prior to a subsequent drawing process.

After being subjected to the end-diameter reduction, pre-treatment process, the starting pipe of the inner pipe was subjected to the drawing process in the following manner to produce the inner pipe 2. First, one end portion having the reduced diameter was inserted into a die having inner grooves with profiles that correspond to the desired outer profiles of the inner pipe. Then, the entire starting pipe of the inner pipe was linearly drawn out through the die by gripping and pulling the end portion that protruded through the die while simultaneously rotating the die.

As shown in FIG. 2, the resulting inner pipe 2 had a profile with two projections 21 that were produced by causing portions of the circular periphery to outwardly project at respective longitudinal positions in a transverse cross sectional view taken along a line that is orthogonal to the axial direction of the inner pipe 2. In such a cross sectional view, the projections have a smoothly-curved, continuous contour line and the tops 210 of the projections 21 have an arch-like (rounded peak) profile. Both of the projections 21 have a width, W, as viewed in a direction that is orthogonal to the radial direction, which extends through the tops 210 of the projections 21. The width W gradually decreases in the radially outward direction to reach a point or portion that is intended to contact the inner peripheral surface of the outer pipe 10.

FIG. 2 is a transverse cross sectional view of the inner pipe 2 as viewed in the axial direction. FIG. 2 shows (i) an inner circular boundary line 26 that mostly coincides with the inner periphery of the inner pipe 2 and (ii) an outer circular boundary line 27 that contacts or intersects the tops 210 of the projections 21. These boundary lines 26 and 27 are produced as the inner pipe is twisted to form the spiral (helical) projections 21 and peripherally displace the smallest diameter section and the largest diameter section. The inner boundary line 26 coincides with the inscribed circle of the inner pipe 2. The outer boundary line 27 coincides with the circumscribed circle of the inner pipe 2 and also with the inscribed circle of the outer pipe 10 (FIG. 3), which will be described hereinafter.

The diameter, d1, of the inner boundary line 26 (the inscribed circle of the inner pipe 2) is 5 mm and the diameter, d2, of the outer boundary line 27 (the inscribed circle of the outer pipe 10) is 8 mm. The wall thickness of the inner pipe 2 is 1 mm. As shown in FIG. 4, the angle α of the spiral projections 21 defined on the inner pipe 2 extending in direction b relative to direction a (i.e. the longitudinal axis of the inner pipe 2) is about 30°. When this angle is converted into the corresponding spiral pitch of the projections 21 of the inner pipe 2, the spiral pitch is 43.5 mm.

As shown in FIG. 4, the other (i.e. non pre-treated) extruded smooth pipe was then used as the starting material (starting pipe) of the outer pipe 10 (simply referred to as the outer pipe 10 hereinafter). The inner pipe 2 was inserted into the outer pipe 2 to produce the double pipe structure. In this state, the inner diameter of the outer pipe 10 was greater than the outer diameter (outer boundary line 27) of the inner pipe 2. Therefore, the outer pipe 10 was drawn through a die having a circular inner bore with an inner diameter of 9 mm in order to reduce the diameter of the outer pipe 10. This caused the outer pipe 10 and the inner pipe 2 to become integrally assembled, i.e. the projections 21 firmly and fixedly contacted the inner peripheral surface of the reduced-diameter outer pipe 10.

As a result, the double pipe 1 of this example was obtained as shown in FIG. 3. The diameter, d1, of the inscribed circle of the inner pipe 2 and the diameter, d2, of the inscribed circle of the outer pipe 10 substantially maintained the above respective values and the relationship d1/d2 exceeded 0.6.

As was described above, the double pipe 1 is comprised of an inner pipe 2, which was produced by twisting a smooth pipe so as to produce spiral projections 21 with a particular profile, and an outer pipe 10 that is a circular smooth pipe. The tops 210 of the two projections 21 of the inner pipe 2 closely contact the inner peripheral surface of the outer pipe 10, preferably in an at least substantially gas-tight manner. As a result, two outer flow paths are produced between the inner pipe 2 and the outer pipe 10 and are peripherally separated from each other. That is, two spiral outer flow paths 31 were formed.

As compared to flow path(s) that linearly extend along the longitudinal axis of a double pipe, these two (spiral) outer flow paths 31 of the double pipe 1 are longer for a double pipe having the same length. Therefore, the double pipe structure 1 of the present example has an increased heat exchange efficiency as compared to a double pipe structure having one or more linear flow paths.

The interior of the inner pipe 2 serves as an inner flow path 32 and its outer wall (i.e. the inner peripheral wall of the inner pipe 2) has spiral or helical grooves defined therein. Thus, when fluid (e.g., a refrigerant or other reagent) is caused to flow through the inner flow path 21, a suitably turbulent flow will be generated, which also increases the heat exchange efficiency of the double pipe 1.

The inner pipe 2 and the outer pipe 10 are held in close contact and are integrated with each other in the double pipe 1. Thus, even if a heat exchange cycle system incorporating this double pipe 1 vibrates during operation, the inner pipe 2 will not collide with (rattle against) the outer pipe 10 in the double pipe 1, thereby reliably preventing or at least minimizing any noise generation caused by such rattling.

Furthermore, the diameter, d1, of the inscribed circle of the inner pipe 2 and the diameter, d2, of the inscribed circle of the outer pipe 10 satisfy the relationship 0.6<d1/d2. As a result, even if the double pipe 1 is subjected to an intense machine bending process so as to produce a radius of curvature R up to 100 mm (e.g., approximately between 55 mm and 35 mm), adjacent projections 21 will not contact, or come close to contacting, each other, which would otherwise cause the cross sectional areas of one or both of the outer flow paths 31 to close or narrow in an undesirable manner.

The total cross sectional area, S1, of the outer flow paths 31 of the double pipe 1 was 11 mm² and the cross sectional area, S2, of the inner flow path 32 within the inner pipe 2 was 22 mm². In other words, the required relationship S2/S1≧1.5 was satisfied. Thus, this design is suitable for causing a refrigerant whose volume varies, due to changes in physical properties, to flow through the double pipe 1.

Thus, the double pipe 1 of this example has a structure that prevents or at least substantially minimizes noise generation (rattling) during operation and is suitable for machine bending at the time of installation. Furthermore, the double pipe 1 has a structure that is capable of achieving superior heat exchange performance, i.e. it has very advantageous thermodynamic properties.

Example 2

As shown in FIG. 5, the double pipe 102 of this example includes an inner pipe 2 having a profile with four projections 21 that are separated at regular intervals in the circumferential direction and were produced by partly deforming the outer periphery of a starting pipe having a circular cross section so as to create the radially-outward-extending projections 21. Therefore, as shown in FIG. 5, four outer flow paths 31 were formed between the outer pipe 10 and the inner pipe 2 and were partitioned at four circumferential positions that are separated at regular intervals, i.e. equidistantly.

In this example as well, all four projections 21 have a width, W, that gradually decreases in the radially-outward direction. The double pipe 102 of this example was prepared in substantially the same manner as was described above for the double pipe 1 of Example 1 with the following exceptions: the starting material (starting pipe) for the outer pipe (A3003) had an outer diameter of 21 mmø and a wall thickness of 1.2 mm, and the starting material (pipe) for the inner pipe (A3003) had an outer diameter of 19 mmø and a wall thickness of 1.2 mm.

The specific dimensions of the double pipe 102 of this example were such that, after completion of the manufacturing process, the outer pipe 10 had an outer diameter of 20.4 mm and a wall thickness of 1.2 mm. The inner pipe 2 had an inner diameter, d1, of 12.7 mm (the diameter of the inscribed circle) along the parts thereof that were free from the projections 21 and an outer diameter, d2, of 18 mm (the diameter of the inscribed circle of the outer pipe 10) along the parts thereof where the projections 21 were formed. Furthermore, the outer diameter, d3, of the inner pipe along the parts that were free from the projections 21 was 15.1 mm and the wall thickness of the inner pipe 2 was 1.2 mm. Consequently, the relationship 0.6<d1/d2 was satisfied.

The total cross sectional area, S1, of the outer flow paths of the double pipe 102 was 70 mm² and the cross sectional area, S2, of the inner flow path 32 within the inner pipe 2 was 130 mm². Therefore, the required relationship S2/S1≧1.5 was satisfied.

The double pipe 102 of this example provides advantages similar to those of the double pipe of Example 1.

Example 3

As shown in FIG. 6, the double pipe 103 of this example includes an inner pipe 2 having a profile with eight projections 21 that were separated at regular intervals (equidistantly in the circumferential direction) and were produced by partly deforming the outer periphery of a starting pipe having a circular cross section so as to produce radially-outward-extending projections 21. Therefore, as shown in FIG. 6, eight outer flow paths 31 were formed between the outer pipe 10 and the inner pipe 2 and were partitioned at eight circumferential positions that were separated at regular intervals.

In this example as well, all eight projections had a width, W, that gradually decreases in the radially-outward direction. The double pipe 103 of this example was prepared in substantially the same manner as was described above for the double pipe 1 of Example 1 with the following exceptions: the starting pipe for the outer pipe (A3003) had an outer diameter of 23 mmø and a wall thickness of 1.3 mm, and the starting pipe for the inner pipe (A3003) had an outer diameter of 21 mmø and a wall thickness of 1.2 mm.

The specific dimensions of the double pipe 103 of this example were such that, after completion of the manufacturing process, the outer pipe 10 had an outer diameter of 22 mm and a wall thickness of 1.3 mm. The inner pipe 2 had an inner diameter, d1, of 13.6 mm (the diameter of the inscribed circle) along the parts thereof that were free from the projections 21 and an outer diameter, d2, of 19.4 mm (the diameter of the inscribed circle of the outer pipe 10) along the parts thereof where the projections 21 were formed. Furthermore, the outer diameter, d3, of the inner pipe along the parts that were free from the projections 21 was 16 mm and the wall thickness of the inner pipe 2 was 1.2 mm. Consequently, the relationship 0.6<d1/d2 was satisfied.

The total cross sectional area, S1, of the outer flow paths 31 of the double pipe 102 was 83 mm² and the cross sectional area, S2, of the inner flow path 32 in the inner pipe 2 was 152 mm². Therefore, the required relationship S2/S1≧1.5 was satisfied.

The double pipe 103 of this example provides advantages similar to those of the double pipe of Example 1.

Example 4

As shown in FIG. 7, the double pipe 104 of this example includes an inner pipe 2 having a profile with eight projections 21 that are separated at regular intervals in the circumferential direction and were produced by partly deforming the outer periphery of a starting pipe having a circular cross section so as to produce radially-outward-extending projections 21. Therefore, as shown in FIG. 7, eight outer flow paths 31 were formed between the outer pipe 10 and the inner pipe 2 and were partitioned at eight circumferential positions that were separated at regular intervals (i.e. equidistantly in the circumferential direction). Thus, the above-described arrangement of this example was basically or generally the same as that of Example 3 (FIG. 6).

However, as shown in FIG. 7, adjacent projections are disposed closer to each other at positions directly underneath their tops, as compared with Example 3, so that the projections 21 of the present Example appear more wave-like or similar to undulations than the (sharper) peaks of Examples 1-3. In this example as well, all eight projections had a width, W, that gradually decreases in the radially-outward direction.

The double pipe 104 of this example was prepared in substantially same manner as was described above for the double pipe 1 of Example 1 with the following exceptions: the starting pipe for the outer pipe (A3003) had an outer diameter of 25 mmø and a wall thickness of 1.5 mm, and the starting pipe for the inner pipe (A3003) had an outer diameter of 19.2 mmø and a wall thickness of 1.2 mm.

The specific dimensions of the double pipe 104 of this example were such that, after completion of the manufacturing process, the outer pipe 10 had an outer diameter of 22.2 mm and a wall thickness of 1.5 mm. The inner pipe 2 had an inner diameter, d1, of 13.4 mm (the diameter of the inscribed circle) along the parts thereof that were free from the projections 21. The outer diameter, d2, of the inner pipe 2 was 19.4 mm (the diameter of the inscribed circle of the outer pipe 10) along the parts thereof where the projections 21 were formed and the wall thickness of the inner pipe 2 was 1.2 mm. Therefore, the relationship 0.6<d1/d2 was satisfied.

The total cross sectional area, S1, of the outer flow paths of the double pipe 102 was 58 mm² and the cross sectional area, S2, of the inner flow path 32 within the inner pipe 2 was 177 mm². Therefore, the required relationship S2/S1≧1.5 was satisfied.

The double pipe 104 of this example provides advantages similar to those of the double pipe of Example 1.

Test Examples

A total of eight (double pipe) Test Samples T1 through T8 made of the same material as that of Example 4 were prepared by changing the diameter, d1, of the inscribed circle of the inner pipe and the diameter, d2, of the inscribed circle of the outer pipe. All of the Test Samples were subjected to a bending test to observe the influence (effect) of the bending operation on the projections 21. FIG. 18 provides Table 1, which shows the values of d1, d2 and d1/d2 of Test Samples T1 through T8.

As Comparative Examples, two Test Samples T9 and T10 were prepared as conventional double pipes 9 having an outer pipe 90 that was a smooth pipe with a circular cross section and an inner pipe 92 that was a twisted multi-coiled pipe. The basic configuration of Test Samples T9 and T10 is shown in FIG. 8, but they had different dimensions as shown in Table 1. In Test Samples T9 and T10, all the projections 921 had a width, W, that gradually increases toward the outside and then gradually decreases in the form of a circular arc as viewed in a direction orthogonal to the radial direction extending through the tops of the projections 921. Both the inner pipe 92 and the outer pipe 90 of Test Samples T9 and T10 were made of phosphorous-deoxidized copper (C1220) with a wall thickness of 1 mm. Table 1 shows the diameter, d1, of the inscribed circle of the inner pipe 92 and the diameter, d2, of the inscribed circle of the outer pipe 90 as well as the value of the ratio of d1/d2 for both Test Sample T9 and Test Sample T10.

As illustrated in FIG. 9, each of the samples was bent using a bending machine so as to result in a radius of curvature, R, of 45 mm, 55 mm or 100 mm at the innermost periphery; then, the bent part was cut and the cut surface was observed. For these visual observations, emphasis was placed on whether the projections of the inner pipe had been deformed to a sufficiently large extent that fluid flow in the outer flow paths 31 was restricted or impeded due to a narrowing or closing of the paths 31. Table 1 also shows the results of these observations. FIG. 10 is a sketch of Test Sample T7 as a representative sample in which the outer flow paths 31 were neither closed nor substantially narrowed (although the undulated profile of the inside relative to the cut position was not shown). FIG. 11 is a sketch of Test Sample T10 as a representative sample in which some of the outer flow paths 932 were closed or narrowed (although the undulated profile of the inside relative to the cut position was not shown).

As is apparent from Table 1 and FIG. 10, all of Test Samples T1 through T8 had a value exceeding 0.6 for the ratio of d1/d2 and did not exhibit the phenomenon, in which some or all of the projections 21 were crushed during the bending operation, thereby closing or narrowing the corresponding outer flow paths 31, even though the inner pipe 2 and the outer pipe 10 were slightly deformed and some of the projections 21 were also slightly deformed if the radius of curvature R was relatively small and equal to 45 mm or 55 mm. On the other hand, as is apparent from Table 1 and FIG. 11, Test Samples T9 and T10 had a value less than 0.6 for the ratio of d1/d2 and exhibited the phenomenon, in which some of the projections 921 of the inner pipe 92 were deformed to such a large extent that defective areas 95 were observed, where some or all of the outer flow paths 931 were closed or narrowed due the deformations of a plurality of the projections 921, even though the radius of curvature R was not only 55 mm but also 100 mm, which is a relatively large value (i.e. a relatively small amount of bending).

Example 5

As shown in FIG. 14, the double pipe 105 of this example includes an inner pipe 2 with a profile having eight projections 21 that are separated at regular intervals (are equidistant in the circumferential direction) and were produced by partly deforming the outer periphery of a starting material (starting pipe) having a circular cross section so as to produce radially-outward-extending projections 21. Therefore, as shown in FIG. 13, eight outer flow paths 31 were formed between the outer pipe 10 and the inner pipe 2 and were partitioned at eight circumferential positions that were separated at regular intervals (i.e. equidistantly in the circumferential direction). In this example as well, all eight projections 21 had a width, W, that gradually decreases in the radially-outward direction.

As shown in FIGS. 12 and 13, the outer pipe 10 of the double pipe 105 is substantially in the form of a smooth pipe. The inner peripheral surface of the outer pipe 10 has a circular cross section. The outer periphery of the pipe 10 is mostly or generally circular, but has four indentations or grooves 4 respectively extending length-wise (e.g., straight) in the axial direction at four locations on the outer circumference of the outer pipe 10.

The grooves 4 are formed on the outer surface of the outer pipe 10 while the outer pipe 10 and the inner pipe 2 are being subjected to a drawing process that reduces the diameter of the outer pipe. As a result of forming the grooves 4, the tops 210 of the projections 21 of the inner pipe 2 will firmly or fixedly come into contact with the inner surface of the outer pipe 10, whereby the inner pipe 2 and the outer pipe 10 become integral such that no relative movement therebetween is possible. In addition, the grooves 4 rarely have a negative influence on the bending process and/or on the heat exchange performance.

The specific dimensions of the double pipe 105 of this example were such that, after completion of the manufacturing process, the outer pipe 10 had an outer diameter of 22 mm and a wall thickness of 1.6 mm. The inner pipe 2 had an inner diameter, d1, of 13 mm (the diameter of the inscribed circle) along the parts thereof that were free from the projections 21, an outer diameter, d2, of 18.8 mm (the diameter of the inscribed circle of the outer pipe 10) along the parts thereof where the projections 21 were formed, and a wall thickness of 1.35 mm. Therefore, the relationship 0.6<d1/d2 was satisfied.

The total cross sectional area S1 of the outer flow paths 31 of the double pipe 105 and the cross sectional area S2 of the inner flow path 32 in the inner pipe 2 satisfy the relationship S2/S1≧1.5.

Example 6

As shown in FIGS. 15-17, the double pipe 106 of this example includes an inner pipe 2 having a profile with eight projections 21 that are separated at regular intervals (equidistantly in the circumferential direction) and were produced by partly deforming the outer periphery of a starting pipe having a circular cross section so as to produce radially-outward-extending projections 21.

Therefore, as shown in FIG. 16, eight outer flow paths 31 were formed between the outer pipe 10 and the inner pipe 2 and were partitioned at eight circumferential positions that are separated or spaced at regular (equidistant) intervals in the circumferential direction of the outer pipe 10. In this example as well, all eight projections 21 had a width, W, that gradually decreases in the radially-outward direction. It is also noted that, in the Examples 1-5, the cross section of projections 21 was a smooth or continuous curve. However, in Example 6, the cross section of projections 21 is two intersecting straight lines (i.e. a sharp or pointed peak, rather than the rounded peak of Examples 1-5).

As shown in FIGS. 15 and 16, the outer pipe 10 of the double pipe 106 is substantially in the form of a smooth pipe. The inner peripheral surface of the outer pipe 10 has an at least substantially circular cross section. The outer periphery of the pipe 10 is mostly or generally circular, but has four indentations or grooves 4 respectively extending length-wise (e.g., straight) in the axial direction at four locations on the outer circumference of the outer pipe 10.

The grooves 4 are formed on the outer surface of the outer pipe 10 while the outer pipe 10 and the inner pipe 2 are being subjected to a drawing process that reduces the diameter of the outer pipe 10. As a result of forming the grooves 4, the tops 210 of the projections 21 of the inner pipe 2 will firmly or fixedly come into contact with the inner surface of the outer pipe 10, such that the inner pipe 2 and the outer pipe 10 become integral and no relative movement therebetween is possible. In addition, the grooves 4 rarely have a negative influence on the bending process and/or on the heat exchange performance.

The specific dimensions of the double pipe 106 of this example were such that, after completion of the manufacturing process, the outer pipe 10 had an outer diameter of 22 mm and a wall thickness of 1.0 mm. The inner pipe 2 had an inner diameter, d1, of 15.7 mm (the diameter of the inscribed circle) along the parts thereof that were free from the projections 21 and an outer diameter, d2, of 20.4 mm (the diameter of the inscribed circle of the outer pipe 10) along the parts thereof where the projections 21 were formed and a wall thickness of 0.5 mm. Consequently, the relationship 0.6<d1/d2 was satisfied.

The required relationship S2/S1≧1.5 between the total cross sectional area, S1, of the outer flow paths 31 of the double pipe 106 and the cross sectional area, S2, of the inner flow path 32 in the inner pipe 2 was satisfied.

Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed above may be utilized separately or in conjunction with other features and teachings to provide improved double pipes for heat exchangers and methods for manufacturing and using the same.

Moreover, combinations of features and steps disclosed in the above detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.

All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter.

REFERENCE NUMBER LIST

• 1, 102, 103, 104, 105, 106: double pipe for heat exchanger
• 10: external pipe
• 2: internal pipe
• 21: projection
• 210: top
• 31: outer flow path
• 32: inner flow path
• 4: indentation (groove)

Claims

1. A double pipe for a heat exchanger comprising:

an inner pipe having a plurality of spiral projections extending radially outward from an outer peripheral surface of the inner pipe, and

an outer pipe disposed around the inner pipe, at least an inner peripheral surface thereof being at least substantially smooth and having an at least substantially circular cross section,

wherein the inner peripheral surface of the outer pipe contacts all of the projections of the inner pipe so as to define a plurality of peripherally-separated, outer flow paths between the outer peripheral surface of the inner pipe and the inner peripheral surface of the outer pipe,

the double pipe is configured such that fluid flowing within an inner flow path defined by the inner pipe exchanges heat with fluid flowing through the outer flow paths, and

an inscribed circle of the inner pipe has a first diameter (d1), an inscribed circle of the outer pipe has a second diameter (d2), and the relationship 0.6<d1/d2 is satisfied.

2. The double pipe according to claim 1, wherein the inner pipe has 2-12 of the spiral projections.

3. The double pipe according to claim 2, wherein the outer pipe has an outer diameter between 10-30 mm.

4. The double pipe according to claim 3, wherein the inner pipe has a wall thickness of 0.5-2.0 mm.

5. The double pipe according to claim 4, wherein the spiral projections each form an angle with a longitudinal axis of the inner pipe that is between 10°-70°.

6. The double pipe according to claim 5, wherein both the inner pipe and the outer pipe are made of an aluminum alloy or a copper alloy.

7. The double pipe according to claim 6, wherein the outer flow paths have a combined first cross sectional area (S1), the inner flow path has a second cross sectional area (S2) and the relationship S2/S1≧1.5 is satisfied.

8. The double pipe according to claim 7, wherein the spiral projections have a width as viewed in a direction orthogonal to a radial direction of the double pipe that extends through the tops of the projections, and the width gradually decreases towards the outside in the radial direction.

9. The double pipe according to claim 8, wherein a plurality of longitudinally-extending grooves are defined in an outer peripheral surface of the outer pipe at spaced intervals around a circumferential direction of the outer pipe.

10. The double pipe according to claim 1, wherein the outer pipe has an outer diameter between 10-30 mm.

11. The double pipe according to claim 1, wherein the inner pipe has a wall thickness of 0.5-2.0 mm.

12. The double pipe according to claim 1, wherein the spiral projections each form an angle with a longitudinal axis of the inner pipe that is between 10°-70°.

13. The double pipe according to claim 1, wherein both the inner pipe and the outer pipe are made of an aluminum alloy or a copper alloy.

14. The double pipe according to claim 1, wherein the outer flow paths have a combined first cross sectional area (S1), the inner flow path has a second cross sectional area (S2) and the relationship S2/S1≧1.5 is satisfied.

15. The double pipe according to claim 1, wherein the spiral projections have a width as viewed in a direction orthogonal to a radial direction of the double pipe that extends through the tops of the projections, and the width gradually decreases towards the outside in the radial direction.

16. The double pipe according to claim 1, wherein a plurality of longitudinally-extending grooves are defined in an outer peripheral surface of the outer pipe at spaced intervals around a circumferential direction of the outer pipe.

17. A double pipe for a heat exchanger having a double pipe structure formed by arranging an inner pipe inside of an outer pipe, the double pipe being configured such that fluid flowing within the inner pipe exchanges heat with fluid flowing between the inner pipe and the outer pipe;

wherein the inner pipe has a profile formed by partly deforming its originally circular outer periphery in cross section so as to exhibit a plurality of longitudinally straight and narrow radially-outward-extending projections and then twisting the inner pipe having the plurality of longitudinally straight and narrow radially-outward-extending projections so as to form a plurality of spiral and narrow radially-outward-extending projections;

the outer pipe is formed of a smooth pipe, at least an inner peripheral surface thereof has a circular cross section;

the inner peripheral surface of the outer pipe contacts tops of the projections of the inner pipe so as to define a plurality of peripherally-separated, outer flow paths; and

a diameter (d1) of an inscribed circle of the inner pipe and a diameter (d2) of an inscribed circle of the outer pipe satisfy the relationship 0.6<d1/d2.

18. The double pipe according to claim 17, wherein:

the inner pipe has 2 to 12 of the spiral projections;

the outer pipe has an outer diameter of between 10-30 mm; and

the inner pipe has a wall thickness of between 0.5-2.0 mm.

19. The double pipe according to claim 17, wherein:

the spiral projections each form an angle with a longitudinal axis of the inner pipe that is between 10°-70°; and

the projections have a width, as viewed in a direction orthogonal to a radial direction extending through the tops of the projections, that gradually decreases toward the outside in the radial direction.

20. The double pipe according to claim 17, wherein a cross sectional area (S1) of the outer flow paths and a cross sectional area (S2) of the inner flow path within the inner pipe satisfy the relationship S2/S1≧1.5.