Double-wall-tube heat exchanger

The present invention relates to a double-wall-tube heat exchanger and, more particularly, to a double-wall-tube heat exchanger which has an outer tube, and an inner tube provided in and spaced apart from the outer tube.

Herein, the term “condenser” refers to not only an ordinary condenser but also a subcooling condenser, which has a condensing section and a subcooling section.

A conventionally proposed refrigeration system for use in a car air conditioner includes a compressor; a condenser having a condensing section and a subcooling section; an evaporator; an expansion valve serving as a pressure-reducing device; a vapor-liquid separator; and an intermediate heat exchanger disposed between the condenser and the evaporator and adapted to perform heat exchange between a high-temperature refrigerant from the subcooling section of the condenser and a low-temperature refrigerant from the evaporator (as disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-162241). In the refrigeration system described in the publication, the refrigerant which has been subcooled in the subcooling section of the condenser is further cooled in the intermediate heat exchanger by the low-temperature, low-pressure refrigerant from the evaporator. By this procedure, the cooling performance of the evaporator is improved.

The intermediate heat exchanger used in the refrigeration system described in the above-mentioned publication has an outer tube, and an inner tube disposed in and spaced apart from the outer tube; the inner tube has grooves which are formed on its outer wall surface, by deforming its wall, in such a manner as to extend in its longitudinal direction; a clearance between the outer tube and the inner tube serves as a high-temperature refrigerant flow path through which the high-temperature refrigerant from the condenser flows; and the interior of the inner tube serves as a low-temperature refrigerant flow path through which the low-temperature refrigerant from the evaporator flows.

However, the intermediate heat exchanger described in the above-mentioned publication involves the following problem: the area of heat transfer between the high-temperature refrigerant flow path and the low-temperature refrigerant flow path is small, resulting in insufficient heat exchange performance.

An object of the present invention is to solve the above-mentioned problem and to provide a double-wall-tube heat exchanger exhibiting excellent heat exchange performance.

To achieve the above object, the present invention comprises the following modes.

1) A double-wall-tube heat exchanger comprising an outer tube, and an inner tube disposed in and spaced apart from the outer tube. A clearance between the outer tube and the inner tube and the interior of the inner tube serve as respective refrigerant flow paths. The inner tube has a plurality of interior fins formed on the inner circumferential surface thereof. The interior fins project radially inward; extend in the longitudinal direction of the inner tube; and are arranged at circumferential intervals. The inner tube has a plurality of elongated projections formed on the outer circumferential surface thereof. The elongated projections project radially outward; extend in the longitudinal direction; and are arranged at circumferential intervals. The fin height of the interior fins is greater than the projecting height of the elongated projections.

2) A double-wall-tube heat exchanger according to par. 1), wherein a radial clearance between the inner circumferential surface of the outer tube and a portion of the outer circumferential surface of the inner tube where the elongated projections are not formed is 0.4 mm to 1.2 mm inclusive.

Par. 2) specifies a radial clearance of 0.4 mm to 1.2 mm inclusive between the inner circumferential surface of the outer tube and the portion of the outer circumferential surface of the inner tube where the elongated projections are not formed, for the following reason. If the radial clearance is excessively small, pressure loss increases sharply in the refrigerant flow path formed between the inner tube and the outer tube. On the other hand, if the radial clearance is excessively large, the flow velocity of a refrigerant drops in the refrigerant flow path formed between the inner tube and the outer tube, potentially resulting in a drop in heat transfer coefficient.

3) A double-wall-tube heat exchanger according to par. 1), wherein a clearance between projecting ends of the elongated projections of the inner tube and the inner circumferential surface of the outer tube is 0 mm to 0.5 mm inclusive.

Par. 3) specifies a clearance of 0 mm to 0.5 mm inclusive between the projecting ends of the elongated projections of the inner tube and the inner circumferential surface of the outer tube, for the following reason. If the clearance is excessively large, in the case where the double-wall-tube heat exchanger has a bend(s), wrinkles are apt to be formed on the outer tube in a bending process.

4) A double-wall-tube heat exchanger comprising an outer tube, and an inner tube disposed in and spaced apart from the outer tube. A clearance between the outer tube and the inner tube and the interior of the inner tube serves as respective refrigerant flow paths. The inner tube has a plurality of interior fins formed on the inner circumferential surface thereof. The interior fins project radially inward; extend in the longitudinal direction of the inner tube; and are arranged at circumferential intervals. The outer tube has a plurality of elongated projections formed on the inner circumferential surface thereof. The elongated projections project radially inward; extend in the longitudinal direction of the outer tube; and are arranged at circumferential intervals.

5) A double-wall-tube heat exchanger according to par. 4), wherein a radial clearance between a portion of the inner circumferential surface of the outer tube where the elongated projections are not formed and the outer circumferential surface of the inner tube is 0.4 mm to 1.2 mm inclusive.

Par. 5) specifies a radial clearance of 0.4 mm to 1.2 mm inclusive between the portion of the inner circumferential surface of the outer tube where the elongated projections are not formed and the outer circumferential surface of the inner tube, for the following reason. If the radial clearance is excessively small, pressure loss increases sharply in the refrigerant flow path formed between the inner tube and the outer tube. On the other hand, if the radial clearance is excessively large, the flow velocity of a refrigerant drops in the refrigerant flow path formed between the inner tube and the outer tube, potentially resulting in a drop in heat transfer coefficient.

6) A double-wall-tube heat exchanger according to par. 4), wherein a clearance between projecting ends of the elongated projections of the outer tube and the outer circumferential surface of the inner tube is 0 mm to 0.5 mm inclusive.

Par. 6) specifies a clearance of 0 mm to 0.5 mm inclusive between the projecting ends of the elongated projections of the outer tube and the outer circumferential surface of the inner tube, for the following reason. If the clearance is excessively large, in the case where the double-wall-tube heat exchanger has a bend(s), wrinkles are apt to be formed on the outer tube in a bending process.

7) A double-wall-tube heat exchanger according to par. 1) or 4), wherein the interior fins of the inner tube have a fin thickness of 0.2 mm to 2.0 mm inclusive.

Par. 7) specifies a fin thickness of 0.2 mm to 2.0 mm inclusive for the interior fins of the inner tube, for the following reason. If the fin thickness is excessively thin, the fin efficiency of the interior fins drops, and working may become difficult. If the fin thickness is in excess of 2.0 mm, the effect of improving the fin efficiency of the interior fins is impaired, and working may become difficult. In consideration of extrusion workability in forming the inner tube by extrusion, and bending workability in the case where the double-wall-tube heat exchanger has a bend(s), the fin thickness of the interior fins of the inner tube is more preferably 0.3 mm to 0.7 mm inclusive.

8) A double-wall-tube heat exchanger according to par. 1) or 4), wherein the interior fins have a fin height of 1.0 mm to 3.0 mm inclusive.

Par. 8) specifies a fin height of 1.0 mm to 3.0 mm inclusive for the interior fins of the inner tube, for the following reason. If the fin height is excessively low, the area of heat transfer between the inner tube and a refrigerant which flows through the refrigerant flow path in the inner tube fails to become sufficiently large; as a result, heat transfer performance fails to be sufficiently improved. If the fin height is excessively high, in the case where the double-wall-tube heat exchanger has a bend(s), the interior fins may be buckled in a bending process, potentially blocking the refrigerant flow path in the inner tube.