Heat transfer tube and heat exchanger using same

Abstract

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

Description

[0001]The present application is based on Japanese patent application No. 2006-038531, the entire contents of which are incorporated herein by reference.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention relates to a heat transfer tube and a heat exchanger using the heat transfer tube, and particularly, to a heat transfer tube for a water-refrigerant heat exchanger used in a natural refrigerant heat pump water heater (which may herein be referred to as simply “heat pump water heater”), and a heat exchanger using the heat transfer tube.[0004]2. Description of the Related Art[0005]Conventionally well-known heat pump water heaters using a natural refrigerant (e.g., carbon dioxide refrigerant) employ generally two kinds of heat exchangers, i.e., a heat radiator and a heat absorber. The two heat exchangers use, as a heat transfer tube, refrigerant tubes for the heat radiator and for the heat absorber, respectively.[0006]In the heat pump water heaters, t...

AI Technical Summary

Benefits of technology

[0061]The embodiments of the present invention have the following advantages:

[0062](1) In a prior-art heat transfer tube (a plain tube, an inner-grooved tube, etc.), there is the problem of very low heat transfer performance due to a very small water-flow-rate in a water-refrigerant heat exchanger of a heat pump water heater, leading to a laminar flow in the heat transfer tube. Also, a prior-art heat transfer tube using a corrugated tube does not define unevenness-to-outside-diameter ratio Hc / OD, and is indefinite in heat transfer performance effect. In contrast to these, according to the corrugated heat transfer tube in the present embodiments, the unevenness-to-outside-diameter ratio Hc / OD can be sufficiently large at low cost even compared to an inner-grooved tube, and the heat transfer performance can be substantially enhanced by the turbulence effect of fluid crossing unevenness defined by this Hc / OD. Particularly, it is possible to realize twice or more the performance compared to a plain tube, in a low Reynolds number Re range (e.g., 1000-5000, particularly 1000-3000) difficult to enhance the performance in the prior-art product.

[0063](2) According to the corrugated heat transfer tube in the present embodiments, because the twist angle βc which the corrugated grooves make with the tube axis can be 40° or higher, which is difficult to form in the inner-grooved tube, it is possible to increase the frequency of fluid crossing unevenness, and thereby promote fluid turbulence effect. Also, by adjusting the relationship between the number of threads and the corrugation pitch Pc, it is possible to make the twist angle βc large at low cost compared to the inner-grooved tube, etc.

[0064](3) According to the corrugated heat transfer tube in the above third to sixth embodiments, because it is possible to maximize enhancement in the heat transfer performance of the water tube and the heat transfer area of the water tube relative to the refrigerant, the heat exchanger efficiency is enhanced. Further, according to the above third and fifth embodiments, it is possible to ensure enhancement in the heat transfer performance of the refrigerant in addition to the heat transfer performance of the water tube.

[0065](4) It is possible to relatively easily provide a large leak detection portion, in comparison to the inner-grooved tube. Specifically, although leak detection groove formation typically requires use of an inner-grooved tube with high fins as a leak detection tube, because the corrugated heat transfer tube is used as the inner tube, it is possible to make the leak detection grooves large (at low cost), and thereby use a plain tube as the leak detection tube 12.

[0066](5) According to the above fifth and sixth embodiments, the corrugated outer tube allows enhancement in flexibility for bending of the heat exchanger.

[0067](6) According to the above seventh embodiment, because the outer tube through which a refrigerant flows is helically wound along the corrugated grooves of the corrugated heat transfer tube, it is possible to have flexibility for bending of the heat exchanger, and increase effective contact area (effective heat transfer area) between the outer tube and the water tube (the heat transfer tube around which is wound by the outer tube), compared to the case where the outer tube is wound around a plain tube or an inner-grooved tube.

Problems solved by technology

In such a double tube heat exchanger, in the inner tube through which a refrigerant flows, hole formation may be caused by corrosion, which leads to a mixing of the water and refrigerant.

In the heat exchanger disclosed in JP-A-2004-360974, however, there are the problems that the plural-tube-helically-twisting step itself is complicated and costly (twisting a hollow tube that tends to deform (collapse, break, etc.) is not so easy compared to twisting a solid wire), and that the treatment (structure) of the heat exchanger ends, in which the first heat transfer tube and the plural-tube second heat transfer tube are separated from each other, is complicated.

There is also the problem that, when the above-mentioned leak detection portion is provided, it is necessary to cause each of the second heat transfer tube to comprise a double tube, which leads to an even higher cost.

Also, in JP-A-2002-228370, simply forming the core tube in a corrugated shape or inserting the torsion sheet in the core tube may make no desired heat transfer performance, and an increase in cost or pressure loss.

Further, because of limitations on an inner-grooved tube fabrication method, it is difficult to form a shape change to cause a turbulence effect in a laminar flow region by the small flow-rate.

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