Method for the production of tubes for heat exchangers from precipitation-hardened alloys by high-frequency induction welding

Abstract

Described herein is an embodiment of a method for the production of tubes for heat exchangers, including the steps of: a) producing an ingot with rectangular cross section (plate) b) hot-rolling said plate so as to form a blank c) rapidly cooling said blank by spraying water inside a purposely designed cooling tunnel d) cold-rolling said blank until a strip or skelp is obtained e) forming the tube starting from the skelp to pass from the plane geometry of the skelp to the cylindrical geometry of the tube and welding the tube; in which the ingot is formed starting from an alloy that can undergo precipitation hardening, and the tube is welded by high-frequency induction welding.

Description

PRIORITY CLAIM [0001] This application claims priority from European patent application No. 06425498.0, filed Jul. 19, 2006, which is incorporated herein by reference. TECHNICAL FIELD [0002] An embodiment of the present invention relates to a method for producing tubes for heat exchangers, in particular for applications in the sector of air conditioning and refrigeration, or more generally for applications that call for high-level thermo-mechanical properties. BACKGROUND [0003] Generally, the tubes used in the air-conditioning and refrigeration (ACR) sector are made starting from Cu-DHP (deoxidized high-residual phosphorus) alloys, without oxygen, in which to guarantee deoxidization a relatively high content of residual phosphorus, comprised usually between 0.015 and 0.04 wt %, is maintained. The presence of phosphorus enables elimination of phenomena of brittleness in a reducing environment, improvement of the cold plastic deformability, and above all increase in the suitability fo...

AI Technical Summary

Benefits of technology

[0013] An embodiment of the present invention is a method for the production of tubes for heat exchangers that are suited to working at the high pressures dictated by the use of new thermal fluids, that does not present the drawbacks described above and, in particular, that is relatively simple and inexpensive to implement, that makes use of alloys that are already available, and that obtains tubes that are relatively thin but with good mechanical characteristics and have purposely provided internal grooves such as to provide in use high heat-exchange coefficients.

[0014] In particular, according to an embodiment of the invention, the tubes are produced starting from precipitation-hardened alloys, but exploiting a welding process that guarantees maintenance of the characteristics of strength and heat-exchange efficiency also in the weld bead, such as high-frequency induction welding. Furthermore, according to a method of an embodiment of the present invention, the weld bead that is obtained using said welding method is of dimensions of the same order of magnitude as the possible internal groove of the surface of the tube and, hence, concurs in a homogeneous way to increasing the turbulence and, consequently, the heat-exchange coefficient of the tubes during operation.

Problems solved by technology

Likewise, if the copper already contained oxygen to start with, the strength of the joint would be in any case impaired.

From the operating standpoint, however, said coolants call for higher operating pressures.

The use of the typical ACR tubes in heat exchangers that operate at such high operating pressures would necessarily lead to a considerable increase in the thickness of the tube, with consequent marked increases of cost, also in the light of the constant increase in the price of the raw material over the last few years.

This drawback remains, albeit to a lesser extent, even working the tube appropriately in order to increase the heat-exchange efficiency, for example rendering the internal surface of the tube appropriately corrugated, resorting to profiles with even complex geometries, which increase the turbulence of the fluid that flows within the tubes and consequently lead to higher heat-exchange coefficients.

Amongst alloys that present this type of behavior are the Cu—Fe—P and Cu—Fe—Ni—P systems, upon which, by means of appropriate thermal treatment, mechanical properties can be bestowed that are markedly better than those of pure copper, of which they maintain, however, the electrical and thermal conductivity substantially unvaried.

The use of said alloys, instead of Cu-DHP alloys, for making tubes by means of longitudinal welding, for example starting from a grooved skelp, appears, however, problematical.

It is on the other hand evident that, if an attempt were made to form tubes directly with said alloys, for example via extrusion, a series of practical problems would be encountered, such as that of obtaining internal grooves with an intersecting profile, or else that of forgoing said grooves, with consequent reduction in the heat-exchange coefficient.

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