Method for making brazed aluminum heat exchanger and apparatus

Abstract

Disclosed is a heat exchanger comprising a boiling passage and cooling passage defined by opposite sides of metal walls. The heat exchanger is made with high strength alloys of aluminum that have improved creep resistance that maintain tensile strength and yield strength after use in this invention. Layers of brazing material between the metal walls and a spacer member bond components of the heat exchanger together. An enhanced boiling layer (EBL) comprising metal particles bonded to each other and to a boiling side of the metal wall provides nucleate boiling pores to improve heat transfer. The EBL has a melting temperature that is higher than the melting temperature of the brazing material. Also disclosed is a process for assembling the heat exchanger.

Description

BACKGROUND OF THE INVENTION[0001]This invention relates to an improved method for making a metal heat exchanger with high heat transfer efficiency. Specifically, this invention relates to the use of High Flux-coated aluminum sheets in the fabrication of a brazed heat exchanger.[0002]Two designs of heat exchanger are presently in general use for reboiler-condensers in cryogenic, refinery and chemical applications. One type of heat exchanger in current use is a vertical shell and tube heat exchanger. To achieve a sufficiently high degree of heat transfer at relatively low temperature differences with this design, enhanced boiling layers (EBL) are used. An EBL typically has a structure comprising a multitude of pores that provide boiling nucleation sites to facilitate boiling. An EBL is applied to the inside of the tubes, and longitudinal flutes are provided on the outside of the tubes to facilitate heat transfer.[0003]A common heat exchanger used in cryogenic, refinery and chemical ap...

AI Technical Summary

Benefits of technology

[0006]We provide an improved method for making a brazed metal heat exchanger and the resulting apparatus. An enhanced boiling layer (EBL) is provided on the walls of the boiling passages. The melting temperature of the brazing material is lower than the melting temperature of the metal particles in the enhanced boiling layer. In an embodiment, the metal in the enhanced boiling layer and / or the brazing layer is an alloy of a first metal and a second metal which alloy has a lower melting temperature than that of the first metal. Different second metals can be used in the EBL and in the brazing material so long as the second metal provides an alloy with a lower melting temperature. In an embodiment, the concentration of the second metal in the brazing material is greater than in the EBL. Even when the brazing temperature gets within about 8.3 Celsius degrees (15 Fahrenheit degrees) of the melting point of the metal in the EBL for an extended period of time, the EBL unexpectedly retains its porosity, and thus its effectiveness. In an embodiment, the condensing passages contain fins to facilitate heat transfer.

[0008]The use of certain alloys and tempers of aluminum has been found to provide superior mechanical properties in the sheets used for the brazed metal heat exchanger, and thereby facilitate fabrication of the exchanger.

[0011]We found that a porous aluminum EBL, containing Al—Si brazing alloy powder, and developed for application to Aluminum Alloy 3003 wall material, would readily bond to the 037x alloys without loss in coating strength or adhesion.

Problems solved by technology

Such brazed aluminum heat exchangers described in these patents have not been commercialized because EBLs are typically brazed at 565° to 593° C.

Maintaining the integrity and effectiveness of the EBL, particularly the porous structure provided by the mutually bonded metal particles, during the second hotter heat treatment to effect brazing has been difficult.

This difficulty accounts for the lack of commercially available brazed heat exchangers with EBL in the boiling passages.

However, since that time the inventors herein have discovered that these prior art brazed aluminum heat exchangers suffer from poor mechanical properties following the initial brazing process.

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