Heat exchange – Tubular structure – With discrete heat transfer means
Reexamination Certificate
2002-06-25
2003-11-11
McKinnon, Terrell (Department: 3743)
Heat exchange
Tubular structure
With discrete heat transfer means
C029S890053, C138S038000
Reexamination Certificate
active
06644394
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to heat exchangers, and more specifically, to heat exchanger tubes having braze alloy flow barriers. The invention also relates to a method of making the braze alloy flow barriers on the heat exchanger tube along with an apparatus for performing the method.
BACKGROUND OF THE INVENTION
During the manufacture of a heat exchanger, the heat exchanger tubes are typically attached to manifolds and/or fins by means of furnace brazing. A typical brazing operation is controlled atmospheric brazing. During controlled atmospheric brazing of aluminum heat exchangers, braze alloy that has been clad to various components (i.e., fin stock and manifolds) becomes molten and forms the required joints via flow by capillary forces. When the heat exchanger tubing is aluminum, dissolution and erosion of the aluminum tube walls can occur as a result of excessive molten braze alloy flowing along the tubes. The flow of molten braze alloy along the tube is facilitated by longitudinal die-lines. Die-lines are formed in the surface of the extruded tube during the extrusion process. These die-lines may be characterized as very small grooves that are a result of the tool (die)—tube interaction during the extrusion process. The die-lines extend longitudinally along the surface of the tube. Excessive flow of molten braze metal is generally undesirable, particularly when localized within a die-line or group of die-lines.
The braze alloy is a low melting temperature aluminum alloy that is typically high in silicon (7-13%). The brazing operation typically occurs at a temperature of approximately 600-615° C. During the brazing process, the silicon from the braze alloy will diffuse into the tube alloy and effectively lower its melting temperature at the surface. If excessive molten metal flow takes place in a concentrated region (such as along die-lines), the braze alloy will lower the temperature of the tube wall and allow it to melt and flow along with the molten braze alloy. Hence, as the tube wall is being dissolved by the braze alloy, the flow of the braze alloy erodes the tube wall. The erosion of the tube has consequences which can vary. For example, as a minimum the tube will lose thickness and in the worst case develop an opening through the tube into the passageways within the tube thus rendering the tube defective.
Therefore, excessive flowing of molten braze alloy along heat exchanger tubes is undesirable and can cause damage to the heat exchanger tubes. Therefore, it is desirable to minimize and/or prevent the flow of molten braze alloy along heat exchanger tubes during the furnace brazing process.
SUMMARY OF THE INVENTION
The present invention provides heat exchanger tubes that minimize and/or prevent concentrated and/or localized flow of braze alloy along the tube and/or die-lines during the brazing process. A heat exchanger tube according to the principles of the present invention has a plurality of striae in the surface of the tube that extend along a portion of the width of the surface. The plurality of striae interrupt and restrict the flow of molten braze alloy along the length of the tube. The striae thereby reduce the incidence of excessive tube wall erosion.
A heat exchanger tube with the plurality of flow barriers or striae can be fabricated by passing the tube along a cylindrical roller having an outer surface with a plurality of projections that extend radially outward from the surface. The projections form the plurality of striae on the tube as the surface of the tube is passed along the outer surface of the roller.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
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patent: 3693710 (1972-09-01), Drosnin
patent: 6056048 (2000-05-01), Takahashi et al.
patent: 6173762 (2001-01-01), Ishida et al.
patent: 2002/0195233 (2002-12-01), Thors et al.
patent: WO 00/70291 (2000-11-01), None
Kraft Frank F.
Skrzypchak Dave W.
Brazeway, Inc.
Harness & Dickey & Pierce P.L.C.
McKinnon Terrell
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