Stock material or miscellaneous articles – Structurally defined web or sheet – Including variation in thickness
Patent
1990-01-23
1992-03-03
Robinson, Ellis P.
Stock material or miscellaneous articles
Structurally defined web or sheet
Including variation in thickness
205134, 205224, 428167, 428178, 428181, 428182, 428188, 428906, 29110, 291211, 29890, 264 56, 264 67, 2041291, 422168, 422180, 502439, 502527, B32B 300, C25D 500, B01D 5000
Patent
active
050931784
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
In power engineering it is a common goal to achieve within a limited volume a transfer of energy between a gas flow and a solid body or a chemical reaction in a gas aided by a catalyst fixed to the surface of a solid body. The solid body must then be shaped with a maximum contact surface with the gas flow without too high flow resistance. The solid body is then often shaped with a large number of parallel channels, separated by thin walls, thereby dividing the gas flow into a large number of partial flows with turbulent flow pattern.
Examples of use of such flow dividers are found among heat exchangers, mufflers, catalyst carriers for chemical industry and for emission control in vehicles. Other examples are for gas flow direction in furnaces, burners and wind tunnels.
For use at high temperatures above 700 degrees C. two types of material are mainly used, none fully satisfactory.
Ceramic materials, such as aluminium oxide, can be extruded as a slurry to form bodies with parallel channels as disclosed in patents EP 294.106 and EP 275.162, and thereafter be converted by heat treatment to water-free alumina, but have disadvantages due to built-in stresses, fragility, difficulty in handling prior to the heat treatment and difficulty to shape the channel entries for low flow resistance. They may need complex mounting devices for enclosure in metal, as shown in U.S. Pat. No. 3,966,419.
Metals are easy to shape, both by extrusion and by winding together grooved, corrugated or pleated strips as shown in U.S. Pat. No. 4,719,680, but at elevated temperatures they are mechanically unstable by creep deformation, and chemically unstable by reaction with the gases.
SUMMARY OF THE INVENTION
The present invention concerns a method of making flow dividers, where the shaping is done in a metallic state, and a conversion to ceramic material is carried out after the shaping. This results in greater liberty in choice of shape, and lower cost.
It is well known that anodic oxidation in an electrolyte containing oxalic acid permits conversion of the whole thickness of a thin-walled aluminium object to hydrated aluminium oxide. It is also known that heat treatment of hydrated aluminium oxide can convert it to non-hydrated aluminium oxide in the alpha-alumina modification, which is tough and wear resistent. It is also known to combine these known steps to fabricate simply shaped items such as loudspeaker membranes from alpha-alumina, according to the patent applications DE 35 42 202 and DE 35 46 548.
The shapes of metallic flow dividers, extruded as well as wound, comprise portions with larger thickness than average, such as where two walls meet, and narrow passages where the current density and the oxidation rate are less than average, such as where two layers touch when wound together, or far into the central channels. If such bodies are anodized with the known methods, it would be impossible to avoid metallic remnants where the thickness is large or the oxidation rate low, because these portions lose contact with the current source when the thinner easily oxidated portions have been converted. During a subsequent heat treatment, the metal would melt and damage the shape of the body.
According to the invention, the flow divider with parallel channels is shaped in a first step by extrusion or by winding together corrugated, pleated or grooved strips. If desired, the mechanical stability of a wound divider can be improved by thermal bonding in heated inert gas.
In a second step, the channel exits or entrances are chemically deburred to lower the flow resistance.
In a third step, the aluminium metal is converted to hydrated aluminium oxide through anodic oxidation in an electrolyte containing oxalic acid, with a current source connected to the upper end of the flow divider and the flow divider is slowly lowered into the electrolyte, with the channels vertically oriented while the current flows. All parts of the flow divider are then successively converted to hydrated aluminium oxide, with the not yet
REFERENCES:
patent: 4719680 (1988-01-01), Cyron
Sjogren G. H. Pedro
Sundstrom Erik
Loney Donald J.
Robinson Ellis P.
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