Metal working – Method of mechanical manufacture – Heat exchanger or boiler making
Reexamination Certificate
1998-09-03
2001-05-15
Rosenbaum, I Cuda (Department: 3726)
Metal working
Method of mechanical manufacture
Heat exchanger or boiler making
C219S069170, C264S293000
Reexamination Certificate
active
06230408
ABSTRACT:
SUMMARY OF THE INVENTION
The invention concerns a method for fabrication of micro heat exchangers, in which the surfaces of single metallic layers are structured and the structured layers are stacked one on top of the other and joined together to form a micro structural body with channel-like passageways.
BACKGROUND OF THE INVENTION
Heat exchangers find application in chemical and process engineering for the transfer of heat between two media. The known plate type heat exchangers consist of a plurality of profiled sheets, which are assembled into a sheet pack. The wall thickness of the individual sheets, for strength reasons, is generally 0.4 to 0.8 mm. The sheet packs are mounted in a frame and clamped together with several tightening screws.
The heat-transferring media are guided through four plate openings at the corners of the sheets. Every two openings are alternately sealed off from the rest of the flow space so that the two media alternately flow through the intervals between the plates. By closing off individual passageways, it is possible to collect up all plates or certain plate packs in succession, in order to achieve a multiple path design (VDI Heat Atlas, 7thEd., 1994, Ob 18, 19).
There are familiar heat exchangers which work by the counterflow principle. Moreover, heat exchangers with a cross flow design are known.
Miniaturization of heat exchangers enables an enlargement of the outer surfaces available for heat exchange processes as compared to the total volume of the media flow. In this way, high capacities of heat transfer between the media are achieved.
A method for fabrication of fine structure bodies such as heat exchangers is known from DE 37 09 278 A1, in which grooves with constant cross section along their length are introduced into the surface of clarnpable metal foils and the foils are layered one on top of the other and joined together to form a microstructure body with channel-like passageways. In the known method, the grooves are worked into the foils with shaping diamonds. The method is not suitable for fabrication of microstructures with very large aspect ratios, i.e., the ratio between the height of the structure and its lateral dimensions, or structures with low surface roughness. Moreover, a free lateral structurizability is not provided, since the shaping diamonds can only form grooves with constant cross section along their length.
DE 39 15 920 A1 describes a micro heat exchanger with two layers of semiconductor material structured by etching. As a drawback, it turns out that metals are not accessible to this structurization.
DETAILED DESCRIPTION OF THE INVENTION
The purpose of the invention is to indicate a method which allows one to cheaply produce micro heat exchangers in large numbers.
This purpose is accomplished, according to the invention, with the features of Patent claim
1
, as well as Patent claim
2
.
In a first processing step, a structurization which is a positive or negative image of the structurization of the individual layers is worked into a body. On the surface of this body, a galvanic layer is deposited in a second step. In a variant of the method, the resulting metallic microstructure, which is the negative of the original structure, is used as a shaping insert. In a second process variant, the resulting metallic microstructure, which is the negative of the original structure, is used as an electrode in an electroerosion method. In this case, the structurization of the electrode is copied in a metallic layer. The resulting metallic layer, structurized inversely to the structurization of the electrode, is used as a shaping insert. In a last step, the individual layers are shaped with the shaping insert obtained by the first or second process variant and assembled to produce the microstructure body of the heat exchanger.
The advantage of this method lies in that different materials can be used for the fabrication of the first structurization and for the shaping insert. The first structurization can occur, for example, in easily workable plastic. According to the first process variant, the shaping insert can then be produced galvanically in a hard metal alloy. According to the second process variant, the electrode used for the spark erosion machining can be galvanically deposited and this electrode can then be copied in a hard metal alloy.
The advantage of the galvanic shaping consists in that the properties of the starting structure, such as dimension, precision, and surface roughness, remain intact. A high dimensional precision is of advantage, because it enables channels with very small dimensions, i.e., a high surface/volume ratio. Furthermore, the thickness of the walls between the channels can be considerably reduced, which ensures a high heat transfer rate. This is especially important for micro heat exchangers, in which the heat transfer occurs within the individual layers. Little surface roughness and high structural precision is advantageous, because it reduces in particular the flow resistance in small channels.
The second process variant is of advantage inasmuch as the electroerosion method can produce hard metal embossing stamps, e.g., for the embossing of aluminum, which have a long lifetime, since the stamp made from hard metal has little wear.
The structurization can be rather easily worked into a plastic body or the plastic layer of a plastic-coated body. Yet it is also possible, in theory, to work the structurization into a metallic body. The thickness of the plastic body or the thickness of the plastic layer depend on the structurization method.
When it is only necessary to create low structural height up to around 100 &mgr;m and/or when less demands are placed on the precision, the body is structured by a photolithography process. In the familiar photolithography process, the body is coated with a photoresist and a mask is deposited on the body. Next, the photoresist is exposed to UV light. The physical and chemical changes in the photoresist allow a selective development and, thus, formation of the structurization in the body.
Yet the structurization can also be incorporated in the body by an x-ray deep lithography method. In the known x-ray deep lithography method, an x-ray resist is exposed to x-rays. The changes in the x-ray resist will be confined to an exact region by virtue of the highly parallel x-ray light and less diffiaction at the mask because of the small wavelength and thus enable a more selective development as compared to photolithography. The structure created in this way, besides having high precision in the submicrometer range, also has considerably less surface relief as compared to machining processes. It is possible to achieve relatively large structural peaks up to a few millimeters. Thus, narrow walls can be created for broad channels.
A three-dimensional microstructurization, i.e., the formation of valleys of different width, as well as different depth, can be accomplished by working the body with laser light. Laser ablation does not require a development step, as with the photolithography or x-ray deep lithography method.
When the structurization is worked into the surface of a plastic body, the surface of the plastic body must be metallized before depositing the galvanic layer. The metallization can be done by high-vacuum coating.
Preferably, the body is a substrate provided with a metallic layer and a plastic cover layer, and the structurization is incorporated into the plastic layer such that the metal layer is exposed at the bottom of the valleys. The galvanic layer can then be deposited on the exposed metal layer in the valleys. After filling the valleys, the surface of the body is metallized and a galvanic cover layer is deposited onto the surface of the body. According to the first process variant, the galvanically deposited body obtained in this way is used as a shaping insert. According to the second process variant, the galvanically deposited body serves as the electrode in a spark erosion process for structurization of preferably a hard metal body. The body so struct
Ehrfeld Wolfgang
Michel Frank
Richter Thomas
Weber Lutz
Cuda Rosenbaum I
Hudak & Shunk Co. L.P.A.
Institut Fur Mikrotechnik Mainz GmbH
Nguyen Trinh
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