Method Of Fabricating A Heat Exchanger Having Two Separate Passageways Therein

Abstract

A method of manufacturing a heat exchanger for exchanging heat between helium flows in a temperature range below 2K, comprising two end plates having inlet and outlet apertures, and a stack of very thin foils arranged between the end plates with passageways that extend transverse to the planes of the foils and separate ducts in the planes of the foils which communicate with the passageways. During manufacture the foils are attached with supports provided between foils, which supports are removed by means of a rinsing liquid after the foils have been soldered to each other.

Description

BACKGROUND OF THE INVENTION

The invention relates to a method of manufacturing a heat exchanger which is suitable for the exchange of heat between two helium flows in the temperature range below 2K. Two end plates are provided with an inlet aperture for one of the flows and an outlet aperture for the other helium flow. A stack of further plates are arranged between said end plates, the stack comprising four through ducts which extend transverse to the plate direction and which communicate two by two with the apertures in the end plates and which further bound a number of separated parallel ducts which extend mainly in the plate direction and which alternately communicate with two of the through ducts.

A heat exchanger of the kind set forth is known from German Offenlegungsschrift 1,501,669. This known heat exchanger is intended for cooling highly viscous media such as curds. Consequently, the heat exchanger is of substantial proportions and its construction is very robust.

The heat exchanger relating to the method according to the present invention is intended for exchanging heat between two helium flows at temperatures below 2K. At these very low temperatures, the exchange of heat is effected mainly by the exchange of phonons (quantized atom vibrations). Because the acoustic velocities in the helium and in the metal wall differ considerably, the refractive index for phonons in helium is much larger than that in the metal wall. For the passage of phonons from helium to the wall, this gives rise to substantial reflection. The heat exchange between helium and the metal wall is very poor due to the occurrence of this surface resistance. In order to obtain adequate heat exchange at these low temperatures, use was made thus far of heat exchangers having very large surfaces. For example, heat exchangers are known for this purpose which consist of a copper block containing two chambers which are filled with copper balls which are sintered to each other. These chambers are connected in the helium flows which have to exchange heat.

These heat exchangers are not very satisfactory in view of their large dimensions which give rise to long refrigeration times of the refrigerators in which they are used.

SUMMARY OF THE NEW INVENTION

The method according to the invention has for its object to provide a heat exchanger offering a substantially better exchange of heat, extremely low flow resistance and small volume. To this end, the method is characterized in that the stack of plates is formed by a number of very thin metal foils which are proportioned such that the thickness of the separating walls between the ducts is 2-10 .mu., the thickness dimension of the ducts being between 40.mu. and 100.mu.. A support having the same thickness dimension as the parallel ducts is provided during the stacking of the foils, at least at the area where each of the parallel ducts communicates with a through duct, this support being made of a metal which can be selectively removed at a later stage by means of a rinsing liquid which does not attack the foils. Subsequently the end plates are firmly pressed against the stack of foils and the assembly is subjected to a thermal treatment during which the foils are connected to each other by soldering or diffusion. Then the heat exchanger thus formed is rinsed with the said rinsing liquid and the supports are removed.

In a heat exchanger manufactured by the method according to the invention, use is made of foils which have a very small thickness but which are nevertheless vacuum tight. With the very small thickness dimension of the separating wall (2-10.mu.), the phonons can now readily penetrate through this separating wall as a result of two effects, i.e., the phonon tunnel effect and the free-path length effect. Thanks to these effects a striking improvement of the exchanging of heat is achieved. Furthermore, the heat exchanger thus obtained has a small volume and a low flow resistance.

A difficulty encountered in the manufacture is the vacuum-tight connection of the foils. This connection can be obtained by soldering, in which case the foils must be provided with a layer of solder, or by diffusion. In both cases the foils must be firmly pressed against each other. At the areas where the parallel ducts communicate with the passage, this pressing against each other cannot be adequately realized and at these areas leaks occur because of insufficient pressing.

This problem is eliminated in the invention by providing a support, in at least these areas, which ensures the transfer of force. This support is made of a metal which can be etched away at a later stage by means of a rinsing liquid, without the foils being attacked. In this manner very good adhesion of the foils is achieved.

However, if the foils are soldered to each other, sometimes a problem arises in that the solder flows along the support, so that clogging occurs with the result that the rinsing liquid cannot flow through.

In order to eliminate this problem, in a further preferred embodiment of the invention, each of the supports to be provided is formed by a frame which can consist of a number of parts and which extends along the entire inner edge of the relevant parallel duct.

It was found that such a frame completely stops the flowing of the soldering material into the ducts, so that clogging no longer occurs.

In a further preferred embodiment of the method according to the invention, each of the foils is formed by growing a layer having a thickness of less than 5 .mu. on a suitable substrate in an electrochemical manner. The areas for the through ducts first are covered with a lacquer, after which a further layer of lacquer is provided on the areas where the parallel duct extends and the through channels extend further, after which further growing of the foils up to a thickness of between 40 .mu. and 100 .mu. takes place.

The invention will be described in detail hereinafter with reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exposed view of a heat exchanger.

FIG. 2 shows the heat exchanger of FIG. 1 in the operating condition.

FIGS. 3 and 4 show how supports can be used for assembly.

FIGS. 5 and 6 diagrammatically illustrate how a foil can be made.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 diagrammatically shows the construction of a heat exchanger. It comprises two end pieces 1 and 2, each of which is provided with an inlet aperture, 3 and 4, respectively, for a helium flow forward and rearward and with an outlet aperture, 5 and 6, respectively. Arranged between the end plates 1 and 2 is a stack 7 of copper foils. These foils are divided into closed foils 8 having a thickness of approximately 2 .mu., and foils 9 having a rectangular aperture. By means of apertures 10, 11, 12 and 13, four through ducts or passageways are obtained which communicate with the apertures 3, 4, 5 and 6, respectively, in the end plates defining forward flow via inlet aperture 3, passageways 10 and 13, ducts 14, and outlet aperture 6; and rearward flow via inlet aperture 4, passageways 11 and 12, ducts 15, and outlet aperture 5

The rectangular apertures in the foils 9 form parallel ducts 14 and 15, the ducts 14 communicating with the passageways 10 and 13, and the ducts 15 with the passageways 11 and 14. In this manner a heat exchanger configuration is obtained comprising a number of parallel ducts for the one helium flow and a number of parallel ducts for the other helium flow.

Two methods can be used to obtain adhesion of the end plates to the foils, i.e., soldering and diffusion.

In both cases all foils must be forcibly pressed against each other. This can be effected over the entire circumference with the exception of the areas 16 because there three foil parts are situated which are bound by a duct part. Consequently, at these areas no forces can be transferred and leakage is liable to occur.

In order to prevent such leakage, supports 20 can be provided at the areas 18 as shown in FIG. 3 in the case of diffusion of the foils. It is thus achieved that the foils are properly pressed together also at the areas 16. During the relevant thermal treatment, the foils will be properly diffused together also at these areas.

If the foils are soldered to each other, in which case the foils 9 are provided with a layer of soldering material in advance, it is more advantageous to construct the supports in the form of a frame 22, as shown in FIG. 4, which extends along the entire inner edge of the relevant parallel duct. This frame offers the important advantage that the flowing of soldering material into the duct space is completely counteracted, so that these ducts cannot become clogged. So as to ensure proper transfer of forces at the areas 18, the frame 22 has such a thickened portion 23 at these areas that only a narrow gap remains.

The supports as well as the supporting frames are made, for example, of soft steel or aluminium, respectively, i.e., materials which can be removed by means of an acid or a lye after completion of diffusion or soldering, respectively. To this end, it is important that the acid or the lye can be rinsed through, so the supports and supporting frames may not completely block the passage.

After completion of these operations, a heat exchanger as shown in FIG. 2 is obtained. This heat exchanger excels by its favourable heat exchanging properties, small volume and low flow resistance. Furthermore, the use of the method according to the invention ensures a very reliable and leak-free heat exchanger.

The number of parallel ducts in which the helium flows exchange heat can be arbitrarily chosen, depending on the circulating helium flows and the relevant temperature range. In many cases 100 or more ducts will be used.

As was already stated in the preamble, the use of very thin separating walls between the ducts and the resultant phonon tunnel effect and the free-path length effect ensure a very favourable exchanging of heat with low flow resistance and small volume.

In the stack of the heat exchanger shown in FIG. 2, two kinds of foils are used, i.e., closed foils having a thickness of approximately 2 .mu. and foils having an aperture and a thickness of approximately 40 .mu.. It was found that foils can be manufactured in which these two are combined. The manufacture thereof is diagrammatically shown in FIGS. 5 and 6.

On a brass plate 30 layers of lacquer are first provided at the areas 31, 32, 33 and 34. Subsequently, a copper layer 35 is electrochemically grown to a thickness of approximately 2 .mu.. After that, a lacquer layer 36 is provided ant the copper foil is grown further to approximately 40 .mu..

By means of these foils the number of soldered or diffused connections between the foils is reduced to one half.

Claims

What is claimed is:

1. A method of manufacturing a heat exchanger through which fluid can flow in forward and rearward directions comprising the steps of stacking alternately thin metal sheets of thickness in the range of 2-10.mu. and thick metal sheets of thickness in the range 40-100.mu., said thick sheets having apertures therein which with said adjacent thin sheets define ducts extending generally parallel to the planes of said sheets with height of the ducts equal to the thickness of said thick sheets, said ducts being forward flow ducts spaced alternately, and rearward flow ducts situated intermediate each two forward-flow ducts, said thick and thin sheets having further apertures that are aligned along axes transverse to said planes of the sheets to define at least two separate passageways for said forward and rearward flows respectively, said forward flow passageway and duct being in communication, and said rearward flow passageway and duct being in communication, placing in each of said ducts and between and contacting the adjacent thin sheets a support having thickness substantially the same as that of said thick sheet, each support being a material that is etchable by an etching solvent that will not dissolve said metal sheets, pressing said sheets together in the direction of said axes, heat treating said pressed stack of sheets to bond together said sheets by soldering, etching away said supports by rinsing same with said etching solvent.

2. A method of manufacturing a heat exchanger through which fluid can flow in forward and rearward directions comprising the steps of stacking alternately thin metal sheets of thickness in the range of 2-10.mu. and thick metal sheets of thickness in the range of 40-100.mu., said thick sheets having apertures therein which with said adjacent thin sheets define ducts extending generally parallel to the planes of said sheets with height of the ducts equal to the thickness of said thick sheets, said ducts being forward flow ducts spaced alternately, and rearward flow ducts situated intermediate each two forward-flow ducts, said thick and thin sheets having further apertures that are aligned along axes transverse to said planes of the sheets to define at least two separate passageways for said forward and rearward flows respectively, said forward flow passageway and duct being in communication, and said rearward flow passageway and duct being in communication, placing in each of said ducts and between and contacting the adjacent thin sheets a support having thickness substantially the same as that of said thick sheet, each support being a material that is etchable by an etching solvent that will not dissolve said metal sheets, pressing said sheets together in the direction of said axes, heat treating said pressed stack of sheets to bond together said sheets by diffusion, and etching away said supports by rinsing same with said etching solvent.

3. A method according to claim 1 wherein each of said supports comprises a frame extending along and spaced inward from the edge of said aperture that defines the corresponding duct.

4. A method according to claim 1, comprising the further steps of forming said thick and thin sheets of metal which have predetermined area by providing on a substrate a first layer of lacquer covering zones where said passageway apertures are to be located in a thin sheet, then electrochemically forming upon said substrate a first layer of copper to a maximum thickness of 5.mu. covering an area equal to said sheet area and defining a first thin sheet, then providing a second layer of lacquer on said first thin sheet in a zone where said duct aperture is to be located in a thick sheet, then electrochemically forming upon said first thin sheet a second layer of copper to a thickness in the range of 40 to 100.mu. thick defining a first thick sheet, repeating these steps alternatingly, until the desired number of thick and thin sheets are formed, and then dissolving all the lacquer layers.

5. A method according to claim 1 wherein said thin sheets have thickness of approximately 2.mu. and said thick sheets have thickness of approximately 40.mu..