GB1581262A

GB1581262A - Rotor for regenerative moisture and/or heat exchanger

Info

Publication number: GB1581262A
Application number: GB29639/77A
Authority: GB
Original assignee: Carl Munters AB
Current assignee: Carl Munters AB
Priority date: 1976-07-21
Filing date: 1977-07-14
Publication date: 1980-12-10
Also published as: SE407455B; NO143714B; NO772595L; CS198261B2; FR2358913A1; FR2358913B1; NO143714C; JPS6127680B2; DE2732989A1; SE7608329L; NO143714L; CA1116163A; DE2732989C2; JPS5313252A

Description

(54) ROTOR FOR REGENERATIVE MOISTURE AND/OR HEAT EXCHANGER (71) We, AKTIEBOLAGET CARL MUNTERS, a Swedish Company, of Industrivagen 2, S-191 47 Sollentuna, Sweden, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a rotor for a regenerative exchanger for the transfer of moisture and preferably also heat between two gas streams, particularly air streams.

An important area of application for a regenerative exchanger is in supplying fresh air to premises; the incoming fresh air and the outgoing exhaust air exchanging moisture and heat in a rotor, so that, for example, in the winter the outgoing exhaust air provides the incoming fresh air with heat and moisture. Each stream of air is provided with its own individual inlet and outlet which is separated from the other stream of air so that the respective streams of air are directed to individual zones of the rotor. The rotor consists of a plurality of sheets which are alternately flat and corrugated and which form a network of continuous, parallel channels for both streams of air. The corrugated sheets support the flat sheets along the ridges of the corrugations which results in the channels being separated laterally. The rotor is normally cylindrical, the channels running parallel with the axis of rotation.

Previously such rotors were manufactured of foils or sheets of fibrous, incombustible material, such as asbestos paper, or material made highly porous in another way, such as earthenware. Sheets of this type of material then act as carriers for a hygroscopic substance, preferably a hygroscopic salt solution, lithium chloride being the most usual substance used.

A rotor of the type described above incorporating highly porous foils or sheets can possess exceptional characteristics, e.g.

incombustibility, a high capacity for moisture transfer and good mechanical strength.

However, a large number of operations are required to provide a rotor with these properties, and this results in the rotor being relatively .expensive and time-consuming to manufacture. The material from which the sheets are constructed has to be impregnated with several substances in order to give the mechanical strength required, particularly when wet, and this impregnation has to be performed after the sheeting has been corrugated and fashioned into the form of a rotor.

Moreover the rotor's peripheral surfaces have to be finished once these manufacturing stages have been completed; grinding and milling being required to achieve the requisite flatness and accuracy.

If the sheets are made of aluminium foil manufacturing costs can be reduced considerably, particularly if the manufacturing technique described in our copending application no. 19568/77 (Serial No. 1570734 is used. Such a rotor can be given fireproof properties and mechanical strength that are completely acceptable but it lacks one property that is important in a moisture exchange - hygroscopicity.

An object of this invention is to produce a rotor which is totally or partially made of aluminium foil and which possesses excellent hygroscopic properties while keeping the manufacturing process simple and the costs of material low.

According to the present invention there is provided a rotor for a regenerative exchanger for the transfer of moisture and/or heat, between two gas streams, the rotor comprising layers of aluminium foil, as hereinafter defined, which layers are alternately flat and corrugated to form a plurality of continuous channels for said gas streams, the surface of the foils having a porous hygroscopic coating of aluminium hydroxide obtained by precipitation from a solution of an alkali aluminate applied to the surfaces.

The invention also includes a method of manufacturing a rotor for a regenerative exchanger for the transfer of moisture, and/or heat between two gas streams composed of layers of aluminium foil, as hereinafter defined, which layers are alternately flat and corrugated to form a plurality of continuous channels for said gas streams the method comprising treating the foils with an aluminate solution so that the surfaces thereof are given a porous hygroscopic structure.

Preferably the rotor is manufactured using a method in which two strips of aluminium foil, which can be 0.03 - 0.1 mm thick, are used. One strip is provided with corrugations of a height of 1 - 3 mm and is attached to the other strip, which is flat, using a simple adhesive, e.g. the one described in detail in the copending patent application mentioned above, the strips then being formed into a cylindrical rotor of the required size by winding.

The aluminium foils are treated with an aqueous solution of an alkali aluminate which may be potassium, sodium or lithium aluminate or a mixture of these. Preferably the treatment with the alkali aluminate solution consists, either of immersing the completed rotor into a bath of 15 - 20% aluminate solution, or such a solution is applied to the rotor in a sufficiently large quantity to effect the treatment e.g. by pouring the solution over the rotor.

The concentration is preferably 16 - 17% aluminate in the solution and, when effected in a bath, the bath is suitably at room temperature or somewhat lower such as 18 and the treatment time is relatively short e.g. 3 minutes. In this way the channels of the rotor are at least partially filled by the aluminate solution so that the surfaces of the foils are being etched so that a hygroscopic coating later can adhere to it.

This hygroscopic coating is created in subsequent treatment step as follows.

The rotor is lifted from the impregnation bath or the supply of solution stopped so that substantially all the solution leaves the channels excepts for a film or a skin of the solution which remains on the surfaces of the rotor.

This film is more easily retained by turning the rotor immediately after emptying of the solution so that the channels are brought to a horizontal position. The liquid in the channels is then converted generating heat and subjecting the foils of the rotor to a substantial rise in temperature so that aluminium being a part of the retained treatment liquid, will be deposited on the surfaces and adheres as a hygroscopic coating consisting mainly of hydroxides and at the same time hydrogen gas will escape. This subsequent step continues for longer than the immersion step, preferably as long as water remains in the channels. When the reaction has ceased the rotor is washed to remove unwanted water soluble residuals. However, before the washing it is advantageous to slowly cool the rotor in order to prevent separation of the coating and from the foil carriers as a result of thermal stresses. The coating may be further strengthened by allowing the foils of the rotor time to age in a moist atmosphere e.g.

during a time of two days (48 hours). In this way an increase of the grain size in the coating adhering to the foils is obtained and the risk of decomposition is further minimized.

As stated above, in the first treatment step an etching of the surface of the foil is accomplished to give the same a possibility to form a secure seat for the porous hygroscopic coating of aluminium hydroxide. However, whilst the effect on the thickness of aluminium foil itself is insignificant in the first step, the second treatment step provides the addition of a porous hygroscopic coating of about 10 - 20 ,um on each side of the foil.

The increase of weight on a foil with a thickness of e.g. 50 ,um may be about 10%. The main components of the coating are aluminium hydroxides which are taken from the impregnation solution and not by any conversion of the aluminium foil itself.

The treatment of the invention should preferably be carried out after the rotor has been formed. This method has proved to have an effect that is of value for the strength of the rotor. It appears that some type of bridging takes place between the porous coatings at the contact surfaces between adjacent foil layers and this results in a strengthening of the joints between the corrugated sheets.

The hygroscopic coating can be concentrated by repeated immersion in the aluminate solution in the first treatment step. In addition a crushed or pulverised solid adsorption medium, e.g. silica gel, can be introduced into the aluminate solution at some stage of the immersion process. Such a powder attaches itself surprisingly well to the surface of the foil without reducing its sorption properties to any great extent.

The hygroscopic properties obtained using the method described above are based on adsorption and have proved to result in moisture transfer properties comparable with those obtained in rotors manufactured using highly porous foil made of asbestos or earthenware with lithium chloride as the hygroscopic substance, as mentioned in the opening section.

In many cases, where the rotor is used in ventilation apparatus there is a risk that pollution in the exhaust air, e.g. fats or oils, can cover the surfaces with a thin film which can restrict the diffusion of the moisture to and from the hygroscopic coating, if this consists of a solid adsorption medium. It is easy for its fine pores and capillaries, in which the moisture condenses, to become blocked and put out of action. If, on the other hand, the hygroscopic substance consists of a salt solution, the entire wet surface becomes active while the liquid tends to seep in and break through the film of contamination.

In similar operating solutions a hygros copic capacity provided by a salt solution is preferable.

However, untreated aluminium foil does not have the capability of retaining a suffi cient quantity of the salt solution on its surface. It has now been shown that the porous coating obtained by the treatment of aluminium foil with alkali aluminate is capable of retaining a greater quantity of salt solution than an untreated surface.

Moreover, treatment with aluminate provides a base onto which other coats that will further increase the porosity can be bonded.

A description has already been given of such a coating where the addition of a powder to, for example sodium aluminate, during the immersion process provided an additional coating. The water absorbency capability of foil surfaces can be considerably increased by this method of treatment. The rotor, once treated with aluminate, can also be immersed in water-glass and then exposed to carbon dioxide. This gives an additional coating of chemically precipitated silicon dioxide which also increases porosity.

One vital question when using a salt solution as the hygroscopic substance is the degree of corrosion that can occur. Extensive tests have shown that lithium chloride cannot be used; corrosion attacks far too rapidly.

However lithium chloride can be used in solution with an inhibitor such as lithium hydroxide which makes the salt solution less corrosive.

Another lithium salt, lithium nitrate, has proved to result in, practically speaking, no corrosion of aluminium foil treated with aluminate. At the same time lithium nitrate gives the foil extrememly good hygroscopic properties within the relative moisture range for the simultaneous moisture and tempera ture exchange in question here, i.e. a relative moisture in excess of 10 - 20%. Calcium bromide and sodium chloride have also proved to be considerably less corrosive than lithium chloride on surfaces treated with aluminate, although they are not so benign as lithium nitrate. Calcium bromide has shown itself to by hygroscopically particularly suit able.

The substances and techniques described above which are intended to provide the rotor with adsorbing and absorbing proper ties are inexpensive and of such a nature that the principles of the manufacturing method described in the copening patent application 19568/77 (Serial No. 1570734) can be retained. Together these result in manufacturing costs which considerably undercut the costs which were necessary to manufacture high-efficiency transfer rotors in the past.

In this specification the term "aluminium foil" is to be construed broadly as including sheets consisting of a carrier of non-metallic material such as fibers of glass or cellulose or plastic foils which are provided with a layer or coating of aluminium.

WHAT WE CLAIM IS: 1. A rotor for a regenerative exchanger for the transfer of moisture and/or heat, between two gas streams, the rotor comprising layers of aluminium foil, as hereinbefore defined, which layers are alternately flat and corrugated to form a plurality of continuous channels for said gas streams, the surface of the foils having a porous hygroscopic coating of aluminium hydroxide obtained by precipitation from a solution of an alkali aluminate applied to the surfaces.

2. A method of manufacturing a rotor for a regenerative exchanger for the transfer of moisture, and/or heat between two gas streams composed of layers of aluminium foil, as hereinbefore defined, which layers are alternately flat and corrugated to form a plurality of continuous channels for said gas streams the method comprising treating the foils with an aluminate solution so that the surfaces thereof are given a porous hygroscopic structure.

3. A method according to Claim 2 wherein the aluminate solution is sodium aluminate.

4. A method according to Claim 2 or 3 wherein the aluminate treatment is effected after the rotor has been formed.

5. A method according to Claim 2, 3 or 4 wherein an additional coating of an inorganic powder is effected by admixing such a powder to the aluminate solution.

6. A method according to Claim 5 wherein the powder consists of a solid adsorption medium, such as silica gel.

7. A method according to any one of the Claims 2 to 6 wherein to the surface layer is added a hygroscopic substance which is a solution of lithium nitrate.

8. A method according to any one of the Claims 2 to 6 wherein to the surface is added a hygroscopic substance being a salt solution such as lithium chloride and an inhibitor making the salt solution less corrosive, such as lithium hydroxide.

9. A method according to Claim 2 wherein the channels of the rotor are entirely or practically entirely filled with an aluminate solution during a first, short period of time and that the aluminium surface of the rotor thereafter is held in contact with only a film of the solution during a second, longer period of time.

10. A method of manufacturing a rotor

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Claims

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fine pores and capillaries, in which the moisture condenses, to become blocked and put out of action. If, on the other hand, the hygroscopic substance consists of a salt solution, the entire wet surface becomes active while the liquid tends to seep in and break through the film of contamination.

In similar operating solutions a hygros copic capacity provided by a salt solution is preferable.

However, untreated aluminium foil does not have the capability of retaining a suffi cient quantity of the salt solution on its surface. It has now been shown that the porous coating obtained by the treatment of aluminium foil with alkali aluminate is capable of retaining a greater quantity of salt solution than an untreated surface.

Moreover, treatment with aluminate provides a base onto which other coats that will further increase the porosity can be bonded.

A description has already been given of such a coating where the addition of a powder to, for example sodium aluminate, during the immersion process provided an additional coating. The water absorbency capability of foil surfaces can be considerably increased by this method of treatment. The rotor, once treated with aluminate, can also be immersed in water-glass and then exposed to carbon dioxide. This gives an additional coating of chemically precipitated silicon dioxide which also increases porosity.

One vital question when using a salt solution as the hygroscopic substance is the degree of corrosion that can occur. Extensive tests have shown that lithium chloride cannot be used; corrosion attacks far too rapidly.

However lithium chloride can be used in solution with an inhibitor such as lithium hydroxide which makes the salt solution less corrosive.

Another lithium salt, lithium nitrate, has proved to result in, practically speaking, no corrosion of aluminium foil treated with aluminate. At the same time lithium nitrate gives the foil extrememly good hygroscopic properties within the relative moisture range for the simultaneous moisture and tempera ture exchange in question here, i.e. a relative moisture in excess of 10 - 20%. Calcium bromide and sodium chloride have also proved to be considerably less corrosive than lithium chloride on surfaces treated with aluminate, although they are not so benign as lithium nitrate. Calcium bromide has shown itself to by hygroscopically particularly suit able.

The substances and techniques described above which are intended to provide the rotor with adsorbing and absorbing proper ties are inexpensive and of such a nature that the principles of the manufacturing method described in the copening patent application

19568/77 (Serial No. 1570734) can be retained. Together these result in manufacturing costs which considerably undercut the costs which were necessary to manufacture high-efficiency transfer rotors in the past.

In this specification the term "aluminium foil" is to be construed broadly as including sheets consisting of a carrier of non-metallic material such as fibers of glass or cellulose or plastic foils which are provided with a layer or coating of aluminium.

WHAT WE CLAIM IS: 1. A rotor for a regenerative exchanger for the transfer of moisture and/or heat, between two gas streams, the rotor comprising layers of aluminium foil, as hereinbefore defined, which layers are alternately flat and corrugated to form a plurality of continuous channels for said gas streams, the surface of the foils having a porous hygroscopic coating of aluminium hydroxide obtained by precipitation from a solution of an alkali aluminate applied to the surfaces.
2. A method of manufacturing a rotor for a regenerative exchanger for the transfer of moisture, and/or heat between two gas streams composed of layers of aluminium foil, as hereinbefore defined, which layers are alternately flat and corrugated to form a plurality of continuous channels for said gas streams the method comprising treating the foils with an aluminate solution so that the surfaces thereof are given a porous hygroscopic structure.
3. A method according to Claim 2 wherein the aluminate solution is sodium aluminate.
4. A method according to Claim 2 or 3 wherein the aluminate treatment is effected after the rotor has been formed.
5. A method according to Claim 2, 3 or 4 wherein an additional coating of an inorganic powder is effected by admixing such a powder to the aluminate solution.
6. A method according to Claim 5 wherein the powder consists of a solid adsorption medium, such as silica gel.
7. A method according to any one of the Claims 2 to 6 wherein to the surface layer is added a hygroscopic substance which is a solution of lithium nitrate.
8. A method according to any one of the Claims 2 to 6 wherein to the surface is added a hygroscopic substance being a salt solution such as lithium chloride and an inhibitor making the salt solution less corrosive, such as lithium hydroxide.
9. A method according to Claim 2 wherein the channels of the rotor are entirely or practically entirely filled with an aluminate solution during a first, short period of time and that the aluminium surface of the rotor thereafter is held in contact with only a film of the solution during a second, longer period of time.
10. A method of manufacturing a rotor

for a regenerative exchanger according to Claim 2 and substantially as hereinbefore described.
11. A rotor made in accordance with the method of any one of Claims 2 to 10.