NO143714B - ROTOR FOR REGENERATIVE MOISTENCE RESP. HEAT EXCHANGES AND PROCEDURE FOR THIS MANUFACTURING - Google Patents

Description

The present invention relates to a rotor for a regenerative exchanger of moisture and preferably also heat between two gas (air) flows composed of thin layers, which are alternately flat and corrugated and form a number of parallel through channels for the two media, the rotor's corrugated and possibly also flat layers consisting of aluminum foils, the surfaces of which have a porous hygroscopic coating of aluminum hydroxide. The corrugated layers collide with the flat layers along corrugation ribs, whereby the channels are separated from each other laterally. The rotor usually has a cylindrical shape, whereby the channels run parallel to the axis of rotation and open out at the two flat sides of the rotor.

An important area of application for the regenerative exchanger is ventilators for the supply of fresh air to premises, as

and the exhaust air exchanges moisture and heat in the rotor, so that, for example, in winter the exhaust air emits heat and moisture to the supply air.

Each air stream thereby has inlets and outlets which are separated from each other and which are in connection with each zone of the rotor's side surfaces where the channels open out.

In the manufacture of such rotors, foils or layers of fibrous, non-combustible material such as asbestos paper or highly porous material such as ceramics have been used up to now. Layers of this type then serve as a carrier for a hygroscopic material, preferably a hygroscopic salt solution, where lithium chloride is the most commonly occurring material.

A rotor of the kind described above with highly porous foils or layers can have extremely good properties such as non-flammability, high moisture transfer capability and good mechanical strength. To achieve all these properties, however, a large number of work operations are required, which make these rotors relatively expensive and time-consuming to produce. Thus, one must impregnate the foil material with a plurality of means, in order to achieve the necessary mechanical strength, especially in a moist state, and this impregnation must take place after the foil material has been corrugated and wound up into a rotor shape. Furthermore, after these treatment steps, the rotor's boundary surfaces must be processed by e.g. grinding and milling in order to achieve the necessary flatness and dimensional accuracy.

By making the layers of aluminum foil, production costs can be significantly reduced, especially if you use it

manufacturing technique which is described in patent application 7605703-3. Such a rotor can be given perfectly acceptable fire properties and mechanical strength, but lacks an essential property for a moisture exchanger, namely hygroscopicity.

It is the purpose of the invention to give a rotor completely or partially made of aluminum foil excellent hygroscopic properties while maintaining a simplified manufacturing process and low material costs.

This is achieved according to the invention as stated in the claims.

According to a special, valuable embodiment of the invention, the rotor is produced from two webs or strips of aluminum foil, which can have a thickness of 0.03 - 0.1 mm. One web is corrugated to a wave height of 1 - 3 mm and joined by a suitable binder, e.g. as described in more detail in the aforementioned patent application, with the planar path, after which this so-called single wave is wound up into a cylindrical rotor of the desired size.

According to the invention, the aluminum foils are now treated with a water solution of alkali aluminate in the form of potassium sodium or lithium aluminate or a mixture of these. The treatment or impregnation with the solution takes place in a first step by dipping the ready-made rotor in a bath in a 15 - 20% alumina solution or pouring it heavily with this solution. The concentration is preferably 16 - 17% aluminate in the solution. The bath has room temperature or something below this, such as 18° and the time is relatively short, e.g. 3 min. During this step, the rotor's channels must be at least substantially filled with the alumina top solution. During the treatment step, the foil rafts are etched so that a porous coating can then be stuck to them.

This coating is formed in a subsequent treatment step after the rotor has been lifted from the impregnation bath so that most of the solution has left the channels. However, a film or film of the solution must remain on the surfaces of the rotor, which preferably happens by bringing the channels to a horizontal position immediately after emptying. The liquid thus supplied from the impregnation bath in the rotor's channels is now converted while generating heat as the foil is exposed to a strong increase in temperature. Thereby, aluminum contained in the liquid is precipitated and adheres as a hygroscopic coating of mainly hydroxides on the surfaces of the foils and at the same time hydrogen is released. The reaction takes place for a longer time than the dipping, preferably as long as water remains in the channels. After the reaction has ended, the rotor is rinsed clean of water-soluble residual products. Before rinsing, it is advantageous to slowly cool the rotor to prevent breakage of the contact surface between the coating and the foil carrier due to thermal stresses. The coating is further strengthened by allowing the rotor's foils to age for a period of e.g. two days in a humid atmosphere. This results in an increase in the grain size in the coating adhering to the foils and the risk of destruction is further reduced.

In the first treatment step, as mentioned, an etching of the foil surface is provided to give it the opportunity to form a secure attachment for the porous coating of aluminum hydroxides. The influence of the aluminum foil itself is therefore very insignificant, while the second treatment step gives a porous increase of 10 - 20 on each side of the foil. The weight increase by e.g. 50 f*™ thick foil can be approx. 10%. The main components of the coating are made up of a number of different aluminum hydroxides which are thus taken from the impregnation solution and thus not by conversion of the aluminum foil itself.

Preferably, the treatment according to the invention is carried out after the rotor has been wound. Namely, it has been shown that there is an effect of value for the strength of the rotor by this method in such a way that a strengthening of the adhesive joint between the flat and the corrugated webs takes place. A form of bridging between the porous layers in the contact surfaces between the foils seems to take place.

The hygroscopic coating can be strengthened by repeated dipping in aluminate solution in the first treatment step. Furthermore, a finely divided or pulverized solid adsorbent, such as silica gel, can be added to the aluminate solution in one of the dipping steps. Such a powder thereby adheres surprisingly well to the foil surface, without its sorption properties being significantly blocked.

The hygroscopic properties achieved in the aforementioned manner are based on adsorption and have been shown to provide moisture transfer properties comparable to those obtained in rotors made from the highly porous foil materials mentioned in the introduction of asbestos or ceramics with lithium chloride as hygroscopic substance.

In many application cases in connection with ventilation is

where there is a risk that pollutants in the exhaust air from e.g. fat or oil coats the surfaces with a thin film, which can partially or completely prevent the diffusion of water to and from the hygroscopic coating, if this consists of a solid sorbent. Its fine pores and capillaries, in which the water condenses, can then be easily blocked and inactivated. If, on the other hand, the hygroscopic agent consists of a salt solution, the entire wetted surface becomes active, while the liquid tends to penetrate and break through the contaminated membrane.

In such operating cases, a hygroscopicity, achieved by a salt solution, is preferable.

However, an untreated aluminum foil cannot retain a sufficient amount of salt solution on its surface. It now turns out that the porous layer obtained by treating the aluminum foil with alkali metal aluminate means that it is able to retain a larger amount of salt solution than an untreated surface. The aluminate treatment also provides attachment for other coatings that further increase the porosity. Such has already been described, where mixing in a powder by dipping in a sodium aluminate solution provides an extra coating. A foil surface treated in this way has a substantially enhanced ability to absorb water. You can also dip the aluminate-treated rotor in a glass of water and then expose it to carbon dioxide. This results in an extra coating of chemically precipitated silicon dioxide, which also

increases porosity.

A question of vital importance, when using a salt solution as a hygroscopic substance, is the extent to which corrosion of the aluminum foil can take place. Extensive tests have shown that lithium chloride cannot be used; the corrosion attacks become far too rapid.

It has been shown that another lithium salt, lithium nitrate, gives practically no corrosion attack on the aluminate-treated aluminum foil, at the same time that the lithium nitrate gives the foil extremely good hygroscopic properties within the spectrum of relative humidities, which is relevant for simultaneous humidity and temperature exchange, i.e. at relative humidities above 10 - 20%.

Calcium bromide and sodium chloride also cause significantly lower corrosion attacks than lithium chloride on the aluminate-treated surface, although they are not as low as with lithium nitrate. From a hygroscopic point of view, calcium bromide in particular gives good results.

The means and the technique described above to provide advertising or absorbent properties for the rotor, are cheap and allow the manufacturing process described in patent application 7605703-3 to be retained in its main features. All in all, this leads to manufacturing costs that are significantly less than what has hitherto had to be reckoned with in the case of previously highly efficient moisture-transferring rotors.

The term aluminum foil also includes a layer, which consists of

a carrier of non-metallic material, e.g. fibers of glass or cellulose or foil of plastic, which is provided with an overcoat or coating of aluminium.

Claims (4)

1. Rotor for a regenerative exchanger of moisture and preferably also heat between two gas (air) streams, composed of thin layers which are alternately flat and corrugated, and which form parallel, continuous channels for the two streams, the rotor's corrugated and possibly also its flat layer consists of aluminum foils, the surfaces of which have a porous, hygroscopic coating of aluminum hydroxide, characterized in that the porous coating is formed by precipitation during the development of heat from an alkali-aluminate solution containing at least 15% by weight of alkali aluminate which is applied to the surfaces. 2. Method for producing a rotor for a regenerative exchanger of moisture and preferably also heat between two gas (air) streams, which rotor is composed of thin layers which are alternately flat and corrugated, and which form, parallel, continuous channels for the two streams, the corrugated and possibly also the flat layers being made of aluminum foils, where the surfaces of said foils are given a porous, hygroscopic coating of aluminum hydroxide by treatment with an aqueous alkaline solution, characterized in that the rotor is treated in one or more stages with an alkali aluminate solution containing at least 15% by weight of alkali aluminate so that a film of the solution is left on the aforementioned surfaces, as precipitation of the porous coating occurs during heat generation and while the rotor's channels are kept horizontal. 3. Method as stated in claim 2, characterized in that an additional coating of an inorganic hygroscopic powder is provided in one of the treatment steps by mixing such a powder into the aluminate solution. 4. Method as stated in claim 3, characterized in that the powder consists of a solid adsorbent, such as silica gel.