This invention concerns a rotating energy recovery unit. More particularly, the invention concerns applying, onto the surface of a metal foil used for production of a rotating energy recovery unit, a desiccant structured to quickly absorb moisture and to quickly release moisture.

It is known to use energy recovery units in ventilating, heating and air conditioning of buildings. In the field, such systems are termed HVAC systems - Heating, Ventilation, Air Conditioning. In HVAC-systems, it is known to use rotating air-to-air energy recovery units. A rotating energy recovery unit comprises a rotating wheel, which will have a first portion being in contact with the incoming air supply stream of the HVAC- system, and a second portion being in contact with the exhaust air stream of the HVAC-system. The air flows through channels formed axially in the wheel. The wheel rotates at a velocity of ca. 20 rpm, and such that the first portion comes into contact with the exhaust air stream and the second portion with the incoming air supply stream. In this manner, energy is transmitted from the most energy-rich air flow to the air flow containing the least energy.

In the field, a distinction is made between latent energy and sensible energy. The latent energy of the air is comprised of the moisture content, and the sensible energy of the air is comprised of the temperature of the air. A rotating wheel comprised of a metal, such as aluminium, will be heated by the warmest air flow and will transmit this sensible energy to the coldest air flow. So-called rotating total enthalpy air-to-air energy recovery units transmit both sensible and latent energy. This is achieved by virtue of the surface in the channels of the wheel being coated with a desiccant. For example, this desiccant may be a dry film comprising a zeolite, a silica gel or activated alumina, or it may be an oxidized surface comprising, for example, an aluminium oxide.

Hereinafter, a rotating wheel in an energy recovery unit, where the wheel recovers sensible energy, will be termed a heat recovery wheel. A heat recovery wheel, which is provided with a desiccant for allowing latent energy to be recovered too, is termed a full energy recovery wheel (enthalpy recovery wheel/total energy recovery wheel).

When the desired indoor air temperature is higher than the outdoor air temperature, it is favourable, in terms of energy conservation, to use an energy recovery unit. A heat recovery wheel will heat the incoming air supply stream with energy taken from the exhaust air stream. This reduces the total heating requirement of the building. During heating of the incoming air supply stream, the relative moisture content of the incoming air supply stream will decrease. This air will therefore be perceived as dry, and even too dry, for persons in the building. Given that moisture is supplied to the air in the building, the absolute amount of moisture in the exhaust air stream flow will be larger than the absolute amount of moisture in the incoming air flow. Upon using a full energy recovery wheel, moisture may be transmitted from the outgoing air flow to the incoming air flow. The moisture content in the indoor air will therefore be perceived as more comfortable, and at least less dry.

When the desired indoor air temperature is lower than the outdoor air temperature, it is also favourable, in terms of energy conservation, to use an energy recovery unit. A heat recovery wheel will harvest heat energy from inflowing warm air and transmit it to the discharging, cooler air. This cools down the incoming air supply stream and reduces the load on the air conditioning installation of the building. Due to its relative high temperature, the incoming air supply stream may contain too much moisture. Upon using a full energy recovery wheel, the moisture may be transmitted from the incoming air flow to the outgoing air flow. This further reduces the load on the air conditioning installation of the building.

A rotating heat recovery wheel or full energy recovery wheel rotates at a velocity which may be 20 rpm. By so doing, a portion of the heat-/full energy recovery wheel is in contact with the first air flow for about 1.5 seconds pr. revolution and, similarly, in contact with the second air flow for about 1.5 seconds pr. revolution. A heat recovery wheel should therefore be comprised of a material having good thermal conductivity properties, such as e.g. aluminium. The desiccant in a full energy recovery wheel should be able to quickly absorb moisture and quickly release moisture.

A desiccant, such as activated alumina (Al ₂ 0 ₃ ), has a pore size distribution of between 8 and 70 angstroms, and silica gel has a pore size distribution of between 8 and 100 angstroms. Oxidized aluminium surfaces have an even larger pore size distribution. A desiccant, such a natural zeolites having the general formula Na ₂ 0 ^, Al ₂ 0 ₃ ' nSi0 ₂ ^, xH ₂ 0, may have a relatively large pore size distribution, whereas artificial zeolites may have a small pore size distribution and small pores, for example of 3 to 5 angstroms. Water molecules have a diameter of 2.8 angstroms.

A rotating heat recovery wheel or full energy recovery wheel may have a diameter of between 10 cm and 5 m. The axial extent of the wheels may be between 25 mm and 38 cm. The wheel may be manufactured from rolled aluminium foil being of 50 pm up to 100 pm thickness, for example of about 70 pm thickness. The wheel is manufactured by alternately placing a layer of plane aluminium foil and a layer of corrugated aluminium foil. In this manner, axial channels are formed extending from the one plane side of the wheels onto the other plane side of the wheel. Patent publication US 4769053 shows, in detail, how such a wheel may be constructed.

A full energy recovery wheel is manufactured by virtue of coating the aluminium foil with a desiccant before assembling the foil into a wheel. The foil to be corrugated must also be coated with the desiccant before corrugation. It is generally difficult to handle aluminium foil being thinner than 0.1 mm. It is particularly difficult to handle thin aluminium foil when being heated, owing to the fact that the yield strength or tensile strength of the aluminium decreases with increasing temperatures, and at a temperature of above 240 - 250 °C, preparation becomes very difficult. Coating of aluminium is carried out by virtue of conveying the aluminium through a bath so as to form a coating on both sides, or by applying the coating by means of rollers or by spraying onto the one side. After the application, the aluminium is conveyed through an oven for drying and curing of the coating. The dry coating may have a thickness of between 2 pm and 20 pm, for example a thickness of between 3 pm and 12 pm, and especially a thickness of between 3 pm and 5 pm.

A desiccant, for example a zeolite, silica gel or activated alumina, may be coated onto aluminium foil by means of a suitable binding agent. For example, suitable binding agents comprise polyurethane, nitrile phenols, water-based binders and alkyd-based resins, as described in patent publication US 4769053. The coating may be carried out in two steps, wherein the binding agent is applied at first in order to form an adhesive - surface, and wherein the desiccant is applied in a second step in, for example, a fluid- ized powder bath. By so doing, the binding agent is prevented from covering the pores in the desiccant and making it less effective. Patent publication US 5496397 sets out to solve this problem by adding a temporary pore-filling agent to the mixture of the binding agent and the desiccant, and wherein the pore-filling agent disappears from the pores in a subsequent drying process. At certain operational conditions, water vapour will condense and form water drops in the axial channels in the recovery wheel. Insofar as the cross-sectional area of each channel is small, for example having a height of 1.3 mm to 2.5 mm and having a largest width of 2.5 to 5 mm, the water drops will increase the hydraulic resistance for the air flow through the channels of the rotor. The water drops will be blown out of the channels by incoming air supply stream or exhaust air stream.

Incoming air supply stream and exhaust air stream through the recovery wheel contains dust particles that may constitute nourishment to micro-organisms, for example bacteria and fungi. In half the time of one revolution, a desiccant will contain moisture which may be available to micro-organisms. A basis for microbial proliferation may therefore exist in a recovery wheel. Patent publication US7279543 teaches about a method of applying an antibacterial polymer, for example one that comprises diami- nobenzoic acid, onto an object by means of a vapour deposition- or polymerization technique under low pressure. The object may have a complicated shape, for example surfaces in a heat exchanger. Patent publication KR 1020070099806 teaches about a coating comprising an antibacterial substance taken from fermented lactic acid bacterial products. The coating is used in heat exchangers in ventilation systems. Patent publication KR 20060135336 teaches to coat a fan in an air conditioning installation with an antibacterial coating. The antibacterial coating comprises nano-particles.

Practice goes to show that known application methods for a coating comprising desic- cants may provide inferior adhesion to the metal. Corrugation of the aluminium foil after application of the coating may cause the coating to become undone, especially where angles or comers are formed in the material. Practice also goes to show that, over time, the coating becomes undone from the substratum. In particular, it may become undone if the rotor must be cleaned. Such cleaning may be carried out through vacuum-cleaning, flushing with pressurized soapy water, or with compressed air.

The object of the invention is to remedy or to reduce at least one of the disadvantages of the prior art, or at least to provide a useful alternative to the prior art.

The object is achieved by virtue of features disclosed in the following description and in the subsequent claims.

In a first aspect, the invention concerns a metal foil structured in a manner allowing it to be used in a full energy recovery wheel, wherein the metal foil is provided with a coating, and the coating comprises a desiccant. The desiccant is comprised of a silane- based agent.

The metal foil may be comprised of aluminium. The silane-based agent may be comprised of a metal surface pre-treatment agent. The coating may further comprise a bioactive component. The bioactive component may comprise a biocide as an active working agent. The bioactive component may comprise an active working agent which impedes biological activity, a so-called bacteriostat, fungistat or algistat. The bioactive component may comprise at least one active working agent chosen from a group comprised of a bacteriostat, a bactericide, a fungistat, a fungicide, an algistat and an algicide. The coating may further comprise a pH-regulator.

The invention further concerns a full energy recovery wheel comprising a plane metal foil and a corrugated metal foil. The plane metal foil and the corrugated metal foil may be provided with a coating comprising a desiccant, wherein the desiccant may be comprised of a silane-based agent. The plane metal foil and the corrugated metal foil of the full energy recovery wheel may be comprised of aluminium.

In a second aspect, the invention concerns a method for applying a coating onto a degreased and washed metal foil in a bath, wherein the thickness of the coating is determined by means of wiping rollers, and wherein the applied coating is dried by virtue of conveying the coated metal foil through an oven. The bath comprises a silane-based agent. The silane-based agent may be comprised of a metal surface pre- treatment agent. The bath may further comprise a bioactive component. The bioactive component may comprise at least one active working agent chosen from a group comprised of a bacteriostat, a bactericide, a fungistat, a fungicide, an algistat and an algicide.

In a third aspect, the invention concerns use of a silane-based metal surface pre- treatment agent as a desiccant in a coating on a metal foil used in a full energy recovery wheel.

Hereinafter, an example of a preferred embodiment is described. Example 1

A 25 cm wide and 50 pm thick pre-treated aluminium foil was spooled onto a spool having initial spool diameter of 15 cm. The pre-treatment comprised degreasing the aluminium foil and rinsing it with demineralised water. The foil was fed from the spool onto and through a bath for application of a coating onto both sides of the foil. The bath was comprised of a silane-based metal surface pre-treatment agent. Oxsilan® MM-0705 from Chemetall GmbH diluted with demineralised water, in accordance with the direction of the manufacturer, was used as a silane-based metal surface pre- treatment agent. The pH-value of the bath was checked regularly with a pH-meter. According to the direction of the manufacturer, the pH-value of the bath is to be between pH 4.5 and 6.5, and the pH may be lowered by adding Gardobond-Additive H7215, and the pH may be increased by adding Gardobond-Additive H7202 from Chemetall GmbH.

The foil having a velocity of up to 30 m/min was conveyed from the bath and through wiping rollers of a type known per se and adjusted so as to leave the applied coating 4 pm thick on both sides of the foil. Then the foil was conveyed through an electric IR heater and was heated to approximately 200 °C. Thereafter the foil was conveyed through a precooler, conveyed across a water-cooled cylinder and spooled onto a receiving spool.

The applied metal surface pre-treatment agent exhibited good adhesion to the aluminium foil and did not become undone during corrugation of the foil.

Example 2

The same type of foil as described in example 1 was pre-treated, applied a coating, dried and spooled in the same manner as described in example 1.

The bath was comprised of Oxsilan® MM-0705 from Chemetall GmbH diluted with de- mineralised water in accordance with the direction of the manufacturer. Parmetol DF 19 Forte from Schulke & Mayr GmbH was admixed to the bath. Parmetol comprised 1 % of the user solution in the bath. Parmetol contains two bioactive components, diuron, CAS-No. 330-54-1, and carbendazim, CAS-No. 10605-21-7. Parmetol contains 15 - < 25 % diuron and 8 - 12 % carbendazim.

The applied mixture of the metal surface pre-treatment agent and the bioactive component exhibited good adhesion to the aluminium foil and did not become undone during corrugation of the foil.

Further tests have shown that a user solution admixed with Parmetol, as described in example 2, to 2 %, 3 %, 4 % and 5 %, respectively, of the user solution, also exhibited good adhesion to the aluminium foil and did not become undone during corrugation of the foil.