The present invention relates to a desorbable sorption filter, in particular for a heating or air-conditioning system of a motor vehicle, and to a method for producing the sorption filter.
A present day motor vehicle air-conditioning system serves, inter alia, to increase comfort in terms of air temperature, atmospheric humidity and air quality. In addition to the functions of ventilation, heating and cooling of the passenger compartment, an important role of the system is to remove solid and gaseous odorous and irritating substances and pollutants as well as atmospheric humidity, a function which affects not only the comfort but also the safety and health of the passengers during travel.
To clean the air, particle filters are used for the solid constituents, and sorption filters, for example, activated carbon filters, are used for separating out undesirable gases.
Foamed polyurethane filters laden with activated carbon are known. These filters are designed as simple disposable filters and have to be exchanged regularly, in order to maintain a sufficient filter action over a prolonged period. Therefore, they have to be arranged at a readily accessible point inside the air-conditioning system. In addition, these filters do not comply with the growing trends toward maintenance-free vehicles and preservation of resources. Finally, exceeding the maintenance intervals is harmful to health.
For this reason, desorbable filter designs have been proposed. With these it is possible, in principle, to considerably extend the filter service life, so that it is possible to produce a lifetime filter, i.e., a filter which does not have to be exchanged throughout the life of a motor vehicle. For this purpose, the filter structure has to be heated to temperatures of around 200xc2x0 C. at certain intervals, in order to expel the ad- and/or absorbed pollutants. Currently three possible routes are essentially being followed with designs of this type, but all these routes have various drawbacks.
DE 43 04 077 and DE 195 12 844 disclose filters which can be desorbed indirectly by means of hot air. However, the energy consumption required for desorption is unacceptably high on account of large amounts of air being heated. Moreover, the hot air means that a large amount of heat is entrained, and consequently relatively large parts of the system have to be able to withstand high temperatures.
Furthermore, activated carbon filters in which the activated carbon can be heated directly by the flow of current are known. Currently, these directly heatable activated carbon fabric filters are very expensive and difficult to produce. Moreover, their mechanical stability in operation is low. Although this problem can be alleviated by constructing a laminate comprising activated carbon fabric, supporting fabric and/or a nonwoven, this increases the costs and the flow resistance. In addition, providing good electrical contact between the fabric and metallic current supply conductors over a prolonged period of time is very difficult. Furthermore, over the course of time, oxidation may cause the electrical resistance to rise, which may lead to inhomogeneity in the current density distribution. In this case, temperature monitoring becomes more difficult and there is an increased risk of xe2x80x9chot spotsxe2x80x9d being formed. A further drawback is the relatively small amount of carbon in thin fabrics, requiring low flow velocities and relatively short adsorption/desorption cycles. High gas concentrations may cause the small amount of carbon to become saturated excessively quickly under certain circumstances, which may cause the filtering capacity to be lost. This has the effect of reducing comfort, for example, in the event of odor-intensive pollution peaks, as occur frequently in traffic.
Finally, DE 42 25 272 A1 and DE 195 17 016 A1, for example, disclose sorption filters in which carbon granules are applied to electrically heatable metal support structures by means of special adhesives. DE 42 25 272 A1 discloses an adsorption filter which comprises a two- or three-dimensional support structure and adsorber grains fixed thereto. This support structure comprises an electrically conductive material and is heated by current flowing through it. In this way, it is possible to desorb the adsorber material. DE 195 17 016 A1 describes an air-treatment system, in particular for a vehicle interior, having at least two activated carbon filter units. In each case one unit is electrically heated, in an alternating manner, and the pollutants are carried away by an outgoing air stream. At the same time, the air stream which is to be treated is passed over the other filter unit, and is ultimately supplied to the vehicle interior. The activated carbon filter units have metallic support bodies around which the air flows and which are coated with activated carbon. The metallic support bodies are designed as electrical heating resistors, so that when current flows through them they are heated to a sufficient extent for the activated carbon to be desorbed. The disclosures of these documents are incorporated by reference herein.
Sorption filters of this type, in which the carbon can be heated by means of metal supports through which current can flow, can be produced at lower cost and are easier to make into a geometrically stable, pleated form. However, these have the drawback that the carbon granules, which are usually applied by special coating processes, in the small amounts of carbon applied, exhibit adsorption kinetics which are too low, therefore leading to high levels of immediate failure. The reasons for this is that the gases which are to be adsorbed have long diffusion paths to the micropores in the interior of the grains, and there are large air gaps between the individual activated carbon particles. Moreover, the adhesion of the individual carbon particles to the supports by means of an adhesive is often insufficient at elevated desorption temperatures of around 200xc2x0 C. The use of an adhesive also causes fundamental problems because, either during the coating operation or during the heating operation to temperatures of around 200xc2x0 C., foreign substances may evaporate out of the adhesive and may irreversibly contaminate the activated carbon. In operation, there are also problems with the heat insulation of the particles and with the long-term stability of the adhesion of the particles.
It is therefore one object of the invention to provide an improved, directly heatable sorption filter which in particular allows inexpensive production, good electrical contact and has good adsorption kinetics while dispensing with the use of adhesive systems altogether.
In accordance with one aspect of the present invention there has been provided a desorbable sorption filter, comprising: a metallic support comprised of a conductive metal, whereby the support can be heated by flow of electric current, and wherein the support includes a plurality of holding elements protruding therefrom; and an organic or inorganic adsorber mounted at or on the support, wherein the adsorber is of generally planar design and is mechanically connected to the support by means of the holding elements arranged on the support.
In accordance with another aspect of the invention, there has been provided a method for producing a sorption filter as described above, comprising: providing the adsorber; producing the support with the holding elements by stamping out junctions and deforming free ends of the support material which are formed as a result of the stamping; laying the adsorber onto the support and, optionally, a protective layer under the support; and
connecting the support and adsorber by deforming the holding elements.
According to still another aspect of the invention, there has been provided a motor vehicle heating or air-conditioning system, comprising a heating or air-conditioning unit having an air inlet and an air outlet, and a sorption filter mounted in at least one of said air inlet and said air outlet, wherein the sorption filter comprises a filter as defined above.
Further objects, features and advantages of the present invention will become apparent from the detailed description of preferred embodiments that follows, when considered together with the accompanying figures of drawing.