The invention relates to a method of filtering mediums such as liquid, which flows through heat exchangers such as plate heat exchangers.

The invention moreover relates to a filter unit for filtration of mediums such as liquids flowing through heat exchangers such as plate heat exchangers.

From the German patent document nr. DT2613835 a method and a filtering system are known, which filtrates the cooling medium in a heat exchanger.

The technique, which is described in the German patent document, includes washing of the filter with the purpose of removing the impurities from the filter, which have been scraped off.

It has been found, however, that this prior art involves some drawbacks, as the washing often does not clean the filter adequately efficient, whereby filth and impurities have a tendency to be accumulated in the filter, which results in reduced efficiency and a risk of breakdown of the heat exchanger.

From JP11257892 A a filter unit is known, containing a conical filter build into a cylindrical pipe, where the filter is scraped off by a single conical rotating scraper, and where the cylindrical pipe can be placed in the inlet of a heat exchanger.

The technique, which is described in the Japanese patent document has however got several drawbacks, including, that the filter is space- consuming, in that it must be placed outside a heat exchanger and in addition the filtering effect is problematic, in that the surface of the filter itself is limited by the pipe diameter as well as the tapered surface of the filter. Accordingly, an object of the invention is to improve the known method and the known filtering system.

The object of the invention is achieved by a method of the type stated in the introductory portion of claim 1 , which is characterized in that the filtration is performed in one or more filtering units integrated in a heat exchanger, which are cleaned by a rotating scraping process.

Hereby, it is thus possible to effectively remove accumulated impurities from a filter, in that the impurities are scraped away from the filter. At the same time the technique is space-saving in that the filter is build into the heat exchanger itself and the filter can have a large effective surface.

As stated in claim 2 it is furthermore a distinctive feature of the invention that the cleaning of a filter unit is performed by rotating two or more scrapers, which touch the internal surface of the filter sieve.

Hereby is achieved the advantage that the cleaning efficiency can be adapted to the specific need and it is moreover achieved, when relating one or more scrapers including especially 3 or more, that the rotating part can become self-stabilizing and self-centered.

In claim 3 is stated that it also is a special characteristic of the invention that the scrapers are pressed against the internal surface of the filter sieve with a pressure determined by a device such as a spring.

When the scrapers are pressed against the internal surface of the filter sieve by pressure from e.g. a spring, it is achieved that the pressure from the scrapers against the filter in principle becomes unchanged even though the scrapers are worn down. Claim 4 describes that it is also a characteristic of the invention that the scrapers are tilted in relation to the longitudinal axis of the filter unit.

Hereby it is ensured that the impurities, which are scraped off are guided towards the one end of the filter unit where they are piled up when the scrapers are turned. The impurities, which are piled up can then be removed via a drain.

In claim 5 it is specified that it is in addition a distinctive feature of the invention that the scrapers are mounted radially from an axis of rotation, which is placed centrally in the longitudinal axis of the filter unit.

Because the scrapers are mounted on a rotating shaft, which runs through the whole filter in its longitudinal direction, it is achieved that the construction can be manufactured in an uncomplicated way and made robust and thereby high durability and low maintenance costs are ensured.

Additional appropriate forms of embodiment for the method are specified in claims 6 to 9.

As mentioned, the invention also relates to a filter unit.

This filter unit is characterized in that the filter unit is build into the heat exchanger and provided with two or more scrapers, which scrape off the internal surface of the filter sieve by rotating around a central pivot shaft, and where the scraping elements are placed with equidistant distance along the pivot shaft and with an equidistant angle seen in a cross section of the pivot shaft.

Hereby it becomes possible to produce heat exchangers with a built-in / intergrated efficient self-cleaning filter, which ensures stable operation of the heat exchangers with very low maintenance costs. The invention will now be explained more fully with reference to the drawings, in which Fig. 1 shows a heat exchanger with a built-in self-cleaning filter unit. Fig. 2 shows, seen in a cross section, a self-cleaning filter unit. Fig. 3 shows an integrally built self-cleaning filter unit. Fig. 4 shows a separated self-cleaning filter unit. Fig. 5 shows a separated scraper. Fig. 1 shows in 1 a plate heat exchanger with inlet 3 and outlet 4 of the cooling medium with a built-in self-cleaning filter unit 2. The heat exchanger can, naturally, also be used in applications where a heating medium is being used instead of a cooling medium, and the heat exchanger has naturally also input and output of a medium such as a liquid with which the energy is going to interact with. The filter 2 is on fig. 1 shown built-in to the inlet of the coolant but can in the same way be built-in to one of the heat exchanger's additional inputs. The principle shown in fig. 1 will typically be used if the cooling medium of the heat exchanger is e.g. sea- or lake water, which is often the case in maritime and industrial heat exchangers. The self-cleaning filter unit, which is shown integrated in the heat exchanger in fig. 1 , is shown magnified in fig. 2.

In fig. 2 the medium such as seawater is conveyed into the filter unit via the inlet 3 and after passage of the filter surface 17, the medium is drained off as shown by 5. The filter area 17 can e.g. be made of a fine-meshed rust¬ proof material. In the longitudinal direction of the filter a shaft 3 is mounted, on which a number of scraping elements 6 are mounted that touch the internal side of the filter area 17. When the shaft 7 is turned by the attached engine 9, the scraping elements 6 will effectively scrape off the internal side of the filter area 17, whereby impurities, which would elsewise be able to plug up the filter, are removed.

As it appears in fig. 2, the scraping elements are tilted in relation to the pivot shaft 7, whereby the scraped off impurities are guided into the direction of the outlet branch 8, when the scraping elements are turning.

The scraped off impurities will thereby pile up at the outlet branch 8, from where the impurities can be removed from the filter.

Fig. 3 shows a total filter unit, where a valve is fitted on the outlet branch 8, which, when activated, can remove the scraped off impurities from the filter 17.

Fig. 4 shows a separated filter unit with the filter itself 17, which is internally scraped off by the scraping elements 6, which are fitted on the shaft 7, which can rotate by activating the engine 9. Impurities, which have been scraped off can be removed from the filter unit by activating the valve 10.

When, as shown in fig. 4, there are mounted several scraping elements in the filter unit, it is appropriate to mount these with equidistant distance in the longitudinal direction of the shaft 7, and with and equidistant angle seen in the cross-sectional view of the shaft 7.

It is obvious that a self-cleaning filter covered by the invention can be made with only one scraping element, but by supplying the shaft 7 with several small, equidistantly placed scraping elements, these will be able to stabilize and centralize the rotation of the shaft 7 so effectively that the filter unit could function well with just one end-bearing supporting the shaft 7.

The self-centering effect is achieved by using several identical scraping elements, which press with the same power against the inner surface of the filter. The scraping elements are placed with equidistant distance along the rotating shaft and with an equidistant angle seen in a cross section of the rotating shaft.

Besides stabilizing the rotation, the self-centering effect has got the advantage that the filter purges homogeneously across the whole surface of the filter. In addition the mechanical wear becomes uniform across the whole surface of the filter, which ensures low maintenance costs.

Fig. 5 shows a separated scraping element, where the scraper head 6, which touches the filter that is going to be scraped off, is pressed against the surface of the filter with a well-defined force from the spring 13. The spring 13 is placed in the pipe section 17, which is provided with a keyway 14, in which the pin 12, which maintains the scraper 6, can slide. The scraper 6 is fixed in the lateral direction at the shown guideway 15. The whole scraping element is fitted on the shaft 7, which, as previously described, can be brought to rotating powered by an engine.

Tests have shown that it is appropriate to construct the scraper 6 in a polymeric material such as polyetheretherketone (PEEK), which combines an efficient scraping quality with a good resistance against wear.

When the scraper 6 with time is worn as a result of the friction against the filter, the spring-based construction ensures, as shown in fig. 5, that the pressure from the scraper against the filter, which is going to be cleaned, is held approximately constant, even though the scraper 6 is being worn.

Hereby it is achieved, that the self-cleaning efficiency of the filter is not reduced as a result of wearing-down of the scrapers 6.

If a sensor device is e.g. build into the pipe section 16, which can detect the degree of wearing-down of the scraper 6, the system will be able to automatically submit a signal to the surroundings, when the time is ready for the scrapers 6 to be replaced.

The application of the homogeneous modular scraping elements have in addition got the advantage that filters can easily be made in a random length just by adapting the number of scraper elements.

It is part of the present invention that scrapers 6 can have other shapes than the one shown in fig. 5, including being shaped as brushes.

For a given filter unit with a known filter area and scraping element it will also be possible to deliver and/or submit a signal when it is time for replacement of worn out scrapers, as long as the integrated number of rotations of the scrapers is known, in that the wear will be proportional with the number of rotations.

The scraping process itself can be controlled by e.g. a computer and thereby run fully automatic, without the use of manual labour. The filter unit can with advantage be made in rust-proof materials such as rust-proof steel alloys, where the scrapers though as previously described with advantage can be made of a polymeric material.