Background Environment-related problems are of growing concern and it is becoming more and more desirable to purify fumes originating from burning of traditional fuels, such as oil or woord. In addition, the use of wood and other bio-fuels as sources of heat is becoming increasingly common, both for fuelling boilers in central-heating installations and for fuelling open fireplaces and the like.

Traditionally, no direct purifacation is effected in installations where fuels of this kind are burned; instead particles of soot and other pollutants are allowed to escape directly to the environment.

Consequently, there is a need for a simple and cheap device for purifying flue gases, which may be applied to existing combustion systems in a simple manner without requiring expensive and complex reconstruction and which may be used especially in small installations, such as in one family houses but also in larger installations, such as multi-family houses, blocks of flats or industrial premises.

Object of the invention Consequently, it is an object of the present invention to provide a filtering device for purifying flue gases, which is simple and inexpensive to produce and which may be conveniently applied on existing combustion systems.

This object is achieved by means of a filtering device as defined in the appended claims.

Brief Description of the Drawings For exemplifying purposes, the invention will be described in the following in more detail by means of one embodiment and with reference to the accompanying drawings, wherein: Fig 1 is a lateral view of the invention in accordance with one embodiment; Fig 2 is a partly broken view of the device of Fig 1, mounted on a chimney; Fig 3 is a view from above of the device of Fig 1 and Fig 4 is a view from below of the device of Fig 1.

Description of Preferred Embodiments A filtering device in accordance with a preferred embodiment of the invention is shown in Fig 1. This filtering device comprises a housing in the form of a box 1. The lower bottom of the box is formed with an opening dimensioned to allow the device to be applied from above around a chimney 2 (see Fig 2). The box further comprises a lid 3, which covers an upper maintenance opening. In its closed position, the lid 3 essentially covers the entire maintenance opening. Preferably, the lid 3 is provided at one of its sides with hinges 4 or similar means so as to allow the lid to be pivoted upwards, in the manner of a hatch, into an open position, as illustrated in Fig 1, and to be then pivoted back to the closed position, as shown in Fig 2. Preferably, chains or similar means are also provided between the lid and the box in order to prevent the lid from being pivoted beyond a maximum open position.

In addition, the lid is provided with an upper opening 6, which preferably is of considerably reduced

size as compared with the lower openings. As in accordance with the shown embodiment, this opening may be located essentially centrally on the lid 3 and be provided with a pipe 7. Furthermore, the pipe 7 may be provided above its mouth but spaced some distance therefrom, with a protection hat 8 for the purpose of preventing rainwater and the like from entering into and dripping down the pipe.

Furthermore, the device is provided in accordance with the invention with a fan 12 designed to suck the fumes upwards, through the pipe 7. Advantageously, the fan is a small electrical fan of e. g. 0.5 W, which is placed inside the pipe 7.

A filter 9 is disposed between the upper and the lower openings. Preferably, the filter is mounted in a filter holder, which for example could comprise a fine- mesh net 10. In accordance with the shown embodiment this net may be anchored to the lid, for instance via stays 11, such that the filter will accompany the lid as the latter is pivoted to its open position, Alternatively, the filter could of course be arranged inside the box 1 itself. Preferably, the extension of the filter 9 is such that the latter screens off a large part of the route transversely between the lower and upper openings, in order to force at least most of the flue gases to pass through the filter and to thus ensure that the flue gas particles are efficiently trapped inside the filter. In case the filter becomes heavily particle-laden and consequently less permeable, an alternative route of passage should, however, be available to the flue gases.

This alternative route preferably could be a narrow gap arranged between the filter material and the inner wall of the housing. In this manner, the flue gases may by- pass the filter but at the same time their preferred choice of route will be through the filter, as this route is the shorter one. Preferably, the area not screened-off by the filter amounts to at least 15% and at the most to

25% of the inner cross-sectional area of the housing. In addition, the inner cross-sectional area of the housing at the level where the filter is positioned should be at least 75% of the upper surface area of the chimney. It goes without saying that the areas not covered by the filter could be disposed differently, such as at one or only a few of the sides of the filter material, or even be enclosed by the filter material.

Preferably, the filter consists of an organic and combustible filter material, such as plane filter of HI type available from Scandfilter. One requirement that the filter material necessarily must meet is, however, that it should be heat-resistant.

Because the housing may be opened, a feature which in accordance with the embodiment is achieved by the division thereof into one base part in the form of a box and one upper part in the form of a lid 3, the filter is easily accessible from the exterior. This is desirable, since it facilitates filter-changes. Also, this arrangement offers the chimney sweeper access to the chimney pipe without having to remove the device.

Preferably, the filter is changed at regular intervals, such as every third month, and preferably in connection with sweeping of the chimney.

It is preferable to provide the device with a coarser-mesh net 13, disposed inside the box 1, below the filter 9. This coarse-mesh net or grid prevents large- size parts, such a filter parts separating from the filter, from falling downwards, and in addition also particularly prevents large-size particles, especially burning particles of this kind, from being carried upwards, into contact with the filter.

The device in accordance with the invention preferably is secured to the chimney 2 in the following manner. The lower opening is placed about the chimney from above and the box is fastened to the chimney neck for example by means of screws 14. The box should fit

comparatively narrowly about the chimney neck, and in addition it advantageously should have a certain extension along the chimney neck in order to prevent flue gases from escaping by any other route than through the upper opening 6. Possibly, the extension of the box could be such that the box covers the entire chimney neck all the way down to the level of the roof of the house.

In addition, the device could be provided with a handle 15 allowing it to be easily applied on and dismounted from the chimney, as also with locking means 16 to prevent unauthorised and undesirable opening of the hatch.

The fan could be in a permanent on-condition. It is however, preferable, that it is of an on/off type. For this purpose a switch may be provided in the area of the furnace to make it possible to activate the fan before the firing starts. This is an important feature, since it is particularly during the early stage of the firing, when the temperature is low, that the amount of pollutants is at its maximum.

The on/off condition of the fan may also be arranged to be controlled automatically, in which case a sensor, designed to detect the occurrence of flue gases, heat, opening of the boiler door or the like, may be positioned in close vicinity to hear the furnace or the flue gas passage in order to activate the fan automatically when the firing is about to start or has just started.

In addition, the device may be provided with automatic detection means, which are arranged to indicate when the filter is fully laden and therefore need to be replaced and to signal this fact to the user, for instance by illumination of an emergency lamp.

The present invention provides a device, which is simple and inexpensive to produce, which is simple to mount on chimneys of existing installations, and which is easy to maintain. In this manner it becomes possible to purify a considerably larger amount of flue gas emissions

than may be done today. Another advantage provided by the device is the reduced heat losses obtained on account of the upper opening being smaller than chimney mouth, a feature which also leads to some cost savings.

The invention has been described herein by means of one embodiment. However, a large number of modifications are possible, such as positioning the filter in the base part instead of in the lid part, locating the areas not covered by the filter elsewhere or possibly even omitting such areas entirely, or using different filter materials, and so on. These and other obvious varieties of the invention must be regarded to be within the scope of protection of the invention as the latter is defined in the appended claims.