Abstract:
A filter device is proposed for cleaning gas entraining foreign bodies, including at least one filter unit having at least one filter surface on a raw gas side to which a raw gas stream containing foreign bodies can be supplied, wherein filtration aids can be supplied to the raw gas stream and/or the filter surface, and wherein filtration aids and/or foreign matter attached to the filter surface can be cleaned off. The filter device additionally includes a fluidized bed arrangement in which a carrier fluid stream can be generated such that cleaned-off filtration aids and/or foreign matter can be held at least in part as filtration aerosol in a surrounding of the filter unit and/or can re-attach to a filter surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the national stage entry under 35 U.S.C. 371 of PCT/EP2011/065286, filed on Sep. 5, 2011, which claims the benefit of the Sep. 10, 2010 priority date of German application DE 10 2010 045 000.6. 
     FIELD OF THE INVENTION 
     Background 
     The present invention relates to a filter device for cleaning gas entraining foreign matter or bodies, in particular to a filter device employing dry filtration, as well as to a corresponding method for filtering gas entraining foreign matter or bodies. 
     Filter devices employing dry filtration are used in a variety of applications for cleaning exhaust air or other exhaust gases (in the following generally referred to as exhaust air) arising in installations in a wide range of industries. A very specific advantage of dry filters over wet filter devices, e.g. rinsing systems, is the relatively simple processing of the filtered-out foreign bodies arising, as these do not arise bound in liquid phases. 
     However, there are problems present when there are foreign bodies contained in exhaust air that are of sticky nature. This is the case, for example, when the exhaust air contains tarry dusts, or in the case of exhaust air from painting/varnishing facilities, especially wet painting/varnishing facilities. The reason therefor is that such sticky foreign bodies preferentially settle in the pores of the filter, gradually clogging the same. Due to their sticky nature, they tightly attach to the filter surface and thus can no longer be removed from the filter surface by use of cleaning systems employing pressurized air as usually applied for dry filters. The filter thus rapidly loses its cleaning capability and has to be replaced. 
     It is known to add filtration aids when dry filters are used for filtering exhaust air containing foreign matter of sticky nature. This may be effected upstream of the dry filter, by injection of filtration aids in the raw gas stream fed to the dry filter. It is also possible to coat a filter surface on the raw gas side of the dry filter with filtration aids before the same is contacted with foreign bodies (so-called “precoating”). The filtration aids are intended to establish a bond with the sticky foreign bodies in the exhaust air and make sure that agglomerates made up with filtration aids and foreign bodies attach to the substrate surface, which agglomerates can be removed easily using the conventional cleaning methods. An example of the addition of filtration aids directly in the raw gas stream upstream of the dry filter is disclosed in DE 42 11 465 C2. Precoating of the filter surfaces with filtration aids is known e.g. from DE 197 15 195 A1. Moreover, in both documents there are measures provided for collecting material arising after cleaning off of the filter and for returning the same to the raw gas stream, optionally after processing thereof. 
     DE 199 24 130 A1 and DE 103 61 266 A1 disclose a method in which material arising after cleaning off of the filter unit is collected in a collecting container, and the collected material is stirred in the collecting container by blowing-in pressurized air, such that it re-attaches to the filter surface. 
     SUMMARY 
     It is an object of the present invention to make available an improved filter device of the type described, which in particular permits more efficient utilization of filtration aids. In addition thereto, a corresponding method for filtering gas entraining foreign bodies is to be indicated. 
     To meet this object, the invention suggests a filter device for cleaning gas entraining foreign bodies, as well as a corresponding method for filtering gas entraining foreign bodies, comprising: there is provided at least one filter unit having at least one filter surface on the untreated or raw gas side of the same to which raw gas side a raw gas stream containing foreign bodies can be supplied, wherein filtration aids can be supplied to the raw gas stream and/or the filter surface on the raw gas side. Filtration aids and/or foreign matter attached to the filter surface on the raw gas side can be cleaned off. In addition thereto, there is provided a fluidized bed arrangement in which a carrier fluid stream can be generated such that cleaned-off filtration aids and/or foreign matter can be held at least in part as filtration aerosol in a surrounding of the filter unit and/or can re-attach to a filter surface on the raw gas side. The term “filtration aerosol” is used here in quite general form for designating a mixture of solid and/or liquid suspended particles in the carrier fluid stream, without implying a limit of the diameter of the suspended particles. Up to which size the particles may grow before they are removed from the aerosol layer due to their gravity, is dependent mainly upon the properties of the carrier fluid stream. With a suitable adjustment of the carrier fluid stream, it is easily possible to keep particles suspended that have a diameter of more than 100 μm. 
     The filter surface can be cleaned off in the sense that material adhering to the filter surface can detached again from the filter surface in a cleaning-off process which as a rule is performed periodically. The time intervals as a rule will be selected in consideration of the loading of the filter surface and will be chosen such that a predetermined pressure loss across the filter stage is not exceeded. 
     The addition of filtration aids—examples for such filtration aids are rock flour (crushed rock of silt size) or other mineral dusts—results in the formation of agglomerates made up with filtration aids and foreign bodies attached to the same. Such agglomerates are by far less sticky than the pure foreign bodies, so that such agglomerates are considerably easier to detach from the filter surface than the pure foreign bodies. Efficient use of filter devices employing dry filters thus is successfully possible also for cleaning exhaust air containing sticky foreign bodies, as in exhaust air in wet or dry painting or varnishing facilities. 
     For a sufficient agglomeration of the foreign bodies with the filtration aids to take place in sufficiently rapid manner, it is expedient to add a comparatively large amount of filtration aids. This ensures in addition that the entire filter surface is coated with a layer of filtration aids, possibly with foreign bodies attached thereto, within a short period of time, thus protecting the filter surface. However, this approach means that only a smaller percentage of the added filtration aids has formed agglomerates within one single cleaning-off cycle. There are of course endeavors being made for using the auxiliary materials as efficiently as possible, i.e. to convert possibly all filtration aids added into agglomerates of filtration aids with attached foreign bodies. This has created the idea of repeatedly reusing the mixture of filtration aids and foreign bodies that results upon cleaning-off of the filter, until an as complete as possible transformation, i.e. covering of filtration aids with foreign bodies, has taken place. 
     The renewed supply of material removed in a cleaning-off operation to the raw gas or the filter surface, however, has turned out problematic as the material removed from the filter surface and collected on the floor or in a separate container often forms caked structures that are difficult to separate again. Even the blowing-in of pressurized air into the material collected on the floor, with the aim of stirring this material to such an extent that it attaches again to the filter surface, has only partially satisfactory results. The invention therefore suggests an up to now completely novel approach. Instead of intercepting the material removed from the filter surface in a cleaning-off operation and collecting the same in a container or on the floor of the raw gas space, this material is to be kept as close as possible to the filter surface. This permits on the one hand that this material quickly re-establishes contact with the raw gas stream and on the other hand remains in contact with the filter surface. In this manner, it is possible for further agglomerates to form, or for already present agglomerates to grow further by attachment of additional foreign bodies or other agglomerates. Moreover, it is also possible for the cleaned-off material to rapidly re-attach to the filter surface and thus provide a protective coating. The processes described can take place in virtually continuous manner. Shutting-down of the device for the purpose of cleaning-off and newly coating of the filter surface is not absolutely necessary any more. 
     It has turned out that it is possible to hold the material detached from the filter surface with the aid of a fluidized bed. The material forms an aerosol layer in the fluidized bed that is held “suspended” by a carrier fluid stream. In the “fluidized bed”, operating in the manner of an arrangement known as swirl bed, a stream of the carrier fluid, which as a rule is directly upwardly, i.e. counter to the force of gravity, flows through the individual particles of solid phase formed of foreign bodies, filtration aids and agglomerates formed by the same, and cooperates with the same such that the individual particles are held in a fluidized state and, as it were, are “suspended”. This fluidized bed arrangement can be adjusted such that the solid phase formed of foreign bodies, filtration aids and agglomerations formed by the same can establish optimum contact with additional foreign bodies entrained by the raw gas stream and thus very efficiently forms further agglomerates or causes already formed agglomerates to grow further. When the fluidized solid phase is held near the filter surface, it is moreover possible to successfully coat the same again with a protective layer within a short period of time after a cleaning-off operation. 
     By way of the stream of the carrier fluid it is possible to adjust which maximum mass density agglomerates of filtration aids and foreign bodies are allowed to have in order to still permit the same to be carried in fluidized form. When the agglomerates exceed this critical mass density, they gradually fall down further and finally drop out of the fluidized bed completely. Already saturated agglomerates, for which further use for having additional foreign bodies attached thereto or for coating the filter surface is no longer desirable, can thus be successfully removed from the filter device by dropping out. To the extent in which spent agglomerates drop out of the fluidized bed, unspent filtration aids can be added in continuous manner. Thus, there is no need for stopping the filter device for replacing spent filtration aids. 
     The fluidized bed arrangement in operation preferably forms a filtration aerosol layer of filtration aids, foreign bodies and agglomerates of filtration aids with foreign bodies attached thereto, which in vertical direction extends from a lower limit at the height of or slightly below the bottom side of the filter unit to an upper limit at the height of or slightly above the upper side of the filter unit or even still further. In an example, the filtration aerosol layer can extend up to 90 cm, preferably up to 60 cm and most preferably up to 50 cm below the bottom side of the filter unit. 
     As carrier fluid, there can be used in particular a carrier gas (e.g. air) that is present on the raw gas side of the filter unit. The carrier fluid may comprise the raw gas stream itself and in the simplest case can be constituted by the gas establishing the raw gas stream. In such cases, the fluidized bed arrangement preferably comprises a swirler arrangement through which an upwardly directed, preferably turbulent flow can be generated on the raw gas side, in particular in the filtration aerosol layer. The swirler arrangement is to enhance the tendency of forming vortices, that is present in the raw gas stream anyway, such that a fluidized bed forms in the filtration aerosol layer, which takes care that the filtration aerosol is kept stably held in this layer, while there is nevertheless good mixing taking place within the layer in order to be able to promote agglomeration and to efficiently form a protective coating on the filter surface, respectively. 
     The fluidized bed arrangement can preferably comprise a carrier fluid introduction arrangement in addition, through which the raw gas side of the filter unit can be acted upon by carrier fluid. The carrier fluid can establish the carrier fluid stream in addition to the raw gas stream. Furthermore, the carrier fluid introduction arrangement can also be designed to cause swirling of the carrier fluid introduced. Moreover, it is expedient to introduce the carrier fluid in such a manner that an upwardly directed carrier fluid stream results. An advantage of using a carrier fluid stream generated by a swirler/carrier fluid introduction arrangement consists in that the carrier fluid stream can be adjusted—e.g. by a corresponding design and/or control of the swirler/carrier fluid introduction arrangement—such that particles that are heavier than a predetermined critical mass density are no longer held suspended and drop out of the filtration aerosol layer. 
     The swirler/carrier fluid introduction arrangement may comprise e.g. an annular conduit arranged below the filter unit and provided with a nozzle arrangement comprising at least one—preferably several—nozzles. The annular conduit in operation may be acted upon by carrier fluid such that in operation a carrier fluid stream is formed downstream of the nozzle arrangement, which is directed towards the filter unit and acts counter to the force of gravity. As an alternative, there may be, underneath the filter unit, a fluidizing floor or base member provided with fluidizing openings (e.g. slots). This fluidizing floor is subjected to carrier fluid from its bottom side so that the carrier fluid flows through the fluidizing openings and, downstream of the fluidizing floor, forms a carrier fluid stream directed towards the filter unit and acting counter to the force of gravity. 
     The swirler/carrier fluid introduction arrangement can be provided as a means that is continuously in operation. When the carrier fluid introduction arrangement generates a sufficiently strong carrier fluid stream, the intensity of the carrier fluid stream can be set largely independently of the raw gas stream. 
     In an embodiment the filter device can comprise a housing defining at least one raw gas space facing the raw gas side of the filter unit, and having at least one raw gas inflow opening leading into the raw gas space. The raw gas stream enters the raw gas space through said raw gas inflow opening. The housing then as a rule will define a clean gas space as well, facing the clean gas side of the filter unit and having an outflow opening for filtered clean gas leading into the clean gas space. 
     It has turned out that, in an expedient embodiment, the raw gas inflow opening may be arranged approx. at the height of or slightly below the filter unit. This is expedient in particular when there is a carrier fluid stream from below, since this carrier fluid stream then mixes with the raw gas stream below the filter unit, entraining the foreign bodies contained therein. As the carrier fluid stream also carries the filtration aids, this provides for efficient agglomeration between filtration aids and foreign bodies. 
     In a further embodiment, the housing may have at least one additional inflow opening leading into the raw gas space, through which the filtration aids can be supplied. The filtration aids supplied can be pure filtration aids or filtration aids loaded with slight amounts of foreign bodies. It is expedient when the additional inflow opening is disposed as well at the height of or slightly below the filter unit. Filtration aids can be supplied through this additional inflow opening in predetermined intervals or as a continuous stream of filtration aids in order to replace material discharged from the fluidized bed. A possibly required initial coating of the filter device can be effected through the additional inflow opening as well. Optionally, this additional inflow opening can also be used for supplying slightly contaminated filtration aids arising after cleaning-off of the filter unit and branched off from the fluidized bed. The latter are filtration aids that are slightly contaminated with foreign bodies which, however, have foreign bodies attached or agglomerated thereto only to such a minor extent that still further foreign bodies can attach thereto. 
     As an alternative or in addition, there may also be provided a further filtration aid supply means opening into the raw gas stream upstream of the filter unit. By way of this further filtration aid supply means, there may be supplied fresh filtration aids to the raw gas stream on the one hand, but on the other hand also such filtration aids that are branched off from the fluidized bed and are slightly contaminated with foreign bodies. 
     In an embodiment, the inflow opening for the raw gas stream can be arranged such that the raw gas stream merges with the carrier fluid stream at an angle of approx. 90 degrees. It is particularly expedient when the raw gas stream joins the carrier fluid stream underneath the filter unit. 
     The filter device in addition may comprise a collecting container for non-reusable particles, which is associated with the raw gas side. Non-reusable particles are “saturated” agglomerates of filtration aids and foreign bodies that are covered or loaded with foreign bodies to such a large extent that further attachment of foreign bodies is no longer efficiently possible. Such saturated agglomerates have considerably higher mass density than the foreign bodies alone and also than filtration aids that are not or just weakly loaded with foreign bodies. Depending on the adjustment of the carrier fluid stream, it is possible to adjust a maximum mass density of particles that can still be held suspended in the fluidized bed. As soon as the agglomerates become heavier than this mass density, they will drop out of the filtration aerosol layer and can be collected in the collecting container therebelow, which preferably has a disposal opening at a lowest point through which material can be discharged from the collecting container. This can take place using e.g. a vacuum conveying means. 
     The collecting container can be arranged e.g. below a housing portion disposed underneath the filter unit and having a substantially funnel-shaped configuration. 
     For facilitating discharge of the agglomerations from the collecting container that have become unusable, the collecting container may have a fluidizing arrangement in addition, through which material in the collecting container can be acted upon by gas so that the flowability of the same is retained. In this regard, the fluidizing arrangement can be operated continuously or briefly during discharge of material collected in the collecting container. 
     The collecting container furthermore can have a conveying means associated therewith through which material can be withdrawn from the collecting container and supplied to the filtration aerosol layer and/or the raw gas stream upstream of the filter unit. Such a conveying means may be of assistance in particular in a restart situation of the filter device as it permits particles, which upon deactivation of the device and the accompanying inevitable breakdown of the carrier fluid stream drop from the filtration aerosol layer into the collecting container, to be withdrawn from the collecting container and be returned to the filtration aerosol layer and the raw gas stream, respectively, when the device is restarted. For, the material from the filtration aerosol layer that is located in the collecting container after deactivation of the device, as a rule will still be usable as filtration aid. Without the conveying means, this material would have to be disposed of. The conveying means can be operated pneumatically with positive pressure, e.g. in the form of a low pressure injector, with negative pressure as vacuum conveying means or in fluidized form with pressure in the manner of a solid pump, in particular a solid diaphragm pump. 
     Cleaning-off can be effected by a cleaning-off unit associated with the filter unit. The cleaning-off means may comprise a pressurized-air cleaning-off unit that is arranged above the filter unit on the clean gas side of the same such that the filter unit can have pressurized air applied thereto in accordance with the counterflow principle in order to remove material deposited on the filter unit on the raw gas side. When the filter unit has a plurality of filter elements, the pressurized-air cleaning-off unit can be controlled such that only part of the filter elements is cleaned off in alternating, successive manner each, so that an as uniform as possible amount of cleaned-off filtration aids/foreign bodies results that have to be kept suspended. 
     The filter unit preferably has at least one filter element that is in the form of a rigid-body filter. 
     In an embodiment, the filter element may have a base body of sintered material, preferably containing sintered polyethylene (PE) particles as main constituent. The base body can be provided with a surface coating containing polytetrafluoroethylene (PTFE) particles. 
     In an alternative embodiment, the filter element can be formed of an arrangement of PE bodies, in particular PE tubes, with a filter membrane of PTFE being laminated onto each of the PE bodies. 
     In an embodiment the housing may have at least one lateral housing opening that can be closed by a corresponding lid and is arranged at the height of the filter elements and through which the filter elements can be inserted into the upper and lower filter module, respectively. The housing may be e.g. of cylindrical or rectangular configuration in a portion surrounding the filter unit and merge with a lower funnel-shaped portion that is possibly followed by the collecting container at the lowest location thereof. 
     Feasible filtration aids are in particular rock flour (crushed rock of silt size), e.g. limestone powder, and other mineral dusts. 
     Upstream of the filter unit there can be arranged in addition a pre-separator stage—preferably comprising a rinsing system or a cyclone separator—which already separates coarse contaminations. 
     There may also be an additional filter stage—preferably comprising a depth-loading filter—arranged downstream of the filter unit. By regular examination of the downstream filter stage, failure or deterioration of the filter unit can be detected, and the filter unit possibly can be replaced. The downstream filter stage moreover prevents that uncleaned exhaust air can leave the installation. 
     Preferred fields of application of the filter device described are apparatus and devices in which exhaust air is created containing sticky and/or tarry foreign matter. It has turned out that the device according to the invention can be installed and is of valuable service especially in installations of one of the types indicated hereinafter:
         a device for eliminating air pollutions in a wet painting or varnishing facility or plant,   a device for eliminating air pollutions in a dray painting or varnishing facility or plant,   a device for eliminating air pollutions in a laser beam welding facility or plant or other welding fume extraction system,   a device for eliminating air pollutions in flue gases, in particular in flue gases arising in combustion processes (in such combustion processes, there are frequently developed tarry or soot-like products that have to be removed from the exhaust gas as pollutions);   a device for eliminating air pollutions in a cavity sealing facility or plant,   a device for eliminating air pollutions in a plant for adhesively joining metal parts;   a device for eliminating gaseous pollutions in an exhaust gas stream with addition of chemisorptive filtration aids that chemically react with the gaseous pollutions (e.g. sulfur dioxide, SO 2 , or HCl gas contained in exhaust gas can be effectively deposited on a dry filter by addition of calcium hydroxide, Ca(OH) 2 , as filtration aid, with the chemisorption taking place simultaneously with the removal of solid or liquid aerosols from the exhaust gas).       

     According to the invention, there is suggested furthermore a method of cleaning gas entraining foreign bodies in which the device described hereinbefore is utilized. This method involves in particular supplying a raw gas stream containing foreign bodies to at least one filter unit having at least one filter surface on a raw gas side, with filtration aids being supplied to the raw gas stream and/or the filter surface. Furthermore, filtration aids and/or foreign bodies attached to the filter surface are cleaned off, and a carrier fluid stream is generated such that cleaned-off filtration aids and/or foreign bodies can be held at least in part as filtration aerosol in a surrounding of the filter unit and can re-attach to a filter surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail hereinafter by way of embodiments shown in the drawings, in which: 
         FIG. 1  shows a side view of a filter device for cleaning gas entraining foreign bodies in accordance with an embodiment; and 
         FIG. 2  shows the device of  FIG. 1  in a side view rotated by 90 degrees with respect to the view of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  each show, in side views rotated by 90 degrees in relation to each other, a filter device  10  for cleaning gas entraining foreign bodies according to an embodiment. The device  10  comprises a filter unit  12  (not shown in  FIG. 1 ,  FIG. 2  outlines one of the filter elements  14  of filter unit  12 ). The filter unit  12  is mounted above a raw gas inflow opening  16  in an upper part of a housing  18 , which has been omitted for clarity. The filter unit  12  comprises several filter elements  14  provided in the form of rigid-body filters and mounted to a common upper support and extending parallel to each other in vertical direction, as schematically illustrated in  FIG. 2  which shows one of the filter elements  14  in the mounting location of the same. 
     In the lower part of the housing  18 , enclosing a raw gas space  15 , as illustrated in  FIGS. 1 and 2 , there are formed an additional inflow opening  20  as well as a flap  22  in addition to the raw gas inflow opening  16 . All of these openings  16 ,  20 ,  22  are arranged substantially at the same height in an annular upper portion  18  a of lower housing part  18 . In a portion  18   b  following below said portion  18   a , the housing  18  adopts the shape of a funnel with downwardly tapering side walls. Following on the bottom side of housing  18 , there is provided a collecting container  24  for collecting no longer usable material before such material is discharged through a disposal opening  26  located at the lowest point of the collecting container  24  and through a disposal funnel  28  into a vacuum conveying means  30 , and is disposed of, as indicated by arrow  32  in  FIG. 2 . Disposal funnel  28  normally is closed at its lowest location by a valve  34  and is opened briefly only when material is to be discharged from the collecting container  24 . For ensuring disposal of the material that is collected in the collecting container  24  and as a rule has foreign bodies attached thereto to a very large extent and possibly resides there for relatively long periods of time, there is provided in the collecting container  24  an obliquely extending fluidizing floor  36  having air supplied thereto via a connection  38 . Said connection  38  has a fan connected thereto, that is shown only schematically at  40 , by means of which pressurized air is fed into fluidizing floor  36 . The air stream generated in fan  40  is adjusted such that the material collecting in the collecting container  24  on the one hand is stirred such that it has good flowability and thus can be discharged easily via disposal opening  26 , but that on the other hand this material cannot get back from the collecting container  24  into the housing  18  or the raw gas space  15 , respectively. 
     The raw gas stream, schematically illustrated by arrow  44 , in which foreign bodies are entrained that are to be eliminated by means of device  10 , enters the raw gas space  15  enclosed by housing  18  via the raw gas inflow opening  16 , with said space  15  being confined at its top side by the raw gas side of filter unit  12 . The raw gas stream  44 , upon entry into said raw gas space  15 , is conveyed to filter unit  12 . On the side of the housing  18  opposite the raw gas inflow opening  16 , there is provided an additional inflow opening  20  through which filtration aids, as a rule rock flour, are fed from a supply container, not shown, into raw gas space  15 . The stream of filtration aids is indicated in  FIG. 1  by arrow  45 . 
     In a lower portion of the funnel-shaped housing portion  18   b , there is provided a connection  48  connected to an annular conduit  46  extending horizontally through housing  18   b . Annular conduit  46  is located above collecting container  24  and in particular is always located above the material collected in collecting container  24 . Connected to connection  48  is a further fan  50  that is also shown only schematically in  FIG. 2 . Fan  50 , like fan  40 , may comprise a side channel blower. In a preferred embodiment, fan  50  is operated continuously during operation of the device  10 . 
     Annular conduit  46  has several nozzles  52   a ,  52   b ,  52   c ,  52   d  formed therein, which are illustrated schematically in  FIG. 2 . Via these nozzles  52   a ,  52   b ,  52   c ,  52   d , constituting a nozzle assembly  52 , pressurized air generated via fan  50  and introduced into annular conduit  46  via connection  48 , is issued and upon deflection by the inner walls of the funnel-shaped housing portion  18   b  forms a carrier fluid stream  54  that is directed substantially vertically upwardly. This carrier fluid stream  54  is outlined in the figures by arc-shaped curves in broken lines. The carrier fluid stream  54 , starting from the nozzle assembly  52 , moves upwardly at a velocity that is determined by the gas pressure generated by fan  50  and by the geometric arrangement of nozzles  52   a ,  52   b ,  52   c ,  52   s  as well as the arrangement thereof in relation to housing  18 . Annular conduit  46  with nozzle assembly  52 , connection  48  and fan  50  are part of a fluidized bed arrangement  55  generating the carrier fluid stream  54  in operation of fan  50 . 
     At the height of the raw gas inflow opening  16 , the carrier fluid stream  54  is united with the raw gas stream  44 . This has the effect that foreign bodies entrained in the raw gas stream  44  are carried upwardly along with the carrier fluid stream  54  and thus are transported to filter unit  12 . If necessary, filtration aids are introduced via the additional inflow opening  20 , with the flow of filtration aids  45  being mixed as well with the carrier flow stream  54  and the filtration aids thus being entrained by the carrier fluid stream  54  and being transported upwardly towards filter unit  12 . On their way to filter unit  12 , these streams  54 ,  44  and  45  mix. This has the result that foreign bodies entrained in the raw gas stream  44  collide with the filtration aids and form agglomerations which then attach to the filter surfaces of filter unit  12 . 
     Filter unit  12  has a pressurized-air cleaning-off unit associated therewith, which is not shown in the drawings and is located on the clean gas side of filter unit  12  above filter elements  14 . In certain intervals, the pressurized-air cleaning-off unit acts on a particular filter element  14  so that the same experiences a pressure surge from its clean gas side. The pressure surge has the effect that material attached on the raw gas side of the respective filter element  14 , which as described hereinbefore comprises either pure filtration aids or agglomerations of filtration aids with foreign bodies attached thereto, is detached from the filter element  14  and falls down due to its gravity. 
     As soon as the cleaned-off particles (as mentioned, either filtration aids or agglomerations of filtration aids with foreign bodies attached thereto), upon having been detached from filter element  14 , come under the influence of the carrier fluid stream  54 , they do not only experience a force in downward direction due to their gravity, but also a force in upward direction due to the upward flow of the carrier fluid. This has the effect that these particles, at least as long as they are not in excess of a certain critical mass density, do not move further downwardly, but move again upwardly until gravity and the force exerted by the carrier fluid stream  54  are balanced. The consequence is that, at a specific height covering a distance from about the height of the filter unit  12  down to the height of the raw gas inflow opening  16 , a relatively stable filtration aerosol layer is formed. In this filtration aerosol layer, the filtration aids, the foreign bodies contained in the raw gas as well as the agglomerations formed by them are held suspended. This holds at least for agglomerations that are not yet fully saturated, the mass density of which has not yet become greater than the critical mass density due to too much loading thereof with foreign bodies. 
     The flow velocity of the carrier fluid stream  54  and/or the flow velocity of the raw gas stream  44  preferably is selected such that turbulence is established in the carrier fluid stream  54  at least as from its unification with the raw gas stream  44 . The filtration aerosol layer then will remain stable in its entirety, however, the individual particles of this layer will be well mixed by the turbulence, which enhances efficient agglomeration and new coating of the filter surfaces with filtration aids. 
     As soon as cleaning-off of each filter element  14  of filter unit  12  has been effected, there will be a sufficient number of particles of filtration aid material in the filtration aerosol layer for effecting further efficient agglomeration with foreign bodies entrained in the raw gas stream. Supply of fresh filtration aids via the additional inflow opening  20  then may basically be dispensed with. However, such particles in device  10  that have become heavier due to the attachment of numerous foreign bodies to the filtration aid material are no longer held in the filtration aerosol layer and fall down into collecting container  24 . This effect is indeed desired as such particles are no longer capable of allowing further foreign bodies to attach and thus are basically unusable for further use as filtration aid. The thus caused continuous loss of filtration aids during operation can be balanced by fresh filtration aids that are supplied via the additional inflow opening  20 . 
     Initial operation of device  10  is best effected in that fan  50  is put into operation first, thus causing the carrier fluid stream  54  to be established. Thereafter, filtration aid is supplied via the additional inflow opening  20 . This causes a stable filtration aerosol layer to be formed on the one hand and on the other hand that a first protective coating of filtration aid material is already formed on the filter surfaces of filter unit  12 . Subsequently, the raw gas stream  44  is added via raw gas inflow opening  16 . As soon as the pressure loss across a respective filter element  14  of filter unit  12  becomes greater during further operation than is specified for operation (or after a specific time of operation), cleaning off of this filter element  12  takes place. This provides for further material for the filtration aerosol layer. The addition of fresh filtration aids can be dispensed with at the latest as of the time when all filter elements have been cleaned off once, apart from the addition of filtration aids for compensating losses of spent filtration aids that are discharged from the filtration aerosol layer. 
     Device  10  comprises furthermore an injector means  58 . This injector means  58  has a fan  60  feeding a pressurized air stream  62  to a low-pressure injector  64  and from there into collecting container  24 . On the side of collecting container  24  opposite to the low-pressure injector  64 , a low-pressure riser  66  opens into collecting container  24 . The low-pressure riser  66  extends upwardly and opens into the raw gas space  15  approximately at the height of the raw gas inflow opening  16 . As an alternative, the low-pressure riser  66  could open into the raw gas stream  44  also at a further upstream location. 
     Injector means  58  is of assistance when the device  10  is restarted after an interruption of operation. For, when device  10  is shut down and even if the filter surfaces are covered completely, the filtration aerosol layer as a rule will still contain filtration aid material that is not yet fully saturated and thus as such could still be used further. However, as soon as fan  50  is deactivated, the carrier fluid stream  54  collapses, with the result that all material in the filtration aerosol layer falls into collecting container  24  and from there actually would have to be disposed of via disposal opening  26 . In this situation, the low-pressure injector means  58  permits the material collected in collecting container  24 , which actually is still usable, to be withdrawn from collecting container  24  and returned to the filtration aerosol layer when device  10  is restarted. To this end, fan  50  is activated first, causing the carrier fluid stream  54  to be established. Following this, fan  40  is activated so as to stir up the material located on the floor of collecting container  24  and cause the same to enter into a fluidized state in which it can be carried out easily from collecting container  24 . Thereafter, fan  60  is activated, and the material located in collecting container  24  is supplied via riser line  66  to the filtration aerosol layer. Disposal opening  26  is closed at that time. As soon as the collecting container  24  has been emptied, fan  60  can be deactivated again, and the device  10  can be operated as described hereinbefore. 
     As an alternative to the low-pressure injector means  58 , there could also be provided a vacuum conveying means in order to withdraw the material present in collecting container  24  by suction and supply the same to the raw gas stream  44  upstream of the housing or convey the same directly to the raw gas stream  15  in the vicinity of raw gas inflow opening  16 . As a further alternative, conveying could be effected in fluidized form using pressure, e.g. by means of a solid diaphragm pump.