APPARATUS FOR AIR CLEANING IN CONJUNCTION WITH SPRAY PAINTING.

TECHNICAL FIELD The present invention relates to an apparatus for cleaning air which has been polluted or contaminated by sticky or glutinous particles such as paint or lacquer particles obtained during paint spraying. The apparatus is of the kind comprising a filter means including a movably arranged material web, and means for evacuating the polluted or contaminated air through said web so that the pollutants or contaminants are separated from the air passing through. The apparatus is particularly suitable for being a part of a defining surface in a paint spraying space, e.g. a wall in a spray booth or the like.

BACKGROUND ART

An apparatus of the kind mentioned above is previ¬ ously known, e.g. from the U.S. PS 2,841,073. This known apparatus includes a disposable, highly absorbent fabric cloth or curtain, hanging as an inner filter wall in a spray booth and through which air charged with paint is sucked. The fabric curtain, which thus has a convention¬ al felted or porous structure, starts from an upwardly arranged storage roll and its lower portion is received in a downwardly arranged collecting trough. When an ex¬ posed length of the fabric curtain has taken up a given quantity of paint, a new dray.section is rolled from the storage roll simultaneously as the consumed portion is collected in the collection trough. When the entire roll has been used, a new one is put in place and the filter material web polluted by the paint it has taken up, and collected in the collecting trough, is thrown away.

Although there is the advantage with an apparatus of this kind of avoiding the usual work with paint- -separating curtains or cascades of water, to which are added various chemicals, the disadvantages involved, such as a nonuniform effect due to increasing clogging of the filter material web and fequently repeated ex¬ changes of the latter with accompanying high costs, have meant that the apparatus has not obtained any practical importance. It has also been previously proposed mere air .. filters, i.e. filters for removing dust etc. from ven¬ tilation air, these filters comprising an endless, circulating, oil-soaked filter material web through which the ventilation air is sucked (cf. U.S, PS 2.144.451, for instance). The filter material web, which is caused to pass through an oil trough for removing collected dust particles and for -being soaked with oil, has a conventional filter structure, i.e. it includes a felted or porous material such as aluminium wool, of not unconsiderable thickness. The filter material web is built up of filter material sections which are con¬ nected to each other via stiff impermeable transverse members, whereby the filter material web can accommodate itself to the return means or turnstiles over which it passes.

An air filter of this kind is impossible to use in conjunction with paint spraying, since in such a case it would be clogged up by the large amount of sticky or glutinous paint or laquer particles which are taken up. It can also be noted that air filters of this kind now have not even any practical use of importance for clean¬ ing ventilating air.

OBJECT OF THE INVENTION

The object of the present invention is to provide an improved apparatus of the kind given in the intro-

duction, with which there is gained a number of advanta¬ ges such as:

No risk of the filter means being clogged;

Simplified maintenance due to self-cleaning effects and small waste amounts which can easily be destroyed;

Extremely good separation capacity;

Very uniform evacuation;

Very low noise level; Very low material consumption;

No use of chemicals is required;

Low energy consumtion;

Low operating costs.

SUMMARY OF THE INVENTION The above-mentioned object is achieved by the apparatus in accordance with the invention being given the characterizing features disclosed in the accompaying claims.

The apparatus in accordance with the invention is thus essentially distinguished in that the filter means comprises an endless band or strip-like materialweb, which has a substantially smooth surface with direct perforations intended for the passage of air, these per¬ forations being relatively very small, and which is arranged for circulating round at least a first and a second return or deflecting means, a first or front ex¬ posed web part being intended for the passage of eva-

., , , . -, , , , part or cuated polluted air and preferably a second or rear/part portion of the material web being arranged for connection to said air evacuation means so that the air is sucked out further through the second part or part portion of the material web; in that means are provided for supply¬ ing a liquid dissolving or taking up contaminants en¬ trained in the air flowing through the web, said liquid supply means preferably including a container containing

said liquid, through which the material web is arranged to pass; in that means are arranged for controlled drive of the endless material web for circulating round said return or deflecting means; and in that means are arranged for coaction with the web for removing from the web contaminants separated from the air and taken up by the liquid.

The contaminants" ^* separated from the air will substantially be taken up by the liquid on the material web, and removal thus substantially means the removal of contaminated liquid from the web, before the latter,, provided with new liquid, continues once again to run over the air filtering and particle collecting passage. One and the same liquid is preferably utilized, this liquid being circulated, i.e. the removed contaminated liquid is collected for re-use. This means very _. low liquid consumption and can suitably be carried out by means of the previously-mentioned liquid container. In conjuction with the collection, taken-up contaminants can settle for later removal. This purification of the liquid can also be performed in a separate purification or sedimentation vessel, to which collected liquid is taken either continuously or at given time intervals, depending on how contaminated the liquid is. The means for removing contaminated liquid from the material web are arranged to advantage in a liquid container, especially submerged in the liquid therein, and can include to advantage stripping or scraping means coacting with the outside of the web. Although mechanical scrapers are generally to be preferred, means of the liquid flushing type can also be utilized.

The material web is suitably thin (with a typical thickness of the order of magnitude of 1 mm) and easily flexible in its circulation direction. The rear surface of the web is also suitably substantially smooth or uniform, and the perforations in the web are suitably

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distinct through-holes, preferably in a compact or solid material. The web shall thus not have a felted or similar structure, e.g. such as would enable paint particles to fasten in it and clog it up. The material web utilized should have a surface structure allowing easy removal of contaminants taken up. It should be particularly well-suited to coaction with scraper or stripping means, and for this purpose it can suitably have a completely smooth, glossy and homogenous surface, which has the necessary orifices or perforat¬ ions for the direct passage of air. The material should not be affected by the liquid or solvent in the paint taken up, but should be wetted by the liquid.

To advantage, the web can have the form of a band or strip of suitable width, consisting of, or coated with a plastics material. The plastics material is ad- vantageously polytetrafluoroethylene (PTFE; Teflon ). It is however also possible to use perforated thin steel sheet, for example, with or without a special surface coating.

The perforations of the web, or the orifices made in an equivalent way, which allow air to pass through the web, are preferably made so that evacuation will be substantially uniform over the whole of the exposed material web portion. In this way there is generally obtained very advantageous uniform airflow conditions, e.g. in the spray booth in which the apparatus is incor¬ porated. This evenly distributed evacuation can usually be obtained by having substantially equally-sized holes distributed substantially uniformly over the exposed portion of the material web. However, for obtaining special marginal effects with respect to the flow con¬ ditions, for example, it is possible to have a different size of hole at the edge or marginal areas of the expo- sed portion of the web. It is also possible to allow some parts of the web to be unperforated, e.g. where .

the web may require support against the suction action, such as at the middle of the web if the web has great width.

The favourable airflow conditions achieved by the apparatus in accordance with the invention means that smaller quantities of air can be evacuated with retain¬ ed good separating effect, as well as operation with lower airflow speeds, this being advantageous from the point of view of noise, energy and investment. It is thus possible to utilize more;slender dimensioned evacuat¬ ion fans and ducts than previously. It is thus not necessary to process greater amounts of evacuated air than what is required according to current regulations. Both with regard to the separating effect as well as the noise and the suction effect, the size of the holes and the total hole area is, of course, of import¬ ance. It has been found suitable to have a total hole area which is between about 2 and about 20 , particular¬ ly between about 5 and about 10%, and preferably about 7% of the total exposed material web surface. The holes can suitably have a largest opening dimension, especial¬ ly diameter, which is between about 0.5 and about 3 mm, preferably- about 1 mm. A suitable airflow rate through the holes can be between about 3 m/s and about 15 m/s, typically 7-8 m/s.

It should be emphasized that the flow of air, on passing into the holes in the web in the apparatus in accordance with the invention- is of a nature such that a special separating effect is achieved. As a result of the special flow conditions, the air mass loaded with particles moves substantially linearly towards the ex¬ posed material web. The air is deflected and sucked through the holes in the material web first when it is very close to the web. However, the majority of the particles entrained in the air do not have time to be deflected sufficiently for being sucked into the holes,

but strike, and remain on the liquid-coated web between the holes. To a certain extent, the particles moving substantially straight towards holes in the material web will be taken up by the liquid in and around the holes. Particles passing through the holes have been found to have a tendency to be deposited around the rear opening of the holes, as a result of the air sucked through the holes being deflected back towards the rear side of the web to some extent _? The particles, which nevertheless accompany the air to the rear part of the web, will be separated there by effects similar to those already described for the front part of the web.

Since the air which is sucked out by the air evacuation means thus suitably first passes through a front or upper exposed part or part portion of the end- les's material web, and thereafter through a second or rear part or part portion, .the majority of the contami¬ nants carried by the evacuated air will be taken up on the exposed web part simultaneously as a further very good post-filtration takes place at the second web part. It has been found that this gives extremely effective total filtration. Thus, at least 90-951 of the conta¬ minants such as paint can be taken up at the front part of the web, and in total more than 99% of the contami- nants can be taken up in the filter. If supplementary filtration should be desirable, a more conventional extra filter can be arranged, as will be accounted for later.

In order to obtain a good separating effect, it is important that the web is evenly, and suitably over- flowingly, provided with liquid. The liquid (which pre¬ ferably circulates in the apparatus) should not contain too many previously collected contaminants, when it is supplied to the endless web. Although the nature of the liquid is of importance in this connection, it is often sufficient for the supply of liquid to arrange the

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previously-mentioned liquid container so that the material web passes through the container after having taken up contaminants, partly to be liberated from heavily contaminated liquid and partly to take up new liquid. This new liquid accompanies the web during its course through the rear passage, over return or deflect¬ ing means and via the front passage back again to the liquid container, where it arrives with entrained par¬ ticles. The separated contaminants can settle in the container protected by the liquid. In some cases it is thus possible to use separated paint again.

If so desired, separate or extra liquid supply can, of course, be arranged in one or more places. For example means can be provided for flushing or drenching the web with liquid before or in conjunction with the exposure of the web, so that its front exposed part obtains such a rich supply of liquid that large quantities of conta¬ minants can be taken up without any problems.

To advantage, the web can be arranged to coact with a return or deflecting means (preferably arranged to be at least partially submerged in the liquid. in a liquid container) which is formed, e.g. as a roller, such that when the web passes over the return or de¬ flecting means, liquid is pressed through the perfo- rations in the web from its inside to its outside as a result of the wedging action between the return or deflecting means and the inside of the web, whereby a selfcleaning effect is obtained, which means that con¬ taminants deposited in the web perforations and on the rear of the web to a large extent are brought out onto the outside or front side of the material web for mechanical removal if required, e.g. by a following scraping or stripping means. Although this self-cleaning effect has the greatest importance in the front portion or part of the web when it passes through a liquid con¬ tainer, for example, a similar effect has importance

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for bringing contaminants at an upper return means from the front side to the rear side of a second or rear material web part.

With the object of improving the removal of con- taminants from the web, means can also be arranged for forcing liquid through the web at an increased pressure from its inside to its outside, suitably in conjunction with the web passing through the liquid in a liquid container, whereby contaminants deposited in the per- forations and at the perforations on the rear side of the material web are brought out to the outside of the material web. These means for forcing liquid through the web can comprise a liquid delivery nozzle connecting to a return or deflecting means and to the inside of the material web, and which define a wedge-shaped space therebetween, and means for feeding liquid at increased pressure to said liquid delivery nozzle.

It is also possible to allow a roller-shaped re¬ turn or deflecting means to be provided with stud-like projections which are arranged for engaging in the through-holes of the web to clean them when the material web passes by. A separate means, with similar construct¬ ion and effect, can also be arranged.

The utilized liquid should have a suitable vis- cosity, so that it can accompany upwardly-going parts of the endless material web, and so that it is only in¬ significantly entrained by the air flowing through the material web. The liquid should also wet the web well, and have a specific weight differing from the specific weight of collected contaminants, so that the latter can easily be separated from the liquid in a liquid container. The liquid should further not react with contaminants such as paint or solvent included in the paint or unfavourably act on the material web or other elements with which it comes into contact. Neither should the liquid be volatile.

To advantage, the liquid can be of the oil type, or a solvent for the contaminants. Organic oils have been found to be particularly suitable, more specifical¬ ly oils of the hydrocarbon type, preferably substan- tially made up from unsubstituted hydrocarbons. Paraffin oil is a particularly preferred liquid. Another example of a liquid is castor oil.

With regard to the drive of the endless material web, the drive preferably should be continuous, while the process giving rise to pollution is in progress, but intermittent operation is in itself possible when conditions so allow, and the liquid distribution on the material web will still be satisfactory.

A suitable operating speed for the web has been found to be between about 0.05 m/s and about 0.5 m/s, especially between about 0.1 m/s and about 0.3 m/s.

As previously mentioned, an extra filter can be arranged so that the evacuated air passes through it after first having passed through the rear part of the material web. The extra filter is suitably arranged before possible fan means and outlet ducts. This filter will catch both remaining particles and entrained liquid. In view of the nature of the latter, it can give the extra filter a self-cleaning effect, in as far as the liquid, after having saturated the filter, flows down from it into a collecting vessel underneath, or directly into the liquid container, while carrying with it particles deposited on the extra filter.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the apparatus in accordance with the invention, as well as some modifications thereof, are illustrated in the accompanying drawings. In the drawings, Fig. 1 is a front view of a spray booth in¬ cluding an apparatus in accordance with the present invention, certain cover plate parts being removed for the purpose of clarification; Fig. 2 is a partial

longitudinal sectional view of the booth of Fig. 1; Fig. 3 is a schematic, partial, sectional view illu¬ strating a modification of the apparatus of Figs. 1 and 2; Fig. 4 is a schematic, partial, sectional view illustrating another modification of the apparatus of

Fig. 1 and 2; Fig. 5 is a schematic, partial, sectional view illustrating a still further modification of the apparatus of Fig. 1 and 2; and Fig. 6 is a schematic view of a modification of the apparatus in accordance with the invention involving circulation of the utilized liquid with separation of collected paint in a separate sedimentation vessel.

DESCRIPTION OF EMBODIMENT

In Figs. 1 and 2 there is shown an apparatus in accordance with the invention, incorporated in a spray booth, the apparatus in accordance with the invention constituting the rear or inner wall 1 in the spray booth, which further includes side walls 3 and 5, a floor 7 and a ceiling portion 9. The vertically arranged apparatus includes a filter cassette with an endless material web 11,12 in the form of a band with a width such as to extend sub- ^■ stantially over the whole width of the booth. The band is taken over and guided by an upper, horizontally ex- tending, return and driving roller 13, and a bottom horizontally extending return roller 14. Rollers 13,14 are rotatably mounted in, and connect two like cassette cheek plates, of which only one 15 is shown in Figs. 1 and 2. At the top, cheek plates 15 (and thereby the filter cassette) are suspended from brackets 16 in the ceiling portion 9 of the booth, and are easily removab¬ le from the brackets.

The bottom portion of the filter cassette, in¬ cluding the lower return roller 14, hangs down in paraffin oil 20 in a container 21 , arranged on the spray

booth floor 7, this container extending across the whole of the spray booth width.

The band 11,12 comprises solid PFTE with a smooth surface, and has reinforced edge or marginal areas and evenly distributed through-holes 10, except in the mar¬ ginal areas, with a typical hole size of about 1 mm and with a total hole area of about 7% of the exposed surface

For guiding the front vertical part 11 and rear parallel part 12 of the endless band, the cheek plates are provided with inwardly projecting flanges 17,18 ex¬ tending substantially along the whole length of the cheek plates between rollers 13,14. A front flange 17 projects behind the front band part 11 at a distance from the front edge of cheek plate 15 somewhat exceed- ing the band thickness. A rear flange 18 projects behind, the rear band part 12 level with the rear edge of cheek plate 15. Flanges 17,18 project approximately to where the exposed or operative area of the filter band begins, i.e. the flanges guide the marginal areas of the band. For defining and framing-in the exposed operative portion of the endless band's front part (said portion constituting substantially the whole of the front part) above the liquid container 21 , as well as for fixing the filter cassette, there are two side cover plates 23,24 and an upper cover plate 25, which joint onto the side walls 3,5 and ceiling portion 9, respectively, of the spray booth. The side cover plates cover the margin¬ al areas of the endless land and adjoin the front edges of cheek plates 15. Upper cover plate 25 has an upper outwardly angled portion connecting to the ceiling thus giving favourable airflow conditions, and a lower portion connecting to the front edges of cheek plates 15 and extending downwards to just under upper return roller 13 close to the endless band's front part 11. On its rear side, the filter cassette is connected to an evacuation system via a connection means, made as

a box-like frame, the side members 27 of which (only one member visible in Fig. 2) are arranged to engage against the rear edges of cheek plates 15, and the upper and lower cross members 28 and 29, respectively, of which are arranged to be close to the endless band's rear part 12. The through-opening of the connection means correspondsto the exposed portion of the endless band's front part.

The connection means is connected to an evacuat- ion duct 31 having an evacuation propeller fan 32 via vertically arranged extra filter 33. There is a collect¬ ion container 35 arranged under the extra filter for collecting oil and possible entrained paint accumulat¬ ing at the filter and dropping down from it. Means are arranged for driving upper roller 13 to drive the endless band in the direction indicated by arrow 40. These driving means include a first driving wheel 41 secured on the shaft of roller 13 and on the other side of cheek plate 15, and a second driving wheel 42 which is coupled to an electric motor 44 via coupling means 43, not shown in closer detail. Driving wheels 41 and 42 are made such that they simply and securely go into driving mesh with each other when the filter cassette is hung up in brackets 16 and is caused to come into engagement against connection means 27,28,29 and is fixed by means of cover plates 23,24,25.

For removing taken-up paint from endless band 11, 12 when it passes through liquid container 21, a scraper or stripping means 47 is arranged on the filter cassette for coaction with the band when it passes round lower return roller 14. The scraper is flexible and engages lightly against the band at a point shortly after the band has come into contact with the roller. In this way the removal of paint from the band will be facili- tated by the wedging action occurring in the wedgeshaped space 49 between the smooth roller and the approaching

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band, and which forces out the paint taken up in the band holes to the outside of the band. The scraper, which can be made from rubber material, extends over the whole width of the endless band. Of course, the scraper also could be suspended in the liquid container itself, on the bottom of which separated paint 50 is collected. This separated paint can easily be removed either continuously or at suitable intervals.

It should be emphasized here that the separated paint 50 is shielded under oil 20, and therefore does not dry or oxidize, and in certain cases it can be used again. The protective effect of oil 20, as well as the fact that separated paint is rapidly taken down into container 21 , furthermore means that there can be small discharge from the apparatus of solvent incorporated in the paint.

In operation, air is sucked through the moving, endless perforated band 11,12, as is indicated by arrows in Fig. 2. The band is soaked by paraffin oil, which accompanies the band when its rear part 12 goes up out of the paraffin oil 20 in container 21. By the band being soaked by oil, there are no problems with friction where the band rubs against other parts (e.g. flanges 17,18 as a result of the backward pull of the suction on both parts of the band) .

Circulation of the oil in the apparatus means that the apparatus is continuously subjected to a purification process, which together with the previous¬ ly described self-cleaning effects for the band 11,12 and extra filter 33 results in that service of the appa¬ ratus is extremely simplified. The obtained automatic concentration of the separated contaminants furthermore means that destruction thereof (if such is necessary) is- facilitated. Destruction is also facilitated by the fact that the oil-soaked contaminants can be quite simply burnt up in many cases.

A modified embodiment of liquid container 21 and scraper 47 is illustrated very schematically in Fig. 3; the removal of paint 51 taken up on band 11 by means of wedging and scraping action being illustrated at the same time (at 52 and 53, resp.)

Liquid container 21 in this embodiment has a backwardly and downwardly decreasing area towards a rearwardly placed collection pocket 55, from which . collected paint 50 is arranged to be sucked away via a pipeline 57, e.g. for further separation and re-use. Scraper 47 includes a scraper blade 63 attached to a transverse support and screen element 61. Element 61 is pivotably arranged on a shaft 65 journalled in the side walls of container 21, shaft 65 being eccent- rically arranged such that scraper blade 63 engages against band 11 by gravity. A downwardly directed screen plate 67 is attached to the forward portion of element 61, and opposingly arranged with respect to scraper blade 63. Screen plate 67 together with a horizontal screen plate 69 projecting from the rear wall of the container 21 under roller 14, contributes to cause paraffin oil, liberated from paint, to flow up on the rear side of roller 14 and is conveyed on the rear part of the endless band. The liquid level in container 21 is such that roller 14 is covered by .... liquid and that there is some circulation of the liquid in the container, whereby deposition of paint 50 on the bottom of the container is facilitated.

In Fig. 4, there is similarly very schematically shown a modification of the apparatus of Figs. 1 and 2, meaning that liquid 20 in container 21 is forced through the front part 11 of the band at a slightly increased pressure for the purpose of cleaning holes, just when front part 11 approaches roller 14 and scrap- er means 47, i.e. when the band is at the previously-

- entioned wedge-shaped area 49. For this purpose there

is provided a trough-like or slot-like elongate nozzle 71 so that its longitudinal edges having flexible lips 73,74 engage against band part 11 and roller 14, resp. Nozzle 71 is supplied with liquid via a pipe 75 from a pump 77, the inlet pipe 79 of which sucks liquid from container 21.

In Fig. 5, there is very schematically shown, how an apparatus of Figs. 1 and 2 can be supplemented by means for supplying extra liquid to the endless band's front part 11. Thus, a liquid container 80 ex¬ tending along the length of roller 13 is attached to upper cover plate 25, the container having an overflow edge 81. Container 80 is fed via a pipe 82 and a pump (not shown) with liquid from container 21 , so that liquid flows out over edge 81 and down onto the endless band in conjunction with the latter passing between roller 13 and the bottom edge of plate 25. A very uni¬ form distribution of a lavish amount of liquid is here¬ by obtained on the endless band's forward part 11. An alternative embodiment of the liquid container is schematically illustrated in Fig. 6, special means being connected to the container for separating or sedimenting the paint particles taken up by the liquid and subsequent recycling of clean liquid to the contain- er. Said special means include a pipe 83 connected to the tapering bottom portion of container 21 for dis¬ charge of contaminated liquid from the liquid container, a pump 84 connected to pipe 83, a pipe 85 connected to the outlet side of the pump and opening out into a closed sedimentation vessel 87, and a pipe 89 connected to the sedimentation vessel for recycling liquid 20 liberated from paint particles 50, said pipe 89 open¬ ing out in a plate 91 sloping backwards and upwards from the rear wall of liquid container 21. The plate is adapted to extend under an extra filter (not shown here) of a kind already mentioned, for catching liquid

dropping down therefrom, recycling of clean liquid from the pipe taking place at a level such that the liquid dropping down from the extra filter is flushed by clean liquid recycled to the upper part of the liquid container via plate 91.

The upper part of sedimentation vessel 87 is cylindrical and has an inner cylindrical screening plate 93 connected to, and downwardly projecting from the lid portion 95 of the vessel, so that a relatively narrow annular gap 97 is formed between plate 93 and the wall of vessel 87. Pipe 85 opens out tangentially in gap 97 at the top thereof, and pipe 89 is connected to the middle of lid portion 95, the opening of pipe 89 being surrounded by a tubular strainer means 99, extending below the lower edge of plate 93. When pump 84 is in operation, liquid polluted with paint particles is thus pumped from container 21 into gap 97, in which it sinks downwards while rotating and flows past the lower edge of screening plate 93 and inwards and upwards in vessel 87. Simultaneously the corresponding amount of liquid is forced out into pipe 89 inside strainer means 99. When pump 84 is not in operation, there is a sedimenta¬ tion of paint particles towards the bottom of vessel 87, as well as some , deposition of paint particles to- wards the bottom of container 21. As will be understood, the operating conditions of the pump can be controlled (rate of liquid circulation; continuous or intermittent operation) so that there is adjustment to prevailing operating conditions and so that the best sedimentation effect is achieved in the sedimentation vessel.

When a given quantity of separated paint 50 is collected at the bottom of the sedimentation vessel, which can be downwardly conical, for instance, this paint can be simply tapped off by bottom emptying means 96, a suitable quantity ^" of clean liquid being ^' refilled, e.g. in the liquid container or upwardly in the sediment¬ ation vessel.

It is, of course, possible to utilize two or more sedimentation vessels, which are used alternatingly for the same liquid container, and after changing of the vessels, the one from which paint sediments are to be emptied can stand undisturbed a given time before this is done.

It will be understood that with a liquid circu¬ lation system of the kind described above, in combinat¬ ion with the construction of the apparatus in other respects, there is achieved that the spray booth itself can be utilized practically without any interruption in operation for cleaning etc . , as well as that the liquid container can contain a smaller amount of liquid. The invention is naturally not limited to the described embodiments, but further alterations and modifications are possible within the scope of the following claims.

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