WO1990007982A1

WO1990007982A1 - Arrangement for cleaning ventilation air polluted with paint particles

Info

Publication number: WO1990007982A1
Application number: PCT/SE1990/000039
Authority: WO
Inventors: Sune Bengtsson; Hans Lannefors
Original assignee: Fläkt Ab
Priority date: 1989-01-20
Filing date: 1990-01-17
Publication date: 1990-07-26
Also published as: SE8900214D0; SE8900214L; AU4946190A

Abstract

An arrangement for cleaning ventilation air coming from a spray booth (1) for painting e.g. car bodies (2), and polluted with paint particles comprises a duct (7) which is located downstream of the spray booth and in which means (8) for separating the paint particles of the ventilation air is so arranged as to be located immediately underneath the floor (5) of the spray booth (1). The means (8) consists of at least one discharge electrode (10) serving as ion source, and at least one movable gas-permeable collecting electrode (12). Further, the collecting electrode is arranged transversely of the direction of flow of the ventilation air through the duct (7).

Description

ARRANGEMENT FOR CLEANING VENTILATION AIR POLLUTED

WITH PAINT PARTICLES

The present invention relates to an arrangement for cleaning ventilation air coming from a spray booth for painting e.g. car bodies, and polluted with paint par¬ ticles, comprising a duct which is located downstream of the spray booth and in which means is provided for sepa¬ rating the paint particles of the ventilation air. Venturi separators, so-called slot-type Venturis, have been used for a long time for separating paint par¬ ticles from ventilation air coming from spray booths. Se¬ paration is carried out by contacting the polluted venti¬ lation air with a circulating flow of water, part of the paint particles colliding with the water droplets and be¬ ing intercepted by them. See e.g. SE 196,718.

One drawback of venturi separators is that high-level noise is produced therein in connection with the rapid in¬ crease in velocity of the ventilation air passing through the venturi.

Another drawback inherent in venturi separators is that the ventilation air, after passing through the ven¬ turi separator, will contain both paint particles and wa¬ ter droplets as well as solvents evaporated in the spray booth.

A further drawback of venturi separators is that they must be supplemented with large, complicated and expensive plants for handling the flow of water coming from the ven¬ turi separator and polluted with paint particles. Such plants comprise, inter alia, settling tanks for sedimenta¬ tion of the paint slurry of the polluted water, systems for supplying different chemicals, such as flocculants and detackifiers, to the polluted water, equipment for remov¬ ing the sedimented paint sludge from the settling tanks, systems for recycling the purified water to the spray booth etc. See e.g. US 4,564,464. Also, since substantial flows of water are involved, the costs of operation of these plants become considerable.

In order that the objects, e.g. car bodies, which are painted in a spray booth should be provided with an ac- ceptable surface layer, it is required, inter alia, that the ventilation air has a certain temperature and a cer¬ tain relative humidity. Generally, the ventilation air is conditioned to about 23°C and a relative humidity of 60-70%. This means that much energy is required in cold climatic zones for conditioning the ventilation air, if it is not recylced to the spray booth.

Since, as pointed out above, the ventilation air hav¬ ing passed through the venturi separator contains both paint particles and water droplets as well as solvents evaporated in the spray booth, it must be further cleaned before it can be recycled to the spray booth. Otherwise, the remaining paint particles and water droplets would have a devastating effect on the quality of the surface layer of the objects painted in the spray booth. If pain - ing should be carried out by human beings instead of ro¬ bots, it is also vital that the above-mentioned solvent vapours be removed from the ventilation air.

Even if the ventilation air is recycled, it must be supplied with a certain amount of energy before it can be recycled into the spray booth to compensate for the cool¬ ing taking place in the venturi separator. In the venturi separator, the air can be cooled to 17-18°C and be given a relative humidity of 90-100%.

SE 8601290-3, for instance, describes a method for recycling the major portion of the ventilation air to the spray booth. This is achieved by conducting the ventila¬ tion air leaving the venturi separator through an electro¬ static precipitator operating according to the wet separa¬ tion method, before recycling it to the spray booth. This precipitator efficiently separates the particulate pollu¬ tants of the ventilation air, that is paint particles and water droplets, whereas not the solvent vapours contained therein. In this case, it is not necessary to remove the solvent vapours from the ventilation air since painting robots are used in the spray booth.

By using the above-mentioned technique, the energy consumption of the painting plant is reduced. However, since a conventional venturi separator is still used, this technique suffers from most of the shortcomings mentioned above. Further, using an electrostatic precipitator in¬ volves additional installation and running costs for the painting plant.

DE 37 05 634 describes a method of cleaning polluted ventilation air from a spray booth by directly conducting it into a conventional electrostatic precipitator operating according to the wet separation method. According to this prior art method, the venturi sepa¬ rator has thus been replaced by an electrostatic precipi¬ tator, which means that no annoying sound is produced, though efficient separation of the particulate pollutants in the ventilation air is obtained. Since the ventilation air is still sprayed with wa¬ ter, this technique also suffers from the above-mentioned drawbacks resulting from the use of process water, i.e. the costs for cleaning polluted process water and in¬ creased energy consumption because of cooling and humidi- fication of the ventilation air.

Another drawback of this technique is that the elec¬ trostatic precipitator used has a large volume, which means, for instance, that this separating device cannot be installed in an existing purification plant. The object of the present invention thus is to pro¬ vide an efficient, simple and relatively compact arrange¬ ment for cleaning ventilation air coming from a spray booth and polluted with paint particles, which device is both inexpensive to construct and operate and can also be installed by modifying an existing purification plant. According to the present invention, this object is achieved by means of a device which is of the type stated in the introduction to this specification and is charac¬ terised in that said means is so arranged in the duct as to be located immediately underneath the floor of the spray booth, and that said means comprises at least one discharge electrode serving as ion source, and at least one movable gas-permeable collecting electrode arranged transversely of the direction of flow of the ventilation air through the duct.

The collecting electrode preferably is designed as an endless belt.

The collecting electrode preferably is adapted to pass through means provided on one or both sides of the duct for mechanical and/or chemical cleaning of the col¬ lecting electrode.

The collecting electrode may be adapted to pass through means provided on one or both sides of the duct for coating the collecting electrode with a proctective surface layer with respect to paint particles.

The discharge electrode preferably is designed as pointed projections facing in the direction of flow of the ventilation air.

The discharge electrode may be movable. The discharge electrode may be designed as an endless belt.

The discharge electrode may be adapted to pass throug means provided on one or both sides of the duct for mecha¬ nical and/or chemical cleaning of the discharge electrode. Preferably, at least one field or control electrode having a potential not exceeding that of the discharge ele trode is disposed downstream of the collecting electrode. The field electrode preferably is designed as a row of rods extending parallel to the collecting electrode. The collecting electrode designed as an endless belt preferably is adapted to be passed twice by the polluted ventilation air, two parallel field electrodes preferably being provided in the space between the entry side of the collecting electrode and the exit side thereof.

A further field electrode preferably is provided downstream of the exit side of the collecting electrode. The invention will be described in more detail here- inbelow with reference to the accompanying drawing which is a vertical section of a painting plant comprising a spray booth and an arrangement according to the invention for cleaning ventilation air coming from the spray booth and polluted with paint particles.

The painting plant shown in the drawing thus com¬ prises a spray booth 1 through which car bodies 2 are ad¬ vanced while being sprayed with paint from stationary spray nozzles 3. These nozzles are automatically activated as a car body passes underneath. The paint may equally well be applied by means of robots, however not by humans, since the ventilation air is recycled without any previous removal of solvent vapours taken up in the spray booth. The car bodies 2 rest on a conveyor 4 travelling through the spray booth 1 immediately above the floor grating 5 thereof. The ceiling 6 of the spray booth is perforated and may also be provided with special air nozzles.

Below the floor grating 5, the spray booth passes into a duct 7 in which an electrostatic precipitator 8 operating according to the dry separation method is pro¬ vided transversely of the direction of flow of the venti¬ lation air, see arrows 9, through the duct.

The electrostatic precipitator consists of a sta- tionary discharge electrode 10 serving as ion source and designed as a grating or netting at the junctions of which pointed projections 11 are provided, and of a movable col¬ lecting electrode 12. The collecting electrode is made up of a number of perforated plates forming an endless belt passing over two rollers 13, 14. At least one of these rollers is driven by an electric motor (not shown). As appears from the drawing, the rollers 13 and 14 are arranged each in a chamber 15 and 16, respectively, provided on both sides of the duct 7. Each chamber is pro¬ vided with means for cleaning the collecting electrode. For each chamber, these means consist of a pair of first scrapers 17 located where the collecting electrode enters the chamber from the duct 7 through a slot 18 formed in the wall of the duct, a nozzle 19 provided after the first pair of scrapers 17 for spraying solvent on to the col- lecting electrode 12, a pair of cleaning brushes 20 dis¬ posed after the nozzle 19, and a pair of second scrapers 21 provided where the collecting electrode leaves the chamber and passes into the duct 7 through a slot 22 form¬ ed in the wall of the duct. Further, the bottoms of the chambers are funnel-shaped to facilitate discharge therefrom of the particulate pol¬ lutants removed from the collecting electrode during the cleaning operation. Of course, the outlets of the funnels communicate with a suitable collecting and handling system. Field electrodes 23 in the form of rows of rods ex¬ tending parallel to the collecting electrode are provided both in the space between the entry side of the collecting electrode 12 and the exit side thereof, and below this latter side. In the illustrated embodiment, two rows have been arranged in the above-mentioned space whereas only one row has been arranged below the exit side. This confi¬ guration is of course not the only conceivable one, but the number of field electrodes arranged between the two sides of the collecting electrode and below the exit side thereof may be varied in different ways. It is of course also possible to arrange several collecting, discharge and field electrodes below each other.

As further appears from the drawing, the collecting electrode 12 is grounded while the discharge electrode 10 and the field electrodes 23 are connected each to the ne¬ gative pole of a d.c. source 24 and 25, respectively. The positive poles of the d.c. sources are connected to the collecting, electrode and jointly grounded. The voltage U2 of the d.c. source 25 exceeds or is equal to the voltage Ul of the d.c. source 24. The discharge electrode has, for instance, a potential of -35 kV while the field electrodes have a potential of -45 kV.

Downstream of the electrostatic precipitator 8 the duct 7 passes into a conduit 26 which, just before its upper end, branches into two conduits 26a and 26b. The conduit 26a recycles the major portion of the ventilation air into the upper part of the spray booth 1 through an air supply unit 27 in which fresh conditioned air is sup¬ plied to the ventilation air through a conduit 29. The conduit 26b, on the other hand, emits the remaining venti¬ lation air into the atmosphere through an incinerator 28 for the combustion of the solvent vapours of the ventila¬ tion air.

The operation of the painting plant will be described in more detail hereinbelow with reference to the drawing. The ventilation air flows into the spray booth through the perforated ceiling 6 and when passing through the spray booth it captures such paint particles as have not adhered to the car bodies 2 during the painting operation. The air then flows on down through the floor 5 of the spray booth, to the electrostatic precipitator 8. In the electrostatic precipitator, the paint par¬ ticles entrained are charged by electrons emitted from the pointed projections 11 of the discharge electrode 10, and are then carried by the ventilation air and the electric field produced between the discharge electrode and the outer entry side of the collecting electrode 12, against said outer entry side. The paint particles that are not captured by the plates of the collecting electrode, but flow through the perforations of the plates into the space between the entry and exit sides of the collecting elec- trode, will be carried back on to the inner entry side by the electric field produced between said inner entry side and the topmost field electrode 23, in a direction con- trary to the direction of flow of the ventilation air. The paint particles which have not yet been captured by the collecting electrode 12 will be applied to the inner exit side by the ventilation air and the electric field pro- duced between the central field electrode 23 and said in¬ ner exit side. The remaining paint particles flowing out through the perforations in the plates of the collecting electrode will be carried back against the outer exit side by the electric field produced between said outer exit side and the lowermost field electrode 23, in a direction contrary to the direction of flow of the ventilation air.

After the ventilation air has passed through the electrostatic precipitator 8, there are practically no paint particles left in the ventilation air which can be recycled to the spray booth through the conduit 26 by a fan (not shown), without previously having to pass through additional separating means. Since only about 10% of the air flow is drawn off through the conduit 26b, the venti¬ lation air will be recycled about 10 times before passing through the incinerator 28, which means that the solvent content of the ventilation air is concentrated to such an extent that the solvent vapours can be combusted in the incinerator substantially without any supply of additional fuel. The paint particles adhering to the collecting elec¬ trode 12 are moved into the chambers 15 and 16 as the col¬ lecting electrode passes over the rollers 13 and 14. In the chambers, the first pairs of scrapers 17 first scrape off the loose paint particles, whereupon the nozzles 19 spray a suitable solvent on to the collecting electrode to dissolve paint particles adhering to it. These paint par¬ ticles are then removed from the collecting electrode by the cleaning brushes 20, whereupon any remaining paint particles are scraped off by the second pairs of scrapers 21. The paint particles removed from the collecting elec¬ trode 12 will then drop by gravity to the funnel-shaped bottoms of the chambers and from there on to the collect¬ ing and handling system mentioned above.

The invention is of course not restricted to the em¬ bodiment described above, but may be modified in several different ways within the scope of the accompanying claims. In those cases where there is a risk of the paint par¬ ticles also adhering to the discharge electrode, it is ad¬ vantageous to design the discharge electrode as a movable endless belt which is cleaned in the same manner and by such means as have been described above in connection with the cleaning of the collecting electrode.

For the same reason, the field electrodes may of course also be movable.

It is also conceivable to provide in the chambers 15 and 16, for instance after the second pairs of scrapers, means for coating the collecting electrode with a thin oil film to prevent the paint particles from adhering to it. In those cases where the ventilation air contains a relatively small amount of paint particles, it would be sufficient to arrange only one cleaning chamber on one of the sides of the duct 7.

Finally, the collecting electrode may consist of a netting or grating instead of a number of perforated plates.

Claims

1. Arrangement for cleaning ventilation air co ing from a spray booth (1) for painting e.g. car bodies (2), and polluted with paint particles, comprising a duct (7) which is located downstream of the spray booth and in which means (8) is provided for separating the paint particles of the ventilation air, c h a r a c t e r i s e d in that said means (8) is so arranged in the duct (7) as to be lo¬ cated immediately underneath the floor (5) of the spray booth (1), and that said means comprises at least one dis¬ charge electrode (10) serving as ion source, and at least one movable gas-permeable collecting electrode (12) ar- ranged transversely of the direction of flow of the ven¬ tilation air through the duct (7).

2. Arrangement as claimed in claim 1, c h a r a c ¬ e r i s e in that the collecting electrode (12) is designed as an endless belt.

3. Arrangement as claimed in claim 1 or 2, c h a ¬ r a c t e r i s e d in that the collecting electrode (12) is adapted to pass through means (17, 19, 20, 21) provided on one or both sides of the duct (7) for mechanical and/or chemical cleaning of the collecting electrode.

4. Arrangement as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the collecting electrode (12) is adapted to pass through means provided on one or both sides of the duct (7) for coating the collecting elec¬ trode with a protective surface layer with respect to paint particles.

5. Arrangement as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the discharge electrode (10) is designed as pointed projections (11) facing in the direction of flow of the ventilation air.

6. Arrangement as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that the discharge electrode (10) is movable.

7. Arrangement as claimed in claim 6, c h a r a c ¬ t e r i s e d in that the discharge electrode (10) is de¬ signed as an endless belt.

8. Arrangement as claimed in claim 6 or 7, c h a - r a c t e r i s e d in that the discharge electrode (10) is adapted to pass through means provided on one or both sides of the duct (7) for mechanical and/or chemical clean¬ ing of the discharge electrode.

9. Arrangement as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that at least one field electrode (23) having a potential not exceeding that of the discharge electrode (10) is disposed downstream of the collecting electrode (12).

10. Arrangement as claimed in claim 9, c h a r a c - t e r i s e d in that the field electrode (23) is designed as a row of rods extending parallel to the collecting elec¬ trode (12).

11. Arrangement as claimed in claims 2 and 10, c h a r a c t e r i s e d in that the collecting electrode (12) designed as an endless belt is adapted to be passed twice by the polluted ventilation air, and that two parallel fiel electrodes (23) are provided in the space between the entry side of the collecting electrode (12) and the exit side thereof.

12. Arrangement as claimed in claim 11, c h a r a c ¬ t e r i s e d in that a further field electrode (23) is provided downstream of the exit side of the collecting electrode (12).