GB2160120A - Scrubbing gases - Google Patents

Abstract

An installation is provided for cleansing a gas polluted by solid and/or liquid particles, comprising a first streaming sheet washing device (4) disposed close to the work enclosure (1), a second venturi and sprayed jet washing device (20) disposed downstream of the first washing device (4) and a centrifugal separating device (29) disposed downstream of the second washing device (20). <IMAGE>

Description

SPECIFICATION Improvements to installation for cleansing a gas polluted by solid and/or liquid particles The invention relates to installations for cleansing a gas polluted by solid andior liquid particles and taken from a work enclosure in which objects are subjected to a surface treatment.

The invention applies more particularly, but not exclusively, to installations for cleansing air polluted by a suspension of fine paint particles and taken from a paint spray unit in which objects, particularly motor vehicle body-works, are coated with layers of paint.

It has already been proposed to provide such installations with a streaming sheet washer device in which the polluted gas from the work enclosure passes through washing liquid sheets, said washing device being disposed close to the work enclosure.

If such a streaming sheet washer device is used alone, so as to obtain efficient cleansing, a cumbersome washing device is required (multiplicity and series arrangement of the streaming sheets) and noisy (dimensions of the streaming sheets, small passage sections for the gas).

Now, requirements are becoming more and more strict, not only in so far as cleansing efficiency is concerned but also in so far as the noise nuisance is concerned in the environment. A solution using such a streaming sheet washer device alone is therefore unthinkable, all the more so since the constructional and operating costs thereof are high.

There exist other types of washer devices, such as venturi and sprayed jet washing devices in which the polluted gas passes through at least one venturi into which is discharged at the level of the necked portion, at least one jet of sprayed washing liquid intended to produce a washing liquid mist through which the polluted gas passes.

However, if washing devices of this type are used alone, problems of clogging up are soon met with, particularly when it is a question of a gas polluted by a suspension of fine paint particles.

Finally; separator devices are known operating by difference of weight, more particularly by centrifugation, for collecting the solid and/or liquid particles of a polluted gas but, as in the previous case, these devices become quickly clogged up if the material forming such particles cannot be readily eliminated, which is of course the case when these particles are fine paint particles.

The aim of the invention is precisely is to avoid the above mentioned disadvantages, related to the different washing or separating devices used individually.

According to the invention, the installation comprises in combination, - a first streaming sheet washing device in which the polluted gas coming from the working enclosure passes through sheets of washing liquid, said device being disposed close to the work enclosure, said device being adapted so that its collecting efficiency is such that it collects substantially the whole of the particles with average dimensions greater than 6 microns and about half the particles having average dimensions less than 6 microns, - a second venturi and sprayed jet washing device in which the partially cleansed gas from the first washing device passes through at least one venturi into which is discharged, at the level of the necked portion, at least one jet of sprayed washing liquid, said device being adapted so that its collecting efficiency is such that it collects substantially the whole of the particles having average dimensions less than 6 microns still present in the partially cleansed gas, - and a centrifugal separating device in which the cleansed gas charged with washing liquid droplets which collected particles of average dimensions less than 6 microns in the second washing device is rid of these droplets which are deposited by centrifugation on the walls of said separating device and stream by gravity along these walls.

It will then be readily understood that, with the combination of these two washing devices and this separating device, - the first washing device, because of its relatively low efficiency (100% of particles with dimensions greater than 6 microns and only 50% of the particles with dimensions less than 6 microns), requires little space, is the cause of acceptable noise and is relatively inexpensive to construct and operate, - the second washing device, because it treats air already cleansed (containing only 50% of particles with dimesnions less than 6 microns), does not raise problems of rapid clogging up, - and the separating device, because it collects droplets of washing liquid which have only picked up particles of small dimensions and in relatively small amounts (50% of the particles with dimensions less than 6 microns), does not raise any clogging up problem, said droplets being removed by streaming without any risk of sticking to the walls.

In a preferred arrangement of the invention, the installation further comprises, downstream of said operating device, a gas purifying device, using more particularly a physico-chemical effect, for removing at least partially solvent vapors which the gas might still contain when cleansed and rid of the drops of washing liquid.

With such an arrangement, the thus purified gas may be either discharged directly into the atmosphere, or be recycled, at least partially, back to the work enclosure; in this latter case, savings in energy are obtained for the thus recycled gas presents temperature and hygrometry conditions which may be favorable for the treatment carried out in said enclosure, which then avoids having to bring the whole of the gas fed into this enclosure previously up to said temperature and hygrometry conditions, which is particularly advantageous in the case of an installation in which the treatment enclosure is formed by a paint spray cabin or unit.

According to another arrangement of the invention, a part of the washing liquid fed into the second washing device may come from the separating device.

The invention consists, apart from the arrangements discussed above, of certain other arrangements which are preferably used at the same time and which will be more explicitly discussed hereafter.

The invention will in any case be well understood from the complement of description which follows with the accompanying drawings, which complement and drawings are relative to a preferred embodiment of the invention and present of course no limitative character.

Figure 1 of these drawings shows a simplified diagram of an installation constructed in accordance with the invention.

Figure 2 shows a simplified diagram of an installation constructed in accordance with a particularly advantageous arrangement of the invention.

In these Figures 1 and 2, a paint spray cabin 1 is shown in which motor vehicle body-works 2 are coated with layers of paint, for example by means of a set of spray guns (not shown).

This paint cabin 1 is fed with clean air, brought previously up to the desired conditions of temperature and hygrometry, through a duct 39; for example this air is at 20 C and has a relative humidity of 70%.

Below this paint cabin is disposed a first streaming sheet washing device 4 in which the paint particle loaded air passes through sheets of washing liquid.

To this end, the first washing device 4 comprises a sloping floor 5 supplied with washing water through lateral overflow channels 6, themselves supplied with water through piping 7.

Through orifices 8 are formed in the sloping floor 5 and allow streaming sheets to be formed through which the paint particle polluted air passes. Other streaming sheets are formed between a transverse wall 11 and vertical walls 10 at the level of the passages 9 existing between this transverse wall 11 and these vertical walls 10.

The air polluted by the fine paint particles passes through the sheets of washing water and, through intimate contact with the washing water, is partially cleansed.

The first washing device 4 is adapted so as to - collect substantially the whole of the paint particles with average dimensions greater than 6 mi crons, - and to connect about half of the paint particles having average dimensions less than 6 microns.

The first washing device 4 may be produced in a relatively simple way (so have low constructional and operating costs), and cause only an acceptable noise nuisance (less than 80 dBa).

A system for removing the droplets of washing water is provided at the outlet of this first washing device 4, said system comprising a passage 13 in which the air is suddenly accelerated and the water droplets fall downstream of passage 13 by gravity and are collected in a tank 15. The water is then removed from this tank 15 through piping 16 which runs into the general pipe 17 for removing the water from the first washing device 4.

A system for treating the water (not shown) may then be provided, which may allow total or partial recycling of the treated water to the pipe 7 supplying droplets 6.

The partially cleansed air from this first washing device 4 is fed through a duct 19 to a second venturi and sprayed jet washing device 20. This partially cleansed air passes through a venturi 21 into which is discharged, at the level of the necked or constricted portion 22, a jet of sprayed washing water.

The washing water jet thus sprayed is projected orthogonally into the air flow and forms a dense sheet, ensuring an intimate contact between the water and the air which still contains about half of the paint particles of average dimensions less than 6 microns.

The second washing device 20 is adapted so that substantially the whole of these particles are then collected.

The washing water coming from a reservoir 27 is fed to the neck 22 of the venturi 21 by means of a pipe 25 and a pump 26.

The air thus cleansed by the first and second washing devices 4 and 20 but charged with droplets of washing water which, in the second washing device 20, collected the paint particles of average dimensions less than 6 microns, is then fed through a duct 28 to a centrifugal separating device 29.

This centrifugal separating device 29 comprises a tangential entry 30 to which duct 28 is connected.

This tangential entry 30 causes a rotational movement of the air containing the water droplets, and this rotational movement is accelerated in a zone 31 having an annular passage 30 formed between a central core 33 and a concentric vertical wall 34.

An upper vertical wall 35 collects the droplets of water which then stream along this upper vertical wall 35 and are collected at the level of the concentric vertical wall 34. This collected water is advantageously recycled to the reservoir 27 supplying the second washing device 20. Said reservoir being moreover supplied with fresh water.

By means of a duct 37 and a fan 38 the air from the separating device 29 may be discharged to a chimney 40 (Figure 1) or to a purifying device 41 (Figure 2), using more especially a physico- chemical effect, for removing the solvent vapors which the air may still contain after cleansing and removal of the droplets of washing water.

This purifying device 41 may be of the active charcoal, or organic oil spraying, or plasma field, or a Beta radiation field type.

Once the air has thus been purified, it is advantageous to recycle it wholly or partially to the duct 39 supplying the paint cabin 1: in fact, the temperature and hygrometry conditions of the air at the outlet of the purifying device 41 are very close to those required (20 C and 70 of relative humidity) for supplying the paint cabin 1.

The installation thus formed has a certain number of advantages among which may be mentioned those summed up below: - it is possible to provide continuous removal of the polluted gas from the work enclosure, with a high efficiency; the efficiency with particles having average dimensions less than 6 microns may be close to 95% then, in the case of a paint cabin with a concentration of 20mg of paint per m3 of air at the outlet of the cabin, a concentration at the output of the separator device may be obtained of 1 mg of paint per m3 of air, - the noise generated in the work enclosure by the flow of extracted gas is reduced to below a threshold of 80 dBa; this reduction is made possible by the fact that the pressure loss in the first washing device is purposely limited (of the order of 1600 Pa for a conventional washing device, this pressure loss is brought down to 800 Pa in the case of the invention), the reduction in efficiency of this first washing device which results therefrom being offset by the presence of the second washing device situated downstream; - - the second washing device is inexpensive to construct and also inexpensive to operate (particularly its water consumption); moreover, it does not generate noise pollution and its position, downstream of the first washing device, results in only slight clogging up thereof; - because of the efficient removal of the solid and/or liquid particles in the two washing devices and removal of the droplets of washing water in the separating device, a purifying device may be provided more particularly for removing the solvents, without any risk of choking for this purifying device; - the recycling, at least partial, of the purified gas from the purifying device into the work enclosure leads to substantial energy saving; in the particular case of the paint cabin, the air fed into this cabin must be at about 20 C and have a relative humidity of the order of 70%, these conditions being substantially those of the purified air which may there- fore be recycled directly into the paint cabin; - the consumption of washing liquid in the second washing device may be considerably reduced by recycling into this second washing device at least a part of the washing liquid coming from the centrifugal separating device; - the positioning of the different elements of the installation may be determined as a function of the different requirements, only the first washing device being situated close to the work enclosure, generally below.

Claims (7)

1. An installation for cleansing a gas polluted by solid and/or liquid particles, said polluted gas being taken from a work enclosure in which objects are subjected to a surface treatment, characterized by the fact that it comprises in combination: - - a first streaming sheet washing device (4) in which the polluted gas from the work enclosure (1) passes through sheets of washing liquid, said device (4) being disposed close to the work enclosure (1), said device (4) being adapted so that its collecting efficiency is such that it collects substantially the whole of the particles with average dimensions greater than 6 microns, and about half the particles having average dimensions less than 6 microns, - a second venturi and sprayed jet washing device (20) in which the partially cleansed gas from the first washing device (4) passes through at least one venturi (21) into which is discharged, at the level of the necked portion (22), at least one jet of sprayed washing liquid, said device (20) being adapted so that its collecting efficiency is such that it collects substantially the whole of the particles with average dimensions less than 6 microns still present in the partially cleansed gas, - and a centrifugal separating device (29) in which the gas which has been cleansed but is still laden with droplets of washing liquid which have collected particles with average dimensions less than 6 microns in the second washing device (20) is rid of these droplets which are deposited by centrifugation on the walls of said separating device and stream by gravity along these walls.

2. The installation according to claim 1, characterized by the fact that it further comprises, downstream of the separating device (29), a gas purifying device (41), using more especially a physico- chemical effect, for removing at least partially solvent vapors which the gas may still contain after cleansing and removal of the droplets of washing liquid.

3. The installation according to claim 1, characterized by the fact that the cleansed gas rid of the washing liquid droplets is recycled, at least partially, into the work enclosure.

4. The installation according to claim 2, characterized by the fact that the cleansed gas rid of the washing liquid droplets and purified is recycled, at least partially into the work enclosure.

5. The installation according to any one of claims 1 to 4, characterized by the fact that at least part of the washing liquid collected in the separating device is recycled into the second washing device.

6. The installation according to any one of claims 1 to 5, characterized by the fact that the work enclosure is formed by a paint cabin.

7. An installation substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.