IE41926B1 - Improvements in or relating to electrostatic powder-coating installations - Google Patents

Abstract

1493637 Electrostatic spray booth GEMA AG APPARATEBAU 12 Nov 1975 [14 Nov 1974 23 July 1975] 46791/75 Heading B2L In an electrostatic spray booth 10 of the type wherein excess powder from a spray gun 17 is removed therefrom through a moving filter strip 23 by suction of a blower 34, the filtered powder is removed from the upper run 22 of the strip outside the booth by a suction nozzle 44, the removed powder then being drawn by suction blower 49 via line 47 through a cyclone separator 48 wherein most of the powder removed from the air stream and passed via sieve 54 to supply container 19. Powder not separated at 48 is recycled to the booth via line 58 and discharge tube 59. Fresh powder is added to the system from container 41 onto filter strip 23 upstream of the suction nozzle 44 and downstream of corona discharge electrode 37. The suction nozzle 44 may be provided with a device illustrated in Figs. 2 and 3 (not shown) for pulsating the air in suction duct 47 so as to prevent powder deposition therein. The suction blower 49 is preferably of the injector type [Fig. 4 (not shown)] and comprises two parts (71), (72) defining an annular gap (76) therebetween through which compressed air is blown into a constriction (82) thereby drawing-in air from line 50.

Description

The present invention relates to electrostatic powder-coating installations of the kind which have a booth in which are arranged means to deliver eletrostatically charged powder, said means being connected to a supply container, and in which part of the space within the booth is bounded by the first face of a filter which is movable through the booth, the other face of the filter being adjacent the suction side of suction means which act on the said space through the filter, and in which a cleaning arrangement is associated with the filter to remove from it excess powder deposited thereon. Hereinafter, such installations will be referred to as of the kind described.

Because of the advantages which it offers in comparison with wet spraying (e.g. greater depth of coating per pass, absence of solvent vapours), it is hardly conceivable that present-day production techniques could dispense with the coating of articles by means of electrostatically charged powder (referred to below as electrostatic powder coating for short). There are however two factors involved in electrostatic powder coating to which particular attention has to be paid if this type of coating is to remain economically competitive with wet spraying.

Both factors are connected with the fact that a relatively large proportion of the electrostatically charged powder which is released either completely misses the article to be coated or else does not adequately adhere thereto, for example because the coating has already reached the required thickness. This proportion may be termed excess powder. On the one hand, one part of this excess powder collects on the parts of the installation in a similar way to dust and when the nature of the powder changes, when colour is changed for instance, it has to be removed, which takes time. On the other hand, another part of the excess powder is removed from the installation by a suction means forming part of the installation so that it may, if required, be fed back for re-use.

Hence, the first factor to which attention has to be paid in the time expended on cleaning the installation, bearing in mind that this time represents downtime or deadtime for the installation, and the second factor is the recovery and re-use of the excess powder, bearing in mind that this powder represents a considerable item in costs.

An installation of the kind described is known in which the major proportion of the excess powder settles on the 41936 filter as a result of the action of the suction means and so there is considerably less fouling of the rest of the installation by powder deposits. The cleaning arrangement removes the excess powder from the filter and feeds it to a collecting container. In this way, although the excess powder is not lost, it is still far from being capable of re-use in its existing form since it has lost a considerable proportion of its ability to flow freely. Also the known installation is incapable of continuous operation since, firstly, the collecting container has to be changed for an empty one every so often, and secondly the recovered powder, after being re-processed, has to be emptied back into the supply container, possibly together with fresh powder.

It is an object of the invention to provide an installation of the kind described in such a way that there is virtually no need for the powder extracted by the filter to be re-processed, with the result that it can be fed to the supply container directly and continuously in a true closed circuit with minimal risk of contamination.

Accordingly the present invention consists in an electrostatic powder coating installation of the kind described, wherein the cleaning arrangement has a suction nozzle which is directed towards one face of the filter and which is connected to the suction side of a suction blower via an interposed separator, the outlet of the separator being connected to the supply container.

In an installation according to the invention the powder is thus removed from the filter by suction, i.e. pneumatically, which necessarily means that it is carried away in the suction duot in suspension so to speak, in which - 4 41926 case the turbulence which arises preserves the powder's ability to flow freely, or should this in fact be necessary, restores to it this ability. In addition, due to the connection between the outlet of the separator and the supply container, a true closed circuit is set up for the excess powder and this permits continuous operation.

The filter, which advantageously takes the form of a filter strip, may extend beyond the booth in its direction of movement, the suction nozzle then advantageously being arranged in the area where the section of the filter band external to the booth is located. A particular advantage of this arrangement is that it is especially easy to add fresh powder and to mix it with the excess powder circulating in the closed circuit.

If desired, an outlet of a feed device for fresh powder, which outlet is directed towards the first face of the filter, may be arranged upstream of the suction nozzle along the line of movement of the filter. Consequently, not only is the excess powder sucked through the suction nozzle, but also the fresh powder which is deposited on the same face of the filter, so that in the course of this process the two types of powder are automatically intimately mixed with one another.

Between the booth and the suction nozzle may be arranged a device which acts on the filter so as to suppress electrostatic charges. In this way it is possible to remove any residual electrostatic charge there may still be on the excess powder deposited on the filter. Also, to keep the connection between the suction nozzle and the suction-blower free from deposits of the excess powder which flows through it in suspension, it is possible to provide.means for generating surges in the flow. As a consequence, the suction action of the suction nozzle becomes a pulsating action and favourable conditions are created for the formation of turbulence in the connecting duct.

Between the outlet of the separator and the supply container may be provided a sieve device, for the particular purpose of removing, by sieving the powder, any contamination which may have been picked up, prior to entry into the supply container and, if necessary to restore the powder's ability to flow freely.

Free-flowing characteristics in the powder will be more readily achieved in the separator the less exacting is the separator in separating out the powder from the flow of air. In other words, its free-flowing characteristics will scarcely be affected if the separator fails to separate out with 100% efficiency. On the other hand this means that the air pumped by the suction blower will still contain a residual amount of powder. It is therefore advantageous to connect the pressure side of the suction blower to the interior of the booth. Consequently, there is as it were a second, closed circuit formed, wherein any powder which may not have been separated out on the occasion of the first passage through the separator travels back to the filter, is removed therefrom by the suction nozzle, and is fed back once more to the separator.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which show certain embodiments thereof by way of example and in which:Fig. 1 is a schematic side view, partly in section, of an electrostatic powder coating installation, Figs. 2 and 3 show embodiments of a component of the installation shown in Fig. 1, and Fig. 4 is a schematic view of a modified embodiment.

Referring now to the drawings, the installation shown includes a booth 10 which is bounded by a top 11, end-walls 12 and 13, and side-walls which are not shown in the drawing. The internal space contained within the booth 10 is marked 14. As indicated by the double broken lines 15, a conveyor means may be provided which runs through the booth 10 and by means of which the articles to be coated may be conveyed, in the direction of arrow 16, through an archlike opening (not shown) in end-wall 12 into the internal space 14 and out again through a similar opening in end-wall 13. In the interior 14 of booth 10 is arranged at least one spray nozzle, in the form of a spray gun 17 for example, the outlet end of which is naturally directed towards the articles to be coated. The spray gun 17 is connected, via a connecting line (e.g. a hose 18) to a supply container 19 and by means of an input line 20 to a voltage source 21. Via input line 18 the gun 17 is supplied with the powder which is to be electrostatically charged, which is air-carried, and via input line 20 it is supplied with the electrical energy needed for the electrostatic charging process. Input line 20 may be arranged in or on hose 18.

The floor of the booth 10 is formed by the upper face of the upper run 22 of an endless filter strip 23, which runs on return rollers 24, 25 in the manner of a conveyor belt. One of the return rollers 24, 25 is driven, via a chain drive or a shaft 26, by a geared motor 27 in the direction of arrow 28.

The lower face of run 22 of the filter strip 23 rests 419 36 on the one hand on a laterally projecting flange 29 which runs round the open end of a suction trough 30 and on the other hand on a grille 31 which may be formed from fine wire-mesh and which covers over the open end of trough 30 in the same plane as flange 29. The space 32 within trough 30 is connected by means of a connecting duct 33 to the suction side of a blower 34. From what is said above it will be apparent that when the blower 34 is operating it acts on the internal space 14, through the part of the upper run which spans trough 30, in such a way that there is always a slight partial vacuum in this space and a slight flow of air in the direction of arrows 35. Due to this flow of air, excess powder which is projected by the spray gun 17 but fails to adhere to the article to be coated will be caused to settle mainly on the upper face of the upper run 22 of the filter. By selecting a filter strip 23 of the appropriate permeability, the powder can be prevented from passing through the strip.

During its movements, the upper run 22 of the filter strip 23 leaves the space 14 within the booth 10 at 36, while continuing to carry with it the powder whioh it has collected in the course of its travel. Outlet 36 is provided with a corona discharge electrode 37 which irradiates the upper face of run 22. Electrode 37 is powered by a high-voltage source 39 via a connecting lead 38. The corona discharge electrode 37 serves to remove any residual charge which may still exist on the powder present on the upper face of run 22. Electrode 37 could equally well be replaced by another device which produces an ionized i.e. an electrically conductive, area around it, e.g. a low-power beta emitter.

In the direction of travel 23^ of the upper run 22, and following the corona discharge electrode 37, is arranged a feed device which is referred to in general by reference numeral 40. Feed device 40 possesses a filling funnel 41 which supplies a metering screw 43 driven by a motor 42. The metering screw extends for virtually the entire width of the filter strip 23. The purpose of the feed device 40 is to supply the installation with fresh powder, either of the same or a different type, as has yet to be described.

Following the feed device 40 is a suction nozzle 44 which has a suction slot 45 which likewise extends for practically the entire width of the filter strip 23. Suction nozzle 44 is connected via a means 46 for producing surges of air, to a connecting duct 47 which leads to a separator 48 in the form of a cyclone. Separator 48 is connected via a suction duct 50 to the suction side of a blower 49, so that when the blower 49 is operating, a strong suction effect exists in connecting duct 47. The means 46 for generating air surges serves the purpose of making the suction effect exerted by blower 49 pulsating rather than continuous, which makes it more difficult for the powder removed by suction nozzle 44 from the filter strip 23 to settle on the inner wall of connecting duct 47, if it does not entirely prevent this. Separator 48 has an outlet 51 through which the powder which is separated out in separator 48 by means of the suction air is fed continously to a vibrating or rotary sieve 54, contained in a housing 61, by a valve, particularly a sluice valve in the form of, for example, a compartmented wheel 52 which is driven by a motor 53. Instead of preceding the sieve 54, the valve may follow the sieve. The compartment wheel 52 and the vibrating or rotary sieve 54 are arranged in a closed housing, which,among other things, serves to protect the powder on sieve 54 against contamination and, if there is no compartmented wheel 52, acts as an air-lock to maintain the partial vacuum prevailing in the separator 48 and to prevent incorrect air-flow from being caused. The powder, after having been treated, that is to say, freed as far as required of possible contaminants and rendered freeflowing again, by sieve 54 makes its way, via an outlet 55 for sieved material, directly and continuously to the supply container 19. Any coarser fractions in the powder which is removed by the suction nozzle 44 from the upper run 22 which are not broken down again by the action of the sieve 54, pass via a waste duct 56 to a waste container 57.

The pressure side of blower 49 is connected to a pressure duct 58 which in turn leads to the interior 14 of booth 10 and there opens into a distributor pipe 59 having a plurality of openings 60. The reason for this arrangement is, inter alia, that generally speaking the separator 48 cannot be so designed that it separates out all the powder in the stream of powder-laden air from connecting duct 47, due to the fact that this would create a risk of the powder particles separated out caking together in the separator 48, which would be an obstacle to immediate re-use, i.e. delivery into the supply container. Since as a result the separator 48 is deliberately allowed not to separate out all the powder from the flow arriving from connecting duct 47, a part of this powder travels via suction duct 50 to pressure duct 58 and thus to distributor pipe 59, where it makes its way back into the interior 14 of booth 10 and thus again becomes subject to 419 26 the suction effect of blower 34. As a result, it is again deposited on the upper face of the upper run 22 of filter strip 23. It should be noted that the throughput of blower 34 is at least twice and preferably many times greater than the throughput of suction blower 49, so that even when the pressure duct 58 opens into the internal space 14, this has only a negligible effect on the partial vacuum in the internal space.

From the foregoing it will be apparent that between return roller 24 and the driven return roller 25 the upper run 22 of the filter strip travels through roughly the following regions. In a first region a an increasing amount of excess powder from spray gun 17 settles on the said upper run. In the next region b, this settling of excess powder from spray gun 17 continues, but to it is added a residual amount of excess powder which had not previously been separated out in the separator 48. In regions a and b the powder is fixed in the filter material forming the filter strip 23 by the suction effect which exists as the filter strip is passing over the open end of trough 30. It cannot therefore be blown about on leaving booth 10, i.e. at the transition to region c. Also, the corona discharge electrode 37 acts in region c and this ensures that any residual charges which may possibly exist are removed. Assuming the feed device 40 to be in operation, the layer of powder on the upper run receives an increment of fresh powder, so that in region d of the upper run is found powder from a total of three sources. In region e the upper run is then freed of all the powder on it by suction nozzle 44, the three fractions being vigorously whirled about and mixed together before arriving at separator 48. Region e is followed 410 26 by a final region £, the significance of which will be explained more fully below.

Fig. 2 is a schematic view of a means 46 for producing air surges. Xn it can be seen a part of. the upper run 22, the suction nozzle 44 with its suction slot 45, and the beginning of the connecting duct 47. In a cup-shaped widened portion 62 which follows on directly from the suction nozzle 44 is formed a sort of conical valve seating 63 which co-operates with a closure member or float 65 which is pre-loaded by a weak pressure spring 64. When the installation is switched off, the float 65 normally almost seals the opening formed at the valve seating 63. As soon as the partial vacuum in connecting duct 47 is sufficiently great, float 65 lifts off valve seating 63 and a powerful flow of air is set up.

However, because of this, the pressure differential between the two sides of the valve seating 63 'drops and spring 64 is in a position to overcome this pressure differential and to force the float 65 back towards the valve seating 63. Hence, in time, the float 65 is brought to a state of oscillation, as a result of which the flow in connecting duct 47, and also, though to a lesser extent, the flow in suction duct 50, in pressure duct 58 and in separator 48, will be pulsating. Consequently it will at least be made more difficult, particularly for fine fractions of the powder taken up by the suction nozzle 44, to settle on the inner walls of these ducts.

Approximately the same effect can be achieved with the means shown in Fig. 3.In this means an air-infiltration opening is provided in connecting duct 47 immediately following suction nozzle 44, this opening normally being closed by a flap 67. If the flap is briefly opened periodically in the direction indicated by arrow 68, there is a powerful surge of air, due to the fact that the flowsection of opening 66 is considerably larger than the flowsection between the upper run 22 and slot 45. Of course, the time for which flap 67 is open is so calculated that during it the upper run only travels a distance which is considerably less than the width of slot 45.

The mode of operation of the apparatus described may be summarised as follows: The articles to be coated are moved through booth 10 in the direction of arrow 16. The spray gun 17 produces a cloud of electrostatically charged powder particles in the internal space 14, which cloud, owing to the fact that a slight partial vacuum prevails in the internal space 14, is confined to this space. A large proportion of the electrostatically charged particles are thus deposited on the articles to be coated and leave the booth while adhering to these articles. The proportion of powder which does not find its way to the articles settles on the upper run of the filter strip and thus arrives under the suction nozzle 44, where it is recovered and fed back to the supply container 19 via the separator. With the exception of that part of the powder which remains adhering to the articles, the powder thus travels round a first closed circuit, starting from the supply container 19 and passing through the hose 18 to the spray gun 17, from there into the internal space 14 and after settling on run 22 to the suction nozzle 44, then through the connecting duct 47 to the separator 48 and finally back through outlet 51 and sieve 54 to the supply container 19. It should be noted that the circuit is closed and is practically only accessible in regions c and e along the upper run. A further powder circuit is followed by that part of the powder which is not separated out by separator 48. This circuit begins at suction duct 50, then leads through suction blower 49, through pressure duct 58 and distributor pipe 59 and into the internal space 14, and then, after deposition takes place on run 22, continues to suction nozzle 44 and via connecting duct 47 leads back to the separator 48. This closed circuit too is practically only accessible on regions c to e of the upper run. To replace the powder which is used up in actually coating the articles, fresh powder toe is also deposited on the upper run 22 by the feed device 40. Hence, the supply container 19 is not topped up directly but the fresh powder is first deposited on the excess powder and is then picked up by the suction nozzle 44, whereby vigorous intermixing takes place. By comparing the quantity of powder delivered to the installation With the quantity actually used up in the coating operation, it has been found that powder losses in the installation described are well below 1% and it can therefore justifiably be said that recovery and immediate re use are complete, not forgetting that these operations take place continuously.

The installation also offers notable advantages as regards changing the nature of the powder. When such a change is made, first of all articles cease to be fed into the booth. Hose 18 is then detached from the supply container and blown out with compressed air. The remaining parts of the installation continue to operate. The powder still in the installation passes through one of the two circuits previously described until it finally comes to rest in the supply container which is no longer connected to hose 18. At the same time the filter strip may be subjected to a deeply penetrating cleaning operation in region f of the upper run, by means of a powerful industrial vacuum dust-extractor for example. Feed device 40 is not operating at this time of course. Supply container 19 can then be replaced by an empty one, and after this the feed device 40 can be loaded with the fresh powder and hose 18 can be connected to the new supply container 19. After a brief starting-up period the installation becomes fully supplied with the new colour without any blemishes due to powder particles of the old colour being apparent to the naked eye in the coatings which are formed. Apart from the short change-over period and that for cleaning the filter strip 23 there is practically no down-time.

The purpose of the modified embodiment shown in Fig. is further to reduce the time required for cleaning, to simplify the cleaning itself, and to make the installation more reliable in operation, though without increasing its technical complications as compared with the installation in Fig. 1.

The essential difference, as will be explained below, lies in the fact that the suction blower 49 takes the form of a compressed-air-operated injector blower.

When the suction blower 49 is in this form, the section of the circuit which consists of the suction nozzle 44, the separator 48, the suction blower 49, and possibly the pressure duct 58 from the suction blower 49 to the interior 14 of the booth 10, has no mechanically moving parts (such as an impeller rotor) at all, which has the advantageous consequence that reliability is improved and the effort required for cleaning is very considerably reduced. 419 26 In Fig. 4 can be seen the booth 10 and its internal space 14 the bottom of which is formed by the upper run 22 of the endless filter strip 23 which is drawn through the booth in the direction of arrow 23^. The face of run 22 remote from the internal space 14 stretches over the suction trough 30, which in turn is connected by connecting duct 33 to the high-throughput blower 34, which is not shown in this instance. Near the end of the upper run 22 the filter strip is associated with the suction nozzle 44, from Which connecting duct 47 leads to the separator 48. From separator 48, suction duct 50 leads to the suction side of suction blower 49, the pressure side of which connects back up with the interior 14 of booth 10 via pressure duct 58 and distributing pipe 59.

The suction blower 49 shown is one of the large class of injector blowers, the blower in the present instance being one which relies on the so-called Coanda effect. It has a body which in the present case is made up of a first body part and a second body part 72, the parts being connected together co-axially by flanges by means of bolts 74 with a sealing and distance ring 73 interposed. Body 70, or in other words the two parts 71 and 72 of the body, enclose a nozzle chamber 75 which, looked at from the inlet end, first converges to a narrowest point 82 and then diverges again. Upstream of the narrowest point 82, an annular gap 76 opens into the nozzle chamber 75, this gap being bounded on one side by the acute-angled rim 77 of body part 71 and on the other by the obtuse-angled rim 78 of body part 72. Annular gap 76 communicates with an annular chamber 79 formed in body part 72 and annular chamber 79 is connected to a source 81 of 41936 compressed air via a connecting duel 80.

An example of the type of blower 49 which has been described is that known among specialists and in the trade by the name Air-Mover.

As already mentioned, in the case of the blower described the amount of compressed air flowing into the nozzle chamber 75 depends, inter alia, on the width of the annular gap 76. However, the amount of air conveyed between its inlet and outlet by blower 49 depends not only on the compressed air entering nozzle chamber 75 but also on the resistance which has to be overcome both at the suction end and at the pressure end. In order to be able to find the optimum setting for blower 49 in each individual case, it is advantageous to make the width of annular gap 76 adjustable. This can for example be done by the exchanging sealing and distance ring 73 for a thicker (shown shaded at 731) or thinner one or, possibly by using' a threaded sleeve, making the two parts 71 and 72 of the body such that they can be displaced co-axially relative to one another while keeping annular chamber 79 sealed off from the outside, in which case there would be no need for sealing and distance ring 73.In the first case the fineness with which the width of annular gap 76 can be adjusted depends on the number of distance rings of different thicknesses which are available, while in the second case the width of the annular gap 76 can be adjusted without graduations.

It is of course also possible for an injector blower operating on the principle of a jet pump to be used. It is advantageous for ducts 50 and 58 to be connected to the blower shown by quick-action couplings so that the blower is very easy to clean. All that need be done is to release the quick-action couplings and to run a brush of a similar type to a bottle-cleaning brush through nozzle chamber 75, Virtually no deposits can form in annular gap 76. during operation since there is in it a very fast flow in the direction of nozzle chamber 76 which prevents any deposition. On the other hand, very fine fractions may settle on the walls bordering on the nozzle chamber 75 but these are considerably easier to clean than for example the impeller rotor of a centrifugal fan.

Claims (26)

1. An electrostatic powder coating installation of the kind described, wherein the cleaning arrangement has a suction nozzle which is directed towards one face of the filter and which is connected to the suction side of a suction blower via an interposed separator, the outlet of the separator being connected to the supply container.

2. An installation according to Claim 1, wherein the filter is in the form of a filter strip, and extends beyond the booth in the direction in which it travels, the suction nozzle being arranged in the area occupied by a region of the filter strip situated outside the booth.

3. An installation according to Claim 1 or Claim 2, wherein, in the direction in which the filter travels, the suction nozzle is preceded by an outlet of a feed device for fresh powder, which outlet is directed towards the first face of the filter.

4. An installation according to Claim 2 or Claim 3, wherein a device for removing electrostatic charges which acts on the filter is arranged between the booth and the suction nozzle.

5. An installation according to Claim 4, wherein the device for removing electrostatic charges comprises a corona discharge electrode.

6. An installation according to Claim 4 when dependant on Claim 3, wherein the device for removing electrostatic charges is arranged between the booth and the outlet of the feed device.

7. An installation according to any one of the preceding Claims, wherein means for generating surges in the flow through the suction nozzle are provided between the suction nozzle and the separator.

8. An installation according to Claim 7, wherein a valve seating which is able to be sealed periodically by a vibratable float, is provided in a duct which connects the suction nozzle and the separator.

9. An installation according to Claim 7, wherein an air-infiltration opening which is controlled by a reciprocable flap is arranged in the connecting duct between the suction nozzle and the separator.

10. An installation according to any one of the preceding Claims, wherein, between the outlet from the separator and the supply container there is arranged a sieve, particularly to improve the free-flowing characteristics of the powder coming from the said outlet of the separator by sieving, prior to entry into the supply container.

11. An installation according to Claim 10, wherein the sieve is preceded or followed by a valve.

12. An installation according to Claim 11, wherein the valve comprises a sluice valve in the form of a compartment wheel.

13. An installation according to Claim 3 or any one of Claims 4 to 12 when dependent on Claim 3, wherein the feed device has an adjustable metering arrangement.

14. An installation according to any one of the preceding Claims, wherein the pressure side of the suction blower is connected to the space within the booth.

15. An installation according to Claim 14, wherein a distributor pipe which has a plurality of outlet openings and which is connected to the pressure side of the suction 20 419 26 blower, is arranged in the internal space near the point where the filter strip leaves the booth.

16. An installation according to Claim 14, wherein the throughput of the suction means is greater than that of the suction blower.

17. An installation according to any one of the preceding Claims, wherein the suction blower is in the form of a compressed-air operated injector blower.

18. An installation according to Claim 17, wherein the pressure side of the injector blower is connected to the space within the booth.

19. An installation according to Claim 17 or 18, wherein the injector blower consists of a blower which relies on the Coanda effect.

20. An installation according to Claim 18, wherein the injector blower is arranged with its pressure side immediately adjoining one wall of the booth in the area where the path of movement of the filter strip through the booth terminates.

21. An installation according to Claim 17, wherein both the duct which leads to the suction side of the injector blower and the duct which leads away from its pressure side, are connected to the blower by quick-action couplings.

22. An installation according to Claim 19, wherein the blower has a nozzle chamber with a constriction, into which chamber an annular gap connected to a source of compressed air opens upstream of the constriction.

23. An installation according to Claim 22, wherein the width of the annular gap is adjustable and fixable in an adjusted position.

24. An installation according to Claim 23, wherein the width of the annular gap can be adjusted without graduations.

25. An installation for the purpose set forth, 5 substantially as hereinbefore described with reference to Figures 1, 2 and 3 of the accompanying drawings.

26. An installation for the purpose set forth, substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.