US 3918641 A
An electrostatic powder coating installation incorporating a cabin in which there are arranged means connected with a supply container for the delivery of electrostatically charged powder. A part of the internal compartment of the cabin is limited by one side or face of a filter moving through the cabin, the other face of which confronts the suction side of a suction device effective through the filter at the aforementioned internal compartment. A cleaning device is operatively associated with the filter in order to remove therefrom excess powder deposited thereon. The cleaning device embodies a suction nozzle directed towards the one face of the filter and which while interposing a separator is operatively connected with the suction side of a suction blower. The outlet of the separator is connected with a supply container. The method of operating the installation entails depositing fresh powder upon a section of the filter located in front of the suction nozzle for the purpose of replacing the powder consumed during the coating operation.
Claims available in
Description (OCR text may contain errors)
United States Patent Lehmann et al. Nov. 11, 1975 ELECTROSTATIC POWDER COATING Primary E.\'an1inerM. Henson Wood, Jr.
INSTALLATION Assistant E.\'uminerMichael Mar  Inventors: Ernst Lehmann; Robert Prinzin Attorney. Agent, or F1rmWerner W. Kleeman both of St. Gall, Switzerland  ABSTRACT  Assgnee' g 1 dApparatebau An electrostatic powder coating installation incorpo- W1 Zer rating a cabin in which there are arranged means con-  Filed: Dec. 20, 1974 nected with a supply container for the delivery of electrostatically charged powder. A part of the internal  Appl' 534675 compartment of the cabin is limited by one side or face of a filter moving through the cabin, the other  Foreign Application Priority Data face of which confronts the suction side of a suction Nov. 14, 1974 Switzerland 15177/74 device effective through the filter at the aforementioned internal compartment. A cleaning device is op- [52 US. Cl. 239/15; 118/621; 317/3 eratively associated with the filter in Order to remove 51 Int. c1.'- BOSB 5/02 therefrom excess Powder deposited thereon The 5 Field f Search 239/3 15; 317/3; 117/17; cleaning device embodies a suction nozzle directed 11 21 towards the one face of the filter and which while interposing a separator is operatively connected with the 5 References Cited suction side of a suction blower. The outlet of the sep UNITED STATES PATENTS arator is connected with a supply container. The a 1 X method of operatmg the lnstallatlon entalls deposmng i i 4 1 3 5 fresh powder upon a section of the filter located in front of the suction nozzle for the purpose of replacing the powder consumed during the coating operation.
16 Ciaims, 3 Drawing Figures U.S. Patent N0v. 11,- 1975 3,918,641
ELECTROSTATIC POWDER COATING INSTALLATION BACKGROUND OF THE INVENTION The present invention relates to a new and improved construction of electrostatic powder coating installation.
The coating of articles or the like by means of electrostatically charged powder briefly hereinafter referred to as electrostatic powder coatingat the present time cannot hardly be dispensed with in fabrication or manufacturing operations in consideration of the presently available advantages in contrast to wet spraying (for example, greater layer thickness per working operation, no solvent vapors, just to mention a few). Hence it is particularly incumbent to especially take into account two aspects during electrostatic powder coating in order to maintain such coating technique economically competitive with wet spraying.
Both aspects are entwined with the circumstance that a relatively great proportion of the released electrostatically charged powder in fact does not even impinge upon the article or object to be coated or does not sufficiently adhere thereto, for instance because the desired layer thickness has already been attained. This proportion can be designated as excess powder. A part of this excess powder deposits like dust at the parts or components of the installation and upon changing the quality of the powder for instance changing the powder colormust be removed by performing an extremely time consuming work. On the other hand, a further part of this excess powder is removed by a suction device belonging to the installation out of the installation itself in order to possibly again be delivered for reuse.
Hence, there is applicable on the one hand the aspect of the time needed for cleaning the installation, since this time constitutes downtime or deadtime for themstallation and, on the other hand, there is also to be considered the aspect of reclaiming and reusing the excess powder since such constitutes a considerable cost factor.
A state-of-the-art installation by means of which it has been attempted to satisfy both of the above-mentioned aspects possesses a cabin in which there are arranged means coupled with a supply container for the purpose of delivering electrostatically charged powder. A part of the internal space of the cabin is bounded by one face ofa filter moving through the cabin, the other face of which is in confronting relationship with the suction side of a suction device which is effective through the filter at the aforementioned internal space. A cleaning device is associated in operative coaction with the filter for removing therefrom excess powder which has deposited thereon.
With this already proposed prior art installation the greatest portion of the excess powder falls onto the filter under the action of the suction device, so that the remaining parts of the installation are considerably less contaminated by depositions. By means ofthe cleaning device the excess powder is removed from the filter and delivered to a collecting container. In this way while the excess powder is not lost, still it is incapable of being re-cmployed in the then existing state since it has lost a considerable proportion of its trickling or friable propertiesv Also the proposed equipment does not permil of a continuous working operation since, firstly, the collecting container from time-to-time must be ex- '2 changed for an empty container, and the reclaimed powder after having been re-proccssed, if desired together with fresh powder. must be deposited into the supply container.
SUMMARY OF THE INVENTION Hence it is a primary object of the present invention to provide an improved electrostatic powder coating installation which is not associated with the aforementioned drawbacks and limitations of the prior art proposals.
Another and more specific object of the invention is directed to constructing an installation of the previ ously mentioned type in such a manner that re-processing of the powder removed from the filter is practically superfluous, so that it can be delivered in a truly closed circulation system directly and continuously to the supply container with a minimum danger of contamination.
Now in order to implement these and still further objects of the invention which will become more readily apparent as the description proceeds the installation of this development is manifested by the features that the cleaning device is equipped with a suction nozzle directed towards the one face ofthe filter, this suction nozzle, while interposing a separator, is connected with the suction side of a suction blower, the outlet of the separator being operatively coupled with the supply container.
The removal of the powder from the filter by means of the installation of this invention occurs therefore by carrying out a suction operation, i.e., pneumatically, which automatically provides the result that the removal in the suction line takes place to a certain extent in suspension, whereby the thus occurring turbulence maintains the friability or trickling properties of the powder or. again establishes such properties in the event such is even necessary. Additionally due to the connection between the outlet of the separator and the supply container there is established a truly closed circulation system for the excess powder which permits of a continuous working operation.
The filter advantageously constructed as a filter band can extend out of the cabin in its direction of movement, the suction nozzle then advantageously being arranged at the region of the section of the filter band located externally of the cabin. With this construction it is especially beneficial that the replenishment of fresh powder and its admixing with the excess powder circulating throughout the closed circulation system is extremely simple- In the direction of movement of the filter'there can be arranged an outlet of an infeed device for fresh powder which is disposed ahead of the suction nozzle, i.e. upstream thereof when considered with respect to the direction of movement of the filter band. Furthermore, the outlet of the infeed device just men tioned above is arranged to be directed towards the one face or side of the filter. Consequently, there is not only sucked through the suction nozzle the excess powder, rather also the fresh powder deposited upon the same side or face of the filter, so that during the course of this operation both of the powder components are automatically intimately admixed with one another.
Between the cabin and the suction nozzle there can be arranged a device acting at the filter for removal of the electrostatic charge. In this way it is possible to remove any existing electrostatic residual charge from the excess powder deposited upon the filter. In order to be also able to protect the connection prevailing be tween the suction nozzle and the suction blower against deposition of the excess powder which flows through in suspension. it is possible to provide means for generating flow or current surges. In this way the suction action of the suction nozzle takes on a pulsating characteristic and in the connection line or conduit there is favored the formation of turbulence or vortexes.
Between the outlet of the separator and the supply container there can be provided a sieve mechanism. especially for the purpose of removing possibly entrained contaminants by sieving the powder prior to entry into the supply container and. if necessary, to again rcestablish the trickling or friable properties of the powder.
In the separator such friable or pulverulent properties of the powder will' be obtained to a better degree the less intense the separator separates the powder out of the air current. In other words, this means that the friable behavior will hardly be impaired when the degree of separation of the separator is not 100 percent. On the other hand this leads to the result that the air conveyed by the suction blower is associated with a residual proportion of powder. It is therefore advantageous to couple the pressure side of the suction blower with the internal compartment or space of the cabin. Consequently, there is formed to a certain extent a second closed circulation system, wherein the powder which possibly was not separated out during the first passage through the separator again arrives at the filter. removed therefrom by the suction nozzle and again delivered to the separator.
BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now the drawing. the exemplary illustrated embodiment of installation shown therein will be understood to comprise a cabin which is delimited or bounded by a ceiling or cover 11, end walls 12 and 13 and side walls which have been conveniently omitted from the drawing to preserve clarity inillustration and to reveal internal structure. The internal space or compartment which is enclosed by the cabin 10 has been conveniently designated by reference character 14. As indicated by means of the double broken lines 15 there "can be provided a conveyor mechanism which extends through the cabin 10 and by means of which there are delivered in the direction of the arrow 16 through a gantry-like opening (not shown) the articles or the like to be coated into the internal space or compartment 14, the gantry-like opening being conveniently provided for instance in the end wall 12. Further the articles can again be removed out of the cabin 10 by means of a similar opening which in this case is provided at the end wall 13.
Within the internal compartment or space 14 of the cabin 10 there is arranged at least onc spray nozzle. for instance. in the form of a spray gun or pistol 17, the outlet end of which obviously is directed towards the articles to be coated. The spray pistol 17 is connected via a connection line, for instance a hose 18 or equivalent, with a supply container 19 and by means of an infeed or delivery line 20 with a voltage source 21. By means of the infeed or delivery line 18 there is delivered to the spray pistol 17 through the agency of conveying air the powder which is to be electrostatically charged. By means of the infeed line 20 there is furnished the electrical energy needed for electrostatic charging. In this connection it is mentioned that the infeed line 20 can be arranged in or at the hose 18.
Continuing it is to be observed that the floor of the cabin 10 is formed by the upper face of the upper run 22 of an endless filter band 23 or equivalent structure, which is guided about deflecting rollers 24, 25 in the manner of a conveyor band. As regards the deflecting rollers 24, 25 it is possible for one. such as the deflecting roller 25 to be driven through the agency of a chain drive or a shaft 26 by means of a gear or transmission motor 27 or the like in the direction of the arrow 28.
The lower face or side of the run 22 of the filter band 23 bears on the one hand upon a flange 29 which laterally protrudes and surrounds the open side of a suction vat or trough 30 and, on the other hand, bears upon a grate 31 formed for instance of a fine wire mesh, and which grate spans the open side of the vat or trough 30 and is substantially coplanar to the flange or flange means 29. Internal compartment 32 of the vat 30 is connected by means of a connection line 33 with the suction side of a blower 34. From what has been discussed above it will be apparent that during operation ofthe blower 34 such acts through the part of the upper run of the filter band which spans the vat or trough 30 at the internal compartment 14 in the sense that within such internal compartment there always prevails a lower negative pressure and a slight air flow in the direction of the arrows 35. By means of this air flow excess powder. delivered by the spray pistol 17, yet not adhering at the article to be coated, will be caused to deposit primarily at the upper face of theupper run 22 of the filter. By suitably selecting the permeability of the filter band 23 it is possible to prevent penetration of the powder through the same.
During the course of its movement the upper run 22 of the filter band 23 leaves the internal compartment 14 of the cabin 10 at the location 36 and carries along therewith the powder which has collected thereon during the course of the throughpassage of such filter band. The outlet 36 is provided with a corona electrode 37 which radiates the upper face of the filter band run 22. This electrode 37 is powered by a high-voltage source 39 through the agency of a connection line 'or conductor 38. The corona electrode 37 serves the purpose of removing the residual charge adhering to the powder possibly present at the upper face of the run 22. This corona electrode 37, of course. could be equally replaced by another device which produces an ionized surrounding region. i.e., electrically conductive surrounding region, for instance by using a weak beta radiating device.
In the direction of travel 23 of the upper run 22, and following the corona electrode 37, there is arranged a delivery or infeed device which has been designated in its entirety by reference character 40. This infeed device 40 possesses a filling funnel 4i or the like which supplies a dosing worm 43 driven by a motor 42. The dosing worm 43 in turn extends practically over the entire width of the filter band 23. This dosing device or dosing mechanism 40 serves the purpose of delivering to the installation new powder, whether such be of the same or a different quality, as shall still be more fully described hereinafter.
The delivery mechanism or device 40 has arranged thereafter a suction nozzle 44 possessing a suction slot 45 which likewise extends practically over the entire width of the filter band 23. This suction nozzle 44 is connected via a device 46 for producing air surges or pulses with a connection conduit or line 47 leading to a separator 48 in the form of a cyclone, by way of example. The separator 48 is coupled through the agency of a suction conduit 50 with the suction side of a blower 49, so that upon operation of the suction blower 49 there prevails a pronounced suction action in the con nection conduit 47. The device 46 for generating the air surges serves the purpose of insuring that the suction action emanating from the blower 49 is not continuous rather pulsating, whereby there is rendered more difficult in any case, if not completely avoided, a deposition of the powder removed by the suction nozzle 44 from the filter band at the inner wall of the connection conduit 47.
This separator 48 possesses an outlet or discharge 51 by means of which there is continuously delivered the powder separated out at the separator 48 by means of the suction air, for instanc through the agency of a bucket wheel or sluice impeller 52 driven by a motor 53, to an oscillating or rotary sieve 54. Both the bucket wheel or impeller 52 as well as also the oscillating or rotary sieve 54 are arranged in a closed housing, which among other things. serves 'the purpose of protecting against contamination the powder located upon the sieve 54, and, in the event there is omitted the impeller wheel 52, serves as sluice, in order to maintain the negative pressure prevailing in the separator 48 and preventing the generation of any false air currents. The powder which has been prepared by the sieve or filter ,54, that is to say, to the extent necessary freed of possible contaminants and again rendered friable, arrives through the agency of a sieved material outlet'or discharge 55 directly and continuously in the supply container 19. Possibly existing coarser fractions of the powder removed by the suction nozzle 44 from the upper run 22, and which are not again comminuted by the action of the sieve 54, arrive via a waste conduit 56 in a waste or refuse container 57.
The pressure side of the blower 49 is connected with a pressure conduit 58 which, in turn, leads to the internal compartment 14 of the cabin l and there opens into a distributor conduit or pipe 59 having a multiplicity of openings 60. The reason for this arrangement, among other things, resides in the fact that the separa tor 48 as a general rule cannot be designed such that it completely separates out the powder from the powderair current existing in the connection conduit 47, because the danger thereby exists that the separated out powder particles will cake together or conglomerate in the separator 48, which prc hibits the direct reuse, i.e., the delivery into the supply container. Since the separator 48 as will be recognized cannot separate out all of the powder out of the flow emanating from the connection conduit 47, a part of this powder arrives via the suction conduit 50 in the pressure conduit 58 and thus at the distributor conduit 59, where such powder again arrives in the internal compartment 14 of the cabin l0 and thus is subjected to the suction action emanating from the blower 34. Consequently, this powder again is deposited at the upper face of the upper run 22 of the filter band 23. It is to be observed that the conveying capacity of the blower 34 amounts to a multiple of the conveying capacity of the suction blower 49, so that even when the pressure line or conduit 58 opens into the internal compartment 14, the negative pressure in this internal compartment thus will be only slightly affected.
From the above discussion it will be recognized that the upper run 22 of the filter band 23 passes approximately the following sections between the deflecting roller 24 and the driven deflecting roller 25. In a first section a there is deposited upon such upper run to an increasing degree excess powder coming out of the spray pistolor gun 17. In the next following section b this deposition of excess powder emanating from the spray pistol l7 continues, whereby however there is added a residual amount of excess powder which was not previously separated out in the separator 48. In the sections a and b this powder is fixedly held in the filter material of the filter band 23 due to the suction action which prevails upon the travel of the filter band over the open side of the vat or trough 30. Hence upon exit out of the cabin 10, i.c., upon the transition to the section 0 this powder cannot be whirled about. In the section 0 there is additionally effective the corona electrode 37 which insures that any possibly present residual charges are removed. If the delivery or infeed mechanism 40 is in operation the powder layer appearing at the upper run is supplemented by fresh powder, so that at the section d of the upper run there is present powder from the total of three regions. In the section e the upper run is thus freed of all present powder by the suction nozzle 44, and the three fractions are forcefully brought to turbulence or whirled about prior to reaching the separator 48. Following the section e is a last section f, the significance of which will be explained more fully hereinafter.
In FIG. 2 there is schematically illustrated a possible device or mechanism 46 for producing air surges or pulses. There will be recognized a part of the upper run 22, the suction nozzle 44 with its suction slot 45, as well as the starting portion of the connection conduit or line 47. In a substantially pot-shaped widened portion 62 which directly merges with the suction nozzle 44 there is formed a type of conical valve seat 63 which coacts with a closure element or floating body 65 prebiased by a weak pressure spring 64 or the like. When the installation is switched off the floating body 65 normally almost closes the through-passage formed by the valve seat 63. As soon as the nagative pressure in the connection conduit 47 is sufficient, then the floating body 65 raises off of the valve seat 63, so that a powerful air current is formed. As a result the pressure differential between both sides of the valve seat 63 drops and the spring 64 is capable of overcoming such pressure differential and again forcing the floating body 65 in the direction of the valve seat 63. Hence with time the floating body 65 is brought into a state of oscillation, so that the flow in the connection conduit 47 and together therewith even if to a lesser extent the flow in the suction conduit 50 and in the pressure conduit 58 as well as in the separator 48 will be pulsating. Consequently, the deposit, especially the fine fractions of the '7 powder taken up by the suction nozzle 44, at the inner wall ofsuch conduits at least will be rendered more difficult.
' Approximately the same action can he realized with the equipment shown in FIG. 3. With this device a false air opening 66 is provided in the connection conduit 47 directly at the connection with the suction nozzle 44, this false air opening normally being closed by a flap member 67 or equivalent structure. if this flap 67 is periodically and briefly opened in the direction of the arrow 68, then, since the through flow cross-section of the false air opening 66 is considerably greater than the through flow cross-section between the upper run 22 and the slot 45, there prevails a powerful air surge. Of course, the opening time of the flap or flap member 67 is calculated such that in this time the upper run only passes through a path of travel which is considerably smaller than the width of the slot 45.
The mode of operation of the described apparatus can be considered as follows: The articles to be coated are moved through the cabin in the direction of the arrow 16. The spray gun or pistol 17 generates in the internal compartment 14 a cloud of electrostatically charged powder particles, and which cloud, owing to the fact that a slight negative pressure prevails in the internal compartment 14, is limited to such internal compartment 14. A large proportion of theelectrostatically charged particles thus deposit upon the articles to be coated and while adhering to such articles leave the cabin. The proportion of the powder which has not found its way to the articles deposits upon the upper run of the filter band and thus arrives beneath the suction nozzel 44 where it is reclaimed and via the separator 48 again delivered to the supply container 19. With the exception of that part of the powder which remains adherent to the articles, the powder accordingly moves through a first closed cycle or circulation system, starting from the supply container 19 through the hose [8 to the spray gun 17, from that location into the internal compartment or space 14 and after depositing on the run 22 arrives at the suction nozzle 44, then through the connection conduit 47 to the separator 48 and through the outlet 51 and the sieve 54 again arrives back at the supply container 19. It is to be observed that this circulation system is closed and practically only accessible in the sections 0 to e at the upper run. A further powder circulation system is passed through by that part of the powder which is not separated out by the separator 48. This'circulation system begins at the suction conduit 50, thereafter leads through the suction blower 49, through the pressure conduit 58, the distributor conduit 59 into the internal compartment 14, and after depositing on the run 22 continues up to the suction nozzle 44 and via the connection conduit 47 leads back to the separator 48. Also this closed cycle or circulation system is practically only accessible at the sec tions 0 to e of the upper run. In order to replace the powder consumed by the actually occurred coating of the articles or objects, fresh powder is likewise deposited upon the upper run 22 by means ofthe delivery device 40. Hence, the supply container 19 is not directly refilled, rather the fresh powder initially is deposited upon the excess powder and then taken up through the suction nozzle 44 whereby there occurs a powerful or forceful admixing. If there is compared the quantity of powder delivered to the installation with the quantity actually consumed by the coating operation, then it has been found that the loss in powder with the described 8 installation is considerably below 1%, so that it canbc justifiably stated that the reclaiming and direct reuse is complete, whereby it is to be observed that these operations take place continuously.
Also upon changing the quality of the powder the installation affords notable advantages. With such change initially no articles are delivered for instance any longer to the cabin. Then the hose conduit 18 is separated from the supply container and compressed air blown therethrough. The remaining parts of the installation operate as previously described. The powder still located in the installation passes through one ofthe two previously described circulation systems until it finally arrives at the supply container which is no longer coupled with the hose 18. At the same time it is possible for instance with the aid of a powerful industrial vacuum cleaner to subject the filter band 23 at the section f of the upper run to a cleaning action which deeply penetrates such upper run of the filter band. Of course at this point in time the delivery mechanism 40 is not in operation. Consequently, the supply container 19 can be replaced by an empty one, whereafter the delivery mechanism 40 can be charged with the new powder and the hose conduit 18 can be connected to the new supply container 19. After a brief start-up time the installation is faultlessly charged with the new color without there being discernible with the naked eye at the formed coatings a contamination by powder particles of the preceding color. Apart from the short conversion time and the cleaning of the filter band 23 there are practically no deadtimes.
While there is shown and described present preferred embodiments of the application, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.
Accordingly, what is claimed is:
1. An electrostatic powder coating installation comprising a cabin, a supply container, means operatively coupled with the supply container for delivering electrostatically charged powder into the interior of the cabin, said cabin being provided with an internal compartment, a filter movable in a predetermined direction of travel through the cabin, said filter having opposed faces, one face of said filter bounding a part of the internal compartment of the cabin, a suction device having a suction side and effective through the filter at said internal compartment, the other face of said filter confronting the suction side of the suction device, a cleaning device operatively associated with the filter for removing excess powder which has deposited on the filter, said cleaning device possessing a suction nozzle directed towards the one face of the filter. a suction blower having a suction side, a separator, the suction nozzle being connected by means of the separator with the suction side of the suction blower, said separator having an outlet flow communicating with the supply container.
2. The installation as defined in claim I, wherein said filter comprises a filter band which moves along said predetermined direction of travel and extends out of said cabin in the direction of travel of said filter band. said suction nozzle being arranged at the region of a section of the filter band located externally of the cabin.
3. The installation as defined in claim 1, further including a delivery mechanism having an outlet for fresh powder, said outlet of the delivery mechanism being 9 directed towards the one face of the filter. and wherein the delivery mechanism is arranged ahead ofthc suction nozzle with respect to the direction of travel of the filter.
-4. The installation as defined in claim 2, further including means acting upon the filter for the removal of electrostatic charges and arranged between the cabin and the suction nozzle.
5. The installation as defined in claim 4, wherein said means for the removal of electrostatic charges comprises a corona electrode.
6. The installation as defined in claim 4, further including a delivery mechanism having an outlet for fresh powder, said outlet being directed towards the one face of the filter, said delivery mechanism being arranged ahead of the suction nozzle with respect to the direction of travel of the filter, and wherein the means for the removal of the electrostatic charges is arranged between the cabin and the outlet of the delivery mechanism.
7. The installation as defined in claim 1, further including means for generating current surges in the suction nozzle, said generating means being arranged between the suction nozzle and the separator.
8. The installation as defined in claim 7, further including a connection conduit dispose-d between the suction nozzle and the separator, said connection conduit having a throughpassage and being provided with an oscillating floating body for periodically throttling said throughpassage.
9. The installation as defined in claim 7, further including a connection conduit arranged between the 10 suction nozzle and the separator, means defining a false air opening provided in the connection conduit and controlled by a to-and-fro movable closure element.
10. The installation as defined in claim 1, wherein a sieve mechanism is arranged between the outlet of the separator and the supply container, said sieve mechanism due to its sieving action improving the friability of the powder arriving out of said outlet of the separator prior to its entry into the supply container. 7
11. The installation as defined in claim 10, wherein a sluice impeller is positioned to cooperate with said sieve mechanism.
12. The installation as defined in claim 11, wherein said sluice impeller is selectively arranged upstream or downstream of the sieve mechanism.
13. The installation as defined in claim 3, wherein the delivery mechanism possesses a regulatable dosing mechanism. I Y
14. The installation as defined in claim 1, wherein said suction blower has a pressure side, and means for connecting the pressure side with the internal compartment of the cabin.
15. The installation as defined in claim 14, further including a distributor conduit having a number of outlet openings arranged in the internal compartment of the cabin at the region of the exit of the filter band out of the cabin, the pressure side of said suction blower being connected with said distributor conduit.
16. The installation as defined in claim 14, wherein I the conveying capacity of the suction device is a multiple of the conveying capacity of the suction blower.