No. 529 590 PATENT No. 529 590

[emblem] Swiss Confederation Swiss Office for Intellectual Property S

International Classification: B 05 b 5/08 Application number: 10689/71 Filing date: July 20, 1971, 5:30 PM Patent issued: October 31, 1972 Patent published: December 15, 1972

Parent Patent

Gema AG Apparatebau, St. Gallen

System for Supplying an Electrostatic Coating Device

with Powdered Coating Material

Robert Prinzing, St. Gallen, is the inventor.

The invention relates to a system for supplying an electrostatic coating device with powdered coating material, in which system a powder, which is recovered in an automatic filter device, and fresh powder are fed by way of a sieve machine to the powder container of the coating device.

When atomizers are used for electrostatic powder coating, not all of the atomized powder remains adhering to the object that is to be coated. For efficiency reasons the excess powder is collected, cleaned in a filter device, and re-atomized following the addition of fresh powder. In larger coating systems, the powder is recovered automatically. In this case an exhaust fan draws the powder-containing air out of the atomizing booth through a suction pipe into the filter device; and the recovered powder is drawn from the collecting container of the filter device and is fed through a lock to the sieve machine. For the addition of fresh powder the standard procedure is for a hose, which is attached to the suction pipe, to draw fresh powder from a supply tank and then to feed this fresh powder into the filter device. Furthermore, the suction pipe exhibits a butterfly valve in the flow direction upstream of the hose connection, so that depending on the position of the butterfly valve, powder-containing air is drawn from the booth or fresh powder is drawn from the supply tank into the filter device. The design of such systems is perhaps simple, but they have the drawback that the coating process has to be interrupted during the intake of fresh powder, because the booth is no longer vented to an adequate extent when the suction pipe is shut off. This state leads to serious problems especially if the coating process is supposed to run totally automatically according to a specified program. It has been demonstrated that the powder is more likely to ignite and explode in the filter device - and consequently at a higher rate in terms of number of incidences - than at other locations in the coating system with powder recovery, because in the filter device the powder can easily reach the critical concentration required for an explosion. If during an explosion the entire powder supply that is in the filter device ignites, this can readily lead to the destruction of the device. Consequently the prior art powder recovery systems are unstable in operation. If the coating material is a relatively highly hygroscopic powder, then a longer residence time of the powder in the filter device will have an adverse effect, because in the filter device the powder is exposed - to a particularly intensive degree - to the influence of the atmospheric air and can absorb a large amount of moisture.

The object of the invention is to provide a powder supply system, which is intended for an electrostatic coating device and which allows, independently of the addition of fresh powder, a steady air flow, conveying used powder to the filter device, to be maintained in a suction pipe, in order to guarantee a constant negative pressure in a booth, connected to the suction pipe, and, thus, to make possible a coating process without any interruption. In addition, said powder supply system can be operated without any problems and also automatically in such a way that no significant amount of recovered powder accumulates in the filter device. Nevertheless, it is guaranteed that the powder container of the coating device will be provided with an adequate supply, independently of the holding capacity of said powder container.

The invention solves this problem in that a collecting container of the automatic filter device and a fresh powder conveying line, which is intended for connecting to a fresh powder supply tank, are connected by way of a branching of a pipe to the feed line of a separator, which is assigned to the sieve machine and operated by a pneumatic conveyor; and the branching of a pipe exhibits shut-off means for selectively shutting off the one or the other inflow. Therefore, the powder that accumulates in the booth can be drawn out of the booth in the customary way with an exhaust fan by way of the filter device and the suction pipe, but independently of the addition of fresh powder, which is carried out with a separate pneumatic conveying device. Hence, given an essentially constant power output of the exhaust fan, the booth maintains an adequately constant negative pressure. In order to keep the supply of recovered powder, exposed to the atmospheric air, at an amount that is safe in the event of an explosion in the filter device, the inventive method for operating the system provides that by actuating the shut-off means at the branching of a pipe the inflow of recovered powder and the inflow of fresh powder are enabled alternatingly, and the fresh powder inflow is blocked, when the collecting container has collected a defined amount of recovered powder.

The branching of a pipe can exhibit automatically operated shut-off means, and the actuation of the shut-off means can be controlled by a control unit. The control unit can be preferably a timer, by means of which the shut-off means of the branching of a pipe can be set, according to a time program, to the inflow of the recovered powder and the inflow of the fresh powder. In order to ensure at the same time that the powder supply in the collecting container of the filter device does not rise above the defined amount, the collecting container can be assigned a sensor, which is configured to detect an upper fill level and which emits a signal, when the recovered powder in the collecting container has reached the upper level. For an automatic operation, the signal of the sensor can deactivate the control by means of the control unit with the timer; and the shut-off means can be set to the inflow of the recovered powder into the branching of a pipe. In order to ensure in the case of an automatic operation with time-controlled shut-off means that the powder flows continuously into the powder container of the coating device, the collecting container can also exhibit a sensor, which is intended for detecting a bottom fill level and which emits a signal, when the recovered powder in the collecting container has reached the bottom level. Therefore, the signal of this sensor for the bottom level makes it possible to set the shut-off means of the branching of a pipe to the inflow of fresh powder. With the aid of these two sensors the shut-off means of the branching of a pipe can also be controlled so as to maintain an almost constant composition of the coating material composed of recovered powder and fresh powder.

If a large amount of powder is atomized without interruption, it is possible under some circumstances that the fresh powder silo can be totally emptied during the coating operation. Then in this case the powder container of the coating device would be provided to an inadequate extent with only recovered powder. In order to prevent this situation during an automatic operation, the fresh powder conveying line can be attached to the outflow of a supply tank, which is designed as a silo, in the supply system; and the supply tank may exhibit a separator, which is operated by a pneumatic conveyor and to the feed line of which is attached a suction line, which in intended for connecting to a fresh powder reserve container. The separator of the fresh powder supply tank is attached preferably to the existing pneumatic conveyor of the separator, assigned to the sieve machine. In this case a shut-off valve is disposed between the conveyor and the separator of the fresh powder supply tank. This shut-off valve can be controlled automatically by the signals from the level sensors, assigned to the supply tank. A shut-off valve is also connected in an expedient way between the conveyor and the separator of the sieve machine, in order, for example, to be able to interrupt the conveying process of the recovered powder and fresh powder during the filling operation of the supply tank, if in the event of low powder consumption the powder supply in the container of the coating device has reached the maximum allowable level. This valve, too, can be controlled automatically by means of level sensors, mounted on the powder container of the coating device.

The embodiments of the powder supply system, according to the invention, are illustrated as schematic drawings in the figures.

Figure 1 depicts a system supplied with fresh powder from a supply tank, a cyclone separator and an exhaust fan as a conveyor for the recovered powder and fresh powder; and

Figure 2 depicts a system for a totally automatic operation with automatic refilling of the fresh powder supply tank from a reserve container.

The coating devices, depicted in the drawings, include at least one electrostatic powder atomizer 1 of the conventional design; a high voltage and control unit 2, to which the powder atomizer is attached with a cable 3; and a powder container 4, having, for example, a conical shape, from the outlet of which runs a powder line, preferably a flexible hose 5, to the atomizer 1. The powder atomizer 1 is located in the booth K, which is only suggested in the drawing with a symbol. The objects G, which are to be coated, are conveyed through the booth K by a conveyor chain F.

A suction pipe 7 runs from the booth K to an automatic filter device 6, which contains, for example, the filter hoses. An exhaust fan 8 draws air from the booth K through the filter device 6 and the suction pipe 7, so that the booth always maintains a certain slight negative pressure and, thus, a pressure gradient from the outside to the inside is maintained. During the coating process the air, which is drawn out of the booth, is loaded with the coating powder, which falls off and which is recovered in the filter device 6 and is collected in a container 9, which is attached to the conical bottom of the filter device.

The one inflow 10a of a pipe branching 10 is connected to the collecting container 9. To the other inflow 10b of the pipe branching 10 is connected a fresh powder conveying line 13. An additional conveying line 15 connects the outflow 10c of the pipe branching 10 to a separator 16, which delivers the recovered powder and fresh powder by way of a lock 17 into the sieve machine 18, which is mounted on the powder container 4 of the coating device. The separator 16 is attached to a pneumatic conveyor 19. To this extent the construction of the design of the powder supply system, depicted in Figure 1 and Figure 2, is identical.

In the simpler design, depicted in Figure 1, the pipe branching 10 is attached to the collecting container 9 of the filter device 6 and to the fresh powder conveying line 13 by means of one valve 11 and 12 respectively. The fresh powder conveying line 13 dips into a supply tank with fresh powder. Said supply tank can be, for example, the shipping container. The separator 16 is a relatively inexpensive cyclone separator, and to this cyclone separator is attached an exhaust fan as the pneumatic conveyor 19.

Since there will always be powder particles in the exhaust air of the exhaust fan, the exhaust air outlet of the exhaust fan is connected to the filter device 6 by means of a pipeline 20, in order to filter additionally this exhaust air. The collecting container 9 exhibits a level sensor 22a of a conventional design. This level sensor emits a signal, when the powder supply in the collecting container has reached a defined upper level. In the case of especially simple installations the two shut-off valves 11 and 12 of the pipe branching 10 can be operated by hand. In this case when the level sensor 22a emits a signal, the valve 11 for the inflow of recovered powder is opened; and the valve 12 for the inflow of fresh powder is blocked. However, the two shut-off valves 11 and 12 can be operated preferably automatically and controlled by a control unit 21. In the installation, according to Figure 1, the control unit 21 is designed as a timer and opens and closes the two valves 11 and 12 alternatingly, so that during a certain period of time recovered powder, then fresh powder and then again recovered powder etc. are always drawn into the separator 16. The switching times can be set as a function of the required powder. When the demand for powder is constant, such a timer is totally adequate. If, however, an irregularity occurs in the powder consumption, then it can happen that the powder supply in the collecting container rises as far as to the upper level. Then owing to the signal, emitted by the sensor 22a, the time control unit is turned off; and the valve 11 for the recovered powder is opened. Therefore, an accumulation of a large amount of powder in the filter device is totally ruled out. The pressure in the filter device 6 and in the suction pipe 7 is monitored - as customary in such installations - by means of a pressure transducer 23 and is held preferably automatically constant.

In the fully automatic supply system (shown, for example, in Figure 2) where the supply tank is refilled with fresh powder, the pipe branching 10' contains a shut-off member 11' for alternatingly opening and/or closing the two inflows 10'a and 10'b. The separator 16 at the sieve machine 18 is, for example, a filter separator, which is connected to an alternatingly sucking and counter-blowing conveyor 19' by way of an air line 24 and over a shut-off valve 25. The fresh powder conveying line 13 connects the inflow 10'b of the pipe branching 10' to the outflow 14'a of a supply tank 14', which is configured in the shape of a conical silo. The supply tank 14' exhibits a second separator 28, which is also attached to the pneumatic conveyor 19' by way of an air line 27 and a shut-off valve 26, so that it is also possible to operate the separator 16 or the separator 28 - optionally also both of them together - by means of the conveyor 19', as a function of the blocking position of the two valves 25, 26.

The two shut-off valves 25 and 26 as well as the shut-off member 11' of the pipe branching 10' can be operated automatically, and in particular, as a function of the control signals.

The collecting container 9 of the filter device 6 exhibits two sensors 22a and 22b, of which the one sensor 22a emits a control signal, when the powder supply in the collecting container 9 has reached an upper fill level; and the other sensor 22b emits a control signal, when the powder supply has dropped to a bottom level. Owing to the control signals the shut-off member 10' is controlled in such a manner that, when the sensor 22a for the upper fill level emits a signal, the first inflow 10'a is opened, and the second inflow 10'b is blocked. That is, only the powder from the collecting container 9 can pass through the pipe branching 10' to the separator 16. Upon a signal of the second sensor 22b, the first inflow 10'a is blocked, and the second inflow 10'b is opened, so that only the fresh powder from the supply tank 14' can pass through the pipe branching 10' to the separator.

The shut-off valve 25, which is disposed in the air line pipe 24 of the first separator 16, is controlled by a sensor 32, which is mounted on the powder container 4 of the coating device and which emits a signal, when the fill level in the powder container has reached an upper level. When the powder in the container 4 reaches the upper level, then a signal of the sensor 32 causes the shut-off valve 25 to close, so that the separator 16 is withdrawn from service. There can also be, in addition, a sensor for a bottom fill level. The signal of this additional sensor provides that in any case the shut-off valve 25 is opened, and the separator 16 is active, when the powder level in the container 4 reaches the bottom level.

Even the fresh powder supply tank 14' has two level sensors 31a and 31b, by means of which the shut-off valve 26 in the air line pipe 27 of the second separator 28 is controlled. If the fresh powder supply in the supply tank 14' falls to the bottom level, the sensor 31b controls the shut-off valve 26 so as to move into the open position. Then the fresh powder is drawn from the reserve container 30 through the suction line 29 into the separator 28; and the powder supply in the supply tank 14' is refilled. If the powder supply in the supply tank 14' reaches the upper level, then in any case a signal of the sensor 31a causes the shut-off valve 26 to be moved into the blocking position, thus stopping the refilling of powder. Suitable embodiments of the sensors 25, 26 and 27 are well-known, as are the control units for the shut-off means of the pipe branching and the shut-off valves.

For the explanation of the automatic operating mode of the system, shown in Figure 2, it is assumed that the powder container 4 of the supply tank 14' and the reserve container 30 contain powder, whereas the collecting container 9 at the filter device 6 is empty. Upon switching on the powder supply, the valve 25 is opened; the valve 26 is blocked; and the pipe branching 10' has an open inflow 10'b for the fresh powder. The fresh powder passes through the first separator 16 and the sieve machine 18 into the powder container 4 of the coating device. As the coating process progresses, the recovered powder accumulates in the collecting container 9. If the recovered powder in the collecting container 9 reaches the upper level, then the pipe branching 10' switches automatically in response to a signal of the sensor 22a. After all of the fresh powder in the conveying line 15 has been drawn off, only the recovered powder is conveyed into the powder container 4, and, in particular, only until the powder level in the collecting container 9 has reached the bottom level, whereupon then a signal of the sensor 22b causes the pipe branching 10' to be moved back again. The recovered powder and fresh powder accumulate layer by layer in the powder container 4 of the coating device and are thoroughly mixed by an agitator, provided in the powder container.

The above described system can be easily adapted to the respective existing requirements. Thus, for example, for an enhanced performance the separator 28 for the refilling of fresh powder can be attached to a dedicated conveyor. In the case of coating devices that exhibit lower performance and in which the powder supply, which can be stored in the fresh powder supply tank, will suffice, the reserve container 30 and the second separator 28 can be omitted. A wide range of variations is also possible for the control of the pipe branching 10' and the shut-off valves 25 and 26. Thus, for example, there may be timers, which in response to the signals of the sensors for the respective bottom level turn off following the passage of a set time, so that the sensors for the respective upper level can be kept solely as safety measures or may be totally omitted.

Patent Claims

I. System for supplying an electrostatic coating device with powdered coating material, in which system a powder, which is recovered in an automatic filter device, and fresh powder are fed by way of a sieve machine to the powder container of the coating device, characterized in that a collecting container (9) of the automatic filter device (6) and a fresh powder conveying line (13), which is intended for connecting to a fresh powder supply tank (14 and/or 14'), are connected by way of a pipe branching (10 and/or 10') to the feed line (16a) of a separator (16), which is assigned to the sieve machine (18) and operated by a pneumatic conveyor (19 and/or 19'); and the pipe branching exhibits shut-off means (11, 12 and/or 11') for selectively shutting off the one or the other inflow (10a, 10b).

II. Method for operating the system, as claimed in patent claim I, characterized in that by actuating the shut-off means (11, 12, and/or 11') at the pipe branching (10 and/or 10') the inflow of recovered powder and the inflow of fresh powder are enabled alternatingly, and the fresh powder inflow is blocked, when the collecting container has collected a defined amount of recovered powder, in order to hold the supply of recovered powder, exposed to the atmospheric air, at an amount that is safe in the event of an explosion in the filter device.

Dependent Claims

1. System, as claimed in patent claim I, characterized in that the pipe branching (10 and/or 10') exhibits automatically operated shut-off means (11, 12 and/or 11'), and the actuation of the shut-off means is controlled by a control unit (21).

2. System, as claimed in the dependent claim 1, characterized in that the control unit (21) is a timer, by means of which the shut-off means (11, 12 and/or 11') of the pipe branching (10 and/or 10') are set, according to a time program, to the inflow of the recovered powder and the inflow of the fresh powder.

3. System, as claimed in the patent claim I, characterized in that the collecting container (9) of the filter device (6) is assigned a sensor (22a), which is configured to detect an upper fill level and which emits a signal, when the recovered powder in the collecting container has reached the upper level.

4. System, as claimed in the dependent claims 2 and 3, characterized in that the signal of the sensor (22) deactivates the control of the shut-off means (11, 12 and/or 11') by means of the control unit (21); and the shut-off means are set to the inflow of the recovered powder into the pipe branching (10 and/or 10').

5. System, as claimed in the patent claim I, characterized in that the collecting container (9) of the filter device (6) is assigned a sensor (22b), which is configured to detect a bottom fill level and which emits a signal, when the recovered powder in the collecting container has reached the bottom level.

6. System, as claimed in the dependent claims 1, 3 and 5, characterized in that the shut-off means (11, 12 and/or 11') of the pipe branching (10 and/or 10') are set through the control unit (21) by means of a signal of the sensor (22a) for the upper fill level to the inflow of the recovered powder and by means of a signal of the sensor (22b) for the bottom fill level to the inflow of fresh powder.

7. System, as claimed in the patent claim I, characterized in that the fresh powder conveying line (13) is attached to the outflow (14'a) of a fresh powder supply tank (14'), which is designed as a silo; and the supply tank (14') exhibits a separator (28), which is operated by a pneumatic conveyor and to the feed line (28a) of which is attached a suction line (29), which is intended for connecting to a fresh powder reserve container (30), said separator serving to maintain the fresh powder supply in the supply tank (14').

8. System, as claimed in the dependent claim 7, characterized in that the first separator (16), assigned to the sieve machine (18), and the second separator (28), assigned to the fresh powder supply tank (14'), are attached to a common pneumatic conveyor (19'), a shut-off valve (26) being connected between the conveyor (19') and the second separator (28).

9. System, as claimed in the dependent claim 8, characterized in that the shut-off valve (26), which is disposed between the conveyor (19') and the second separator (28), can be operated automatically and is controlled by a sensor (31b), which scans a bottom fill level in the fresh powder supply tank (14'), for the purpose of opening.

10. System, as claimed in the dependent claim 9, characterized in that the fresh powder supply tank (14') has a sensor (31a) for scanning an upper fill level; and the shut-off valve (26) is controlled by the sensor (31a) for the upper fill level for the purpose of closing.

11. System, as claimed in the dependent claim 8, characterized in that a shut-off valve (25) is provided between the pneumatic conveyor (19') and the first separator (16), assigned to the sieve machine (18), in order to be able to turn off the first separator when the conveyor is in operation.

12. System, as claimed in the dependent claim 11, characterized in that the shut-off valve (25) for the first separator (16) can be operated automatically and is controlled by signals, indicating the fill states in the powder container (4) of the coating device.

13. System, as claimed in the patent claim 1 or the dependent claim 7, characterized in that the separator(s) (16 and/or 16, 28) is a cyclone separator or are cyclone separators; and that the pneumatic conveyor (19) is an exhaust fan, the exhaust air of which is recycled to the filter device (6).

Gema AG Apparatebau

Representative: E. Blum & Co., Zurich

1 Sheet

key to the figures:

German English

Steuergerät control unit