Patent Application
20070235558
The invention relates to a system for continuously metering and transporting powders, to the use of the system, and to a coating powder sprayer installation including the system.
In the field of spraying coating powders, it is known to supply a pneumatic or rotary sprayer with coating powder from a tank in which the coating powder is fluidized with air, whereas a dip tube penetrates the fluidized powder bed and a Venturi aspiration system is mounted on the upper portion of the dip tube which allows to aspirate a portion of the fluidized powder. The tank may be vibrated to improve fluidization. This type of equipment can achieve only a low powder flowrate and limited powder transportation distances, which may compromise certain applications. Moreover, contact of certain essential portions of the equipment with the coating powder to drive movement of the powder results in premature wear of those portions, as a consequence of which the value of the powder flowrate obtained tends to drift. As a result of this, the powder flowrate supplied to the sprayer cannot really be guaranteed, and costly and time-consuming preventive maintenance operations must be undertaken.
To remedy those flowrate and transportation distance limitations, it is known in this field to use a closed reservoir that is supplied sequentially with powder and in which the coating powder is fluidized and conveyed to the sprayer by discharging it into a pipe whose mouth faces a drive air ejector. The function of this ejector is to pressurize the closed reservoir and to meter the powder to be transported. The ejector is sometimes placed directly in the powder bed, as in the Applicant's CSV216 equipment. It may equally be placed close to the mouth of the tube, in which case the powder is aspirated via a dip tube. That type of equipment eliminates the flowrate and transportation distance limitations referred to above but does not provide a complete remedy to wear of certain portions because of the presence of an ejector. Furthermore, control of the powder flowrate over a wide range is not a simple matter.
A pressurized pot as described in FR-A-1 279 167 may be used to transport a powder continuously, at a high flowrate and over long distances, without using a Venturi system subject to fast wear. However, in this system, no control of the product flowrate can be implemented, this flowrate varying as the pot is emptied, since it is difficult to control both the air flowrate necessary to fluidize the powder and the pressure inside the pot. It is also difficult to maintain the coating powder in the fluidized state ready for use without starting to pressurize the pot and discharge the powder. Moreover, to limit head losses in the flow, the dip tube and the pipes to which it is connected must-have a relatively large diameter if the powder is to be transported over a long distance, of the order of several meters, and at a high flowrate.
EP-A-1 454 675 discloses the use of a pressurized pot to supply a sprayer with dense fluidized coating powder. The powder flowrate supplied to the sprayer may be controlled by establishing and controlling the pressure in the pot, in particular by means of a vent to the atmosphere. An independent air injector system is provided for stopping the flow of the air/powder mixture and cleaning the sprayer supply pipe when necessary. That kind of equipment and the dense transportation method cannot be used to supply a sprayer over a long distance and are incompatible with successive stopping and restarting of supply.
The above problems arise in any system for continuously metering and transporting powders, for example systems for transporting food, pharmaceutical, or agricultural powders.
In these applications, as in the field of coating powder sprayer installations, it is often important to transport a powder continuously, i.e. without pulsations or sudden fluctuations liable to compromise the operation of the equipment being supplied with the powder, as well as controlling the powder flowrate and avoiding the systematic use of pipes of large diameter, which is not always compatible with the applications envisaged, in particular when supplying mobile equipments.
On these lines, the invention relates to a system for continuously metering and transporting powder from a closed reservoir, the system comprising means for fluidizing at least a portion of the powder in the reservoir, a tube dipping into the fluidized powder and discharging to the outside of the reservoir, and means for pressurizing the reservoir. This system is characterized in that it further comprises supply means for continuously supplying gas for pressurizing the reservoir to a chamber for mixing the gas with the fluidized powder leaving the tube, the supply means being equipped with or constituting a constriction to the flow of the pressurizing gas, and a hose for transporting the powder mixed with the gas is connected to the downstream end of the mixing chamber.
Thanks to the invention, the head loss induced by the constriction on the path of the pressurizing gas to the mixing chamber establishes a pressure difference between the pressure inside the reservoir and the pressure in the mixing chamber. This pressure difference is in fact applied across the dip tube, with the result that it determines the fluidized powder flowrate in the tube when the density of the powder is controlled in the region in which the dip tube is in contact with the powder. In other words, determining, and where applicable controlling, the head loss caused by the pressurizing gas supply means can be used to control the fluidized powder flowrate in the dip tube if the density of the powder is controlled. Reinjecting the pressurizing gas into the mixing chamber at the outlet of the dip tube means that the fluidized powder can be diluted at will by adjusting the pressurizing gas flowrate and/or the constriction. Diluting the powder by addition of gas in this way facilitates its continuous transportation at a high flowrate over large distances.
The invention also relates to a particular use of the system referred to above and more specifically its use to supply a sprayer with coating powder.
The invention further relates to a coating powder sprayer installation that comprises a coating powder sprayer and a system as defined above for supplying the sprayer with coating powder. An installation of the above kind is easier to install and operate than prior art installations and the quality of the coating obtained is improved because the coating powder flowrate can be controlled sufficiently accurately to optimize the operation of the sprayer.
The invention can be better understood and other advantages of the invention become more clearly apparent in the light of the following description of one embodiment of a coating powder sprayer installation of the invention and of seven embodiments of a powder transportation system of the invention, which description is given by way of example only and with reference to the appended drawings, in which:
The installation I shown in
The sprayer 2 is connected to the HT unit 3 by an HT cable 6 and to the pot.4 by a hose 7. The sprayer 2 is mounted on an arm 8 extending through a window 9 in a wall 10 of a coating booth C. The arm 8 is movable vertically, as shown by the double-headed arrow F1, and supported by a mast 11 extending vertically from a base 12 of a reciprocator 13.
In operation, a cloud of coating powder is directed from the sprayer 2 towards the objects O along the electrostatic field lines.
Alternatively, the sprayer may not be of the electrostatic type, in which case the path of the powder constituting the coating powder is determined essentially by pneumatic and gravitational forces.
The path of the hose 7 varies in time because of the vertical movement of the arm 8 and the hose cannot have too large a diameter if it is not to impede movement of the mobile portion of the reciprocator.
As seen in
The bottom 101 is equipped with a porous plate 104 above a distribution chamber 105 supplied with air at a controlled pressure or with a controlled flowrate by a pipe 106 in the bottom 101 discharging to the outside via a connector 107 to which is connected a pipe C1 supplied with compressed air by a regulated compressed air supply S1.
A plate 108 fixed to the bottom 101 supports a vibrator 109 for transmitting vibrations to the pot 4 as a whole to agitate the fluidized mixture in order to facilitate its fluidization and to prevent the formation of preferential flows in or clumping of the powder.
When the chamber 105 is supplied with compressed air, the air flows through the plate 104, as indicated by the arrows F2, which fluidizes the coating powder in the interior volume V4 of the pot 4 and creates a bed L4 Of fluidized coating powder that extends above the plate 103 to a height H4 that depends on the quantity of coating powder present in the volume V4 and on the pressure and the flowrate of the compressed air supply to the chamber 105.
A dip tube 110 extends downwards from the lid 102 to the vicinity of the plate 104. It has a relatively small inside diameter d110 and passes through the lid 102 inside a sleeve 111 that projects upward from the lid 102 and carries a connector 112 to which the hose 7 is connected.
The tube 110 has a lower end 110A and an upper end 110B.
The upper portion of the tube 110 is inside a central bore 113 of the sleeve 111 which is cylindrical and of circular section and the inside diameter d113 of which is strictly greater than the outside diameter d110 of the tube 110. A ring 114 around the tube 110 is engaged in the bore 113 to hold the tube 110 in position in the bore 113.
Above the bore 113, the sleeve 111 has a second bore 115 aligned with an axis Z-Z′ common to the components 110, 113 and 114 and the diameter d115 of which is greater than the diameter d113.
The inside diameter d116 of a liner 116 in the bore 115 is greater than the diameter D110 of the tube 110.
A second pipe C2 connects a compressed air supply S2 to a volume W4 that is a portion of the volume V4 that is not occupied by the fluidized coating powder bed L4, i.e. the portion thereof that lies between the upper surface of the bed L4 and the inside face of the lid 102, on a height H′4.
Because of the supply of compressed air to the volume W4, there is a pressure P4 in this portion W4 of the volume V4 which exerts on the upper surface of the bed L4 a force that tends to expel a portion of the coating powder into the end 110A of the tube 110.
A third pipe C3 connects the supply S to a tap 117 on the sleeve 111 in a direction that is globally radial with respect to the axis Z-Z′ and discharges into the bore 113 at a distance from the end 110B.
The pipe C3 has an adjustable constriction R3 that creates a head loss ΔP between its upstream and downstream ends.
There are two different fluid flows to be considered at the upper end 110B of the tube 110. The first flow is a flow of air coming from the pipe C3 via the constriction R3. The second flow is a flow of the mixture of powder and fluidization air rising up the tube 110.
At the end 110B of the tube 110 and in most of the interior volume V6 of the liner 116, there is a pressure P6 which, for simplicity, may be considered to be the pressure in each of the two flows. The pressure P6 depends directly on the flow of the mixture of powder and air downstream of the end 110B of the tube 110, i.e. in the interior volume V6 of the liner 116, the hose 7 and the sprayer 2. The pressure P6 is stable if the flows and usage conditions downstream of the end 110B have stabilized under steady state conditions.
The volume V6 constitutes a chamber in which a flow E1 of fluidized coating powder from the bed L4 is mixed with a flow E2 of gas from the pipe C3.
Considering only the flow E2 of gas through the constriction R3, the constriction induces a head loss ΔP that depends directly on the flowrate of air in the pipe C3 and in the constriction R3. Under steady state conditions, once the reservoir 4 is pressurized, the head losses in the pipe C2 may be considered negligible compared to ΔP. Similarly, the head losses in the portions of the pipe C3 respectively upstream and downstream of the constriction R3 may be considered negligible compared to ΔP, as can the head losses in the annular space inside the bore 113 around the tube 110. In the light of the foregoing remarks, the following equation applies, where P4 is the air pressure in the volume W4 and ΔP depends on the flowrate of gas in the constriction R3:
P4=P6+ΔP
The pressure P′4 in the fluidized bed L4 in the vicinity of the end 110A of the tube 110 may be estimated as follows:
P′4=P4+ρgh4
where ρ is the density of the fluidized bed L4, g is the acceleration due to gravity and h4 is the height of the tube 110 in the bed L4 above the end 110A of the tube 110.
Under the above conditions, the pressure difference ΔP110 between the ends of the tube 110 may be expressed as follows:
ΔP110=P′4−P6=P4+ρgh4−P6=ΔP+ρgh4
The pressure difference between the inlet and the outlet of the tube 110 is therefore equal to the pressure difference created by the constriction R3 plus a factor depending on the height of the fluidized bed L4, which factor may be calculated.
Under the above conditions, it is possible to control the pressure difference ΔP110 by controlling the head loss ΔP induced by the flow E2 of gas in the constriction R3, ignoring the variation of the height H4 and on condition that the density ρ is maintained substantially constant.
Controlling the pressure difference ΔP110 enables the mass flowrate of fluidized powder in the tube 110 to be controlled because that flowrate is a one-to-one function of the pressure difference ΔP110, the characteristics of the sprayed powder and the geometrical characteristics of the tube 110. Controlling the mass flowrate of powder is therefore easier in proportion to the degree to which the term ΔP that defines ΔP110 is dominant over the term ρgh4, which circumvents any variations of the height H4.
To vary the head loss induced by the flow of gas in the constriction R3 and thereby to modify the mass flowrate of powder discharged through the tube 110, it is possible to operate on the gas flowrate in the constriction R3 and/or on the geometry of that constriction in the case of a variable constriction. If the constriction is fixed, controlling the gas flowrate in it controls the flowrate of powder discharged through the tube 110 and conveyed to the sprayer 2.
The volume V6 constitutes a mixing chamber for mixing the flow E1 of fluidized powder and the flow E2 of air from the pipe C3, the fluidized coating powder itself being a mixture of powder and fluidizing air from the chamber 105.
Note that the flow E2 discharges around the end 110B of the tube 110 concentrically with the axis of the tube 110, which regulates the flowrate of the mixture of coating powder and air and prevents sudden fluctuations in the flow downstream of the end 110B.
A second tap 118 optionally provided on the sleeve 111 also discharges into the bore 113. The second tap is connected by a pipe C4 to a compressed air supply S3 and conveys to the vicinity of the end 110B of the tube 110 air for further diluting the air/powder mixture produced in the chamber V6.
The taps 117 and 118 are on the upstream side of the downstream end 110B of the tube 110, which prevents the pipes C3 and C4 from being soiled by preventing unwanted return flow of the powder towards them.
The liner 116 may be interchangeable and its inside diameter d116 selected as a function of the inside diameter d7 of the hose 7. In practice, the diameter d116 is preferably made substantially equal to the inside diameter d7 of the hose 7.
The bore 113 and/or the liner 116 may be cylindrical with straight generatrices or carry on their inside face a thread or a helical raised pattern to improve the mixing of air and powder by rotational stirring or a Vortex effect. A thread 116a is partially represented in chain-dotted line in
The length L116 of the liner 116, which is equal to the length of the mixing chamber V6, is at least three times the diameter d116 and preferably about ten times that diameter, which enables good homogenization of the air/powder mixture from the tube 110 and the air from the bore 113. If a Vortex effect or rotational stirring is used, the ratio L116/d116 may be approximately 5/1.
The invention supplies a continuous mixture of air and coating powder to the sprayer 2 over a long distance and at a high and controlled flowrate, even though the tube 110 and the hose 7 have small diameters and the hose 7 is relatively long and adapted to deform as a function of the movements of the arm 8.
In a second embodiment of the invention, shown in
The downstream end 310B of the tube 310 discharges into a mixing chamber V6 above the pot 204.
A pipe C3 with an adjustable constriction R3 connects the volume W4 and the mixing chamber V6 through the lid 302; as before, this means that the flowrate of the coating powder/fluidizing gas mixture flowing in the pipe 310 can be controlled by means of the head loss produced by the constriction R3 in the pipe C3.
A vent 318 on the lid 302 vents the volume W4 to the atmosphere, in particular when the equipment to which the pot 204 is connected by a hose 207 is not operating or before it is filled manually with coating powder.
In a third embodiment of the invention, shown in
A bed L4 of agrofood powder, for example sugar or flour, is therefore produced, leaving a free volume W4 in the upper portion of the pot 404 that is supplied with pressurizing air by a pipe C2 connected to a regulated compressed air supply S2.
A hose 407 connects the pot 404 to a station at which the powder is used (not shown).
E1 denotes the flow of the powder/diluting air mixture in the pipe 510.
As is more particularly clear from
The pressure P6 in the chamber V6 is lower than the pressure P4 in the upper portion of the pot 404 by an amount determined, amongst other things, by the geometry of the pipe C3 and by the flowrate of the gas flowing in the pipe. Accordingly, controlling the flowrate of the gas flowing in the pipe C3 controls the flowrate of the flow E1 of fluidized mixture flowing in the pipe 510. The flowrate of the gas flowing in the pipe C3 in fact depends on the flowrate of the fluidizing gas supplied via the pipe C1 and on the flowrate of the pressurizing gas conveyed from the reservoir 404 by the pipe C2.
If the fluidizing gas flowrate is constant, a substantially constant mass per unit volume of fluidized powder is obtained and the only parameters to be controlled in order to control the powder flowrate in the pipe 510 are the pressure and/or the flowrate of the gas for pressurizing the reservoir via the pipe C2. The constriction formed by the pipe C3 may be designed so that the fluidizing gas flowrate is insufficient of itself to generate flow in the pipe 510, which means that the fluidized powder bed L4 may be continuously fluidized without discharging powder into the pipe 110 and the powder remains instantly available for “pumping” as and when required.
In an embodiment of the invention that is not shown, the section of the pipe C3 or of its inlet region may be adjustable so that the air flowrate in the pipe and therefore the head loss and the flowrate in the tube 510 may be modulated.
In the
In fifth and sixth embodiments of the invention, shown in
The reservoir 4 is equipped with a vibrator 109 and a weighing system 150 for continuously determining the weight of powder contained in the reservoir. The weighing system sends a signal Σ1 to a control unit U of the installation incorporating the reservoir 4. It is therefore possible to monitor the consumption of a sprayer supplied from the reservoir 4 by comparing the weight of the powder at the start and the end of application. It is also possible to monitor the flowrate of powder consumed by the sprayer by integrating, over a shorter or longer time period, the variations in the weight of the powder detected by the weighing system.
A cylindrical baffle 160 is supported inside the reservoir 4 by non shown lugs bearing on the wall 103. The baffle 160 is cylindrical, of circular section and concentric with the tube 110. It delimits two volumes in the interior volume V4 of the reservoir 4, namely a volume V160 in the form of a column inside the baffle 160 and a volume W160 inside the reservoir and surrounding the baffle 160.
The plate 104 is disk-shaped with a radius similar to the inside radius of the baffle 160. The lower edge 160A of the baffle 160 is at a non-zero height h160 relative to the bottom 101. Fluidizing air passing through the plate 104, as shown by the arrow F2, creates a fluidized powder bed L4 in the volume V160, the bed L4 being supplied continuously with coating powder stored in non-fluidized form in the volume W160 and flowing under its own weight towards the centre of the plate 104. The arrows F16 indicate path of the coating powder to be fluidized from the volume W160 to the volume V160.
The air flowrate through the plate 104 is adjusted so that the bed L4 extends to the upper edge 160B of the baffle 160, a portion of the coating powder overflowing under its own weight towards the volume W160, as indicated by the arrow F17.
The height h4 of the fluidized bed L4 above the lower end 110A of the tube 110 is therefore constant to the extent that the height H4 of the fluidized bed is itself constant. This prevents fluctuations in the quantity of coating powder directed to the mouth 110A of the tube 110.
In other words, the baffle 160 maintains in the reservoir 4 a column L4 of fluidized powder of constant or virtually constant height H4, despite consumption of the powder and despite the fact that the reservoir is not replenished during application of the powder. The volume W160 provides in the reservoir 4 a reserve of powder to be fluidized that compensates the consumption of the powder.
A vibrator 109 is fitted in the lower portion of the reservoir 4 to facilitate movement of the powder to be fluidized in the direction of the arrows F16.
The
Other means may be provided to maintain a predetermined height of fluidized coating powder above the mouth 110A of the tube 110. For example, means for manually or automatically adjusting the position of the tube 110 relative to the lid of the pressurized pot may be provided so that the lower end 110A is immersed to a substantially constant height h4 in the fluidized bed L4. It is equally possible to maintain the total height H4 of the fluidized powder bed L4 by maintaining the overall quantity of fluidized coating powder in the reservoir substantially constant, the reservoir being supplied with the powder continuously or virtually continuously.
A weighing system may also be used with the reservoir of the
Using a weighing system like the weighing system 150 supplies information as to the weight of the powder in the reservoir for estimating the height of fluidized powder and to ensure filling of the reservoir to a maximum fluidized powder height that in practice corresponds to a maximum weight of powder that the reservoir is able to contain. This kind of weighing system also enables real-time estimation of the changing height of the fluidized powder for automatically correcting the flowrate supplied to the sprayer by the system, if necessary. This compensates any drift in the flowrate resulting from a variation in the height of fluidized powder that may occur in the first four embodiments of the system that have no baffle like the baffle 160.
The invention has been described in applications to a coating powder installation and to transporting agrofood powders. It is not limited to those applications, however, although the application to coating powder installations is highly advantageous. In particular, the invention may be used in the pharmaceutical field to transport medication in powder form or in the agriculture field to transport herbicides, fungicides or fertilizers in powder form.
The nature of the fluidizing gas and the pressurizing gas may be adapted as a function of the nature of the powder to be transported.
In the first two embodiments, the constriction R3 is variable. It may be fixed, however. Controlling the total flowrate of gas entering the reservoir and the flowrate of the gas passing through the constriction then enables the flowrate of powder taken up from the reservoir to be controlled. Where appropriate, the constriction may be changed as a function of the characteristics of the powder to be transported and the characteristics of the installation downstream of the mixing chamber.
The constriction of the third and fourth embodiments may be adjustable.
It may consist of a tube of relatively small inside diameter, a diaphragm or any other appropriate means.
Regardless of the embodiment concerned, the constriction in or constituting the pipe for supplying gas to the mixing chamber may be adjustable or fixed.
In practice, a fixed constriction facilitates control of the powder flowrate because it is a relatively simple matter to govern the flowrate and/or the pressure of the gas for pressurizing the reservoir, for example using a solenoid valve. Furthermore, as the gas passing through this kind of constriction may be lightly laden with coating powder particles, especially in the case of the
In practice, to enable separation of powder flowrate control and fluidization, the flowrate of the fluidizing gas may be negligible compared to the flowrate of the gas for pressurizing the reservoir. In particular, this means that the coating powder may remain in the fluidized bed form pending pumping it into the reservoir, without the air flowrate necessary for producing this fluidization being sufficient to create a head loss across the constriction and cause pumping of the powder.
In the invention as shown, in certain items of equipment all of the powder is fluidized. The invention applies equally to the situation in which only a portion of the powder is fluidized. Furthermore, all the embodiments of the invention include a system known in the art for supplying the reservoir with powder, either continuously or sequentially. A simple option is for the lid of the reservoir to be removed periodically and powder to be tipped into the reservoir by an operative.
The invention has been described using separate air supplies S1, S2 and/or S3. Those supplies are in practice supplied from a common main network and pressure or flowrate regulating means are provided on the upstream side of the supplies S1, S2 and/or S3 so that they can be managed independently. Two supplies or all three supplies could instead be combined.
In a seventh embodiment, shown in
The pressurized pot 4 has a cylindrical wall 103 and its internal volume is divided by a porous plate 104 into a distribution chamber 105 and a volume for producing a fluidized bed L4 of coating powder. The chamber 105 is supplied with compressed air at a controlled pressure from a compressed air supply S1, the compressed air passing through the plate 104, as shown by the arrows F2, to fluidize the bed L4.
A tube 110 dips from the lid 102 of the pot 4 into the fluidized bed L4 and draws off a portion of the powder, as explained above. The tube 110 is connected at the top to a flexible hose 117 that extends from the upper end 110B of the tube 110 to a volume V6 defined inside the arm 120 in which the dense phase powder passing through the tube 110 and the hose 117 is mixed with additional air. To this end, the volume V6 is supplied via a pipe C3 of relatively small diameter from a pipe C2 connected to a second compressed air supply S2.
The pipe C2 is also connected to the interior volume V4 of the pot 4 above the fluidized bed L4 by a pipe section C′3.
Downstream of the volume V6, the coating powder mixed with air flows in a hose 7 supplying the sprayer 2.
In practice, the length of the hose 117 may be of the order of 6 to 8 m and the length of the hose 7 greater than 10 cm and less than 2 m. The length of the hose 7 could nevertheless be increased to up to 50% of the total length of the flow path between the pot 4 and the sprayer 2.
The pipe C3 constitutes means for continuously supplying pressurizing gas from the supply S2 to the mixing chamber formed by the volume V6. Given its length and its diameter, which in practice is less than 5 mm, the pipe C3 induces in the flow of air from the supply S2 a head loss caused by the constriction that it forms.
According to an aspect of the invention that is not shown, the pipe C3 could also be equipped with a variable constriction like the constriction R3 in the embodiments shown in
Compared to the first embodiment described, the seventh embodiment corresponds to a situation in which a dense coating powder is transported over a relatively great distance, namely the length of the hose 117, and, following dilution, over a relatively short distance, namely the length of the hose 7. In all other respects this embodiment is similar to the first embodiment.
The features of the various embodiments of the present invention may be combined with each other.
