WO2006075211A2

WO2006075211A2 - Dosing device for a particle blasting apparatus

Info

Publication number: WO2006075211A2
Application number: PCT/IB2005/003977
Authority: WO
Inventors: Pierre Rochat
Original assignee: Exa Sa
Priority date: 2005-01-14
Filing date: 2005-12-12
Publication date: 2006-07-20
Also published as: GB0500649D0; WO2006075211A3

Abstract

A dosing device for controlled delivery of a particulate material comprising a particulate material container, a particulate material metering device, a feed conduit adapted to deliver particulate material from the container to the metering device, and a particulate material delivery conduit adapted to receive metered particulate material from the metering device, wherein the feed conduit comprises means for providing a tortuous flow path for the particulate material through the conduit. Methods of controlled delivery of a particulate material and apparatus comprising the dosing device are also provided.

Description

DOSING DEVICE FOR A PARTICLE BLASTING APPARATUS

INTRODUCTION

The invention relates to a dosing device for controlled delivery of a particulate material . In particular, the invention relates to a particle blasting apparatus which includes a dosing device for controlled delivery of particulate material to a blasting nozzle .

With the exception of a few systems , most of the blasting media equipment utilise pressure differential technology . Essentially, this technology uses a pressurized container where aggregate and high pressure air are mixed and forced through an adjustable (or non adjustable) metering device into a pressurised delivery conduit where the mixture of air and aggregate is accelerated towards a blasting nozzle . In such a system, the air pressure used for accelerating the aggregate must be lower than the pressure "pushing" the aggregate into adjustable (or non adjustable) metering device in order to prevent the generation of back pressure that will either stop the aggregate being delivered into the delivery conduit (and main air flow) or even pushing back the aggregate back into the pressurised container .

A number of problems exist with this arrangement . First , a high volume of air is required to mix with the aggregate in the tank and may stay in contact with the aggregate for a long period or time . As most , if not all , the types of aggregate used in blasting operations have a tendency to be sensitive to humidity, the particles of the aggregate tend to bind together, blocking up the adjustable (or non adjustable) metering device . This problem is further complicated by the difficulties involved in providing dry air for pressurising the system, these difficulties including the size and expense of a dryer which is capable of drying the volumes of air involved

Even with an effective air dryer, when using fine aggregate particle (300μm and lower) , the pressure has a tendency to mechanically compact and bind the particles together . When the aggregate came to a "restriction" (conical lower parts of the container) and the adjustable (or non adjustable) metering device, the high concentration of particle, highly compacted by high air pressure, have the effect of blocking the aggregate in the bottom of the tank and forming a plug over and into the adjustable (or non adjustable) metering device, blocking the apparatus and preventing operation. In such a situation, the machine has to be stopped, and the machine opened and manually unblocked.

These problems dictate that the "pressure differential" blasting equipment utilise a relatively wide bore feed conduit between the product container and the metering device in order to reduce the risk of blocking. The result is a high consumption of aggregate (from 50kg/ hour up to over 200 kg/hour) . While this is not a great problem when conventional abrasive blasting media is used, it is not acceptable for precision blasting which uses a new generation of fine, non aggressive, low-abrasive aggregate (i . e . EXA HDO) that provide optimal performance at a dosage rate of 5 to 25 kg/hour . A further problem with the high usage rate inherent in using convention pressure differential is that, after the blasting operation, high quantities of aggregate need to be recuperated and cleaned, resulting in a considerable cost due to the manpower required.

It is an object of the invention to overcome at least one of the above-referenced problems . STATEMENTS OF INVENTION

According to the invention, there is provided a dosing device for controlled delivery of a particulate material comprising:

- a particulate material container; - a particulate material metering device; - a feed conduit adapted to deliver particulate material from the container to the metering device; and - a particulate material delivery conduit adapted to receive metered particulate material from the metering device,

wherein the feed conduit comprises means for providing a tortuous flow path for the particulate material through the conduit .

In one embodiment of the invention, the means for providing a tortuous flow path comprises a screw, wherein the particulate material flows down the screw in a helical manner along the threads of the screw. Generally, the flow path is slanted. Typically, the screw and the feed conduit are separate, wherein the screw is dimensioned such that the conduit snugly embraces the screw. It will be appreciated that the screw and conduit may be integrally formed, or the same effect could be obtained by providing a helical bore arrangement within a solid body. It is important to note that the screw is not adapted to turn within the conduit, rather it remains static and the particulate material flows along the threads in a helical manner under the force of gravity and/or the force of pressure from the pressurised particulate material container . Obviously, the rate of flow of product along the screw may be varied by using screws of different dimensions , i . e . different pitch of threads , depth of threads , number of turns , etc .

Typically, the screw is dimensioned to deliver particulate material to the metering device at a desired rate. While any given screw will have a fixed flow rate, the use of a removable screw allows different screws to be used in different application, and in this regard screws may be chosen that provide a fixed maximum flow rates of anything in the range of less than 1 and greater than 100kg/hour (wt/wt) . A preferred maximum flow rate for general blasting operations is about 25 kg/hour . When the dosing device is used for microblasting, i . e . dental application, the screw will be dimensioned to provide a fixed maximum flow rate of anything between 100 and 1000g/hour .

It will be appreciated that other means for providing a tortuous flow path may be used. For example, a series of slanted baffles are envisaged which would provide the same effect of "slow" delivery of material from the particulate material container to the metering device . In one embodiment of the invention, the metering device comprises a valve which is adjustable to vary the rate of flow of the particulate material into the delivery conduit . Thus , if the screw is chosen to provide a maximum flow of 25 kg/hour (wt/wt) , then the metering device is adjustable to vary to rate of flow of product into the delivery conduit over the range 0 to 25 kg/hour .

In a preferred embodiment of the invention, the delivery conduit comprises a turbulence chamber adapted to receive particulate material from the metering device, and means for generating a depression within the turbulence chamber . Typically, the depression within the turbulence chamber is generated by a three-part system comprising the turbulence chamber, a nozzle of narrow bore located upstream of the turbulence chamber, and adapted to deliver pressurised air into the turbulence chamber, and a conduit for receiving a mixture of pressurised air and particulate material, the conduit having a bore larger than that of the nozzle .

In one embodiment, the depression generating means is adapted to generate a depression within the turbulence chamber of between -0.5 to -1.0 Bar, ideally about -0.7 Bar . The generation of a depression within the turbulence chamber has the effect of mixing the air and the particulate material , and also accelerates the thus-formed mixture of air and particulate material downstream along the delivery conduit .

In a preferred embodiment of the invention, the dosing device further includes a main air flow conduit which is adapted to introduce pressurised air into the delivery conduit downstream of the turbulence chamber . The purpose of the main air flow conduit is to further accelerate the mixture of air and particulate material downstream along the delivery conduit .

The invention also relates to a particle blasting apparatus comprising a dosing device according to the invention, a source of compressed air, and a blasting nozzle in fluid communication with the delivery conduit .

The invention also relates to a method of operating a particle blasting apparatus according to the invention, comprising the steps of :

- choosing the means for providing a tortuous flow path on the basis of a desired maximum flow rate of particulate product; - adjusting the metering device to achieve the desired rate of delivery of particulate material into the delivery conduit; - pressurising the product material container; and - providing pressurised air to the delivery conduit such that a mixture of air and particulate product is accelerated along the delivery conduit to the blasting nozzle .

Typically, the means for providing a tortuous path is provided by a screw.

Suitably, the pressurised air provided to the delivery conduit is such as to generate a depression in a turbulence chamber of the delivery conduit of between -0.5 and -1.0 Bar, ideally about -0.7 Bar .

Ideally, the method involves the introduction of pressurised air from a main air conduit into the delivery conduit downstream of the turbulence chamber, and wherein the pressure and volume of air applied in the product material container is less than the pressure and volume of air in the main air flow conduit .

Generally, the pressure at the depression added to the pressure in the particulate material container is greater than the pressure in the main air flow conduit .

In a second aspect, the invention relates to a dosing device for controlled delivery of a particulate material comprising: - a particulate material container; - a particulate material metering device; - a feed conduit adapted to deliver particulate material from the container to the metering device; and - a particulate material delivery conduit adapted to receive metered particulate material from the metering device,

wherein the delivery conduit comprises a turbulence chamber adapted to receive particulate material from the metering device, and means for generating a depression within the turbulence chamber, and wherein the device includes a main air flow conduit which is adapted to introduce pressurised air into the delivery conduit downstream of the turbulence chamber.

Typically, the depression within the turbulence chamber is generated by three-part system comprising the turbulence chamber, a nozzle of narrow bore located upstream of the turbulence chamber, and adapted to deliver pressurised air into the turbulence chamber, and a receiving conduit for receiving a mixture of pressurised air and particulate material , the receiving conduit having a larger bore than the nozzle . In one embodiment, the depression generating means is adapted to generate a depression of between -0.5 to -1.0 Bar, typically between -0.7 and -0.9 Bar. In one embodiment , the feed conduit comprises means for providing a tortuous flow path for the particulate material through the conduit . Ideally, the means for providing a tortuous flow path comprises a screw, wherein the particulate material flows down the screw in a helical manner along the threads of the screw. Ideally, the screw and the feed conduit are separate, wherein the screw is dimensioned such that the feed conduit snugly embraces the screw.

In one embodiment, the screw is dimensioned to deliver particulate material to the metering device at a rate of about 25 kg/hour (wt/wt) .

In one embodiment of the invention, the metering device comprises a valve which is adjustable to vary the rate of flow of the particulate material into the delivery conduit .

The invention also relates to a method of operating a particle blasting apparatus according to the second aspect of the invention, comprising the steps of : - adjusting the metering device to achieve the desired rate of delivery of particulate material into the delivery conduit ; - pressurising the product material container; and - providing pressurised air to the delivery conduit upstream of the turbulence chamber such that a depression is created in the turbulence chamber and mixture of air and particulate product is accelerated along the delivery conduit ; and - providing pressurised air to the main air flow conduit which further accelerates the mixture of air and particulate product along the delivery conduit towards the blasting nozzle,

wherein the pressure and volume of air applied to the product material container is less than the pressure and volume of air applied to the main air flow conduit .

Typically, the pressurised air provided to the delivery conduit is such as to generate a depression in a turbulence chamber of the delivery conduit of between -0.5 and -1.0 Bar, ideally about -0.7 Bar .

In a particularly preferred embodiment of both aspect of the invention, the pressure in the particulate material container (Pl ) added to the pressure (depression) in the turbulence chamber is greater than the pressure in the main air flow conduit (P4 ) .

The invention will be more clearly understood from the following description of some embodiments thereof , given by way of example only, with reference to the accompanying drawings in which:

Fig . 1 is an elevational , partly cut-away, view of a dosing device according to the invention;

Fig. 2 is a side elevational , partly cut-away, view of the dosing device of Fig. 1 showing a screw and valve in position;

Fig. 3 is an elevational, partly cut-away, view of a dosing device according to an alternative embodiment of the invention; and

Fig . 4 is an elevational , partly cut-away, view of the dosing device of Fig. 3 showing a screw and valve in position.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings , and initially to Figures 1 and 2 , there is illustrated a dosing device according to the invention. The dosing device comprises a feed conduit 1 formed in a dosing device body 2 which provides fluid communication between a particulate material container (not shown) and a metering device 6 , 7. The metering device 6 , 7 functions to meter the flow of particulate material from the feed conduit 1 to a delivery conduit below the metering device .

In more detail , the feed conduit 1 includes a screw 8 which fits snugly within the conduit 1. The dimensions of the screw are chosen to provide a maximum flow rate in a helical manner along the threads of the screw of about 25 kg/hour .

The metering device comprises a cylindrical socket 7 having input aperture 7.1 and a diametrically opposed larger output aperture 7.2. A valve (or tap) 6 comprises a cylindrical plug which is dimensioned to fit within the socket 7 in a tight but sliding manner . The valve 6 includes a through hole (not shown) which when aligned with the input and output apertures allows particulate material to pass into the delivery conduit below. Adjustment of the valve 6 by rotation allows the flow rate through the valve to be varied in the range 5 to 25 kg/hour (wt/wt) , or even fully prevented.

The delivery conduit includes a turbulence chamber 5 located directly below and adjacent the output aperture 7.2 of the metering device, and a venturi system consisting of a pressurised air delivery conduit 3 , and a receiving conduit 4 for receiving a mixture of pressurised air and particulate material from the turbulence chamber 5. The pressurised air delivery conduit 3 has a nozzle 3a of narrow bore (about 2mm) located adjacent the turbulence chamber 5. The receiving conduit 4 has a bore of about 5mm. Although not shown, the mixture of particulate material and pressurised air is accelerated along the receiving conduit 4 by means of a source of pressurised air which is provided downstream of the turbulence chamber 5 through a main air flow conduit .

Referring to Figs 2 to 4 , there is illustrated a further embodiment of the invention in which parts similar to those illustrated with reference to Figs 1 and 2 are assigned the same reference numerals . In this embodiment , the downstream elements of the delivery conduit are illustrated. Thus , the main air flow conduit 10 provides pressurised air (P4 ) into the delivery conduit downstream of the turbulence chamber 5. The downstream part of the delivery conduit widens to a bore of about 20mm, which is the same bore as that of the main air flow conduit 10. The bore of the delivery conduit remains at about 20mm until it reaches the nozzle .

Example

In an example of the use of the device of the invention, the particulate material container (not shown) is pressurised (Pl ) to a pressure of about 1 Bar . Material in the container flows through the conduit along the threads of the screw 1 is a helical manner. The dimensions of the screw 1 are chosen such that the maximum flow rate along the screw is about 25 kg/hour . Adjustment of the metering device 6 , 7 allows the ultimate flow rate into the turbulence chamber to be accurately varied to between 0 and 25 kg/hour .

The working pressure is defined by setting the pressure in the main air flow conduit (P4 ) . The pressure (P2 ) in the pressurised air delivery conduit 3 is set such that a depression (Dep) of about -0.7 Bar is achieved in the turbulence chamber 5. Generally, a pressure (P2 ) of between 2 and 4 Bar is sufficient to achieve such a depression. This results in a cumulative pressure in the receiving conduit (Pl + Dep = P3 ) of about 1.7 Bar . In order to prevent the main air flow from forcing the mixture of air and particulate material back upstream, the pressure at P3 needs to be greater than the pressure in the main air flow conduit P4. Given that P3 is about 1.7 Bar, a working pressure of about 1.5 may be chosen for P4.

Thus , in effect , the pressure in main air flow conduit, P4 (which is approximately equal to the working pressure) must always be less than the pressure in the particulate material container (Pl ) added to the depression in the turbulence chamber (Dep) , i . e . :

P4 < [ Pl + Dep]

During operation of the device, the depression will be set at a pressure of between -0.5 and -1.0 Bar, usually about -0.7 Bar, and generally will not be varied. During or prior to operation, the pressures Pl and P4 may be varied to achieve the desired working pressure . If the pressure P4 needs to be changed, the pressure Pl will automatically be changed in a proportional manner, ensuring that the formula P4 < [Pl + Dep] is adhered to .

The generation of a depression in the turbulence chamber helps mix the particulate material with the air, and also assists in the acceleration of the mixture of product and air into the downstream parts of the delivery conduit towards the main air flow. The depression effect created in the turbulence chamber also prevents the generation of any back-pressure in the turbulence chamber, and allows both the pressure, and volume, of air supplied to the particulate material container be minimised. This has the direct effect of minimising the amount of agglomeration or "caking" which occurs in the feed conduit and 1 and metering device 6 , 7.

The invention also has a significant effect on the volumes of air used. With conventional pressure differential systems , the pressure required in the particulate material container was greater, thus the volumes of air required were greater ( typically about 60% to 70% of total air came from the pressurised product container . However, with the dosing device of the present invention, the "push" required from the particulate material container is considerably less , thus less pressure and less air are required. With the dosing device of the invention, the amount of air required in the product container is only about 20% to 40% of the total air . This means that less moisture is in contact with the particulate material , which results in less agglomeration and caking of the particulate material .

The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail without departing from the spirit of the invention. Thus , for example, while this specification refers to the use of pressurised air, it will be appreciated that any suitable fluid may be used to pressurise the system, including both liquid and gasses . Further, while the specification refers to the blasting material as particulate material , this term is not intended to limit the types of materials that the dosing device of the invention may be used with, other than the limitation that the material must be flowable .

Claims

1. A dosing device for controlled delivery of a particulate material comprising:

2. A dosing device as claimed in Claim 1 in which the means for providing a tortuous flow path comprises a screw, wherein the particulate material flows down the screw in a helical manner along the threads of the screw.

3. A dosing device as claimed in Claim 2 in which the screw and the feed conduit are separate, wherein the screw is dimensioned such that the conduit snugly embraces the screw.

4. A dosing device as claimed in any of Claims 2 to 4, wherein the screw is removable and replacable with a screw of different dimensions .

5. A dosing device as claimed in Claim 4 , wherein the screw is dimensioned to deliver particulate material to the metering device at a rate of about 25 kg/hour (wt/wt) .

6. A dosing device as claimed in any preceding Claim in which the metering device comprises a valve which is adjustable to vary the rate of flow of the particulate material into the delivery conduit .

7. A dosing device as claimed in any preceding Claim, wherein the delivery conduit comprises a turbulence chamber adapted to receive particulate material from the metering device, and means for generating a depression within the turbulence chamber .

8. A dosing device as claimed in Claim 7 wherein the depression within the turbulence chamber is created by a three-part system comprising the turbulence chamber, a nozzle of narrow bore located upstream of the turbulence chamber, and adapted to deliver pressurised air into the turbulence chamber, and a conduit for receiving a mixture of pressurised air and particulate material , the conduit having a bore larger than that of the nozzle .

9. A dosing device as claimed in any of Claims 7 to 8 in which the depression generating means is adapted to generate a depression of between -0.5 to -1.0 Bar .

10. A dosing device as claimed in any of Claims 7 to 9 and further including a main air flow conduit which is adapted to introduce pressurised air into the delivery conduit downstream of the turbulence chamber .

11. A particle blasting apparatus comprising a dosing device according to any of Claims 1 to 10 , a source of compressed air, and a blasting nozzle in fluid communication with the delivery conduit .

12. A method of operating a particle blasting apparatus as claimed in Claim 11 , comprising the steps of : a . choosing the means for providing a tortuous flow path on the basis of a desired maximum flow rate of particulate product; b. adjusting the metering device to achieve the desired rate of delivery of particulate material into the delivery conduit; c . pressurising the product material container; and d. providing pressurised air to the delivery conduit such that a mixture of air and particulate product is accelerated along the delivery conduit to the blasting nozzle .

13. A method as claimed in Claim 12 in which the means for providing a tortuous path is provided by a screw.

14. A method as claimed in Claim 12 or 13 in which the pressurised air provided to the delivery conduit is such as to generate a depression in a turbulence chamber of the delivery conduit of between -0.5 and -1.0 Bar .

15. A method as claimed in Claim 14 in which the pressurised air provided to the delivery conduit is such as to generate a depression in a turbulence chamber of the delivery conduit of about -0.7 Bar .

16. A method as claimed in any of Claims 12 to 15 , wherein the dosing device includes a main air flow conduit which is adapted to introduce pressurised air into the delivery conduit downstream of the turbulence chamber, and wherein the pressure in the product material container is less than the pressure in the main air flow conduit .

17. A dosing device for controlled delivery of a particulate material comprising : a . a particulate material container; ■ b . a particulate material metering device; c . a feed conduit adapted to deliver particulate material from the container to the metering device; and d. a particulate material delivery conduit adapted to receive metered particulate material from the metering device,

wherein the delivery conduit comprises a turbulence chamber adapted to receive particulate material from the metering device, and means for generating a depression within the turbulence chamber, and wherein the device includes a main air flow conduit which is adapted to introduce pressurised air into the delivery conduit downstream of the turbulence chamber .

18. A dosing device as claimed in Claim 17 wherein the depression within the turbulence chamber is generated by a three-part system comprising the turbulence chamber, a nozzle of narrow bore located upstream of the turbulence chamber, and adapted to deliver pressurised air into the turbulence chamber, and a receiving conduit for receiving a mixture of pressurised air and particulate material , the receiving conduit having a bore larger than that of the nozzle .

19. A dosing device as claimed in any of Claims 17 or 18 in which the depression generating means is adapted to generate a depression of between -0.5 to -1.0 Bar.

20. A dosing device as claimed in any of Claims 17 to 19 wherein the feed conduit comprises means for providing a tortuous flow path for the particulate material through the conduit .

21. A dosing device as claimed in Claim 20 in which the means for providing a tortuous flow path comprises a screw, wherein the particulate material flows down the screw in a helical manner along the threads of the screw.

22. A dosing device as claimed in Claim 21 in which the screw and the feed conduit are separate, wherein the screw is dimensioned such that the feed conduit snugly embraces the screw.

23. A dosing device as claimed in any of Claims 21 or 22 , wherein the screw is removable and replaceable with a screw of different dimensions .

24. A dosing device as claimed in Claim 23 , wherein the screw is dimensioned to deliver particulate material to the metering device at a rate of about 25 kg/hour (wt/wt) .

25. A dosing device as claimed in any of Claims 17 to 24 in which the metering device comprises a valve which is adjustable to vary the rate of flow of the particulate material into the delivery conduit .

26. A particle blasting apparatus comprising a dosing device according to any of Claims 17 to 25 , a source of compressed air, and a blasting nozzle in fluid communication with the delivery conduit .

27. A method of operating a particle blasting apparatus as claimed in Claim 26 , comprising the steps of :

a . adjusting the metering device to achieve the desired rate of delivery of particulate material into the delivery conduit ; b . pressurising the product material container; and c . providing pressurised air to the delivery conduit upstream of the turbulence chamber such that a depression is created in the turbulence chamber and mixture of air and particulate product is accelerated along the delivery conduit ; and d. providing pressurised air to the main air flow conduit which further accelerates the mixture of air and particulate product along the delivery conduit towards the blasting nozzle ,

wherein the pressure applied to the product material container is less than the pressure applied to the main air flow conduit .

28. A method as claimed in Claim 27 in which the pressurised air provided to the delivery conduit is such as to generate a depression in a turbulence chamber of the delivery conduit of between - 0.5 and -1.0 Bar .

29. A method as claimed in Claim 28 in which the pressurised air provided to the delivery conduit is such as to generate a depression in a turbulence chamber of the delivery conduit of about - 0.7 Bar .

30. A method as claimed in any of Claims 27 to 29 wherein the pressure in the particulate material container added to the pressure in the turbulence chamber is less than the pressure in the main air flow conduit .