GB2250460A

GB2250460A - Blasting with particulate material

Info

Publication number: GB2250460A
Application number: GB9017136A
Authority: GB
Inventors: Robert Sidney Jackson
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-08-04
Filing date: 1990-08-04
Publication date: 1992-06-10
Also published as: WO1992002349A3; GB9017136D0; WO1992002349A2; EP0497935A1

Abstract

Apparatus for performing a process of cleaning surfaces of buildings comprises a chamber (11) partitioned by a pyramidal barrier means (33) into lower and upper zones (35a, 35b). Low velocity gas is impelled from a perforated body (15) to fluidize a bed of particulate material (13) particles of a certain energy escaping to a handpiece (21) by way of escape means (19). The bed is maintained in a steady state by the descent under gravity of particulate material (37) upheld by the barrier means (33), the material (37) and the material (13) forming a continuum at the peripheral communicating passage (41) of the barrier means. Gas at high velocity along passage (25) and six passages (95) in a swirl imparting member (79) entrains particles carried by the low velocity gas along passage (23) at the junction (27) of the passage (23) with the passages (95). As the particulate material attains the junction it is immediately accelerated by the high velocity gas to its discharge velocity from the handpiece.

Description

The Removal of Material from Surfaces This invention relates to a method of and apparatus for removing material from a target surface by contact of particulate material projected thereat from a handpiece.

Methods and apparatus heretofore employed in the cleaning of masonry, particularly sandstone masonry, and involving so-called sand-blasting techniques have been responsible for causing significant damage in the subject structure. The often brutal treatment afforded by the bombardment of particular material, at a time varying rate of discharge, has effected not only removal of the undesirable surface accretions but has resulted in the creation of fissures in the fabric of the structure beneath, the damage inflicted leading sometimes to continuous deterioration in the fabric.The damage produced using such methods has been of such serious character that certain municipal authorities responsible for the maintenance of buildings have declared a moratorium suspending further cleaning using such methods until they have been developed to provide a sufficiently gentle treatment taking the surface of the stone back to an undamaged clean state whilst effecting this in an economical manner.

According to the present invention a method of removing material from a target surface by contact of particulate material projected thereat from a handpiece, comprises: causing particulate material to be entrained in a stream of relatively low velocity gas which is conducted along first passage means provided in the said handpiece to a junction position whereat the particulate material is accelerated to its projection velocity from the handpiece by merger of the particle bearing low velocity gas with relatively high velocity gas flowing in second passage means provided in the handpiece.

According to the invention also, a method of removing material from a target surface by contact of particulate material projected thereat from a handpiece, comprises: entraining in a flow of relatively low velocity gas, particulate material continuously being liberated during fluidization of a bed of such material by the flow therethrough of the said low velocity gas; and conducting the particle bearing low velocity gas flow along first passage means provided in the handpiece to a junction position whereat the particulate material conveyed to the said junction by the particle bearing low velocity gas stream, is accelerated, as it attains the said junction, to its projection velocity from the handpiece.

Preferably, the said first and second passage means are such and are so arranged with respect to one another at the said junction therebetween that'a substantially reduced pressure promoting the flow of low velocity gas along the first passage means is created at the said junction. Preferably, also, particulate material is supplied to the bed of such material in such manner and at such rate as to maintain the fluidized bed in a substantially steady state throughout.

According to the invention, also, apparatus which is for use in removing material from a target surface by contact of particulate material projected thereat, comprises: a chamber adapted to be charged to form a bed of particulate material therewithin; means operable to impel gas at such relatively low velocity through such a bed as to render the bed to a fluidized state occupying a certain space, with the particles having an energy distribution determined by the energy input rate of the said low velocity gas; control means operable to vary the velocity of the low velocity gas so as to establish a desired energy distribution between the particles of the fluidized bed; particle escape-passage defining means associated with the said space; and a handpiece having therein first and second passage means merging at a junction the said first passage means communicating with the said particle escape-passage defining means, and the said second passage means being adapted to be connected to a source of relatively high velocity gas; and in which, the said first and second passage means of the handpiece are such and are so arranged with respect to one another that, in operation, particulate material carried, by the low velocity gas, from the said space, by way of the escape-passage defining means, and conveyed thereby through the said first passage means is accelerated by merger of the particle bearing low velocity gas with relatively high velocity gas flowing in the said second passage means as it attains the said junction, to its projection velocity from the handpiece.

Preferably, the particle escape-passage defining means comprises a perforate hollow body; and the chamber wall has an aperture through which the said perforate hollow body communicates with the handpiece.

Preferably the apparatus has second control means operable, when the handpiece is connected with the said second passage means thereof to the said high velocity gas source, to vary the velocity of the said high velocity gas to a desired value.

Preferably the apparatus has particulate material feed means operable when, in use of the apparatus, the chamber is charged with a bed of fluidized particulate material continuously to admit particulate material to the said fluidized bed in replacement for particulate material being lost from the bed, in such manner and at such rate as to maintain the fluidized bed in a substantially steady state.

Suitably, the said feed means comprises barrier means partitioning the said chamber into upper and lower zones, the lower zone to be charged to form the said bed of particulate material to be fluidized, the upper to serve as a depletable reservoir of such particulate material the said barrier means serving to uphold particulate material introduced into the said upper zone and being arranged, adapted, and constructed such as to cause such particulate material as may be upheld thereby to descend to the said lower zone at a rate and in a manner such that a bed of particulate material fluidized in the lower zone is maintained substantially in steady state regardless substantially of loss of particulate material therefrom by way of the escape-passage defining means to the handpiece.

Preferably, the said barrier means is arranged, adapted and constructed such as to provide a space at its periphery being a space communicating between the said upper and lower zones by way of which particulate material upheld by the barrier means is able to descend to the lower zone.

Preferably, the barrier means has an upper surface inclined with respect to the vertical direction of the chamber; the said communicating space is at the periphery of the inclined upper surface; and such particulate material as may be upheld by the barrier means descends to the lower zone down the said inclined surface to the said periphery thereof.

Preferably, the said barrier means has an axis extending in the vertical direction of the said chamber; and the said inclined surface has a symmetry with respect to said vertical axis. Suitably, the chamber sidewall has a rectangular inner surface; and the said barrier means has a pyramidally inclined surface with a rectangularly extending periphery.

Preferably, the said barrier means comprises a body of sheet material; and the perforate hollow body, being the escape-passage defining means, is within the space defined in the lower zone by the body of sheet material.

Preferably, the said perforate hollow body is secured within the said space dependant from the body of sheet material; the hollow body is in communication with an aperture in the wall of the said upper chamber zone by way of conduit means and extending to the said chamber wall aperture from the hollow body through an aperture through the sheet material. Suitably, the perforate hollow body comprises a perforate tubular member.

Preferably the apparatus has, at the bottom of the chamber, low velocity gas admission means in the form of a hollow body having a perforate upwardly facing wall.

Preferably, the lower zone of the chamber is in the form of a pocket the periphery of which extends around the inner surface of the chamber sidewall in particle-sealed relationship therewith; and the said perforate upwardly facing wall of the hollow body constitutes the bottom-most portion of the wall of the said pocket.

Preferably, the said pocket has a symmetry with respect to the vertical axis of the chamber.

Preferably, the said pocket periphery is in particle sealed contact with the chamber sidewall at a position adjacent to the periphery of the barrier means.

Preferably, the apparatus handpiece has a body part and a nozzle part removably secured thereto at the forward end thereof; a passage, being the said first passage means of the handpiece, extends lengthwise through the said handpiece forward end; the said second passage means of the handpiece extends lengthwise through the handpiece forward end and is circumferentially disposed with respect to the said first passage means; and the nozzle part is provided with a cavity adjacent its connection with the body part, being the aforesaid junction between the said first and second passage means.

Preferably, the said forward end of the body part is constituted as a swirl-imparting part removably secured to the rear portion of the body part; the nozzle part is removably secured to the said swirl-imparting part at the forward end thereof; the second passage means extends through the swirl-imparting part as a multiplicity of separate passages communicating with the said nozzle part cavity and formed so as to impart to high velocity gas flow to the said cavity a velocity component at an angle to low velocity gas flow to the said cavity through the first passage means of the said swirl-imparting part.

Preferably, the handpiece is provided with means operable, when the said handpiece second passage means is connected to a source of high velocity gas to cause said high velocity gas to flow therealong and to actuate the controllable pressure reduction valve means to cause gas at a low velocity determined by the setting of the said valve means to flow in the said handpiece first passage means.

The foregoing and other features of a method and apparatus for removing material from a target surface, are hereinafter described with reference to the accompanying drawings, in which: Figs. 1 and 2 are diagrams representing the chamber, or powder hopper of the apparatus, Fig. 1 showing, diagrammatically, a section taken on I-I of Fig. 2 and Fig. 2 a section on II-II of Fig. 1, Fig. 3 is a diagram showing a longitudinal section through the handpiece, or gun, of the apparatus; Fig. 4 is a diagram showing a scrap end view of the handpiece of Fig. 2; Fig. 5 shows diagrammatically a section taken on V-V of Fig. 3; and Fig. 6 is a diagrammatic system diagram of the apparatus.

The apparatus comprises: a lidded chamber, being a powder hopper 11 adapted to be charged to form a bed 13 of particulate material therewithin; means 15 operable to impel gas at such relatively low velocity through such a bed as to render it to a fluidized state, occupying a certain space S, with particles having an energy distribution determined by the energy input rate of the low velocity gas; control means, a regulator 17 operable inter alia to vary the velocity of the low velocity gas so as to establish a desired energy distribution between the particles of the fluidized bed; particle escape-passage defining means 19 associated with the said space S; and a handpiece 21 having therein first and second passage means 23, 25, respectively, merging at a junction 27, the said first passage means 23 communicating with the said escapepassage defining means 19 and the said second passage means 25 being adapted to be connected to a source (not shown) of relatively high velocity gas. The first and second passage means 23, 25, are such and are so arranged with respect to one another that, in operation, with the second passage means 25 of the handpiece 21 connected to a high velocity gas source, suitably a compressor, particulate material carried, by the low velocity gas, from the space S by way of the escape-passage defining means 19 and conveyed thereby through the handpiece first passage means 23 is accelerated as it attains the junction 27 to its projection velocity from the handpiece.

The apparatus has second control means 29 operable, when the handpiece is connected with the second passage means 25 to the high velocity gas source, to vary the velocity of the high velocity gas to a desired value. As will be more fully noted hereinafter, high velocity gas supplied by way of the second passage means 25 of the handpiece to the junction 27 creates at the cavity a reduced pressure affecting the velocity of the low velocity gas. This interaction is naturally taken into account in setting the values of the gas velocities.

Particulate material feed control means 31 is provided operable when, in use of the apparatus, the chamber 11 is charged with a bed of fluidized particulate material 13, continuously to admit further particulate material to the said fluidized bed 13 in replacement for particulate material being lost from the bed, in such manner and at such rate as to maintain the fluidized bed in a substantially steady state throughout use of the apparatus.

More particularly, the feed control means 31 comprises barrier means 33 partitioning the chamber 11 into lower and upper zones 35a, 35b, the lower zone 35a to be charged to form the said bed 13 of particulate material to be fluidized, the upper 35b to serve as a depletable reservoir for such particulate material as 37. The barrier means 33 which upholds, temporarily, particulate material introduced into the upper zone 35b, is such as to cause particulate material 37 upheld thereby to descend to the lower zone at a rate and in a manner such that the bed 13 of particulate material fluidized, in operation of the apparatus, in the lower zone 35a is maintained substantially in steady state regardless substantially of loss of particulate material therefrom by way of the escape-passage defining means 19.

Specifically, the barrier means 33 is so arranged, adapted and constituted as to provide at its periphery 39, a space 41 communicating between the upper and lower zones 35b, 35a, through which particulate material 37 upheld by the barrier means 33 may descend to the lower zone 35a.

In particular the chamber 11 is rectangular in crosssection; and the barrier means 33 is in the form of a pyramid, open at it base 43, and formed from sheet material. The pyramidal barrier means 33 is supported within the chamber 11 spaced from the inner surface of the chamber sidewall 45 by four straps 47 one at each side of the pyramid, adjacent the base 43 thereof.

As will be gathered, particulate material 37 upheld temporarily upon the upper inclined surface 49 of the pyramid makes its way downwardly of the said inclined surface to descend to the lower zone 35a through the peripherally extending space 41.

As shown, the particulate material of the fluidized bed 13 and the particulate material 37 of the reservoir 35b form a continuuTn around a void, the space S, defined by the pyramidal barrier means 33. Particulate material descending from the reservoir 35b to the bed 13 is in exact balance with the particulate material leaving the bed 13 by way of the escape passage means 19. It is by this means that the fluidized bed 13 is maintained in the steady state during operation of the apparatus.

The perforate hollow body 19 within the space S is, specifically, within that part of the space S defined by envelope of the pyramidal barrier means 31. The body 19 being, in the embodiment depicted, a curved perforate tube, extends at one end 51 thereof, through an aperture in the sheet pyramidal barrier means; and a low pressure hose 53 leading to the first passage means 23 of the handpiece extends from the end 51 through an aperture in the chamber sidewall 45.

The lower zone 35a is, in the depicted embodiment, in the form of an inverted pyramidal pocket, the periphery 55 of the wall 57 of which extends around the inner surface of the chamber sidewall 45 in particle-sealed relationship therewith; and means operable to impel gas 15 in the form of a hollow body perforate across its upwardly facing wall 61 thereby to form an orifice plate, constitutes the bottom-most portion of the pocket wall 57. A hose 71 conveys gas at low pressure to the hollow body 59.

The chamber lid 111 is removably secured to the chamber body. The lid 111 is fitted with a pressure relief valve 73 against the possibility of over-pressure in the chamber.

The handpiece 21 has a handled body part 75 and a nozzle part 77 removably secured to the body part 75 at the forward part 79 thereof.

The first passage means 23 is constituted by a rigid tube extending through two aligned bores 81a, 81b, formed lengthwise of the body part 75, the bores 81a, Slb, being through forward and rear parts 79, 83, respectively, of a barrel portion 85 of the body part 75, the parts 79, 83 being interrupted by the presence of the passage means 25 which extends, as shown, the length of the handle portion 87 and the intermediate section 89 of the barrel portion 85.

The nozzle part 77 has a cavity 91 tapering conically from the front face 93 of the forward part 79 of the barrel portion 85, the cavity 91 constituting the junction of the said first and second passage means 23, 25.

The forward part 79 of the barrel portion 85 is detachably secured to the intermediate section 89 thereof.

The part 79 is pierced from end to end with, in the embodiment, six passages 95 circumferentially distributed about the tube 23. The passages 95 make an angle with the longitudinal axis of the barrel portion 85 at their forward terminal portions in order to create in high velocity gas flow therethrough a swirl component to the gas as it enters the cavity 91.

The body part 75 of the handpiece 21 carries a pneumatic valve 97 connected to a regulator 99, monitored by pressure gauge 99', by way of a control line hose 101; and a handpiece trigger mechanism 103 is biased by a spring 105 to a position such that, until the bias is overcome, the valve 97 is held in the open condition, passing to an outlet tube (not shown) therefrom gas received at the valve 97 by way of the hose 101. The said outlet tube (not shown) is secured to the body part 75 with its free end adjacent to the tip of the handpiece.

The regulator 99 relays gas to the hose 101 from the high pressure hose 103 by way of a conduit 105 and restrictor 1070 From the foregoing it will be understood that when the apparatus is active but not in use, there is a restricted and regulated gas flow through the valve 97 to the tip of the handpiece which gas flow may be employed to clear dust and other debris liberated from the target surface after a period of use of the apparatus.

A high pressure hose 104 from which gas is bled to the valve 97 by way of the conduits 105 and 101 is adapted to be connected to the the air compressor by way of the primary pressure regulator and moisture trap 29.

A piston valve 109 has its ports, 109a, 109b, connected in series with the pressure hose 104. The valve 109 is spring biased by an associated spring 111 to the closed condition.

Downstream of the valve 109 a conduit 113 leads from the high pressure hose 104 to the regulator 17. The regulator 17 is monitored by a pressure gauge 17b.

In operation with a charge of particulate material in the lower zone 35a of the chamber 11 and a mass of such material upheld by the baffle means, and with the hose 104 connected to a working compressor, high velocity air regulated at, say, 80 psi by the regulator 29 is bled to the normally open valve 97 and, from thence, to the handpiece tip through the restrictor 107 and regulator 99.

When the trigger 103 is depressed, against the bias of the spring 105, pressure build up in the hose 104 is applied to the normally closed piston valve 109 opening that valve against the bias of the spring 111. Air at relatively high velocity is relayed by the hose 104 to the handpiece 21 passing by way of the six passages 95, forming part of the second passage means 25, of the swirl part 95 to the junction cavity 91 of the handpiece creating, by venturi effect, a reduced pressure at the outlet from the first passage means 23.

Air bled from the hose 104 to pass by way of the conduit 113 to the regulator 17, as a result of the opening of the valve 109, is relayed regulated at, say, 10 psi to produce a low velocity air stream as previously mentioned flowing by way of the conduit 71 to the hollow body 15 by which low velocity gas is impelled through the fluidized bed.

Air emerging from the body 59 upwardly through the orifices in the upper wall thereof is effective to maintain in the fluidized state, the charge of particulate material 13 in the lower zone 35a.

The kinetic energy of the low velocity airstream, which energy is variable by adjustment of the regulator 17 in relation to the high velocity air flow as determined by the setting of the regulator 29, determines the energy distribution between the fluidized particles of the bed of particulate material. Particulate material having kinetic energy such that it has random motion in the vicinity of the perforated tube 19 is carried by the low pressure air flowing into the said tube to be conveyed to the handpiece by way of the rigid tube 23. The relatively low velocity of the air passing to the tube 23 and hence of the particulate material conveyed thereby is determined by the pressure differential between the space S into which more energetic particles are lifted by air flow from the body 59 and the reduced pressure created by the high velocity air flow through the six passages 95 of the swirl member 79 into the conical cavity 91 of the nozzle part 77.

The swirl member 79 has a cylindrical outer shell part 79a and a removable cylindrical core part 79b pierced by the six passages 95. The core part 79b may be one of a set of such parts differing in the angle given to passages in relation to the longitudinal axis of the part. The particle discharge pattern produced as a result of the angled passages 95 is of generally annular form. And the distribution of particulate material discharged may be varied by suitable choice of core part. The freedom of choice of core parts provides an extra parameter in the determination of the removal of material at the target surface it being desirable to employ different core parts accordingly as particle size may be varied.

As particulate material reaches the junction, the cavity 91, at the forward end of the tube 23 with the second passage means 25 it is accelerated by the airstream flowing into the cavity from the passages 95 to attain substantially the high velocity at which the particulate material is projected at the target surface.

The apparatus hereinbefore described possesses an inherent high degree of flexibility in its operation, ensuring the discharge of particulate material from the handpiece at a predetermined discharge rate thereby to enable fine control of the process of removing material from the target surface using particulate material within a wide range of particle sizes. Depending on the particle size, a swirl imparting part 79 of appropriate passage dimensions and configuration may be chosen according to particle size. The nozzle 77 is desirably fabricated from a very hard material, typically tungsten carbide, in order to extend the useful life of the nozzle under abrasive attack from high velocity particulate material therethrough. Typically the particle size may have any value in the range 5 to 250 microns.

As for the space 41 at the periphery of the baffle means, this should be as small as may be necessary to maintain the fluidized bed in a substantially steady state whilst not being so small as to encourage bridging of the space 41 by particulate matter of the mass upheld by the baffle means. It has been found that the space 41 may have a width of between 1/8" and 1/4" whilst maintaining satisfactory feed to the bed beneath.

Claims

1. A method of removing material from a target surface by contact of particulate material projected thereat from a handpiece, which comprises: causing particulate material to be entrained in a stream of relatively low velocity gas which is conducted along first passage means provided in the said handpiece to a junction position whereat the particulate material is accelerated to its projection velocity from the handpiece by merger of the particle bearing low velocity gas with relatively high velocity gas flowing in second passage means provided in the handpiece.

2. A method of removing material from a target surface by contact of particulate material projected thereat from a handpiece, which comprises: entraining in a flow of relatively low velocity gas, particulate material continuously being liberated during fluidization of a bed of such material by the flow therethrough of the said low velocity gas; and conducting the particle bearing low velocity gas flow along first passage means provided in the handpiece to a junction position whereat the particulate material conveyed to the said junction by the particle bearing low velocity gas stream, is accelerated, as it attains the said junction, to its projection velocity from the handpiece.

3. A method as claimed in claim 2 in which: the said first and second passage means are such and are so arranged with respect to one another at the said junction therebetween that a substantially reduced pressure promoting the flow of low velocity gas along the first passage means is created at the said junction.

4. A method as claimed in claim 2 or 3 in which: particulate material is supplied to the bed of such material in such manner and at such rate as to maintain the fluidized bed in a substantially steady state throughout.

5. Apparatus which is for use in removing material from a target surface by contact of particulate material projected thereat, and which comprises: a chamber adapted to be charged to form a bed of particulate material therewithin; means operable to impel gas at such relatively low velocity through such a bed as to render the bed to a fluidized state occupying a certain space, with the particles having an energy distribution determined by the energy input rate of the said low velocity gas; control means operable to vary the velocity of the low velocity gas so as to establish a desired energy distribution between the particles of the fluidized bed; particle escape-passage defining means associated with the said space; and a handpiece having therein first and second passage means merging at a junction the said first passage means communicating with the said particle escape-passage defining means, and the said second passage means being adapted to be connected to a source of relatively high velocity gas; and in which, the said first and second passage means of the handpiece are such and are so arranged with respect to one another that, in operation, particulate material carried, by the low velocity gas, from the said space, by way of the escape-passage defining means, and conveyed thereby through the said first passage means is accelerated by merger of the particle bearing low velocity gas with relatively high velocity gas flowing in the said second passage means as it attains the said junction, to its projection velocity from the handpiece.

5. Apparatus as claimed in claim 5 in which: the particle escape-passage defining means comprises a perforate hollow body; and the chamber wall has an aperture through which the said perforate hollow body communicates with the handpiece.

7. Apparatus as claimed in claim 5 or 6 and which has: second control means operable, when the handpiece is connected with the said second passage means thereof to the said high velocity gas source, to vary the velocity of the said high velocity gas to a desired value.

8. Apparatus as claimed in claim 5, 6 or 7 which has: particulate material feed means operable when, in use of the apparatus, the chamber is charged with a bed of fluidized particulate material continuously to admit particulate material to the said fluidized bed in replacement for particulate material being lost from the bed, in such manner and at such rate as to maintain the fluidized bed in a substantially steady state.

9. Apparatus as claimed in claim 8 in which: the said feed means comprises barrier means partitioning the said chamber into upper and lower zones, the lower zone to be charged to form the said bed of particulate material to be fluidized, the upper to serve as a depletable reservoir of such particulate material the said barrier means serving to uphold particulate material introduced into the said upper zone and being arranged, adapted, and constructed such as to cause such particulate material as may be upheld thereby to descend to the said lower zone at a rate and in a manner such that a bed of particulate material fluidized in the lower zone is maintained substantially in steady state regardless substantially of loss of particulate material therefrom by way of the escape-passage defining means to the handpiece.

10. Apparatus as claimed in claim 9 in which: the said barrier means is arranged, adapted and constructed such as to provide a space at its periphery being a space communicating between the said upper and lower zones by way of which particulate material upheld by the barrier means is able to descend to the lower zone.

11. Apparatus as claimed in claim 10 in which: the barrier means has an upper surface inclined with respect to the vertical direction of the chamber; the said communicating space is at the periphery of the inclined upper surface; and such particulate material as may be upheld by the barrier means descends to the lower zone down the said inclined surface to the said periphery thereof.

12. Apparatus as claimed in claim 11 in which: the said barrier means has an axis extending in the vertical direction of the said chamber; and the said inclined surface has a symmetry with respect to said vertical axis.

13. Apparatus as claimed in claim 12 in which: the chamber sidewall has a rectangular inner surface; and the said barrier means has a pyramidally inclined surface with a rectangularly extending periphery.

14. Apparatus as claimed in claim 12 or 13 in which: the said barrier means comprises a body of sheet material; and the perforate hollow body, being the escape-passage defining means, is within the space defined in the lower zone by the body of sheet material.

15. Apparatus as claimed in claim 14 in which: the said perforate hollow body is secured within the said space dependant from the body of sheet material; the hollow body is in communication with an aperture in the wall of the said upper chamber zone by way of conduit means and extending to the said chamber wall aperture from the hollow body through an aperture through the sheet material.

16. Apparatus as claimed in claim 15 in which: the perforate hollow body comprises a perforate tubular member.

17. Apparatus as claimed in any of claims 5 to 16 which has at the bottom of the chamber, low velocity gas admission means in the form of a hollow body having a perforate upwardly facing wall.

18. Apparatus as claimed in claim 17 in which: the lower zone of the chamber is in the form of a pocket the periphery of which extends around the inner surface of the chamber sidewall in particle-sealed relationship therewith; and the said perforate upwardly facing wall of the hollow body constitutes the bottom-most portion of the wall of the said pocket.

19. Apparatus as claimed in claim 18 in which: the said pocket has a symmetry with respect to the vertical axis of the chamber.

20. Apparatus as claimed in claim 15 or 16 when appendant to any of claims 12 to 16 in which the said pocket periphery is in particle sealed contact with the chamber sidewall at a position adjacent to the periphery of the barrier means.

21. Apparatus as claimed in any of claims 5 to 20 in which: the handpiece has a body part and a nozzle part removably secured thereto at the forward end thereof; a passage, being the said first passage means of the handpiece, extends lengthwise through the said handpiece forward end; the said second passage means of the handpiece extends lengthwise through the handpiece forward end and is circumferentially disposed with respect to the said first passage means; and the nozzle part is provided with a cavity adjacent its connection with the body part, being the aforesaid junction between the said first and second passage means.

22. Apparatus as claimed in claim 21 in which: the said forward end of the body part is constituted as a swirl-imparting part removably secured to the rear portion of the body part; the nozzle part is removably secured to the said swirl-imparting part at the forward end thereof; the second passage means extends through the swirlimparting part as a multiplicity of separate passages communicating with the said nozzle part cavity and formed so as to impart to high velocity gas flow to the said cavity a velocity component at an angle to low velocity gas flow to the said cavity through the first passage means of the said swirl-imparting part.

23. Apparatus as claimed in claim 23 in which: the handpiece is provided with means operable, when the said handpiece second passage means is connected to a source of high velocity gas to cause said high velocity gas to flow therealong and to actuate the controllable pressure reduction valve means to cause gas at a low velocity determined by the setting of the said valve means to flow in the said handpiece first passage means.