GB2455571A

GB2455571A - Inter-connecting nozzle impactor component arrangement

Info

Publication number: GB2455571A
Application number: GB0724452A
Authority: GB
Inventors: Michael Jefferson Smurthwaite
Original assignee: WESTECH INSTR HOLDINGS PLC
Current assignee: WESTECH INSTR HOLDINGS PLC
Priority date: 2007-12-15
Filing date: 2007-12-15
Publication date: 2009-06-17
Also published as: GB0724452D0

Abstract

A cascade impactor including stacked body members 9, 10, 11 having inter-fitting parts which include respective surfaces of rotation about an axis, and which are provided with circumferentially spaced co-operating formations which each include an engagement formation provided on one of the body members, possibly in the form of lug (34, Fig 4) and a receiving formation on the other body member. The receiving formation including an entry portion, with an inlet 8, which the engagement formation can enter on movement of the body members together in the direction of the axis, and a helically-extending portion of predetermined length along which the engagement formation can move to secure the body members together. A filter member 212 may also be present within member 11, and a base member 13 is also included. A circular get plate 32, with a plurality of apertures is provided by body member 11 underneath body member 10. Impaction plates 36, 36a include wall members 37, 37a.

Description

Ref: Al 2083GB Title: Connection of body members for impactor

Description of Invention

This invention relates to impactors, for measuring the size distribution and concentration of particles (which may be liquid droplets or solid particles), in an aerosol of the particles suspended in a gas. More particularly, the invention relates to the construction of a body for such an Impactor wherein body members which form the impactor itself, and related body members such as inlet and outlet members, are connected to one another.

An impactor is a well-known device, comprising a jet or a number of jets through which a stream of the particle-laden gas (aerosol) is caused to flow.

The stream impinges on a surface facing the jet, some of the particles in the stream impacting on the surface and remaining thereon. The surface may be treated to enhance retention thereon of impacting particles, e.g. by being greased or by being coated or covered with a suitable substrate. The jet(s) may be of circular, square, rectangular, or any other suitable shape, and the impaction surface may be flat or curved, or in the form of a dish or cup, and of any suitable material for example, e.g. glass, metal or plastics, possibly with a covering as aforesaid.

A jet or number of jets and an appropriate impaction surface together constitute a single stage impactor and divide the particles of the stream into two fractions namely those remaining on the impaction surface and those which are retained in the gas stream. A number of impaction stages may be arranged in succession as a cascade impactor, with the gas passing from stage to stage carrying particles with it, the particles being removed at each stage in discreet size ranges according to the size of jet(s) at each stage and the distance between the jet(s) and the impaction surface. Particles not retained at a particular stage are camed onwards to the next stage(s).

In conventional cascade impactors, the most important characteristic is the collection efficiency of each stage with respect to a particular size of particle; the collection efficiency is defined as the fraction of particles passing through the jet(s) that are collected on the impaction surface of that stage. The ideal device would collection 100% of particles greater than the so-called cut size of a stage, and all smaller particles are carried onto the next stage. In practice, impactors are not ideal and the particle size that has a 50% collection efficiency at a stage is defined as the cut-point of that stage (D50 cut point). In any event, a number of stages arranged successively to form a complete impactor enable the particle size distribution of an aerosol to be characterised.

The size of the or each jet and the distance from it or them to the impaction surface (the "jet to plate ratio") determine the cut size of an impaction stage.

Successive stages of a cascade impactor will have smaller and smaller jets, increasing the velocity of gas and particles travelling through the impactor, possibly there being a final filter stage to remove any remaining particles not already removed by impaction. It is essential that the velocity of gas travelling through the jet(s) in each stage, and hence velocity of the particles, governing the cut point of each stage, is controlled during, operation, and typically a cascade impactor is operated at a constant flow rate during a measurement cycle by drawing the particle-laden gas through it using a suction pump.

Ingress of external gas or air which can alter the velocity of flow through the jets or disturb flow patterns within the device must be prevented.

After a test flow through a cascade impactor has taken place, the impactor has to be dismantled stage by stage to enable the amount of particles retained in each stage by impaction to be measured, and hence the particle size distribution in the gas stream to be analysed. Since a large number of tests may be performed in succession, the impactor may be dismantled and reassembled several times in a day. Hence whatever construction is adopted for a body of the impactor should facilitate dismantling and reassembly of body members which form the successive stages, whilst meeting the requirement for sealing as mentioned above. Known impactors and cascade impactors use springs and/or clamping mechanisms to secure body members together, in conjunction with appropriate seals between the body members to ensure gas-tightness. Seals such as 0-rings, flat gaskets, or combinations and variations thereof have been utilised. Screw threads have been used to join stages or components of a cascade impactor together, including seals of compressible material between body parts which are screw threaded together. However, the force exerted by springs or clamping mechanisms, and the extent to which the seals are compressed, is undefined, so there can be small variations in the spacing between adjacent body parts. Thus, variation is possible in the jet-to-plate ratio and hence in the cut point of any stage. Such mechanisms are also difficult and time consuming to dismantle and assemble, and suffer difficulties such as the collection surfaces tending to stick to the seals.

Accordingly, it is an object of the present invention to address these problems associated with known constructions of impactors, and provide a connection which is convenient to dismantle and reassemble whilst meeting the sealing requirement in an improved manner.

According to one aspect of the invention, we provide an impactor including body members having interfitting parts which include respective surfaces of rotation about an axis, and which are provided with circumferentially spaced co-operating formations which each include an engagement formation provided on one of the body members and a receiving formation on the other body member, the receiving formation including an entry portion which the engagement formation can enter on movement of the body members together in the direction of the axis, and a helically-extending portion of predetermined length along which the engagement formation can move for a limited distance to secure the body members together.

Preferably, an annular sealing element is disposed between facing surfaces of the body members, which sealing element is compressed as the engagement formations are moved from the entry portions of the receiving formations along the helically-extending portions thereof.

The sealing element may be an 0-ring.

The length (circumferential extent) of each helically-extending portion of the receiving formations may, by way of example, be such that approximately 25 to 45 degrees of relative angular movement between the body members, about the axis, is required fully to secure the body members together after the engagement formation has been entered into the entry portion of each receiving formation.

Preferably, when the engagement formations have been moved along the helically-extending portions of the receiving formations to the ends thereof,.

they are retained by friction in that position. Such friction arises between the engagement formations and receiving formations, and between the sealing element and the surfaces of the body members between which the sealing element is disposed. When the sealing element is compressed substantial forces arise in the direction lengthwise of the axis of the impactor, and hence the frictional forces are increased so that the angular movement between the body members, which would be necessary to disengage their co-operating formations, is resisted.

However, it would be within the scope of the invention for there to be some additional means for retaining the co-operating formations in engagement. For example, there may be an additional catch device for retaining the body members in a relative angular position of full engagement of their co-operating formations. Possibly the receiving formations may each have a non-helical portion engaged by the respective engagement formation, e.g. a circumferential (non-inclined) portion, or a return portion in the manner of a ubayoner fitting.

Each engagement formation may comprise a radially-extending pin provided on a body member. Conveniently such a pin may extend radially inwardly from an inwardly-facing surface of the one body member, the corresponding receiving formation being provided in a radially outwardly-facing surface of the other body member.

The impactor may be a cascade impactor, comprising a plurality of body members for successive impactor stages of the impactor, and co-operating formations as aforesaid may be provided on all such body members of the cascade impactor. Body members forming inlet and outlet members for gas flow through the impactor are, preferably, also provided with engagement formations or receiving formations, as aforesaid, as appropriate.

An impactor, preferably a cascade impactor, in accordance with the invention presents a number of advantages over an impactor with previously-known methods of connecting body members together. The limited angular movement necessary between each pair of body members to assemble and dismantle them enables this to be done rapidly, compared with known clamping mechanisms or screw-threaded connections between body members. The limited extent of the helically-extending portions of the receiving formation means that a required spacing between body members can be established rapidly and maintained, so that the jet-to-plate ratio is accurately set. The limited angular movement between body members minimises the potential for repetitive strain injury in personnel assembling or dismantling impactors, particularly compared with impactors where body members are screw-threaded together. A tightly-controlled degree of compression of the sealing elements between body members can be maintained, so that they are compressed sufficiently to ensure gas-tightness but not over-compressed which might damage the sealing elements necessitating their replacement.

The invention will now be described by way of example with reference to the accompanying drawings of which: Figure 1 is a transverse cross-section through a cascade impactor in accordance with the invention; Figure 2 is a transverse section through a first body member of the impactor; Figure 3 is a plan view of a second body member of the impactor; Figure 4 is a transverse section, to an enlarged scale, of part of the body member of figure 3; Figures, 5, 6 and 7 are respectively a transverse section through, an elevation of, and a perspective view of a further body member forming a base of the impactor.

Referring firstly to figure 1 of the drawings, this shows, in transverse section, a cascade impactor in accordance with the Invention. It comprises a stack of a number of impactor stages through which a stream of particle-laden gas is drawn in the manner described above. It has a body made from a number of body members assembled to one another, namely an inlet member indicated generally at 8, a body member indicated generally at 9, a body member indicated generally at 10, a filter member indicated generally at 11, and a base member indicated generally at 13. The body members 9, 10 and 11 together make up two impactor stages.

The impactor as illustrated is vertical in configuration and generally cylindrical in overall shape, with an upright central longitudinal axis indicated at A. In many cases, impactors are operated in this orientation. References herein to uupwardly "downwardly", and cognate expressions, refer to such an orientation. Nevertheless, it is to be understood that the components could be arranged to lie in some other orientation.

Aspects of the body member 10 are shown in figures 3 and 4. Certain parts thereof are also very similar to the body member 9 shown in figure 2, so the following descnption is also applicable to the latter body member. These body members each comprise an annular extenor wall 12 with cylindrical external and internal surfaces 14, 16, respectively. The wall 12 and its surfaces 14, 16 as with all other circular features of the body members, are centred on the axis A. At its lowermost free end, the wall 12 is relieved internally by a recess with a radially inwardly- facing cylindrical surface 20. At its upper end, the wall 12 has an axially upwardly facing annular surface 22 followed by an annular upstand 24 with a cylindrical radially outwardly facing surface 26. The surface 26 is dimensioned to fit closely within the internal cylindrical surface 20 of the wall 12. so the upstand 26 can enter the open lower end of the internal wall 16 of the housing member 9 on top of the housing member 10. Radially inwardly of the upstand 24 there is an inwardly extending wall portion 28, followed by a downwardly extending cylindrical wall portion 30 at the lower end of which a circular jet plate 32 extends across the body member. The jet plate 32 is provided with a plurality of apertures as indicated at 32, whose size, shape and spacing are determined in accordance with the size of particle intended to be collected by an impaction plate carried by the body member 11 beneath the body member 10. The wall portion 28 of the body member 10 is provided with three circumferentially spaced lugs 34 for carrying an impaction plate for collecting particles passing through the jet plate of the body member 9 above the body member 10: such impaction plate is the base 36 of a dish-like element with an annular side wall 37, with the impaction plate 36 facing the jet plate 32 of the body member 9 and at a predetermined spacing therefrom. A similar such impaction plate 36, 37 is carried by lugs as 36 on the body member 11 beneath the body member 10.

The body member 9 differs from the body member 10 in that, because it is the body member immediately beneath the input member 11, it does not have to carry an impaction plate. Therefore, instead of the support lugs 34, the upstand 24, as indicated at 24a in figure 2, extends radially inwardly to a chamfer 24 leading into the inwardly-facing annular surface 31 of wall portion 30.

The body member 8 which forms an inlet member for the impactor comprises a circular body 200 with an axially downwardly facing annular groove 202 adjacent its outer periphery. The groove 202 is dimensioned so as to receive the upstand 24, 24 of the body member 9. The inlet member 8 has a frusto-conical portIon 204 leading into a spigot 206 for connection to a source of particle-laden gas which is to be drawn through the impactor.

The body member 11 supports the impaction member 36, 37a as aforesaid, but does not have a jet plate as 32. instead it has a perforated transverse surface 210 above which is supported a filter element 212 which may be a filter paper disposed within a carrier ring 214. This serves to trap and retain any particles which are not collected by the previous two impaction stages.

The body member 13, which is the base member of the impactor, as shown in figures 5 to 7, has a main part 100 which is of somewhat disc-like form, with a central downwardly-extending outlet port 102 leading into a radial outlet passage 104 for connection, e.g. by a suitable flexible hose, to a suction pump for drawing gas through the base member after having passed through the inlet member then the impactor stages. A fitting 106 for connection to a flexible hose is shown in figure 7.

The body members 9, 10 and 11, as well as the inlet body member 8 and base member 13, are connected to one another by respective sets of co-operating formations. These comprise receiving formations at the uppermost end of each body member, for engagement by engagement formations provided at the lowermost end of a superposed body member. The inlet body member 8, being the uppermost member of the impactor, is provided only with engagement formations, while the base member 13 being the lowermost body member of the impactor, has Only the receiving formations.

The receiving formations are clearly visible on the base member 13 in figures 5, 6 and 7, and parts thereof on the body member 10 are visible in figures 3 and 4. Figures 3 and 7 show the presence of three receiving formations 38 spaced equally circumferentially about the respective body members, although possibly the circumferential spacing could be unequal if it were desired to enable adjacent body members to be connected together in one particular angular orientation relative to one another. Each receiving formation 38 comprises an axially-extending entry portion 40 followed by a circumferentialfy extending helical groove portion 42 of shallow helix angle. The engagement formations comprises pins or peg as indicated at 46, extending radially inwardly of the wall portion 20, the radial depth of the protruding part of each engagement formation 46 corresponding to the radial depth of the receiving formations 38.

To assemble adjacent body members to one another, they are presented to one another in the direction of axis A so that the engagement formations 46 enter the entry portions 40 of the receIving formation 38. Then the two body members are moved angularly relative to one another about the axis A so that the pins 46 move along the helically-extending portions 42 of the receiving formations, until they reach the closed ends of the portions 42. It will be apparent that this causes the body members to adopt a very closely-determined axial position relative to one another, so that the jet plate 32 of one body member is spaced at the required distance from the impact surface carried by the body member beneath.

For sealing between adjacent body members, each body member has a formation receiving a sealing element. For the body member 10, and also for the body members 11 and 13, the sealing element is an 0-ring accommodated in an annular recess 50 at the radially innermost part of whether the upwardly-facing surface 22. When adjacent body members are connected together as above described, the 0-ring is compressed between the body members to a pre-determinecj extent. The friction established between the body members and the 0-ring is such that they will remain in the connected position relative to one another in use, without the necessity of any retaining formation or the like to hold the body members in that angular position, wherein the engagement formations 46 have reached the closed ends of the helically-extending portions 42 of the receiving formations. Nevertheless, it would be within the broadest scope of the invention for there to be some additional retaining means. In the case of the inlet member 8, an 0-ring 52 is accommodated in an annular groove in the inlet member and engages the surface 31 of the body member 9.

An annular groove 50 in the base member 13, for accommodating an 0-ring engaging the body member 11, is visible in figures 5 and 7.

The total circumferential extent of each of the receiving formations 38 may be such as to require an angular movement of some 25 to 45 degrees of one body member relative to another to secure them together and disconnect them from one another. The circumferential extent of the entry portion 40 of each receiving formation is somewhat greater than the diameter of each of the engagement formations 46, so highly accurate alignment of adjacent body members is not required to enable them to be assembled to one another.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. Ref: Al 2083GB Title: Connection of body members for impactor

Description of Invention

The sealing element may be an 0-ring.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

Claims 1. An impactor including body members having interfitting parts which include respective surfaces of rotation about an axis, and which are provided with circumferentially spaced co-operating formations which each include an engagement formation provided on one of the body members and a receiving formation on the other body member, the receiving formation including an entry portion which the engagement formation can enter on movement of the body members together in the direction of the axis, and a helically-extending portion of predetermined length along which the engagement formation can move upon relative angular movement about the axis of the body members, to secure the body members together.
2. An impactor according to claim I further comprising an annular sealing element disposed between facing surface of the body members, which sealing element is compressed in the course of movement of the engagement formations along the helically-extending portions of the receiving formations.
3. An impactor according to claim 2 wherein the sealing element comprises an 0-ring.
4. An impactor according to any one of the preceding claims wherein the length of each helically-extending portion of the receMng formations provides for approximately 25 to 45 degrees of relative angular movement between the body members,
5. An impactor according to any one of the preceding claims wherein the co-operating formations are retained in engagement with one another by friction between contacting surfaces thereof.
6. An impactor according to any one of the preceding claims wherein each engagement formation comprises a radially-extending pin provided on a body member.
7. An impactor according to claim 6 wherein each pin extends radially inwardly from an inwardly facing surface of the body member, the receiving formation therefor being provided in a radially outwardly- facing surface of the other body member.
8. An impactor according to any one of the preceding claims comprising a plurality of body members for successive impactor stages, all such body members being provided with co-operating formations for engagement with further body members on either side.
9. An impactor according to claim 8 wherein body members forming inlet and outlet members for gas flow through the impactor are provided with engagement formations or receiving formations for co-operation with formations on impactor stage body members.
10. A body member for an impactor according to any one of the preceding claims, the body member including: receiving formations for co-operation with engagement formations on another body member, each receiving formation including an entry portion which the engagement formation can enter and a portion of predetermined length extending helically of the body member along which the engagement formation can move upon movement of the body members angularly relative to one another; and/or engagement formations for co-operating with said receiving formations on another body member.
11. A body member according to claim 11 for forming an impactor stage of the impactor, and having both receiving formations and the engagement formations for co-operation with respective further body members.
12. A body member according to claim 11 for forming an inlet member of the impactor and having only the receiving formations or the engagement formations.
13. A body member according to claim 11 for forming an outlet member of the impactor and having only the receiving formations or the engagement formations.
14. An impactor, or a body member therefor, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.
15. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.

15. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.

Claims 1. An impactor including body members having interfitting parts which include respective surfaces of rotation about an axis, and which are provided with circumferentially spaced co-operating formations which each include an engagement formation provided on one of the body members and a receiving formation on the other body member, the receiving formation including an entry portion which the engagement formation can enter on movement of the body members together in the direction of the axis, and a helically-extending portion of predetermined length along which the engagement formation can move upon relative angular movement about the axis of the body members, to secure the body members together.

2. An impactor according to claim I further comprising an annular sealing element disposed between facing surface of the body members, which sealing element is compressed in the course of movement of the engagement formations along the helically-extending portions of the receiving formations.

3. An impactor according to claim 2 wherein the sealing element comprises an 0-ring.

4. An impactor according to any one of the preceding claims wherein the length of each helically-extending portion of the receMng formations provides for approximately 25 to 45 degrees of relative angular movement between the body members, 5. An impactor according to any one of the preceding claims wherein the co-operating formations are retained in engagement with one another by friction between contacting surfaces thereof.

6. An impactor according to any one of the preceding claims wherein each engagement formation comprises a radially-extending pin provided on a body member.

7. An impactor according to claim 6 wherein each pin extends radially inwardly from an inwardly facing surface of the body member, the receiving formation therefor being provided in a radially outwardly- facing surface of the other body member.

8. An impactor according to any one of the preceding claims comprising a plurality of body members for successive impactor stages, all such body members being provided with co-operating formations for engagement with further body members on either side.

9. An impactor according to claim 8 wherein body members forming inlet and outlet members for gas flow through the impactor are provided with engagement formations or receiving formations for co-operation with formations on impactor stage body members.

10. A body member for an impactor according to any one of the preceding claims, the body member including: receiving formations for co-operation with engagement formations on another body member, each receiving formation including an entry portion which the engagement formation can enter and a portion of predetermined length extending helically of the body member along which the engagement formation can move upon movement of the body members angularly relative to one another; and/or engagement formations for co-operating with said receiving formations on another body member.

11. A body member according to claim 11 for forming an impactor stage of the impactor, and having both receiving formations and the engagement formations for co-operation with respective further body members.

12. A body member according to claim 11 for forming an inlet member of the impactor and having only the receiving formations or the engagement formations.

13. A body member according to claim 11 for forming an outlet member of the impactor and having only the receiving formations or the engagement formations.

14. An impactor, or a body member therefor, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.