Patent Publication Number: US-10767662-B2

Title: Multi-stage vacuum ejector with molded nozzle having integral valve elements

Description:
PRIORITY 
     This application is a U.S. national stage application of International Application No. PCT/EP2013/077121, filed Dec. 18, 2013, which claims priority to United Kingdom Application No. 1223420.9, filed Dec. 21, 2012, each of which is incorporated by reference in its entirety into this application. 
     TECHNICAL FIELD 
     The present invention relates to vacuum ejectors driven by compressed air. 
     BACKGROUND ART 
     Vacuum pumps are known which use a source of compressed air (or other high-pressure fluid) in order to generate a negative pressure or vacuum in a surrounding space. Compressed-air driven ejectors operate by accelerating the high pressure air through a drive nozzle and ejecting it as an air jet at high speed across a gap between the drive nozzle and an outlet flow passage or nozzle. Fluid medium in the surrounding space between the drive nozzle and outlet nozzle is entrained into the high-speed flow of compressed air, and the jet flow of entrained medium and air originating from the compressed-air source is ejected through the outlet nozzle. As the fluid in the space between the drive and outlet nozzles is ejected in this way, a negative pressure or vacuum is created in the volume surrounding the air jet which this fluid or medium previously occupied. 
     For any given compressed-air source (which may also be called the drive fluid), the nozzles in the vacuum ejector may be tailored either to produce a high-volume flow, but not to obtain as high a negative pressure (i.e., the absolute pressure will not fall as low), or to obtain a higher negative pressure (i.e., the absolute pressure will be lower), but without achieving as high a volume flow rate. As such, any individual pair of a drive nozzle and outlet nozzle will be tailored either towards producing a high-volume flow rate or achieving a high negative pressure. 
     A high negative pressure is desirable in order to generate the maximum pressure differential with ambient pressure, and so generate the maximum suction forces which can be applied by the negative pressure, for example for lifting applications. At the same time, a high-volume flow rate is necessary in order to ensure that a volume to be evacuated can be emptied sufficiently quickly to allow for repetitive actuation of the associated vacuum device, or equally in order to convey a sufficient volume of material, in vacuum conveyer applications. 
     In order to achieve both a high ultimate vacuum level and a high overall volume flow rate, so-called multi-stage ejectors have been devised, which comprise three or more nozzles arranged in series within a housing, each adjacent pair of nozzles in the series defining a respective stage across which a negative pressure is generated in the gap between the adjacent two nozzles. Again, in general, any individual pair of nozzles in the series may be tailored either towards producing a high-volume flow rate or achieving a high negative pressure, for a given source of compressed air. 
     In such multi-stage ejectors, the earliest stages produce the highest levels of negative pressure, i.e., the lowest absolute pressures, whilst the subsequent stages provide successively lower negative pressure levels, i.e., higher absolute pressures, but increase the overall volume throughput of the ejector device. In order to apply the generated vacuum across the multiple stages to a desired vacuum device or volume to be evacuated, the successive stages are typically connected to a common collection chamber, whilst valves are provided to each successive stage, at least after the first, drive stage, so that the subsequent stages can be closed off from the collection chamber once the negative pressure in that chamber has been reduced below the negative pressure which the second and subsequent stages are able to generate. 
     The drive stage is so-called because it is the only stage connected to the source of pressurised fluid (compressed air), and so drives the flow of pressurised fluid through all of the subsequent stages and nozzles in the series, before the drive fluid and entrained fluid is ejected from the vacuum ejector. 
     In order to provide for the entrainment of fluid across each successive stage, the series of nozzles present a through-channel with gradually increasing sectional opening area, through which the stream of high-speed fluid is fed in order to entrain air or other medium in the surrounding volume into the high-speed jet flow. The nozzles between each stage form the outlet nozzle of one stage and the inlet nozzle of the next stage, and are configured to successively accelerate the flow of air and other medium in order to direct a high-speed jet of the fluid across each successive stage. 
     Although different pressurised fluids may be utilised as the drive fluid, multi-stage ejectors of the present type are typically driven by compressed air, and most usually are used to entrain air as the medium to be evacuated from the volume surrounding the jet flow through each gap in the series of nozzles, across the respective stages. 
     One design of multi-stage ejector which has found commercial success is to present the series of nozzles in a coaxial arrangement within a substantially cylindrical housing which incorporates a series of suction ports therein in communication with each stage of the ejector, the suction ports being provided with suitable valve members for selectively communicating each stage with a surrounding volume of air. So presented, the cylindrical body is formed as a so-called ejector cartridge, which, when installed inside a housing module, or within a suitably dimensioned bore hole, can be used to evacuate the surrounding chamber, which is in turn fluidly coupled to the vacuum device to which the negative pressure is to be applied. 
     Such a device is disclosed in PCT International Publication No. WO 99/49216 A1, in the name of PIAB AB, and is shown in  FIGS. 14 and 15  of the present application. 
     As shown in  FIG. 14 , the ejector cartridge  1  comprises four jet-shaped nozzles  2 ,  3 ,  4  and  5  which define a through-channel  6  with gradually increasing cross-sectional opening area. The nozzles are arranged end-to-end in series with respective slots  7 ,  8  and  9  between them. 
     The nozzles  2 ,  3 ,  4  and  5  are formed in respective nozzle bodies, which are designed to be assembled together to form an integrated nozzle body  1 . Through openings  10  are arranged in the wall of the nozzle body, to provide flow communication with an outer surrounding space. 
     Turning to  FIG. 15 , it can be seen how the ejector cartridge  1  may be mounted within a bore hole or housing, in which the outer surrounding space corresponds to a chamber V to be evacuated. Each of the through openings  10  is provided with a valve member  11  in order to selectively permit the flow of air or other fluid from the surrounding space V into the space or chamber between each adjacent pair of nozzles. As shown in  FIG. 15 , the ejector cartridge  1  has been mounted in a machine component  20 , in which the bore hole has been drilled or otherwise formed. The ejector cartridge  1  extends from an inlet chamber i to an outlet chamber u, and is arranged to evacuate the three separate chambers constituting the outer surrounding space V, each of which is separated from the adjacent chamber by an O-ring  22 . Although not shown, each of the chambers constituting the outer surrounding space V is connected to a common collection chamber or suction port, in order to apply the generated negative pressure to an associated vacuum-operated device, such as a suction cup. 
     Although such multi-stage ejector arrangements are beneficial in providing both a high-volume flow rate and a high level of negative pressure, there is necessarily still some degree of compromise in the design of each successive stage in the ejector, in order to obtain an overall desired performance characteristic for the multi-stage ejector as a whole. Accordingly, it has also been proposed to provide a further so-called booster nozzle, provided in parallel with the drive nozzle of the multi-stage ejector, where the booster nozzle is specifically designed to obtain the highest possible level of vacuum, but does not form part of the series of coaxially arranged nozzles which make up the multi-stage ejector. In this way, the booster nozzle can be configured to obtain the highest possible level of vacuum, whilst the parallel multi-stage ejector nozzle series can be arranged to obtain a high-volume throughput, which enables a high negative pressure (low absolute pressure) to be obtained within the volume to be evacuated within an acceptably short period of time. 
     Such an arrangement is disclosed in U.S. Pat. No. 4,395,202, as shown in  FIG. 13  of the present application. In this arrangement, there is provided a set of ejector nozzles  12 ,  13 ,  14 ,  15  arranged successively for evacuation of associated chambers  5 ,  6 ,  7 , which are in mutual communication with a vacuum collecting compartment  16  through respective ports  18 ,  19  and  20 . Valves,  21 ,  22  and  23  are respectively provided to the ports  18 ,  19  and  20 . 
     An additional pair of nozzles  24  and  25  is provided in parallel to the drive nozzle  12  of the multi-stage ejector, and is arranged in a separate booster chamber  4 , connected to the collecting chamber  16  via a port  17 . The booster stage is comprised of a pair of nozzles  24  and  25 , with the inlet nozzle  24  being connected, together with the drive nozzle  12  of the multi-stage ejector, to the inlet chamber  3 , which is supplied with compressed air. The pair of nozzles  24  and  25  across the booster stage serves to generate the highest possible vacuum (lowest negative pressure) in the booster chamber  4 . The jet of compressed air which is generated by the nozzle  24  is ejected out of the booster stage through nozzle  25 , into the same chamber  5  across which the drive nozzle  12  propels the drive jet of compressed air. In this way, the air expelled out of the booster stage is entrained into the drive jet flow to be expelled from the multi-stage ejector. Furthermore, the vacuum generated by the drive stage of the multi-stage ejector is applied to the exit of nozzle  25 , so that the pressure differential across the booster stage is increased whereby the vacuum level which can be generated by the booster stage can be increased, i.e., the absolute pressure which can be obtained is reduced. 
     In operation of the vacuum ejector, the series of nozzles  12 ,  13 ,  14  and  15  of the multi-stage ejector is able to produce a high volume flow rate so as quickly to generate a vacuum to a low absolute pressure in the collecting chamber  16  within a short period of time by entraining fluid from each of the chambers  5 ,  6  and  7  and the collecting chamber  16  into the jet streams formed by each successive stage of the ejector. The booster stage functions in parallel to the multi-stage ejector, but typically produces a low volume flow rate, and so does not contribute significantly to the initial vacuum formation process. As the vacuum level in the collecting chamber  16  increases (i.e., as the absolute pressure falls), the associated valve members  23 ,  22  and  21  will close in turn, as the pressure in the vacuum collecting chamber  16  drops below the pressure in the associated chamber  7 ,  6  or  5 , respectively. Eventually, the pressure in the collection chamber  16  will fall below the lowest pressure that any of the stages of the multi-stage ejector is able to generate, so that all of the valves are closed, and all further evacuation will then be done by the booster stage, which provides suction to the collection chamber  16  via suction port  17 . 
     Such multi-stage ejectors and ejector cartridges as described above have found commercial success in a number of different industries, and in particular in the manufacturing industry, where such vacuum ejectors may be connected to suction cups and used for picking and placing components during an assembly process. 
     As the demands for high vacuum levels (i.e. low absolute pressures) in processes such as de-gassing, de-humidifying, filling of hydraulic systems, forced filtration, etc., continue to increase, there is increasing demand for vacuum ejectors which are able to repeatedly provide a high level of negative pressure (i.e., a low absolute pressure) in order to carry out the above and other processes. 
     Coupled with this, there is an increasing drive towards smaller-sized ejectors, which are able to provide the desired evacuation capability at remote locations on the machinery (i.e., at the ends of mechanical arms, and significant distances from the ultimate source of compressed air) without negatively impacting on the overall dimensions of the machine. In particular, there is a desire for ejector devices having a small footprint, and so able to apply a vacuum to increasingly compact working areas. 
     SUMMARY OF THE INVENTION 
     The invention provides a multi-stage ejector for generating a vacuum from a source of compressed air by passing said compressed air through a series of nozzles, accelerating said compressed air, and entraining air so as to form a jet flow in one or more stages and generate a vacuum across each stage, the multi-stage ejector comprising a drive stage; a second stage; and a converging-diverging nozzle provided in said series of nozzles between said drive stage and said second stage for receiving jet flow from said drive stage and accelerating said jet flow to form a second stage air jet and directing said second stage air jet into an inlet of an outlet nozzle of the second stage, wherein said converging-diverging nozzle is formed in a moulded nozzle piece mounted in said multi-stage ejector. 
     A method of making a multi-stage ejector cartridge for generating a vacuum from a source of compressed air by passing said compressed air through a series of nozzles, accelerating said compressed air, and entraining air so as to form a jet flow in at least a drive stage and a second stage and generate a vacuum across each of these stages, the method comprising mounting a moulded nozzle piece including a converging-diverging nozzle between the drive stage and the second stage. 
     Relative to the prior art discussed above, the invention renders the manufacturing of an at least equally efficient multi-stage ejector more cost-efficient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To enable a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:— 
         FIG. 1A  shows a longitudinal, axial sectional view through a first embodiment of an ejector cartridge according to the present invention, as seen in a direction perpendicular to the direction of airflow through the ejector cartridge; 
         FIG. 1B  shows a perspective side view of the ejector cartridge of  FIG. 1A , from the same direction as  FIG. 1A ; 
         FIG. 2  shows a longitudinal, axial sectional view of a second embodiment of an ejector cartridge according to the present invention, similar to the embodiment of  FIG. 1A , but having separate flap valves in place of the unitary valve member of  FIG. 1A , as seen in a direction perpendicular to the direction of airflow through the ejector cartridge; 
         FIG. 3A  shows a longitudinal, axial sectional view of the unitary ejector housing body, defining the second stage and exit nozzle, of the ejector cartridge of  FIGS. 1A and 2 , as seen in a direction perpendicular to the direction of airflow through the ejector cartridge; 
         FIG. 3B  shows a longitudinal, axial sectional view of the unitary drive stage housing piece, including the second stage nozzle, of  FIGS. 1A and 2 , as seen in a direction perpendicular to the direction of airflow through the ejector cartridge; 
         FIG. 3C  shows a longitudinal, axial sectional view of the drive nozzle piece of  FIGS. 1A and 2 , as seen in a direction perpendicular to the direction of airflow through the ejector cartridge; 
         FIG. 4  shows an enlarged partial longitudinal, axial sectional view detailing one form of a drive nozzle which may be used in the drive nozzle arrays of the ejectors disclosed herein, as seen in a direction perpendicular to the direction of airflow through the drive nozzle; 
         FIG. 5A  shows a longitudinal, axial sectional view of a second embodiment of an ejector cartridge according to the present invention, shown along the sectional line A-A of  FIG. 5B ; 
         FIG. 5B  shows an axial end view of the ejector cartridge of  FIG. 5A  seen from the exit end of the cartridge; 
         FIG. 6  again details a longitudinal, axial sectional view of the ejector cartridge of  FIG. 5A , as seen in a direction perpendicular to the direction of airflow through the ejector, indicating the relationship between the grouping of the ejector array nozzles and the inner diameter of the second stage converging-diverging nozzle; 
         FIG. 7A  shows a longitudinal, axial sectional view of the unitary ejector housing body, defining the drive stage, second stage and exit nozzle, of the ejector cartridge of  FIG. 5A , as seen in a direction perpendicular to the direction of airflow through the ejector; 
         FIG. 7B  shows a longitudinal, axial sectional view as seen in a direction perpendicular to the direction of airflow through it, and an axial end view from the exit end of, the second stage nozzle piece of  FIG. 5A , incorporating an integral valve member therewith; 
         FIG. 7C  shows a longitudinal, axial sectional side view as seen in a direction perpendicular to the direction of airflow through it, and axial end view from the exit end of, the drive nozzle piece of the ejector cartridge of  FIG. 5A ; 
         FIG. 8  shows an isometric sectional view, through a plane containing its longitudinal axis, which is parallel to the direction of airflow through it, of the ejector cartridge of  FIG. 5A , detailing how the second stage nozzle piece and drive nozzle piece are mounted into the ejector housing body; 
         FIG. 9  shows a longitudinal, axial sectional view, as seen in a direction perpendicular to the direction of airflow through the ejector, of an alternative embodiment of a unitary ejector housing body similar to that of  FIG. 5A , but having a modified diverging nozzle section, which may be used in place of the ejector housing of  FIG. 5A . 
         FIG. 10  shows a schematic comparison between the flow development through a multi-stage series of nozzles having a single drive nozzle and a multi-stage series of nozzles having a drive nozzle array including four drive nozzles; 
         FIGS. 11A to 11C  illustrate an embodiment of an ejector, having the ejector cartridge of  FIG. 1A  mounted in an ejector housing module and connected to a mounting plate, with  FIG. 11A  showing an underside view of the ejector housing module detailing the inlet, outlet and suction ports;  FIG. 11B  showing a longitudinal, axial sectional view through the ejector housing module, as seen in a direction perpendicular to the direction of airflow through the ejector, detailing how the cartridge of  FIG. 1A  is mounted into the housing module, and  FIG. 11C  showing a top plan view of the ejector housing module, including the location of mounting holes for connecting the housing module to the mounting plate; 
         FIG. 12  shows a longitudinal, axial sectional view, as seen in a direction perpendicular to the direction of airflow through the ejector cartridge, of an ejector with a similar ejector housing module to that of  FIGS. 11A to 11C , but in which the ejector cartridge of  FIG. 5A  is mounted in place of the ejector cartridge of  FIG. 1A , and further having a booster ejector module mounted between the mounting plate and the ejector housing module; 
         FIG. 13  shows a prior art ejector unit including a booster stage incorporated into a common housing in parallel with the in-line series of multi-stage ejector nozzles; and 
         FIGS. 14 and 15  show sectional views of a prior art ejector cartridge, with  FIG. 15  illustrating a cartridge being mounted into a housing unit of an ejector. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described with reference to the accompanying Figures. Like reference numerals have been used to refer to like features throughout the description of the various embodiments. 
       FIGS. 1A and 1B  show a first embodiment of an ejector according to the present invention. The embodiment of  FIGS. 1A and 1B  is configured as an ejector cartridge  100 . Such a cartridge is intended to be installed within an ejector housing module, or within a bore or chamber formed in an associated piece of equipment, which defines the volume to be evacuated by the ejector cartridge. 
     Although the most preferred embodiment of the ejector, as shown in the drawings, is designed to work with air as the drive fluid, and as the fluid to be evacuated, the ejector will be applicable to any gas as the drive fluid, and any gas as the fluid to be evacuated. The drive fluid will have a primary direction of movement, or flow, through the ejector. This direction is parallel to the longitudinal axis of the ejector, shown horizontally in the drawings, and starting from the inlet  114 . In the following, this direction will be referred to as the direction of airflow. 
     Ejector cartridge  100  is a multi-stage ejector having a first, drive stage  100 A and a second stage  100 B, for generating a respective vacuum across each stage. 
     The drive stage comprises a drive nozzle array  110 , which is arranged to accelerate compressed air supplied to the inlet  114  of the drive nozzle array  110 , so as direct a jet flow of high speed air into the inlet of a second stage nozzle  132 . Second stage nozzle  132  is, likewise, arranged to project a jet flow of air into an exit nozzle  146  of the ejector cartridge. 
     Unlike with the ejector cartridge shown in  FIGS. 14 and 15  of the present application, which has a single drive nozzle, the ejector cartridge  100  includes a drive nozzle array  110 , which has plurality of drive nozzles  120 . The drive nozzles  120  are each configured to generate an air jet of high speed air across the drive stage of the ejector cartridge  100 , and are grouped so that the individual jet flows generated by each of the drive nozzles  120  will all be fed together in common into the inlet  131  of the second stage nozzle  132 . 
     In  FIG. 1A, 111  indicates a view onto nozzle array  110 , as seen from second stage drive nozzle  132 . Even though the view  111  is shown in the second stage nozzle,  132 , this is done for illustrative purposes only. As shown schematically in  FIG. 1A , the drive nozzle array  110  includes four drive nozzles  120 , which are grouped together in a two-by-two matrix in such a way that the outlets of the four drive nozzles, when viewed in an axial direction along centre axis CL of the ejector cartridge  100 , will all lie within a boundary perimeter essentially equal to the smallest inner diameter of the second stage nozzle  132 . This is shown in  FIG. 1A  by a circle drawn part way along the length of the second stage nozzle  132 , corresponding to the inner cross-section of the second stage nozzle perpendicular to the centre axis CL, and having four smaller circles drawn within its perimeter, which shows how the outlet positions of four drive nozzles  120  could be arranged so that they are all aligned with the inlet of the second stage nozzle in the direction of the centre axis CL. It will be appreciated that this larger circle and the four smaller circles do not represent a structural feature part way along the second stage nozzle  132 , but are a projection of the drive nozzle array grouping onto the cross-section of the second stage nozzle, made for purposes of illustrating the relative concentric and coaxial alignment of these components along centre axis CL. The same applies for the similar circular groupings shown part way along the second stage nozzles in  FIGS. 2 and 6 . 
     Subsequent to the drive nozzle array, in the direction of airflow through the ejector, are the second stage nozzle  132  and the exit nozzle  146 . These nozzles are each provided as single, converging-diverging nozzles, provided in series with the drive nozzle array  110  along the centre axis CL. Accordingly, when compressed air is supplied to the inlet  114  of the drive nozzle piece  112  at the inlet of the ejector cartridge  100 , a high-speed air jet will be generated by each of the nozzles  120 , so as to form a jet flow in which the drive air jets are directed together in common into the inlet  131  of the second stage nozzle  132 . In this way, air or other fluid medium in the volume between the drive nozzle array  110  and the inlet  131  of the second stage nozzle  132 , in particular the volume surrounding each of the drive jets generated by the respective drive nozzles  120 , will be entrained into the jet flow, and driven into the second stage nozzle  132 . 
     The consumption and the feed pressure of the supplied compressed air can vary in accordance with ejector size and desired evacuation characteristics. For smaller ejectors, a consumption range from about 0.1 to about 0.2 Nl/s (normalized litres per second) at feed pressures of from about 0.1 to about 0.25 MPa will usually be sufficient, and large ejectors typically consume from about 1.25 to about 1.75 Nl/s at about 0.4 to about 0.6 MPa. Ranges in between for sizes in between are possible and common. Without wishing to be bound to these particular ranges, compressed air as used herein is to be understood to have such properties. 
     The fluid in the jet flow exiting the drive stage is then accelerated in the second stage converging-diverging nozzle  132 , so as to generate an air jet across the second stage  100 B, which is in turn directed into the inlet of the exit nozzle  146 . In the same way, air or other fluid medium in the volume surrounding the air jet generated by the second stage nozzle  132  will be entrained into the jet flow, and ejected from the ejector cartridge  100  through the exit nozzle  146 . 
     When fluid is entrained into the respective jet flows in the first stage  100 A and second stage  100 B, a suction force is generated which will tend to draw further fluid media from the surrounding environment into the ejector cartridge  100  through the suction ports  142  and  144  which are disposed around the body of the ejector cartridge  100 , respectively associated with each of the first stage  100 A and the second stage  100 B. As described above, the drive stage  100 A will generate a higher value of negative pressure (i.e., a lower absolute pressure) than the second stage  100 B. Accordingly, a valve member  135  is provided to selectively open and close the suction ports  144  of the second stage  100 B. The valve member  133  closes off the suction ports  144  when the negative pressure generated in the surrounding volume exceeds that which can be generated in the second stage  100 B. Closing the ports prevents any backflow of the air being evacuated by the drive stage  100 A; backflow would result from this air re-entering the volume to be evacuated out of the second stage  100 B through the suction port  144  under a condition of reverse flow. 
     In the embodiment of  FIG. 1A , the valve member  125  is provided as a unitary body which extends around the whole inner circumference of the second stage  100 B of the vacuum ejector cartridge  100 , in order to selectively open and close the suction ports  144  according to the pressure difference between the negative pressure generated in the second stage  100 B and the external vacuum condition in the surrounding volume. As an alternative, as shown in  FIG. 2 , a number of separate flap-valve members, or one member having a number of separate valve flaps  135 , can be provided, one associated with each of the suction ports  144 . 
     As will be apparent from  FIG. 1B , the ejector cartridge  100  is formed as a substantially rotationally symmetric body, forming a body of revolution about the centre axis CL, with the exception of the drive nozzle array  110  and the suction ports  142  and  144 . Although the drive nozzle array  110  and the portions including suction ports  142  and  144  do not, strictly-speaking, form bodies of revolution, they may be disposed with rotational symmetry about said axis of rotation CL, thus representing only minor discontinuities in what is otherwise a body of revolution about the centre axis CL. 
     As shown in  FIGS. 1A and 1B , the ejector cartridge  100  is a substantially cylindrical ejector cartridge having a substantially circular cross-sectional shape along its length in the plane perpendicular to the centre axis CL, i.e., perpendicular to the direction of airflow through the ejector cartridge  100 . However, it will be appreciated that it is not essential for the ejector cartridge  100 , or the components thereof, to be formed with a circular cross-section, and the various nozzles, in particular, can be formed with square or other non-circular cross-sections, should this be suitable for a particular application. Nevertheless, a substantially cylindrical or tubular form is preferred for the ejector cartridge  100 , since this permits the ejector cartridge  100  to be installed most easily within a borehole or other ejector housing module, utilising appropriate seals such as the O-rings  112   a  and  140   a  shown in  FIGS. 1A and 1B . 
     Turning to the particular construction of the ejector cartridge  100  of  FIGS. 1A and 1B , it can be seen that the ejector cartridge is constituted by a two-part housing, consisting of second stage housing piece  140  and drive stage housing piece  130 . A drive nozzle piece  112 , defining the drive nozzle array  110 , is mounted into the inlet end of the drive stage housing piece  130 . The valve member  135  is, in this embodiment, formed as a separate member, and is mounted to the drive stage housing piece  130  in a corresponding, and preferably circumferential, groove formed in that housing, so as to be assembled into the ejector cartridge  100  when the drive stage housing piece  130  is inserted into the inlet end of second stage housing piece  140 . 
     With reference also to  FIGS. 3A to 3C , the components of the ejector cartridge  100  will be described in more detail. 
     The second stage housing piece  140  includes an inlet portion, which has receiving structure  145  arranged to receive the drive stage housing piece  130  which, in turn, receives the drive nozzle array  110 . As will be appreciated from  FIG. 1A , the valve member  135  engages with the receiving structure  145  and serves to provide a seal between the second stage housing piece  140  and the drive stage housing piece  130 , when the drive stage housing piece  130  is mounted into the inlet end of the second stage housing piece  140 . 
     Second stage housing piece  140  defines a converging-diverging nozzle  146 , which constitutes the exit nozzle of the ejector cartridge  100 . This converging-diverging nozzle  146  includes a converging inlet section  147 , a straight section  148  and a diverging section  149 . Straight section  148  could be slightly diverging, too. The second stage housing piece  140  also defines the second stage suction ports  144 , through which air or other fluid medium in the surrounding volume is sucked into the second stage so as to be ejected from the ejector cartridge  100  through exit nozzle  146 . 
     A particular feature of the exit nozzle  146  is that the diverging section  149  includes a stepwise expansion in diameter  150 , formed part way along the diverging section  149 , in this example nearer to the outlet end of the nozzle  146  than to the inlet of the diverging section  149 ; in the illustrated embodiment, the expansion is near to the outlet end of the exit nozzle  146 . The first section  149   a  of the diverging nozzle section  149  extends from the straight section  148  with a divergence angle which may be substantially constant, up to the point where the stepwise expansion in diameter is provided at a sharp corner  151 . Preferably, the sharp corner  151  is defined by an undercut in the diverging section  149  of the nozzle  146 . At the stepwise expansion in diameter  150 , the wall of the diverging section reverses direction to form the sharp corner  151 , where the wall changes from diverging whilst extending in an axial direction towards the exit end of the ejector cartridge  100 , to being diverging whilst extending in an axial direction towards the inlet end of the ejector cartridge  100 , for a short distance, before reversing back to again diverge whilst extending in the axial direction towards the outlet end of the cartridge  100 . The last reversal back into a diverging shape is optional in that the second portion  149   b  as shown in the Figures may initially, i.e. immediately downstream of the sharp corner, may reverse back to continue in a cylindrical, straight-walled shape, before it continues in a diverging shape shortly before the outlet end of the cartridge  100 . The shape of the nozzle  146  will be selected in accordance with the desired characteristics of the ejector, keeping in mind that the shape serves to render the change from the flow and pressure conditions in the nozzle to the expansion of the flow into ambient pressure less abrupt. In this manner, the design of the outlet end of the cartridge  100  can advantageously used to influence pressure and flow rate conditions in the drive nozzle. As a result the skilled person will have greater freedom in designing the drive nozzle. 
     As shown in  FIG. 3A , the stepwise change in diameter can be measured by comparing the diameter Di immediately before the stepwise expansion, at the sharp corner  151 , with the diameter Do immediately after the stepwise expansion, at the point  152  which is radially in-line with point  151 , but on the second diverging portion  149   b  of the diverging section  149 . A stepwise change in diameter serves to trip the fluid flow in the diverging section  149   b  of the nozzle  146 , so as to generate a turbulent outlet flow along the nozzle wall, thereby reducing the friction at the outlet of the nozzle  146  and correspondingly improving the efficiency with which the ejector cartridge  100  can generate a vacuum from a given source of compressed air. 
     The ratio Di to Do is preferably between 6 to 7 and 20 to 21, and most preferably is about 94 to 105. 
     Turning to  FIG. 3B , there is shown the drive stage housing piece  130 , which defines an inlet section in which suction ports  142  are formed, through which air or other surrounding medium may be sucked into the drive stage to be ejected through the second stage nozzle and the exit nozzle of the ejector cartridge  100 . The drive stage housing piece  130  includes an annular groove  139 , for receiving the valve body  135  therein. Equally, the annular groove  139  may be provided as a series of separate grooves, for receiving individual valve members  135 , for the respective suction openings  144 . 
     The drive stage housing piece  130  also forms a nozzle body, in which the converging-diverging second stage nozzle  132  is defined, having a converging inlet section  136 , a straight middle section  137  and a diverging outlet section  138 . The second stage nozzle defines an inlet  131  and an outlet  133 . Furthermore, the second stage nozzle piece  130  defines a receiving structure  134 , such as in the form of an annular groove, for mounting the drive nozzle piece  112  into the inlet end of the drive stage housing piece  130 . In this way, a notch or equivalent engaging structure may be provided on the drive nozzle piece  112 , to engage with the groove  134 , or otherwise an annular O-ring seal  112   b  may be provided so as to couple the drive nozzle piece  112  and the drive stage housing piece  130  together by being mutually received in respective grooves of these two components. 
     Turning to  FIG. 3C , the drive nozzle piece  112  is shown, provided with such an O-ring  112   b  for forming a sealed interconnection with receiving structure such as annular groove  134  at the inlet end of the drive stage housing piece  130 . The drive nozzle piece  112  is provided with the drive nozzle array  110 , which includes a plurality of drive nozzles  120 . The drive nozzle piece  112  includes an inlet  114 , to which the compressed air supply is provided for supplying compressed air to the drive nozzles  120  in order to generate respective air jets of high speed air from each drive nozzle  120 . The fluid flow produced by the drive jets and any fluid medium entrained therein may in general be termed as jet flow or drive jet flow. 
       FIG. 4  shows an enlarged cross-sectional view through a drive nozzle  120 . In this case, the drive nozzle  120  is formed with a circular cross-section, as viewed in the axial direction of each nozzle, although non-circular cross-sections are also possible, with equivalent fluid dynamic effect. 
     Each of the drive nozzles  120  may be formed in the drive nozzle piece  112  in the manner shown in  FIG. 4 , so as to have a straight-walled inlet flow section  122  and a diverging outlet flow section  124 . The straight-walled inlet flow section is neither converging nor diverging, and is provided with a radiused, rounded or chamfered edge or edges at the inlet  121 . The diverging outlet flow section  124  extends from the outlet end of the straight-walled section  122  so as to exhibit a decreasing degree of divergence along its length towards the exit end of the drive nozzle. That is to say, that the diverging section  124  is most divergent at the inlet end of the outlet flow section  124 , where it extends from the straight-walled portion  122 , and is least divergent at the outlet end of that section  124 . The diverging section  124  may also comprise a further straight-walled section  126  at the exit end of diverging outlet flow section  124 . As viewed in cross-section, in a direction perpendicular to the direction of air flow through the drive nozzle  120 , the diverging section  124  has the shape of a segment of an ellipse lying with its foci on the longitudinal centre axis of the straight-walled inlet flow section  122 , and extends from the most-diverging end to the least-diverging end of the diverging nozzle section  124 . 
     If a straight-walled section  126  is provided at the exit of the drive nozzle  120 , this section preferably has a length le which is 12% or less, preferably 10% or less, than the overall length LN of the drive nozzle as a whole. 
     In contrast with the radiused, rounded or chamfered edge or edges of the inlet  121  of the drive nozzle  120 , the exit of the drive nozzle  120  provides a sharp edge at substantially 90° to the end face of the nozzle body  112  in which the drive nozzle  120  is formed. This serves to help produce a coherent jet of high-speed air exiting from the drive nozzle  120 , when compressed air is provided to the drive nozzle inlet  121  and accelerated through the drive nozzle  120 . 
     Such acceleration is provided primarily in the diverging section  124  of the nozzle  120 , which provides a diameter expansion from an inner diameter di at the outlet of the inlet flow section  122  to an inner diameter do at the exit of diverging outlet flow section  124 . The ratio between the inner diameter di at the outlet end of the inlet flow section  122  and the inner diameter do at the exit of the nozzle  120  will be selected in accordance with the desired characteristics of the ejector. If an ejector is designed to what is commonly referred to as “high flow”, then do will be smaller relative to di, for instance do≈1.3·di. If an ejector is designed to what is commonly referred to as “high vacuum”, then do will be greater relative to di, for instance do≈2·di. Thus, typical ranges between the inner diameter di at the outlet end of the inlet flow section  122  and the inner diameter do at the exit of the nozzle  120  are between 1 to 1.2 and 1 to 2.2 (1/1.2≤di/do≤1/2.2). 
     Irrespective of the presence or absence of a straight-walled section  126 , and independent of the axial length chosen for the diverging outlet flow section  124 , the axial length of the straight-walled inlet flow section  122  may preferably be about 5 times the inner diameter di at the outlet end of the inlet flow section  122 . The axial length of the diverging outlet flow section  124 , either on its own or including a straight-walled section  126  if the latter is provided, may preferably be at least twice the inner diameter do at the exit of the nozzle  120 , independent of the axial length chosen for the straight-walled inlet flow section  122 . Alternatively, the axial length of the straight-walled inlet flow section  122  may be about 5 times the inner diameter di at the outlet end of the inlet flow section  122 , and the axial length of the diverging outlet flow section  124 , including a straight-walled section  126 , may be at least twice the inner diameter do at the exit of the nozzle  120 . 
     As shown in  FIGS. 1A, 2 and 3C , the drive nozzles  120  are provided in the drive nozzle array  110  so as to be aligned substantially in parallel to one another, that is with the longitudinal centre axis of each of the nozzles  120  being axially aligned in parallel with the centre axis CL of the ejector cartridge  100 . Of course, the drive nozzles  120  in the drive nozzle array  110  may equally be provided with a slight divergence or convergence, in order to tailor the shape of the co-formed jet flow that is projected from the nozzle array  110  towards the inlet  131  of the second stage nozzle  132 , a slight convergence being preferred over a slight divergence. 
     Equally, although these Figures show nozzle array  110  consisting of four drive nozzles, arranged in a two-by-two matrix, this is not any limitation on the present invention, which may include any number of drive nozzles  120 , such as, specifically, two, three, four, five or six drive nozzles, arranged in a suitable grouping in the drive nozzle array  110 . For example: three nozzles may be arranged at the points of a triangle; four nozzles can be arranged, as shown, at the corner of a square; five nozzles can be arranged at the corners of a pentagon, or at the corners of a square with one in the centre of the square; and six nozzles can be variously grouped, including at the corners of a hexagon. 
     An even larger number of drive nozzles  120  is, of course, also possible and contemplated for the drive nozzle array  110 , according to purpose. It is also contemplated that the design of each drive nozzle might be varied in order to control the co-formed drive jet flow—for example, in a grouping having a centre nozzle with multiple surrounding nozzles, the centre nozzle might be configured to give a higher-speed air jet with a lower volume flow rate than each of the surrounding nozzles. 
     Turning to  FIGS. 5A, 5B, 6, 7A to 7C and 8 , there is shown a second embodiment of an ejector according to the present invention. The embodiment of  FIGS. 5A, 5B, 6, 7A to 7C and 8  is also configured as an ejector cartridge  200 . 
     The ejector  200  is similar in construction and operation to the ejector  100 , and the description above of the features, components, operation and use of the ejector  100  applies equally to the ejector  200 , except where further features or variations are particularly explained. Again, ejector cartridge  200  includes a first, drive stage  200 A and a second stage  200 B. 
       FIG. 5B  is an axial end view, facing towards the exit end of the ejector  200 , which clearly shows the outlets of the drive nozzles  220  arranged in a grouping so as to face into and along the axial passage defined by the second stage nozzle  232  and the exit nozzle  246 .  FIG. 5A  shows the section A-A of  FIG. 5B , which contains the centre axis CL, about which the ejector cartridge  200  substantially forms a body of revolution. Again, the body of the ejector cartridge  200  is substantially cylindrical, with the exception of the suction ports  242  and  244 , and the diverging section of the exit nozzle. 
     The construction of the ejector cartridge  200  is substantially the same as that of ejector cartridge  100 , with the main exception that the ejector cartridge  200  is formed to have a single housing piece  240  constituting both the drive stage  200 A and the second stage  200 B. The second stage nozzle is formed as a separate second stage nozzle piece  230 , which is arranged to be inserted into the housing  240  from the inlet end thereof, prior to inserting the drive nozzle piece  212  also into the inlet end of the housing piece  240 . 
     It will be apparent that the second stage nozzle body  230  is simply press-fitted into the second stage  200 B part of housing  240 , whereas the drive nozzle piece  212  is provided with an inter-engaging annular ridge  212   b , configured to engage into the annular groove  234  provided as receiving structure at the inlet of the housing piece  240 . 
     As seen more clearly in  FIGS. 6 and 7C , the drive nozzle piece  212  includes rods or posts  216 , which extend forwardly from a radially outer flange section of the drive nozzle piece  212 , and abuttingly engage the rear side of the second stage nozzle piece  230 , so as to hold it axially in place within the ejector housing  240 . These posts or rods  216  function both to secure the second stage nozzle piece  230  in position within the ejector housing piece  240 , and also to maintain a desired spacing between the exit of the ejector nozzles  220  of ejector nozzle array  210  and the inlet  231  to the second stage converging-diverging nozzle  232 . 
     It will otherwise be appreciated that the ejector cartridge  200  is arranged to operate in the same manner as ejector cartridge  100 , with compressed air being supplied to the inlet  214  of drive nozzle array  210  at the inlet of ejector cartridge  200 , and accelerated through drive nozzles  220  of drive nozzle array  210  so as to emerge as respective drive air jets, directed together in common into the inlet  231  of the second stage nozzle  232 . This array of drive air jets again entrains fluid in the surrounding volume into the drive jet flow, creating a suction which will draw surrounding fluid in through the suction ports  242  formed in the housing  240  at the first drive stage  200 A. The compressed air and entrained fluid medium is then accelerated in the second stage nozzle  232  to emerge as a second stage air jet, which is directed in turn into the exit nozzle  246 . Exit nozzle  246  is again defined by the housing piece  240  as a converging-diverging nozzle. As before, the high-speed air jet through the second stage  200 B entrains air or other fluid medium in the volume surrounding the second stage air jet into the second stage jet flow and ejects it from the ejector  200  through the exit nozzle  246 . This creates a suction force at the suction ports  244 , thereby drawing in fluid medium from any surrounding volume. A valve member  235  is again provided, in order to selectively open and close the second stage suction ports  244 , in dependence on the relative levels of negative pressure in the second stage  200 B and the surrounding volume. In this embodiment, the valve member  235  is formed as an integral component of the second stage nozzle piece, with which it forms a unitary moulded body. The valve  235  will open when the pressure in the second stage  200 B is below the pressure in the surrounding volume, and will close when the pressure in the surrounding volume falls below the pressure in the second stage  200 B. 
     Again, as may be taken from  FIG. 6 , the drive nozzles  220  are arranged in a grouping which permits the air jets from all of the drive nozzles  220  to be directed together into the inlet  231  of the second stage nozzle  232 . This is shown schematically in  FIG. 6  by way of the drive nozzle grouping being shown as smaller circles arranged in a two-by-two matrix inside each of two adjacent larger circles which, correspond to the inner diameter of the second stage nozzle  232 . The left-hand grouping in  FIG. 6  corresponds to the alignment of the drive nozzles  220  as shown in  FIG. 6 , whereas the right-hand grouping shows how the nozzles remain within the confines of the perimeter of the second stage nozzle  232 , even if the grouping is rotated through a 45° angle. In this way, it can be seen how the multiple nozzles of the drive nozzle array  210  are able to direct their respective drive jets together into the common inlet  231  of the second stage nozzle  232 . As noted above, the two adjacent circles containing the drive nozzle groupings drawn in the middle channel of the second stage nozzle in  FIG. 6  do not represent structural features part way along the second stage nozzle  132 , but are a projection of possible drive nozzle array groupings onto the cross-section of the second stage nozzle, made for purposes of illustrating the relative alignment of these components along centre axis CL. 
     Referring to  FIG. 7A , the housing piece  240  is shown, having an inlet end with a receiving structure  234  in the form of an annular groove for receiving the drive nozzle piece  212 . First, drive stage suction ports  242  and second stage suction ports  244  are also shown, provided as openings in the otherwise substantially cylindrical body of the housing piece  240 . At its distal end, the housing piece  240  defines the converging-diverging exit nozzle  246  of the ejector cartridge  200 , including converging inlet section  247 , straight-walled section  248  and diverging outlet section  249 . As with the embodiment of  FIGS. 1, 2 and 3A , the diverging portion  249  of exit nozzle  246  is provided, near the outlet end, with a stepwise expansion in diameter  250 , dividing the diverging section  249  into first and second diverging sections  249   a  and  249   b , respectively. At the stepwise expansion in diameter  250 , there is formed an undercut, at which the wall of the diverging section  249 , as viewed in cross-section in the direction perpendicular to the direction of air flow through the exit nozzle  246 , reverses from diverging whilst extending in the axial direction towards the outlet of the ejector cartridge  200  to diverging whilst extending in the axial direction towards the inlet of the ejector cartridge  200 , before reversing again to be diverging whilst extending in the axial direction towards the outlet end of the ejector cartridge  200 . This reversal in the direction of the wall of the diverging section  249  creates a sharp corner  251 , at the stepwise expansion  250 . This stepwise expansion in diameter may have the same dimensional relationships as the stepwise expansion in diameter  150  for the outlet section  149  in the exit nozzle  146  for the ejector cartridge  100  described above. 
     It is also possible for the diverging section  249  to be provided with more than one stepwise expansion in diameter. Turning to  FIG. 9 , an ejector housing piece  270  is shown which represents an alternative embodiment to the ejector housing piece  240 , and which may be used in place of ejector housing piece  240  in the ejector cartridge  200 . As with ejector housing piece  240 , ejector housing piece  270  includes receiving structure  234  at its inlet end for receiving the ejector nozzle piece  212 , suction ports  242  and  244 , and receiving structure  245  between the suction ports, for receiving the second stage nozzle piece  230 . Again, ejector housing piece  270  defines a converging-diverging nozzle  246  at its outlet end, to provide the exit nozzle  246  for the ejector cartridge  200 . This exit nozzle  246  includes a converging inlet section  247 , a straight-walled middle section  248  and a diverging outlet section  249 . However, in this instance, the diverging outlet section  249  is divided into first, second and third diverging sections  249   a ,  249   b  and  249   c . Stepwise expansions in diameter  250  and  255  are provided at two positions along the length of the diverging section  249 , separately the diverging section into the first, second and third diverging sections  249   a ,  249   b  and  249   c . The stepwise expansion in diameter  250  is formed near to the outlet end of the diverging section  249 , the same as in  FIG. 7A . An intermediate stepwise expansion in diameter  255  is further provided, formed again by an undercut in the wall of the diverging section  249  of the outlet nozzle  246 . The undercut forms a sharp corner  256  at the position of the stepwise expansion at the end of the first section  249   a , at which point the nozzle wall, as viewed in cross-section in a direction perpendicular to the direction of air flow through the nozzle, reverses from diverging whilst extending in an axial direction towards the outlet of the nozzle to diverging whilst extending in an axial direction towards the inlet of the nozzle, before reversing again to be diverging whilst extending in the axial direction towards the outlet of the nozzle. 
     The angle of the diverging wall of the exit nozzle  246  in diverging section  249  is substantially the same in all three sections  249   a ,  249   b  and  249   c , although it will be appreciated that more or less divergent angles may be used towards the exit end of the nozzle. Again, the purpose of the stepwise expansions in diameter  250 ,  255  in the diverging section  249  of exit nozzle  246  is to trip the air flow into a turbulent air flow, so as to reduce the friction at the nozzle wall that is experienced by the air passing through the exit nozzle  246 , and so influence resistance to air flow through the ejector cartridge  200  as a whole. 
     As seen in  FIG. 9 , the intermediate stepwise expansion  255  does not provide for as large an increase in diameter as the stepwise expansion  250  provided near to the outlet end of the nozzle  246 . Thus, the increase in diameter between the sharp corner  256  and the point  257  on the inner wall of the nozzle  246  radially in line with the sharp corner  256 , but in the second divergent section  249   b , is smaller than the step in diameter between the sharp corner  251  at the second stepwise expansion in diameter  250 , to the point  252  which is radially in line with the sharp corner  251  on the wall of the third diverging nozzle section  249   c.    
     Returning to  FIG. 7A , it will be seen that the ejector housing piece  240  also includes a receiving structure  245 , in the form of a shoulder, for receiving the second stage nozzle piece  230 . Second stage nozzle piece  245 , as shown in  FIG. 7B , is provided with a radially outer flange at its inlet end to abut with the corresponding shoulder formed in the receiving structure  245  of nozzle piece  240 . 
     The second stage nozzle piece  230  shown in  FIG. 7B  furthermore defines the converging-diverging second stage nozzle  232 , including converging inlet section  236 , straight-walled middle section  237  and diverging outlet section  238 , extending between the inlet  231  and outlet  233  of the second stage nozzle  232 . In the second stage nozzle piece  230  of  FIG. 7B , the valve member  235  is integrally formed with the nozzle piece  230 , so as to provide for the selective opening and closing of the second stage suction ports  244  in the ejector housing piece  240  or  270  of the ejector cartridge  200 . To facilitate flexibility in the valve member  235 , openings  260  may be provided near to the base of the valve member  235 , so as to allow the valve member  235  to open and close more easily with respect to the suction ports  244 . 
       FIG. 7B  shows, in one view, a cross-sectional view of the nozzle piece  230  in a direction perpendicular to the direction of air flow through the nozzle piece  230 , and also shows the nozzle piece  230  in an axial end view, as seen from the outlet end  233  of the nozzle  232 . In this latter view, a plurality of teeth  262  can also be seen, which are formed near to the base of the valve member  235 , on the outside of the second stage nozzle body  230 . Teeth  262  are arranged to engage with corresponding teeth which may be provided in the engaging structure  245  of the ejector housing piece  240  or  270 . These teeth are provided to facilitate rotational alignment of the second stage nozzle body  230  with the ejector housing piece  240  or  270  of the ejector cartridge  200 . Such alignment will often not be necessitated, in particular given the rotationally-symmetric form of the ejector cartridge  200 . However, in certain embodiments, the ejector housing piece  240  or  270  may be provided with second stage suction ports  244  which are not evenly distributed around the circumference of the ejector housing, or the second stage nozzle piece  230  may be provided with separate valve members  235  corresponding to each of the suction ports  244 , necessitating alignment between the valve members  235  and the respective suction ports  244  which they are to selectively open and close. 
     It will be appreciated that no sealing member is provided in order to prevent air leaking around the second stage nozzle piece  230  between the first, drive stage  200 A and the second stage  200 B. This is in view of the fact that the second stage nozzle piece  230  is intended to be made from a relatively soft and conforming rubber or plastic, which will conform to the inner dimension of the ejector housing piece  240  or  270  to form an airtight seal therewith. In cooperation with the posts or rods  216  provided on the drive nozzle piece  212 , which hold the second stage nozzle piece  230  axially in position, this will provide a secure seal around the inlet end of the second stage nozzle piece  230 . 
     Turning to  FIG. 7C , the drive nozzle piece  212  is shown, again in a cross-sectional view seen in a direction perpendicular to the direction of airflow through the drive nozzle piece  212 , and viewed in the axial direction looking from the outlet end of the drive nozzles  220 . Drive nozzle piece  212  has an inlet  214  for receiving compressed air from a compressed air supply, and for providing the compressed air to the plurality of drive nozzles  220  in the drive nozzle array  210 . Drive nozzles  220  of the drive nozzle array  210  may be formed in the same way as drive nozzle  120  shown in  FIG. 4 . 
     The drive nozzle piece  212  is formed with an annular ridge  212   b  (or a series of projections arranged in a ring around the circumference of the drive nozzle piece  230 ) which is sized to engage with an annular groove  234  of the receiving structure at the inlet end of ejector housing piece  240  or  270 , so as to secure the drive nozzle piece  212  into the housing piece  240  of the ejector cartridge  200 . It will be appreciated that, in place of the annular ridge  212   b , the drive nozzle piece  212  could be provided with an annular groove, and an elastomeric O-ring could be provided in the groove of the drive nozzle piece to engage with the groove  234  of the ejector housing piece  240  or  270 , when the drive nozzle piece  212  is fitted therein, so as to secure the two pieces together. It will also be appreciated that there is no need to provide an airtight seal at the receiving structure  234 , since the necessary sealing between the ejector cartridge  200  and the outside volume to be evacuated is obtained through the use of elastomeric seal  212   a  (as may be understood with reference to  FIG. 12 , to be discussed further below). Equally, the ridge  212   b  could be formed as a groove, and a ridge provided in place of the groove of the receiving structure  234  of the ejector housing piece  240  or  270 , to be received in the groove of the drive nozzle piece  212 . 
     The secure snap-fitting of the drive nozzle piece  212  into the inlet end of the ejector housing piece  240  or  270  further secures the second stage nozzle piece  230  in place, as the rods or posts  216 , which extend from the drive nozzle piece  212  in a forward axial direction, are arranged to press against the back surface of the second stage nozzle piece  230  to secure it against the shoulder provided in the receiving structure  245  of the ejector housing piece  240  or  270 . The second stage nozzle piece  230  is thus axially secured in place, and is also spaced the desired axial distance from drive nozzle array  210 . It will readily be appreciated that the use of rods or posts  216 , in addition to providing the necessary structural stability, also provides for the unobstructed flow of air or other fluid medium surrounding the ejector cartridge  200  into the drive stage  200 A through the suction ports  242 . 
     Turning to  FIG. 9 , there is shown a cross-sectional perspective view of the ejector cartridge  200 , which details how the second stage nozzle piece  230  and drive nozzle piece  212  are mounted into the ejector housing  240  and arranged to provide for an axial flow of high speed air generated by the drive nozzles  220  and directed successively through the second stage nozzle  232  and the exit nozzle  246 .  FIG. 9  also illustrates how air flow through the suction ports  242  and  244  can be entrained into the jet flow created by the air jets produced by the drive nozzles  220  and the second stage nozzle  232  in the respective first, drive stage  200 A and second stage  200 B. 
     Turning to  FIG. 10 , this figure shows a comparison between a single drive jet flow generated by a single drive nozzle and allowed to expand in an axial sequential flow through a second stage nozzle and an exit nozzle in side-by-side relation to a multiple drive jet flow as may be generated by the ejector cartridges  100  and  200 , which have four drive nozzles  120 ,  220  in the respective drive nozzle arrays  110 ,  210 . As can be appreciated from this representative illustration, the development of the fluid flow through the second stage nozzle and exit nozzle for the multiple drive jet flow example is substantially the same as for the single drive jet flow example of the conventional ejectors. 
     Even so, it has been found that the multiple drive nozzle arrangement allows an ejector cartridge to produce a superior performance in terms of the negative pressure which is generated and the volume flow rate through the ejector cartridge than for a single drive nozzle multi-stage ejector of the construction shown in  FIGS. 14 and 15  of the present application. Put another way, in order to obtain the same performance as a multi-stage ejector of the design of  FIGS. 14 and 15 , a multi-stage ejector according to the present invention, having multiple drive nozzles, is able to generate the same performance using a smaller quantity of compressed air, thereby providing a greater level of efficiency. Additionally, for ejectors of equivalent performance, the ejectors of the present invention, having multiple drive nozzles in the drive nozzle array, are shorter and have a smaller footprint than ejectors of the design shown in  FIGS. 14 and 15 . In particular, both designs of ejector may have a substantially equivalent diameter for the same level of performance, but the ejector cartridge of  FIGS. 14 and 15  require a three-stage arrangement in order to obtain the same levels of performance which the ejector cartridges of the present invention, as exemplified by the embodiments  100  and  200  described above, are able to achieve with only a two-stage arrangement. Accordingly, for equivalent performance, the ejector cartridges according to the present invention can be made smaller in size and of reduced footprint as compared with the ejector cartridges of the prior art. 
     With reference to the above embodiments of the ejector cartridges  100  and  200 , it will be appreciated that the second stage nozzle piece  130 ,  230  and the drive nozzle piece  112 ,  212  may be received within the corresponding receiving structures into which they are fitted not only via the press-fit or snap-fit arrangements as illustrated in the accompanying drawings, but equally by any alternative form of mating or threaded engagement, or furthermore by being glued, welded or otherwise fixed into place. 
     As regards the manufacturing of the components of the ejector cartridges  100  and  200 , it is preferred that the ejector cartridge housing pieces  130 ,  140 ,  240  or  270 , and the drive nozzle pieces  112 ,  212  be formed by a one-shot moulding process using a suitable plastics material, as will be known to the skilled person. 
     In the case of the unitary, integrally moulded second stage nozzle piece  230 , the material has to provide the necessary flexibility to allow the valve member  235  to open and close the suction ports  244 , whilst at the same time being structurally rigid enough so that the desired flow development will occur through the converging-diverging nozzle  232 . As such, the second stage nozzle piece  230  is preferably formed from a relatively compliant material, being either a plastic or rubber, and preferably being made from a suitable thermoplastic elastomer formulation, such as the thermoplastic polyurethane elastomer (TPE(U)) available from BASF under the trade designation Elastollan®, S-series, from a soft thermoplastic vulcanizate (TPV) such as Santoprene™ TPV 8281-65MED as available from ExxonMobil Chemical Europe, from NBR or other suitable materials. Common fluor rubber or FPM rubber would be another suitable material. 
     The specific material to be used for moulding the second stage ejector piece  230  will, in practice, be determined by the intended use for the ejector cartridge  200 . Specifically, it is envisaged to use TPE(U) for most applications, but to use standard type Viton® A, B or F as available from E. I. du Pont de Nemours and Company where chemical resistance is important. 
     It is envisaged that the drive nozzles  120  and  220  may be formed in the drive nozzle pieces  112 ,  212  during the moulding process by which the nozzle pieces  112 ,  212  are formed. Equally, the drive nozzles  120  and  220  may be formed in an already-moulded nozzle piece  112 ,  212 , such as by boring, where sufficient dimensional accuracy is not possible at the time of moulding of the drive nozzle piece  112 ,  212 . As for the second stage nozzle  132 ,  232  and the exit nozzle  146 ,  246 , it is envisaged that these will be formed as part of the moulding process by which the respective components  130 ,  230 ,  140 ,  240  are formed, without need of subsequent manufacturing steps. 
     With reference now to  FIGS. 11A to 11C , there is shown an example of how an ejector cartridge  100  (equivalently, the ejector cartridge  200 ) may be mounted into a housing module  1000 , for use in a vacuum pump or similar. 
       FIG. 11B  shows the ejector  100  mounted into an internal bore  1012 ,  1040 ,  1060  formed in housing module  1000 . O-ring seals  112   a  and  140   b  provide a seal, respectively, between the drive nozzle piece  112  and an inlet bore  1012  of the housing module  1000 , and between an outside of the second stage ejector housing piece  140  and the inside of the bore defined in the housing module, so as to separate the bore into an intermediate vacuum chamber  1040  and an exit chamber  1060 . The housing module  1000  is provided with an inlet chamber  1020 , to which a compressed air source is to be connected in order to provide the ejector cartridge  100  with a supply of compressed air. Inlet bore  1012  is connected into the inlet chamber  1020 , so that the compressed air is supplied to the inlet  114  of the drive nozzle piece  112 . In operation, the compressed air forms a stream of high speed jet flow through the ejector  100 , which creates a suction force at the suction ports  142  and  144 , at the drive stage and second stage, respectively, of the ejector  100 , before the compressed air and any entrained fluid from the surrounding volume is ejected through the exit nozzle  146  into exit chamber  1060 . A muffler or alternative stop member  1100  is provided in the opening of the housing module bore, so as to close off the exit chamber  1060  to contain the fluid ejected from the ejector  100  and to suppress noise caused by this high speed jet flow of air exiting from the exit nozzle  146  of the ejector  100 . Stop member  1100  is provided with arms or rods  1110  arranged to secure the ejector cartridge  100  axially in place in the bore of housing module  1000 . The stop member  1100  may be secured in place using a suitable sealing member such as elastomeric O-ring  1100   a , or may be otherwise threaded, secured, welded or glued in place in a sealing fashion in order to close off the bore of the housing module  1000 . 
     The air ejected from ejector  100  is, instead of being expelled to atmosphere on exit from the ejector  100 , conveyed away from the housing module  1000  through exit port  1046 , formed in the base of the housing module  1000 . In this way, compressed air is supplied into the housing module through the inlet port  1014 , and the compressed air and any entrained fluid evacuated from the surrounding volume is expelled from the housing module  1000  through the exit port  1046 . Housing module  1000  is furthermore provided with suction ports  1042  and  1044 , which are arranged to connect the volume in the vacuum chamber  1040  which surrounds the first and second stage suction ports  142  and  144  of the ejector  100  with a volume to be evacuated. The volume to be evacuated may comprise, for example, one or more suction cups or other suction devices, or any other vacuum-operated machinery. 
     In the example shown in  FIG. 11B , the housing module  1000  is connected along its base surface to a connection plate  1200  of a vacuum-operated device, the connection plate  1200  being provided with ports  1214 ,  1242 ,  1244  and  1246  which correspond to the ports  1014 ,  1042 ,  1044  and  1046  formed in the base of the housing module  1000 . Elastomeric seals, such as O-rings  1014   a ,  1042   a ,  1044   a  and  1046   a  are provided between the corresponding ports of the housing module  1000  and the ports  1214 ,  1242 ,  1244  and  1246  of the connector plate  1200 . Port  1214  of the connector plate  1200  is connected to a compressed air supply, for supplying compressed air through the inlet port  1014  into the inlet chamber  1020  of the housing module  1000 . Likewise, air expelled through the outlet  1046  of the housing module  1000  is carried away through the outlet passage  1246  in connector plate  1200 . Similarly, ports  1242  and  1244  in connector plate  1200  connect the vacuum generated by the ejector  100  to the volume to be evacuated, with air or other fluid medium in the volume to be evacuated being drawn through the ports  1242 ,  1244  in connector plate  1200 , through the suction inlets  1042  and  1044  in the housing module  1000  and into the vacuum chamber  1040  formed in the bore surrounding the first and second stages  100 A,  100 B of the ejector cartridge  100 . 
     In the early stages of vacuum generation, a large differential pressure will exist across the second stage  100 B of the ejector cartridge  100  and the valve member or members  135  will open so that fluid medium will be entrained through the suction inlet  144  and into the second stage jet flow, as well as simultaneously being entrained into the drive section  100 A through the suction ports  142 . However, as the vacuum in the volume to be evacuated increases, so that a higher negative pressure (i.e., a lower absolute pressure) is generated, the pressure differential across the valve members  135  will reduce, until these valve members close, at which point only the drive stage  100 A will provide suction to the chamber  1040  through the suction port  142 , which in turn provides suction through the suction ports  1042  and  1044  of the housing module to the ports  1242 ,  1244  of the connecting plate  1200 . 
     By mounting the ejector cartridge in a housing module in this way, the vacuum generated by the ejector cartridge  100  can be selectively applied, via the connecting plate  1200 , to associated connected vacuum-operated equipment, as desired. 
       FIG. 11A  shows the disposition of the inlet port  1014 , suction ports  1042 ,  1044  and outlet port  1046  of the housing module  1000 . It will be appreciated that the position of the inlet port, outlet port and suction ports in the housing module  1000  does not necessarily correspond to the location of the inlet  114 , suction ports  142 ,  144 , and ejector exit nozzle  146  of the ejector cartridge  100 , but instead necessarily corresponds to the position of the inlet port  1214 , suction ports  1242 ,  1244  and outlet port  1246  of the connector plate  1200  to which the housing module  1000  is to be attached. However, since the suction ports  142 ,  144  are arranged to evacuate the entire vacuum chamber  1040  which surrounds the first and second stages  100 A and  100 B of the ejector cartridge  100 , it is not necessary to provide alignment between the suction ports  142 ,  144  of the ejector cartridge  100  and the suction ports  1042 ,  1044  of the housing module  1000 , provided that there is a suitable location within the bore of the housing module  100  where the elastomeric O-ring  140   b  is able to seal off the bore of the housing module to form the vacuum chamber  1040  and exit chamber  1060 . 
     Turning to  FIG. 11C , there is illustrated an arrangement of connectors for interconnecting one or more modular housing units together, using bores, such as threaded bores  1050  provided in the housing module  1000 , each threaded bore  1050  being provided with a recessed area  1055  surrounding the bore opening at its upper end, to permit a connecting member, such as a screw or bolt, to be recessed relative to the upper surface of the housing module  1000 . Such connector holes may also be used to attach the housing module  1000  to the connector plate  1200 , as appropriate. 
     One use for such a modular housing arrangement is shown in  FIG. 12 , in which the ejector  100  has been replaced, merely by way of example, by ejector cartridge  200  in the housing module  1000 . However, in this example, the housing module  1000  is not connected directly to the connector plate  1200 , but is instead connected onto a booster module  2000 , which houses a booster ejector  300 , the booster module  2000  being in turn connected to a connector plate  1200 . In this example, the connector plate  1200  includes an inlet port  1214 , a single suction port  1242 , and an outlet port  1246 . 
     The housing module  1000  is otherwise as described in respect of  FIG. 11 , with the exception that the suction port  1042  is provided with a valve member  1350 , which permits selective opening and closing of the suction port  1042  between the vacuum chamber  1040  of housing module  1000  and the booster stage of booster ejector  300 . 
     Booster module  2000  includes an inlet chamber  2020  for receiving compressed air from the inlet port  1214  of the connector plate  1200  through a corresponding inlet port  2014 . The inlet chamber  2020  of the booster module  2000  is connected to an inlet bore  2012  of the booster module  2000 , in which the booster ejector  300  is mounted, in order to supply compressed air to the inlet of the booster ejector  300 . This bore in which the booster ejector  300  is mounted may, for example, be formed by drilling into the booster module  2000  from the side adjacent to the inlet chamber  2020 , and so a stop member  2100  is provided in order to seal off the borehole opening. The inlet chamber  2020  also provides an outlet port  2015 , which connects inlet chamber  2020  to the inlet port  1014  of the housing module  1000  in order to simultaneously supply compressed air to the inlet of the ejector cartridge  200 . 
     The booster module  2000  includes a suction port  2042  for applying suction to the suction port  1242  of the connector plate  1200  from a vacuum chamber  2030 . Vacuum chamber  2030  is likewise connected to the vacuum chamber  1040  of the housing module via a port  2033  in the booster module  2000  and the suction port  1042  in the housing module  1000 . In this way, the vacuum generated by the ejector cartridge  200  can be applied to the volume to be evacuated by drawing the air or other fluid medium to be evacuated through the suction port  1242  of the connection plate  1200 , through the suction port  2042 , through the vacuum chamber  2030 , through the ports  2030  and  1042 , through the vacuum chamber  1040  and into the suction ports  242  and  244  of the ejector cartridge  200 . In practice, this will happen during the early stages of supplying compressed air to the ejector arrangement shown in  FIG. 12 , as the ejector cartridge  200  is able to entrain a substantially larger volume of air into the drive stage  200 A and second stage  200 B than is the booster cartridge  300 . However, once the vacuum produced in the volume to be evacuated drops below the highest negative pressure value (i.e., the lowest absolute pressure) which the ejector  200  can generate, the valve  1350  will close, to prevent a backflow of air from the evacuation chamber  1040  surrounding the ejector  200  into the chamber  2030  which surrounds the booster ejector  300 . 
     Booster ejector  300  comprises a pair of nozzles, being a drive nozzle  320  and an exit nozzle  346 , which together form a booster stage, across which a high vacuum (low absolute pressure) is obtained. Specifically, drive nozzle  320  directs a high speed air jet into the inlet of the converging-diverging nozzle  346 , thereby entraining air or other fluid medium in the volume surrounding the air jet into the booster jet flow and so creating a vacuum at the suction port  342  which is connected to the chamber  2030  to be evacuated and which is in turn connected to the suction port  2042  of the booster module which is sealed to the suction port  1242  of the connector plate  1200 , so as to evacuate a connected volume to be evacuated. 
     The booster drive nozzle  320  may have a similar configuration to the drive nozzles  120  and  220  as described above, but is specifically designed to achieve a high vacuum level (low absolute pressure), in combination with the converging-diverging nozzle  346  which is formed of a converging section  347 , straight-walled middle section  348  and diverging exit section  349 . The fluid expelled by nozzle  346  from the outlet of the booster ejector  300  is discharged into a chamber  2040  in the booster module  2000 , which is in turn connected, via an outlet port  2045 , to the suction port  2044  of the housing module  1000 . In this way, the air which is ejected through the booster ejector  300  is subsequently entrained into the jet flow of the ejector cartridge  200  via the suction ports  242  and/or  244 , and then ejected out of the ejector cartridge  200  into the ejection chamber  1060 , through the outlet port  1046  and an associated port  2047  of the booster module, through an outlet passage  2060  of the booster module  2000 , through an outlet port  2046  of the booster module and out through the outlet port  2046  of the connector plate  1200 . 
     As will be appreciated, the booster drive nozzle  320  is formed as part of a nozzle body  312 , which is press fitted or otherwise secured in the bore  2012  provided in the booster module  2000 . The booster exit nozzle  346  is likewise formed as part of a booster outlet nozzle piece  340 , which is also press fitted or otherwise secured in the bore formed in the booster module  2000  which defines the exit chamber  2040 . Respective elastomeric seals, such as O-rings  340   a  and  312   a , seal off each end of the booster ejector  300 , so as to define the evacuation chamber  2030  to be evacuated by the booster ejector  300 . As shown in  FIG. 12 , elastomeric seals, such as O-rings  1014   a ,  1042   a ,  1044   a ,  1046   a ,  2014   a ,  2042   a  and  2046   a  are provided at the respective inlet and outlet ports of the housing module  1000  and the booster module  2000 , to provide airtight seals between the adjacent ports and connected chambers. 
     With the arrangement shown in  FIG. 12 , the ejector cartridge  200  can provide a high level of vacuum within a short space of time, and this is supplemented by the booster cartridge  300  so as to further increase the negative pressure (i.e., further reduce the absolute pressure) which is applied to the volume to be evacuated, to which the housing module  1000  and booster module  2000  are connected via port  1242  of the connector plate  1200 . 
     It is also to be noted that the suction provided by the ejector cartridge  200  to the suction port  1044  reduces the pressure in the exit chamber  2040  at the outlet of the booster ejector  300 , such that the pressure differential across the booster ejector  300 , between the inlet chamber  2020  and the outlet chamber  2040 , is increased. This, in turn, can be used to obtain a further increase in the vacuum level (i.e., a further reduction in the absolute pressure) which the booster ejector  300  is able to achieve.