Patent Publication Number: US-2022213892-A1

Title: A pump and a method of pumping a gas

Description:
CROSS-REFERENCE OF RELATED APPLICATION 
     This application is a Section 371 National Stage Application of International Application No. PCT/GB2020/050338, filed Feb. 13, 2020, and published as WO 2020/165592A1 on Aug. 20, 2020, the content of which is hereby incorporated by reference in its entirety and which claims priority of British Application No. 1902106.2, filed Feb. 15, 2019 
    
    
     FIELD 
     The field of the invention relates to pumps and methods of pumping a gas. 
     BACKGROUND 
     Different types of pumps for pumping gases are known. These include entrapment type pumps, where a gas is captured on a surface inside the pump prior to being removed; kinetic or momentum transfer pumps such as turbomolecular pumps where the molecules of the gas are accelerated from the inlet side towards the outlet or exhaust side, and positive displacement pumps, where gas is trapped and moved from the inlet towards the outlet of the pump. 
     Positive displacement pumps provide moving pumping chambers generally formed between one or more rotors and a stator, the movement of the rotors causing the effective pumping chamber to move. Gas received at an inlet enters and is trapped in the pumping chamber and moved to an outlet. In some cases the volume of the gas pocket reduces during movement to improve efficiency. Such pumps include roots and rotary vane type pumps. In order to draw the gas into the chamber, the chamber generally expands and to expel the gas from the chamber, the chamber volume generally contracts. This change in volume can be achieved for example in a rotary vane pump by blades that extend in and out of the pump chamber using devices such as springs, which are themselves subject to wear, or by using two synchronised rotors in a roots or screw pump which cooperate with each other and a stator to move a pocket of gas and generate the volumetric changes between inlet and outlet. An additional rotor requires an additional shaft, bearings and timing methods such as gears to synchronise the rotor movements. 
     Furthermore, in order to minimise or at least reduce leakage and move the gas efficiently while it is trapped the moving parts need to form a close seal with each other and with the static parts which form the trapped volume of gas. Some pumps use a liquid such as oil to seal between the surfaces of the trapped volume whilst others rely on tight non-contacting clearances which can lead to increased manufacturing costs and can also lead to pumps that are sensitive to locking or seizure if the parts come into contact or where particulates or impurities are present in the fluid being pumped. 
     It would be desirable to provide a pump that is resistant to wear, offers low power consumption, a relatively small pumping mechanism and is relatively inexpensive to manufacture and operate. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
     SUMMARY 
     A first aspect of the present invention provides a pump for pumping a gas, said pump comprising: a pump housing element and a further element; one of said pump housing and said further element comprising a protrusion extending towards the other element, said other element comprising at least one liquid opening; said protrusion, pump housing and further element forming a path from a gas inlet to a gas outlet; wherein said further element is concentrically mounted within a bore of said pump housing and said pump housing and further element are mounted rotatably with respect to each other; and said protrusion comprises a helix, a cross section through an axial plane of said helix varying such that said helix is narrower at a point towards said other element and wider at an intersection of said protrusion with said one of said pump housing and said further element from which said protrusion extends; and said at least one liquid opening is configured such that liquid output from said at least one liquid opening forms a liquid blade, said liquid blade being operable to drive gas along said path from said gas inlet to said gas outlet on rotation of one of said elements. 
     The inventor of the present invention recognised that were the elements of a pump to be configured with liquid opening(s) such that liquid output through the openings formed a surface or blade between the elements of the pump, then on rotation of one of the elements with respect to the other, the liquid blade could be used to drive the gas through the pump. Furthermore, if the elements of the pump were configured such that a helical protrusion extending between them formed a path from a gas inlet to a gas outlet, then such a path could lead the gas from the inlet to the outlet driven by the liquid blade extending between the elements and across the path. This would have the potential to provide a simple, compact, low power, low cost arrangement and the problems that arise due to friction and wear between contacting surfaces and the cost involved in manufacturing tolerances for tight clearances would be avoided or at least mitigated. 
     Such a blade could be formed by driving a liquid through one or more liquid openings. Arranging the liquid opening(s) on one of the elements allows a stream of liquid to form a liquid surface or blade between the elements. Such a liquid blade is by its nature, deformable, low cost, and able to provide good sealing between surfaces of the trapped volume without the need for tight manufacturing tolerances. Furthermore, such a blade is not subject to wear itself and provides very little wear on the surfaces that it contacts. 
     The blade is formed of a flowing liquid such that the liquid forming the blade is continuously replenished. A surface of the blade acts along with a surface of the elements, and protrusion to confine, trap, isolate or enclose the gas to be pumped. Rotation of one of the elements causes the trapped gas to be moved from a gas inlet to a gas outlet along a path defined by the protrusion. Gas to be pumped is located on either side of the blade. 
     One or other of the elements may be mounted to rotate, or both may be mounted to rotate in opposite directions. In this regard relative motion between the elements is required, it is unimportant which is the rotating element. In some embodiments, the pump housing element is mounted to be stationery and the further element is mounted to rotate. 
     However, a potential problem with such a design is that without careful design of the shape of the helical protrusion, portions of the protrusion may mask the outer wall from the blade and leakage paths along this masked area may arise. In order to address this potential problem the helical protrusion is configured such that it has an axial cross section that is tapered, so that it is wider at the wall to which it is attached than it is at its free end. A helical thread with an axial cross section that is not tapered, provides a radial cross section that is substantially rectangular and such a shape may mask portions of the wall from which it extends from the liquid blade. Such masking is detrimental to the pump performance providing a leakage path along the helix from the gas outlet to the gas inlet. A thread that is wider at the wall edge provides a horizontal cross section that is also wider towards the wall limiting the masking effect of this profile on the liquid blade. 
     Although the variation in the width of the protrusion can have a number of values in some embodiments, said protrusion is at least twice as wide at a point 10% along the protrusion from the element from which it extends as it is at a point 95% along the protrusion and in some embodiments more than four times as wide. 
     In some embodiments, an upper surface of said at least one protrusion has a form corresponding to half a parabola. 
     It may be advantageous if the protrusion has a curved form so that liquid is not thrown of the surface but tends to adhere to it. A substantially parabolic form of the upper surface provides a radial cross section of a form that may have a trailing edge (back or final edge encountered by the blade when rotating) that is substantially straight and extends at an angle from the wall to which it is attached. It should be noted that an axial cross section or plane is one that is parallel to and passes through the axis of the pump about which the rotating element rotates, and a radial cross section or plane is one perpendicular to the axis, which in operation of most embodiments of the pump is generally horizontal. 
     In some embodiments, said at least one protrusion extends from said pump housing element and has a cross section through a radial plane such that said protrusion does not mask said pump housing element from a water blade extending tangentially out from said further element. 
     The inventors of the present invention recognised that in order for the pump to pump effectively the shape of the protrusion should be such that the protrusion does not mask the liquid blade from reaching the outer wall at certain points as this could provide a leakage path along the whole length of the helix. When determining how such a leakage path could be avoided or at least inhibited the inventors recognised that the direction of the liquid blade will vary depending on operating conditions and if the protrusion could be designed to be effective for the extreme conditions then the intermediary conditions should also be covered. Thus, the protrusion is configured such that the pump housing wall is not masked by the protrusion from a tangential blade, which relates to the condition of no pressure difference across the blade. 
     In some embodiments, said at least one protrusion extends from said pump housing element and has a cross section through a radial plane such that said protrusion does not mask said pump housing element from a water blade extending at right angles from said further element. 
     At higher pressure differences the blade may extend substantially perpendicular to the inner further element and in which case the protrusion can be designed with this in mind. 
     In some embodiments, a lower surface of said at least one protrusion is flat, such a configuration providing a protrusion that does not mask the outer wall from a perpendicular liquid blade. 
     In other embodiments, said protrusion has a cross section in an axial plane of a substantially parabolic form. 
     It may be convenient to form the protrusion as a symmetrical protrusion with the upper and lower surfaces having the same parabolic form. Such a protrusion forms an effective seal with the liquid blade along the length of the helix. 
     In some embodiments, a trailing edge with respect to the direction of rotation of said cross section of said protrusion in said radial plane is at an acute angle with respect to a tangent to said pump housing wall that is between a maximum angle where said trailing edge is parallel to said tangent of said further element and up to 15% less than said maximum angle. 
     As noted previously the angle of the trailing edge should be selected to inhibit the radial cross section of the protrusion from masking the outer wall from the blade. A trailing edge that is parallel to a tangent from the inner rotating element provides an acceptable performance even at limit conditions, however, the protrusion may be formed with a slightly larger area and with a slightly smaller acute angle. 
     In some embodiments, a leading edge with respect to the direction of rotation of said cross section of said protrusion in said radial plane lies between a curved line curved in the direction of rotation and with a radius of curvature equal to half a distance between said pump housing element and said further element and a line extending away from said trailing edge and parallel to a tangent of said further element. 
     For the purposes of this patent application the leading edge of the helical thread is taken to be the portion of the thread in that radial plane that the blade contacts first when rotating and the trailing edge is the portion that it contacts when moving beyond that portion of the thread. 
     The required shape of the leading edge to avoid masking of the wall by the protrusion may be configured in dependence upon the higher pressure conditions of operation. The limit case corresponding to operation at maximum pressure conditions corresponds to a blade that has a curved surface with a radius of half the gap between the elements. A protrusion with a leading edge with a corresponding shape will be acceptable even at the limit condition. This form relates to the smallest cross section to provide acceptable sealing, and it can be enlarged by extending the leading edge away from the direction of rotation until it is parallel to a tangent of the inner element. 
     In effect a cross section that matches the profile of the radial or horizontal cross section of the protrusion to the trajectory of the droplets of the blade at the limit conditions will provide a protrusion that does not mask the outer wall from the blade and provides acceptable pumping. The profile can be enlarged although a sloping leading edge should be maintained. 
     Where the radial cross section has both the leading edge and trailing edges parallel to a tangent of the inner element, then the radial cross section forms a segment of a circle in the annular gap between the two elements. 
     In some embodiments, a lower surface of said at least one protrusion is flat and this provides a leading edge with respect to the direction of rotation of the radial cross section that is substantially perpendicular to a tangent to said further element. 
     Such an arrangement is easy to machine and where the upper surface is curved and wider at its intersecting end than its free end provides a protrusion with acceptable performance. 
     In some embodiments, said at least one liquid opening is formed on a surface of said element that is mounted to rotate. 
     The element that is mounted to rotate is generally termed the rotor and it may be advantageous if this is the element having the liquid openings as the rotating motion may help in the expelling of liquid from the liquid openings to form the blades. 
     In some embodiments, a cross section of said path formed by said protrusion, pump housing and further element decreases from said gas inlet to said gas outlet. 
     In some cases the cross section of the path may decrease between the inlet and outlet, either continually or along portions of the path. The resulting reduction in volume leads to volumetric compression of the gas as it is pumped which not only aids in the expelling of gas from the chamber but also reduces the power required for pumping a given volume of gas. 
     In some embodiments said inner component is eccentrically mounted within said bore of said outer component, while in others said inner component is concentrically mounted within said bore of said outer component. 
     Eccentrically mounting the inner component means that when there is relative rotation the gas pocket formed by the pump housing elements and liquid blade will change in volume around the circumference of the elements. This eccentric mounting requires the blades to change in size as the element(s) rotate, but this will happen naturally. There is no requirement for mechanical or sliding parts such as springs and solid blades to create the changing size of the blades. 
     The liquid outlet(s) may be arranged in a number of ways. There may be a plurality of liquid outlets arranged adjacent to each other, or there may be a single outlet in a slot form. In some embodiments, the slot or plurality of outlets has a longitudinal form running substantially parallel to an axis of the elements. Such an arrangement provides a blade substantially perpendicular to the radius of the pumping chamber. 
     In other embodiments the slot or adjacent outlets may be angled with respect to the axis of the elements and in some cases may form a helix such that a helical liquid blade is formed between the stator and rotor. 
     A pump configured to generate such a blade may be used in conjunction with a helical protrusion on the surface of the other component. A helical protrusion provides a pump that acts in a similar way to a screw pump. Such a protrusion can be used in conjunction with an axial liquid blade or with a helical blade. 
     In some embodiments, an angle of said helix changes from said gas inlet towards said gas outlet such that a pitch of said helix reduces towards said gas outlet. 
     Reduction of the pitch of the helix towards the gas outlet provides volumetric compression to the gas as it is pumped which not only aids in the expelling of gas from the chamber but also reduces the power required for pumping a given volume of gas. 
     In some embodiments, at least one of said pump housing and further element are tapered such that a distance between said stator and said rotor reduces towards said gas outlet. 
     One way of providing a pumping chamber which reduces in size between the inlet and outlet is to provide a tapering such that the distance between the elements reduces towards the gas outlet. In some embodiments it is the non rotating element or stator that is tapered, while in others it is the rotating element or rotor that is tapered. Tapering of the stator that does not rotate is often the simplest way of generating the reduction in size of the pumping chamber towards the gas outlet. 
     In some embodiments said at least one of said pump housing and further element that are tapered are non axi-symmetrically tapered. 
     Although generally the taper will be axi-symmetric, in some embodiments it is the bore of the outer component that is non axi-symmetrically tapered towards said gas outlet, while in other embodiment the inner component may have an increasing diameter. 
     In some embodiments, said plurality of liquid outlets provide a plurality of streams of liquid which form a plurality of liquid blades between said pump housing element and further element. 
     Although, the pump may comprise a single liquid opening to form a single liquid blade in some embodiments it comprises a plurality of liquid openings. Liquid from the plurality of openings may form a single blade or the openings may be arranged such that liquid expelled from them forms a plurality of blades. 
     In some embodiments, at least one set of said plurality of liquid openings are arranged adjacent to each other and streams output from said at least one set of said plurality of liquid openings combine to form a single liquid blade. 
     In some cases there may be a plurality of openings and a set of these may form a single blade. Where there is only one blade this set may comprise all the liquid openings, while in other embodiments, there may be several sets each set arranged to form their own blade. Although a liquid blade may be formed from a single liquid outlet in the form of say a slot, in some embodiments it may be formed by a plurality of adjacent openings that are close enough together for the streams of liquid through each to coalesce and form a single blade. Having a plurality of openings rather than a single slot may improve the structural integrity of the elements that they are arranged on and thereby improve the mechanical integrity of the pump. 
     For the purposes of this patent application where the term rotor is used this refers to the pump housing or further element that rotates and where the term stator is used this refers to the element that the rotor rotates with respect to. Furthermore, the gas to be pumped may be a vapour, or a gas vapour mixture, or a gas having particles entrained within it. 
     In some embodiments, the rotor is rotatably mounted within a bore of the stator and the stream of liquid forming the liquid blade between the rotor and the stator bore is operable to drive the gas through the pump on rotation of the rotor within the stator bore. 
     Rotation of the rotor provides relative motion between the surfaces enclosing the gas pocket, such that in some embodiments the liquid surface drives the gas along a pumping path from a gas inlet to a gas outlet. This relative motion along with, in some embodiments, a change in volume of the gas pocket can be provided without any appreciable wear on the surfaces confining the gas pocket as at least one is formed from a liquid blade and due to its deformable nature its surface shape and size will adapt to the distance between the rotor and stator during rotation. 
     In some embodiments said pump comprises a driving mechanism for exerting a driving force on the liquid to drive said liquid from said liquid source through said at least one liquid opening. 
     Although the driving force exerted on the liquid may come from a source external to the pump, the pump may for example be connected to an external pressurised liquid source, in some embodiments the pump itself comprises a driving mechanism for exerting this driving force on the liquid. 
     Although the liquid openings may be formed on the surface of a rotatable inner component, in some embodiments they are formed on the surface of a stationery outer component. This may have the advantage of allowing a simpler way of supplying pressurised liquid to the pump. 
     In some embodiments, said pump further comprises a liquid reservoir, said inner component being rotatably mounted and comprising a hollow body having an opening at a lower end extending into said liquid reservoir, an internal diameter of said hollow rotor increasing from said lower end. 
     One way of providing the driving force to the liquid where the liquid outlet(s) are on the rotating inner component is to use a hollow component and to spin this hollow component. In such an embodiment, the spinning of the hollow rotor will cause liquid within the hollow rotor body to be forced by centrifugal action against the outer circumference of the hollow rotor body and out through the one or more liquid outlets forming a liquid stream. Where the liquid outlets are arranged appropriately this liquid stream will form the liquid blade extending to the stator bore. 
     Where said hollow rotor has an opening at a lower end extending into the liquid reservoir, an internal diameter of the hollow rotor increasing from said lower end will help liquid to rise up within the rotor and be expelled through the liquid outlet(s) on spinning of the rotor. In this way at the lower end that is immersed in the liquid reservoir there is a smaller diameter and the diameter increases up the hollow body. This causes liquid pushed by a centrifugal force against the inner surface of the hollow body to rise up the increasing internal diameter towards the top of the rotor body. The increase in diameter may be a sloped increase or it may be a stepped increase or it may be a combination of the two. It may also be complemented by vanes on the internal surface of the rotor to support the acceleration of the liquid towards the larger diameter. The liquid is thrown out towards the inner surface of the hollow body and rises up pushed up by the acceleration and pressure of the following liquid. The speed of rotation will affect how high the liquid is pushed up the hollow body, as will other parameters such as the density of the liquid. Appropriate speeds and sizes of rotor can be selected according to the desired flow rate of the liquid to be pumped through the outlets to form the blades or vanes. It should be noted that sufficient liquid should be supplied from the reservoir into the hollow rotor body to maintain an uninterrupted stream of liquid between the rotor and the stator in order for the gas to be effectively pumped. This again will depend on the parameters such as the rotating speed of the rotor and also the size and number of outlets, and the height of the rotor. 
     In some embodiments, said pump comprises at least one hydrodynamic bearing to support at least one end of said rotating element. 
     Rotors of pumps are supported on bearings and typically these are roller bearings or ball bearings which can be expensive parts, requiring lubrication and subject to wear. A hydrodynamic bearing which utilises a liquid film between a cylindrical shaft and bore may be appropriate for this type of pump. In some cases the hydrodynamic bearing is filled with liquid from the same liquid source as the pump blades making efficient use of the liquid supply and mechanical features already used in the pump and avoiding the use of additional components or a different lubricant liquid. 
     Although the pump may be a number of things such as a compressor, in some embodiments it comprises a vacuum pump. Pumps according to embodiments, make particularly effective vacuum pumps allowing gas to be transported in an efficient manner with low wear and a low initial cost. 
     A second aspect of the present invention provides a wet scrubber for reducing pollutants pumped from an abatement system, said wet scrubber comprising a pump according to first aspect of the present invention. 
     Abatement systems are often used in conjunction with wet scrubbers which provide a stream of liquid to react with gases or remove particulates from the gases that are pumped from the abatement system. A pump that uses a liquid surface to move the gas may be used either in conjunction with an additional liquid scrubbing source or on its own, providing both the liquid source and the pumping required to move the gas and to remove particulates from it. 
     A third aspect of the present invention provides a method of pumping a gas, said method comprising: outputting liquid from at least one liquid opening on one of a pump housing element or a further element, the other of said pump housing and said further element comprising a protrusion, said protrusion, pump housing and further element forming a path from a gas inlet to a gas outlet; wherein said protrusion comprises a helix, a cross section through an axial plane of said helix varying such that said helix is narrower at a point towards said other element and wider at an intersection of said protrusion with said one of said pump housing and said further element from which said protrusion extends; and rotating one of said pump housing element or said further element such that liquid output from said at least one liquid opening forms a liquid blade and drives gas along said path from said gas inlet to said gas outlet on rotation of one of said elements. 
     Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. 
     Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. 
     The Summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detail Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a screw pump type embodiments with a helical protrusion on the stator bore and liquid openings on the rotor; 
         FIG. 2  shows a cross section through a radial plane, of a thread not according to an embodiment; 
         FIG. 3  schematically shows the directions of the liquid blades at different operating pressures; 
         FIG. 4  shows a helical thread of a pump according to an embodiment; 
         FIG. 5  shows a helical thread of a pump according to another embodiment; 
         FIG. 6  shows a cross section in the radial plane of the thread of  FIG. 5 ; 
         FIG. 7  shows a helical thread of a pump according to a further embodiment; 
         FIG. 8  shows liquid openings on an inner component for forming longitudinal liquid blades according to embodiments; 
         FIG. 9  shows liquid openings on an inner component for forming a helical liquid blade according to an embodiment; and 
         FIG. 10  show a hollow shaft in a reservoir with liquid blades formed by liquid expelled from liquid openings during rotation of the shaft. 
     
    
    
     DETAILED DESCRIPTION 
     Before discussing the embodiments in any more detail, first an overview will be provided. 
     Embodiments provide a pump comprising liquid blades that are high velocity surfaces formed of liquid, which surfaces emulate some of the solid mechanical surfaces which are found in conventional vacuum pumps and which are used as the physical boundaries to isolate and move pockets of gas. The liquid may be water, other liquids may be used for example to change characteristics of the pump such as vapour pressure or process compatibility. 
     The size and shape of the liquid surfaces will adapt to the relative position of the other pump elements unlike a rigid solid surface found in conventional pumps and will also provide a good seal with other surfaces provided they are suitably shaped without either causing appreciable wear on these surfaces or relying on tight tolerances or being sensitive to particulates in any gas or fluid flow being pumped. 
     The liquid “blades” are formed from a continuous stream of liquid originating from holes or slots. In some embodiments they are in a rotating element that forms the rotor of the pump. The streams of liquid travel at high velocity towards the other pump housing element. The pressure required to drive the liquid from one element to the other under high velocity can be achieved through centrifugal action of the rotating element, through a pressurised liquid source, or through a combination of the two. 
     Protrusions formed between the elements define a helical path that liquid blade(s) can drive gas along on rotation of one of the pumping house elements. The protrusions form a path from an inlet at one longitudinal end of the pump to an outlet at the opposing longitudinal end. 
       FIG. 1  shows an embodiment where the further element  10  is mounted within the pump housing element  20 . In this embodiment, the protrusion  25  is a thread extending from the inner surface of pump housing element  20 . This internal thread  25  is in the form of a helix. This can be used in conjunction with the inner components of  FIG. 8  having longitudinal slots or that of  FIG. 96  having helical slots. 
     In this embodiment the inner component  10  is rotatably mounted with the lower end in a liquid reservoir  30 . On rotation of the inner component or rotor  10 , liquid rises up the hollow shaft and is output through liquid openings to form longitudinal liquid blades  40  which sweep gas along a helical path defined by thread  25 , stator bore  20  and rotor  10  from gas inlet  50  to gas outlet  52 . In effect the liquid surfaces  40  create trapped ‘pockets’ along the thread form and as the liquid surfaces rotate the pockets move from the gas inlet towards the gas outlet. The shape of the thread may be adapted to the curvature of the liquid surface to provide appropriate sealing across the channel. 
     Although in this embodiment the thread is on the stator and the rotor rotates, where the helical path is formed by a mechanical thread or protrusion on the surface of one of the components, it is only relative motion between the two components that is required and as such, the thread could be on the rotor and the stator could have the liquid outlets. In this regard the stator is the fixed part and the rotor the rotating part, the rotor may be the inner component or it may be the outer component. In the latter case, the stator is a cylinder within the rotating outer component. In this embodiment, the stator and rotor may be concentrically mounted. It should be noted that where the liquid openings are on the static component then a different way of driving the liquid from the liquid openings will be required, such as by connection to a pressurised liquid source. 
     Although in this embodiment the liquid outlets are shown as slots extending vertically, they may be a plurality of adjacent liquid outlets following this formations, or they may have a different formation, albeit they will extend along the longitudinal axis of the component between the gas inlet  50  and gas outlet  52 . 
     An advantage of having a mechanical thread  25  is that there may be an increased tolerance to back migration of the liquid when it hits the opposing surface, driving the liquid towards the outlet and achieving higher pressure ratios across the pump. 
     One potential problem with such an arrangement is illustrated with respect to  FIG. 2 .  FIG. 2  schematically shows a cross section through the pump of  FIG. 1 . In this case with a simple non-tapered screw thread the water blade  40  may be blocked by the surface profile of the thread  25  before reaching the wall. 
       FIG. 2  schematically shows how water expelled from an outlet by a rotating body will continue to move in the direction of travel of the rotating body such that it will form a blade which if other forces are discounted, is in a direction tangential to the rotor as shown by line  40 . As the blade rotates from point  1  to point  2 , a conventional thread  25  with a substantially uniform narrow rectangular radial cross section will block the blade  40  from reaching the stator wall at certain positions of the blade, and this will cause an area  60  which does not seal. This area will follow the path of the screw helix from top to bottom and may cause a leakage path between the top and bottom of the stator. 
     Embodiments seek to address this by matching the surface profile of the protrusion or thread  25  to the extremes of the blade profile to allow the water blade to reach the outer wall under the different pressure conditions that the pump may operate under. 
       FIG. 3  schematically shows how the profile of the blade may vary with differing pressure differences (PD) between the blades, the possible blade profiles  40   a  and  40   b  being shown. Blade edge  40   a  schematically represents the case with no pressure difference between the blades, that is the condition at startup for example. Blade edge  40   b  schematically represents the case of a higher maximum pressure difference across the blades. It should be noted that these representations are schematic and do not take into account other factors that affect the shape of the blade such as the rotational movement of the rotor. In reality the shape of blade  40   b  may be more of a tear drop or semi-circular shape the curve extending in the direction of rotation of the rotor. 
     Bearing the different potential geometries of the blade in mind, the channel design should be such as to create a seal even with DP=0. Were this not the case then generating the initial pressure difference would be difficult. Blade edge  40   a  assumes one extreme case to create a seal DP=0 mbar, while line  40   b  represents the other extreme case to create a seal e.g. DP=maximum. The location of the blade at other pressure differences will lie between the two extremes. 
       FIG. 4  shows different views of a helical thread according to an embodiment where the cross section (and the initial portion) of the thread is/are adapted to avoid or at least inhibit the potential leakage path formed by the blades being blocked by the thread profile from reaching the opposing wall. 
       FIG. 4 a    shows an isometric view of the thread. 
       FIG. 4 b    shows in the lower figure a cross section through the lines B-B of the upper figure. As can be seen when taking a horizontal or radial cross section through this inclined thread, the portion of the thread closer to the stator wall is thicker and thus, a section through the thread shows this section extending further than the section towards the centre. Thus, in the horizontal or radial plane, the cross section of the inclined thread forms a shape of a circle segment, which shape that does not block the blade from reaching the wall. This is in contrast to the cross section of the inclined substantially linear thread of  FIG. 2 , that leads to a “shadow” where no water reaches the wall, the shape does not form a shadow for the water blade 
       FIG. 4C  shows the tapered profile of the screw thread in the axial or vertical plane. As can be seen the profile is symmetrical about a mid line and has a substantially parabolic profile. The mid line is perpendicular to the wall of the stator. This profile provides a surface which a vertical water blade will pass over without pulling away from the thread surface and reach the edge wall and thereby inhibit any leakage of gas. 
       FIG. 5  shows a further example which illustrates the minimum permissible cross sectional profile of screw thread so that it matches the two extremes of the transmission path of a particle ejected from the nozzle. The trailing edge with respect to the direction of rotation which is clockwise in this example, has an edge that is parallel to a tangent of the inner rotating element. This corresponds to the water blade path where there is no pressure difference so particle travels along the tangent. This cross section defines the predominately parabolic vertical profile of the upper surface of the thread shown in detail C. At the other extreme of maximum pressure difference the particle travels with a radius of curvature equal to half the gap, this defines the profile of the lower surface. 
       FIG. 6  shows in more detail a radial cross section of the protrusion of minimum area, whereby the trailing edge (second edge encountered when rotating clockwise) follows the line of a water blade at zero pressure difference and the leading edge (first edge of protrusion encountered by blade) follows the form of the water blade at maximum pressure difference. 
     The protrusion shown in  FIGS. 5 and 6  provide an example of a thread, with the leading and trailing edges being defined by the path of the blades at the extreme ends of operation of the pump and the horizontal or radial cross section of the protrusion being at a minimum or lower value. The area of the protrusion can be extended by making the angle of the trailing edge more acute at the apex with the outer wall and by making the leading edge extend further in a counter clockwise direction. 
     It may for example be advantageous to make the leading edge perpendicular to the tangent of the inner element, as this corresponds to a flat lower surface of the protrusion which may make it easier to machine. This is shown in  FIG. 7 . Alternatively a symmetrical protrusion may have advantages, the blade tending to adhere to rounded surfaces, and in this case the cross section will have the shape of a circle segment form of  FIG. 4 , while the axial cross section of the protrusion will have an outer parabolic form. 
     In some embodiments the liquid openings on the inner component  10  may have a longitudinal form as shown in  FIG. 9  to provide axial blades that drive the gas along the helical path formed by the thread  25 . In other embodiments the inner component  10  may have liquid openings in a helical form to provide a helical blade. 
     Where the liquid openings have a helical form to form helical blades, then the helical form of the thread and blades progress in opposite directions, such that if the helical thread descends in a clockwise direction, the helical blades descend in an anti-clockwise direction. 
       FIGS. 8 and 9  show different arrangements of liquid openings  15  on the inner components of pumps of embodiments. In  FIG. 8  the openings  15  are arranged longitudinally in an axial direction along the inner component  10  and in operation provide longitudinal blades for sweeping gas along a path defined by protrusions on the outer component. Each blade may be formed by one longitudinal slot or by a plurality of liquid openings arranged along a length of the inner component. A plurality of blades may be provided at different circumferential positions of the inner component. 
       FIG. 9  shows an alternative embodiment where the liquid opening is a helix and provides in operation a helical blade. In the embodiment shown the helix is formed from one helical slot, while in other embodiments, it may be formed from a plurality of openings arranged along a helical path. 
     The liquid blades are formed by driving liquid through the openings. This may be done in a number of ways, by for example using a pressurised liquid source. However, in some embodiments where the liquid openings are on the rotor of the pump, the force for driving the liquid is provided by the driving mechanism used to rotate the rotor. 
       FIG. 10  shows how on rotation of a hollow rotor  10  within a liquid reservoir  30 , liquid is driven through liquid openings to form liquid blades.  FIG. 10  shows a cross section through a substantially circular hollow shaft  10  which is configured to rotate in a substantially circular stator bore  20 . The shaft forms the rotor  10  of the pump and has an outside diameter that is smaller than the stator bore  20  inside diameter. The axes of the shaft and stator are orientated vertically and the base of the hollow open ended shaft is submerged in a liquid reservoir  30 . 
       FIG. 10  shows the liquid  32  from liquid reservoir  30  rising up the shaft  10  on rotation of the rotor. The hollow bore of the shaft  10  has an internal increase in diameter  12  positioned below the liquid reservoir level which serves when the shaft rotates to accelerate the liquid through centrifugal force and pump it up the inside of the shaft then out of holes or elongated slots (not shown) in the shaft to form a contiguous liquid surface  40  between the shaft or rotor  10  and the stator inner bore  20 . The liquid flows back down the inner wall of the stator bore  20  into the reservoir  30 . This is on a continuous cycle basis, such that the liquid, in some embodiments water, that contacts the stator inner bore  20  travels down the bore under gravity and replenishes the reservoir. Note that the arrows depict the direction of flow of the liquid to create a single surface or blade  40 . 
     The liquid inside the shaft is forced through the holes/slots under centrifugal force and travels towards the stator bore to form the plurality of liquid surfaces  40 , these form blades that drive the gas through the pump as the rotor  10  rotates. 
     Although in many of the embodiments described above the liquid circulation providing the liquid surface is generated by a rotating rotor providing a centrifugal force on the liquid, in some embodiments an alternative way of generating the liquid circulation is used, namely that of a high pressure liquid source. 
     Such a high pressure liquid supply or pump could be used separately or in conjunction with regulated shaft rotation—enabling independent variability of both fluid velocity and shaft frequency according to pumping performance requirements allowing controllable efficiency and pump tuning. 
     In some embodiments, the pump may be used in a wet scrubbing environment so that the pumping function may be integrated into the wet scrubbing, the liquid blades being an advantage in such an embodiment. In this regard, by placing one of the liquid blade pumps in line with process gas flow the pump may be used for wet scrubbing in addition to vacuum generation—for example on the outlet (or inlet) of an abatement system. 
     Where a means to drive the shaft is required such as a motor and frequency inverter or belt drive, such a drive system may preferentially be positioned at the top of the shaft to reduce risk of liquid leaking into the drive means. 
     In summary, embodiments function effectively where a circulation of liquid that meets or exceeds the emission from the liquid outlets can be achieved. This helps sustain the blades as a continuous surface. It should be noted that many parameters such as the size of the liquid outlets, the type of liquid used, the liquid velocity, the distance between elements and the length of blade and the speed of rotation all affect the formation and maintenance of the liquid surfaces. Thus, these features should be selected depending on the properties required of a particular pump, such as power consumption, pumping capacity and compression. 
     Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. 
     Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.