IMPROVEMENTS IN MECHANICAL PUMPS

Abstract: A mechanical pump having a pumping chamber 10 within which is positioned a pair of intermeshing rotors 5,8, preferably of the claw type, with each rotor mounted for rotation on respective shafts and the first rotor 8 being associated with an inlet 9 to the pumping chamber 10 and the second rotor 5 being associated with an outlet 3 from the pumping chamber, wherein the second rotor 5 has a cavity 6 in its surface immediately adjacent the outlet 3.

Description: The present invention relates to mechanical pumps and in particular to mechanical vacuum pumps incorporating at least one pair of intermeshing rotors, especially rotors of the type known as "claw" rotors. When intermeshing claw type rotors are employed in mechanical vacuum pumps or compressors for use with gases or vapours which tend to condense or liquefy during the pumping or compression process, a hydraulic hammering effect is sometimes experienced. This hydraulic hammering effect is caused by the inability of the claw type rotors to expel liquid sufficiently rapidly from their swept volume. The liquid thus accumulates to give the hammering effect which can lead to mechanical failure. A typical claw type rotor mechanism is illustrated in Figures 1a to 1d of the accompanying drawings which shows a pair of rotors 5,8 mounted on respective shafts 1A,1B for rotation about the shafts in the direction shown by the arrows with the claws 7,13 closely engaging the walls of a chamber 2. Any liquid formed in the volume swept by the rotors 5,8 or from vapour or entrained in gas entering through an inlet port 9 in the wall of the pumping chamber 10, tends to move radially outwards under centrifugal force towards the stator walls 2 away from the outlet port 3 which is located in the side wall 4 adjacent the centre of the rotor 5. The claws scoop the liquid as they rotate, and the claw 7 of the rotor 8 which is associated with the inlet port 9 throws the liquid towards the outlet port 3 as it rotates from the positions 1a through 1b to 1c of Figure 1. However, in the critical position between positions 1b and 1c, the outlet port 3 is closed and this prevents expulsion of the liquid from the pumping chamber 10. The liquid is thereby trapped between the rotors 5, 8 and creates an hydraulic hammering effect which can lead to mechanical failure of the pump. The present invention is concerned with the provision a mechanical pump having at least one pair of intermeshing rotors in which any hydraulic hammering effect can be mitigated or prevented by providing a cavity in a leading edge of a rotor to retain the liquid during critical parts of the pump cycle. Although certain notches on the surface of rotors in pumps of this type have previously been divided - see US Patent Specification No. 3 472 445 and French Patent Application Publication No. 2 109 798 - none have had the ability to retain the liquid therein and have been used for different reasons. In accordance with the present invention, there is provided a mechanical pump comprising first and second intermeshing rotors (5, 8), each rotor being mounted for rotation on respective shafts (1A, 1B) and located in a pumping chamber (2), an inlet (9) to the pumping chamber with which the first rotor is associated and an outlet (3) from the pumping chamber with which the second rotor (5) is associated, a cavity (6) being present in the surface of the second rotor (5) adjacent the outlet (3) which cavity (6) communicates with the outlet during each revolution of the second rotor, and the cavity being substantially cup-shaped, characterised in that the cavity extends in a radial direction and is positioned such that, in use of the pump, liquid present in the chamber is urged into, and retained within, the cavity during rotation of the second rotor and is ejected therefrom by centrifugal forces during communication between the cavity and outlet. The invention is primarily, but not exclusively, concerned with pumps having a "claw" type rotor profile. The nature of the cavity must be such that any condensed liquid which is present in particular in the volume of gas or vapour being "swept" by the rotors is urged into the cavity and ejected therefrom when the swept volume communicates with the chamber outlet. Ideally, the pump is one in which the inlet to the pumping chamber is formed as a port in a first wall of the pumping chamber and the outlet from the pumping chamber is formed as a port in an opposite wall of the pumping chamber. Preferably the cavity is positioned in a side of the second rotor which engages the wall of the chamber containing the outlet. Most preferably the position of the cavity is such that the condensed liquid is urged into the cavity by centrifugal force. In general the shape of the cavity is not important but preferably the shape is such that condensed liquid is retained within the cavity despite the rotation of the rotor and the centrifugal forces caused thereby. This can be achieved, for example, by arranging for the cavity to be substantially cup-shaped and extending in a direction such that centrifugally driven liquid is urged to the base of the cup before being deposited in the chamber outlet. For the avoidance of any doubt, the pump may comprise a plurality of individual chambers each having its own pair of rotors, some or, preferably, all of which may have cavities in accordance with the invention. In practice, it is generally preferable for the shafts on which the rotors are mounted to be orientated vertically. It is also preferable for the inlet to be positioned in a wall in the top of the chamber and for the outlet to be positioned in a wall at the bottom of the chamber. For a better understanding of the invention, reference will now be made, by way of example only, to the accompanying schematic drawings in which:- Figures 1a to 1d are transverse cross-sectional sketches through a pumping chamber of a known mechanical pump employing intermeshing claw type rotors and illustrating different relative positions of the rotors during a pumping operation; Figures 2a to 2e are transverse cross-sectional sketches through a pumping chamber of a mechanical pump employing intermesjing claw type rotors embodying the present invention and which illustrate different relative positions of the rotors during a pumping operation; and Figure 3 is a perspective view of the rotors illustrated in Figure 2a. Figure 4 is a top view of two separate rotors for use in pumps of the invention showing differently shaped cavities therein. As shown in Figures 1a to 1d, the pumping chamber 10 of a mechanical pump contains intermeshing claw type rotors 5, 8 each mounted on a shaft (not shown) in a manner known per se . The rotor 5 rotates in a clockwise sense as indicated by the arrows whilst rotor 8 rotates in an anti-clockwise sense. When used to pump a vapour which during the pumping operation condenses to a liquid, said liquid will be trapped in the space 12 between the rotors 5, 8at a time when the outlet port 3 is closed. As a consequence, the liquid can create a hammering effect between the rotors 5, 8 which can lead to mechanical failure. Referring now to Figures 2a to 2e and Figure 3, the arrangement of the pumping chamber 10 and the rotors 5, 8 is substantially identical to that of the known mechanical pump and like parts will be identified by the same reference numerals. The rotor 5 which is associated with the outlet port 3 is formed with a cavity 6 on its surface immediately adjacent the side wall 4 in which the outlet port 3 is formed. The shape and location of the cavity 6 and its relation to the outlet port 3 is illustrated in Figures 2a to 2e and Figure 3. When the shafts on which the rotors 5, 8 are located are in the vertical orientation, the liquid tends to collect under gravity on the bottom sidewall 4 in which the outlet port 3 is located. This liquid is thrown into the cavity 6 in position 2b to 2c, in particular by the action of claw 7 of rotor 8, and under centrifugal force is discharged into the outlet port 3 as it passes over it in the position 2e back to 2a. The cavity 6 expels a quantity of liquid, each revolution, sufficient to prevent build-up of liquid in the pumping chamber to such an extent that a hydraulic lock and resulting hammering could occur in position 2b to 2c between rotors 5 and 8. Cavity 6 also forms part of the outlet passage through which pumped gases, vapours and the liquid are discharged. It will be appreciated that in a multi-stage claw type rotor pump which in its normal operative position has the axes of the rotors vertical, a cavity 6 is provided in the rotor face associated with the outlet port at each stage. In all cases, the position of the cavity is such that it does not interfere with the basic operation of the pump. Finally, with reference to Figure 4, there is shown two different shapes which can usefully be employed in pumps of the invention.

Claims: A mechanical pump comprising first and second intermeshing rotors (5, 8), each rotor being mounted for rotation on respective shafts (1A, 1B) and located in a pumping chamber (2), an inlet (9) to the pumping chamber with which the first rotor is associated and an outlet (3) from the pumping chamber with which the second rotor (5) is associated, a cavity (6) being present in the surface of the second rotor (5) adjacent the outlet (3) which cavity (6) communicates with the outlet during each revolution of the second rotor, and the cavity being substantially cup-shaped, characterised in that the cavity extends in a radial direction and is positioned such that, in use of the pump, liquid present in the chamber is urged into, and retained within, the cavity during rotation of the second rotor and is ejected therefrom by centrifugal forces during communication between the cavity and outlet. A pump according to Claim 1 in which each rotor is of the "claw" type. A pump according to Claim 1 or Claim 2 in which the shaft in which the rotors are mounted are orientated substantially vertically. A pump according to any preceding claim in which the pump comprises a plurality (two or more) of individual pumping chambers having their own pair of rotors and each second rotor of which has a cavity.

Inventors: WYCLIFFE, HENRYK
Applicants: THE BOC GROUP PLC
Application year: 1989
Main classification: F04C18
IPC codes: F04C2, F04C18