Twin shaft pumps and a method of pumping

Abstract

A twin shaft pump may include two cooperating rotors configured to rotate in opposite directions about parallel axes of rotation; a stator comprising a stator bore in which the rotors are mounted to rotate. The stator bore includes a central part between the two axes of rotation, and an outer part outside of the two axes, the rotors being configured to have cooperating dimensions with the stator bore such that an outer edge of each rotor that is remote from the other rotor seals with the stator bore when rotating in at least a portion of the outer part. A fluid inlet is provided in the stator bore, at least a portion of the fluid inlet being in the central part of the stator bore between the axes of rotation. A fluid outlet is provide in an opposing surface of the stator bore, the fluid outlet being in the central part of the stator bore. The fluid inlet and fluid outlet are arranged such that on rotation of the rotors, the rotors each move a pumping chamber between the fluid inlet and the fluid outlet; wherein at least a portion of the fluid inlet is arranged to extend beyond the central part of the stator bore.

Claims

1. A twin shaft pump comprising two cooperating rotors configured to rotate in opposite directions about parallel axes of rotation; a stator comprising a stator bore in which the two cooperating rotors are mounted to rotate; the stator bore comprising a central part between the parallel axes of rotation, and an outer part outside of the parallel axes, the two cooperating rotors being configured to have cooperating dimensions with the stator bore such that an outer edge of each rotor that is remote from the other rotor seals with the stator bore when rotating in at least a portion of the outer part; a fluid inlet in the stator bore, at least a portion of the fluid inlet being in the central part of the stator bore between the parallel axes of rotation; a fluid outlet in an opposing surface of the stator bore, the fluid outlet being in the central part of the stator bore; the fluid inlet and the fluid outlet being arranged such that on rotation of the two cooperating rotors, the two cooperating rotors each move a pumping chamber between the fluid inlet and the fluid outlet; wherein at least a portion of the fluid inlet is arranged to extend beyond the central part of the stator bore, and wherein the fluid outlet and the fluid inlet are arranged such that an opposing outer surface of each of the two cooperating rotors moves beyond an edge of the fluid outlet prior to the outer surface of the rotor sealing with the stator bore beyond the fluid inlet, such that the pumping chambers between the stator bore and each of the two cooperating rotors is in fluid communication with both the fluid inlet and the outlet for a fraction of each rotor rotation.

2. The pump according to claim 1, wherein the fluid inlet is arranged to extend beyond the central part such that an outer edge of each of the two cooperating rotors starts to seal with the stator bore beyond the fluid inlet when at an angle of rotation of between 5° and 25° after a top dead center position, the top dead center position being a rotor position where a diameter of the rotor is perpendicular to a line joining the parallel axes of rotation.

3. The pump according to claim 2, wherein the fluid inlet is arranged to extend beyond the central part such that an outer edge of each of the two cooperating rotors starts to seal with the stator bore beyond the fluid inlet when at an angle of rotation of between 10° and 20° after a top dead center position.

4. The pump according to claim 1, wherein the fluid inlet is symmetrical about a plane mid-way between the parallel axes of rotation, and is arranged such that the fluid inlet extends beyond the central part on both sides.

5. The pump according to claim 1, wherein the fluid outlet is configured such that it smaller than the fluid inlet.

6. The pump according to claim 1, wherein the fluid outlet is arranged such that during rotation the rotor moves beyond an edge of the fluid outlet bringing one of the pumping chambers into fluid communication with the fluid outlet when at angle of rotation of between 5° and 20° beyond a bottom dead center position, the bottom dead center position being where a diameter of the rotor is perpendicular to a line joining the parallel axes of rotation.

7. The pump according to claim 6, wherein the fluid outlet is arranged such that the rotor moves beyond an edge of the fluid outlet bringing one of the pumping chambers into fluid communication with the fluid outlet at an angle of rotation of between 5° and 15° beyond the bottom dead center position.

8. The pump according to claim 1, wherein the fluid outlet is symmetrical about a plane mid-way between the parallel axes of rotation.

9. The pump according to claim 1, wherein the pump comprises a roots pump.

10. The pump according to claim 1, wherein the pump comprises a high speed pump.

11. The pump according to claim 10, wherein the pump is configured for a speed of operation of between 5,000 and 18,000 RPM.

12. The pump according to claim 10, wherein the pump is configured for a maximum velocity of a tip of the rotor during operation of between 60 and 120 m/s.

13. The pump according to claim 1, wherein the pump comprises a multi-stage pump.

14. The pump according to claim 1, wherein the pump comprises a single stage pump.

15. A method of high speed pumping comprising: rotating two cooperating rotors within a stator bore of a stator of a twin shaft roots pump in opposite directions at a rotational speed greater than 5,000 RPM, rotation of the cooperating rotors each moving a pumping chamber between a fluid inlet and a fluid outlet; starting to seal the pumping chambers from a fluid inlet when respective rotors move beyond an angle of between 5° and 25° after a top dead center position, the top dead center position being a rotor position where a diameter of the rotor is perpendicular to a line joining parallel axes of rotation of the two cooperating rotors; and starting to open the pumping chambers to a fluid outlet when respective rotors move beyond 5° and 20° of a bottom dead center position, the bottom dead center position being where a diameter of the rotor is perpendicular to a line joining the parallel axes of rotation, wherein the fluid outlet and the fluid inlet are arranged such that an opposing outer surface of each of the two cooperating rotors moves beyond an edge of the fluid outlet prior to the outer surface of the rotor sealing with the stator bore beyond the fluid inlet, such that the pumping chambers between the stator bore and each of the two cooperating rotors is in fluid communication with both the fluid inlet and the outlet for a fraction of each rotor rotation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present disclosure will now be described further, with reference to the accompanying drawings.

(2) FIG. 1 illustrates a twin shaft roots pump according to the prior art.

(3) FIG. 2 illustrates a twin shaft roots pump according to an embodiment.

DETAILED DESCRIPTION

(4) Before discussing the embodiments in any more detail, first an overview will be provided.

(5) To enable a twin shaft pump such as a roots blower to operate effectively at high tip speeds improved inlet conductance to the rotors is provided. This is achieved in embodiments by increasing the size of the inlet and thereby delaying the closing of the inlet, and in some cases correspondingly delaying the opening of the exhaust. The inlet may be delayed by more than the exhaust, so that in some embodiments both are open for a brief time. This may be acceptable for high speed operation where due to the high rotor speeds, the exhaust fluid is unable to reach the inlet during the brief period that they are both open.

(6) FIG. 1 shows a twin shaft roots pump according to the prior art. The twin shaft roots pump according to the prior art has two rotors 10 and 12, operable to rotate about parallel rotational axis 30 and 32 within a stator bore 20. Gas inlet 40 and gas outlet 50 are configured such that the edges align with the axes of rotation 30, 32, such that the point of transition between the inlets and outlets being open is the top dead centre A or bottom dead centre B positions of each rotor. Rotor 10 is shown in this position and in this position pumping chamber 15 between rotor 10 and stator bore 20 is sealed from both the inlet 40 and the outlet 50. Further rotation of the rotor in the anti-clockwise direction moves the pumping chamber 15 around to the gas outlet 50 where gas is expelled. During this rotation gas is sucked in via inlet 40 and is itself captured within a new pumping chamber 15 when the rotor 10 has moved through 180 degrees; at this point rotor tip seals just beyond the fluid inlet 40. In this way, gas is moved from the inlet 40 to the outlet 50. Rotor 12 rotates in the opposite clockwise direction and moves gas in a corresponding way.

(7) Although conventional twin shaft roots pumps are able to operate at relatively high speeds, when the speed is increased beyond a certain amount it has been found that there is not a corresponding increase in capacity. The inventor determined that this was due to problems with supplying enough gas at the inlet. In effect inlet conductance of a conventional pump is not able to supply gas at a sufficient rate for the increased pumping speeds. Embodiments of the disclosure have addressed this by providing a pump such as that shown in FIG. 2.

(8) FIG. 2 shows a pump according to an embodiment. The pump of FIG. 2 is similar to the prior art pump of FIG. 1, but the gas inlet 40 has been extended beyond the central part 60 of the stator bore that lies between the rotational axes 30, 32 into the outer parts 62 of the stator bore, which lie beyond these rotational axes 30, 32. This increase in gas inlet size provides a corresponding delay in closing the inlet and allows additional gas to be swept into the pump as the rotors rotate, providing increased inlet conductance and alleviating the limiting factor for increasing capacity with increasing rotational speed.

(9) Owing to this increase in the gas inlet size when the rotor is in the top dead centre position A as is shown for rotor 10, then at this point the inlet is open, that is there is no seal between the stator bore 20 and rotor 10, such that pumping chamber 15 is in fluid communication with inlet 40. In effect, there is an inlet delay A-A′ of a few degrees of rotation before the rotor 10 seals with the stator bore 20 when compared to the pump of FIG. 1.

(10) As can be envisaged if the exhaust were the same size as a conventional exhaust then there would be some rotational angles where the pumping chamber is in fluid communication with both the gas inlet 40 and the gas outlet 50. In this embodiment, in order to mitigate against this, the exhaust 50 has also been provided with a rotational delay B-B′ in closing, in this case by decreasing its size. Thus in the bottom dead centre position B, rotor 12 has not yet reached the exhaust or gas outlet and thus still seals with the stator bore 20 such that pumping chamber 15 is not at this point in fluid communication with the gas outlet 50. Once rotor 10 has rotated a little further beyond the angle of the exhaust delay B-B′, then gas outlet 50 will start to be opened by the rotor 12 and pumping chamber 15 will be in fluid communication with it. If the inlet and exhaust delays are matched, then the closing of the inlet will be synchronised with the opening of the exhaust and the pumping chamber will be sealed for a moment such that the inlet and outlet are not in fluid communication via the pumping chamber 15. However, in some embodiments and indeed in this embodiment, the exhaust delay B-B′ is made to be smaller than the inlet delay A-A′ such that there will be a brief moment when the pumping chamber 15 is in fluid communication with both the inlet 40 and the exhaust 50.

(11) An advantage of not matching the inlet and exhaust delay is that the gas outlet does not need to be reduced in size by as much as the gas inlet is increased in size. Although compression of the gas during pumping does allow the exhaust to be smaller than the inlet without affecting capacity, there is a limit beyond which the reduction in the exhaust may itself become a limiting factor. Thus, having a design which allows the inlet to be increased in size by more than the outlet can be advantageous. Such a design is particularly applicable for high speed operation. As can be seen the overlap in the inlet and outlet being open occurs for a few angles of rotation of the rotor in every rotation. In high speed operation this will only occur for a short amount of time, such that the time period during which there is a fluid flow path between the inlet and outlet will be small enough that latency effects and the prevailing flow direction of the gas or fluid being pumped is sufficient to avoid any significant amount of flow between the outlet and inlet. Thus, this flow path will not be detrimental to pumping performance and the advantage of an increase in inlet size, and a lower decrease in outlet size is provided. Thus in some embodiments, the inlet delay A-A′ is made to be larger than the exhaust delay B-B′.

(12) In other embodiments, and in particular for designs that are configured to operate at lower speeds, synchronising the opening and closing of the inlet and outlet such that there is a moment where the pumping chamber 15 is sealed from both inlet and outlet and no backflow path is present, may be found to be advantageous. In such a design, the gas inlet and exhaust delays will be equal.

(13) In summary to improve a twin shaft pump's high speed operation improved inlet conductance to the rotors is provided. Embodiments achieve this by creating a wider inlet, delaying the closing of the inlet, and allowing more time for the gas to enter the rotors and more area through which the gas can flow.

(14) The exhaust opening may also be delayed and this results in a narrow exhaust area, however due to the compression achieved in the pump this does not result in a conductance problem. The inlet may be delayed by more than the exhaust, so both are open for a brief time. This may be acceptable at high rotor speeds, exhaust gas being unable to reach the inlet in the short time before it has closed.

(15) Although illustrative embodiments of the disclosure have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the disclosure 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 disclosure as defined by the appended claims and their equivalents.

REFERENCE SIGNS

(16) 10, 12 rotors 20 stator bore 30, 32 axes of rotation 40 fluid inlet 50 fluid outlet 60 central part of pump 62 outer part of pump