FIELD OF THE INVENTION

The invention relates to the field of claw pumps.

BACKGROUND

Claw pumps that use two cooperating rotors to move and compress fluid from an inlet to an outlet are known. A claw pump has two claw-type rotors within a stator that rotate in opposite directions to each other in a non-contact manner with a narrow clearance between the rotors. The two claw-type rotors form a pocket between them. This pocket has a larger volume when alongside the inlet, the volume decreasing as the rotors rotate towards the outlet owing to the profile of the rotors. A gas within the decreasing size pocket is compressed prior to being discharged through the outlet.

Claw pumps have conventionally been operated as dry pumps with suction, compression and exhaust being performed continuously without using either a lubricant or sealing liquid to create a vacuum condition or pressurized air. The advantage of a dry pump is that it provides clean evacuation and discharge.

Although claw pumps have many advantages, the size of these pumps is limited by the amount of cooling that can be provided to the pumps. Where larger pumps are required other designs such as liquid ring pumps may be used. Liquid ring pumps are simple and reliable pumps. However, they have a relatively low efficiency and therefore consume a lot of power.

It would be desirable to provide a claw pump that is suitable for large industrial scale applications. SUMMARY

A first aspect of the present invention provides a claw pump, comprising: a stator element; two cooperating rotor elements mounted on parallel shafts and each having a claw shaped profile; a gas feed inlet for inputting a gas flow into said pump; an outlet for outputting said gas flow from said pump; and a liquid input configured to admit liquid into said gas flow during pumping operations in order to provide persistent cooling of said pump during said pumping operations.

The inventor of the present invention recognised that claw pumps are efficient and can be used with advantage in many situations. However, owing to the problems associated with cooling them they have hitherto been limited to small scale applications. The inventor has addressed this by adding a liquid to the gas being pumped, this liquid providing effective and persistent cooling to the pump allowing pumps on a much larger scale to be produced.

Conventionally claw pumps have been operated as dry pumps and this has provided the attendant advantage of low contamination. The idea of adding a liquid to the gases being pumped, is not one readily contemplated. The introduction of liquid causes contamination of the exhaust and can, in some cases, lead to contamination of the process being pumped. Thus, conventionally where liquid has been used to clean or wash dry pumps this is not done during pumping operations but rather during a separate cleaning cycle.

Furthermore, in addition to the potential problem of contamination, adding liquid to a pump during normal pumping operations provides the potential for hydraulic lock and cavitation problems. This is a particular issue with claw pumps that operate on the principle of close clearances between the rotors. These close clearances are vulnerable to hydraulic lock if a liquid is introduced.

The inventor recognised that in many circumstances some contamination of the exhaust may be acceptable. Furthermore, many of the problems that might arise due to the close clearances could be mitigated by increasing these clearances. In this regard, as the liquid improves the sealing, it is possible to increase the clearances of a claw pump when operated with liquid present in the gas flow and still maintain or even improve the pumping efficiency. In this way potential problems with hydraulic lock may be avoided or at least mitigated. Furthermore, increasing the clearances provides the additional advantages of making the pump more tolerant to particulates and also to manufacturing tolerances. Thus, claw pumps adapted to have a liquid input and operate with liquid present in the gas flow are provided, such claw pumps may operate on a larger scale and at higher powers than conventional dry claw pumps. It should be noted that the clearances within a pump will depend on the fluids being pumped, the scale of the pump, the required pumping pressure differential and power efficiency.

However, in general, adding a liquid to the gas being pumped improves sealing and therefore allows clearances to be increased while maintaining pumping effectiveness. Typical clearances for dry claw pumps are greater than 0.05mm and less than 0.2mm, while claw pumps according to embodiments may have clearances of approximately double these values, from 0.1 mm to 0.3mm. In some larger scale pumps the clearances may rise to 0.4mm.

The liquid may be provided at the gas inlet or at another point in the pump chamber. The introduction of the liquid during pumping operations cools the pump allowing the pump to be used on a larger scale and for higher power operations. In this regard a dry claw pump is limited to operations of up to say 30KW, while a pump according to an embodiment may be used up to l OOKW and in some cases up to 500KW. The liquid is admitted continually that is either continuously or at regular frequent intervals in order to provide persistent cooling of the pump. In addition to providing cooling, persistent introduction of liquid also improves sealing of running clearances and helps in the flushing of any debris.

The cooling effect of the liquid provides more uniform temperatures throughout the pump, reducing differential expansions and helping to avoid problems such as the pump seizing. In effect a pump that can be operated on a similar scale to a liquid ring pump is provided but with a more efficient pumping mechanism and thus, lower power consumption. Although the liquid may be admitted into the pump at a number of different points, in some embodiments it is admitted at the gas feed inlet. Having the liquid inlet at the gas feed inlet avoids the need for an additional inlet to the pumping chamber, and owing to the claw pump's operation, means that liquid is sent in either direction by the rotating claws. In this regard, as the liquid is used to cool the pump it is important that the liquid is present through as much of the pumping chamber as possible and thus, admitting the liquid at the gas feed inlet and sending it in the two directions of the rotating rotors, is not only practical but also provides the benefit of cooling throughout the pumping chamber. In some embodiments, said gas feed inlet comprises a feed line and an opening into said pump, said liquid input comprising a nozzle in said feed line for delivering a spray of liquid into said gas flow.

Although the liquid inlet could have a number of forms, in some embodiments it comprises a nozzle that delivers a spray of liquid into the gas flow. This again helps distribute the liquid throughout the system which helps provide more uniform cooling and sealing of the clearance gaps.

In some embodiments, said liquid input comprises a flow rate control means configured to admit a controlled quantity of liquid into said gas.

It may be advantageous to control the amount of liquid input into the gas flow. In some embodiments the flow rate is controlled in dependence on the flow of gas. In some embodiments, the flow rate control means not only controls the amount of liquid input but also the nature of the input such that the liquid may be continuously input or it may be input continually at regular intervals. In some embodiments, said flow rate control means is configured to control said quantity to a value between 0.1 % and 2% of the swept volume of said pump, preferably between 0.1 % and 0.6% of the swept volume of the pump. The amount of liquid input to the pump should be controlled in order to provide effective cooling and in some cases sealing and debris removal. It has been found that a flow rate of from 2% down to 0.1 % of the swept volume is sufficient to provide many of the required effects and indeed a flow rate of 0.6% down to 0.1 % of the swept volume can be particularly advantageous. A flow rate of less than 0.1 % of the swept volume does not provide sufficient cooling in most cases.

In some embodiments, said flow rate control means is operable to provide a variably controllable quantity of liquid. In some cases the flow rate control means may provide a constant controlled quantity of liquid and this may be acceptable where the pump has certain predetermined operating conditions. In other cases, it may be advantageous to provide a quantity of liquid that can be controllably varied, such that the quantity of liquid can be selected according to the particular operating conditions.

The flow rate control means may comprise a number of things for example it may be a valve, a restriction through which the liquid flow and/or an orifice plate. It should be clear to a skilled person that any means of controlling the liquid flow rate could be used.

In some embodiments, said two parallel shafts run along parallel longitudinal axes, and at least a portion of said outlet is located in a radial outer position beyond a radial mid-point of said rotors. Claw pumps generally have their outlet towards a radially inner position as the outlet is opened and closed by the rotors rotating and the rotors are thickest towards the centre. However, where there is liquid admitted to the pump then this liquid will be thrown towards the outer circumference of the pumping chamber during rotation of the rotors and in order to effectively evacuate the liquid from the pumping chamber it is advantageous that at least a portion of the outlet is located in a radially outer position beyond a radial mid-point of the rotors. Preferably, beyond 80% of the radius of said rotor.

In some embodiments, said outlet is located in an axial end surface of said pump and is located such that it is intermittently covered and uncovered by one of said rotors as said rotor rotates, said at least a portion of said outlet being

intermittently covered by a radial outer portion of said one of said rotors.

In order for the rotor to effectively cover and uncover a portion of the outlet that is in a radial outer position it may be necessary to adapt the rotor. In some embodiments, said one of said rotors comprises a disc on an end of said rotor in proximity to said axial end surface comprising said outlet, said disc extending towards an outer radius of said rotor, said disc comprising an orifice extending towards an outer edge of said disc and configured such that said disc with orifice periodically covers and uncovers said outlet as said one of said rotors rotates. A practical way of adapting the rotor is to provide the rotor with a disc on the end that is in proximity to the axial end surface of the pump that has the outlet, and to provide the disc with an orifice that extends towards the outer edge of the disc so that the disc with the orifice periodically covers and uncovers the outlet as the rotor rotates. In order to periodically cover and uncover the outlet which may extend beyond 80% of the radius of the rotors, the orifice and the disc should extend in a similar way so that they co-operate with this outlet.

In some embodiments said pump comprises two outlets, one outlet located in each axial end surface of said pump such that it is intermittently covered and uncovered by one of said rotors as said rotor rotates, said at least a portion of said two outlets being intermittently covered by a radial outer portion of said one of said rotors. In order to improve evacuation efficiency it may be advantageous to have an outlet on either axial end face. Where there are two outlets one in each axial end face then in some

embodiments said one of said rotors comprises a disc on each axial end, said discs extending towards an outer radius of said rotor, said discs each comprising an orifice extending towards an outer edge of said disc and configured such that each of said discs with orifice periodically covers and uncovers a corresponding one of said outlets as said one of said rotors rotates.

Although there may only be a single outlet in one axial end face, or two outlets one on either axial end face, in other cases there may be additional outlets. The outlet(s) may each, or a subset may, comprise non-return valves which open when the pressure within the pumping chamber is greater than the pressure outside the pumping chamber and close automatically when the pressures equalise or the pressure outside the pumping chamber exceeds that within the pumping chamber. In some embodiments, said plurality of outlets are located on said radial outer surface at different positions along a length of said pump.

Alternatively and/or additionally to an outlet being on the axial end of the pump one or more outlets may be on the radial outer surface of the pump and these outlets are covered by a non-return valve. This non-return valve may be a reed valve or some other spring loaded cover.

In some embodiments, where the plurality of outlets are located on the radial outer surface, then they will be located at different positions along a length of pump. In some embodiments, the pump further comprises a liquid separation unit in fluid communication with said outlet, for separating said liquid from a gas and liquid mixture output from said pump. Having added a liquid to the gas being pumped, then in many cases the liquid may need to be separated from the output gas. This is particularly the case where the output gas is supplied for use in some process. Thus, there may be a liquid separation unit perhaps in the form of a centrifugal separator that acts on the gas output from the outlet and separates the liquid from it.

In some embodiments, said pump further comprises a recirculation unit for recirculating at least a portion of said liquid separated from said gas and liquid mixture, said liquid recirculation unit further comprising at least one of a heat exchanger and a filter.

A further reason for separating the liquid may be that it is to be recycled and used again within the pump and if this is the case the pump will comprise a

recirculation unit that recirculates at least a portion of the liquid separated from the gas and liquid mixture. The liquid recirculation unit may comprise a heat exchanger and/or a filter. In this regard, the liquid in addition to cooling and helping seal the pump may also remove debris and thus, where it is being recirculated a filter will be advantageous to remove the debris. Furthermore, as it is used to cool the pump then a heat exchanger to cool the liquid before it is re- input to the pump may be useful. In some cases, where only a portion of the liquid is recycled then a heat exchanger may not be required as the liquid may be mixed with new cooler liquid before being re-input to the pump.

In some embodiments, the pump comprises a vacuum pump. Where the pump is a vacuum pump then the liquid that is input may be water. Water has the advantage of having a relatively high heat capacity, being nontoxic and readily available. The liquid selected should be compatible with the gas being pumped and the process that the pump is used in conjunction with. Thus, where it is important that certain toxic contaminants are not introduced, water may be a good choice of liquid. In other embodiments, the pump comprises a compressor.

Where the pump is a compressor then the recirculation unit may be configured to recirculate oil. Water may not be a suitable liquid for use in a compressor as damp compressed air may not be desirable, while oil is readily removable from the compressed gas and helps lubricate the pump. It should be noted that oil may also be a suitable liquid for use with some vacuum pumps.

Although the claw pump may have rotors with different forms of claws, in some embodiments said rotors each comprise a two hooked claw profile. This is a particularly effective profile for providing compression on a volume of gas that is relatively large compared to the size of the pump.

In some embodiments, said shafts mounting said rotors extend from one end of said rotors and are supported and driven from a portion of said shaft extending from said one end of said rotors.

The mounting of the shafts may be done in a cantilevered fashion making it easier to access the rotors without disturbing the bearings, gears and motors. In some embodiments, said claw pump comprises a single stage claw pump.

Claw pumps are generally used as single stage devices as a single stage will provide compression due to the rotating gas pocket that decreases in size between the inlet and outlet. Thus, they do not require multiple stages to provide compression as is the case, for example, with a roots pump. Although claw pumps are generally used as single stage pumps and operate efficiently in this way, embodiments may be applied to different types of claw pumps including multiple stage claw pumps.

A second aspect of the present invention provides a method of operating a claw pump comprising: rotating the rotors to provide a flow of gas into said claw pump; supplying a liquid to said flow of gas being sucked into said claw pump so as to provide persistent cooling of said claw pump during a pumping operation;

outputting said liquid and gas mixture at an outlet. A method of operating a claw pump, such as a pump according to a first aspect of the present invention is provided. The rotors are rotated in opposite directions and compress and move the input fluids to an outlet. The liquid added to the gas in a persistent manner provides continual cooling of the claw pump during the pumping operation.

In some embodiments, the method further comprises separating at least some of said liquid from said output liquid and gas mixture; and recirculating at least some of said separated liquid. In some embodiments, said pump is operated as part of a process and said liquid is a liquid compatible with said process.

Although the liquid can be any type of liquid that is compatible with the process and provides the required cooling, in some embodiments the liquid comprises one of water, ethanol or oil. Each of water, ethanol and oil have different advantages and may be applicable to different operations of the pumps. Water has a high heat capacity, is non-toxic, readily available, cheap and easy to dispose of. Oil has high lubricating properties and is relatively easy to separate from the exhaust gas, while ethanol may be suitable choice where the process is such that ethanol vapour may be evolved from the process. In some embodiments, said step of supplying said liquid comprises supplying said liquid in a quantity of between 0.1 % and 2% of said swept volume of said pump, preferably between 0.1 % and 0.6%. 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.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 shows in cross section a claw pump according to the prior art;

Figure 2 shows in cross section a claw pump and the rotors of a claw pump according to an embodiment;

Figure 3 shows a longitudinal section through the pump of Figure 2;

Figure 4 shows a cross section through a pump according to an embodiment including a radially mounted outlet; and

Figure 5 schematically shows a pump including a liquid separation and recirculation unit.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided. Embodiments provide a claw pump suitable for large scale industrial applications. A liquid, which may be water but could be any process compatible liquid is injected into the pump, preferably at the inlet, to seal the running clearances, cool the mechanism and flush debris from the pump. Owing to the presence of liquid within the operating pump the running clearances can be increased relative to a dry pump and this makes the machine more tolerant to particulates passing through the pump and to variations in the manufacturing tolerances. It also makes it less susceptible to hydraulic locking.

The cooling that the liquid provides leads to a more uniform temperature throughout the pump which reduces differential expansions and thus potential seizing of the pump. As the liquid is very effective at cooling the pump

embodiments allow much larger pumps to be designed than would be the case with a dry claw pump. Furthermore, the liquid flushes particulates and debris through the pump keeping the pump clean. Typically the amount of liquid injection would be between 0.1 & 2% of the swept volume of the machine preferably between 0.1 & 0.5%.

In some embodiments, the liquid is separated from the gas once the fluid has been output from the pump and the separated liquid is re-circulated perhaps after filtering and heat exchange. Embodiments use a two hook claw design as this does not require an inlet valve and has good inlet conductance. Other claw designs could also be used. The claw mechanism is important as it is able to compress a gas volume before it is exhausted through a valve system. This greatly increases the efficiency of the pump when compared with non-compression stages such as a roots mechanism.

Figure 1 shows a claw pump according to the prior art. Claw pump 10 is a single stage two hook claw pump having two rotors 40, 42 which rotate in opposite directions and are mounted on parallel longitudinal shafts within stator 80. The pump 10 has an inlet 20 for the input of gas and the rotors 40, 42 rotate forming a pocket between them which carries the gas from the gas inlet 20 towards a gas outlet 30. Owing to the shape of the rotors the pocket holding the gas becomes smaller as they rotate thereby compressing the gas. The gas is forced out through outlet 30 as rotor 40, 42 pushes over the top of the outlet.

Rotor 42 acts to cover and uncover outlet 30 as it rotates. Outlet 30 is located in a radially inner position of the pump as at this position rotor 42 is wide and can act to close and open a relatively large opening 30. The opening 30 should be relatively large in order to provide efficient evacuation of the pump. Figure 1 also shows a view along the length of the rotor. Figure 2 shows a pump 10 similar to that of the prior art but adapted according to an embodiment. Gas inlet 20 which is located substantially centrally between the two rotors and extends along much of the longitudinal axis of the pump, has liquid inlet 22 projecting into the gas flow. Liquid inlet 22 comprises one or more nozzles for spraying liquid into the gas flow as it arrives in the pump. Rotor 42 has a disc 50 on its end next to the axial end surface of the pump housing which comprises the pump outlet 130. The presence of this disc allows outlet 130 shown in a dotted line on the axial end face of the pump housing to be

significantly larger than the outlet 30 of the prior art and yet still be periodically closed by the rotation of rotor 42. In addition to its increased size outlet 130 extends from a radially inner position to a radially outer position and this is also made possible by the presence of the disc 50 which itself extends to this radially outer position. In effect the presence of the disc means that the outlet 130 is no longer confined by the shape of the rotor and in particular can extend beyond a radially inner position. In this way, the outlet 130 can be enlarged and can be extended to the radially outer portion of the axial end surface. An enlarged opening provides for more effective evacuation of the pumped fluid and in particular, by extending the opening to a radially outer position, liquid within the gas and liquid mixture that is thrown towards the radially outer position will be effectively evacuated. Although only one outlet 130 at one axial end of the pump is shown in this Figure, in some embodiments an outlet may be provided at both axial ends. In such a case each end of the rotor 42 will comprise a disc 50. The operation of the claw pumps of both Figures 1 and 2 provide a port 30, 130 on an end face that is periodically covered and uncovered by one of the rotating rotors, such that the rotor acts as an exhaust valve for the pump. As seen in Figure 1 the port 30 was conventionally positioned at the inner radius of the rotor and not near the outer radius of the rotor. Although this worked for previous applications, in embodiments where liquid is introduced into the pumped fluid then in order to increase the volume of liquid that can be discharged it is advantageous if the port is close to the outer radius of the rotator as this is where much of the liquid is during rotation of the claw. Such a port 130 is achieved by the embodiment of Figures 2 and 3 where a disc 50 is mounted on one end of the rotor that covers and uncovers the port. As can be seen from both Figure 2 and Figure 3 the disc on rotor 42 is relatively thin and passes over the top of rotor 40. The disc extends to the outer radius of the rotor and allows the port 130 also to extend this far. This not only increases the port area but gives a substantial area close to the outer radius thereby increasing the volume of liquid that can be expelled. The large port area also allows greater gas flow which enables pumps in some embodiments to operate with a single port thus simplifying the design. As can be seen from Figure 3 the claw rotors are cantilevered from a gear box making the system easy for servicing and cleaning.

Figures 2 and 3 show one embodiment where the outlet 130 has been changed to improve liquid evacuation. Figure 4 shows an alternative embodiment where the outlet 30, 130 in the end face is replaced by one or more additional outlets 32 provided along the length of stator 80, each outlet having a reed valve 34. In other embodiments these outlet(s) 32 may be used in conjunction with an outlet 130 in the end face.

This embodiment has an advantage over a fixed port design in that the valves can open when the pressure in the swept volume reaches exhaust pressure which avoids any significant over compression of the gas, which may otherwise happen during high flows situations where the outlet is a fixed port. A further advantage is reduced noise level as backflows or sudden expansion of exhaust gas is not generated.

In general reed valves or in fact any non-return valves are not used on dry pumps as they become very hot in the exhaust gas flows and suffer from fatigue. In this design, the liquid cools the reeds and cushions their movement and improves the sealing making a reed valve a good choice for exhaust valve where a liquid has been added to the pumped gas. Where these pumps are used as vacuum pumps they may be used to produce a vacuum of down to 20 millibars with water cooled to a temperature of 15°C being used as the cooling liquid or down to 80 millibars where the input liquid is at a temperature of 40°C. Figure 5 shows a pump system comprising a pump 10, a liquid separation unit 60 and a re-circulation unit 70. The liquid separation unit 60 is connected to the exhaust of the pump to remove liquid from the exhaust gas output from the pump 10. The liquid separation unit 60 is a known liquid separation unit and may use a separation mechanism such as a centrifugal separator to remove the liquid.

The liquid separation unit 60 may be used on its own in such a pump system or it may be used in conjunction with a liquid recirculation unit 70. A liquid

recirculation unit may be particularly advantageous where the liquid has significant supply and/or disposal costs associated with it. Where recirculation is used, a filtering mechanism may be present in the liquid recirculation unit to remove any debris. One or more heat exchangers may also be used to cool the liquid before re-use in the pump. As the liquid is used to cool the pump and as it also helps to remove debris then a filter and heat exchanger in the recirculation unit help to improve pump performance where the liquid is recirculated.

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.

REFERENCE SIGNS

10 claw pump

20 gas inlet

22 liquid inlet

30, 32, 130 outlet 34 reed valve

40, 42 rotor

50 disc

60 liquid separation unit 70 liquid recirculation unit 80 stator