GB2215403A - Rotary compressors - Google Patents

Abstract

A rotary compressor of Wankel type comprises an epitrochoidal stator (4) and a trochoidal rotor (14) which rotates within the stator about its own axis which itself rotates about an axis through the centre of the stator. The tips (16) of the rotor contact the stator defining three compression cells whose volume progressively increases and decreases as the rotor rotates. Each compression cell communicates with an inlet (20) as its volume increases and with an outlet (24) including a non-return valve (26) as its volume decreases. Leakage between ares of high and low pressure is prevented by movable seals (21) mounted in slots (19) at the stator cusps and biased by springs (23) into sliding contact with the rotor. Leakage may be further prevented by the provision of movable seals at the tips of the rotor or by injecting oil at 34 into the air being compressed. The oil is subsequently separated from the compressed air and recycled. <IMAGE>

Description

ROTARY COMPRESSORS The present invention relates to rotary compressors and is concerned with that type of compressor which is of oil sealed type, that is to say the type in which oil is injected into the compression space and is subsequently removed from the compressed air and then reused.

Various types of positive displacement oil sealed rotary compressors are known of which one is the socalled sliding vane eccentric rotor compressor in which a plurality of vanes are slidably accommodated in equispaced longitudinal slots formed in the periphery of a rotor which is mounted to rotate about an eccentric axis within a cylindricaistator. The outer edges of the vanes are maintained in contact with the inner surface of the stator and divide the crescent-shaped space between the rotor and stator into a plurality, typically eight, of compression cells whose volume progressively increases and then decreases as the rotor rotates. An air inlet and outlet are positioned respectively to communicate with each cell as its volume increases and when its volume has reached its minimum value.

Oil is injected into each cell as its volume is increasing and serves not only to lubricate the vanes but also to form a seal between the vanes and the stator and between the vanes and the two end plates which close the ends of the stator and also to remove the heat which is generated by virtue of friction and by virtue of the compression to which the air is subjected.

The vanes are maintained "in contact" with the stator by centrifugal force but they are in practice spaced from the stator by a small clearance gap which is occupied by oil which,sels the gap. The width of this gap is determined solely by the magnitude of the centrifugal force, i.e. by the speed of rotation of the rotor and by the properties of the oil. At the typical speed of rotation of the rotor of 1500rpm the width of the clearance gap is typically of the order of 10Pn This is considerably less than is required for sealing purposes bearing in mind the presence of the oil and results in considerable power being dissipated by virtue of friction at the tips pf the vanes.

Frictional energy is also dissipated at the base and sides of the vanes by virtue of friction with the slots within which they reciprocate radially.

At a speed of 1500 rpm typically about 10% of the input energy is dissipated by friction with the vanes and it is found that this percentage increases at a rate greater than the square of the speed of rotation of the rotor. Thus whilst it is frequently desirable to be able to operate such a compressor at an increased speed the substantially increased frictirnilpower loss which would be incurred effectively renders this impossible on the grounds of efficiency which is an increasingly important consideration in the field of compressors.

If it is required to reduce the output from a sliding vane eccentric rotor compressor it is possible simply to throttle the inlet. However, this does not reduce and in fact slightly increases the pressure differentials across the vanes and the total power consumed by the compressor running on, say, 10% load is well in excess of 50% of full load power. It is possible to reduce the power consumed more substantially when running on reduced load but this considerably increases the capital cost of the compressor and necessitates complex control technology.

Accordingly it is an object of the present invention to provide a rotary compressor which can be operated at a higher speed than eccentric rotor sliding vane compressors without incurring an efficiency penalty and whose compressed air output can be altered by means which are both simple and cheap and do not necessitate the incorporation of complex electronic or mechanical controls.

Rotary piston or "Wankel" type compressors are known and it is to this type of compressor that the invention relates. The invention thus relates specifically to a rotary compressor comprising a stator within which is a rotor connected by gearing to a shaft which is mounted to rotate about a fixed axis, the rotor being rotatable about a movable axis which is itself rotatable about the fixed axis whereby rotation of the shaft results in rotation of the rotor about both the fixed and movable axes, the rotor having two or more lobes which together with the stator define two or more compression cells and whose tips remain in sealing contact with the stator whereby rotation of the rotor results in a progressive increase and then decrease in the volume of each compression cell, the compressor having an inlet arranged to communicate with each compression cell whilst its volume is increasing and an outlet arranged to communicate with each compression cell whilst its volume is decreasing, the outlet including a non-return valve arranged to permit air to leave the stator only when the pressure upstream of the valve exceeds that downstream of the valve.

Known compressors of this type all have a relatively low efficiency which is caused by leakage of gas through the gap between the surface of the rotor and the or each cusp. It will be appreciated that, in use, there is a substantial pressure differential across the or each cusp and that whilst there is a reliable seal when a tip seal on the rotor is passing across the cusp this is not the case at all other times.

It is thus a further object of the present invention to provide a compressor of the type referred to in which leakage across the or each cusp is substantially eliminated whereby the efficiency of the compressor is substantially higher than that of known compressors of this type.

According to one aspect of the present invention the compressor is of oil flooded type and includes an oil sump, means for injecting oil from the sump into each compression cell whilst its volume is increasing and means for removing substantially all the oil from the compressed air and returning it to the sump.

According to a further aspect of the present invention, which may be provided additionally or alternatively to the first aspect, the compressor includes a movable seal in the stator at the or each cusp which is biased into sliding contact with the surface of the rotor.

The compressor in accordance with the present invention thus incorporates a rotor/stator unit of generally similar type to that used in a Wankel engine and may therefore be termed a planetary rotation rotary piston compressor. Wankel engines are of course well known per se but they have suffered from substantial reliability problems due to the difficulty of providing a reliable tip seal between the tips of the rotors and the interior of the stator.

These problems have been largely solved in recent years but only with considerable complexity and at considerable expense by providing complex multi-part tip seals. The ends of a Wankel engine are closed by end plates and in order to seal the piston with respect to the end plates it is necessary to provide complex arcuate seals set into grooves in the end faces of the piston and the machining of these grooves and the positioning of the seals has been found to be extremely complex and expensive. Whilst it is of course known that various engines can be run as compressors or pumps and vice versa it is found that the injection of oil into the compression spaces of the compressor in accordance with the present invention substantially eliminates the problems which have been experienced with Wankel engines. Thus the seals which reliably seal between the tips of the lobes and the stator and between the stator and the end plates are unnecessary since the presence of a relatively large volume of oil ensures that a reliable seal is maintained even if the clearance between the components to be sealed is as much as 0.1mum. Accordingly, the manufacturing tolerances can be very much greater than in a Wankel engine and the complex seals can be eliminated and the relatively large clearances will nevertheless be reliably sealed by the oil.

The clearance between the tips of the rotor loads and the stator is determined solely by the geometry and dimensions of these components and is not altered by the speed of rotation of the rotor, as in the case of a sliding vane eccentric rotor compressor By virtue of the fact that these clearances are substantially larger than in a sliding vane eccentric rotor compressor and remain so at all speeds the frictional losses are very much smaller and the compressor can be operated at a very much higher speed, eg. 3000 rpm or faster without incurring any disproportionate efficiency penality as regards increased frictional lusses thereby obtaining the advantage of increased output from the same unit.

The rotor may be of two lobe type or it may have four or more lobes but it is preferred that it has three lobes in which event it will be of trochoidal shape and the stator of epitrochoidal shape. In general, the rotor will have n lobes and the stator n-l cusps which together define n compression cells. There will in general be n-l inlets and n-l outlets, an inlet and an outlet being situated adjacent one another but on opposite sides of a stator cusp,. In a Wankel engine no importance -is placed on the reliability of the seal between the cusps and the rotor but in the construction of the present invention such a seal is of the greatest importance since the pressure differential across it will be equal to the full output pressure of the compressor.It is therefore preferred that the peripheral surface of the rotor is smooth and continuous between the tips of the lobes to facilitate the maintenance of a reliable seal and it will be appreciated that this is in stark contrast to a Wankel engine in which the peripheral surface of the rotor is generally recessed to provide a minimum volume for each cell, that is to say, a minimum volume of the combustion chamber.

As mentioned above, the or each outlet includes a non-return valve arranged to permit air to leave the stator only when the pressure upstream of the valve exceeds that downstream of the valve. This is in sharp contrast to a sliding vane eccentric rotor compressor in which the stator outlets are permanently unobstructed.

In the latter type of compressor the pressure within each compression cell progressively rises to a value somewhat above the delivery pressure of the. compressor and is then abruptly brought into communication with the discharge space of the compressor as the leading vane defining it passes over the stator outlet ports. However, it will be appreciated tat in a compressor in accordance with the present invention each compression cell is inherently in communication with an outlet over substantially the entire time during which its volume is decreasing. Without the presence of a nonreturn valve, air would flow backwards into each compression cell at the beginning of each compression phase and then have to be recompressed.

Each inlet may constitute a single inlet port or a plurality of inlet ports spaced apart over the length of the stator or a single elongate port which extends over the majority or all of the length of the stator.

Similarly, each outlet may constitute a single valved port or a plurality of valved ports or a single elongate valved port which extends over the majority or all of the length of the stator, The inlet and/or the outlet may aleternatively be positioned in one or both of the end plates which close the stator. The piston of a Wankel engine generally has a short axial length but it will be appreciated that having regard to the substantially lower pressures which are involved the rotor of a compressor in accordance with the present invention may be rather longer, e.g.

in excess of 20 cm or 30 cm. Its ends will of course be closed by two end plates which are sealed to the end faces of the rotor by virtue of the oil which is continuously present within the stator. The volume of oil injected into each compression cell is typically between 1 and 2% of the maximum volume of that cell.

The oil sump may comprise a separate container remote from the stator but it is preferred that the stator is within a casing whose lower portion constitutes the oil sump. -The oil may be injected into the stator by any conventional means, such as a separate oil pump, but it is preferred that it is injected by virtue of the pressure differentials existing within the compressor. It is therefore preferred that the oil in the sump is subject, in use, to the compressor delivery pressure which serves to force the oil from the sump and through one or more oil injectors in the stator wall.

As mentioned above, by virtue of the presence of the oil which serves to maintain a reliable seal between the tips of the lobes and the stator and between the stator cusps and the rotor and between the rotor end faces and the end plates no complex sealing arrangements are required.

In the simplest form of the invention the peripheral surface of the rotor including the tips of the lobes comprises a single component, e.g. it is machined from a single casting.

The means by which the rotor is geared to the central shaft, e.g. an internally tothedor splined ring, may constitute an integral part of this component or a separate insert secured in position.

However, it is also possible for the tips of the rotor lobes to carry a separate seal but in this event the seal may be simpler than that in a Wankel engine. Alternatively, the tip of each rotor lobe may have a longitudinal slot formed in it in which there is a sliding vane generally similar to that in a sliding vane eccentric rotor compressor. These vanes will maintain a reliable seal with the stator by virtue of the centrifugal force to which they are subjected but their use will not produce the same problem as regards frictional losses as in an eccentric rotor sliding vane compressor.The reason for this is that the vanes need have only a very small radial dimension, e.g. less than 10 or 5 mm, and by virtue of the fact that they will therefore be subjected to a substantially reduced torque as a result of the pressure differential between adjacent cells to which they are exposed they may also have a substantially reduced peripheral dimension.

This means that the mass of the vanes will be very substantially less than that in a sliding vane eccentric rotary compressor and thus the magnitude of the centrifugal force to which they are subjected will also be very substantially less. Thus at a given speed of rotation the contact pressure between the vanes and the stator wall will be substantially reduced.

As mentioned above, as an alternative or an addition to the injection of a substantial volume of oil there may be a seal at each cusp which is biased into contact with the surface of the rotor. This seal may again be of a form similar to a vane of a sliding vane compressor and accommodated in a slot in the stator but it will have to include biasing means, such as a spring behind it in the slot, to press it into contact with the rotor. The provision of such a seal will eliminate the primary cause of inefficiency in such compressors, namely leakage between the cusps and the rotor. However, it is still desirable to have some oil, albeit a smaller amount, in the compression space both for lubrication purposes and to promote a seal between the rotor and stator.

It is preferred that the inlet of the compressor includes one or more throttle valves arra-nged to throttle the inlet and thus, in use, to reduce the outlet of the compressor. The throttle valve may be manually actuatable but is preferably automatically actuated in dependence on the load to which the compressor is subjected. The magnitude of this load is indicated by the magnitude of the compressor output pressure since as the load falls the output pressure rises. Accordingly, the throttle valve is preferably actuated by a servo valve which is subjected to the output pressure in a manner similar to the known unloader valve in the sliding vane eccentric rotor compressor. If the inlet is throttled a smaller mass of air is admitted into each compression cell and the output of the compressor is therefore reduced.By virtue of the presence of the non-return valve at the outlet the compressor therefore performs less work and the energy consumed by the compressor is therefore a simple function of its output in stark contrast to the sliding vane eccentric rotor compressor in which the power consumed decreases only slightly as the output is reduced.

Further features and details of the present invention will be apparent from the following description of one specific embodiment which is given with reference to the accompanying diagrammatic transverse sectional view of a compressor in accordance with the present invention.

The compressor comprises an outer casing 2 within which is a fixed stator 4 of epitrochoidal shape.

Extending centrally within the stator 4 is a shaft 6 which is rotatable with respect to the housing and, in use, is connected to a drive motor (not shown). The end of the shaft 6 carries splines 8 which mesh with similar splines 10 formed on the interior of an aperture 12 centrally disposed in an elongate trochoidal rotor 14. The tips 16 of the three lobes of the rotor are each spaced 0.1 mm or less from the wall of the stator. The rotor and stator together define three spaces or compression cells. Rotation of the shaft 6 causes rotation of the rotor 14 about an axis which itself rotates about the axis of the shaft 6 by virtue of the engagement of the splines 8 and 10.

During the rotation of the rotor the spacing of the tips 16 of the lobes of the rotor from the stator wall remains substantially constant.

The internal wall of the stator 4 has two opposed cusps 18. Downstream of each cusp in the direction of rotation of the rotor, which is anticlockwise in the Figure, is an inlet port 20 which communicates with the atmosphere via a respective filter 22. It will be appreciated that the two filters may of course be combined into a single filter. In a corresponding position immediately upstream of each cusp is an outlet port 24 which is provided with a non-return valve 26 which is arranged to permit compressed air to leave the stator only when the pressure in the compression cell communicating with the valve is greater than the pressure prevailing within the interior of the casing 2.

Situated substantially at each cusp 18, there is a slot 19 in the stator which slidably accommodates a blade or seal 21 which extends the entire length of the stator. Situated behind each seal 21 is a spring 23 which presses it into sliding contact with the rotor.

The seals 21 maintain a seal with the rotor and prevent gas leakage from the high pressure outlet side to the low pressure inlet side.

As the rotor rotates the volume of each compression cell successively progressively increases and then decreases. Shortly after the tips 16 of each lobe passes a cusp 18 the compression cell behind it comes into communication with an inlet port 20. The volume of the compression cell then progressively increases and air is drawn into it through the inlet port. As the volume of the cell then begins to decrease its communuication with the inlet port is interrupted and it begins to communicate with an outlet port 24.By virtue of the progressively reducing volume of the cell the pressure within it progressively increases and when this pressure becomes equal to or slightly greater than that in the interior of the casing 2 the non-return valve 26 opens and the air is progressively forced into the interior of the casing 2 whereby a further reduction in the volume of that compression cell produces an increase in pressure of the air within that cell and within the casing 2.

The lower portion of the casing 2 constitutes an oil sump containing a volume of oil 28. The sump communicates via a pipe 30 with an oil cooler 32 which in turn communicates via a further pipe (not shown) with two oil injectors 34 or series of longitudinally spaced oil injectors situated about 450 upstream of the two outlet ports 24. In use, the pressure within the casing 2 acts on the oil 28 and forces it via the cooler through the injectors 34 into the interior of the stator.

At the upper end of the casing 2 is a tubular oil separating or coalescing element 36, e.g. of glass fibre material, whose lower end is closed by a plate 38 which defines an oil collection space 40. The oil collection space communicates via a pipe 42 with the inlets of the compressor. The space within the coalescing element 36 communicates with the output line 44 of the compressor via a minimum pressure valve 46 including a piston 48 biased into the closed position by a spring 50 whose characteristic is such that the piston 48 will only open to permit air to leave the casing once the pressure within the casing has reached a minimum value of e.g. 2 bar.

As mentioned above, oil is injected into each compression cell during rotation of the rotor and the injectors are so positioned that oil is so injected during a major proportion of the time that each cell is increasing in volume and a major proportion of the time during which it is decreasing in volume. The volume of oil is such that it occupies the clearance between the tips 16 of the lobes and the rotor 4 thereby effecting a reliable seal at these positions. As each tip 16 moves around the stator it constantly "wipes" oil from the stator wall thereby ensuring that sufficient oil is always present at the desired positions for lubricating and sealing purposes. Similarly, the seals 21 "wipe" oil from the peripheral surface of the rotor which is smooth and continuous to effect a reliable seal. The oil also removes from tne stator heat which is produced by virtue of the compression and thus renders the compression more nearly isothermal and thus more efficient. After the compressed air with entrained oil droplets leaves the outlets 24 it flows towards the coalescing element 36. A substantial proportion of the oil droplets fall downwardly towards the sump 28 or coalesce on the wall of the stator or the casing and trickle down to the sump and the remainder are coalesced as the compressed air flows through the element 36 and collects in the oil collecting space 40.

By virtue of the pressure differential between the interior of the casing and the inlet the oil in the space 40 is returned through the line 42 into the stator through an inlet port 20. The air leaving the compressor outlet 44 is thus substantially oil-free.

By virtue of the fact that there is a relatively large and substantially constant clearance between the tips 16 of the rotor and the stator wall which is occupied only by oil the energy dissipated due to friction is relatively small and the compressor may be run at high speeds without disproportionately increasing the frictionally dissipated energy.

If the compressed air load to which the compressor is subjected is less than the normal throughput of the compressor its speed of rotation may be reduced or its inlet may be throttled. For this purpose, the two inlets 20 are provided with respective throttle valves 52 which are conveniently automatically actuated in dependence on the pressure within the casing by a servo valve (not shown).

In a modified construction which is not illustrated, the tips 16 of the rotor are each provided with a shallow radial slot which slidably accomodates a sealing vane which is maintained in sealing contact with the stator by centrifugal force. Additionally, the end faces of the rotor are provided with arcuate slots which extend between the rotor tips 16 and accommodate outwardly biased sealing strips which maintain a sliding seal with the end plates of the stator. The provision of these additional seals will improve the efficiency of the compressor and/or permit the volume oil present in the stator to be considerably reduced.

Claims (10)

1. A rotary compressor comprising a stator, whose internal wall has one or more cusps within which is a rotor connected by gearing to a shaft which is mounted to rotate about a fixed axis, the rotor being rotatable about a movable axis which is itself rotatable about the fixed axis whereby rotation of the shaft results in rotation of the rotor about both the fixed and movable axes, the rotor having two or more lobes which together with the stator define two or more compression cells and whose tips remain in sealing contact with the stator whereby rotation of the rotor results in a progressive increase and then decrease in the volume of each compression cell, the compressor having an inlet arranged to communicate with each compression cell whilst its volume is increasing and an outlet arranged to communicate with each compression cell whilst its volume is decreasing, the outlet including a non-return valve arranged to permit air to leave the stator only when the pressure upstream of the valve exceeds that downstream of the valve, the compressor including a movable seal in the stator at the or each cusp which is biased into sliding contact with the surface of the rotor.

2. A compressor as claimed in claim 1 in which the rotor is of trochoidal shape and the stator of epitrochoidal shape and that stator has two peripherally spaced inlets and two peripherally spaced outlets.

3. A compressor Nas claimed in claim 1 one claim 2 in which the peripheral surface of the rotor is smooth and continuous between the tips of the lobes and remains in sealing contact with the cusps of the stator wall.

4. A compressor as claimed in any one of the preceding claims in which the peripheral surface of the rotor including the tips of the lobes comprises a single component.

5. A compressor as claimed in any one of claims 1, 2 and 3 in which the rotor carries a movable seal at each tip which is biased into sliding contact with the surface of the stator.

6. A compressor as claimed in any one of the preceding claims in which the inlet includes a throttle valve arranged to throttle the inlet and thus, in use, to reduce the output of the compressor.

7. A compressor as claimed in any one of the preceding claims, including an oil sump, means for injecting oil from the sump into each compression cell whilst its volume is increasing and means for removing substantially all the oil from the compressed air and returning it to the sump.

8. A compressor as claimed in claim 7 in which the stator is contained within a casing whose lower portion constitutes the oil sump.

9. A compressor as claimed in claim 7 or claim 8 in which the oil in the sump is subject, in use, to the compressor delivery pressure which serves to force the oil from the sump and through one or more oil injectors in the stator wall.

10. A rotary compressor substantially as specifically herein described with reference to the accompanying drawing.