WO1983001491A1

WO1983001491A1 - Rotary piston compressors and expanders

Info

Publication number: WO1983001491A1
Application number: PCT/BR1982/000015
Authority: WO
Inventors: Roger Boyd Walker
Original assignee: Roger Boyd Walker
Priority date: 1981-10-16
Filing date: 1982-10-15
Publication date: 1983-04-28

Abstract

A rotary piston compressor which, due to its geometry, has eliminated virtually all dead space and therefore is of increased efficiency and has a high capacity for handling liquids in liquid/gas mixtures. The specification describes the compressor, together with air and vapour cycle refrigeration machines which utilize the high liquid capacity to cool the working fluid by liquid mixing. There is also described a liquid separator which may be incorporated within the machine itself, and a device for controlling the compression ratio of the compressor by means of a variable unloading disc set into the compressor casing end wall.

Description

Title: "Rotary Piston Compressors and Expanders".

The present invention concerns rotary piston positive displacement compressor and expander machines, and is particularly concerned with improvements in the efficiency of such machines and the use of such machines in air and vapour-cycle refrigeration circuits.

The devices considered the most advanced of their type are described in US Patents 3,790,315, 3,799,712 and UK Patent 1,321,485, to Atlas Copco Aktiebolag, and US Patents 3,472,445, 3,535,060, and 4,224,016 to A.Ξ. Brown. All these compressors deliver the compressed air through ports in the parallel side walls of a figure-of-eight shaped compression chamber, which ports are uncovered by the rotation of the main rotary piston. The air being compressed moves parallel to the side wall until - expelled through these ports, which normally have to be rather small to provide a reasonable compression ratio, and have to be sharp edged. This inevitably implies that the air is subject to an abrupt 90° change of direction through sharp edged, usually small ports, causing turbulence and pumping losses that increase markedly with rotation velocity.

In addition to the above problems, these compressors have to deliver, laterally, the last part of the compressed air trapped between the rotary pistons at an extremely high velocity; that is if the rotary pistons are to have any reasonable width, and are to turn at the high speeds desirable for their efficient operation. The result is that there tends to be a rapid, and wasteful pressure build-up in the chamber at the end of delivery which has been relieved by cutting notches in the main rotor, at the base of the teeth. These notches, which indeed serve to reduce these throttling losses, do, however, introduce αeaα spaces into the compressor geometry in which compressed working fluid is tranferred to the low-pressure side where it expands without doing any useful work and dissipates the compressio energy as heat. An alternative approach has been to make the compressor piston, rotors thin, which reduces these throttling problems at the cost of a proportional reduction in the fluid handling capacity of the compressor. For example, the capacity of the above compressors is of the order of about one third of that of a machine of the same size constructed according to the teachings of the present inventio . An. additional consequence of this same throttling effect is that this type of compressor tends to be very sensitive_,, to the presence of. liquids in the fluid flow, whi produce a destructive liquid shock at the end of delivery unless some part of the active piston is relieved by cuttin notches, or chamfering. This solves the liquid shock pr^oblem but again at the cost of losses in dead spaces or b leakage.

An alternative form to the above compressor is to employ a main rotary piston on a hollow shaft. The compresse fluid is delivered through holes or slots cut in the wall o main rotary piston, at the base of the working tooth, or teeth. This configuration effectively solves the problems caused by the forced lateral movement of the fluid at the end of delivery, although at the cost of dead spaces in the ports in the wall of the main rotary piston, where pockets of compressed fluid are wastefully carried over to the low pressure side of the compressor. A compressor such or described in US 3,545,895 would also suffer throttling loss in the holes or slots cut in the rotor shaft, which are necessarily restricted in their area of cross-section by th nature of the compressor's geometry, as well as by structur considerations. A solution to the problems posed by the inherent limitations in the size of the slots in the wall of the shaft of the main rotor is implicit in the proposal for a projection on the gate rotor which enters into and effectively closes the slot in the wall of the hollow main rotor. However in this document CLabus, 1976, US No. 3,945,777) there is no disclosure, and no teaching, that even suggests the application of this idea to a compressor or expander without dead spaces and with smooth fluid flow at all points. These are essential elements of the present invention.

In the present invention the projections on the gate rotor piston enter into and pass through the slots in the wall of the rotary piston, which provides a greater sealing area between the gate rotor projections and the stationary sleeve valve. This feature does not exist in prior art.

The objects of the present invention are composed of several aspects, the first of which is to provide a parallel axis rotary piston compressor or expander that: (1) is oil free in its working area; (2) has no dead spaces whatsoever; C3) does not oblige the working fluid to undergo any abrupt changes in direction; (4) has not point that in any way restricts the smooth flow of the working fluid; C5) is able to accept relatively large proportions of liquid mixed with the compressible fluid without suffering any liquid shock, even at high rotational speeds; (6) has an improved sealing geometry.

In order to derive maximum performance in heat cycle applications of the present machines, a second aspect of the invention is to incorporate into the machine itself a separator for liquid entrained in the working fluid. As a mixture of compressed fluid and liquid is delivered to the sleeve valve inside the hollow main piston shaft, it is caused to swirl by the internal shape of the sleeve valve.

The swirling compressed fluid, while still inside the sleeve valve, is then passed through a smoothly contoured throat which,_, by the well-known cyclone effect, increases the rotational velocity of the fluid and separates liquid droplets from the gas as vapour flow. The rapidly swirli compressed fluid is then passed through a suitably shaped chamber to recover most of the swirl energy. The applica is unaware of any previous attempt to incorporate such a liquid separator into a rotary piston compressor.

A third aspect of the invention, relating to th control of the flow in a rotary piston machine, concerns or more rotatable discs or rings, or segments thereof, located in the end walls of such compressor. Brown in 19 US. 3,723,031, Nilsson, in 1969, US. 3,468,294 and Schibb in 1963, US 3,108,740 disclose such discs, which all cont the flow in piston compressors by allowing a part of the fluid trapped in the compression chamber to escape, throu a hole o_ς,.a slot in the unloading disc or ring, into a du behind the disc, and return to the inlet of the compresso The present invention dispenses with this return duct by using a thick unloading ring or disc with a groove cut in the surface that faces into the compression chamber. The fluid escapes from the front of the tooth on the main roto into the groove, and back into the un-compressed chamber behind the tooth. The amount of fluid thus unloaded is regulated by rotating the unloading ring or disc by a suitable device, such as a gear pinion linked to a lever o control wheel.

The above described aspects of the invention combine to enhance the overall efficiency and practicality of the rotary piston machine, and it is by combining two such machines that an improved refrigeration apparatus may be devised. Thus, a fourth aspect of the present inventio relates to the mounting of a compressor and an expander of this type on the same pair of parallel shafts together wit a heat exchanger, appropriate ducting and control devices, to form an open cycle air refrigeration system, or a close As in any Joule cycle heat engine, the overall efficiency of such a refrigeration system is extremely sensitive to the individual efficiencies of its compressor and expander. Only when these efficiencies reach the order of 95% can the open air cycle refrigerator begin to compete in energy efficiency with vapour compression cycles, and then only for certain applications. Additionally, only when the air cycle machines are inherently very simple and inexpensive, and there are no mechanical or fluid-dynamic restrictions to fluid flow at high rotational speeds can air cycle machines compete with vapour compression machines in capital cost. No positive displacent machine known to the existing art fully satisfies these criteria. The closest in the existing art is a set oscillating vane - compressor-expander machines and systems patented by Edwards in 1972, US 3,686,893 1978, and US 4,088,426, which function on a different principle from the present invention.

The increased efficiencies of the machines, and their ability to work with liquid/vapour mixtures makes them particularly suitable for use in closed cycle vapour compression refrigerators , which application forms a fifth aspect of the present invention. It is well-known to increase the efficiency and the capacity of a conventional vapour compression refrigeration cycle - comprising compressor, condenser, throttling expansion device, evaporator and control and by-pass devices - by substituting the throttling, insenthalpic expansion device by a means to recover the expansion work. Such a means may be based on heat transfer devices (Vatril, 1981, US. 4,304,099) , or on mechanical, quasi-isentropic expansion devices. Both continuous flow and positve displacement devices have been proposed. A positive displacement semi-isentropic expansion device rotating in unison with the compressor has two advantages: it precisely meters the amount of liquid flow; it simultaneously increases the amount of refrigeration and reduces the amount of work required. However, such devices are raraly used in practice because the modest gains are more thari offset by the increased cost and inconvenience of a separate expander. In the patent to Masser, 1980, US. 4,235,079, the expansion devices disclosed either entail a separate expander with a distinct housing and several movi parts, or they entail oscillating vane pumps with vapour compression on one side and, for the same mass flow, liqui expansion on the other. This inevitably means that the compressor would have to be under-size, or the expander over-size, due to the orders-o -magnitude difference in dinsities of the liquid and the vapour phases.

The expansion device proposed in this patent simp comprises a second pair of pistons constituted by a slender ring or slender disc, each bolted on to the end of a respective compressor piston, and rotating within a chamber formed for the purpose within a spacer. ^' Extra holes are formed in the sleeve valve and face plate to admit the liquid and withdraw the expanded liquid/vapour mixture. Such a simple mechanism, costing little more than an expansion valve, represents a significant advance in the ar both for the increased refrigeration efficiency and for its contribution to the control device to be disclosed below.

The performance of a vapour compression refrigera tion cycle may be controlled by the temperature of refrigeration and by the amount of refrigeration. The present invention offers the possibility of controlling the cycle by liquid injection, in that the combination of a positive displacement liquid expander with a device to inje metered amounts of liquid into the vapour entering the compressor produces a very sensitive temperature control device: small increases in the proportion of liquid injecte produce significant reductions in the temperature of refrigeration.

Prior temperature control devices which employ adjustable flows of liquid mixed with the vapour entering the compressor, or injected directly into the compressor, are principally concerned with devices to cool, and regulat the tem erature of the oil in compressors, for example in Shibbye, 1982, US Patent Re. 30,869. A "liquid back" device for metered mixing of liquid with the vapour entering the compressor has been disclosed by Nozawa, 1981, in US. 4,261,180, but that employs liquid mixing as a means principally to improve compressor sealing, to cool the compressor components, and to cool, by evaporation the compressed vapour. The invention does not teach the use of a metered proportion of liquid as the main means for regulating the refrigeration cycle temperature; as it does not specify the use of a positive displacement expansion valve, or of some other form of expansion valve in which the flow rate varies very little with differences in pressure-

A thermostatically metered mixture of liquid with the vapour entering the compressor has been proposed as means to control superheat of the vapour discharged from the compressor by Miller in 1977, US. 4,049,410. This patent teaches control of temperature by a thermostatically %_» - operated expansion valve.

Several refrigeration systems that mix metered proportions of liquid wit the gas being compressed are known, for example that of Kelly, 1981, US. 4.258,553, but as part of a system to regulate the overall thermal output rather than the refrigeration temperature. These devices have not been proposed in association with positive displace_ ment expanders. "

The final aspect of the present invention proposes a refrigeration system in which the compressed, condensed liquid is expanded in a positive displacement expander, running at the same speed as the compressor, and the temperature of refrigeration is regulated by varying the proportion of liquid mixed with the compressor intake and/or injected into the compressor.

The present invention will now be described in detail, with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view of a rotary piston compressor, sectioned in the plane of the axes of its shaf ts .

Figure 2 is a sectional view of the compressor figure^" 1, taken in the plane II-II of figure 1.

Figures 3 and 4 show axial and transverse secti respectively of an alternative valve arrangement of the compressor of figures 1 and 2. Figure 4 is a section on line IV-IV of figure 3, and figure 3 is a section on line III-III of figure 4.

Figure 5 is a part-section of a compressor of th type shown in figures 1 and 2, showing a control device fo regulating the flow rate of the compressor.

Figure 6 is a transverse section on plane VI-VI figure 5.

Figure 7 is a section of the sleeve valve, taken on line VII-VII of figure 5.

Figure 8 is an axial section of a combined compressor-expander machine using the compressor of figure 1 and 2.

Figure 9 is a section on line IX-IX of figure 8. Figure 10 is a section on line X-X . of figure 8.

Figure 11 is a schematic view, similar to figure 8, showing an alternative sleeve valve arrangement.

Figure 12 and 13 are detail cross-sections of th sleeve valve of figure 11, taken on lines XII-XII and XIII XIII respectively.

Figure 14 shows the compressor-expander pair- applied to a vapour refrigeration cycle, in axial section.

Figures 15, 16 and 17 show transverse cross-sect of the machine, taken on lines XV-XV, XVI-XVI, XVII-XVII, respectively.

Figure 1 and figure 2, show two views of the sam basic rotary piston machine, in this case working as a compressor with a relatively constant compression ratio. The moving parts of the compressor are a main shaft (1) wh receives or transmits torque, on which is mounted the main rotor (2) , and a secondary shaft (3) on which is mounted t gate rotor (4) . Spur gears (5) are mounted on both these

Y shafts and constrain them to rotate at equal speeds in opposite senses. The two rotors rotate in sealing proximity with each other and with, the casing (6) , and with a stationaiy sleeve valve { ! ) which is located inside the hollow main rotor (_.2) . A device for injecting and mixing a liquid (8) with the fluid being compressed may be fixed to the compressor inlet. Such liquid injection improves the sealing of the compressor and further increases operational efficiency by removing part or almost all of the heart of compression for the working fluid during compression.-

The working fluid is drawn into th.e compressor through the inlet port C9) ; in the direction shown by arrows (10) . The fluid trapped in the compression chamber (11) has, in the position shown in figure 2, just begun to be compressed. After both pistons have rotated some 130° in the direction shown by the arrows (12) , the transfer port C13) in the wall of the sleeve valve will open and the compressed fluid and any accompanying liquid will be transferred through the high-pressure duct (.14) and out of the compressor. (The volumetric compression ratio of the compressor illustrated in figures 1 and 2 is about 6:1) .

The profile of the main rotor is defined by two teeth (_.15) whose end faces (16) rotate in sealing proximity with the casing (6) . The leading faces (17) of these teeth are volutes precisely defined by the evolution of th_.e edges C18) of the projecting teeth (19) on the gate rotor. The trailing faces (20) of the teeth on the main rotor have a profile that is convenient to manufacture and gives sufficient thickness for the teeth to provide adequate sealing. Next follow cylindrical segments (21) that rotate in sealing proximity with cylindrical segments (22) of the gate rotor. When the segments (21) of the main rotor and the segments (.22) of the gate rotor have equal diameters, as in figure 2, they rotate in rolling contact to improve sealing. Following the cylindrical segments (21) of the gate rotor are precisely defined double curves (23) which terminate in ports (24) cut in the wall of the hollow main rotor. The double curves (.23) on the main rotor rotate i sealing proximity with precisely defined double curves C25 on the^' leading teeth C191 of the gate rotor C41. Recesses C26) at the base of the teeth C191 of the gate rotor (4) provide clearance for the teeth C15) of the main rotor (2) The teeth C19) of the gate rotor (4) enter the ports (24) on the main rotor to a small degree (up to about 2% of the radius of the tips C18) of th.e teeth (19)), thus increasin the sealing area between the peripheries (27) of the gate rotor teeth C19) and a sleeve valve (7) inside the hollow main rotor shaft. The thickness of the gate rotor teeth C19) is determined by the compression ratio, which determi the width of the ports (-24) on the main rotor.

Both main rotor C2) and gate rotor (.4) have parallel sides that rotate in sealing contact with the casing C6) . The width of the rotors is limited by structu and manufacturing considerations rather than by fluid han considerations.

Figure 3 and figure 4 show an alternative form o sleeve valve to the sleeve valve (7) shown in use in the compressor of figures 1 and 2. The object of this valve i to separate liquid droplets from the compressed gas. This valve passes the entering compressed fluid through a shape throat (30) so as to impart a swirl to the mixture of compressed gas and liquid droplets within the cylindricall shaped interior (31) of the sleeve valve. This swirl caus some separation of the liquid droplets from the gas. The swirl is greatly magnified by passing the swirling compres gas through a narrow throat (32) , which separates most of rest of the liquid droplets from the gas. The liquid drop lets are collected in a channel (33) and drained off in a tube (34) . Most of the swirl energy imparted to the gas may be recovered in a vortex chamber (35) .

A similar effect may be achieved by simply extending the parallel-sided sleeve valve tube (31) sufficiently so that the smaller droplets have time to migrate to the walls of the tube and thence to the channel (33) ^'.

Figures 5, 6 and 7 show an unloading device incorporated into the same basic compressor shown in figur

1 and 2. An end wall of the casing (6) is substituted by grooved disc (40) and a device that can rotate it through to 180°. In this illustration the device is a shaft and pinion (41) whose teeth engage with teeth (42) cut in the periphery of the grooved disc (40) . One side of the disc

(40) is maintained in sealing proximity with one side of th main piston (2) and the gate piston (4) . In this side of the disc is cut a groove (43) so shaped as to permit the working fluid that would have been trapped and compressed in the compression chamber (11) by the advancing tooth

(15) of the main piston to escape around the tooth, via th groove (431 and back into that part of the compressor casin

(10) where it is not subject to compression. The proportio of the charge of working fluid thus released depends on the angular position of th.e grooved disc.

The profile of the groove (43) as shown in figure 6 is defined by two circular arcs (44) and (45) , a curve (4 that approximates the curbe (17) on the front of the main piston rotor tooth (15) , and a curve (47) whose form need not be precisely defined. The angle subtended by the groove at the main shaft centre is normally less than 180°. The valves employed in these unloading compressor may be . variable sleeve valves or a hybrid of a sleeve valve (48) and leaf valve (49) , as shown in cross section figure 7.

Such a valve, which operates as a manual sleeve valve under full load, eliminates most of the blow-back of compressed fluid into the compression chamber at reduced loads that would normally occur in such a compressor equipped with a simple sleeve valve.

Figures 8, 9 and 10 illustrate three cross sectio of an open-cycle air refrigeration machine that may incorporate means for water injection and which comprises a compressor, an expander with a variable admission valve, a The compressor-expander machine comprises a pair of rotary piston machines of the type illustrated in figure 1 and figure 2 mounted side by side on the same pair of rotating shafts; the main rotor shaft (511, and the gate rotor shaf (52) . The rotation of these two shafts is precisely coordinated by a pair of equal diameter gears (53) . Insid the hollow main rotor pair C54) , (55) is a double sleeve valve (56) , (57) and a division (58) which separates the compressed, heated and water-carrying air leaving the compressor, from the cooled, water-free air entering the expander. The amount of compressed air admitted to the expander may be varied by moving part of the sleeve of the expander valve (59) , which movement is accomplished by a gear and pinion and a lever (60) . The refrigerator intake air is atmospheric air ( and enters the compressor inlet (62) where it may be mixed with droplets of water (63) . - The mixture of air and water is drawn into the compression chamber (64) where it is compressed and thereby heated. How much it is heated depe on the compression ratio and on the proportion of water mi with the air. An alternative route for injecting the wate into the compressor is through holes in the periphery of t compressor casing (65) . The compressed mixture of air and water droplets is transferred to the interior of the compressor sleeve valve (56) , then through a duct (67) and out of sleeve valve into a duct (68) which conveys it ^"to a heat exchanger (69) and a water separator (70) . If sufficient water has been injected, the heat exchanger is not needed. The water-free compressed air at approximatel ambient temperature is returned through duct (71) to the duct (72) which conducts it into the variable sleeve valve (57) of the expander. The compressed air is expanded in t expansion chamber (73) where it delivers work to the shaft (52) , (51) and is cooled. The cooled air (74) is delivere to the refrigerated space via the outlet (75) . The compression pressure, and hence the temperature of refrigeration of the cycle is determined by the adjustment (60)' of the variable sleeve (59) of the valve of the expander. As the volume of compressed air admitted to the expander is reduced, the cycle pressure rises and the refrigeration temperature falls. An alternative sleeve valve for this machine, which incorporates a water separator, is shown in figures 11, 12, and 13. This water separator functions on the principles illustrated in figures 3, and 4, as .described above. The mixture of compressed air and water droplets enters the compressor sleeve valve (76) tangentially, which imparts a swirl in the direction shown by the arrow (77) . This swirl causes the water droplets to migrate to the sleeve valve inside wall and to collect in the water drain channel (78) . The rate of swirl is greatly increased, and the remaining smaller water droplets are separated, as the compressed air passes through the throat (79) into the expander variable sleeve valve, (80) where the rate of sv/irl is reduced and part of the swirl energy is recovered.

The compressed air is then ejected through the variable sleeve valve port (81) into the expansion chamber. The water, which is under pressure, is removed through the tube (82) . This pressurized water may be used to lubricate the bearings (83, 84) (figure 8) .

Figures 14, 15, 16, 17 illustrate the incorporation of the compressor, control disc, and an expander into a vapour compressor refrigeration system. The refrigeration system comprises a compressor (90) and fluid injection device (91) , an unloading disc (92) , an expander (93) , a condenser (94) , a liquid separator (95) , an evaporator (96) , as well as the necessary control devices (not shown) . This partcular machine illustrates a combination of compressor, control disc and expander to demonstrate how simply these kinds of control disc and expansion devices may be added on to the compressors. Practical refrigeration machines may be compressor-expanders, or unloading compressors, or simple compressors. The fluid injection is necessary to improve the compressor sealing, cool the compressor and improve the overall cycle efficiency, as well as for the control function specifically described below.

The machine illustrated in figures 14, 15, 16 an 17 comprises a main shaft (97) which is driven by an elect or other, motor. Attached to the main shaft is the hollow main rotor (98) of the compressor, which is the same type illustrated in figure 2. Attached to the end of the main rotor is a sleeve (991 on which is mounted the main rotor the expander (1001. Inside the hollow main rotor are loca two stationary sleeve valves mounted on the same sleeve; t compressor sleeve valve (1011 and the expander sleeve valv (102) . The compressor sleeve valve may be of the type sho in figure 7. At one end of the rotor, in the casing part 108d, is located the compressor unloading disc (92) , whose actuating mechanism is not shown.

The gate rotor shaft (103) rotates on a fixed pi (104) at an equal speed and in the opposite sense to the main rotor, due to the action of the gears. (105) . The compressor gate rotor (106) and the expander gate rotor (107) are mounted on this shaft (103) . It should be noted that there are no protruding teeth, on this expander gate rotor (107) , in order to confine all the flow of the expanded vapour/liquid mixture to the main rotor side of t casing and facilitate the flow of the liquid. To improve the flow of compressed liquid into the expander, a volume (not shown) may be provided close to the inlet into the expander, the volume containing vapour or some compressibl substance.

The casing (108) enclosing the rotors comprises six parts, corresponding to the various parts of the machi i.e. motor casing and bearings 108a, gears 108b, compresso rotors 108c, unloading disc 108d, expander rotors 108c, an end cover and sleeve valve mountings 108f.

When the refrigeration system is functioning the compressed and heated vapour, mixed with some liquid, leav the duct (109) inside the compressor sleeve valve (101) (the machnie should be in an orientation so as to prevent liquid from accumulating within the sleeve valve) and enters the duct (110) which takes it to the condenser (94) , from whsch the condensed liquxα ^"returns by ducL (111) L- - the expander inlet valve (102) . In the expander the liquid is cooled, and some evaporates to give a low-pressure mixture of liquid and vapour. The mixture passes through the tube (1121 to the liquid separator (95) (the liquid separator may be incorporated into the expander casing 108e) . From the separator, part of the liquid flows through tube (113) to the mixer (91), and the rest of the liquid-vapour mixture flows through the tube (114) to the evaporator (96) . From the evaporator the vapour flows through the tube (115) to the mixer (91) where it is mixed with a metered proportion of liquid. The proportion of liquid mixed with the vapour is used to regulate the temperature of refrigeration of the cycle. For a given displacement of the compressor, an increase in the proportion of liquid entering the compressor markedly reduces the evaporator pressure and the temperature of refrigeration. The fact that this type of compressor can deal with large proportions of liquid in the working fluid without malfunction allows it to handle more than half of the working fluid as a liquid, and hence achieve substantial unloading of the compressor.

Claims

Claims :

1. A rotary pistcr compr^oe=sor comprising a pair o pistons mounted on parallel shafts and constrained to rotat at equal speeds in opposite directions in sealing engagemen with each other and with, a casing a first port being provid in the casing for the entry of a fluid, and a second port being provided in one of the pistons for the exit of said fluid in a radial direction, the said one piston also havin a radially outwardly extending tooth formed therealong, the tooth having a leading face and a trailing face and the second port being formed at the radially inner end of the leading face of the tooth, and the other of said rotary pistons having a radially outwardly extending tooth having leading and a trailing face, and a blind recess formed at t base of the trailing face to accommodatethe tooth of the one rotary piston, a stationary hollow shaft extending axia within s^Ld one piston and having an axial slot penetrating its sidewall, characterized in that the pistons (2,4). secon port (24) , and slot (131 are so arranged that the radially inner edge of the leading face (17) of the one piston (2) and the radially outer edge of the trailing face of the oth piston (4) meet just as the axial slot (13) and the second., port (24) begin to coincide.

2. A rotary piston compressor according to claim 1 characterized in that the axial slot (13) is arranged to lead tangentially into the interior of the hollow shaft to impart swirl to the working fluid.

3. A rotary piston compressor according to claim 2 characterized in that a throat (32) is provided within the hollow shaft, and a longitudinal channel (33) is formed in the interior surface of the shaft wall for collection of liquid particles separated from a gaseous or vapour working fluid.

4. A rotary piston compressor according to claim 1 characterized in that at one axial end of said one piston, a rotatable disc (40) is mounted flush with the casing, the greater than the thickness of the tooth of said one piston.

5. A compressor according to claim 4, characterized in that the disc (40) has gear teeth (42) formed on a part of its periphery, and is selectively rotatable by means of

5 a drive pinion.

6. A compressor according to claim 1, characterized in that the second port is provided with a resilient leaf valve (49) .

7. An air cycle refrigeration machine, characterized 10 in that it comprises a rotary piston compressor as described in claim 1 and a rotary piston expander mounted on the same pair of parallel shafts (51, 52) , the inlet of the compressor being provided with means (62) to mix a liquid with the working fluid, the working fluid being led to a heat

15 exchanger and liquid separator, and working fluid being led then to the expander, via a hollow shaft (57)-. and an inlet port, th. xpander being substantially the same in size and configuration as the compressor but working in reverse and discharging through and exit port (75) in the machine

20 casing.

8. An air cycle refrigeration machine according to claim 7, characterized in that liquid is mixed with the .. working fluid in sufficient quantities to extract substantially all the heat generated by compression therefore,

25 and the working fluid and liquid mixture is separated in a vortex separator mounted entirely within hollow shaft on which one of the compressor and one of the expander rotors are mounted, the vortex separator comprising first and second substantially coaxial cylindrical (76, 80) situated

30 within the compressor and expander rotors, respectively, and connected by a throat (79) , the first chamber including a longitudinal water collection channel (78) leading to a discharge duct (82) .

9. A vapour cycle re rigeration apparatus compris-

35 ing a compressor according to claim 1 and characterized by having an expander mounted en the same pair of parallel shafts and rotating in distinct chambers within the same -13-

casihg, the compressed vapour being led from the compresso via a hollow shaft to a condenser, and the condensed worki fluid returning to the expander via the same hollow shaft the expander being of reduced dimensions in relation to th compressor and comprising a first rotor (100) having a num of radially outwardly extending teeth and a second rotor (107) having a like number of corresponding recesses, the condensed working fluid being admitted to the expander via ports formed at the bases of the teeth of the first rotor, the expanded fluid being led from the expander to a liquid separator (95) and the vapour thence passing to an evapora (96) and returning to the compressor inlet, a part of the separated liquid being mixed with the vapour at the compressor inlet (.91) or in the compression chamber.

10. A vapour cycle refrigeration machine accordi to claim 9, characterized in that the compressor has a variable compression ratio.

11. A vapour cycle refrigeration machine accordi to claim 9 or claim 10, characterized in that the amount of liquid mixed with the vapour is used to control the refrigeration temperature.

12. A compressor substantially as herein describ and illustrated in any of figures 1 to 7 of the accompanyi drawings.

13. A refrigeration machine substantially as her described and illustrated in figures 8 to 13 or 14 to 17 o the accompanying drawings.

If