GB2120727A - Rotary positive-displacement fluid-machines - Google Patents

Abstract

A machine for a compressible fluid, which may be a compressor or an expander, comprises sliding vanes 14 mounted in a casing 2 and biassed towards a rotor 1 by eg. adjustable springs 17, the rotor being furnished with separate ducts for the admission and discharge of the working fluid to and from the inter-vane chambers a...f, one of said ducts having portions 27, 28, 29, 31, 32, 33 and the other duct having portions 34a, 34b, 36. <IMAGE>

Description

SPECIFICATION Rotary positive displacement machine This invention relates to a rotary positive displacement machine through which a compressible fluid (e.g. air, water vapour, or organic refrigerant vapour) is to pass. The following description is mainly directed to an expander, i.e. the machine expands the fluid from a higher pressure to a lower pressure, but the invention is equally applicable to a compressor.

Positive displacement expanders such as vane-type air motors and reciprocating steam engines often suffer from poor mechanical and thermodynamic efficiency because of significant friction between moving parts and excessive leakage through internal clearances.

The present invention provides a rotary positive displacement machine through which a compressible fluid is to pass, the machine comprising a rotor mounted in a casing, slidable vanes projecting inwards from the casing towards the rotor, and biassing means for urging the vanes into contact with the periphery of the rotor, the periphery of the rotor, the casing, and the vanes together defining separate chambers which change in volume cyclically as the rotor turns relative to the casing, the rotor having two separate ducts for the compressible fluid, the ducts communicating with the periphery of the rotor at different positions such that one duct communicates with a chamber whose volume is increasing while the other duct communicates with a chamber whose volume is decreasing.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through a rotary positve displacement expander; Figure 2 is a cross-section through the expander; and Figure 3 is a graph of pressure versus volume.

The expander comprises a rotor 1 mounted in a casing 2. The rotor has a body 3 keyed on a shaft 4 mounted in bearings 6 in the casing. The casing has a body 7 with a right circular cylindrical bore 8 coaxial with the shaft 4. The body 7 may be made integrally in one piece, but in the preferred embodiment illustrated it is made up of separate segments 9. End plates 11 of the casing are clamped against the ends of the casing body 7 by end members 1 2 which are bolted to the segments 9 through the end plates 11.

Each adjacent pair of segments 9 define between them a slot 1 3 in which a vane 14 slides. The slots 1 3 are equally spaced around the bore 8 and are at an angle to the radius; alternatively the slots may be radial. Each vane 14 is generally rectangular but has two trapezium-shaped recesses 16, each recess receiving a compression spring 1 7 which bears against a screw 1 8 and is accommodated in an enlarged cavity 1 9 between the segments 9. The springs 1 7 urge the vanes 14 to remain in contact with the periphery of the rotor body 3 up to a given maximum speed of rotation. The biasing force of the springs is preferably adjustable, e.g. by means of the screws 1 8.

The slots 1 3 may be provided with inserts of low coefficient of friction material (not shown) to reduce the sliding friction of the vanes. Ducts (not shown) may be provided in the casing body 7 and/or in the vanes to communicate the local pressure in the casing bore adjacent a vane to the space between that vane and the bottom of the slot in which that vane slides. The vane tips may be specially contoured or sealed to reduce leakage between the vanes and the rotor. The vane ends adjacent the end plates 11 may also incorporate seals. Since the vanes reciprocate without rotation, the vane seais would be unlikely to exhibit excessive wear. The expander illustrated has six vanes, but the number of vanes may be more or less than this depending on the desired characteristics of the expander.

The rotor body 3, between the end plates 11, has the shape of a right cylinder whose profile is a lobe-shaped closed curve. This curve is non-reentrant, i.e. it is not concave at any point. Because of this, the peripheral surface of the rotor body may readily be finished to any desired degree of smoothness and parallelism, thus reducing friction and leakage at the vane tips. The rotor body can also be accurately aligned in the axial direction on the shaft 4.

The rotor body 3 is tangential to the casing bore 8 at a point 21 on its peripheral profile.

From this point 21, going in the clockwise direction (Fig. 2), the spacing of the profile from the bore 8 increases graduaily along a convex curve over an angular extent of about 250 to a point 22. From the point 21, going in the anticlockwise direction, the spacing of the profile from the bore 8 increases gradually along a convex curve over an angular extent of about 60 to a point 23. Between the points 22 and 23, which is the region where the profile is at its maximum distance from the bore 8, the profile has a straight portion, i.e. the peripheral surface is flat here.

The periphery of the rotor body, the bore 8 of the casing, the end plates 11, and the vanes 14 together define six separate chambers a two fwhich change in colume cyclically as the rotor 1 turns relative to the casing 2.

As seen in Fig. 2, with the rotor turning anticlockwise, the chamber a has just passed its minimum volume and is beginning to expand, the chambers b and c are expanding, the chamber d is just about to reach its maximum volume, the chamber e has passed its maximum volume and is contracting, and the chamber f is just about to reach its minimum volume.

The casing 2 has an axial inlet port 24 which is outside a seal 26 through which the rotor shaft 4 passes. The port 24 communicates with a duct portion 27 extending along the shaft 4 and communicating, in turn, with a branching radial portion 28 which leads into a segment-shaped cavity 29. This cavity communicates with the periphery of the rotor body through a main inlet 31 (consisting of one or more ports) which has a minor forward extension 32 and a major rearward extension 33. Ahead of the tangent point 21 outlet duct portions 34a, 34b in the periphery of the rotor body communicate with an internal rotor cavity 36 which runs from end to end of the rotor body.At each end the rotor body has a boss 37 which engages an annular seal 38 clamped between the end plate 11 and the end member 1 2. In the end member 1 2 there is an annular space 39 into which the rotor cavity 36 opens; this space 39 communicates with exhust ports 41 leading to an exhaust manifold 42.

Although it is preferable to exhaust the compressible fluid from both ends of the rotor as described above, in certain situations it may be desirable to exhaust from only one end (which may be the same end as the inlet or the opposite end), in which case the rotor cavity 36 would only extend to that one end.

Furthermore, in some cases a single exhaust port 41 port may be sufficient.

The arrangement of the inlet ducting 27, 28, 29, 31, 32, 33, the exhaust ducting 34a, 34b, 36 and the shape of the rotor periphery are such that the rotor 1 is dynamically balanced; in particular its centre of gravity lies on the axis of the shaft 4. Any slight imbalance can be corrected by local removal of material from the rotor.

The expander operates in the same manner as any conventional positive displacement expansion device, i.e. there are four phases in the working cycle of each of the chambers a to f, as follows (see Fig. 3): (i) an admission or inlet phase in which high pressure gas or vapour is admitted to the chamber until a cut-off point is reached (chamber a); (ii) an expansion phase in which the high pressure gas trapped in the inlet volume expands to a point immediately prior to the exhaust opening (chambers b, c, d); (iii) an exhaust phase in which the expanded gas is rejected (chamber e); (iv) a recompression phase in which any residual exhaust gas is recompressed to a point immediately prior to the inlet opening.

The compressible fluid under high pressure enters the expander through the inlet port 24 and finds its way through the internal ducting 27, 28, 29, 31, 32, 33 in the rotor 1 to the chamber of near-minimum volume formed between two adjacent vanes 14, the periphery of the rotor body 3, the casing bore 8, and the end plates 111. The pressure distribution around the rotor body 3 in such that a torque is developed which causes the rotor to rotate.

As it does so the inlet 31, 32, 33 in the rotor periphery is cut off from communication with the chamber in question by the leading vane as the inlet passes under it. The trapped fluid expands in the increasing volume of the chamber as the rotor rotates until, at maximum volume, the outlet ports 34a begin to pass under the trailing vane. The expanded gas escapes through the cavity 36 to the end spaces 39 and into the exhaust manifold 42, being squeezed out by the reducing volume of the chamber in question. Any remaining gas is recompressed to the inlet condition.

Because of the presence of the tangential point 21 it is permissible for the outlet ports 34b ahead of that point and the inlet extension 32 behind that point to be in simultaneous communication with the same chamber (chamber fin Fig. 2). This allows one to extend the inlet phase and the exhaust phase, thereby enhancing efficiency. A5 can be seen from Fig. 2, the inlet ducting and the exhaust ducting communicate with two adjacent chambers simultaneously at ali lies.

The expander can be used in any application where positive displacement expanders are currently used. In particular its application is suitable where high efficiency is important, such as in an organic Rankine cycle for the conversion of solar energy to mechanical energy, or the utilization of low-grade waste heat to produce mechanical energy. Efficient operation is also to be expected with wet expansion, such as that which occurs when using saturated steam; this should therefore permit an efficient steam cycle to be used for the thermo-mechanical conversion.

Claims (10)

1. A rotary positive displacement machine through which a compressible fluid is to pass, the machine comprising a rotor mounted in a casing, slidable vanes projecting inwards from the casing towards the rotor, and biassing means for urging the vanes into contact with the periphery of the rotor, the periphery of the rotor, the casing, and the vanes together defining separate chambers which change in volume cyclically as the rotor turns relative to the casing, the rotor having two separate ducts for the compressible fluid, the ducts communicating with the periphery of the rotor at different positions such that one duct communicates with a chamber whose volume is increasing while the other duct communicates with a chamber whose volume is decreasing.

2. A machine as claimed in claim 1, in which the rotor is tangential to the casing in a region of the rotor periphery between the ducts.

3. A machine as claimed in claim 1 or in which at least one of the ducts communicates with two adjacent chambers simultaneously at all times.

4. A machine as claimed in any of claims 1 to 3, in which one of the ducts has a portion extending along the rotation axis of the rotor from one end of the rotor, where the said axial portion communicates with a port in the casing, the duct having a radial portion which branches from the axial portion.

5. A machine as claimed in any of claims 1 to 4, in which one of the ducts opens at one end of the rotor into annular space which is defined between the rotor and the casing and which communicates with at least one port in the casing.

6. A machine as claimed in any of claims 1 to 5, in which the profile of the periphery of the rotor is a closed non-reentrant curve.

7. A machine as claimed in claim 6, in which the curve has a straight portion in a region in which it is at its maximum distance from the interior of the casing.

8. A machine as claimed in any of claims 1 to 7, in which the centre of gravity of the rotor lies on the rotation axis of the rotor.

9. A machine as claimed in any of claims 1 to 8, in which the casing comprises segments, each pair of adjacent segments defining between them a slot in which one of the vanes slides.

10. A rotary positive displacement machine, substantially as described with reference to, and as shown in, the accompanying drawings.