EP0057198B1

EP0057198B1 - Rotary compressors

Info

Publication number: EP0057198B1
Application number: EP81902040A
Authority: EP
Inventors: Richard John Gray
Original assignee: Hydrovane Compressor Co Ltd
Current assignee: Hydrovane Compressor Co Ltd
Priority date: 1980-07-31
Filing date: 1981-07-24
Publication date: 1984-03-21
Also published as: DE3162800D1; WO1982000500A1; ES504314A0; IT8149002A0; GB2081383A; IT1142786B; ES8205959A1; EP0057198A1; GB2081383B

Description

The present invention relates to rotary oil mist compressors and is particularly concerned with such compressors of sliding vane eccentric rotor type.
The term oil mist compressor is used herein to refer to those compressors in which oil is injected into the air to be compressed and is subsequently separated from the compressed air and recycled for re-use.
Conventionally, sliding vane eccentric rotor compressors remove the entrained oil droplets from the compressed air in two stages. In the primary stage, the separation is effected by causing the compressed air to impinge against a surface thus coalescing into larger droplets which run down into a sump within the compressor. The surface may be afforded by the wall of a labyrinthine passage or by an impingement shield situated adjacent the outlet ports in the rotor/stator unit. The compressed air then passes through an aperture or passageway into a secondary separation space containing a plurality of coalescing elements, of e. g. ceramic material of felt, through which the compressed air passes thus coalescing substantially all the remaining entrained oil droplets. This oil drips down to the floor of the secondary separation space and from there is returned for re-use.
British Patent No. 1 134 224 illustrates a typical arrangement in which the oil is returned from the secondary separation space to the atmospheric side of the compressor inlet valve by a passageway, and is then drawn into the compressor by the inflowing air. This passageway tends to return not only the oil from the secondary separator but also compressed air. The delivery pressure of such compressors is typically about 7 bars, and this full pressure differential is present between the ends of the passageway. In order to prevent excessive volumes of air being recycled from the secondary separation space to the inlet, which reduces the output and is wasteful of energy, the passageway includes an oil return valve including a small orifice which ensures that only oil is recycled or throttles the air flow and thus reduces the power wasted. Substantially the entire pressure drop occurs across this orifice, which must therefore be of extremely small diameter to fulfil its function adequately.
This construction suffers from the disadvantage that, due to its small size, the orifice is subject to blocking by contaminant particles in the oil, e. g. particles which have broken loose from the coalescing elements. Should a blockage occur, the coalesced oil will gradually flood the secondary separation space and thereafter will be lost through the compressor outlet. The orifice size chosen is generally a compromise between the size desired for maximum efficiency and the size required to give satisfactory reliability in service. Typically this orifice has a diameter of 0.5 mm.
A further disadvantage of the known construction is that the oil tends to atomise when it is forced through the small orifice, and the oil spray produced tends sometimes to pass out of the compressor inlet and is therefore lost and gives the compressor the appearance of smoking.
According to the present invention a rotary oil mist compressor of the type including a compression space in which, in use, air is progressively compressed, a sump which, in use, contains oil, oil injection means to inject oil from the sump into the compression space, primary separation means to coalesce and remove a proportion of the entrained oil droplets from the compressed air, secondary separation means to remove substantially the remainder of the oil droplets, an oil space remote from the sump in which, in use, the oil coalesced by the secondary separation means collects and an oil return passage communicating with the said oil space and with the said compression space to return the oil to the compression space, the oil return passage including an oil return valve having a throttling orifice is characterised in that the oil return passage communicates directly with the compression space at a point between the inlet and the outlet where, in use, the pressure is between 50 and 90 % of the normal working pressure and that the throttling orifice of the oil return valve has a diameter of between 1 and 2 mm.
Thus in a compressor according to the present invention the oil that is recycled within the compressor by the pressure generated by it is returned not under the action of the full compressor pressure as previously but under the action of only a proportion of it, i. e. 10 to 50 % of it. This enables the orifice of the oil return valve to be considerably larger than was previously the case, i. e. between 1 and 2 mm whilst still fulfilling the same function and this reduces or substantially eliminates the risk of the aperture becoming blocked by contaminant particles. It was previously necessary to provide a filter upstream of the oil return valve to reduce the risk of the valve becoming blocked, but in a compressor according to the invention it may be possible to eliminate this filter altogether. In addition the risk of the compressor inlet « smoking due to oil droplets which are reatomised by the oil return valve coming out of the inlet is almost entirely removed because the oil is returned directly into the compression space.
In the preferred embodiment of the invention, which is a sliding vane eccentric rotor compressor, the primary separation means serves to coalesce a proportion of the entrained oil droplets by causing the compressed air to impinge against at least one surface, e. g. the primary separation means constitutes an impingement shield disposed around the stator against which the compressed air impinges, thus causing a substantial proportion of the entrained oil droplets to coalesce and drip down into the compressor sump, and the secondary separation means includes at least one porous coalescing element through which, in use, the compressed air passes. Preferably the rotor stator unit defining the compression space is situated within a compressor casing and the secondary separation means is situated within a separate separator casing secured to the compressor casing. The space within the separator casing below the or each porous coalescing element constitutes a space in which the coalesced oil connects and it is from this space that the oil is returned to the compression space under the action of the differential between the compressor delivery pressure and the pressure prevailing at the point at which the oil return passage communicates with the compression space.
This system of returning coalesced oil is not restricted to returning oil from the bottom of the secondary separation casing, but may be applied to returning oil from any point at which it accumulates. In a preferred embodiment the compressor has two or more oil spaces in which coalesced oil collects, and an oil return passage associated with each space each of which incorporates a respective oil return valve, the passages communicating with a single common oil return passage at a point downstream of the oil return valves.
In a preferred embodiment the secondary separation coalescing elements communicate with the primary separation stage by means of a common separator manifold so constructed that oil droplets tend to coalesce in it. This reduces the separation load on the secondary separation stage but results in two spaces in which coalesced oil collects and must be subsequently returned to the rotor stator unit.
Further features and details of the invention will be apparent from the accompanying description of a sliding vane eccentric rotor compressor in accordance with the present invention which is given by way of example with reference to the accompanying drawing which is a transverse section through the compressor from which certain inessential integers have been omitted for the sake of simplicity.
The compressor has a main separator casing 2, the lower portion of which constitutes an oil sump 3 und and which contains a stator 4 within which is an eccentric rotor 6 carrying a plurality of sliding vanes 8. Situated around the stator is an impingement shield schematically illustrated at 10.
The rotor and stator together define a crescent shaped working space, and in use the rotor rotates anti-clockwise as seen in the Figure, and the outer edges of the vanes are kept in contact with the interior surface of the stator by centrifugal force. Air is sucked into the working space through an inlet, not shown, which extends from about the 10 o'clock to the 5 o'clock positions and is progressively compressed. Oil is withdrawn from the sump and injected into the working space through injection means, which are not shown, thus ensuring an adequate gas seal between the vanes and the stator and the compressor end plates. The compressed air with entrained oil droplets exits through a series of ports in the stator and then impinges against the impingement shield, which constitutes a primary separation means, and a large proportion of the entrained oil droplets coalesce against it and then drip down into the sump 3. The compressed air then passes into the main volume of the compressor casing 2.
Secured to the left-hand side of the compressor casing is a secondary separator casing 14. Within the casing 14 is one or more, and preferably two, vertically disposed coalescing elements 16, of e. g. ceramic material, which constitute a secondary separation means. Within each coalescing element 16 is a coaxially disposed closed-bottomed metallic tube 18 whose wall has a plurality of apertures 20 formed in it and whose upper end communicates with a common separator manifold 22. The interior of the separator manifold communicates with the space within the main separator casing 2 through a thermally actuated shut-off valve which is not illustrated, but which is described and illustrated in British patent No. 1218769.
Thus the compressed air within the compressor casing passes into the thermally actuated shut-off valve and then turns through 90° to flow through apertures in the side wall of the valve into the separator manifold 22. From there the compressed air turns through a further 90° to enter into the or each metallic tube 18 and then passes through the coalescing elements and thence upwards within the separator casing 14 to the compressed air outlet 24 substantially free of entrained oil droplets.
The acceleration and turbulence caused by the right angled bends through which the compressed air flows results in a further proportion of the entrained oil droplets coalescing and being deposited within the separator manifold 22. The accumulated oil passes under the action of a pressure differential through apertures 26 in a hollow oil return bolt 28 which passes through the manifold 22 and from there is returned to the rotor stator unit, as described in more detail below. The oil droplets coalesced by the elements 16 drips down to the floor of the separator casing 14, into a bore 30, thence into a common oil return bore 32, and a further oil return bore 36 and is returned to the rotor stator unit.
The oil return bolt 28 and the oil return bore 36 communicate with a common chamber 38 by means of a respective oil return valve constituted by a hollow screw 40 having an axial passage 42 within it. The chamber 38 communicates with the working space within the rotor stator unit by means of a common oil return drilling 44 formed in one of the compressor end plates. The drilling 44 communicates with the working space at a point about 30° upstream of the outlets 12. In the present case the compressor has a rated output pressure of about 7 bars, and the drilling 44 therefore communicates with the working space at point where the pressure is already considerable, in this case about 5 bars. The oil upstream of the oil return screws 40 is substantially at delivery pressure, i. e. 7 bars, and the pressure drop across the oil return screws is therefore only about 2 bars, instead of the full working pressure as was previously the case. To enable the screws to pass the requisite volume of oil at this reduced pressure differential the diameter of the apertures is about 1.5 mm. Should the screws be passing air instead of, or as well as oil, the volume will not be excessive due to the fact that the pressure differential is reduced. However, the increase in size of the throttling apertures 40 reduces or substantially eliminates the risk of these apertures becoming blocked. Separate throttling orifices are used for each oil return pathway since the oil upstream of the orifices may be at slightly different pressures, and this will substantially eliminate the risk of oil being forced under pressure from one oil collection point to another rather than to the rotor stator unit.

Claims

1. A rotary oil mist compressor including a compression space in which, in use, air is progressively compressed, a sump (3) which, in use, contains oil, oil injection means to inject oil from the sump into the compression space, primary separation means (10) to coalesce and remove a proportion of the entrained oil droplets from the compressed air, secondary separation means (16, 22) to remove substantially the remainder of the oil droplets, an oil space remote from the sump and in which, in use, the oil coalesced by the secondary separation means (16, 22) collects and an oil return passage (28, 36, 44) communicating with the said oil space and with the said compression space to return the oil to the compression space, the oil return passage (28, 36, 44) including an oil return valve (40) having a throttling orifice (42), characterised in that the oil return passage (44) communicates directly with the compression space at a point between the inlet and the outlet where, in use, the pressure is between 50 and 90 % of the normal working pressure and that the throttling orifice (42) of the oil return valve (40) has a diameter of between 1 and 2 mm.

2. A compressor as claimed in Claim 1, characterised in that it is of sliding vane eccentric rotor type, that the primary separation means (10) serves to coalesce a proportion of the entrained oil droplets by causing the compressed air to impinge against at least one surface (10) and the secondary separation means (16, 22) includes at least one porous coalescing element (16) through which, in use, the compressed air passes.

3. A compressor as claimed in Claim 2, characterised in that the rotor stator unit (4, 6) defining the compression stage is situated within a compressor casing (2) and the secondary separation stage (16) is situated within a separate separator casing (14) secured to the compressor casing (2).

4. A compressor as claimed in Claim 2, characterised in that the oil space is situated below the secondary separation stage (16) and the oil coalesced by this stage collects in the oil space.

5. A compressor as claimed in Claim 2, characterised in that it has two oil spaces in which coalesced oil collects and an oil return passage (28, 36) associated with each oil space each of which incorporates a respective oil return valve (40), the passages (28, 36) communicating at a point (38) downstream of the oil return valves (40).

6. A compressor as claimed in Claim 5, characterised in that the secondary separation stage includes two coalescing elements (16) which communicate with the primary separation stage by means of a common separator manifold (22) so constructed that, in use, oil droplets tend to coalesce in it, the space in the separator casing (14) below the coalescing elements (16) constituting a first oil space and the separator manifold (22) constituting a second oil space.