Abstract:
A centrifugal separator for separating oil suspended in air and especially intended for use with aircraft engines includes a rotatable chamber filled with a relatively rigid porous material and is driven by a hollow shaft. An inlet for the mixture of air and oil is provided in one end wall of the chamber and separate outlets for oil and air are provided in the outer and inner cylindrical walls of the chamber respectively. Oil droplets are formed in the rigid porous material and ejected out through the oil outlet under centrifugal forces while air relatively free from oil exits the air outlet and enters into the inside of the hollow shaft under the pressure difference. A cut-away area is formed in the rigid porous material to facilitate ejection from the separator of the oil droplets formed in the porous material to clear the passages of the porous material and facilitate a continuous movement of the mixture of air and oil mist.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus for separating a liquid in suspension, and more particularly to an apparatus for air/oil separation especially but not exclusively for use in gas turbine engine oil systems. 
     BACKGROUND OF THE INVENTION 
     Oil systems, particularly gas turbine engine oil systems require separators for separating air and oil from the mixture produced during operation. These mixtures vary from oil emulsified with air to air contaminated by droplets of oil. For example, the compressed air streams used in gas turbine engines to pressure labyrinth seals for the engine main bearings in order to avoid excessive loss of a lubricating oil, almost invariably become contaminated with oil in the form of droplets suspended in the air. Contamination of the compressed air with oil mist is particularly disadvantageous because, loss of contaminated air from the labyrinth seals in the compressor causes fouling of the engine parts and produces noxious and unpleasant products in air drawn from the compressor for cabin pressurization. A further disadvantage is, of course, the increased loss of the lubricating oil from the engine oil reservoir, necessitating larger capacity reservoirs and adding to the weight of the engine. It has been a particular problem that even relatively small inefficiencies in such separators lead to the loss of a significant quantity of lubricating oil during each hour of operation of the engine. 
     Centrifugal separators have been extensively used in the aircraft industry in attempts to remove the majority of oil mist from compressed air streams. An example is described in U.S. Pat. No. 4,714,139 issued to Lorenze et al. on Dec. 22, 1987 in which an air/oil separator is used in a gas turbine power plant especially for an aircraft that includes a pump used for the separation of the air and the oil, out of the air/oil mixture. The pump is so constructed that centripetal and centrifugal forces are employed for the air/oil separation. The air passes by centripetal force through a sponge type filter structure and out through a hollow central shaft mounting the pump wheel or forming an integral one-piece component with the pump wheel. The oil does not travel through the filter but is reversed in its travel direction by centrifugal force for return into the lubricant circulating system. 
     Another example is U.S. Pat. No. 4,755,103 which issued to Streifinger on Jul. 5, 1988. Streifinger describes means for separating an air/oil mixture and for returning oil droplets to a circulation having oil consuming devices, which includes a porous air-permeable element built into the hollow main shaft between two oil impermeable walls whereby outer feed openings for the air/oil mixture are arranged in the upstream wall which correspond to openings in the main shaft while an air discharge opening is arranged through the downstream wall in its center which is in communication with the vent line. The oil is prevented from flowing out of the air discharge opening by centrifugal force and it is thrown back into the bearing chamber and is fed back to the tank by way of the suction line. 
     The disadvantage of the above prior art lies in that the oil separated from the air/oil mixture travels in a reverse direction relative to the air/oil mixture flow in the sponge type filter or the porous air-permeable element, and exits from the inlet for admitting the air/oil mixture, thereby creating a blockage and increasing the delta pressure across the air/oil separator, resulting in loss of efficiency. 
     Smith describes, in U.S. Pat. No. 4,049,401 issued on Sep. 20, 1977, a centrifugal separator for separating suspensions of oil mist in air and especially intended for use with aircraft engines. The separator comprises a rotatable chamber filled with a relatively rigid porous material and driven by a hollow shaft. An inlet for the suspension is provided in one end wall of the chamber and separate outlets for oil and air in the form of an apertured chamber outer wall and apertures in the shaft respectively. A buffer is positioned within the chamber between two different grades of porous material to facilitate radial movement of the oil droplets in the porous material. The oil droplets are directed away from the air/oil mixture flow path, thereby reducing the blockage and therefore the delta pressure across the air/oil separator. However, as pointed out by Smith, it has proven possible to dispense with the buffer and the differing grades of porous material without any significant loss of efficiency. In other words, the buffer and differing grades of porous material have not improved the efficiency of the separator. 
     Therefore, there is a need for an improved air/oil separator for more efficient separation of the air/oil mixture. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide an air/oil separator with a relatively higher efficiency of separation. 
     It is another object of the present invention to provide an improved form of separator suitable for use in aircraft and lending itself to the construction of a modular and interchangeable separator that is simply constructed and relatively economical to manufacture. 
     In accordance with the present invention there is provided an apparatus adapted for separating a liquid suspended in a gas, comprising a rotatable vessel defining a chamber and a packing within the chamber adapted to rotate with the vessel. The rotatable vessel has an inlet at a first end of the chamber for admitting the gas having the liquid suspended into the chamber, an outlet for the liquid in the vessel in a radially outer part of the chamber and an outlet for the gas in the vessel in a radially inner part of the chamber at a second end thereof. The packing includes a substantially rigid matrix adapted to inhibit collapse under centrifugal forces during rotation of the vessel, the matrix including interstices defining a plurality of flow passages permeable to the liquid and gas for both axial and radial movement of the liquid and gas therethrough. A cut-away area is formed in the packing to facilitate ejection from the packing of liquid droplets formed in the passages of the packing, whereby the passages are relatively cleared to facilitate a continuous movement of a succeeding portion of the mixture of the liquid and gas therethrough. The cut-away area preferably extends inwardly and radially from an outer periphery of the packing. It is also preferable that the cut-away area is axially located close to the first end of the chamber, and axially aligns with the liquid outlet of the chamber. 
     In one embodiment of the present invention the cut-away area comprises a plurality of bores circumferentially spaced apart from one another, extending radially from the outer periphery of the packing. 
     In another embodiment of the present invention the cut-away area comprises an annular groove extending radially from the outer periphery of the packing. 
     In accordance with another aspect of the present invention, an air/oil separating arrangement is provided for a machine having a rotatable hollow shaft. In the arrangement a vessel defining a separating chamber is mounted to the hollow shaft and adapted to rotate together with the hollow shaft. The vessel has an inlet at a first end of the separating chamber for admitting a mixture of air and oil mist under a pressure differential between outside of the chamber and inside of the hollow shaft, an outlet in a radial inner part of the chamber in communication with the inside of the hollow shaft for exhausting the air into the hollow shaft under the pressure differential, and an outlet in the vessel in a radial outer part of the chamber for expelling oil droplets out of the chamber under centrifugal forces during rotation of the chamber. A packing is filled within the chamber adapted for rotation with the chamber. The packing has a substantially rigid matrix adapted to inhibit collapse under the centrifugal forces. The matrix includes interstices defining a plurality of flow passages permeable to the air and oil mist for both axial and radial movement of the air and oil therethrough. A cut-away area formed in the packing radially extending from an outer periphery of the packing to facilitate ejection from the packing of the oil droplets formed in the passages of the packing, whereby the passages are cleared to facilitate a continuous movement of a following portion of the mixture of the air and oil mist. 
     The cut-away area is preferably located axially close to but spaced apart from an end of the packing adjacent to the inlet. It is also preferable that the oil outlet is axially located close to the first end of the chamber and axially aligns with the cut-away area of the packing while the air outlet is axially located close to a second end of the chamber. 
     Preferably, the vessel defining the separating chamber is sealingly mounted to the hollow shaft, and an annular space formed between the radially inner part of the chamber and the hollow shaft communicates with the inside of the hollow shaft through at least one aperture through a wall of the hollow shaft so that a pressure at the air outlet of the chamber is maintained lower than the pressure at the inlet and the oil outlet of the chamber. 
     The air/oil separator according to the present invention advantageously provides an efficient solution with a relatively simple structure for separating oil suspended in air. A major proportion of the oil suspended in the air forms oil droplets in the passages of the packing at a first stage after entering the packing. The major proportion of oil droplets formed in the passages at the first stage will partially block the passages and slow down the continuous movement of a following portion of the mixture in the passages although the oil droplets are eventually ejected away radially by centrifugal forces. The cut-away area formed close to the inlet is used as an early exit from the passages in the packing and a temporary reservoir for the major proportion of the oil droplets so that the oil droplets formed in the passages at the first stage are collected in the cut-away area and quickly ejected from the packing to clear the passages and facilitate a continuous movement of a following portion of the mixture of air and oil. 
     Other advantages and features of the invention will be better understood with reference to the preferred embodiments described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration the preferred embodiments thereof in which: 
     FIG. 1 is a longitudinally cross-sectional view of an air/oil separator mounted on a hollow shaft according to one preferred embodiment of the invention; 
     FIG. 2 is a cross-sectional view of the air/oil separator taken along line  2 — 2  in FIG.  1  and rotated 90° counterclockwise; 
     FIG. 3 is a longitudinally cross-sectional view of the air/oil separator taken along line  3 — 3  and detached from the hollow shaft; and 
     FIG. 4 is a longitudinally cross-sectional view of a air/oil separator according to another preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, particularly to FIGS. 1 through 3, a rotatable centrifugal separator  10  is mounted on a hollow shaft  12  of a gear box for a jet engine. The hollow shaft  12  is rotatably supported in bearings, only one of which is shown and indicated at numeral  14 , and is driven from a main shaft of the engine (not shown) by way of gear  16 . A mixture including oil mist suspended in air is passed via the gear box casing (not shown) to the inlet  18  for the separator  10 . 
     The separator  10  includes a rotatable chamber  20  formed by a rear wall  22 , a front walls  24 , outer cylindrical wall  26  and inner cylindrical  28 . The rear wall  22  and the outer cylindrical wall  26  are made as an integral piece while the front wall  24  and the inner cylindrical wall  28  are made as another integral piece. The rear wall  22  includes an annular and axial flange  30 , and the front wall  24  includes an annular flange  32  having a radial section  34  so that the assembly of chamber  20  is radially supported on the hollow shaft  12  through the flanges  30  and  32 . 
     Inside the chamber  20  there is provided a packing  36  snugly fit between and frictionally engaging the outer and inner cylindrical walls  26  and  28  so that the assembly of the chamber  20  is secured together. The assembly of chamber  20  is axially restrained on the hollow shaft  12  through the radial section  34  of the flange  32  which is forced to abut an annular shoulder  38  of the hollow shaft  12  by, for example, a nut (not shown) via the bearing  14  and an annular spacer  40 . The assembly of chamber  20  is rotatable together with the hollow shaft  12 , connected to the hollow shaft  12  by any well known means, such as a key and a notch, not shown. 
     An annular air outlet  42  is formed between the rear wall  22  and the end of the inner cylindrical wall  28  because the inner cylindrical wall  28  is shorter than the outer cylindrical wall  26 . An oil outlet  44  is provided in the outer cylindrical wall  26 . Seals are provided between the chamber  20  and the hollow shaft  12  one of which, as an example, is shown and indicated at numeral  46 , whereby a pressure difference is maintained between the outside of the chamber  20  and the inside of the hollow shaft  12 . An annular space  48  is formed between the inner cylindrical wall  28  of the chamber  20  and the hollow shaft  12  to communicate with the inside of the chamber  20  through the outlet  42  and the inside of the hollow shaft  12  through the apertures  50  in the wall of the hollow shaft  12  to maintain the pressure at the outlet  42  lower than the pressure at the inlet  18 , thereby causing a flow of the mixture of air and oil mist entering the inlet  18 , and a flow of air exiting from the outlet  42  to enter the inside of the hollow shaft  12 . The air generally will not escape from the outlet  44  when passing through the packing  36  because the air flows towards the outlet  42  under the pressure difference between the inlet  18  and the outlet  42  while there is no pressure difference between the inlet  18  and the outlet  44 . On the other hand, the mixture of the air and oil may enter the outlet  44  because of the pressure difference between the outlet  44  and the outlet  42 . Nevertheless, a majority of the mixture of air and oil mist will be admitted to the chamber  20  through the inlet  18  but not the outlet  44  because the oil droplets formed in the passages of the packing  36  radially move toward the outlet  44  under centrifugal forces resulting in a blockage to the mixture of air and oil mist entering outlet  44 . 
     A most satisfactorily permeable and substantially rigid matrix material to form the packing  36 , is the product Retimet™ made by the Dunlop Company Ltd. Retimet™ includes a mesh of metal formed by plating a metal onto a synthetic open-celled formed structure and subsequently leaching out and/or otherwise removing the synthetic material. By substantially rigid it is meant that the matrix is capable of withstanding the centrifugal and other loads imposed on it during operation of the separator without suffering a significant amount of deformation which would tend to close the mesh and unduly restrict the flow of the mixture of air and oil therethrough. One particular advantage that stems from the use of Retimet™ is that it can be radially machined approximately to size and is sufficiently deformable to allow it to be sized by compressing prior to its insertion into the chamber  20 . 
     The matrix material, nevertheless, could be formed of any suitable mesh material, for example, wire gauze or expanded mesh, a plurality of pierced plates, or alternatively a bristle structure. The essential feature of the matrix is that it should provide both axial and radial passages therethrough and a relatively large surface area of the passages for oil particles to collect on so that oil may migrate radially outwards and air radially inwards while the mixture of the air and oil mist travel axially through the matrix. It is suggested that a matrix having a higher delta pressure should be selected for use when the oil concentration in the mixture of air and oil mist to be separated, is higher. 
     In order to improve the efficiency of the separator  10 , a plurality of radial bores  52  are drilled in the outer periphery of the packing  36  and circumferentially spaced apart from one another as shown in FIG.  2 . The radial bores  52  are located axially close to the front wall  24  of the chamber  20 . The distance between the center of each radial bore  52  and the outer surface of the front wall  24 , as indicated by letter “D” in FIG. 3 is about two times the diameter of the radial bore  52  when the thickness of the front wall  24  measures between ½ and the total diameter of a radial bore  52 . 
     In accordance with the deployment of radial bores  52 , the outlet  44  is formed with a plurality of apertures  44 a drilled in the outer cylindrical wall  26  of the chamber  20  and circumferentially spaced apart from one another. Each of the apertures  44   a  has a diameter equal to that of the radial bores  52  and aligns with the respective radial bores  52 . The inlet  18  is accordingly formed with a plurality of apertures  18   a,  numbering equally to the radial bores  52 , drilled in the front wall  24  and circumferentially spaced apart from one another. Each of the apertures  18   a  has a diameter which is equal to the distance from the outer surface of the outer cylindrical wall  26  to the bottom of the radial bore  52  as indicated at letter “d” in FIG.  3 . It is suggested that the distance “d” is about 3 times of the diameter of a radial bore  52 . The center line of each aperture  18   a  is radially spaced apart from the outer surface of the outer cylindrical wall  26  a distance of “S” which is equal to or slightly greater than the radius of the aperture  18   a  plus the thickness of the outer cylindrical wall  26  so that the apertures  18   a  are not blocked by the outer cylindrical wall  26 . When the chamber  20  is assembled with the packing  36 , the center line of the radial bores  52  is angled, as indicated by “A” as shown in FIG. 2, with a radium of the front wall  24  passing the center of the corresponding apertures  18   a  in the direction opposite to rotation “R” so that the radial bores  52  are substantially tangent to the inlet apertures  18   a  as shown in FIG.  2 . 
     In operation, the mixture of air and oil mist enters the inlet apertures  18   a  under pressure and generally moves axially through the packing  36 . The inlet apertures  18   a  are located at a distance from the axis of rotation of the separator  10 , where the centrifugal force field acting on the oil mist is relatively strong. Therefore, the heavy oil droplets formed in the passages of the packing  36  are thrown radially to the outer periphery of packing  36 . The mixture of air and oil mist when just entering the packing  36  through the inlet apertures  18   a  is oil rich and a major proportion of oil droplets are formed in the passage in the packing  36 . The major proportion of the oil droplets rotate together with, but slower than the packing  36  because of their inertia. Thus, the movement of the oil droplets is offset from the axial direction and the droplets are collected in the radial bores  52  which are circumferentially behind the respective inlet apertures  18   a  relative to the direction of the rotation. The oil droplets collected in the radial bores  52  are rapidly thrown out of the rotating chamber  20  through the outlet apertures  44   a  by centrifugal forces. After the major proportion of the oil droplets are ejected from the radial bores  52 , the mixture of air and oil mist in the passages in the packing  36  downstream of the radial bores  52  is relatively oil lean. Therefore, the remainder of the oil mist in the mixture forms a relatively smaller quantity of oil droplets in the passages of the packing  36  downstream of the radial bores  52  so that the passages in the packing  36  downstream of the radial bores  52  are relatively cleared to facilitate a continuous movement of following portion of the mixture of the air and oil mist. 
     The oil droplets formed from the remainder of the oil mist in the passages in the packing  36  downstream of the radial bores  52  are driven by the centrifugal forces to move radially and outwardly towards the outer cylindrical wall  26  of the chamber  20 , and eventually move along the outer cylindrical wall  26  and exit from the outlet apertures  44   a.  Air relatively free from oil mist then leaves the separator  10  through the air outlet  42  and the annular space  48  to enter the apertures  50  in the wall of the hollow shaft  12 , and is carried to a point of use by the hollow shaft  12 . 
     An air/oil separator  10   a  according to another preferred embodiment of the invention as shown in FIG. 4 generally has a structure similar to the air/oil separator  10  shown in FIGS. 1-3 and the parts similar to those equivalents in FIG. 3 are indicated by the same numerals and will not be redundantly described. 
     The cut-away area of the packing  36  of the air/oil separator  10   a,  being different from the plurality of radial bores  52  of the air/oil separator  10  shown in FIG. 1, is formed by an annular groove  52   a  radially extending from the outer periphery of the packing  36  to simplify the machining of the packing  36 . The depth and axial position of the annular groove  52   a  are similar to those of the radial bores  52  of the air/oil separator  10 , whereby the annular groove  52   a  will simulate the same advantage as the radial bores  52  of the air/oil separator  10  at a lower manufacturing cost. This structure also makes assembly of the air/oil separator  10   a  easier. Unlike the angular relationship indicated by “A” in FIG. 1, between the radial bores  52  and the inlet apertures  18   a,  there is no such relationship required between the inlet apertures  18   a  and the annular groove  52   a  because the annular groove  52   a  is circumferentially continuous. The number of outlet apertures  44   b  can be determined without matching the number of the radial bores  52 . In this particular embodiment, five outlet apertures  44   b  are provided, equally and circumferentially spaced apart from one another (not shown). The diameter of each of the outlet apertures  44   b  is about ½ of the width of the annular groove  52   a.    
     The outer cylindrical wall  26  of the chamber  20  in this embodiment, being different again from that of the air/oil separator  10 , is formed with an upstream section  26   a  having a relatively larger diameter and a downstream section  26   b  having a relatively smaller diameter. The mixture of air and oil mist under the pressure difference moves axially and radially, inwardly from the inlet apertures  18   a  towards the annular outlet  42  so that the air with the remainder of the oil mist to be separated moving in the passages in the downstream section of the packing  36  is generally in an inner radial portion of the packing  36 . Therefore the diameter reduced downstream section  26   b  of the packing  36  will not substantially affect the efficiency of the separation of while the weight of the separator is thereby reduced. In addition the stepped outer cylindrical wall configuration facilitates the oil droplets formed in the passages downstream of the annular groove  52   a  to flow back along the outer cylindrical wall  26  towards the outlet apertures  44   b.    
     Modifications and improvements to the above described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.