Improvements in and relating to compressors

Abstract: There is disclosed a compressor an impeller wheel (12) including a plurality of vanes or blades (14) each of which includes a leading edge (16), a trailing edge (18) and an outer free edge (20), said wheel (12) being mounted for rotation within a stationary housing (10), the housing (10) including an inner wall (28) and an outer wall (22), at least part of the inner surface (32) of the inner wall (28) being in close proximity to, and of similar contour to, the outer free edges (20) of the blades or vanes (14), and forming an inlet (30) to said impeller wheel (12) in a region adjacent the leading edges (16) of said blades or vanes (14), said outer wall (22) forming a gas intake (24) surrounding said inner wall (28) and extending in an axial direction, said gas intake connecting with a chamber (34), formed between said inner and outer walls (28 and 22) in a region preferably at least partly surrounding said blades or vanes (14), and communication being provided through said inner wall (28) between said chamber (34) and the inner surface (32) of said inner wall (28) whereby gas may pass between the area swept by the vanes or blades (14) and the gas intake (24) via the chamber (34). The compressor may be included in a multi-stage compressor and/or may be used in a turbocharger for an internal combustion engine.

Description: The present invention relates to compressors e.g. axial and centrifugal compressors and multi-stage versions thereof. Compressors normally comprise an impeller wheel, carrying a plurality of blades or vanes, and mounted on an axis for rotation within a stationary housing. Rotation of this impeller wheel causes gas (usually air) to be drawn into the impeller wheel and to be discharged to a passage or passages for transferring the compressed gas to its destination. In the case of a centrifugal compressor the gas is discharged centrifugally and in the case of an axial compressor the gas is discharged axially. In the case of a turbine drivon compressor in e.g. a turbocharger, the compressor impeller wheel and the turbine wheel are mounted on a common axis so that rotation of the turbine wheel causes rotation of the impeller wheel. It has been proposed in U.S. Specification No. 4,248,566 to form an annular control slot in the stationary housing so as to allow an inflow of gas from outside the housing to the impeller wheel under high r.p.m. conditions nf compressor operation and to allow gas flow to bleed from the impeller wheel to the exterior of the housing when the wheel is operating at lower r:p.m. whereby to flow stabilize the impeller wheel at part r.p.m. operation. Such an arrangement however provides stable operation over only a relatively narrow range of engine r.p.m. and there is now a requirement to increase the engine r.p.m. range over which compressors can operate in stable manner. This is achieved in accordance with the present invention by providing communication between the chamber in which the compressor wheel rotates and an annular chamber formed in the gas intake to the impeller wheel and preferably at least partly surrounding the impeller wheel. The air is thus not bled to the exterior of the housing, and thus atmosphere, nor drawn in from atmosphere separately from the normal gas intake to the compressor (as in US-A 4 248 566), but is bled back to the normal intake or Is drawn from the normal intake. US-A 4 248 566 describes a compressor arrangement comprising an Impeller wheel induding a plurality of blades or vanes each of which includes a leading edge, a trailing edge and an outer free edge, said wheel being mounted for rotation within a stationary housing, which housing includes an inner wall and an outer wall, at least part of the inner surface of the inner wall being in dose proximity to, and of similar contour to, the outer free edges of the blades or vanes, said outer wall forming a gas intake extending in an axial direction. The arrangement indudes an annular control slot formed in the housing which allows an inflow of gas from outside the housing to the impeller wheel when it is running at high r.p.m. and allows gas to bleed from the impeller wheel to the oxterior of the housing when it is running at low r.p.m. This arrangement however provides stable operation over only a relatively narrow range of engine r.p.m. In the arrangement of the present invention the air is not bled to the exterior of the housing (i.e. the atmosphere) nor drawn in from the atmosphere separately from the normal gas intake to the compressor, but is drawn from and bled back to the normal intake to the compressor. This arrangement provides stable operation over a wider r.p.m. range. German Patent 1087747 shows a typical approach for blowing off excess air delivered by a compressor when a significant reduction in delivery output is required, for example in the process industry. In the German patent a valve 12 opens to let excess airflow from the compressor discharge through a bypass pipe 11 in cases where the output from the compressor diffuser 6 is not as great as in the unbypassed condition. According to the present invention there is disclosed a compressor comprising an impeller wheel including a plurality of vanes or blades each of which includes a leading edge, a trailing edge and an outer free edge, said wheel being mounted for rotation within a stationary housing, which housing includes an inner wall and an outer wall, at least part of the inner surface of the inner wall being in close proximity to, and of similar contour to, the outer free edges of the blade or vanes, said outer wall forming a gas intake extending in an axial direction, said gas intake surrounding said inner wall, said inner wall forming an inlet to said impeller wheel in a region adjacent the leading edges of said blades or vanes, a chamber formed between said inner and outer walls in a region at least partly surrounding said blades or vanes, a communication being provided through said inner wall between said chamber and the inner surface of said inner wall, said communication providing a bidirectional flowpath through said inner wall, between said chamber and the inner surface of said inner wall which flowpath is open at all times and gas movement in one direction or in the other direction through said flowpath is in response to the pressure differential between said chamber and the area swept by the vanes or blades, one end of said flowpath opening into the inner surface of the inner wall at a position not more than 34% along the meridional length from the leading edge of the blades or vanes, and the total cross-sectional area of said opening of the flowpath to the inner surface of the inner wall being at least 13% of the inducer annular area (i.e. the frontal area of the impeller wheel at the leading edge minus of the hub area), and communication being provided between said gas intake and said chamber, which communication is open at all times. The communication between the chamber and the inner surface of the inner wall may be an annular slot extending around the inner wall and bridged by a series of connecting webs or may be a plurality of holes. In the event that the communication comprises a plurality of holes then it is preferred that the number of such holes is not equal to, nor a multiple of, nor a factor of, the number of blades or vanes on the impeller wheel. Excitation may well occur in the event that the number of such holes is equal to, a multiple of, or a factor of, the number of blades or vanes. The preferred number of holes (subject to the above condition) is from 29 to 43. Preferably the total area of the holes or the slot at the inner surface of the inner wall is from 13 to 23% of the inducer annular area (I.e. the frontal area of the impeller wheel at the leading edge minus the hub area). In the case of centrifugal compressors the holes or slot are preferably located at a point along the meridional length just upstream of the point of minimum static pressure, and more preferably at a point some 65 to 75% of the distance from the leading edge of the blades to the minimum static pressure point The point of location of the slot or holes is thus typically some 22 to 34% along the meridional length from the leading edges of the blades or vanes. In the case of axial compressors the holes or slots are preferably located some 15 to 25% along the length of the outer free edges of the blades from the leading edges. The final exact selection of the various preferred features (e.g. slot or holes, area, and position of hole or slot) for optimum benefit depends upon the particular compressor and its use. During high flow and high r.p.m. operation of the compressor the pressure at the impeller end of the slot or holes is less than the pressure at the chamber end of the slot or holes and air thus flows through the slot or holes from the annular chamber to the impeller wheel thereby increasing the amount of air reaching the impeller wheel. During operation of the compressor near its surge line however, the pressure at the impeller end of the slot or holes increases to above that at the chamber end of the slots or holes and thus air bleeds out of the area swept by the impeller wheel, through the slot or holes and through the annular chamber, thereby reducing the amount of air in the impeller wheel. The air bleeding out of the impeller wheel is thus recirculated to the inlet. This stabilizes compressor operation, moving the surge line to lower flow over the entire r.p.m. range of the compressor. Use of the compressor of the present invention enables compressor operation over a wider range of engine r.p.m. than was previously possible. The compressor of the present invention is especially useful when forming part of a turbocharger for an internal combustion engine particularly where an air deaner is provided upstream of the air intake to the compressor. This latter preference is because the air deaner results in the air pressure in the intake being depressed below atmospheric to a greater extent than without an air deaner and thus results in even better operation of the compressor of the invention due to the pressure differential between the two ends of the slot of holes at low flow (i.e. near surge) being greater. In a multi-stage compressor a number of compressors e.g. axial, centrifugal or both are connected in series so that the outlet from one compressor leads to the inlet of the next compressor in the series. One or more of the compressors in series may be in accordance with the invention. The invention will now be further described by way of example with reference to the accompanying drawings in which :- Figure 1 is a graph of pressure against mass flow in a compressor ; Figure 2 is a cross-section through part of a compressor in accordance with one embodiment of the present invention ; Figure 3 is a cross-section through part of a compressor in accordance with another embodiment of the present invention ; Figure 4 is a cross-section through part of a compressor in accordance with a further embodiment of the present invention ; Figure 5 is a cross-section through part of a compressor in accordance with yet a further embodiment of the present invention ; Figure 6 is a cross-section through a multi-stage compressor in accordance with the present invention. Referring to Figure 1 there is shown a graph plotting pressure against mass flow in a single stage centrifugal compressor. The area between the lines D and E which is shown by shading, indicates a typical engine r.p.m. range over which a compressor not incorporating the present invention will operate. There is however a requirement to increase the engine r.p.m. range to cover an area between the lines D and B on the graph and it is therefore necessary to a;ter the characteristics of the compressor in order to move the surge line from the line marked S 1 to the line marked S 2 . This can be achieved by use of the present invention. Similar results can be achieved with an axial compressor. Referring now to Figure 2, there is shown a cross-section view of a single stage centrifugal compressor comprising a housing 10 having an impeller wheel 12 mounted in conventional manner for rotation therein. The wheel indudes a plurality of blades or vanes 14 of conventional design and each induding a leading edge 16, a trailing edge 18 and an outer free edge 20. The housing includes an outer wall 22, defining an intake 24 for gas such as air, and a passageway or passageways 26 for carrying compressed gas from the impeller wheel 12 to its destination e.g. the inlet manifold of an internal combustion engine. An inner wall 28 defines an inlet 30 to the impeller and an inner surface 32 of said inner wall 28 is in dose proximity to and of extremely similar contour to, the outer free edges 20 of the blades or vanes 14. The inner wall 28 extends a short distance upstream from the blades 14 of the impeller wheel 12 whereby to form an annular space or chamber 34 between the walls 22 and 28. The annular chamber 34 partly surrounds the impeller wheel 12. An annular slot 36 is formed in the wall 28 and a series of webs 38 serve to bridge the annular slot at intervals round its circumference. The slot 36 is located along the meridional length (line A on the drawing) at a point just upstream of the point of minimum static pressure. This point is preferably some 65 to 75% of the distance from the leading edges 16 of the blades or vanes 14 to the point of minimum static pressure and is typically 22 to 34% of the impeller blade length. In the arrangement shown in Figure 1 the slot is located some 73% of the distance from the leading edge 16 of the blades 14 to the point of minimum static pressure and is 30% of the length of the impeller blades 14 from the leading edges 16 of the blades. The total area of the slot is normally of the order of 13 to 23% of the inducer annular area In the arrangement shown the total area of the slot is 15% of the inducer annular area. In operation the impeller wheel 12 is rotated e.g. by a turbine wheel (not shown) attached to a common axis with the compressor wheel and this causes air to be drawn into the impeller wheel 12 through intake 24 and inlet 30. The air is compressed by the impeller wheel 12 and Is then fed to its ultimate destination via passageway or passageways 26. The pressure in the chamber 34 is normally lower than atmospheric pressure and during high flow and high r.p.m. operation the pressure in the area swept by the impeller wheel is less than in the chamber 34 and thus air flows through the slot 36 from the chamber 34 to the impeller wheel 12 thereby increasing the amount of air reaching the impeller wheel, and increasing its maximum flow capacity. As the flow through the impeller wheel 12 drops or as r.p.m. of the impeller wheel drops so the amount of air drawn into the wheel 16 through the slot 36 decreases until equilibrium is reached. Further drop in impeller wheel flow or r.p.m. results in the pressure in the area swept by the impeller wheel being greater than in the chamber 34 and thus air flows through the slot 36 from the impeller 12 to the chamber 34. The air bled out of the impeller wheel 12 is recirculated to the air intake and thereby back to the inlet 30. Increase in flow or r.p.m. of the impeller wheel causes the reverse to happen, i.e. a decrease in the amount of air bled from the impeller wheel followed by equilibrium followed by air being drawn into the impeller wheel 12 via the slot 36. This particular arrangement results in improved stability of the compressor at all speeds and a shift in the characteristics of the compressor. For example, the surge line is moved as shown in Figure 1 from S 1 to S 2 and the maximum flow capacity is moved from line F 1 to F 2 as shown in Figure 1. The compressor can thus be matched to engines with a wider speed range than can conventional compressors. Referring now to Figure 3 there is shown an alternative embodiment in which the slot 36 is replaced by a series of holes 40. In this case there is of course no need for the webs 38 of the arrangement of Figure 2. The positioning of the holes 40 along the meridional length and area of the holes at the inner surface 32 is similar to the positioning and area of the slot 36 in Figure 2. The number of holes should be arranged so that it is not equal to, nor a multiple of, nor a factor of the number of blades on the compressor wheel. If the number of holes is a multiple of or a factor of the number of blades then excitation can be induced. In the arrangement shown in Figure 3 the number of holes 40 is 29 and the number of blades is 16. Referring now to Figure 4 there is shown a further alternative embodiment of the invention in which the chamber 34 is formed by a series of blind bores 42 in the wall of the housing. The inner and outer walls 28 and 22 respectively are thus connected between these bores 42. The bores may be connected either to an annular slot similar to slot 36 in Figure 2 or to a series of holes similar to those holes 40 in Figure 3. Referring now to Figure 5 there is shown an arrangement In which the chamber 34 is formed partly in the housing 10 and partly by an annular slot 44 (with connecting webs) or series of holes 44 formed in a ring 46 which may be aluminium or plastic. The chamber 34, as in other embodiments, communications with the impeller wheel 12 via a series of holes or a slot. Referring now to Figure 6, there is shown a multi-stage compressor, comprising an axial compressor 100, and two centrifugal compressors 102 and 104 arranged in series. Axial compressor 100 indudes an impeller wheel 12 having a series of vanes or blades 106 each of which includes a leading edge 108, a trailing edge 110 and an outer free edge 112. Air compressed by compressor 100 is fed via axial outlet 114 to the inlet 116 of centrifugal compressor 102. Axial compressor 100 includes inner and outer walls 28 and 22 respectively defining an annular space or chamber 34 as in the arrangement of Figures 2 and 3. In addition, a series of holes 40 (which could alternatively be a slot 36) is provided as in the device of Figure 3. Operation is similar to that of the device of Figures 2 and 3 with air bleeding from the impeller wheel 12 to the chamber 34 near surge and with air being drawn from the chamber 34 to the impeller at high flow and high r.p.m. The two centrifugal compressors 102. 104 are each individually similar to one of the compressors described in connection with one of Figures 2 to 5. The outlet from compressor 102 is connected to the inlet to compressor 104.

Claims: A compressor comprising an impeller wheel (12) including a plurality of vanes or blades (14) each of which includes a leading edge (16), a trailing edge (18) and an outer free edge (20), said wheel (12) being mounted for rotation within a stationary housing (10), which housing (10) includes an inner wall (28) and an outer wall (22), at least part of the inner surface (32) of the inner wall (28) being in close proximity to, and of similar contour to, the outer free edges (20) of the blade or vanes (14), said outer wall (22) forming a gas intake (24) extending in an axial direction, said gas intake (24) surrounding said inner wall (28), said inner wall (28) forming an inlet (30) to said impeller wheel (12) in a region adjacent the leading edges (16) of said blades or vanes (14), a chamber (34) formed between said inner and outer walls (28 and 22) in a region at least partly surrounding said blades or vanes (14), a communication (36,40) being provided through said inner wall (28) between said chamber (34) and the inner surface of said inner wall (28), said communication (36,40) providing a bidirectional flowpath (36,40) through said inner wall (28), between said chamber (34) and the inner surface (32) of said inner wall (28) which flowpath is open at all times and gas movement in one direction or in the other direction through said flowpath (36,40) is in response to the pressure differential between said chamber (34) and the area swept by the vanes or blades (14), one end of said flowpath opening into the inner surface of the inner wall (28) at a position not more than 34% along the meridional length from the leading edge of the blades or vanes (14), and the total cross-sectional area of said opening of the flowpath (36,40) to the inner surface of the inner wall being at least 13% of the inducer annular area (i.e. the frontal area of the impeller wheel (12) at the leading edge minus of the hub area), and communication being provided between said gas intake (24) and said chamber (34), which communication is open at all times. A compressor as claimed in claim 1, characterised in that the bidirectional flowpath between the chamber (34) and the inner surface (32) of the inner wall (28) is an annular slot (36) extending around the inner wall and bridged by a series of connecting webs (38), or a plurality of holes (40). A compressor as claimed in claim 2, characterised in that the bidirectional flowpath comprises a plurality of holes (40) and the number of such holes (40) is not equal to, nor a multiple of, nor a factor of, the number of blades or vanes (14) of the impeller wheel (12). A compressor as claimed in claim 2 or 3, characterised in that the bidirectional flowpath comprises from twenty-nine to forty-three holes (40). A compressor as claimed in claim 2, 3 or 4, characterised in that the total area of the holes (40) or the slot (36) at the inner surface (32) of the inner wall (28) is from 13 to 23% of the inducer annular area (i.e. the frontal area of the impeller wheel (12) at the leading edge minus the hub area). A compressor as claimed in any one of claims 2 to 5, characterised in that the compressor is a centrifugal compressor and the holes (40) or slot (36) are located at a point along the meridional length just upstream of the point of minimum static pressure. A compressor as claimed in any one of claims 2 to 6, characterised in that the holes (40) or slot (36) are located at a point some 65 to 75% of the distance from the leading edge (16) of the blades (14) to the minimum static pressure point. A compressor as claimed in any one of claims 2 to 5, characterised in that the compressor is an axial compressor and the holes (40) or slots (36) are located some 15 to 25% along the length of the outer free edges (20) of the blades (14) from the leading edges. A multi-stage compressor characterised by comprising a number of compressors connected in a series so that the outlet from one compressor leads to the inlet of the next compressor in the series, in which one or more of the compressors in series is a compressor as claimed in any one of the preceding claims. A turbocharger for an internal combustion engine characterised by including a compressor as claimed in any one of the preceding claims.

Inventors: FISHER FRANK BYRON, LANGDON PAUL JOSEPH, FISHER, FRANK BYRON, LANGDON, PAUL JOSEPH
Applicants: HOLSET ENGINEERING CO, HOLSET ENGINEERING COMPANY LIMITED
Application year: 1986
Main classification: F04D27
IPC codes: F04D29, F04D27