GB2485835A

GB2485835A - Axially overlapping compressor impeller stages

Info

Publication number: GB2485835A
Application number: GB1020131.7A
Authority: GB
Inventors: Adrian Graham Alford; Julian Oliver Reed
Original assignee: Corac Group PLC
Current assignee: Corac Group PLC
Priority date: 2010-11-29
Filing date: 2010-11-29
Publication date: 2012-05-30
Also published as: GB201020131D0

Abstract

An impeller 1 for a compressor has two series connected stages 2, 7. The impeller has two sets of blades, one associated with each stage. The sets of blades axially overlap such that the total axial length of the bladed section of the impeller is less than the sum of the axial lengths of the two sets of blades. The two sets of vanes may be mounted on opposing sides of a blade supporting body. A third impeller stage may be provided, each adjacent pair of sets of blades being mounted on a common blade supporting body. At least some of the blades may be splitter blades. An air flow passage between the two sets of blades may cross the air flow inlet to the first set of blades.

Description

IMPELLER

Field of the invention

The invention relates to an impeller for a compressor of radial or mixed-flow configuration. The invention also relates to a compressor comprising such an impeller.

Background to the invention

A centrifugal or mixed-flow impeller generally consists of a hub or disc comprising a set of radially disposed blades or vanes projecting from the side of the hub or disc. The impeller is supported by bearings in a casing and is rotated, usually at high rotational speed, to accelerate a fluid such as air, another gas ora liquid and move it through a piping system. Centrifugal action causes the fluid to flow radially outwardly along the blades or vanes which increases the pressure of the fluid which then passes into a diffuser.

Selecting the right impeller type and dimensions for a specific application is typically based on a number of factors including the type of fluid, the rotational speed, the required flow rate and the required operating conditions such as temperature and pressure.

It is known to provide a single-stage centrifugal compressor for accelerating a fluid comprising a shaft driven by a motor, the shaft supporting at one end a single-sided impeller in which vanes project from one side of the hub, or a double-sided impeller in which two hubs are arranged back to back with their vanes projecting in opposite directions and a fluid is circulated through the impeller such that both sets of vanes are used to accelerate the fluid. Shrouds are provided, either rotating or stationary, to minimise or prevent leakage of a fluid around the impeller vanes.

One of the main factors affecting the pressure of the fluid achievable by the impeller is the blade tip speed. The fluid pressure can be increased, for example, by running the motor at a faster rotational speed to increase the rotational speed of the shaft supporting the impeller and the tip speed. The diameter of the impeller may also be increased so that the tip speed is increased for a given rotational speed of the motor and impeller shaft.

It is also known to increase the fluid pressure achievable by a compressor by employing a number of series connected impeller stages through which the fluid flows sequentially. It is known, for example, to provide a two-stage compressor having two single-sided impellers, one at each end of a common supporting shaft, or two single-sided impellers axially spaced from one another at the same end of the shaft. It is also known to provide a multi-stage compressor having more than two single-sided impellers, at least two of which are axially spaced from one another at the same end of the supporting shaft.

Examples of such compressors are disclosed in EP0359514. A diffuser and an intercooler can be provided between adjacent impeller stages to reduce the temperature of the fluid entering a subsequent impeller stage.

A problem of these known compressors is that the provision of two or more axially-spaced impellers at the same end of the supporting shaft to increase the fluid pressure leads to an increase in the required length and mass of the shaft and thereby an increase in the axial length and physical size of the compressor.

It is generally appreciated that, particularly in the case of high speed direct-drive compression systems, the need to prevent or control the modes of vibration of the shaft is the limiting factor in the achievement of a high efficiency of operation.

For a given shaft power and output pressure, a single-shaft, two-stage compressor will require a certain motor and shaft rotational speed to achieve a tolerable adiabatic efficiency. This rotational speed will often be above the critical speed of the shaft and impeller arrangement leading to unacceptable modes of vibration of the shaft and this situation cannot be improved within the constraints of known materials and shaft and impeller construction methods.

A known solution to this problem is to provide a compressor in which a common shaft supports at one end a first impeller providing a first impeller stage and at its other end, axially spaced second and third impellers providing second and third impeller stages.

The fluid is then circulated through the impeller stages in series to achieve the required pressure. The addition of the third impeller can reduce the rotational speed that is necessary to achieve a tolerable value of adiabatic efficiency. However, it will tend to increase the required mass and length of the shaft such that operation of the compressor continues to be limited by the aforementioned shaft dynamics problems to the same or a greater degree than those suffered by known single shaft, two-stage compressors. A third impeller also reduces the natural frequency of the shaft, requiring the compressor to be run at a slower rotational speed thereby reducing the impeller blade tip speed and so limiting or eliminating any advantage associated with adding the third impeller. The third impeller stage also adds significant cost and complexity to the compressor.

Summary of the invention

In accordance with the broadest aspect of the invention, there is provided an impeller for a compressor having two series connected stages, the impeller having two sets of blades associated with the respective stages, wherein the two sets of blades overlap one another axially such that the total axial length of the bladed section of the impeller is less than the sum of the axial lengths of the two sets of blades.

Nesting of the blades of the two impeller stages as proposed by the invention reduces the physical size of the impeller compared to the use of an equivalent number of separate axially spaced conventional impellers whilst permitting fluid pressure to be increased by an amount comparable to that achieved by an equivalent number of axially separate impellers of the same diameter.

According to a second aspect of the invention, there is provided an impeller for a compressor of radial or mixed-flow configuration, the impeller comprising: a first impeller stage consisting of a first set of vanes projecting from a first side of a first vane-supporting body rotatable about an impeller axis of rotation; a second impeller stage consisting of a second set of vanes projecting from a first side of a second vane-supporting body rotatable about the impeller axis of rotation; the first and second impeller stages being arranged so that the second set of vanes extends between the first side of the second vane-supporting body and a second side of the first vane-supporting body and couples the first and second vane-supporting bodies for simultaneous rotation of about the impeller axis of rotation; wherein the second side of the first vane-supporting body is adapted so as to receive at least part of the axial length of the second vane-supporting body and the second set of vanes such that the second impeller stage is at least partially nested or embedded within the first impeller stage.

The terms "vane" and "blade" are used interchangeably to refer to the parts of the impeller that project from the vane-supporting body to impinge on a fluid and accelerate it radially.

The term "vane-supporting body" is used to refer to the main body of an impeller such as a hub or disc from which the vanes or blades of the impeller project.

The term "splitter blades" is used to refer to blades projecting from the vane-supporting body that are shorter than other blades of the set of blades, and extend to the full tip diameter.

The impeller may comprise one or more additional impeller stages, each adjacent pair of impeller stages being coupled for simultaneous rotation by a set of vanes extending between and coupling the vane-supporting bodies of the adjacent pair of impeller stages and arranged such that the second side of one of the vane-supporting bodies of the adjacent pair is adapted so as to receive at least part of the axial length of the other vane-supporting body of the adjacent pair so that the adjacent impeller stages are at least partially nested or embedded within one another.

At least some of the vanes of at least one of the sets of vanes may be splitter blades.

Some of the blades which may include the splitter blades may be canted over at one end. This enhances the ability of the impeller stages to accelerate a fluid radially into a diffuser during use of the impeller by increasing the circumferential separation between the tips of the remaining blades and reducing friction on the fluid.

Preferably, the first and second impeller stages are formed integrally with one another. More preferably, adjacent impeller stages may be machined from a single billet of material. This minimises the mass and maximises the strength of the impeller, minimises the resistance to a fluid being accelerated by the impeller and minimises the number of steps required to manufacture the impeller.

The invention also provides a compressor of radial or mixed-flow configuration comprising: a motor and a shaft, the shaft supporting at least one end one or more of the impellers of any of the preceding claims or a combination of one or more of the impellers of any of the proceeding claims.

The invention addresses the aforementioned drawbacks of the prior art by combining or nesting two impeller stages to provide a more compact two-stage impeller having a shorter axial length and a reduced weight compared with known individual axially-spaced first and second impeller stages.

This results in a more compact compressor in which the shaft length and mass is minimised compared to known compressors having an equivalent number of impeller stages that are axially spaced from one another along the shaft. The invention permits the addition of further impeller stage for little or no increase in mass or shaft length making the compression system less susceptible to the aforementioned shaft dynamics problems associated with known compression systems.

The invention therefore enables the provision of a compression system within the constraints of available shaft and impeller materials and known shaft and impeller construction methods which has an enhanced adiabatic efficiency compared to known systems.

Preferably, the compressor comprises a casing for the at least one impeller, the casing defining a flow path for a fluid and including a first flow channel for directing a fluid from an inlet towards the first set of vanes, a first stage diffuser for receiving the fluid from the first set of vanes, a second flow channel for redirecting the fluid from the first stage diffuser towards the second set of vanes, a second stage diffuser for receiving the fluid from the second set of vanes and directing the fluid into a volute casing to an outlet.

Preferably the diffuser of the first compressor stage is water-cooled.

The casing of the compressor is arranged such that the first and second flow channels cross one another in the region of the casing between the exit of the first stage diffuser and the second set of vanes which is a relatively low velocity region of the impeller casing.

A compressor of the invention may be used with an induction or switched reluctance motor but is particularly suited for use with a permanent magnet motor.

The impeller and compressor of the invention are particularly suitable for the provision of high pressure air, such as 7-bar oil-free air, for a wide variety of industrial purposes. The compressor is not however restricted to such applications and can be used for fluids other than air and may for example be used in a refrigeration system. The impeller may further be used in a high pressure blower and in any other machines that currently use impellers of known type, particularly those in which it is essential or desirable to minimise the mass, physical size and complexity of the machine.

Brief Description of the Drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Figure 1 is a top perspective view of an impeller embodying the present invention; Figure 2 is an underneath perspective view of the impeller of Figure 1; Figure 3 is side view of the impeller of Figures 1 and 2; Figure 4 is a section view through the impeller of Figure 3; Figure 5 is a section view through a two-stage impeller arrangement comprising the impeller of Figures 1 to 4 and showing the fluid flow through the first and second impeller stages; and Figure 6 is a schematic illustration of a compressor comprising the impeller arrangement of figure 5.

Detailed Description

Figures 1 to 4 show an impeller I comprising a first or outer impeller stage 2 consisting of a first set of vanes 3 projecting from a first side 4 of a first vane-supporting body 5. The first vane-supporting body 5 is rotatable about an impeller axis of rotation 6.

The impeller also comprises a second or inner impeller stage 7 consisting of a second set of vanes 8 projecting from a first side 9 of a second vane-supporting body 10. The second vane-supporting body is rotatable about the same impeller axis of rotation 6. The second vane-supporting body is generally in the form of a hollow, truncated cone having a continuously curved wall from which the first set of vanes extends.

The first 2 and second 7 impeller stages are arranged so that the second set of vanes 8 extends between the first side 9 of the second vane-supporting body 10 and a second side 11 of the first vane-supporting body 5 and couples the first and second vane-supporting bodies for simultaneous rotation about the impeller axis of rotation 6.

The impeller is arranged so that the second impeller stage 7 is at least partially nested or embedded within the first impeller stage 2. The first vane-supporting body is effectively hollowed out to accommodate at least a portion of the axial length of the second impeller stage and the second set of vanes. This reduces the required axial length and physical size of the first and second impeller stages and an associated machine compared to known compressors in which independent first and second impeller stages axially spaced from one another along the common shaft are provided.

The second vane-supporting body 10 can be coupled to a supporting shaft using any suitable connection method that will be readily apparent to the skilled person. The impeller may also be integrally formed with the supporting shaft. The second vane-supporting body 10 may include a through or blind hole 12 to reduce its weight or facilitate mechanical connection.

The first vane-supporting body 5 forms a rotating shroud for the second set of vanes 8 of the second vane-supporting body 10 arranged to rotate simultaneously with the second vane-supporting body. The second set of vanes 8 are therefore enclosed by the first and second vane-supporting bodies which prevents leakage of a fluid around the second set of vanes and reduces the effects of friction between the fluid and the impeller casing in which the impeller rotates thereby further increasing the efficiency of operation of a machine which includes the impeller 1.

At least some of the vanes of at least one of the first 3 and second 8 set of vanes are curved, preferably continuously curved to enhance the transition of a fluid from an axial to a substantially radial direction. Splitter blades 3a are also provided in the first and second pluralities of vanes to facilitate the impingement of the vanes on the fluid and the acceleration of a fluid in the radial direction.

The impeller I comprising the first and second vane-supporting bodies and the first and second pluralities of vanes is preferably integrally formed as a single component. It can, for example be manufactured as a casting, or be machined from a single billet of material using 5-axis machining. It is envisaged that the impeller could alternatively be manufactured as two separate impeller stages that are coupled together for simultaneous rotation, for example, by one of the following joining methods: a splined connection, welding, brazing or shrink fitting. Other suitable joining methods will be readily apparent to the skilled person. Whether it is an integrally formed component or one in which the first and second stages are joined, the first and second impeller stages and the associated pluralities of vanes are rotatable simultaneously by a single supporting shaft.

The impeller can also comprise one or more additional impeller stages. One or more additional vane-supporting bodies can be provided, the impeller being arranged so that the vane-supporting bodies of adjacent pairs of impeller stages are coupled for simultaneous rotation by the intermediate set of vanes projecting between the first side of one vane-supporting body and the second side of the adjacent vane-supporting body.

Each adjacent pair of impeller stages are nested or embedded so as to reduce the total axial length of the impeller compared to known individual axially-spaced impellers. The coupling of each adjacent pair impeller stages ensures they all rotate simultaneously with the supporting shaft. The adjacent impeller stages can be formed integrally as a single component, joined using a suitable joining method as previously described or formed integrally with the supporting shaft.

The impeller is preferably manufactured from titanium or a titanium alloy. The impeller may alternatively be manufactured from another suitable material, such as stainless steel, aluminium, an aluminium alloy or a high-strength, low-mass alloy, as will be readily apparent to the skilled person.

Figure 5 is a section view through a two-stage impeller arrangement 13 comprising the impeller I of the invention. Figure 5 also shows the flow path of a fluid through the first 2 and second 7 impeller stages of the impeller.

The arrangement includes a first stage inlet casing 14 defining a fluid flow inlet 15, a first stage diffuser 16, a second stage diffuser 17, a volute casing 18 and an annular cavity 19.

In use of the preferred embodiment of the impeller arrangement 14, a fluid flows into the casing through an inlet 15 and is funnelled or otherwise directed by a first fluid flow channel towards the first set of vanes 3 of the first impeller stage 2 of the impeller I. The fluid is accelerated radially by the first set of vanes into a first stage diffuser 16. The fluid is then recirculated by a second fluid flow channel through the second set of vanes 8 of the second impeller stage 7 and accelerated further by the second set of vanes radially into a second stage diffuser 17 and then into a spiral volute casing 18. The fluid then flows around the volute casing to a fluid outlet.

In an alternative embodiment not shown in the Figures, the impeller casing is arranged so that the fluid is directed first through the second set of blades 8 of the second impeller stage 7 and then recirculated through the first set of blades 3 of the first impeller stage 2. In this embodiment fluid flows nto the casing through an inlet 15 and is funnelled or otherwise directed by the first fluid flow channel towards the second set of vanes 8 of the second impeller stage 7 of the impeller 1. The fluid is accelerated radially by the second set of vanes 8 into a diffuser. The fluid is then recirculated by a second fluid flow channel to the first set of vanes 3 of the first impeller stage 2 by which it is further accelerated into another diffuser and then into a spiral volute casing. The fluid then flows around the volute casing to a fluid outlet.

In either embodiment, a cavity 19 is provided in the impeller casing through which a cooling medium, preferably cooling water, is circulated to cool the fluid flowing through the first stage diffuser. Cooling the first stage diffuser, particularly by water cooling, reduces the temperature of the gas entering the second stage, reducing the compression work and therefore increasing the isentropic efficiency of the overall compression process. (NB -it will also slightly increase the pressure ratio of the second stage.) In either embodiment the first and second fluid flow channels which direct fluid to the first and second impeller stages cross one another inside the impeller casing. The need for the flow channel to cross may be avoided by providing a separate diffuser and collector for each of the first and second impeller stages. However, this is likely to result in an increase in the physical size and weight of the compression system.

In the preferred embodiment in which the fluid is directed first through the first impeller stage 2 and then recirculated through the second impeller stage 7, the crossing of the fluid flow channel occurs at the location generally marked 20 in Figure 5 in the region of the casing between the exit of the first stage diffuser and the second set of vanes. This is a relatively lower velocity region of the impeller casing. In the alternative embodiment in which the fluid is directed first through the second impeller stage 7 and then recirculated through the first impeller stage 2, crossing of the fluid flow channel occurs in the region of the casing proximate the first and second stage diffusers which may be a relatively higher velocity region of the impeller casing.

Figure 6 shows a compressor 21 with three stages of which the first stage is of conventional design and the second and third stages employ an impeller of the invention.

The compressor comprises an air filter 22, an inlet 23, an integral electric motor 24 driving a common shaft 25. Electric motor 24 is preferably a permanent magnet motor but may alternatively be an induction or switched reluctance motor.

The common shaft 25 supports at one end a first stage impeller arrangement including a single-sided impeller 26 a casing 27 and a volute casing 28. The common supporting shaft 25 supports at its other end the impeller arrangement discussed above in relation to Figure 5 including the impeller 1 according to the present invention defining second and third impeller stages. The compressor also includes an intercooler 29, an aftercooler 30, a non-return valve 31 and flow lines 32, 33, 34 and 35.

The intercooler 29 is positioned between an outlet from the volute casing 28 of the first impeller stage and the inlet 15 of the impeller casing 14 of the impeller I defining the nested second and third impeller stages. The intercooler is fluidly connected to the volute casing 28 and inlet 15 by flow lines 32 and 33 respectively.

The aftercooler 30 is positioned between the outlet of the volute casing 18 of the nested second and third impeller stages and the non-return valve 31. The aftercooler is fluidly connected to the volute casing 18 and the non-return valve 31 by flow lines 34 and 35 respectively.

The intercooler 29 reduces the temperature of a fluid flowing through the system to increase the efficiency of operation of the compressor. The aftercooler 30 reduces the compressor package discharge temperature to that required for use of the compressed gas.

In operation, the electric motor 24 rotates the common shaft 25 thereby rotating the first impeller 26 and the impeller I defining the nested second and third impeller stages at high rotational speed. A fluid such as air is drawn into the compressor through the air filter 22 and the inlet 23 and is accelerated radially by the set of vanes of the first impeller stage 26 into a diffuser and volute casing 28.

The intercooler 29 cools the fluid flowing from the volute casing 28 of the first impeller stage to the inlet 15 of the casing 14 of the nested second and third stage impeller stages by heat exchange with a cooling medium pumped through the intercooler from an inlet 36 to an outlet 37.

The fluid is then circulated through the second and third impeller stages in the manner described above with reference to Figure 5, the second and third impeller stages being understood to be equivalent the first and second impeller stages described above in relation to Figures 1 to 5.

The aftercooler 30 cools the fluid flowing from volute casing 18 of the nested second and third impeller stages to the non-return valve 31 by heat exchange with a cooling medium pumped through the aftercooler from an inlet 38 to an outlet 39.

The non-return valve 31 prevents the fluid returning from the outlet through the nested second and third impeller stages.

A number of alternative embodiments will be readily apparent, the main variation being the number of impeller stages and the nature of the impellers provided at one or both ends of the shaft 25 of the or each motor of a compression system. For example, one or more impeflers 1 described above with reference to Figures 1 to 5, any or all of which comprise two or more nested impeller stages, may be provided at one end of the shaft and one or more known single or double-sided impellers may be provided at the other end of the shaft. Alternatively one or more impellers I described above with reference to Figures 1 to 5, any or all of which comprise two or more nested impeller stages, may be provided at both ends of the common shaft 25. The compression system may also be setup so that the fluid is circulated through the nested adjacent impeller stages in any particular order which may be the same or different for any nested impellers included in the compression system.

A compressor or other machine including one or more impellers of the invention can therefore be tailored to the requirements of a particular application such as the required fluid pressure and the constraints on the physical size, mass, cost and complexity of the machine.

Claims

CLAIMS1. An impeller for a compressor having two series connected stages, the impeller having two sets of blades associated with the respective stages, wherein the two sets of blades overlap one another axially such that the total axial length of the bladed section of the impeller is less than the sum of the axial lengths of the two sets of blades.
2. An impeller for a compressor of radial or mixed-flow configuration, the impeller comprising: a first impeller stage consisting of a first set of vanes projecting from a first side of a first vane-supporting body rotatable about an impeller axis of rotation; a second impeller stage consisting of a second set of vanes projecting from a first side of a second vane-supporting body rotatable about the impeller axis of rotation; the first and second impeller stages being arranged so that the second set of vanes extends between the first side of the second vane-supporting body and a second side of the first vane-supporting body and couples the first and second vane-supporting bodies for simultaneous rotation about the impeller axis of rotation; wherein the second side of the first vane-supporting body is adapted so as to receive at least part of the axial length of the second vane-supporting body and the second set of vanes such that the second impeller stage is at least partially nested or embedded within the first impeller stage.
3. The impeller of claim 1 or 2 comprising one or more additional impeller stages, each adjacent pair of impeller stages being coupled for simultaneous rotation by a set of vanes extending between and coupling the vane-supporting bodies of the adjacent pair of impeller stages, arranged such that the second side of one of the vane-supporting bodies of the adjacent pair is adapted so as to receive at least part of the axial length of the other vane-supporting body of the adjacent pair so that adjacent pairs of impeller stages are at least partially nested or embedded within one another.
4. The impeller of any preceding claim in which at least some of the vanes of at least one of the first and second pluralities of vanes are splitter blades.
5. The impeller of any preceding claim in which the first and second impeller stages or each adjacent pair of impeller stages are integrally formed.
6. The impeller of claim 5 machined from a single billet of material.
7. A compressor of radial or mixed-flow configuration comprising a motor and a shaft, the shaft supporting at least one end one or more of the impellers of any of the preceding claims or a combination of one or more of the impellers of any of the proceeding claims.
8. The compressor of claim 7 further comprising a casing for the at least one impeller, the casing defining a flow path for a fluid including: a first fluid flow channel for directing a fluid from an inlet towards the first set of vanes, a first stage diffuser for receiving the fluid from the first set of vanes, a second fluid flow channel for redirecting the fluid from the first stage diffuser towards the second set of vanes, a second stage diffuser for receiving the fluid from the second set of vanes and directing the fluid into a volute casing to an outlet.
9. The compressor of claim 7 further comprising a casing for the at least one impeller, the casing defining a flow path for a fluid including a first fluid flow channel for directing a fluid from an inlet towards the second set of vanes, a first stage diffuser for receiving the fluid from the second set of vanes, a second fluid flow channel for redirecting the fluid from the first stage diffuser towards the first set of vanes, a second stage diffuser for receiving the fluid from the first set of vanes and directing the fluid into a volute casing to an outlet.
10. The compressor of claim 8 or 9, where in the first stage diffuser is adapted be cooled by a supply of cooling fluid.
11. The compressor of claim 8, 9 or 10 wherein the casing is arranged such that the first and second fluid flow channels cross one another in the region of the casing between an exit of the first stage diffuser and the second set of vanes.
12. The compressor of any of claims 7 to 11 configured to provide 5 -13 barg air for industrial use.
13. A refrigeration system comprising a motor and a shaft, the shaft supporting at at least one end one or more of the impellers of any of the preceding claims or a combination of one or more of the impellers of any of the proceeding claims.
14. An impeller substantially as hereinbefore described with reference to the figures 1 to 4, 14. A compressor of radial or mixed-flow configuration substantially as hereinbefore described with reference to Figures 5 and 6.