Centrifugal compressor with recirculation structure

A centrifugal compressor for a chiller system includes a casing having an inlet portion and an outlet portion, a recirculation structure including a recirculation path and a recirculation discharge cavity, an impeller disposed downstream of the recirculation discharge cavity, the impeller being attached to a shaft rotatable about a shaft rotation axis, a motor arranged to rotate the shaft in order to rotate the impeller, and a diffuser disposed in the outlet portion downstream of the impeller. The recirculation structure is configured and arranged to impart a swirl to a flow of refrigerant into the inlet portion, with a velocity of a recirculation flow caused by the swirl being higher than a velocity of the flow of the refrigerant in the inlet portion.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to a centrifugal compressor in a chiller system. More specifically, the present invention relates to a centrifugal compressor with a recirculation structure of refrigerant.

Background Information

A chiller system is a refrigerating machine or apparatus that removes heat from a medium. Commonly a liquid such as water is used as the medium and the chiller system operates in a vapor-compression refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. As a necessary byproduct, refrigeration creates waste heat that must be exhausted to ambient or, for greater efficiency, recovered for heating purposes. A conventional chiller system often utilizes a centrifugal compressor, which is often referred to as a turbo compressor. Thus, such chiller systems can be referred to as turbo chillers. Alternatively, other types of compressors, e.g. a screw compressor, can be utilized.

In a conventional (turbo) chiller, refrigerant is compressed in the centrifugal compressor and sent to a heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as a condenser because the refrigerant condenses in this heat exchanger. As a result, heat is transferred to the medium (liquid) so that the medium is heated. Refrigerant exiting the condenser is expanded by an expansion valve and sent to another heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as an evaporator because refrigerant is heated (evaporated) in this heat exchanger. As a result, heat is transferred from the medium (liquid) to the refrigerant, and the liquid is chilled. The refrigerant from the evaporator is then returned to the centrifugal compressor and the cycle is repeated. The liquid utilized is often water.

A conventional centrifugal compressor basically includes a casing, an inlet guide vane, an impeller, a diffuser, a motor, various sensors and a controller. Refrigerant flows in order through the inlet guide vane, the impeller and the diffuser. Thus, the inlet guide vane is coupled to a gas intake port of the centrifugal compressor while the diffuser is coupled to a gas outlet port of the impeller. The inlet guide vane controls the flow rate of refrigerant gas into the impeller. The impeller increases the velocity of refrigerant gas. The diffuser works to transform the velocity of refrigerant gas (dynamic pressure), given by the impeller, into (static) pressure. The motor rotates the impeller. The controller controls the motor, the inlet guide vane and the expansion valve. In this manner, the refrigerant is compressed in a conventional centrifugal compressor.

When the pressure next to the compressor discharge is higher than the compressor discharge pressure, the fluid tends to reverse or even flow back in the compressor. This happens when the lift pressure (condenser pressure—evaporator pressure) exceeds the compressor lift capability. This phenomenon, called surge, repeats and occurs in cycles. The compressor loses the ability to maintain its lift when surge occurs and the entire system becomes unstable. A collection of surge points during varying compressor speed or varying inlet gas angle is called a surge surface. In normal conditions, the compressor operates in the right side of the surge surface. However, during startup/operation in part load, the operating point will move towards the surge line because flow is reduced. If conditions are such that the operating point approaches the surge line, flow recirculation occurs in the impeller and diffuser. The flow separation will eventually cause a decrease in the discharge pressure, and flow from suction to discharge will resume. Surging can cause damage to the mechanical impeller/shaft system and/or to the thrust bearing due to the rotor shifting back and forth from the active to the inactive side. This is defined as the surge cycle of the compressor.

Therefore, techniques have been developed to control surge. See for example U.S. Pat. No. 4,248,055 and U.S. Patent Application Publication No. 2013/0180272.

SUMMARY OF THE INVENTION

In a centrifugal compressor, a compressor controller can control various parts to control surge. For example, the inlet guide vane and/or the discharge diffuser vane can be controlled or the speed of the compressor can be adjusted to control surge. However, these systems can limit the operation range of the compressor, and thus, can reduce performance of the compressor.

Therefore, one object of the present invention is to provide a centrifugal compressor that prevents surge without reducing performance of the compressor.

Another object of the present invention is to provide a centrifugal compressor that controls surge without overly complicated construction.

Yet another object of the present invention is to provide a centrifugal compressor that regulates a refrigerant flow while minimizing efficiency loss and allows an overall greater range of the refrigerant flow.

One or more of the above objects can basically be attained by providing a centrifugal compressor adapted to be used in a chiller system, the centrifugal compressor including a casing having an inlet portion and an outlet portion, a recirculation structure including a recirculation path and a recirculation discharge cavity, an impeller disposed downstream of the recirculation discharge cavity, the impeller being attached to a shaft rotatable about a shaft rotation axis, a motor arranged to rotate the shaft in order to rotate the impeller, and a diffuser disposed in the outlet portion downstream of the impeller. The recirculation structure is configured and arranged to impart a swirl to a flow of refrigerant into the inlet portion, with a velocity of a recirculation flow caused by the swirl being higher than a velocity of the flow of the refrigerant in the inlet portion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially toFIG. 1, a chiller system10, which includes a compressor22with a recirculation structure50, is illustrated in accordance with a first embodiment of the present invention. The chiller system10is preferably a water chiller that utilizes cooling water and chiller water in a conventional manner. The chiller system10illustrated herein is a single stage chiller system. However, it will be apparent to those skilled in the art from this disclosure that the chiller system10could be a multiple stage chiller system including two or more stages.

The chiller system10basically includes a controller20, the compressor22, a condenser24, an expansion valve26, and an evaporator28connected together in series to form a loop refrigeration cycle. In addition, various sensors S and T are disposed throughout the circuit of the chiller system10as shown inFIG. 1. The chiller system10is conventional except that the compressor22has the recirculation structure50in accordance with the present invention.

Referring toFIGS. 1, 2A and 2B, in the illustrated embodiment, the compressor22is a centrifugal compressor. The centrifugal compressor22of the illustrated embodiment basically includes a casing30, an optional inlet guide vane32, an impeller34, a diffuser/volute36, a discharge nozzle37, a motor38and a magnetic bearing assembly40as well as various conventional sensors. The controller20receives signals from the various sensors and controls the inlet guide vane32, the motor38and the magnetic bearing assembly40in a conventional manner. Refrigerant flows in order through the inlet guide vane32, the impeller34and the diffuser/volute36. The inlet guide vane32controls the flow rate of refrigerant gas into the impeller34in a conventional manner. The impeller34increases the velocity of refrigerant gas. The motor speed determines the amount of increase of the velocity of refrigerant gas. The diffuser/volute36increases the refrigerant pressure. The motor38rotates the impeller34via a shaft42. The magnetic bearing assembly40magnetically supports the shaft42. In this manner, the refrigerant is compressed in the centrifugal compressor22. The centrifugal compressor22of the illustrated embodiment includes the inlet guide vane32. However, the inlet guide vane32is optional, and the recirculation structure50in accordance with the present invention can be applied to a centrifugal compressor which does not include an inlet guide vane.

Referring toFIG. 2B, the magnetic bearing assembly40is conventional, and thus, will not be discussed and/or illustrated in detail herein. Rather, it will be apparent to those skilled in the art that any suitable bearing can be used without departing from the present invention. As seen inFIG. 2B, the magnetic bearing assembly40preferably includes a first radial magnetic bearing44, a second radial magnetic bearing46and an axial (thrust) magnetic bearing48. In any case, at least one radial magnetic bearing44or46rotatably supports the shaft42. The thrust magnetic bearing48supports the shaft42along a rotational axis X by acting on a thrust disk45. The thrust magnetic bearing48includes the thrust disk45which is attached to the shaft42.

The centrifugal compressor22illustrated inFIG. 2Ais a single stage compressor, while the centrifugal compressor22illustrated inFIG. 2Bis a two-stage compressor including a first stage impeller34aand a second stage impeller34b. As mentioned above, the recirculation structure50in accordance with the present invention can be applied to a single stage compressor and a multiple stage compressor including two or more stages.

Referring toFIGS. 1 and 17, the controller20is an electronic controller that includes a magnetic bearing control section71, a compressor variable frequency drive72, a compressor motor control section73, an inlet guide vane control section74(optional), an expansion valve control section75, and a recirculation structure control section76.

In the illustrated embodiment, the control sections are sections of the controller20programmed to execute the control of the parts described herein. The magnetic bearing control section71, the compressor variable frequency drive72, the compressor motor control section73, the inlet guide vane control section74(optional), the expansion valve control section75, and the recirculation structure control section76are coupled to each other, and form parts of a centrifugal compressor control portion that is electrically coupled to an I/O interface of the compressor22. However, it will be apparent to those skilled in the art from this disclosure that the precise number, location and/or structure of the control sections, portions and/or controller20can be changed without departing from the present invention so long as the one or more controllers are programed to execute control of the parts of the chiller system10as explained herein.

The controller20is conventional, and thus, includes at least one microprocessor or CPU, an Input/output (I/O) interface, Random Access Memory (RAM), Read Only Memory (ROM), a storage device (either temporary or permanent) forming a computer readable medium programmed to execute one or more control programs to control the chiller system10. The controller20may optionally include an input interface such as a keypad to receive inputs from a user and a display device used to display various parameters to a user. The parts and programming are conventional, and thus, will not be discussed in detail herein, except as needed to understand the embodiment(s).

First Embodiment

Referring now toFIGS. 2-10, the detailed structure of the recirculation structure50of the centrifugal compressor22according to the first embodiment will be explained. The casing30of the centrifugal compressor22has an inlet portion31aand an outlet portion31b. As best shown inFIG. 6, the recirculation structure50includes a recirculation path52and a recirculation discharge cavity54. The recirculation path52of the recirculation structure50is disposed inside the casing30in this embodiment. The recirculation path52introduces refrigerant from the diffuser/volute36of the compressor22, and the introduced refrigerant is discharged from the recirculation discharge cavity54, as explained in more detail below.

As best understood fromFIG. 6, a plurality of recirculation discharge guide vanes56are disposed to surround the recirculation discharge cavity54. The recirculation discharge guide vanes56are circumferentially arranged with respect to a shaft rotation axis X of the shaft42. The recirculation discharge guide vanes56are located between the inlet guide vane32and the impeller34along the direction parallel to the shaft rotation axis X. As mentioned above, however, the inlet guide vane32is optional, and the recirculation structure50in accordance with the present invention can be applied to a centrifugal compressor which does not include an inlet guide vane.

In the illustrated embodiment, the recirculation structure50further includes an annular plate58. The recirculation discharge guide vanes56are disposed on the annular plate58to be spaced from each other substantially equally. Each of the recirculation discharge guide vanes56is rotatably attached onto the annular plate58using a vane shaft60. Each of the recirculation discharge guide vanes56is connected to a rotating mechanism (not shown) which rotates each of the recirculation discharge guide vanes56. The rotating mechanism is conventional, and thus, will not be discussed and/or illustrated in detail herein. Rather, it will be apparent to those skilled in the art that any suitable rotating mechanism can be used without departing from the present invention. The rotating mechanism is coupled to the recirculation structure control section76of the controller20. The angle of each recirculation discharge guide vane56is adjustable by rotating the recirculation discharge guide vanes56with the rotating mechanism. The recirculation structure control section76of the controller20is configured to control the angle of each recirculation discharge guide vane56.

As shown inFIG. 8, each of the recirculation discharge guide vanes56is rotatable about a shaft rotation axis Y of the vane shaft60. The shaft rotation axis Y of the vane shaft60is substantially parallel to the shaft rotation axis X of the shaft42. The plurality of recirculation discharge guide vanes56can be connected to a linking mechanism (not shown). The linking mechanism is conventional, and thus, will not be discussed and/or illustrated in detail herein. Rather, it will be apparent to those skilled in the art that any suitable linking mechanism can be used without departing from the present invention. In the illustrated embodiment, the plurality of recirculation discharge guide vanes56are linked with one another by the linking mechanism so that the angles of the plurality of recirculation discharge guide vanes56are adjusted simultaneously. For example, the angles of the plurality of recirculation discharge guide vanes56can be adjusted gradually from the open state as shown inFIG. 9Ato the closed state as shown inFIG. 9C.

Referring toFIGS. 6 and 7, the recirculation path52includes a recirculation pipe. The recirculation pipe52extends from the diffuser/volute36of the compressor22toward the plurality of recirculation discharge guide vanes56in the first embodiment. An annular groove62is disposed in the casing30to connect the recirculation pipe52and the plurality of recirculation discharge guide vanes56. The annular groove62extends the whole inner circumference of the casing30. The refrigerant introduced from the diffuser/volute36of the compressor22via the recirculation pipe52passes through the annular groove62and flows toward the plurality of recirculation discharge guide vanes56. The plurality of recirculation discharge guide vanes56increase the velocity of the refrigerant and create a swirl of the refrigerant. The swirl of the refrigerant is discharged from the recirculation discharge cavity54and mixed into the main flow of the refrigerant in the inlet portion31aof the casing31of the compressor22. In this manner, the recirculation structure50imparts a swirl to the flow of refrigerant in the inlet portion31a, with the velocity of the recirculation flow caused by the swirl being higher than the velocity of the flow of the refrigerant in the inlet portion31a. The recirculation flow of the refrigerant can be controlled by adjusting the angles of the recirculation discharge guide vanes56.

Also, the direction of the recirculation flow can be controlled by adjusting the angles of the recirculation discharge guide vanes56. More specifically, the direction of the recirculation flow can be controlled to be in the same direction as the rotation direction of the impeller34as shown by arrow A inFIG. 6. In this case, a significant ability to reduce the main flow of the refrigerant is predicted with minimum efficiency and pressure rise penalties. Alternatively, the direction of the recirculation flow can be controlled to be in the opposite direction to the rotation direction of the impeller34as shown by arrow B inFIG. 6. In this case, an increase head or pressure rise will result with a small efficiency penalty.

Second Embodiment

Referring toFIGS. 11A-11D, the recirculation structure50in accordance with the second embodiment will be explained.

The recirculation structure50in the second embodiment further includes an interlocking plate64which has a similar shape to the annular plate58except that the interlocking plate64has a plurality of recesses66adapted to receive the plurality of recirculation discharge guide vanes56disposed on the annular plate58as illustrated inFIG. 11B. In the second embodiment, the recirculation discharge guide vanes56are fixedly attached to the annular plate58so as to fit properly in the recesses66of the interlocking plate64. The interlocking plate64is connected to a linear actuator (not shown) so that the interlocking plate64can be moved axially along the direction parallel to the shaft rotation axis X of the shaft42of the motor38. The linear actuator is conventional, and thus, will not be discussed and/or illustrated in detail herein. Rather, it will be apparent to those skilled in the art that any suitable linear actuator can be used without departing from the present invention.

As shown inFIG. 11C, the interlocking plate64can be moved axially in a direction where the annular plate58and the interlocking plate64are close with each other. In this close position as shown inFIG. 11C, the plurality of recesses66of the interlocking plate64receive the plurality of recirculation discharge guide vanes56on the annular plate58. Also, as shown inFIG. 11D, the interlocking plate64can be moved axially in a direction where the annular plate58and the interlocking plate64are separate from each other. In this separate position as shown inFIG. 11D, the plurality of recirculation discharge guide vanes56on the annular plate58are released from the plurality of recesses66of the interlocking plate64. This axial movement of the interlocking plate64allows the flow area of the recirculation flow to vary in the axial direction, and thus, the recirculation flow can be further controlled with this axial movement of the interlocking plate64. Alternatively, the annular plate58may be connected to a linear actuator. In this case, the axial movement of the annular plate58allows the flow area of the recirculation flow to vary in the axial direction, and thus, the recirculation flow can be further controlled with this axial movement of the annular plate58. Both of the interlocking plate64and the annular plate58can be configured to move axially.

Third Embodiment

Referring toFIGS. 12-15, the recirculation structure50in accordance with the third embodiment will be explained.

The recirculation structure50in the third embodiment further includes a rotating manifold plate70having a shape as illustrated inFIGS. 14A-14C. The plurality of recirculation discharge guide vanes56are attached to the annular plate58to be stationary in this embodiment. The plurality of recirculation discharge guide vanes56are disposed at a substantially same interval with each other such that channels68are defined between each of the plurality of recirculation discharge guide vanes56. The plurality of recirculation discharge guide vanes56occupy substantially half of the flow area of the refrigerant in the radial direction as illustrated inFIGS. 15A-15C. The rotating manifold plate70is arranged to be rotatable about an axis which is coincident with the shaft rotation axis X of the shaft42of the motor38. As the rotating manifold plate70rotates, the rotating manifold plate70closes off the channels68between each of the plurality of recirculation discharge guide vanes56, as explained in more detail below.

When the rotating manifold plate70is in a fully open position as illustrated inFIG. 15A, the rotating manifold plate70aligns with the plurality of recirculation discharge guide vanes56in the radial direction and the channels68between each of the plurality of recirculation discharge guide vanes56are fully opened. When the rotating manifold plate70is in a 50% open position as illustrated inFIG. 15B, the rotating manifold plate70occupies 50% of the channels68between each of the plurality of recirculation discharge guide vanes56. When the rotating manifold plate70is in a fully closed position as illustrated inFIG. 15C, the channels68between each of the plurality of recirculation discharge guide vanes56are fully closed with the rotating manifold plate70. With this arrangement, the rotating manifold plate70gradually open/close the channels68between each of the plurality of recirculation discharge guide vanes56. The rotation of the rotating manifold plate70allows the flow area of the recirculation flow to vary in the radial direction, and thus, the recirculation flow can be further controlled. In the illustrated embodiment, the rotating manifold plate70rotates with respect to the annular plate58. Alternatively, the annular plate58can be rotated with respect to the stationary plate70.

Modified Embodiment

In the first embodiment, the recirculation pipe52of the recirculation structure50is disposed inside the casing30as illustrated inFIGS. 6 and 7. In a modified embodiment of the first embodiment, the recirculation pipe52′ is disposed outside the casing30as illustrated inFIG. 16. For example, the recirculation pipe52′ can be provided to extend from the discharge nozzle37of the compressor22toward the plurality of recirculation discharge guide vanes56. The recirculation pipe52′ includes a valve53to adjust the flow of the refrigerant passing through the recirculation pipe52. The valve is conventional, and thus, will not be discussed and/or illustrated in detail herein. Rather, it will be apparent to those skilled in the art that any suitable valve can be used without departing from the present invention. The modified embodiment can also apply to the second embodiment and the third embodiment as explained above.

In terms of global environment protection, use of new low GWP (Global Warming Potential) refrigerants such like R1233zd, R1234ze are considered for chiller systems. One example of the low global warming potential refrigerant is low pressure refrigerant in which the evaporation pressure is equal to or less than the atmospheric pressure. For example, low pressure refrigerant R1233zd is a candidate for centrifugal chiller applications because it is non-flammable, non-toxic, low cost, and has a high COP compared to other candidates such like R1234ze, which are current major refrigerant R134a alternatives. The compressor22having the recirculation structure50in accordance with the present invention is useful with any type of refrigerant including low pressure refrigerant such as R1233zd.

General Interpretation of Terms

The term “detect” as used herein to describe an operation or function carried out by a component, a section, a device or the like includes a component, a section, a device or the like that does not require physical detection, but rather includes determining, measuring, modeling, predicting or computing or the like to carry out the operation or function.