System and method for extending the operating range of a dynamic compressor

Abstract

The system includes a dynamic compressor and a controller having a processor and a memory. The compressor includes a first compressor stage having a first variable inlet guide vane (VIGV) and a second compressor stage having a second VIGV. The memory stores instructions programming the processor to operate the compressor at a current speed, a first position of the first VIGV, and a second position of the second VIGV to compress a working fluid and determine if a condition is met. If the condition is not met, the processor is programmed to continue operating the compressor at the current speed, the first position of the first VIGV, and the second position of the second VIGV. If the condition is met, the processor is programmed to change the second position of the second VIGV to a third position and maintain the first position of the first VIGV.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 17 / 585,736, filed January 27, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] Field of Disclosure The field of the disclosure relates to control systems, and more particularly to control systems for dynamic compressors. [Background technology]

[0003] Dynamic compressors, including centrifugal compressors, are commonly used in process industries and heating, ventilation, and air conditioning (HVAC) systems. The compressor is operatively connected to a motor via a shaft that supports multiple compressor stages. The motor rotates the compressor stages via the shaft at selected rotational speeds and load conditions to compress the refrigerant to a specified demand. The motor speed and load can be controlled to operate the compressor under a wide range of operating conditions. The compressor's operating range is limited by a surge region at low flow rates and a choke region at high flow rates. Knowledge of the compressor's exact operating point helps avoid operation in surge or choke.

[0004] In a multi-stage compressor, the refrigerant enters each compressor stage at different pressures and volumetric flow rates. Therefore, each stage must be designed to be "matched" to the other stages so that it can effectively handle the fluid it receives from the previous stage, and the compressor can operate safely and efficiently under a wide range of operating conditions.

[0005] In addition to the dynamic compressor, most HVAC systems include a condenser, expansion device, and evaporator fluidly coupled to the dynamic compressor in a closed loop. In some applications, the basic cycle is modified with additional components or alternative configurations to improve system performance and efficiency. For example, some HVAC systems use an economization loop, in which a portion of the low-temperature liquid refrigerant is diverted from downstream of the condenser through a heat exchanger or flash tank to cool the main flow. Downstream of the heat exchanger or flash tank, an additional portion of the flow may be diverted to an intermediate-temperature evaporator. The diverted portion, or economization flow, is then injected as a low-temperature, intermediate-pressure gas between compressor stages, improving overall system efficiency. In other systems, flow is removed from between compressor stages and diverted to an additional condenser to enable additional applications.

[0006] In such systems, adding or removing flow between compressor stages results in the stages no longer being aerodynamically matched. Without proper stage matching, a stage downstream of injection may experience undesirable flow conditions at different rates than the rest of the compressor, compromising the performance, efficiency, and safety of the overall machine. Thus, it is desirable to maintain aerodynamic matching between stages, regardless of whether mass flow rates between the stages change.

[0007] This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and / or claimed below. This discussion is believed to be helpful in providing the reader with background information to better understand the various aspects of the present disclosure. As such, it should be understood that these statements are to be read in this light, and not as admissions of prior art. Summary of the Invention

[0008] One aspect of the present disclosure relates to a system including a dynamic compressor operable to compress a working fluid and a controller coupled to the dynamic compressor. The dynamic compressor includes a first compressor stage having a first variable inlet guide vane (VIGV) and a second compressor stage having a second VIGV. The controller includes a processor and a memory. The memory stores instructions that program the processor to operate the dynamic compressor at a current speed, a first position of the first VIGV, and a second position of the second VIGV to compress the working fluid and determine whether a condition is met. If the condition is not met, the instructions stored in the memory program the processor to continue operating the compressor at the current speed, the first position of the first VIGV, and the second position of the second VIGV. If the condition is met, the instructions stored in the memory program the processor to change the second position of the second VIGV to a third position different from the second position and maintain the first position of the first VIGV.

[0009] Another aspect of the present disclosure relates to a controller for a dynamic compressor having a first compressor stage and a second compressor stage. The controller includes a processor and a memory. The memory stores instructions that program the processor to operate the dynamic compressor at a current speed, a first position of a first VIGV of the first compressor stage, and a second position of a second VIGV of the second compressor stage to compress a working fluid and determine whether a condition is met. If the condition is not met, the instructions stored in the memory program the processor to continue operating the compressor at the current speed, the first position of the first VIGV, and the second position of the second VIGV. If the condition is met, the instructions stored in the memory program the processor to change the second position of the second VIGV to a third position different from the second position and maintain the first position of the first VIGV.

[0010] Another aspect of the present disclosure relates to a method for extending the operating range of a dynamic compressor having a first compressor stage and a second compressor stage compressing a working fluid, the method including: operating the dynamic compressor at a current speed, a first position of a first VIGV of the first compressor stage, and a second position of a second VIGV of the second compressor stage to compress the working fluid; and determining whether a condition is met. If the condition is not met, the method further includes continuing to operate the compressor at the current speed, the first position of the first VIGV, and the second position of the second VIGV; and, if the condition is met, changing the second position of the second VIGV to a third position different from the second position and maintaining the first position of the first VIGV.

[0011] Various refinements exist in the features described in connection with the above-described aspects of the present disclosure. Additional features may also be incorporated into the above-described aspects of the present disclosure. These refinements and additional features may exist individually or in any combination. For example, various features described below in connection with any of the illustrated embodiments of the present disclosure may be incorporated into any of the above-described aspects of the present disclosure, alone or in any combination. [Brief explanation of the drawings]

[0012] [Figure 1] A perspective view of the assembled dynamic compressor. [Figure 2] 2 is a cross-sectional view of the dynamic compressor of FIG. 1 taken along line 2-2 with the external conduit removed. [Figure 3] FIG. 3 is a schematic diagram of a first exemplary HVAC system in which the dynamic compressor shown in FIGS. 1 and 2 can be installed. [Figure 4] FIG. 3 is a schematic diagram of a second exemplary HVAC system in which the dynamic compressor shown in FIGS. 1 and 2 can be installed. [Figure 5] FIG. 3 is a schematic diagram of a third exemplary HVAC system in which the dynamic compressor shown in FIGS. 1 and 2 can be installed. [Figure 6] FIG. 10 is a schematic diagram of a fourth exemplary HVAC system in which the dynamic compressor shown in FIGS. 1 and 2 can be installed. [Figure 7] FIG. 3 is a block diagram of the control system for the dynamic compressor shown in FIGS. 1 and 2. [Figure 8] 3 is an operating map of the dynamic compressor shown in Figures 1 and 2. [Figure 9] 3 is an operating map of the second compressor stage of the dynamic compressor shown in Figures 1 and 2, showing the current operating points before and after injecting flow upstream of the second compressor stage. [Figure 10] 3 is an operating map of the second compressor stage of the dynamic compressor shown in Figures 1 and 2, showing the current operating points before and after removing flow upstream of the second compressor stage. [Figure 11] 3 is an operating map of the second compressor stage of the dynamic compressor shown in FIGS. 1 and 2 when injecting flow upstream of the second compressor stage and moving the second VIGV from its second position to a position different from its second position. [Figure 12] 3 is an operating map of the second compressor stage of the dynamic compressor shown in FIGS. 1 and 2 when flow is removed from upstream of the second compressor stage and the second VIGV is moved from its second position to a position different from its second position. [Figure 13] A method for extending the operating range of the dynamic compressor shown in Figures 1 and 2. [Figure 14] 3 is a map of predetermined operating points of the dynamic compressor shown in Figures 1 and 2. [Figure 15] 3 is a flowchart of an exemplary control algorithm for determining the limiting speed of the second compressor stage of the dynamic compressor shown in FIGS. 1 and 2 when flow is added or removed from upstream of the second compressor stage. DETAILED DESCRIPTION OF THE INVENTION

[0013] Corresponding reference characters indicate corresponding parts throughout the drawings.

[0014] For simplicity, examples are described with respect to centrifugal compressors. However, the methods and systems described herein can be applied to any suitable dynamic compressor. HVAC system performance and efficiency can be improved by diverting a portion of the main flow to auxiliary loops and circulation components. In such systems, flow can be injected or removed between compressor stages so that each stage has a different mass flow rate. However, aerodynamic matching between stages must be maintained regardless of such flow changes to avoid operation in undesirable conditions. A control strategy can be used to determine if stages become unmatched and adjust the variable inlet guide vanes (VIGVs) at the inlet of each stage to restore proper stage matching.

[0015] Referring to FIG. 1, a two-stage refrigerant compressor is generally designated 100. The compressor 100 is operable to compress a working fluid (e.g., a refrigerant) and includes a compressor housing 102 defining at least one sealed cavity within which each stage of refrigerant compression occurs. The compressor 100 includes a first refrigerant inlet 110 for introducing refrigerant vapor into a first compressor stage (not labeled in FIG. 1), a first refrigerant outlet 114, a refrigerant transfer conduit 112 for transferring compressed refrigerant from the first compressor stage to a second compressor stage, a second refrigerant inlet 118 for introducing refrigerant vapor into the second compressor stage (not labeled in FIG. 1), and a second refrigerant outlet 120. The refrigerant transfer conduit 112 is operably connected at opposite ends to the first refrigerant outlet 114 and the second refrigerant inlet 118, respectively. The refrigerant transfer conduit 112 further includes a port 122 for adding or removing flow between the first and second compressor stages. A second refrigerant outlet 120 delivers compressed refrigerant from the second compressor stage to a refrigeration system in which the compressor 100 is incorporated (FIGS. 3-6).

[0016] Referring to FIG. 2 , the compressor housing 102 encloses a first compressor stage 124 and a second compressor stage 126 at opposite ends of the compressor 100. The first compressor stage 124 includes a first compression mechanism 106 configured to impart kinetic energy to a refrigerant entering through a first refrigerant inlet 110. In some embodiments, the first compression mechanism 106 is an impeller. The kinetic energy imparted to the refrigerant by the first compression mechanism 106 is converted to an increase in refrigerant pressure as the refrigerant's velocity slows as it is transported into a sealed cavity (e.g., a diffuser) formed within a volute 132. The first compressor stage 124 further includes a first variable inlet guide vane (VIGV) 134 disposed at the first refrigerant inlet 110 upstream of the first compression mechanism 106. The first VIGV 134 includes a plurality of vanes whose position can be controlled to introduce a pre-whirl into the gaseous refrigerant entering the first refrigerant inlet 110 .

[0017] Similarly, the second compressor stage 126 includes a second compression mechanism 116 configured to impart kinetic energy to the refrigerant transferred from the first compressor stage 124 through a second refrigerant inlet 118. In some embodiments, the second compression mechanism 116 is an impeller. The kinetic energy imparted to the refrigerant by the second compression mechanism 116 is converted into an increase in refrigerant pressure as the refrigerant slows as it is transferred into a sealed cavity (e.g., a diffuser) formed within the volute 132. The compressed refrigerant leaves the second compressor stage 126 through a second refrigerant outlet 120 (not shown in FIG. 2 ). The second compressor stage 126 further includes a second variable inlet guide vane (VIGV) 136 disposed upstream of the second compression mechanism 116 from the second refrigerant inlet 118. The second VIGV 136 includes a plurality of vanes whose positions can be controlled to introduce a pre-swirl into the gaseous refrigerant entering the second refrigerant inlet 118 .

[0018] The first compression mechanism 106 and the second compression mechanism 116 are connected to opposite ends of the shaft 104. The shaft 104 is operatively connected to a motor 108 positioned between the first compression mechanism 106 and the second compression mechanism 116, which rotate at a selected rotational speed to compress the refrigerant exiting a second refrigerant outlet 120 (not shown in FIG. 2 ) to a preselected pressure. The compressor 100 may incorporate any suitable motor, including, but not limited to, an electric motor.

[0019] 3 is a schematic diagram of a first exemplary HVAC system 300 in which the compressor 100 of FIGS. 1 and 2 may be installed. The system 300 has a single closed refrigerant loop 310 including the compressor 100, a condenser 320, a first expansion device 330, and an evaporator 340. Refrigerant enters the compressor 100 as a low-pressure, low-temperature gas through the first refrigerant inlet 110. The first compressor stage 124 and the second compressor stage 126 add kinetic energy to the refrigerant, converting it into a pressure increase, and the refrigerant exits the compressor 100 through the second refrigerant outlet 120 as a high-pressure, high-temperature gas. The refrigerant enters the condenser 320, which is fluidly coupled to the second compressor stage 126, and is converted into a heat Q out is removed and the refrigerant gas is converted into a high-pressure, high-temperature liquid.

[0020] The condenser 320 is fluidly coupled to a first expansion device 330, which reduces the pressure of the refrigerant. In some embodiments, the pressure is reduced until the current temperature of the liquid refrigerant reaches its boiling point temperature at that pressure, causing a portion of the liquid refrigerant to boil and turn into a gas, resulting in a two-phase mixture. The first expansion device 330 can be a fixed orifice, a thermal expansion valve, an electronic expansion valve, or any type of expansion device that enables the HVAC system 300 to function as described herein. The first expansion device 330 is fluidly coupled to an evaporator 340, which receives at its inlet a low-pressure, low-temperature liquid refrigerant or a two-phase mixture of liquid and gaseous refrigerant. In the evaporator 340, the refrigerant is converted to a heat Q in The evaporator 340 absorbs the refrigerant and changes phase from a liquid to a gas. The evaporator 340 is fluidly coupled to the first compressor stage 124 and the cycle begins again.

[0021] Figure 4 is a schematic diagram of a second exemplary HVAC system 400 in which the compressor 100 of Figures 1 and 2 may be installed. System 400 has a primary refrigerant loop 410 that includes compressor 100, condenser 320, a first stream 492 of heat exchanger 490, first expansion device 330, and evaporator 340. System 400 also has a secondary refrigerant loop 460 that is fluidly connected to a portion of primary refrigerant loop 410 and is controlled by an economization valve 470, which is described in more detail herein.

[0022] The secondary refrigerant loop 460 includes an economization valve 470, a second expansion device 480, a second stream 494 of a heat exchanger 490, a second compressor stage 126, and a condenser 320. In the embodiment shown in Figure 4, the components of the secondary refrigerant loop 460 are fluidly coupled in the order listed, with the condenser 320 further coupled to the second expansion device 480 to close the secondary refrigerant loop 460.

[0023] The economization valve 470 can be fully open, partially open, or fully closed, and its state determines whether refrigerant flows through the secondary refrigerant loop 460. That is, when the economization valve 470 is fully closed, all of the refrigerant flows through the primary refrigerant loop 410, and the system 400 operates substantially similarly to the system 300 shown in FIG. 3. When the economization valve 470 is open, the liquid refrigerant exiting the condenser 320 splits into two streams: most of the refrigerant flows through the primary refrigerant loop 410, and the remainder is diverted to the secondary refrigerant loop 460. The economization valve 470 can be a solenoid valve, an electronic expansion valve, or any type of valve that enables the system 400 to function as described herein.

[0024] Economization valve 470, when open, is fluidly coupled to second expansion device 480 and reduces the pressure of the liquid economizer stream until the current temperature of the liquid refrigerant is at its boiling point temperature at that pressure. The refrigerant in the secondary refrigerant loop becomes a two-phase mixture as it enters heat exchanger 490, causing some of the liquid refrigerant to boil and turn to a gas. The second expansion device can be sized and selected to bypass a specific amount of refrigerant (e.g., 0-20 percent of the total mass flow, or any amount of refrigerant flow that enables system 400 to function as described herein) through secondary refrigerant loop 460 when economization valve 470 is open.

[0025] In some embodiments, second expansion device 480 is a thermal expansion valve (TXV) that adjusts the refrigerant flow rate through secondary refrigerant loop 460 based on the heat load of heat exchanger 490. The TXV operates in combination with a valve 496 located downstream of second flow 494 of heat exchanger 490. A membrane within the TXV is movable to balance the refrigerant pressure within the bulb with the refrigerant pressure upstream of heat exchanger 490. The movement of the membrane is coupled to a needle that sets the position of the valve, thereby controlling the amount of refrigerant flowing through secondary refrigerant loop 460. In further embodiments, second expansion device 480 can be a fixed orifice, an electronic expansion valve, or any type of expansion device that enables system 400 to function as described herein.

[0026] The refrigerant exits second expansion device 480 and enters second stream 494 of heat exchanger 490 as a low-pressure liquid or two-phase mixture. Second stream 494 is in thermal communication with first stream 492, which carries high-pressure liquid refrigerant from condenser 320 of primary refrigerant loop 410. Thermal contact between the two streams 492, 494 cools the refrigerant in first stream 492 and warms and boils the refrigerant in second stream 494. The cooled refrigerant in first stream 492 exits heat exchanger 490 as a low-temperature, high-pressure liquid, and the boiled refrigerant in second stream 494 exits heat exchanger 490 as a low-temperature, intermediate-pressure gas. Heat exchanger 490 can be a counterflow heat exchanger, a cross-flow heat exchanger, a parallel-flow heat exchanger, a shell-and-tube heat exchanger, a mixing chamber, or any type of heat exchanger that enables system 400 to function as described herein. In further embodiments, a flash tank may be used instead of or in addition to heat exchanger 490.

[0027] The low temperature, intermediate pressure gas exiting the second stream 494 of the heat exchanger 490 is then injected into the refrigerant transfer conduit 112 of the compressor 100 and mixes with the refrigerant stream of the primary refrigerant loop 410 before reaching the second compressor stage 126. The primary refrigerant loop 410 and the secondary refrigerant loop 460 merge at the second compressor stage 126 and diverge again after the refrigerant leaves the condenser 320.

[0028] FIG. 5 illustrates a third exemplary HVAC system 500 in which the compressor 100 of FIGS. 1 and 2 can be installed. The system 500 includes a low-temperature primary refrigerant loop 510, a medium-temperature secondary refrigerant loop 560, and a tertiary refrigerant loop 580 fluidly coupled thereto. Downstream of the condenser 320, the refrigerant flow is throttled in a first expansion device 530, reducing the pressure until a portion of the liquid refrigerant boils, forming a two-phase mixture. A flash tank 590 separates the two-phase refrigerant mixture into liquid and gas portions, which are split into the primary loop 510 and the tertiary loop 580, respectively. In certain embodiments, a heat exchanger may be used instead of or in addition to the flash tank 590. The liquid refrigerant in the primary loop 510 is throttled in a second expansion device 532 and split again, with a portion of the refrigerant continuing along the primary loop 510 and the remainder splitting into the secondary loop 560. In the secondary loop 560, the refrigerant passes through the medium-temperature evaporator 540, where the refrigerant is boiled and converted to a gas, providing cooling to the medium-temperature space. In the primary loop 510, the liquid refrigerant 510 is throttled in the third expansion device 330 before entering the low-temperature evaporator 340, where the refrigerant is boiled and converted to a gas, providing cooling to the low-temperature space. The refrigerant then enters the first compressor stage 124, where it is compressed to the pressure of the medium-temperature evaporator 540. The gaseous refrigerant in the tertiary loop 580 is throttled in the fourth expansion device 534 to the pressure of the medium-temperature evaporator 540. The gaseous refrigerant in the secondary loop 560 and the tertiary loop 580 are combined and injected into the refrigerant transfer conduit 112 between the first compressor stage 124 and the second compressor stage 126.

[0029] FIG. 6 illustrates a fourth exemplary HVAC system 600 in which the compressor 100 of FIGS. 1 and 2 can be installed. The system 600 includes a primary refrigerant loop 610, which includes the compressor 100, the condenser 320, the first expansion device 330, and the evaporator 340. The system also includes a secondary refrigerant loop 660 fluidly connected to a portion of the primary refrigerant loop 610 and controlled by a valve 670. When the valve 670 is fully closed, all of the refrigerant flows through the primary loop 610, and the system 600 operates substantially similarly to the system 300 illustrated in FIG. 3. When the valve 670 is open, a portion of the refrigerant flow is diverted from the refrigerant transfer conduit 112 between the first compressor stage 124 and the second compressor stage 126 through the secondary loop 660. The diverted flow passes through an auxiliary condenser 620 where it is throttled by a second expansion device 630 and rejoins the primary loop 610 before entering the evaporator 340 .

[0030] FIG. 7 illustrates an exemplary embodiment of a system 700 including a dynamic compressor 100. The compressor 100 includes a compressor housing 102, a compression mechanism 707, a motor 108, a speed sensor 717, a pressure sensor 709, and a controller 710. In this embodiment, the dynamic compressor 100 is a two-stage centrifugal compressor, and the compression mechanism 707 is an impeller for each stage. In another embodiment, the dynamic compressor 100 is an axial compressor, and the compression mechanism 707 is an axial rotor. The speed sensor 717 measures the rotational speed of the compressor 100, and the pressure sensor 709 measures the pressure at various points along the compressor flow path, including the refrigerant inlet and refrigerant outlet. Additional sensors, including, but not limited to, temperature sensors, flow sensors, current sensors 708, voltage sensors, rotational speed sensors, and any other suitable sensors, may be installed in the compressor 100 to provide data regarding its operation. Compressor 100 is not limited to a particular configuration within system 700 and may be configured similarly to compressor 100 described in Figures 1 and 2, or may be configured differently. System 700 further includes an unloading device 701, a variable frequency drive (VFD) 716, and a user interface 715.

[0031] A controller 710 is operatively connected to the compressor 100 and controls operation of the compressor 100 based in part on the measured parameters described above. The controller 710 includes a processor 711, a memory 712, and an unload interface 714. The memory 712 stores a map 1000 (see FIG. 14 ) of predetermined operating points 50 of the compressor 100, which may be stored in any suitable data structure, such as a table or a matrix. The map 1000 may include predetermined operating points for only the first compressor stage 124, for only the second compressor stage 126, or for the entire compressor 100. The memory 712 also stores instructions that are executed by the processor 711 to operate the compressor 100 to compress the working fluid, determine if the flows of the first compressor stage 124 and the second compressor stage 126 have become mismatched, and adjust the unloading devices 701 at the inlets 110, 118 of each compressor stage 124, 126 as needed to restore proper stage matching. The map 1000 of a given operating point 50 and the method 1300 for determining if the compressor stages 124, 126 are matched are described in more detail below.

[0032] The system 700 includes an interface for connecting the controller 710 to a VFD 716 and a motor interface 713 for connecting the VFD 716 to the motor 108. In certain embodiments, the VFD 716 operates under the control of the controller 710. In further embodiments, the VFD 716 is part of the controller 710. The system 700 also includes an unload interface 714 for connecting the controller 710 to an unload device 701.

[0033] The controller 710 is operatively coupled to the unloading device 701 via an unloading interface 714, which removes and / or reduces the load on the compressor 100 during start-up and shutdown routines, during detected surge events, and when directed by the controller 710. In an exemplary embodiment, the unloading device 701 is a variable inlet guide vane (VIGV) at the inlet of each impeller stage ( FIG. 2 ). In other embodiments, the unloading device 701 may be a variable diffuser. The controller 710 is configured to control at least one operating parameter of the unloading device 701, such as the position of each VIGV.

[0034] In another embodiment, the unloading device 701 is a bypass valve. A bypass valve, such as a refrigerant bypass valve, provides an alternate path for gas, thereby limiting the pressure rise of the compressor 100 and preventing potential surge events, regardless of how slowly the motor 108 accelerates during startup or decelerates during shutdown. In another embodiment, the unloading device 701 is an expansion valve. In yet another embodiment, the unloading device 701 may be a variable orifice or diameter valve, such as a servo valve, or a fixed orifice or diameter valve, such as a solenoid valve or pulse-width modulated (PWM) valve configured to open and close according to a duty cycle. While many types of unloading devices are described herein, the unloading device 701 may be any suitable device or combination of devices that reduces the load on the compressor 100. The unloading device 701 may also be used as a loading device to increase the load on the compressor 100.

[0035] System 700 further includes a user interface 715 configured to output (e.g., display) and / or receive (e.g., from a user) information related to system 700. In some embodiments, user interface 715 is configured to receive activation and / or deactivation input from a user to activate and deactivate (i.e., turn on and off) or enable operation of system 700. Additionally, in some embodiments, user interface 715 is configured to output information related to one or more operational characteristics of system 700, including, but not limited to, warning indicators such as severity alerts, occurrence alerts, fault alerts, motor speed alerts, and other suitable information.

[0036] User interface 715 may include any suitable input and output devices that enable user interface 715 to function as described herein. For example, user interface 715 may include input devices, including, but not limited to, a keyboard, a mouse, a touchscreen, a joystick, a throttle, buttons, switches, and / or other input devices. Additionally, user interface 715 may include, but is not limited to, a display (e.g., a liquid crystal display (LCD) or an organic light-emitting diode (OLED) display), a speaker, indicator lights, gauges, and / or other output devices. Furthermore, user interface 715 may be part of another component, such as a system controller (not shown). Other embodiments do not include user interface 715.

[0037] The controller 710 is generally configured to control the operation of the compressor 100. The controller 710 may control operation through programming and instructions from another device or controller, or may be integrated with the system 700 through a system controller. In some embodiments, for example, the controller 710 receives user input from a user interface 715 and controls one or more components of the system 700 in response to such user input. For example, the controller 710 may control the motor 108 based on user input received from the user interface 715. In some embodiments, the system 700 may be controlled by a remote control interface. For example, the system 700 may include a communications interface (not shown) configured for connection to a wireless control interface that enables remote control and activation of the system 700. The wireless control interface may be embodied on a portable computing device, such as a tablet or smartphone.

[0038] The controller 710 may generally include any suitable computer and / or other processing device, including any suitable combination of computers, processing devices, etc., communicatively coupled to each other and capable of operating independently or in connection with each other (e.g., the controller 710 may form all or part of a controller network). The controller 710 may include one or more modules or devices, one or more of which may be housed within the system 700 or located remotely from the system 700. The controller 710 may be part of or separate from the compressor 100, or may be part of a system controller in an HVAC system. The controller 710 and / or components of the controller 710 may be integrated or incorporated within other components of the system 700. The controller 710 includes one or more processors 711 and associated memory devices 712 configured to perform various computer-implemented functions (e.g., performing the calculations, decisions, and functions disclosed herein).

[0039] As used herein, the term "processor" refers not only to integrated circuits, but also to controllers, microcontrollers, microcomputers, programmable logic controllers (PLCs), application-specific integrated circuits, and other programmable circuits. Additionally, the memory device 712 of the controller 710 generally is or can include memory elements, including, but not limited to, computer-readable media (e.g., random access memory (RAM)), computer-readable non-volatile media (e.g., flash memory), floppy disks, compact disk read-only memories (CD-ROMs), magneto-optical disks (MODs), digital versatile disks (DVDs), and / or other suitable memory elements. Such memory devices 712 generally can be configured to store appropriate computer-readable instructions that, when executed by the processor 711, configure or cause the controller 710 to perform various functions described herein, including, but not limited to, controlling the system 700, controlling the operation of the motor 108, receiving input from and providing output to an operator via the user interface 715, controlling the unload device 701, and / or various other suitable computer-implemented functions.

[0040] Referring to FIG. 8, an operating envelope or operating map 800 of an exemplary centrifugal dynamic compressor 100 is shown. The operating map 800 is a graphical representation of one of a plurality of maps 1000 of predetermined operating points 50 stored in memory 712. The operating map 800 graphically displays compressor performance in terms of flow rate, head, and speed. The operating map 800 shows the head versus inlet mass flow rate at the design point of the compressor 100 as a percentage of those values. Head is the total pressure ratio of the outlet pressure to the inlet pressure. The inlet mass flow rate is a measure of the amount of working fluid, such as a refrigerant, flowing through the compression mechanism 707. The operating map 800 shows a plurality of compressor speed lines 807. In this example, there are five speed lines 807 ranging from 70% of design speed to 110% of design speed, each separated by 10%. Although these particular speed lines are shown in this example, any number of speed lines at any different percentage of the compressor's design speed can be shown for any type of compressor.

[0041] Surge limit line 804 indicates the minimum flow rate before a surge occurs in surge region 806 (i.e., to the left of surge limit line 804). Surge control line 803 roughly indicates the minimum flow rate at which compressor 100 can safely operate without risk of entering a surge. Surge control line 803 is defined by surge margin 805 from surge limit line 804. By operating to the right of surge control line 803, compressor 100 avoids a surge. Similarly, choke line 801 indicates that by operating to the right of it, compressor 100 operates in a choked flow.

[0042] A first operating point 809 of compressor 100 is displayed on operating map 800 as the intersection of the speed line, the inlet mass flow rate value, and the total pressure ratio value. For example, the first operating point 809 shown on operating map 800 is 112% inlet mass flow rate, 90% head, and 100% speed, although any number of operating points can be displayed for any type of compressor. The operating point defines the current operating parameters of compressor 100, and operating map 800 indicates how close the current operating point is to operating in an unstable condition (i.e., surge) or inefficient condition (i.e., choke).

[0043] The first operating point 809 shown in Figure 8 may represent the operation of the compressor 100 when the first compressor stage 124 and the second compressor stage 126 receive the same mass flow rate. For example, the first operating point 809 may represent the operation of the compressor 100 installed in the first exemplary HVAC system 300 shown in Figure 3. Alternatively, the first operating point 809 may represent the operation of the compressor 100 installed in the second exemplary HVAC system 400 shown in Figure 4 when the economization valve 470 of the HVAC system 400 is closed, i.e., when refrigerant circulates only through the primary loop 410 and no economizer flow is added between the compressor stage 124 and the compressor stage 126. The first operating point 809 may also represent the operation of the compressor 100 installed in the fourth exemplary HVAC system 600 shown in FIG. 6 when the valve 670 is closed, i.e., when refrigerant circulates only through the primary loop 610 and no flow is removed from between the compressor stage 124 and the compressor stage 126.

[0044] FIG. 9 illustrates an operating map 900 for the second compressor stage 126 of the dynamic compressor 100. A second operating point 909 may represent the operation of the second compressor stage 126 when the first compressor stage 124 and the second compressor stage 126 receive the same mass flow rate, as in the configuration described above. A third operating point 913 may represent the operating condition of the second compressor stage 126 when additional flow is added between the compressor stage 124 and the compressor stage 126. For example, the third operating point 913 may represent the operation of the compressor 100 installed in the system 400 when the economization valve 470 is open. The third operating point 913 may also represent the operation of the compressor 100 installed in the system 500 shown in FIG. 5.

[0045] The second compressor stage 126 cannot achieve the same pressure rise for a higher mass flow rate at the same compressor speed. As a result, the current operating point of the second compressor stage 126 shifts to the right along the 100% speed line from the second operating point 909 to the third operating point 913. The third operating point 913 therefore indicates that the second compressor stage 126 operates at the same speed as the second operating point 909, but with a higher inlet mass flow rate and a lower head. When the third operating point 913 of the second compressor stage 126 shifts beyond the second stage choke line 901, the second compressor stage 126 operates in a choked flow. This causes the entire compressor 100 to operate in a choked flow, reducing its performance and efficiency.

[0046] 10 , the fourth operating point 919 is another example representation of the operation of the second compressor stage 126 when the first compressor stage 124 and the second compressor stage 126 receive the same mass flow rate, as in the configuration described above. The fifth operating point 923 represents the operating condition of the second compressor stage 126 when flow between the first compressor stage 124 and the second compressor stage 126 is removed. For example, the fifth operating point 923 may represent the operation of the compressor 100 installed in the fourth example HVAC system 600 shown in FIG. 6 when the valve 670 is open and flow between the second compressor stage 124 and the second compressor stage 126 is removed. Reducing the mass flow rate through the second compressor stage 126 shifts its operating point to the left along the 100% speed line from the fourth operating point 919 to the fifth operating point 923. Thus, fifth operating point 923 shows second compressor stage 126 operating at the same speed as fourth operating point 919, but with a reduced inlet flow rate. If fifth operating point 923 of second compressor stage 126 shifts beyond second stage surge control line 903 or surge limit line 904, second compressor stage 126 is at risk of surge. Surge in second compressor stage 126 can disrupt the entire machine and potentially lead to structural damage.

[0047] The degradation in performance and operating range shown in Figures 9 and 10 can be mitigated by adjusting the position of the second VIGV 136, thereby shifting the operating map of the second compressor stage 126 so that the second compressor stage 126 does not choke or surge entirely in front of the compressor 100. Figures 9 and 10 show an operating map 900 of the second compressor stage 126 when the first VIGV 134 and the second VIGV 136 are in the same position. In other words, the first position of the first VIGV 134 and the second position of the second VIGV 136 are the same position.

[0048] FIG. 11 shows an operating map 1100 of the second compressor stage 126 when the second VIGV 136 is moved to a third position different from the second position. Operating map 1100 is superimposed on operating map 900 shown in FIG. 9, which includes second operating point 909 and third operating point 913. Adjusting the second VIGV 136 to the third position changes the pre-swirl added to the refrigerant gas entering the second compressor stage 126, shifting its operating envelope to the right and widening its choked range. Because the speed of the second compressor stage 126 remains the same, the current operating point is shifted upward from the third operating point 913 to a sixth operating point 1113, the new 100% speed line for the second compressor stage 126. Because the sixth operating point 1113 is to the left of the new choke line 1101, the second compressor stage 126 no longer operates in choked flow. This allows the HVAC system 400, 500 to enjoy the benefits of an economization loop or booster system without compromising the performance and operating range of the compressor 100.

[0049] 12, the third position of the second VIGV 136 may also be selected to shift the operating envelope of the second compressor stage 126 to the left, thereby extending the surge range of the compressor 100. FIG. 12 shows an operating map 1200 of the second compressor stage 126 when the second VIGV 136 is adjusted to the third position, which extends the surge range of the compressor 100. The operating map 1200 is overlaid on the operating map 900 shown in FIG. 9, which includes a fourth operating point 919 and a fifth operating point 923. The current operating point is shifted downward from the fifth operating point 923 to a seventh operating point 1223, to the new 100% speed line of the second compressor stage 126. Because the seventh operating point 1223 is to the right of the new surge control line 1203 , the second compressor stage 126 is no longer at risk of surging before the rest of the compressor 100 .

[0050] The memory 712 stores instructions that program the processor 711 to extend the operating range of the compressor 100 as described above. An exemplary method 1300 is shown in FIG. 13. The processor 711 operates the compressor 100 at a current speed, a first position of the first VIGV 134, and a second position of the second VIGV 136 to compress the working fluid (1302). In some embodiments, the first position of the first VIGV 134 and the second position of the second VIGV 136 are the same position. While operating the dynamic compressor 100 (1302), the processor 711 determines whether a condition is met. If the condition is not met, the instructions stored in the memory 712 program the processor 711 to continue operating the compressor 100 at the current speed, the first position of the first VIGV 134, and the second position of the second VIGV 136 (1310). If the condition is met, the instructions stored in the memory 712 program the processor 711 to change the second position of the second VIGV 136 to a third position different from the second position and maintain the first position of the first VIGV 134 (1312).

[0051] In the exemplary method shown in FIG. 13, determining whether a condition is met includes determining whether a valve in fluid communication with compressor 100 is open (1304). In certain embodiments, the valve is economization valve 470 of system 400 shown in FIG. 4. In other embodiments, the valve may be valve 670 of system 600 shown in FIG. 6. A valve is considered open when it is fully or partially open, fluidly connecting a portion of the primary refrigerant loop to the secondary refrigerant loop at the valve. If the valve is not open, the condition is not met and compressor 100 continues to operate at the current condition (1310).

[0052] If the valve is open, the processor 711 is further programmed to determine 1306 a limiting speed for the second compressor stage 126. In embodiments where the valve is the economization valve 470 of the system 400 shown in FIG. 4, the limiting speed for the second compressor stage 126 may be the choke speed of the second compressor stage 126. In such embodiments, if the processor 711 determines 1308 that the choke speed of the second compressor stage 126 is greater than or equal to the current speed of the dynamic compressor 100, the condition is not met and the compressor continues to operate at the current conditions 1310. If the processor 711 determines 1308 that the choke speed of the second compressor stage 126 is less than the current speed of the dynamic compressor 100, the condition is met, and the second position of the second VIGV 136 is changed 1312 to a third position different from the second position, and the first position of the first VIGV 134 is maintained. Thus, the condition is met when the economization valve 470 is open and the choke speed of the second compressor stage 126 is less than the current speed of the dynamic compressor 100.

[0053] In embodiments where the valve is valve 670 of system 600 shown in FIG. 6 , the speed limit of second compressor stage 126 may be the surge control speed of second compressor stage 126. In such embodiments, if processor 711 determines that the surge control speed of second compressor stage 126 is less than or equal to the current speed of dynamic compressor 100, the condition is not met and the compressor continues to operate at the current conditions (1310). If processor 711 determines that the surge control speed of second compressor stage 126 is greater than the current speed of dynamic compressor 100 (1309), the condition is met and the second position of second VIGV 136 is changed to a third position different from the second position (1312), and the first position of first VIGV 134 is maintained. Thus, the condition is met when the valve 670 is open and the surge control speed of the second compressor stage 126 is greater than the current speed of the dynamic compressor 100 .

[0054] Method 1300 may be used in embodiments in which memory 712 further stores a map 1000 of predetermined operating points 50 for compressor 100. FIG. 14 is a representative illustration of map 1000 of predetermined operating points 50 stored in memory 712. Each predetermined operating point 50 is shown as the intersection of a compressor speed value and a stage pressure ratio value. An inlet mass flow rate is defined for each predetermined operating point 50. Map 1000 includes a range of predetermined operating points 50 up to points along a mechanical surge line 1020 and a mechanical choke line 1030. Memory 712 stores a surge point mass flow rate for each predetermined surge point and a choke point mass flow rate for all predetermined choke points. Map 1000 does not include points above surge line 1020 or below choke line 1030. This is because points above surge line 1020 or below choke line 1030 should be avoided and are thus not "operating points." Other embodiments may include inlet mass flow rates at points above the surge line 1020 or below the choke line 1030.

[0055] In the map 1000, the predetermined operating points 50 are in the speed range of 10% to 35% and the pressure ratio range of 5% to 50%, with each point spaced 5% apart on both axes. While these specific operating points 50 are shown in this example, any number of operating points can be displayed at any value and with any resolution for any type of compressor. The speed, pressure ratio, inlet mass flow rate, and VIGV position values ​​for each predetermined operating point 50 can be generated by simulating the operation of the dynamic compressor 100 on a computer, testing the dynamic compressor 100 in a controlled environment, a combination of simulation and testing, or any other suitable method for predetermining the speed, pressure ratio, inlet mass flow rate, and VIGV position values ​​for each predetermined operating point 50.

[0056] 14 may include a predetermined operating point 50 for only the second compressor stage 126. In addition to a mechanical choke line 1032 and a surge control line 1020, the map 1000 also shows a choke line 1050 for the second compressor stage 126 when flow is added between the compressor stage 124 and the compressor stage 126, and a surge line 1040 for the second compressor stage 126 when flow is removed between the compressor stage 124 and the compressor stage 126. In such an embodiment, determining 1306 the limit speed of the second compressor stage 126 includes obtaining the value of the predetermined operating point 50 for the second compressor stage 126 from the map 1000 of predetermined operating points 50.

[0057] The predetermined operating point 50 obtained from the map 1000 may indicate the choke speed or surge control speed of the second compressor stage 126 at the current operating point 1009, or the operating points 50 may be used to graphically determine the choke speed or surge control speed at the current operating point. For example, the choke speed of the second compressor stage 126 at the current operating point 1009 is indicated by 1034, and the surge control speed is indicated by 1024. The predetermined operating point 50 may have the same speed, pressure ratio, and inlet mass flow rate as the current operating point 1009 of the second compressor stage 126. In a further embodiment, the predetermined operating point 50 closest to the current operating point 1009 may be obtained. In a further embodiment, a new point corresponding to the current operating point 1009 may be interpolated from the predetermined operating point 50. A method for mass flow rate interpolation using multiple predetermined operating points is disclosed in U.S. Patent Application No. 17 / 243,787, the entire contents of which are incorporated herein by reference.

[0058] Map 1000 may alternatively include predetermined operating points for the entire compressor 100. In such an embodiment, the limit speed of the second compressor stage 126 may be calculated rather than directly obtained. FIG. 15 shows a flowchart of an example control algorithm 1510, 1520 for determining 1308, 1309 the limit speed of the second compressor stage 126 when adding or removing flow between compressor stages 124 and 126. The compressor choke speed is the speed at which the compressor 100 chokes for a given pressure rise and can be determined graphically from map 1000. For example, referring to FIG. 14, the compressor choke speed 1034 for the current operating point 1009 is approximately 33% of the compressor design speed. The choke speed of the second compressor stage 126 can be calculated and compared to the current speed of the compressor 100 when the economization valve 470 of the system 400 is open. 15, the choke speed of the second compressor stage 126 can be calculated from the compressor choke speed. For example, the choke speed of the second compressor stage 126 can be calculated as the difference between the compressor choke speed obtained from the map 1000 for a given operating point 50 and the product of the compressor pressure ratio PR and a given constant k: N choke,2S =N choke,compressor -PR*k In some embodiments, the predetermined constant k is 500.

[0059] The compressor surge control speed, above which the compressor 100 may surge for a given pressure rise, can be determined graphically from the map 1000. For example, referring to FIG. 14, the compressor surge control speed 1024 for the current operating point 1009 is approximately 17% of the compressor design speed. The surge control speed of the second compressor stage 126 can be calculated and compared to the current speed of the compressor 100 when the valve 670 of the system 600 is open. In the control algorithm 1520 shown in FIG. 15, the surge control speed of the second compressor stage 126 can be calculated from the compressor surge control speed. For example, the surge control speed of the second compressor stage can be calculated as the sum of the compressor surge control speed obtained from the map 1000 for the predetermined operating point 50 and the product of the compressor pressure ratio PR and a predetermined constant k. N surge control,2S =N surge control,compressor +PR*k In some embodiments, the predetermined constant k is 500.

[0060] The technical advantages of the methods and systems described herein are: (a) cycle modifications can improve HVAC system efficiency by increasing system capacity and efficiency without compromising compressor performance and operating range; and (b) independent control of VIGV at each compressor stage can extend the compressor operating range in either direction.

[0061] As used herein, the terms "about," "substantially," "essentially," and "approximately," when used in conjunction with a range of dimensions, concentrations, temperatures, or other physical or chemical properties or characteristics, are intended to cover the variation that may exist at the upper and / or lower limits of the range of the property or characteristic, including, for example, variation resulting from rounding, measurement method, or other statistical variation.

[0062] When introducing elements of the present disclosure or embodiments thereof, the articles "a," "an," "the," and "said" mean that there are one or more elements. The terms "comprising," "including," "containing," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of specific orientation terms (such as "top," "bottom," "side," etc.) is for convenience of description and does not require a particular orientation of the items being described.

[0063] Because various changes may be made in the above structures and methods without departing from the scope of this disclosure, all matter contained in the above description and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Claims

1. 1. A system comprising:

1. A dynamic compressor operable to compress a working fluid, comprising: a first compressor stage having first variable inlet guide vanes (VIGVs) controllable to selectively introduce pre-swirl into a working fluid entering the first compressor stage; a second compressor stage having a second VIGV controllable to selectively introduce pre-swirl into the working fluid entering the second compressor stage; a motor configured to drive the first compressor stage and the second compressor stage at the same speed; a valve in fluid communication with the dynamic compressor for selectively directing the flow of working fluid to an inlet of the second compressor stage without the flow passing through the first compressor stage; a controller connected to the dynamic compressor; The controller includes a processor and a memory, the memory including: operating the dynamic compressor at a current speed, a first position of the first VIGV, and a second position of the second VIGV to compress the working fluid; determining whether a condition is met while the dynamic compressor is operating; if the condition is not met, continuing to operate the dynamic compressor at the current speed, the first position of the first VIGV, and the second position of the second VIGV; When the condition is satisfied, instructions are stored to program the second position of the second VIGV to be changed to a third position different from the second position, and to maintain the first position of the first VIGV; determining whether the condition is met includes determining whether the valve is open. system.

2. 2. The system of claim 1, wherein the memory further stores instructions programming the processor to determine a limiting speed of the second compressor stage when the valve is open.

3. the limiting speed is a choke speed of the second compressor stage, and determining whether the condition is met includes:

3. The system of claim 2, further comprising determining that the condition is met when the valve is open and the choke speed of the second compressor stage is less than the current speed of the dynamic compressor.

4. the limiting speed is a surge control speed of the second compressor stage, and determining whether the condition is met includes:

3. The system of claim 2, further comprising determining that the condition is met if the valve is open and the surge control speed of the second compressor stage is greater than the current speed of the dynamic compressor.

5. 3. The system of claim 2, wherein the memory further stores a map of predetermined operating points for the dynamic compressor.

6. the map of predetermined operating points includes an operating point of the second compressor stage; The system of claim 5 , wherein determining the limit speed of the second compressor stage includes obtaining an operating point value for the second compressor stage from the map of predetermined operating points.

7. 6. The system of claim 5, wherein determining the limit speed of the second compressor stage comprises calculating the limit speed of the second compressor stage based on a compressor pressure ratio and a value of the limit speed obtained from a map of the predetermined operating point, and based on the compressor pressure ratio and a predetermined constant.

8. The system of claim 1 , wherein the first location of the first VIGV and the second location of the second VIGV are the same location.

9. The system described in claim 1 further comprising a primary refrigerant loop and a secondary refrigerant loop comprising at least a portion of the primary refrigerant loop, the secondary refrigerant loop branching off from the primary refrigerant loop at the valve, and the secondary refrigerant loop connecting the outlet of the first compressor stage to the inlet of the second compressor stage via the valve without passing through the first compressor stage.

10. The primary refrigerant loop comprises: a condenser fluidly coupled to the second compressor stage; a first expansion device fluidly coupled to the condenser; an evaporator fluidly coupled to the first expansion device and the first compressor stage; The secondary refrigerant loop comprises: a second expansion device fluidly coupled to the condenser; 10. The system of claim 9, comprising a heat exchanger fluidly coupled to the second expansion device, the condenser, and the second compressor stage.

11. A controller for a dynamic compressor having a motor, a first compressor stage driven by the motor, and a second compressor stage driven by the motor, the controller comprising: a processor; a memory, the memory including: operating the dynamic compressor at a current speed, a first position of a first variable inlet guide vane (VIGV) of the first compressor stage, and a second position of a second VIGV of the second compressor stage to compress a working fluid, the first VIGV being controllable to selectively introduce pre-swirl into the working fluid entering the first compressor stage, and the second VIGV being controllable to selectively introduce pre-swirl into the working fluid entering the second compressor stage; determining whether a condition is met while the dynamic compressor is operating; if the condition is not met, continuing to operate the compressor at the current speed, the first position of the first VIGV, and the second position of the second VIGV; When the condition is satisfied, instructions are stored to program the second position of the second VIGV to be changed to a third position different from the second position, and to maintain the first position of the first VIGV; determining whether a condition is met includes determining whether a valve is open, the valve being in fluid communication with the dynamic compressor for selectively directing the flow of working fluid to an inlet of the second compressor stage without the flow of working fluid passing through the first compressor stage; controller.

12. Determining whether the condition is met includes: The controller of claim 11 including determining a limiting speed of the second compressor stage when the valve is open.

13. the limiting speed is a choke speed of the second compressor stage, and determining whether the condition is met includes:

13. The controller of claim 12, further comprising determining that the condition is met when the valve is open and the choke speed of the second compressor stage is less than the current speed of the dynamic compressor.

14. the limiting speed is a surge control speed of the second compressor stage, and determining whether the condition is met includes:

13. The controller of claim 12, further comprising determining that the condition is met if the valve is open and the surge control speed of the second compressor stage is greater than a current speed of the dynamic compressor.

15. 13. The controller of claim 12, wherein the memory further stores a map of predetermined operating points for the dynamic compressor.

16. the map of predetermined operating points includes an operating point of the second compressor stage; The controller of claim 15 , wherein determining the speed limit of the second compressor stage includes obtaining an operating point value for the second compressor stage from the map of predetermined operating points.

17. 16. The controller of claim 15, wherein determining the limit speed of the second compressor stage comprises calculating the limit speed of the second compressor stage based on a compressor pressure ratio and a value of the limit speed obtained from a map of the predetermined operating point, and based on the compressor pressure ratio and a predetermined constant.

18. 1. A method of extending an operating range of a dynamic compressor compressing a working fluid, the dynamic compressor having a motor, a first compressor stage driven by the motor, and a second compressor stage driven by the motor, the method comprising: operating the dynamic compressor at a current speed, a first position of a first variable inlet guide vane (VIGV) of the first compressor stage, and a second position of a second VIGV of the second compressor stage to compress a working fluid, wherein the first VIGV is controllable to selectively introduce pre-swirl into the working fluid entering the first compressor stage and the second VIGV is controllable to selectively introduce pre-swirl into the working fluid entering the second compressor stage; determining whether a condition is met while operating the dynamic compressor; if the condition is not met, continuing to operate the compressor at the current speed, the first position of the first VIGV, and the second position of the second VIGV; and if the condition is satisfied, changing the second position of the second VIGV to a third position different from the second position, and maintaining the first position of the first VIGV; determining whether the condition is met includes determining whether a valve is open, the valve being in fluid communication with the dynamic compressor for selectively directing the flow of working fluid to an inlet of the second compressor stage without the flow passing through the first compressor stage; method.

19. The step of determining whether the condition is satisfied includes: and determining a limiting speed of the second compressor stage if the valve is open.

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