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 Application The present application claims priority from U.S. Patent Application No. 17 / 585,736 filed on January 27, 2022, the entire disclosure of which is incorporated herein by reference.

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

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

[0004] In a multi-stage compressor, refrigerant enters each compressor stage at different pressure and volumetric flow rates. Accordingly, each stage must be designed to "match" the other stages, so that each stage can effectively process the fluid received 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, expander, and evaporator fluid-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 economy loop that diverts a portion of the cryogenic liquid refrigerant 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 a medium-temperature evaporator. This diverted portion, or economy flow, is then injected between the compressor stages as a cryogenic, medium-pressure gas to improve the overall system efficiency. In other systems, the flow is removed from between the compressor stages and diverted to an additional condenser to enable additional uses.

[0006] In such systems, adding or removing flow between compressor stages can cause the stages to no longer aerodynamically match. Without proper stage matching, the downstream stages of the injection may experience undesirable flow conditions under different conditions than the rest of the compressor, potentially compromising the overall performance, efficiency, and safety of the machine. Therefore, it is desirable to maintain aerodynamic matching between stages, regardless of changes in mass flow rates between them.

[0007] This section aims to introduce to the reader various aspects of the technology that may be related to the various aspects of the disclosure described and / or claimed below. This description is intended to help the reader provide background information to better understand the various aspects of the disclosure. Therefore, it should be understood that these descriptions should be read in this context and not as an endorsement of prior art. [Overview of the Initiative]

[0008] One aspect of the present disclosure relates to a system comprising a dynamic compressor operable to compress a working fluid and a controller connected 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 memory. The memory stores instructions that program the processor to operate the dynamic compressor at the 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 memory program the processor to continue operating the compressor at the current speed, a first position of the first VIGV, and a second position of the second VIGV. If the conditions are met, the instruction stored in memory programs the processor to change the second position of the second VIGV to a third position different from the second position, while maintaining 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 memory. The memory stores instructions that program the processor to operate the dynamic compressor at the current speed, at a first position of the first VIGV of the first compressor stage, and at a second position of the second VIGV of the second compressor stage to compress the working fluid and to determine whether a condition is met. If the condition is not met, the instructions stored in memory program the processor to continue operating the compressor at the current speed, at a first position of the first VIGV, and at a second position of the second VIGV. If the condition is met, the instructions stored in memory program the processor to change the second position of the second VIGV to a third position different from the second position, and to 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 that compress a working fluid. The method includes the steps of: compressing a working fluid by operating the dynamic compressor at a current speed, a first position of the first VIGV of the first compressor stage, and a second position of the second VIGV of the second compressor stage; and determining whether a condition is met. The method further includes, if the condition is not met, continuing to operate the compressor at a current speed, a first position of the first VIGV, and a 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] The features described in relation to the embodiments of this disclosure described above are subject to various improvements. Further features may also be incorporated into the embodiments of this disclosure described above. These improvements and additional features may exist individually or in any combination. For example, the various features described below in relation to any of the illustrated embodiments of this disclosure may be incorporated individually or in any combination into any of the embodiments of this disclosure described above. [Brief explanation of the drawing]

[0012] [Figure 1] This is a perspective view of the assembled dynamic compressor. [Figure 2] Figure 1 is a cross-sectional view of the dynamic compressor after it has been cut along line 2-2 and the external conduit has been removed. [Figure 3] Figures 1 and 2 show schematic diagrams of a first exemplary HVAC system in which a dynamic compressor can be installed. [Figure 4] Figures 1 and 2 show schematic diagrams of a second exemplary HVAC system in which a dynamic compressor can be installed. [Figure 5] Figures 1 and 2 show schematic diagrams of a third exemplary HVAC system in which a dynamic compressor can be installed. [Figure 6] Figures 1 and 2 show a schematic diagram of a fourth exemplary HVAC system in which a dynamic compressor can be installed. [Figure 7] Figures 1 and 2 are block diagrams of the control system for the dynamic compressor. [Figure 8] Figures 1 and 2 show the operation maps of the dynamic compressor. [Figure 9] Figures 1 and 2 show the operation map of the second compressor stage of the dynamic compressor, indicating the current operating point before and after the injection of flow upstream of the second compressor stage. [Figure 10] Figures 1 and 2 show the operation map of the second compressor stage of the dynamic compressor, indicating the current operating point before and after the flow is removed upstream of the second compressor stage. [Figure 11] Figures 1 and 2 show the operation maps of the second compressor stage of the dynamic compressor when flow is injected upstream of the second compressor stage and the second VIGV is moved from its second position to a different position. [Figure 12] Figures 1 and 2 show the operation maps of the second compressor stage of the dynamic compressor when the flow is removed from upstream of the second compressor stage and the second VIGV is moved from its second position to a different position. [Figure 13] This is a method for extending the operating range of the dynamic compressor shown in Figures 1 and 2. [Figure 14] Figures 1 and 2 show maps of predetermined operating points for the dynamic compressor. [Figure 15] This is a flowchart of an exemplary control algorithm for determining the limit speed of the second compressor stage of the dynamic compressor shown in Figures 1 and 2, when flow is added or removed from upstream of the second compressor stage. [Modes for carrying out the invention]

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

[0014] For the sake of brevity, an example is described with reference to a centrifugal compressor. However, the methods and systems described herein can be applied to any suitable dynamic compressor. The performance and efficiency of HVAC systems can be improved by diverting a portion of the main flow to auxiliary loops and circulating components. In such systems, flow can be injected or extracted between compressor stages such that each stage has a different mass flow rate. However, to avoid operation under undesirable conditions, it is necessary to maintain aerodynamic matching between stages, with or without such flow modifications. A control strategy can be used to determine whether stages have become flow mismatched and adjust the variable inlet guide vanes (VIGV) at the inlet of each stage to restore proper stage matching.

[0015] Referring to FIG. 1, a two-stage refrigerant compressor is generally indicated at 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, in which refrigerant compression for each stage is performed. 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 operatively 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 compressor stage and the second compressor stage. The 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 to 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 apply kinetic energy to refrigerant flowing in through the 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 into an increase in refrigerant pressure, as the refrigerant velocity decreases when being transferred 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 positions are controllable, and the positions of the vanes introduce pre-whirl to the gaseous refrigerant entering the first refrigerant inlet 110.

[0017] Similarly, the second compressor stage 126 includes a second compression mechanism 116, which is configured to add kinetic energy to the refrigerant transferred from the first compressor stage 124, flowing in 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 velocity of the refrigerant slows down as it is transferred to a sealed cavity (e.g., a diffuser) formed within the volute 132. The compressed refrigerant is discharged from the second compressor stage 126 through a second refrigerant outlet 120 (not shown in Figure 2). The second compressor stage 126 further includes a second variable inlet guide vane (VIGV) 136 located upstream of the second compression mechanism 116 at the second refrigerant inlet 118. The second VIGV136 includes multiple vanes, and the position of the vanes can be controlled to introduce 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 operably 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 the second refrigerant outlet 120 (not shown in Figure 2) to a predetermined pressure. The compressor 100 can incorporate any suitable motor, including but not limited to an electric motor.

[0019] Figure 3 is a schematic diagram of a first exemplary HVAC system 300 in which the compressor 100 of Figures 1 and 2 can be installed. The system 300 has a single closed refrigerant loop 310 including the compressor 100, a condenser 320, a first expansion unit 330, and an evaporator 340. The refrigerant enters the compressor 100 from the first refrigerant inlet 110 as a low-pressure, low-temperature gas. 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 from the second refrigerant outlet 120 as a high-pressure, high-temperature gas. The refrigerant enters the condenser 320, which is fluid-coupled to the second compressor stage 126, and heat Q out The refrigerant gas is removed and converted into a high-pressure, high-temperature liquid.

[0020] The condenser 320 is fluid-coupled to a first expansion device 330 that 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 at that pressure, and as some of the liquid refrigerant boils and turns into a gas, the refrigerant becomes a two-phase mixture. The first expansion device 330 may be a fixed orifice, a thermal expansion valve, an electronic expansion valve, or any type of expansion device that allows the HVAC system 300 to function as described herein. The first expansion device 330 is fluid-coupled to an evaporator 340, which receives a low-pressure, low-temperature liquid refrigerant, or a two-phase mixture of liquid and gaseous refrigerant at its inlet. In the evaporator 340, the refrigerant is heated to heat Q in It absorbs and undergoes a phase change from liquid to gas. The evaporator 340 is fluid-coupled to the first compressor stage 124, and the cycle restarts.

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

[0022] The secondary refrigerant loop 460 includes an economization valve 470, a second expansion device 480, a second flow 494 of the 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 fluid-coupled in the order listed, and the condenser 320 is 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 the refrigerant flows through the secondary refrigerant loop 460. That is, when the economization valve 470 is fully closed, all the refrigerant flows through the primary refrigerant loop 410, and system 400 operates substantially the same as system 300 shown in Figure 3. When the economization valve 470 is open, the liquid refrigerant discharged from the condenser 320 splits into two flows, with the majority of the refrigerant flowing through the primary refrigerant loop 410 and the remainder 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 allows system 400 to function as described herein.

[0024] When the economization valve 470 is open, it is fluid-coupled to the second expansion unit 480, reducing the pressure of the liquid economizer flow until the current temperature of the liquid refrigerant reaches its boiling point at that pressure. The refrigerant in the secondary refrigerant loop becomes a two-phase mixture as some of the liquid refrigerant boils and turns into gas when it enters the heat exchanger 490. The second expansion unit can be sized and selected to divert a specific amount of refrigerant (e.g., 0–20 percent of the total mass flow rate, or any amount of refrigerant flow rate that the system 400 can function as described herein) through the secondary refrigerant loop 460 when the economization valve 470 is open.

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

[0026] The refrigerant exits the second expansion unit 480 and enters the second flow 494 of the heat exchanger 490 as a low-pressure liquid or two-phase mixture. The second flow 494 is thermally connected to the first flow 492, which carries the high-pressure liquid refrigerant from the condenser 320 of the primary refrigerant loop 410. The thermal contact between the two flows 492, 494 cools the refrigerant in the first flow 492 and heats and boils the refrigerant in the second flow 494. The cooled refrigerant in the first flow 492 exits the heat exchanger 490 as a low-temperature, high-pressure liquid, and the boiled refrigerant in the second flow 494 exits the heat exchanger 490 as a low-temperature, medium-pressure gas. The heat exchanger 490 can be a counterflow heat exchanger, a crossflow heat exchanger, a parallel flow heat exchanger, a shell-and-tube heat exchanger, a mixing chamber, or any type of heat exchanger in which the system 400 can function as described herein. In further embodiments, a flash tank may be used instead of, or in addition to, the heat exchanger 490.

[0027] The low-temperature, medium-pressure gas discharged from the second flow 494 of the heat exchanger 490 is then injected into the refrigerant transfer conduit 112 of the compressor 100 and mixed with the refrigerant flow 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 then branch off again after the refrigerant has been discharged from the condenser 320.

[0028] Figure 5 shows a third exemplary HVAC system 500 in which the compressor 100 of Figures 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 to them. Downstream of the condenser 320, the refrigerant flow is throttled by a first expansion device 530, reducing the pressure until a portion of the liquid refrigerant boils and a two-phase mixture is formed. A flash tank 590 separates the two-phase refrigerant mixture into liquid and gaseous portions, which branch 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 by a second expansion device 532 and branched again, with a portion of the refrigerant continuing along the primary loop 510 and the remainder branching into the secondary loop 560. In the secondary loop 560, the refrigerant passes through the medium-temperature evaporator 540, where it boils and is converted into 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 it boils and is converted into a gas, providing cooling to the low-temperature space. Next, the refrigerant 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 refrigerants 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] Figure 6 shows a fourth exemplary HVAC system 600 in which the compressor 100 of Figures 1 and 2 can be installed. The system 600 includes a primary refrigerant loop 610, which includes the compressor 100, a condenser 320, a first expansion unit 330, and an evaporator 340. The system also includes a secondary refrigerant loop 660, which is fluid-connected to a portion of the primary refrigerant loop 610 and controlled by a valve 670. When the valve 670 is fully closed, all refrigerant flows through the primary loop 610, and the system 600 operates substantially the same as the system 300 shown in Figure 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 the auxiliary condenser 620, which is narrowed by the second expansion device 630, and rejoins the primary loop 610 before entering the evaporator 340.

[0030] Figure 7 shows 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 the impeller for each stage. In other embodiments, the dynamic compressor 100 is an axial flow compressor, and the compression mechanism 707 is an axial flow 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 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 on the compressor 100 to provide data on its operation. The compressor 100 is not limited to a specific configuration within the system 700 and may be configured similarly to the compressor 100 described in Figures 1 and 2, or in a different manner. The system 700 further includes an unloading device 701, a variable frequency drive (VFD) 716, and a user interface 715.

[0031] The controller 710 is operably connected to the compressor 100 and controls the operation of the compressor 100, partly based on the measurement 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 Figure 14) of a plurality of predetermined operating points 50 of the compressor 100, which can be stored in any suitable data structure such as a table or matrix. The map 1000 includes a plurality of predetermined operating points of only the first compressor stage 124, a plurality of predetermined operating points of only the second compressor stage 126, or a plurality of predetermined operating points of the entire compressor 100. The memory 712 also stores instructions executed by the processor 711. These instructions operate the compressor 100 to compress the working fluid, determine whether the flow in the first compressor stage 124 and the second compressor stage 126 is no longer synchronized, and adjust the unloading devices 701 at the inlets 110 and 118 of each compressor stage 124 and 126 as necessary to restore proper stage matching. The method 1300 for determining whether the map 1000 of a predetermined operating point 50 matches the compressor stages 124 and 126 will be described in more detail below.

[0032] The system 700 includes an interface for connecting the controller 710 to the 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 unloading device 701.

[0033] The controller 710 is operably coupled to the unloading device 701 via an unload interface 714, which removes and / or reduces the load on the compressor 100 during start-up and stop routines, during detected surge events, and when instructed by the controller 710. In an exemplary embodiment, the unloading device 701 is a variable inlet guide vane (VIGV) located at the inlet of each impeller stage (Figure 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 other embodiments, the unloading device 701 is a bypass valve. A bypass valve, such as a refrigerant bypass valve, provides an alternative gas path, thereby limiting the pressure rise in the compressor 100 and preventing potential surge events, regardless of how slowly the motor 108 accelerates at startup or how slowly it decelerates at shutdown. In other embodiments, 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 modulation (PWM) valve, configured to control opening and closing according to a duty cycle. While many types of unloading devices are described here, 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 load device to increase the load on the compressor 100.

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

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

[0037] The controller 710 is generally configured to control the operation of the compressor 100. The controller 710 controls the operation through programming and commands from another device or controller, or is 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 can 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 communication 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 include any suitable computer and / or other processing devices, including any suitable combination of computers, processing devices, etc., that can be communicated together and operate independently or in conjunction with each other (for example, 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 separately from the system 700. The controller 710 may be part of the compressor 100, or it may be separate, or it may be part of a system controller in the HVAC system. The controller 710 and / or components of the controller 710 may be integrated or incorporated into 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 implementation functions (e.g., calculations, decisions, and function executions 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. Furthermore, the memory device 712 of the controller 710 may include, but not be 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 memory (CD-ROM), magneto-optical disks (MODs), digital multipurpose disks (DVDs), and / or other suitable memory elements. Such a memory device 712 may generally be configured to store suitable computer-readable instructions, and when the instructions are executed by the processor 711, the controller 710 may be configured or made to perform various functions described herein, including, but not limited to, control of the system 700, control of the operation of the motor 108, receive input from the user interface 715, provide output to the operator via the user interface 715, control of the unloading device 701, and / or various other suitable computer implementation functions.

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

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

[0042] The first operating point 809 of the compressor 100 is displayed on the operation 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 the operation map 800 has an inlet mass flow rate of 112%, a head of 90%, and a speed of 100%, but any number of operating points can be displayed for any type of compressor. The operating points define the current operating parameters of the compressor 100, and the operation map 800 shows how close the current operating point is to operating in an unstable condition (i.e., surge) or an inefficient condition (i.e., choke).

[0043] The first operating point 809 shown in Figure 8 can represent the operation of 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 can represent the operation of compressor 100 installed in the first exemplary HVAC system 300 shown in Figure 3. Alternatively, the first operating point 809 can represent the operation of 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 the refrigerant circulates only through the primary loop 410 and no economizer flow is added between compressor stage 124 and compressor stage 126. The first operating point 809 can also represent the operation of the compressor 100 installed in the fourth exemplary HVAC system 600 shown in Figure 6, when the valve 670 is closed, i.e., when the refrigerant circulates only through the primary loop 610 and the flow is not removed between the compressor stage 124 and the compressor stage 126.

[0044] Figure 9 shows the operation map 900 of the second compressor stage 126 of the dynamic compressor 100. The second operating point 909 can 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. The third operating point 913 can represent the operating conditions of the second compressor stage 126 when flow is added between the compressor stage 124 and the compressor stage 126. For example, the third operating point 913 can represent the operation of the compressor 100 installed in system 400 when the economy valve 470 is open. The third operating point 913 can also represent the operation of the compressor 100 installed in system 500 shown in Figure 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. Thus, the third operating point 913 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 choke flow. This causes the entire compressor 100 to operate in choke flow, reducing its performance and efficiency.

[0046] Referring to Figure 10, the fourth operating point 919 is another exemplary 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 the flow between compressor stage 124 and compressor stage 126 is removed. For example, the fifth operating point 923 can represent the operation of compressor 100 installed in the fourth exemplary HVAC system 600 shown in Figure 6 when valve 670 is open and the flow between compressor stage 124 and compressor stage 126 is removed. Reducing the mass flow rate through the second compressor stage 126 shifts its operating point to the left from the fourth operating point 919 to the fifth operating point 923 along the 100% velocity line. Therefore, the fifth operating point 923 indicates that the second compressor stage 126 is operating at the same speed as the fourth operating point 919, but with a reduced inlet flow rate. If the fifth operating point 923 of the second compressor stage 126 shifts beyond the surge control line 903 or surge limit line 904 of the second stage (compressor stage), the second compressor stage 126 is at risk of a surge. If a surge occurs in the second compressor stage 126, the entire machine may become disrupted and could lead to structural damage.

[0047] The performance and operating range degradation 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 and preventing the second compressor stage 126 from choking or surgeing as a whole in front of the compressor 100. Figures 9 and 10 show the 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] Figure 11 shows the operation map 1100 of the second compressor stage 126 when the second VIGV 136 is moved to a third position different from the second position. Operation map 1100 is superimposed on operation map 900 shown in Figure 9, which includes the second operating point 909 and the 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 the choke range. As 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 the sixth operating point 1113, up to the new 100% speed line of 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 with choke flow. This allows HVAC systems 400 and 500 to enjoy the benefits of an economy loop or booster system without compromising the performance and operating range of the compressor 100.

[0049] Referring to Figure 12, the third position of the second VIGV 136 can also be selected such that the operating envelope of the second compressor stage 126 is shifted to the left, extending the surge range of the compressor 100. Figure 12 shows the operating map 1200 of the second compressor stage 126 when the second VIGV 136 is adjusted to the third position that extends the surge range of the compressor 100. Operating map 1200 is superimposed on operating map 900 shown in Figure 9, which includes the fourth operating point 919 and the fifth operating point 923. The current operating point is shifted downward from the fifth operating point 923 to the seventh operating point 1223, to the new 100% speed line of the second compressor stage 126. Because the seventh operating point 1223 is located to the right of the new surge control line 1203, the second compressor stage 126 is no longer at risk of surges occurring before the rest of the compressor 100.

[0050] Memory 712 stores instructions programmed to the processor 711 to extend the operating range of the compressor 100 as described above. An exemplary method 1300 is shown in Figure 13. The processor 711 operates the compressor 100 at the current speed, the first position of the first VIGV 134, and the 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 the dynamic compressor 100 is operating (1302), the processor 711 determines whether a condition is met. If the condition is not met, the instructions stored in 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 conditions are met, the instruction stored in memory 712 programs the processor 711 to change the second position of the second VIGV136 to a third position different from the second position, while maintaining the first position of the first VIGV134 (1312).

[0051] In the exemplary method shown in Figure 13, the step of determining whether the condition is met includes determining whether a valve that is in fluid communication with the compressor 100 is open (1304). In a particular embodiment, the valve is the economy valve 470 of the system 400 shown in Figure 4. In other embodiments, the valve may be the valve 670 of the system 600 shown in Figure 6. The valve is considered open if it is fully or partially open and a portion of the primary refrigerant loop is fluidly connected to the secondary refrigerant loop by the valve. If the valve is closed, the condition is not met and the compressor 100 continues to operate under the current conditions (1310).

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

[0053] In an embodiment where the valve is valve 670 of system 600 shown in Figure 6, the limiting speed of the second compressor stage 126 may be the surge control speed of the second compressor stage 126. In such an embodiment, if the processor 711 determines that the surge control speed of the second compressor stage 126 is less than or equal to the current speed of the dynamic compressor 100, the condition is not met and the compressor continues to operate under the current conditions (1310). If the processor 711 determines that the surge control speed of the second compressor stage 126 is greater than the current speed of the dynamic compressor 100 (1309), the condition is met, the second position of the second VIGV 136 is changed to a third position different from the second position (1312), and the first position of the first VIGV 134 is maintained. Thus, the condition is met when 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 an embodiment in which memory 712 further stores a map 1000 of predetermined operating points 50 of the compressor 100. Figure 14 is a typical diagram of the map 1000 of predetermined operating points 50 stored in memory 712. Each predetermined operating point 50 is shown as the intersection of the compressor speed value and the stage pressure ratio value. An inlet mass flow rate is defined for each predetermined operating point 50. Map 1000 includes predetermined operating points 50 in the range up to points along the mechanical surge line 1020 and the mechanical choke line 1030. Memory 712 stores the surge point mass flow rate for each predetermined surge point and the choke point mass flow rate for all predetermined choke points. Map 1000 does not include points above the surge line 1020 or below the choke line 1030. This is because points above the surge line 1020 or below the choke line 1030 should be avoided and are therefore not "operating points". In other embodiments, the inlet mass flow rate may include a point above the surge line 1020 or below the choke line 1030.

[0055] In map 1000, the predetermined operating points 50 are located in the range of 10% to 35% for speed and 5% to 50% for pressure ratio, with each point 5% away from both axes. In this example, these specific operating points 50 are shown, but any number of operating points can be displayed for any type of compressor at any value and resolution. The values ​​for speed, pressure ratio, inlet mass flow rate, and VIGV position 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, combining simulation and testing, or by other suitable methods for predetermining the values ​​for speed, pressure ratio, inlet mass flow rate, and VIGV position for each predetermined operating point 50.

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

[0057] A predetermined operating point 50 obtained from map 1000 may represent the choke speed or surge control speed of the second compressor stage 126 at the current operating point 1009, or these 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 shown as 1034, and the surge control speed is shown as 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 of mass flow rate interpolation using a plurality of predetermined operating points is disclosed in U.S. Patent Application No. 17 / 243,787, the entirety of which is incorporated herein by reference.

[0058] Map 1000 may alternatively include predetermined operating points for the entire compressor 100. In such embodiments, the limit speed of the second compressor stage 126 can be calculated rather than directly obtained. Figure 15 shows flowcharts of exemplary control algorithms 1510, 1520 for determining the limit speed of the second compressor stage 126 when adding or removing flow between compressor stage 124 and compressor stage 126 (1308, 1309). The compressor choke speed is the speed at which the compressor 100 chokes in response to a given pressure increase and can be determined graphically from map 1000. For example, referring to Figure 14, the compressor choke speed 1034 at 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 with the current speed of the compressor 100 when the economization valve 470 of system 400 is open. In the control algorithm 1510 shown in Figure 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 can be calculated as the difference between the compressor choke speed obtained from the map 1000 of a predetermined operating point 50 and the product of the compressor pressure ratio PR and a predetermined 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 exceeds the speed at which compressor 100 can surge with a given pressure increase, but can be determined graphically from map 1000. For example, referring to Figure 14, the compressor surge control speed 1024 at the current operating point 1009 is approximately 17% of the compressor design speed. When valve 670 of system 600 is open, the surge control speed of the second compressor stage 126 can be calculated and compared with the current speed of compressor 100. In the control algorithm 1520 shown in Figure 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 product of the compressor surge control speed obtained from map 1000 at a predetermined operating point 50, 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 as follows: (a) By modifying the cycle, the efficiency of the HVAC system can be improved by increasing the system capacity and efficiency without compromising the compressor performance and operating range; and (b) By individually controlling the VIGV at each compressor stage, the operating range of the compressor can be extended in either direction.

[0061] As used herein, the terms “about,” “substantially,” “essentially,” and “approximately,” when used in conjunction with ranges of dimensions, concentrations, temperatures, or other physical or chemical properties or features, are intended to cover any variations that may exist at the upper and / or lower limits of a range of a property or feature, including, for example, variations resulting from rounding, measurement methods, or other statistical variations.

[0062] When describing elements of this disclosure or its embodiments, the articles “a, an,” “the,” and “said” indicate that there are one or more elements. The terms “comprising,” “including,” “containing,” and “having” are intended to be comprehensive and mean that there may be additional elements other than those listed. The use of terms indicating a particular orientation (such as “top,” “bottom,” “side,” etc.) is for explanatory convenience and does not require a specific orientation of the item being described.

[0063] Various modifications can be made to the above configuration and method without departing from the scope of this disclosure. Therefore, all matters included in the above description and shown in the attached drawings are to be interpreted as illustrative and not to be limiting.

Claims

1. A system, and said system is A dynamic compressor capable of operating to compress a working fluid, A first compressor stage having a first variable inlet guide vane (VIGV) that can be controlled to selectively introduce pre-swirl into the working fluid flowing into the first compressor stage, A second compressor stage having a second VIGV that can be controlled to selectively introduce pre-swirl into the working fluid flowing into the second compressor stage, A dynamic compressor including a motor configured to drive the first compressor stage and the second compressor stage at the same speed, A valve that is in fluid communication with the dynamic compressor and selectively directs the flow of the working fluid to the inlet of the second compressor stage without passing through the first compressor stage, The dynamic compressor includes a controller connected to the dynamic compressor, The controller includes a processor and memory, and the memory contains the processor, The dynamic compressor is operated at the current speed, the first position of the first VIGV, and the second position of the second VIGV to compress the working fluid, wherein the first position of the first VIGV and the second position of the second VIGV are the same position. While the dynamic compressor is operating, it is necessary to determine whether the conditions are met. If the above conditions are not met, the dynamic compressor will continue to operate at the current speed, the first position of the first VIGV, and the second position of the second VIGV, and If the above conditions are met, an instruction is stored to program the second position of the second VIGV to be changed to a third position different from the second position, and the first position of the first VIGV to be maintained. The determination of whether the above conditions are met includes determining whether the valve is open; if the valve is not open, the above conditions are not met; if the valve is open, the above conditions are met. system.

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

3. The aforementioned speed limit is the choke speed of the second compressor stage, and determining whether the above conditions are met is: The system according to 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 slower than the current speed of the dynamic compressor.

4. The aforementioned speed limit is the surge control speed of the second compressor stage, and determining whether the aforementioned conditions are met is: The system according to claim 2, further comprising determining that the condition is met when 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. The system according to claim 2, wherein the memory further stores a map of predetermined operating points of the dynamic compressor.

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

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

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

9. The system according to claim 1, further comprising a primary refrigerant loop and a secondary refrigerant loop including at least a portion of the primary refrigerant loop, wherein the secondary refrigerant loop branches off from the primary refrigerant loop at a valve, and the secondary refrigerant loop connects the outlet of the second 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 is A condenser fluid-coupled to the second compressor stage, A first expansion device fluid-coupled to the condenser, The system includes the first expansion device and an evaporator fluid-coupled to the first compressor stage, The aforementioned secondary refrigerant loop is A second expansion device fluid-coupled to the condenser, The system according to claim 9, comprising the second expansion device, the condenser, and a heat exchanger fluidly coupled to 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, wherein the controller Processor and The memory includes the processor, The dynamic compressor is operated at the current speed, with the first variable inlet guide vane (VIGV) of the first compressor stage in a first position, and the second VIGV of the second compressor stage in a second position to compress the working fluid, wherein the first VIGV is controllable to selectively introduce pre-swirl into the working fluid flowing into the first compressor stage, and the second VIGV is controllable to selectively introduce pre-swirl into the working fluid flowing into the second compressor stage, and the first position of the first VIGV and the second position of the second VIGV are the same position. While the dynamic compressor is operating, determine whether the conditions are met. If the above conditions are not met, the compressor will continue to operate at the current speed, the first position of the first VIGV, and the second position of the second VIGV. If the above conditions are met, an instruction is 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, where if the valve is closed the condition is not met, and if the valve is open the condition is met, and the valve is in fluid communication with the dynamic compressor to selectively direct the flow of the working fluid to the inlet of the second compressor stage without the flow of the working fluid passing through the first compressor stage. controller.

12. Determining whether the above conditions are met is, The controller according to claim 11, comprising determining the limiting speed of the second compressor stage when the valve is open.

13. The aforementioned speed limit is the choke speed of the second compressor stage, and determining whether the above conditions are met is: The controller according to 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 slower than the current speed of the dynamic compressor.

14. The aforementioned speed limit is the surge control speed of the second compressor stage, and determining whether the aforementioned conditions are met is: The controller according to claim 12, further comprising determining that the condition is met when the valve is open and the surge control speed of the second compressor stage is greater than the current speed of the dynamic compressor.

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

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

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

18. A method for extending the operating range of a dynamic compressor that compresses a working fluid, wherein the dynamic compressor includes a motor, a first compressor stage driven by the motor, and a second compressor stage driven by the motor, and the method is as follows: A step of compressing a working fluid by operating the dynamic compressor at the current speed, at a first position of the first variable inlet guide vane (VIGV) of the first compressor stage, and at a second position of the second VIGV of the second compressor stage, wherein the first VIGV is controllable to selectively introduce pre-swirl into the working fluid flowing into the first compressor stage, the second VIGV is controllable to selectively introduce pre-swirl into the working fluid flowing into the second compressor stage, and the first position of the first VIGV and the second position of the second VIGV are the same position. The steps include determining whether the conditions are met while the dynamic compressor is in operation, If the above conditions are not met, the step of continuing to operate the compressor at the current speed, the first position of the first VIGV, and the second position of the second VIGV, If the above conditions are met, the step of 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 above conditions are met includes determining whether the valve is open, where if the valve is closed the above conditions are not met, and if the valve is open the above conditions are met, and the valve is in fluid communication with the dynamic compressor to selectively direct the flow of working fluid to the inlet of the second compressor stage without the flow of working fluid passing through the first compressor stage. method.

19. The step of determining whether the above conditions are met is: The method according to claim 18, comprising the step of determining the limiting speed of the second compressor stage when the valve is open.

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