Patent ID: 12247773

Corresponding reference characters indicate corresponding parts throughout the drawings.

DETAILED DESCRIPTION

For conciseness, examples will be described with respect to a centrifugal compressor. However, the methods and systems described herein may be applied to any suitable dynamic compressor. The performance and efficiency of an HVAC system can be improved by diverting portions of the main flow through supplemental loops and cycle components. In such systems, flow can be injected or removed between compressor stages such that each stage has a different mass flow rate. However, aerodynamic matching between stages should be maintained both with and without such flow modifications in order to avoid operation in undesirable conditions. A controls strategy can be used to determine when stages are no longer flow-matched and adjust a variable inlet guide vane (VIGV) at each stage's inlet to restore proper stage matching.

Referring toFIG.1, a two-stage refrigerant compressor is indicated generally at100. The compressor100is operable to compress a working fluid (e.g., refrigerant), and includes a compressor housing102that forms at least one sealed cavity within which each stage of refrigerant compression is accomplished. The compressor100includes a first refrigerant inlet110to introduce refrigerant vapor into the first compressor stage (not labeled inFIG.1), a first refrigerant exit114, a refrigerant transfer conduit112to transfer compressed refrigerant from the first compressor stage to the second compressor stage, a second refrigerant inlet118to introduce refrigerant vapor into the second compressor stage (not labeled inFIG.1), and a second refrigerant exit120. The refrigerant transfer conduit112is operatively connected at opposite ends to the first refrigerant exit114and the second refrigerant inlet118, respectively. The refrigerant transfer conduit112further includes a port122for adding or removing flow between the first and second compressor stages. The second refrigerant exit120delivers compressed refrigerant from the second compressor stage to a cooling system in which compressor100is incorporated (FIGS.3-6).

Referring toFIG.2, the compressor housing102encloses a first compressor stage124and a second compressor stage126at opposite ends of the compressor100. The first compressor stage124includes a first compression mechanism106configured to add kinetic energy to refrigerant entering via the first refrigerant inlet110. In some embodiments, the first compression mechanism106is an impeller. The kinetic energy imparted to the refrigerant by the first compression mechanism106is converted to increased refrigerant pressure as the refrigerant velocity is slowed upon transfer to a sealed cavity (e.g., a diffuser) formed within the volute132. The first compressor stage124additionally includes a first variable inlet guide vane (VIGV)134disposed upstream of the first compression mechanism106in the first refrigerant inlet110. The first VIGV134includes a plurality of vanes whose position can be controlled to introduce pre-whirl into the gaseous refrigerant entering the first refrigerant inlet110.

Similarly, the second compressor stage126includes a second compression mechanism116configured to add kinetic energy to refrigerant transferred from the first compressor stage124entering via the second refrigerant inlet118. In some embodiments, the second compression mechanism116is an impeller. The kinetic energy imparted to the refrigerant by the second compression mechanism116is converted to increased refrigerant pressure as the refrigerant velocity is slowed upon transfer to a sealed cavity (e.g., a diffuser) formed within the volute132. Compressed refrigerant exits the second compressor stage126via the second refrigerant exit120(not shown inFIG.2). The second compressor stage126additionally includes a second variable inlet guide vane (VIGV)136disposed upstream of the second compression mechanism116in the second refrigerant inlet118. The second VIGV136includes a plurality of vanes whose position can be controlled to introduce pre-whirl into the gaseous refrigerant entering the second refrigerant inlet118.

The first compression mechanism106and second compression mechanism116are connected at opposite ends of a shaft104. The shaft104is operatively connected to a motor108positioned between the first compression mechanism106and second compression mechanism116such that the first compression mechanism106and second compression mechanism116are rotated at a rotation speed selected to compress the refrigerant to a pre-selected pressure exiting the second refrigerant exit120(not shown inFIG.2). Any suitable motor may be incorporated into the compressor100including, but not limited to, an electrical motor.

FIG.3is a schematic diagram of a first example HVAC system300in which the compressor100ofFIGS.1and2may be installed. The system300has a single, closed refrigerant loop310that includes the compressor100, a condenser320, a first expansion device330, and an evaporator340. Refrigerant enters the compressor100at the first refrigerant inlet110as a low-pressure, low-temperature gas. The first and second compressor stages124,126add kinetic energy to the refrigerant and convert it to pressure rise, and the refrigerant exits the compressor100at the second refrigerant exit120as a high-pressure, high-temperature gas. The refrigerant enters the condenser320, which is fluidly coupled to the second compressor stage126, and heat Qoutis removed to convert the refrigerant gas into a high-pressure, high-temperature liquid.

The condenser320is fluidly coupled to the first expansion device330, which reduces the pressure of the refrigerant. In some embodiments, the pressure may be reduced until the liquid refrigerant's current temperature becomes the boiling point temperature at that pressure, and the refrigerant becomes a two-phase mixture as some of the liquid refrigerant boils and turns into a gas. The first expansion device330may be a fixed orifice, a thermal expansion valve, an electronic expansion valve, or any type of expansion device that allows the HVAC system300to function as described herein. The first expansion device330is fluidly coupled to the evaporator340, which receives low-pressure, low-temperature liquid refrigerant or a two-phase mixture of liquid and gaseous refrigerant at its inlet. In the evaporator340, the refrigerant absorbs heat Qin to change phase from a liquid to a gas. The evaporator340is fluidly coupled to the first compressor stage124, and the cycle begins again.

FIG.4is a schematic diagram of a second example HVAC system400in which the compressor100ofFIGS.1and2may be installed. The system400has a primary refrigerant loop410that includes the compressor100, the condenser320, a first stream492of a heat exchanger490, the first expansion device330, and the evaporator340. The system400also has a secondary refrigerant loop460that is fluidly connected to a portion of the primary refrigerant loop410and controlled by an economization valve470, which will be described in further detail herein.

The secondary refrigerant loop460includes the economization valve470, a second expansion device480, a second stream494of the heat exchanger490, the second compressor stage126, and the condenser320. In the embodiment illustrated inFIG.4, the components of the secondary refrigerant loop460are fluidly coupled in the order in which they are listed, with the condenser320being additionally coupled to the second expansion device480to close the secondary refrigerant loop460.

The economization valve470can be fully open, partially open, or fully closed, and its status determines whether refrigerant will flow through the secondary refrigerant loop460. That is, when the economization valve470is fully closed, all of the refrigerant will flow through the primary refrigerant loop410, and the system400will operate in substantially the same way as the system300illustrated inFIG.3. When the economization valve470is open, the liquid refrigerant exiting the condenser320separates into two streams, with the majority of refrigerant flowing through the primary refrigerant loop410, and the remainder being diverted through the secondary refrigerant loop460. The economization valve470can be a solenoid valve, electronic expansion valve, or any type of valve that allows the system400to function as described herein.

When open, the economization valve470is fluidly coupled to the second expansion device480, which reduces the pressure of the liquid economizer flow until the liquid refrigerant's current temperature becomes the boiling point temperature 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 a gas as it enters the heat exchanger490. The second expansion device can be sized and selected to divert a particular amount of refrigerant through the secondary refrigerant loop460when the economization valve470is open, for example, 0 to 20 percent of the total mass flow, or any amount of refrigerant flow that allows the system400to function as described herein.

In some embodiments, the second expansion device480is a thermal expansion valve (TXV) that adjusts the amount of refrigerant flow through the secondary refrigerant loop460based on the thermal load of the heat exchanger490. The TXV works in combination with a bulb496located downstream of the second stream494of the heat exchanger490. A membrane inside the TXV is movable to balance the refrigerant pressure inside the bulb with the refrigerant pressure upstream of the heat exchanger490. The movement of the membrane is coupled to a needle that sets the position of the valve, thereby controlling the amount of refrigerant that flows through the secondary refrigerant loop460. In further embodiments, the second expansion device480can also be a fixed orifice, an electronic expansion valve, or any type of expansion device that allows the system400to function as described herein.

The refrigerant exits the second expansion device480and enters the second stream494of the heat exchanger490as a low-pressure liquid or two-phase mixture. The second stream494comes into thermal communication with the first stream492, which carries high-pressure liquid refrigerant from the condenser320in the primary refrigerant loop410. The thermal contact between the two streams492,494cools the refrigerant in the first stream492and warms the refrigerant in the second stream494, causing it to boil. The cooled refrigerant in the first stream492exits the heat exchanger490as a lower-temperature, high-pressure liquid, and the boiled refrigerant in the second stream494exits the heat exchanger490as a low-temperature, intermediate-pressure gas. The heat exchanger490may 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 allows the system400to function as described herein. In further embodiments, a flash tank may be used instead of or in addition to the heat exchanger490.

The low-temperature, intermediate-pressure gas exiting the second stream494of the heat exchanger490is then injected into the refrigerant transfer conduit112of the compressor100to be mixed with the refrigerant flow of the primary refrigerant loop410before it reaches the second compressor stage126. The primary and secondary refrigerant loops410,460converge at the second compressor stage126, and diverge once again after the refrigerant exits the condenser320.

FIG.5is a third example HVAC system500in which the compressor100ofFIGS.1and2may be installed. System500includes a low temperature, primary refrigerant loop510, a medium-temperature, secondary refrigerant loop560, and a tertiary refrigerant loop580fluidly coupled thereto. Downstream of the condenser320, the refrigerant flow is throttled through a first expansion device530, reducing its pressure until some of the liquid refrigerant boils off, creating a two-phase mixture. A flash tank590separates the two-phase refrigerant mixture into liquid and gaseous fractions, which respectively diverge into the primary and tertiary loops510,580. In certain embodiments, a heat exchanger may be used instead of or in addition to the flash tank590. The liquid refrigerant in the primary loop510is throttled through a second expansion device532and once again diverges, with a portion of the refrigerant continuing along the primary loop510and the remainder separating into the secondary loop560. In the secondary loop560, the refrigerant flows through a medium-temperature evaporator540, where the refrigerant is boiled and converted to a gas, providing refrigeration to a medium-temperature space. In the primary loop510, the liquid refrigerant510is throttled through a third expansion device330before entering a low-temperature evaporator340, where the refrigerant is boiled and converted to a gas, providing refrigeration to a low-temperature space. The refrigerant then enters the first compressor stage124, where it is compressed to the pressure of the medium-temperature evaporator540. The gaseous refrigerant in the tertiary loop580is throttled through a fourth expansion device534to the pressure of the medium-temperature evaporator540. The gaseous refrigerant in the secondary and tertiary loops560,580are combined and injected into the refrigerant transfer conduit112between the first and second compressor stages124,126.

FIG.6shows a fourth example HVAC system600in which the compressor100ofFIGS.1and2may be installed. System600includes a primary refrigerant loop610that includes the compressor100, the condenser320, the first expansion device330, and the evaporator340. The system also includes a secondary refrigerant loop660that is fluidly connected to a portion of the primary refrigerant loop610and controlled by a valve670. When the valve670is fully closed, all refrigerant flows through the primary loop610, and the system600operates in substantially the same way as the system300illustrated inFIG.3. When the valve670is open, a portion of the refrigerant flow is diverted from the refrigerant transfer conduit112between the first and second compressor stages124,126and through the secondary loop660. The diverted flow passes through a supplemental condenser620throttled by a second expansion device630and rejoins the primary loop610prior to entering the evaporator340.

FIG.7shows an example embodiment of a system700including the dynamic compressor100. The compressor100includes a compressor housing102, a compression mechanism707, a motor108, a speed sensor717, pressure sensors709and a controller710. In the present embodiment, the dynamic compressor100is a two-stage centrifugal compressor, and the compression mechanism707is an impeller in each stage. In other embodiments, the dynamic compressor100may be an axial compressor, and the compression mechanism707may be an axial rotor. The speed sensor717measures the rotational speed of the compressor100, and the pressure sensors709measure pressure at various points along the compressor flow path, including at the refrigerant inlet and the refrigerant exit. Additional sensors may be installed in the compressor100to provide data on its operation, including but not limited to temperature sensors, flow sensors, current sensors708, voltage sensors, rotational rate sensors, and any other suitable sensors. The compressor100is not limited to a specific construction in the system700and may be constructed similarly to the compressor100described inFIGS.1and2or may be constructed in a different manner. The system700further includes an unloading device701, a variable frequency drive (VFD)716, and a user interface715.

A controller710is operatively connected to the compressor100to control its operation, based in part on the measured parameters described above. The controller710includes a processor711, a memory712, and an unloading interface714. The memory712stores a map1000(see, e.g.,FIG.14) of a plurality of predetermined operating points50of the compressor100which can be stored in any suitable data structure, such as a table, a matrix, or the like. The map1000may include a plurality of predetermined operating points of the first compressor stage124alone, the second compressor stage126alone, or the compressor100as a whole. The memory712additionally stores instructions that are executed by the processor711to operate the compressor100to compress the working fluid, determine when the first and second compressor stages124,126are no longer flow-matched, and adjust the unloading device701at the inlet110,118of each compressor stage124,126to restore proper stage matching, if necessary. The map1000of predetermined operating points50and a method1300of determining whether the compressor stages124,126are matched are discussed in greater detail further below.

The system700includes an interface for connection of the controller710to the VFD716and a motor interface713for connection of the VFD716to the motor108. In certain embodiments, the VFD716operates under the control of the controller710. In further embodiments, the VFD716is a part of the controller710. The system700further includes an unloading interface714for connection of the controller710to the unloading device701.

The controller710is operatively coupled to the unloading device701through the unloading interface714, which removes and/or reduces the load on the compressor100during start-up and shut-down routines, during detected surge events, and when otherwise instructed by the controller710to do so. In the example embodiment, the unloading device701is a variable inlet guide vane (VIGV) at the inlet of each impeller stage (FIG.2). In other embodiments, the unloading device701may be a variable diffuser. The controller710is configured to control at least one operating parameter of the unloading device701, such as a position of each VIGV.

In other embodiments, the unloading device701is a bypass valve. Bypass valves, such as refrigerant bypass valves, provide an alternative path for the gas, thereby limiting the pressure rise of the compressor100and preventing any potential surge events, no matter how slowly the motor108is accelerating during start-up or decelerating during shut-down. In other embodiments, the unloading device701is an expansion valve. In still other embodiments, the unloading device701may be a variable orifice or diameter valve, such as a servo valve, and a fixed orifice or diameter valve, such as a solenoid valve or a pulse-width-modulated (PWM) valve configured to control opening and closing according to a duty cycle. Although many types of unloading devices are described here, the unloading device701may be any suitable device, or combination of devices, that reduce the load on the compressor100. The unloading device701may additionally be used as a loading device to increase the load on the compressor100.

The system700further includes a user interface715configured to output (e.g., display) and/or receive information (e.g., from a user) associated with the system700. In some embodiments, the user interface715is configured to receive an activation and/or deactivation input from a user to activate and deactivate (i.e., turn on and off) or otherwise enable operation of the system700. Moreover, in some embodiments, the user interface715is configured to output information associated with one or more operational characteristics of the system700, including, for example and without limitation, warning indicators such as severity alerts, occurrence alerts, fault alerts, motor speed alerts, and any other suitable information.

The user interface715may include any suitable input devices and output devices that enable the user interface715to function as described herein. For example, the user interface715may include input devices including, but not limited to, a keyboard, mouse, touchscreen, joystick(s), throttle(s), buttons, switches, and/or other input devices. Moreover, the user interface715may include output devices including, for example and without limitation, a display (e.g., a liquid crystal display (LCD) or an organic light emitting diode (OLED) display), speakers, indicator lights, instruments, and/or other output devices. Furthermore, the user interface715may be part of a different component, such as a system controller (not shown). Other embodiments do not include a user interface715.

The controller710is generally configured to control operation of the compressor100. The controller710controls operation through programming and instructions from another device or controller or is integrated with the system700through a system controller. In some embodiments, for example, the controller710receives user input from the user interface715, and controls one or more components of the system700in response to such user inputs. For example, the controller710may control the motor108based on user input received from the user interface715. In some embodiments, the system700may be controlled by a remote control interface. For example, the system700may include a communication interface (not shown) configured for connection to a wireless control interface that enables remote control and activation of the system700. The wireless control interface may be embodied on a portable computing device, such as a tablet or smartphone.

The controller710may generally include any suitable computer and/or other processing unit, including any suitable combination of computers, processing units and/or the like that may be communicatively coupled to one another and that may be operated independently or in connection within one another (e.g., controller710may form all or part of a controller network). Controller710may include one or more modules or devices, one or more of which is enclosed within system700, or may be located remote from system700. The controller710may be part of compressor100or separate and may be part of a system controller in an HVAC system. Controller710and/or components of controller710may be integrated or incorporated within other components of system700. The controller710may include one or more processor(s)711and associated memory device(s)712configured to perform a variety of computer-implemented functions (e.g., performing the calculations, determinations, and functions disclosed herein).

As used herein, the term “processor” refers not only to integrated circuits, but also to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application-specific integrated circuit, and other programmable circuits. Additionally, memory device(s)712of controller710may generally be or include memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s)712may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s)711, configure or cause the controller710to perform various functions described herein including, but not limited to, controlling the system700, controlling operation of the motor108, receiving inputs from user interface715, providing output to an operator via user interface715, controlling the unloading device701and/or various other suitable computer-implemented functions.

Referring toFIG.8, an operating envelope or operating map800of the example dynamic centrifugal compressor100is shown. This operating map800is one graphical representation of the map1000of a plurality of predetermined operating points50stored by the memory712. The operating map800graphically displays a compressor's performance in terms of flows, heads, and speeds. The operating map800shows head vs. inlet mass flow rate as a percentage of their values at the design point of the compressor100. The head is a total pressure ratio of exit pressure to inlet pressure. Inlet mass flow rate is a measure of the amount of a working fluid, such as a refrigerant, flowing through the compression mechanism707. The operating map800shows a plurality of compressor speed lines807. In this example, there are five speed lines807that range from 70% design speed to 110% design speed, with each line separated by a 10% difference. Although these particular speed lines are shown in this example, any number of speed lines at any different percentages of the compressor design speed may be shown for any type of compressor.

A surge limit line804indicates the minimum flow before surge occurs in the surge region806(i.e., to the left of surge limit line804). A surge control line803roughly indicates the minimum flow under which the compressor100can safely operate without risk of slipping into surge. The surge control line803is defined by a surge margin805from the surge limit line804. By operating to the right of the surge control line803, the compressor100should avoid surging. Similarly, the choke line801indicates that operation to its right will result in the compressor100operating with choked flow.

A first operating point809of the compressor100is shown on the operating map800as the intersection of a speed line, inlet mass flow rate value, and total pressure ratio value. For example, the first operating point809shown in operating map800is at 112% inlet mass flow rate, 90% head, and 100% speed, though any number of operating points may be shown for any type of compressor. The operating point defines the current operating parameters of the compressor100, and the operating map800indicates how close the current operating point is to operating in an unstable condition (i.e., surge) or an inefficient condition (i.e., choke).

The first operating point809shown inFIG.8may represent operation of the compressor100when the first and second compressor stages124,126receive the same mass flow. For example, the first operating point809may represent operation of the compressor100as installed in the first example HVAC system300shown inFIG.3. Alternatively, the first operating point809may represent operation of the compressor100as installed in the second example HVAC system400shown inFIG.4when the economization valve470of the HVAC system400is closed; that is, when refrigerant only circulates through the primary loop410, and there is no economizer flow added between compressor stages124,126. The first operating point809may also represent operation of the compressor100as installed in the fourth example HVAC system600shown inFIG.6when the valve670is closed; that is, when refrigerant only circulates through the primary loop610, and no flow is removed from between compressor stages124,126.

FIG.9shows an operating map900of the second compressor stage126of the dynamic compressor100. A second operating point909may represent operation of the second compressor stage126when the first and second compressor stages124,126receive the same mass flow, as in the configurations described above. A third operating point913may represent the second compressor stage's126operating conditions when flow is added between compressor stages124,126. For example, the third operating point913may represent operation of the compressor100as installed in system400when the economization valve470is opened. The third operating point913may also represent operation of the compressor100as installed in system500illustrated inFIG.5.

The second compressor stage126is unable to achieve the same pressure rise for a higher mass flow at the same compressor speed. As a result, the current operating point of the second compressor stage126shifts to the right along the 100% speed line from second operating point909to third operating point913. Accordingly, the third operating point913indicates operation of the second stage126at the same speed as second operating point909, but with a greater inlet mass flow and a lower head. If the third operating point913of the second compressor stage126shifts past a second stage choke line901, the second compressor stage126will operate with choked flow. This causes the entire compressor100to operate with choked flow, degrading its performance and efficiency.

With reference toFIG.10, a fourth operating point919is another example representation of the operation of the second compressor stage126when the first and second compressor stages124,126receive the same mass flow, as in the configurations described above. A fifth operating point923represents the second compressor stage's126operating conditions when flow is removed between compressor stages124,126. For example, the fifth operating point923may represent operation of the compressor100as installed in the fourth example HVAC system600shown inFIG.6when the valve670is open and flow is removed from between compressor stages124,126. Reducing the mass flow through the second compressor stage126shifts its operating point to the left along the 100% speed line, from the fourth operating point919to the fifth operating point923. Accordingly, the fifth operating point923indicates operation of the second stage126at the same speed as fourth operating point919, but with a reduced inlet flow. If the fifth operating point923of the second compressor stage126shifts past a surge control line903or a surge limit line904of the second stage, the second compressor stage126will be at risk of slipping into surge. A surge event in the second compressor stage126disrupts the machine as a whole, leading to possible structural damage.

The decline in performance and operating range illustrated inFIGS.9and10can be mitigated by adjusting the position of the second VIGV136, thereby shifting the operating map of the second compressor stage126so the second compressor stage126will neither choke nor surge before the compressor100as a whole.FIGS.9and10show the operating map900of the second compressor stage126when the first and second VIGVs134,136have the same position. In other words, the first position of the first VIGV134and the second position of the second VIGV136are the same position.

FIG.11shows an operating map1100of the second compressor stage126when the second VIGV136has been moved to a third position different than the second position. The operating map1100is overlaid on the operating map900shown inFIG.9, including the second and third operating points909,913. Adjusting the second VIGV136to the third position changes the pre-whirl added to the refrigerant gas entering the second compressor stage126, shifting its operating envelope to the right to extend its choke range. Since the speed of the second compressor stage126remains the same, the current operating point is shifted upwards to the new 100% speed line of the second compressor stage126, from the third operating point913to a sixth operating point1113. Since the sixth operating point1113is to the left of the new choke line1101, the second compressor stage126will no longer operate with choked flow. This allows the HVAC system400,500to reap the benefits of an economization loop or booster system without compromising the performance and operating range of the compressor100.

With reference toFIG.12, the third position of the second VIGV136may also be chosen such that the operating envelope of the second compressor stage126shifts to the left to extend the surge range of the compressor100.FIG.12illustrates an operating map1200of the second compressor stage126when the second VIGV136has been adjusted to a third position that extends the surge range of the compressor100. The operating map1200is overlaid on the operating map900shown inFIG.9, including the fourth and fifth operating points919,923. The current operating point is shifted downwards to the new 100% speed line of the second compressor stage126, from the fifth operating point923to a seventh operating point1223. Since the seventh operating point1223is to the right of the new surge control line1203, the second compressor stage126will no longer be at risk of surging before the rest of the compressor100.

The memory712stores instructions that program the processor711to extend the operating range of the compressor100as described above. An example method1300is shown inFIG.13. The processor711operates1302the compressor100at a current speed, a first position of the first VIGV134and a second position of the second VIGV136to compress the working fluid. In some embodiments, the first position of the first VIGV134and the second position of the second VIGV136are the same position. While operating1302the dynamic compressor100, the processor711determines if a condition is satisfied. If the condition is not satisfied, the instructions stored in the memory712program the processor711to continue1310to operate the compressor100at the current speed, the first position of the first VIGV134, and the second position of the second VIGV136. If the condition is satisfied, the instructions stored in the memory712program the processor711to change1312the second position of the second VIGV136to a third position different than the second position and maintain the first position of the first VIGV134.

In the example method shown inFIG.13, determining if a condition is satisfied includes determining1304if a valve in fluid communication with the compressor100is open. In certain embodiments, the valve may be the economization valve470of the system400shown inFIG.4. In other embodiments, the valve may be the valve670of the system600shown inFIG.6. The valve is considered open if it is fully or partially open such that a portion of the primary refrigerant loop is fluidly connected to the secondary refrigerant loop at the valve. If the valve is not open, the condition is not satisfied, and the compressor100continues1310to operate at its current conditions.

If the valve is open, the processor711is additionally programmed to determine1306a limiting speed of the second compressor stage126. In embodiments where the valve is the economization valve470of the system400shown inFIG.4, the limiting speed of the second compressor stage126may be a choke speed of the second compressor stage126. In such embodiments, if the processor711determines1308that the choke speed of the second compressor stage126is greater than or equal to the current speed of the dynamic compressor100, the condition is not satisfied, and the compressor continues1310to operate at its current conditions. If the processor711determines1308that the choke speed of the second compressor stage126is less than the current speed of the dynamic compressor100, the condition is satisfied, and the second position of the second VIGV136will change1312to a third position different than the second position and maintain the first position of the first VIGV134. Thus, the condition is satisfied when the economization valve470is open and the choke speed of the second compressor stage126is less than the current speed of the dynamic compressor100.

In embodiments where the valve is the valve670of the system600shown inFIG.6, the limiting speed of the second compressor stage126may be a surge control speed of the second compressor stage126. In such embodiments, if the processor711determines1309that the surge control speed of the second compressor stage126is less than or equal to the current speed of the dynamic compressor100, the condition is not satisfied, and the compressor continues1310to operate at its current conditions. If the processor711determines1309that the surge control speed of the second compressor stage126is greater than the current speed of the dynamic compressor100, the condition is satisfied, and the second position of the second VIGV136will change1312to a third position different than the second position and maintain the first position of the first VIGV134. Thus, the condition is satisfied when the valve670is open and the surge control speed of the second compressor stage126is greater than the current speed of the dynamic compressor100.

The method1300may be used in embodiments where the memory712further stores a map1000of predetermined operating points50of the compressor100.FIG.14is a representative illustration of a map1000of predetermined operating points50stored by the memory712. Each predetermined operating point50is 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 point50. The map1000includes predetermined operating points50in a range up to and including points along the machine surge line1020and the machine choke line1030. The memory712stores a surge point mass flow for each predetermined surge point and a choke point mass flow for every predetermined choke point. The map1000does not include any points above the surge line1020or below the choke line1030, because points above the surge line1020or below the choke line1030are to be avoided and are thus not “operating points.” In other embodiments, the inlet mass flow rate of points above the surge line1020or below the choke line1030may be included.

In the map1000, the predetermined operating points50range between 10% and 35% speed, and between 5% and 50% pressure ratio, with each point separated by 5% on both axes. Although these particular operating points50are shown in this example, any number of operating points at any values and with any resolution may be shown for any type of compressor. The speed, pressure ratio, inlet mass flow rate, and VIGV position values of each predetermined operating point50may be generated by simulating operation of the dynamic compressor100on a computer, testing the dynamic compressor100in a controlled environment, a combination of simulation and testing, or by any other suitable method for predetermining the speed, pressure ratio, inlet mass flow rate, and VIGV position values of each predetermined operating point50.

The map1000shown inFIG.14may include predetermined operating points50of the second compressor stage126alone. In addition to the machine choke line1032and surge control line1020, the map1000also shows the choke line1050of the second compressor stage126when flow is added between compressor stages124,126, as well as the surge line1040of the second compressor stage126when flow is removed between compressor stages124,126. In such embodiments, determining1306the limiting speed of the second compressor stage126includes retrieving a value of a predetermined operating point50of the second compressor stage126from the map1000of predetermined operating points50.

The predetermined operating points50retrieved from the map1000may indicate the choke speed or the surge control speed of the second compressor stage126at the current operating point1009, or they may be used to graphically determine the choke speed or surge control speed at the current operating point. For example, the choke speed of second compressor stage126at the current operating point1009is indicated at1034, and the surge control speed is indicated at1024. The predetermined operating points50may have the same speed, pressure ratio, and inlet mass flow as the current operating point1009of the second compressor stage126. In further embodiments, the predetermined operating points50closest to the current operating point1009may be retrieved. In further embodiments, a new point corresponding to the current operating point1009may be interpolated from the predetermined operating points50. A method of mass flow interpolation using a plurality of predetermined operating points is disclosed in U.S. patent application Ser. No. 17/243,787 which is incorporated by reference herein in its entirety.

The map1000may alternatively include predetermined operating points of the compressor100as a whole. In such embodiments, the limiting speed of the second compressor stage126can be calculated instead of directly retrieved.FIG.15shows a flow chart of example control algorithms1510,1520for determining1308,1309the limiting speed of the second compressor stage126when flow is added or removed between compressor stages124,126. The compressor choke speed, which is the speed at which the compressor100will choke for a given pressure rise, can be determined graphically from the map1000. For example, and with reference toFIG.14, the compressor choke speed1034of the current operating point1009is around 33% of the compressor design speed. If the economization valve470of system400is open, the choke speed of the second compressor stage126can be calculated and compared to the current speed of the compressor100. In the control algorithm1510shown inFIG.15, the choke speed of the second compressor stage126can be calculated from the compressor choke speed. For example, the second stage choke speed can be calculated as the difference between the compressor choke speed retrieved from the map1000of predetermined operating points50and the product of the compressor pressure ratio PR and a predetermined constant k:

Nchoke,2⁢S=Nchoke,compressor-PR*k
In some embodiments, the predetermined constant k is 500.

The compressor surge control speed, which is the speed past which the compressor100may surge for a given pressure rise, can be determined graphically from the map1000. For example, and with reference toFIG.14, the compressor surge control speed1024of the current operating point1009is around 17% of the compressor design speed. If the valve670of system600is open, the surge control speed of the second compressor stage126can be calculated and compared to the current speed of the compressor100. In the control algorithm1520shown inFIG.15, the surge control speed of the second compressor stage126can be calculated from the compressor surge control speed. For example, the second stage surge control speed can be calculated as the sum of the compressor surge control speed retrieved from the map1000of predetermined operating points50and the product of the compressor pressure ratio PR and a predetermined constant k:

Nsurge⁢control,2⁢S=Nsurge⁢control,compressor+PR*k
In some embodiments, the predetermined constant k is 500.

Technical benefits of the methods and systems described herein are as follows: (a) HVAC system efficiency can be improved with cycle modifications that increase the system's capacity and efficiency without compromising the performance and operating range of the compressor, and (b) the operating range of the compressor can be extended in either direction by controlling the VIGV at each compressor stage separately.

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 characteristics is meant to cover variations that may exist in the upper and/or lower limits of the ranges of the properties or characteristics, including, for example, variations resulting from rounding, measurement methodology or other statistical variation.

When introducing elements of the present disclosure or the embodiment(s) thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the 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 terms indicating a particular orientation (e.g., “top,” “bottom,” “side,” etc.) is for convenience of description and does not require any particular orientation of the item described.

As various changes could be made in the above constructions and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense.