Start-Stop Control System and Method for Gas Foil Bearing Machines

Abstract

The HVAC system includes an unloading device, a centrifugal compressor, a gas foil bearing, a VFD, and a controller. The controller is programmed to start the centrifugal compressor from a stopped condition by activating the unloading device to remove a load from the centrifugal compressor, accelerating a motor to a first speed above a lift-off speed of the centrifugal compressor and below an operating speed of the centrifugal compressor, running the motor for a period of time, activating the unloading device to apply a load to the centrifugal compressor, and accelerating the motor to the operating speed.

Description

[Technical Field]

[0001] (Reference to Related Application) This application claims priority to U.S. Patent Application No. 17 / 009,535, filed September 1, 2020, which is incorporated herein by reference in its entirety.

[0002] (Technical field) The field of the disclosure relates generally to control systems, and more particularly to control systems for machines including gas foil bearing assemblies. [Background technology]

[0003] Gas foil bearing (GFB) machines are used, among other things, in HVAC applications such as two-stage refrigerant centrifugal compressors. The HVAC compressor has a drive shaft operatively connected to a motor between impeller stages supported by gas foil bearings. The drive shaft can be positioned between the impeller stages so that the impeller rotates at a rotational speed to compress the refrigerant to a selected pressure in the HVAC system. The compressor bearing typically includes one or more features that reduce friction between the compressor bearing and the drive shaft. Once the shaft rotates fast enough, the gas pushes the foils away from the shaft so that contact does not occur. The shaft and gas foil bearing are separated by the high pressure of the gas generated by the rotation, which draws the gas into the bearing via viscous effects. A high speed of the shaft relative to the gas foil bearing is required to initiate an air gap, and once this is achieved, no wear occurs. These bearings have several advantages over known bearings, including weight reduction due to elimination of the oil system, stable operation at high speeds and temperatures, low power loss at high speeds, and long life with low maintenance.

[0004] Current gas foil bearings deform in response to the pressure generated within the compressor. Wear and tear on gas foil bearings occur during startup and shutdown operations. More specifically, operating a GFB machine below lift-off speed and compressor surge events accelerate the wear of bearings and bearing coatings. Surge is a characteristic behavior of centrifugal compressors that can occur when the inlet flow is reduced so that the head generated by the compressor is insufficient to overcome the compressor discharge pressure. Once surge occurs, the compressor output pressure is dramatically reduced, causing a reversal of flow within the compressor. When a centrifugal compressor surges, there is an actual reversal of gas flow through the compressor impeller. Surge usually begins in the first stage of a multi-stage compressor and can occur very rapidly. Compressors are particularly susceptible to surge events during startup and shutdown due to their lower operating speeds. The severity of surge events and the damage caused by them increase with compressor speed. Minimizing the time that the GFB machine is operated below its lift-off speed and minimizing the number and severity of surge events experienced by the compressor increases bearing life.

[0005] This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and / or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. Summary of the Invention

[0006] In one aspect, an HVAC system is described that includes an unloading device, a centrifugal compressor, a gas foil bearing, and a controller. The centrifugal compressor includes a compressor housing, a motor having a drive shaft rotatably supported within the compressor housing, and an impeller connected to the drive shaft and operable to compress refrigerant gas upon rotation of the drive shaft. The gas foil bearing is supported by the compressor housing and supports the drive shaft. The controller is connected to the motor and the unloading device. The controller is programmed to start the centrifugal compressor from a stopped condition by activating the unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed that is above a lift-off speed of the gas foil bearing and below an operating speed of the centrifugal compressor, operating the motor at the first speed for a period of time, activating the unloading device to apply a load to the centrifugal compressor, and accelerating the motor to the operating speed. The controller is further programmed to shut down the centrifugal compressor from an operating condition by activating an unloading device to remove the load from the centrifugal compressor, slowing the motor down towards a minimum speed greater than zero, and removing power from the motor when the speed of the motor reaches the minimum speed and allowing the motor to coast to a stop.

[0007] In another aspect, a controller for controlling a centrifugal compressor having a gas foil bearing supporting a shaft of an impeller driven by a motor is described, the controller including a motor interface for connection to the motor, an unload interface for connection to an unload device, a processor, and a memory. The memory contains instructions that, when executed by the processor, cause the controller to start the centrifugal compressor from a stopped condition by activating an unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed that is above a lift-off speed of the gas foil bearing and below an operating speed of the centrifugal compressor, running the motor at the first speed for a period of time, activating the unloading device to apply a load to the centrifugal compressor, and accelerating the motor to the operating speed. The memory further contains instructions that, when executed by the processor, cause the controller to stop the centrifugal compressor from an operating condition by activating the unloading device to remove a load from the centrifugal compressor, decelerating the motor towards a minimum speed that is greater than 0, and removing power from the motor when its speed reaches the minimum speed and allowing the motor to coast to a stop.

[0008] In yet another aspect, a method for controlling a centrifugal compressor having a gas foil bearing supporting a shaft of an impeller driven by a motor is described. The method includes activating an unloading device to remove a load from the centrifugal compressor, accelerating the motor to a first speed above the lift-off speed of the gas foil bearing and below the operating speed of the centrifugal compressor, operating the motor at the first speed for a period of time, activating the unloading device to apply a load to the centrifugal compressor, and starting the centrifugal compressor from a stopped condition by accelerating the motor to the operating speed. The method further includes activating the unloading device to remove the load from the centrifugal compressor, decelerating the motor toward a minimum speed greater than zero, and stopping the centrifugal compressor from an operating condition by removing power from the motor and allowing the motor to coast to a stop when the speed of the motor reaches the minimum speed.

[0009] Various refinements of the features noted in connection with the above-described aspects exist. Additional features may also be incorporated into the above-described aspects. These refinements and additional features may exist individually or in any combination. For example, the various features discussed below in connection with any of the illustrated embodiments may be incorporated into any of the above-described aspects alone or in any combination.

[0010] The following figures illustrate various aspects of the present disclosure. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a perspective view of the assembled compressor.

[0012] [Figure 2] 2 is a cross-sectional view of the compressor of FIG. 1 taken along line 2-2 with the external conduit removed.

[0013] [Figure 3] 3 is a cross-sectional view through the sleeve of the bearing housing shown in FIG. 2 showing the drive shaft supported in a foil bearing assembly held within the sleeve of the bearing housing using a pair of retaining clips.

[0014] [Figure 4] 10 is a cross-sectional view of another embodiment of a bearing housing suitable for use in the compressor of FIG. 1 showing a drive shaft supported in a foil bearing assembly retained within the bearing housing between a retaining lip formed in the bearing housing at one end and a retaining clip at the other end.

[0015] [Figure 5] FIG. 1 is an exploded view of the elements of the foil bearing assembly positioned relative to the bearing housing and drive shaft.

[0016] [Figure 6] FIG. 1 is a block diagram of a start-stop control system for a gas foil bearing (GFB) machine.

[0017] [Figure 7] 1 is a flow chart of a method for starting a centrifugal compressor from a stopped state.

[0018] [Figure 8] 1 is a surge current signature graph for a centrifugal compressor.

[0019] [Figure 9] 4 is a flow chart of a method for shutting down a centrifugal compressor from operation.

[0020] [Figure 10] 10 is a graph of a deceleration curve for estimated surge speed and minimum speed change as the actual speed drops during shutdown of the centrifugal compressor.

[0021] [Figure 11]4 is a flow chart of an exemplary start-up routine for a centrifugal compressor.

[0022] [Figure 12] 4 is a flow chart of an exemplary shutdown routine for a centrifugal compressor.

[0023] [Figure 13] 4 is a flow chart of an exemplary emergency shutdown routine for a centrifugal compressor. DETAILED DESCRIPTION OF THE INVENTION

[0024] Corresponding reference characters indicate corresponding parts throughout the drawings.

[0025] For simplicity, an example will be described with respect to an HVAC compressor. However, the methods and systems described herein may be applied to any suitable gas foil bearing (GFB) machine. In a start-stop control system for a GFB machine, a start-up routine prevents further bearing deformation by disconnecting the load on the compressor and then rapidly accelerating the compressor to a no-load speed above the bearing lift-off speed (~10k RPM) and holding that speed until the initial surge has subsided. In addition, a stop routine further prevents bearing deformation by disconnecting the load and then gradually decelerating the compressor to the estimated surge speed plus a margin and then allowing the compressor to coast to a stop.

[0026] Referring to FIG. 1, a compressor in the form of a two-stage refrigerant compressor is generally designated 100. Compressor 100 generally includes a compressor housing 102 forming at least one sealed cavity within which each stage of refrigerant compression is accomplished. Compressor 100 includes a first refrigerant inlet 110 for introducing refrigerant vapor into a first compression stage (not labeled in FIG. 1), a first refrigerant outlet 114, a refrigerant transfer conduit 112 for transferring compressed refrigerant from the first compression stage to a second compression stage, a second refrigerant inlet 118 for introducing refrigerant vapor into the second compression stage (not labeled in FIG. 1), and a second refrigerant outlet 120. Refrigerant transfer conduit 112 is operatively connected at opposite ends to first refrigerant outlet 114 and second refrigerant inlet 118, respectively. A second refrigerant outlet 120 delivers compressed refrigerant from the second compression stage to a refrigeration system in which the compressor 100 is incorporated. The refrigerant transfer conduit 112 may further include a refrigerant bleed 122 for adding refrigerant as needed in the compressor 100.

[0027] Referring to FIG. 2 , the compressor housing 102 encloses a first compression stage 124 and a second compression stage 126 at opposite ends of the compressor 100. The first compression stage 124 includes a first impeller 106 configured to add kinetic energy to the refrigerant entering through the first refrigerant inlet 110. The kinetic energy imparted to the refrigerant by the first impeller 106 is converted to increased refrigerant pressure (i.e., compression) because the refrigerant velocity is slowed after transfer to a sealed cavity (e.g., a diffuser) formed between the bearing housing 200 and a portion of the outer compressor housing 102. Similarly, the second compression stage 126 includes a second impeller 116 configured to add kinetic energy to the refrigerant transferred from the first compression stage 124 entering through the second refrigerant inlet 118. The kinetic energy imparted to the refrigerant by the second impeller 116 is converted to increased refrigerant pressure (i.e., compression). This is because the refrigerant velocity is slowed after transfer to a sealed cavity (e.g., a diffuser) formed between the bearing housing 200a and the second portion of the outer compressor housing 102. The compressed refrigerant exits the second compression stage 126 via a second refrigerant outlet 120 (not shown in FIG. 2).

[0028] 2, the first stage impeller 106 and the second stage impeller 116 are connected at opposite ends of a drive shaft 104. The drive shaft 104 is operatively connected to a motor 108 positioned between the first stage impeller 106 and the second stage impeller 116 such that the first stage impeller 106 and the second stage impeller 116 are rotated at a selected rotational speed to compress the refrigerant to a preselected pressure that exits the second refrigerant outlet 120. Any suitable motor may be incorporated into the compressor 100, including, but not limited to, an electric motor. The drive shaft 104 is supported by a gas foil bearing assembly 300 positioned within a sleeve 202 of each bearing housing 200 / 200a, as described in additional detail below. Each bearing housing 200 / 200a includes mounting structure (not shown) for connecting the respective bearing housing 200 / 200a to the compressor housing 102, as shown in FIG.

[0029] 2, each bearing housing 200 / 200a supports a drive shaft 104, which protrudes through the bearing housing 200 / 200a on the opposite side of the sleeve 202, with the impeller 106 connected to the protruding end of the drive shaft 104. Referring to FIGS. 3 and 5, a gas foil bearing assembly 300 is positioned within a cylindrical bore 206 in the bearing housing 200. The drive shaft 104 fits closely within the gas foil bearing assembly 300, which includes an outer compliant foil or foil layer 302 positioned adjacent to the inner wall of the sleeve 202, an inner compliant foil or foil layer 306 (also referred to as the "top foil") positioned adjacent the drive shaft 104, and a bump foil or foil layer 310 positioned between the inner foil layer 306 and the outer foil layer 302. The foils or layers 302 / 306 / 310 of the gas foil bearing assembly form an essentially cylindrical tube sized to receive the drive shaft 104 with relatively little or no gap design as dictated by existing foil bearing design methodologies. The components of the foil bearing assembly 300, such as the outer foil layer 302, the inner foil layer 306, and the bump foil layer 310, may be constructed of any suitable material that enables the foil bearing assembly 300 to function as described herein. Suitable materials include, by way of example and not limitation, metal alloys. In some embodiments, for example, the outer foil layer 302, the inner foil layer 306, and the bump foil layer 310 are each constructed of stainless steel (e.g., 17-4 stainless steel).

[0030] 3 , the foil bearing assembly 300 of the illustrated embodiment further includes a pair of foil keepers 312 a / 312 b positioned adjacent opposite ends of the layers 302 / 306 / 310 to prevent axial slippage of the layers 302 / 306 / 310 within the cylindrical bore 206 of the sleeve 202. A pair of foil retaining clips 314 a / 314 b positioned adjacent the foil keepers 312 a / 312 b, respectively, secure the layers 302 / 306 / 310 in a locked axial position within the cylindrical bore 206 (not shown in FIG. 3 ). The foil retaining clips 314 a / 314 b may be removably connected to the bearing housing 200.

[0031] 4, each bearing housing 200 includes a foil retaining lip 214 that is integrally formed (e.g., cast) with the bearing housing 200 and that projects radially inward from the radially inner surface 204 that defines the cylindrical bore 206. In the illustrated embodiment, the foil retaining lip 214 is positioned near the impeller end 216 of the cylindrical bore 206, proximate the impeller 116 (shown in FIG. 2). The foil retaining lip 214 is sized and dimensioned to project a radial distance from the radially inner surface 204 that overlaps at least a portion of the layers 302 / 306 / 310 of the foil bearing assembly 300. The foil retaining lip 214 may extend completely around the circumference of the radially inner surface 204, or the foil retaining lip may include two or more segments that extend over a portion of the circumference of the radially inner surface 204 and are separated by a space that is flush with adjacent radially inner surfaces 204. Bearing housing 200a (not shown in FIG. 4) is similarly formed.

[0032] 4 further includes a single foil retaining clip 314 positioned adjacent an end of the layer 302 / 306 / 310 opposite the foil retaining lip 214 to restrain axial movement of the layer 302 / 306 / 310 within the cylindrical bore 206 of the sleeve 202. In this embodiment, the foil retaining clip 314 snaps into a circumferential groove 212 formed in the radially inner surface 204 of the cylindrical bore 206 near a motor end 218 of the cylindrical bore 206.

[0033] The foil retaining lip 214 may be positioned within any region of the cylindrical bore 206 near the impeller end 216, including, but not limited to, immediately adjacent the opening of the cylindrical bore 206 at the impeller end 216. Alternatively, the foil retaining lip 214 may be positioned within any region of the cylindrical bore 206 near the motor end 218, including, but not limited to, immediately adjacent the opening of the cylindrical bore 206 at the motor end 218. In such an embodiment, the foil retaining clip 314 snaps into the circumferential groove 212 formed in the radially inner surface 204 of the cylindrical bore 206 near the impeller end 216 in a configuration essentially opposite to that shown in FIG.

[0034] 4, the foil bearing assembly 300 is installed within the bearing housing 200 by inserting the foil bearing assembly 300 into the cylindrical bore 206 of the bearing housing 200 at the motor end 218. The foil bearing assembly 300 is then advanced axially into the cylindrical bore 206 toward the impeller end 216 until the layers 302 / 306 / 310 contact the foil retaining lip 214. The foil retaining clip 314 is then snapped into the circumferential groove 212 near the motor end 218 of the cylindrical bore 206 to lock the foil bearing assembly 300 in place.

[0035] In other embodiments, any suitable method may be used to secure the foil bearing assembly 300 within the sleeve 202. Non-limiting examples of suitable methods include fasteners and retaining clips, adhesives, set screws, and any other suitable fastening method.

[0036] The bearing housing 200 / 200a may further serve as a mounting structure for various elements, including, but not limited to, radial bearings, such as the foil bearing assembly 300 described above, thrust bearings, and sensing devices (not shown) used as feedback for passive or active control schemes, such as proximity probes, pressure transducers, thermocouples, key phasers, etc.

[0037] The foil bearing assembly 300 may be provided in any suitable form, including, but not limited to, two layers, three layers, four layers, or additional layers. The bump foil 310 of the foil bearing assembly 300 may be formed from a radially resilient structure to provide a resilient surface for the rotating drive shaft 104 during operation of the compressor 100. The bump foil 310 may be formed from any suitable radially resilient structure, including, but not limited to, an array of deformable bumps or other configurations designed to deform and rebound under intermittent compressive radial loads, and any other resilient, elastic material capable of compressing and rebounding under intermittent compressive radial loads. The bump foil 310 may be connected to at least one adjacent layer, including, but not limited to, at least one of the outer layer 302 and the inner layer 306. In some embodiments, the bump foil 310 may be connected to both the outer layer 302 and the inner layer 306. In other embodiments, the bump foil 310 may be free-floating and may not be connected to any layer of the foil bearing assembly 300.

[0038] Referring to FIG. 6, an exemplary embodiment of a system 400 includes a centrifugal compressor 404. The system 400 includes the compressor 404, which comprises a compressor housing 405, an unloading device 401, a user interface 415, and a controller 410. The compressor 404 includes a motor 406, an impeller 407, and gas foil bearings 409. The system 400 further includes a variable frequency drive (VFD) 416 having a current sensor 408 and a motor interface 413 in communication with the motor 406. The compressor 404, including the compressor housing 405, the motor 406, the impeller 407, and the gas foil bearings 409, may be configured similarly to the compressor 100 described in FIGS. 1-5 or may be configured differently. The compressor 404 is not limited to a particular configuration within the system 400. The compressor 404 includes a controller 410 for controlling the start, stop, and operating routines of the compressor 404. The controller 410 includes a processor 411, a memory 412, and an unload interface 414. The memory 412 includes instructions that are executed by the processor 411 to cause the controller 410 to perform a method.

[0039] The unloading device 401 of the system 400 removes and / or reduces the load on the compressor during start-up and shutdown routines to address surge events and prevent accelerated wear on the gas foil bearings 409. In a centrifugal compressor 404, too low a flow or too high a pressure rise changes the angle of attack in the impeller, resulting in separation and stall. This causes compressor flow instabilities, impacting bearings and gears and, if used, system pressure instabilities. This is caused by reduced intake air density and flow due to issues such as excessive throttling, hot discharge gas, cold return gas, or a clogged condenser intake filter. When these process conditions cause the compressor 404 to operate at low flow rates, ensuring the compressor 404 always handles flow rates greater than the surge value, the unloading device 401 is opened when necessary to allow gas delivered by the compressor 404 to recirculate to the suction. When the unloading device 401 is coupled to the compressor 404, flow is maintained to prevent the compressor 404 from entering a stall / surge cycle. In this example, the unloading device 401 is a bypass valve or blow-off valve, in acceptable applications. A bypass valve, such as a refrigerant bypass valve, provides an alternative path for gas, thereby stopping the compressor 404 from increasing pressure, thus limiting any potential surge, no matter how slowly the compressor motor 406 accelerates during startup or deceleration from shutdown. In another embodiment, the unloading device 401 is an expansion valve. Expansion valves remove pressure from liquid refrigerant to allow it to expand or change state from liquid to vapor in the compressor 404 evaporator and are included in many HVAC systems. Further embodiments of the unloading device 401 include variable orifice or diameter valves, such as servo valves, and fixed orifice or diameter valves, such as solenoid valves and pulse-width modulated (PWM) valves, configured to control opening and closing according to a duty cycle.Other embodiments of the unload device 401 may include, but are not limited to, a variable diffuser or a variable inlet guide vane (VIGV). While many types of unload devices are described herein, the unload device 401 may be any suitable device that reduces the load on the compressor 404. The strategic opening of the unload device 401 during the start-up and shutdown routines of the compressor 404 is directed by the system 400.

[0040] The unloading device 401 is operatively coupled to a controller 410, which is configured to control at least one operating parameter of the unloading device 401, such as the opening of a bypass valve, according to one or more control schemes, as described in detail below. The controller 410 is configured to control the removal or reduction of load on the compressor 404 according to one or more control schemes based on measurements or other data received from a current sensor 408 and to monitor one or more conditions of the compressor 404. The current sensor 408 senses the current of the motor 406, and the controller 410 determines whether the surge of the compressor 404 has stopped if the sensed current of the motor 406 is a substantially constant current. Non-limiting examples of suitable sensors for use in the one or more control schemes include a temperature sensor, a pressure sensor, a flow sensor, a current sensor, a voltage sensor, a rotational speed sensor, and any other suitable sensor. In other embodiments, the controller 410 controls the removal or reduction of load on the compressor 404 according to one or more control schemes without relying on measurements or other data received from sensors, and instead operates based on preset timing.

[0041] In some embodiments, the removal or reduction of the load is controlled in response to a detected condition of the compressor 404. In these embodiments, the compressor system 100 includes at least one unloading device 401 controlled by a controller 410. In some embodiments, the removal or reduction of the load on the compressor 404 is controlled by actuating the at least one unloading device 401 according to one or more feedback control schemes based on the detected condition of the compressor 404. Feedback or closed-loop control schemes used to facilitate the reduction or reduction of the load on the compressor 404 may include, but are not limited to, a PID controller, a PI controller, a fuzzy logic controller, and any other suitable control scheme that may be used to reduce or reduce the load on the compressor 404.

[0042] The control system 400 includes a motor interface 413 for connecting the VFD 416 to the motor 406, an interface 413 for connecting the controller to a drive, and an unload interface 414 for connecting the controller 410 to an unload device 401 so that the processor 411 may execute instructions stored in the memory 412 to reduce or disconnect the load from the compressor 400 during the start-up and stop methods.

[0043] Control system 400 includes a user interface 415 configured to output (e.g., display) and / or receive (e.g., from a user) information related to system 400. In some embodiments, user interface 415 is configured to receive activation and / or deactivation inputs from a user to activate and deactivate (i.e., turn on and off) or otherwise enable operation of system 400. Moreover, in some embodiments, user interface 415 is configured to output information related to one or more operating characteristics of system 400, including, for example, but not limited to, warning indicators, the status of gas foil bearings 409, and any other suitable information.

[0044] User interface 415 may include any suitable input and output devices that enable user interface 415 to function as described herein. For example, user interface 415 may include input devices including, but not limited to, a keyboard, a mouse, a touchscreen, a joystick, a throttle, buttons, switches, and / or other input devices. Additionally, user interface 415 may include output devices including, but not limited to, a display (e.g., a liquid crystal display (LCD) or an organic light-emitting diode (OLED) display), a speaker, indicator lights, gauges, and / or other output devices. Furthermore, user interface 415 may be part of a different component, such as a system controller (not shown). Other embodiments do not include user interface 415.

[0045] In some embodiments, system 400 may be controlled by a remote control interface. For example, system 400 may include a communications interface (not shown) configured to connect to a wireless control interface that allows for remote control and activation of system 400. The wireless control interface may be implemented on a portable computing device, such as a tablet or smartphone.

[0046] The controller 410 is generally configured to control the operation of the compressor 404. The controller 410 may control the operation through instructions and programming from another device or controller, or may be integrated with the control system 400 through a system controller. In some embodiments, for example, the controller 410 receives user input from a user interface 415 and controls one or more components of the system 400 in response to such user input. For example, the controller 410 may control the supply of power to the motor 406 based on the user input received from the user interface 415. Additionally, in some embodiments, the controller 410 may regulate or control the power supplied to the system 400, such as from an energy storage device.

[0047] The controllers 410 may generally include any suitable computers and / or other processing units, including any suitable combination of computers, processing units, and / or the like, which may be communicatively coupled to each other and may operate independently or in connection with each other (e.g., the controllers 410 may form all or part of a controller network). The controller 410 may include one or more modules or devices, one or more of which may be enclosed within the system 400 or located remotely from the system 400. The controller 410 may be part of the compressor 404 or may be separate and may be part of a system controller in an HVAC system. The controller 410 and / or components of the controller 410 may be integrated or incorporated within other components of the system 400. In some embodiments, for example, the controller 410 may be incorporated within the motor 406 or the unload device 401. The controller 410 may include one or more processors 411 and associated memory devices 412 configured to perform various computer-implemented functions (e.g., calculating, determining, and performing the functions disclosed herein). As used herein, the term "processor" refers not only to integrated circuits, but also to controllers, microcontrollers, microcomputers, programmable logic controllers (PLCs), application-specific integrated circuits, and other programmable circuits. Additionally, the memory devices 412 of the controller 410 may generally be or include memory elements, including, but not limited to, computer-readable media (e.g., random access memory (RAM)), computer-readable non-volatile media (e.g., flash memory), floppy disks, compact disk read-only memory (CD-ROM), magneto-optical disks (MODs), digital versatile disks (DVDs), and / or other suitable memory elements.Such memory device 412 may generally be configured to store suitable computer-readable instructions that, when implemented by a processor, configure or cause controller 410 to perform various functions described herein, including, but not limited to, controlling system 400, controlling operation of motor 406, receiving input from user interface 415, providing output to an operator via user interface 415, controlling unload device 401 and / or various other suitable computer-implemented functions.

[0048] Referring to FIG. 7, a method 500 for starting a centrifugal compressor from a stopped state is described. The method 500 may also be referred to as a start-up routine or a start-up method. This start-up method 500, as well as additional methods described herein, may be implemented using the system 400 described above. The processor 411 executes instructions for the method 500, and the memory 412 stores the instructions. The start-up method 500 begins by activating (501) the unloading device 401 to remove load from the centrifugal compressor 404. In an exemplary embodiment, the unloading device is a bypass valve between the discharge and suction of the compressor 404, which opens the valve, thereby reducing the pressure ratio of the compressor 404 and increasing its mass flow rate. Next, the motor 406 is accelerated (502) to a first speed above the lift-off speed of the gas foil bearing 409 and below the operating speed of the centrifugal compressor 404. By rapidly accelerating to the first speed while the compressor 404 is unloaded, the compressor 404 operates below the lift-off speed of the gas foil bearings 409 for a reduced amount of time to help prevent wear on the gas foil bearings 409 and to address potential surge events during startup. The motor 406 is then operated at the first speed for a period of time (503). In an exemplary embodiment, the time period during which the motor 406 is operated is set and predetermined by the manufacturer or by a user via the user interface 415. In other embodiments, the time period is a variable period of time that can be estimated, calculated, or measured, beginning when the motor reaches the first speed and ending when centrifugal compressor surging stops. Regardless of whether the compressor 404 experiences a surge event, the system 400 accelerates the compressor 404 to the first speed above the lift-off speed of the gas foil bearings 409 to minimize wear thereon. In some cases, the startup routine may be free of surge events when the motor 406 is operated at a first speed for a period of time.If a surge event occurs during the start-up method 500, the current sensor 408 senses the current in the motor 406, and the controller 410 determines that the compressor 404 has stopped surging when the sensed current in the motor 406 is a substantially constant current. In some embodiments, the system 400 may incorporate different learning algorithms to monitor and store system measurements during multiple starts and to optimize the time the motor is operated at the first speed based on historical trends stored in memory 412 of surge event duration and severity. In some embodiments, the system 400 monitors and stores measurements associated with all surge events experienced over the life of the compressor 404 and sets the time period as the longest surge event currently stored in memory 412. In this case, each time a new surge event occurs that is longer than the time currently set by the system 400, the time period is reset to be longer than the most recent surge event for future iterations of the start-up method 500. In some embodiments, a machine learning algorithm or neural network may be utilized by the system 400 to predict the frequency and duration of surge events for the compressor 404 based on different simulated environments in which the compressor 404 resides and to set the time period based on these simulations.

[0049] Referring to Figure 8, a surge current signature graph 600 during start-up method 500 is shown, including a speed curve 601 and a motor current curve 602. Figure 8 shows the motor speed 502 being accelerated to a first speed (502) and running the motor 406 at the first speed (503) for a time period 605. While the motor 406 is running at the first speed for time period 605, an area of ​​possible surge 603 is identified by oscillations in the motor current curve 602. The compressor 404 is held at no-load speed until the current oscillation pattern of surge ends (604) and the compressor 404 is indicated for a full start-up.

[0050] Referring back to the start-up method 500 of FIG. 7 , after the motor 406 is operated (503) at a first speed for a time period 605, the unloading device 401 is activated (504) to apply a load to the compressor 404. In some embodiments, applying a load to the compressor 404 includes closing a bypass valve. In FIG. 8 , in all cases of a surge, activating the unloading device (504) to apply a load to the compressor 404 begins only after the surge current oscillation pattern has stopped (604). If there is no surge during start-up, the motor 406 operates at the first speed for a time period 605, typically predetermined by the manufacturer or by the user via the user interface 415. Finally, the motor 406 is then accelerated to an operating speed, and the compressor 404 completes the start-up routine and begins its operating routine. In some embodiments, the controller 410 accelerates the motor 406 to a first speed at a first acceleration rate and accelerates the motor to the operating speed at a second acceleration rate that is less than the first acceleration rate. In other embodiments, the first and second acceleration rates are the same. In other embodiments, the second acceleration rate is less than the first acceleration rate.

[0051] Referring to FIG. 9 , a method 700 for shutting down the centrifugal compressor 404 from operation is described. Method 700 may also be referred to as a shutdown routine or shutdown method. First, the unload device 401 is activated (701) to remove the load from the compressor 404, similar to the starting method 500. The controller 410 may implement the starting method 500, the operation, and the shutdown method 700, or any combination of these three stages of the compressor 404. Next, the motor 406 decelerates (702) toward a minimum speed greater than zero. Finally, once the motor 406 reaches the minimum speed, power is removed (703) from the motor 406, and the motor 406 is then allowed to coast to a stop. In some embodiments, the minimum speed is an estimated surge speed below which a compressor 404 surge, plus a margin, may occur.

[0052] Referring to FIG. 10 , a deceleration curve 800 for estimated surge speed 801 and minimum speed 802 is shown changing as actual speed 803 decreases during shutdown of compressor 404. Actual speed deceleration curve 803 illustrates shutdown method 700 as motor 406 decelerates toward minimum speed 802, which is greater than zero. At the point where actual speed is minimum speed 804, power is removed and motor 406 is allowed to coast to a stop. Estimated surge speed curve 801 is the estimated compressor speed below which there is a risk of a surge event occurring. To facilitate avoidance of a surge event, a margin 805 is added to estimated surge speed curve 801 to provide extra protection for compressor 404 from falling below estimated surge speed curve 801 during deceleration 702. The estimated surge speed plus margin curve 802 is the level at which the controller 410 prevents the speed of the compressor 404 from decreasing toward the minimum speed during deceleration (702). In some embodiments, the estimated surge speed is looked up by the controller 410 from a lookup table and varies with the pressure ratio of the HVAC system in which the compressor 404 resides. In other embodiments, the minimum speed is looked up by the controller 410 from a lookup table and varies with the pressure ratio of the HVAC system in which the compressor 404 resides. As described above with respect to the surge event in the startup method 500, a learning algorithm may be used to calculate the acceleration curves, including the estimated surge speed plus margin curve 802, the estimated surge speed curve 801, and the minimum speed in the shutdown method 700.

[0053] Referring to FIG. 11 , an exemplary embodiment of a start-up method 900 is described. The start-up method 900 is one embodiment of the broader start-up method 500, is intended to be non-limiting, and may be implemented using the system 400 described above. In this embodiment, the motor 406 and gas foil bearings 409 are given a start condition in which their temperatures are below 100°F. Other start conditions may include waiting at least five minutes since the compressor 404 has stopped, the compressor 404 having a power demand (kW) greater than minimum, and the VFD 416 not failing. The start routine begins by requesting a start and checking whether the above-mentioned start conditions are met and will not proceed until the conditions are met. After the conditions are met, the bypass valve is set to 100% and opens to unload the compressor 404. The VFD 416 is enabled, and the motor 406 acceleration is increased to A. max Set to RPM / sec and speed command is N unload RPM and then the start timer is enabled. max is 4500 RPM / sec, and N unload is 10000RPM. The speed increases and N start T to reach RPM start seconds, and the method 900 will not proceed further until this criterion is met. start After more than N seconds start If the RPM is reached, an exemplary embodiment of a stopping method 1000 (shown in FIG. 12) is implemented as described below. start is 0.5 seconds, and N start is 2000 RPM. start N in seconds start When the RPM is reached, the speed is unload The speed is increased to T unload N in seconds unload RPM must be reached and no progress will be made until this criterion is met. unload After more than N seconds unload If the RPM is reached, the stop method 1000 is executed.unload is 5 seconds. unload After the RPM is achieved, a settle timer is enabled for a surge event. The method 900 checks if a surge is detected, and if so, the settle time is T reset In an exemplary embodiment, T reset is 0 seconds. In this case, T surge If the start timer indicates that more than 10 minutes have elapsed, the stop method 1000 is executed. surge If the start timer indicates that less than a minute has elapsed, the method repeats, checking for a surge until no surge is detected. When no surge is detected, the process timer is checked. settle If the start timer indicates that less than T seconds have been alerted, the start timer surge The timer is checked again to see if the time has elapsed. surge If more than 10 minutes have elapsed, the stop method 1000 is executed and the start timer is set to T surge If less than a minute has elapsed, the method repeats the surge detection step. surge is 2 minutes, T sttle is 30 seconds. settle If the processing timer indicates that more than R seconds have elapsed, the bypass valve will open %B open %, and acceleration is set to A min RPM / sec and the speed command is set to the minimum speed N min Then the actual speed is set to N min The start timer is compared with T min Indicates a time greater than a minute and the actual speed is N min If T is not reached, the stop method 1000 is executed. min If the minimum speed is reached below B, automatic control is activated for the compressor 404. open is 50% and R openis 0.5% per second, and A min is 150 RPM / sec, and T min In another embodiment, A max , N unload , T start , N start , N unload , T unload , T reset , T surge , T sttle , B open , R open , A min , N min , and T min may be any other suitable value. The automatic control presented here is a separate set of control algorithms for running the compressor to a stop.

[0054] 12, an exemplary embodiment of a shutdown method 1000 is described. The shutdown method 1000 is one embodiment of the broader stop method 700 and may be implemented using the system 400 described above. In this embodiment, as in the start-up method 900 described above, when a shutdown is requested, the bypass valve is set to 100% to unload the compressor. The stop timer is enabled and the speed command is set to a minimum speed N min The actual speed is set to N min is greater than 1.01, the stop timer is stop It is checked to see if minutes have passed. In this case, T stop If the stop timer indicates that less than one minute has elapsed, the speed command is again min and the actual speed is N min The actual speed is checked to see if it is greater than the value multiplied by the factor. stop If the time is greater than or equal to 1 minute, the VFD command is disabled and the speed command is set to 0 RPM. If the actual speed is less than the minimum speed multiplied by a factor, the VFD is also disabled and the speed command is set to 0 RPM. In an exemplary embodiment, Tstop is 3 minutes and the coefficient is 1.01. Next is T wait The first step is to wait 10 minutes, and then the bypass valve is set to 0%. wait is 2 minutes. The stopping method 1000 then ends when the motor 406 coasts to a stop.

[0055] 13, in addition to the normal stop method 1000, an emergency stop (e-stop) method 1100 may be implemented for the user to override (disable) the HVAC system via the user interface 415 or in the event of an emergency stop. When an e-stop is requested, the VFD command is set to disabled and the speed command is set to 0 RPM. The bypass valve is set to 100% and the T wait After 10 minutes, the bypass valve is set to 0%. wait is 2 minutes. The e-stop method 1100 then ends and the motor 406 coasts to a stop.

[0056] Technical advantages of the methods and systems described herein are: (a) minimizing the time a compressor is below gas foil bearing lift-off speed to prevent bearing wear during start-up and shutdown procedures in an HVAC system; (b) utilizing an unloading device to minimize the number and severity of surge events seen by a compressor in an HVAC system; (c) maintaining the compressor at no-load speed for a period of time to handle surge events during start-up and shutdown procedures; and (d) reducing speed during shutdown to limit the number and severity of surge events.

[0057] When introducing elements of the disclosure or embodiments thereof, the articles "a," "an," "the," and "said" are intended to mean that there is more than one element. The terms "comprising," "including," "containing," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of specific orientational terms (e.g., "top," "bottom," "side," etc.) is for convenience of description and does not require any particular orientation of the items being described.

[0058] Since various changes may be made in the above structures and methods without departing from the scope of the present disclosure, all matter contained in the above description and illustrated in the accompanying drawings should be interpreted as illustrative and not in a limiting sense.

Claims

1. an unload device; a centrifugal compressor; 1. An HVAC system comprising: The centrifugal compressor comprises: A compressor housing; a motor having a drive shaft rotatably supported within the compressor housing; an impeller connected to the drive shaft and operable to compress a refrigerant gas upon rotation of the drive shaft; a gas foil bearing supported by the compressor housing and supporting the drive shaft; a controller connected to the motor and the unload device; The controller activating the unloading device to remove a load from the centrifugal compressor; accelerating the motor to a first speed above a lift-off speed of the gas foil bearing and below an operating speed of the centrifugal compressor; operating the motor at the first speed for a period of time; activating the unloading device to apply the load to the centrifugal compressor; and Accelerating the motor to the operating speed programmed to start the centrifugal compressor from a stopped condition; HVAC system.

2. The controller activating the unloading device to remove a load from the centrifugal compressor; decelerating the motor towards a minimum speed greater than zero; and by removing power from the motor when its speed reaches the minimum speed and allowing the motor to coast to a stop; further programmed to shut down the centrifugal compressor from an operating condition. The HVAC system of claim 1 .

3. The HVAC system of claim 1 , wherein the time period is a predetermined time period.

4. 2. The HVAC system of claim 1, wherein the time period is a variable time period that begins when the motor reaches the first speed and ends when the centrifugal compressor ceases to surge.

5. 5. The HVAC system of claim 4, further comprising a current sensor that senses a current in the motor, and wherein the controller determines that the surge in the centrifugal compressor has stopped when the sensed current in the motor is a substantially constant current.

6. 2. The HVAC system of claim 1, wherein the controller accelerates the motor to the first speed at a first acceleration rate and accelerates the motor to the operating speed at a second acceleration rate that is less than the first acceleration rate.

7. 3. The HVAC system of claim 2, wherein the minimum speed comprises an estimated surge speed below which surge of the centrifugal compressor may occur, plus a margin.

8. 8. The HVAC system of claim 7, wherein the estimated surge speed is retrieved by the controller from a look-up table and varies with pressure ratio of the HVAC system.

9. 8. The HVAC system of claim 7, wherein the minimum speed is retrieved by the controller from a look-up table and varies with a pressure ratio of the HVAC system.

10. The HVAC system of claim 1 , wherein the unloading device includes a refrigerant bypass valve.

11. 1. A controller for controlling a centrifugal compressor having a gas foil bearing supporting a shaft of an impeller driven by a motor, comprising: a VFD including a motor interface for connection to the motor; an unload interface for connection to an unload device; a processor; a memory containing instructions; The instructions, when executed by the processor, cause the controller to: activating the unloading device to remove a load from the centrifugal compressor; accelerating the motor to a first speed above a lift-off speed of the gas foil bearing and below an operating speed of the centrifugal compressor; operating the motor at the first speed for a period of time; activating the unloading device to apply the load to the centrifugal compressor; and Accelerating the motor to the operating speed starting the centrifugal compressor from a stopped condition; controller.

12. The memory contains instructions that, when executed by the processor, cause the controller to: activating the unloading device to remove a load from the centrifugal compressor; decelerating the motor towards a minimum speed greater than zero; and by removing power from the motor when its speed reaches the minimum speed and allowing the motor to coast to a stop; removing the centrifugal compressor from an operating condition; The controller of claim 11.

13. 12. The controller of claim 11, wherein the time period is a variable time period that begins when the motor reaches the first speed and ends when the centrifugal compressor ceases to surge.

14. 14. The controller of claim 13, further comprising a current sensor interface that receives a signal representative of the motor current from a current sensor, the controller determining that the centrifugal compressor surge has stopped when the sensed motor current is a substantially constant current.

15. 12. The controller of claim 11, wherein the controller accelerates the motor to the first speed at a first acceleration rate and accelerates the motor to the operating speed at a second acceleration rate that is less than the first acceleration rate.

16. The controller of claim 12 , wherein the minimum speed comprises an estimated surge speed below which surge of the centrifugal compressor may occur, plus a margin.

17. 17. The controller of claim 16, wherein the estimated surge speed or the minimum speed is retrieved by the controller from a look-up table and varies with a pressure ratio of an HVAC system including the unload device and the centrifugal compressor.

18. 1. A method of controlling a centrifugal compressor having a gas foil bearing supporting a shaft of an impeller driven by a motor, the method comprising: activating an unloading device to remove a load from the centrifugal compressor; accelerating the motor to a first speed above a lift-off speed of the gas foil bearing and below an operating speed of the centrifugal compressor; operating the motor at the first speed for a period of time; activating the unloading device to apply the load to the centrifugal compressor; and Accelerating the motor to the operating speed starting the centrifugal compressor from a stopped condition; method.

19. activating the unloading device to remove a load from the centrifugal compressor; decelerating the motor towards a minimum speed greater than zero; and by removing power from the motor when its speed reaches the minimum speed and allowing the motor to coast to a stop; and further comprising shutting down the centrifugal compressor from an operating condition.

20. The method of claim 18.

20. 20. The method of claim 18, wherein the time period is a variable time period that begins when the motor reaches the first speed and ends when the centrifugal compressor ceases to surge.

21. receiving a signal representative of a current in the motor from a current sensor; determining that the centrifugal compressor surge has ceased when the sensed current in the motor is a substantially constant current.

21. The method of claim 20.

22. 20. The method of claim 18, wherein accelerating the motor to a first speed comprises accelerating the motor to the first speed at a first acceleration rate, and accelerating the motor to the operating speed comprises accelerating the motor to the operating speed at a second acceleration rate that is less than the first acceleration rate.

23. 20. The method of claim 19, wherein the minimum speed comprises an estimated surge speed below which surge of the centrifugal compressor may occur plus a margin, the method further comprising retrieving the estimated surge speed or the minimum speed from a lookup table.

Images