Patent Publication Number: US-11022351-B2

Title: Liquid sensing for refrigerant-lubricated bearings

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Benefit is claimed of U.S. Patent Application No. 62/201,064, filed Aug. 4, 2015, and entitled “Liquid Sensing for Refrigerant-Lubricated Bearings”, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
    
    
     BACKGROUND 
     The disclosure relates to compressor lubrication. More particularly, the disclosure relates to centrifugal compressor lubrication. 
     A typical centrifugal chiller operates with levels of lubricant at key locations in flowing refrigerant. The presence of an oil reservoir, typically with more than a kilogram of oil will cause an overall content of oil to exceed 1.0 percent by weight when the oil accumulation in the reservoir is added to the numerator and denominator of the fraction. The concentration will be relatively low in the condenser (e.g., 50 ppm to 500 ppm). At other locations, the concentrations will be higher. For example the oil sump may have 60+ percent oil. This oil-rich portion is used to lubricate bearings. Thus, flow to the bearings will typically be well over 50 percent oil. At one or more locations in the system, strainers, stills, or other means may be used to withdraw oil and return it to a reservoir. It is desirable to remove the oil from locations where it may interfere with heat transfer or other operations. 
     There has for a long time existed a desire to operate chiller compressors and other rotating machinery and pumps without the need for a dedicated oil system. David C. Brondum, D. C., James E. Materne, J. E., Biancardi, F. R., and Pandy, D. R., “High-Speed, Direct-Drive Centrifugal Compressors for Commercial HVAC Systems,” presented at the 1998 International Compressor Conference at Purdue, 1998; Pandy, D. R. and Brondum, D., “Innovative, Small, High-Speed Centrifugal Compressor Technologies,” presented at the 1996 International Compressor Engineering conference at Purdue, July, 1996; Sishtla, V. M., “Design and Testing of an Oil-Free Centrifugal Water-Cooled Chiller”, International Conference on Compressors and their Systems: 13-15 Sep. 1999, City University, London, UK, conference transactions, The Institution of Mechanical Engineers, 1999, pp. 505-521. In these tests, ceramic balls were used as the rolling element. 
     Jandal et al., WO2014/117012 A1, published Jul. 31, 2014, discloses a refrigerant-lubricated compressor. With such compressors, it is important that relatively high quality (high liquid fraction) refrigerant be delivered to the bearings. 
     SUMMARY 
     One aspect of the disclosure involves a vapor compression system comprising a compressor. The compressor comprises: an inlet and an outlet; and an electric motor. The motor has a stator and a rotor. A plurality of bearings support the rotor. A fluid flowpath extends to the plurality of bearings. A branch from the fluid flowpath extends to the compressor. A liquid sensor is along the branch. 
     In one or more embodiments of any of the foregoing embodiments, the liquid sensor may be along the branch spaced locally above the fluid flowpath. 
     In one or more embodiments of any of the foregoing embodiments, the liquid sensor may be an optical sensor. 
     In one or more embodiments of any of the foregoing embodiments, the liquid sensor may have a glass prism exposed to fluid along the branch. 
     In one or more embodiments of any of the foregoing embodiments, the branch may extend from the fluid flowpath directly back to the compressor. 
     In one or more embodiments of any of the foregoing embodiments, a degas tank may have an inlet and an outlet along the fluid flowpath. The liquid sensor may be mounted on the degas tank above the degas tank outlet. 
     In one or more embodiments of any of the foregoing embodiments, the liquid sensor may be mounted centered at least 25 mm above the outlet of the degas tank. 
     In one or more embodiments of any of the foregoing embodiments, the liquid sensor may be mounted generally horizontally. 
     In one or more embodiments of any of the foregoing embodiments, the liquid sensor may be a liquid level switch. 
     In one or more embodiments of any of the foregoing embodiments, the liquid level switch may be configured to have a closed condition associated with a sufficient liquid exposure. 
     In one or more embodiments of any of the foregoing embodiments, the compressor may be a centrifugal compressor comprising an impeller and the branch may extend to upstream of the impeller. 
     In one or more embodiments of any of the foregoing embodiments, the compressor may be a centrifugal compressor comprising inlet guide vanes and the branch may extend to downstream of the inlet guide vanes. 
     In one or more embodiments of any of the foregoing embodiments, the vapor compression system further comprises a condenser and an evaporator. A suction line is along a flowpath between the evaporator and an inlet of a first stage of the compressor. A flowpath leg extends from a discharge of the first stage to a second stage inlet. A discharge line is along a flowpath between a discharge of the second stage and an inlet of the condenser. 
     In one or more embodiments of any of the foregoing embodiments, a refrigerant charge of the system is one or more of: essentially oil-free; and at least 50% by weight trans 1-chloro, 3,3,3-fluoropropene (R1233zd(E)). 
     In one or more embodiments of any of the foregoing embodiments, the vapor compression system may lack an oil reservoir. 
     In one or more embodiments of any of the foregoing embodiments, a method for using the system comprises running the compressor; and responsive to an insufficiency of liquid detected by the liquid sensor, shutting down the compressor. 
     In one or more embodiments of any of the foregoing embodiments, the method further comprises during the running, drawing vapor along the branch. 
     In one or more embodiments of any of the foregoing embodiments, the compressor may be a centrifugal compressor comprising an impeller and the branch may extend to upstream of the impeller. 
     Another aspect of the disclosure involves a vapor compression system comprising a compressor. The compressor comprises an inlet and an outlet and an electric motor having: a stator; and a rotor; a plurality of bearings supporting the rotor. A fluid flowpath extends to the plurality of bearings. A liquid sensor is on a degas tank. 
     In one or more embodiments of any of the foregoing embodiments, the liquid sensor may be spaced locally above the flowpath. 
     In one or more embodiments of any of the foregoing embodiments, the liquid sensor may be a liquid level switch. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a vapor compression system in a first mode of operation. 
         FIG. 2  is a schematic view of the system in a second mode of operation. 
         FIG. 3  is a partially schematic view of a liquid level sensor in a vapor compression system. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     It is desirable to ensure sufficient liquid refrigerant is delivered to the bearings. This may present issues of both flow rate and quality of refrigerant (e.g., the volume fraction of liquid flowing in the supply line). A sensor may be used configured to detect liquid, detect the absence of a liquid, or otherwise discriminate between the two. One example of a sensor involves optical measurement. 
     An exemplary baseline system locates the liquid sensor immediately along a supply line supplying refrigerant to the bearings. In one example, the supply line may be generally horizontal and the liquid sensor may be mounted generally atop the line (e.g., having an optical port along an upper portion of the line). This may be insufficient to provide a desired sensitivity to maintain sufficiently high refrigerant quality. 
     A system which, in some implementations, may be modified from such a baseline involves means for concentrating vapor proximate the sensor to improve effective sensor sensitivity. Exemplary means comprises a branch off of the refrigerant supply flowpath. To concentrate vapor in the branch, the branch extends upward from the supply flowpath. The exemplary sensor may be located along a side of the branch. Low density of vapor will allow vapor to concentrate in the branch above the liquid. Although the branch may be a blind branch, the exemplary configurations below vent the branch by connecting the branch to a low pressure location such as between the inlet guide vanes and to impeller of a centrifugal compressor. The greater the amount of vapor in the refrigerant supply flow, the greater the amount that can pass into the branch. In an equilibrium condition, more vapor in the flow will lead to more vapor in the branch and thus a lower level of the surface or the liquid in the branch. The height of the surface may thus be used as a proxy for the amount of vapor in the flow. A liquid level sensor at a given height in the branch may thus serve as a proxy for a sensor along the line intended to indicate a particular concentration of vapor. Although 100% liquid in the supply line is desirable, a target may be at least 95% liquid by volume or at least 98%. 
       FIG. 1  shows a vapor compression system  20 . This reflects details of one particular baseline system. Other systems may be subject to similar modifications to add a liquid sensor or replace a baseline liquid sensor.  FIG. 1  shows flow arrows (and thus associated valve conditions) associated with operating conditions that may correspond to a startup condition or, generally, a condition where there is a low pressure difference between condenser and evaporator. Other operating conditions are discussed further below. The exemplary system  20  is a chiller having a compressor  22  driving a recirculating flow of refrigerant. The exemplary compressor is a two-stage centrifugal compressor having a first stage  24  and a second stage  26 . Impellers of the two stages are co-spooled and directly driven by an electric motor  28  having a stator  30  and a rotor  32 . The compressor has a housing or case  34  supporting one or more bearings  36  to in turn support the rotor  32  for rotation about its central longitudinal axis  500  forming a central longitudinal axis of the compressor. As is discussed further below, the bearings are rolling element bearings with one or more circumferential arrays of rolling elements radially sandwiched between an inner race on the rotor (e.g., mounted to a shaft) and an outer race on the housing (e.g., press fit into a bearing compartment). Exemplary rolling elements include balls, straight rollers (e.g., including needles), and tapered rollers. Exemplary bearings are hybrid bearings with steel races and ceramic rolling elements. Exemplary ceramic rolling elements are silicon nitride ceramic balls. Exemplary races are 52100 bearing steel rings and high nitrogen CrMo martensitic steel rings, including Bolder N360 (trademark of BÖHLER Edelstahl GmbH &amp; Co KG, Kapfenberg, Austria) and Cronidur 30 (trademark of Energietechnik Essen GmbH, Essen, Germany). 
     As is discussed further below, the exemplary vapor compression system  20  is an essentially oil or lubricant-free system. Accordingly, it omits various components of traditional oil systems such as dedicated oil pumps, oil separators, oil reservoirs, and the like. However, a very small amount of oil or other material that may typically be used as a lubricant may be included in the overall refrigerant charge to provide benefits that go well beyond the essentially non-existent amount of lubrication such material would be expected to provide. As is discussed further below, a small amount of material may react with bearing surfaces to form protective coatings. Accordingly, even though traditional oil-related components may be omitted, additional components may be present to provide refrigerant containing the small amounts of material to the bearings. In discussing this below, terms such as “oil-rich” may be used. Such terms are understood as used to designate conditions relative to other conditions within the present system. Thus, “oil-rich” as applied to a location in the  FIG. 1  system may be regarded as extremely oil-depleted or oil-free in a traditional system. 
     The exemplary compressor has an overall inlet (inlet port or suction port)  40  and an overall outlet (outlet port or discharge port)  42 . In the exemplary configuration, the outlet  42  is an outlet of the second stage  26 . The inlet  40  is upstream of an inlet guide vane array  44  which is in turn upstream of the first stage inlet  46 . The first stage outlet  48  is coupled to the second stage inlet  50  by an interstage line (interstage)  52 . Although inlet guide vanes (IGVs) are shown only for the first stage, alternative implementations may additionally or alternatively have IGVs for the second stage. In such cases, the line  240  could go to the second stage. Another variation is a single stage compressor with inlet guide vanes. 
     As is discussed further below, additional flows of refrigerant may exit and/or enter the compressor at additional locations. From the discharge port  42 , a main refrigerant flowpath  54  proceeds downstream in a normal operational mode along a discharge line  56  to a first heat exchanger  58 . In the normal operational mode, the first heat exchanger is a heat rejection heat exchanger, namely a condenser. The exemplary condenser is a refrigerant-water heat exchanger wherein refrigerant passes over tubes of a tube bundle which carry a flow of water (or other liquid). The condenser  58  has one or more inlets and one or more outlets. An exemplary primary inlet is labeled  60 . An exemplary primary outlet is labeled  62 . An exemplary outlet  62  is an outlet of a sump  64  at the base of a vessel of the condenser  58 . An outlet float valve assembly  65  may include an orifice at the outlet  62  to serve as an expansion device. Additional sump outlets are shown and discussed below. 
     The exemplary system  20  is an economized system having an economizer  70  downstream of the condenser along the flowpath  54 . The exemplary economizer is a flash tank economizer having an inlet  72 , a liquid outlet  74 , and a vapor outlet  76 . In the exemplary implementation, the vapor outlet  76  is connected to an economizer line  80  defining an economizer flowpath  84  as a branch off the main flowpath  54  returning to an economizer port  86  of the compressor which may be at the interstage (e.g., line  52 ). A control valve  82  (e.g., an on-off solenoid valve may be along the economizer line. An outlet float valve assembly  75  may include an orifice at the liquid outlet  74  to serve as an expansion device. The main flowpath  54  proceeds downstream from the economizer liquid outlet  74  to an inlet  90  of a second heat exchanger  88 . The exemplary heat exchanger  88  is, in the normal operational mode, a heat absorption heat exchanger (e.g., evaporator). In the exemplary chiller implementation, the evaporator  88  is a refrigerant-water heat exchanger which may have a vessel and tube bundle construction wherein the tube bundle carries the water or other liquid being cooled in the normal operational mode. For simplicity of illustration,  FIG. 1  omits details including the inlet and outlet for the flows of water or other heat transfer fluid. The evaporator has a main outlet  92  connected to a suction line  94  which completes the main flowpath  54  returning to the inlet  40 . 
     Several additional optional flowpaths and associated conduits and other hardware are shown branching off from and returning to the main flowpath  54 . In addition to the economizer flowpath  84 , a motor cooling flowpath  100  also branches off from and returns to the flowpath  54 . The exemplary motor cooling flowpath  100  includes a line  102  extending from an upstream end at a port  104  on some component along the main flowpath (shown as the sump  64 ). The line  102  extends to a cooling port  106  on the compressor. The motor cooling flowpath passes through the port  106  into a motor case of the compressor. In the motor case, the cooling flow cools the stator and rotor and then exits a drain port  108 . Along the flowpath  100 , a motor cooling return line  109  returns the flow from the port  108  to the main flowpath. In this example, it returns to a port  110  on the vessel of the evaporator  88 . 
     A more complicated optional system of branch flowpaths may be associated with bearing lubrication. A trunk  120  of a bearing supply flowpath is formed by a line  122  extending from a port  124  located along the main flowpath (e.g., at the sump  64 ). A branch flowpath  126  ultimately passes to one or more ports  130  on the compressor communicating refrigerant to respective associated bearings  36 . One or more ports  134  extend from one or more drains at the bearings to return refrigerant to the main flowpath. In this embodiment, two possible return paths are shown. A first return path or branch  140  passes to a port  142  immediately downstream of the inlet guide vane array  44 . This port  142  is at essentially the lowest pressure condition in the system and thus provides the maximum suction for drawing refrigerant through the bearings. A valve  146  may be along a line  144  along this flowpath branch. The exemplary valve  146  is an electronically controlled on-off valve (e.g., a solenoid valve) under control of a system controller. A second bearing return flowpath/branch  150  is discussed below. 
     Several additional optional features relating to refrigerant supply to the bearing are shown. A subcooler  160  may be provided to cool refrigerant passing to the bearings. The exemplary subcooler  160  is a heat exchanger that receives flow passing along the flowpath  126  and transfers heat from that refrigerant flow to a refrigerant flow passing along a branch  162 . The exemplary branch  162  branches off directly or indirectly from the main flowpath. In the illustrated embodiment, the branch  162  branches off from the flowpath  120  leaving the branch  126 . In alternative implementations, the branch  162  may be fully separate from the flowpath  120  back to the condenser  58 . The branch  162  passes through a subcooler supply line  164  in which an expansion device  166  (e.g., an expansion valve) is located. Expanded refrigerant enters a subcooler inlet of the subcooler  160  and exits a subcooler outlet. In the subcooler  160 , the expanded refrigerant absorbs heat from refrigerant passing along the branch  126  which has entered a main inlet and then exits a main outlet. The exemplary subcooler  160  is a brazed plate heat exchanger. The flowpath  162  returns to the main flowpath. In the exemplary embodiment, the flowpath  162  merges with the flowpath along line  109  and enters the port  110  on the heat exchanger  88 . Alternatively, line  164  can come directly from the condenser independently of line  122  and its associated flowpath. Alternatively, line  162  can drain directly into the evaporator  88  independently of line  109  and its associated flow path. 
     As noted above,  FIG. 1  also shows a second bearing drain flowpath branch  150 . The exemplary flowpath branch  150  joins the flowpath  162  downstream of the subcooler and upstream of the junction with the line  109 . A valve  170  (e.g., similar to  146 ) is located in a line  172  along the flowpath  150  to control flow. In an exemplary  FIG. 1  condition, the valve  170  is closed blocking flow along the branch  150 . 
       FIG. 1  shows further details of exemplary flowpath  126 . The flowpath  126  further branches into a branch  180  and a parallel branch  182  which then remerge. A pump  184  is located along the branch  180  and a one-way check valve  186  located along the branch  182 . In the normal  FIG. 1  operating condition, pressure is sufficient to drive flow downstream through the valve  186 . Accordingly, the pump  184  is not used. If pressure is insufficient (discussed below) the pump  184  may be turned on to drive refrigerant flow for bearing supply. 
     The flowpath  126  proceeds downstream through a filter  190  prior to entering the ports  130 .  FIG. 1  also shows a temperature sensor  192  downstream of the filter  190  for measuring temperature of refrigerant entering the compressor for bearing cooling. 
       FIG. 1  further shows yet another branch  220  for bearing cooling. The exemplary branch  220  bypasses both the optional subcooler and the pump before rejoining the branch flowpath  126  upstream of the filter  90 . The exemplary branch  220  passes along a line  222  extending from a port  224  on the evaporator and passes through a check valve  226  which may be similar to the check valve  186 . 
       FIG. 1  further shows a bypass flowpath or branch  240  for returning refrigerant to the compressor. A line  242  is along the flowpath from an upstream end at a port  244  on the evaporator  88  to a downstream end at a port  246  in similar location to the port  142  downstream of the inlet guide vane array  44 . This may be used to deliver a relative oil-rich flow of refrigerant back to the compressor. Specifically, oil may tend to concentrate in liquid refrigerant in a lower portion of the evaporator. Because the port  244  may be positioned at such a location to deliver oil-rich refrigerant directly to the bearings, flow along the flowpath  240  delivers the oil rich mixture into the compressing flow which, in turn, passes to the condenser  58  and may thus increase the amount of oil available in the flow along the flowpath  120 . Pressure difference between the ports  244  and  246  alone may be insufficient to drive this flow. Accordingly, an eductor  250  or similar pump may be located along the line  242 . The eductor has a suction port or inlet  252  upstream and a discharge port or outlet  254  downstream. A motive flow inlet  256  receives flow along a branch flowpath  260  associated with a branch line  262 . The exemplary branch line  262  and flowpath  260  branch off the main flowpath at essentially compressor discharge conditions (e.g., from upstream of the condenser inlet  60  or from the headspace of the condenser). 
       FIG. 1  shows a degas tank  300  along the bearing supply line and flowpath  126 . The tank has an inlet  302  for receiving liquid refrigerant (e.g., downstream of the filter  190 ). The exemplary inlet  302  is at a bottom of the tank. The exemplary tank is a cylindrical metallic tank oriented with its axis vertically. An exemplary refrigerant outlet  304  is along a sidewall of the tank. An additional port  306  on the tank is connected to a vacuum line  308  and associated flowpath  310  (a branch off the bearing supply flowpath) to draw vapor from the headspace  312  of the tank. The exemplary line  308  and flowpath  310  extend to a low pressure location in the system. An exemplary low pressure location is downstream of the inlet guide vanes such as the port  142 , port  146 , or a similar dedicated port. Other low pressure locations within the compressor (bypassing the compressor inlet) or along the main flowpath upstream of the compressor inlet may be used. Similarly, the refrigerant supply flowpath may branch off the main flowpath at any of several locations appropriate for the particular system configuration. Along the line  308  and flowpath  310 ,  FIG. 1  also shows an exemplary strainer  320  and orifice  322 . The orifice functions to limit flow rate to avoid drawing liquid from the degas tank.  FIG. 1  shows a single sensor for both refrigerant supplies. Alternatively, separate sensor assemblies may be provided for individual bearing refrigerant supply lines. 
       FIG. 1  further shows a liquid level sensor  330  mounted to the tank. The exemplary liquid level sensor  330  is mounted above the ports  302  and  304 . An exemplary mounting is by a height of at least 25 mm (or at least 30 mm or 25 mm to 50 mm or 30 mm to 40 mm) above the outlet port  304  (i.e., the central axis  520  of the sensor is spaced by that much above the upper extremity of the outlet port). The sensor may be oriented horizontally (e.g., with the axis of its cylindrical body and its prism) within about 10° or 5° of horizontal) to avoid trapping of bubbles by the sensor. Thus, the line  308  and flowpath  310  withdraw vapor from above the sensor  330 . Although these are shown extending extend from the bearing supply flowpath directly back to the compressor (instead of rejoining the main flowpath upstream of the suction port), other low-pressure destinations might be used. 
     A number of types and configurations of liquid level sensors exist. The exemplary sensor is an optical sensor as discussed below. The sensor has an operative/sensing end  332  positioned to be exposed to the liquid in a normal situation of sufficient liquid. In this example, the sensor is an optical sensor and the exposure is an optical exposure which may, however, also include physical exposure with the end  332  contacting the fluid (liquid refrigerant and/or vapor) in the tank. The sensor  330  may be used to determine whether the liquid surface  314  has descended below a critical level (whereafter further descent might risk vapor passing through the port  304  and being ingested by the bearings). The determination of the surface  314  descending to this threshold height may trigger a response by the controller  900 . Exemplary responses may include compressor shutdown or may include some form of remedial activity. 
       FIG. 3  shows an alternative configuration wherein a T-fitting  400  is placed with its head  402  (alternatively phrased as arms) along the bearing supply line/flowpath. The leg  404  of the T extends upward to form a small degas tank. The sensor  330  is mounted to a side of the tank.  FIG. 3  shows the operative end  332  of the sensor as being formed by a prism  340 . Again, the sensor is oriented sideways or horizontally (e.g., with its axis within about 10° or 5° of horizontal or of parallel to the head of the T) spaced above the flowpath  126  (i.e. above the local upper extremity along the arms of the T) by a height such as a least 25 mm (or at least 30 mm or 25 mm to 50 mm or 30 mm to 40 mm). 
     The exemplary sensor of  FIGS. 1-3  is a switch positioned to change state when the liquid level transits a certain threshold height relative to the prism  340 . The exemplary liquid level switch is configured to have a closed condition associated with a sufficient liquid exposure (although an open condition version may alternatively be used). An exemplary threshold is approximately halfway up the prism. 
       FIG. 2  shows flow arrows associated with a normal operational mode (e.g., with high pressure difference between condenser and evaporator). Yet other modes are possible and may be dependent upon other system details or modifications thereof (e.g., a defrost dehumidification mode where one heat exchanger is a refrigerant-air heat exchanger or possible other modes where the functions of the two heat exchangers become reversed). 
     The overall circulating refrigerant mixture may comprise: one or more base refrigerants or refrigerant bases (e.g., discussed below); optionally a small amount of an oil material that might normally be regarded as a lubricant; optionally, further additives; and contaminants, if any. 
     Exemplary base refrigerant can include one or more hydrofluoroolefins, hydrochloroolefins, and mixtures thereof (e.g., including hydrochloroflouroolefins). Below HFO is used to synonymously refer to all three of these refrigerant types. Exemplary hydrochloroflouroolefins include chloro-trifluoropropenes. Exemplary chloro-trifluoropropenes, are 1-chloro-3,3,3-trifluoropropene and/or 2-chloro-3,3,3-trifluoropropene, and most particularly trans-1-chloro-3,3,3-trifluoropropene (E-HFO-1233zd, alternatively identified as R1233zd(E)). The hydrofluoroolefins can be a C3 hydrofluoroolefin containing at least one fluorine atom, at least one hydrogen atom and at least one alkene linkage. Exemplary hydrofluoroolefins include 3,3,3-trifluoropropene (HFO-1234zf), E-1,3,3,3-tetrafluoropropene, (E-HFO-1234ze), Z-1,3,3,3-tetrafluoropropene (Z-HFO-1234ze), 2,3,3,3-tetrafluoropropene (HFO-1234yf), E-1,2,3,3,3-pentafluoropropene (E-HFO-1255ye), Z-1,2,3,3,3-pentafluoropropene (Z-HFO-125ye). 
     Exemplary oils are polyol ester (POE) oils. Other possible oils include polyalkylene glycols (PAG), polyvinyl ethers (PVE), alkylbenzenes, polyalpha olefins, mineral oils, and the like as well as mixtures. A relevant consideration is the availability of hydrocarbons that can form an organic protective layer on the bearing surfaces. 
     The trace polyol ester oil (100 ppm) may particularly be of the hindered type excellent in thermal stability. The polyol ester oil is obtained from the condensation reaction between polyhydric alcohols and monohydric fatty acids (e.g., medium molecular weight (C5-C10)). Particular examples of polyhydric alcohols include neopentyl glycol, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, dipentaerythritol, and higher polyether oligomers of pentaerythritol, such as tripentaerythritol and tetrapentaerythritol. Polyol esters can be formed from monohydric fatty acids including n-pentanoic acid, n-hexanoic acid, n-heptanoic acid, n-octanoic acid, 2-methylbutanoicacid, 2-methylpentanoic acid, 2-methylhexanoic acid, 2-ethylhexanoic acid, isooctanoic acid, 3,5,5-trimethylhexanoic acid. 
     The additives may comprise a wide range of functionalities, including: extreme pressure agents; acid capturing agents; defoamers; surfactants; antioxidants; corrosion-inhibitors; plasticizers; metal deactivating agents. These may comprise a wide range or chemistries including: epoxides; unsaturated hydrocarbons or unsaturated halocarbons; phthalates; phenols; phosphates; perfluoropolyethers; thiols; phosphites; siloxanes; tolytriazoles; benzotriazoles; amines; zinc dithiophosphates; and amine/phosphate ester salts. Exemplary individual additive concentrations are no more than 1.0% by weight, more particularly 10 ppm to 5000 ppm or no more than 1000 ppm or no more than 200 ppm. Exemplary aggregate non-oil additive concentrations are no more than 5.0% by weight, more particularly, no more than 2.0% or no more than 1.0% or no more than 5000 ppm or no more than 1000 ppm or no more than 500 ppm or no more than 200 ppm or no more than 100 ppm. 
       FIG. 1  further shows a controller  900 . The controller may receive user inputs from an input device (e.g., switches, keyboard, or the like) and sensors (not shown, e.g., pressure sensors, temperature sensors, and/or flow sensors (e.g. particularly measuring flow to the bearings) at various system locations). The controller may be coupled to the sensors and controllable system components (e.g., valves, the bearings, the compressor motor, vane actuators, and the like) via control lines (e.g., hardwired or wireless communication paths). The controller may include one or more: processors; memory (e.g., for storing program information for execution by the processor to perform the operational methods and for storing data used or generated by the program(s)); and hardware interface devices (e.g., ports) for interfacing with input/output devices and controllable system components. 
     The system may be made using otherwise conventional or yet-developed materials and techniques. 
     The use of “first”, “second”, and the like in the description and following claims is for differentiation within the claim only and does not necessarily indicate relative or absolute importance or temporal order. Similarly, the identification in a claim of one element as “first” (or the like) does not preclude such “first” element from identifying an element that is referred to as “second” (or the like) in another claim or in the description. 
     Where a measure is given in English units followed by a parenthetical containing SI or other units, the parenthetical&#39;s units are a conversion and should not imply a degree of precision not found in the English units. 
     One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied to an existing basic system, details of such configuration or its associated use may influence details of particular implementations. Accordingly, other embodiments are within the scope of the following claims.