Abstract:
A method includes operating a variable-speed compressor in a refrigeration system carrying a refrigerant-oil mixture at a varying compressor speed to provide cooling capacity in proportion to cooling demand on the refrigeration system, determining that increased oil return to the compressor compared to a current oil-return rate is desired, and increasing the speed of the compressor to be disproportionately high relative to the cooling demand to provide increased oil return to the compressor.

Description:
BACKGROUND 
       [0001]    It is often desirable to remove heat from air using a refrigeration system. For example, in a data center with many electronic components operating and generating heat, it is desirable to remove the heat from the data center to help prevent operational problems with the electronic components. 
         [0002]    The amount of heat produced by the electronic equipment typically varies over time, e.g., with the amount of use of the equipment. Thus, variable-speed compressors may be used to help efficiently and cost effectively remove the heat. With oil mixed with refrigerant in a refrigeration system, the amount of oil returned to the compressor varies with the compressor speed. Proper design of oil return to the compressor is important to avoid issues such as excessive pressure loss in tubes of the refrigeration system, low efficiency of the refrigeration system, and low capacity of the refrigeration system. To help return oil to the compressor, tubes may be sized to help return oil during times of low compressor speed, a separate oil return system may be provided, an extra oil replenishing or holding system may be used, and/or compressor oil level may be manually checked and replenished. Such techniques may have associated issues such as high pressure loss, short lifespan of the compressor due to insufficient or excess oil return, high fabrication/installation cost, and/or high maintenance cost. 
       SUMMARY 
       [0003]    In general, in an aspect, the invention provides a refrigeration system including: a condenser configured to condense refrigerant in a refrigerant-oil mixture from first gas-phase refrigerant to first liquid-phase refrigerant; a pressure mechanism in liquid communication with the condenser and configured to receive the first liquid-phase refrigerant from the condenser and to reduce a pressure of the first liquid-phase refrigerant of the mixture to produce second liquid-phase refrigerant; an evaporator in liquid communication with the pressure mechanism and configured to receive the second liquid-phase refrigerant from the pressure mechanism and to transfer heat to the second liquid-phase refrigerant of the mixture to convert the second liquid-phase refrigerant to second gas-phase refrigerant; a compressor subsystem in liquid communication with the evaporator and the condenser and configured to receive the second gas-phase refrigerant from the evaporator and to compress the second gas-phase refrigerant vapor refrigerant of the mixture into the first gas-phase refrigerant and send the first gas-phase refrigerant toward the condenser; and a controller coupled to the compressor subsystem and configured to determine that an amount of oil available to the compressor subsystem is less than desired and to control the compressor subsystem to increase a speed of the compressor subsystem to cause more oil to be provided to the compressor subsystem such that the compressor subsystem has a desirable amount of oil available to the compressor subsystem. 
         [0004]    Implementations of the invention may include one or more of the following features. The compressor subsystem includes a variable-speed compressor. The controller is configured to determine that the amount of oil available to the compressor subsystem is less than desired if the variable-speed compressor&#39;s speed is below a first speed threshold for longer than a first time threshold. The controller is configured to cause the variable-speed compressor to increase its speed if the variable-speed compressor&#39;s speed is below a second speed threshold for longer than a second time threshold, the second speed threshold being lower than the first speed threshold, the second time threshold being shorter than the first time threshold. The controller is configured to cause the variable-speed compressor to make its speed approximately 60% or higher of a maximum speed associated with the variable-speed compressor if the controller determines that the amount of oil available to the compressor subsystem is less than desired. The controller is configured to determine that the amount of oil available to the compressor subsystem is less than desired if an oil level at the variable-speed compressor is below a lower oil-level threshold. The controller is configured to cause the compressor to increase its speed until the oil level at the compressor reaches an upper oil-level threshold that is higher than the lower oil-level threshold. The compressor subsystem comprises a first compressor and a second compressor, the first compressor being a fixed-speed compressor, and wherein the controller is configured to cause the first compressor to operate if the controller determines that the amount of oil available to the compressor subsystem is less than desired and the first compressor is currently off. 
         [0005]    In general, in another aspect, the invention provides a system including: a variable-speed compressor including an input configured to receive a refrigerant-oil mixture with the refrigerant being in gas phase and at a first pressure, the compressor including an output configured to transfer the refrigerant-oil mixture with the refrigerant being in gas phase and at a second pressure that is higher than the first pressure, the compressor being further configured to increase the pressure of the refrigerant received via the input from the first pressure to the second pressure and to supply the refrigerant-oil mixture with the refrigerant at the second pressure to the output; and a controller coupled to the compressor and configured to control the compressor to increase a speed of the compressor from a first speed to a second speed if the compressor speed has been below a first low-speed threshold for longer than a first time, the second speed being substantially faster than the first speed. 
         [0006]    Implementations of the invention may include one or more of the following features. The second speed is at least about 60% of a maximum speed of the compressor. The second speed is at least about 90% of the maximum speed of the compressor. The second speed is about 100% of the maximum speed of the compressor and the controller is configured to control the compressor speed to be the second speed for a second time that is about 10% of the first time or less. The controller is configured to cause the compressor to increase its speed if the compressor speed is below a second low-speed threshold for longer than a third time threshold, the second speed threshold being lower than the first speed threshold, the third time threshold being shorter than the first time threshold. The second speed is about 100% of the maximum speed of the compressor and the second speed is about 10% of the third time or less. 
         [0007]    In general, in another aspect, the invention provides a method including: operating a variable-speed compressor in a refrigeration system carrying a refrigerant-oil mixture at a varying compressor speed to provide cooling capacity in proportion to cooling demand on the refrigeration system; determining that increased oil return to the compressor compared to a current oil-return rate is desired; and increasing the speed of the compressor to be disproportionately high relative to the cooling demand to provide increased oil return to the compressor. 
         [0008]    Implementations of the invention may include one or more of the following features. The method further includes returning the speed of the compressor to a speed proportionate to the cooling demand after increasing the speed of the compressor to be disproportionately high relative to the cooling demand has resulted in the compressor being desirably lubricated. 
         [0009]    In accordance with implementations of the invention, one or more of the following capabilities may be provided. A variable speed compressor can be provided with appropriate amounts of returned oil. Proper amounts of oil may be returned to a compressor despite variations in compressor speed. Cooling efficiency of a variable-speed compressor refrigeration system may be improved. Proper amounts of oil may be returned to a variable-speed compressor refrigeration systems using different tube sizes. A variable-speed compressor refrigeration system can have a simple construction, low cost, and/or high compressor reliability. 
         [0010]    These and other capabilities of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]      FIG. 1  is a block diagram of a data center. 
           [0012]      FIG. 2  is a block diagram of a refrigeration system shown in  FIG. 1 . 
           [0013]      FIG. 3  is a block flow diagram of a process of altering a speed of a compressor shown in  FIG. 2  to provide desired oil return to the compressor. 
           [0014]      FIGS. 4-5  are block diagrams of other refrigeration systems. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    Embodiments of the invention provide techniques for regulating oil return to a variable-speed compressor. For example, a controller can monitor a refrigeration system to determine and/or infer amounts of oil return to a variable-speed compressor. The controller can control the speed of the compressor to interrupt a period of relatively low speed with a period of relatively high speed as desired to increase the oil return to the compressor, and then return to a lower speed. The controller can repeat this as appropriate. This system is exemplary, however, and not limiting of the invention as other implementations in accordance with the disclosure are possible. 
         [0016]    Referring to  FIG. 1 , a system  10  includes a data center  12  that includes equipment racks  14  and a refrigeration system  16 . The racks  14  are, here, arranged in two rows  18 ,  20 , although other configurations are possible. The equipment racks  14  contain electronic components such as servers or other information technology equipment. The electronic components produce heat that is expelled from backs  21  of the racks  14  between the rows  18 ,  20  and the refrigeration system  16  is configured to cool the hot air and provide cool air to fronts  22  of the racks  14 . 
         [0017]    Referring to  FIG. 2 , the refrigeration system  16  includes a condenser  24 , an expansion valve  26 , an evaporator  28 , a variable-speed compressor  30 , and a controller  32 . Not all of the components of the refrigeration system  16  may be contained in a rack. For example, the condenser  24  may be located outside of the rack, e.g., external to the data center  10  outside. The condenser  24 , expansion valve  26 , and evaporator  28  operate in typical fashion, and are not discussed here. Refrigerant lines  34 ,  36 ,  38 ,  40  contain and transport a refrigerant and oil mixture between the devices  24 ,  26 ,  28 ,  30 . Here, the lines/tubes  34 ,  36 ,  38 ,  40  are configured for optimum oil return to the compressor  30  at 80% of maximum compressor speed, although other configurations may be used. The compressor  30  is configured to operate a different speeds to accommodate different cooling capacities as desired (e.g., during peak and off-peak hours of use of, and thus heat production by, the equipment in the racks  14 ). The compressor  30  is coupled to the controller  32  and configured to vary its speed in response to a control signal from the controller  32 . The controller  32  is configured to increase and lower the speed of the compressor in accordance with cooling demand on the refrigeration system  16  by the racks  14 . 
         [0018]    The controller  32  is configured to regulate the speed of the compressor  30  to help ensure adequate oil return to the compressor  30 . Here, the controller  32  includes a timer  42 . The controller  32  is configured to monitor the speed of the compressor  30  and to actuate the timer  42  if the compressor speed reaches or drops below one or more threshold speeds. The timer  42  can track multiple times. Here, the controller  32  is configured to actuate the timer  42  to track amounts of time that the compressor speed drops below low-speed thresholds, e.g., 75 Hz and 50 Hz where a maximum speed is about 85 Hz. The controller  32  is configured to reset the corresponding times tracked by the timer  42  if the compressor speed exceeds 50 Hz or 75 Hz, respectively. The controller  32  is configured to send control signals to the compressor  30  to cause the compressor  30  to increase its speed if the timer  42  reaches a low-speed threshold time since the corresponding compressor speed dropped below the corresponding low-speed threshold. For example, the controller  32  can cause the compressor  30  to increase its speed to about 100% of its maximum speed if the speed has been below 50 Hz for more than 30 minutes, or below 75 Hz for more than 12 hours. The compressor  30  can have its speed increased to other percentages of the maximum speed, e.g., between about 60% and 100%. The controller  32  is configured to cause the compressor  30  to increase its speed to a desired speed that is substantially higher than the current speed and to actuate the timer  42  again. The controller  32  is configured to monitor the timer  42  and, in response to the timer reaching one or more increased-speed threshold times, to cause the compressor  30  to return to its previous speed or whatever speed is dictated by current demand on the refrigeration system  16 . For example, the controller  32  can cause the compressor to run at 100% of its maximum speed for an increased-speed time before returning the compressor speed to a speed dictated by cooling demand. The increased-speed time is preferably less than about 10% of the threshold time for the compressor  30  to be running at a lowest speed threshold before triggering acceleration of the compressor speed, and preferably less than about 8% of this time, e.g., about 7% or two minutes for the example of a 30-minute time threshold for the lower speed threshold of 50 Hz. The increased-speed time is preferably longer if the speed to which the compressor  30  is increased is less than 100% of the maximum speed for the compressor. Thus, the controller  32  can cause the compressor to interrupt periods of relatively low speed with periods of relatively high speed, and then return to a relatively low speed to help ensure adequate oil return to the compressor  30  while providing efficient amounts of cooling corresponding to demands on the refrigeration system  16 . 
         [0019]    The controller  32  may take a variety of forms. For example, the controller  32  may provide digital or analog signals, or combinations of these, to the compressor  30 . The controller  32  may comprise a mechanical device, a thermo-mechanical device, or combinations of these. The controller  32  may comprise a processor and memory that stores computer program code instructions that are readable and executable by the processor to perform functions as described here. Further, the controller  32  may comprise a combination of two or more of the techniques mentioned. 
         [0020]    In operation, referring to  FIG. 3 , with further reference to  FIGS. 1-2 , a process  50  for cooling the data center  12  while helping to ensure adequate oil return to the compressor  30  includes the stages shown. The process  50 , however, is exemplary only and not limiting. The process  50  may be altered, e.g., by having stages added, removed, or rearranged. 
         [0021]    At stage  52 , the controller  32  causes the compressor speed to vary in accordance with (proportional to) cooling demand on the refrigeration system  16 . The controller  32  may receive indicia from various sources, e.g., air temperature sensors, refrigerant pressure sensors, etc. to determine the cooling demand on the system  16  and send control signals to the compressor  30  to vary its speed accordingly. 
         [0022]    At stage  54 , the controller  32  determines whether the compressor speed drops below a first low-speed threshold. For example, the first low-speed threshold may be 75 Hz. If the speed has not gone below the first low-speed threshold, then the controller  32  continues to monitor the compressor speed. If, however, the compressor speed decreases below the first low-speed threshold, then the process  50  proceeds to both stage  56  and stage  64 . 
         [0023]    At stage  56 , the controller  32  resets and starts a first timer using the timer  42 . The controller  32  causes the timer  42  to start a first timer for tracking an amount of time that the compressor  30  runs at a speed less than the first low-speed threshold. 
         [0024]    At stage  58 , an inquiry is made as to whether the compressor speed exceeds the first low-speed threshold. The controller  32  determines whether the compressor speed now exceeds the first low-speed threshold (e.g., by being increased at stage  60  via stage  72  as discussed below). If so, then the process returns to stage  54 , and otherwise proceeds to stage  60 . 
         [0025]    At stage  60 , the controller  32  determines whether the time indicated by the first timer exceeds a first time threshold associated with the first low-speed threshold. For example, the first time threshold may be  12  hours. If the first timer does not exceed the first time threshold, then the process  50  returns to stage  58 , and otherwise proceeds to stage  62 . 
         [0026]    At stage  62 , the compressor speed is increased. The controller  32  sends a control signal to the compressor  30  in response to which the compressor increases its speed, e.g., to 100% of its maximum speed. The controller  32  causes the compressor  30  to operate at this speed for a predetermined amount of time, e.g., two minutes. 
         [0027]    At stage  64 , with the compressor speed being below the first low-speed threshold, the controller  32  determines whether the compressor speed drops below a second, lower low-speed threshold. For example, the second low-speed threshold may be 50 Hz. If the speed has not gone below the second low-speed threshold, then the process  50  proceeds to stage  66 . If, however, the compressor speed decreases below the second low-speed threshold, then the process  50  proceeds to stage  68 . 
         [0028]    At stage  66 , the controller  32  determines whether the compressor speed exceeds the first low-speed threshold. If the compressor speed does not exceeded the first low-speed threshold, then the process  50  returns to stage  64 , and otherwise returns to stage  52 . For example, if the compressor speed is increased at stage  62 , then the process returns from stage  66  to stage  52 . 
         [0029]    At stage  68 , the controller  32  actuates a second timer. The controller  32  causes the timer  42  to actuate a second timer for tracking an amount of time that the compressor  30  runs at a speed less than the second low-speed threshold. 
         [0030]    At stage  70 , the controller  32  determines whether the compressor speed exceeds the second low-speed threshold. If the compressor speed exceeds the second low-speed threshold, then the process  50  returns to stage  64 , and returns to stage  52  if the compressor speed exceeds the second low-speed threshold. 
         [0031]    At stage  72 , the controller  32  determines whether the time indicated by the second timer exceeds a second time threshold associated with the second low-speed threshold. For example, the second time threshold may be 30 minutes. If the second timer does not exceed the second time threshold, then the process  50  returns to stage  70 , and otherwise proceeds to stage  62 . 
         [0032]    Other embodiments are within the scope and spirit of the invention. For example, due to the nature of software, functions described above can be implemented using software, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. 
         [0033]    In exemplary embodiments of refrigeration systems, instead of different compressor speed thresholds and corresponding threshold times, the controller could monitor the compressor speed for an average compressor speed dropping below a threshold speed for a threshold time. For example, the controller is configured to reset its timer if the average compressor speed drops below a low-speed threshold and to send control signals to the compressor to cause the compressor to increase its speed if the timer reaches a low-speed threshold time since the average compressor speed exceeded the threshold speed. Alternatively, the controller could determine whether the average speed over different periods of time has not exceeded corresponding, different average speed values and cause the compressor to increase speed if the average speed violates any speed-time pair. 
         [0034]    Referring to  FIG. 4 , a controller  82  is configured to monitor an oil level indicator  84  in a compressor  80 . In accordance with a current oil level and a desired oil level, the controller  32  can send a control signal to increase its speed such that more oil is returned to the compressor  30 . For example, if the compressor  30  runs at a low speed for a period of time, the oil level in the compressor  30  may drop. The controller  32  is configured to determine when a higher rate of oil return than the present rate is desired, e.g., when the oil level reaches or drops below a lower threshold level and to respond by signaling to the compressor  30  to increase its speed. The controller  32  may cause the compressor  30  to increase its speed to a desired speed for a desired amount of time. Alternatively, the controller  32  may cause the compressor  30  to increase its speed to a desired speed until the oil level reaches a higher threshold level (higher than the lower threshold level), at which time the controller  32  can cause the compressor  30  to return to its previous speed or whatever speed is dictated by current demand on the refrigeration system  16 . Thus, the controller  32  can cause the compressor to interrupt periods of relatively low speed with periods of relatively high speed, and then return to a relatively low speed to help ensure adequate oil return to the compressor  30  while providing efficient amounts of cooling corresponding to demands on the refrigeration system  16 . 
         [0035]    As a further example, a controller could use a timer to periodically cause a compressor to operate at a high speed, regardless of the historical speed of the compressor. For example, the controller could cause the compressor to operate at its maximum speed for two minutes every 30 minutes. The speed and duration of the high-speed interval and the length of time between high-speed intervals are preferably selected to guard against foreseeable unacceptable oil return scenarios. In this example, the durations and speed are selected to equal the shorter time interval, corresponding to the compressor speed below 50 Hz, discussed above. 
         [0036]    Further, a controller could monitor other properties and control a compressor accordingly. For example, referring to  FIG. 5 , a controller  92  can monitor a pressure sensor  94 , a temperature sensor  96 , and a gas speed sensor  98 . The sensors  94 ,  96 ,  98  shown are exemplary only, and other sensors for detecting other characteristics, more of any of the types of sensors shown, and/or different locations for sensing system characteristics, may be used. In this example, the controller  92  can monitor pressure, temperature, and gas speed and determine when a compressor  100  should operate at a higher speed to guard against undesirably low oil return to the compressor  100 . The controller  92  can control the compressor  100  to increase its speed as desired to boost oil return to the compressor  100  appropriately. 
         [0037]    Further, combinations of techniques discussed above for determining when and how to control a compressor speed, and controlling the compressor speed, may be used. 
         [0038]    Further still, the techniques described can be applied to various system configurations. For example, multiple variable speed compressors with a shared oil distribution (e.g., an oil leveling tube or the like) may have their speeds increased to help ensure adequate oil return for the compressors. A multiple compressor setup preferably would us an oil level control like in  FIG. 4 , where the controller gets feedback from an oil level sensor in the compressor. Two or more constant speed compressors with shared oil distribution (with an oil equalization tube) can be regulated to help ensure adequate oil return. For example, if a constant speed compressor is not running and oil return drops to an undesired level, then the compressor can be actuated to its constant speed to provide adequate oil return. Further, techniques described can be applied to systems with a compressor employing unloading to reduce compression capacity of the compressor. Techniques described may also be used with multi-compressor systems that employ unloading and that have shared oil distribution. Techniques described can also be applied to systems with a compressor with a hot gas bypass, multi-compressor systems with a hot gas bypass, unloading, and shared oil distribution, systems with a compressor with varying capacity, multi-compressor systems with varying capacity, unloading, and shared oil distribution, etc. Further combinations of these systems or other cooling devices may be used. Also, the techniques described can be applied to other systems not listed. 
         [0039]    Also, while the description above refers to “the invention,” more than one invention may be disclosed.