Abstract:
A method of lubricant management in a heating ventilation and air conditioning (HVAC) system includes flowing a volume of a compressor lubricant and refrigerant mixture from an evaporator into a lubricant still and stopping the flow of the compressor lubricant and refrigerant mixture into the lubricant still when the mixture fills the lubricant still to a selected level. Compressor lubricant is distilled from the mixture via a thermal energy exchange, and the distillation is stopped when a concentration of compressor lubricant in the lubricant still exceeds a predetermined concentration level. The distillate is urged from the lubricant still.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to heating, ventilation and air conditioning (HVAC) systems. More specifically, the subject matter disclosed herein relates to compressor oil management for HVAC systems. 
         [0002]    HVAC systems, such as chillers, often use a flooded or falling film evaporator to facilitate a thermal energy exchange between a refrigerant in the evaporator and a medium flowing in a number of evaporator tubes positioned in the evaporator. The compressor in such systems requires lubrication, typically via oil, to remain operational. As such, a portion of the oil used to lubricate the compressor intermingles with the flow of refrigerant through the compressor and finds its way into the refrigerant flow to the evaporator. When the system is at full load, the refrigerant in the evaporator is continuously contaminated with between about 1% and 5% oil. At partial load, vapor velocity in the evaporator is not sufficient to carry oil from the evaporator to the suction line, so oil accumulates in the evaporator. It is desired to remove the oil from the evaporator for at least two reasons. First, the oil is needed to lubricate the compressor, so it is desired to return the oil to the compressor to replenish a supply thereat. Without doing so, the oil will eventually be depleted from the compressor oil sump. Second, the oil in the evaporator degrades the performance of the system, in particular, the evaporator. 
         [0003]    Chillers and other HVAC systems often include an oil management system in a effort to ensure a continuous supply of oil to the compressor . Such an oil management system typically includes an ejector, essentially a pump, which is run continuously to remove refrigerant-rich oil from the evaporator. The ejector uses compressor discharge gas as its working fluid to draw the oil-rich refrigerant from the evaporator and transport it, together with the discharge gas, back to the compressor. This operation, in a typical system, results in about 1% to 2% additional energy consumption by the HVAC system. Further, the typical oil management system leaves the evaporator refrigerant charge continuously contaminated with about 1.5% to 3% oil. This continual contamination reduces overall heat transfer performance of the evaporator by about 3% to 10%. Additionally, in HVAC systems utilizing low pressure refrigerants, the oil contamination causes a reduction in refrigerant vapor pressure resulting in up to an additional about 1% in HVAC system energy consumption. 
       BRIEF SUMMARY 
       [0004]    In one embodiment, a heating, ventilation and air conditioning (HVAC) system includes a compressor having a flow of compressor lubricant therein, the compressor compressing a flow of vapor refrigerant therethrough and an evaporator operably connected to the compressor including a plurality of evaporator tubes through which a volume of thermal energy transfer medium is flowed for a thermal energy exchange with a liquid refrigerant in the evaporator. The HVAC system further includes a lubricant management system including a lubricant still receptive of a flow of compressor lubricant and refrigerant mixture from the evaporator. An inlet flow control device is utilized to stop the flow of the mixture into the lubricant still when a mixture level in the still reaches a selected level, and an outlet flow control device is utilized to urge distillate from the lubricant still when a concentration of lubricant in the distillate reaches a selected concentration level. 
         [0005]    In another embodiment, a method of lubricant management in a heating ventilation and air conditioning (HVAC) system includes flowing a volume of a compressor lubricant and refrigerant mixture from an evaporator into a lubricant still and stopping the flow of the compressor lubricant and refrigerant mixture into the lubricant still when the mixture fills the lubricant still to a selected level. Compressor lubricant is distilled from the mixture via a thermal energy exchange, and the distillation is stopped when a concentration of compressor lubricant in the lubricant still exceeds a predetermined concentration level. The distillate is urged from the lubricant still. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a schematic view of an embodiment of a heating, ventilation and air conditioning system; and 
           [0009]      FIG. 2  is a schematic view of an embodiment of an oil management system for an HVAC system. 
       
    
    
       [0010]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing. 
       DETAILED DESCRIPTION 
       [0011]    Shown in  FIG. 1  is a schematic view an embodiment of a heating, ventilation and air conditioning (HVAC) unit, for example, a chiller  10  utilizing a falling film evaporator  12 . A flow of vapor refrigerant  14  is directed into a compressor  16 , such as a centrifugal or screw compressor, and then to a condenser  18  that outputs a flow of liquid refrigerant  20  to an expansion valve  22 . The expansion valve  22  outputs a vapor and liquid refrigerant mixture  24  to, in some embodiments, an economizer  26  and then to a separator  28 , in which portions of vapor refrigerant are separated from liquid refrigerant and returned to the compressor  16 . The liquid refrigerant output by the separator  28  is routed to the evaporator  12 . It is to be appreciated that, in other embodiments, the vapor and liquid refrigerant mixture  24  may be routed directly to the evaporator  12  from the expansion valve  22 . 
         [0012]    A thermal energy exchange occurs between a flow of heat transfer medium flowing through a plurality of evaporator tubes  30  into and out of the evaporator  12  and the liquid refrigerant  20  flowing over the evaporator tubes  30  and into a refrigerant pool  32 , such as in a falling film evaporator, shown. In other embodiments, the evaporator  12  is a flooded evaporator where the evaporator tubes  30  are submerged in the refrigerant pool  32 . As the liquid refrigerant  20  is boiled off in the evaporator  12 , the vapor refrigerant  14  is directed to the compressor  16 . 
         [0013]    The compressor  16  requires a flow of lubricant, such as oil or other liquid lubricant, therethrough to prevent overheating and damage to the compressor  16 . Oil is provided from an oil sump  34  to the compressor  16 . As the compressor  16  operates, a portion of the oil becomes mixed with or entrained in the flow of refrigerant through the chiller  10 . It is desirable to prevent depletion of the oil supply in the oil sump  34  and prevent buildup of oil in the evaporator  12 , which negatively affects evaporator  12  and chiller  10  performance. 
         [0014]    Referring now to  FIG. 2 , and embodiment of an oil management system  36  is shown with the chiller  10 . The oil management system  36  includes an oil still  38 , with an ejector  40  operated intermittently to reduce oil content in the evaporator  12 , while reducing energy consumption of the chiller  10 , compared to prior art chillers having a continuously operating ejector. To begin a cycle of the oil management system  36 , evaporator valve  42  is opened allowing a flow of refrigerant and oil mixture  44  to flow into and fill the oil still  38 , typically via gravity. Evaporator valve  42  is then closed. Oil still valve  46  is opened, forcing warm liquid refrigerant  20  to flow from the condenser  18  to a still heat exchanger  48 , for example a coil. It should be appreciated that hot gas refrigerant  14  from the compressor  16  may be used in place of warm liquid refrigerant  20 . As the liquid refrigerant  20  flows through the still heat exchanger  48 , the refrigerant and oil mixture  44  boils. The liquid refrigerant  20 , after flowing through the still heat exchanger  48  is subcooled by the process and flowed into the separator  28 , or alternatively the evaporator  12 , through the oil still valve  46 . The boiling process in the oil still  38  results in vapor refrigerant, which is vented to the evaporator  12  via still vent  50 . After venting the vapor refrigerant to the evaporator, a high-concentration oil mixture  52 , for example, over  50 % oil, remains in the oil still  38 . When a preset time interval is reached or temperature and/or pressure, or level in the still indicates a high oil concentration, the oil still valve  46  is closed to stop the flow from the condenser  18  to the oil still  38 . The opening and/or closing of valves  46  and  42  may be controlled by, for example, a timer or by a temperature and/or pressure sensor in the oil still  38 . The oil mixture  52  is returned to the compressor  16  by opening an ejector valve  54  to direct compressor discharge gas  56  into the ejector  40 , thereby drawing the oil mixture  52  from the oil still  38  and urging the oil mixture  52  to the compressor  16 . Once the oil mixture  52  is discharged to the compressor  16 , operation of the ejector  40  is stopped by closing the ejector valve  54 . As above, opening and closing of the ejector valve  54  may be done via a timed operation, by sensing an oil level in the oil still  38 , or the like. It should be understood that an oil pump may be used in lieu of an ejector provided that the cost impact to the system is not unfavorable. 
         [0015]    Further, in some embodiments, the frequency of operation of the oil management system  36  may be determined by a need to control an oil concentration in the evaporator  12  around a predetermined set point, for example, about  1 % concentration of oil in the evaporator  12 . In such embodiments, a sensor  58  located in the evaporator  12 , for example, a temperature and pressure sensor, is utilized to determine the oil concentration in the evaporator  12 . It is to be appreciated that other measurements, such as a refractive index measurement, may be used to determine the oil concentration in the evaporator  12 . If the oil concentration exceeds the set point, the operation of the oil management system  36  is triggered by the sensor  58  or other means. Similarly, when the oil concentration no longer exceeds the set point, operation of the oil management system  36  is stopped. 
         [0016]    Intermittent operation of the ejector  40 , as described above, increases chiller  10  performance over prior art systems with continuously operation ejectors, as discharge gas  56  is only routed to the ejector  40  when needed, and can thus flow to the condenser  18  when the ejector valve  54  is closed. Further, the reduction in oil concentration at the evaporator  12  allows for increased evaporator efficiency, which can translate into reduced material costs for the evaporator  12  since comparable chiller  10  performance can be achieved with a smaller evaporator  12 . In some embodiments, chiller  10  energy consumption is reduced by about 0.5 to 1.5% compared to prior art systems with an additional 1% benefit for low pressure systems, those using refrigerant having a liquid phase saturation pressure below about 45 psi (310.3 kPa) at 104° F. (40° C.). An example of low pressure refrigerant is R245fa. Further, in some embodiments, evaporator  12  oil concentrations can be maintained under about 1%, translating into a material savings for evaporator  12  of between about 1% and about 4%. 
         [0017]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.