Patent Publication Number: US-10309698-B2

Title: Oil return management in a HVAC system

Description:
FIELD 
     The disclosure herein relates to heating, ventilation, and air-conditioning (“HVAC”) systems, such as may include a chiller. Generally, methods, systems, and apparatuses are described that are directed to oil return management in the HVAC systems. 
     BACKGROUND 
     A HVAC system can typically include a compressor, heat exchangers such as a condenser and an evaporator, and an expansion device forming a refrigeration circuit. Refrigerant vapor is generally compressed by the compressor, and then condensed into liquid refrigerant in the condenser. The liquid refrigerant is then expanded by the expansion device to become low-pressure low-temperature two-phase refrigerant and is directed into the evaporator; and the two-phase refrigerant can then exchange heat with a process fluid, such as air or water, in the evaporator. The two-phase refrigerant may be vaporized in the evaporator and return to the compressor. The process fluid may then be used for other purposes, such as for example cooling a space of a building. 
     The compressors of the HVAC system, such as a screw compressor, may be lubricated, for example, by oil. The oil can circulate in the refrigerant circuit along with the refrigerant. 
     SUMMARY 
     Embodiments of managing oil return in a HVAC system are provided. Generally, a refrigerant/oil mixture in an evaporator can be directed out of the evaporator into an oil return heat exchanger configured to help vaporize a refrigerant portion of the refrigerant/oil mixture with heat energy. The vaporized refrigerant portion can then entrain the oil portion of the refrigerant/oil mixture to drive the oil portion back to, for example, the compressor. In some embodiments, superheat refrigerant vapor from a condenser can be directed into the oil return heat exchanger as the heat energy to vaporize the refrigerant portion in the refrigerant/oil mixture. 
     In some embodiments, a HVAC system may include an oil return heat exchanger that has an evaporator side configured to receive a refrigerant/oil mixture from the evaporator and a condenser side configured to receive superheat refrigerant vapor from the condenser. The evaporator side and the condenser side may be configured to exchange heat in the oil return heat exchanger. 
     In the evaporator side of the oil return heat exchanger, a refrigerant portion of the refrigerant/oil mixture from the evaporator may be vaporized so that an oil portion of the refrigerant/oil mixture may be entrained by the vaporized refrigerant portion. In some embodiments, the refrigerant/oil mixture flowing out of the oil return heat exchanger may be directed into the compressor by, for example, suction of the compressor. 
     In the condenser side of the oil return heat exchanger, the superheat refrigerant vapor may be condensed into liquid refrigerant. In some embodiments, the condensed liquid refrigerant may be directed back to the condenser after flowing out of the oil return heat exchanger. 
     In some embodiments, the oil return heat exchanger may be physically positioned lower than the evaporator so that gravity can help direct the refrigerant/oil mixture to the oil return heat exchanger. In some embodiments, the evaporator side of the oil return heat exchanger may have an evaporator-side inlet and an evaporator-side outlet, and the evaporator-side outlet may be positioned physically higher than the evaporator-side inlet. 
     In some embodiments, the condenser side of the oil return heat exchanger may have a condenser-side inlet and a condenser-side outlet, and the condenser-side outlet is positioned physically higher than the condenser-side inlet. 
     In some embodiments, the condenser has a condenser oil return heat exchanger outlet and a condenser oil return heat exchanger inlet, and the condenser oil return heat exchanger outlet is positioned physically higher than the condenser oil return heat exchanger inlet. 
     In some embodiments, the oil return heat exchanger may be a brazed-plate heat exchanger. In some embodiments, the compressor may be a screw compressor. 
     A method of managing oil return in a HVAC system may include directing superheat refrigerant vapor into a first side of an oil return heat exchanger, directing a refrigerant/oil mixture into a second side of the oil return heat exchanger and directing the refrigerant/oil mixture flowing out of the oil return heat exchanger to a compressor of the HVAC system. The oil return heat exchanger may be configured to exchange heat between the superheat refrigerant vapor and the refrigerant/oil mixture in the oil return heat exchanger. In some embodiments, the method of managing oil return to the compressor in a HVAC system can include preventing the superheat refrigerant vapor flowing into the first side of the oil return heat exchanger when the HVAC system is operated at a full load condition or relatively high saturated evaporator temperature. 
     Other features and aspects of the embodiments will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the drawings in which like reference numbers represent corresponding parts throughout. 
         FIG. 1  illustrates a schematic diagram of a HVAC system including an oil return heat exchanger, according to one embodiment. 
         FIG. 2  illustrates a schematic diagram of a HVAC system including an oil return heat exchanger, according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A compressor of a HVAC system may be lubricated with oil. In some HVAC systems, such as may include a chiller system, the oil of the compressor may circulate with the refrigerant in the refrigeration circuit, which is typically formed by the compressor, a condenser, an evaporator and an expansion device. Managing oil return to the compressor in the refrigeration circuit may be important to keep a proper oil level in the compressor to, for example, lubricate moving parts of the compressor. If the oil level in the compressor is too low, the compressor may be damaged due to lack of lubrication. Improvements can be made to help manage oil return to the compressor so that the oil level in the compressor can be kept at the proper level. 
     The embodiments as disclosed herein relate to methods, systems, and apparatuses that help manage oil return in, for example, a chiller. The chiller may include a condenser and an evaporator. In some embodiments, a refrigerant/oil mixture can be directed out of the evaporator into an oil return heat exchanger that is configured to help vaporize a refrigerant portion of the refrigerant/oil mixture. In some embodiments, superheat refrigerant vapor from the condenser can be directed into the oil return heat exchanger so that the heat energy of the superheat refrigerant vapor can be used to vaporize the refrigerant portion in the refrigerant/oil mixture. In some embodiments, the oil return heat exchanger can be physically positioned lower than the evaporator so that gravity can help the refrigerant/oil mixture to flow into the oil return heat exchanger. The embodiments as disclosed herein can help vaporize the refrigerant portion in the refrigerant/oil mixture so that an oil portion in the refrigerant/oil mixture can be entrained in the vaporized refrigerant portion and directed, for example, to the compressor by using for example suction of the compressor. In some embodiments, the refrigerant portion may be largely vaporized and the oil portion may be directed to the compressor as oil droplets entrained in the vaporized refrigerant. The embodiments as disclosed herein may also help increase heat transfer efficiency of the evaporator, and or the capacity/efficiency of the compressor. 
     References are made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration of the embodiments in which the embodiments may be practiced. It is to be understood that the terms used herein are for the purpose of describing the figures and embodiments and should not be regarded as limiting the scope of the present application. 
       FIG. 1  illustrates a HVAC system  100  that includes a compressor  110 , a condenser  120 , an expansion device  130  and an evaporator  140 , forming a refrigeration circuit. Refrigerant can be compressed by the compressor  110  and condensed into liquid refrigerant in the condenser  120 . The liquid refrigerant can be expanded by the expansion device  130  then be directed into the evaporator  140  to exchange heat with a process fluid (e.g. air or water). The process fluid can then be used for other applications, such as for example cooling a space of a building. The refrigerant can then return to the compressor  110  to be compressed. The HVAC system  100  may include other components, such as an oil separator  115 , unit controller (not shown) and other components as may be typically employed in a chiller. 
     Oil that lubricates the compressor  110  can be circulated in the refrigerant circuit with the refrigerant. Oil typically has a higher saturation temperature than the refrigerant and typically is in a liquid state when being circulated in the refrigerant circuit with the refrigerant. The evaporator  140 , which may be a falling film type or a flooded type evaporator may tend to collect a relatively large amount oil inside the evaporator  140  under certain conditions. 
     In a falling film type of evaporator, the refrigerant can be vaporized in the evaporator  140 . An oil portion, which may be circulated along with the refrigerant, generally is not vaporized. The oil portion may therefore accumulate inside the evaporator  140 . In a flooded type of evaporator, the evaporator  140  may include a relatively large amount of oil/refrigerant mixture to submerge heat exchange tubes (not shown) in the evaporator  140 . Managing oil return in these types of evaporators may be important for maintaining a proper oil level in the compressor  110  for proper lubrication of the compressor  110 . If the oil is collected in the evaporator  140  and does not return to the compressor  110 , the oil level in the compressor  110  may get low, causing damages to the compressor  110 . The oil collected in the evaporator  140  may also decrease heat exchange efficiency of the evaporator  140 . 
     To help oil collected in the evaporator  140  return to the compressor  110 , the refrigerant/oil mixture in the evaporator  140  can be directed into an oil return heat exchanger  150 . The oil return heat exchanger  150  is generally configured to have a condenser side  156  and an evaporator side  157 . The condenser side  156  is generally configured to receive, for example, refrigerant from the condenser  120  as a heat source to exchange heat with a refrigerant/oil mixture received by the evaporator side  157  from the evaporator  140 . As a result, a refrigerant portion of the refrigerant/oil mixture may be vaporized in the oil return heat exchanger  150 . 
     The heat energy source of the oil return heat exchanger  150  can be superheat refrigerant vapor from the condenser  120 . The superheat refrigerant vapor can be directed out of the condenser  120  from a condenser oil return heat exchanger outlet  122  into a condenser-side inlet  152  of the oil return heat exchanger  150 . The superheat refrigerant vapor can exchange heat with the refrigerant/oil mixture directed out of an evaporator oil return heat exchanger outlet  142  of the evaporator  140  into an evaporator-side inlet  153  of the oil return heat exchanger  150 . The superheat refrigerant vapor can help vaporize the refrigerant portion of the refrigerant/oil mixture inside the oil return heat exchanger  150 . 
     The oil generally has a higher saturation temperature than the temperature of the superheat refrigerant vapor. As a result, the oil portion of the refrigerant/oil mixture can remain in the liquid state after flowing out of the oil return heat exchanger  150 . In some embodiments, when the refrigerant/oil mixture flows out of an evaporator-side outlet  155  of the oil return heat exchanger  150 , the refrigerant portion of the refrigerant/oil mixture can be largely vaporized and the remaining refrigerant/oil mixture can be largely an oil portion in the liquid state. In some embodiments, after flowing through the oil return heat exchanger  150 , the refrigerant/oil mixture directed into the evaporator-side inlet  153  can mainly contain oil droplets at the evaporator-side outlet  155 . The oil portion droplets can be entrained into the compressor  110  by the vaporized refrigerant portion. This may help oil return to the compressor  110 . 
     In some embodiments, the evaporator oil return heat exchanger outlet  142  can be positioned at where an oil concentration in the evaporator  140  is relatively high. In a falling film evaporator, generally the lower portion of the evaporator  140  has a relatively high oil concentration. Accordingly, the evaporator oil return heat exchanger outlet  142  can be positioned on the lower portion of the evaporator  140 . In a flooded evaporator, the liquid level position inside the evaporator  140  has a relatively high oil concentration. Accordingly, the evaporator oil return heat exchanger outlet  142  can be positioned at about the liquid level position in the evaporator  140 . 
     In some embodiments, the oil return heat exchanger  150  can be physically positioned relatively lower than the evaporator  140 , so that gravity can help drain the refrigerant/oil mixture from the evaporator oil return heat exchanger outlet  142  into the oil return heat exchanger  150 . 
     In some embodiments, a density of the refrigerant/oil mixture at the evaporator-side outlet  155  is lower than the density of the refrigerant/oil mixture at the evaporator-side inlet  153 , which can create a pressure differential between the inlet  153  and the outlet  155 . The density/pressure differential of the refrigerant/oil mixture between the outlet  155  and the inlet  153  can help drive the refrigerant/oil mixture to flow from the inlet  153  toward the outlet  155 . In some embodiments, the evaporator-side inlet  153  can be physically positioned lower than the evaporator-side outlet  155 . 
     The condenser  120  includes an upper portion  123  and a lower portion  125 . The upper portion  123  may generally contain superheat refrigerant vapor and the lower portion  125  may generally contain liquid refrigerant. The upper portion  123  has a pressure P 1  that is higher than a pressure P 3  of the lower portion  124 . In some embodiments, the pressure differential between P 1  and P 3  is at or about 3 psi. 
     The condenser oil return heat exchanger outlet  122  can be generally positioned in the upper portion  123  of the condenser  120 , and the condenser oil return heat exchanger inlet  124  can be generally positioned in the lower portion  125  of the condenser  120 . The pressure differential between P 1  and P 3  can help drive superheat refrigerant vapor toward and pass through the oil return heat exchanger  150 . 
     In the oil return heat exchanger  150 , the superheat refrigerant vapor generally releases heat to the refrigerant/oil mixture from the evaporator  140 . As a result, the superheat refrigerant vapor can be condensed into liquid refrigerant, which can be directed back to the condenser oil return heat exchanger inlet  124 . 
     In some embodiments, the condenser-side inlet  152  of the oil return heat exchanger  150  can be physically positioned lower than the condenser-side outlet  154  of the oil return heat exchanger  150 . In some embodiments, a pressure P 2  at the condenser-side outlet  154  is generally smaller than the pressure P 1 . In some embodiments, a pressure differential between the pressure P 1  and the pressure P 2  is smaller than the pressure differential between the pressure P 1  and the pressure P 3 . As a result, the refrigerant can be driven from the condenser oil return heat exchanger outlet  122  into the condenser-side inlet  152  of the oil return heat exchanger  150  as a superheat vapor, then be driven back to the condenser oil return heat exchanger inlet  124  from the condenser-side outlet  154  as refrigerant liquid by the pressure differential. 
     The oil return heat exchanger  150  can be a brazed-plate heat exchanger, with the understanding that other types of the heat exchanger can also be used. The brazed-plate heat exchanger can be relatively compact, which may help, for example, a retrofit application of an existing HVAC system with the oil return heat exchanger  150 . 
     The heat exchanger capacity of the oil return heat exchanger  150  can be configured according to design specifications. In some embodiments, the heat exchanger capacity of the oil return heat exchanger  150  can be configured so that a designed specific oil circulation ration (OCR) (which is defined as the oil mass fraction in the compressor mass flow rate) can be achieved. In some embodiments, the OCR may be, for example, about 0.03%. In some embodiments, the heat exchanger capacity of the oil return heat exchanger  150  can be configured based on the OCR, a peak oil concentration (POC) (which is defined as the highest oil concentration in the refrigerant/oil mixture in the evaporator) in the evaporator  140  and a heat exchanger capacity of the evaporator  140 . In some embodiments, the heat exchanger capacity of the oil return heat exchanger  150  may be configured relative to the heat exchange capacity of the evaporator. In some embodiments, the heat exchanger capacity of the oil return heat exchanger  150  can be configured at about: (OCR)/(POC)*(the heat exchanger capacity of the evaporator  140 ). In some embodiments, the heat exchanger capacity of the oil return heat exchanger  150  may be about 0.5% to 1% of the heat exchanger capacity of the evaporator  140 . 
     The compressor  110  for the HVAC system  100  can be a screw compressor, a centrifugal compressor or other suitable compressors. These types of compressors may require oil for lubrication, and can generally benefit from the embodiments as disclosed herein. The screw compressor may require a relatively large amount of oil for lubrication, therefore the screw compressor may benefit from the embodiments as disclosed herein relatively more than other types of compressors. 
     The condenser  120  of the HVAC system  100  may be an air-cooled condenser or water-cooled condenser. In some embodiments, the condenser  120  can be a water-cooled shell-and-tube condenser. 
     As illustrated in  FIG. 2 , in some embodiments, a solenoid valve  260  can be positioned between a condenser oil return heat exchanger outlet  222  and a condenser-side inlet  252  of an oil return heat exchanger  250  of a HVAC system  200 . The solenoid valve  260  can be configured to have an “on” state that is configured to generally allow refrigerant vapor to flow from the condenser oil return heat exchanger outlet  222  to the condenser-side inlet  252 , and an “off” state that is configured to generally prevent refrigerant flow between the condenser oil return heat exchanger outlet  222  and the condenser-side inlet  252 . By regulating a period of time that the solenoid valve  260  stays in the “on” or “off” state, an amount of superheat refrigerant vapor directed to the oil return heat exchanger  250  can be regulated. By regulating the amount of superheat refrigerant vapor directed to the oil return heat exchanger  250 , the amount of heat energy directed into the oil return heat exchanger  250  can be controlled. As a result, the refrigerant/oil mixture flow in the oil return heat exchanger  250  can also be regulated. The operation of the solenoid valve  260  can be controlled by a controller  270 , with the understanding that the operation of the solenoid valve  260  can also be controlled manually or by other suitable controllers. 
     The solenoid valve  260  may help manage oil return in some operation conditions. For example, when the HVAC system  200  is operated with a relatively high load, such as at or about a full load condition or a relatively high saturated temperature in an evaporator  240 , the OCR in the condenser  220  and/or the evaporator  240  can be relatively low and more oil can return from evaporator  240  to the compressor  210 . In such a condition, the oil return to a compressor  210  may be sufficient to keep a proper oil level in the compressor  210 , thus keep the compressor  210  properly lubricated without engaging the oil return heat exchanger  250 . It may not be necessary to use the oil return heat exchanger  250  to help the oil return to the compressor  210 . The controller  270  can obtain the load condition or the saturated temperature in the evaporator  240  from, for example, a unit controller of the HVAC system  200 . When the controller  270  detects, for example, a full load condition, the controller  270  can set the solenoid valve  260  to the “off” state so as to generally prevent refrigerant flow between the condenser oil return heat exchanger outlet  222  and the condenser-side inlet  252 . 
     With regard to the foregoing description, it is to be understood that changes may be made in detail, without departing from the scope of the present invention. It is intended that the specification and depicted embodiments are to be considered exemplary only, with a true scope and spirit of the invention being indicated by the broad meaning of the claims.