Oil Management in Refrigeration Systems

This application is a continuation application of and claims the benefit of priority to U.S. application Ser. No. 17/467,630, filed on Sep. 7, 2021, the contents of which is hereby incorporated by reference.

This disclosure relates to refrigeration systems, and particularly to oil management in refrigeration systems.

Refrigeration systems are used to cool spaces such as refrigerators, display cases, coolers, and freezers. Refrigeration systems rely on refrigeration cycles of a refrigerant that alternately absorbs and rejects heat as the refrigerant is circulated through the system. Refrigeration systems include one or more compressors that compress the working fluid to increase the pressure of the fluid as part of the refrigeration cycle. Compressors may use oil for different purposes, such as to lubricate components of the compressor. The oil can mix with the working fluid and leave the compressor, which can affect the operation of the compressor and reduce the heat transfer and energy efficiency of the working fluid. The refrigeration system can use different piping configurations to return the oil to the compressor. Methods and equipment for returning the oil to the compressor are sought.

Implementations of the present disclosure include a refrigeration assembly that includes a receiver tank, a heat exchanger, a first piping assembly, and a second piping assembly. The receiver tank has a fluid outlet and a fluid inlet that receives a working fluid. The heat exchanger is disposed within the receiver tank. The heat exchanger has coiled tubing that is fluidly coupled to the fluid inlet and to the fluid outlet. The first piping assembly is disposed between and is fluidly coupled to the fluid inlet and the coiled tubing. The first piping assembly has a first double riser and a first P-trap. The second piping assembly is disposed between and is fluidly coupled to the fluid outlet and the coiled tubing. The second piping assembly includes a second double riser and a second P-trap.

In some implementations, the working fluid includes a mixture of refrigerant and oil, and the first P-trap and the second P-trap are configured to retain oil accumulated during flowing of the refrigerant through the refrigeration assembly. In some implementations, each of the first piping assembly and the second piping assembly flow, during different load conditions of the refrigeration assembly, the oil from the respective P-traps toward the fluid outlet of the heat exchanger coil. In some implementations, the coiled tubing has a first end attached to the first double riser and a second end attached to the second double riser. The first end resides at a first elevation and the second end resides at a second elevation lower than the first elevation.

In some implementations, the first double riser flows oil received from the first P-trap to the coiled tubing. The second P-trap receives oil from the coiled tubing. The second double riser flows oil received from the second P-trap to the fluid outlet of the heat exchanger coil.

In some implementations, the refrigeration assembly operates under a first load condition and a second load condition higher than the first load condition. The first riser of the first double riser increase a flow speed of the working fluid when the first P-trap is substantially blocked by accumulated oil during the first load condition. A second riser of the second double riser increases a flow speed of the working fluid when the second P-trap is substantially blocked by accumulated oil during the first load condition.

In some implementations, the first P-trap retains oil received from the fluid inlet during a low-load condition of the refrigeration assembly, and the second P-trap retains oil received from the coiled tubing during the low-load condition of the refrigeration assembly.

In some implementations, the fluid inlet is fluidly coupled to a supply suction line that has a first diameter. The fluid outlet is fluidly coupled to a return suction line that has a second diameter substantially equal to the first diameter.

In some implementations, the first double riser and the second double riser each have a first riser that has a first diameter and a second riser that has a second diameter larger than the first diameter. The second riser has the respective P-trap, and each of the first riser and second riser are attached to the respective fluid outlet or fluid inlet of the heat exchanger coil.

In some implementations, the receiver tank includes a flash tank of a COrefrigeration assembly. The heat exchanger coil flows COas refrigerant. The receiver tank retains a liquid phase of the COrefrigerant in thermal contact with the heat exchanger coil. The fluid inlet of the flash tank receives COrefrigerant from one or more evaporators, and the fluid outlet routes the COrefrigerant to one or more compressors.

In some implementations, the first piping assembly and the second piping assembly are in thermal contact with the fluid inside the receiver tank such that the working fluid flowing through the first piping assembly and the second piping assembly transfers heat to the liquid inside the receiver tank or the liquid inside the receiver tank transfers heat to the first piping assembly and the second piping assembly.

Implementations of the present disclosure include a refrigeration assembly that includes a receiver tank and a heat exchanger. The receiver tank defines a volume that retains a liquid. The heat exchanger is disposed within the receiver tank and is in thermal contact with the liquid. The heat exchanger directs a working fluid there through and transfers heat from the working fluid to the liquid or vice versa. The heat exchanger includes coiled tubing, a fluid inlet, a piping assembly, and a fluid outlet. The fluid inlet is fluidly coupled to the coiled tubing and is configured to receive the working fluid. The piping assembly is disposed between and is fluidly coupled to the fluid inlet and the coiled tubing. The piping assembly has a riser and an oil trap. The fluid outlet is fluidly coupled to the coiled tubing. The fluid outlet directs the working fluid received from coiled tubing out of the receiver tank.

In some implementations, the riser includes a second coiled tubing in thermal communication with the liquid inside the flash tank. The second coiled tubing is disposed between the fluid inlet and the fluid outlet of the heat exchanger.

In some implementations, the oil trap is disposed downstream of the riser and resides between the riser and the coiled tubing.

In some implementations, the riser is attached, at a fluid connection, to a pipe connected to the outlet. The fluid connection is disposed between the fluid outlet and the coiled tubing.

In some implementations, the working fluid includes a mixture of refrigerant and oil. The oil trap retains oil accumulated during flowing of the refrigerant through the heat exchanger. The piping assembly directs, during different load conditions of the refrigeration assembly, the oil from the oil trap toward the fluid outlet of the heat exchanger. In some implementations, the coiled tubing includes a first end attached to the fluid outlet and a second end attached to the fluid inlet. The first end resides at a first elevation and the second end resides at a second elevation lower than the first elevation. The riser flows oil received from the oil trap to the fluid outlet of the heat exchanger.

In some implementations, the refrigeration assembly includes a second piping assembly attached to and residing between the coiled tubing and the fluid outlet. The first piping assembly is attached to and residing between the coiled tubing and the fluid inlet. The first piping assembly includes a second riser attached to the oil trap, and the second piping assembly including a second oil trap, a third riser, and a fourth riser attached to the oil trap.

In some implementations, the refrigeration assembly operates under a first load condition and a second load condition higher than the first load condition. The first riser increases a flow speed of the working fluid with the first P-trap substantially blocked by accumulated oil during the first load condition. The second riser increases a flow speed of the working fluid with the second P-trap substantially blocked by accumulated oil during the first load condition.

In some implementations, the fluid inlet is fluidly coupled to a supply suction line that has a first diameter. The fluid outlet is fluidly coupled to a return suction line that has a second diameter substantially equal to the first diameter.

Particular implementations of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. For example, the refrigeration assembly of the present disclosure can increase the heat transfer area in a flash tank coil while increasing the flow rate of the oil to the compressor and minimizing the pressure drop of the working fluid throughout the system. Additionally, the refrigeration assembly can keep the superheat stable with proper heat transfer.

Oil logging in the suction lines of a refrigeration systems may be common during low-load operating conditions (e.g., during winter months and at night). To reduce or prevent oil logging in the suction lines and to increase the heat transfer area of a receiver tank, a refrigeration assembly with one or more risers and P-traps inside the receiver tank can be implemented.

shows a schematic diagram (e.g., a piping and instrumentation diagram) of a refrigeration system. The refrigeration systemcan be e.g., a basic commercial COrefrigeration system, an ammonia refrigeration system, or a chilled water refrigeration system. The refrigeration systemincludes a compressoror group of compressors (e.g., transcritical compressors, subcritical compressors, or a combination of the two), one or more gas coolers or condensers, a receiver tank, and an evaporatoror group of evaporators (e.g., medium-temperature display cases, low-temperature display cases, or a combination of the two).

In the example of a COrefrigeration system, the compressorscan flow a medium-temperature discharge working fluid (e.g., CO) in vapor or gas phase to the gas cooler. The gas coolercondenses or cools the medium-temperature working fluid. The vapor or liquid vapor mixture phase of the working fluid flows from the gas coolerto the receiver tank. In some implementations, the liquid vapor mixture phase of the working fluid can flow through a valve(e.g., a high-pressure control valve) that lowers the pressure of the liquid vapor mixture phase of the working fluid before it reaches the receiver tank.

At the receiver tank, the liquid phase of the working fluid (e.g., high-pressure fluid) accumulates at the bottom of the tankand the vapor phase (e.g., medium temperature suction gas) of the working fluid rises to the top of the tank. The medium temperature suction gas can be released to the ambient or directed to another component of the refrigeration system. For example, the medium temperature suction gas can be conveyed from the receiver tank, through a gas line, to the compressors. The gas linecan included a valve(e.g., a flash gas bypass valve) that regulates the pressure of the gas.

The liquid phase of the working fluid flows from the receiver tank, through a liquid line, to the evaporators. The liquid lineincludes an expansion valvethat decreases the pressure of the liquid phase of the working fluid before the working fluid reaches the evaporators. The evaporatorsreceive the working fluid (e.g., a liquid vapor mixture of the working fluid) from the expansion valveto transfer heat to the working fluid. The working fluid evaporates in the evaporators. The vapor phase of the working fluid flows back from the evaporators, through a suction line, to the flash tankand then to the compressors.

The suction lineincludes a supply linethat supplies the working fluid to the tankand a return linethat returns or flows the working fluid from the tankto the compressors. As further described in detail below with respect to, the suction lineincludes or is connected to a heat exchangerdisposed inside the receiver tank. The working fluid inside the heat exchangertransfers heat through a heat transfer surfaceof the heat exchanger to the liquid or condensate inside the tank. The gas phase of the working fluid flows from the tankto the compressors. An oil separatorcan help convey oil back to the compressors, but the oil that escapes the separator can accumulate in the suction linesduring low load conditions of the system. As further described in detail below with respect to, the heat exchangerinside the tank can help flow oil in the suction linesback to the compressors.

depicts a refrigeration systemaccording to a different implementation of the present disclosure. The refrigeration systemis similar to the refrigeration systemin, with the exception of separate groups of evaporators and respective compressors. For example, the refrigeration systemincludes one or more medium-temperature evaporators(e.g., medium-temperature display cases) and one or more low-temperature evaporators(e.g., low-temperature display cases). The medium-temperature evaporatorscan include, for example, refrigerated display cases that display medium-temperature merchandise such as non-frozen products, and the low-temperature display casescan include, for example, refrigerated display cases that display low-temperature merchandise such as frozen products.

The refrigeration systemalso includes one or more transcritical compressorsand one or more subcritical compressorsThe subcritical compressorsreceive a vapor phase of the working fluid from the low-temperature evaporatorsThe transcritical compressorsreceive a vapor phase of the working fluid from the medium-temperature evaporatorsand from the subcritical compressorsThe low-temperature suction lineof the low-temperature evaporatorsis connected to the receiver tank.

For example, medium-temperature discharge gas (or liquid and gas) flows from the condenserto the receiver tank. A first portion of the liquid phase of the working fluid flows from the tankto the low-temperature evaporators(passing first through expansion valves). A second portion of the liquid phase of the working fluid flows from the tankto the medium-temperature evaporatorsAfter passing through the low-temperature evaporatorsthe working fluid, as a low-temperature suction gas, flows through the low-temperature suction lineto the receiver tank, and from the tankto the subcritical compressorsThe suction linecan include an accumulatorthat can meter or prevent the flow of fluid refrigerant and oil back to the compressorsThe working fluid, as a low-temperature discharge gas, flows from the subcritical compressorsto mix with the medium temperature suction gas that flows from the medium-temperature evaporatorsto the transcritical compressorsThe medium temperature suction gas flows through a medium temperature suction lineto the transcritical compressors

depicts a refrigeration systemsimilar to the refrigeration systemin, with the exception of the medium-temperature suction lineof the medium-temperature evaporatorsbeing connected to the receiver tank. For example, the low-temperature suction lineextends from the low-temperature evaporatorsto the subcritical compressorswithout passing through the receiver tank. The medium-temperature suction lineincludes the heat exchangerinside the tank.

depicts a refrigeration systemsimilar to the refrigeration systemsandinrespectively, with the exception of having both suction linesandconnected to the receiver tank. The medium-temperature suction lineis connected to a first heat exchangerdisposed inside the receiver tank. The low-temperature suction lineis connected to a second heat exchangerdisposed inside the receiver tank. Both heat exchangersandcan transfer heat to the working fluid inside the receiver tank.

depicts a refrigeration assemblyaccording to implementations of the present disclosure. The refrigeration assemblyincludes the receiver tank(e.g., a receiver flash tank or vessel or liquid vapor separator) and a heat exchanger(e.g., a heat exchanger coil) disposed within the receiver flash tank. The flash tankdefines an interior volume “V” that retains or stores a first working fluid “F” (e.g., high-pressure condensate). The first working fluid (liquid or liquid vapor mixture) is received into the receiver tankthrough a fluid inlet port. The first working fluid (liquid or liquid vapor mixture) exits the receiver tankthrough a fluid outlet port. The first working fluid “F” can include a liquid-vapor mixture, with the liquid stored at the bottom of the receiver tankto contact the heat exchanger.

The heat exchangeris in thermal communication (e.g., thermal contact) with the first working fluid “F.” For example, the heat exchangeis in thermal communication with the liquid within the tank. The heat exchangertransfers heat from a second working fluid “F” to the first working fluid “F,” and vice versa. For example, as the second working fluid “F” flows along the piping of the heat exchanger, at least a portion of the first fluid “F” can condense and flow down as liquid. In some implementations, the fluid “F” can sub cool the fluid “F” and portion of the vapor phase of the fluid “F.”

The heat exchangerincludes coiled tubing, a fluid inletfluidly coupled to the coiled tubing, and a fluid outletfluidly coupled to the coiled tubing. For example, the fluid inletis fluidly coupled with the coiled tubingby being arranged to communicate the second working fluid “F” to the coiled tubing. Likewise, the fluid outletis arranged to receive the second working fluid “F” from the coiled tubing. The heat exchangeralso includes a first piping assemblythat resides between and that is fluidly coupled to the fluid inletand the coiled tubing.

The second working fluid “F” can include a refrigerant (e.g., CO, ammonia, R134a, water, or a combination of the four) and oil from the compressor. During low-load conditions of the system, the oil may log in the heat exchanger. As further described in detail below with respect to, the piping assemblyhelps flow accumulated or logged oil back to the compressor by implementing a double riser configuration that increases the velocity of the second working fluid during low-load conditions. Because the first piping assemblyis disposed inside the flash tank, the first piping assemblyis in thermal contact with the liquid or vapor or liquid vapor mixture phase of the first working fluid “F” inside the tank. Such configuration increases the heat transfer area of the piping assemblyinside the tank. By increasing the heat transfer area inside the tank, the heat transferred between the second fluid and the first fluid can be incremented, increasing the efficiency of the refrigeration cycle.

The heat exchangercan also include a second piping assemblythat resides between and that is fluidly coupled to the fluid outletand the coiled tubing. The second piping assemblyhelps flow accumulated oil back to the compressor by increasing the velocity of the second fluid “F.” Because the second piping assemblyis disposed inside the flash tank, the second piping assemblyis in thermal contact with the liquid or vapor or liquid vapor mixture phase of the first working fluid “F” inside the tank. Such configuration further increases the heat transfer area of the piping assemblyinside the tank. The first and second piping assembliesandincrease the heat transfer surface or area of the heat exchangerto more effectively transfer heat to and from the first working fluid “F.”

For example, the temperature in a superheat state of the working fluid “F” at the inletmay not be stable and varies due to display case operating conditions (low super heat in most cases), which can damage the compressors. The heat transfer between the working fluids “F” and “F” inside the tank helps to maintain stable temperature/superheat at the outletof the fluid F.

The two piping assembliesandcan be different from each other. For example, the working fluid can enter the heat exchangerthrough the inletat the bottom and the fluid flows up through the inlet double riser to enter the coil tubingat the top. The working fluid flows downward through the coil tubingand to the outlet double riser. The two double risers can be designed such that the working fluid is generally always flowing through the coilso that the full heat transfer takes place. The two double risers can increase the velocity at both the inletand the outletto carry the oil back to the compressors during low-load conditions.

The fluid inletof the receiver tankis attached to and is in fluid communication with supply suction line. The supply suction lineextends from the outlet of an evaporator or display cases or coolers or freezers to the receiver tank. The fluid outletof the receiver tankis attached to and in fluid communication with a return suction line. For example, the return suction linedirects the second working fluid “F” received from the outletof the receiver tankto compressor(s).

In some implementations, the suction linesandcan be sized to maintain the second working fluid “F” flowing at a desired velocity to achieve the desired flow rate of the oil back to the compressor. In some implementations, the first and second piping assembliesandcan flow accumulated fluid/gas back to the compressor while minimizing a pressure drop across the heat exchanger, which allows the suction pipesandto have equal or similar sizes. For example, the supply suction linehas a first diameter (e.g., internal diameter) “d” and the suction linecan have a second diameter (e.g., internal diameter) “d” that is different or substantially equal to the first diameter “d.”

In some implementations, the receiver tankcan be a flash tank of a COrefrigeration assembly. For example, the second working fluid “F” flown in the heat exchanger coilcan include COvapor and the first working fluid “F” in thermal contact with the heat exchanger coilcan include COin liquid or liquid vapor mixture phase.

show the configuration of the two piping assembliesandthat are disposed inside the receiver tank. In some implementations, the heat exchangercan only include one piping assembly. The two piping assembliesandtogether help flow accumulated oil back to the compressor. For example, as depicted in, the first piping assemblycan include a first double riserand a first P-trap or oil trap. The first piping assemblyresides between and is in fluid communication with the fluid inletand the coiled tubing. The second piping assemblycan include a second double riserand a second P-trap or oil trap. The second piping assemblyresides between and is in fluid communication with the fluid outletand the coiled tubing.

The working fluid “F” may include a mixture of refrigerant and oil that, during low-load conditions, may leave behind the oil which then accumulates along the tubing (e.g., due to the relatively low velocity of the refrigerant). The refrigeration systemcan be considered to run at low-load conditions when the system operates at about 5% to 20% of the total load capacity. For example, if the refrigeration systemis designed to remove the heat load of 100,000 BTUs per hour (BTUH), then from about 5,000 BTUH to 20,000 BTUH is considered as low load. During this time, not all compressors will run but one compressor may run at low speed. The first P-trapand the second P-trapretain oil as the refrigerant flows through the heat exchangerduring low-load conditions. For example, the first P-trapcan retain oil received from the fluid inlet, and the second P-trapcan retain oil received from the coiled tubing.

The coiled tubinghas a first endattached to the first double riserand a second endattached to the second double riser. The first endis positioned vertically above the second end. For example, the first endis arranged at a first elevation and the second endis arranged at a second elevation lower than the first elevation.

Each of the first and second double risersandcan include a main riser (e.g., a first riser) and a secondary riser (e.g., a second riser). In some implementations, the main riser can be smaller than the secondary riser. For example, the first double riserincludes a first riserand a second riser. The second risercan include the first P-trap. The first riseris attached to and in fluid communication with the second riser. The second double riserincludes a third riserand a fourth riser. The fourth risercan include the second P-trap. The third riseris attached to and in fluid communication with the fourth riser.

The first risercan have a first inner diameter and the second risercan have a second inner diameter larger than the first inner diameter. Similarly, the third risercan have a third inner diameter and the fourth risercan have a fourth inner diameter larger than the third inner diameter. For example, the first risercan have a diameter of about ⅜ inch to 2⅛ inches, and the second risercan have a diameter of about ½ inch to 2⅝ inches. Similarly, the third risercan have an inner diameter of about ⅜ inch to 2⅛ inches, and the fourth risercan have an inner diameter of about ½ inch to 2⅝ inches. The size (e.g., inner diameters) of the double risers and the coiled tubingcan be oversized to use uniform sizes (e.g., reduce the changes in sizing) across the heat exchanger. The size of the heat exchanger can be designed to keep, for example, during normal load conditions, the velocity of the second fluid “F” at about 1200 feet per minute to return the oil to the compressor.

During full load or normal load conditions, the refrigerant and oil mixture enters the inletand most or all of the fluid/gas and oil mixture flows through the first P-trap, up the second riser, and then through the first double riser. In some implementations, part of the fluid/gas and oil mixture can flow through the first riserand then enter the coil tubingat the inletof the coil tubing. The mixture flows downwards through the coil tubingand exits the coil tubingthrough the outletof the coil tubing. Then, most or all of the fluid/gas and oil mixture flows through the second P-trap, up the fourth riser, and through the second double riser. A part of the fluid/gas and oil mixture flows through the third riserand exits at the outlet.

During partial/low load condition, the fluid/gas and oil mixture enters through the inletand flows to the first P-trap. Due to the low velocity of the mixture, oil accumulates at the P-trapand blocks the gas flow through the first P-trap, which forces the mixture to flow through the first riser. Because the first riseris smaller in diameter when compared to, the mixture increases in velocity through the first riser, thereby carrying the oil to the compressor(s). Similarly, when the mixture enters the second P-trap, oil accumulates in the P-trapand blocks the flow of mixture through the fourth riser. The blockage forces the mixture to flow through the third riserto exit through the fluid outlet. Because the third riserhas a smaller diameter compared to the fourth riser, the mixture increases in velocity and carries the oil to the compressor(s).

In some implementations, when the load increases, the pressure of the mixture is high enough to push the oil from the P-trapsandup the large risersand. In some implementations, when the load increases, the piping assembliesandcan create a pressure differential to drag or suck the oil up the large risersanduntil the larger pipe is unclogged, which allows the system to working normally again.