Patent Publication Number: US-11656009-B2

Title: Cooling system with oil return to accumulator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/803,611 filed Feb. 27, 2020, by Shitong Zha et al., and entitled “COOLING SYSTEM WITH OIL RETURN TO ACCUMULATOR,” which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to a cooling system. 
     BACKGROUND 
     Cooling systems cycle refrigerant to cool various spaces. 
     SUMMARY 
     Cooling systems cycle refrigerant to cool various spaces. For example, in some industrial facilities, cooling systems cycle a primary refrigerant that cools secondary refrigerants. The secondary refrigerants are then cycled to cool different parts of the industrial facility (e.g., different industrial and/or manufacturing processes). These systems typically include a compressor to compress the primary refrigerant and a high side heat exchanger that removes heat from the compressed primary refrigerant. When the compressor compresses the primary refrigerant, oil that coats certain components of the compressor may mix with and be discharged with the primary refrigerant. 
     Depending on the nature of the primary refrigerant, the cooling system may be able to move the oil along with the primary refrigerant through the cooling system such that the oil is eventually cycled back to the compressor. However, when certain primary refrigerants (e.g., carbon dioxide) are used, the oil may get stuck in a portion of the cooling system (e.g., at a low side heat exchanger). As a result, the compressor(s) in the system begin losing oil, which eventually leads to breakdown or failure. Additionally, the components in which the oil gets stuck may also become less efficient as the oil builds in these components. 
     This disclosure contemplates unconventional cooling systems that drain oil from low side heat exchangers to vessels and then uses compressed refrigerant to push the oil in the vessels back towards a compressor. Generally, the cooling systems operate in three different modes of operation: a normal mode, an oil drain mode, and an oil return mode. During the normal mode, a primary refrigerant is cycled to cool one or more secondary refrigerants. As the primary refrigerant is cycled, oil from a compressor may mix with the primary refrigerant and become stuck in a low side heat exchanger. During the oil drain mode, the oil in the low side heat exchanger is allowed to drain into a vessel. During the oil return mode, compressed refrigerant is directed to the vessel to push the oil in the vessel back towards a compressor. In this manner, oil in a low side heat exchanger is returned to a compressor. Certain embodiments of the cooling system are described below. 
     According to an embodiment, a system includes a flash tank, a first low side heat exchanger, an accumulator, a first compressor, a second compressor, an oil reservoir, a first valve, a second valve, and a third valve. The flash tank stores a primary refrigerant. During a first mode of operation, the first and second valves are closed, the third valve is open, the first low side heat exchanger uses primary refrigerant from the flash tank to cool a secondary refrigerant, the accumulator receives primary refrigerant from the first low side heat exchanger, the first compressor compresses primary refrigerant from the accumulator, and the second compressor compresses primary refrigerant from the first compressor. During a second mode of operation, the first valve is open and directs primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to a vessel, the second valve is closed, and the third valve is open and directs primary refrigerant from the vessel to the accumulator. During a third mode of operation, the first and third valves are closed and the second valve is open and directs primary refrigerant from the second compressor to the vessel. The primary refrigerant from the second compressor pushes the oil in the vessel to the oil reservoir. 
     According to another embodiment, a method includes storing, by a flash tank, a primary refrigerant. During a first mode of operation, the method includes closing a first valve and a second valve, opening a third valve, using, by a first low side heat exchanger, primary refrigerant from the flash tank to cool a secondary refrigerant, receiving, by an accumulator, primary refrigerant from the first low side heat exchanger, compressing, by a first compressor, primary refrigerant from the accumulator, and compressing, by a second compressor, primary refrigerant from the first compressor. During a second mode of operation, the method includes opening the first valve, directing, by the first valve, primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to a vessel, closing the second valve, opening the third valve, and directing, by the third valve, primary refrigerant from the vessel to the accumulator. During a third mode of operation, the method includes closing the first and third valves, opening the second valve, directing, by the second valve, primary refrigerant from the second compressor to the vessel, and pushing, by the primary refrigerant from the second compressor, the oil in the vessel to an oil reservoir. 
     According to yet another embodiment, a system includes a high side heat exchanger, a flash tank, a first low side heat exchanger, an accumulator, a first compressor, a second compressor, an oil reservoir, a first valve, a second valve, and a third valve. The high side heat exchanger removes heat from a primary refrigerant. The flash tank stores the primary refrigerant. During a first mode of operation, the first and second valves are closed, the third valve is open, the first low side heat exchanger uses primary refrigerant from the flash tank to cool a secondary refrigerant, the accumulator receives primary refrigerant from the first low side heat exchanger, the first compressor compresses primary refrigerant from the accumulator, and the second compressor compresses primary refrigerant from the first compressor. During a second mode of operation, the first valve is open and directs primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to a vessel, the second valve is closed, and the third valve is open and directs primary refrigerant from the vessel to the accumulator. During a third mode of operation, the first and third valves are closed and the second valve is open and directs primary refrigerant from the second compressor to the vessel. The primary refrigerant from the second compressor pushes the oil in the vessel to the oil reservoir. 
     According to an embodiment, a system includes a flash tank, a first low side heat exchanger, a first accumulator, a first compressor, a second accumulator, a second compressor, a first valve, a second valve, and a third valve. The flash tank stores a primary refrigerant. During a first mode of operation, the first and second valves are closed, the third valve is open, the first low side heat exchanger uses primary refrigerant from the flash tank to cool a secondary refrigerant, the first accumulator receives primary refrigerant from the first low side heat exchanger, the first compressor compresses primary refrigerant from the first accumulator, the second accumulator receives primary refrigerant from the first compressor, and the second compressor compresses primary refrigerant from the second accumulator. During a second mode of operation, the first valve is open and directs primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to a vessel, the second valve is closed, and the third valve is open and directs primary refrigerant from the vessel to the first accumulator. During a third mode of operation, the first and third valves are closed and the second valve is open and directs primary refrigerant from the second compressor to the vessel. The primary refrigerant from the second compressor pushes the oil in the vessel to the second accumulator. 
     According to another embodiment, a method includes storing, by a flash tank, a primary refrigerant. During a first mode of operation, the method includes closing a first valve and a second valve, opening a third valve, using, by a first low side heat exchanger, primary refrigerant from the flash tank to cool a secondary refrigerant, receiving, by a first accumulator, primary refrigerant from the first low side heat exchanger, compressing, by a first compressor, primary refrigerant from the first accumulator, receiving, by a second accumulator, primary refrigerant from the first compressor, and compressing by a second compressor, primary refrigerant from the second accumulator. During a second mode of operation, the method includes opening the first valve, directing, by the first valve, primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to a vessel, closing the second valve, opening the third valve, and directing, by the third valve, primary refrigerant from the vessel to the first accumulator. During a third mode of operation, the method includes closing the first and third valves, opening the second valve, directing, by the second valve, primary refrigerant from the second compressor to the vessel, and pushing, by the primary refrigerant from the second compressor, the oil in the vessel to the second accumulator. 
     According to yet another embodiment, a system includes a high side heat exchanger, a flash tank, a first low side heat exchanger, a first accumulator, a first compressor, a second accumulator, a second compressor, a first valve, a second valve, and a third valve. The high side heat exchanger removes heat from a primary refrigerant. The flash tank stores the primary refrigerant. During a first mode of operation, the first and second valves are closed, the third valve is open, the first low side heat exchanger uses primary refrigerant from the flash tank to cool a secondary refrigerant, the first accumulator receives primary refrigerant from the first low side heat exchanger, the first compressor compresses primary refrigerant from the first accumulator, the second accumulator receives primary refrigerant from the first compressor, and the second compressor compresses primary refrigerant from the second accumulator. During a second mode of operation, the first valve is open and directs primary refrigerant from the first low side heat exchanger and an oil from the first low side heat exchanger to a vessel, the second valve is closed, and the third valve is open and directs primary refrigerant from the vessel to the first accumulator. During a third mode of operation, the first and third valves are closed and the second valve is open and directs primary refrigerant from the second compressor to the vessel. The primary refrigerant from the second compressor pushes the oil in the vessel to the second accumulator. 
     Certain embodiments provide one or more technical advantages. For example, an embodiment allows oil to be drained from a low side heat exchanger and returned to a compressor, which may improve the efficiency of the low side heat exchanger and the lifespan of the compressor. Certain embodiments may include none, some, or all of the above technical advantages. One or more other technical advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    illustrates an example cooling system; 
         FIGS.  2 A- 2 C  illustrate an example cooling system; 
         FIG.  3    is a flowchart illustrating a method of operating an example cooling system; 
         FIGS.  4 A- 4 C  illustrate an example cooling system; and 
         FIG.  5    is a flowchart illustrating a method of operation an example cooling system. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure and its advantages are best understood by referring to  FIGS.  1  through  5    of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     Cooling systems cycle refrigerant to cool various spaces. For example, in some industrial facilities, cooling systems cycle a primary refrigerant that cools secondary refrigerants. The secondary refrigerants are then cycled to cool different parts of the industrial facility (e.g., different industrial and/or manufacturing processes). These systems typically include a compressor to compress the primary refrigerant and a high side heat exchanger that removes heat from the compressed primary refrigerant. When the compressor compresses the primary refrigerant, oil that coats certain components of the compressor may mix with and be discharged with the primary refrigerant. 
     Depending on the nature of the primary refrigerant, the cooling system may be able to move the oil along with the primary refrigerant through the cooling system such that the oil is eventually cycled back to the compressor. However, when certain primary refrigerants (e.g., carbon dioxide) are used, the oil may get stuck in a portion of the cooling system (e.g., at a low side heat exchanger). As a result, the compressor(s) in the system begin losing oil, which eventually leads to breakdown or failure. Additionally, the components in which the oil gets stuck may also become less efficient as the oil builds in these components. 
     This disclosure contemplates unconventional cooling systems that drain oil from low side heat exchangers to vessels and then uses compressed refrigerant to push the oil in the vessels back towards a compressor. Generally, the cooling systems operate in three different modes of operation: a normal mode, an oil drain mode, and an oil return mode. During the normal mode, a primary refrigerant is cycled to cool one or more secondary refrigerants. As the primary refrigerant is cycled, oil from a compressor may mix with the primary refrigerant and become stuck in a low side heat exchanger. During the oil drain mode, the oil in the low side heat exchanger is allowed to drain into a vessel. During the oil return mode, compressed refrigerant is directed to the vessel to push the oil in the vessel back towards a compressor. In this manner, oil in a low side heat exchanger is returned to a compressor. The cooling systems will be described using  FIGS.  1  through  5   .  FIG.  1    will describe an existing cooling system.  FIGS.  2 A- 2 C and  3    describe a first cooling system that drains oil from a low side heat exchanger.  FIGS.  4 A- 4 C and  5    describe a second cooling system that drains oil from a low side heat exchanger. 
       FIG.  1    illustrates an example cooling system  100 . As shown in  FIG.  1   , system  100  includes a high side heat exchanger  102 , low side heat exchangers  104 A and  104 B, cooling systems  106 A and  106 B, and compressor  108 . Generally, system  100  cycles a primary refrigerant to cool secondary refrigerants used by cooling systems  106 A and  106 B. Cooling system  100  or any cooling system described herein may include any number of low side heat exchangers. 
     High side heat exchanger  102  removes heat from a primary refrigerant. When heat is removed from the refrigerant, the refrigerant is cooled. High side heat exchanger  102  may be operated as a condenser and/or a gas cooler. When operating as a condenser, high side heat exchanger  102  cools the refrigerant such that the state of the refrigerant changes from a gas to a liquid. When operating as a gas cooler, high side heat exchanger  102  cools gaseous refrigerant and the refrigerant remains a gas. In certain configurations, high side heat exchanger  102  is positioned such that heat removed from the refrigerant may be discharged into the air. For example, high side heat exchanger  102  may be positioned on a rooftop so that heat removed from the refrigerant may be discharged into the air. This disclosure contemplates any suitable refrigerant being used in any of the disclosed cooling systems. 
     Low side heat exchangers  104 A and  104 B transfer heat from secondary refrigerants from cooling systems  106 A and  106 B to the primary refrigerant from high side heat exchanger  102 . As a result, the primary refrigerant heats up and the secondary refrigerants are cooled. The cooled secondary refrigerants are then directed back to cooling systems  106 A and  106 B to cool components in cooling systems  106 A and  106 B. In the example of  FIG.  1   , low side heat exchanger  104 A transfers heat from a secondary refrigerant from cooling system  106 A to the primary refrigerant from high side heat exchanger  102  and low side heat exchanger  104 B transfers heat from a second refrigerant from cooling system  106 B to the primary refrigerant from high side heat exchanger  102 . Cooling systems  106 A and  106 B may use the same or different secondary refrigerants. 
     Cooling systems  106 A and  106 B may use the secondary refrigerants to cool different things. For example, cooling systems  106 A and  106 B may be installed in an industrial facility and cool different portions of the industrial facility, such as different industrial and/or manufacturing processes. When these processes are cooled, the secondary refrigerants are heated and cycled back to low side heat exchangers  104 A and  104 B, where the secondary refrigerants are cooled again. 
     Primary refrigerant flows from low side heat exchangers  104 A and  104 B to compressor  108 . The disclosed cooling systems may include any number of compressors  108 . Compressor  108  compresses primary refrigerant to increase the pressure of the refrigerant. As a result, the heat in the refrigerant may become concentrated. When the compressor  108  compresses the refrigerant, oil that coats certain components of compressor  108  may mix with and be discharged with the refrigerant. Depending on the nature of the primary refrigerant, cooling system  100  may be able to move the oil along with the primary refrigerant through cooling system  100  such that the oil is eventually cycled back to compressor  108 . However, when certain primary refrigerants (e.g., carbon dioxide) are used, the oil may get stuck in a portion of the cooling system (e.g., at low side heat exchangers  104 A and  104 B). As a result, compressor  108  loses oil, which eventually leads to breakdown or failure. Additionally, the components in which the oil gets stuck may also become less efficient as the oil builds in these components. 
     This disclosure contemplates unconventional cooling systems that drain oil from low side heat exchangers to vessels and then uses compressed refrigerant to push the oil in the vessels back towards a compressor. Generally, the cooling systems operate in three different modes of operation: a normal mode, an oil drain mode, and an oil return mode. During the normal mode, a primary refrigerant is cycled to cool one or more secondary refrigerants. As the primary refrigerant is cycled, oil from a compressor may mix with the primary refrigerant and become stuck in a low side heat exchanger. During the oil drain mode, the oil in the low side heat exchanger is allowed to drain into a vessel. During the oil return mode, compressed refrigerant is directed to the vessel to push the oil in the vessel back towards a compressor. In this manner, oil in a low side heat exchanger is returned to a compressor. The unconventional systems will be described in more detail using  FIGS.  2 A- 2 C,  3 ,  4 A- 4 C, and  5   . 
       FIGS.  2 A- 2 C  illustrate an example cooling system  200 . As seen in  FIGS.  2 A- 2 C , cooling system  200  includes a high side heat exchanger  202 , a flash tank  204 , low side heat exchangers  206 A and  206 B, an accumulator  208 , a compressor  210 , a compressor  212 , an oil separator  214 , valves  216 A and  216 B, valves  218 A and  218 B, valves  220 A and  220 B, vessels  222 A and  222 B, valves  224 A and  224 B, valve  226 , controller  228 , one or more sensors  234 , valves  238 A and  238 B, and an oil reservoir  240 . Generally, cooling system  200  operates in three modes of operation: a normal mode of operation, an oil drain mode of operation, and an oil return mode of operation.  FIG.  2 A  illustrates cooling system  200  operating in the normal mode of operation.  FIG.  2 B  illustrates cooling system  200  operating in the oil drain mode of operation.  FIG.  2 C  illustrates cooling system  200  operating in the oil return mode of operation. By cycling through these modes of operation, cooling system  200  can direct oil in low side heat exchangers  206 A and  206 B towards compressors  210  and  212 . 
     High side heat exchanger  202  operates similarly as high side heat exchanger  102  in cooling system  100 . Generally, high side heat exchanger  202  removes heat from a primary refrigerant (e.g., carbon dioxide) cycling through cooling system  200 . When heat is removed from the refrigerant, the refrigerant is cooled. High side heat exchanger  202  may be operated as a condenser and/or a gas cooler. When operating as a condenser, high side heat exchanger  202  cools the refrigerant such that the state of the refrigerant changes from a gas to a liquid. When operating as a gas cooler, high side heat exchanger  202  cools gaseous refrigerant and the refrigerant remains a gas. In certain configurations, high side heat exchanger  202  is positioned such that heat removed from the refrigerant may be discharged into the air. For example, high side heat exchanger  202  may be positioned on a rooftop so that heat removed from the refrigerant may be discharged into the air. This disclosure contemplates any suitable refrigerant being used in any of the disclosed cooling systems. 
     Flash tank  204  stores primary refrigerant received from high side heat exchanger  202 . This disclosure contemplates flash tank  204  storing refrigerant in any state such as, for example, a liquid state and/or a gaseous state. Refrigerant leaving flash tank  204  is fed to low side heat exchanger(s)  206 A and/or  206 B. In some embodiments, a flash gas and/or a gaseous refrigerant is released from flash tank  204 . By releasing flash gas, the pressure within flash tank  204  may be reduced. 
     Low side heat exchangers  206 A and  206 B may operate similarly as low side heat exchangers  104 A and  104 B in cooling system  100 . System  200  may include any suitable number of low side heat exchangers  206 . Generally low side heat exchangers  206 A and  206 B transfer heat from secondary refrigerants (e.g., water, glycol, etc.) to the primary refrigerant (e.g., carbon dioxide) in cooling system  200 . As a result, the primary refrigerant is heated while the secondary refrigerant is cooled. Low side heat exchangers  206 A and  206 B may include any suitable structure (e.g., plates, tubes, fins, etc.) for transferring heat between refrigerants. For example, low side heat exchangers  206 A and  206 B may be shell tube or shell plate type evaporators commonly found in industrial facilities. 
     Low side heat exchangers  206 A and  206 B then direct cooled secondary refrigerant to cooling systems  106 A and  106 B. In the example of  FIGS.  2 A- 2 C , low side heat exchanger  206 A directs cooled secondary refrigerant to cooling system  106 A and low side heat exchanger  206 B directs cooled secondary refrigerant to cooling system  106 B. Low side heat exchangers  206 A and  206 B may cool different secondary refrigerants. Cooling systems  106 A and  106 B may use different secondary refrigerants. In other words, low side heat exchanger  206 A may cool and cooling system  106 A may use a secondary refrigerant while low side heat exchanger  206 B may cool and cooling system  106 B may use a tertiary refrigerant. 
     Cooling systems  106 A and  106 B may use the cooled secondary refrigerants from low side heat exchangers  206 A and  206 B to cool different things, such as for example, different industrial processes and/or methods. The secondary refrigerants may then be heated and directed back to low side heat exchangers  206 A and  206 B for cooling. System  200  may include any suitable number of cooling systems  106 . 
     Accumulator  208  receives primary refrigerant from one or more of low side heat exchangers  206 A and  206 B. Accumulator  208  may separate a liquid portion from a gaseous portion of the refrigerant. For example, refrigerant may enter through a top surface of accumulator  208 . A liquid portion of the refrigerant may drop to the bottom of accumulator  208  while a gaseous portion of the refrigerant may float towards the top of accumulator  208 . Accumulator  208  includes a U-shaped pipe that sucks refrigerant out of accumulator  208 . Because the end of the U-shaped pipe is located near the top of accumulator  208 , the gaseous refrigerant is sucked into the end of the U-shaped pipe while the liquid refrigerant collects at the bottom of accumulator  208 . 
     Compressor  210  compresses primary refrigerant discharged by accumulator  208 . Compressor  212  compresses primary refrigerant discharged by compressor  210 . Cooling system  200  may include any number of compressors  210  and/or  212 . Both compressors  210  and  212  compress refrigerant to increase the pressure of the refrigerant. As a result, the heat in the refrigerant may become concentrated and the refrigerant may become a high-pressure gas. Compressor  210  compresses refrigerant from accumulator  208  and sends the compressed refrigerant to compressor  212 . Compressor  112  compresses the refrigerant from compressor  210 . When compressors  210  and  212  compress refrigerant, oil that coats certain components of compressors  210  and  212  may mix with and be discharged with the refrigerant. 
     Oil separator  214  separates an oil from the primary refrigerant discharged by compressor  212 . The oil may be introduced by certain components of system  200 , such as compressors  210  and/or  212 . By separating out the oil from the refrigerant, the efficiency of other components (e.g., high side heat exchanger  202  and low side heat exchangers  206 A and  206 B) is maintained. If oil separator  214  is not present, then the oil may clog these components, which may reduce the heat transfer efficiency of system  200 . Oil separator  214  may not completely remove the oil from the refrigerant, and as a result, some oil may still flow into other components of system  200  (e.g., low side heat exchangers  206 A and  206 B). Oil separator  214  directs separated oil to oil reservoir  240 . Oil reservoir  240  stores oil and returns oil back to compressors  210  and  212 . During the oil return mode of operation, oil may be directed from vessels  222 A and  222 B to oil reservoir  240 . 
     Valves  216 A and  216 B control a flow of primary refrigerant from flash tank  204  to low side heat exchangers  206 A and  206 B. System  200  may include any suitable number of valves  216  based on the number of low side heat exchangers  206  in system  200 . Valve  216 A and  216 B may be thermal expansion valves that cool refrigerant flowing through valves  216 A and  216 B. For example, valves  216 A and  216 B may reduce the pressure and therefore the temperature of the refrigerant flowing through valves  216 A and  216 B. Valves  216 A and  216 B reduce pressure of the refrigerant flowing into valves  216 A and  216 B. The temperature of the refrigerant may then drop as pressure is reduced. As a result, refrigerant entering valves  216 A and  216 B may be cooler when leaving valves  216 A and  216 B. When valve  216 A is open, primary refrigerant flows from flash tank  204  to low side heat exchanger  206 A. When valve  216 A is closed, primary refrigerant does not flow from flash tank  204  to low side heat exchanger  206 A. When valve  216 B is open, primary refrigerant flows from flash tank  204  to low side heat exchanger  206 B. When valve  216 B is closed, primary refrigerant does not flow from flash tank  204  to low side heat exchanger  206 B. 
     Valves  218 A and  218 B control a flow of refrigerant and/or oil from low side heat exchangers  206 A and  206 B to vessels  222 A and  222 B. System  200  may include any suitable number of valves  218  based on the number of low side heat exchangers  206  in system  200 . During the oil drain mode of operation, valves  218 A and  218 B may be open to allow refrigerant and/or oil to flow from low side heat exchanger  206 A and  206 B to vessels  222 A and  222 B. During the normal mode of operation and the oil return mode of operation, valves  218 A and  218 B may be closed. In certain embodiments, valve  218 A and  218 B may be solenoid valves. 
     Valves  220 A and  220 B control a flow of refrigerant from compressor  212  to vessels  222 A and  222 B. System  200  may include any suitable number of valves  220  based on the number of low side heat exchangers  206  in system  200 . In certain embodiments, valves  220 A and  220 B may be solenoid valves. During the oil return mode of operation, valves  220 A and  220 B may be open to allow refrigerant from compressor  212  to flow to vessels  222 A and  222 B. That refrigerant pushes oil and/or refrigerant that has collected in vessels  222 A and  222 B towards oil reservoir  240 . During the normal mode of operation and the oil drain mode of operation, valves  220 A and  220 B are closed. 
     Vessels  222 A and  222 B collect oil and/or refrigerant for low side heat exchangers  206 A and  206 B. System  200  may include any suitable number of vessels  222  based on the number of low side heat exchangers  206  in system  200 . By collecting oil in vessels  222 A and  222 B, that oil is allowed to drain from low side heat exchangers  206 A and  206 B, thereby improving the efficiency of low side heat exchangers  206 A and  206 B. During the oil drain mode of operation, oil drains from low side heat exchangers  206 A and  206 B into vessels  222 A and  222 B. During the oil return mode of operation, refrigerant from compressor  212  pushes oil that has collected in vessels  222 A and  222 B towards oil reservoir  240  for return to compressors  210  and  212 . During the normal mode of operation, valves  218 A,  218 B,  220 A,  220 B,  236 A, and  236 B are closed to prevent refrigerant and oil from flowing into vessels  222 A and  222 B. Vessels  222 A and  222 B may include any suitable components for holding and/or storing refrigerant and/or oil. For example, vessels  222 A and  222 B may include one or more of a container/tank and a coil (e.g., a container/tank only, a coil only, a container/tank and a coil arranged in series with one another, a coil disposed within a container/tank, etc.). The container/tank and/or coil may be of any suitable shape and size. 
     Valves  224 A and  224 B control a flow of refrigerant from low side heat exchangers  206 A and  206 B to accumulator  208 . System  200  may include any suitable number of valves  224  based on the number of low side heat exchangers  206  in system  200 . In certain embodiments, valves  224 A and  224 B are check valves that allow refrigerant to flow when a pressure of that refrigerant exceeds a threshold. In this manner, valves  224 A and  224 B direct a flow of refrigerant from low side heat exchangers  206 A and  206 B to accumulator  208  and control a pressure of the refrigerant flowing to accumulator  208 . 
     Valves  236 A and  236 B control a flow of refrigerant from vessels  222 A and  222 B to accumulator  208 . System  200  may include any suitable number of valves  236  based on the number of low side heat exchangers  206  in system  200 . During the oil drain mode of operation, valves  236 A and  236 B may be open to direct refrigerant in vessels  222 A and  222 B to accumulator  208 . For example, during the oil drain mode, refrigerant and oil from low side heat exchanger  206 A and/or  206 B may drain into vessel  222 A and/or  222 B. Valves  236 A and  236 B allow the refrigerant to flow to accumulator  208  while keeping the oil in vessel  222 A and/or  222 B. During the normal mode of operation and the oil return mode of operation, valves  236 A and  236 B are closed. 
     Valves  238 A and  238 B control a flow of oil and refrigerant from vessels  222 A and  222 B to oil reservoir  240 . System  200  may include any suitable number of valves  238  based on the number of low side heat exchangers  206  in system  200 . In particular embodiments, valves  238 A and  238 B are check valves that allow refrigerant to flow when a pressure of that refrigerant exceeds a threshold. During the normal mode of operation and the oil drain mode of operation, the pressure of the oil and refrigerant in vessels  222 A and  222 B may not be sufficiently high to open valves  238 A and  238 B. As a result, oil and/or refrigerant does not flow through valves  238 A and  238 B to oil reservoir  240 . During the oil return mode of operation, pressurized refrigerant from compressor  212  is directed to vessel  222 A and/or  222 B. As a result, the pressure of the oil and/or refrigerant in vessel  222 A and/or  222 B may be sufficiently high to push the oil and/or refrigerant through valve  238 A and/or  238 B to oil reservoir  240 . 
     Valve  226  controls a flow of refrigerant from flash tank  204  to compressor  212 . Valve  226  may be referred to as a flash gas bypass valve because the refrigerant flowing through valve  226  may take the form of a flash gas from flash tank  204 . If the pressure of the refrigerant in flash tank  204  is too high, valve  226  may open to direct flash gas from flash tank  204  to compressor  212 . As a result, the pressure of flash tank  204  may be reduced. 
     Controller  228  controls the operation of cooling system  200 . For example, controller  228  may cause certain valves to open and/or close to transition cooling system  200  from one mode of operation to another. Controller  228  includes a processor  230  and a memory  232 . This disclosure contemplates processor  230  and memory  232  being configured to perform any of the operations of controller  228  described herein. 
     Processor  230  is any electronic circuitry, including, but not limited to microprocessors, application specific integrated circuits (ASIC), application specific instruction set processor (ASIP), and/or state machines, that communicatively couples to memory  232  and controls the operation of controller  228 . Processor  230  may be 8-bit, 16-bit, 32-bit, 64-bit or of any other suitable architecture. Processor  230  may include an arithmetic logic unit (ALU) for performing arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that fetches instructions from memory and executes them by directing the coordinated operations of the ALU, registers and other components. Processor  230  may include other hardware that operates software to control and process information. Processor  230  executes software stored on memory to perform any of the functions described herein. Processor  230  controls the operation and administration of controller  228  by processing information received from sensors  234  and memory  232 . Processor  230  may be a programmable logic device, a microcontroller, a microprocessor, any suitable processing device, or any suitable combination of the preceding. Processor  230  is not limited to a single processing device and may encompass multiple processing devices. 
     Memory  232  may store, either permanently or temporarily, data, operational software, or other information for processor  230 . Memory  232  may include any one or a combination of volatile or non-volatile local or remote devices suitable for storing information. For example, memory  232  may include random access memory (RAM), read only memory (ROM), magnetic storage devices, optical storage devices, or any other suitable information storage device or a combination of these devices. The software represents any suitable set of instructions, logic, or code embodied in a computer-readable storage medium. For example, the software may be embodied in memory  232 , a disk, a CD, or a flash drive. In particular embodiments, the software may include an application executable by processor  230  to perform one or more of the functions described herein. 
     Sensors  234  may include one or more sensors  234  that detect characteristics of cooling system  200 . For example, sensors  234  may include one or more temperature sensors that detect the temperature of refrigerant in cooling system  200 . In certain embodiments, these temperature sensors may detect the temperature of a primary refrigerant in low side heat exchangers  206 A and/or  206 B and a temperature of secondary refrigerant in low side heat exchangers  206 A and  206 B. In some embodiments, sensors  234  include one or more level sensors that detect a level of oil in cooling system  200 . 
     Controller  228  may transition system  200  from one mode of operation to another based on the detections made by one or more sensors  234 . For example, controller  228  may transition cooling system  200  from the normal mode of operation to the oil drain mode of operations when the difference between the detected temperatures of the primary refrigerant and a secondary refrigerant increases above a threshold. As another example, controller  228  may transition cooling system  200  from the normal mode of operation to the oil drain mode of operation when a detected level of oil in cooling system  200  falls below or exceeds a threshold. Controller  228  may transition system  200  between different modes of operation by controlling various components of system (e.g., by opening and/or closing valves). 
     The different modes of operation of cooling system  200  will now be described using  FIGS.  2 A- 2 C .  FIG.  2 A  illustrates cooling system  200  operating in a normal mode of operation. During the normal mode of operation, valves  216 A and  216 B are open to allow primary refrigerant from flash tank  204  to flow to low side heat exchangers  206 A and  206 B. Low side heat exchangers  206 A and  206 B transfer heat from secondary refrigerants to the primary refrigerant. The cooled secondary refrigerant is then cycled to cooling systems  106 A and  106 B. The heated primary refrigerant is directed through valves  224 A and  224 B to accumulator  208 . Accumulator  208  separates gaseous and liquid portions of the received refrigerant. Compressor  210  compresses the gaseous refrigerant from accumulator  208 . Compressor  212  compresses the refrigerant from compressor  210 . Oil separator  214  separates an oil from the refrigerant from compressor  212  and directs the oil to oil reservoir  240 . The oil in oil reservoir  240  is returned to compressors  210  and  212 . Valves  218 A,  218 B,  220 A,  220 B,  236 A, and  236 B are closed. 
     As cooling system  200  operates in the normal mode of operation, oil from compressors  210  and/or  212  may begin to build in low side heat exchangers  206 A and/or  206 B (e.g., because oil separator  214  does not separate all the oil from the refrigerant). As this oil builds, the efficiency of low side heat exchangers  206 A and/or  206 B may decrease. In certain embodiments, the drop in efficiency in low side heat exchangers  206 A and/or  206 B may cause less heat transfer to occur within low side heat exchangers  206 A and/or  206 B. As a result, the temperature differential between the primary refrigerant and the secondary refrigerant in low side heat exchangers  206 A and/or  206 B may increase. One or more sensors  234  may detect a temperature of the primary refrigerant and a temperature of the secondary refrigerant in low side heat exchangers  206 A and/or  206 B. When controller  228  determines that this temperature differential increases above a threshold, controller  228  may determine that the oil building up in low side heat exchangers  206 A and/or  206 B should be drained and returned to compressors  210  and/or  212 . As a result, controller  228  may transition cooling system  200  from the normal mode of operation to the oil drain mode of operation. 
     In certain embodiments, one or more sensors  234  may detect a level of oil in cooling system  200 . For example, one or more sensors  234  may detect a level of oil in low side heat exchangers  206 A and/or  206 B or a level of oil in oil reservoir  240 . Based on the detected levels of oil, controller  228  may transition cooling system  200  from the normal mode of operation to the oil drain mode of operation. For example, if one or more sensors  234  detect that a level of oil in low side heat exchanger  206 A or  206 B exceeds a threshold, controller  228  may determine that the oil in low side heat exchanger  206 A or  206 B should be drained and transition cooling system  200  from the normal mode of operation to the oil drain mode of operation. As another example, if one or more sensors  234  detect that a level of oil in oil reservoir  240  falls below a threshold, controller  228  may determine that low side heat exchanger  206 A or  206 B should be drained and transition cooling system  200  from the normal mode of operation to the oil drain mode of operation. 
       FIG.  2 B  illustrates cooling system  200  operating in the oil drain mode of operation. To transition cooling system  200  from the normal mode of operation to the oil drain mode of operation, controller  228  closes one of valves  216 A and  216 B. In this manner, primary refrigerant stops flowing from flash tank  204  to one of low side heat exchangers  206 A and  206 B. In the example of  FIG.  2 B , valve  216 A is closed and valve  216 B is open. In this manner, primary refrigerant continues to flow to low side heat exchanger  206 B and oil in low side heat exchanger  206 A is allowed to drain. This disclosure contemplates that valve  216 B may instead be closed and valve  216 A remains open during the oil drain mode. Generally, cooling system  200  may drain oil from any suitable number of low side heat exchangers  206  while allowing other low side heat exchangers  206  to operate in a normal mode of operation. 
     During the oil drain mode of operation, controller  228  also opens one of valves  218 A and  218 B and one of valves  236 A and  236 B. In the example of  FIG.  2 B , valve  218 A is open to allow refrigerant and/or oil to drain from low side heat exchanger  206 A through valve  218 A to vessel  222 A. Valve  218 B remains closed. Additionally, valve  236 A is open to allow refrigerant in vessel  222 A to flow to accumulator  208  through valve  236 A. Valve  236 B remains closed. In this manner, oil that has collected in low side heat exchanger  206 A is directed to vessel  222 A by valve  218 A. This disclosure contemplates controller  228  opening any suitable number of valves  218  and  236  during the oil drain mode while keeping other valves  218  and  236  closed so that their corresponding low side heat exchangers  206  may operate in the normal mode of operation. Controller  228  keeps valves  220 A and  220 B closed during the oil drain mode of operation. 
     Controller  228  may transition cooling system  200  from the oil drain mode of operation to the oil return mode of operation after cooling system  200  has been in the oil drain mode of operation for a particular period of time (e.g., one to two minutes). After that period of time, cooling system  200  transitions from the oil drain mode of operation to the oil return mode of operation. 
       FIG.  2 C  illustrates cooling system  200  in the oil return mode of operation. In the example of  FIG.  2 C , controller  228  transitions low side heat exchanger  206 A to the oil return mode of operation. 
     During the oil return mode of operation, valve  216 A remains closed so that low side heat exchanger  206 A does not receive primary refrigerant from flash tank  204 . Valve  218 A is closed so that oil and refrigerant from low side heat exchanger  206 A does not continue draining to vessel  222 A. Valve  236 A is also closed to prevent refrigerant from flowing from vessel  222 A to accumulator  208 . Controller  228  opens valve  220 A, so that valve  220 A directs refrigerant from compressor  212  into vessel  222 A. This refrigerant pushes the oil in vessel  222 A through valve  238 A to oil reservoir  240 . The oil then collects in oil reservoir  240  and is returned to compressors  210  and  212 . Valve  216 B is open and valves  218 B,  220 B, and  236 B are closed so that low side heat exchanger  206 B supplies refrigerant to compressors  210  and  212  that can be directed through valve  220 A. 
     Oil reservoir  240  includes a vent  242  that allows refrigerant collecting in oil reservoir  240  to escape. The refrigerant flows through vent  242  to flash tank  204 . In this manner, refrigerant does not build in oil reservoir  240 . Vent  242  may direct refrigerant from oil reservoir  240  to flash tank  204  during any suitable mode of operation (and not merely during the oil return mode of operation). 
     In particular embodiments, controller  228  transitions cooling system  200  from the oil return mode of operation back to the normal mode of operation after cooling system  200  has been in the oil return mode of operation for a particular period of time (e.g., ten to twenty seconds). To transition the example of  FIG.  2 C  back to the normal mode of operation, controller  228  closes valve  220 A and opens valve  216 A. 
     Although  FIGS.  2 A- 2 C  show cooling system  200  transitioning through the normal mode of operation, the oil drain mode of operation, and the oil return mode of operation to drain and return oil collected in low side heat exchanger  206 A, this disclosure contemplates cooling system  200  transitioning through these three modes of operation for any low side heat exchanger  206  in system  200 . By transitioning through these three modes, oil that is collected in low side heat exchanger  206  may be returned to compressor  210  and/or compressor  212  in particular embodiments. 
       FIG.  3    is a flowchart illustrating a method  300  of operating an example cooling system  200 . In particular embodiments, various components of cooling system  200  perform the steps of method  300 . By performing method  300 , an oil that has collected in a low side heat exchanger  206  may be returned to a compressor  210  or  212 . 
     A high side heat exchanger  202  removes heat from a primary refrigerant (e.g., carbon dioxide) in step  302 . In step  304 , a flash tank  204  stores the primary refrigerant. In step  306 , controller  228  determines whether cooling system  200  should be in a first mode of operation (e.g., a normal mode of operation). For example, controller  228  may determine a difference in the temperature between a primary refrigerant and a secondary refrigerant in low side heat exchanger  206  to determine whether cooling system  200  should be in the first mode of operation. As another example, controller  228  may determine a level of oil in the cooling system  200  to determine whether the cooling system  200  should be in the first mode of operation. 
     If the system  200  should be in the first mode of operation, controller  228  closes valves  218 A and/or  220 A (if they are not already closed) in step  308 . Controller  228  opens a valve  236 A (if it is not already open) in step  310 . In step  312 , low side heat exchanger  206 A uses the primary refrigerant to cool a secondary refrigerant. Accumulator  208  receives the primary refrigerant from low side heat exchanger  206 A in step  314 . Compressor  210  compresses the primary refrigerant from accumulator  208  in step  316 . In step  318 , compressor  212  compresses the primary refrigerant from compressor  210 . 
     If controller  228  determines that cooling system  200  should not be in the first mode of operation, controller  228  determines whether cooling system  200  should be in the second mode of operation (e.g., an oil drain mode of operation) in step  320 . As discussed previously, controller  228  may determine whether cooling system  200  should be in the second mode of operation based on a detected temperature differential and/or oil level. If controller  228  determines that cooling system  200  should be in the second mode of operation, controller  228  opens valve  218 A (if valve  218 A is not already open) in step  322 . In step  324 , controller  228  closes valve  220 A (if valve  220 A is not already closed). In step  326 , controller  228  opens valve  236 A (if valve  236 A is not already open). As a result, oil from low side heat exchanger  206 A is allowed to drain through valve  218 A to vessel  222 A. Refrigerant in vessel  222 A is allowed to flow to accumulator  208  through valve  236 A. 
     If controller  228  determines that cooling system  200  should not be in the first mode or second mode of operation, controller  228  may determine that cooling system  200  should be in a third mode of operation (e.g., an oil return mode of operation). In response, controller  228  closes valves  218 A and  236 A (if valves  218 A and  236 A are not already closed) in step  328 . Controller  228  then opens valve  220 A (if valve  220 A is not already opened) in step  330 . As a result, refrigerant from compressor  212  flows to vessel  222 A through valve  220 A to push oil that is collected in vessel  222 A to oil reservoir  240 . The oil collected in oil reservoir  240  may then be returned to compressor  210  and/or compressor  212 . 
     Modifications, additions, or omissions may be made to method  300  depicted in  FIG.  3   . Method  300  may include more, fewer, or other steps. For example, steps may be performed in parallel or in any suitable order. While discussed as system  200  (or components thereof) performing the steps, any suitable component of system  200  may perform one or more steps of the method. 
       FIGS.  4 A- 4 C  illustrate an example cooling system  400 . As seen in  FIGS.  4 A- 4 C , cooling system  400  includes a high side heat exchanger  202 , a flash tank  204 , low side heat exchangers  206 A and  206 B, accumulators  208 A and  208 B, a compressor  210 , a compressor  212 , an oil separator  214 , valves  216 A and  216 B, valves  218 A and  218 B, valves  220 A and  220 B, vessels  222 A and  222 B, valves  224 A and  224 B, valve  226 , controller  228 , one or more sensors  234 , and valves  238 A and  238 B. Generally, cooling system  400  operates in three modes of operation: a normal mode of operation, an oil drain mode of operation, and an oil return mode of operation.  FIG.  4 A  illustrates cooling system  400  operating in the normal mode of operation.  FIG.  4 B  illustrates cooling system  400  operating in the oil drain mode of operation.  FIG.  4 C  illustrates cooling system  400  operating in the oil return mode of operation. By cycling through these modes of operation, cooling system  400  can direct oil in low side heat exchangers  206 A and  206 B towards compressors  210  and  212 . 
     High side heat exchanger  202  operates similarly as high side heat exchanger  102  in cooling system  100 . Generally, high side heat exchanger  202  removes heat from a primary refrigerant (e.g., carbon dioxide) cycling through cooling system  400 . When heat is removed from the refrigerant, the refrigerant is cooled. High side heat exchanger  202  may be operated as a condenser and/or a gas cooler. When operating as a condenser, high side heat exchanger  202  cools the refrigerant such that the state of the refrigerant changes from a gas to a liquid. When operating as a gas cooler, high side heat exchanger  202  cools gaseous refrigerant and the refrigerant remains a gas. In certain configurations, high side heat exchanger  202  is positioned such that heat removed from the refrigerant may be discharged into the air. For example, high side heat exchanger  202  may be positioned on a rooftop so that heat removed from the refrigerant may be discharged into the air. This disclosure contemplates any suitable refrigerant being used in any of the disclosed cooling systems. 
     Flash tank  204  stores primary refrigerant received from high side heat exchanger  202 . This disclosure contemplates flash tank  204  storing refrigerant in any state such as, for example, a liquid state and/or a gaseous state. Refrigerant leaving flash tank  204  is fed to low side heat exchanger(s)  206 A and/or  206 B. In some embodiments, a flash gas and/or a gaseous refrigerant is released from flash tank  204 . By releasing flash gas, the pressure within flash tank  204  may be reduced. 
     Low side heat exchangers  206 A and  206 B may operate similarly as low side heat exchangers  104 A and  104 B in cooling system  100 . System  400  may include any suitable number of low side heat exchangers  206 . Generally, low side heat exchangers  206 A and  206 B transfer heat from secondary refrigerants (e.g., water, glycol, etc.) to the primary refrigerant (e.g., carbon dioxide) in cooling system  400 . As a result, the primary refrigerant is heated while the secondary refrigerant is cooled. Low side heat exchangers  206 A and  206 B may include any suitable structure (e.g., plates, tubes, fins, etc.) for transferring heat between refrigerants. For example, low side heat exchangers  206 A and  206 B may be shell tube or shell plate type evaporators commonly found in industrial facilities. 
     Low side heat exchangers  206 A and  206 B then direct cooled secondary refrigerant to cooling systems  106 A and  106 B. In the example of  FIGS.  4 A- 4 C , low side heat exchanger  206 A directs cooled secondary refrigerant to cooling system  106 A and low side heat exchanger  206 B directs cooled secondary refrigerant to cooling system  106 B. Low side heat exchangers  206 A and  206 B may cool different secondary refrigerants. Cooling systems  106 A and  106 B may use different secondary refrigerants. In other words, low side heat exchanger  206 A may cool and cooling system  106 A may use a secondary refrigerant while low side heat exchanger  206 B may cool and cooling system  106 B may use a tertiary refrigerant. 
     Cooling systems  106 A and  106 B may use the cooled secondary refrigerants from low side heat exchangers  206 A and  206 B to cool different things, such as for example, different industrial processes and/or methods. The secondary refrigerants may then be heated and directed back to low side heat exchangers  206 A and  206 B for cooling. System  400  may include any suitable number of cooling systems  106 . 
     Accumulator  208 A receives primary refrigerant from one or more of low side heat exchangers  206 A and  206 B. Accumulator  208 A may separate a liquid portion from a gaseous portion of the refrigerant. For example, refrigerant may enter through a top surface of accumulator  208 A. A liquid portion of the refrigerant may drop to the bottom of accumulator  208 A while a gaseous portion of the refrigerant may float towards the top of accumulator  208 A. Accumulator  208 A includes a U-shaped pipe that sucks refrigerant out of accumulator  208 A. Because the end of the U-shaped pipe is located near the top of accumulator  208 A, the gaseous refrigerant is sucked into the end of the U-shaped pipe while the liquid refrigerant collects at the bottom of accumulator  208 A. 
     Compressor  210  compresses primary refrigerant discharged by accumulator  208 A and directs that refrigerant to accumulator  208 B. Accumulator  208 B may separate a liquid portion from a gaseous portion of the refrigerant. For example, refrigerant may enter through a top surface of accumulator  208 B. A liquid portion of the refrigerant may drop to the bottom of accumulator  208 B while a gaseous portion of the refrigerant may float towards the top of accumulator  208 B. Accumulator  208 B includes a U-shaped pipe that sucks refrigerant out of accumulator  208 B. Because the end of the U-shaped pipe is located near the top of accumulator  208 B, the gaseous refrigerant is sucked into the end of the U-shaped pipe while the liquid refrigerant collects at the bottom of accumulator  208 B. Compressor  212  compresses primary refrigerant discharged by accumulator  208 B. 
     Cooling system  400  may include any number of compressors  210  and/or  212 . Both compressors  210  and  212  compress refrigerant to increase the pressure of the refrigerant. As a result, the heat in the refrigerant may become concentrated and the refrigerant may become a high-pressure gas. Compressor  210  compresses refrigerant from accumulator  208 A and sends the compressed refrigerant to accumulator  208 B. Compressor  112  compresses the refrigerant from accumulator  208 B. When compressors  210  and  212  compress refrigerant, oil that coats certain components of compressors  210  and  212  may mix with and be discharged with the refrigerant. 
     Oil separator  214  separates an oil from the primary refrigerant discharged by compressor  212 . The oil may be introduced by certain components of system  400 , such as compressors  210  and/or  212 . By separating out the oil from the refrigerant, the efficiency of other components (e.g., high side heat exchanger  202  and low side heat exchangers  206 A and  206 B) is maintained. If oil separator  214  is not present, then the oil may clog these components, which may reduce the heat transfer efficiency of system  400 . Oil separator  214  may not completely remove the oil from the refrigerant, and as a result, some oil may still flow into other components of system  400  (e.g., low side heat exchangers  206 A and  206 B). 
     Valves  216 A and  216 B control a flow of primary refrigerant from flash tank  204  to low side heat exchangers  206 A and  206 B. System  400  may include any suitable number of valves  216  based on the number of low side heat exchangers  206  in system  400 . Valve  216 A and  216 B may be thermal expansion valves that cool refrigerant flowing through valves  216 A and  216 B. For example, valves  216 A and  216 B may reduce the pressure and therefore the temperature of the refrigerant flowing through valves  216 A and  216 B. Valves  216 A and  216 B reduce pressure of the refrigerant flowing into valves  216 A and  216 B. The temperature of the refrigerant may then drop as pressure is reduced. As a result, refrigerant entering valves  216 A and  216 B may be cooler when leaving valves  216 A and  216 B. When valve  216 A is open, primary refrigerant flows from flash tank  204  to low side heat exchanger  206 A. When valve  216 A is closed, primary refrigerant does not flow from flash tank  204  to low side heat exchanger  206 A. When valve  216 B is open, primary refrigerant flows from flash tank  204  to low side heat exchanger  206 B. When valve  216 B is closed, primary refrigerant does not flow from flash tank  204  to low side heat exchanger  206 B. 
     Valves  218 A and  218 B control a flow of refrigerant and/or oil from low side heat exchangers  206 A and  206 B to vessels  222 A and  222 B. System  400  may include any suitable number of valves  218  based on the number of low side heat exchangers  206  in system  400 . During the oil drain mode of operation, valves  218 A and  218 B may be open to allow refrigerant and/or oil to flow from low side heat exchanger  206 A and  206 B to vessels  222 A and  222 B. During the normal mode of operation and the oil return mode of operation, valves  218 A and  218 B may be closed. In certain embodiments, valve  218 A and  218 B may be solenoid valves. 
     Valves  220 A and  220 B control a flow of refrigerant from compressor  212  to vessels  222 A and  222 B. System  400  may include any suitable number of valves  220  based on the number of low side heat exchangers  206  in system  400 . In certain embodiments, valves  220 A and  220 B may be solenoid valves. During the oil return mode of operation, valves  220 A and  220 B may be open to allow refrigerant from compressor  212  to flow to vessels  222 A and  222 B. That refrigerant pushes oil and/or refrigerant that has collected in vessels  222 A and  222 B towards accumulator  208 B. During the normal mode of operation and the oil drain mode of operation, valves  220 A and  220 B are closed. 
     Vessels  222 A and  222 B collect oil and/or refrigerant for low side heat exchangers  206 A and  206 B. System  400  may include any suitable number of vessels  222  based on the number of low side heat exchangers  206  in system  400 . By collecting oil in vessels  222 A and  222 B, that oil is allowed to drain from low side heat exchangers  206 A and  206 B, thereby improving the efficiency of low side heat exchangers  206 A and  206 B. During the oil drain mode of operation, oil drains from low side heat exchangers  206 A and  206 B into vessels  222 A and  222 B. During the oil return mode of operation, refrigerant from compressor  212  pushes oil that has collected in vessels  222 A and  222 B towards accumulator  208 B for return to compressor  212 . During the normal mode of operation, valves  218 A,  218 B,  220 A,  220 B,  236 A, and  236 B are closed to prevent refrigerant and oil from flowing into vessels  222 A and  222 B. Vessels  222 A and  222 B may include any suitable components for holding and/or storing refrigerant and/or oil. For example, vessels  222 A and  222 B may include one or more of a container/tank and a coil (e.g., a container/tank only, a coil only, a container/tank and a coil arranged in series with one another, a coil disposed within a container/tank, etc.). The container/tank and/or coil may be of any suitable shape and size. 
     Valves  224 A and  224 B control a flow of refrigerant from low side heat exchangers  206 A and  206 B to accumulator  208 A. System  400  may include any suitable number of valves  224  based on the number of low side heat exchangers  206  in system  400 . In certain embodiments, valves  224 A and  224 B are check valves that allow refrigerant to flow when a pressure of that refrigerant exceeds a threshold. In this manner, valves  224 A and  224 B direct a flow of refrigerant from low side heat exchangers  206 A and  206 B to accumulator  208 A and control a pressure of the refrigerant flowing to accumulator  208 A. 
     Valves  236 A and  236 B control a flow of refrigerant from vessels  222 A and  222 B to accumulator  208 A. System  400  may include any suitable number of valves  236  based on the number of low side heat exchangers  206  in system  400 . During the oil drain mode of operation, valves  236 A and  236 B may be open to direct refrigerant in vessels  222 A and  222 B to accumulator  208 A. For example, during the oil drain mode, refrigerant and oil from low side heat exchanger  206 A and/or  206 B may drain into vessel  222 A and/or  222 B. Valves  236 A and  236 B allow the refrigerant to flow to accumulator  208 A while keeping the oil in vessel  222 A and/or  222 B. During the normal mode of operation and the oil return mode of operation, valves  236 A and  236 B are closed. 
     Valves  238 A and  238 B control a flow of oil and refrigerant from vessels  222 A and  222 B to accumulator  208 B. System  400  may include any suitable number of valves  238  based on the number of low side heat exchangers  206  in system  400 . In particular embodiments, valves  238 A and  238 B are check valves that allow refrigerant to flow when a pressure of that refrigerant exceeds a threshold. During the normal mode of operation and the oil drain mode of operation, the pressure of the oil and refrigerant in vessels  222 A and  222 B may not be sufficiently high to open valves  238 A and  238 B. As a result, oil and/or refrigerant does not flow through valves  238 A and  238 B to accumulator  208 B. During the oil return mode of operation, pressurized refrigerant from compressor  212  is directed to vessel  222 A and/or  222 B. As a result, the pressure of the oil and/or refrigerant in vessel  222 A and/or  222 B may be sufficiently high to push the oil and/or refrigerant through valve  238 A and/or  238 B to accumulator  208 B. 
     Valve  226  controls a flow of refrigerant from flash tank  204  to compressor  212 . Valve  226  may be referred to as a flash gas bypass valve because the refrigerant flowing through valve  226  may take the form of a flash gas from flash tank  204 . If the pressure of the refrigerant in flash tank  204  is too high, valve  226  may open to direct flash gas from flash tank  204  to compressor  212 . As a result, the pressure of flash tank  204  may be reduced. 
     Controller  228  controls the operation of cooling system  400 . For example, controller  228  may cause certain valves to open and/or close to transition cooling system  400  from one mode of operation to another. Controller  228  includes a processor  230  and a memory  232 . This disclosure contemplates processor  230  and memory  232  being configured to perform any of the operations of controller  228  described herein. 
     Processor  230  is any electronic circuitry, including, but not limited to microprocessors, application specific integrated circuits (ASIC), application specific instruction set processor (ASIP), and/or state machines, that communicatively couples to memory  232  and controls the operation of controller  228 . Processor  230  may be 8-bit, 16-bit, 32-bit, 64-bit or of any other suitable architecture. Processor  230  may include an arithmetic logic unit (ALU) for performing arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that fetches instructions from memory and executes them by directing the coordinated operations of the ALU, registers and other components. Processor  230  may include other hardware that operates software to control and process information. Processor  230  executes software stored on memory to perform any of the functions described herein. Processor  230  controls the operation and administration of controller  228  by processing information received from sensors  234  and memory  232 . Processor  230  may be a programmable logic device, a microcontroller, a microprocessor, any suitable processing device, or any suitable combination of the preceding. Processor  230  is not limited to a single processing device and may encompass multiple processing devices. 
     Memory  232  may store, either permanently or temporarily, data, operational software, or other information for processor  230 . Memory  232  may include any one or a combination of volatile or non-volatile local or remote devices suitable for storing information. For example, memory  232  may include random access memory (RAM), read only memory (ROM), magnetic storage devices, optical storage devices, or any other suitable information storage device or a combination of these devices. The software represents any suitable set of instructions, logic, or code embodied in a computer-readable storage medium. For example, the software may be embodied in memory  232 , a disk, a CD, or a flash drive. In particular embodiments, the software may include an application executable by processor  230  to perform one or more of the functions described herein. 
     Sensors  234  may include one or more sensors  234  that detect characteristics of cooling system  400 . For example, sensors  234  may include one or more temperature sensors that detect the temperature of refrigerant in cooling system  400 . In certain embodiments, these temperature sensors may detect the temperature of a primary refrigerant in low side heat exchangers  206 A and/or  206 B and a temperature of secondary refrigerant in low side heat exchangers  206 A and  206 B. In some embodiments, sensors  234  include one or more level sensors that detect a level of oil in cooling system  400 . 
     Controller  228  may transition system  400  from one mode of operation to another based on the detections made by one or more sensors  234 . For example, controller  228  may transition cooling system  400  from the normal mode of operation to the oil drain mode of operations when the difference between the detected temperatures of the primary refrigerant and a secondary refrigerant increases above a threshold. As another example, controller  228  may transition cooling system  400  from the normal mode of operation to the oil drain mode of operation when a detected level of oil in cooling system  400  falls below or exceeds a threshold. Controller  228  may transition system  400  between different modes of operation by controlling various components of system (e.g., by opening and/or closing valves). 
     The different modes of operation of cooling system  400  will now be described using  FIGS.  4 A- 4 C .  FIG.  4 A  illustrates cooling system  400  operating in a normal mode of operation. During the normal mode of operation, valves  216 A and  216 B are open to allow primary refrigerant from flash tank  204  to flow to low side heat exchangers  206 A and  206 B. Low side heat exchangers  206 A and  206 B transfer heat from secondary refrigerants to the primary refrigerant. The cooled secondary refrigerant is then cycled to cooling systems  106 A and  106 B. The heated primary refrigerant is directed through valves  224 A and  224 B to accumulator  208 A. Accumulator  208 A separates gaseous and liquid portions of the received refrigerant. Compressor  210  compresses the gaseous refrigerant from accumulator  208 A and directs that refrigerant to accumulator  208 B. Accumulator  208 B separates gaseous and liquid portions of the received refrigerant. Compressor  212  compresses the refrigerant from accumulator  208 B. Oil separator  214  separates an oil from the refrigerant from compressor  212 . Valves  218 A,  218 B,  220 A,  220 B,  236 A, and  236 B are closed. 
     As cooling system  400  operates in the normal mode of operation, oil from compressors  210  and/or  212  may begin to build in low side heat exchangers  206 A and/or  206 B (e.g., because oil separator  214  does not separate all the oil from the refrigerant). As this oil builds, the efficiency of low side heat exchangers  206 A and/or  206 B may decrease. In certain embodiments, the drop in efficiency in low side heat exchangers  206 A and/or  206 B may cause less heat transfer to occur within low side heat exchangers  206 A and/or  206 B. As a result, the temperature differential between the primary refrigerant and the secondary refrigerant in low side heat exchangers  206 A and/or  206 B may increase. One or more sensors  234  may detect a temperature of the primary refrigerant and a temperature of the secondary refrigerant in low side heat exchangers  206 A and/or  206 B. When controller  228  determines that this temperature differential increases above a threshold, controller  228  may determine that the oil building up in low side heat exchangers  206 A and/or  206 B should be drained and returned to compressors  210  and/or  212 . As a result, controller  228  may transition cooling system  400  from the normal mode of operation to the oil drain mode of operation. 
     In certain embodiments, one or more sensors  234  may detect a level of oil in cooling system  400 . For example, one or more sensors  234  may detect a level of oil in low side heat exchangers  206 A and/or  206 B or a level of oil in a reservoir of oil separator  214 . Based on the detected levels of oil, controller  228  may transition cooling system  400  from the normal mode of operation to the oil drain mode of operation. For example, if one or more sensors  234  detect that a level of oil in low side heat exchanger  206 A or  206 B exceeds a threshold, controller  228  may determine that the oil in low side heat exchanger  206 A or  206 B should be drained and transition cooling system  400  from the normal mode of operation to the oil drain mode of operation. As another example, if one or more sensors  234  detect that a level of oil in a reservoir of oil separator  214  falls below a threshold, controller  228  may determine that low side heat exchanger  206 A or  206 B should be drained and transition cooling system  400  from the normal mode of operation to the oil drain mode of operation. 
       FIG.  4 B  illustrates cooling system  400  operating in the oil drain mode of operation. To transition cooling system  400  from the normal mode of operation to the oil drain mode of operation, controller  228  closes one of valves  216 A and  216 B. In this manner, primary refrigerant stops flowing from flash tank  204  to one of low side heat exchangers  206 A and  206 B. In the example of  FIG.  4 B , valve  216 A is closed and valve  216 B is open. In this manner, primary refrigerant continues to flow to low side heat exchanger  206 B and oil in low side heat exchanger  206 A is allowed to drain. This disclosure contemplates that valve  216 B may instead be closed and valve  216 A remains open during the oil drain mode. Generally, cooling system  400  may drain oil from any suitable number of low side heat exchangers  206  while allowing other low side heat exchangers  206  to operate in a normal mode of operation. 
     During the oil drain mode of operation, controller  228  also opens one of valves  218 A and  218 B and one of valves  236 A and  236 B. In the example of  FIG.  4 B , valve  218 A is open to allow refrigerant and/or oil to drain from low side heat exchanger  206 A through valve  218 A to vessel  222 A. Valve  218 B remains closed. Additionally, valve  236 A is open to allow refrigerant in vessel  222 A to flow to accumulator  208 A through valve  236 A. Valve  236 B remains closed. In this manner, oil that has collected in low side heat exchanger  206 A is directed to vessel  222 A by valve  218 A. This disclosure contemplates controller  228  opening any suitable number of valves  218  and  236  during the oil drain mode while keeping other valves  218  and  236  closed so that their corresponding low side heat exchangers  206  may operate in the normal mode of operation. Controller  228  keeps valves  220 A and  220 B closed during the oil drain mode of operation. 
     Controller  228  may transition cooling system  400  from the oil drain mode of operation to the oil return mode of operation after cooling system  400  has been in the oil drain mode of operation for a particular period of time (e.g., one to two minutes). After that period of time, cooling system  400  transitions from the oil drain mode of operation to the oil return mode of operation. 
       FIG.  4 C  illustrates cooling system  400  in the oil return mode of operation. In the example of  FIG.  4 C , controller  228  transitions low side heat exchanger  206 A to the oil return mode of operation. 
     During the oil return mode of operation, valve  216 A remains closed so that low side heat exchanger  206 A does not receive primary refrigerant from flash tank  204 . Valve  218 A is closed so that oil and refrigerant from low side heat exchanger  206 A does not continue draining to vessel  222 A. Valve  236 A is also closed to prevent refrigerant from flowing from vessel  222 A to accumulator  208 A. Controller  228  opens valve  220 A, so that valve  220 A directs refrigerant from compressor  212  into vessel  222 A. This refrigerant pushes the oil in vessel  222 A through valve  238 A to accumulator  208 B. The oil then collects in accumulator  208 B. In certain embodiments, accumulator  208 B includes a hole  402  in the U-shaped pipe through which oil that is collecting at the bottom of accumulator  208 B may be sucked into the U-shaped pipe and be directed to compressor  212 . As a result, the oil that is collected by accumulator  208 B may be returned to compressor  212 . Valve  216 B is open and valves  218 B and  220 B are closed during the oil return mode so that low side heat exchanger  206 B supplies refrigerant to compressors  210  and  212  that can be directed through valve  220 A. 
     In particular embodiments, controller  228  transitions cooling system  400  from the oil return mode of operation back to the normal mode of operation after cooling system  400  has been in the oil return mode of operation for a particular period of time (e.g., ten to twenty seconds). To transition the example of  FIG.  4 C  back to the normal mode of operation, controller  228  closes valve  220 A and opens valve  216 A. 
     Although  FIGS.  4 A- 4 C  show cooling system  400  transitioning through the normal mode of operation, the oil drain mode of operation, and the oil return mode of operation to drain and return oil collected in low side heat exchanger  206 A, this disclosure contemplates cooling system  400  transitioning through these three modes of operation for any low side heat exchanger  206  in system  400 . By transitioning through these three modes, oil that is collected in low side heat exchanger  206  may be returned to compressor  210  and/or compressor  212  in particular embodiments. 
       FIG.  5    is a flowchart illustrating a method  500  of operating an example cooling system  400 . In particular embodiments, various components of cooling system  400  perform the steps of method  500 . By performing method  500 , an oil that has collected in a low side heat exchanger  206  may be returned to a compressor  210  or  212 . 
     A high side heat exchanger  202  removes heat from a primary refrigerant (e.g., carbon dioxide) in step  502 . In step  504 , a flash tank  204  stores the primary refrigerant. In step  506 , controller  228  determines whether cooling system  400  should be in a first mode of operation (e.g., a normal mode of operation). For example, controller  228  may determine a difference in the temperature between a primary refrigerant and a secondary refrigerant in low side heat exchanger  206  to determine whether cooling system  400  should be in the first mode of operation. As another example, controller  228  may determine a level of oil in the cooling system  400  to determine whether the cooling system  400  should be in the first mode of operation. 
     If the system  400  should be in the first mode of operation, controller  228  closes valves  218 A,  220 A, and/or  236 A (if they are not already closed) in step  508 . In step  510 , low side heat exchanger  206 A uses the primary refrigerant to cool a secondary refrigerant. Accumulator  208 A receives the primary refrigerant from low side heat exchanger  206 A in step  512 . Compressor  210  compresses the primary refrigerant from accumulator  208 A in step  514 . In step  516 , accumulator  208 B receives the refrigerant from compressor  210 . In step  518 , compressor  212  compresses the primary refrigerant from accumulator  208 B. 
     If controller  228  determines that cooling system  400  should not be in the first mode of operation, controller  228  determines whether cooling system  400  should be in the second mode of operation (e.g., an oil drain mode of operation) in step  520 . As discussed previously, controller  228  may determine whether cooling system  400  should be in the second mode of operation based on a detected temperature differential and/or oil level. If controller  228  determines that cooling system  400  should be in the second mode of operation, controller  228  opens valve  218 A (if valve  218 A is not already open) in step  522 . In step  524 , controller  228  closes valve  220 A (if valve  220 A is not already closed). In step  526 , controller  228  opens valve  236 A (if valve  236 A is not already open). As a result, oil from low side heat exchanger  206 A is allowed to drain through valve  218 A to vessel  222 A. Refrigerant in vessel  222 A is allowed to flow to accumulator  208 A through valve  236 A. 
     If controller  228  determines that cooling system  400  should not be in the first mode or second mode of operation, controller  228  may determine that cooling system  400  should be in a third mode of operation (e.g., an oil return mode of operation). In response, controller  228  closes valves  218 A and  236 A (if valves  218 A and  236 A are not already closed) in step  528 . Controller  228  then opens valve  220 A (if valve  220 A is not already opened) in step  530 . As a result, refrigerant from compressor  212  flows to vessel  222 A through valve  220 A to push oil that is collected in vessel  222 A to accumulator  208 B. 
     Modifications, additions, or omissions may be made to method  500  depicted in  FIG.  5   . Method  500  may include more, fewer, or other steps. For example, steps may be performed in parallel or in any suitable order. While discussed as system  400  (or components thereof) performing the steps, any suitable component of system  400  may perform one or more steps of the method. 
     Modifications, additions, or omissions may be made to the systems and apparatuses described herein without departing from the scope of the disclosure. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. Additionally, operations of the systems and apparatuses may be performed using any suitable logic comprising software, hardware, and/or other logic. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
     This disclosure may refer to a refrigerant being from a particular component of a system (e.g., the refrigerant from the compressor, the refrigerant from the flash tank, etc.). When such terminology is used, this disclosure is not limiting the described refrigerant to being directly from the particular component. This disclosure contemplates refrigerant being from a particular component (e.g., the low side heat exchanger) even though there may be other intervening components between the particular component and the destination of the refrigerant. For example, the compressor receives a refrigerant from the low side heat exchanger even though there may be valves, vessels, and/or an accumulator between the low side heat exchanger and the compressor. 
     Although the present disclosure includes several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present disclosure encompass such changes, variations, alterations, transformations, and modifications as fall within the scope of the appended claims.