Patent Publication Number: US-10782055-B2

Title: Cooling system

Description:
TECHNICAL FIELD 
     This disclosure relates generally to a cooling system. 
     BACKGROUND 
     Cooling systems may cycle a refrigerant to cool various spaces. For example, a refrigeration system may cycle refrigerant to cool spaces near or around refrigeration loads. After the refrigerant absorbs heat, it can be cycled back to the refrigeration loads to defrost the refrigeration loads. 
     SUMMARY 
     Cooling systems cycle refrigerant to cool various spaces. For example, a refrigeration system cycles refrigerant to cool spaces near or around refrigeration loads. These loads include metal components, such as coils, that carry the refrigerant. As the refrigerant passes through these metallic components, frost and/or ice may accumulate on the exterior of these metallic components. The ice and/or frost reduce the efficiency of the load. For example, as frost and/or ice accumulates on a load, it may become more difficult for the refrigerant within the load to absorb heat that is external to the load. Typically, the ice and frost accumulate on loads in a low temperature section of the system (e.g., freezer cases). 
     In existing systems, one way to address frost and/or ice accumulation on the load is to cycle refrigerant back to the load after the refrigerant has absorbed heat from the load. Usually, discharge from a low temperature compressor is cycled back to a load to defrost that load. In this manner, the heated refrigerant passes over the frost and/or ice accumulation and defrosts the load. This process of cycling hot refrigerant over frosted and/or iced loads is known as hot gas defrost. Existing cooling systems that have a hot gas defrost cycle typically use a stepper valve at the low temperature compressor discharge to increase the pressure of the refrigerant so that the refrigerant can be directed to the flash tank after defrost. However, the pressure difference between the refrigerant at the low temperature compressor and the refrigerant in the flash tank can be small (e.g., 4 bar). As a result, large piping is typically used to limit the pressure drop of the refrigerant during defrost, which can be costly and increase the footprint of the system. 
     This disclosure contemplates a cooling system that performs hot gas defrost while maintaining a larger pressure differential (e.g., 12 bar). The system includes an accumulator that separates refrigerant into liquid and vapor components. After refrigerant is used to defrost a load, the refrigerant is directed to the accumulator. The accumulator separates this refrigerant into liquid and vapor components. The liquid component is directed to the flash tank through an ejector, and the vapor component is directed to a medium temperature compressor. Because the pressure of the refrigerant at the accumulator is lower than the pressure of the refrigerant at the flash tank, the pressure differential of the refrigerant between the low temperature compressor and the accumulator is increased. As a result, smaller piping may be used, which reduces cost and the footprint of the system. Certain embodiments of the cooling system are described below. 
     According to an embodiment, an apparatus includes an ejector, a first load, a second load, a third load, a first compressor, a second compressor, and an accumulator. The ejector directs a refrigerant to a flash tank that stores the refrigerant. The first load uses the refrigerant from the flash tank to cool a first space proximate the first load. The second load uses the refrigerant from the flash tank to cool a second space proximate the second load. The first compressor compresses the refrigerant from the first load. The accumulator separates the refrigerant from the second load into a first liquid portion and a first vapor portion and directs the first liquid portion to the ejector. The ejector directs the first liquid portion to the flash tank. The accumulator directs the first vapor portion to the second compressor. The second compressor compresses the first vapor portion. During a first mode of operation, the third load uses the refrigerant from the flash tank to cool a third space proximate the third load, the first compressor compresses the refrigerant from the third load, and the second compressor compresses the refrigerant from the first compressor. During a second mode of operation, the first compressor directs the refrigerant to the third load to defrost the third load, the accumulator separates the refrigerant that defrosted the third load into a second liquid portion and a second vapor portion, the ejector directs the second liquid portion to the flash tank, and the second compressor compresses the second vapor portion. 
     According to another embodiment, a method includes directing, by an ejector, a refrigerant to a flash tank and storing, by the flash tank, the refrigerant. The method also includes using, by a first load, the refrigerant from the flash tank to cool a first space proximate the first load and using, by a second load, the refrigerant from the flash tank to cool a second space proximate the second load. The method further includes compressing, by a first compressor, the refrigerant from the first load and separating, by an accumulator, the refrigerant from the second load into a first liquid portion and a first vapor portion. The method also includes directing, by the accumulator, the first liquid portion to the ejector, directing, by the ejector, the first liquid portion to the flash tank, directing, by the accumulator, the first vapor portion to a second compressor, and compressing, by the second compressor, the first vapor portion. During a first mode of operation, the method includes using, by a third load, the refrigerant from the flash tank to cool a third space proximate the third load, compressing, by the first compressor, the refrigerant from the third load, and compressing, by the second compressor, the refrigerant from the first compressor. During a second mode of operation, the method includes directing, by the first compressor, the refrigerant to the third load to defrost the third load, separating, by the accumulator, the refrigerant that defrosted the third load into a second liquid portion and a second vapor portion, directing, by the ejector, the second liquid portion to the flash tank, and compressing, by the second compressor, the second vapor portion. 
     According to yet another embodiment, a system includes a high side heat exchanger, an ejector, a first load, a second load, a third load, a first compressor, a second compressor, and an accumulator. The high side heat exchanger removes heat from a refrigerant. The ejector directs the refrigerant from the high side heat exchanger to a flash tank that stores the refrigerant. The first load uses the refrigerant from the flash tank to cool a first space proximate the first load. The second load uses the refrigerant from the flash tank to cool a second space proximate the second load. The first compressor compresses the refrigerant from the first load. The accumulator separates the refrigerant from the second load into a first liquid portion and a first vapor portion and directs the first liquid portion to the ejector. The ejector directs the first liquid portion to the flash tank. The accumulator directs the first vapor portion to the second compressor. The second compressor compresses the first vapor portion. During a first mode of operation, the third load uses the refrigerant from the flash tank to cool a third space proximate the third load, the first compressor compresses the refrigerant from the third load, and the second compressor compresses the refrigerant from the first compressor. During a second mode of operation, the first compressor directs the refrigerant to the third load to defrost the third load, the accumulator separates the refrigerant that defrosted the third load into a second liquid portion and a second vapor portion, the ejector directs the second liquid portion to the flash tank, and the second compressor compresses the second vapor portion. 
     Certain embodiments provide one or more technical advantages. For example, an embodiment reduces the size and cost of piping in a cooling system by directing refrigerant used to defrost a load to an accumulator, rather than directly to a flash tank. As another example, an embodiment reduces the amount of refrigerant in a cooling system and the size of a flash tank in the cooling system by directing refrigerant used to defrost a load to an accumulator, rather than directly to a flash tank. 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; 
         FIG. 2  illustrates an example cooling system; 
         FIG. 3  illustrates an example cooling system; and 
         FIG. 4  is a flowchart illustrating a method of operating an example cooling system. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure and its advantages are best understood by referring to  FIGS. 1 through 4  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, a refrigeration system cycles refrigerant to cool spaces near or around refrigeration loads. These loads include metal components, such as coils, that carry the refrigerant. As the refrigerant passes through these metallic components, frost and/or ice may accumulate on the exterior of these metallic components. The ice and/or frost reduce the efficiency of the load. For example, as frost and/or ice accumulates on a load, it may become more difficult for the refrigerant within the load to absorb heat that is external to the load. Typically, the ice and frost accumulate on loads in a low temperature section of the system (e.g., freezer cases). 
     In existing systems, one way to address frost and/or ice accumulation on the load is to cycle refrigerant back to the load after the refrigerant has absorbed heat from the load. Usually, discharge from a low temperature compressor is cycled back to a load to defrost that load. In this manner, the heated refrigerant passes over the frost and/or ice accumulation and defrosts the load. This process of cycling hot refrigerant over frosted and/or iced loads is known as hot gas defrost. Existing cooling systems that have a hot gas defrost cycle typically use a stepper valve at the low temperature compressor discharge to increase the pressure of the refrigerant so that the refrigerant can be directed to the flash tank after defrost. However, the pressure difference between the refrigerant at the low temperature compressor and the refrigerant in the flash tank can be small (e.g., 4 bar). As a result, large piping is typically used to limit the pressure drop of the refrigerant during defrost, which can be costly and increase the footprint of the system. 
     This disclosure contemplates a cooling system that performs hot gas defrost while maintaining a larger pressure differential (e.g., 12 bar). The system includes an accumulator that separates refrigerant into liquid and vapor components. After refrigerant is used to defrost a load, the refrigerant is directed to the accumulator. The accumulator separates this refrigerant into liquid and vapor components. The liquid component is directed to the flash tank through an ejector, and the vapor component is directed to a medium temperature compressor. Because the pressure of the refrigerant at the accumulator is lower than the pressure of the refrigerant at the flash tank, the pressure differential of the refrigerant between the low temperature compressor and the accumulator is increased. As a result, smaller piping may be used, which reduces cost and the footprint of the system. 
     In certain embodiments, the size and cost of piping in a cooling system are reduced by directing refrigerant used to defrost a load to an accumulator, rather than directly to a flash tank. In some embodiments, the amount of refrigerant in a cooling system and the size of a flash tank in the cooling system are reduced by directing refrigerant used to defrost a load to an accumulator, rather than directly to a flash tank. The cooling system will be described using  FIGS. 1 through 4 .  FIG. 1  will describe an existing cooling system with hot gas defrost.  FIGS. 2 through 4  describe the cooling system with an accumulator and ejector. 
       FIG. 1  illustrates an example cooling system  100 . As shown in  FIG. 1 , system  100  includes a high side heat exchanger  105 , a flash tank  110 , a medium temperature load  115 , low temperature loads  120 A- 120 D, a medium temperature compressor  125 , a low temperature compressor  130 , and a valve  135 . By operating valve  135 , system  100  allows for hot gas to be circulated to a low temperature load  120  to defrost low temperature load  120 . After defrosting low temperature load  120 , the hot gas and/or refrigerant is cycled back to flash tank  110 . This disclosure contemplates cooling system  100  or any cooling system described herein including any number of loads, whether low temperature or medium temperature. 
     High side heat exchanger  105  removes heat from a refrigerant. When heat is removed from the refrigerant, the refrigerant is cooled. This disclosure contemplates high side heat exchanger  105  being operated as a condenser and/or a gas cooler. When operating as a condenser, high side heat exchanger  105  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  105  cools gaseous refrigerant and the refrigerant remains a gas. In certain configurations, high side heat exchanger  105  is positioned such that heat removed from the refrigerant may be discharged into the air. For example, high side heat exchanger  105  may be positioned on a rooftop so that heat removed from the refrigerant may be discharged into the air. As another example, high side heat exchanger  105  may be positioned external to a building and/or on the side of a building. This disclosure contemplates any suitable refrigerant (e.g., carbon dioxide) being used in any of the disclosed cooling systems. 
     Flash tank  110  stores refrigerant received from high side heat exchanger  105 . This disclosure contemplates flash tank  110  storing refrigerant in any state such as, for example, a liquid state and/or a gaseous state. Refrigerant leaving flash tank  110  is fed to low temperature loads  120 A- 120 D and medium temperature load  115 . In some embodiments, a flash gas and/or a gaseous refrigerant is released from flash tank  110 . By releasing flash gas, the pressure within flash tank  110  may be reduced. 
     System  100  includes a low temperature portion and a medium temperature portion. The low temperature portion operates at a lower temperature than the medium temperature portion. In some refrigeration systems, the low temperature portion may be a freezer system and the medium temperature system may be a regular refrigeration system. In a grocery store setting, the low temperature portion may include freezers used to hold frozen foods, and the medium temperature portion may include refrigerated shelves used to hold produce. Refrigerant flows from flash tank  110  to both the low temperature and medium temperature portions of the refrigeration system. For example, the refrigerant flows to low temperature loads  120 A- 120 D and medium temperature load  115 . When the refrigerant reaches low temperature loads  120 A- 120 D or medium temperature load  115 , the refrigerant removes heat from the air around low temperature loads  120 A- 120 D or medium temperature load  115 . As a result, the air is cooled. The cooled air may then be circulated such as, for example, by a fan to cool a space such as, for example, a freezer and/or a refrigerated shelf. As refrigerant passes through low temperature loads  120 A- 120 D and medium temperature load  115 , the refrigerant may change from a liquid state to a gaseous state as it absorbs heat. This disclosure contemplates including any number of low temperature loads  120 And medium temperature loads  115  in any of the disclosed cooling systems. 
     The refrigerant cools metallic components of low temperature loads  120 A- 120 D and medium temperature load  115  as the refrigerant passes through low temperature loads  120 A- 120 D and medium temperature load  115 . For example, metallic coils, plates, parts of low temperature loads  120 A- 120 D and medium temperature load  115  may cool as the refrigerant passes through them. These components may become so cold that vapor in the air external to these components condenses and eventually freeze or frost onto these components. As the ice or frost accumulates on these metallic components, it may become more difficult for the refrigerant in these components to absorb heat from the air external to these components. In essence, the frost and ice acts as a thermal barrier. As a result, the efficiency of cooling system  100  decreases the more ice and frost that accumulates. Cooling system  100  may use heated refrigerant to defrost these metallic components. 
     Refrigerant flows from low temperature loads  120 A-D and medium temperature load  115  to compressors  125  and  130 . This disclosure contemplates the disclosed cooling systems including any number of low temperature compressors  130  and medium temperature compressors  125 . Both the low temperature compressor  130  and medium temperature compressor  125  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. Low temperature compressor  130  compresses refrigerant from low temperature loads  120 A- 120 D and sends the compressed refrigerant to medium temperature compressor  125 . Medium temperature compressor  125  compresses a mixture of the refrigerant from low temperature compressor  130  and medium temperature load  115 . Medium temperature compressor  125  then sends the compressed refrigerant to high side heat exchanger  105 . 
     Valve  135  may be opened or closed to cycle refrigerant from low temperature compressor  130  back to a low temperature load  120 . The refrigerant may be heated after absorbing heat from the other low temperature loads  120  and being compressed by low temperature compressor  130 . The hot refrigerant and/or hot gas is then cycled over the metallic components of the low temperature load  120  to defrost it. Afterwards, the hot gas and/or refrigerant is cycled back to flash tank  110 . There may be additional valves between low temperature compressor  130  and low temperature loads  120 A-D that control to which load  120 A-D is defrosted by the refrigerant coming from low temperature compressor  130 . This process of cycling heated refrigerant over a low temperature load  120  to defrost it is referred to as a defrost cycle. 
     Existing cooling systems that have a hot gas defrost cycle typically use a stepper valve at the low temperature compressor discharge to increase the pressure of the refrigerant so that the refrigerant can be directed to the flash tank after defrost. However, the pressure difference between the refrigerant at the low temperature compressor and the refrigerant in the flash tank can be small (e.g., 4 bar). As a result, large piping is typically used to limit the pressure drop of the refrigerant during defrost, which can be costly and increase the footprint of the system. 
     This disclosure contemplates a cooling system that performs hot gas defrost while maintaining a larger pressure differential (e.g., 12 bar). The system includes an accumulator that separates refrigerant into liquid and vapor components. After refrigerant is used to defrost a load, the refrigerant is directed to the accumulator. The accumulator separates this refrigerant into liquid and vapor components. The liquid component is directed to the flash tank through an ejector, and the vapor component is directed to a medium temperature compressor. Because the pressure of the refrigerant at the accumulator is lower than the pressure of the refrigerant at the flash tank, the pressure differential of the refrigerant between the low temperature compressor and the accumulator is increased. As a result, smaller piping may be used, which reduces cost and the footprint of the system. Embodiments of the cooling system are described below using  FIGS. 2-4 . These figures illustrate embodiments that include a certain number of loads and compressors for clarity and readability. However, this disclosure contemplates these embodiments including any suitable number of loads and compressors. 
       FIG. 2  illustrates an example cooling system  200 . As seen in  FIG. 2 , cooling system  200  includes a high side heat exchanger  105 , an ejector  205 , a flash tank  110 , medium temperature loads  115 A and  115 B, low temperature loads  120 A and  120 B, medium temperature compressor  125 , low temperature compressor  130 , valves  135 A,  135 B,  135 C, and  135 D, an accumulator  210 , a parallel compressor  215 , an oil separator  220 , and valves  225 A,  225 B,  225 C, and  225 D. Generally, accumulator  210  separates a refrigerant used to defrost a load into liquid and vapor portions. Accumulator  210  then directs the liquid portion to ejector  205  in flash tank  110  and the vapor portion to medium temperature compressor  125 . In this manner, the pressure differential between accumulator  210  and low temperature compressor  130  is increased relative to the pressure differential between low temperature compressor  130  and flash tank  110 , which reduces the cost and size of piping used to contain the refrigerant in certain embodiments. 
     High side heat exchanger  105 , flash tank  110 , medium temperature loads  115 A and  115 B, low temperature loads  120 A and  120 B, and low temperature compressor  130  operate similarly in system  200  as they did in system  100 . For example, high side heat exchanger  105  removes heat from a refrigerant. Flash tank  110  stores the refrigerant. Medium temperature loads  115 A and  115 B and low temperature loads  120 A and  120 B use the refrigerant from flash tank  110  to cool spaces proximate those loads. Low temperature compressor  130  compresses the refrigerant from low temperature loads  120 A and  120 B. 
     Ejector  205  receives refrigerant from high side heat exchanger  105  and/or accumulator  210 . Ejector  205  then ejects and/or directs this refrigerant to flash tank  110 . In some systems, the pressure of the ejected refrigerant is controlled and/or adjusted by the pressure of the refrigerant from accumulator  110  and the shape of ejector  205 . 
     Accumulator  210  separates a received refrigerant into liquid and vapor portions. For examples, accumulator  210  receives the refrigerant from medium temperature loads  115 A and  115 B. Accumulator  210  then separates the received refrigerant into a liquid portion  212  and a vapor portion  214 . Accumulator  210  then directs some of liquid portion  212  to ejector  205  and some of the vapor portion  214  to medium compressor  125 . Ejector  205  directs liquid portion  212  to flash tank  110  for storage. Medium temperature compressor  125  compresses vapor portion  214 . Some of liquid portion  212  and vapor portion  214  may remain in accumulator  210  instead of being directed to other components of system  200 . During a defrost cycle, accumulator  210  receives refrigerant that was used to defrost a load. Accumulator  210  separates this refrigerant into liquid portion  212  and vapor portion  214 . Some of liquid portion  212  is then directed to ejector  205  and flash tank  110 , and some of vapor portion  214  is directed to medium temperature compressor  125 . 
     Parallel compressor  215  compresses a flash gas from flash tank  110 . Flash tank  110  may discharge the flash gas to parallel compressor  215 . After parallel compressor  215  compresses the flash gas, parallel compressor  215  directs the compressed flash gas to oil separator  220 . By discharging flash gas, the pressure of the refrigerant in flash tank  110  can be regulated. 
     Oil separator  220  separates an oil from received refrigerant. For example, oil separator  210  may receive refrigerant from parallel compressor  215  and/or medium temperature compressor  125 . Oil separator  220  separates oil from this received refrigerant and directs the refrigerant to high side heat exchanger  105 . By separating oil from the received refrigerant, oil separator  220  prevents the oil from flowing to other components of system  200 . In this manner the oil does not damage other components of system  200 . 
     During a first mode of operation (e.g., a regular refrigeration cycle), medium temperature loads  115 A and  115 B, and low temperature loads  120 A and  120 B use refrigerant from flash tank  110  to cool spaces proximate those loads. The refrigerant used by low temperature loads  120 A and  120 B is directed to low temperature compressor  130 . The refrigerant used by medium temperature loads  115 A and  115 B is directly to accumulator  210 . Low temperature compressor  130  compresses the refrigerant from low temperature load from  120 A and  120 B and directs the compressed refrigerant to medium temperature compressor  125 . Accumulator  210  separates the refrigerant from medium temperature loads  115 A and  115 B into liquid portion  212  and vapor portion  214 . Accumulator  210  then directs some of liquid portion  212  to ejector  205  and some of vapor portion  214  to medium temperature compressor  125 . Medium temperature compressor  125  then compresses the refrigerant from low temperature compressor  130  and accumulator  210 . After compressing the refrigerant, medium temperature compressor  125  directs the refrigerant to oil separator  220  and high side heat exchanger  105 . In this manner, the refrigerant is cycled through system  200  to cool spaces proximate the loads. 
     During a defrost cycle, or a second mode of operation, one or more of the loads is defrosted using the refrigerant from low temperature compressor  130 . Valves  135 A,  135 B,  135 C,  135 D,  225 A,  225 B,  225 C, and/or  225 D are controlled to allow refrigerant to flow from low temperature compressor  130  back to one of the loads to defrost the load. For example, in one defrost cycle, valves  135 C and  225 C can open to allow refrigerant to flow from low temperature compressor  130  through low temperature load  120 A to defrost low temperature load  120 A. In another defrost cycle, valve  135 B and  225 B can open to allow refrigerant to flow from low temperature compressor  130  through medium temperature load  115 B to defrost medium temperature load  115 B. This disclosure contemplates using refrigerant from low temperature compressor  130  to defrost any number of loads and any type of loads. 
     This disclosure contemplates valves  135 A,  135 B,  135 C,  135 D,  225 A,  225 B,  225 C, and  225 D being any type of valve. For example, one or more of these valves may be a check valve that allows refrigerant to flow through the valve when the refrigerant has reached a threshold pressure. As another example, one or more of these valves may be a solenoid valve that can be opened or closed by a control. Using a previous example, valve  135 C may be a solenoid valve and valve  225 C may be a check valve. In this example, during a defrost cycle, valve  135 C opens to allow refrigerant to flow from low temperature compressor  130  to low temperature load  120 A to defrost low temperature load  120 A. The pressure of that refrigerant builds until it is high enough to pass through check valve  225 C and flow to accumulator  210 . When the defrost cycle ends, valve  135 C is closed. In another example, both valves  135 C and  225 C are solenoid valves. During the defrost cycle, both valves  135 C and  225 C are opened to allow refrigerant to flow from low temperature compressor  130  through low temperature load  120 A to defrost low temperature load  120 A. When the defrost cycle ends, valves  135 C and  225 C are closed. 
     After the refrigerant defrosts a load, the refrigerant is directed to accumulator  210 . Accumulator  210  separates that refrigerant into liquid portion  212  and vapor portion  214 . Accumulator  210  then directs some of liquid portion  212  to ejector  205  and flash tank  110  and some of vapor portion  214  to medium temperature compressor  125 . Ejector  205  directs liquid portion  212  to flash tank  110  for storage. Medium temperature compressor  125  compresses vapor portion  214 . Because the pressure of the refrigerant at accumulator  210  is lower than the pressure of the refrigerant at flash tank  110 , the pressure differential between low temperature compressor  130  and accumulator  210  is greater than the pressure differential between low temperature compressor  130  and flash tank  110 . As a result, in certain embodiments, by directing the refrigerant used to defrost the loads to accumulator  210 , the cost and size of piping used to carry that refrigerant is reduced compared to a system that directs the refrigerant directly to flash tank  110  after defrost. Additionally, in some embodiments, by directing the refrigerant used to defrost the loads to accumulator  210  the amount of refrigerant in the system and the size of flash tank  110  can be reduced without negatively impacting the efficiency of system  200 . 
     In certain embodiments, a defrost cycle to defrost a medium temperature load  115  may be different from a defrost cycle to defrost a low temperature load  120 . As a result, during a first defrost cycle, or a second mode of operation, a low temperature load  120  may be defrosted. Then, in a second defrost cycle, or a third mode of operation, a medium temperature load  115  may be defrosted. 
       FIG. 3  illustrates an example cooling system  300 . As seen in  FIG. 3 , system  300  includes a high side heat exchanger  105 , an ejector  205 , a flash tank  110 , medium temperature loads  115 A and  115 B, low temperature loads  120 A and  120 B, low temperature compressor  130 , accumulator  210 , medium temperature compressor  125 , parallel compressor  215 , oil separator  220 , valves  135 A,  135 B,  135 C, and  135 D, and valves  225 A,  225 B,  225 C, and  225 D. Generally, accumulator  210  separates a refrigerant that was used to defrost a load into a liquid portion  212  and a vapor portion  214 . Accumulator  210  then directs some of the liquid portion  212  to ejector  205  and flash tank  110  and some of the vapor portion  214  to medium temperature compressor  125 . Because the pressure of the refrigerant at accumulator  210  is lower than the pressure of the refrigerant at flash tank  110 , the pressure differential between low temperature compressor  130  and accumulator  210  is greater than the pressure differential between low temperature compressor  130  and flash tank  110 . As a result, the size of the piping used to carry the refrigerant may be reduced when the refrigerant used to defrost the loads is directed to accumulator  210  instead of directly to flash tank  110  in certain embodiments. 
     High side heat exchanger  105 , ejector  205 , flash tank  110 , medium temperature loads  115 A and  115 B, low temperature loads  120 A and  120 B, low temperature compressor  130 , medium temperature compressor  125 , accumulator  210 , parallel compressor  215 , oil separator  220 , valves  135 A,  135 B,  135 C and  135 D, and valves  225 A,  225 B,  225 C and  225 D operate similarly as they did in system  200 . For example, high side heat exchanger  105  removes heat from a refrigerant. Ejector  205  directs the refrigerant to flash tank  110 . Flash tank  110  stores the refrigerant. Medium temperature loads  115 A and  115 B and low temperature loads  120 A and  120 B use the refrigerant from flash tank  110  to cool spaces proximate those loads. Low temperature compressor  130  compresses the refrigerant from low temperature loads  120 A and  120 B. Accumulator  210  separates refrigerant into liquid portion  212  and vapor portion  214 . Accumulator  210  then directs some of liquid portion  212  to ejector  205  and flash tank  110  and some of vapor portion  214  to medium temperature compressor  125 . Ejector  205  directs liquid portion  212  to flash tank  110  for storage. Medium temperature compressor  125  compresses vapor portion  214 . Parallel compressor  215  compresses flash gas discharged from flash tank  110 . Oil separator  220  separates oil from refrigerant received from parallel compressor  215  and medium temperature compressor  125 . 
     An important difference between system  300  and system  200  is that medium temperature loads  115 A and  115 B are arranged in series in system  300 , whereas these loads are arranged in parallel in system  200 . In other words, in system  300 , medium temperature load  115 B uses refrigerant from flash tank  110  that has passed through medium temperature load  115 A. After medium temperature load  115 B uses that refrigerant from medium temperature load  115 A to cool a space proximate medium temperature load  115 B, medium temperature load  115 B directs the refrigerant to accumulator  210 . Likewise, medium temperature load  115 A uses refrigerant directly from flash tank  110  to cool a space proximate medium temperature load  115 A and then directs that refrigerant to medium temperature load  115 B. As shown in  FIG. 3 , it is possible to use accumulator  210  to increase the pressure differential of the refrigerant even though medium temperature loads  115 A and  115 B are arranged in series as opposed to in parallel in system  200 . 
     During a first mode of operation, or regular refrigeration cycle, medium temperature loads  115 A and  115 B and low temperature loads  120 A and  120 B use refrigerant to cool spaces proximate those loads. Low temperature loads  120 A and  120 B direct the refrigerant to low temperature compressor  130 . Medium temperature load  115 A directs refrigerant to medium temperature load  115 B. Medium temperature load  115 B directs the refrigerant to accumulator  210 . Low temperature compressor  130  compresses the refrigerant from low temperature loads  120 A and  120 B and directs the refrigerant to medium temperature compressor  125 . Accumulator  210  separates the refrigerant from medium temperature load  115 B into a liquid portion  212  and vapor portion  214 . Accumulator  210  then directs some of the liquid portion  212  to ejector  205  in flash tank  110  and some of vapor portion  214  to medium temperature compressor  125 . Ejector  205  directs liquid portion  212  to flash tank  110  for storage. Medium temperature compressor  125  compresses vapor portion  214  and the refrigerant from low temperature compressor  130  and directs that refrigerant to oil separator  220 . 
     During a second mode of operation, or defrost cycle, low temperature compressor  130  directs refrigerant back to a load to defrost the load. For example, during a low temperature defrost cycle, low temperature compressor  130  directs refrigerant back to low temperature load  120 A. Valves  135 C and  225 C can open to allow refrigerant to flow from low temperature compressor  130  through low temperature load  120 A to defrost low temperature load  120 A. As another example, during a medium temperature defrost cycle, valves  135 A and  225 A can open to allow refrigerant to flow from low temperature compressor  130  through medium temperature load  115 A to defrost medium temperature load  115 A. 
     After the refrigerant defrosts the load, the refrigerant is directed to accumulator  210 . Accumulator  210  separates the refrigerant into liquid portion  212  and vapor portion  214 . Accumulator  210  then directs some of liquid portion  212  to ejector  205  and flash tank  110  and some of vapor portion  214  to medium temperature compressor  125 . Ejector  205  directs liquid portion  212  to flash tank  110  for storage. Medium temperature compressor  125  compresses vapor portion  214 . In this manner, the size and cost of piping used to carry the refrigerant is reduced compared to implementations where refrigerant used to defrost the loads flows directly to flash tank  110 . 
       FIG. 4  is a flowchart illustrating a method  400  of operating an example cooling system. In certain embodiments, various components of system  200  or system  300  perform the steps of method  400 . By performing method  400 , the size and cost of piping used to carry refrigerant is reduced in certain embodiments. 
     In step  405 , an ejector directs the refrigerant to a flash tank. The flash tank stores the refrigerant in step  410 . In step  415 , a first load uses the refrigerant to cool a first space. A second load uses the refrigerant to cool a second space in step  420 . In step  425 , a first compressor compresses the refrigerant from the first load. An accumulator separates the refrigerant from the second load into a first liquid portion and a first vapor portion in step  430 . In step  435 , the accumulator directs the first liquid portion to the ejector. The ejector directs the first liquid portion to the flash tank in steps  440 . In step  445 , the accumulator directs the first vapor portion to a second compressor. The second compressor compresses the first vapor portion in step  450 . 
     During a first mode of operation, such as, for example, a regular refrigeration cycle, a third load uses the refrigerant to cool a third space in step  455 . In step  460 , the first compressor compresses the refrigerant from the third load. The second compressor compresses the refrigerant from the first compressor in step  465 . 
     During a second mode of operation, such as, for example, a defrost cycle, the first compressor directs the refrigerant to the third load to defrost the third load in step  470 . In step  475 , the accumulator separates the refrigerant that defrosted the third load into a second liquid portion and a second vapor portion. The ejector directs the second liquid portion to the flash tank in step  480 . In step  485 , the second compressor compresses the second vapor portion. 
     Modifications, additions, or omissions may be made to method  400  depicted in  FIG. 4 . Method  400  may include more, fewer, or other steps. For example, steps may be performed in parallel or in any suitable order. While discussed as systems  200  and/or  300  (or components thereof) performing the steps, any suitable component of systems  200  and/or  300  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 medium temperature compressor, the refrigerant from the low temperature 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 high side heat exchanger) even though there may be other intervening components between the particular component and the destination of the refrigerant. For example, the flash tank receives a refrigerant from the accumulator even though there is an ejector between the flash tank and the accumulator. 
     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.