Patent Publication Number: US-10767906-B2

Title: Hot gas defrost in a cooling system

Description:
TECHNICAL FIELD 
     This disclosure relates generally to a cooling system, specifically hot gas defrost in 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 OF THE DISCLOSURE 
     According to one embodiment, a system includes a high side heat exchanger, a first load, a second load, a first compressor, a second compressor, and a third compressor. The high side heat exchanger removes heat from a refrigerant. The first load uses the refrigerant to remove heat from a first space proximate the first load. The second load uses the refrigerant to remove heat from a second space proximate the second load. The first compressor compresses the refrigerant from the first load and sends the refrigerant to the first load. The refrigerant defrosts the first load. The second compressor compresses the refrigerant from the second load and the refrigerant from the first load that defrosted the first load. The third compressor compresses the refrigerant from the first compressor. 
     According to another embodiment, a method includes removing heat from a refrigerant using a high side heat exchanger and removing heat from a first space proximate a first load using the refrigerant. The method also includes removing heat from a second space proximate a second load using the refrigerant and compressing the refrigerant from the first load using a first compressor. The method further includes sending the refrigerant compressed at the first compressor to the first load. The refrigerant defrosts the first load and compressing the refrigerant from the second load using a second compressor. The method also includes compressing the refrigerant from the first load that defrosted the first load using the second compressor and compressing the refrigerant from the first compressor using the third compressor. 
     According to yet another embodiment, a system includes a first load, a second load, a first compressor, a second compressor, and a third compressor. The first load uses a refrigerant to remove heat from a first space proximate the first load. The second load uses the refrigerant to remove heat from a second space proximate the second load. The first compressor compresses the refrigerant from the first load and sends the refrigerant to the first load. The refrigerant defrosts the first load. The second compressor compresses the refrigerant from the second load and the refrigerant from the first load that defrosted the first load. The third compressor compresses the refrigerant from the first compressor. 
     Certain embodiments may provide one or more technical advantages. For example, an embodiment reduces the size of the piping used in existing cooling systems. As another example, an embodiment removes a stepper valve used in existing cooling systems. 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; and 
         FIG. 3  is a flowchart illustrating a method of operating the example cooling system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure and its advantages are best understood by referring to  FIGS. 1 through 3  of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     Cooling systems may cycle refrigerant to cool various spaces. For example, a refrigeration system may cycle refrigerant to cool spaces near or around refrigeration loads. These loads may 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 may 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. 
     In existing systems, one way to address frost and/or ice accumulation on the load is to cycle the refrigerant to the load after the refrigerant has absorbed heat from the load. In this manner, the heated refrigerant may pass over the frost and/or ice accumulation and defrost 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 require a stepper valve to build up discharge pressure for hot gas defrost. For example, the stepper valve may increase the pressure of the refrigerant from 28 bar to 40 bar. After the hot gas is used to defrost the load, the gas is pumped to a flash tank that usually stores refrigerant at 36 bar. The small pressure difference between the hot gas supply and the flash tank (for example, 40 bar−36 bar=4 bar) results in the need for large piping to limit the pressure drop across the hot gas/refrigerant line. If the pressure drop across the hot gas/refrigerant is too large, then the pressure at the flash tank may overtake the pressure at the stepper valve and the flow of the hot gas may reverse and/or stop. The large piping increases the material cost of the refrigeration system and it increases the amount of space occupied by the refrigeration system. 
     This disclosure contemplates a cooling system that removes the need for a stepper valve. The cooling system includes a parallel compressor that receives refrigerant from a low temperature compressor. The refrigerant from the low temperature compressor is also cycled back to a low temperature load to defrost the low temperature load. After defrosting, the refrigerant is then cycled to a medium temperature compressor. In this manner, the pressure difference between the hot gas supply and the hot gas return is increased. The increased pressure difference may allow piping of reduced sizing to be used in the cooling system. Reducing the size of the piping may reduce the cost of the system and the space needed to install the system. In some embodiments, reducing the size of the piping may also allow a reduction in the refrigerant charge and the size of a flash tank used in the system. 
     The cooling system will be described using  FIGS. 1 through 3 .  FIG. 1  will describe an existing cooling system with hot gas defrost.  FIGS. 2 and 3  describe the cooling system with improved hot gas defrost. 
       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 , a low temperature load  120 , 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 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 . 
     High side heat exchanger  105  may remove 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, a fluid cooler, 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 fluid cooler, high side heat exchanger  105  cools liquid refrigerant and the refrigerant remains 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. 
     Flash tank  110  may store 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 load  120  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  may include a low temperature portion and a medium temperature portion. The low temperature portion may operate 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 may flow from flash tank  110  to both the low temperature and medium temperature portions of the refrigeration system. For example, the refrigerant may flow to low temperature load  120  and medium temperature load  115 . When the refrigerant reaches low temperature load  120  or medium temperature load  115 , the refrigerant removes heat from the air around low temperature load  120  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 load  120  and medium temperature load  115  the refrigerant may change from a liquid state to a gaseous state as it absorbs heat. 
     The refrigerant may cool metallic components of low temperature load  120  and medium temperature load  115  as the refrigerant passes through low temperature load  120  and medium temperature load  115 . For example, metallic coils, plates, parts of low temperature load  120  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 may flow from low temperature load  120  and medium temperature load  115  to compressors  125  and  130 . This disclosure contemplates system  100  including any number of low temperature compressors  130  and medium temperature compressors  125 . Both the low temperature compressor  130  and medium temperature compressor  125  may be configured 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  may compress refrigerant from low temperature load  120  and send the compressed refrigerant to medium temperature compressor  125 . Medium temperature compressor  125  may compress refrigerant from low temperature compressor  130  and medium temperature load  115 . Medium temperature compressor  125  may then send 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 low temperature load  120 . The refrigerant may be heated after absorbing heat from low temperature load  120  and being compressed by low temperature compressor  130 . The hot refrigerant and/or hot gas is then cycled over the metallic components of low temperature load  120  to defrost those components. Afterwards, the hot gas and/or refrigerant is cycled back to flash tank  110 . 
     Valve  135  includes a stepper valve that increases the pressure of the hot gas and/or refrigerant so that it can be cycled back to low temperature load  120  to defrost low temperature load  120 . For example, the stepper valve may increase the pressure of the hot gas and/or refrigerant from 28 bar to 40 bar. The stepper valve is needed so that the pressure of the hot gas and/or refrigerant can be increased above the pressure of flash tank  110  (the pressure of flash tank  110  may be 36 bar, for example). In this manner, the hot gas and/or refrigerant may be at a high enough pressure to be cycled back into flash tank  110 . 
     In this example, the pressure difference between the hot gas and/or refrigerant and flash tank  110  may be around 4 bar because the stepper valve increases the pressure of the refrigerant to 40 bar and flash tank  110  is held at 36 bar. This difference in pressure of 4 bar is small and results in system  100  needing large piping to limit the pressure drop of the hot gas and/or refrigerant as it defrosts low temperature load  120  and then travels to flash tank  110 . If the pressure drop across the hot gas and/or refrigerant line is too large, then the pressure at flash tank  110  may overcome the pressure at the stepper valve and the flow of hot gas and/or refrigerant may reverse and/or stop The large piping results in increased cost and a larger footprint for system  100 . 
       FIG. 2  illustrates an example cooling system  200 . As shown in  FIG. 2 , system  200  includes a high side heat exchanger  105 , a flash tank  110 , a medium temperature load  115 , a low temperature load  120 , a medium temperature compressor  125 , a low temperature compressor  130 , a parallel compressor  205 , and a valve  210 . System  200  includes several components that are also present in system  100 . These components may operate similarly as they do in system  100 . However, system  200  differs from system  100  in that system  200  includes a different configuration that allows for a reduction in the size of the piping used to carry the hot gas that defrosts low temperature load  120 . 
     Parallel compressor  205  may be a compressor that compresses refrigerant from low temperature compressor  130  and flash gas from flash tank  110 . Parallel compressor  205  sends the compressed refrigerant and/or flash gas to high side heat exchanger  105 . Unlike system  100 , low temperature compressor  130  in system  200  does not send compressed refrigerant directly to medium temperature compressor  125 . 
     Valve  210  may be open and/or closed to allow hot gas and/or refrigerant to be cycled back to low temperature load  120  to defrost low temperature load  120 . After defrosting low temperature load  120 , the hot gas and/or refrigerant may be cycled to medium temperature compressor  125  instead of flash tank  110 . In certain embodiments, the configuration of system  200  may result in a larger pressure differential between the hot gas supply and the hot gas return. Using the numbers from the previous example, the hot gas supply, for example the hot gas coming from low temperature compressor  130 , may be at the pressure of flash tank  110  which is 36 bar. The pressure at medium temperature compressor  125  may be 28 bar resulting in a pressure difference of 8 bar, which is larger than the pressure difference in system  100  of 4 bar. As a result of the larger pressure difference, the size of the piping used to transport the hot gas and/or refrigerant may be reduced. The reduced size decreases the cost of system  200  and it reduces the footprint of system  200 . In some embodiments, the larger pressure difference also means that valve  210  does not need to include a stepper valve. 
     In certain embodiments, system  200  may include additional low temperature loads  120 . For example, system  200  may include a second low temperature load  215  that receives refrigerant from flash tank  110 . The second low temperature load  215  may send refrigerant to low temperature compressor  130  and/or a second low temperature compressor  130 . The compressed refrigerant may then be sent to parallel compressor  205  and/or may be cycled back to low temperature load  120  and/or the second low temperature load  215  to defrost those loads. 
     In certain embodiments, system  200  may include a heat exchanger that transfers heat between refrigerant from high side heat exchanger  105  and refrigerant from medium temperature load  115 . The heat exchanger may also transfer heat between refrigerant from high side heat exchanger  105  and refrigerant that is used to defrost low temperature load  120 . In this manner, the heat of the refrigerant arriving at medium temperature compressor  125  may be regulated. 
     In particular embodiments, system  200  includes an oil separator  220  before high side heat exchanger  105 . The oil separator  220  may separate oils from the refrigerant from medium temperature compressor  125  and parallel compressor  205 . By separating the oil from the refrigerant, it may be easier for high side heat exchanger  105  to remove heat from the refrigerant. Additionally, separating oil from the refrigerant may increase the lifetime and/or efficiency of other components of system  200 . The oil separator  220  may separate the oil from the refrigerant and send the refrigerant to high side heat exchanger  105 . 
     This disclosure contemplates system  200  including any number of components. For example, system  200  may include any number of low temperature loads, medium temperature loads, and air conditioning loads. As another example, system  200  may include any number of low temperature compressors, medium temperature compressors, and parallel compressors. As yet another example, system  200  may include any number of high side heat exchangers  105  and flash tanks  110 . This disclosure also contemplates cooling system  200  using any appropriate refrigerant. For example, cooling system  200  may use a carbon dioxide refrigerant. This disclosure also contemplates system  200  being configured for hot gas defrost on any of medium temperature load(s)  115  and low temperature load(s)  120 . After the hot gas is used to defrost a load, the hot gas may be sent to medium temperature compressor  125 . System  200  may include multiple valves  210  that direct the hot gas to any of medium temperature load(s)  115  and low temperature load(s)  120 . 
       FIG. 3  is a flowchart illustrating a method  300  of operating the example cooling system  200  of  FIG. 2 . Various components of system  200  perform the steps of method  300 . In particular embodiments, performing method  300  may allow for the size of the piping used to transport hot gas and/or refrigerant to be reduced thereby leading to a reduction in cost and a reduction in footprint of system  200 . 
     High side heat exchanger  105  removes heat from a refrigerant in step  305 . In step  310 , low temperature load  120  removes heat from a first space proximate low temperature load  120 . In step  315 , medium temperature load  115  removes heat from a second space proximate medium temperature load  115 . Low temperature compressor  130  compresses the refrigerant from low temperature load  120  in step  320 . In step  325 , the compressed refrigerant from low temperature compressor  130  is used to defrost low temperature load  120 . Medium temperature compressor  125  compresses the refrigerant from medium temperature load  115  in step  330 . In step  335 , medium temperature compressor  125  compresses the refrigerant used to defrost low temperature load  120 . In step  340 , parallel compressor  205  compresses the refrigerant from low temperature compressor  130 . 
     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 various components of cooling system  200  performing the steps, any suitable component or combination of components of system  200  may perform one or more steps of the method. 
     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.