Patent Publication Number: US-2021164711-A1

Title: Cooling system for low temperature storage

Description:
TECHNICAL FIELD 
     The present disclosure relates to a cooling system for a low temperature storage. 
     BACKGROUND ART 
     A cooling system for cooling a low temperature storage may be generally understood as a cooling system for cooling a large warehouse of a factory in which low temperatures, in particular, sub-zero temperatures must be maintained or a food storage (showcase) in which refrigerating/freezing is required. 
     In a process of driving the cooling system, a phenomenon in which frosting of an evaporator included in the system is made may occur. In order to remove the frost, the cooling system needs to perform a defrosting operation. As an example, the defrosting operation may be performed periodically, or may be performed when an evaporation temperature of the evaporator is less than or equal to a set temperature. 
     Conventionally, in order to perform the defrosting operation, the cooling system is configured such that an electric heater is installed at a position adjacent to the evaporator. When the electric heater is driven, heat generated from the electric heater is transferred to the evaporator, so that the frost can be removed. 
     Information on the related prior patent documents is as follows. 
     PRIOR PATENT LITERATURE 
     Korean Patent Registration number: 10-1266936, Registration date: May 16, 2013 
     Title of invention: Eco-friendly storage control device for reducing carbon generation 
     However, according to such a conventional defrosting method using a heater, there were the following problems. 
     First, there is a problem that a cost increases due to consumption in excessive electric energy. 
     Second, the cooling operation through the evaporator is stopped while the defrosting operation by the heater is performed, so that the temperature of the storage is raised, and accordingly, causing a problem in which the freshness of food stored in the storage is lowered. 
     Third, there is a problem that the replacement or repair cost of the heater increases due to frequent failure of the heater. 
     DISCLOSURE 
     Technical Problem 
     The present disclosure has been proposed in order to solve this problem, and an object of the present disclosure is to provide a cooling system for a low temperature storage, capable of performing a defrosting operation of a first evaporator using hot gas. 
     In addition, another object of the present disclosure is to provide a cooling system for a low temperature storage, in which a condensed refrigerant which has undergone defrosting is expanded and evaporated in a second evaporator to simultaneously perform a defrosting operation and a cooling operation. 
     In addition, still another object of the present disclosure is to provide a cooling system for a low temperature storage, which includes a bypass pipe extending from an outlet side of an evaporator to an inlet side of an outdoor heat exchanger to guide the refrigerant passing through the evaporator to an outdoor heat exchanger through the bypass pipe during a defrosting operation to enable the outdoor heat exchanger to function as an evaporator, thereby obtaining an amount of heat required for defrosting from an outdoor air heat source. 
     Technical Solution 
     According to an embodiment of the present disclosure, a cooling system for a low temperature storage includes a first outdoor valve disposed between a compressor and an outdoor heat exchanger to selectively restrict inflow of refrigerant into the outdoor heat exchanger, and a first bypass pipe branched from an inlet side of the first outdoor valve to guide the refrigerant to bypass the outdoor heat exchanger, thereby guiding the refrigerant to bypass the outdoor heat exchanger when the defrosting operation of the cooling system is performed. 
     In addition, the cooling system may further include suction connection pipe branched from an outlet side of the first outdoor valve to guide a refrigerant passing through the outdoor heat exchanger to a suction side of the compressor, thereby using the heat amount of the outside air heat-exchanged in the outdoor heat exchanger as the amount of defrost heat during the defrosting operation. 
     In addition, the cooling system may further include a first evaporator into which a refrigerant flowing through the first bypass pipe is introduced to perform defrost, and a second evaporator disposed at an outlet side of the first evaporator to evaporate a refrigerant passing through the first evaporator to simultaneously perform a defrosting operation of a certain evaporator and a cooling operation of another evaporator. 
     In addition, the cooling system may further include a discharge pipe configured to extend from the outlet side of the compressor to the outdoor heat exchanger, and a liquid pipe disposed at an outlet side of the outdoor heat exchanger, a refrigerant condensed in the outdoor heat exchanger flowing through the liquid pipe. 
     The discharge pipe may include a first branch portion connected to one end of the first bypass pipe, and a second branch portion to which the suction connection pipe is connected, thereby facilitating the configuration of the first bypass pipe and the suction connection pipe. 
     The cooling system may further includes a third branch portion formed in the liquid pipe, and a second bypass pipe connected to the third branch portion, a refrigerant defrosted in the first evaporator flowing through the second bypass pipe, so that the refrigerant which has defrosted some evaporators can easily flow to the outdoor heat exchanger during the defrosting operation. 
     The cooling system may further include a bypass expansion device disposed in the second bypass pipe. 
     The liquid pipe may further include a fourth branch portion to which the other end of the first bypass pipe is connected. 
     The cooling system may further include a first connection pipe connected to the liquid pipe and to which a first defrost valve is disposed, and a third bypass pipe branched from the first connection pipe to guide a refrigerant to the first evaporator. 
     The cooling system may further include a first evaporator outlet pipe connected to the first evaporator, and a second defrost valve connected to the third bypass pipe and the first evaporator outlet pipe. 
     The cooling system may further include a connection pipe branched from the third bypass pipe, a second evaporator outlet pipe connected to the second evaporator, and a third defrost valve connected to the connection pipe and the second evaporator outlet pipe. 
     The cooling system may further include a gas tube disposed at a suction side of the compressor, a refrigerant evaporated in the second evaporator flowing through the gas tube, and the gas tube may include a fifth branch portion to which the suction connection pipe is connected. 
     The cooling system may further include a second outdoor valve disposed in the first bypass pipe. 
     The cooling system may further include a third outdoor valve disposed in the suction connection pipe. 
     The cooling system may further include an outdoor unit in which the compressor and the outdoor heat exchanger are disposed, and an indoor unit in which the first and second evaporators are disposed, and a connection unit disposed between the outdoor unit and the indoor unit and connected to the outdoor unit with three-way pipe and to the indoor unit with three-way pipe. 
     Advantageous Effects 
     According to the present disclosure as described above, since the defrosting operation of the first evaporator may be performed using hot gas, a defrost time is shortened and energy consumption for defrosting may be reduced. 
     In addition, since a condensed refrigerant that has undergone defrosting may be expanded and evaporated in the second evaporator, there is an effect that the defrosting operation and the cooling operation can be simultaneously performed. 
     In addition, a refrigerant which has defrosted the first evaporator is guided to the outdoor heat exchanger, so that the outdoor heat exchanger functions as an evaporator during the defrosting operation, thereby obtaining the amount of heat required for defrosting from the outdoor air heat source to improve the efficiency of the defrosting operation. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cycle diagram showing the configuration of a cooling system according to an embodiment of the present disclosure. 
         FIG. 2  is a cycle diagram showing a flow state of a refrigerant when performing a cooling operation of a cooling system according to an embodiment of the present disclosure. 
         FIG. 3  is a cycle diagram showing a flow state of a refrigerant when performing a defrosting operation of a first evaporator according to an embodiment of the present disclosure. 
         FIG. 4  is a cycle diagram showing a flow state of a refrigerant when performing a defrosting operation of a second evaporator according to an embodiment of the present disclosure. 
     
    
    
     MODE FOR INVENTION 
     Hereinafter, embodiments of the present disclosure will be described in more detail with reference to the drawings. However, the spirit of the present disclosure is not limited to the presented embodiments, and those skilled in the art who understand the spirit of the present disclosure will be able to easily propose other embodiments within the scope of the same idea. 
       FIG. 1  is a cycle diagram showing the configuration of a cooling system according to an embodiment of the present disclosure. 
     Referring to  FIG. 1 , a cooling system  1  according to an embodiment of the present disclosure may include an outdoor unit  10  disposed outdoors, an indoor unit  30  disposed in a storage to supply cool air to maintain a low temperature of the storage, and a connection unit  50  connected between the outdoor unit  10  and the indoor unit  30  to guide the flow of a refrigerant during a defrosting operation of the cooling system  10 . As an example, the cooling system  1  may cool the storage such that that an internal temperature of the storage is maintained below zero. 
     The connection unit  50  may be understood as a “defrosting device”, which includes a plurality of refrigerant pipes and valves for guiding the flow of a refrigerant to enable a defrosting operation. 
     The outdoor unit  10  may be detachably connected to the connection unit  50 . In detail, the outdoor unit  10  and the connection unit  50  may be connected with three-way pipe. The outdoor unit  10  may include a first service valve  175  connected to a liquid pipe  170  and a second service valve  255  connected to a gas pipe  111 . The outdoor unit  10  may further include a third service valve  176  connected to a second bypass pipe  135 . The second bypass pipe  135  may be understood as a liquid pipe through which a liquid refrigerant flows. 
     The connection unit  50  may be provided with three connection portions C 1 , C 2 , and C 3  connected to the outdoor unit  10 . The three connection portions C 1 , C 2 , C 3  include a first connection portion Cl connected to the first service valve  175  of the outdoor unit  10 , a second connection portion C 2  connected to a second service valve  255  of the outdoor unit  10 , and a third connection portion C 3  connected to a third service valve  176  of the outdoor unit  10 . 
     The cooling system  1  may include a first system pipe  175   a  connecting the first service valve  175  and the first connection portion C 1 , a second system pipe  255   a  connecting the second service valve  255  and the second connection portion C 2 , and a third system pipe  176 a connecting the third service valve  176  and the third connection portion C 3 . 
     The connection unit  50  and the indoor unit  30  may be connected with three-way pipe. The connection unit  50  may be provided with three connection portions C 4 , C 5  and C 6  connected to the indoor unit  30 . The three connection portions C 4 , C 5  and C 6  include a fourth connection portion C 4  connected to an evaporator inlet pipe  210  provided in the indoor unit  30 , a fifth connection portion C 5  connected to a first evaporator outlet pipe  227  and a sixth connection portion C 6  connected to a second evaporator outlet pipe  237 . 
     The outdoor unit  10  may include a compressor  110  for compressing a refrigerant, a suction pipe  112  connected to an inlet side of the compressor  110  to guide suction of a refrigerant through the compressor  110 , and a discharge pipe  114  connected to an outlet side of the compressor  110  to guide discharge of the refrigerant compressed by the compressor  110 . 
     The suction pipe  112  may be understood as a configuration of a gas pipe extending from a gas-liquid separator  105  to a suction port of the compressor  110  to guide the flow of the refrigerant. The gas-liquid separator  105  may be disposed at the suction side of the compressor  110  to separate a gaseous refrigerant from a refrigerant and supplying the gaseous refrigerant to the compressor  110 . The suction pipe  112  may guide the refrigerant discharged from the gas-liquid separator  105  to the suction port of the compressor  110 . 
     The outdoor unit  10  may further include a gas pipe  111  extending from the second service valve  255  to the gas-liquid separator  105 . The evaporated gaseous refrigerant may flow through the gas pipe  111 . 
     The discharge pipe  114  may be understood as a pipe extending from the discharge port of the compressor  110  to an outdoor heat exchanger  140 . 
     The outdoor unit  10  may further include an oil separator  115  installed in the discharge pipe  114  to separate oil which is discharged together with the refrigerant from the compressor  110  and a recovery pipe  116  extending from the oil separator  115  to the suction pipe  112 . Oil flowing through the recovery pipe  116  may be recovered into the compressor  110 . An oil amount adjusting device  117  for adjusting (reducing) the flow amount of the recovered oil may be installed in the recovery pipe  116 . For example, the oil amount adjusting device  117  may include a capillary tube. 
     The discharge pipe  114  may be provided with a first check valve  118  that allows only one-way flow of a refrigerant. The first check valve  118  may allow flow of a refrigerant from the compressor  110  to the outdoor heat exchanger  140 , in particular, to a first branch portion  114   a,  and restrict the flow of the refrigerant in the opposite direction. For example, the first check valve  118  may be disposed at the outlet side of the oil separator  115 . 
     The discharge pipe  114  may be installed with a first outdoor valve  121  that selectively allows flow of a refrigerant from the compressor  110  to the outdoor heat exchanger  140 . The first outdoor valve  121  may be installed between the first branch portion  114   a  and a second branch portion  114   b.  That is, the first branch portion  114   a  may be disposed at the inlet side of the first outdoor valve  121  and the second branch portion  114   b  may be disposed at the outlet side of the first outdoor valve  121 . 
     The first branch portion  114   a  may be understood as a point at which the discharge pipe  114  and a first bypass pipe  130  are connected, and the second branch portion  114   b  may be understood as a point at which the discharge pipe  114  and the suction connection pipe  125  are connected. 
     For example, the first outdoor valve  121  may include a solenoid valve which is able to be controlled in on/off manner or an electronic expansion valve of which an opening degree is controllable. 
     An outdoor heat exchanger  140  may be installed at an outlet side of the first outdoor valve  121 . The outdoor heat exchanger  140  is a device that performs heat exchange between a refrigerant and outside air, and an outdoor fan  140   a  for blowing outside air toward the outdoor heat exchanger  140  may be provided at one side of the outdoor heat exchanger  140 . When the outdoor fan  140   a  is driven, the refrigerant flowing through the outdoor heat exchanger  140  may be subjected to heat exchange with outside air. 
     A liquid pipe  170  may be connected to the outlet side of the outdoor heat exchanger  140 . The liquid pipe  170  may extend from the outdoor heat exchanger  140  to a first service valve  175 . 
     A receiver  160  and a second check valve  162  may be installed in the liquid pipe  170 . For example, the second check valve  162  may be disposed at an outlet side of the receiver  160 . 
     The receiver  160  may form a chamber for storing a refrigerant condensed in the outdoor heat exchanger  140 . A liquid refrigerant stored in the chamber may flow toward the first service valve  175 . The second check valve  162  may allow flow of a refrigerant from the outdoor heat exchanger  140  toward the first service valve  175  and restrict the flow of the refrigerant in the opposite direction. 
     A supercooler  164  may be installed at the outlet side of the second check valve  162 . In the supercooler  164 , heat exchange may be made between a main refrigerant condensed in the outdoor heat exchanger  140  and a branch refrigerant branched from the main refrigerant. 
     The outdoor unit  10  may further include an injection pipe  165  that is branched from the liquid pipe  170  and extends to the compressor  110  to guide the branch refrigerant to flow into the compressor  110 . A supercooling expansion device  167  for depressurizing the branch refrigerant may be installed in the injection pipe  165 . 
     Through heat exchange in the supercooler  164 , the main refrigerant may be supercooled, and the branch refrigerant may be vaporized and injected into the compressor  110 . 
     The outdoor unit  10  may further include a first bypass pipe  130  that guide a high-pressure refrigerant (hot gas refrigerant) compressed by the compressor  110  to bypass the outdoor heat exchanger  140 . The first bypass pipe  130  may be connected to the first branch portion  114   a  of the discharge pipe  114 . 
     When a defrosting operation of the cooling system  1  is performed, the hot gas refrigerant compressed by the compressor  110  may be branched from the first branch portion  114   a  and flow through the first bypass pipe  130 . 
     The first bypass pipe  130  may be connected to a fourth branch portion  172  of the liquid pipe  170 . That is, one end of the first bypass pipe  130  may be coupled to the first branch portion  114   a,  and the other end may be coupled to the fourth branch portion  172 . During the defrosting operation of the cooling system  1 , the hot gas refrigerant may be introduced into the first bypass pipe  130  from the first branch portion  114   a,  and then introduced into the liquid pipe  170  from the fourth branch portion  172 . 
     A second outdoor valve  122  that selectively allows flow of a refrigerant through the first bypass pipe  130  may be installed on the first bypass pipe  130 . For example, the second outdoor valve  122  may include a solenoid valve which is able to be controlled in on/off manner or an electronic expansion valve of which an opening degree is controllable. When the cooling operation of the cooling system  1  is performed, the second outdoor valve  122  may be controlled to be closed, and may be controlled to be opened when the defrosting operation is performed. 
     The outdoor unit  10  may further include a second bypass pipe  135  which is branched from the third branch portion  171  of the liquid pipe  170  and extends to the third service valve  176 . One end of the second bypass pipe  135  may be connected to the third branch portion  171 , and the other end may be connected to the third service valve  176 . 
     A bypass expansion device  136  may be installed in the second bypass pipe  135 . For example, the bypass expansion device  136  may include an electronic expansion valve. During the defrosting operation of the cooling system  1 , a refrigerant that has defrosted while passing through the first evaporator  220  or the second evaporator  230  may pass through the second bypass pipe  135  and flow into the outdoor heat exchanger  140 . In this case, the refrigerant may be depressurized in the bypass expansion device  136  and then evaporated in the outdoor heat exchanger  140 . 
     The outdoor unit  10  may further include a suction connection pipe  125  extending from the second branch portion  114   b  of the discharge pipe  114  to the fifth branch portion  113  of the gas pipe  111 . One end of the suction connection pipe  125  may be coupled to the second branch portion  114   b,  and the other end may be coupled to the fifth branch portion  113 . The fifth branch part  113  is a point where the suction connection pipe  125  and the gas pipe  111  are connected, and may be disposed at the inlet side of the gas-liquid separator  105 . 
     The second branch portion  114   b  may be disposed between the first branch portion  114   a  and the outdoor heat exchanger  140  based on refrigerant flow, and the first outdoor valve  121  may be disposed between the first branch portion and the second branch portion  114   a  and  114   b.    
     During the defrosting operation of the cooling system  1 , the refrigerant evaporated from the outdoor heat exchanger  140  may be combined with a gaseous refrigerant flowing into the gas pipe  111  from the fifth branch portion  113  while flowing through the suction connection pipe  125 . The combined refrigerant may be sucked into the compressor  110 . 
     The suction connection pipe  125  may be installed with a third outdoor valve  123  that selectively allows flow of a refrigerant through the suction connection pipe  125 . For example, the third outdoor valve  123  may include a solenoid valve which is able to be controlled in on/off manner or an electronic expansion valve of which an opening degree is controllable. When the cooling operation of the cooling system  1  is performed, the third outdoor valve  123  may be controlled to be closed, and may be controlled to be opened when the defrosting operation is performed. 
     The connection unit  50  may include a first connection pipe  214  extending from the first connection portion Cl to the fourth connection portion C 4 . A first defrost valve  215  for selectively opening the first connection pipe  214  may be installed in the first connection pipe  214 . For example, the first defrost valve  215  may include a solenoid valve capable of being controlled in on/off manner. 
     The connection unit  50  may further include a second connection pipe  217  extending from the fifth connection portion C 5  to the second connection portion C 2 . A second defrost valve  228  may be installed in the second connection pipe  217 . For example, the second defrost valve  228  may include a three-way valve. 
     The connection unit  50  may further include a third connection pipe  218  extending from the sixth connection portion C 6  to the second connection pipe  217 . The third connection pipe  218  may be connected to the second connection pipe  217  at the seventh branch portion  227   a  of the second connection pipe  217 . A third defrost valve  238  may be installed in the third connection pipe  218 . For example, the third defrost valve  238  may include a three-way valve. 
     The connection unit  50  may further include a fourth connection pipe  219  extending from the first connection pipe  214  to the third connection portion C 3 . The fourth connection pipe  219  may be connected to the first connection pipe  214  at the eighth branch portion  214   b  of the first connection pipe  214 . 
     The connection unit  50  may further include a third bypass pipe  240  extending from the first connection pipe  214  to the second defrost valve  228 . A sixth branch portion  214   a  to which the third bypass pipe  240  is connected may be provided in the first connection pipe  214 . During the defrosting operation of the cooling system  1 , hot gas may flow from the sixth branch portion  214   a  of the first connection pipe  214  to the third bypass pipe  240 , and may be introduced into the first evaporator  220  through the second defrost valve  228 . 
     The connection unit  50  may further include a connection pipe  242  branched from the third bypass pipe  240  and connected to the third defrost valve  238 . The first and second ports of the third defrost valve  238  may be connected to the third connection pipe  218 , and the third port may be connected to the connection pipe  242 . The first and second ports of the second defrost valve  228  may be connected to the second connection pipe  217 , and the third port may be connected to the third bypass pipe  240 . 
     The indoor unit  30  may include a plurality of evaporators  220  and  230  for evaporating a refrigerant. The plurality of evaporators  220  and  230  may include a first evaporator  220  and a second evaporator  230 . During the cooling operation of the cooling system  1 , the refrigerant may be evaporated in the first and second evaporators  220  and  230 . On the other hand, during the defrosting operation of the cooling system  1 , one of the first evaporator  220  and the second evaporator  230  may perform defrosting and the other may evaporate a refrigerant. 
     The indoor unit  30  may include an evaporator inlet pipe  210  extending from the fourth connection portion C 4  of the connection unit  50  to the inlet side of the first and second evaporators  220  and  230 . The evaporator inlet pipe  210  may be branched and connected to the first and second evaporators  220  and  230 , individually. The branch pipes connected to the first and second evaporators  220  and  230  may be referred to as “first evaporator branch pipe” and “second evaporator branch pipe”, respectively. 
     A first evaporator expansion device  225  may be installed in the first evaporator branch pipe, and a second evaporator expansion device  235  may be installed in the second evaporator branch pipe. For example, each of the first evaporator expansion device  225  and the second evaporator expansion device  235  may include an electronic expansion valve (EEV) for depressurizing a refrigerant. 
     A first evaporator fan  220   a  may be installed at one side of the first evaporator  220 , and a second evaporator fan  230   a  may be installed at one side of the second evaporator  230 . For example, the first and second evaporator fans  220   a  and  230   a  may be installed on a wall of a storage to blow cool air toward the storage. 
     The indoor unit  30  may further include a first evaporator outlet pipe  227  which is disposed at the outlet side of the first evaporator  220  and extends to the fifth connection portion C 5  of the connection unit  50  and a second evaporator outlet pipe  237  which is disposed at the outlet side of the second evaporator  230  and extends to the sixth connection portion C 6  of the connection unit  50 . 
     During the cooling operation of the cooling system  1 , refrigerants evaporated from the first and second evaporators  220  and  230  may flow into the connection unit  50  through the first and second evaporator outlet pipes  227  and  228 , respectively. 
     On the other hand, during the defrosting operation of the cooling system  1 , in particular, the defrosting operation of the first evaporator  220 , hot gas passing through the third bypass pipe  240  and the second defrost valve  228  may be introduced into the first evaporator  220  through the first evaporator outlet pipe  227 , so that the hot gas is used for defrosting, and is then depressurized in the second evaporator expansion device  235 , and evaporated in the second evaporator  230 . The evaporated refrigerant may flow into the sixth connection portion C 6  of the connection unit  50  through the second evaporator outlet pipe  237 . 
     During the defrosting operation of the cooling system  1 , in particular, the defrosting operation of the second evaporator  230 , hot gas passing through the third bypass pipe  240 , the connection pipe  242 , and the third defrost valve  238  may be introduced into the second evaporator  230  through the second evaporator outlet pipe  237 , so that the hot gas is used for defrosting, and is then depressurized in the first evaporator expansion device  225 , and evaporated in the first evaporator  220 . The evaporated refrigerant may flow into the fifth connection portion C 5  of the connection unit  50  through the first evaporator outlet pipe  227 . 
     Meanwhile, during the defrosting operation of the first evaporator  220  or the second evaporator  230 , at least a part of the refrigerants that have used to defrost the evaporator may be introduced to the connection unit  50  through the fourth connection portion C 4  and flows from the eighth branch portion  214   b  to the fourth connection pipe  219  to flow through the second bypass pipe  135  of the outdoor unit  10 . 
       FIG. 2  is a cycle diagram showing a flow state of a refrigerant when performing a cooling operation of a cooling system according to an embodiment of the present disclosure. 
     Referring to  FIG. 2 , when performing the cooling operation of the cooling system  1  according to the embodiment of the present disclosure, a high-pressure refrigerant compressed by the compressor  110  may pass through the first outdoor valve  121 , which is opened, through the discharge pipe  114  and may be introduced into the outdoor heat exchanger  140  and condensed. In this case, the second outdoor valve  122  and the third outdoor valve  123  may be controlled to be closed. 
     The refrigerant discharged from the outdoor heat exchanger  140  may be supercooled while flowing through the liquid pipe  170  and passing through the supercooler  164 . The refrigerant supercooled in the supercooler  164  may be discharged through the first service valve  175  and may be introduced into the connection unit  50  through the first connection portion C 1 . 
     Meanwhile, the branch refrigerant passing through the supercooler  164  may be injected into the compressor  110  through an injection pipe  165 . 
     The refrigerant introduced into the connection unit  50  may flow through the first connection pipe  214  and may be branched and introduced into the first and second evaporators  220  and  230  through the evaporator inlet pipe  210 . In this case, one port of the second defrost valve  228  to which the third bypass pipe  240  is connected and one port of the third defrost valve  238  to which the connection pipe  242  is connected may be closed so that the flow of the refrigerant to the third bypass pipe  240  and the connection pipe  242  may be restricted. 
     The refrigerant branched from the evaporator inlet pipe  210  may be evaporated in each of the first and second evaporators  220  and  230  to generate cold air, and the generated cold air may be supplied into the inside of the storage by the first and second evaporator fans  220   a  and  230   a.    
     The refrigerant evaporated from the first and second evaporators  220  and  230  may flow through the first and second evaporator outlet pipes  227  and  237 , respectively, and may flow into the connection unit  50 . In detail, the refrigerant flowing through the first evaporator outlet pipe  227  may be introduced into the connection unit  50  through the fifth connection portion C 5  and pass through the second defrost valve  228 . The refrigerant flowing through the second evaporator outlet pipe  237  may be introduced into the connection unit  50  through the sixth connection portion C 6  and may pass through the third defrost valve  238 . The refrigerant that has passed through the third defrost valve  238  may be combined with a refrigerant that has passed through the second defrost valve  228  in the seventh branch portion  227   a  of the second connection pipe  217 . 
     The combined refrigerant may be discharged from the connection unit  50  through the second connection portion C 2  and be introduced into the outdoor unit  10  through the second service valve  255 . The refrigerant introduced into the outdoor unit  10  may flow through the gas pipe  111  and may be sucked into the compressor  110  through the gas-liquid separator  105 . Such a cycle may be repeated, and the storage may be efficiently cooled by the circulation of such a refrigerant cycle. 
       FIG. 3  is a cycle diagram showing a flow state of a refrigerant when performing a defrosting operation of a first evaporator according to an embodiment of the present disclosure. 
     Referring to  FIG. 3 , when the defrosting operation of the cooling system  1  according to an embodiment of the present disclosure is performed, in particular when the defrosting operation of the first evaporator  220  is performed, a high-pressure refrigerant compressed by the compressor  110  may be introduced into the first bypass pipe  130  from the first branch portion  114   a  through the discharge pipe  114 . In this case, since the first outdoor valve  121  is closed and the second outdoor valve  122  is opened, the flow of the refrigerant into the outdoor heat exchanger  140  may be restricted. 
     The refrigerant flowing through the first bypass pipe  130  may be introduced into the liquid pipe  170  from the fourth branch portion  172  and may flow into the first service valve  175 . In this case, since the supercooling expansion device  167  is closed, the flow of the refrigerant from the fourth branch portion  172  to the injection pipe  165  may be restricted. The second check valve  162  may restrict the refrigerant from flowing into the outdoor heat exchanger  140  from the fourth branch portion  172 . 
     The refrigerant discharged from the outdoor unit  10  through the first service valve  175  may be introduced into the connection unit  50  through the first connection portion C 1 , and flow into the third bypass pipe  240  from the sixth branch portion  214   a.  In this case, since the first defrost valve  215  is closed, the flow of the refrigerant to the evaporator inlet pipe  210  may be restricted. 
     The refrigerant flowing through the third bypass pipe  240  may be introduced into the second defrost valve  228  and may be introduced into the first evaporator  220  through the first evaporator outlet pipe  227 . In this case, since one port of the third defrost valve  238  to which the connection pipe  242  is connected is closed, the flow of a refrigerant to the connection pipe  242  may be restricted. 
     The refrigerant flowing into the first evaporator  220  may form a high-pressure hot gas. Accordingly, while the hot gas refrigerant passes through the first evaporator  220 , the first evaporator  220  may be defrosted and the refrigerant may be condensed. At least a part of the refrigerants that have passed through the first evaporator  220  may be depressurized in the second evaporator expansion device  235  and may be evaporated in the second evaporator  230 . In this case, since the first evaporator expansion device  225  is fully opened, the refrigerant may not be depressurized while passing through the first evaporator expansion device  225 . 
     When the refrigerant is evaporated in the second evaporator  230 , cold air is generated, and the generated cold air may be supplied to the internal space of the storage by driving the second evaporator fan  230   a.  As described above, since the second evaporator  230  may perform a cooling operation while the first evaporator  220  is defrosted, a phenomenon in which the internal temperature of the storage is rapidly increased may be prevented. 
     The refrigerant evaporated in the second evaporator  230  may pass through the third defrost valve  238  through the second evaporator outlet pipe  237  and may be discharged from the connection unit  50  through the second connection portion C 2 . The refrigerant discharged from the connection unit  50  may be introduced into the outdoor unit  10  through the second service valve  255  to flow through the gas pipe  111 . The refrigerant may be sucked into the compressor  110  through the gas-liquid separator  105 . 
     On the other hand, a part of the refrigerant which has defrosted the first evaporator  220  may be introduced into the connection unit  50  through the fourth connection portion C 4 , and flow through the fourth connection pipe  219  in the eighth branch portion  214   b.  That is, a part of the refrigerant that have passed through the first evaporator  220  may be introduced into the second evaporator expansion device  235 , and the remaining refrigerant may flow into the fourth connection portion C 4 . In this case, since the first defrost valve  215  is closed, the refrigerant may be restricted from flowing toward the first connection portion C 1 . 
     The refrigerant which has flowed through the fourth connection pipe  219  may be introduced into the outdoor unit  10  through the third service valve  176  and flow through the second bypass pipe  135 . The refrigerant may be depressurized in the bypass expansion device  136  while flowing through the second bypass pipe  135 , and the depressurized refrigerant may be introduced into the outdoor heat exchanger  140  from the third branch portion  171 . That is, the outdoor heat exchanger  140  may function as an evaporator, and in this process, the refrigerant may absorb heat from the outside air, so that the cooling system  1  has the advantage of securing the amount of heat required for defrosting from an external heat source. 
     On the other hand, since the refrigerant depressurized by the bypass expansion device  136  has a low pressure, the refrigerant may be restricted from flowing from the third branch portion  171  into the fourth branch portion  172  through which a high-pressure hot gas flows due to a pressure difference. 
     The refrigerant evaporated from the outdoor heat exchanger  140  may be introduced into the suction connection pipe  125  from the second branch portion  114   b.  That is, since the third outdoor valve  123  installed in the suction connection pipe  125  is opened and the first outdoor valve  121  is closed, the refrigerant passing through the outdoor heat exchanger  140  may flow into the suction connection pipe  125 . 
     The refrigerant in the suction connection pipe  125  may be combined with the refrigerant flowing through the gas pipe  111  in the fifth branch portion  113 . The combined refrigerant may be sucked into the compressor  110  through the gas-liquid separator  105 . Such a cycle may be repeated, and the defrosting operation of a certain evaporator and the cooling operation of the storage may be performed simultaneously or continuously due to the circulation of the refrigerant cycle. 
       FIG. 4  is a cycle diagram showing a flow state of a refrigerant when performing a defrosting operation of a second evaporator according to an embodiment of the present disclosure. 
     Referring to  FIG. 4 , when the defrosting operation of the cooling system  1  according to an embodiment of the present disclosure is performed, in particular when the defrosting operation of the second evaporator  230  is performed, the flow of a refrigerant in the outdoor unit  10  is the same as described with reference to  FIG. 3 , and is different from described with reference to  FIG. 3  in the flow of a refrigerant in the indoor unit  30  and the connection unit  50 . Accordingly, the description with reference to  FIG. 3  is referred to with respect to the same portion as in  FIG. 3 , and a portion different from the description with reference to  FIG. 3  will be mainly described. 
     The refrigerant compressed by the compressor  110  may flow from the first branch portion  114   a  to the first bypass pipe  130  and flow from the fourth branch portion  172  to the liquid pipe  170 . The refrigerant may be discharged from the outdoor unit  10  through the first service valve  175  and may be introduced into the connection unit  50  through the first connection portion C 1 . 
     The refrigerant introduced into the connection unit  50  may flow through the first connection pipe  214 . Since the first defrost valve  215  is closed, the refrigerant may flow from the sixth branch portion  214   a  to the third bypass pipe  240 . 
     The refrigerant in the third bypass pipe  240  may be introduced into the third defrost valve  238  through the branched connection pipe  242 , then discharged from the third defrost valve  238  and introduced into the second evaporator  230 . In this case, since one port of the second defrost valve  228  to which the third bypass pipe  240  is connected is closed, the refrigerant may be restricted from being introduced into the second defrost valve  228 . 
     The refrigerant introduced into the second evaporator  230 , that is, high-pressure hot gas may defrost the second evaporator  230 . At least a part of the refrigerants that have passed through the second evaporator  230  may be depressurized in the first evaporator expansion device  225  and may be evaporated in the first evaporator  220 . In this case, since the second evaporator expansion device  235  is fully opened, the refrigerant may not be depressurized while passing through the second evaporator expansion device  235 . 
     When the refrigerant is evaporated in the first evaporator  220 , cold air is generated, and the generated cold air may be supplied to the internal space of the storage by driving the first evaporator fan  220   a.  As described above, since the first evaporator  220  may perform a cooling operation while the second evaporator  230  is defrosted, a phenomenon in which the internal temperature of the storage is rapidly increased may be prevented. 
     The refrigerant evaporated in the first evaporator  220  may pass through the second defrost valve  228  through the first evaporator outlet pipe  227  and may be discharged from the connection unit  50  through the second connection portion C 2 . The refrigerant discharged from the connection unit  50  may be introduced into the outdoor unit  10  through the second service valve  255 , flow through the gas pipe  111  and be sucked into the compressor  110 . 
     On the other hand, a part of the refrigerant which has defrosted the second evaporator  230  may be introduced into the connection unit  50  through the fourth connection portion C 4 , and flow through the fourth connection pipe  219  in the eighth branch portion  214   b.  That is, a part of the refrigerant that have passed through the second evaporator  230  may be introduced into the first evaporator expansion device  225 , and the remaining refrigerant may flow into the fourth connection portion C 4 . In this case, since the first defrost valve  215  is closed, the refrigerant may be restricted from flowing toward the first connection portion C 1 . 
     The refrigerant which has flowed through the fourth connection pipe  219  may be introduced into the outdoor unit  10  through the third service valve  176  and flow through the second bypass pipe  135 . The refrigerant may be depressurized in the bypass expansion device  136  while flowing through the second bypass pipe  135 , and the depressurized refrigerant may be introduced into the outdoor heat exchanger  140  from the third branch portion  171 . That is, the outdoor heat exchanger  140  may function as an evaporator, and in this process, the refrigerant may absorb heat from the outside air, so that the cooling system  1  has the advantage of securing the amount of heat required for defrosting from an external heat source. 
     On the other hand, since the refrigerant depressurized by the bypass expansion device  136  has a low pressure, the refrigerant may be restricted from flowing from the third branch portion  171  into the fourth branch portion  172  through which a high-pressure hot gas flows due to a pressure difference. 
     The refrigerant evaporated from the outdoor heat exchanger  140  may be introduced into the suction connection pipe  125  from the second branch portion  114   b.  That is, since the third outdoor valve  123  installed in the suction connection pipe  125  is opened and the first outdoor valve  121  is closed, the refrigerant passing through the outdoor heat exchanger  140  may flow into the suction connection pipe  125 . 
     The refrigerant in the suction connection pipe  125  may be combined with the refrigerant flowing through the gas pipe  111  in the fifth branch portion  113 . The combined refrigerant may be sucked into the compressor  110  through the gas-liquid separator  105 . Such a cycle may be repeated, and the defrosting operation of a certain evaporator and the cooling operation of the storage may be performed simultaneously or continuously due to the circulation of the refrigerant cycle.