Patent Publication Number: US-11649990-B2

Title: Compressor system for cryocooler and auxiliary cooling device

Description:
RELATED APPLICATIONS 
     The content of Japanese Patent Application No. 2020-033085, on the basis of which priority benefits are claimed in an accompanying application data sheet, is in its entirety incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     Certain embodiments of the present invention relate to a compressor system for a cryocooler and an auxiliary cooling device. 
     Description of Related Art 
     An oil-lubricated helium compressor with a dual aftercooler is proposed (for example, refer to the related art). Two after coolers that cool helium and an oil, that is, a water-cooled aftercooler and an air-cooled aftercooler are built in the compressor. The air-cooled aftercooler is disposed in series or in parallel with the water-cooled aftercooler. By operating a fan of the air-cooled aftercooler, redundancy in a case where a cooling water circuit of the water-cooled aftercooler is blocked is provided. 
     SUMMARY 
     The present inventor has examined the compressor described above and has recognized the followings. As a matter of fact, emergency situations in which a cooling fan is to be operated is usually rare. In a case where the frequency of operation is extremely low, a risk in which the sticking of the cooling fan occurs can be high. When the sticking occurs, the fan cannot blow wind. For this reason, there is a concern over the reliability of redundancy using the cooling fan. In addition, the air-cooled aftercooler has a size corresponding thereto. When the air-cooled aftercooler is built in, the size of the compressor becomes larger, and manufacturing costs can increase. 
     According to an embodiment of the present invention, there is provided a compressor system for a cryocooler including a compressor unit that includes a compressor main body compressing a refrigerant gas of the cryocooler and a liquid-cooled heat exchanger cooling, through heat exchange with a cooling liquid, at least one of the refrigerant gas compressed by the compressor main body and an oil lubricating the compressor main body, a supply line through which the cooling liquid is supplied from a main chiller to the liquid-cooled heat exchanger, a collecting line through which the cooling liquid is collected from the liquid-cooled heat exchanger to the main chiller, and a backup chiller that is provided outside the compressor unit, circulates the cooling liquid to the liquid-cooled heat exchanger in place of the main chiller or together with the main chiller, and includes a circulation pump and a cooler cooling the cooling liquid on an inlet side or an outlet side of the circulation pump. 
     According to another embodiment of the present invention, there is provided an auxiliary cooling device for a compressor unit for a cryocooler including a supply line through which a cooling liquid is supplied from a main chiller to a liquid-cooled heat exchanger built in the compressor unit, a collecting line through which the cooling liquid is collected from the liquid-cooled heat exchanger to the main chiller, and a backup chiller that is provided outside the compressor unit, circulates the cooling liquid to the liquid-cooled heat exchanger in place of the main chiller or together with the main chiller, and includes a circulation pump and a cooler cooling the cooling liquid on an inlet side or an outlet side of the circulation pump. 
     Any combination of the components described above and a combination obtained by switching the components and expressions of the present invention between methods, devices, and systems are also effective as an embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram schematically showing a compressor system for a cryocooler according to an embodiment. 
         FIG.  2    is a diagram schematically showing a modification example of the compressor system for a cryocooler according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     It is desirable to provide redundancy in cooling in a compressor system for a cryocooler. 
     Hereinafter, an embodiment for carrying out the present invention will be described in detail with reference to the drawings. In the description and drawings, the same or equivalent components, members, and processes will be assigned with the same reference symbols, and redundant description thereof will be omitted as appropriate. The scales and shapes of the illustrated parts are set for convenience in order to make the description easy to understand, and are not to be understood as limiting unless stated otherwise. The embodiment is merely an example and does not limit the scope of the present invention. All characteristics and combinations to be described in the embodiment are not necessarily essential to the invention. 
       FIG.  1    is a diagram schematically showing a compressor system for a cryocooler according to the embodiment. A compressor system  100  includes a compressor unit  102  and an auxiliary cooling device  10 . The compressor system  100  configures a cryocooler  106  together with a cold head  104 . In addition, a main chiller  70  is provided in order to cool the compressor unit  102 . A cooling system for the compressor unit  102  is configured by the main chiller  70  and the auxiliary cooling device  10 . 
     The compressor unit  102  is configured to collect a refrigerant gas of the cryocooler  106  from the cold head  104 , to pressurize the collected refrigerant gas, and to supply the refrigerant gas to the cold head  104  again. The cold head  104  is also called an expander and has a room temperature section  104   a  and a low-temperature section  104   b  which is also called a cooling stage. The compressor unit  102  and the cold head  104  configure a refrigeration cycle of the cryocooler  106 , and thereby the low-temperature section  104   b  is cooled to a desired cryogenic temperature. The refrigerant gas is also called a working gas, and other suitable gases may be used although a helium gas is typically used. 
     Although the cryocooler  106  is, for example, a single-stage or two-stage Gifford-McMahon (GM) cryocooler, the cryocooler may be a pulse tube cryocooler, a Stirling cryocooler, or other types of cryocoolers. Although the cold head  104  has a different configuration depending on the type of the cryocooler  106 , the compressor unit  102  can use the configuration described below regardless of the type of the cryocooler  106 . 
     In general, both of the pressure of a refrigerant gas supplied from the compressor unit  102  to the cold head  104  and the pressure of a refrigerant gas collected from the cold head  104  to the compressor unit  102  are significantly higher than the atmospheric pressure, and can be called a first high pressure and a second high pressure, respectively. For convenience of description, the first high pressure and the second high pressure are also simply called a high pressure and a low pressure, respectively. Typically, the high pressure is, for example, 2 to 3 MPa. The low pressure is, for example, 0.5 to 1.5 MPa, and is, for example, approximately 0.8 MPa. 
     The compressor unit  102  includes a compressor main body  110 , an oil line  112 , an oil separator  114 , and an adsorber  116 . In addition, the compressor unit  102  includes a discharge port  118 , a suction port  120 , a discharge flow path  122 , a suction flow path  124 , a storage tank  126 , a bypass valve  128 , and a liquid-cooled heat exchanger  130 . The liquid-cooled heat exchanger  130  includes a refrigerant gas cooling unit  130   a  and an oil cooling unit  130   b . Further, the compressor unit  102  includes a compressor casing  132  that accommodates each of components of the compressor unit  102 , including the compressor main body  110 , the oil separator  114 , and the liquid-cooled heat exchanger  130 . 
     The compressor main body  110  is configured to internally compress a refrigerant gas sucked from a suction port thereof and to discharge the refrigerant gas from a discharge port. The compressor main body  110  may be, for example, a scroll type pump, a rotary type pump, or other pumps that pressurize the refrigerant gas. The compressor main body  110  may be configured to discharge the refrigerant gas at a fixed and constant flow rate. Alternatively, the compressor main body  110  may be configured to change the flow rate of the refrigerant gas to be discharged. The compressor main body  110  is called a compression capsule in some cases. 
     An oil is used in the compressor main body  110  for the sake of cooling and lubrication, and a sucked refrigerant gas is directly exposed to the oil in the compressor main body  110 . Accordingly, the refrigerant gas is delivered from the discharge port in a state where the oil is slightly mixed. 
     The oil line  112  includes an oil circulation line  112   a  and an oil return line  112   b . The oil circulation line  112   a  is configured such that an oil flowing out from the compressor main body  110  flows into the compressor main body  110  again through the oil cooling unit  130   b . An orifice that controls the flow rate of the oil flowing inside is provided in the oil circulation line  112   a . In addition, a filter that removes dust included in the oil may be provided in the oil circulation line  112   a . The oil return line  112   b  connects the oil separator  114  to the compressor main body  110  in order to return the oil collected by the oil separator  114  to the compressor main body  110 . In the middle of the oil return line  112   b , the filter that removes dust included in the oil separated out by the oil separator  114  and the orifice that controls the amount of the oil returning to the compressor main body  110  may be provided. 
     The oil separator  114  is provided in order to separate an oil, which is mixed in the refrigerant gas as passing through the compressor main body  110 , out from the refrigerant gas by causing the refrigerant gas. The adsorber  116  is provided in order to remove, for example, a vaporized oil and other contaminants remaining in the refrigerant gas from the refrigerant gas through adsorption. The oil separator  114  and the adsorber  116  are connected in series. In the discharge flow path  122 , the oil separator  114  is disposed on a compressor main body  110  side, and the adsorber  116  is disposed on a discharge port  118  side. 
     The discharge port  118  is an outlet of a refrigerant gas that is provided in the compressor casing  132  in order to deliver the refrigerant gas, which is pressurized to a high pressure by the compressor main body  110 , from the compressor unit  102 , and the suction port  120  is an inlet of the refrigerant gas that is provided in the compressor casing  132  in order for the compressor unit  102  to receive the low-pressure refrigerant gas. The discharge port  118  and the suction port  120  are connected to a high pressure port  108   a  and a low pressure port  108   b  of the cold head  104 , respectively, via a refrigerant gas pipe. The high pressure port  108   a  and the low pressure port  108   b  are provided in the room temperature section  104   a  of the cold head  104 . In the compressor unit  102 , a gas discharge port of the compressor main body  110  is connected to the discharge port  118  by the discharge flow path  122 , and the suction port  120  is connected to a gas suction port of the compressor main body  110  by the suction flow path  124 . 
     The storage tank  126  is provided as a volume for removing pulsation included in a low-pressure refrigerant gas returning from the cold head  104  to the compressor unit  102 . The storage tank  126  is disposed on the suction flow path  124 . 
     The bypass valve  128  connects the discharge flow path  122  to the suction flow path  124  to bypass the compressor main body  110 . For example, the bypass valve  128  branches off from the discharge flow path  122  between the oil separator  114  and the adsorber  116 , and is connected to the suction flow path  124  between the compressor main body  110  and the storage tank  126 . The bypass valve  128  is provided in order to control the flow rate of a refrigerant gas and/or in order to equalize the discharge flow path  122  and the suction flow path  124  when the compressor unit  102  is stopped. 
     Therefore, a refrigerant gas to be collected from the cold head  104  to the compressor unit  102  flows from the low pressure port  108   b  into the suction port  120  of the compressor unit  102 . The refrigerant gas is collected into the gas suction port of the compressor main body  110  via the storage tank  126  on the suction flow path  124 . The refrigerant gas is compressed and pressurized by the compressor main body  110 . The refrigerant gas to be delivered from the discharge port of the compressor main body  110  exits the compressor unit  102  from the discharge port  118  via the refrigerant gas cooling unit  130   a , the oil separator  114 , and the adsorber  116  which are on the discharge flow path  122 . The refrigerant gas is supplied from the high pressure port  108   a  into the cold head  104 . 
     The liquid-cooled heat exchanger  130  is built in the compressor unit  102  as a main cooling device for the compressor unit  102 . The liquid-cooled heat exchanger  130  is configured to cool a refrigerant gas compressed by the compressor main body  110  and an oil lubricating the compressor main body  110  through heat exchange with a cooling liquid or a cooling fluid. Typically, the cooling liquid is cooling water such as tap water and industrial water. 
     The refrigerant gas cooling unit  130   a  is disposed on the discharge flow path  122  in order to cool a high-pressure refrigerant gas heated by compression heat generated with the compression of a refrigerant gas in the compressor main body  110 . In the embodiment, the refrigerant gas cooling unit  130   a  is disposed between the gas discharge port of the compressor main body  110  and the oil separator  114  on the discharge flow path  122 . The oil cooling unit  130   b  is disposed on the oil circulation line  112   a  in order to cool an oil flowing in the oil circulation line  112   a.    
     A cooling liquid inlet port  134  and a cooling liquid outlet port  136  are provided in the compressor casing  132 . An inlet side of the liquid-cooled heat exchanger  130  is connected to the cooling liquid inlet port  134 , and an outlet side of the liquid-cooled heat exchanger  130  is connected to the cooling liquid outlet port  136 , forming an internal cooling liquid flow path of the compressor unit  102 . The cooling liquid flows from the cooling liquid inlet port  134  into the compressor unit  102 , and is supplied to the liquid-cooled heat exchanger  130 . The cooling liquid used in cooling a refrigerant gas and an oil in the liquid-cooled heat exchanger  130  is discharged outside the compressor unit  102  from the liquid-cooled heat exchanger  130  through the cooling liquid outlet port  136 . The refrigerant gas cooling unit  130   a  and the oil cooling unit  130   b  are connected in series. On the internal cooling liquid flow path of the compressor unit  102 , the refrigerant gas cooling unit  130   a  is disposed on a cooling liquid inlet port  134  side, and the oil cooling unit  130   b  is disposed on a cooling liquid outlet port  136  side. 
     Although the liquid-cooled heat exchanger  130  is configured to cool both of a refrigerant gas and an oil in the embodiment, the invention is not limited thereto. The liquid-cooled heat exchanger  130  may be configured to cool only one of the refrigerant gas and the oil. In this case, for example, the compressor unit  102  may have two liquid-cooled heat exchangers, that is, may have a heat exchanger that cools the refrigerant gas and a heat exchanger that cools the oil. 
     A cooling liquid is supplied from the main chiller  70  to the compressor unit  102  through the cooling liquid inlet port  134 . The cooling liquid used in cooling is collected in the main chiller  70  from the compressor unit  102  through the cooling liquid outlet port  136 . 
     The main chiller  70  is configured to adjust the temperate of a cooling liquid and to circulate the cooling liquid. The cooling liquid is cooled by the main chiller  70  to, for example, a temperature that is lower than the room temperature and higher than a freezing point (0° C. in the case of water) of the cooling liquid. The main chiller  70  may be, for example, a known water chiller. The main chiller  70  does not need to be provided as a dedicated cooling liquid source for the compressor unit  102 , and rather can be shared by a plurality of devices that need a cooling liquid. Accordingly, the main chiller  70  may be connected to various devices used in factories, hospitals, or other locations where the cryocooler  106  is provided and used to provide a cooling liquid to the devices. 
     The auxiliary cooling device  10  includes a supply line  12  through which a cooling liquid is supplied from the main chiller  70  to the liquid-cooled heat exchanger  130  and a collecting line  14  through which the cooling liquid is collected from the liquid-cooled heat exchanger  130  to the main chiller  70 . The supply line  12  connects a cooling liquid supply port  71  of the main chiller  70  to the cooling liquid inlet port  134  of the compressor unit  102 . The collecting line  14  connects a cooling liquid collecting port  72  of the main chiller  70  to the cooling liquid outlet port  136  of the compressor unit  102 . 
     Each of the supply line  12  and the collecting line  14  may be an appropriate pipe or an appropriate flow path which is suitable for transporting a cooling liquid, such as a flexible pipe and a rigid pipe. Each of ends of the supply line  12  and the collecting line  14  may be a joint that can be attached or detached, such as a self-sealing coupling. In this case, it is easy to attach or detach the auxiliary cooling device  10  to the main chiller  70  and the compressor unit  102 , which is convenient. 
     The auxiliary cooling device  10  includes a backup chiller  20  that is provided outside the compressor unit  102  and circulates a cooling liquid to the liquid-cooled heat exchanger  130  in place of the main chiller  70  or together with the main chiller  70 . 
     The backup chiller  20  includes a circulation pump  22  and a cooler  24  connected to the circulation pump  22  in series. In the embodiment, the cooler  24  cools a cooling liquid, on the inlet side of the circulation pump  22 . However, without being limited thereto, the cooler  24  may cool the cooling liquid, on the outlet side of the circulation pump  22 . 
     The backup chiller  20  is provided in parallel with the main chiller  70  with respect to the compressor unit  102 . The backup chiller  20  includes a connecting line  16  that connects the supply line  12  and the collecting line  14  to each other. The circulation pump  22  and the cooler  24  are provided on the connecting line  16 . 
     The circulation pump  22  circulates a cooling liquid from the collecting line  14  to the supply line  12 . Insofar as a pump has a pumping ability to recover a pressure loss of the collecting line  14  with respect to the supply line  12  and is appropriate for properties of the cooling liquid such as the type or composition of the cooling liquid, a known pump can be used as the circulation pump  22  as appropriate. 
     For example, the cooler  24  is a liquid-cooled heat exchanger. Accordingly, the liquid-cooled heat exchanger  130  of the compressor unit  102  is also called a first liquid-cooled heat exchanger and the cooler  24  is also called a second liquid-cooled heat exchanger. The cooler  24  is configured to cool a first cooling liquid through heat exchange between the first cooling liquid collected from the liquid-cooled heat exchanger  130  and a second cooling liquid flowing in a second cooling liquid line  26 . 
     The second cooling liquid line  26  may be a non-circulating type that exhausts a cooling liquid used in cooling to the outside (for example, sewage), and the second cooling liquid may be cooling water such as tap water and industrial water. Alternatively, the second cooling liquid line  26  may be a circulating type, and may be connected to the main chiller  70  such that cooling water is circulated by the main chiller  70 . The second cooling liquid line  26  may be a second water chiller provided separately from the main chiller  70 . Alternatively, the second cooling liquid line  26  may be configured to allow, for example, a cooling oil and other cooling liquids or cooling fluids to be circulated. 
     Each of the supply line  12  and the collecting line  14  is separably connected to the main chiller  70  on a main chiller  70  side with respect to the connecting line  16 . The supply line  12  and the collecting line  14  may be disconnected from the main chiller  70  by closing a valve to be described later. Alternatively, by removing the supply line  12  and the collecting line  14  from the cooling liquid supply port  71  and the cooling liquid collecting port  72  respectively, the supply line  12  and the collecting line  14  may be disconnected from the main chiller  70 . 
     The backup chiller  20  includes a set of first valves  28  and a set of second valves  30 . Both of the first valves  28  and the second valves  30  are, for example, on/off valves. Instead of a combination of the first valves  28  and the second valves  30 , a three-way valve may be provided. 
     On the connecting line  16 , one of the set of first valves  28  is provided on a supply line  12  side, the other one is provided on a collecting line  14  side, and the circulation pump  22  and the cooler  24  are disposed between the two first valves  28 . The first valves  28  are opened and closed in synchronization with each other. When both of the first valves  28  are open, the backup chiller  20  is connected to the liquid-cooled heat exchanger  130 . When both of the first valves  28  are closed, the backup chiller  20  is disconnected from the liquid-cooled heat exchanger  130 . 
     One of the set of second valves  30  is provided on the supply line  12  and the other one is provided on the collecting line  14 . Both of the two second valves  30  are disposed on the main chiller  70  side with respect to the connecting line  16 . The second valves  30  are also opened and closed in synchronization with each other. When both of the second valves  30  are open, the main chiller  70  is connected to the liquid-cooled heat exchanger  130 . When both of the second valves  30  are closed, the main chiller  70  is disconnected from the liquid-cooled heat exchanger  130 . Alternatively, the second valves  30  may be check valves that are disposed to prevent backflow in the supply line  12  and the collecting line  14  respectively. 
     The auxiliary cooling device  10  may include a bypass line  18  that connects the supply line  12  and the collecting line  14  to each other on the main chiller  70  side with respect to the connecting line  16 . The bypass line  18  is a part of a flow path that bypasses the liquid-cooled heat exchanger  130  and the backup chiller  20  and circulates a cooling liquid to the main chiller  70 . 
     A third valve  32  is provided on the bypass line  18 . The third valve  32  is, for example, an on/off valve. When the third valve  32  is open, a flow of a cooling liquid which has passed through the bypass line  18  from the collecting line  14  to the supply line  12  is allowed, and the flow of the cooling liquid that has passed through the bypass line  18  is blocked when the third valve  32  is closed. The third valve  32  may be a check valve that allows the flow of the cooling liquid from the collecting line  14  to the supply line  12  and blocks backflow. 
     In a case where the bypass line  18  is provided in the auxiliary cooling device  10 , the second valves  30  are disposed on a backup chiller  20  side with respect to the bypass line  18 . Therefore, a flow path of a cooling liquid from the cooling liquid supply port  71  of the main chiller  70  to the cooling liquid collecting port  72  through the bypass line  18  is formed when the second valves  30  are closed and the third valve  32  is open. That is, a cooling liquid circulation path for the main chiller  70 , which does not pass through the liquid-cooled heat exchanger  130  of the compressor unit  102 , is formed by the bypass line  18 . 
     Each of the connecting line  16  and the bypass line  18  may be attachable or detachable to or from the supply line  12  and the collecting line  14 . In addition, each of the connecting line  16  and the bypass line  18  may be an appropriate pipe or an appropriate flow path which is suitable for transporting a cooling liquid, such as a flexible pipe and a rigid pipe. 
     Components of the auxiliary cooling device  10 , such as the backup chiller  20  and the bypass line  18 , may be provided as one unit accommodated in a casing like the compressor unit  102 . Compared to a case where the parts are individually prepared, an operation of attaching the auxiliary cooling device  10  to the compressor unit  102  and the main chiller  70  is easy. 
     Various types of sensors are provided in the compressor system  100 . For example, the compressor unit  102  includes a first temperature sensor  138  that measures the temperature of a cooling liquid. The first temperature sensor  138  is provided, for example, on the outlet side of the liquid-cooled heat exchanger  130 , that is, between the liquid-cooled heat exchanger  130  and the cooling liquid outlet port  136  on the internal cooling liquid flow path of the compressor unit  102 . In addition thereto or instead thereof, another cooling liquid temperature sensor that measures the temperature of the cooling liquid may be provided on the inlet side of the liquid-cooled heat exchanger  130 . 
     In addition, the compressor unit  102  may include a second temperature sensor  140  that measures the temperature of a refrigerant gas. The second temperature sensor  140  may be provided on the discharge flow path  122 , for example, between the refrigerant gas cooling unit  130   a  and the oil separator  114 . In addition thereto or instead thereof, another refrigerant gas temperature sensor that measures the temperature of the refrigerant gas may be provided between the discharge port of the compressor main body  110  and the refrigerant gas cooling unit  130   a . The compressor unit  102  may include a third temperature sensor  142  that measures the temperature of an oil. The third temperature sensor  142  may be provided on the oil circulation line  112   a , between an oil inlet of the compressor main body  110  and the oil cooling unit  130   b.    
     The backup chiller  20  includes a sensor  34  that measures the temperature of a cooling liquid. The sensor  34  is disposed on the supply line  12 . The sensor  34  is disposed on a compressor unit  102  side with respect to the connecting line  16 . Accordingly, not only a cooling liquid supplied from the backup chiller  20  to the compressor unit  102  but also a cooling liquid supplied from the main chiller  70  to the compressor unit  102  can be measured. The sensor  34  may measure the flow rate or the pressure of the cooling liquid, instead of or in addition to the temperature of the cooling liquid. In other words, the sensor  34  may be configured by one or a plurality of sensors, and can include, for example, at least one of a temperature sensor, a flow rate sensor, and a pressure sensor. In addition to the sensor  34  or instead of the sensor  34 , another sensor that measures the temperature, the flow rate, or the pressure of the cooling liquid may be provided on the collecting line  14 . 
     A controller  40  that activates the backup chiller  20  is provided in the backup chiller  20 . The controller  40  is configured to receive, from at least one sensor, a sensor signal indicating measurement results by the sensor, and to activate the backup chiller  20  based on the measurement results. The controller  40  is configured to control components of the backup chiller  20 , such as the turning on and off of the circulation pump  22  and the opening and closing of the first valves  28 . 
     For example, the controller  40  may activate the backup chiller  20  based on the temperature of a cooling liquid, which is measured by the first temperature sensor  138 . In this case, the controller  40  receives a first temperature sensor signal indicating the measured temperature of the cooling liquid from the first temperature sensor  138 , and compares the measured temperature with a temperature threshold. Ina case where a cooling liquid temperature is higher than the threshold, the temperature threshold is set to a value at which a cooling liquid is evaluated to have an excessively high temperature. 
     As one reason why a cooling liquid temperature measured by the first temperature sensor  138  exceeds the temperature threshold, for example, a case where the temperature of a cooling liquid supplied from the main chiller  70  to the compressor unit  102  is excessively high (that is, the cooling failure or malfunction of the main chiller  70 ) is assumed. 
     Thus, the controller  40  activates the backup chiller  20  in a case where the measured temperature exceeds the temperature threshold. On the other hand, the controller  40  does not activate the backup chiller  20  in a case where the measured temperature does not exceed the temperature threshold. 
     In order to activate the backup chiller  20 , the controller  40  switches the circulation pump  22  from off to on to start a cooling liquid delivering operation by the circulation pump  22 , and opens the first valves  28 . In a case where the second cooling liquid line  26  is also a circulating type, the controller  40  may also switch a circulation pump of the second cooling liquid line  26  from off to on. When stopping the operation of the backup chiller  20 , the controller  40  switches on the circulation pump  22 , and closes the first valves  28 . 
     In this case, the controller  40  may close the second valves  30  and disconnect the main chiller  70  from the compressor unit  102 . At the same time, the controller  40  may open the third valve  32 . In this manner, the main chiller  70  can be disconnected from the compressor unit  102  without obstructing the flow of a cooling liquid in the main chiller  70 , and can use the backup chiller  20  in place of the main chiller  70 . When the main chiller  70  is disconnected from the compressor unit  102 , the main chiller  70  may be inspected and repaired. 
     In order to activate the backup chiller  20 , the controller  40  may be other sensors. It is conceivable that the temperature of a refrigerant gas or an oil in the compressor unit  102  has a correlation with the temperature of a cooling liquid collected from the liquid-cooled heat exchanger  130  or supplied to the liquid-cooled heat exchanger  130 . For example, it is conceivable, as a result of cooling failure of the main chiller  70 , that the cooling capacity of the liquid-cooled heat exchanger  130  is insufficient and the temperature of the refrigerant gas or the oil increases. Therefore, the controller  40  may activate the backup chiller  20  based on the temperature of the refrigerant gas, which is measured by the second temperature sensor  140 . The controller  40  may activate the backup chiller  20  based on the temperature of the oil, which is measured by the third temperature sensor  142 . The controller  40  may activate the backup chiller  20  based on a temperature measured by at least one temperature sensor of the first temperature sensor  138 , the second temperature sensor  140 , and the third temperature sensor  142 . 
     In addition, in order to activate the backup chiller  20 , the controller  40  may use the sensor  34  disposed outside the compressor unit  102 . As described above, the sensor  34  may measure the temperature of a cooling liquid, and the controller  40  may activate the backup chiller  20  based on the temperature of the cooling liquid, which is measured by the sensor  34 . 
     Alternatively, the sensor  34  may measure the flow rate or the pressure of a cooling liquid. In a case where the flow rate or the pressure of the cooling liquid is lower than the flow rate or the pressure of the threshold, as one reason for that, it is conceivable that the cooling liquid is insufficiently supplied from the main chiller  70 . The threshold is set to be a value lower than the flow rate or the pressure in the supply line  12  (or the collecting line  14 ) when the cooling liquid is normally supplied from the main chiller  70 . Accordingly, the controller  40  may activate the backup chiller  20  based on the flow rate or the pressure of the cooling liquid, which is measured by the sensor  34 . The controller  40  may compare flow rate or a pressure with the threshold of the flow rate or the pressure of the cooling liquid, which is measured by the sensor  34 , and activate the backup chiller  20  in a case where the measured value is lower than the threshold. On the other hand, the controller  40  does not activate the backup chiller  20  in a case where the measured value exceeds the threshold. 
     In a case of activating the backup chiller  20  only in an emergency such as the cooling failure or malfunction of the main chiller  70 , the frequency of such a situation is usually assumed to be significantly low. The backup chiller  20  is activated in many cases after a so-called dormant period in which operation is stopped for a long period of time. 
     Thus, the controller  40  may activate the backup chiller  20  at any timing (for example, periodically). As described above, the activation of the backup chiller  20  by the controller  40  is not limited to being performed based on measurement results by at least one sensor provided inside or outside the compressor unit  102 . 
     The controller  40  may receive, from at least one sensor, a sensor signal indicating measurement results by the sensor, and monitor the backup chiller  20  based on the measurement results. For example, the controller  40  compares a cooling liquid temperature measured by the first temperature sensor  138  with the temperature threshold. The controller  40  determines that the backup chiller  20  is normal in a case where the measured temperature does not exceed the temperature threshold. The controller  40  determines that there is failure in the backup chiller  20  in a case where the measured temperature exceeds the temperature threshold. In this manner, it is possible to confirm that the backup chiller  20  operates normally. An unexpected situation, in which as a result of overlooking malfunction occurred during a long operation stopped period, the backup chiller  20  cannot be operated in a case where the backup chiller is to be operated in place of the main chiller  70 , can be avoided. 
     In order to confirm the operation of the backup chiller  20 , the controller  40  may activate the backup chiller  20 , close the second valves  30 , and disconnect the main chiller  70  from the compressor unit  102 . At the same time, the controller  40  may open the third valve  32 . The operation of the backup chiller  20  can be confirmed by disconnecting the main chiller  70  from the compressor unit  102 , without obstructing the flow of a cooling liquid in the main chiller  70 . In a case where operation failure has occurred in the backup chiller  20 , the backup chiller  20  can be repaired or replaced independently while continuing cooling by the main chiller  70  (that is, while the compressor unit  102  and the cryocooler  106  continue operating). This leads to the reliability improvement of the compressor system  100 . 
     In a case where the controller  40  is configured to activate the backup chiller  20  based on measurement results by a sensor provided in the compressor unit  102 , such as the first temperature sensor  138 , the controller  40  may configure a part of a compressor controller that comprehensively controls the operation of the compressor system  100 . Alternatively, in a case where the controller  40  is configured to activate the backup chiller  20  based on measurement results by a sensor provided outside the compressor unit  102 , such as the sensor  34 , the controller  40  may be separately provided from the compressor controller. 
     The controller  40  is realized by an element or a circuit including a CPU and a memory of a computer as a hardware configuration and is realized by a computer program as a software configuration, but is shown in the drawings as a functional block realized in cooperation therewith. It is clear for those skilled in the art that the functional blocks can be realized in various manners in combination with hardware and software. 
     Automatically activating the backup chiller  20  through control by the controller  40  is not essential. An operator of the compressor system  100  may manually operate the circulation pump  22 , switch a valve, and activate the backup chiller  20 . 
     The backup chiller  20  may not only be used in place of the main chiller  70  but also be (simultaneously) operated together with the main chiller  70 . Such a combined use of the main chiller  70  and the backup chiller  20  may be performed not only when the cooling failure of the main chiller  70  has occurred but also when the main chiller  70  operates normally. The cooling capacity of the compressor system  100  can be temporarily enhanced by adding the cooling capacity of the backup chiller  20  to the cooling capacity of the main chiller  70 . 
     As described hereinbefore, in the embodiment, redundancy in cooling the compressor unit  102  is caused by using the backup chiller  20  in place of the main chiller  70  or together with the main chiller  70 , in order to circulate a cooling liquid to the liquid-cooled heat exchanger  130  of the compressor unit  102 . By operating the backup chiller  20 , it is possible to deal with a decrease or loss of cooling capacity caused by the aged degradation or malfunction of the main chiller  70 . Alternatively, the cooling capacity of the compressor system  100  can be temporarily enhanced by simultaneously operating the main chiller  70  and the backup chiller  20 . In this manner, the cooling function of the compressor unit  102  is stabilized, and the operation continuity and reliability of the compressor unit  102  and the cryocooler  106  are improved. 
     In addition, while two water-cooled and air-cooled aftercoolers are built in a compressor in a configuration of the related art, the liquid-cooled heat exchanger  130  is disposed in the compressor unit  102  and the backup chiller  20  is provided outside the compressor unit  102  in the compressor system  100  according to the embodiment. For this reason, only the liquid-cooled heat exchanger  130  is included as a standard device, and the compressor unit  102  can be designed in a form of not including the backup chiller  20 . The structure of the compressor unit  102  is simplified, leading to cost reduction. The backup chiller  20  can be retrofitted optionally as necessary. 
     Since the backup chiller  20  is provided outside the compressor unit  102 , a degree of freedom in selecting a place to be provided increases. The main chiller  70  is often placed at a remote place from the compressor unit  102  (for example, another room), and the main chiller  70  and the compressor unit  102  are connected to each other by a relatively long cooling liquid pipe. The backup chiller  20  can be disposed by appropriately selecting a place that does not interfere with other devices, such as an empty space from the route of the cooling liquid pipe. 
     In the embodiment, the backup chiller  20  is a liquid-cooled type. Accordingly, problems peculiar to an air-cooled cooler, such as sticking of a cooling fan, do not occur. 
       FIG.  2    is a diagram schematically showing a modification example of the compressor system for a cryocooler according to the embodiment. Also in the embodiment shown in  FIG.  2   , as similarly shown in  FIG.  1   , the compressor system  100  includes the backup chiller  20  that is provided outside the compressor unit  102  and circulates a cooling liquid to the compressor unit  102  in place of the main chiller  70  or together with the main chiller  70 . The backup chiller  20  includes the circulation pump  22  and the cooler  24 . However, the cooler  24  is an air-cooled type, and has a cooling fan disposed to blow wind to the connecting line  16  in order to cool the cooling liquid flowing in the connecting line  16 . 
     In order to confirm the operation of the backup chiller  20 , the controller  40  may monitor the backup chiller  20  based on the motor voltage or the current of the cooling fan, instead of the sensor  34  or together with the sensor  34 . In addition, the cooling fan may be capable of switching between normal rotation and reverse rotation. In this case, the controller  40  may cause the cooling fan to rotate reversely when it is determined that there is failure in the backup chiller  20 . Even when the cooling fan is stuck or clogged with dust, there is a possibility of being eliminated or alleviated by reversely rotating the fan. 
     The present invention has been described based on the embodiment. It is clear for those skilled in the art that the present invention is not limited to the embodiment, various design changes are possible, various modification examples are possible, and such modification examples are also within the scope of the present invention. Various characteristics described related to one embodiment are also applicable to the other embodiment. A new embodiment generated through combination also has the effects of each of the combined embodiments. 
     Although the backup chiller  20  and the main chiller  70  are connected in parallel with each other with respect to the liquid-cooled heat exchanger  130  of the compressor unit  102  in the embodiment described above, the present invention is not limited thereto. In one embodiment, the backup chiller  20  and the main chiller  70  may be connected in series. In this case, the backup chiller  20  may be provided on the supply line  12  (or the collecting line  14 ). 
     Without being limited to the liquid-cooled or air-cooled cooler described above, the cooler  24  of the backup chiller  20  may be, for example, another type of cooler, such as cooling a cooling liquid by a cooling element (for example, a Peltier element). 
     Although the present invention has been described using specific phrases based on the embodiment, the embodiment merely shows one aspect of the principles and applications of the present invention, and many modification examples and changes in disposition are allowed without departing from the gist of the present invention defined in the claims. 
     It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.