Patent Publication Number: US-2023145115-A1

Title: Heat pump system and controller for controlling operation of the same

Description:
TECHNICAL FIELD 
     The present invention relates to a heat pump system and a controller for controlling operation of a heat pump system. 
     BACKGROUND ART 
     EP 3 115 714 A1 proposes a heat pump system configured to perform a refrigerant recovery operation. In the refrigerant recovery operation, refrigerant is recovered from a utilization-side piping section to a heatsource-side piping section by operating a compressor while an on-off valve disposed in a liquid refrigerant pipe is closed and an on-off valve disposed in a gas refrigerant pipe is open. In the above system, the on-off valve disposed in the gas refrigerant pipe is closed after the refrigerant recovery operation. 
     However, with the above configuration, the refrigerant in the heatsource-side piping section would flow back to the utilization-side piping section through the gas refrigerant pipe before the on-off valve is closed. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] EP 3 115 714 A1 
       
    
     SUMMARY OF INVENTION 
     The object of the present invention is to provide a heat pump system and a controller for controlling operation of a heat pump system that can prevent refrigerant which has been recovered to a heatsource-side piping section by a refrigerant recovery operation from flowing back to a utilization-side piping section. 
     A first aspect of the present invention provides a heat pump system, comprising: a compressor; a heatsource-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a utilization-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a high-pressure refrigerant pipe connected to each of a discharge port of the compressor and the heatsource-side heat exchanger; a liquid refrigerant pipe connected to each of the heatsource-side heat exchanger and the utilization-side heat exchanger; a low-pressure refrigerant pipe connected to each of the utilization-side heat exchanger and a suction port of the compressor; a liquid-side on-off valve disposed in the liquid refrigerant pipe; an expansion mechanism disposed in the liquid refrigerant pipe; a gas-side on-off valve disposed in the low-pressure refrigerant pipe; and a controller configured to control the heat pump system to perform a refrigerant recovery operation for recovering refrigerant from a utilization-side piping section to a heatsource-side piping section by operating the compressor while the liquid-side on-off valve is closed and the gas-side on-off valve is open, the utilization-side piping section extending between the liquid-side on-off valve and the gas-side on-off valve and including at least the utilization-side heat exchanger, the heatsource-side piping section extending between the gas-side on-off valve and the liquid-side on-off valve and including at least the compressor, wherein the controller is configured to, in the refrigerant recovery operation, control the heat pump system such that the gas-side on-off valve starts closing when a predetermined valve-close condition is satisfied during the compressor is operating for recovering refrigerant, and such that the operation of the compressor for recovering refrigerant stops after the closing of the gas-side on-off valve started. 
     As soon as the operation of the compressor for recovering refrigerant stops, pressure at the suction port of the compressor starts increasing, and this pressure increase propagates in the low-pressure refrigerant pipe. Thus, if the operation of the compressor for recovering refrigerant stops when the gas-side on-off valve is still fully open, the refrigerant easily flows back to the utilization-side piping section through the low-pressure refrigerant pipe. In this regard, the heat pump system with the above configuration stops the operation of the compressor after the closing of the gas-side on-off valve started. Hence, it is possible to prevent refrigerant recovered by the refrigerant recovery operation from flowing back to the utilization-side piping section. 
     According to a preferred embodiment of the heat pump system mentioned above, the heat pump system further comprises a refrigerant leakage detector configured to detect an occurrence of refrigerant leakage in the utilization-side piping section, wherein the controller is configured to control the heat pump system to perform the refrigerant recovery operation when the occurrence of refrigerant leakage has been detected. 
     With the above configuration, it is possible to evacuate refrigerant from the utilization-side piping section when a refrigerant leakage has occurred in utilization-side piping section. Thereby, further refrigerant leakage can be prevented, and repair of the leakage point can be safely performed. 
     According to another preferred embodiment of the heat pump system mentioned above, the gas-side on-off valve is an electric valve. 
     An electric valve is configured to rotate a motor to move a needle inside the valve and close a passage. Thus, although it is easy to control the start timing and speed of the closing of an electric valve, it takes relatively long to complete its closure. In this regard, the heat pump system according to the present invention can start closing the gas-side on-off valve earlier. Hence, the refrigerant back flow through the gas-side on-off valve can be effectively prevented. 
     According to further another preferred embodiment of any one of the heat pump systems mentioned above, at least the utilization-side heat exchanger is disposed in a utilization-side unit; and at least the compressor, the gas-side on-off valve, and the controller are disposed in a heatsource-side unit which is separated from the utilization-side unit. 
     The heat pump system in which elements are separated into a utilization-side unit and a heatsource-side unit is advantageous for various situations such as an air-conditioning system for multiple target spaces. With the above configuration, the compressor, the gas-side on-off valve, and the controller are disposed in the same unit. Thus, even if the heat pump system is separated into a utilization-side unit and a heatsource-side unit, the controller can control the gas-side on-off valve and the compressor from a close location. Thereby, it is possible to prevent the refrigerant recovered in the heatsource-side unit from flowing back to the utilization-side unit certainly. 
     According to further another preferred embodiment of any one of the heat pump systems mentioned above, Cv value of the gas-side on-off valve is greater than Cv value of liquid-side on-off valve. 
     In general, the diameter of the low-pressure refrigerant pipe is greater than that of the liquid refrigerant pipe, and Cv value of the gas-side on-off valve is thus greater than Cv value of liquid-side on-off valve. Meanwhile, the greater Cv value of a valve is, the longer it takes to complete closure of the valve. In this regard, the heat pump system according to the present invention can start closing the gas-side on-off valve earlier. Hence, the refrigerant back flow through the gas-side on-off valve can be effectively prevented. 
     According to further another preferred embodiment of any one of the heat pump systems mentioned above, a bypass pipe connected to the liquid refrigerant pipe at a point between the heatsource-side heat exchanger and the liquid-side on-off valve and connected to the low-pressure refrigerant pipe at a point between the gas-side on-off valve and the compressor; a bypass expansion mechanism disposed in the bypass pipe; and an accumulator interposed in the low-pressure refrigerant pipe at a point between the bypass pipe and the compressor, wherein the controller is configured to control, in the refrigerant recovery operation, the bypass expansion mechanism to open. 
     With the above configuration, it is possible to draw refrigerant from the utilization-side piping section to the heatsource-side piping section while circulating the drawn refrigerant within the heatsource-side piping section. Moreover, the refrigerant can be accumulated not only in the heatsource-side heat exchanger but also in the accumulator. Thus, it is possible to increase an amount of refrigerant to be recovered. Furthermore, volume of the heat source side heat exchanger can also be determined from its required heat exchange capacity, regardless of the amount of refrigerant to be recovered. Thus, size and design of the heat source side heat exchanger can be optimized. 
     According to further another preferred embodiment of any one of the heat pump systems mentioned above, the controller is configured to, in the refrigerant recovery operation, control the heat pump system such that the operation of the compressor for recovering refrigerant stops after the closing of the gas-side on-off valve is completed. 
     With the above configuration, even if the movement of the compressor quickly stops and the low-pressure refrigerant pipe of the heatsource-side piping section is short, it is possible to prevent the refrigerant back flow. 
     According to further another preferred embodiment of any one of the heat pump systems mentioned above, the heat pump system further comprises a suction pressure detector configured to detect pressure of refrigerant flowing in the low-pressure pipe, wherein the predetermined valve-close condition includes that the pressure of refrigerant flowing in the low-pressure pipe is below a first predetermined suction pressure value. 
     With the above configuration, it is possible to close off the flow of refrigerant in the low-pressure refrigerant pipe when the pressure in the low-pressure refrigerant pipe has become low, i.e. when it is supposed that refrigerant has been mostly recovered from the utilization-side piping section to the heatsource-side piping section. Thereby, it is possible to close the gas-side on-off valve earlier and thus stop the operation of the compressor earlier, while recovering most refrigerant. 
     According to further another preferred embodiment of any one of the heat pump systems mentioned above that has the low-pressure gas state detector, the predetermined valve-close condition further includes that the pressure of refrigerant flowing in the low-pressure pipe has been kept below the first predetermined suction pressure value for a second predetermined time. 
     With the above configuration, it is possible to close off the flow of refrigerant in the low-pressure refrigerant pipe on condition that the low pressure in the low-pressure refrigerant pipe has become sufficiently low, i.e. when refrigerant has been sufficiently recovered from the utilization-side piping section to the heatsource-side piping section. Thereby, it is possible to close the gas-side on-off valve earlier and thus stop the operation of the compressor earlier, while sufficiently recovering refrigerant. 
     According to further another preferred embodiment of any one of the heat pump systems mentioned above, the controller is configured to, in the refrigerant recovery operation, control the refrigerant compressor such that operation of the refrigerant compressor stops when a predetermined compressor-stop condition is satisfied after the gas-side on-off valve has started closing, the predetermined compressor-stop condition including at least one of: a first condition that change rate of pressure of refrigerant flowing in the high-pressure refrigerant pipe is below a first predetermined change rate value and change rate of pressure of refrigerant flowing in the low-pressure refrigerant pipe is below a second predetermined change rate value which is equal to or different from the first predetermined change rate value; a second condition that pressure of refrigerant flowing in the low-pressure refrigerant pipe is below a second predetermined suction pressure value which is lower than the first predetermined suction pressure value; a third condition that a third predetermined time has elapsed after the closing of the gas-side on-off valve was completed; and a fourth condition that a fourth predetermined time has elapsed after the closing of the gas-side on-off valve started. 
     With the above configuration, it is possible to stop the operation of the compressor to complete the refrigerant recovery operation at an appropriate timing. For instance, it is possible to stop the operation of the compressor when the heat pump system is in a state where refrigerant can be prevented from flowing back from the heatsource-side piping section to the utilization-side piping section via the low-pressure refrigerant pipe. As mentioned above, the gas-side on-off valve is started closing before the operation of the compressor is stopped regardless of the timing of the compressor stop. 
     A second aspect of the present invention provides a controller for controlling operation of a heat pump system, the heat pump system comprising: a compressor; a heatsource-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a utilization-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a high-pressure refrigerant pipe connected to each of a discharge port of the compressor and the heatsource-side heat exchanger; a liquid refrigerant pipe connected to each of the heatsource-side heat exchanger and the utilization-side heat exchanger; a low-pressure refrigerant pipe connected to each of the utilization-side heat exchanger and a suction port of the compressor; a liquid-side on-off valve disposed in the liquid refrigerant pipe; an expansion mechanism disposed in the liquid refrigerant pipe; and a gas-side on-off valve disposed in the low-pressure refrigerant pipe, the controller being configured to control the heat pump system to perform a refrigerant recovery operation for recovering refrigerant from a utilization-side piping section to a heatsource-side piping section by operating the compressor while the liquid-side on-off valve is closed and the gas-side on-off valve is open, the utilization-side piping section extending between the liquid-side on-off valve and the gas-side on-off valve and including at least the utilization-side heat exchanger, the heatsource-side piping section extending between the gas-side on-off valve and the liquid-side on-off valve and including at least the compressor, wherein the controller is configured to, in the refrigerant recovery operation, control the heat pump system such that the gas-side on-off valve starts closing when a predetermined valve-close condition is satisfied during the compressor is operating for recovering refrigerant, and such that the operation of the compressor for recovering refrigerant stops after the closing of the gas-side on-off valve started. 
     As soon as the operation of the compressor for recovering refrigerant stops, pressure at the suction port of the compressor starts increasing, and this pressure increase propagates in the low-pressure refrigerant pipe. Thus, if the operation of the compressor for recovering refrigerant stops when the gas-side on-off valve is still fully open, the refrigerant easily flows back to the utilization-side piping section through the low-pressure refrigerant pipe. In this regard, the controller with the above configuration stops the operation of the compressor after the closing of the gas-side on-off valve started. Hence, it is possible to prevent refrigerant recovered by the refrigerant recovery operation from flowing back to the utilization-side piping section. Furthermore, it is also possible to achieve the above effects in an existing heat pump system just by applying the controller according to the present invention to the existing heat pump system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic configuration view of a heat pump system according to a preferred embodiment of the present invention. 
         FIG.  2    is a block diagram indicating a functional configuration of a controller shown in  FIG.  1   . 
         FIG.  3    is a first part of a flow chart indicating a process of a refrigerant recovery operation performed by the controller. 
         FIG.  4    is a second part of the flow chart indicating the process of the refrigerant recovery operation. 
         FIG.  5    is a table showing examples of conditions used as a compressor-stop condition. 
         FIG.  6    is a schematic configuration view of a first modification of the heat pump system according to the preferred embodiment. 
         FIG.  7    is a schematic configuration view of a second modification of the heat pump system according to the preferred embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A preferred embodiment of a heat pump system according to the present invention (hereafter referred to as “the present embodiment”) will be described with reference to the drawings. For instance, the heat pump system according to the present embodiment is an air-conditioning system capable of a cooling operation and a heating operation by using R32 refrigerant. 
     &lt;Circuit Configuration of System&gt; 
       FIG.  1    is a schematic configuration view of a heat pump system according to the present embodiment. 
     As shown in  FIG.  1   , the heat pump system  100  comprises, a compressor  210 , a mode switching mechanism  220 , a heatsource-side heat exchanger  230 , a utilization-side heat exchanger  240 , and an accumulator  250 . The heatsource-side heat exchanger  230  may be provided with a heatsource-side fan  231 , and the utilization-side heat exchanger  240  is provided with a utilization-side fan  241 . 
     The heat pump system  100  also comprises a discharge-side refrigerant pipe  310 , a first gas refrigerant pipe  320 , a liquid refrigerant pipe  330 , a second gas refrigerant pipe  340 , and a suction-side refrigerant pipe  350 . The discharge-side refrigerant pipe  310  is connected to each of a discharge port of the compressor  210  and the mode switching mechanism  220 . The first gas refrigerant pipe  320  is connected to each of the mode switching mechanism  220  and the heatsource-side heat exchanger  230 . The liquid refrigerant pipe  330  is connected to each of the heatsource-side heat exchanger  230  and the utilization-side heat exchanger  240 . The second gas refrigerant pipe  340  is connected to each of the utilization-side heat exchanger  240  and the mode switching mechanism  220 . The suction-side refrigerant pipe  350  is connected to each of the mode switching mechanism  220  and a suction port of the compressor  210 . The accumulator  250  is interposed in the suction-side refrigerant pipe  350 . 
     The heat pump system  100  further comprises a heatsource-side expansion mechanism  410 , a liquid-side on-off valve  420 , a liquid-side stop valve  430 , a utilization-side expansion mechanism  440 , a gas-side stop valve  450 , and a gas-side on-off valve  460 . The heatsource-side expansion mechanism  410 , the liquid-side on-off valve  420 , the liquid-side stop valve  430 , and the utilization-side expansion mechanism  440  are disposed in the liquid refrigerant pipe  330  in this order along a direction from the heatsource-side heat exchanger  230  towards the utilization-side heat exchanger  240 . The gas-side stop valve  450  and the gas-side on-off valve  460  are disposed in the second gas refrigerant pipe  340  in this order along a direction from the utilization-side heat exchanger  240  towards the mode switching mechanism  220 . The heatsource-side expansion mechanism  410  and the utilization-side expansion mechanism  440  each corresponds to an expansion mechanism according to the present invention. 
     The heat pump system  100  further comprises a refrigerant heat exchanger  260 , a bypass pipe  360 , and a bypass expansion mechanism  470 . The refrigerant heat exchanger  260  is arranged to the liquid refrigerant pipe  330  at a location between the heatsource-side expansion mechanism  410  and the liquid-side on-off valve  420 . The bypass pipe  360  is connected to each of the liquid refrigerant pipe  330  and the suction-side refrigerant pipe  350  in parallel with the utilization-side heat exchanger  240 . More specifically, the bypass pipe  360  is connected to the liquid refrigerant pipe  330  at a point between the heatsource-side expansion mechanism  410  and the refrigerant heat exchanger  260 , and connected to the suction-side refrigerant pipe  350  at a point between the mode switching mechanism  220  and the accumulator  250 . A part of the bypass pipe  360  is arranged in the refrigerant heat exchanger  260 . The bypass expansion mechanism  470  is disposed in the bypass pipe  360  at a point between the liquid refrigerant pipe  330  and the refrigerant heat exchanger  260 . 
     The heat pump system  100  further comprises a discharge-side refrigerant state detector  510 , an ambient temperature detector  520 , a refrigerant leakage detector  530 , and a suction-side refrigerant state detector  540 . The discharge-side refrigerant state detector  510  is attached to the discharge-side refrigerant pipe  310 . The ambient temperature detector  520  is disposed in the vicinity of the heatsource-side heat exchanger  230 . The refrigerant leakage detector  530  is arranged in the vicinity of the utilization-side heat exchanger  240 . The suction-side refrigerant state detector  540  is attached to the suction-side refrigerant pipe  350  at a point between the accumulator  250  and the compressor  210 . The suction-side refrigerant state detector  540  corresponds to each of an evaporation temperature detector and a suction pressure detector according to the present invention. 
     The heat pump system  100  further comprises a controller  600 . The controller  600  is connected to each of the above machineries by wired/wireless communication paths (not shown). 
     The heat pump system  100  may have a heatsource-side unit  110  and a utilization-side unit  120  as separated units. For instance, the heatsource-side unit  110  is a unit disposed outside, and the utilization-side unit  120  is a unit disposed in or close to a target space to be air-conditioned. In this case, at least the compressor  210 , the gas-side on-off valve  460 , the liquid-side on-off valve  420 , and the controller  600  are disposed in the heatsource-side unit  110 , and at least the utilization-side heat exchanger  240  is disposed in a utilization-side unit  120 . 
     In the present embodiment, the liquid refrigerant pipe  330  and the second gas refrigerant pipe  340  extend between the heatsource-side unit  110  and the utilization-side unit  120 . The utilization-side expansion mechanism  440 , utilization-side heat exchanger  240 , the utilization-side fan  241 , and the refrigerant leakage detector  530  among the above-mentioned machineries are arranged in the utilization-side unit  120 , and the other machineries are arranged in the heatsource-side unit  110 . The controller  600  may be connected to the machineries in the utilization-side unit  120  via a sub-controller (not shown) arranged in the utilization-side unit  120 . It can be said that the sub-controller in the utilization-side unit  120  is a part of the controller  600 . 
     &lt;Functions of Mechanisms&gt; 
     The compressor  210  has a suction port and a discharge port, and configured to suction refrigerant via the suction port, compress the suctioned refrigerant internally, and discharge the compressed refrigerant from the discharge port. 
     The mode switching mechanism  220  is configured to switch between a cooling mode connection and a heating mode connection. By the cooling mode connection, the mode switching mechanism  220  connects the discharge-side refrigerant pipe  310  and the first gas refrigerant pipe  320  to each other to form a high-pressure refrigerant pipe, and connects the suction-side refrigerant pipe  350  and the second gas refrigerant pipe  340  to each other to form a low-pressure refrigerant pipe. By the heating mode connection, the mode switching mechanism  220  connects the discharge-side refrigerant pipe  310  and the second gas refrigerant pipe  340  to each other to form a high-pressure refrigerant pipe, and connects the suction-side refrigerant pipe  350  and the first gas refrigerant pipe  320  to each other to form a low-pressure refrigerant pipe. Here, the high-pressure refrigerant pipe is a pipe (a flow path) connected to each of the discharge port of the compressor  210  and the heatsource-side heat exchanger  230 , and the low-pressure refrigerant pipe is a pipe (a flow path) connected to each of the utilization-side heat exchanger  240  and the suction port of the compressor  210 . The mode switching mechanism  220  may be a four-way selector valve. 
     The heatsource-side heat exchanger  230  is configured to allow refrigerant to flow therein from the first gas refrigerant pipe  320  to the liquid refrigerant pipe  330  and vice versa. The heatsource-side heat exchanger  230  is also configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough. In the present embodiment, the heatsource-side heat exchanger  230  is configured to allow outdoor air to pass therethrough. The heatsource-side fan  231  is configured to promote the flow of the air passing through the heatsource-side heat exchanger  230 . 
     The utilization-side heat exchanger  240  is configured to allow refrigerant to flow therein from the liquid refrigerant pipe  330  to second gas refrigerant pipe  340  and vice versa. The utilization-side heat exchanger  240  is also configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough. In the present embodiment, the utilization-side heat exchanger  240  is configured to allow indoor air in the target space and/or outdoor air to pass therethrough. The utilization-side fan  241  is configured to promote the flow of the air passing through the utilization-side heat exchanger  240 . The air which has passed through the utilization-side heat exchanger  240  is supplied to the target space. 
     The accumulator  250  is configured to separate gas refrigerant from the refrigerant flown into the accumulator  250  and forward the separated gas refrigerant. The accumulator  250  is also configured to accumulate excess refrigerant in the heat pump circuit of the heat pump system  100 . 
     The refrigerant heat exchanger  260  is configured to cause heat exchange between refrigerant flowing in the liquid refrigerant pipe  330  and refrigerant which has flown into the bypass pipe  360  and has been decompressed and expanded by the bypass expansion mechanism  470 . The refrigerant heat exchanger  260  may have two flow channels which form a part of the liquid refrigerant pipe  330  and a part of the bypass pipe  360 , respectively, and have thermal conductance therebetween. 
     The heatsource-side expansion mechanism  410  is configured to decompress and expand refrigerant flowing therethrough when the heatsource-side expansion mechanism  410  is partly open. More specifically, the heatsource-side expansion mechanism  410  is configured to, under control by the controller  600 , decompress and expand refrigerant flowing in the liquid refrigerant pipe  330  from the utilization-side heat exchanger  240  towards the heatsource-side heat exchanger  230  during the heat pump system  100  is in the heating operation. The heatsource-side expansion mechanism  410  may be an electric expansion valve. 
     The liquid-side on-off valve  420  is configured to regulate a flow of refrigerant therethrough. More specifically, the liquid-side on-off valve  420  is configured to, under control by the controller  600 , close off the flow of refrigerant in at least a part of the liquid refrigerant pipe  330  when the liquid-side on-off valve  420  is fully closed. The liquid-side on-off valve  420  may be an electric expansion valve. 
     The liquid-side stop valve  430  is configured to stop a flow of refrigerant therethrough when manually operated to close. The liquid-side stop valve  430  is kept fully open unless manually operated to close. The liquid-side stop valve  430  may be a service valve configured to be switched between an open state and a close state while allowing refrigerant to be charged to and discharged from the heat pump circuit therethrough. 
     The utilization-side expansion mechanism  440  is configured to decompress and expand refrigerant flowing therethrough when the utilization-side expansion mechanism  440  is partly open. More specifically, the utilization-side expansion mechanism  440  is configured to, under control by the controller  600 , decompress and expand refrigerant flowing in the liquid refrigerant pipe  330  from the heatsource-side heat exchanger  230  towards the utilization-side heat exchanger  240  during the heat pump system  100  is in the cooling operation. The utilization-side expansion mechanism  440  may be an electric expansion valve. 
     The gas-side stop valve  450  is configured to stop a flow of refrigerant therethrough when manually operated to close. The gas-side stop valve  450  is kept fully open unless manually operated to close. The liquid-side stop valve  430  may be a service valve configured to be switched between an open state and a close state while allowing refrigerant to be charged to and discharged from the heat pump circuit therethrough. 
     The gas-side on-off valve  460  is configured to regulate a flow of refrigerant therethrough. More specifically, the gas-side on-off valve  460  is configured to, under control by the controller  600 , close off the flow of refrigerant in at least a part of the liquid refrigerant pipe  330  when the gas-side on-off valve  460  is fully closed. The gas-side on-off valve  460  may be an electric expansion valve. 
     In general, the diameter of the second gas refrigerant pipe  340  is greater than that of the liquid refrigerant pipe  330 . Thus, Cv value of the gas-side on-off valve  460  is greater than Cv value of liquid-side on-off valve  420 . For instance, Cv value of the gas-side on-off valve  460  is over five times larger than Cv value of the liquid-side on-off valve  420 . Cv value of the gas-side on-off valve  460  may be 5, and Cv value of the liquid-side on-off valve  420  may be 0.6. In this case, Cv value of the heatsource-side expansion mechanism  410  may be 0.3. 
     The bypass expansion mechanism  470  is configured to decompress and expand refrigerant flowing therethrough when the bypass expansion mechanism  470  is partly open. More specifically, the bypass expansion mechanism  470  is configured to, under control by the controller  600 , decompress and expand refrigerant flowing in the bypass pipe  360  from the liquid refrigerant pipe  330  towards the suction-side refrigerant pipe  350  during the heat pump system  100  is operating in the cooling operation and a refrigerant recovery operation mentioned later. The bypass expansion mechanism  470  may be an electric expansion valve. 
     In the following descriptions, the heatsource-side expansion mechanism  410 , the liquid-side on-off valve  420 , the utilization-side expansion mechanism  440 , the gas-side on-off valve  460 , and the bypass expansion mechanism  470  are collectively called “the control valves” as necessary. 
     The discharge-side refrigerant state detector  510  is configured to detect pressure and/or temperature of refrigerant flowing in the discharge-side refrigerant pipe  310 , and transmit discharge-side refrigerant information indicating the detected pressure (hereinafter referred to as “the discharge pressure Pc”) and/or the detected temperature (hereinafter referred to as “the discharge temperature Tdi”) to the controller  600  continuously or regularly. Alternatively, or additionally, the discharge-side refrigerant state detector  510  may transmit the discharge-side refrigerant information when the detected discharge pressure Pc and/or discharge temperature Tdi has changed by a predetermined amount, and/or upon receiving a request from the controller  600 . The discharge-side refrigerant state detector  510  may be a capacitive pressure sensor and/or a thermistor. 
     The ambient temperature detector  520  is configured to detect temperature of the fluid (the outdoor air) which passes through the heatsource-side heat exchanger  230 , and transmit ambient temperature information indicating the detected temperature (hereinafter referred to as “the ambient temperature Ta”) to the controller  600  continuously or regularly. Alternatively, or additionally, the ambient temperature detector  520  may transmit the ambient temperature information when the detected temperature Ta has changed by a predetermined amount, and/or upon receiving a request from the controller  600 . The ambient temperature detector  520  may be a thermistor disposed in an air-flow path of the outdoor air flowing through the heatsource-side heat exchanger  230  on the upstream side of the heatsource-side heat exchanger  230 . In other words, the ambient temperature detector  520  is configured to detect temperature of fluid which is subject to heat exchange with refrigerant in the heatsource-side heat exchanger  230 . 
     The refrigerant leakage detector  530  is configured to detect an occurrence of refrigerant leakage in the utilization-side unit  120  and transmit refrigerant leakage information to the controller  600  continuously or regularly. The refrigerant leakage information is information indicating whether or not a refrigerant leakage in the utilization-side unit  120  (hereinafter referred to simply as “the refrigerant leakage”) has occurred. Alternatively, or additionally, the refrigerant leakage detector  530  may transmit the refrigerant leakage information when the refrigerant leakage has occurred. 
     The refrigerant leakage detector  530  may be a semi-conductor gas sensor reactive to the refrigerant used in the heat-pump system  100 . In this case, the refrigerant leakage detector  530  detects a concentration of the refrigerant in an air surrounding the refrigerant leakage detector  530 , and outputs a detection value indicating the detected concentration as the refrigerant leakage information. Whether or not the detection value is greater than a predetermined threshold indicates whether the refrigerant leakage has occurred. The refrigerant leakage detector  530  is disposed in the utilization-side unit  120  or the target space. In a case where refrigerant which is heavier than an air, such as R32 refrigerant, the refrigerant leakage detector  530  is preferably disposed on or close to an inner bottom surface of an air chamber (not shown) in which utilization-side heat exchanger  240  is arranged. 
     The suction-side refrigerant state detector  540  is configured to detect pressure of refrigerant flowing in the suction-side refrigerant pipe  350  and detect evaporation temperature of refrigerant flowing in the suction-side refrigerant pipe  350 . The suction-side refrigerant state detector  540  is further configured to transmit suction-side refrigerant information indicating the detected pressure (hereinafter referred to as “the suction pressure Pe”) and the detected evaporation temperature TeS to the controller  600  continuously or regularly. Alternatively, or additionally, the suction-side refrigerant state detector  540  may transmit the suction-side refrigerant information when the detected suction pressure Pe and/or evaporation temperature TeS has changed by a predetermined amount, and/or upon receiving a request from the controller  600 . 
     The suction-side refrigerant state detector  540  may include a capacitive pressure sensor configured to detect pressure of refrigerant flowing in the suction-side refrigerant pipe  350 , and a thermistor configured to detect temperature of refrigerant flowing in the suction-side refrigerant pipe  350 . The suction-side refrigerant state detector  540  may further include a storage media and a calculator. In this case, the storage memory stores a table information indicating known correlation between pressure of the refrigerant and evaporation temperature TeS of the refrigerant at the pressure in advance. The calculator calculates the evaporation temperature TeS of the refrigerant based on the detected pressure and the table. Yet, this calculation may be performed by the controller  600 . 
     In the following descriptions, the discharge-side refrigerant state detector  510 , the ambient temperature detector  520 , the refrigerant leakage detector  530 , and the suction-side refrigerant state detector  540  are collectively called “the sensors” as necessary. 
     The controller  600  is configured to switch the mode switching mechanism  220  between the cooling mode connection and the heating mode connection in accordance with an instruction made by a user or an external controller, and control the cooling operation and the heating operation of the heat pump system  100 . 
     In the cooling operation, the controller  600  controls the machineries of the heat pump system  100  such that refrigerant discharged from the compressor  210  flows through the heatsource-side heat exchanger  230 , each of the utilization-side heat exchanger  240  and the bypass pipe  360 , and the accumulator  250  in this order, and is suctioned to the compressor  210 . The arrows show in  FIG.  1    indicates a flow direction of refrigerant during the heat pump system  100  is in the cooling operation. In the cooling operation, the heatsource-side unit  110  functions as a condenser, and the utilization-side unit  120  functions as an evaporator. 
     In the heating operation, the controller  600  controls the machineries such that refrigerant discharged from the compressor  210  flows through the utilization-side heat exchanger  240 , the heatsource-side heat exchanger  230 , and the accumulator  250  in this order, and is suctioned to the compressor  210 . It can be said that the first gas refrigerant pipe  320  is a part of the suction-side refrigerant pipe  350  and the second gas refrigerant pipe  340  is a part of the discharge-side refrigerant pipe  310  when the mode switching mechanism  220  is in the heating mode connection. In the heating operation, the heatsource-side unit  110  functions as an evaporator, and the utilization-side unit  120  functions as a condenser. 
     The controller  600  is further configured to control the heat pump system  100  to perform a refrigerant recovery operation when an occurrence of the refrigerant leakage has been detected. The refrigerant recovery operation is an operation for recovering refrigerant from a utilization-side piping section  102  to a heatsource-side piping section  101  by operating the compressor  210  while the liquid-side on-off valve  420  is closed and the gas-side on-off valve  460  is open. Here, the heatsource-side piping section  101  is a piping section extending between the gas-side on-off valve  460  and the liquid-side on-off valve  420  and including at least the compressor  210 . The heatsource-side piping section  101  also includes the heatsource-side heat exchanger  230 . The utilization-side piping section  102  is a piping section extending between the liquid-side on-off valve  420  and the gas-side on-off valve  460  and including at least the utilization-side heat exchanger  240 . 
     In the present embodiment, the heatsource-side piping section  101  includes a part of the second gas refrigerant pipe  340  that is connected to the mode switching mechanism  220 , the mode switching mechanism  220 , the suction-side refrigerant pipe  350 , the accumulator  250 , the compressor  210 , the discharge-side refrigerant pipe  310 , the first gas refrigerant pipe  320 , the heatsource-side heat exchanger  230 , a part of the liquid refrigerant pipe  330  that is connected to the heatsource-side heat exchanger  230 , the heatsource-side expansion mechanism  410 , the refrigerant heat exchanger  260 , the bypass pipe  360 , and the bypass expansion mechanism  470 . The utilization-side piping section  102  includes a part of the liquid refrigerant pipe  330  that is connected to the utilization-side heat exchanger  240 , the liquid-side stop valve  430 , the utilization-side expansion mechanism  440 , a part of the second gas refrigerant pipe  340  that is connected to the utilization-side heat exchanger  240 , and the gas-side stop valve  450 . 
     In the refrigerant recovery operation, the controller  600  controls the machineries of the heat pump system  100  such that refrigerant present in the utilization-side piping section  102  is drawn towards the suction port of the compressor  210  via the second gas refrigerant pipe  340  and then circulated within the heatsource-side piping section  101  through the heatsource-side heat exchanger  230 , the bypass pipe  360 , and the accumulator  250 . The refrigerant is accumulated mainly in the accumulator  250  and the heatsource-side heat exchanger  230  during being circulated within the heatsource-side piping section  101 . 
     The controller  600  is further configured to, in the refrigerant recovery operation, control the compressor  210  such that, when the ambient temperature Ta is higher than or equal to a predetermined ambient temperature value Ta_th, increase rate of the compressor rotation speed is low compared with increase rate of the compressor rotation speed of when the ambient temperature Ta is lower than the predetermined ambient temperature value Ta_th. Here, the “compressor rotation speed” means rotation speed of the compressor  210 , which is expressed the number of rotations per minute for instance. The increase rate of the compressor rotation speed is an increased amount of the compressor rotation speed per unit time for instance. 
     The controller  600  is further configured to, in the refrigerant recovery operation, control the heat pump system  100  such that the gas-side on-off valve  460  starts closing when a predetermined valve-close condition is satisfied during the compressor  210  is operating for recovering refrigerant. The controller  600  is also configured to control the heat pump system  100  such that the operation of the compressor  210  for recovering refrigerant stops after the closing of the gas-side on-off valve  460  started. The details regarding the controller  600  is explained hereinafter. 
     &lt;Functional Configuration of Controller&gt; 
     The controller  600  includes an arithmetic circuit such as a CPU (Central Processing Unit), a work memory used by the CPU such as a RAM (Random Access Memory), a recording medium storing control programs and information used by the CPU such as a ROM (Read Only Memory), and a timer, although they are not shown. The controller  600  is configured to perform information processing and signal processing by the CPU executing the control programs to control operation of the heat pump system  100 . Thus, functions of the controller  600  are achieved by execution of the programs. 
       FIG.  2    is a block diagram indicating a functional configuration of the controller  600 . 
     As shown in  FIG.  2   , the controller  600  has a storage section  610 , an information input section  620 , a normal operation controller  630 , an information output section  640 , and a refrigerant recovery controller  650 . 
     The storage section  610  stores information in a form readable by the refrigerant recovery controller  650 . The stored information may include conditions and values used by the normal operation controller  630  and the refrigerant recovery controller  650 . The stored information may be prepared in advance based on experiments or the like. 
     The information input section  620  is configured to acquire, from the sensors, information necessary for controlling the operation of the heat pump system  100 , and transfer the acquired information to the refrigerant recovery controller  650 . The information input section  620  may further transfer the acquired information to the normal operation controller  630 . The information to be acquired includes the discharge-side refrigerant information, the ambient temperature information, the refrigerant leakage information, and the suction-side refrigerant information mentioned above. The information input section  620  may include a wired/wireless communication interface for communicating with each of the sensors. The information input section  620  may transmit requests to the sensors requesting for information under control by the refrigerant recovery controller  650 . 
     The normal operation controller  630  is configured to control the cooling operation and the heating operation of the heat pump system  100 . For the cooling operation, the normal operation controller  630  is configured to control the mode switching mechanism  220  to switch to or maintain the cooling mode connection, control the heatsource-side expansion mechanism  410 , the liquid-side on-off valve  420 , and the gas-side on-off valve  460  to fully open, and control the utilization-side expansion mechanism  440  and the bypass expansion mechanism  470  to be partly open. For the heating operation, the normal operation controller  630  is configured to control the mode switching mechanism  220  to switch to or maintain the heating mode connection, control the gas-side on-off valve  460 , the utilization-side expansion mechanism  440 , and the liquid-side on-off valve  420  to fully open, control the heatsource-side expansion mechanism  410  to be partly open, and control the bypass expansion mechanism  470  to be fully closed. The normal operation controller  630  is also configured to control the compressor  210 , the heatsource-side fan  231 , and the utilization-side fan  241  to operate for both the cooling operation and the heating operation. The normal operation controller  630  may include a wired/wireless communication interface for communicating with each of the mode switching mechanism  220 , the control valves, the compressor  210 , the heatsource-side fan  231 , and the utilization-side fan  241 . 
     Regarding the control of the compressor  210 , the normal operation controller  630  is configured to control the compressor rotation speed such that the evaporation temperature TeS approaches a target evaporation temperature value TeS_tgt. The target evaporation temperature value TeS_tgt is used regardless of whether the heat pump system  100  is in the cooling operation or the refrigerant recovery operation, but the value of the target evaporation temperature value TeS_tgt is different as explained later. The normal operation controller  630  is also configured to monitor whether the discharge pressure Pc is kept below a predetermined threshold, and decrease the compressor rotation speed when the discharge pressure Pc has exceeded the predetermined threshold (i.e. a drooping control is performed). 
     The normal operation controller  630  may also be configured to control the heat pump system  100  under control by the refrigerant recovery controller  650  during the refrigerant recovery operation. 
     The information output section  640  is configured to output information to a user of the heat pump system  100  or an external device such as an information output device under control by the refrigerant recovery controller  650 . The information output section  640  may include a display device, an electric light, a loudspeaker, a wired/wireless communication interface for transmitting information to an external device. Thus, the information output section  640  is configured to output the information by image, light, sound, communication signal or the like. 
     The refrigerant recovery controller  650  is configured to perform the refrigerant recovery operation, by using the normal operation controller  630  for instance. The refrigerant recovery controller  650  has a leakage detection section  651 , a temperature detection section  652 , an acceleration rate switching section  653 , and a timing control section  654 . 
     The leakage detection section  651  is configured to detect an occurrence of the refrigerant leakage based on the refrigerant leakage information from the refrigerant leakage detector  530 . For instance, the leakage detection section  651  is configured to determine that the refrigerant leakage has occurred when the concentration of refrigerant detected by the refrigerant leakage detector  530  is greater than a predetermined concentration value. Yet, this determination may be performed by the refrigerant leakage detector  530  or the information input section  620 . A moving average of time-series data of the detected concentration may be used for the above determination. The leakage detection section  651  may passively receive the refrigerant leakage information continuously or regularly transmitted by the refrigerant leakage detector  530 , or actively acquire the refrigerant leakage information by regularly sending a request to the refrigerant leakage detector  530 . 
     The temperature detection section  652  is configured to obtain the ambient temperature information from the ambient temperature detector  520 . The temperature detection section  652  may passively receive the ambient temperature information continuously or regularly transmitted by the ambient temperature detector  520 , or actively acquire the ambient temperature information by sending a request to the ambient temperature detector  520  when the leakage detection section  651  has determined that the refrigerant leakage has occurred. 
     The acceleration rate switching section  653  is configured to set a target increase rate value Rv_tgt based on whether the acquired ambient temperature Ta is higher than or equal to the predetermined ambient temperature value Ta_th. More specifically, when the ambient temperature Ta is higher than or equal to than the predetermined ambient temperature value Ta_th, the acceleration rate switching section  653  is configured to set the target increase rate value Rv_tgt so as to be low compared with the target increase rate value Rv_tgt of when the ambient temperature Ta is lower than the predetermined ambient temperature value Ta_th. 
     The timing control section  654  is configured to perform the refrigerant recovery operation controlling the timings of events in the refrigerant recovery operation. In particular, the timing control section  654  is configured to control the compressor  210  to increase the compressor rotation speed by the set target increase rate value Rv_tgt, and control the gas-side on-off valve  460  to close and control the compressor  210  to stop the operation for recovering refrigerant after the closing of the gas-side on-off valve  460  started. The functions of the timing control section  654  are detailed in the following explanations on the operation by the controller  600 . 
     &lt;Operation by Controller&gt; 
     The leakage detection section  651  of the controller  600  repeats a determination whether the refrigerant leakage has occurred during the compressor  210  is not operating, during the cooling operation, and during the heating operation. When an occurrence of the refrigerant leakage has been detected, the controller  600  starts the refrigerant recovery operation. 
     If an occurrence of the refrigerant leakage has been detected during the compressor  210  is not operating and the mode switching mechanism  220  is not in the cooling mode connection, the controller  600  controls the mode switching mechanism  220  to switch to the cooling mode connection, and then starts the refrigerant recovery operation. If an occurrence of the refrigerant leakage has been detected during the cooling operation, the controller  600  controls the compressor  210  to stop, and then starts the refrigerant recovery operation. If an occurrence of the refrigerant leakage has been detected during the heating operation, the controller  600  controls the mode switching mechanism  220  to switch to the cooling mode connection, controls the compressor  210  to stop, and then starts the refrigerant recovery operation. In any cases, the controller  600  is configured to control the mode switching mechanism  220  to maintain the cooling mode connection during the refrigerant recovery operation is performed. 
     When an occurrence of the refrigerant leakage has been detected, the refrigerant recovery controller  650  may output alarm information via the information output section  640  to notify the user of the occurrence of the refrigerant leakage. It is preferable that the refrigerant recovery controller  650  transmits a signal to the utilization-side unit  120  such that the alarm information is also outputted from a display device, an electric light, a loudspeaker or the like (not shown) of the utilization-side unit  120 . 
       FIG.  3    is a first part of a flow chart indicating the process of the refrigerant recovery operation performed by the controller  600 , and  FIG.  4    is a second part of the flow chart. 
     In step S 1100 , the timing control section  654  of the controller  600  controls the heatsource-side expansion mechanism  410  to fully open and controls the bypass expansion mechanism  470  to be fully open. Here, the gas-side on-off valve  460  should already be open, and the compressor  210  is still stopped. Thereby, refrigerant can smoothly circulate within the heatsource-side piping section  101  when the operation of the compressor  210  is started afterwards. 
     In step S 1200 , the timing control section  654  controls the liquid-side on-off valve  420  to close. Thereby, refrigerant can be prevented from flowing into the utilization-side piping section  102  via the liquid refrigerant pipe  330  when the operation of the compressor  210  is started afterwards. 
     In step S 1300 , the timing control section  654  sets a lower value to the target evaporation temperature value TeS_tgt which is used for controlling the compressor rotation speed, compared with a value normally used in the cooling operation. More specifically, the timing control section  654  changes the target evaporation temperature value TeS_tgt from a first target evaporation temperature value TeS_ 1  to a second target evaporation temperature value TeS_ 2 . The first target evaporation temperature value TeS_ 1  is a default value, and the second target evaporation temperature value TeS_ 2  is a value lower than the first target evaporation temperature value TeS_ 1 . For instance, the first target evaporation temperature value TeS_ 1  is −6 degree Celsius which is used in the normal cooling operation, and the second target evaporation temperature value TeS_ 2  is −30 degree Celsius. Thereby, the compressor  210  can keep operating in the refrigerant recovery operation even if the evaporation temperature TeS becomes low. Yet, the measure for keeping the compressor  210  operating is not limited to this. 
     In step S 1400 , the timing control section  654  controls the utilization-side expansion mechanism  440  to open. Thereby, refrigerant can smoothly flow out from the utilization-side piping section  102  when the operation of the compressor  210  is started afterwards. It is preferable that the utilization-side expansion mechanism  440  is gradually opened. 
     In step S 1500 , the timing control section  654  controls the compressor  210  to start operating. Thereby, refrigerant present in utilization-side piping section  102  can start being drawn towards the heatsource-side piping section  101  via the second gas refrigerant pipe  340 . It is preferable that operation of the compressor  210  starts only after a first predetermined time T_ 1  has elapsed after operation of the compressor  210  stopped. For instance, the first predetermined time T_ 1  is 1 minute. Thereby, it is possible to reliably complete the preparations of the control valves before the operation of the compressor  210  is started. 
     By the above steps S 1100  to S 1500 , if the compressor can start operating in a state where the liquid-side on-off valve is closed and the heatsource-side expansion mechanism  410 , the bypass expansion mechanism  470 , the utilization-side expansion mechanism  440 , and the gas-side on-off valve  460  are open. Yet, the measure for preparing such a state of the control valves are not limited to the above steps S 1100  to S 1400 . 
     In step S 1600 , the temperature detection section  652  acquires the ambient temperature Ta, and the acceleration rate switching section  653  determines whether the acquired ambient temperature Ta is lower than the predetermined ambient temperature value Ta_th. A moving average of time-series data of the detected ambient temperature Ta may be used for the above determination. If the ambient temperature Ta is lower than the predetermined ambient temperature value Ta_th (S 1600 : Yes), the process proceeds to step S 1700 . If the ambient temperature Ta is higher than or equal to the predetermined ambient temperature value Ta_th (S 1600 : No), the process proceeds to step S 1800 . For instance, the predetermined ambient temperature value Ta_th is 35 degree Celsius. 
     In step S 1700 , the acceleration rate switching section  653  sets a first predetermined increase rate value Rv_ 1  to the target increase rate value Rv_tgt. 
     In step S 1800 , the acceleration rate switching section  653  sets a second predetermined increase rate value Rv_ 2  to the target increase rate value Rv_tgt. Here, the second predetermined increase rate value Rv_ 2  is lower than the first predetermined increase rate value Rv_ 1 . 
     In step S 1900 , the timing control section  654  controls the compressor  210  such that the compressor rotation speed starts increasing by the target increase rate value Rv_tgt with the set value. The timing control section  654  may control the compressor  210  to start rotating at a predetermined frequency and then increase the compressor rotation speed by increasing the frequency by a predetermined step at a predetermined interval. The predetermined step may be determined for each interval based on the evaporation temperature TeS or the like. In this case, the target increase rate value Rv_tgt may be used as an upper limit of the increased step in each interval. In other words, the timing control section  654  may set an upper limit to the step of frequency to be increased in each interval in the step S 1800 , whereas setting substantially no upper limit in the step S 1700 . 
     The compressor  210  is controlled to increase the frequency gradually such that the evaporation temperature TeS approaches the target evaporation temperature value TeS_tgt as mentioned above. Yet, during the refrigerant recovery operation, the evaporation temperature TeS would not get to the target evaporation temperature value TeS_tgt since the target evaporation temperature value TeS_tgt has been lowered in the step S 1300 . Thus, the compressor  210  keeps operating while increasing its rotation speed. When the process proceeded to the step S 1800 , it takes longer than when the process proceeded to the step S 1700  until the compressor rotation speed reaches the same speed. 
     By the above steps S 1600  to S 1900 , it is possible to increase the compressor rotation speed while making the increase speed of the compressor rotation speed slower when the ambient temperature Ta is relatively high. 
     In step S 2000 , the timing control section  654  determines whether a predetermined valve-close condition is satisfied. The predetermined valve-close condition is a condition indicating that refrigerant has been recovered from the utilization-side piping section  102  to the heatsource-side piping section  101  sufficiently. 
     In the present embodiment, the predetermined valve-close condition is that the suction pressure Pe has been kept below a first predetermined suction pressure value Pe_ 1  for a second predetermined time T_ 2 . For this determination, the timing control section  654  acquires the suction pressure Pe and determine whether the above predetermined valve-close condition is satisfied. A moving average of time-series data of the detected suction pressure Pe may be used for the above determination. For instance, the first predetermined suction pressure value Pe_ 1  is 3.0 kilopascal and the second predetermined time T_ 2  is 30 seconds. Yet, the duration for the second predetermined time T_ 2  may be excluded from the above predetermined valve-close condition. 
     If the suction pressure Pe is not below the first predetermined suction pressure value Pe_ 1 , or the suction pressure Pe is below the first predetermined suction pressure value Pe_ 1  but it has not been kept for the second predetermined time T_ 2  yet (S 2000 : No), the determination of the step S 2200  is repeated. If the suction pressure Pe has been kept below the first predetermined suction pressure value Pe_ 1  for the second predetermined time T_ 2  (S 2000 : Yes), the process proceeds to step S 2100 . 
     In step S 2100 , the timing control section  654  controls the gas-side on-off valve  460  to start closing. Thereby, the gas-side on-off valve  460  is closed to prevent refrigerant from flowing back from the heatsource-side piping section  101  to the utilization-side piping section  102  via the second gas refrigerant pipe  340 , even if the operation of the compressor  210  is stopped afterwards. It is preferable that the gas-side on-off valve  460  is gradually closed. For instance, the timing control section  654  controls the gas-side on-off valve  460  to start closing by sending a shut-off signal to the gas-side on-off valve  460 . The shut-off signal may be a pulse signal with pulse-number decreasing to zero. 
     In step S 2200 , the timing control section  654  determines whether a predetermined compressor-stop condition is satisfied. The predetermined compressor-stop condition is a condition indicating that it is possible to prevent refrigerant from flowing back from the heatsource-side piping section  101  to the utilization-side piping section  102  via the second gas refrigerant pipe  340  even if the operation of the compressor  210  is stopped, and/or that the operation of the compressor  210  needs to be stopped for safety reason or the like. If the predetermined compressor-stop condition is not satisfied (S 2200 : No), the determination of the step S 2200  is repeated. If the predetermined compressor-stop condition is satisfied (S 2200 : Yes), the process proceeds to step S 2300 . 
       FIG.  5    is a table showing examples of the compressor-stop condition. For instance, the compressor-stop condition includes at least one of first to fourth conditions shown in  FIG.  5   . 
     The first condition is that change rate of the discharge pressure Pc (hereinafter referred to as “the discharge pressure change rate IRpcl”) is below a predetermined discharge pressure change rate value Rpc_th, and change rate of the suction pressure Pe (hereinafter referred to as “the suction pressure change rate IRpel”) is below a predetermined suction pressure change rate value Rpe_th. The predetermined suction pressure change rate value Rpe_th may be equal to or different from the predetermined discharge pressure change rate value Rpc_th. Here, the discharge pressure change rate IRpcl may be an absolute value of changed amount of the discharge pressure Pc per unit time, and the suction pressure change rate IRpel may be an absolute value of changed amount of the suction pressure Pe per unit time. For instance, both the predetermined discharge pressure change rate value Rpc_th and the predetermined suction pressure change rate value Rpe_th are 0.2 kgf/cm 2  per second. A moving average of time-series data of the detected discharge pressure Pc and a moving average of time-series data of the detected suction pressure Pe may be used for determination of this condition. 
     The second condition is that the suction pressure Pe is below a second predetermined suction pressure value Pe_ 2  which is lower than the first predetermined suction pressure value Pe_ 1  used in the step S 2000 . For instance, the second predetermined suction pressure value Pe_ 2  is 1.0 kilopascal. A moving average of time-series data of the detected suction pressure Pe may be used for determination of this condition. 
     The third condition is that a third predetermined time T_ 3  has elapsed after the closing of the gas-side on-off valve  460  was completed. For instance, the third predetermined time T_ 3  is 2 minutes. Yet, the third predetermined time T_ 3  may be zero. The timing control section  654  may detect the completion of the closing of the gas-side on-off valve  460  by using a sensor. 
     The fourth condition is that a fourth predetermined time T_ 4  has elapsed after the closing of the gas-side on-off valve  460  started in step S 2100 . It is preferable that the fourth predetermined time T_ 4  is longer than a time period that it takes for the gas-side on-off valve  460  to close. 
     The timing control section  654  may use only one of the above first to fourth conditions. Alternatively, the timing control section  654  may use a combination of any two or more of the above first to fourth conditions as an AND condition (a logical conjunction) or an OR condition (a logical disjunction). Yet, the predetermined compressor-stop condition is not limited to these. In any case, the timing control section  654  is configured to acquire the information necessary for determining the predetermined compressor-stop condition. 
     In step S 2300  of  FIG.  4   , the timing control section  654  controls the utilization-side expansion mechanism  440  to close. 
     In step S 2400 , the timing control section  654  controls the compressor  210  to stop operating, and controls the heatsource-side expansion mechanism  410  and the bypass expansion mechanism  470  to close. For instance, the timing control section  654  controls the compressor  210  to stop operating by controlling a power supply to the compressor  210  to stop. 
     By the above steps S 2000  to S 2400 , it is possible to stop operation of the compressor  210  and close the control valves when the refrigerant recovery operation can be or should be finished. Then, the refrigerant recovery operation is terminated. The gas-side on-off valve  460  starts closing before the operation of the compressor  210  when the refrigerant recovery operation is to be finished. The timing control section  654  may terminate the refrigerant recovery operation when a predetermined termination condition has been satisfied, regardless of the above predetermined valve-close condition and predetermined compressor-stop condition. Yet, even in such a case, it is desirable that the timing control section  654  controls the compressor  210  to stop after the closure of the gas-side on-off valve  460  has been started, or more preferably after the closure of the gas-side on-off valve  460  has been completed. 
     When the refrigerant recovery operation has been terminated, the refrigerant recovery controller  650  may output termination information via the information output section  640  to notify the user of the termination of the refrigerant recovery operation. It is preferable that the refrigerant recovery controller  650  transmits a signal to the utilization-side unit  120  such that the termination information is also outputted from the display device, the electric light, the loudspeaker or the like of the utilization-side unit  120 . 
     After the termination of the refrigerant recovery operation, the user or a maintenance person of the heat pump system  100  may repair a refrigerant leaking point of the utilization-side unit  120 . Since most of refrigerant has been evacuated from the utilization-side piping section  102 , the repair can be safely performed. 
     &lt;Advantageous Effect&gt; 
     As described above, the heat pump system  100  is configured to, in the refrigerant recovery operation, control the heat pump system  100  such that the gas-side on-off valve  460  starts closing when a predetermined valve-close condition is satisfied during the compressor  210  is operating for recovering refrigerant, and such that the operation of the compressor  210  for recovering refrigerant stops after the closing of the gas-side on-off valve  460  started. Thereby, it is possible to prevent refrigerant which has been recovered to a heatsource-side piping section  101  from flowing back to the utilization-side piping section  102 . 
     &lt;Modifications&gt; 
     The configurations and operations of the heat pump system  100  and/or the controller  600  are not limited to the configurations and operations explained above, unless departing from the scope of the present invention as defined in the appended claims. For instance, some of the elements of the heat pump system  100  and some of the operational steps performed by the controller  600  can be omitted. 
     For example, in a case of a refrigeration system, i.e. where the heating operation is not required, the mode switching mechanism  220  and the heatsource-side expansion mechanism  410  can be omitted. In a case where required performance of the heat pump system  100  not high, the refrigerant heat exchanger  260  can be omitted. In a case where there is no bypass pipe is connected to each of the liquid refrigerant pipe  330  and the suction-side refrigerant pipe  350  in parallel with the utilization-side heat exchanger  240 , the accumulator  250  can be omitted. In a case where sufficient air flow through the heatsource-side heat exchanger  230  and/or utilization-side heat exchanger  240  is ensured, the heatsource-side fan  231  and/or the utilization-side fan  241  can be omitted. In a case where the heat pump system  100   a  is formed as a single unit, the liquid-side stop valve  430  and the gas-side stop valve  450  can be omitted. 
     The controller  600  may perform the determination whether the refrigerant leakage has occurred only when a predetermined condition is satisfied. For instance, the controller  600  may repeat the determination only during the compressor  210  is not operating. If an occurrence of the refrigerant leakage is indicated by a user operation, the refrigerant leakage detector  530  can be omitted. Moreover, the refrigerant recovery operation may be triggered by other events, such as an input of an instruction requesting a start of the refrigerant recovery operation regardless of whether the refrigerant leakage has occurred. The steps of the controller  600  pertaining to the omitted elements can be omitted. One or more of the sensors do not necessary for the process by the controller  600  can be omitted. 
       FIG.  6    is a schematic configuration view of a heat pump system as a first modification of the heat pump system  100  according to the present embodiment. 
     As shown in  FIG.  6   , the heat pump system  100   a  comprises the compressor  210 , the heatsource-side heat exchanger  230 , the utilization-side heat exchanger  240 , the accumulator  250  disposed at a point between the bypass pipe  360  and the compressor  210 , the discharge-side refrigerant pipe  310  connected to the heatsource-side heat exchanger  230 , the liquid refrigerant pipe  330 , the suction-side refrigerant pipe  350  connected to utilization-side heat exchanger  240 , the bypass pipe  360 , the utilization-side expansion mechanism  440 , the gas-side on-off valve  460 , the bypass expansion mechanism  470 , the ambient temperature detector  520 , and a controller  600   a  corresponding to the controller  600 . The utilization-side expansion mechanism  440  may be disposed at a point between the heatsource-side heat exchanger  230  and the bypass pipe  360 . In this configuration, the discharge-side refrigerant pipe  310  corresponds to the high-pressure refrigerant pipe according to the present invention, and the suction-side refrigerant pipe  350  corresponds to the low-pressure refrigerant pipe according to the present invention. Meanwhile, as mentioned above, the heat pump system  100   a  does not necessarily comprise the other elements explained in the present embodiment using  FIG.  1   . In addition, further elements can be omitted. 
       FIG.  7    is a schematic configuration view of a heat pump system as a second modification of the heat pump system  100  according to the present embodiment. 
     As shown in  FIG.  7   , the heat pump system  100   b  does not have the bypass pipe  360  and the bypass expansion mechanism  470 , and the accumulator  250 , compared with the first modification. Even if these elements are omitted, the refrigerant can be drawn from the utilization-side piping section  102  towards the heatsource-side piping section  101 , and the drawn refrigerant can be accumulated mainly in the heatsource-side heat exchanger  230 . The controller  600   b  of the heat pump system  100   b , which corresponds to the controller  600 , needs to perform less steps. 
     Other modifications of the present embodiment are also possible. For instance, the controller  600  may set three or more of different predetermined increase rate value Rv_ 1 , Rv_ 2 , Rv_ 3 , . . . corresponding to different predetermined ambient temperature values Ta_th 1 , Ta_th 2 , . . . the ambient temperature detector  520  may acquire the temperature of the outdoor air from an external device such as a weather information server by wired/wireless communication. In this case, the ambient temperature detector  520  need not be arranged in the vicinity of the heatsource-side heat exchanger  230 . If the discharge pressure is unlikely to be excessively high during the refrigerant recovery operation, the target increase rate value Rv_tgt need not be changed depending on the ambient temperature T 1 . In this case, the ambient temperature detector  520  can be omitted. 
     The refrigerant leakage detector  530  may be configured to detect an occurrence of refrigerant leakage in any part of the utilization-side piping section  102 . The controller  600  may be disposed outside the heatsource-side piping section  101 . The controller  600  may also be distanced away from the other part of the heat pump system  100 . The fluid which passes thorough the heatsource-side heat exchanger  230  and the fluid which passes thorough the utilization-side heat exchanger  240  may be fluid other than air, such as water. Refrigerant other than R32 refrigerant may be used. 
     A plurality of the utilization-side units  120  may be connected to the heatsource-side unit  110 . In this case, the liquid-side on-off valve  420  may be disposed for each of sub liquid refrigerant pipes branched from the liquid refrigerant  330  towards the utilization-side units  120 , and the gas-side on-off valve  460  may be disposed for each of sub gas refrigerant pipes branched from the second gas refrigerant pipe  340  towards the utilization-side units  120 . It is preferable that the liquid-side on-off valves  420  and the gas-side on-off valves  460  are disposed within or close to the heatsource-side unit  110 . The refrigerant recovery operation is performed when an occurrence of refrigerant leakage has been detected in any of the utilization-side units  120  or any of the corresponding utilization-side piping sections  102 . It is preferable that, among the liquid-side on-off valves  420  and the gas-side on-off valves  460 , only the gas-side on-off valve  460  corresponding to the utilization-side unit  120  in which a refrigerant leakage has occurred is open during the refrigerant recovery operation. 
     While only selected embodiments and modifications have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, unless specifically stated otherwise, the size, shape, location or orientation of the various components can be changed as needed and/or desired so long as the changes do not substantially affect their intended function. Unless specifically stated otherwise, components that are shown directly connected or contacting each other can have intermediate structures disposed between them so long as the changes do not substantially affect their intended function. The functions of one element can be performed by two, and vice versa unless specifically stated otherwise. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100 ,  100   a ,  100   b : Heat Pump System 
               101 : Heatsource-Side Piping Section 
               102 : Utilization-Side Piping Section 
               110 : Heatsource-Side Unit 
               120 : Utilization-Side Unit 
               210 : Compressor 
               220 : Mode Switching Mechanism 
               230 : Heatsource-Side Heat Exchanger 
               231 : Heatsource-Side Fan 
               240 : Utilization-Side Heat Exchanger 
               241 : Utilization-Side Fan 
               250 : Accumulator 
               260 : Refrigerant Heat Exchanger 
               310 : Discharge-Side Refrigerant Pipe (High-Pressure Refrigerant Pipe) 
               320 : First Gas Refrigerant Pipe (High-Pressure Refrigerant Pipe, Low-Pressure Refrigerant Pipe) 
               330 : Liquid Refrigerant Pipe 
               340 : Second Gas Refrigerant Pipe (Low-Pressure Refrigerant Pipe, High-Pressure Refrigerant Pipe) 
               350 : Suction-Side Refrigerant Pipe (Low-Pressure Refrigerant Pipe) 
               360 : Bypass Pipe 
               410 : Heatsource-Side Expansion Mechanism (Expansion Mechanism) 
               420 : Liquid-Side On-Off Valve 
               430 : Liquid-Side Stop Valve 
               440 : Utilization-Side Expansion Mechanism (Expansion Mechanism) 
               450 : Gas-Side Stop Valve 
               460 : Gas-Side On-Off Valve 
               470 : Bypass Expansion Mechanism 
               510 : Discharge-Side Refrigerant State Detector 
               520 : Ambient Temperature Detector 
               530 : Refrigerant Leakage Detector 
               540 : Suction-Side Refrigerant State Detector (Evaporation Temperature Detector, Suction Pressure Detector) 
               600 ,  600   a ,  600   b : Controller 
               610 : Storage Section 
               620 : Information Input Section 
               630 : Normal Operation Controller 
               640 : Information Output Section 
               650 : Refrigerant Recovery Controller 
               651 : Leakage Detection Section 
               652 : Temperature Detection Section 
               653 : Acceleration Rate Switching Section 
               654 : Timing Control Section