Patent Publication Number: US-2023134655-A1

Title: Refrigeration cycle device

Description:
FIELD 
     The present disclosure relates to a refrigeration cycle device. 
     BACKGROUND 
     Patent Literature 1 described below discloses a refrigeration device for container. The refrigeration device includes a condenser positioned outside a refrigerator, an evaporator positioned inside the refrigerator, a hot-gas bypass path connecting a discharge pipe of a compressor and an inlet of the evaporator, a three-way proportional valve provided at a branch point thereof, and an injection bypass path connecting a liquid line and a suction line via an injection solenoid valve. During a defrost operation, this refrigeration device causes a discharge gas refrigerant to flow into the evaporator through the three-way proportional valve and the hot-gas bypass path, and opens the injection solenoid valve to replenish refrigerant from the liquid line to the suction line. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] JP H6-347143A 
       
    
     SUMMARY 
     Technical Problem 
     Since in the aforementioned conventional refrigeration device, liquid refrigerant flows into the suction line from the liquid line during a defrost operation, there is a likelihood that the liquid refrigerant is sucked into the compressor. 
     The present disclosure is made to solve the problem as described above and has an object to provide a refrigeration cycle device that is advantageous in reliably preventing liquid refrigerant from being sucked into a compressor. 
     Solution to Problem 
     A refrigeration cycle device according to the present disclosure includes: a compressor to compress refrigerant; a suction passage connecting to a suction port of the compressor; a discharge passage connecting to a discharge port of the compressor; an air heat exchanger to exchange heat between the refrigerant and air; a utilization heat exchanger to exchange heat between the refrigerant and a heat medium; a first refrigerant passage connecting the utilization heat exchanger to the discharge passage; a second refrigerant passage connecting the air heat exchanger to the suction passage; a receiver to store therein liquid refrigerant that is the refrigerant in liquid phase; a first expansion valve; a second expansion valve; a third refrigerant passage connecting the utilization heat exchanger to the first expansion valve; a fourth refrigerant passage connecting the first expansion valve to the receiver; a fifth refrigerant passage connecting the receiver to the second expansion valve; a sixth refrigerant passage connecting the second expansion valve to the air heat exchanger; a hot-gas bypass passage connecting the discharge passage to the sixth refrigerant passage; a hot-gas bypass valve provided on the hot-gas bypass passage; an internal heat exchanger to exchange heat between the liquid refrigerant inside the receiver and the refrigerant passing through the suction passage, or between the refrigerant passing through the fourth refrigerant passage and the refrigerant passing through the suction passage; a liquid bypass passage including an inlet portion connected to the fourth refrigerant passage, the fifth refrigerant passage, or a lower portion of the receiver, and an outlet portion connected to the suction passage upstream of the internal heat exchanger; and a liquid bypass valve provided on the liquid bypass passage. 
     Advantageous Effect of Invention 
     According to the present disclosure, it becomes possible to provide the refrigeration cycle device that is advantageous in reliably preventing the liquid refrigerant from being sucked into the compressor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram showing a refrigeration cycle device according to embodiment 1. 
         FIG.  2    is a diagram showing a flow of refrigerant during a heating operation of the refrigeration cycle device according to embodiment 1. 
         FIG.  3    is a diagram showing a flow of the refrigerant at a time of a hot-gas defrost operation of the refrigeration cycle device according to embodiment 1. 
         FIG.  4    is one example of a functional block diagram of the refrigeration cycle device according to embodiment 1. 
         FIG.  5    is a flowchart showing an example of a process at a time of executing the hot-gas defrost operation. 
         FIG.  6    is a timing chart showing an operation example of each of actuators from the heating operation until the operation transitions to the hot-gas defrost operation and returns to the heating operation. 
         FIG.  7    is a diagram showing a refrigeration cycle device according to embodiment 2. 
         FIG.  8    is one example of a functional block diagram of the refrigeration cycle device according to embodiment 2. 
         FIG.  9    is a diagram showing a flow of the refrigerant at a time of a heating operation of the refrigeration cycle device according to embodiment 2. 
         FIG.  10    is a diagram showing a flow of the refrigerant at a time of a hot-gas defrost operation of the refrigeration cycle device according to embodiment 2. 
         FIG.  11    is a flowchart showing an example of a process at a time of executing the hot-gas defrost operation according to embodiment 2. 
         FIG.  12    is a timing chart showing an operation example of each of actuators from the heating operation according to embodiment 2 until the operation transitions to the hot-gas defrost operation and returns to the heating operation. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments are described with reference to the drawings. Common or corresponding elements in each of the drawings are assigned with the same reference signs, and explanation thereof is simplified or omitted. 
     Embodiment 1 
       FIG.  1    is a diagram showing a refrigeration cycle device  1  according to embodiment 1. As shown in  FIG.  1   , the refrigeration cycle device  1  of the present embodiment includes a compressor  2  configured to compress refrigerant. A substance used as the refrigerant is not particularly limited, and may be any one of CO 2 , HFC, and HFO, for example. Further, the refrigeration cycle device  1  may use flammable refrigerant. Flammable refrigerant has an advantage of having a small impact on global warming. As the flammable refrigerant, there are cited hydrocarbon refrigerants such as R290 (propane) and R600a (isobutane), for example. 
     A suction passage  3  connects to a suction port of the compressor  2 . A discharge passage  4  connects to a discharge port of the compressor  2 . An air heat exchanger  5  is positioned outdoor. Hereinafter, outdoor air is referred to as “outside air”. The air heat exchanger  5  exchanges heat between the refrigerant and outside air. The air heat exchanger  5  has a refrigerant flow path. The air heat exchanger  5  has a structure in which air can pass. When a blower  23  works in the illustrated example, outside air flows through the air heat exchanger  5 . 
     A utilization heat exchanger  6  exchanges heat between the refrigerant and a heat medium. The heat medium is a medium for conveying heat to a heat demand section (not illustrated) that is equipment or a place that uses heat. The heat medium in the present embodiment is a liquid. The liquid heat medium may be, for example, water, or may be brine, other than water. The utilization heat exchanger  6  in the present embodiment has a refrigerant flow path and a heat medium flow path. In the illustrated example, the heat medium flow path of the utilization heat exchanger  6  is connected to the heat demand section via a heat medium circuit  100 . The heat medium circuit  100  has a heat medium pump  101 . When the heat medium circulates in the heat medium circuit  100  by an operation of the heat medium pump  101 , the heat medium that passes through the utilization heat exchanger  6  is supplied to the heat demand section. The heat medium that passes through the heat demand section returns to the utilization heat exchanger  6 . The heat medium circuit  100  may include a valve not illustrated for controlling a flow rate or a circulation route of the heat medium. 
     The heat demand section may include indoor heating equipment for heating a room. The indoor heating equipment may include at least one of a floor heating panel that is installed under a floor of a room, a radiator, a panel heater, and a fan convector that are installed in the room, for example. The heat demand section may include a heat storage tank. The heat storage tank may be a hot water storage tank that stores hot water. The heat medium heated by the utilization heat exchanger  6  may be stored in the heat storage tank, or hot water heated by exchanging heat with the heat medium heated by the utilization heat exchanger  6  may be stored in the hot water storage tank. The heat demand section may include indoor cooling equipment for cooling a room. The indoor cooling equipment may include a fan coil, for example. The heat demand section may be used in both the indoor heating equipment and the indoor cooling equipment. 
     Note that the heat medium in the present embodiment is not limited to the liquid but may be gas. For example, the heat medium may be indoor air that is air inside the room. In this case, a blower (not illustrated) that generates an air flow may be included so that the indoor air that passes through the utilization heat exchanger  6  is blown into the room. 
     A first refrigerant passage  7  connects one end of the refrigerant flow path of the utilization heat exchanger  6  to the discharge passage  4 . A second refrigerant passage  8  connects one end of the refrigerant flow path of the air heat exchanger  5  to the suction passage  3 . 
     In the present disclosure, refrigerant in a liquid phase state is referred to as a “liquid refrigerant”, and refrigerant in a gaseous phase state is referred to as a “gas refrigerant”. A receiver  9  is provided to store liquid refrigerant. Inside the receiver  9 , a liquid level  90  of the liquid refrigerant is formed. An inner space of the receiver  9  above the liquid level  90  is filled with a gas refrigerant. 
     The refrigeration cycle device  1  further includes a first expansion valve  11  and a second expansion valve  12 . The first expansion valve  11  and the second expansion valve  12  each have a first port and a second port. A third refrigerant passage  13  connects the other end of the refrigerant flow path of the utilization heat exchanger  6  to the first port of the first expansion valve  11 . A fourth refrigerant passage  14  connects the second port of the first expansion valve  11  to the receiver  9 . A fifth refrigerant passage  15  connects the receiver  9  to the first port of the second expansion valve  12 . A sixth refrigerant passage  16  connects the second port of the second expansion valve  12  to the other end of the refrigerant flow path of the air heat exchanger  5 . 
     In the illustrated example, a tip end opening  14   a  of the fourth refrigerant passage  14  is located in a lower portion in the receiver  9  and is located under the liquid level  90 . A tip end opening  15   a  of the fifth refrigerant passage  15  is located in the lower portion in the receiver  9  and is under the liquid level  90 . 
     A hot-gas bypass passage  17  connects the discharge passage  4  to the sixth refrigerant passage  16 . One end of the hot-gas bypass passage  17  is connected to a branch portion  4   a  provided on the discharge passage  4 . The other end of the hot-gas bypass passage  17  is connected to a branch portion  16   a  provided on the sixth refrigerant passage  16 . A hot-gas bypass valve  18  is provided on the hot-gas bypass passage  17 . 
     An internal heat exchanger  19  in the present embodiment exchanges heat between the liquid refrigerant inside the receiver  9  and the refrigerant passing through the suction passage  3 . The internal heat exchanger  19  is provided inside the receiver  9 . The internal heat exchanger  19  is located under the liquid level  90  of the liquid refrigerant. The refrigerant passing through the suction passage  3  is heated by the liquid refrigerant inside the receiver  9  when passing through the internal heat exchanger  19 . 
     A liquid bypass passage  20  has an inlet portion  20   a  connected to the fifth refrigerant passage  15 , and an outlet portion  20   b  connected to the suction passage  3  upstream of the internal heat exchanger  19 . The inlet portion  20   a  is connected to a branch portion provided on the fifth refrigerant passage  15 . The outlet portion  20   b  is connected to a branch portion provided on the suction passage  3 . A liquid bypass valve  21  is provided on the liquid bypass passage  20 . 
     The refrigeration cycle device  1  can perform a heating operation. The heating operation is an operation that heats the heat medium by causing the refrigerant discharged from the compressor  2  to flow into the utilization heat exchanger  6 . For example, in a system in which the heat demand section includes the indoor heating equipment, an indoor-heating can be performed by supplying the heat medium heated by the utilization heat exchanger  6  by a heating operation to the indoor heating equipment. Alternatively, in a system in which the heat demand section includes a heat storage tank such as a hot water storage tank, a heat accumulating operation that accumulates the heat medium or hot water heated by the heating operation into the heat storage tank can be performed. 
       FIG.  2    is a diagram showing a flow of the refrigerant during the heating operation of the refrigeration cycle device  1  according to embodiment 1. As shown in  FIG.  2   , the flow of the refrigerant during the heating operation is as follows. The hot-gas bypass valve  18  and the liquid bypass valve  21  are closed, and the refrigerant does not flow into the hot-gas bypass passage  17  and the liquid bypass passage  20 . A high-temperature and high-pressure refrigerant discharged from the compressor  2  flows into the utilization heat exchanger  6  through the discharge passage  4  and the first refrigerant passage  7 . The high-pressure refrigerant cooled by the heat medium in the utilization heat exchanger  6  flows into the first expansion valve  11  through the third refrigerant passage  13 . The high-pressure refrigerant is decompressed and expanded by the first expansion valve  11  to become a medium-pressure refrigerant. The medium-pressure refrigerant flows into the receiver  9  from the first expansion valve  11  through the fourth refrigerant passage  14 . The low-pressure refrigerant flowing through the suction passage  3  to pass through the internal heat exchanger  19  cools the liquid refrigerant in the receiver  9 . The medium-pressure liquid refrigerant in the receiver  9  flows into the second expansion valve  12  through the fifth refrigerant passage  15 . The medium-pressure liquid refrigerant is decompressed and expanded by the second expansion valve  12  to become a gas-liquid two-phase low-temperature and low-pressure refrigerant. The low-temperature and low-pressure refrigerant flows into the air heat exchanger  5  through the sixth refrigerant passage  16 . The low-temperature and low-pressure refrigerant evaporates by absorbing heat of outside air in the air heat exchanger  5 . The low-pressure refrigerant flows into the suction passage  3  through the second refrigerant passage  8  from the air heat exchanger  5 . The low-pressure refrigerant flowing in the suction passage  3  is heated by the medium-pressure refrigerant inside the receiver  9  when passing through the internal heat exchanger  19  on the way, and thereafter sucked by the compressor  2 . 
     If the heating operation is performed under a condition that the temperature of the outside air is low, moisture contained in the outside air may become frost and adhere to the air heat exchanger  5 . The refrigeration cycle device  1  can execute a hot-gas defrost operation for removing the frost adhering to the air heat exchanger  5 .  FIG.  3    is a diagram showing a flow of the refrigerant at a time of a hot-gas defrost operation of the refrigeration cycle device  1  according to embodiment 1. At the time of the hot-gas defrost operation, the hot-gas bypass valve  18  is opened, the second expansion valve  12  is closed, the first expansion valve  11  is continuously or intermittently opened, and the liquid bypass valve  21  is continuously or intermittently opened. 
     As shown in  FIG.  3   , the flow of the refrigerant at the time of the hot-gas defrost operation is as follows. Most of the high-temperature and high-pressure refrigerant discharged to the discharge passage  4  from the compressor  2  passes through the branch portion  4   a , the hot-gas bypass passage  17 , the hot-gas bypass valve  18 , the branch portion  16   a , and the sixth refrigerant passage  16 , and flows into the air heat exchanger  5 . Frost is melted and removed by the heat of the refrigerant, that is, hot gas flowing into the air heat exchanger  5 . The refrigerant passing through the air heat exchanger  5  is sucked by the compressor  2  after passing through the second refrigerant passage  8 , the suction passage  3 , and the internal heat exchanger  19 . The rest of the high-temperature and high-pressure refrigerant discharged to the discharge passage  4  from the compressor  2  flows into the utilization heat exchanger  6  through the first refrigerant passage  7 . The refrigerant flowing into the utilization heat exchanger  6  is cooled by the heat medium and condenses. The condensed liquid refrigerant stays inside the utilization heat exchanger  6 . The liquid refrigerant inside the utilization heat exchanger  6  passes through the third refrigerant passage  13  and the first expansion valve  11  and flows into the receiver  9 . The liquid refrigerant inside the receiver  9  flows to the fifth refrigerant passage  15  from the tip end opening  15   a . The liquid refrigerant passes through the inlet portion  20   a , the liquid bypass passage  20 , and the outlet portion  20   b , and joins the refrigerant flowing through the suction passage  3 . 
     If the liquid refrigerant accumulates inside the utilization heat exchanger  6  at the time of the hot-gas defrost operation, the flow rate of the refrigerant circulating from the compressor  2  to the air heat exchanger  5  will become insufficient, as a result of which, a defrosting ability will be reduced. In contrast to this, the present embodiment provides the following effect at the time of the hot-gas defrost operation. The liquid refrigerant accumulated inside the utilization heat exchanger  6  can be supplied to the suction passage  3  through the first expansion valve  11  and the liquid bypass passage  20 . Therefore, shortage of the flow rate of the refrigerant circulating from the compressor  2  to the air heat exchanger  5  can be reliably prevented, so that a high defrosting ability can be maintained. The liquid refrigerant flowing into the suction passage  3  from the outlet portion  20   b  of the liquid bypass passage  20  is heated by the internal heat exchanger  19  and evaporated. Therefore, the liquid refrigerant can be reliably prevented from being sucked by the compressor  2 . Therefore, according to the present embodiment, there is provided an advantage that an accumulator for preventing the liquid refrigerant from being sucked by the compressor  2  does not have to be provided on a suction side of the compressor  2 . Accordingly, as illustrated, an accumulator is not provided on the suction passage  3 . In other words, the refrigerant passing through the internal heat exchanger  19  is sucked by the compressor  2  without passing through an accumulator. 
     The refrigeration cycle device  1  may further include controlling circuitry  50  configured to execute the heating operation and the hot-gas defrost operation. By adding the controlling circuitry  50 , there is provided an advantage that the operation of the heating operation and the operation of the hot-gas defrost operation can be automated. At the time of the heating operation, the controlling circuitry  50  closes the hot-gas bypass valve  18  and the liquid bypass valve  21 . At the time of the hot-gas defrost operation, the controlling circuitry  50  opens the hot-gas bypass valve  18  and closes the second expansion valve  12 . Further, at the time of the hot-gas defrost operation, the controlling circuitry  50  continuously or intermittently opens the liquid bypass valve  21 . The controlling circuitry  50  may perform control so that an opening degree of the first expansion valve  11  at the time of the hot-gas defrost operation becomes smaller than an opening degree of the first expansion valve  11  at the time of the heating operation. During execution of the hot-gas defrost operation, the controlling circuitry  50  may keep the first expansion valve  11  open or may control the first expansion valve  11  so that it repeatedly opens and closes. During execution of the hot-gas defrost operation, the controlling circuitry  50  may keep the liquid bypass valve  21  open, or may control the liquid bypass valve  21  so that it repeatedly opens and closes. 
     The refrigeration cycle device  1  may further include a refrigerant circuit switching valve  22  that switches between a forward cycle circuit and a reverse cycle circuit. The forward cycle circuit shown in  FIG.  2    is a circuit in which the refrigerant discharged from the compressor  2  flows into the utilization heat exchanger  6  through the first refrigerant passage  7 . Though not illustrated, the reverse cycle circuit is a circuit in which the refrigerant discharged from the compressor  2  flows into the air heat exchanger  5  through the second refrigerant passage  8 . By adding the refrigerant circuit switching valve  22 , it becomes possible to perform a cooling operation using the reverse cycle circuit. A cooling operation is an operation that cools the heat medium in the utilization heat exchanger  6 . For example, in the system in which the heat demand section includes the indoor cooling equipment, an indoor-cooling can be performed by supplying the heat medium cooled by the cooling operation to the indoor cooling equipment from the utilization heat exchanger  6 . 
     In the illustrated example, the refrigerant circuit switching valve  22  includes an a-port, a b-port, a c-port and a d-port. The a-port of the refrigerant circuit switching valve  22  is connected to the discharge port of the compressor  2  by the discharge passage  4 . The b-port of the refrigerant circuit switching valve  22  is connected to the suction port of the compressor  2  by the suction passage  3 . The c-port of the refrigerant circuit switching valve  22  is connected to the utilization heat exchanger  6  by the first refrigerant passage  7 . The d-port of the refrigerant circuit switching valve  22  is connected to the air heat exchanger  5  by the second refrigerant passage  8 . 
     The refrigerant circuit switching valve  22  switches the refrigerant flow path by moving a valve body, for example. At the time of the heating operation in  FIG.  2   , the refrigerant circuit switching valve  22  causes the a-port to communicate with the c-port, and causes the b-port to communicate with the d-port, whereby the forward cycle circuit is formed. Thereby, the discharge passage  4  is connected to the first refrigerant passage  7 , and the suction passage  3  is connected to the second refrigerant passage  8 . A state of the refrigerant circuit switching valve  22  at the time of the hot-gas defrost operation in  FIG.  3    is also the same as described above. 
     At a time of the cooling operation, the refrigerant circuit switching valve  22  causes the a-port to communicate with the d-port, and causes the b-port to communicate with the c-port, whereby the reverse cycle circuit is formed. Thereby, the discharge passage  4  is connected to the second refrigerant passage  8 , and the suction passage  3  is connected to the first refrigerant passage  7 . A flow of the refrigerant at the time of an operation by the reverse cycle circuit is as follows. The high-temperature and high-pressure refrigerant discharged from the compressor  2  flows into the air heat exchanger  5  through the discharge passage  4  and the second refrigerant passage  8 . The high-pressure refrigerant is cooled by outside air in the air heat exchanger  5 . The cooled high-pressure refrigerant flows into the second expansion valve  12  through the sixth refrigerant passage  16  from the air heat exchanger  5 . The high-pressure refrigerant is decompressed and expanded by the second expansion valve  12  and becomes a medium-pressure refrigerant. The medium-pressure refrigerant flows into the receiver  9  through the fifth refrigerant passage  15  from the second expansion valve  12 . The medium-pressure liquid refrigerant flows into the first expansion valve  11  through the fourth refrigerant passage  14  from the receiver  9 . The medium-pressure liquid refrigerant is decompressed and expanded by the first expansion valve  11  and becomes a gas-liquid two-phase low-pressure refrigerant. The low-pressure refrigerant flows into the utilization heat exchanger  6  through the third refrigerant passage  13 . The low-pressure refrigerant evaporates in the utilization heat exchanger  6 , and thereby the heat medium is cooled. The low-pressure refrigerant passing through the utilization heat exchanger  6  flows into the suction passage  3  from the first refrigerant passage  7 . The low-pressure refrigerant flowing through the suction passage  3  is heated by the medium-pressure refrigerant in the receiver  9  when passing through the internal heat exchanger  19  on the way, and thereafter is sucked by the compressor  2 . The liquid refrigerant in the receiver  9  is cooled by the low-pressure refrigerant flowing through the suction passage  3  and passing through the internal heat exchanger  19 . 
     The refrigeration cycle device  1  of the present disclosure may not include the refrigerant circuit switching valve  22  and may not be able to execute the reverse cycle operation. When the refrigerant circuit switching valve  22  is not included, the discharge passage  4  can be configured to directly connect to the first refrigerant passage  7  and the suction passage  3  can be configured to directly connect to the second refrigerant passage  8 . 
     As shown in  FIG.  1   , the refrigeration cycle device  1  may further include at least one of a discharge pressure sensor  24 , a discharge temperature sensor  25 , a suction pressure sensor  26 , a suction temperature sensor  27 , a first temperature sensor  28 , a second temperature sensor  29 , a third temperature sensor  30 , and an outside air temperature sensor  31 . The discharge pressure sensor  24  installed on the discharge passage  4  detects a compressor discharge pressure that is a pressure of the refrigerant discharged from the compressor  2 . The discharge temperature sensor  25  installed on the discharge passage  4  detects a compressor discharge temperature that is a temperature of the refrigerant discharged from the compressor  2 . The suction pressure sensor  26  installed on the suction passage  3  detects a compressor suction pressure that is a pressure of the refrigerant to be sucked by the compressor  2 . The suction temperature sensor  27  installed on the suction passage  3  downstream of the internal heat exchanger  19  detects a compressor suction temperature that is a temperature of the refrigerant to be sucked by the compressor  2 . The first temperature sensor  28  installed on the sixth refrigerant passage  16  between the branch portion  16   a  and the air heat exchanger  5  detects a temperature of the refrigerant between the air heat exchanger  5  and the second expansion valve  12 . The second temperature sensor  29  installed on the second refrigerant passage  8  detects a temperature of the refrigerant between the refrigerant circuit switching valve  22  and the air heat exchanger  5 . The third temperature sensor  30  installed on the fourth refrigerant passage  14  detects a temperature of the liquid refrigerant between the utilization heat exchanger  6  and the first expansion valve  11 . The outside air temperature sensor  31  detects a temperature of outside air before flowing into the air heat exchanger  5 . 
       FIG.  4    is one example of a functional block diagram of the refrigeration cycle device  1  according to embodiment 1. As shown in  FIG.  4   , each of the compressor  2 , the first expansion valve  11 , the second expansion valve  12 , the hot-gas bypass valve  18 , the liquid bypass valve  21 , the refrigerant circuit switching valve  22 , the blower  23 , the discharge pressure sensor  24 , the discharge temperature sensor  25 , the suction pressure sensor  26 , the suction temperature sensor  27 , the first temperature sensor  28 , the second temperature sensor  29 , the third temperature sensor  30 , and the outside air temperature sensor  31  may be electrically connected to the controlling circuitry  50 . Each of functions of the controlling circuitry  50  may be realized by processing circuitry. The processing circuitry of the controlling circuitry  50  may include at least one processor  51  and at least one memory  52 . At least the one processor  51  may realize each of the functions of the controlling circuitry  50  by reading and executing a program stored in at least the one memory  52 . The processing circuitry of the controlling circuitry  50  may include at least one dedicated piece of hardware. The controlling circuitry  50  may perform control so that a rotation speed of the compressor  2  becomes variable by inverter control, for example. The controlling circuitry  50  may perform control so that a rotation speed of the blower  23  becomes variable by the inverter control, for example. 
     The hot-gas bypass valve  18  is preferably configured by a solenoid valve that is switchable only between opening (full open) and closing (full close), and has a small pressure loss, for example. 
     The liquid bypass valve  21  preferably has a function as an expansion valve capable of adjusting a flow rate by adjusting an opening degree thereof, for example. 
     An operation of the heat medium pump  101  of the heat medium circuit  100  may be controlled by a controller except for the controlling circuitry  50 . For example, a controller included by an air-conditioning device or a hot-water supply device that uses a heat medium may control the operation of the heat medium pump  101 . 
     In the present embodiment, the refrigeration cycle device  1  may be configured to execute the hot-gas defrost operation without stopping the flow of the heat medium in the utilization heat exchanger  6 . According to the present embodiment, it is not necessary to stop the heat medium pump  101  when executing the hot-gas defrost operation, so that a control operation becomes simple. When the heat medium continues to flow into the utilization heat exchanger  6  during the hot-gas defrost operation, the high-temperature and high-pressure refrigerant from the compressor  2  is cooled and condensed by the heat medium, and therefore, liquid refrigerant is easily generated in the utilization heat exchanger  6 . According to the present embodiment, the liquid refrigerant in the utilization heat exchanger  6  can be supplied to the suction passage  3  through the first expansion valve  11  and the liquid bypass passage  20 . Therefore, the liquid refrigerant can be prevented from accumulating in the utilization heat exchanger  6 , so that shortage of the flow rate of the refrigerant circulating from the compressor  2  to the air heat exchanger  5  can be reliably prevented, and a high defrosting ability can be maintained. 
     A heating power [W] is an amount of heat that is given to the heat medium in the utilization heat exchanger  6  per unit time at the time of the heating operation. At the time of the heating operation, the controlling circuitry  50  may adjust the rotation speed of the compressor  2  so as to obtain a predetermined heating power corresponding to a load on the heat medium circuit  100 . 
     A superheat degree of the refrigerant to be sucked by the compressor  2  is referred to as a “suction superheat degree” below. A superheat degree of the refrigerant discharged from the compressor  2  is referred to as a “discharge superheat degree” below. A saturation temperature corresponding to a compressor suction pressure is referred to as a “suction saturation temperature” below. A saturation temperature corresponding to a compressor discharge pressure is referred to as a “discharge saturation temperature” below. The controlling circuitry  50  can calculate the suction saturation temperature by using a detected pressure of the suction pressure sensor  26 . The controlling circuitry  50  can calculate the discharge saturation temperature by using a detected pressure of the discharge pressure sensor  24 . The controlling circuitry  50  can calculate the suction superheat degree from a difference between a detected temperature of the suction temperature sensor  27  and the suction saturation temperature. In the present embodiment, the suction pressure sensor  26  and the suction temperature sensor  27  correspond to detectors that detect the suction superheat degree. The controlling circuitry  50  can calculate the discharge superheat degree from a difference between the detected temperature of the discharge temperature sensor  25  and the discharge saturation temperature. In the present embodiment, the discharge pressure sensor  24  and the discharge temperature sensor  25  correspond detectors that detect the discharge superheat degree. 
     At the time of the heating operation, the controlling circuitry  50  may control an opening degree of the first expansion valve  11  so that a supercooling degree of the refrigerant flowing out from the utilization heat exchanger  6  becomes close to a target. The controlling circuitry  50  may calculate the supercooling degree from a difference between the discharge saturation temperature and a detected temperature of the third temperature sensor  30 . For example, when the controlling circuitry  50  increases the opening degree of the first expansion valve  11 , a flow rate of the refrigerant passing through the utilization heat exchanger  6  increases, and the supercooling degree decreases. 
     At the time of the heating operation, the controlling circuitry  50  may control an opening degree of the second expansion valve  12  so that the suction superheat degree or the discharge superheat degree becomes close to a target. The controlling circuitry  50  may control the second expansion valve  12  by using either the suction superheat degree or the discharge superheat degree. 
     In the following explanation, a temperature of the refrigerant flowing out from the air heat exchanger  5  is referred to as a “refrigerant outlet temperature of the air heat exchanger  5 ”, and a temperature of the refrigerant flowing into the air heat exchanger  5  is referred to as a “refrigerant inlet temperature of the air heat exchanger  5 ”. At the time of the heating operation, the controlling circuitry  50  may control the opening degree of the second expansion valve  12  so that an evaporation superheat degree becomes close to a target. The evaporation superheat degree corresponds to a difference between the refrigerant outlet temperature of the air heat exchanger  5  detected by the second temperature sensor  29  and the refrigerant inlet temperature of the air heat exchanger  5  detected by the first temperature sensor  28 . 
     When the controlling circuitry  50  increases the opening degree of the second expansion valve  12  at the time of the heating operation, the flow rate of the refrigerant passing through the air heat exchanger  5  increases, and each of the suction superheat degree, discharge superheat degree and evaporation superheat degree decreases. At the time of the heating operation, the controlling circuitry  50  may operate the blower  23  at a predetermined rotation speed. 
       FIG.  5    is a flowchart showing an example of a process at a time of executing the hot-gas defrost operation.  FIG.  6    is a timing chart showing an operation example of each of actuators from the heating operation until the operation transitions to the hot-gas defrost operation and returns to the heating operation. Hereinafter, the examples shown in  FIG.  5    and  FIG.  6    are described. 
     When a surface temperature of the air heat exchanger  5  goes down to below freezing due to a low outside air temperature during execution of the heating operation, frost is formed on the surface of the air heat exchanger  5 , and as a result, heat transfer performance of the air heat exchanger  5  is deteriorated. As the amount of frost formation increases, the evaporation temperature with respect to the outside air temperature is lowered. The evaporation temperature mentioned here refers to a saturation temperature of the refrigerant that evaporates in a pipe of the air heat exchanger  5 . When a difference between the outside air temperature and the temperature of the liquid refrigerant in the air heat exchanger  5  becomes larger than a reference value, the controlling circuitry  50  may perform control to transition from the heating operation to the hot-gas defrost operation. The reference value may be approximately 10 K, for example. 
     When transitioning from the heating operation to the hot-gas defrost operation, the controlling circuitry  50  first controls the operation so that the rotation speed of the compressor  2  becomes equal to a minimum rotation speed Fcmin, as step S 101  in  FIG.  5   . Next, the controlling circuitry  50  stops the blower  23 , as step S 102 . Next, the controlling circuitry  50  opens the hot-gas bypass valve  18 , as step S 103 . Next, the controlling circuitry  50  controls the operation so that an opening degree of the liquid bypass valve  21  becomes slight opening (P 3 - 1 ), as step S 104 . The slight opening (P 3 - 1 ) preferably corresponds to a minimum opening degree at which the refrigerant flows through the liquid bypass valve  21 . Next, the controlling circuitry  50  controls the operation so that the opening degree of the first expansion valve  11  becomes slight opening (P 1 - 2 ), as step S 105 . The slight opening (P 1 - 2 ) preferably corresponds to a minimum opening degree at which the refrigerant flows through the first expansion valve  11 . Next, the controlling circuitry  50  controls the operation so that the opening degree of the second expansion valve  12  becomes fully closed, as step S 106 . This prevents the refrigerant from flowing into the second expansion valve  12 . The process from step S 101  to step S 106  described above corresponds to a defrost preparation process in  FIG.  6   . 
     After step S 106 , the controlling circuitry  50  controls the operation so that the rotation speed of the compressor  2  becomes equal to a target rotation speed Fc 2 , as step S 107 . The target rotation speed Fc 2  may be a fixed value. The controlling circuitry  50  may adjust a value of the target rotation speed Fc 2  so that the compressor discharge pressure becomes constant, for example. Next, the controlling circuitry  50  controls the operation so that the opening degree of the liquid bypass valve  21  becomes equal to a target opening degree (P 3 - 2 ), as step S 108 . At this time, the controlling circuitry  50  may adjust a value of the target opening degree (P 3 - 2 ) so that the suction superheat degree or the discharge superheat degree becomes close to a target. By a process of step S 107  and step S 108  described above, the hot-gas defrost operation starts. 
     Since the opening degree of the first expansion valve  11  is the slight opening (P 1 - 2 ) at the time of the hot-gas defrost operation, only a small amount of the high-temperature and high-pressure refrigerant discharged to the discharge passage  4  from the compressor  2  flows into the utilization heat exchanger  6 , and most of the refrigerant flows into the hot-gas bypass passage  17 . The high-temperature and high-pressure refrigerant flowing into the hot-gas bypass passage  17  is decompressed to be the low-pressure gas refrigerant when passing through the hot-gas bypass valve  18 , and thereafter flows into the air heat exchanger  5 . Since the second expansion valve  12  is fully closed at this time, the low-pressure gas refrigerant flows into a pipe that forms the refrigerant flow path of the air heat exchanger  5 , and exchanges heat with frost adhering to a surface of fins joined to the pipe. The frost receives heat of the refrigerant and melts. The refrigerant is cooled by the frost. The refrigerant leaving the air heat exchanger  5  passes through the second refrigerant passage  8  and the refrigerant circuit switching valve  22 , and flows into the suction passage  3 . The refrigerant flowing into the suction passage  3  joins the refrigerant from the liquid bypass passage  20 . The joining refrigerant is heated by exchanging heat with the refrigerant in the receiver  9  in the internal heat exchanger  19 , and thereafter is sucked by the compressor  2  again. In this way, a hot-gas defrost circuit that circulates the refrigerant to the compressor  2 , the hot-gas bypass valve  18 , the air heat exchanger  5 , the refrigerant circuit switching valve  22  and the internal heat exchanger  19  in this order is formed. 
     Since the opening degree of the first expansion valve  11  is the slight opening (P 1 - 2 ) at the time of the hot-gas defrost operation, a small amount of high-pressure refrigerant flows into the utilization heat exchanger  6  through the first refrigerant passage  7  from the discharge passage  4 . Heat is exchanged between the heat medium continuing to flow into the utilization heat exchanger  6  and the high-pressure refrigerant, and thereby the high-pressure refrigerant is cooled and liquefied. The liquefied refrigerant passes through the first expansion valve  11  and flows into the receiver  9 . Since the liquid bypass valve  21  opens, the liquid refrigerant from the receiver  9  passes through the liquid bypass passage  20  and flows into the suction passage  3 . 
     During execution of the hot-gas defrost operation, the controlling circuitry  50  determines whether the refrigerant outlet temperature of the air heat exchanger  5  detected by the second temperature sensor  29  is higher than the reference temperature, as step S 109 . The reference temperature is a temperature for determining an end of the hot-gas defrost operation, and may be 0° C. at which frost melts, or a temperature higher than 0° C. When the refrigerant outlet temperature of the air heat exchanger  5  is the reference temperature or less, it can be determined that frost cannot be removed yet, and therefore the controlling circuitry  50  returns to a process in step S 107  and continues the hot-gas defrost operation. In contrast to this, when the refrigerant outlet temperature of the air heat exchanger  5  is higher than the reference temperature, it can be determined that frost has been removed. In this case, the controlling circuitry  50  proceeds to a process in step S 110  to end the hot-gas defrost operation and restart the heating operation. 
     As step S 110 , the controlling circuitry  50  controls the operation so that the rotation speed of the compressor  2  becomes equal to the minimum rotation speed Fcmin. Next, as step S 111 , the controlling circuitry  50  controls the operation so that the opening degree of the first expansion valve  11  becomes an initial opening degree (P 1 - 3 ) of the heating operation. Next, as step S 112 , the controlling circuitry  50  controls the operation so that the opening degree of the second expansion valve  12  becomes an initial opening degree (P 2 - 3 ) of the heating operation. Next, the controlling circuitry  50  closes the hot-gas bypass valve  18 , as step S 113 . Next, the controlling circuitry  50  fully closes the liquid bypass valve  21 , as step S 114 . A process from step S 110  to step S 114  described above corresponds to a return process in  FIG.  6   . The heating operation restarts by the return process. After the restart of the heating operation, the controlling circuitry  50  operates the blower  23  at a predetermined rotation speed again and controls the operation of each of the compressor  2 , the first expansion valve  11 , and the second expansion valve  12  as in the explanation of the heating operation described above. 
     The refrigeration cycle device  1  of the present embodiment may be configured to be able to further execute a reverse cycle defrost operation. The reverse cycle defrost operation is an operation that removes frost from the air heat exchanger  5  by circulating the refrigerant into the reverse cycle circuit similarly to the aforementioned cooling operation. In the reverse cycle defrost operation, the air heat exchanger  5  is used as a condenser, and the utilization heat exchanger  6  is used as an evaporator. 
     When the reverse cycle defrost operation is executed in the system using a heat medium that can freeze like water, for example, the heat medium in the utilization heat exchanger  6  is cooled by heat of evaporation of the refrigerant to a temperature below the freezing point and may freeze. If the heat medium in the utilization heat exchanger  6  is frozen and expands in volume, there arises a likelihood that the utilization heat exchanger  6  is broken. If the utilization heat exchanger  6  is broken, there is a likelihood that a wall that partitions the heat medium flow path and a refrigerant flow path is broken to leak the refrigerant into the heat medium circuit  100 , or leak the refrigerant into the atmosphere. In order to reliably prevent such an event from occurring, a temperature sensor (not illustrated) that detects the temperature of the heat medium may be provided, and the controlling circuitry  50  may be configured to execute the hot-gas defrost operation when the temperature of the heat medium is lower as compared with a reference, and execute the reverse cycle defrost operation when the temperature of the heat medium is higher as compared with the reference, when melting the frost adhering to the air heat exchanger  5 . Thereby, the hot-gas defrost operation is executed when the heat medium in the utilization heat exchanger  6  is likely to freeze, and the reverse cycle defrost operation is executed when the heat medium in the utilization heat exchanger  6  is unlikely to freeze, so that both can be more properly used. 
     In the refrigeration cycle device  1  using flammable refrigerant, it is particularly important to reliably prevent leakage of the refrigerant into the heat medium circuit  100  or leakage of the refrigerant into the atmosphere. According to the present embodiment, the hot-gas defrost operation is executable, and therefore, the heat medium in the utilization heat exchanger  6  can be reliably prevented from freezing. Therefore, leakage of the refrigerant into the heat medium circuit  100  or leakage of the refrigerant into the atmosphere can be reliably prevented, so that it is suitable to use of flammable refrigerant. 
     The opening degree of the first expansion valve  11  at the time of the hot-gas defrost operation can be an opening degree at which the liquid refrigerant staying in the utilization heat exchanger  6  can be moved to the receiver  9  side through the fourth refrigerant passage  14 . In the present embodiment, the opening degree of the first expansion valve  11  at the time of the hot-gas defrost operation is smaller than the opening degree of the first expansion valve  11  at the time of the heating operation. This can reliably prevent the flow rate of the refrigerant from the compressor  2  to the utilization heat exchanger  6  from becoming larger than necessary at the time of the hot-gas defrost operation. Therefore, reduction in the flow rate of the refrigerant in the hot-gas bypass circuit can be reliably prevented, so that a high defrost ability is obtained. 
     The following problem will arise if the amount of the refrigerant in the hot-gas defrost circuit becomes insufficient as a result that the condensed liquid refrigerant accumulates in the utilization heat exchanger  6  during the hot-gas defrost operation. A density of the refrigerant to be sucked by the compressor  2  is reduced, and the flow rate of the refrigerant is reduced. Further, since the discharge pressure of the compressor is reduced and the discharge temperature is lowered, the defrost ability is reduced. As a result, a time required for defrosting becomes longer. In contrast to this, according to the present embodiment, the refrigerant can be supplied to the hot-gas defrost circuit from the receiver  9  through the liquid bypass valve  21 , so that the amount of the refrigerant in the hot-gas defrost circuit can reliably prevented from becoming insufficient. 
     It is assumed that the refrigerant is supplied to the hot-gas defrost circuit from the receiver  9  by opening the second expansion valve  12  and causing the liquid refrigerant to flow into the air heat exchanger  5  through the sixth refrigerant passage  16  from the receiver  9  during the hot-gas defrost operation. In this case, the gas refrigerant from the hot-gas bypass passage  17  and the liquid refrigerant from the receiver  9  mix, and thereby, the temperature of the refrigerant flowing into the air heat exchanger  5  is lowered. As a result, an amount of heat used for defrosting of the air heat exchanger  5  decreases. In contrast to this, according to the present embodiment, the liquid refrigerant is caused to flow into the suction passage  3  of the compressor  2 , so that the amount of the refrigerant of the hot-gas frost circuit can be adjusted without decreasing the amount of heat used for defrosting of the air heat exchanger  5 . 
     At the time of the hot-gas defrost operation, the controlling circuitry  50  may control the operation or the opening degree of the liquid bypass valve  21  so that the suction superheat degree becomes close to a target. In doing so, it is possible to more reliably prevent the amount of the liquid refrigerant flowing into the suction passage  3  from the liquid bypass passage  20  from becoming too large, and therefore, it is possible to more reliably prevent the compressor  2  from sucking the liquid refrigerant. 
     At the time of the hot-gas defrost operation, the controlling circuitry  50  may control the operation or the opening degree of the liquid bypass valve  21  so that the discharge superheat degree becomes close to a target. In doing so, it is possible to more reliably prevent the amount of the liquid refrigerant flowing into the suction passage  3  from the liquid bypass passage  20  from being too large, and therefore, it is possible to more reliably prevent the compressor  2  from sucking the liquid refrigerant. 
     As described above, according to the present embodiment, the amount of the refrigerant in the hot-gas defrost circuit can be adjusted by one actuator (the liquid bypass valve  21 ) and one control target (the suction superheat degree or discharge superheat degree). Therefore, it is possible to more simplify the control, and it is possible to make the hot-gas defrost operation more stable. 
     In the present disclosure, instead of the illustrated example, the inlet portion  20   a  of the liquid bypass passage  20  may be directly connected to the lower portion of the receiver  9 . In this case, at the time of the hot-gas defrost operation, the liquid refrigerant flowing out to the liquid bypass passage  20  from the lower portion of the receiver  9  can be caused to flow into the suction passage  3 . 
     In the present disclosure, instead of the illustrated example, the inlet portion  20   a  of the liquid bypass passage  20  may be connected to the fourth refrigerant passage  14 . In this case, at the time of the hot-gas defrost operation, the liquid refrigerant in the fourth refrigerant passage  14  can be caused to flow into the suction passage  3  through the liquid bypass passage  20 . Directly connecting a pipe to a vessel like the receiver  9  tends to be more costly than connecting the pipe to a pipe. When the inlet portion  20   a  of the liquid bypass passage  20  is connected to the fourth refrigerant passage  14 , or when the inlet portion  20   a  of the liquid bypass passage  20  is connected to the fifth refrigerant passage  15  as in the illustrated example, it is not necessary to directly connect the pipe forming the liquid bypass passage  20  to the receiver  9 , which is advantageous in cost reduction. 
     As a modified example, the refrigeration cycle device  1  may include an internal heat exchanger that exchanges heat between the refrigerant passing through the fourth refrigerant passage  14  and the refrigerant passing through the suction passage  3 , in place of the illustrated internal heat exchanger  19 . In other words, the internal heat exchanger may be outside the receiver  9 . By the modified example, a similar effect to that of the illustrated embodiment is also obtained. 
     At the time of the hot-gas defrost operation, the controlling circuitry  50  may control the operation or the opening degree of the liquid bypass valve  21  according to the refrigerant outlet temperature of the air heat exchanger  5 . The gas refrigerant is cooled in the air heat exchanger  5  at the time of the hot-gas defrost operation, and therefore, when the amount of the refrigerant in the hot-gas defrost circuit is large, there is a possibility that the refrigerant condenses on a downstream side of the air heat exchanger  5 . When the refrigerant condenses on the downstream side of the air heat exchanger  5 , there is a possibility that the internal heat exchanger  19  cannot sufficiently evaporate the liquid refrigerant in the suction passage  3 . In the light of this, at the time of the hot-gas defrost operation, the controlling circuitry  50  may adjust the amount of the refrigerant in the hot-gas defrost circuit by controlling the operation or the opening degree of the liquid bypass valve  21  so that the refrigerant flowing out from the air heat exchanger  5  is brought into a state of superheated gas. This can more reliably prevent the liquid refrigerant from being sucked by the compressor  2 . A difference between the refrigerant outlet temperature of the air heat exchanger  5  detected by the second temperature sensor  29  and the suction saturation temperature corresponds to the superheat degree of the refrigerant flowing out from the air heat exchanger  5 . The controlling circuitry  50  may control the operation or the opening degree of the liquid bypass valve  21  so that the superheat degree of the refrigerant flowing out from the air heat exchanger  5  becomes close to the target. For example, when the controlling circuitry  50  increases the opening degree of the liquid bypass valve  21 , the amount of the refrigerant in the hot-gas defrost circuit increases, so that the superheat degree of the refrigerant flowing out from the air heat exchanger  5  is reduced. When the controlling circuitry  50  reduces the opening degree of the liquid bypass valve  21  on the contrary to this, the superheat degree of the refrigerant flowing out from the air heat exchanger  5  increases. 
     At the time of the hot-gas defrost operation, the controlling circuitry  50  may control the operation or the opening degree of the liquid bypass valve  21  according to a temperature difference between the refrigerant inlet temperature of the air heat exchanger  5  detected by the first temperature sensor  28 , and the refrigerant outlet temperature of the air heat exchanger  5  detected by the second temperature sensor  29 . When the controlling circuitry  50  increases the opening degree of the liquid bypass valve  21 , the temperature difference increases, and when the controlling circuitry  50  reduces the opening degree of the liquid bypass valve  21 , the temperature difference reduces. 
     At the time of the hot-gas defrost operation, the controlling circuitry  50  may temporarily increase the opening degree of the first expansion valve  11  according to the temperature of the liquid refrigerant flowing out from the utilization heat exchanger  6 . When the high-pressure gas refrigerant is cooled by the heat medium flowing in the utilization heat exchanger  6 , the temperature of the refrigerant is lowered to a lower temperature than the discharge saturation temperature. The lower the refrigerant temperature, the more refrigerant is condensed in the utilization heat exchanger  6 . In the light of this, the controlling circuitry  50  preferably moves the liquid refrigerant staying in the utilization heat exchanger  6  to the receiver  9  by temporarily increasing the opening degree of the first expansion valve  11  when the supercooling degree of the liquid refrigerant flowing out from the utilization heat exchanger  6  becomes larger than the reference. This can always store the liquid refrigerant in the receiver  9 , and therefore adjustment of the amount of the refrigerant in the hot-gas defrost circuit becomes easier. 
     Further, a temperature sensor that detects the temperature of the heat medium flowing into the utilization heat exchanger  6  or the temperature of the heat medium flowing out from the utilization heat exchanger  6  may be installed. At the time of the hot-gas defrost operation, the controlling circuitry  50  determines that the condensation amount of the refrigerant in the utilization heat exchanger  6  becomes large when a difference between the heat medium temperature detected by the temperature sensor, and the temperature of the liquid refrigerant flowing out from the utilization heat exchanger  6  becomes smaller than the reference and may temporarily increase the opening degree of the first expansion valve  11 . 
     Embodiment 2 
     Next, embodiment 2 is described with reference to  FIG.  7    to  FIG.  12   , a difference from embodiment 1 described above is mainly described, and common explanation is simplified or omitted. Further, elements that are common to or correspond to the elements described above are assigned with the same reference signs. 
       FIG.  7    is a diagram showing a refrigeration cycle device  32  according to embodiment 2. As shown in  FIG.  7   , the refrigeration cycle device  32  according to present embodiment 2 further includes a bypass heating heat exchanger  33  as compared with the refrigeration cycle device  1  according to embodiment 1. The bypass heating heat exchanger  33  heats liquid refrigerant passing through a liquid bypass passage  20  by a heat medium. The bypass heating heat exchanger  33  includes a heat medium passage  33   a  and a refrigerant passage  33   b . Heat is exchanged between the heat medium passing through the heat medium passage  33   a  and the refrigerant passing through the refrigerant passage  33   b . In the illustrated example, the heat medium passing through the utilization heat exchanger  6  flows into the heat medium passage  33   a  of the bypass heating heat exchanger  33 . As a modified example, the heat medium passing through the heat medium passage  33   a  of the bypass heating heat exchanger  33  may be configured to flow into the utilization heat exchanger  6 . 
     The liquid bypass passage  20  has a first passage  20   c  that connects an inlet portion  20   a  to an inlet of the refrigerant passage  33   b  of the bypass heating heat exchanger  33 , and a second passage  20   d  that connects an outlet of the refrigerant passage  33   b  of the bypass heating heat exchanger  33  to an outlet portion  20   b.    
     The refrigeration cycle device  32  includes a liquid bypass expansion valve  34 . The liquid bypass expansion valve  34  corresponds to the liquid bypass valve  21  in embodiment 1. The liquid bypass expansion valve  34  is configured by an expansion valve capable of adjusting a flow rate. The liquid bypass expansion valve  34  is positioned on the second passage  20   d . An operation and a function of the liquid bypass expansion valve  34  are the same as or similar to the operation and the function of the liquid bypass valve  21  in embodiment 1. The liquid bypass expansion valve  34  is configured to decompress the refrigerant from the bypass heating heat exchanger  33  at a time of a hot-gas defrost operation. 
     The refrigeration cycle device  32  further includes a liquid bypass solenoid valve  35 . The liquid bypass solenoid valve  35  is positioned on the first passage  20   c . The liquid bypass solenoid valve  35  is preferably a valve that can be switched only between being open and closed. The liquid bypass solenoid valve  35  is configured to decompress the liquid refrigerant passing through the first passage  20   c  at the time of the hot-gas defrost operation. 
     The refrigeration cycle device  32  further includes a liquid bypass temperature sensor  36 . The liquid bypass temperature sensor  36  is positioned in the second passage  20   d  between the liquid bypass expansion valve  34  and the outlet portion  20   b.    
       FIG.  8    is one example of a functional block diagram of the refrigeration cycle device  32  according to embodiment 2. As shown in  FIG.  8   , each of the liquid bypass expansion valve  34 , the liquid bypass solenoid valve  35 , and the liquid bypass temperature sensor  36  is electrically connected to a controlling circuitry  50 . 
       FIG.  9    is a diagram showing a flow of the refrigerant at a time of a heating operation of the refrigeration cycle device  32  according to embodiment 2. As shown in  FIG.  9   , at the time of the heating operation, the liquid bypass expansion valve  34  and the liquid bypass solenoid valve  35  are closed, and the refrigerant does not flow into the liquid bypass passage  20 . The flow of the refrigerant at the time of the heating operation is the same as that of embodiment 1. 
       FIG.  10    is a diagram showing a flow of the refrigerant at a time of a hot-gas defrost operation of the refrigeration cycle device  32  according to embodiment 2. As shown in  FIG.  10   , at the time of the hot-gas defrost operation, the refrigeration cycle device  32  operates as follows. The liquid bypass expansion valve  34  and the liquid bypass solenoid valve  35  are opened. A liquid refrigerant in a receiver  9  flows into the liquid bypass passage  20  from the inlet portion  20   a  after flowing into a fifth refrigerant passage  15  from a tip end opening  15   a . The liquid refrigerant passing through the liquid bypass passage  20  is heated by receiving heat of a heat medium of a heat medium circuit  100  when passing through the bypass heating heat exchanger  33 . At least a part of the liquid refrigerant may evaporate while passing through the bypass heating heat exchanger  33 . According to the present embodiment, at the time of the hot-gas defrost operation, the liquid refrigerant in the liquid bypass passage  20  can be caused to flow into a suction passage  3  after being heated by the bypass heating heat exchanger  33 . Therefore, together with heating by an internal heat exchanger  19 , the liquid refrigerant can be evaporated more reliably before being sucked by the compressor  2 . As a result, the liquid refrigerant can be prevented from being sucked by the compressor  2  more reliably without providing an accumulator on the suction passage  3 . 
     The refrigerant from the inlet portion  20   a  of the liquid bypass passage  20  flows into the bypass heating heat exchanger  33  after being decompressed to a medium pressure by the liquid bypass solenoid valve  35 . The refrigerant passing through the bypass heating heat exchanger  33  flows into the suction passage  3  after being further decompressed to a low pressure by the liquid bypass expansion valve  34 . 
     The liquid bypass expansion valve  34  can adjust a flow rate of the refrigerant by adjustment of an opening degree thereof. In general, an expansion valve may not be able to completely shut off a flow of refrigerant even if a valve body is set to a minimum opening degree. In the present embodiment, the liquid bypass solenoid valve  35  is further installed, so that when the liquid bypass solenoid valve  35  is closed, the flow of the refrigerant can reliably be shut off. Therefore, it is possible to more reliably prevent the liquid refrigerant from bypassing from the liquid bypass passage  20  to the suction passage  3  at the time of the heating operation, for example. 
     According to the present embodiment, the liquid bypass solenoid valve  35  is installed between the inlet portion  20   a  of the liquid bypass passage  20  and the bypass heating heat exchanger  33 , and therefore, closing the liquid bypass solenoid valve  35  ensures that the refrigerant does not build up in the bypass heating heat exchanger  33 . 
     The liquid bypass solenoid valve  35  preferably has a bore diameter thereof selected so that it can decompress the refrigerant. The liquid bypass solenoid valve  35  decompresses the refrigerant, and thereby can make a temperature of the refrigerant flowing into the bypass heating heat exchanger  33  lower than a temperature of the heat medium flowing in the heat medium circuit  100 . As a result, it is possible to evaporate the refrigerant more reliably in the bypass heating heat exchanger  33 . 
     A first expansion valve  11  has a bore diameter thereof selected so that it can control the flow of the refrigerant at the time of the heating operation. Accordingly, when the first expansion valve  11  is slightly opened at the time of the hot-gas defrost operation, there is a possibility that the first expansion valve  11  cannot decompress the refrigerant. If the first expansion valve  11  cannot decompress the refrigerant, and the liquid bypass solenoid valve  35  does not exist, there is a possibility that the refrigerant cannot evaporate in the bypass heating heat exchanger  33 . In contrast to this, according to the present embodiment, the liquid bypass solenoid valve  35  decompresses the refrigerant before entering the bypass heating heat exchanger  33 , and thereby the temperature of the refrigerant reliably becomes lower than the temperature of the heat medium flowing in the heat medium circuit  100 . Therefore, the refrigerant can be reliably evaporated in the bypass heating heat exchanger  33 . 
     According to the present embodiment, the liquid bypass expansion valve  34  installed downstream of the bypass heating heat exchanger  33  decompresses the refrigerant, and thereby the temperature of the refrigerant of the bypass heating heat exchanger  33  can be made higher than a suction saturation temperature. Therefore, it is possible to more reliably prevent the refrigerant temperature of the bypass heating heat exchanger  33  from becoming too low, and therefore, it is possible to more reliably prevent the heat medium like water from being frozen in the bypass heating heat exchanger  33 . 
     In this way, according to the present embodiment, the bypass heating heat exchanger  33  is installed between the liquid bypass solenoid valve  35  and the liquid bypass expansion valve  34 , in the refrigerant flow in the liquid bypass passage  20 , and thereby the bypassing refrigerant can be evaporated at a more appropriate temperature. 
     Note that the liquid bypass solenoid valve  35  can be omitted. For example, if the first expansion valve  11  can decompress the refrigerant at the time of the hot-gas defrost operation, the liquid bypass solenoid valve  35  may not be provided. 
     The bypass heating heat exchanger  33  heats the refrigerant by a heat medium on a utilization side flowing in the heat medium circuit  100 . The heat medium on the utilization side is heated at the time of the heating operation directly before the hot-gas defrost operation, and therefore, has a high temperature to some extent. Therefore, the heat medium on the utilization side has a sufficient amount of heat required to evaporate the refrigerant in the bypass heating heat exchanger  33 . 
       FIG.  11    is a flowchart showing an example of a process at a time of executing the hot-gas defrost operation according to embodiment 2.  FIG.  12    is a timing chart showing an operation example of each of actuators from the heating operation according to embodiment 2 until the operation transitions to the hot-gas defrost operation and returns to the heating operation. Concerning an example shown in  FIG.  11    and  FIG.  12   , a difference from the example shown in  FIG.  5    and  FIG.  6    of embodiment 1 is described hereinafter. 
     Step S 201  to step S 203  in  FIG.  11    are the same as step S 101  to step S 103  in  FIG.  5   . After opening a hot-gas bypass valve  18  as step S 203 , the controlling circuitry  50  opens the liquid bypass solenoid valve  35  as step S 204 . Next, the controlling circuitry  50  controls the operation so that the opening degree of the liquid bypass expansion valve  34  becomes slightly opening (P 3 - 1 ), as step S 205 . Step S 206  to step S 208  in  FIG.  11    are similar to step S 105  to step S 107  in  FIG.  5   . A process from step S 201  to step S 207  corresponds to a defrost preparation process in  FIG.  12   . 
     Next, the controlling circuitry  50  controls the operation so that the opening degree of the liquid bypass expansion valve  34  becomes equal to a target opening degree (P 3 - 2 ), as step S 209 . At this time, the controlling circuitry  50  may adjust a value of the target opening degree (P 3 - 2 ) so that the suction superheat degree or discharge superheat degree becomes close to a target. By a process of step S 208  and step S 209 , the hot-gas defrost operation starts. 
     Step S 210  to step S 214  in  FIG.  11    are similar to step S 109  to step S 113  in  FIG.  5   . Next, the controlling circuitry  50  closes the liquid bypass solenoid valve  35  as step S 215 . Next, the controlling circuitry  50  fully closes the liquid bypass expansion valve  34  as step S 216 . A process from step S 211  to step S 216  corresponds to a return process in  FIG.  12   . 
     REFERENCE SIGNS LIST 
     
         
           1  refrigeration cycle device 
           2  compressor 
           3  suction passage 
           4  discharge passage 
           4   a  branch portion 
           5  air heat exchanger 
           6  utilization heat exchanger 
           7  first refrigerant passage 
           8  second refrigerant passage 
           9  receiver 
           11  first expansion valve 
           12  second expansion valve 
           13  third refrigerant passage 
           14  fourth refrigerant passage 
           14   a  tip end opening 
           15  fifth refrigerant passage 
           15   a  tip end opening 
           16  sixth refrigerant passage 
           16   a  branch portion 
           17  hot-gas bypass passage 
           18  hot-gas bypass valve 
           19  internal heat exchanger 
           20  liquid bypass passage 
           20   a  inlet portion 
           20   b  outlet portion 
           20   c  first passage 
           20   d  second passage 
           21  liquid bypass valve 
           22  refrigerant circuit switching valve 
           23  blower 
           24  discharge pressure sensor 
           25  discharge temperature sensor 
           26  suction pressure sensor 
           27  suction temperature sensor 
           28  first temperature sensor 
           29  second temperature sensor 
           30  third temperature sensor 
           31  outside air temperature sensor 
           32  refrigeration cycle device 
           33  bypass heating heat exchanger 
           34  liquid bypass expansion valve 
           35  liquid bypass solenoid valve 
           36  liquid bypass temperature sensor 
           50  controlling circuitry 
           51  processor 
           52  memory 
           90  liquid level 
           100  heat medium circuit 
           101  heat medium pump