Refrigeration cycle device

Abstract

This refrigeration cycle device comprises an outdoor unit that includes a compressor constituting a refrigerant circuit, and a control device that executes refrigerant recovery operation in which the compressor is operated to recover, into the outdoor unit, a refrigerant circulating through the refrigerant circuit, wherein, in the refrigerant recovery operation, the control device operates the compressor at a first frequency as first stage control, operates the compressor at a second frequency lower than the first frequency as second stage control following the first stage control, and stops the compressor when low-pressure pressure of the refrigerant circuit falls below a stop threshold.

Description

Refrigeration cycle equipment

[0001] The present disclosure relates to a refrigeration cycle device that performs a refrigerant recovery operation.

[0002] A refrigeration cycle apparatus is known in the art that operates a compressor to perform a refrigerant recovery operation in which refrigerant circulating in a refrigerant circuit is recovered to the outdoor unit side (for example, Patent Document 1). Patent Document 1 discloses that, during the refrigerant recovery operation, the compressor is operated at a maximum rotation speed, and when a low-pressure detected by a pressure sensor provided on the suction side of the compressor falls below a threshold, the compressor is stopped and the refrigerant recovery operation is terminated.

[0003] International Publication No. 2017 / 056789

[0004] It is generally known that during refrigerant recovery operation, the low-pressure pressure drops before the refrigerant is recovered. Therefore, even if the low-pressure pressure is detected to be below a threshold and the refrigerant recovery operation is terminated based on the detection result, the refrigerant may not be sufficiently recovered. In particular, when the compressor frequency during the refrigerant recovery operation is set to operate at the maximum rotation speed, as in the refrigeration cycle device of Patent Document 1, the low-pressure pressure drops quickly, resulting in a large amount of unrecovered refrigerant.

[0005] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a refrigeration cycle device that reduces the amount of unrecovered refrigerant while preventing the refrigerant recovery operation time from becoming excessively long during refrigerant recovery operation.

[0006] The refrigeration cycle device according to the present disclosure includes an outdoor unit having a compressor that constitutes a refrigerant circuit, and a control device that performs a refrigerant recovery operation in which the compressor is operated to recover the refrigerant circulating through the refrigerant circuit into the outdoor unit. In the refrigerant recovery operation, the control device operates the compressor at a first frequency as a first stage of control, and operates the compressor at a second frequency lower than the first frequency as a second stage of control subsequent to the first stage of control, and stops the compressor when the low pressure of the refrigerant circuit falls below a stop threshold.

[0007] In the refrigerant recovery operation of the present disclosure, the compressor is operated at a first frequency as a first-stage control, and then, in a second-stage control following the first-stage control, the compressor is operated at a second frequency lower than the first frequency. Lowering the compressor frequency through the second-stage control slows the decrease in low-pressure, allowing the refrigerant recovery operation to continue. Furthermore, because the first-stage control is performed before the second-stage control, the time required for the refrigerant recovery operation is shortened compared to when the compressor is operated at the second frequency throughout the entire refrigerant recovery operation. In this way, the refrigeration cycle apparatus of the present disclosure can reduce the amount of unrecovered refrigerant during the refrigerant recovery operation while preventing the refrigerant recovery operation from becoming excessively long.

[0008] 1 is a circuit diagram showing a refrigeration cycle apparatus according to embodiment 1. FIG. 2 is a hardware configuration diagram showing an example configuration of a control device according to embodiment 1. FIG. 3 is a functional block diagram of a refrigeration cycle apparatus according to embodiment 1. FIG. 4 is a diagram for explaining control of equipment in a refrigerant recovery operation according to embodiment 1. FIG. 5 is a flowchart showing the operation of the control device according to embodiment 1. FIG. 6 is a diagram for explaining control of equipment in a refrigerant recovery operation according to embodiment 2. FIG. 7 is a flowchart showing the operation of the control device according to embodiment 2. FIG. 8 is a diagram for explaining control of equipment in a refrigerant recovery operation according to a modified example of embodiment 2.

[0009] Embodiment 1. A refrigeration cycle apparatus 1 according to embodiment 1 will be described below with reference to the drawings. The refrigeration cycle apparatus 1 is, for example, an air conditioning apparatus that adjusts the temperature of a space to be air-conditioned, such as a room. As will be described in detail later, the refrigeration cycle apparatus 1 can perform normal operation such as cooling operation and heating operation, and refrigerant recovery operation. First, the configuration of each device included in the refrigeration cycle apparatus 1 will be described.

[0010] FIG. 1 is a circuit diagram showing a refrigeration cycle apparatus 1 according to a first embodiment. As shown in FIG. 1 , the refrigeration cycle apparatus 1 includes an outdoor unit 2 that is provided outside a space to be air-conditioned and supplies heat or cold to an indoor unit 3, and an indoor unit 3 that is provided in the space to be air-conditioned and adjusts the temperature of the space to be air-conditioned. As shown in FIG. 1 , the outdoor unit 2 includes a compressor 21, a flow switching device 22, an outdoor heat exchanger 23, and an outdoor expansion valve 24. The indoor unit 3 includes an indoor expansion valve 31 and an indoor heat exchanger 32. The compressor 21, the flow switching device 22, the outdoor heat exchanger 23, the outdoor expansion valve 24, the indoor expansion valve 31, and the indoor heat exchanger 32 are connected by piping to form a refrigerant circuit 10 through which a refrigerant circulates.

[0011] The compressor 21 draws in a refrigerant in a low-temperature and low-pressure state, compresses the drawn refrigerant, and discharges it in a high-temperature and high-pressure state. The compressor 21 has an inverter (not shown) that changes the frequency F based on instructions from a control device 70 (described later), and can change the amount of refrigerant discharged per unit time.

[0012] The flow switching device 22, which is, for example, a four-way valve, switches the flow direction of the refrigerant in the refrigerant circuit 10. The outdoor heat exchanger 23, which is, for example, a fin-and-tube heat exchanger, exchanges heat between the refrigerant and the outdoor air. The outdoor heat exchanger 23 acts as a condenser during cooling operation and as an evaporator during heating operation. The outdoor expansion valve 24, which is, for example, an electronic expansion valve, reduces the pressure of the refrigerant to expand it.

[0013] The indoor expansion valve 31, which is, for example, an electronic expansion valve, reduces the pressure of the refrigerant to expand it. The indoor heat exchanger 32, which is, for example, a fin-and-tube heat exchanger, exchanges heat between the refrigerant and the indoor air. The indoor heat exchanger 32 functions as an evaporator during cooling operation and as a condenser during heating operation.

[0014] The refrigeration cycle apparatus 1 has a liquid shutoff valve 51 and a gas shutoff valve 52. The liquid shutoff valve 51 and the gas shutoff valve 52 are solenoid valves or motor-operated valves, and are provided, for example, in the outdoor unit 2. The liquid shutoff valve 51 and the gas shutoff valve 52 are valves that collect the refrigerant inside the outdoor unit 2 during a refrigerant recovery operation, which will be described later, and prevent the refrigerant from returning to the indoor unit 3 side.

[0015] The liquid shutoff valve 51 is provided between the outdoor expansion valve 24 and the indoor expansion valve 31. The open / closed state of the liquid shutoff valve 51 is controlled by the control device 70. The liquid shutoff valve 51 is controlled to an open state during normal operation. The liquid shutoff valve 51 is controlled to a closed state during refrigerant recovery operation. The open state of the liquid shutoff valve 51 means a state in which refrigerant is allowed to flow from the outdoor unit 2 to the indoor unit 3 during cooling operation, and a state in which refrigerant is allowed to flow from the indoor unit 3 to the outdoor unit 2 during heating operation. The closed state of the liquid shutoff valve 51 means a state in which refrigerant flow from the outdoor unit 2 to the indoor unit 3 is blocked.

[0016] The gas shutoff valve 52 is provided between the indoor heat exchanger 32 and the flow path switching device 22. The open / closed state of the gas shutoff valve 52 is controlled by the control device 70. The gas shutoff valve 52 is controlled to an open state during normal operation. The gas shutoff valve 52 is also controlled to an open state during refrigerant recovery operation and is controlled to a closed state when the refrigerant recovery operation ends. The open state of the gas shutoff valve 52 means a state in which the refrigerant is allowed to flow from the indoor unit 3 to the outdoor unit 2 during cooling operation and a state in which the refrigerant is allowed to flow from the outdoor unit 2 to the indoor unit 3 during heating operation. In addition, during refrigerant recovery operation, the open state of the gas shutoff valve 52 means a state in which the refrigerant is allowed to flow from the indoor unit 3 to the outdoor unit 2. The closed state of the gas shutoff valve 52 means a state in which the refrigerant is blocked from flowing from the outdoor unit 2 to the indoor unit 3.

[0017] The refrigeration cycle apparatus 1 has a suction pressure sensor 61 and a control device 70. The suction pressure sensor 61 is a pressure sensor provided in a pipe on the suction side of the compressor 21. The suction pressure sensor 61 detects the suction pressure of the refrigerant sucked into the compressor 21 and periodically transmits the detection result to the control device 70. The suction pressure is an example of a pressure on the low-pressure side of the refrigerant circuit 10 (hereinafter, sometimes simply referred to as low-pressure pressure PL).

[0018] The control device 70 controls each device of the refrigeration cycle apparatus 1. As will be described in detail later, the control device 70 of the first embodiment is characterized by the control of the frequency F when operating the compressor 21 in the refrigerant recovery operation. An example of a hardware configuration of the control device 70 will be described. FIGS. 2 and 3 are hardware configuration diagrams showing an example of the configuration of the control device 70 according to the first embodiment. When the various functions of the control device 70 are performed by hardware, the control device 70 is configured by a processing circuit 81 as shown in FIG. 2. The processing circuit 81 corresponds to, for example, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a combination thereof.

[0019] Furthermore, when the various functions of the control device 70 are implemented by software, the control device 70 is configured by a processor 82 such as a CPU and a memory 83, as shown in FIG. 3. FIG. 3 shows that the processor 82 and the memory 83 are communicably connected to each other via a bus 84. When each function is implemented by software, the functions of the calculation means 15 and the determination means 16 are realized by software, firmware, or a combination of software and firmware. The software and firmware are written as programs and stored in the memory 83. The processor 82 realizes the functions of each means by reading and executing the programs stored in the memory 83. The memory 83 is, for example, a volatile or non-volatile semiconductor memory.

[0020] (Cooling Operation) Returning to FIG. 1 , the operation of the refrigeration cycle apparatus 1 during normal operation will now be described. First, the cooling operation will be described. In FIG. 1 , the flow of refrigerant during cooling operation is indicated by solid arrows. The refrigeration cycle apparatus 1 performs cooling operation by switching the flow path switching device 22 so that the discharge side of the compressor 21 is connected to the outdoor heat exchanger 23. During cooling operation, the refrigerant drawn into the compressor 21 is compressed by the compressor 21 and discharged in a high-temperature, high-pressure gas state. The high-temperature, high-pressure gas refrigerant discharged from the compressor 21 passes through the flow path switching device 22 and flows into the outdoor heat exchanger 23, which functions as a condenser. The refrigerant that flows into the outdoor heat exchanger 23 exchanges heat with outdoor air, condenses, and liquefies. The liquid refrigerant passes through the outdoor expansion valve 24 and the indoor expansion valve 31, where it is decompressed and expanded to become a low-temperature, low-pressure, two-phase gas-liquid refrigerant. The refrigerant in a gas-liquid two-phase state flows into the indoor heat exchanger 32, which functions as an evaporator. The refrigerant that flows into the indoor heat exchanger 32 exchanges heat with the indoor air and evaporates, becoming gaseous. At this time, the indoor air is cooled, thereby cooling the room. The evaporated refrigerant, now in a low-temperature, low-pressure gas state, then passes through the flow switching device 22 and is drawn into the compressor 21.

[0021] (Heating Operation) Next, the heating operation will be described. In FIG. 1 , the refrigerant flow during heating operation is indicated by dashed arrows. The refrigeration cycle apparatus 1 performs heating operation by switching the flow path switching device 22 so that the discharge side of the compressor 21 is connected to the indoor heat exchanger 32. During heating operation, the refrigerant drawn into the compressor 21 is compressed by the compressor 21 and discharged in a high-temperature, high-pressure gas state. The high-temperature, high-pressure gas refrigerant discharged from the compressor 21 passes through the flow path switching device 22 and flows into the indoor heat exchanger 32, which functions as a condenser. The refrigerant that flows into the indoor heat exchanger 32 exchanges heat with the indoor air, condenses, and liquefies. This heats the indoor air, heating the room. The liquid refrigerant passes through the indoor expansion valve 24, where it is decompressed and expanded to become a low-temperature, low-pressure, two-phase gas-liquid refrigerant. The two-phase gas-liquid refrigerant flows into the outdoor heat exchanger 23, which functions as an evaporator. The refrigerant that has flowed into the outdoor heat exchanger 23 exchanges heat with the outdoor air, evaporates, and gasifies. The evaporated refrigerant then passes through the flow switching device 22 and is drawn into the compressor 21.

[0022] Next, the control details of the control device 70 will be described using FIG. 4 . FIG. 4 is a functional block diagram of the refrigeration cycle apparatus 1 according to the first embodiment. As shown in FIG. 4 , the control device 70 controls the compressor 21, the flow path switching device 22, the outdoor expansion valve 24, the indoor expansion valve 31, the liquid shutoff valve 51, and the gas shutoff valve 52. Specifically, the control device 70 switches the connection direction of the flow path switching device 22 so that cooling operation or heating operation is performed depending on the operation mode selected by the user. The control device 70 also detects the indoor temperature using a temperature sensor (not shown) provided in the indoor unit 3 and controls the frequency F of the compressor 21 and the apertures of the outdoor expansion valve 24 and the indoor expansion valve 31 so that the indoor temperature becomes the set temperature set by the user. Note that the control device 70 may also be configured to control the frequency F of the compressor 21 and the apertures of the outdoor expansion valve 24 and the indoor expansion valve 31 based on the detection result of a pressure sensor (not shown) provided in the piping on the discharge side of the compressor 21 so that the detection result falls within a predetermined control range. The pressure sensor detects the discharge pressure of the refrigerant discharged from the compressor 21 and periodically transmits the detection result to the control device 70. Then, the control device 70 controls the frequency F of the compressor 21 and the open / closed states of the liquid shutoff valve 51 and the gas shutoff valve 52 during the refrigerant recovery operation.

[0023] The refrigerant recovery operation and the control of each device during the refrigerant recovery operation will be described in detail. First, the refrigerant recovery operation is an operation for recovering the refrigerant circulating through the refrigerant circuit 10 into the outdoor unit 2. The refrigerant recovery operation is performed, for example, when a refrigerant leak from the refrigerant circuit 10 is detected. The refrigerant leak is detected by a dedicated gas sensor provided inside the indoor unit 3. The dedicated gas sensor may be installed indoors rather than inside the indoor unit 3, as long as it is electrically connected to the indoor unit 3. However, the refrigerant recovery operation may also be performed when a refrigerant leak is not detected, for example, when the refrigeration cycle apparatus 1 is relocated.

[0024] Next, control of each device during refrigerant recovery operation will be described using Figure 5. Figure 5 is a diagram for explaining control of devices during refrigerant recovery operation according to the first embodiment. The control device 70 controls the connection orientation of the flow path switching device 22 to the same connection orientation as during cooling operation, regardless of the connection orientation during normal operation. The control device 70 also changes the liquid cutoff valve 51 to a closed state and keeps the gas cutoff valve 52 open. Then, to end the refrigerant recovery operation, the control device 70 changes the gas cutoff valve 52 to a closed state while keeping the liquid cutoff valve 51 closed.

[0025] The control device 70 controls the frequency F of the compressor 21 in two stages: a first stage and a second stage. In the first stage, the control device 70 sets the frequency F of the compressor 21 to a first frequency F1 at the start of the refrigerant recovery operation. The first frequency F1 is, for example, a frequency at which the compressor 21 operates at the maximum rotation speed (hereinafter referred to as the maximum frequency). The control device 70 also periodically receives the detection results of the suction pressure sensor 61 during the refrigerant recovery operation. In the second stage, the control device 70 changes the frequency F of the compressor 21 to a second frequency F2 when the detected low pressure PL falls below a deceleration threshold Pdw. The deceleration threshold Pdw is a value higher than a stop threshold Pst (described later), for example, 1.0 MPa. The second frequency F2 is a value lower than the first frequency F1, for example, 30 Hz to 40 Hz. When the detected low pressure PL becomes less than the stop threshold Pst, the control device 70 determines to terminate the refrigerant recovery operation and stops the compressor 21. The stop threshold Pst is, for example, 0.5 MPa. The frequency F of the compressor 21 during normal operation is, for example, 65 Hz.

[0026] By operating the compressor 21 with the flow switching device 22, the liquid shutoff valve 51, and the gas shutoff valve 52 controlled as described above, refrigerant is recovered from the low-pressure side of the refrigerant circuit 10 to the high-pressure side of the refrigerant circuit 10. At this time, gas refrigerant that has passed through the indoor heat exchanger 32, which functions as an evaporator, flows from the indoor unit 3 to the outdoor unit 2. Furthermore, liquid refrigerant that has passed through the outdoor heat exchanger 23, which functions as a condenser, does not flow from the outdoor unit 2 to the indoor unit 3 but remains in the outdoor unit 2. Note that the low-pressure side of the refrigerant circuit 10 is the section from the gas shutoff valve 52 to the suction side of the compressor 21, and the high-pressure side of the refrigerant circuit 10 is the section from the discharge side of the compressor 21 to the gas shutoff valve 52. Furthermore, by controlling the gas shutoff valve 52 to a closed state at the end of the refrigerant recovery operation, backflow of the refrigerant recovered in the outdoor unit 2 to the indoor unit 3 is suppressed.

[0027] Furthermore, by reducing the frequency F of the compressor 21 to the second frequency F2, the rate at which the low-pressure pressure PL decreases is slowed down, which prevents the low-pressure pressure PL from rapidly becoming less than the stop threshold value Pst, and allows the refrigerant recovery operation to continue.

[0028] Next, the control flow of the refrigerant recovery operation will be described using FIG. 6 . FIG. 6 is a flowchart illustrating the operation of the control device 70 according to the first embodiment. As an example, when a refrigerant leak is detected, the refrigerant recovery operation is initiated. First, the control device 70 determines whether the operation mode in the immediately preceding normal operation is cooling operation (step S1). If the operation mode is heating (step S1: NO), the control device 70 changes the connection orientation of the flow path switching device 22 to the same connection orientation as in cooling operation (step S2). If the operation mode is cooling operation (step S1: YES) or if the operation mode is switched to cooling operation, the control device 70 closes the liquid shutoff valve 51 (step S3). Then, the control device 70 determines whether the compressor 21 is operating (step S4). If the compressor 21 is stopped (step S4: NO), the control device 70 starts the compressor 21 (step S5). Note that the order of steps S1 and S2, step S3, and steps S4 and S5 may be reversed. For example, step S3 may be performed before steps S1 and S2, or steps S4 and S5 may be performed before steps S1 and S2.

[0029] If the compressor 21 is operating (step S4: YES) or if the compressor 21 has been started, the control device 70 sets the frequency F of the compressor 21 to a first frequency F1 (step S6). Once the frequency F of the compressor 21 is set to the first frequency F1, the control device 70 determines whether the low-pressure pressure PL is less than the deceleration threshold Pdw (step S7). If the low-pressure pressure PL is equal to or greater than the deceleration threshold Pdw (step S7: NO), the control device 70 maintains the frequency F of the compressor 21 at the first frequency F1 until the low-pressure pressure PL becomes less than the deceleration threshold Pdw. If the low-pressure pressure PL is less than the deceleration threshold Pdw (step S7: YES), the control device 70 sets the frequency F of the compressor 21 to a second frequency F2 (step S8).

[0030] When the frequency F of the compressor 21 is set to the second frequency F2, the control device 70 determines whether the low-pressure pressure PL is less than the stop threshold Pst (step S9). If the low-pressure pressure PL is equal to or greater than the stop threshold Pst (step S9: NO), the control device 70 maintains the frequency F of the compressor 21 at the second frequency F2 until the low-pressure pressure PL becomes less than the stop threshold Pst. If the low-pressure pressure PL is less than the stop threshold Pst (step S7: YES), the control device 70 stops the compressor 21 (step S10). Then, the control device 70 closes the gas shutoff valve 52 (step S11) to terminate the refrigerant recovery operation.

[0031] In the refrigerant recovery operation of the first embodiment, the compressor 21 is operated at a first frequency F1 as a first-stage control, and then, in a second-stage control following the first-stage control, the compressor 21 is operated at a second frequency F2 lower than the first frequency F1. Lowering the frequency F of the compressor 21 through the second-stage control prevents the low-pressure pressure PL from rapidly falling below the stop threshold Pst, allowing the refrigerant recovery operation to continue. Furthermore, because the first-stage control is performed before the second-stage control, the time required for the refrigerant recovery operation is shortened compared to when the compressor 21 is operated at the second frequency F2 throughout the entire refrigerant recovery operation. In this way, the refrigeration cycle apparatus 1 of the first embodiment can reduce the amount of unrecovered refrigerant during the refrigerant recovery operation while preventing the refrigerant recovery operation from being excessively long.

[0032] Embodiment 2. Embodiment 2 differs from Embodiment 1 in that the control of the frequency F of the compressor 21 during the refrigerant recovery operation has three or more stages. In Embodiment 2, the same parts as those in Embodiment 1 are denoted by the same reference numerals and description thereof will be omitted, and the description will focus on the differences from Embodiment 1.

[0033] Control of each device during the refrigerant recovery operation will be described using FIG. 7 . FIG. 7 is a diagram illustrating control of devices during the refrigerant recovery operation according to the second embodiment. The control device 70 controls the frequency F of the compressor 21 in a first to Nth stage (N≧3) until the low-pressure pressure PL reaches the stop threshold Pst. While FIG. 7 illustrates a case in which the control device 70 controls the frequency F of the compressor 21 in six stages, control in three to five stages, or seven or more stages may also be performed. The number of stages to be controlled, i.e., the value of N, is stored in advance in the control device 70. However, when the low-pressure pressure PL detected after the second stage of control falls below the stop threshold Pst, the refrigerant recovery operation is terminated, and control in subsequent stages is not performed.

[0034] As the first stage of control, the control device 70 sets the frequency F of the compressor 21 to a first frequency F1 at the start of the refrigerant recovery operation. The first frequency F1 is, for example, the maximum frequency.

[0035] When the low pressure PL detected during the first stage control becomes less than the first deceleration threshold Pdw1, the control device 70 transitions to the second stage control. The first deceleration threshold Pdw1 is a value higher than the stop threshold Pst. The first deceleration threshold Pdw1 may be a value higher than the deceleration threshold Pdw described in the first embodiment. In other words, in the second embodiment, the transition to the second stage control may be made earlier than in the first embodiment.

[0036] In the following description, the stage at the time of control is represented by a counter n (n≦N). First, the second-stage control will be described, i.e., the case where n is 2. In the second-stage control, the control device 70 changes the frequency F of the compressor 21 to a second frequency F2. The second frequency F2 is set based on the second deceleration threshold Pdw2. Specifically, the second frequency F2 is determined by a function using the second deceleration threshold Pdw2 as an argument. The second deceleration threshold Pdw2 is a value higher than the stop threshold Pst and lower than the low pressure PL last detected during the first-stage control by a predetermined decrease value (e.g., 0.1 MPa). The function determines a lower value as the argument value decreases. The second frequency F2 is a value lower than the first frequency F1, which is the maximum frequency. The second frequency F2 may be different from the second frequency F2 described in the first embodiment.

[0037] When the low pressure PL detected during the second stage control becomes less than the second deceleration threshold Pdw2, the control device 70 transitions to the third stage control.

[0038] The same applies when n is 3 or greater. In the nth stage, the control device 70 changes the frequency F of the compressor 21 to the nth frequency Fn. The nth frequency Fn is set based on the nth deceleration threshold Pdwn. Specifically, the nth frequency Fn is determined by a function that uses the nth deceleration threshold Pdwn as an argument. The nth deceleration threshold Pdwn is a value higher than the stop threshold Pst and lower than the low pressure PL last detected during the (n-1)th stage control by a predetermined reduction value (e.g., 0.1 MPa). In other words, the control at each stage aims to reduce the low pressure PL by the reduction value. As the low pressure PL decreases, the counter n increases, i.e., the control stage progresses, and the nth deceleration threshold Pdwn decreases as the control stage progresses. As described above, the function determines a lower value as the argument value decreases. Therefore, as the nth deceleration threshold Pdwn decreases, the nth frequency Fn becomes smaller than the (n-1)th frequency F(n-1).

[0039] When the low pressure PL detected during the nth stage control becomes less than the nth deceleration threshold Pdwn, the control device 70 transitions to the (n+1)th stage control. As described above, the nth frequency Fn is smaller than the (n-1)th frequency F(n-1). In other words, the frequency of a later stage is smaller than the frequency of a previous stage. Therefore, in the second embodiment, as the control stage progresses, the time it takes to reach the deceleration threshold increases at each stage, and therefore the control of a later stage is performed for a longer period of time than the control of a previous stage. Therefore, as a whole, the refrigerant recovery operation is performed for a longer period of time as the control of a later stage is performed.

[0040] When the low pressure PL detected after the second stage control becomes less than the stop threshold Pst, the control device 70 determines to terminate the refrigerant recovery operation and stops the compressor 21. The stop threshold Pst is the same value as in the first embodiment, for example, 0.5 MPa. The low pressure PL gradually decreases as the frequency F of the compressor 21 decreases stepwise.

[0041] By controlling the frequency F of the compressor 21 in multiple stages until the low-pressure pressure PL reaches the stop threshold Pst, the rate at which the low-pressure pressure PL decreases is slower than in Embodiment 1. Therefore, the low-pressure pressure PL is more reliably prevented from rapidly falling below the stop threshold Pst than in Embodiment 1, and the refrigerant recovery operation can be continued.

[0042] Next, the control flow of the refrigerant recovery operation will be described using FIG. 8 . FIG. 8 is a flowchart showing the operation of the control device 70 according to the second embodiment. Steps S21 to S25 are similar to steps S1 to S5 in the first embodiment, and therefore will not be described here. The control device 70 sets the frequency F of the compressor 21 to a first frequency F1 and sets the counter n to 1 (step S26). Next, the control device 70 determines whether the low-pressure pressure PL is less than the first deceleration threshold Pdw1 (step S27). If the low-pressure pressure PL is equal to or greater than the first deceleration threshold Pdw1 (step S27: NO), the control device 70 waits until the low-pressure pressure PL becomes less than the first deceleration threshold Pdw1.

[0043] If the low-pressure pressure PL becomes less than the first deceleration threshold Pdw1 (step S27: YES), the control device 70 increments the counter n by 1 and sets the frequency F of the compressor 21 to the nth frequency Fn (step S28). The nth frequency Fn here is a value based on the nth deceleration threshold Pdwn. Next, the control device 70 determines whether the low-pressure pressure PL is less than the stop threshold Pst (step S29). If the low-pressure pressure PL is equal to or greater than the stop threshold Pst (step S29: NO), the control device 70 determines whether the low-pressure pressure PL is less than the nth deceleration threshold Pdwn (step S30). If the low-pressure pressure PL is equal to or greater than the nth deceleration threshold Pdwn (step S30: NO), the control device 70 continues the nth stage of control until either the condition of step S29 or S30 is met.

[0044] If the low-pressure pressure PL is less than the n-th deceleration threshold Pdwn, the control device 70 transitions to the (n+1)-th stage control subsequent to the current n-th stage control (step S28). If the low-pressure pressure PL is less than the stop threshold Pst (step S29: YES), the control device 70 stops the compressor 21 (step S31). Then, the control device 70 closes the gas shutoff valve 52 (step S32) and ends the refrigerant recovery operation.

[0045] As described above, in the refrigerant recovery operation of the second embodiment, the frequency F of the compressor 21 is controlled in multiple stages until the low-pressure pressure PL reaches the stop threshold Pst. As a result, the rate at which the low-pressure pressure PL decreases is even slower than in the first embodiment. Therefore, the refrigeration cycle apparatus 1 of the second embodiment more reliably prevents the low-pressure pressure PL from rapidly becoming less than the stop threshold Pst than in the first embodiment, and by continuing the refrigerant recovery operation, the amount of refrigerant that remains unrecovered during the refrigerant recovery operation can be further reduced.

[0046] Furthermore, in the refrigerant recovery operation of the second embodiment, the later the control stage, the longer the time period for which control is performed. Therefore, the compressor 21 is operated at the low frequency F for a longer period of time compared to when control for all stages is performed for the same period of time. This further slows down the rate at which the low-pressure pressure PL decreases. Therefore, the refrigeration cycle apparatus 1 of the second embodiment can further reduce the amount of unrecovered refrigerant during the refrigerant recovery operation.

[0047] The above is a description of the embodiments of the present disclosure. However, the present disclosure is not limited to the configurations of the above embodiments and various modifications and combinations are possible within the scope of the technical concept thereof. For example, in the first and second embodiments, the maximum frequency is set as the first frequency F1. However, the first frequency F1 is not limited to this case as long as it is higher than the second frequency F2. The first frequency F1 may be determined based on the low pressure PL during normal operation immediately before the refrigerant recovery operation is performed. Specifically, when the low pressure PL immediately before the refrigerant recovery operation is performed is less than the threshold pressure, the control device 70 sets the first frequency F1 to a lower value than when the low pressure PL immediately before the refrigerant recovery operation is performed is equal to or greater than the threshold pressure. This can slow the rate of decrease of the low pressure PL even when the low pressure PL is already low at the start of the refrigerant recovery operation, thereby further reducing the amount of refrigerant not recovered during the refrigerant recovery operation.

[0048] Furthermore, in the first and second embodiments, the low-pressure pressure PL is detected using the suction pressure sensor 61, but this is not limiting. For example, the low-pressure pressure PL may be obtained by converting the surface temperature of the piping on the suction side of the compressor 21, i.e., the temperature of the gas refrigerant drawn into the compressor 21. Alternatively, the evaporator pressure (indoor heat exchanger pressure) obtained by converting the surface temperature of the evaporator heat transfer tube may be used as the low-pressure pressure PL.

[0049] In addition, in the control of each stage, the frequency F of the compressor 21 may be reduced at a constant rate or by a constant value.

[0050] FIG. 9 is a diagram illustrating the control of equipment during a refrigerant recovery operation according to a modification of the second embodiment. As shown in FIG. 9 , the function for determining the frequency F of the compressor 21 described in the second embodiment may determine the frequency F such that the decrease in frequency F from the previous control stage is smaller in later control stages. That is, the frequency F of the compressor 21 at each stage is determined so that the difference between the (n+1)th frequency F(n+1) and the nth frequency Fn is smaller than the difference between the nth frequency Fn and the (n-1)th frequency F(n-1). In this case, the compressor 21 operates at a lower frequency F for a longer period of time than when the decrease in frequency F is the same in each stage. This further slows down the rate at which the low-pressure pressure PL decreases. Therefore, the refrigeration cycle apparatus 1 according to the second embodiment can further reduce the amount of unrecovered refrigerant during the refrigerant recovery operation. Note that the control during the refrigerant recovery operation according to the modification of the second embodiment may be performed in combination with the control during the refrigerant recovery operation according to the second embodiment.

[0051] In the second embodiment, the nth deceleration threshold Pdwn is described as being a value lower than the low pressure PL last detected during the (n-1)th stage of control by a predetermined decrease value. In other words, in the second embodiment, the nth deceleration threshold Pdwn is described as being a value that varies depending on the decrease in the low pressure PL. However, the nth deceleration threshold Pdwn does not fluctuate based on the low pressure PL, but may be a fixed value that is specific to each counter n, that is, for each stage of control. In this case, the nth deceleration threshold Pdwn is also a value lower than the (n-1)th deceleration threshold Pdw(n-1).

[0052] In the second embodiment, the control transitions to the (n+1)th stage control when the low pressure PL detected during the nth stage control becomes less than the nth deceleration threshold Pdwn. However, the control may transition to the next stage when a time individually set for each stage control has elapsed. That is, the control device 70 transitions to the (n+1)th stage control when the nth time has elapsed since the start of the nth stage control. Here, the nth time is longer than the (n-1)th time. That is, in this case, as in the second embodiment, the control of a later stage is performed for a longer period than the control of an earlier stage. Therefore, as a whole, the control of a later stage is performed for a longer period of time during the refrigerant recovery operation. In this case, the control device 70 sets the nth frequency Fn based on the low pressure PL last detected during the (n-1)th stage. Specifically, the nth frequency Fn is determined by a function that uses the low pressure PL last detected during the (n-1)th stage as an argument. Here, the function again sets a lower value as the argument value becomes smaller. Therefore, as the low pressure PL decreases due to the refrigerant recovery operation, the nth frequency Fn becomes smaller than the (n-1)th frequency F(n-1).

[0053] REFRIGERATION CYCLE DEVICE, 2 OUTDOOR UNIT, 3 INDOOR UNIT, 10 REFRIGERANTE CIRCUIT, 21 COMPRESSOR, 22 FLOW CHANNEL SWITCHING DEVICE, 23 OUTDOOR HEAT EXCHANGER, 24 OUTDOOR EXPANSION VALVE, 31 INDOOR EXPANSION VALVE, 32 INDOOR HEAT EXCHANGER, 51 LIQUID SHUTOFF VALVE, 52 GAS SHUTOFF VALVE, 61 SUCTION PRESSURE SENSOR, 70 CONTROL DEVICE, 81 PROCESSING CIRCUIT, 82 PROCESSOR, 83 MEMORY, 84 BUS.

Claims

1. A refrigeration cycle apparatus comprising: an outdoor unit having a compressor that constitutes a refrigerant circuit; and a control device that executes a refrigerant recovery operation in which the compressor is operated to recover refrigerant circulating through the refrigerant circuit into the outdoor unit, wherein, in the refrigerant recovery operation, the control device operates the compressor at a first frequency as a first stage of control, operates the compressor at a second frequency lower than the first frequency as a second stage of control subsequent to the first stage of control, and stops the compressor when a low pressure in the refrigerant circuit falls below a stop threshold.

2. The refrigeration cycle device according to claim 1, wherein the control device executes the second stage control when the low pressure falls below a deceleration threshold higher than the stop threshold during the first stage control.

3. The control device operates the compressor at a third frequency lower than the second frequency as a third stage control subsequent to the second stage, executes the second stage control when the low pressure falls below a first deceleration threshold higher than the stop threshold during the first stage control, and executes the third stage control when the low pressure falls below a second deceleration threshold higher than the stop threshold and lower than the first deceleration threshold during the second stage control. A refrigeration cycle device as described in claim 1.

4. The refrigeration cycle device according to claim 3, wherein the control device sets the second deceleration threshold value based on the low pressure last measured during the first stage control.

5. A refrigeration cycle device according to claim 3 or 4, wherein the control device sets the second frequency based on the second deceleration threshold value.

6. A refrigeration cycle device as described in any one of claims 3 to 5, wherein the control device sets a frequency so that a difference between the second frequency and the third frequency is smaller than a difference between the first frequency and the second frequency.

7. The refrigeration cycle device according to any one of claims 1 to 6, wherein the control device sets a frequency at which the compressor is driven at a maximum rotation speed as the first frequency.

8. A refrigeration cycle device as described in any one of claims 1 to 6, wherein the control device sets the first frequency to a lower value when the low pressure immediately before the refrigerant recovery operation is performed is less than a threshold pressure than when the low pressure immediately before the refrigerant recovery operation is performed is equal to or greater than the threshold pressure.

9. A refrigeration cycle device as claimed in any one of claims 1 to 8, comprising: an indoor unit; and a shutoff valve which allows or shuts off the flow of refrigerant between the outdoor unit and the indoor unit, wherein the control device closes the shutoff valve when the refrigerant recovery operation is terminated.

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