JP7705079B2 - Refrigeration Cycle System - Google Patents

Description

This disclosure relates to a refrigeration cycle system.

Patent Document 1 describes a liquid return prevention device for a refrigeration system in which a low-pressure receiver is placed between the compressor and evaporator of a refrigerant circulation system, in which an oil return opening is formed in the refrigerant piping on the compressor side of the low-pressure receiver to return lubricating oil accumulated in the low-pressure receiver to the compressor, and a connecting pipe is provided between the bottom surface of the low-pressure receiver and the evaporator to allow liquid refrigerant accumulated in the low-pressure receiver to flow down to the evaporator when the compressor is stopped.

Japanese Patent Application Publication No. 63-172870

For example, as in the low pressure receiver described in Patent Document 1, a storage container capable of storing a liquid refrigerant (sometimes called a "liquid refrigerant") may be provided on a circuit through which a refrigerant circulates in a refrigeration cycle system. In such a storage container, lubricating oil that circulates through the circuit together with the refrigerant is also stored in order to ensure lubrication within the compressor. If the liquid refrigerant and the lubricating oil are incompatible, they are separated into an upper layer and a lower layer in the storage container, and the lubricating oil is supplied to the compressor by an oil supply mechanism such as the oil return opening described in Patent Document 1.
Depending on the combination of the liquid refrigerant and the lubricating oil, the densities of the two may be reversed at a certain temperature, resulting in an upper layer and a lower layer being interchanged. When the upper layer and the lower layer are interchanged, it becomes difficult for a conventional oil supply mechanism to supply oil to the compressor, resulting in poor lubrication of the compressor.
The present disclosure proposes a refrigeration cycle system that suppresses poor lubrication in a compressor even when the environmental temperature reaches a temperature at which the density of a lubricating oil and the density of a refrigerant in a liquid state are reversed.

A refrigeration cycle system according to a first aspect includes a compressor lubricated by a lubricating oil and compressing a refrigerant, a radiator that passes the refrigerant compressed by the compressor and extracts and radiates heat from the refrigerant passing through, an evaporator that passes the refrigerant compressed by the compressor and radiates heat and reduces its pressure, and exchanges heat between the refrigerant passing through and an object, an electric valve disposed between the radiator and the evaporator and regulating the pressure of the refrigerant passing through the evaporator, a storage container disposed between the evaporator and the compressor and capable of storing the refrigerant and the lubricating oil, and a control unit that controls a state of the refrigerant, wherein the control unit reduces the opening degree of the electric valve when the environmental temperature at which the storage container is installed reaches an inversion temperature at which the density of the lubricating oil and the density of the refrigerant in a liquid state are inverted. In this case, even when the environmental temperature reaches a temperature at which the density of the lubricating oil and the density of the refrigerant in a liquid state are inverted, poor lubrication in the compressor is suppressed.
A refrigeration cycle system of a second aspect is the refrigeration cycle system of the first aspect, characterized in that the control unit is capable of controlling the frequency of motion for compression in the compressor to be higher than before the control to superheat the refrigerant flowing into the storage container.
A refrigeration cycle system according to a third aspect is the refrigeration cycle system according to the first or second aspect, further comprising a temperature sensor for measuring an environmental temperature in which the storage container is placed.
A refrigeration cycle system according to a fourth aspect is the refrigeration cycle system according to the third aspect, characterized in that the control unit reduces the opening of the motor-operated valve based on a measurement value of the temperature sensor.
A refrigeration cycle system according to a fifth aspect is the refrigeration cycle system according to the first or second aspect , characterized in that the lubricating oil is a polyalkylene glycol.

1 is a diagram showing a schematic configuration example of an air conditioning system to which the present embodiment is applied; FIG. 2 is a diagram showing a configuration example of an air conditioning unit according to the present embodiment. FIG. 4 is a diagram showing the relationship between temperature and density of a liquid refrigerant and a lubricant oil. FIG. 1 is a diagram explaining the suction of lubricating oil stored in a low-pressure receiver, in which (a) is a schematic diagram of a low-pressure receiver equipped with an oil return pipe, and (b) is a schematic diagram of a low-pressure receiver equipped with an oil return hole in the middle of the compressor side piping. 5 is a flowchart showing an example of switching between a normal control mode and a reverse rotation control mode according to the present embodiment.

First Embodiment
(Air conditioning system 1)
FIG. 1 is a diagram showing a schematic configuration example of an air conditioning system 1 to which the present embodiment is applied.
As shown in the figure, the air conditioning system 1 to which this embodiment is applied comprises an air conditioning unit 10 including a refrigerant circuit through which a refrigerant circulates, and a control unit 90 that controls various devices (described later using FIG. 2) included in the air conditioning unit 10. The control unit 90 is connected to each device of the air conditioning unit 10 by wire or wirelessly, and is capable of transmitting control signals to each device.
The air conditioning system 1 is an example of a refrigeration cycle system in the present embodiment.

The air conditioning system 1 provides a cooling function for cooling a space by cooling the air taken in and supplying it to the space as cold air. More specifically, the air conditioning system 1 cools the air by extracting heat from the air through heat exchange between a refrigerant passing through a heat exchanger (described later using FIG. 2) provided in the air conditioning unit 10 and the air, which is an example of an object, and then supplies the cooled air as cold air from an air outlet of an indoor unit (described later using FIG. 2) or the like to cool the space.
The air conditioning system 1 also provides a heating function for heating a space by heating the air taken in and supplying it to the space as hot air. More specifically, the air conditioning system 1 heats the air by providing heat to the air through heat exchange between the air and a refrigerant passing through a heat exchanger provided in the air conditioning unit 10. The heated air is then supplied to the space from an air outlet of the indoor unit or the like to heat the space.

(Control unit 90)
The control unit 90 controls the various devices included in the air conditioning unit 10 by sending control signals to each device included in the air conditioning unit 10. In addition, the state of the refrigerant circulating through the refrigerant circuit of the air conditioning unit 10 is controlled in response to the control of the various devices by the control unit 90. The control unit 90 performs control in response to, for example, a measurement value of a temperature sensor (described later using FIG. 2) included in the air conditioning unit 10.
The control unit 90 may also perform control in response to an operation input from a user, such as a temperature setting or an air volume setting, input via an operation panel or controller that accepts operations from the user. Furthermore, the control unit 90 may perform control in response to a measurement value of a temperature sensor that measures the temperature of the space to be cooled/heated.
The control unit 90 may acquire information related to the operation of each device included in the air conditioning unit 10, such as an effective value of the operation relative to a control value, and perform control according to the acquired information. The control unit 90 may also control the amount of cold air/hot air supplied to the space by the air conditioning system 1, the wind direction, etc.

(Configuration of air conditioning unit 10)
FIG. 2 is a diagram showing an example of the configuration of the air conditioning unit 10 according to the present embodiment.
As shown in the figure, the air conditioning unit 10 of the air conditioning system 1 according to this embodiment includes a first refrigerant circuit 31 and a second refrigerant circuit 32. The first refrigerant circuit 31 is disposed across the indoor unit 20 and the outdoor unit 30.

The indoor unit 20 is equipped with an indoor heat exchanger 21 .
During heating operation, the indoor heat exchanger 21 functions as a radiator that heats the heat exchange partner by radiating heat from the refrigerant, heating the heat exchange partner air to produce warm air. During cooling operation, the indoor heat exchanger 21 functions as a cooler that cools the heat exchange partner by absorbing heat from the refrigerant, cooling the heat exchange partner air to produce cold air. In addition, when functioning as a radiator, the refrigerant radiates heat and is cooled, and when functioning as a cooler, the refrigerant absorbs heat and is heated. The indoor heat exchanger 21 is an example of an evaporator that exchanges heat between the passing refrigerant and an object.
Additionally, the indoor unit 20 may include an indoor fan (not shown) and the like.

The first refrigerant circuit 31 is connected to the indoor unit 20 by piping having a first shut-off valve 47 and a second shut-off valve 48. The second refrigerant circuit 32 functions as an assist circuit that enhances the capacity of the first refrigerant circuit 31.
The first refrigerant circuit 31 according to the present embodiment circulates carbon dioxide as an example of a refrigerant, and the second refrigerant circuit 32 circulates propane as an example of a refrigerant.

The first refrigerant circuit 31 is composed of a first compressor 41, a first sub-accumulator 42, a four-way switching valve 43, a first outdoor heat exchanger 44, a cascade heat exchanger 45 shared with the second refrigerant circuit 32, a first electric valve 46, a first shut-off valve 47, the indoor heat exchanger 21, a second shut-off valve 48, and a low-pressure receiver 100 connected in sequence.
The second refrigerant circuit 32 is configured by connecting, in sequence, a second compressor 51, a second sub-accumulator 52, a cascade heat exchanger 45 shared with the first refrigerant circuit 31, a second outdoor heat exchanger 53, and a second electric valve 54.
The first refrigerant circuit 31 and the second refrigerant circuit 32 are not limited to the above-mentioned configuration. For example, the first refrigerant circuit 31 and the second refrigerant circuit 32 may be configured to include a filter, a heat sink, an oil separator, etc. Also, the first refrigerant circuit 31 and the second refrigerant circuit 32 may be configured to include a pressure sensor/temperature sensor that detects the pressure/temperature of the refrigerant at each point in the circuit, a high-pressure switch that is a protective detector, etc.

The first compressor 41 has a discharge side connected to a first port (P1) of the four-way switching valve 43, and a suction side connected to the first sub-accumulator 42. The first sub-accumulator 42 separates the refrigerant into gas and liquid, and causes only the gaseous refrigerant to be sucked into the first compressor 41. The first compressor 41 compresses the sucked gaseous refrigerant and discharges it from the discharge side. The temperature of the discharged refrigerant increases due to the acquisition of heat of compression associated with the compression (sometimes referred to as "heated compression").
In addition, the first compressor 41 according to this embodiment is controlled in terms of, for example, its motion frequency and the amount of refrigerant sucked in/discharged in response to a control signal from the control unit 90. The "motion frequency" refers to the frequency of the motion (operation) of the parts for compressing the refrigerant performed in the compressor. Specifically, for example, it is the frequency of the oscillation of the oscillator in an oscillatory compressor, or the frequency of the rotation of the rotor in a scroll compressor or a rotary compressor.
The first compressor 41 is lubricated by a lubricating oil, which will be described later. More specifically, the lubricating oil ensures lubrication so that the movement of the components for compressing the refrigerant described above is not hindered. Examples of the lubricating oil include polyalkylene glycol (PAG).

The four-way switching valve 43 has a first port (P1), a second port (P2), a third port (P3), and a fourth port (P4), and can switch between a state in which the first port (P1) and the second port (P2) are connected and the third port (P3) and the fourth port (P4) are connected, and a state in which the first port (P1) and the fourth port (P4) are connected and the second port (P2) and the third port (P3) are connected.
During cooling operation, the four-way switching valve 43 is in a state where the first port (P1) and the second port (P2) are connected and the third port (P3) and the fourth port (P4) are connected, and during heating operation, the four-way switching valve 43 switches to a state where the first port (P1) and the fourth port (P4) are connected and the second port (P2) and the third port (P3) are connected.

The first outdoor heat exchanger 44 exchanges heat between the refrigerant and the outside air. The first outdoor heat exchanger 44 functions as a cooler during heating operation and as a radiator during cooling operation. The first outdoor heat exchanger 44 may be configured to include an outdoor fan or the like. The first outdoor heat exchanger 44 is an example of an evaporator that exchanges heat between the refrigerant passing through it and an object.
The cascade heat exchanger 45 exchanges heat between the first refrigerant circuit 31 and the second refrigerant circuit 32. The cascade heat exchanger 45 is, for example, a double-pipe heat exchanger in which two pipes having different diameters are combined in an inner and outer double configuration. The cascade heat exchanger 45 may also be another type of heat exchanger, such as a plate heat exchanger.

The first motor-operated valve 46 includes a valve, such as a ball valve, and a motor for driving the valve, and adjusts the pressure of the refrigerant passing through by adjusting the opening of the valve with the motor. More specifically, the first motor-operated valve 46 is provided between the pipe on the cascade heat exchanger 45 side and the pipe on the first shutoff valve 47 side, and reduces the pressure of the refrigerant flowing from one pipe by throttling and expanding the refrigerant according to the opening of the valve, and then flows it to the other pipe. The temperature of the refrigerant flowing to the other pipe decreases as the pressure is reduced by the throttling and expansion (sometimes referred to as "reduced pressure and temperature").
The opening degree of the first motor-operated valve 46 is adjusted by controlling the driving of each motor in response to a control signal from the control unit 90. Note that, as the valve whose opening degree can be controlled by the control unit 90, in addition to the motor-operated valve, an electromagnetic valve in which the valve is driven by a solenoid may be used.

The low-pressure receiver 100 is a container capable of storing the inflowing refrigerant, stores the liquid refrigerant among the inflowing refrigerant, and discharges the refrigerant in a gaseous state (sometimes referred to as "gas refrigerant") to circulate again. In the above-mentioned first compressor 41, if the liquid refrigerant is sucked in and compressed, it can cause a decrease in compression efficiency and malfunction. For this reason, the low-pressure receiver 100 separates the inflowing refrigerant into liquid refrigerant and gas refrigerant and stores the liquid refrigerant, thereby preventing the liquid refrigerant from being sucked into the first compressor 41. The low-pressure receiver 100 is an example of a storage container according to this embodiment.
Here, the outdoor unit 30 according to this embodiment includes a temperature sensor 101 that measures the temperature of the environment in which the low-pressure receiver 100 is installed (hereinafter, may be referred to as "environmental temperature"). Note that, while FIG. 2 shows the low-pressure receiver 100 and the temperature sensor 101 arranged side by side inside the outdoor unit 30, the arrangement of the temperature sensor 101 is not limited thereto. The temperature sensor 101 may be attached, for example, to the outside of the casing/housing of the outdoor unit 30. Also, for example, a thermistor may be used as the temperature sensor 101. The low-pressure receiver 100 and the temperature sensor 101 will be described in detail later.

The second compressor 51, the second sub-accumulator 52, the second outdoor heat exchanger 53, and the second electric valve 54 in the second refrigerant circuit 32 have the same configuration as the first compressor 41, the first sub-accumulator 42, the first outdoor heat exchanger 44, and the first electric valve 46 in the first refrigerant circuit 31, respectively.

(Refrigerant flow during cooling operation)
An example of the flow of refrigerant in the first refrigerant circuit 31 and the second refrigerant circuit 32 during cooling operation will be described. During cooling operation, the four-way switching valve 43 is in a state in which the first port (P1) and the second port (P2) are in communication with each other, and the third port (P3) and the fourth port (P4) are in communication with each other.

In the first refrigerant circuit 31, the refrigerant is first compressed by the first compressor 41. The compressed refrigerant passes through the four-way switching valve 43 and enters the first outdoor heat exchanger 44. The first outdoor heat exchanger 44 functions as a radiator during cooling operation. The refrigerant leaving the first outdoor heat exchanger 44 enters the cascade heat exchanger 45. During cooling operation, the cascade heat exchanger 45 functions as a radiator in the first refrigerant circuit 31. The refrigerant leaving the cascade heat exchanger 45 is decompressed when passing through the first electric valve 46, and enters the indoor heat exchanger 21 through the first shutoff valve 47. The indoor heat exchanger 21 functions as a cooler during cooling operation. The refrigerant leaving the indoor heat exchanger 21 passes through the second shutoff valve 48, the four-way switching valve 43, the low-pressure receiver 100, and the first sub-accumulator 42, and enters the first compressor 41 again.

The flow of refrigerant in the second refrigerant circuit 32 will be described. In the second refrigerant circuit 32, the refrigerant is first compressed by the second compressor 51. The compressed refrigerant enters the second outdoor heat exchanger 53. The second outdoor heat exchanger 53 functions as a radiator during cooling operation. The refrigerant leaving the second outdoor heat exchanger 53 is decompressed as it passes through the second electric valve 54 and enters the cascade heat exchanger 45. During cooling operation, the cascade heat exchanger 45 functions as a cooler in the second refrigerant circuit 32. The refrigerant leaving the cascade heat exchanger 45 passes through the second sub-accumulator 52 and enters the second compressor 51 again.

The air conditioning unit 10 according to this embodiment forms a binary circuit with a first refrigerant circuit 31 and a second refrigerant circuit 32. More specifically, in the air conditioning unit 10 during cooling operation, the cascade heat exchanger 45 functions as a radiator in the first refrigerant circuit 31 and as a cooler in the second refrigerant circuit 32. In this case, the refrigerant flowing through the first refrigerant circuit 31 is heated and compressed by the first compressor 41, cooled by the first outdoor heat exchanger 44, further cooled by the cascade heat exchanger 45, reduced in pressure and cooled by the first motor-operated valve 46, and cools the air in the indoor heat exchanger 21.

(Refrigerant flow during heating operation)
Next, a description will be given of the flow of refrigerant in the first refrigerant circuit 31 and the second refrigerant circuit 32 during heating operation. During heating operation, the four-way selector valve 43 is in a state in which the first port (P1) and the fourth port (P4) are communicated with each other, and the second port (P2) and the third port (P3) are communicated with each other.

In the first refrigerant circuit 31, the refrigerant is first heated and compressed by the first compressor 41. The compressed refrigerant passes through the four-way switching valve 43 and the second shutoff valve 48, and enters the indoor heat exchanger 21. The indoor heat exchanger 21 functions as a radiator during heating operation. The refrigerant discharged from the indoor heat exchanger 21 passes through the first shutoff valve 47, enters the indoor unit 20, and is reduced in pressure and temperature when passing through the first electric valve 46. The reduced pressure refrigerant passes through the cascade heat exchanger 45, and enters the first outdoor heat exchanger 44. The first outdoor heat exchanger 44 functions as a cooler during heating operation. The refrigerant discharged from the first outdoor heat exchanger 44 passes through the four-way switching valve 43, the low-pressure receiver 100, and the first sub-accumulator 42, and enters the first compressor 41 again.
The cascade heat exchanger 45 may be configured to perform heat exchange even during heating operation. In this case, the cascade heat exchanger 45 functions as a cooler in the first refrigerant circuit 31.

(Storage of lubricating oil)
In the first refrigerant circuit 31, lubricating oil for ensuring lubrication within the first compressor 41 circulates together with the refrigerant. For this reason, the lubricating oil that has flowed in together with the liquid refrigerant is stored in the low-pressure receiver 100.
If the lubricating oil stored in the low-pressure receiver 100 continues to flow in and be stored without being taken out, the amount of lubricating oil supplied to the first compressor 41 gradually decreases. Eventually, it becomes difficult to supply oil to the first compressor 41, and poor lubrication may occur in the first compressor 41. For this reason, the low-pressure receiver 100 needs to be provided with an oil supply mechanism that can take out the stored lubricating oil and supply it to the first compressor 41. On the other hand, as described above with reference to FIG. 2, it is not preferable for liquid refrigerant to be sucked into the first compressor 41. Therefore, it is preferable for the oil supply mechanism to be capable of taking out the lubricating oil and suppressing the taking out of the liquid refrigerant.

Here, if the liquid refrigerant and the lubricating oil are incompatible, they will separate into an upper layer and a lower layer in the low-pressure receiver 100. Depending on the combination of the liquid refrigerant and the lubricating oil, their densities will reverse at a certain temperature (sometimes called the "inversion temperature"), and the upper layer and the lower layer will switch places.

Fig. 3 is a diagram showing the relationship between temperature and density of the liquid refrigerant and the lubricating oil. In Fig. 3, the horizontal axis represents temperature (°C) and the vertical axis represents density (kg/m ³ ). Fig. 3 shows an example in which carbon dioxide is used as the liquid refrigerant and polyalkylene glycol is used as the lubricating oil.
As shown in Fig. 3, at temperatures higher than -20°C, the density of the lubricating oil is greater than that of the liquid refrigerant, so the liquid refrigerant forms an upper layer and the lubricating oil forms a lower layer. On the other hand, at temperatures below -20°C, the density of the lubricating oil is less than that of the liquid refrigerant, so the lubricating oil forms an upper layer and the liquid refrigerant forms a lower layer. Note that -20°C is an example of an inversion temperature.

In this specification, "immiscible" does not mean that the liquid refrigerant and the lubricating oil are completely insoluble in each other, but means that they are poorly soluble in each other at least to the extent that they separate into layers in the low-pressure receiver 100. In addition, it does not mean that they are incompatible in all temperature ranges, but rather that they are incompatible at normally expected environmental temperatures.

The air conditioning system 1 to which this embodiment is applied has a configuration that allows the lubricating oil stored in the low-pressure receiver 100 to be supplied to the first compressor 41 even when the environmental temperature is below -20°C.

1 to 4, configuration examples 100-1 and 100-2 of the low-pressure receiver 100 and removal of lubricating oil stored in the low-pressure receiver 100 will be described. In the following, the configuration examples 100-1 and 100-2 of the low-pressure receiver 100 may be referred to as the "low-pressure receiver 100" without distinction. In addition, unless otherwise specified, the description will be given taking the cooling operation of the air conditioning unit 10 as an example.
4A is a schematic diagram of a low-pressure receiver 100-1 equipped with an oil return pipe 140, and FIG. 4B is a schematic diagram of a low-pressure receiver 100-2 equipped with an oil return hole 151 in the compressor side piping 150. In the low-pressure receiver 100-2, components similar to those in the low-pressure receiver 100-1 may be given common names and symbols and descriptions thereof may be omitted.

As shown in Figure 4 (a), the low-pressure receiver 100-1 includes a storage section 110 capable of storing liquid refrigerant, a heat exchanger side pipe 120 that allows the refrigerant flowing from the indoor heat exchanger 21 side to flow into the storage section 110, a compressor side pipe 130 that discharges the gas refrigerant in the storage section 110 to the first compressor 41 side, and an oil return pipe 140 provided on the bottom surface of the storage section 110.
4(a) , when the environmental temperature is higher than −20° C. and no reversal of the liquid refrigerant and the lubricating oil occurs, the low-pressure receiver 100-1 can suck out the lubricating oil in the lower layer through the oil return pipe 140 and supply it to the first compressor 41. However, when the environmental temperature is lower than −20° C. and a density reversal occurs, if the low-pressure receiver 100-1 is not provided with a configuration for supplying the stored lubricating oil to the first compressor 41, as shown in the right-hand side of the page, “reversed state,” the lubricating oil becomes the upper layer, making it difficult to suck out the lubricating oil through the oil return pipe 140 and difficult to supply to the first compressor 41.

4(b), the low-pressure receiver 100-2 includes a storage section 110, a heat exchanger-side piping 120, and a compressor-side piping 150 that extends so as to pass near the bottom surface of the storage section 110. An oil return hole 151 is provided in the compressor-side piping 150 in a portion that passes near the bottom surface of the storage section 110.
4B, when the environmental temperature is higher than -20°C and no reversal of the liquid refrigerant and the lubricating oil occurs, the low-pressure receiver 100-2 can suck out the lubricating oil in the lower layer through the oil return hole 151 and supply it to the first compressor 41. However, when the environmental temperature is lower than -20°C and a density reversal occurs, if the low-pressure receiver 100-2 is not provided with a configuration for supplying the stored lubricating oil to the first compressor 41, the lubricating oil becomes the upper layer, as shown in the right-hand side of the page, "reversed state," and it becomes difficult to suck out the lubricating oil through the oil return hole 151 and to supply it to the first compressor 41.

In this way, if there is no configuration capable of supplying lubricating oil stored at an environmental temperature of -20°C or below to the first compressor 41, it may become difficult to extract the lubricating oil when the environmental temperature falls below -20°C.
Therefore, in the air conditioning system 1 of this embodiment, the lubricating oil stored in the low-pressure receiver 100 can be supplied to the first compressor 41 even when the ambient temperature falls below -20°C, and the control unit 90 is capable of controlling the refrigerant flowing into the low-pressure receiver 100 to be superheated when the ambient temperature falls below -20°C.

For example, when the measured value of the temperature sensor 101 becomes −20° C. or lower, the control unit 90 according to this embodiment reduces the opening degree of the first electric valve 46 compared to before the temperature became −20° C. or lower, increases the pressure of the refrigerant passing through the indoor heat exchanger 21, and increases the temperature of the refrigerant. This causes the refrigerant to be superheated, and the refrigerant flowing into the low-pressure receiver 100 can become a gas refrigerant.
Furthermore, for example, when the measured value of the temperature sensor 101 becomes -20°C or lower, the control unit 90 increases the operating frequency of the first compressor 41 compared to before the temperature became -20°C or lower, thereby increasing the pressure of the refrigerant passing through the indoor heat exchanger 21 and raising the temperature of the refrigerant. This control can also superheat the refrigerant, and make the refrigerant flowing into the low-pressure receiver 100 a gas refrigerant.

(Switching control modes)
In this embodiment, the control of the various devices included in the air conditioning unit 10 by the control unit 90 includes a normal control mode and a reverse rotation control mode. When there is no reverse rotation of the liquid refrigerant and the lubricant in the low-pressure receiver 100, the control unit 90 controls the various devices in the normal control mode. On the other hand, when there is a possibility of a reverse rotation of the liquid refrigerant and the lubricant, for example, when the environmental temperature is −20° C. or lower, the control unit 90 controls the various devices in the reverse rotation control mode. In the reverse rotation control mode, the control unit 90 controls the refrigerant flowing into the low-pressure receiver 100 so that it is superheated. Switching between the normal control mode and the reverse rotation control mode will be described below with reference to FIG. 5.

FIG. 5 is a flow chart showing an example of switching between the normal control mode and the reverse rotation control mode according to this embodiment.
When the operation of the air conditioning unit 10 is started, the control unit 90 controls the various devices included in the air conditioning unit 10 in normal control mode (step S1001). In the normal control mode, the control unit 90 controls the various devices included in the air conditioning unit 10 so that the outlet temperature of the first outdoor heat exchanger 44, which functions as a radiator during cooling operation, is constant, for example.

Next, the control unit 90 determines whether the measurement value of the temperature sensor 101 is equal to or lower than the reversal temperature (step S1002). If the measurement value of the temperature sensor 101 is equal to or higher than the reversal temperature (NO in step S1002), the process returns to step S1001, and the control unit 90 controls the various devices included in the air conditioning unit 10 in the normal control mode.

On the other hand, if the measured value of the temperature sensor 101 is equal to or lower than the reversal temperature (YES in step S1002), the control unit 90 switches the operation mode and controls the various devices included in the air conditioning unit 10 in the reverse control mode (step S1003). The control unit 90 performs control such that the refrigerant flowing into the low-pressure receiver 100 is superheated, for example, by reducing the opening of the first electric valve 46.

Next, the control unit 90 judges whether the measurement value of the temperature sensor 101 is equal to or lower than the reversal temperature (step S1004). If the measurement value of the temperature sensor 101 is equal to or lower than the reversal temperature (YES in step S1004), the process returns to step S1003, and the control unit 90 controls the various devices included in the air conditioning unit 10 in the reverse rotation control mode.
On the other hand, if the measurement value of the temperature sensor 101 is equal to or higher than the inversion temperature (NO in step S1004), the control unit 90 switches the operation mode and controls the various devices included in the air conditioning unit 10 in the normal control mode (step S1005).

In this way, the control unit 90 according to this embodiment controls the refrigerant flowing into the low-pressure receiver 100 to be superheated when the measurement value of the temperature sensor 101 falls below the reversal temperature. As a result, gas refrigerant flows into the low-pressure receiver 100, making it difficult for liquid refrigerant to accumulate, and even if the measurement value of the temperature sensor 101 falls below the reversal temperature, it is possible to suck out the lubricating oil through the oil return pipe 140 or the oil return hole 151.

5, the condition for switching the control mode is set based on the measurement value of the temperature sensor 101, but is not limited to this. For example, the control unit 90 may switch the control mode based on the low pressure or the suction temperature of the refrigerant.
Furthermore, the above-described control is an example of control for superheating the refrigerant flowing into the low-pressure receiver 100, and other control may be performed.

Second Embodiment
The second embodiment is configured to supply lubricating oil stored in the low-pressure receiver 100 to the first compressor 41 even when the ambient temperature falls below -20°C, and has an oil suction mechanism that can suck out the lubricating oil and supply it to the first compressor 41 even when the liquid refrigerant in the low-pressure receiver 100 is in a layer below the lubricating oil.

For example, the outdoor unit 30 according to the second embodiment has a configuration in which both the liquid refrigerant and the lubricating oil can be sucked out in the low-pressure receiver 100. More specifically, for example, in the low-pressure receiver 100-2 shown in FIG. 4(b), the compressor side piping 150 has a plurality of holes at different heights from the bottom surface of the storage section 110, including the oil return hole 151, and the liquid refrigerant and the lubricating oil can be sucked out by sucking out from the plurality of holes. The sucked out liquid refrigerant and the lubricating oil are heat exchanged with a high-temperature section in the air conditioning system 1 that is at least higher than -20°C, more preferably with a high-temperature section that is higher than the evaporation temperature of the sucked out liquid refrigerant, and are supplied to the first compressor 41. This allows the lubricating oil to be supplied to the first compressor 41, and the suction of the liquid refrigerant in the first compressor 41 can be suppressed.

Third Embodiment
The third embodiment is configured so that the lubricating oil stored in the low-pressure receiver 100 can be supplied to the first compressor 41 even when the ambient temperature is below -20°C, and the liquid refrigerant and lubricating oil stored in the low-pressure receiver 100 can be heated to eliminate reverse flow.

For example, the outdoor unit 30 according to the third embodiment has a heater capable of raising the temperature of the liquid stored inside the low-pressure receiver 100. The heater is provided, for example, close to or in contact with the side or bottom surface of the storage section 110 of the low-pressure receiver 100. The heater is turned on according to the control of the control section 90, and heats the liquid stored inside the low-pressure receiver 100 at least until it exceeds the reversal temperature. In this case, the control section 90 may turn on the heater, for example, in response to the measured value of the temperature sensor 101 being −20° C. or lower. This makes it possible to raise the temperature of the liquid refrigerant and lubricant stored in the low-pressure receiver 100 and eliminate the reversal.

For example, the outdoor unit 30 according to the third embodiment may exchange heat between the refrigerant compressed by the first compressor 41 and the low-pressure receiver 100 or the liquid refrigerant stored in the low-pressure receiver 100, thereby raising the temperature of the stored liquid refrigerant. More specifically, the piping from the first compressor 41 to the four-way switching valve 43 may be extended, and the extended piping may be brought into contact with or wrapped around the storage section 110 of the low-pressure receiver 100, so that heat exchange may be performed between the refrigerant compressed by the first compressor 41 and the low-pressure receiver 100. Also, a part of the extended piping may be inserted into the storage section 110 of the low-pressure receiver 100 so that the extended piping passes through the inside of the storage section 110 of the low-pressure receiver 100, so that heat exchange may be performed between the refrigerant compressed by the first compressor 41 and the low-pressure receiver 100.
Furthermore, for example, the outdoor unit 30 according to the third embodiment may exchange heat between the low-pressure receiver 100 or the liquid refrigerant stored in the low-pressure receiver 100 and the exhaust heat from the first compressor 41 or the second compressor 51, thereby raising the temperature of the stored liquid refrigerant.
Even with these configurations, the heat acquired by the liquid refrigerant through heat exchange can raise the temperature of the liquid refrigerant and lubricating oil stored in the low-pressure receiver 100. When heat exchange is performed between the refrigerant compressed by the first compressor 41 and the low-pressure receiver 100 or the liquid refrigerant stored in the low-pressure receiver 100, a temperature sensor such as a thermistor may be provided inside the storage section 110 of the low-pressure receiver 100 to enable measurement of the temperature of the liquid refrigerant stored in the low-pressure receiver 100.

＜Other＞
In the above embodiment, the refrigeration cycle system is applied to the air conditioning system 1, but the scope of application is not limited. The refrigeration cycle system may be applied to various devices that cool objects, such as freezers, refrigerators, and ice makers, by utilizing the heat absorption in the cooler. The refrigeration cycle system may be applied to various devices that heat objects, such as heaters, water heaters, and water heaters, by utilizing the heat dissipation in the radiator.

Although carbon dioxide and propane are given as examples of the refrigerant circulating through each refrigerant circuit, the type of refrigerant is not limited. For example, in the first refrigerant circuit 31, a mixed refrigerant in which carbon dioxide is mixed with other components may be used, or a single refrigerant or a mixed refrigerant that does not contain carbon dioxide may be used. However, the refrigerant circulating through the first refrigerant circuit 31 is incompatible with the lubricating oil of the first compressor 41.
Additionally, the inversion temperature is determined according to the combination of the refrigerant and the lubricant, and is not limited to the above-mentioned -20°C.

In addition, in the above-described embodiment, an example was described in which the liquid refrigerant is in the upper layer and the lubricating oil is in the lower layer at temperatures higher than the inversion temperature. However, depending on the combination of refrigerant and lubricating oil, there may be cases in which the lubricating oil is in the upper layer and the liquid refrigerant is in the lower layer at temperatures higher than the inversion temperature. In this case, too, by applying the above-described embodiment, a configuration can be created in which the lubricating oil can be supplied to the first compressor 41 even at temperatures below the inversion temperature.

Furthermore, although the air conditioning unit 10 has been described as constituting a binary circuit, it may be configured as a single circuit, for example, without providing the second refrigerant circuit 32 and the cascade heat exchanger 45. In addition, the configuration of each refrigerant circuit is not limited to that described above, and other configurations may be adopted.

In the above embodiment, an example has been described in which the first motor-operated valve 46 is used to enable the control unit 90 to control the opening degree. If control by the control unit 90 is not performed, a capillary tube, an orifice plate, or the like may be used instead of the first motor-operated valve 46.

Although the embodiment has been described above, it will be understood that various changes in form and details are possible without departing from the spirit and scope of the claims.
For example, some of the components may be omitted, or other functions may be added to each component. Furthermore, for example, a component included in one example configuration may be replaced with a component included in another example configuration, or a component included in one example configuration may be added to another example configuration.

1...Air conditioning system, 10...Air conditioning unit, 21...Indoor heat exchanger, 31...First refrigerant circuit, 32...Second refrigerant circuit, 90...Control unit, 100...Low pressure receiver, 101...Temperature sensor

Claims (5)

a compressor lubricated by lubricating oil and compressing a refrigerant;
a radiator through which the refrigerant compressed by the compressor passes and extracts heat from the passing refrigerant and radiates the heat;
an evaporator through which the refrigerant compressed by the compressor, dissipates heat, and is decompressed, passes and exchanges heat between the passing refrigerant and an object;
an electric valve disposed between the radiator and the evaporator for adjusting a pressure of the refrigerant passing through the evaporator;
a storage container provided between the evaporator and the compressor and capable of storing the refrigerant and the lubricating oil;
A control unit for controlling the state of the refrigerant,
The control unit, when the environmental temperature in which the storage container is installed reaches an inversion temperature at which the density of the lubricating oil and the density of the refrigerant in a liquid state are inverted, reduces the opening of the electric valve so that the refrigerant flowing into the storage container becomes a gas refrigerant . The refrigeration cycle system according to claim 1, characterized in that the control unit is capable of controlling the frequency of the compression motion in the compressor to be greater than that before the control of superheating the refrigerant flowing into the storage container. The refrigeration cycle system according to claim 1 or 2, characterized in that it is provided with a temperature sensor that measures the environmental temperature in which the storage container is installed. The refrigeration cycle system according to claim 3, characterized in that the control unit reduces the opening of the motor-operated valve based on the measurement value of the temperature sensor. The refrigeration cycle system according to claim 1 or 2, characterized in that the lubricating oil is polyalkylene glycol.