air conditioner

Abstract

To suppress a heat exchanger from being frosted while securing heat exchange capacity of the heat exchanger.SOLUTION: An air conditioner 1 includes: a refrigerant circuit 5 in which a main expansion valve 15, a first outdoor heat exchanger 16, an evaporator expansion valve 17, and a second outdoor heat exchanger 18 are connected in order so that a nonazeotropic refrigerant can be circulated therein; temperature rise amount calculating means for calculating a rise amount of a temperature rising while the nonazeotropic refrigerant flows in the first outdoor heat exchanger 16 when the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 function as evaporators; and opening control means for controlling the evaporator expansion valve 17 so that an opening of the evaporator expansion valve 17 is larger when the rise amount is smaller than a temperature difference threshold than when the rise amount is equal to or larger than the threshold.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The technology of the present disclosure relates to an air conditioner. [Background technology]

[0002] Non-azeotropic refrigerants used in the refrigeration cycle of air conditioners have the property of increasing in temperature during the evaporation process. Therefore, in air conditioners using non-azeotropic refrigerants, the temperature difference between the refrigerant and the outside air at the outlet side of the evaporator becomes small, resulting in a decrease in the amount of heat exchanged in the evaporator. Air conditioners are known that have a second expansion valve installed between a front-stage evaporator and a rear-stage evaporator, to which refrigerant decompressed by a first expansion valve is supplied (Patent Document 1). Even if the temperature of the refrigerant passing through the front-stage evaporator increases, this type of air conditioner can reduce the temperature of the refrigerant supplied to the rear-stage evaporator by decompressing the refrigerant using the second expansion valve. Therefore, this type of air conditioner can achieve a large temperature difference between the refrigerant and the outside air, thereby preventing a decrease in the amount of heat exchanged. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-222357 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in this type of air conditioner, when the refrigerant circulation rate is high, the pressure loss (flow path resistance) in the front-stage evaporator increases, so the refrigerant temperature does not increase even after passing through the front-stage evaporator. In such cases, when the refrigerant circulation rate is high, the refrigerant is decompressed by the second expansion valve, which causes the temperature of the refrigerant supplied to the rear-stage evaporator to drop excessively, which can lead to frost formation on the rear-stage evaporator.

[0005] The disclosed technology has been made in consideration of the above points, and aims to provide an air conditioner that suppresses frost formation on the heat exchanger. [Means for solving the problem]

[0006] An air conditioner according to one aspect of the present disclosure comprises a refrigerant circuit in which a first expansion valve, a first heat exchanger, a second expansion valve, and a second heat exchanger are connected in sequence, and in which a non-azeotropic refrigerant circulates; a temperature rise calculation means for calculating the amount of temperature rise of the non-azeotropic refrigerant as it flows through the first heat exchanger when the first heat exchanger and the second heat exchanger function as evaporators; and an opening control means for controlling the second expansion valve so that the opening of the second expansion valve is larger when the amount of rise is smaller than a predetermined threshold value compared to when the amount of rise is equal to or greater than the threshold value. [Effects of the Invention]

[0007] The disclosed air conditioner can suppress frost formation on the heat exchanger. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a circuit diagram showing an air conditioner according to an embodiment. [Figure 2] FIG. 2 is a schematic diagram showing a specific connection relationship between the first outdoor heat exchanger, the evaporator expansion valve, the second outdoor heat exchanger, and the bypass circuit. [Figure 3] FIG. 3 is a block diagram showing an air conditioner according to the embodiment. [Figure 4] FIG. 4 is a flowchart showing the first pressure reduction control switching operation. [Figure 5] FIG. 5 is a flowchart showing the second pressure reduction control switching operation. [Figure 6] FIG. 6 is a Mollier diagram showing an example of a state change of the non-azeotropic refrigerant when single-stage pressure reduction control is executed. [Figure 7] FIG. 7 is a th diagram showing an example of a state change of the non-azeotropic refrigerant when single-stage pressure reduction control is executed. [Figure 8]FIG. 8 is a Mollier diagram showing an example of a state change of the non-azeotropic refrigerant when the two-stage pressure reduction control is executed. [Figure 9] FIG. 9 is a th diagram showing an example of a state change of the non-azeotropic refrigerant when the two-stage pressure reduction control is executed. [Figure 10] FIG. 10 is a Mollier diagram showing another example of the state change of the non-azeotropic refrigerant when the two-stage pressure reduction control is executed. [Figure 11] FIG. 11 is a th diagram showing another example of the state change of the non-azeotropic refrigerant when the two-stage pressure reduction control is executed. [Figure 12] FIG. 12 is a Mollier diagram showing another example of the state change of the non-azeotropic refrigerant when the single-stage pressure reduction control is executed. [Figure 13] FIG. 13 is a th diagram showing another example of the state change of the non-azeotropic refrigerant when the single-stage pressure reduction control is executed. DETAILED DESCRIPTION OF THE INVENTION

[0009] An air conditioner according to an embodiment of the present disclosure will be described in detail below with reference to the drawings. Note that the following description does not limit the technology of the present disclosure. In addition, in the following description, the same components are given the same reference numerals, and duplicated descriptions will be omitted. [Example]

[0010] FIG. 1 is a circuit diagram showing an air conditioner 1 of an embodiment. The air conditioner 1 includes an outdoor unit 2 and an indoor unit 3. The outdoor unit 2 is installed outdoors. The indoor unit 3 is installed in a room that is cooled or heated by the air conditioner 1. The air conditioner 1 includes a refrigerant circuit 5 through which a non-azeotropic refrigerant circulates. An example of a non-azeotropic refrigerant is a refrigerant containing refrigerant R32, such as R454C (R32 + R1234yf). The refrigerant circuit 5 includes a compressor 11, a four-way valve 12, an indoor heat exchanger 14, a main expansion valve 15 (first expansion valve), a first outdoor heat exchanger 16 (first heat exchanger), an evaporator expansion valve 17 (second expansion valve), and a second outdoor heat exchanger 18 (second heat exchanger). The compressor 11, four-way valve 12, main expansion valve 15, first outdoor heat exchanger 16, evaporator expansion valve 17, and second outdoor heat exchanger 18 are disposed inside the outdoor unit 2. The indoor heat exchanger 14 is disposed inside the indoor unit 3. The compressor 11 is equipped with a suction pipe 21 and a discharge pipe 22. The compressor 11 compresses gas-phase refrigerant supplied via the suction pipe 21 and discharges the compressed gas-phase refrigerant to the discharge pipe 22.

[0011] The four-way valve 12 has a first connection port 121, a second connection port 122, a third connection port 123, and a fourth connection port 124. The first connection port 121 is connected to the suction pipe 21 and is connected to the compressor 11 via the suction pipe 21. The second connection port 122 is connected to the discharge pipe 22 and is connected to the compressor 11 via the discharge pipe 22. The third connection port 123 is connected to the second outdoor heat exchanger 18. The fourth connection port 124 is connected to the indoor heat exchanger 14. The four-way valve 12 can be switched to a heating position or a cooling position. When the four-way valve 12 is switched to the heating position, the second connection port 122 is connected to the fourth connection port 124, and the third connection port 123 is connected to the first connection port 121. When the four-way valve 12 is switched to the cooling position, the second connection port 122 is connected to the third connection port 123 and the fourth connection port 124 is connected to the first connection port 121.

[0012] The indoor heat exchanger 14 exchanges heat between the refrigerant passing through the indoor heat exchanger 14 and the air in the room where the indoor unit 3 is installed. The indoor heat exchanger 14 is connected to a main expansion valve 15. The main expansion valve 15 adjusts the flow rate of the refrigerant passing through the main expansion valve 15 according to its opening, and reduces the pressure of the refrigerant passing through the main expansion valve 15 according to its opening. The main expansion valve 15 is connected to a first outdoor heat exchanger 16. The first outdoor heat exchanger 16 exchanges heat between the refrigerant passing through the first outdoor heat exchanger 16 and the outdoor air outside where the outdoor unit is installed. The first outdoor heat exchanger 16 is connected to an evaporator expansion valve 17. The evaporator expansion valve 17 adjusts the flow rate of the refrigerant passing through the evaporator expansion valve 17 according to its opening, and reduces the pressure of the refrigerant passing through the evaporator expansion valve 17 according to its opening. The evaporator expansion valve 17 is connected to a second outdoor heat exchanger 18. The second outdoor heat exchanger 18 exchanges heat between the refrigerant passing through the second outdoor heat exchanger 18 and the outside air outside where the outdoor unit is installed.

[0013] The air conditioner 1 further includes a bypass circuit 23. The bypass circuit 23 is disposed inside the outdoor unit 2. A bypass passage 24 is formed in the bypass circuit 23. One end of the bypass passage 24 is connected to a flow path 25 between the first outdoor heat exchanger 16 and the evaporator expansion valve 17, and the other end of the bypass passage 24 is connected to a flow path 26 between the evaporator expansion valve 17 and the second outdoor heat exchanger 18. In other words, the first outdoor heat exchanger 16 is connected to the second outdoor heat exchanger 18 via the bypass passage 24, bypassing the evaporator expansion valve 17.

[0014] The bypass circuit 23 includes a bypass valve 27. The bypass valve 27 is provided midway through the bypass circuit 23. The flow path 25 and the flow path 26 are connected or cut off via the bypass circuit 23 by opening or closing the bypass valve 27.

[0015] The air conditioner 1 further includes a discharge temperature sensor 31, an inlet temperature sensor 32, and an intermediate temperature sensor 33. The discharge temperature sensor 31 is provided in the discharge pipe 22 near the compressor 11, and measures the temperature of the high-pressure gas-phase refrigerant discharged from the compressor 11. The inlet temperature sensor 32 is provided in the refrigerant pipe 34 that connects the main expansion valve 15 and the first outdoor heat exchanger 16, and is provided near the first outdoor heat exchanger 16. The inlet temperature sensor 32 measures the inlet temperature (evaporator inlet temperature), which is the temperature of the refrigerant flowing through the refrigerant pipe 34. The intermediate temperature sensor 33 is provided midway through the flow path 25, and measures the intermediate temperature (evaporator intermediate temperature), which is the temperature of the refrigerant flowing through the flow path 25.

[0016] 2 is a schematic diagram showing the specific connection relationship between the first outdoor heat exchanger 16, the evaporator expansion valve 17, the second outdoor heat exchanger 18, and the bypass circuit 23. The second outdoor heat exchanger 18 has multiple flow paths formed in parallel. The second outdoor heat exchanger 18 includes a flow divider 35 and a branch pipe 36. The flow divider 35 is provided between the evaporator expansion valve 17 and the second outdoor heat exchanger 18, and is connected to the evaporator expansion valve 17 via a flow path 26 and to the multiple flow paths of the second outdoor heat exchanger 18. That is, the evaporator expansion valve 17 is provided between the first outdoor heat exchanger 16 and the flow divider 35. The branch pipe 36 is provided between the second outdoor heat exchanger 18 and the third connection port 123 of the four-way valve 12, and is connected to the multiple flow paths of the second outdoor heat exchanger 18 and is also connected to the third connection port 123 of the four-way valve 12.

[0017] FIG. 3 is a block diagram showing an air conditioner 1 of the embodiment. The air conditioner 1 further includes a control device 41. The control device 41 is a computer and includes a storage device 42 and a CPU (Central Processing Unit) 43. The storage device 42 stores computer programs installed in the control device 41 and stores information used by the CPU 43. The CPU 43 executes the computer programs installed in the control device 41 to process information and acquire information from the storage device 42. The CPU 43 further controls the compressor 11, the four-way valve 12, the main expansion valve 15, the evaporator expansion valve 17, and the bypass valve 27, and acquires information from the discharge temperature sensor 31, the inlet temperature sensor 32, and the intermediate temperature sensor 33. The storage device 42 stores a target discharge temperature, a lower limit temperature, a temperature difference threshold (threshold value), and a frosting limit temperature.

[0018] The computer program installed in the control device 41 includes a plurality of computer programs that respectively cause the control device 41 to realize a plurality of means. The plurality of means include a four-way valve switching means 44, a rotation speed control means 45, a temperature rise amount calculation means 46, an opening control means 47, and a bypass valve opening / closing means 48.

[0019] The four-way valve switching means 44 controls the four-way valve 12 so that the flow paths within the four-way valve 12 are switched to the cooling position when the air conditioner 1 is in cooling operation. The four-way valve switching means 44 controls the four-way valve 12 so that the flow paths within the four-way valve 12 are switched to the heating position when the air conditioner 1 is in heating operation. The rotation speed control means 45 calculates a target rotation speed based on the temperature difference between a set temperature set by the user and the room temperature, and controls the compressor 11 so that the rotation speed of the compressor 11 is equal to the calculated target rotation speed.

[0020] When the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 are functioning as evaporators, the temperature rise calculation means 46 measures the inlet temperature using the inlet temperature sensor 32 and the intermediate temperature using the intermediate temperature sensor 33. The temperature rise calculation means 46 calculates the temperature rise based on the intermediate temperature and the frost limit temperature. The temperature rise is equal to the value obtained by subtracting the frost limit temperature from the intermediate temperature. The frost limit temperature is the temperature of the refrigerant flowing into the second outdoor heat exchanger 18, and is the lower limit value below which frost does not occur, and is determined in advance through experiments, etc. An example of the lower limit temperature is -1°C. The opening control means 47 controls the opening of the main expansion valve 15 and the opening of the evaporator expansion valve 17. The bypass valve opening / closing means 48 opens and closes the bypass valve 27.

[0021] The operations performed by the air conditioner 1 include cooling operation and heating operation. [Cooling operation] The cooling operation is performed, for example, when a user operates the air conditioner 1. When the air conditioner 1 performs the cooling operation, the control device 41 fully opens the evaporator expansion valve 17, opens the bypass valve 27, and connects the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 via the bypass path 24.

[0022] When the air conditioner 1 performs cooling operation, the control device 41 further controls the four-way valve 12 to switch the flow path within the four-way valve 12 to the cooling position. The control device 41 controls the compressor 11 to compress the low-pressure gas-phase refrigerant supplied via the suction pipe 21. The low-pressure gas-phase refrigerant is compressed by the compressor 11 to become high-pressure gas-phase refrigerant. The compressor 11 further discharges the high-pressure gas-phase refrigerant to the discharge pipe 22. The high-pressure gas-phase refrigerant discharged to the discharge pipe 22 is supplied to the second outdoor heat exchanger 18 because the four-way valve 12 is switched to the cooling position.

[0023] The high-pressure gas-phase refrigerant supplied from the four-way valve 12 to the second outdoor heat exchanger 18 is supplied to multiple flow paths of the second outdoor heat exchanger 18 via branch pipes 36 and flows along the multiple flow paths. The second outdoor heat exchanger 18 exchanges heat between the high-pressure gas-phase refrigerant flowing through the multiple flow paths and outside air, cooling the high-pressure gas-phase refrigerant and heating the outside air. The high-pressure gas-phase refrigerant cooled by the second outdoor heat exchanger 18 is supplied to flow path 26 via a flow divider 35. The high-pressure gas-phase refrigerant supplied to flow path 26 is supplied to the first outdoor heat exchanger 16 via the evaporator expansion valve 17, and because the bypass valve 27 is open, is supplied to the first outdoor heat exchanger 16 via the bypass path 24 and passes through the flow paths of the first outdoor heat exchanger 16. The first outdoor heat exchanger 16 exchanges heat between the high-pressure gas-phase refrigerant passing through the first outdoor heat exchanger 16 and the outside air, cooling the high-pressure gas-phase refrigerant and heating the outside air. The high-pressure gas-phase refrigerant is cooled in the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 to become a supercooled high-pressure liquid-phase refrigerant. That is, the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 function as condensers when the air conditioner 1 performs cooling operation. The high-pressure liquid-phase refrigerant is supplied to the main expansion valve 15.

[0024] The control device 41 intermittently acquires the discharge temperature using the discharge temperature sensor 31. It controls the opening of the main expansion valve 15 to adjust the flow rate of refrigerant flowing from the first outdoor heat exchanger 16 to the indoor heat exchanger 14 so that the discharge temperature becomes equal to the target discharge temperature. The target discharge temperature is calculated from the condensing pressure, evaporating pressure, the rotation speed of the compressor 11, etc., and is the target value of the discharge temperature at which the refrigerant sucked into the compressor 11 becomes in an optimal state. In addition, the high-pressure liquid-phase refrigerant is decompressed by the main expansion valve 15 to become a low-pressure two-phase gas-liquid refrigerant. The low-pressure two-phase gas-liquid refrigerant is supplied to the indoor heat exchanger 14.

[0025] The indoor heat exchanger 14 exchanges heat between the low-pressure gas-liquid two-phase refrigerant supplied from the main expansion valve 15 and the air in the room where the indoor unit 3 is installed, heating the low-pressure gas-liquid two-phase refrigerant and cooling the air in the room. The low-pressure gas-liquid two-phase refrigerant is heated by the indoor heat exchanger 14 and becomes a low-pressure gas-phase refrigerant. That is, the indoor heat exchanger 14 functions as an evaporator when the air conditioner 1 performs cooling operation. The indoor unit 3 cools the room by cooling the indoor air with the indoor heat exchanger 14. The low-pressure gas-phase refrigerant is supplied to the four-way valve 12. With the internal flow path of the four-way valve 12 switched to the cooling position, the four-way valve 12 supplies the low-pressure gas-phase refrigerant supplied from the indoor heat exchanger 14 to the compressor 11 via the suction pipe 21. While the air conditioner 1 is performing cooling operation, the control device 41 calculates a target rotation speed based on the temperature difference between the set temperature set by the user's operation and the room temperature, and controls the compressor 11 so that the rotation speed of the compressor 11 is equal to the target rotation speed.

[0026] [Heating operation] The heating operation is performed, for example, when the air conditioner 1 is operated by a user. When the air conditioner 1 performs the heating operation, the control device 41 controls the four-way valve 12 and switches the flow path within the four-way valve 12 to the heating position. The control device 41 controls the compressor 11 to compress the low-pressure gas-phase refrigerant supplied via the suction pipe 21. The low-pressure gas-phase refrigerant is compressed by the compressor 11 to become high-pressure gas-phase refrigerant. The compressor 11 further discharges the high-pressure gas-phase refrigerant to the discharge pipe 22. The high-pressure gas-phase refrigerant discharged to the discharge pipe 22 is supplied to the indoor heat exchanger 14 because the four-way valve 12 is switched to the heating position.

[0027] The indoor heat exchanger 14 exchanges heat between the high-pressure gas-phase refrigerant supplied from the four-way valve 12 and the air in the room where the indoor unit 3 is installed, cooling the high-pressure gas-phase refrigerant and heating the indoor air. The indoor unit 3 heats the room by using the indoor heat exchanger 14 to heat the air in the room. The high-pressure gas-phase refrigerant is cooled by the indoor heat exchanger 14 and becomes a subcooled high-pressure liquid-phase refrigerant. In other words, the indoor heat exchanger 14 functions as a condenser when the air conditioner 1 performs heating operation. The high-pressure liquid-phase refrigerant is supplied to the main expansion valve 15. The control device 41 intermittently obtains the discharge temperature using the discharge temperature sensor 31. The control device 41 controls the opening of the main expansion valve 15 to adjust the flow rate of refrigerant flowing from the first outdoor heat exchanger 16 to the indoor heat exchanger 14 so that the discharge temperature is equal to the target discharge temperature. The high-pressure liquid-phase refrigerant supplied to the main expansion valve 15 is decompressed by the main expansion valve 15 and becomes a low-pressure gas-liquid two-phase refrigerant. The low-pressure gas-liquid two-phase refrigerant is supplied to the first outdoor heat exchanger 16.

[0028] The first outdoor heat exchanger 16 exchanges heat between the low-pressure gas-liquid two-phase refrigerant supplied from the main expansion valve 15 and outside air, cooling the outside air and heating the low-pressure gas-liquid two-phase refrigerant. The low-pressure gas-liquid two-phase refrigerant heated by the first outdoor heat exchanger 16 is supplied to a flow path 26 via an evaporator expansion valve 17 and then to the flow path 26 via a bypass path 24. The low-pressure gas-liquid two-phase refrigerant supplied to the flow path 26 is supplied to multiple flow paths of the second outdoor heat exchanger 18 via a flow divider 35 and flows along the multiple flow paths. The second outdoor heat exchanger 18 exchanges heat between the low-pressure gas-liquid two-phase refrigerant flowing through the multiple flow paths and outside air, cooling the outside air and heating the low-pressure gas-liquid two-phase refrigerant. The low-pressure gas-liquid two-phase refrigerant is heated by the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 and becomes low-pressure gas-phase refrigerant. That is, the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 function as evaporators when the air conditioner 1 performs heating operation.

[0029] The low-pressure gas-phase refrigerant heated by the second outdoor heat exchanger 18 is supplied from multiple flow paths to the four-way valve 12 via branch pipes 36. The low-pressure gas-phase refrigerant supplied from the second outdoor heat exchanger 18 is supplied to the compressor 11 via the suction pipe 21 because the flow paths in the four-way valve 12 are switched to the heating position. While the air conditioner 1 is performing heating operation, the control device 41 calculates a target rotation speed based on the temperature difference between the room temperature and a set temperature set by a user operation, and controls the compressor 11 so that the rotation speed of the compressor 11 becomes equal to the target rotation speed.

[0030] The air conditioner 1 executes one of single-stage pressure reduction control and two-stage pressure reduction control while the heating operation is being performed. [Single-stage pressure reduction control] The single-stage pressure reduction control is executed at the beginning of heating operation. In the single-stage pressure reduction control, the control device 41 fully opens the evaporator expansion valve 17 and opens the bypass valve 27 to connect the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 via the bypass path 24. The control device 41 also intermittently acquires the discharge temperature using the discharge temperature sensor 31. The control device 41 controls the aperture of the main expansion valve 15 to adjust the flow rate of the refrigerant flowing from the indoor heat exchanger 14 to the first outdoor heat exchanger 16 so that the discharge temperature becomes equal to the target discharge temperature.

[0031] The air conditioner 1 executes a first pressure reduction control switching operation while single-stage pressure reduction control is being executed. FIG. 4 is a flowchart showing the first pressure reduction control switching operation. In the first pressure reduction control switching operation, the control device 41 intermittently acquires the inlet temperature using the inlet temperature sensor 32 and the intermediate temperature using the intermediate temperature sensor 33. The control device 41 determines whether the inlet temperature is lower than a lower limit temperature (step S1). The lower limit temperature indicates the lower limit value of the inlet temperature at which frost does not form in the first outdoor heat exchanger 16 when single-stage pressure reduction control is being executed, and is determined in advance through experiments, etc. An example of the lower limit temperature is −1° C. When the inlet temperature is lower than the lower limit temperature (step S1, Yes), the control device 41 determines that there is a possibility of frost forming in the first outdoor heat exchanger 16 and determines that it is necessary to increase the inlet temperature.

[0032] When the inlet temperature is lower than the lower limit temperature (step S1, Yes), the control device 41 determines whether the inlet temperature is lower than the intermediate temperature (step S2), and determines which of the temperature glide and pressure loss has a greater effect on the difference between the inlet temperature and the intermediate temperature. The effect of temperature glide indicates that the temperature of the refrigerant increases when the non-azeotropic refrigerant passes through the first outdoor heat exchanger 16. In other words, the temperature glide has an effect that increases the temperature difference. The effect of pressure loss indicates that when the non-azeotropic refrigerant passes through the first outdoor heat exchanger 16, the refrigerant is decompressed due to pressure loss in the first outdoor heat exchanger 16, causing the refrigerant's temperature to decrease. In other words, the pressure loss has an effect that decreases the temperature difference. The effect of pressure loss increases as the flow velocity of the non-azeotropic refrigerant increases.

[0033] When the inlet temperature is lower than the intermediate temperature (step S2, Yes), the control device 41 determines that the effect of the temperature glide is greater than the effect of the pressure loss. When the inlet temperature is lower than the intermediate temperature (step S2, Yes), that is, when the effect of the temperature glide is greater than the effect of the pressure loss, the control device 41 calculates the intermediate temperature.

[0034] The control device 41 determines whether the amount of temperature rise is equal to or greater than a threshold value (step S3). The temperature rise amount is calculated by the temperature rise amount calculation means 46 based on the intermediate temperature and the frost limit temperature. The temperature rise amount is equal to the value obtained by subtracting the frost limit temperature from the intermediate temperature. The threshold value indicates the upper limit of the amount of temperature rise at which frost does not form in the second outdoor heat exchanger 18 when two-stage pressure reduction control is executed at the current intermediate temperature, and is determined in advance through experiments, etc. When the amount of temperature rise is equal to or greater than the threshold value (step S3, Yes), the control device 41 starts two-stage pressure reduction control (step S4).

[0035] [Two-stage pressure reduction control] In two-stage pressure reduction control, the control device 41 closes the bypass valve 27 to block the bypass path 24. The control device 41 further intermittently acquires the discharge temperature using the discharge temperature sensor 31, the inlet temperature using the inlet temperature sensor 32, and the intermediate temperature using the intermediate temperature sensor 33. Instead of controlling the aperture of the main expansion valve 15 so that the discharge temperature becomes equal to the target discharge temperature, as was done in single-stage pressure reduction control, the control device 41 controls the aperture of the main expansion valve 15 to adjust the flow rate of refrigerant flowing from the indoor heat exchanger 14 to the first outdoor heat exchanger 16 so that the inlet temperature becomes equal to the lower-limit temperature. The control device 41 controls the opening of the evaporator expansion valve 17 to a preset opening, and then controls the opening of the evaporator expansion valve 17 so that the discharge temperature is equal to a target discharge temperature, thereby adjusting the flow rate of the refrigerant flowing from the first outdoor heat exchanger 16 to the second outdoor heat exchanger 18, and reducing the pressure of the refrigerant supplied from the first outdoor heat exchanger 16 to the second outdoor heat exchanger 18. This makes it possible to reduce again the temperature of the refrigerant that rose while flowing through the first outdoor heat exchanger 16, making it possible to increase the temperature difference between the refrigerant and the outside air, and suppressing a decrease in the amount of heat exchange.

[0036] The air conditioner 1 executes a second pressure reduction control switching operation while two-stage pressure reduction control is being executed. Figure 5 is a flowchart showing the second pressure reduction control switching operation. In the second pressure reduction control switching operation, the control device 41 intermittently acquires the intermediate temperature using the intermediate temperature sensor 33. The control device 41 determines whether the intermediate temperature is equal to or lower than a threshold value (step S11). If the intermediate temperature is equal to or lower than the threshold value (Yes in step S11), the control device 41 determines that there is a high possibility of frost forming on the second outdoor heat exchanger 18 if the two-stage pressure reduction control is continued, and starts single-stage pressure reduction control (step S12).

[0037] FIG. 6 is a Mollier diagram showing an example of the state change of the non-azeotropic refrigerant when single-stage pressure reduction control is executed. The pressure of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 is determined when the second outdoor heat exchanger 18 functions as an evaporator and the pressure loss of the first outdoor heat exchanger 16 is negligibly small. The slope of line 51 on the Mollier diagram, which shows the relationship between the pressure and specific enthalpy of the non-azeotropic refrigerant during the evaporation process when the non-azeotropic refrigerant passes through the first outdoor heat exchanger 16, is approximately zero. An isothermal line 52, which connects points on the Mollier diagram where the non-azeotropic refrigerant temperature is equal, slopes downward to the right and monotonically decreases as the specific enthalpy increases when the non-azeotropic refrigerant is in a two-phase gas-liquid state. Therefore, even if the pressure remains nearly constant during the evaporation process, the refrigerant temperature changes in the two-phase region. Three points on line 51 in FIG. 6 indicate the change in specific enthalpy as the non-azeotropic refrigerant passes through the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18. In this example, the temperature of the refrigerant before flowing into the first outdoor heat exchanger 16 is 0°C. The temperature of the refrigerant at the midpoint in the evaporation process including the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 is 4°C. The temperature of the refrigerant flowing out from the second outdoor heat exchanger 18 is 6°C.

[0038] That is, the temperature of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 may rise as shown in Fig. 7 when the pressure loss in the first outdoor heat exchanger 16 is negligibly small. Fig. 7 is a th diagram showing an example of the state change of the non-azeotropic refrigerant when single-stage decompression control is executed. When the temperature of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 rises, the temperature of the non-azeotropic refrigerant flowing through the second outdoor heat exchanger 18 increases, and the temperature difference between the temperature of the non-azeotropic refrigerant flowing through the second outdoor heat exchanger 18 and the outside air becomes smaller. For this reason, the amount of heat exchanged between the refrigerant and the air in the second outdoor heat exchanger 18 may decrease.

[0039] FIG. 8 is a Mollier diagram showing an example of the state change of the non-azeotropic refrigerant when two-stage pressure reduction control is executed in a state where the refrigerant flow rate is low and the pressure loss of the refrigerant during the evaporation process is small. When two-stage pressure reduction control is executed, the non-azeotropic refrigerant that has passed through the first outdoor heat exchanger 16 is supplied to the evaporator expansion valve 17 and reduced in pressure. The temperature of the non-azeotropic refrigerant is reduced as shown in FIG. 9 by the evaporator expansion valve 17 reducing its pressure. FIG. 9 is a th diagram showing an example of the state change of the non-azeotropic refrigerant when two-stage pressure reduction control is executed. The temperature difference between the temperature of the non-azeotropic refrigerant flowing through the second outdoor heat exchanger 18 and the outside air increases as the temperature of the non-azeotropic refrigerant decreases. As the temperature difference between the non-azeotropic refrigerant and the outside air increases, the amount of heat exchanged between the non-azeotropic refrigerant and the outside air in the second outdoor heat exchanger 18 increases. That is, even when the temperature of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 rises, the air conditioner 1 executes two-stage pressure reduction control to lower the temperature of the non-azeotropic refrigerant flowing into the second outdoor heat exchanger 18, thereby ensuring a temperature difference between the temperature of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 and the outdoor air temperature, and thereby increasing the heat exchange rate in the second outdoor heat exchanger 18. The air conditioner 1 can appropriately heat the room while suppressing frost formation, without lowering the inlet temperature of the first outdoor heat exchanger 16 below the lower limit temperature, by lowering the temperature of the non-azeotropic refrigerant flowing into the second outdoor heat exchanger 18 and suppressing a decrease in the heat exchange rate in the second outdoor heat exchanger 18. In this example, the temperature of the refrigerant before flowing into the first outdoor heat exchanger 16 is 2°C. The temperature of the refrigerant decompressed by the evaporator expansion valve 17 is 3°C. The temperature of the refrigerant flowing out of the second outdoor heat exchanger 18 is 4°C.

[0040] 10 is a Mollier diagram showing another example of the state change of the non-azeotropic refrigerant when two-stage decompression control is executed in a state where the refrigerant flow rate is high and the pressure loss of the refrigerant during the evaporation process is large. When the first outdoor heat exchanger 16 functions as an evaporator, the pressure loss (flow path resistance) when the non-azeotropic refrigerant passes through the first outdoor heat exchanger 16 becomes large when the flow rate of the non-azeotropic refrigerant circulating through the refrigerant circuit 5 is large. For example, the flow rate of the non-azeotropic refrigerant circulating through the refrigerant circuit 5 is large when the rotation speed of the compressor 11 is high. When the pressure loss when passing through the first outdoor heat exchanger 16 is large, the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 is reduced in pressure according to the magnitude of the pressure loss. Therefore, when the pressure loss when passing through the first outdoor heat exchanger 16 is large, the slope of line 53 on the Mollier diagram, which shows the relationship between the pressure and specific enthalpy of the non-azeotropic refrigerant during the evaporation process when the non-azeotropic refrigerant passes through the first outdoor heat exchanger 16, is negative, unlike when the pressure loss when passing through the first outdoor heat exchanger 16 is small (line 51 in FIG. 8). Furthermore, when the flow rate of the non-azeotropic refrigerant circulating through the refrigerant circuit 5 is increased, the slope becomes negative and larger than the slope of isotherm 52 once the flow rate exceeds a certain level. Therefore, the temperature of the refrigerant decreases during the evaporation process due to the effect of pressure reduction by the evaporator expansion valve 17. In this example, the temperature of the refrigerant before flowing into the first outdoor heat exchanger 16 is 0°C. The temperature of the refrigerant decompressed by the evaporator expansion valve 17 is -6°C. The temperature of the refrigerant flowing out of the second outdoor heat exchanger 18 is -8°C.

[0041] When two-stage pressure reduction control is executed, the non-azeotropic refrigerant decompressed by the first outdoor heat exchanger 16 is supplied to the evaporator expansion valve 17 and further decompressed by the evaporator expansion valve 17. The second outdoor heat exchanger 18 functions as an evaporator, and the pressure loss when the non-azeotropic refrigerant passes through the second outdoor heat exchanger 18 increases as the flow rate of the non-azeotropic refrigerant circulating through the refrigerant circuit 5 increases. An example of a case where the flow rate of the non-azeotropic refrigerant circulating through the refrigerant circuit 5 is high is when the compressor 11 is operating at a high rotation speed. When the pressure loss when passing through the second outdoor heat exchanger 18 is large, the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 is reduced in accordance with the magnitude of the pressure loss. For this reason, the slope of line 54, which shows the pressure of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 on the Mollier diagram, is negative, unlike when the pressure loss when passing through the second outdoor heat exchanger 18 is small (line 55 in FIG. 8 ). Furthermore, when the flow rate of the non-azeotropic refrigerant circulating through the refrigerant circuit 5 is increased, the slope of line 54 becomes negative and becomes larger than the slope of isothermal line 52 when the flow rate exceeds a certain level. Therefore, the temperature of the refrigerant decreases during the evaporation process due to the pressure loss and the pressure reduction caused by the evaporator expansion valve 17.

[0042] 11 is a th diagram showing another example of a state change of the non-azeotropic refrigerant when two-stage pressure reduction control is executed. When the pressure loss when passing through the first outdoor heat exchanger 16 is large, the temperature of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 decreases. The temperature of the non-azeotropic refrigerant supplied from the first outdoor heat exchanger 16 to the evaporator expansion valve 17 further decreases as the non-azeotropic refrigerant is decompressed by the evaporator expansion valve 17. The temperature of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 further decreases when the pressure loss when passing through the second outdoor heat exchanger 18 is large. Therefore, when the air conditioner 1 executes two-stage pressure reduction control, the temperature of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 decreases, which may promote frost formation on the second outdoor heat exchanger 18.

[0043] FIG. 12 is a Mollier diagram showing another example of a state change of the non-azeotropic refrigerant when single-stage pressure reduction control is executed. When single-stage pressure reduction control is executed, the non-azeotropic refrigerant supplied from the first outdoor heat exchanger 16 to the second outdoor heat exchanger 18 is not significantly reduced in pressure because the evaporator expansion valve 17 is fully open and the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 are connected via the bypass passage 24. Therefore, the temperature of the refrigerant decreases during the evaporation process due only to the influence of pressure loss. In this example, the temperature of the refrigerant before flowing into the first outdoor heat exchanger 16 is 0°C. The temperature of the refrigerant at the midpoint of the evaporation process, including the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18, is −2°C. The temperature of the refrigerant flowing out of the second outdoor heat exchanger 18 is −4°C.

[0044] 13 is a th diagram showing another example of a state change of the non-azeotropic refrigerant when single-stage pressure reduction control is executed. When single-stage pressure reduction control is executed, the temperature of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 does not drop significantly from the temperature of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 because the non-azeotropic refrigerant supplied from the first outdoor heat exchanger 16 to the second outdoor heat exchanger 18 is not significantly reduced in pressure. Therefore, the temperature of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 when single-stage pressure reduction control is executed is higher than the temperature of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 when two-stage pressure reduction control is executed. Therefore, even if the temperature of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 drops, the air conditioner 1 executes single-stage pressure reduction control by fully opening the evaporator expansion valve 17 and opening the bypass valve 27 when the amount of temperature increase is equal to or less than the threshold. This makes it possible to suppress a decrease in the temperature of the second outdoor heat exchanger 18, and to suppress frost formation on the second outdoor heat exchanger 18.

[0045] [Effects of the air conditioner 1 of the embodiment] The air conditioner 1 of the embodiment includes a refrigerant circuit 5, a temperature rise calculation means 46, and an opening control means 47. A main expansion valve 15, a first outdoor heat exchanger 16, an evaporator expansion valve 17, and a second outdoor heat exchanger 18 are connected in this order in the refrigerant circuit 5, and a non-azeotropic refrigerant circulates through the refrigerant circuit 5. The temperature rise calculation means 46 calculates a temperature rise, which is the amount of temperature rise that occurs while the non-azeotropic refrigerant flows through the first outdoor heat exchanger 16 when the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 function as evaporators. The opening control means 47 controls the evaporator expansion valve 17 so that the opening of the evaporator expansion valve 17 is larger when the temperature rise is smaller than a temperature difference threshold value compared to when the temperature rise is equal to or greater than the threshold value.

[0046] When the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 function as evaporators, the temperature of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 and the second outdoor heat exchanger 18 may rise and fall. In the air conditioner 1 of the embodiment, when the temperature of the non-azeotropic refrigerant passing through the first outdoor heat exchanger 16 rises, the evaporator expansion valve 17 reduces the pressure of the non-azeotropic refrigerant that has passed through the first outdoor heat exchanger 16, thereby lowering the temperature of the non-azeotropic refrigerant flowing into the second outdoor heat exchanger 18. In the air conditioner 1 of the embodiment, the temperature of the non-azeotropic refrigerant flowing into the second outdoor heat exchanger 18 decreases, thereby increasing the heat exchange amount of the second outdoor heat exchanger 18 without lowering the evaporator inlet temperature below the lower limit temperature, and suppressing frost formation. In the air conditioner 1 of the embodiment, when the temperature rise is below a threshold value, the evaporator expansion valve 17 is fully opened, thereby suppressing a decrease in the temperature of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 and suppressing frost formation on the second outdoor heat exchanger 18.

[0047] The air conditioner 1 of the embodiment further includes an inlet temperature sensor 32 that detects an inlet temperature indicating the temperature of the refrigerant flowing out of the main expansion valve 15, and an intermediate temperature sensor 33 that detects an intermediate temperature indicating the temperature of the refrigerant flowing out of the first outdoor heat exchanger 16. At this time, the amount of temperature rise is calculated based on the inlet temperature and the intermediate temperature, and is equal to a value obtained by subtracting the inlet temperature from the intermediate temperature. At this time, the air conditioner 1 of the embodiment can appropriately switch between single-stage pressure reduction control and two-stage pressure reduction control based on the inlet temperature and intermediate temperature measured by the inlet temperature sensor 32 and the intermediate temperature sensor 33, respectively, and can appropriately improve the heat exchange capacity of the second outdoor heat exchanger 18 and appropriately suppress frost formation on the second outdoor heat exchanger 18.

[0048] In the above-described embodiment, the temperature rise calculation means 46 of the air conditioner 1 calculates the temperature rise based on the inlet temperature and intermediate temperature measured by the inlet temperature sensor 32 and the intermediate temperature sensor 33, respectively. However, the temperature rise may be calculated without using the inlet temperature and intermediate temperature. For example, the temperature rise calculation means 46 may use the temperature rise estimated based on the rotation speed of the compressor 11 controlled by the rotation speed control means 45. Specifically, the rotation speed threshold value is determined in advance through experiments or the like as the rotation speed of the compressor 11 at which the temperature rise is likely to exceed the temperature difference threshold value. Even in such a case, the air conditioner 1 can appropriately switch between single-stage pressure reduction control and two-stage pressure reduction control, thereby appropriately improving the heat exchange performance of the second outdoor heat exchanger 18 and appropriately suppressing frost formation on the second outdoor heat exchanger 18.

[0049] The air conditioner 1 of this embodiment further includes a bypass passage 24, a bypass valve 27, and bypass valve opening / closing means 48. Because the evaporator expansion valve 17 creates flow path resistance even when fully open, the refrigerant that passes through the evaporator expansion valve 17 is reduced in pressure due to pressure loss. Therefore, the bypass passage 24 supplies the refrigerant that flows out of the first outdoor heat exchanger 16 to the second outdoor heat exchanger 18, bypassing the evaporator expansion valve 17. The bypass valve 27 is provided in the bypass passage 24. The bypass valve opening / closing means 48 controls the bypass valve 27 so that the refrigerant flows through the bypass passage 24 when the amount of temperature rise is smaller than the temperature difference threshold. The bypass valve opening / closing means 48 controls the bypass valve 27 so that the refrigerant does not flow through the bypass passage 24 when the amount of temperature rise is equal to or greater than the temperature difference threshold. Even when an evaporator expansion valve 17 is provided, the air conditioner 1 of the embodiment can prevent the refrigerant supplied to the second outdoor heat exchanger 18 from falling below a lower limit temperature (-1°C) and prevent frost from forming on the second outdoor heat exchanger 18.

[0050] Although the air conditioner 1 in the above-described embodiment is equipped with the bypass circuit 23, the bypass circuit 23 may be omitted. Even if the bypass circuit 23 is omitted, the air conditioner 1 can reduce the temperature of the non-azeotropic refrigerant passing through the second outdoor heat exchanger 18 by the evaporator expansion valve 17 reducing the pressure of the non-azeotropic refrigerant, thereby improving the heat exchange performance of the second outdoor heat exchanger 18. The air conditioner 1 can also suppress a decrease in the temperature of the second outdoor heat exchanger 18 and suppress frost formation on the second outdoor heat exchanger 18 by fully opening the evaporator expansion valve 17. Furthermore, although the air conditioner 1 in the above-described embodiment fully opens the evaporator expansion valve 17 when the amount of temperature rise is smaller than the temperature difference threshold, it does not necessarily have to be fully opened. Specifically, when two-stage pressure reduction control is being executed, the valve may be controlled to an opening smaller than full opening so that the temperature of the refrigerant flowing into the second outdoor heat exchanger 18 is equal to or higher than a temperature at which frost does not form (for example, -1°C).

[0051] Furthermore, the second outdoor heat exchanger 18 of the air conditioner 1 of the embodiment is formed with multiple flow paths through which a non-azeotropic refrigerant flows. The multiple flow paths are connected to the flow paths formed in the first outdoor heat exchanger 16 via a flow divider 35, and the evaporator expansion valve 17 is provided between the first outdoor heat exchanger 16 and the flow divider 35. In this case, the air conditioner 1 of the embodiment does not need to be provided with multiple evaporator expansion valves connected to the multiple flow paths of the second outdoor heat exchanger 18, respectively, and by providing a single evaporator expansion valve 17, manufacturing costs can be reduced.

[0052] Incidentally, although multiple flow paths are formed in the second outdoor heat exchanger 18 of the air conditioner 1 in the embodiment described above, only one flow path may be formed. Even when only one flow path is formed in the second outdoor heat exchanger 18, the air conditioner 1 can improve the heat exchange performance of the second outdoor heat exchanger 18 by having the evaporator expansion valve 17 depressurize the non-azeotropic refrigerant, and can suppress frost formation on the second outdoor heat exchanger 18 by fully opening the evaporator expansion valve 17.

[0053] Although the embodiments have been described above, the embodiments are not limited to the above content. Furthermore, the above-described components include those that can be easily imagined by a person skilled in the art, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the above-described components can be combined as appropriate. Furthermore, at least one of various omissions, substitutions, and modifications of the components can be made without departing from the spirit of the embodiments. [Explanation of symbols]

[0054] 1: Air conditioner 5: Refrigerant circuit 14: Indoor heat exchanger 15: Main expansion valve 16: 1st outdoor heat exchanger 17: Evaporator expansion valve 18:Second outdoor heat exchanger 24: Bypass road 27: Bypass valve 32: Inlet temperature sensor 33: Intermediate temperature sensor 35: Flow divider 46: Temperature rise calculation means 47: Opening control means 48: Bypass valve opening / closing means

Claims

1. a refrigerant circuit in which a first expansion valve, a first heat exchanger, a second expansion valve, and a second heat exchanger are connected in sequence, and in which a non-azeotropic refrigerant circulates; a temperature rise calculation means for calculating a temperature rise amount of the non-azeotropic refrigerant while the non-azeotropic refrigerant flows through the first heat exchanger when the first heat exchanger and the second heat exchanger function as evaporators; an opening control means for controlling the second expansion valve so that, when the increase amount is smaller than a predetermined threshold value, the opening amount of the second expansion valve is larger than when the increase amount is equal to or larger than the threshold value; An air conditioner equipped with the above.

2. an intermediate temperature sensor for detecting an intermediate temperature, which is the temperature of the refrigerant flowing out from the first heat exchanger; a frost limit temperature, which is a temperature of the refrigerant flowing into the second heat exchanger and is a lower limit value at which frost does not occur; The amount of increase is equal to the value obtained by subtracting the frost limit temperature from the intermediate temperature. The air conditioner according to claim 1.

3. The temperature rise amount calculation means calculates the temperature rise amount based on the rotation speed of a compressor that compresses the non-azeotropic refrigerant. The air conditioner according to claim 1.

4. a bypass passage for supplying the refrigerant flowing out of the first heat exchanger to the second heat exchanger while bypassing the second expansion valve; a bypass valve provided in the bypass passage; a bypass valve control means for controlling the bypass valve so that the refrigerant flows through the bypass passage when the amount of increase is smaller than the threshold value, and controlling the bypass valve so that the refrigerant does not flow through the bypass passage when the amount of increase is equal to or greater than the threshold value; The air conditioner according to any one of claims 1 to 3, further comprising:

5. The second heat exchanger is formed with a plurality of flow paths through which a refrigerant flows, the plurality of flow paths are connected to a flow path formed in the first heat exchanger via a flow divider; The second expansion valve is provided between the first heat exchanger and the flow divider. The air conditioner according to any one of claims 1 to 4.

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