Transport refrigeration apparatus and transport container

Abstract

To operate a refrigerating device for transportation performing a two stage compression refrigerating cycle in an appropriate state.SOLUTION: The transport refrigeration system (10) performs a cooling operation for cooling inside air. In the refrigerant circuit (30), the refrigerant having passed through the radiator (33) is distributed to the second passage (62) and the third passage (63). The low-stage compressor (31) the refrigerant having passed through the evaporator (34) of the third passage (63). The high-stage compressor (32) the refrigerant discharged from the low-stage compressor (31) and the refrigerant flowing through the second passage (62). The displacement ratio (= low-stage displacement / high-stage displacement) during the cooling operation is 0.67 or more and 3.42 or less.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to transport refrigeration units and transport containers. [Background technology]

[0002] Patent Document 1 discloses a transport refrigeration unit that cools the interior of a transport container or the like. This transport refrigeration unit is equipped with a compression device including a first compression stage (low stage) and a second compression stage (high stage), and performs a so-called two-stage compression refrigeration cycle. Furthermore, paragraph 0022 of Patent Document 1 states that the first compression stage and the second compression stage may each be configured with an independent compressor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2011 / 091014 Summary of the Invention [Problem to be solved by the invention]

[0004] In the transport refrigeration system of Patent Document 1, if the first compression stage (low stage) and the second compression stage (high stage) are each configured with independent compressors, it is possible to individually adjust the displacement of the low-stage compressor that constitutes the first compression stage and the high-stage compressor that constitutes the second compression stage. The displacement of a compressor is calculated by multiplying the displacement volume of the compressor by the rotational speed of the compressor.

[0005] However, with regard to transport refrigeration systems, the desirable ranges for controlling the displacement of the low-stage compressor and the displacement of the high-stage compressor have not been considered, which has led to the risk that transport refrigeration systems using a two-stage compression refrigeration cycle may not be able to operate in an appropriate state.

[0006] An object of the present disclosure is to operate a transport refrigeration system that uses a two-stage compression refrigeration cycle in an appropriate state. [Means for solving the problem]

[0007] A first aspect of the present disclosure is directed to a transport refrigeration system (10) that includes a refrigerant circuit (30) that circulates carbon dioxide as a refrigerant to perform a refrigeration cycle, and that performs a cooling operation to cool the air inside a transport container (1). The refrigerant circuit (30) includes a radiator (33) that exchanges heat between the refrigerant and outside air, a first passage (61) through which all of the refrigerant flowing out from the radiator (33) flows, a second passage (62) through which part of the refrigerant that has passed through the first passage (61) flows, a third passage (63) through which the remainder of the refrigerant that has passed through the first passage (61) flows, an evaporator (34) that is provided in the third passage (63) and exchanges heat between the refrigerant and the inside air, a low-stage compressor (31) that draws in the refrigerant that has flowed out from the evaporator (34), and a high-stage compressor (32) that draws in the refrigerant discharged from the low-stage compressor (31) and the refrigerant flowing through the second passage (62). In this embodiment, the volume of the refrigerant sucked into the low-stage compressor (31) per unit time is the low-stage displacement, the volume of the refrigerant sucked into the high-stage compressor (32) per unit time is the high-stage displacement, the value obtained by dividing the low-stage displacement by the high-stage displacement is the displacement ratio, and the displacement ratio during the cooling operation is 0.67 or more and 3.42 or less.

[0008] In the first aspect, the transport refrigeration unit (10) performs a cooling operation. In the cooling operation, the refrigerant circuit (30) performs a refrigeration cycle, and the evaporator (34) cools the air inside the compartment. In the refrigerant circuit (30) during the cooling operation, the refrigerant passing through the first passage (61) is distributed to the second passage (62) and the third passage (63). The low-stage compressor (31) draws the refrigerant that has flowed into the third passage (63) and passed through the evaporator (34), compresses the drawn refrigerant, and discharges it. The high-stage compressor (32) draws the refrigerant discharged by the low-stage compressor (31) and the refrigerant flowing through the second passage (62), compresses the drawn refrigerant, and discharges it. The displacement ratio during the cooling operation is equal to or greater than 0.67 and equal to or less than 3.42. Therefore, the transport refrigeration unit (10) can be operated in an appropriate state during the cooling operation.

[0009] A second aspect of the present disclosure is the first aspect, further comprising a controller (80) that controls the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32) individually, and the controller (80) sets the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) to values ​​such that the displacement ratio during the cooling operation is 0.67 or more and 3.42 or less.

[0010] In the second aspect, the controller (80) separately controls the rotation speed of the low-stage compressor (31) and the rotation speed of the high-stage compressor (32). In the transport refrigeration system (10), the controller (80) separately controls the rotation speed of the low-stage compressor (31) and the rotation speed of the high-stage compressor (32), so that the displacement ratio during the cooling operation is equal to or greater than 0.67 and equal to or less than 3.42.

[0011] A third aspect of the present disclosure is the first aspect, further comprising a controller (80) that controls the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32) individually, and the controller (80) controls the rotational speed of the low-stage compressor (31) based on a physical quantity that correlates with the evaporation temperature of the refrigerant in the evaporator (34), and sets the rotational speed of the high-stage compressor (32) to a value that makes the displacement ratio equal to or greater than 0.67 and equal to or less than 3.42.

[0012] In a third aspect, the controller (80) controls the rotation speed of the low-stage compressor (31) and the rotation speed of the high-stage compressor (32) separately. The controller (80) controls the rotation speed of the low-stage compressor (31) based on a predetermined physical quantity. The controller (80) also sets the rotation speed of the high-stage compressor (32) to a value that results in a displacement ratio of 0.67 to 3.42 at the current rotation speed of the low-stage compressor (31).

[0013] A fourth aspect of the present disclosure is any one of the first to third aspects, wherein the low-stage compressor (31) and the high-stage compressor (32) have the same volume of refrigerant drawn in per rotation, the rotational speed ratio is a value obtained by dividing the rotational speed of the low-stage compressor (31) by the rotational speed of the high-stage compressor (32), and the rotational speed ratio during the cooling operation is 0.67 or more and 3.42 or less.

[0014] In the cooling operation of the transport refrigeration unit (10) of the fourth aspect, the rotational speed ratio is 0.67 or more and 3.42 or less. The low-stage displacement is calculated by multiplying the "volume of refrigerant drawn into the low-stage compressor (31) per rotation" by the "rotational speed of the low-stage compressor (31)." The high-stage displacement is calculated by multiplying the "volume of refrigerant drawn into the high-stage compressor (32) per rotation" by the "rotational speed of the high-stage compressor (32)." Therefore, when the rotational speed ratio is 0.67 or more and 3.42 or less, the displacement ratio is 0.67 or more and 3.42 or less.

[0015] A fifth aspect of the present disclosure is any one of the first to fourth aspects, further comprising a heat exchanger (46) that is provided in the third passage (63) upstream of the evaporator (34) and that cools the refrigerant flowing through the third passage (63) by exchanging heat with the refrigerant flowing through the second passage (62).

[0016] In the fifth embodiment, the refrigerant flowing through the third passage (63) is cooled in the heat exchanger (46) and then flows into the evaporator (34).

[0017] A sixth aspect of the present disclosure is any one of the first to fourth aspects, further comprising: a gas-liquid separator (42) that separates the refrigerant that has passed through the first passage (61) into a gas refrigerant and a liquid refrigerant, sends the gas refrigerant to the second passage (62), and sends the liquid refrigerant to the third passage (63); and a heat exchanger (41) that cools the refrigerant flowing through the first passage (61) by exchanging heat with the refrigerant flowing through the second passage (62).

[0018] In the sixth aspect, the refrigerant flowing through the first passage (61) is cooled in the heat exchanger (41) and then flows into the gas-liquid separator (42). The gas-liquid separator (42) separates the refrigerant into gas refrigerant and liquid refrigerant. The gas refrigerant in the gas-liquid separator (42) flows into the second passage (62). The liquid refrigerant in the gas-liquid separator (42) flows into the third passage (63).

[0019] A seventh aspect of the present disclosure is the sixth aspect, wherein the displacement ratio during the cooling operation is equal to or greater than 0.7 and equal to or less than 1.93.

[0020] In the cooling operation of the transport refrigeration unit (10) of the seventh embodiment, the displacement ratio is equal to or greater than 0.7 and equal to or less than 1.93.

[0021] An eighth aspect of the present disclosure is any one of the first to seventh aspects, wherein the refrigerant circuit (30) selectively performs a two-stage compression operation in which both the low-stage compressor (31) and the high-stage compressor (32) are operated to perform a refrigeration cycle, and a single-stage compression operation in which one of the low-stage compressor (31) and the high-stage compressor (32) is operated and the other is stopped to perform a refrigeration cycle.

[0022] The refrigerant circuit (30) of the eighth aspect selectively performs two-stage compression and single-stage compression. In the cooling operation of the transport refrigeration system (10), the displacement ratio when the refrigerant circuit (30) is performing the two-stage compression operation falls within a predetermined range.

[0023] A ninth aspect of the present disclosure is a transport container (1) including a transport refrigeration unit (10) according to any one of the first to eighth aspects, and a container body (2) to which the transport refrigeration unit (10) is attached and which forms an interior space (5) for accommodating cargo.

[0024] In the ninth aspect, a transport container (1) is formed by a transport refrigeration unit (10) and a container body (2). [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a perspective view of a transport refrigeration unit according to a first embodiment. [Figure 2] FIG. 2 is a schematic vertical cross-sectional view of the transport refrigeration unit of the first embodiment and a transport container equipped with the transport refrigeration unit. [Figure 3] FIG. 3 is a piping diagram showing the refrigerant circuit of the transport refrigeration system of the first embodiment. [Figure 4] FIG. 4 is a Mollier diagram (pressure-enthalpy diagram) showing the refrigeration cycle performed by the refrigerant circuit of the first embodiment. [Figure 5] FIG. 5 is a block diagram showing the configuration of a controller of the transport refrigeration system of the first embodiment. [Figure 6] FIG. 6 is a flowchart showing the operation performed by the controller of the first embodiment. [Figure 7] FIG. 7 is a graph showing the relationship between the input to the low-stage compressor and the high-stage compressor and the pressure ratio. [Figure 8] FIG. 8 is a diagram showing the operable range of the transport refrigeration unit of the first embodiment. [Figure 9] FIG. 9 is a piping diagram showing a refrigerant circuit of a transport refrigeration system according to the second embodiment. [Figure 10] FIG. 10 is a Mollier diagram (pressure-enthalpy diagram) showing the refrigeration cycle performed by the refrigerant circuit of the second embodiment. [Figure 11] FIG. 11 is a diagram showing the operable range of the transport refrigeration unit of the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0026] First Embodiment A first embodiment will be described. This embodiment is a transport container (1) equipped with a transport refrigeration unit (10).

[0027] -Shipping container- As shown in Figure 1, the transport container 1 includes a container body 2 and a transport refrigeration unit 10. The transport container 1 is a reefer container capable of controlling the temperature inside.

[0028] The transport container 1 of this embodiment is primarily used for marine transportation. The transport container 1 is loaded onto a ship or the like for transport. However, the use of the transport container 1 is not limited to marine transportation. The transport container 1 may also be used for land transportation. In this case, the transport container 1 is transported by automobile such as a truck, or by rail.

[0029] -Container body- As shown in FIG. 2, the container body (2) is formed in the shape of a hollow box. The container body (2) is formed horizontally long. An opening is formed at one longitudinal end of the container body (2). The opening of the container body (2) is closed by a transport refrigeration unit (10). The container body (2) forms an interior space (5) for storing cargo.

[0030] -Transportation refrigeration equipment- As shown in Fig. 2, the transport refrigeration unit (10) is attached to an opening of the container body (2). The transport refrigeration unit (10) includes a casing (11), a refrigerant circuit (30), and a controller (80). The transport refrigeration unit (10) adjusts the temperature of the air in the interior space (5).

[0031] <Casing> The casing (11) includes a partition wall (12) and a partition plate (15).

[0032] An internal flow path (20) is formed inside the partition wall (12). An external chamber (23) is formed outside the partition wall (12). The internal flow path (20) and the external chamber (23) are separated by the partition wall (12).

[0033] The partition wall (12) includes an outer wall (13) and an inner wall (14). The outer wall (13) is located outside the container body (2). The inner wall (14) is located inside the container body (2).

[0034] The exterior wall (13) closes the opening of the container body (2). The exterior wall (13) is attached to the periphery of the opening of the container body (2). The lower part of the exterior wall (13) bulges toward the inside of the container body (2). The exterior chamber (23) is formed by the lower part of the exterior wall (13).

[0035] The interior wall (14) faces the exterior wall (13). The interior wall (14) has a shape that conforms to the exterior wall (13). The interior wall (14) is disposed at a distance from the exterior wall (13). A heat insulating material (16) is provided between the interior wall (14) and the exterior wall (13).

[0036] The partition plate (15) is disposed inside the container body (2) relative to the interior wall (14). An internal flow path (20) is formed between the partition wall (12) and the partition plate (15). An air inlet (21) is formed between the upper end of the partition plate (15) and the top plate of the container body (2). An air outlet (22) is formed between the lower end of the partition plate (15) and the lower end of the partition wall (12). The internal flow path (20) is formed from the air inlet (21) to the air outlet (22).

[0037] <Refrigerant circuit> The refrigerant circuit (30) is a closed circuit filled with a refrigerant. The refrigerant circuit (30) circulates the refrigerant to perform a vapor compression refrigeration cycle. The refrigerant circuit (30) includes an external heat exchanger (33) and an internal heat exchanger (34). The refrigerant circuit (30) will be described in detail later.

[0038] The external heat exchanger (33) is disposed in an upper portion of the external chamber (23). The external heat exchanger (33) is a fin-and-tube heat exchanger that exchanges heat between the refrigerant and the external air. The external heat exchanger (33) has a generally rectangular cylindrical shape. The internal heat exchanger (34) is disposed in the internal flow path (20). The internal heat exchanger (34) is a fin-and-tube heat exchanger that exchanges heat between the refrigerant and the internal air.

[0039] <External fan> The transport refrigeration unit (10) includes one external fan (26). The external fan (26) is a propeller fan. The external fan (26) is disposed in the external chamber (23). The external fan (26) is disposed inside a cylindrical external heat exchanger (33). The external fan (26) sends external air to the external heat exchanger (33).

[0040] <Interior fan> The transport refrigeration unit (10) includes an internal fan (27). The internal fan (27) is a propeller fan. The internal fan (27) is disposed in the internal flow path (20). The internal fan (27) is disposed above the internal heat exchanger (34). The internal fan (27) sends internal air to the internal heat exchanger (34).

[0041] <Electrical equipment box> As shown in Fig. 1, the transport refrigeration unit (10) has an electrical component box (28). The electrical component box (28) is disposed in an upper portion of the exterior chamber (23). Electrical components such as an inverter board and a control board are housed inside the electrical component box (28).

[0042] - Cooling operation of transport refrigeration equipment - The transport refrigeration unit (10) performs a cooling operation to cool the air inside the unit (10). Here, an outline of the cooling operation of the transport refrigeration unit (10) will be described.

[0043] In the cooling operation of the transport refrigeration unit (10), a refrigeration cycle is performed in the refrigerant circuit (30). In the refrigerant circuit (30), the external heat exchanger (33) functions as a radiator, and the internal heat exchanger (34) functions as an evaporator. In the external heat exchanger (33), the refrigerant radiates heat to the external air. In the internal heat exchanger (34), the refrigerant absorbs heat from the internal air and evaporates. The internal heat exchanger (34) cools the internal air.

[0044] The interior air of the container body (2) circulates between the interior space (5) and the interior flow path (20). The interior air of the interior space (5) flows into the interior flow path (20) through the air inlet (21). The interior air flowing through the interior flow path (20) is cooled by the interior heat exchanger (34). The interior air cooled by the interior heat exchanger (34) is supplied to the interior space (5) through the air outlet (22). In this way, during the cooling operation of the transport refrigeration unit (10), the interior air of the interior space (5) is cooled, and the air temperature of the interior space (5) is maintained at a predetermined target temperature.

[0045] -Refrigerant circuit- 3, the refrigerant circuit (30) is a closed circuit filled with a refrigerant. The refrigerant filled in the refrigerant circuit (30) of this embodiment is carbon dioxide.

[0046] The refrigerant circuit (30) includes a low-stage compressor (31), a high-stage compressor (32), an external heat exchanger (33), an internal heat exchanger (41), a gas-liquid separator (42), a first electric valve (51), a second electric valve (52), and a third electric valve (53).

[0047] The discharge pipe of the low-stage compressor (31) is connected to a suction pipe of the high-stage compressor (32). The discharge pipe of the high-stage compressor (32) is connected to one end of the external heat exchanger (33). The other end of the external heat exchanger (33) is connected to an inlet of the gas-liquid separator (42) via an internal heat exchanger (41) and a first electric valve (51). The gas outlet of the gas-liquid separator (42) is connected to a suction pipe of the high-stage compressor (32) via a second electric valve (52) and the internal heat exchanger (41). The liquid outlet of the gas-liquid separator (42) is connected to one end of the internal heat exchanger (34) via a third electric valve (53). The other end of the internal heat exchanger (34) is connected to a suction pipe of the low-stage compressor (31).

[0048] <1st pipe, 2nd pipe, 3rd pipe> In the refrigerant circuit (30), a pipe connecting the other end of the external heat exchanger (33) and the inlet of the gas-liquid separator (42) is a first pipe (61). The first pipe (61) forms a first passage through which all of the refrigerant flowing out of the internal heat exchanger (34) flows. A first motor-operated valve (51) is disposed in the first pipe (61) downstream of the internal heat exchanger (41).

[0049] In the refrigerant circuit (30), the pipe connecting the gas outlet of the gas-liquid separator (42) and the suction pipe of the high-stage compressor (32) is the second pipe (62). The second pipe (62) forms a second passage through which a part of the refrigerant that has passed through the first pipe (61) flows. A second motor-operated valve (52) is disposed in the second pipe (62) upstream of the internal heat exchanger (41).

[0050] In the refrigerant circuit (30), the pipe connecting the liquid outlet of the gas-liquid separator (42) and the suction pipe of the low-stage compressor (31) is the third pipe (63). The third pipe (63) forms a third passage through which the remainder of the refrigerant that has passed through the first pipe (61) flows. A third motor-operated valve (53) is disposed in the third pipe (63) upstream of the internal heat exchanger (34).

[0051] <Low-stage compressor, high-stage compressor> The low-stage compressor 31 and the high-stage compressor 32 are both hermetic scroll compressors. The low-stage compressor 31 and the high-stage compressor 32 have the same displacement volume. The displacement volume is the volume of fluid drawn into the compressor during one rotation.

[0052] The low-stage compressor (31) and the high-stage compressor (32) are not limited to scroll compressors, but may be any compressors formed by positive displacement fluid machines.

[0053] <External heat exchanger, internal heat exchanger> As described above, each of the external heat exchanger (33) and the internal heat exchanger (34) is a fin-and-tube heat exchanger that exchanges heat between the refrigerant and air. The external heat exchanger (33) exchanges heat between the refrigerant and external air (outside air). The internal heat exchanger (34) exchanges heat between the refrigerant and internal air.

[0054] <Internal heat exchanger> The internal heat exchanger (41) is a heat exchanger that exchanges heat between refrigerants. In this embodiment, the internal heat exchanger (41) is a plate-type heat exchanger. The internal heat exchanger (41) has a first flow path (41a) and a second flow path (41b). The first flow path (41a) of the internal heat exchanger (41) is connected to the first pipe (61). The second flow path (41b) of the internal heat exchanger (41) is connected to the second pipe (62). The internal heat exchanger (41) exchanges heat between the refrigerant flowing through the first flow path (41a) and the refrigerant flowing through the second flow path (41b).

[0055] <Gas-liquid separator> The gas-liquid separator (42) is a container-shaped member that separates the refrigerant in a two-phase gas-liquid state that flows in through an inlet into a liquid refrigerant and a gas refrigerant. In the gas-liquid separator (42), the liquid refrigerant accumulates in the lower part of the gas-liquid separator (42) and flows out through a liquid outlet formed in the bottom part of the gas-liquid separator (42). In the gas-liquid separator (42), the gas refrigerant accumulates in the upper part of the gas-liquid separator (42) and flows out through a gas outlet formed in the upper part of the gas-liquid separator (42).

[0056] <Motor-operated valve> The first motor-operated valve (51), the second motor-operated valve (52), and the third motor-operated valve (53) are each a so-called electronic expansion valve. The electronic expansion valve is a motor-operated valve with a variable opening. The electronic expansion valve includes a valve element and a stepping motor that drives the valve element. When the valve element is moved by the stepping motor, the opening of the electronic expansion valve changes.

[0057] (Bypass pipe, check valve) The refrigerant circuit (30) includes a low-stage bypass pipe (66) and a high-stage bypass pipe (67).

[0058] The low-stage bypass pipe (66) has one end connected to a suction pipe of the low-stage compressor (31) and the other end connected to a discharge pipe of the low-stage compressor (31). The low-stage bypass pipe (66) is provided with a check valve (66a). The check valve (66a) allows refrigerant to flow from one end of the low-stage bypass pipe (66) to the other end and prevents refrigerant from flowing in the opposite direction.

[0059] The high-stage bypass pipe (67) has one end connected to a suction pipe of the high-stage compressor (32) and the other end connected to a discharge pipe of the high-stage compressor (32). The high-stage bypass pipe (67) is provided with a check valve (67a). The check valve (67a) allows refrigerant to flow from one end of the high-stage bypass pipe (67) to the other end and prevents refrigerant from flowing in the opposite direction.

[0060] Sensor The refrigerant circuit (30) is provided with a low pressure sensor (71), an intermediate pressure sensor (72), and a high pressure sensor (73). Although not shown, the refrigerant circuit (30) is also provided with a plurality of temperature sensors.

[0061] The low-pressure sensor (71) is connected to a suction pipe of the low-stage compressor (31) and measures the pressure of the refrigerant sucked into the low-stage compressor (31). The intermediate-pressure sensor (72) is connected to a suction pipe of the high-stage compressor (32) and measures the pressure of the refrigerant sucked into the high-stage compressor (32). The high-pressure sensor (73) is connected to a discharge pipe of the high-stage compressor (32) and measures the pressure of the refrigerant discharged from the high-stage compressor (32).

[0062] - Refrigerant circuit operation - The refrigerant circuit (30) selectively performs a two-stage compression operation that performs a two-stage compression refrigeration cycle and a single-stage compression operation that performs a single-stage compression refrigeration cycle.

[0063] <Two-stage compression operation> The two-stage compression operation is performed when the difference between the set temperature inside the refrigerator and the outside air temperature is relatively large.

[0064] In the two-stage compression operation of the refrigerant circuit (30), both the low-stage compressor (31) and the high-stage compressor (32) operate. The low-stage compressor (31) draws in refrigerant evaporated in the internal heat exchanger (34). The low-stage compressor (31) compresses the drawn in refrigerant and discharges the compressed refrigerant. The high-stage compressor (32) draws in the refrigerant discharged from the low-stage compressor (31) and the refrigerant that has passed through the second pipe (62). The high-stage compressor (32) compresses the drawn in refrigerant and discharges it. The refrigerant discharged from the high-stage compressor (32) flows into the external heat exchanger (33).

[0065] <Single-stage compression operation> The single-stage compression operation is performed when the difference between the set temperature inside the refrigerator and the outside air temperature is relatively small.

[0066] In the single-stage compression mode of the refrigerant circuit (30), the low-stage compressor (31) is inactive and the high-stage compressor (32) is active. The high-stage compressor (32) draws the refrigerant evaporated in the internal heat exchanger (34) through the low-stage bypass pipe (66). The high-stage compressor (32) compresses the drawn refrigerant and discharges it. The refrigerant discharged from the high-stage compressor (32) flows into the external heat exchanger (33).

[0067] In the single-stage compression operation of the refrigerant circuit (30), the high-stage compressor (32) may be stopped and the low-stage compressor (31) may be operated. In this case, the refrigerant discharged from the low-stage compressor (31) flows into the external heat exchanger (33) through the high-stage bypass pipe (67).

[0068] -Two-stage compression refrigeration cycle- The two-stage compression refrigeration cycle performed by the refrigerant circuit (30) will be described in detail with reference to Fig. 4. Fig. 4 is a Mollier diagram (pressure-enthalpy diagram) showing the refrigeration cycle. In the refrigeration cycle shown in Fig. 4, the high pressure of the refrigeration cycle is higher than the critical pressure (7.2 MPa) of the refrigerant (carbon dioxide), and the intermediate pressure of the refrigeration cycle is lower than the critical pressure of the refrigerant (carbon dioxide).

[0069] The refrigerant in the state of point A is compressed by the low-stage compressor (31) to reach the state of point B. In the refrigerant circuit (30), the refrigerant in the state of point B discharged from the low-stage compressor (31) and the refrigerant in the state of point J that has passed through the second pipe (62) join together to reach the refrigerant in the state of point C. The refrigerant in the state of point C is compressed by the high-stage compressor (32) to reach the state of point D.

[0070] The refrigerant in the state of Point D dissipates heat to the outside air (outside air) in the external heat exchanger (33) and reaches the state of Point E. The refrigerant in the state of Point E passes through the first pipe (61) and flows into the first flow path (41a) of the internal heat exchanger (41), where it is cooled by the refrigerant flowing through the second flow path (41b) and reaches the state of Point F. The refrigerant in the state of Point F is decompressed by the first electric valve (51) and reaches the state of Point G (a two-phase gas-liquid state). The refrigerant in the state of Point G flows into the gas-liquid separator (42) and is separated into a refrigerant in the state of Point H (saturated liquid refrigerant) and a refrigerant in the state of Point I (saturated gas refrigerant).

[0071] The refrigerant in the state of point H flows from the gas-liquid separator (42) into the third pipe (63), where it is decompressed by the third motor-operated valve (53) and reaches the state of point K. The refrigerant in the state of point K absorbs heat from the air in the internal heat exchanger (34) and evaporates, reaching the state of point A.

[0072] The refrigerant in the state of point I flows from the gas-liquid separator (42) into the second pipe (62). The refrigerant in the state of point I flowing through the second pipe (62) passes through the second electric valve (52), flows into the second flow path (41b) of the internal heat exchanger (41), absorbs heat from the refrigerant flowing through the first flow path (41a), and reaches the state of point J.

[0073] -Controller- As shown in Fig. 5, the controller (80) includes a microcomputer (81) and a memory device (82). The memory device (82) is a semiconductor memory. The memory device (82) stores software for operating the microcomputer (81). The controller (80) is housed in the electrical component box (28).

[0074] The controller (80) receives measurement values ​​from the high-pressure sensor (73), the intermediate-pressure sensor (72), and the low-pressure sensor (71). Measurement values ​​from a temperature sensor provided in the transport refrigeration system (10) are also input to the controller (80). The controller (80) uses the input sensor measurement values ​​to individually control the rotation speeds of the low-stage compressor (31) and the high-stage compressor (32). The controller (80) also individually controls the openings of the first motor-operated valve (51), the second motor-operated valve (52), and the third motor-operated valve (53).

[0075] <Low-stage compressor control> The controller (80) controls the rotation speed of the low-stage compressor (31) based on the measurement value of the low-pressure sensor (71).

[0076] Specifically, the controller (80) controls the rotation speed of the low-stage compressor (31) so that the measurement value of the low-pressure sensor (71) becomes equal to the target low pressure. When the measurement value of the low-pressure sensor (71) is lower than the target low pressure, the controller (80) reduces the rotation speed of the low-stage compressor (31). When the measurement value of the low-pressure sensor (71) is higher than the target low pressure, the controller (80) increases the rotation speed of the low-stage compressor (31).

[0077] The controller (80) determines the target low pressure based on a set temperature Ts, which is a set value of the inside temperature. Specifically, the controller (80) sets the target low pressure to a saturation pressure corresponding to a temperature (Ts-ΔT) that is lower than the set temperature Ts by a predetermined value.

[0078] As described above, the low-pressure sensor (71) measures the pressure of the refrigerant sucked into the low-stage compressor (31). The pressure of the refrigerant sucked into the low-stage compressor (31) is substantially equal to the pressure (evaporation pressure) of the refrigerant in the internal heat exchanger (34) functioning as an evaporator. The evaporation pressure of the refrigerant is a physical quantity correlated with the evaporation temperature of the refrigerant. Therefore, the controller (80) controls the rotation speed of the low-stage compressor (31) based on the physical quantity correlated with the evaporation temperature of the refrigerant in the internal heat exchanger (34).

[0079] <High-stage compressor control> The controller (80) controls the rotation speed of the high-stage compressor (32) based on the measurement value of the intermediate pressure sensor (72). The control operation of the controller (80) for the high-stage compressor (32) will be described with reference to the flowchart of FIG.

[0080] In the process of step ST1, the controller (80) determines whether the temperature of the internal space (5) is stable. Specifically, the controller (80) determines whether the condition that "the temperature of the discharged air remains within the target temperature range for a predetermined time (e.g., 30 minutes)" is met. If this condition is met, the controller (80) determines that the temperature of the internal space (5) is stable, and performs the process of step ST2. If this condition is not met, the controller (80) determines that the temperature of the internal space (5) is not stable, and performs the process of step ST8.

[0081] The blown air temperature is the temperature of the air blown out from the air outlet (22) of the transport refrigeration unit (10). The target temperature range is, for example, the set temperature Ts±0.5°C.

[0082] In the process of step ST2, the controller (80) acquires the measurement value of the high-pressure sensor (73) and the measurement value of the low-pressure sensor (71). The measurement value of the high-pressure sensor (73) is the high pressure HP of the refrigeration cycle. The measurement value of the low-pressure sensor (71) is the low pressure LP of the refrigeration cycle. After this process is completed, the controller (80) performs the process of step ST3.

[0083] In the process of step ST3, the controller (80) determines whether the condition "HP / LP>6" is met. If this condition is met, the controller (80) performs the process of step ST4. If this condition is not met, the controller (80) performs the process of step ST5.

[0084] In the process of step ST4, the controller (80) sets the value of the intermediate pressure MP of the refrigeration cycle at which the pressure ratio RP is 0.7 as the target intermediate pressure. The pressure ratio RP is a value calculated by the following equation 1. The controller (80) calculates the intermediate pressure MP at which the pressure ratio RP is 0.7 using equation 1 and the high pressure HP and low pressure LP obtained in the process of step ST2. RP = (MP - LP) / (HP - LP) (Equation 1)

[0085] In the process of step ST5, the controller (80) determines whether the condition "HP / LP>2" is met. If this condition is met, the controller (80) performs the process of step ST6. If this condition is not met, the controller (80) performs the process of step ST7.

[0086] In the process of step ST6, the controller (80) sets the value of the intermediate pressure MP of the refrigeration cycle at which the pressure ratio RP is 0.6 as the target intermediate pressure. The controller (80) calculates the intermediate pressure MP at which the pressure ratio RP is 0.6 by using Equation 1 and the high pressure HP and the low pressure LP acquired in the process of step ST2.

[0087] In the process of step ST7, the controller (80) sets the value of the intermediate pressure MP of the refrigeration cycle at which the pressure ratio RP is 0.5 as the target intermediate pressure. The controller (80) calculates the intermediate pressure MP at which the pressure ratio RP is 0.5 by using Equation 1 and the high pressure HP and the low pressure LP acquired in the process of step ST2.

[0088] In the process of step ST8, the controller (80) sets the target intermediate pressure to a predetermined pressure (for example, 6.5 MPa) that is slightly lower than the critical pressure (7.2 MPa) of carbon dioxide, which is the refrigerant.

[0089] After the process of step ST4, step ST6, step ST7, or step ST8 is completed, the controller (80) performs the process of step ST9. In the process of step ST9, the controller (80) acquires a measurement value of the intermediate pressure sensor (72). The measurement value of the intermediate pressure sensor (72) is the intermediate pressure MP of the refrigeration cycle. After the process of step ST9 is completed, the controller (80) performs the process of step ST10.

[0090] In the process of step ST10, the controller (80) controls the rotation speed of the high-stage compressor (32) based on the measurement value of the intermediate pressure sensor (72) and the target intermediate pressure set in the process of step ST4, step ST6, step ST7, or step ST8. When the measurement value of the intermediate pressure sensor (72) is lower than the target intermediate pressure, the controller (80) reduces the rotation speed of the high-stage compressor (32). When the measurement value of the intermediate pressure sensor (72) is higher than the target intermediate pressure, the controller (80) increases the rotation speed of the high-stage compressor (32).

[0091] <Control of motor-operated valve> As described above, the controller (80) controls the opening degrees of the first electric valve (51), the second electric valve (52), and the third electric valve (53) individually.

[0092] The controller (80) controls the opening of the first motor-operated valve (51) so that the high pressure HP of the refrigeration cycle becomes a target high pressure when the high pressure HP of the refrigeration cycle is higher than the critical pressure of the refrigerant. When the high pressure HP of the refrigeration cycle is higher than the target high pressure, the controller (80) increases the opening of the first motor-operated valve (51). When the high pressure HP of the refrigeration cycle is lower than the target high pressure, the controller (80) decreases the opening of the first motor-operated valve (51).

[0093] The controller (80) adjusts the aperture of the second electric valve (52) so that the degree of superheat of the refrigerant at the outlet of the second flow path (41b) of the internal heat exchanger (41) becomes equal to a first target degree of superheat. When the degree of superheat of the refrigerant at the outlet of the second flow path (41b) of the internal heat exchanger (41) is higher than the first target degree of superheat, the controller (80) increases the aperture of the second electric valve (52). When the degree of superheat of the refrigerant at the outlet of the second flow path (41b) of the internal heat exchanger (41) is lower than the first target degree of superheat, the controller (80) decreases the aperture of the second electric valve (52).

[0094] The controller (80) adjusts the opening degree of the third motor-operated valve (53) so that the degree of superheat of the refrigerant at the outlet of the internal heat exchanger (34) becomes equal to a second target degree of superheat. When the degree of superheat of the refrigerant at the outlet of the internal heat exchanger (34) is higher than the second target degree of superheat, the controller (80) increases the opening degree of the third motor-operated valve (53). When the degree of superheat of the refrigerant at the outlet of the internal heat exchanger (34) is lower than the second target degree of superheat, the controller (80) decreases the opening degree of the third motor-operated valve (53).

[0095] -Displacement ratio- In the transport refrigeration system (10) of this embodiment, the controller (80) controls the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32). During the cooling operation of the transport refrigeration system (10), the displacement ratio is 0.7 to 1.93. Here, the reason why the displacement ratio during the cooling operation is desirably 0.7 to 1.93 will be explained.

[0096] <Identifying the refrigeration cycle> When the set value of the inside temperature (set temperature Ts), the temperature of the outside air (outside air temperature Ta), and the pressure ratio RP are specified, the refrigeration cycle performed by the refrigerant circuit (30) is specified. As described above, the pressure ratio RP is defined by Equation 1.

[0097] Specifically, when the set value of the temperature inside the cabinet (set temperature Ts), the temperature of the air outside the cabinet (outside air temperature Ta), and the pressure ratio RP are specified, points A to K that specify the refrigeration cycle shown in Figure 4 are specified as shown below.

[0098] (High pressure HP, medium pressure MP, low pressure LP) In the cooling operation, the transport refrigeration unit (10) is controlled so that the temperature of the air blown out from the air outlet (22) becomes a set temperature Ts. The evaporation temperature Te of the refrigerant in the internal heat exchanger (34) becomes a temperature (Ts-ΔT) that is lower than the set temperature Ts by a predetermined value. Therefore, the low pressure LP of the refrigeration cycle is a saturation pressure corresponding to the temperature (Ts-ΔT).

[0099] In the cooling operation, the controller (80) of the transport refrigeration system (10) changes ΔT within a range of 5° C. to 12° C. depending on the cooling load of the interior space, etc. Therefore, when specifying the refrigeration cycle, ΔT=12° C. is set when the difference between the set temperature Ts and the outside air temperature Ta is relatively large, and ΔT=5° C. is set when the difference between the set temperature Ts and the outside air temperature Ta is relatively small.

[0100] When the high-pressure HP of the refrigeration cycle is higher than the critical pressure of the refrigerant, the higher the high-pressure HP of the refrigeration cycle, the greater the cooling capacity obtained by the refrigeration cycle. Therefore, when the high-pressure HP of the refrigeration cycle is higher than the critical pressure of the refrigerant, the high-pressure HP will be slightly lower than the design upper limit pressure of the refrigerant circuit.

[0101] The intermediate pressure MP of the refrigeration cycle is calculated using the low pressure LP and high pressure HP of the refrigeration cycle, the pressure ratio RP, and Equation 1.

[0102] (Point A) Point A indicates the refrigerant drawn into the low-stage compressor (31). The state of point A is substantially the same as the state of the refrigerant at the outlet of the internal heat exchanger (34) functioning as an evaporator. The pressure at point A is the low pressure LP of the refrigeration cycle. Therefore, assuming that the degree of superheat of the refrigerant at the outlet of the internal heat exchanger (34) is 5°C, the temperature and pressure at point A are identified, and as a result, point A is identified.

[0103] (Point B) Point B indicates the refrigerant discharged from the low-stage compressor (31). The pressure at point B is the intermediate pressure MP of the refrigeration cycle. Assuming that the efficiency of the low-stage compressor (31) is 0.7, the specific entropy sB at point B is the specific entropy sA at point A divided by 0.7 (sB=sA / 0.7). Therefore, the pressure and specific entropy at point B are identified, and as a result, point B is identified.

[0104] (Point E) Point E indicates the refrigerant at the outlet of the external heat exchanger (33) functioning as a radiator. In the external heat exchanger, the refrigerant exchanges heat with the outside air (outside air). Therefore, the temperature of the refrigerant at the outlet of the external heat exchanger (33) is higher than the outside air temperature Ta by a predetermined value (assumed to be 5°C here). The pressure at point E is the high pressure HP of the refrigeration cycle. Therefore, the temperature and pressure at point E are identified, and as a result, point E is identified.

[0105] (Point H, Point I) In the gas-liquid separator (42) of the refrigerant circuit (30), the refrigerant in the state of point G (gas-liquid two-phase state) is separated into a refrigerant in the state of point H (saturated liquid refrigerant) and a refrigerant in the state of point I (saturated gas refrigerant). The pressures at points H and I are the intermediate pressure MP of the refrigeration cycle. Therefore, point H indicating the saturated liquid refrigerant state and point I indicating the saturated gas refrigerant state are identified.

[0106] (Point K) Point K indicates the refrigerant that has passed through the third electric valve (53). The state of point K is substantially the same as the state of the refrigerant that flows into the internal heat exchanger (34) that functions as an evaporator. The specific enthalpy at point K is equal to the specific enthalpy at point H. The pressure at point K is the low pressure LP of the refrigeration cycle. Therefore, the pressure and specific enthalpy at point K are identified, and as a result, point K is identified.

[0107] (Point J) Assuming that the degree of superheat of the refrigerant at the outlet of the second flow path (41b) of the internal heat exchanger (41) is 10°C, the temperature at point J is 10°C higher than the temperature at point I. The pressure at point J is the intermediate pressure MP of the refrigeration cycle. Therefore, the temperature and pressure at point J are identified, and as a result, point J is identified.

[0108] (Point F, Point G) A part of the refrigerant that has passed through the first pipe (61) flows into the second pipe (62), and the rest flows into the third pipe (63). Therefore, the mass flow rate M1 of the refrigerant in the first pipe (61) is equal to the sum of the mass flow rate M2 of the refrigerant in the second pipe (62) and the mass flow rate M3 of the refrigerant in the third pipe (63) (M1=M2+M3).

[0109] In the first flow path (41a) of the internal heat exchanger (41), the state of the refrigerant changes from point E to point F. The mass flow rate of the refrigerant in the first flow path (41a) is equal to the mass flow rate M1 of the refrigerant in the first pipe (61). On the other hand, in the second flow path (41b) of the internal heat exchanger (41), the state of the refrigerant changes from point I to point J. The mass flow rate of the refrigerant in the second flow path (41b) is equal to the mass flow rate M2 of the refrigerant in the second pipe (62).

[0110] In the internal heat exchanger (41), the amount of heat released by the refrigerant in the first flow path (41a) is equal to the amount of heat absorbed by the refrigerant in the second flow path (41b). Therefore, in the internal heat exchanger (41), the following formula 2 holds: In formula 2, hE is the specific enthalpy at point E, hF is the specific enthalpy at point F, hI is the specific enthalpy at point I, and hJ is the specific enthalpy at point J. (hJ-hI)×M2=(hE-hF)×M1 (Equation 2)

[0111] The specific enthalpy hF at point F is equal to the specific enthalpy hG at point G (hF=hG). Furthermore, M2 / M1 is the quality fraction of the refrigerant at point G. The specific enthalpy hG at point G and the quality fraction M2 / M1 of the refrigerant at point G are correlated. Therefore, the specific enthalpy hF at point F and the specific enthalpy hG at point G can be determined using the correlation between the specific enthalpy hG and the quality fraction M2 / M1 of the refrigerant at point G and Equation 2.

[0112] The pressure at point F is the high pressure HP of the refrigeration cycle. Therefore, the specific enthalpy and pressure of point F are identified, and as a result, point F is identified. Also, the pressure at point G is the intermediate pressure MP of the refrigeration cycle. Therefore, the specific enthalpy and pressure of point G are identified, and as a result, point G is identified.

[0113] (Point C) Point C indicates the refrigerant drawn into the high-stage compressor (32). The high-stage compressor (32) draws the refrigerant discharged from the low-stage compressor (31) (refrigerant at point B) and the refrigerant that has passed through the second pipe (62) (refrigerant at point J). The mass flow rate of the refrigerant discharged from the low-stage compressor (31) is equal to the mass flow rate M3 of the refrigerant in the third pipe (63). Therefore, the following equation 3 holds true for the refrigerant at point C: hC×(M2+M3)=hB×M3+hJ×M2 (Formula 3)

[0114] The mass flow rate M1 of the refrigerant in the first pipe (61) is the sum of the mass flow rate M2 of the refrigerant in the second pipe (62) and the mass flow rate M3 of the refrigerant in the third pipe (63) (M1=M2+M3). Therefore, Equation 3 can be transformed into Equation 4. hC=hB×(M3 / M1)+hJ×(M2 / M1) (Formula 4)

[0115] (M2 / M1) is the dryness fraction of the refrigerant at point G. (M3 / M1) is the wetness fraction of the refrigerant at point G. Since point G is identified, the dryness fraction and wetness fraction of the refrigerant at point G are also identified. Therefore, the specific enthalpy hC of the refrigerant at point C is calculated using equation 4. The pressure at point C is the intermediate pressure MP of the refrigeration cycle. Therefore, the pressure and specific enthalpy at point C are identified, and as a result, point C is identified.

[0116] (Point D) Point D indicates the refrigerant discharged from the high-stage compressor (32). The pressure at point D is the high pressure HP of the refrigeration cycle. Assuming that the efficiency of the high-stage compressor (32) is 0.7, the specific entropy sD at point D is the specific entropy sC at point C divided by 0.7 (sD=sC / 0.7). Therefore, the pressure and specific entropy at point D are identified, and as a result, point D is identified.

[0117] <Coefficient of Performance> The coefficient of performance COP of the refrigeration cycle shown in FIG. 4 is calculated by the following equation 5. The specific enthalpy hK at point K is the specific enthalpy of the refrigerant at the inlet of the internal heat exchanger (34) functioning as an evaporator. The specific enthalpy hA at point A is the specific enthalpy of the refrigerant at the outlet of the internal heat exchanger (34) functioning as an evaporator and is also the specific enthalpy of the refrigerant at the inlet of the low-stage compressor (31). The specific enthalpy hB at point B is the specific enthalpy of the refrigerant at the outlet of the low-stage compressor (31). The specific enthalpy hC at point C is the specific enthalpy of the refrigerant at the inlet of the high-stage compressor (32). The specific enthalpy hD at point D is the specific enthalpy of the refrigerant at the outlet of the high-stage compressor (32). COP=(hA-hK)×M3 / {(hB-hA)×M3+(hD-hC)×M1} (Formula 5)

[0118] Equation 5 can be transformed into the following equation 6. (M3 / M1) is the wetness of the refrigerant at point G. Therefore, the coefficient of performance of the refrigeration cycle shown in FIG. 4 is calculated based on the following equation 6. As described above, (M3 / M1) is the wetness of the refrigerant at point G. COP=(hA-hK)×(M3 / M1) / {(hB-hA)×(M3 / M1)+(hD-hC)} (Formula 6)

[0119] <Optimal pressure ratio> In the refrigerant circuit (30) of this embodiment, when the set temperature Ts and the outside air temperature Ta are constant, the high pressure HP and the low pressure LP of the refrigeration cycle are also substantially constant. On the other hand, in the refrigerant circuit (30) of this embodiment, the rotation speeds of the low stage compressor (31) and the high stage compressor (32) are controlled separately. Therefore, in the refrigerant circuit (30) of this embodiment, when the set temperature Ts and the outside air temperature Ta are constant, the intermediate pressure MP of the refrigeration cycle can be adjusted.

[0120] As described above, the pressure ratio RP is a value calculated by Equation 1. When the high pressure HP and low pressure LP of the refrigeration cycle are constant, the intermediate pressure MP of the refrigeration cycle increases as the pressure ratio RP increases.

[0121] 7 shows the input to the low-stage compressor (31) and the input to the high-stage compressor (32) when the pressure ratio RP is changed, when the high-pressure HP and the low-pressure LP of the refrigeration cycle are constant. As shown in FIG. 7, when the high-pressure HP and the low-pressure LP of the refrigeration cycle are constant, the input WL to the low-stage compressor (31) and the input WH to the high-stage compressor (32) change according to the pressure ratio RP. Specifically, the input WL to the low-stage compressor (31) increases as the pressure ratio RP increases. On the other hand, the input WH to the high-stage compressor (32) decreases as the pressure ratio RP increases. Therefore, as shown by the solid line in FIG. 7, there exists a pressure ratio RP at which the sum (WL+WH) of the input WL to the low-stage compressor (31) and the input WH to the high-stage compressor (32) is smallest.

[0122] When the sum (WL+WH) of the input WL to the low-stage compressor (31) and the input WH to the high-stage compressor (32) changes, the coefficient of performance COP of the refrigeration cycle also changes. Therefore, in the refrigeration cycle performed by the refrigerant circuit (30) of this embodiment, when the high pressure HP and the low pressure LP of the refrigeration cycle are constant, there exists a pressure ratio RP at which the coefficient of performance COP is highest. The pressure ratio RP at which the coefficient of performance COP is highest is referred to as the "optimum pressure ratio."

[0123] When the high pressure HP and low pressure LP of the refrigeration cycle are specified, the corresponding optimal pressure ratio is determined. Therefore, when the set temperature Ts and the outside air temperature Ta are specified, the corresponding optimal pressure ratio is determined.

[0124] <Optimal displacement ratio> The displacement ratio RV is calculated by dividing the displacement VL of the low-stage compressor (31) by the displacement VH of the high-stage compressor (32) (RV=VL / VH).

[0125] The displacement VL of the low-stage compressor (31) is the volume of refrigerant sucked per unit time by the low-stage compressor (31). Therefore, the displacement VL of the low-stage compressor (31) is calculated by dividing the mass flow rate of the refrigerant passing through the low-stage compressor (31) (= the mass flow rate M3 of the refrigerant in the third pipe (63)) by the density DA of the refrigerant at the inlet (point A in FIG. 4) of the low-stage compressor (31) (VL=M3 / DA).

[0126] The displacement VH of the high-stage compressor (32) is the volume of refrigerant sucked per unit time by the high-stage compressor (32). Therefore, the displacement VH of the high-stage compressor (32) is calculated by dividing the mass flow rate of the refrigerant passing through the high-stage compressor (32) (=the mass flow rate M1 of the refrigerant in the first pipe (61)) by the density DC of the refrigerant at the inlet (point C in FIG. 4) of the high-stage compressor (32) (VH=M1 / DC).

[0127] Once the set temperature Ts, the outside air temperature Ta, and the pressure ratio RP are specified, the corresponding displacement ratio RV can be calculated. As described above, once the set temperature Ts and the outside air temperature Ta are specified, the corresponding optimal pressure ratio can be specified, and further, the displacement ratio RV corresponding to the optimal pressure ratio can be specified. The displacement ratio RV corresponding to the optimal pressure ratio is the optimal displacement ratio. In this way, once the set temperature Ts and the outside air temperature Ta are specified, the corresponding optimal displacement ratio can be specified.

[0128] The optimum displacement ratio is the displacement ratio that maximizes the coefficient of performance COP of the refrigeration cycle at the specified set temperature Ts and outside air temperature Ta.

[0129] <Displacement ratio range> The hatched region in Figure 8 is the operable region of the transport refrigeration system (10) of this embodiment. The transport refrigeration system (10) of this embodiment performs cooling operation when the set temperature Ts and the outside air temperature Ta are within the operable region. In the transport refrigeration system (10) of this embodiment, the high pressure HP of the refrigeration cycle becomes equal to or higher than the critical pressure of the refrigerant when the outside air temperature is 25°C or higher, and becomes lower than the critical pressure of the refrigerant when the outside air temperature is lower than 25°C.

[0130] Figure 8 shows the optimal displacement ratio for some combinations of set temperature Ts and outside air temperature Ta within the operable range. The optimal displacement ratio decreases as the set temperature Ts increases. The optimal displacement ratio also decreases as the outside air temperature Ta decreases, both in the range where the high pressure HP of the refrigeration cycle is equal to or greater than the critical pressure of the refrigerant and in the range where it is less than the critical pressure of the refrigerant.

[0131] The optimum displacement ratio reaches a maximum value of 1.93 when the set temperature Ts is −30° C. and the outside air temperature Ta is 25° C., and reaches a minimum value of 0.7 when the set temperature Ts is 10° C. and the outside air temperature Ta is 30° C. Therefore, in the transport refrigeration system (10) of this embodiment, by setting the displacement ratio during cooling operation to be 0.7 or more and 1.93 or less, the coefficient of performance COP of the refrigeration cycle performed by the refrigerant circuit (30) can be maintained high.

[0132] -Feature (1) of the first embodiment- In the cooling operation of the transport refrigeration system (10), the controller (80) controls the rotation speed of the low-stage compressor (31) and the rotation speed of the high-stage compressor (32) separately, so that when the high pressure HP and the low pressure LP of the refrigeration cycle are constant, the intermediate pressure MP of the refrigeration cycle is adjusted.

[0133] In the cooling operation of the transport refrigeration system (10) of this embodiment, the displacement ratio is 0.7 or more and 1.93 or less. Therefore, in the transport refrigeration system (10) of this embodiment, the intermediate pressure MP of the refrigeration cycle is at an appropriate value throughout the entire operable range of the transport refrigeration system (10) shown in Figure 8, and the coefficient of performance COP of the refrigeration cycle performed by the refrigerant circuit (30) is maintained high.

[0134] -Feature (2) of the first embodiment- In the cooling operation of the transport refrigeration system (10), the controller (80) controls the rotation speed of the low-stage compressor (31) based on the measurement value of the low-pressure sensor (71), and controls the rotation speed of the high-stage compressor (32) based on the measurement value of the intermediate-pressure sensor (72). Thus, in the transport refrigeration system (10) of this embodiment, the controller (80) controls the rotation speed of the low-stage compressor (31) and the rotation speed of the high-stage compressor (32) separately, thereby setting the low pressure LP and the intermediate pressure MP of the refrigeration cycle to appropriate values.

[0135] -Feature (3) of the first embodiment- The low-stage displacement is calculated by multiplying the "displacement volume of the low-stage compressor (31)" by the "rotational speed of the low-stage compressor (31)." The high-stage displacement is calculated by multiplying the "displacement volume of the high-stage compressor (32)" by the "rotational speed of the high-stage compressor (32)." The displacement ratio is a value obtained by dividing the low-stage displacement by the high-stage displacement. The rotational speed ratio is a value obtained by dividing the rotational speed of the low-stage compressor (31) by the rotational speed of the high-stage compressor (32).

[0136] In the transport refrigeration system (10) of this embodiment, the low-stage compressor (31) and the high-stage compressor (32) have the same displacement volume. Therefore, in the transport refrigeration system (10) of this embodiment, the displacement volume ratio matches the rotational speed ratio. Therefore, in the transport refrigeration system (10) of this embodiment, the displacement volume ratio and the rotational speed ratio during cooling operation are each between 0.7 and 1.93.

[0137] Second Embodiment A second embodiment will be described. In this embodiment, the configuration of the transport refrigeration unit (10) in the transport container (1) of the first embodiment is modified. The transport refrigeration unit (10) of this embodiment differs from the transport refrigeration unit (10) of the first embodiment in the refrigerant circuit (30) and the controller (80).

[0138] Here, the transport refrigeration system (10) of this embodiment will be described, focusing mainly on the differences from the transport refrigeration system (10) of embodiment 1. In principle, the description of the transport refrigeration system (10) of this embodiment that is common to the transport refrigeration system (10) of embodiment 1 will be omitted.

[0139] -Refrigerant circuit- 9, the refrigerant circuit (30) of the present embodiment does not include the first electric valve (51) and the gas-liquid separator (42) provided in the refrigerant circuit (30) of the first embodiment. Furthermore, the refrigerant circuit (30) of the present embodiment includes an internal heat exchanger (46) connected to the second pipe (62) and the third pipe (63), instead of the internal heat exchanger (41) of the first embodiment connected to the first pipe (61) and the second pipe (62).

[0140] <1st pipe, 2nd pipe, 3rd pipe> In the refrigerant circuit (30) of this embodiment, the pipe connected to the other end of the external heat exchanger (33) is the first pipe (61). One end of the first pipe (61) is connected to the other end of the external heat exchanger (33). The first pipe (61) forms a first passage through which all of the refrigerant flowing out of the internal heat exchanger (34) flows.

[0141] In the refrigerant circuit (30) of this embodiment, the pipe connecting the other end of the first pipe and the suction pipe of the high-stage compressor (32) is the second pipe (62). The second pipe (62) forms a second passage through which a portion of the refrigerant that has passed through the first pipe (61) flows. A second electric valve (52) is disposed in the second pipe (62) upstream of the internal heat exchanger (46).

[0142] In the refrigerant circuit (30) of this embodiment, the pipe connecting the other end of the first pipe (61) and the suction pipe of the low-stage compressor (31) is the third pipe (63). The third pipe (63) forms a third passage through which the remainder of the refrigerant that has passed through the first pipe (61) flows. A third motor-operated valve (53) is disposed in the third pipe (63) upstream of the internal heat exchanger (34).

[0143] <Internal heat exchanger> The internal heat exchanger (46) is a heat exchanger that exchanges heat between refrigerants. In this embodiment, the internal heat exchanger (46) is a plate-type heat exchanger. The internal heat exchanger (46) has a first flow path (46a) and a second flow path (46b). The first flow path (46a) of the internal heat exchanger (46) is connected to the third pipe (63). The second flow path (46b) of the internal heat exchanger (46) is connected to the second pipe (62). The internal heat exchanger (46) exchanges heat between the refrigerant flowing through the first flow path (46a) and the refrigerant flowing through the second flow path (46b).

[0144] - Refrigerant circuit operation - The refrigerant circuit (30) of this embodiment selectively performs a two-stage compression operation for performing a two-stage compression refrigeration cycle and a single-stage compression operation for performing a single-stage compression refrigeration cycle, similar to the refrigerant circuit (30) of embodiment 1. As in embodiment 1, in the two-stage compression operation, both the low-stage compressor (31) and the high-stage compressor (32) operate, and in the single-stage compression operation, either the low-stage compressor (31) or the high-stage compressor (32) operates.

[0145] -Two-stage compression refrigeration cycle- The two-stage compression refrigeration cycle performed by the refrigerant circuit (30) of this embodiment will be described in detail with reference to Fig. 10. Fig. 10 is a Mollier diagram (pressure-enthalpy diagram) showing a refrigeration cycle. In the refrigeration cycle shown in Fig. 10, the high pressure of the refrigeration cycle is higher than the critical pressure (7.2 MPa) of the refrigerant (carbon dioxide), and the intermediate pressure of the refrigeration cycle is lower than the critical pressure of the refrigerant (carbon dioxide).

[0146] The refrigerant in the state of point A is compressed by the low-stage compressor (31) to reach the state of point B. In the refrigerant circuit (30), the refrigerant in the state of point B discharged from the low-stage compressor (31) and the refrigerant in the state of point I that has passed through the second pipe (62) join together to reach the refrigerant in the state of point C. The refrigerant in the state of point C is compressed by the high-stage compressor (32) to reach the state of point D.

[0147] The refrigerant in the state of point D dissipates heat to the outside air (outside air) in the external heat exchanger (33) and reaches the state of point E. Part of the refrigerant in the state of point E flowing through the first pipe (61) flows into the second pipe (62), and the rest flows into the third pipe (63).

[0148] The refrigerant flowing through the third pipe (63) in the state of point E flows into the first flow path (46a) of the internal heat exchanger (46) and is cooled by the refrigerant flowing through the second flow path (46b) to reach the state of point F. The refrigerant in the state of point F is decompressed by the third electric valve (53) to reach the state of point G (a two-phase gas-liquid state). The refrigerant in the state of point G absorbs heat from the air inside the compartment in the internal heat exchanger (34) and evaporates to reach the state of point A.

[0149] The refrigerant flowing through the second pipe (62) in the state of point E passes through the second electric valve (52), and then flows into the second flow path (46b) of the internal heat exchanger (46), where it absorbs heat from the refrigerant flowing through the first flow path (46a) and reaches the state of point I.

[0150] -Controller- The controller (80) of this embodiment controls the low stage compressor (31), the high stage compressor (32), the second electric valve (52), and the third electric valve (53) in the same manner as the controller (80) of the first embodiment.

[0151] The controller (80) of this embodiment controls the rotation speed of the low-stage compressor (31) based on the measurement value of the low-pressure sensor (71). The controller (80) of this embodiment controls the rotation speed of the high-stage compressor (32) based on the measurement value of the intermediate-pressure sensor (72). The controller (80) of this embodiment performs the operation shown in the flowchart of Fig. 6 to control the rotation speed of the high-stage compressor (32), similar to the controller (80) of the first embodiment.

[0152] The controller (80) of this embodiment adjusts the aperture of the second motor-operated valve (52) so that the degree of superheat of the refrigerant at the outlet of the second flow path (46b) of the internal heat exchanger (46) becomes a first target degree of superheat. The controller (80) of this embodiment adjusts the aperture of the third motor-operated valve (53) so that the degree of superheat of the refrigerant at the outlet of the internal heat exchanger (34) becomes a second target degree of superheat.

[0153] -Displacement ratio- In the transport refrigeration system (10) of this embodiment, the controller (80) controls the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32). During the cooling operation of the transport refrigeration system (10), the displacement ratio is 0.67 to 3.42. Here, the reason why the displacement ratio during the cooling operation is desirably 0.67 to 3.42 will be described.

[0154] <Identifying the refrigeration cycle> When the set value of the inside temperature (set temperature Ts), the temperature of the outside air (outside air temperature Ta), and the pressure ratio RP are specified, the refrigeration cycle performed by the refrigerant circuit (30) is specified. As described above, the pressure ratio RP is defined by Equation 1.

[0155] Specifically, when the set value of the temperature inside the cabinet (set temperature Ts), the temperature of the air outside the cabinet (outside air temperature Ta), and the pressure ratio RP are specified, points A to I that specify the refrigeration cycle shown in Figure 10 are specified as shown below.

[0156] (High pressure HP, medium pressure MP, low pressure LP) The high pressure HP, the intermediate pressure MP, and the low pressure LP of the refrigeration cycle are identified by carrying out the same processing as that described in the first embodiment.

[0157] (Point A, Point B) Point A indicates the refrigerant drawn into the low-stage compressor (31). Point B indicates the refrigerant discharged from the low-stage compressor (31). Points A and B are identified by performing the same process as that described in the first embodiment (the process for identifying points A and B in FIG. 4).

[0158] (Point E) Point E indicates the refrigerant at the outlet of the external heat exchanger (33) functioning as a radiator. The state of point E is substantially equal to the temperature of the refrigerant at the inlet of the first flow path (46a) of the internal heat exchanger (46). Point E is identified by the same process as that described in the first embodiment (the process for identifying point E in FIG. 4).

[0159] (Point H) Point H indicates the refrigerant at the outlet of the second electric valve (52). The specific enthalpy at point H is equal to the specific enthalpy at point E. The pressure at point H is the intermediate pressure MP of the refrigeration cycle. Therefore, the pressure and specific enthalpy at point H are identified, and as a result, point H is identified.

[0160] (Point F) Point F indicates the refrigerant at the outlet of the first flow path (46a) of the internal heat exchanger (46). The state of point F is substantially the same as the state of the refrigerant flowing into the third electric valve (53). Assuming that the temperature of the refrigerant at the outlet of the first flow path (46a) of the internal heat exchanger (46) is the same as the temperature of the refrigerant at the inlet of the second flow path (46b), the temperature of point F is the same as the temperature of point H. The pressure at point F is the high pressure HP of the refrigeration cycle. Therefore, the temperature and pressure of point F are identified, and as a result, point F is identified.

[0161] (Point I) Point I indicates the refrigerant at the outlet of the second flow path (46b) of the internal heat exchanger (46). The pressure at point I is the intermediate pressure MP of the refrigeration cycle.

[0162] The minimum value SH1 of the degree of superheat at point I is 0° C. (SH1=0). The maximum value of the temperature at point I is the temperature at point E. Point E represents the refrigerant at the inlet of the first flow path (46a) of the internal heat exchanger (46). Therefore, the maximum value SH2 of the degree of superheat at point I is (the temperature at point E)−(the saturation temperature of the refrigerant at the intermediate pressure MP). Therefore, the degree of superheat at point I is set to a value equal to or greater than SH1 and equal to or less than SH2.

[0163] Once the degree of superheat at point I is identified, the temperature at point I is identified. Therefore, the pressure and temperature at point I are identified, and as a result, point I is identified.

[0164] (Point C) A part of the refrigerant that has passed through the first pipe (61) flows into the second pipe (62), and the rest flows into the third pipe (63). Therefore, the mass flow rate M1 of the refrigerant in the first pipe (61) is equal to the sum of the mass flow rate M2 of the refrigerant in the second pipe (62) and the mass flow rate M3 of the refrigerant in the third pipe (63) (M1=M2+M3).

[0165] In the first flow path (46a) of the internal heat exchanger (46), the state of the refrigerant changes from point E to point F. The mass flow rate of the refrigerant in the first flow path (46a) is equal to the mass flow rate M3 of the refrigerant in the third pipe (63). On the other hand, in the second flow path (46b) of the internal heat exchanger (46), the state of the refrigerant changes from point H to point I. The mass flow rate of the refrigerant in the second flow path (46b) is equal to the mass flow rate M2 of the refrigerant in the second pipe (62).

[0166] In the internal heat exchanger (46), the amount of heat released by the refrigerant in the first flow path (46a) is equal to the amount of heat absorbed by the refrigerant in the second flow path (46b). Therefore, in the internal heat exchanger (46), the following formula 7 holds. In formula 7, hE is the specific enthalpy at point E, hF is the specific enthalpy at point F, hH is the specific enthalpy at point H, and hI is the specific enthalpy at point I. Furthermore, formula 7 can be transformed to obtain formula 8. (hI-hH)×M2=(hE-hF)×M3 (Equation 7) M3 / M2=(hI-hH) / (hE-hF) (Formula 8)

[0167] The refrigerant at point C is a mixture of the refrigerant at point B and the refrigerant at point I. Therefore, the following formula 9 holds for the refrigerant at point C. hB is the specific enthalpy at point B, and hC is the specific enthalpy at point C. Furthermore, by transforming formula 9, formula 10 is obtained. hI×M2+hB×M3=hC×(M2+M3) (Formula 9) hI+hB×(M3 / M2)=hC×(1+M3 / M2) (Formula 10)

[0168] The specific enthalpy hC at point C is calculated using Equation 8 and Equation 10. The pressure at point C is the intermediate pressure MP of the refrigeration cycle. Therefore, the pressure and specific enthalpy at point C are identified, and as a result, point C is identified.

[0169] <Coefficient of Performance> The coefficient of performance COP of the refrigeration cycle shown in FIG. 10 is calculated by the following equation 11. The specific enthalpy hG at point G is the specific enthalpy of the refrigerant at the inlet of the internal heat exchanger (34) functioning as an evaporator. The specific enthalpy hA at point A is the specific enthalpy of the refrigerant at the outlet of the internal heat exchanger (34) functioning as an evaporator and is also the specific enthalpy of the refrigerant at the inlet of the low-stage compressor (31). The specific enthalpy hB at point B is the specific enthalpy of the refrigerant at the outlet of the low-stage compressor (31). The specific enthalpy hC at point C is the specific enthalpy of the refrigerant at the inlet of the high-stage compressor (32). The specific enthalpy hD at point D is the specific enthalpy of the refrigerant at the outlet of the high-stage compressor (32). COP=(hA-hG)×M3 / {(hB-hA)×M3+(hD-hC)×(M2+M3)} (Formula 11)

[0170] Equation 11 can be transformed into the following Equation 12. (M3 / M2) is calculated by Equation 8. Therefore, the coefficient of performance of the refrigeration cycle shown in FIG. 10 is calculated based on Equation 12 below. COP=(hA-hG)×(M3 / M2) / {(hB-hA)×(M3 / M2)+(hD-hC)×(1+M3 / M2)} (Equation 12)

[0171] As described above, in the process of identifying the refrigeration cycle, the degree of superheat at point I is set to a value between SH1 and SH2. Changing the degree of superheat at point I changes the coefficient of performance (COP) of the refrigeration cycle. Therefore, the coefficients of performance (COP) of the refrigeration cycle for a plurality of "degrees of superheat at point I" are calculated, and the highest COP among them is set as the coefficient of performance (COP) of the refrigeration cycle performed by the refrigerant circuit (30) of this embodiment.

[0172] <Optimal pressure ratio> As in the refrigeration cycle performed by the refrigerant circuit (30) of the first embodiment, in the refrigeration cycle performed by the refrigerant circuit (30) of the present embodiment, when the high pressure HP and the low pressure LP of the refrigeration cycle are constant, there exists a pressure ratio RP at which the coefficient of performance COP is maximized. The pressure ratio RP at which the coefficient of performance COP is maximized is referred to as the “optimum pressure ratio.”

[0173] When the high pressure HP and low pressure LP of the refrigeration cycle are specified, the corresponding optimal pressure ratio is determined. Therefore, when the set temperature Ts and the outside air temperature Ta are specified, the corresponding optimal pressure ratio is determined.

[0174] <Optimal displacement ratio> As with the transport refrigeration system (10) of the first embodiment, the transport refrigeration system (10) of the present embodiment can identify an optimal displacement ratio corresponding to a set temperature Ts and an outside air temperature Ta by specifying the set temperature Ts and the outside air temperature Ta. The optimal displacement ratio is the displacement ratio that maximizes the coefficient of performance COP of the refrigeration cycle at the specified set temperature Ts and the specified outside air temperature Ta.

[0175] <Displacement ratio range> The hatched region in Figure 11 is the operable region of the transport refrigeration system (10) of this embodiment. The transport refrigeration system (10) of this embodiment performs cooling operation when the set temperature Ts and the outside air temperature Ta are within the operable region. In the transport refrigeration system (10) of this embodiment, the high pressure HP of the refrigeration cycle becomes equal to or higher than the critical pressure of the refrigerant when the outside air temperature is 25°C or higher, and becomes lower than the critical pressure of the refrigerant when the outside air temperature is lower than 25°C.

[0176] Figure 11 shows the optimal displacement ratio for some combinations of set temperature Ts and outside air temperature Ta within the operable range. The optimal displacement ratio decreases as the set temperature Ts increases. The optimal displacement ratio also decreases as the outside air temperature Ta decreases, both in the range where the high pressure HP of the refrigeration cycle is equal to or greater than the critical pressure of the refrigerant and in the range where it is less than the critical pressure of the refrigerant.

[0177] The optimum displacement ratio reaches a maximum value of 3.42 when the set temperature Ts is −30° C. and the outdoor air temperature Ta is 25° C., and reaches a minimum value of 0.67 when the set temperature Ts is 30° C. and the outdoor air temperature Ta is 30° C. Therefore, in the transport refrigeration system (10) of this embodiment, by setting the displacement ratio during the cooling operation to be 0.67 or more and 3.42 or less, the coefficient of performance COP of the refrigeration cycle performed by the refrigerant circuit (30) can be maintained high.

[0178] -Feature (1) of the second embodiment- In the cooling operation of the transport refrigeration system (10), the controller (80) controls the rotation speed of the low-stage compressor (31) and the rotation speed of the high-stage compressor (32) separately, so that when the high pressure HP and the low pressure LP of the refrigeration cycle are constant, the intermediate pressure MP of the refrigeration cycle is adjusted.

[0179] In the cooling operation of the transport refrigeration system (10) of this embodiment, the displacement ratio is 0.67 or more and 3.42 or less. Therefore, in the transport refrigeration system (10) of this embodiment, the intermediate pressure MP of the refrigeration cycle is at an appropriate value throughout the entire operable range of the transport refrigeration system (10) shown in Figure 11, and the coefficient of performance COP of the refrigeration cycle performed by the refrigerant circuit (30) is maintained high.

[0180] -Feature (2) of the second embodiment- In the transport refrigeration system (10) of this embodiment, the low-stage compressor (31) and the high-stage compressor (32) have the same displacement volume. Therefore, similar to the transport refrigeration system (10) of the first embodiment, in the transport refrigeration system (10) of this embodiment, the displacement volume ratio matches the rotational speed ratio. Therefore, in the transport refrigeration system (10) of this embodiment, the displacement ratio and the rotational speed ratio during the cooling operation are each equal to or greater than 0.67 and equal to or less than 3.42.

[0181] Although the embodiments and modifications have been described above, it will be understood that various modifications in form and detail are possible without departing from the spirit and scope of the claims. Furthermore, elements of the above embodiments, modifications, and other embodiments may be combined or substituted as appropriate. Furthermore, the terms "first," "second," "third," etc. in the specification and claims are used to distinguish the terms to which these terms are attached, and do not limit the number or order of the terms. [Industrial Applicability]

[0182] As described above, the present disclosure is useful for transport refrigeration devices and transport containers. [Explanation of symbols]

[0183] 1 shipping container 2. Container body 5. Interior space 10 Transport refrigeration equipment 30 Refrigerant circuit 31 Low-stage compressor 32 High-stage compressor 33 External heat exchanger (radiator) 34 Internal heat exchanger (evaporator) 41 Internal heat exchanger (heat exchanger) 42 Gas-liquid separator 46 Internal heat exchanger (heat exchanger) 61 1st piping (1st passage) 62 2nd piping (2nd passage) 63 3rd piping (3rd passage) 80 Controller

Claims

1. A transport refrigeration system (10) includes a refrigerant circuit (30) that performs a refrigeration cycle by circulating carbon dioxide as a refrigerant, and performs a cooling operation to cool the air inside a transport container (1), The refrigerant circuit (30) includes: a radiator (33) for exchanging heat between the refrigerant and outside air; a first passageway (61) through which all of the refrigerant flowing out of the radiator (33) flows; a second passageway (62) through which a portion of the refrigerant that has passed through the first passageway (61) flows; a third passageway (63) through which the remainder of the refrigerant that has passed through the first passageway (61) flows; an evaporator (34) provided in the third passageway (63) for exchanging heat between the refrigerant and the inside air; a low-stage compressor (31) that sucks the refrigerant flowing out of the evaporator (34); a high-stage compressor (32) that sucks the refrigerant discharged from the low-stage compressor (31) and the refrigerant flowing through the second passage (62); the volume of the refrigerant sucked into the low-stage compressor (31) per unit time is the low-stage displacement, the volume of the refrigerant sucked into the high-stage compressor (32) per unit time is the high-stage displacement, a displacement ratio obtained by dividing the low stage displacement by the high stage displacement; The displacement ratio during the cooling operation is equal to or greater than 0.67 and equal to or less than 3.

42. Transport refrigeration equipment.

2. a controller (80) for individually controlling the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32); The controller (80) sets the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) to values ​​that make the displacement ratio during the cooling operation equal to or greater than 0.67 and equal to or less than 3.

42.

2. The transport refrigeration system of claim 1.

3. a controller (80) for individually controlling the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32); The controller (80) controlling the rotation speed of the low-stage compressor (31) based on a physical quantity correlated with the evaporation temperature of the refrigerant in the evaporator (34); The rotational speed of the high-stage compressor (32) is set to a value that makes the displacement ratio equal to or greater than 0.67 and equal to or less than 3.

42.

2. The transport refrigeration system of claim 1.

4. The low-stage compressor (31) and the high-stage compressor (32) each have the same volume of refrigerant drawn in per rotation, a rotational speed ratio obtained by dividing the rotational speed of the low-stage compressor (31) by the rotational speed of the high-stage compressor (32); The rotational speed ratio during the cooling operation is 0.67 or more and 3.42 or less. The transport refrigeration unit according to any one of claims 1 to 3.

5. a heat exchanger (46) provided in the third passage (63) upstream of the evaporator (34) for cooling the refrigerant flowing through the third passage (63) by exchanging heat with the refrigerant flowing through the second passage (62); The transport refrigeration unit according to any one of claims 1 to 3.

6. a gas-liquid separator (42) that separates the refrigerant that has passed through the first passage (61) into a gas refrigerant and a liquid refrigerant, and sends the gas refrigerant to the second passage (62) and the liquid refrigerant to the third passage (63); a heat exchanger (41) for exchanging heat between the refrigerant flowing through the first passage (61) and the refrigerant flowing through the second passage (62) to cool the refrigerant The transport refrigeration unit according to any one of claims 1 to 3.

7. The displacement ratio during the cooling operation is equal to or greater than 0.7 and equal to or less than 1.

93.

7. The transport refrigeration system of claim 6.

8. The refrigerant circuit (30) a two-stage compression operation in which both the low-stage compressor (31) and the high-stage compressor (32) are operated to perform a refrigeration cycle; A single-stage compression operation is selectively performed in which one of the low-stage compressor (31) and the high-stage compressor (32) is operated and the other is stopped to perform a refrigeration cycle. The transport refrigeration unit according to any one of claims 1 to 3.

9. A transport refrigeration unit (10) according to any one of claims 1 to 3; a container body (2) to which the transport refrigeration unit (10) is attached and which forms an interior space (5) for accommodating cargo; Shipping container.

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