Refrigeration equipment and transport containers for transport

Abstract

To operate a transport refrigeration system using a two-stage compression refrigeration cycle under appropriate conditions. A transport refrigeration unit (10) performs a cooling operation to cool the air inside the refrigerant circuit (30). In the refrigerant circuit (30), refrigerant that has passed through a radiator (33) is distributed to a second passage (62) and a third passage (63). The low-stage compressor (31) draws refrigerant that has passed through an evaporator (34) of the third passage (63). The high-stage compressor (32) draws refrigerant discharged from the low-stage compressor (31) and refrigerant flowing through the second passage (62). During the cooling operation, the displacement ratio (= low-stage displacement / high-stage displacement) is 0.67 or more and 3.42 or less.

Description

Technical Field

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[0001] The present disclosure relates to a refrigeration device for transportation and a container for transportation.

Background Art

[0002] Patent Document 1 discloses a refrigeration device for transportation that cools the interior of a container for transportation or the like. This refrigeration device for transportation includes 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. Further, in paragraph 0022 of Patent Document 1, it is described that each of the first compression stage and the second compression stage may be configured by an independent compressor.

Prior Art Documents

Patent Documents

[0003] <000001​​​​​​​​​​​​​​​​​​​​The purpose of this disclosure is to enable transport refrigeration equipment that performs a two-stage compression refrigeration cycle to operate in an appropriate manner. [Means for solving the problem]

[0007] A first aspect of this disclosure relates 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 performs a cooling operation to cool the air inside a transport container (1). The refrigerant circuit (30) includes a heat exchanger (33) for the refrigerant to exchange heat with the outside air, a first passage (61) through which all of the refrigerant that has flowed out of the heat exchanger (33) flows, a second passage (62) through which a portion of the refrigerant that has passed through the first passage (61) flows, a third passage (63) through which the remaining refrigerant that has passed through the first passage (61) flows, an evaporator (34) provided in the third passage (63) for the refrigerant to exchange heat with the air inside the chamber, a low-stage compressor (31) that sucks in the refrigerant that has flowed out of the evaporator (34), and a high-stage compressor (32) that sucks in the refrigerant discharged by the low-stage compressor (31) and the refrigerant flowing through the second passage (62). In this embodiment, the volume of refrigerant drawn in by the low-stage compressor (31) per unit time is the low-stage displacement amount, the volume of refrigerant drawn in by the high-stage compressor (32) per unit time is the high-stage displacement amount, the value obtained by dividing the low-stage displacement amount by the high-stage displacement amount is the displacement amount ratio, and the displacement amount ratio during the cooling operation is 0.67 or more and 3.42 or less.

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

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

[0010] In the second embodiment, the controller (80) individually controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32). In the transport refrigeration system (10), the controller (80) individually controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32), so that the displacement ratio during cooling operation is between 0.67 and 3.42.

[0011] A third aspect of the present disclosure is, in the first aspect, further comprising a controller (80) for individually controlling the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32), wherein the controller (80) controls the rotational speed of the low-stage compressor (31) based on a physical quantity correlated 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 such that the displacement ratio is 0.67 or more and 3.42 or less.

[0012] In the third embodiment, the controller (80) individually controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32). The controller (80) controls the rotational speed of the low-stage compressor (31) based on a predetermined physical quantity. The controller (80) also controls the rotational speed of the high-stage compressor (32) of The displacement ratio is set to a value such that it is between 0.67 and 3.42 at the rotational speed of the low-stage compressor (31) at that time.

[0013] A fourth aspect of the present disclosure is that, in any one of the first to third aspects described above, the low-stage compressor (31) and the high-stage compressor (32) each inhale the same volume of refrigerant per revolution, the rotational speed ratio is the 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 system (10) of the fourth embodiment, the rotational speed ratio is between 0.67 and 3.42. The low-stage displacement is calculated by multiplying the "volume of refrigerant drawn in by the low-stage compressor (31) per revolution" by the "rotational speed of the low-stage compressor (31)". The high-stage displacement is calculated by multiplying the "volume of refrigerant drawn in by the high-stage compressor (32) per revolution" by the "rotational speed of the high-stage compressor (32)". Therefore, when the rotational speed ratio is between 0.67 and 3.42, the displacement ratio is between 0.67 and 3.42.

[0015] A fifth aspect of the present disclosure, in any one of the first to fourth aspects, includes a heat exchanger (46) provided upstream of the evaporator (34) in the third passage (63), which 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) before flowing into the evaporator (34).

[0017] A sixth aspect of the present disclosure, in any one of the first to fourth aspects described above, includes a gas-liquid separator (42) that separates the refrigerant that has passed through the first passage (61) into a gaseous refrigerant and a liquid refrigerant, sends the gaseous refrigerant to the second passage (62) and 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 embodiment, 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 incoming refrigerant into gaseous refrigerant and liquid refrigerant. The gaseous refrigerant from the gas-liquid separator (42) flows into the second passage (62). The liquid refrigerant from the gas-liquid separator (42) flows into the third passage (63).

[0019] A seventh aspect of this disclosure is the sixth aspect described above, wherein the displacement ratio during the cooling operation is 0.7 or more and 1.93 or less.

[0020] In the cooling operation of the transport refrigeration device (10) of the seventh embodiment, the displacement ratio is 0.7 or more and 1.93 or less.

[0021] An eighth aspect of the present disclosure is that, in any one of the first to seventh aspects described above, 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 embodiment selectively performs a two-stage compression operation and a single-stage compression operation. . Import During the cooling operation of the refrigeration unit (10), the displacement ratio when the refrigerant circuit (30) is performing a 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 device (10) according to any one of the first to eighth aspects described above, and a container body (2) to which the transport refrigeration device (10) is attached and which forms an internal space (5) for accommodating goods.

[0024] In the ninth aspect, the transport container (1) is constituted by the transport refrigeration device (10) and the container body (2).

Brief Description of the Drawings

[0025] [Figure 1] FIG. 1 is a perspective view of the transport refrigeration device according to Embodiment 1. [Figure 2] FIG. 2 is a schematic longitudinal sectional view of the transport refrigeration device according to Embodiment 1 and a transport container provided with the transport refrigeration device. [Figure 3] FIG. 3 is a piping diagram showing the refrigerant circuit of the transport refrigeration device according to Embodiment 1. [Figure 4] FIG. 4 is a Mollier diagram (pressure-enthalpy diagram) showing the refrigeration cycle performed by the refrigerant circuit according to Embodiment 1. [Figure 5] FIG. 5 is a block diagram showing the configuration of the controller of the transport refrigeration device according to Embodiment 1. [Figure 6] FIG. 6 is a flowchart showing the operation performed by the controller according to Embodiment 1. [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 region of the transport refrigeration device according to Embodiment 1. [Figure 9] FIG. 9 is a piping diagram showing the refrigerant circuit of the transport refrigeration device according to Embodiment 2. [Figure 10] FIG. 10 is a Mollier diagram (pressure-enthalpy diagram) showing the refrigeration cycle performed by the refrigerant circuit according to Embodiment 2. [Figure 11] FIG. 11 is a diagram showing the operable region of the transport refrigeration device according to Embodiment 2. ]>

Modes for Carrying Out the Invention

[0026] Embodiment 1 Embodiment 1 will now be described. This embodiment is a transport container (1) equipped with a transport refrigeration device (10).

[0027] -Shipping container- As shown in Figure 1, the transport container (1) comprises a container body (2) and a transport refrigeration unit (10). The transport container (1) is a reefer container that allows for temperature control inside the container.

[0028] The transport container (1) of this embodiment is mainly used for maritime transport. This transport container (1) is transported loaded onto ships or the like. However, the use of the transport container (1) is not limited to maritime transport. The transport container (1) may also be used for land transport. In this case, the transport container (1) is transported by automobiles such as trucks or by rail.

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

[0030] -Transportation refrigeration equipment- As shown in Figure 2, the transport refrigeration unit (10) is installed in the opening of the container body (2). The transport refrigeration unit (10) comprises a casing (11), a refrigerant circuit (30), and a controller (80). The transport refrigeration unit (10) regulates the temperature of the air (internal air) in the internal space (5).

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

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

[0033] The bulkhead (12) comprises an outer wall (13) and an inner wall (14). The outer wall (13) is located on the outside of the container body (2). The inner wall (14) is located on the inside of the container body (2).

[0034] The outer wall (13) closes the opening of the container body (2). The outer wall (13) is attached to the periphery of the opening of the container body (2). The lower part of the outer wall (13) bulges inward toward the inside of the container body (2). The outer chamber (23) is formed by the lower part of the outer 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 positioned at a distance from the exterior wall (13). An insulating material (16) is provided between the interior wall (14) and the exterior wall (13).

[0036] The partition plate (15) is positioned inside the container body (2) relative to the interior wall (14). An internal airflow channel (20) is formed between the partition wall (12) and the partition plate (15). An air intake (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 airflow channel (20) extends from the air intake (21) to the air outlet (22).

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

[0038] The external heat exchanger (33) is located at the top 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 outside air. The external heat exchanger (33) is generally rectangular in shape. The internal heat exchanger (34) is located 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 inside air.

[0039] <External fan> The transport refrigeration unit (10) is equipped with one external fan (26). The external fan (26) is a propeller fan. The external fan (26) is located in the external chamber (23). The external fan (26) is also located inside the 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) is equipped with an internal fan (27). The internal fan (27) is a propeller fan. The internal fan (27) is positioned in the internal airflow path (20). The internal fan (27) is also positioned above the internal heat exchanger (34). The internal fan (27) supplies internal air to the internal heat exchanger (34).

[0041] <Electrical component box> As shown in Figure 1, the transport refrigeration unit (10) has an electrical component box (28). The electrical component box (28) is located at the top of the outer compartment (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. This section outlines the cooling operation of the transport refrigeration unit (10).

[0043] During 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 heat radiator, and the internal heat exchanger (34) functions as an evaporator. In the external heat exchanger (33), the refrigerant releases heat to the outside 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 air inside the container body (2) circulates between the internal space (5) and the internal airflow channel (20). The air inside the internal space (5) flows into the internal airflow channel (20) through the air intake (21). The air flowing through the internal airflow channel (20) is cooled by the internal heat exchanger (34). The air cooled by the internal heat exchanger (34) is supplied to the internal space (5) through the air outlet (22). In this way, during the cooling operation of the transport refrigeration system (10), the air inside the internal space (5) is cooled, and the temperature inside the internal space (5) is maintained at a predetermined target temperature.

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

[0046] The refrigerant circuit (30) includes a low-stage compressor (31), a high-stage compressor (32), and an external heat exchanger (33), inside the warehouse heat exchanger (34) The system includes an internal heat exchanger (41), a gas-liquid separator (42), a first motorized valve (51), a second motorized valve (52), and a third motorized valve (53).

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

[0048] <First piping, second piping, third piping> In the refrigerant circuit (30), the piping connecting the other end of the external heat exchanger (33) and the inlet of the gas-liquid separator (42) is the first piping (61). The first piping (61) forms the first passage through which all of the refrigerant that has flowed out of the internal heat exchanger (34) flows. In the first piping (61), the first electric valve (51) is located downstream of the internal heat exchanger (41).

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

[0050] In the refrigerant circuit (30), the piping connecting the liquid outlet of the gas-liquid separator (42) and the suction pipe of the low-stage compressor (31) is the third piping (63). The third piping (63) forms a third passage through which the remaining refrigerant that has passed through the first piping (61) flows. In the third piping (63), a third electric valve (53) is located 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 fully enclosed scroll compressors. The low-stage compressor (31) and the high-stage compressor (32) have equal displacement volumes. Displacement volume is the volume of fluid drawn in during one rotation of the compressor.

[0052] Note that the low-stage compressor (31) and the high-stage compressor (32) are not limited to scroll compressors. The low-stage compressor (31) and the high-stage compressor (32) can be any compressor composed of positive displacement fluid machinery.

[0053] <External heat exchanger, internal heat exchanger> As described above, the external heat exchanger (33) and the internal heat exchanger (34) are fin-and-tube heat exchangers that exchange heat between the refrigerant and the air. The external heat exchanger (33) exchanges heat between the refrigerant and the outside air (outside air). The internal heat exchanger (34) exchanges heat between the refrigerant and the inside 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 component that separates the gas-liquid two-phase refrigerant flowing in from the inlet into liquid refrigerant and gaseous refrigerant. In the gas-liquid separator (42), the liquid refrigerant accumulates at the bottom of the gas-liquid separator (42) and flows out through a liquid outlet formed at the bottom of the gas-liquid separator (42). In the gas-liquid separator (42), the gaseous refrigerant accumulates at the top of the gas-liquid separator (42) and flows out through a gas outlet formed at the top of the gas-liquid separator (42).

[0056] <Electric valve> The first electric valve (51), the second electric valve (52), and the third electric valve (53) are all so-called electronic expansion valves. An electronic expansion valve is an electric valve with a variable opening. An electronic expansion valve comprises a valve body and a stepping motor that drives the valve body. When the valve body 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 the suction pipe of the low-stage compressor (31) and the other end connected to the discharge pipe of the low-stage compressor (31). A check valve (66a) is provided in the low-stage bypass pipe (66). The check valve (66a) allows the flow of refrigerant from one end of the low-stage bypass pipe (66) to the other end, and prevents the flow of refrigerant in the reverse direction.

[0059] The advanced bypass pipe (67) has one end connected to the suction pipe of the advanced compressor (32) and the other end connected to the discharge pipe of the advanced compressor (32). A check valve (67a) is provided in the advanced bypass pipe (67). The check valve (67a) allows the flow of refrigerant from one end to the other of the advanced bypass pipe (67) and prevents the flow of refrigerant in the reverse direction.

[0060] <Sensor> The refrigerant circuit (30) is equipped with a low-pressure sensor (71), an intermediate-pressure sensor (72), and a high-pressure sensor (73). Although not shown in the diagram, the refrigerant circuit (30) is also equipped with multiple temperature sensors.

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

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

[0063] <Two-stage compression operation> The two-stage compression operation is performed when the difference between the set internal temperature and the outside 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) are in operation. The low-stage compressor (31) draws in the 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 by the low-stage compressor (31) and the refrigerant that has passed through the second piping (62). The high-stage compressor (32) compresses the drawn-in refrigerant and discharges it. The refrigerant discharged by the high-stage compressor (32) flows into the external heat exchanger (33).

[0065] <Single-stage compression operation> Single-stage compression is performed when the difference between the set internal temperature and the outside temperature is relatively small.

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

[0067] In addition, during single-stage compression operation of the refrigerant circuit (30), the high-stage compressor (32) may be deactivated and the low-stage compressor (31) may operate. In that case, the refrigerant discharged by 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 explained in detail with reference to Figure 4. Figure 4 is a Mollier diagram (pressure-enthalpy diagram) showing the refrigeration cycle. In the refrigeration cycle shown in Figure 4, the high pressure of the refrigeration cycle is higher than the critical pressure of the refrigerant (carbon dioxide) (7.2 MPa), and the intermediate pressure of the refrigeration cycle is lower than the critical pressure of the refrigerant (carbon dioxide).

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

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

[0071] The refrigerant in state H flows from the gas-liquid separator (42) into the third pipe (63), is depressurized by the third electric valve (53), and becomes state K. The refrigerant in state K absorbs heat from the air inside the storage chamber in the internal heat exchanger (34) and evaporates, becoming state A.

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

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

[0074] The controller (80) receives the measured values ​​from the high-pressure sensor (73), the intermediate-pressure sensor (72), and the low-pressure sensor (71). The controller (80) also receives the measured values ​​from the temperature sensor installed in the transport refrigeration unit (10). The controller (80) uses the measured values ​​from the input sensors to individually control the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32). The controller (80) also individually controls the opening degrees of the first electric valve (51), the second electric valve (52), and the third electric valve (53).

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

[0076] Specifically, the controller (80) controls the rotational speed of the low-stage compressor (31) so that the value measured by the low-pressure sensor (71) reaches the target low pressure. If the value measured by the low-pressure sensor (71) is lower than the target low pressure, the controller (80) reduces the rotational speed of the low-stage compressor (31). If the value measured by the low-pressure sensor (71) is higher than the target low pressure, the controller (80) increases the rotational speed of the low-stage compressor (31).

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

[0078] As described above, the low-pressure sensor (71) measures the pressure of the refrigerant drawn into the low-stage compressor (31). The pressure of the refrigerant drawn into the low-stage compressor (31) is substantially equal to the pressure of the refrigerant (evaporation pressure) in the internal heat exchanger (34) which functions 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 rotational 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] <Control of high-stage compressors> The controller (80) controls the rotational speed of the high-stage compressor (32) based on the measurement value from the intermediate pressure sensor (72). The control operation of the controller (80) over the high-stage compressor (32) will be explained with reference to the flowchart in Figure 6.

[0080] In step ST1, the controller (80) determines whether the temperature inside the storage space (5) is stable. Specifically, the controller (80) determines whether the condition that "the temperature of the blown-out air remains within the target temperature range for a predetermined period of time (for example, 30 minutes)" is met. If this condition is met, the controller (80) determines that the temperature inside the storage space (5) is stable and proceeds to step ST2. If this condition is not met, the controller (80) determines that the temperature inside the storage space (5) is not stable and proceeds to step ST8.

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

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

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

[0084] In step ST4, the controller (80) sets the intermediate pressure MP of the refrigeration cycle to the target intermediate pressure, where the pressure ratio RP = 0.7. The pressure ratio RP is calculated using the following formula 1. The controller (80) uses formula 1 and the high-pressure HP and low-pressure LP obtained in step ST2 to calculate the intermediate pressure MP where the pressure ratio RP = 0.7. RP = (MP - LP) / (HP - LP) (Equation 1)

[0085] In 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 in step ST6. If this condition is not met, the controller (80) performs the process in step ST7.

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

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

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

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

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

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

[0092] The controller (80) controls the opening of the first electric valve (51) so that the high pressure HP of the refrigeration cycle reaches the target high pressure when the high pressure HP of the refrigeration cycle is higher than the critical pressure of the refrigerant. If the high pressure HP of the refrigeration cycle is higher than the target high pressure, the controller (80) increases the opening of the first electric valve (51). If the high pressure HP of the refrigeration cycle is lower than the target high pressure, the controller (80) decreases the opening of the first electric valve (51).

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

[0094] The controller (80) adjusts the opening of the third electric valve (53) so that the degree of superheating of the refrigerant at the outlet of the internal heat exchanger (34) is equal to the second target superheating degree. If the degree of superheating of the refrigerant at the outlet of the internal heat exchanger (34) is higher than the second target superheating degree, the controller (80) increases the opening of the third electric valve (53). If the degree of superheating of the refrigerant at the outlet of the internal heat exchanger (34) is lower than the second target superheating degree, the controller (80) decreases the opening of the third electric 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 between 0.7 and 1.93. Here, we will explain why it is desirable to maintain a displacement ratio of 0.7 and 1.93 or less during the cooling operation.

[0096] <Identification of the refrigeration cycle> Given that the set value of the internal 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, given that the set value of the internal temperature (set temperature Ts), the temperature of the outside air (outside air temperature Ta), and the pressure ratio RP are specified, points A to K that identify the refrigeration cycle shown in Figure 4 are identified as follows.

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

[0099] During cooling operation, the controller (80) of the transport refrigeration unit (10) changes ΔT within a range of 5°C to 12°C depending on the cooling load of the storage space. Therefore, when specifying the refrigeration cycle, ΔT was set to 12°C when the difference between the set temperature Ts and the ambient temperature Ta was relatively large, and ΔT was set to 5°C when the difference between the set temperature Ts and the ambient temperature Ta was relatively small.

[0100] When the high pressure (HP) of a 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 a refrigeration cycle is higher than the critical pressure of the refrigerant, the high pressure (HP) will be slightly lower than the design upper pressure limit of the refrigerant circuit.

[0101] The intermediate pressure MP of a 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 represents the refrigerant being drawn into the low-stage compressor (31). The state at point A is substantially equivalent to the state of the refrigerant at the outlet of the internal heat exchanger (34), which functions as an evaporator. The pressure at point A is the low-pressure LP of the refrigeration cycle. Therefore, assuming that the degree of superheating of the refrigerant at the outlet of the internal heat exchanger (34) is 5°C, the temperature and pressure at point A are determined, and consequently, point A is identified.

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

[0104] (Point E) Point E represents the refrigerant at the outlet of the external heat exchanger (33), which functions 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 (assuming 5°C here). The pressure at point E is the high pressure HP of the refrigeration cycle. Thus, the temperature and pressure at point E are determined, 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 state G (gas-liquid two-phase state) is separated into the refrigerant in state H (saturated liquid refrigerant) and the refrigerant in state I (saturated gas refrigerant). The pressures at points H and I are the intermediate pressure MP of the refrigeration cycle. Therefore, point H, which represents the saturated liquid refrigerant state, and point I, which represents the saturated gas refrigerant state, are identified.

[0106] (Point K) Point K represents the refrigerant that has passed through the third electric valve (53). The state at point K is substantially equivalent to the state of the refrigerant flowing into the internal heat exchanger (34), which 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 determined, and consequently, point K is determined.

[0107] (Point J) Assuming the degree of superheating 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 determined, and consequently, point J is identified.

[0108] (Point F, Point G) A portion of the refrigerant that has passed through the first pipe (61) flows into the second pipe (62), and the remainder 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 piping (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 piping (62).

[0110] In the internal heat exchanger (41), the amount of heat dissipated 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 equation 2 holds true. In equation 2, hE is the specific enthalpy of point E, hF is the specific enthalpy of point F, hI is the specific enthalpy of point I, and hJ is the specific enthalpy of 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). Also, M2 / M1 is the dryness of the refrigerant at point G. The specific enthalpy hG at point G and the dryness of the refrigerant at point G, M2 / M1, are correlated. Therefore, using the correlation between the specific enthalpy hG and the dryness M2 / M1 for the refrigerant at point G, and equation 2, the specific enthalpy hF at point F and the specific enthalpy hG at point G can be determined.

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

[0113] (Point C) Point C indicates the refrigerant being drawn into the high-stage compressor (32). The high-stage compressor (32) draws in the refrigerant discharged by 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 by the low-stage compressor (31) is equal to the mass flow rate M3 of the refrigerant in the third pipe (63). Therefore, for the refrigerant at point C, the following equation 3 holds true. 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 of the refrigerant at point G. (M3 / M1) is the wetness of the refrigerant at point G. Since point G is identified, the dryness and wetness 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 represents the refrigerant discharged from the high-stage compressor (32). The pressure at point D is the high-pressure HP of the refrigeration cycle. Assuming the efficiency of the high-stage compressor (32) is "0.7", the specific entropy sD at point D is the value obtained by dividing the specific entropy sC at point C by 0.7 (sD = sC / 0.7). Therefore, the pressure and specific entropy at point D are determined, and as a result, point D is identified.

[0117] <Coefficient of Performance> The coefficient of performance (COP) of the refrigeration cycle shown in Figure 4 is calculated by the following formula 5. The specific enthalpy hK at point K is the specific enthalpy of the refrigerant at the inlet of the internal heat exchanger (34) which functions 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) which functions as an evaporator, and 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 Equation 6 below. (M3 / M1) is the moisture content of the refrigerant at point G. Therefore, the coefficient of performance of the refrigeration cycle shown in Figure 4 is calculated based on Equation 6 below. As mentioned above, (M3 / M1) is the moisture content 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, if the set temperature Ts and the ambient temperature Ta are constant, the high pressure HP and low pressure LP of the refrigeration cycle are also substantially constant. On the other hand, in the refrigerant circuit (30) of this embodiment, the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32) are controlled individually. Therefore, in the refrigerant circuit (30) of this embodiment, the intermediate pressure MP of the refrigeration cycle can be adjusted when the set temperature Ts and the ambient temperature Ta are constant.

[0120] As mentioned 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 higher the pressure ratio RP, the higher the intermediate pressure MP of the refrigeration cycle.

[0121] Figure 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, assuming that the high-pressure HP and low-pressure LP of the refrigeration cycle are constant. As shown in Figure 7, when the high-pressure HP and 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 Figure 7, there is a pressure ratio RP at which the sum of the input WL to the low-stage compressor (31) and the input WH to the high-stage compressor (32) (WL+WH) is smallest.

[0122] When the sum of the input WL to the low-stage compressor (31) and the input WH to the high-stage compressor (32) (WL+WH) 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, there exists a pressure ratio RP at which the coefficient of performance (COP) is highest when the high pressure HP and low pressure LP of the refrigeration cycle are constant. This pressure ratio RP at which the coefficient of performance (COP) is highest is defined as the "optimal pressure ratio".

[0123] By identifying the high-pressure (HP) and low-pressure (LP) pressures of the refrigeration cycle, the corresponding optimal pressure ratios can be determined. Therefore, by identifying the set temperature (Ts) and ambient temperature (Ta), the corresponding optimal pressure ratios can be determined.

[0124] <Optimal displacement ratio> The displacement ratio RV is defined as the value obtained by dividing the displacement amount VL of the low-stage compressor (31) by the displacement amount 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 drawn in by the low-stage compressor (31) per unit time. Therefore, the displacement VL of the low-stage compressor (31) is the value obtained by dividing the mass flow rate of the refrigerant passing through the low-stage compressor (31) (= mass flow rate of refrigerant in the third pipe (63) M3) by the density DA of the refrigerant at the inlet of the low-stage compressor (31) (point A in Figure 4) (VL = M3 / DA).

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

[0127] By identifying the set temperature Ts, ambient temperature Ta, and pressure ratio RP, the corresponding displacement ratio RV can be calculated. As described above, by identifying the set temperature Ts and ambient temperature Ta, the corresponding optimal pressure ratio can be identified, and furthermore, the displacement ratio RV corresponding to the optimal pressure ratio can be identified. The displacement ratio RV corresponding to the optimal pressure ratio is the optimal displacement ratio. Thus, by identifying the set temperature Ts and ambient temperature Ta, the corresponding optimal displacement ratio can be identified.

[0128] The optimal displacement ratio is the ratio that maximizes the coefficient of performance (COP) of the refrigeration cycle at the specified set temperature Ts and ambient temperature Ta.

[0129] <Range of displacement ratio> The hatched area in Figure 8 represents the operable range 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 ambient temperature Ta are within the operable range. Furthermore, in the transport refrigeration system (10) of this embodiment, the high pressure HP of the refrigeration cycle becomes equal to or greater than the critical pressure of the refrigerant when the ambient temperature is 25°C or higher, and the high pressure HP of the refrigeration cycle becomes lower than the critical pressure of the refrigerant when the ambient temperature is below 25°C.

[0130] Figure 8 shows the optimal displacement ratio for some combinations of set temperature Ts and ambient temperature Ta that fall within the operable range. The optimal displacement ratio decreases as the set temperature Ts increases. Furthermore, the optimal displacement ratio decreases as the ambient temperature Ta decreases in both the region where the high pressure HP of the refrigeration cycle is above the critical pressure of the refrigerant and the region where it is below the critical pressure of the refrigerant.

[0131] The optimal displacement ratio reaches a maximum of 1.93 when the set temperature Ts = -30°C and the ambient temperature Ta = 25°C, and a minimum of 0.7 when the set temperature Ts = 10°C and the ambient temperature Ta = 30°C. Therefore, in the transport refrigeration system (10) of this embodiment, by keeping the displacement ratio between 0.7 and 1.93 during cooling operation, the coefficient of performance (COP) of the refrigeration cycle performed by the refrigerant circuit (30) can be kept high.

[0132] -Features of Embodiment 1 (1)- During the cooling operation of the transport refrigeration system (10), the controller (80) individually controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32). Therefore, when the high pressure HP and 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 between 0.7 and 1.93. Therefore, in the transport refrigeration system (10) of this embodiment, the intermediate pressure MP of the refrigeration cycle becomes 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 kept high.

[0134] -Features of Embodiment 1 (2)- During the cooling operation of the transport refrigeration system (10), the controller (80) controls the rotational speed of the low-stage compressor (31) based on the measurement value of the low-pressure sensor (71) and controls the rotational speed of the high-stage compressor (32) based on the measurement value of the intermediate-pressure sensor (72). Therefore, in the transport refrigeration system (10) of this embodiment, the controller (80) individually controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32), thereby setting the low-pressure LP and intermediate-pressure MP of the refrigeration cycle to appropriate values.

[0135] -Features of Embodiment 1 (3)- The displacement amount of the low-stage compressor (31) is calculated by multiplying the displacement volume of the low-stage compressor (31) by the rotational speed of the low-stage compressor (31). The displacement amount of the high-stage compressor (32) 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 the value obtained by dividing the displacement amount of the low-stage compressor by the displacement amount of the high-stage compressor. The rotational speed ratio is the 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 equal displacement volumes. Therefore, in the transport refrigeration system (10) of this embodiment, the displacement volume ratio matches the rotational speed ratio. Consequently, in the transport refrigeration system (10) of this embodiment, the displacement volume ratio and the rotational speed ratio during cooling operation are both between 0.7 and 1.93.

[0137] Embodiment 2 Embodiment 2 will now be described. This embodiment is a modified version of Embodiment 1 in which the configuration of the transport refrigeration system (10) is changed in the transport container (1). The transport refrigeration system (10) of this embodiment differs from that of Embodiment 1 in its refrigerant circuit (30) and controller (80).

[0138] This section will mainly describe the differences between the transport refrigeration system (10) of this embodiment and the transport refrigeration system (10) of Embodiment 1. Regarding the transport refrigeration system (10) of this embodiment and the transport refrigeration system (10) of Embodiment 1, explanations will generally be omitted.

[0139] - Refrigerant Circuit - As shown in Figure 9, in the refrigerant circuit (30) of this embodiment, the first electric valve (51) and the gas-liquid separator (42) provided in the refrigerant circuit (30) of Embodiment 1 are omitted. In addition, in the refrigerant circuit (30) of this embodiment, instead of the internal heat exchanger (41) of Embodiment 1 which is connected to the first pipe (61) and the second pipe (62), an internal heat exchanger (46) is provided which is connected to the second pipe (62) and the third pipe (63).

[0140] <First piping, second piping, third piping> In the refrigerant circuit (30) of this embodiment, the piping connected to the other end of the external heat exchanger (33) is the first piping (61). One end of the first piping (61) is connected to the other end of the external heat exchanger (33). The first piping (61) forms a first passage through which all of the refrigerant that has flowed out of the internal heat exchanger (34) flows.

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

[0142] In the refrigerant circuit (30) of this embodiment, the third pipe (63) is the pipe connecting the other end of the first pipe (61) to the suction pipe of the low-stage compressor (31). The third pipe (63) forms a third passage through which the remaining refrigerant that has passed through the first pipe (61) flows. In the third pipe (63), a third electric valve (53) is located 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, like the refrigerant circuit (30) of Embodiment 1, selectively performs a two-stage compression operation to perform a two-stage compression refrigeration cycle and a single-stage compression operation to perform a single-stage compression refrigeration cycle. Similar to Embodiment 1, both the low-stage compressor (31) and the high-stage compressor (32) operate in the two-stage compression operation, while only one of the low-stage compressor (31) or the high-stage compressor (32) operates in the single-stage compression operation.

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

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

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

[0148] The refrigerant in state E flowing through the third pipe (63) flows into the first channel (46a) of the internal heat exchanger (46), is cooled by the refrigerant flowing through the second channel (46b), and becomes state F. The refrigerant in state F is depressurized by the third electric valve (53) and becomes state G (gas-liquid two-phase state). The refrigerant in state G absorbs heat from the internal air in the internal heat exchanger (34) and evaporates, becoming state A.

[0149] The refrigerant flowing through the second pipe (62) in state 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 becomes state 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 Embodiment 1.

[0151] The controller (80) of this embodiment controls the rotational speed of the low-stage compressor (31) based on the measurement value of the low-pressure sensor (71). The controller (80) of this embodiment also controls the rotational speed of the high-stage compressor (32) based on the measurement value of the intermediate-pressure sensor (72). As an operation to control the rotational speed of the high-stage compressor (32), the controller (80) of this embodiment performs the operation shown in the flowchart of Figure 6, similar to the controller (80) of Embodiment 1.

[0152] In this embodiment, the controller (80) adjusts the opening of the second electric valve (52) so that the degree of superheating of the refrigerant at the outlet of the second flow path (46b) of the internal heat exchanger (46) becomes the first target degree of superheating. In addition, the controller (80) in this embodiment adjusts the opening of the third electric valve (53) so that the degree of superheating of the refrigerant at the outlet of the internal heat exchanger (34) becomes the second target degree of superheating.

[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 between 0.67 and 3.42. Here, we will explain why it is desirable to maintain a displacement ratio of 0.67 to 3.42 during the cooling operation.

[0154] <Identification of the refrigeration cycle> Given that the set value of the internal 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, given that the set value of the internal temperature (set temperature Ts), the temperature of the outside air (outside air temperature Ta), and the pressure ratio RP are specified, points A to I that identify the refrigeration cycle shown in Figure 10 are identified as follows.

[0156] (High pressure HP, intermediate pressure MP, low pressure LP) The high-pressure HP, intermediate-pressure MP, and low-pressure LP of the refrigeration cycle are determined by performing the same process as described in Embodiment 1.

[0157] (Point A, Point B) Point A indicates the refrigerant being drawn into the low-stage compressor (31). Point B indicates the refrigerant being discharged from the low-stage compressor (31). Points A and B are identified by performing the same process as described for Embodiment 1 (the process for identifying points A and B in Figure 4).

[0158] (Point E) Point E indicates the refrigerant at the outlet of the external heat exchanger (33) which functions 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 performing the same process as described for Embodiment 1 (the process for identifying point E in Figure 4).

[0159] (Point H) Point H represents 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 determined, and consequently, point H is determined.

[0160] (Point F) Point F represents the refrigerant at the outlet of the first flow path (46a) of the internal heat exchanger (46). The state at 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 equal to the temperature of the refrigerant at the inlet of its second flow path (46b), the temperature at point F is equal to the temperature at point H. The pressure at point F is the high pressure HP of the refrigeration cycle. Therefore, the temperature and pressure at point F are determined, and as a result, point F is determined.

[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 superheat value SH1 at point I is 0°C (SH1=0). The maximum 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 superheat value SH2 at point I is (temperature at point E) - (saturation temperature of the refrigerant at intermediate pressure MP). Thus, the superheat value at point I is set to a value between SH1 and SH2.

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

[0164] (Point C) A portion of the refrigerant that has passed through the first pipe (61) flows into the second pipe (62), and the remainder 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 piping (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 piping (62).

[0166] In the internal heat exchanger (46), the amount of heat released by the refrigerant in the first channel (46a) is equal to the amount of heat absorbed by the refrigerant in the second channel (46b). Therefore, in the internal heat exchanger (46), the following equation 7 holds true. In equation 7, hE is the specific enthalpy of point E, hF is the specific enthalpy of point F, hH is the specific enthalpy of point H, and hI is the specific enthalpy of point I. Furthermore, by rearranging equation 7, equation 8 is obtained. (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, equation 9 below holds true for the refrigerant at point C. hB is the specific enthalpy of point B, and hC is the specific enthalpy of point C. Furthermore, by rearranging equation 9, we obtain equation 10. hI×M2+hB×M3=hC×(M2+M3) (Formula 9) hI+hB×(M3 / M2)=hC×(1+M3 / M2) (Formula 10)

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

[0169] <Coefficient of Performance> The coefficient of performance (COP) of the refrigeration cycle shown in Figure 10 is calculated by the following formula 11. The specific enthalpy hG at point G is the specific enthalpy of the refrigerant at the inlet of the internal heat exchanger (34) which functions 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) which functions as an evaporator, and 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 Equation 12 below. (M3 / M2) is calculated by Equation 8. Therefore, based on Equation 12 below, the coefficient of performance of the refrigeration cycle shown in Figure 10 is calculated. 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 superheating degree at point I is set to a value between SH1 and SH2. Changing the superheating degree at point I changes the coefficient of performance (COP) of the refrigeration cycle. Therefore, the coefficient of performance (COP) of the refrigeration cycle is calculated for multiple "superheating degrees at point I," and the highest coefficient of performance (COP) among them is taken as the coefficient of performance of the refrigeration cycle performed by the refrigerant circuit (30) of this embodiment.

[0172] <Optimal pressure ratio> Similar to the refrigeration cycle performed by the refrigerant circuit (30) of Embodiment 1, in the refrigeration cycle performed by the refrigerant circuit (30) of this embodiment, there exists a pressure ratio RP at which the coefficient of performance COP is highest when the high pressure HP and low pressure LP of the refrigeration cycle are constant. This pressure ratio RP at which the coefficient of performance COP is highest is called the "optimal pressure ratio".

[0173] By identifying the high-pressure (HP) and low-pressure (LP) pressures of the refrigeration cycle, the corresponding optimal pressure ratios can be determined. Therefore, by identifying the set temperature (Ts) and ambient temperature (Ta), the corresponding optimal pressure ratios can be determined.

[0174] <Optimal displacement ratio> Similar to the transport refrigeration system (10) of Embodiment 1, in the transport refrigeration system (10) of this embodiment, the optimal displacement ratio can be determined by identifying the set temperature Ts and the ambient temperature Ta. The optimal displacement ratio is the displacement ratio that maximizes the coefficient of performance (COP) of the refrigeration cycle at the identified set temperature Ts and ambient temperature Ta.

[0175] <Range of displacement ratio> The hatched area in Figure 11 represents the operable range 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 ambient temperature Ta are within the operable range. Furthermore, in the transport refrigeration system (10) of this embodiment, the high pressure HP of the refrigeration cycle becomes equal to or greater than the critical pressure of the refrigerant when the ambient temperature is 25°C or higher, and the high pressure HP of the refrigeration cycle becomes lower than the critical pressure of the refrigerant when the ambient temperature is below 25°C.

[0176] Figure 11 shows the optimal displacement ratio for some combinations of set temperature Ts and ambient temperature Ta that fall within the operable range. The optimal displacement ratio decreases as the set temperature Ts increases. Furthermore, the optimal displacement ratio decreases as the ambient temperature Ta decreases in both the region where the high pressure HP of the refrigeration cycle is above the critical pressure of the refrigerant and the region where it is below the critical pressure of the refrigerant.

[0177] The optimal displacement ratio reaches a maximum of 3.42 when the set temperature Ts = -30°C and the ambient temperature Ta = 25°C, and a minimum of 0.67 when the set temperature Ts = 30°C and the ambient temperature Ta = 30°C. Therefore, in the transport refrigeration system (10) of this embodiment, by keeping the displacement ratio between 0.67 and 3.42 during cooling operation, the coefficient of performance (COP) of the refrigeration cycle performed by the refrigerant circuit (30) can be kept high.

[0178] -Features of Embodiment 2 (1)- During the cooling operation of the transport refrigeration system (10), the controller (80) individually controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32). Therefore, when the high pressure HP and 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 between 0.67 and 3.42. Therefore, in the transport refrigeration system (10) of this embodiment, the intermediate pressure MP of the refrigeration cycle becomes 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 kept high.

[0180] -Features of Embodiment 2 (2)- In the transport refrigeration system (10) of this embodiment, the low-stage compressor (31) and the high-stage compressor (32) have equal displacement volumes. Therefore, similar to the transport refrigeration system (10) of Embodiment 1, in the transport refrigeration system (10) of this embodiment, the displacement volume ratio matches the rotational speed ratio. Consequently, in the transport refrigeration system (10) of this embodiment, the displacement volume ratio and the rotational speed ratio during cooling operation are both between 0.67 and 3.42.

[0181] While embodiments and modifications have been described above, it will be understood that various changes in form and details are possible without departing from the spirit and scope of the claims. Furthermore, elements of the embodiments, modifications, and other embodiments described above may be combined or substituted as appropriate. In addition, the designations "first," "second," "third," etc. in the specification and claims are used to distinguish the phrases to which these designations are given, and do not limit the number or order of such phrases. [Industrial applicability]

[0182] As described above, this disclosure is useful for transport refrigeration equipment 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 Controllers

Claims

1. A transport refrigeration device (10) that includes a refrigerant circuit (30) that circulates carbon dioxide as a refrigerant to perform a refrigeration cycle, and performs a cooling operation to cool the air inside a transport container (1), The cooling operation is performed under both operating conditions where the high pressure of the refrigeration cycle performed by the refrigerant circuit (30) is equal to or greater than the critical pressure of carbon dioxide, and operating conditions where the high pressure of the refrigeration cycle performed by the refrigerant circuit (30) is less than the critical pressure of carbon dioxide. The above refrigerant circuit (30) includes: A heat exchanger (33) for exchanging heat between the above refrigerant and the outside air, The first passage (61) through which all of the refrigerant that has flowed out from the heat sink (33) flows, A second passage (62) through which a portion of the refrigerant that has passed through the first passage (61) flows, The third passage (63) through which the remaining refrigerant that has passed through the first passage (61) flows, An evaporator (34) is provided in the third passage (63) above to exchange heat between the refrigerant and the air inside the chamber, A low-stage compressor (31) that draws in the refrigerant that has flowed out from the evaporator (34) above, A high-stage compressor (32) is provided that draws in the refrigerant discharged by the low-stage compressor (31) and the refrigerant flowing through the second passage (62). The above transport refrigeration system (10) includes a controller (80) that individually controls the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32). The volume of refrigerant drawn in per unit time by the low-stage compressor (31) is the low-stage displacement amount. The volume of refrigerant drawn in by the above-mentioned high-stage compressor (32) per unit time is the high-stage displacement amount. The value obtained by dividing the above low-stage displacement amount by the above high-stage displacement amount is the displacement amount ratio. 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. The above transport refrigeration device (10) is A low-pressure sensor (71) measures the pressure of the refrigerant drawn into the low-stage compressor (31) mentioned above, An intermediate pressure sensor (72) measures the pressure of the refrigerant drawn into the above-mentioned high-stage compressor (32), The system includes a high-pressure sensor (73) for measuring the pressure of the refrigerant discharged from the above-mentioned high-stage compressor (32), The pressure ratio RP is obtained by dividing the difference between the measured value MP of the intermediate pressure sensor (72) and the measured value LP of the low pressure sensor (71) (MP-LP) by the difference between the measured value HP of the high pressure sensor (73) and the measured value LP of the low pressure sensor (71) (HP-LP). The above controller (80) is The value of the pressure ratio RP is determined based on the value obtained by dividing the measured value HP of the high-pressure sensor (73) by the measured value LP of the low-pressure sensor (71) (HP / LP). Based on the defined pressure ratio RP, the measured value LP from the low-pressure sensor (71), and the measured value HP from the high-pressure sensor (73), the target intermediate pressure is calculated. The rotational speed of the high-stage compressor (32) is controlled so that the measured value MP of the intermediate pressure sensor (72) becomes the target intermediate pressure. Refrigeration equipment for transportation.

2. A transport refrigeration device (10) that includes a refrigerant circuit (30) that circulates carbon dioxide as a refrigerant to perform a refrigeration cycle, and performs a cooling operation to cool the air inside a transport container (1), The cooling operation is performed under both operating conditions where the high pressure of the refrigeration cycle performed by the refrigerant circuit (30) is equal to or greater than the critical pressure of carbon dioxide, and operating conditions where the high pressure of the refrigeration cycle performed by the refrigerant circuit (30) is less than the critical pressure of carbon dioxide. The above refrigerant circuit (30) includes: A heat exchanger (33) for exchanging heat between the above refrigerant and the outside air, The first passage (61) through which all of the refrigerant that has flowed out from the heat sink (33) flows, A second passage (62) through which a portion of the refrigerant that has passed through the first passage (61) flows, The third passage (63) through which the remaining refrigerant that has passed through the first passage (61) flows, An evaporator (34) is provided in the third passage (63) above to exchange heat between the refrigerant and the air inside the chamber, A low-stage compressor (31) that draws in the refrigerant that has flowed out from the evaporator (34) above, A high-stage compressor (32) is provided that draws in the refrigerant discharged by the low-stage compressor (31) and the refrigerant flowing through the second passage (62). The above transport refrigeration system (10) includes a controller (80) that individually controls the rotational speeds of the low-stage compressor (31) and the high-stage compressor (32). The volume of refrigerant drawn in per unit time by the low-stage compressor (31) is the low-stage displacement amount. The volume of refrigerant drawn in by the above-mentioned high-stage compressor (32) per unit time is the high-stage displacement amount. The value obtained by dividing the above low-stage displacement amount by the above high-stage displacement amount is the displacement amount ratio. The above controller (80) is The rotational speed of the lower stage compressor (31) is controlled based on a physical quantity correlated with the evaporation temperature of the refrigerant in the evaporator (34). The rotational speed of the above-mentioned high-stage compressor (32) is set to a value such that the displacement ratio is 0.67 or more and 3.42 or less. The above transport refrigeration device (10) is A low-pressure sensor (71) measures the pressure of the refrigerant drawn into the low-stage compressor (31) mentioned above, An intermediate pressure sensor (72) measures the pressure of the refrigerant drawn into the above-mentioned high-stage compressor (32), The system includes a high-pressure sensor (73) for measuring the pressure of the refrigerant discharged from the above-mentioned high-stage compressor (32), The pressure ratio RP is obtained by dividing the difference between the measured value MP of the intermediate pressure sensor (72) and the measured value LP of the low pressure sensor (71) (MP-LP) by the difference between the measured value HP of the high pressure sensor (73) and the measured value LP of the low pressure sensor (71) (HP-LP). The above controller (80) is The value of the pressure ratio RP is determined based on the value obtained by dividing the measured value HP of the high-pressure sensor (73) by the measured value LP of the low-pressure sensor (71) (HP / LP). Based on the defined pressure ratio RP, the measured value LP from the low-pressure sensor (71), and the measured value HP from the high-pressure sensor (73), the target intermediate pressure is calculated. The rotational speed of the high-stage compressor (32) is controlled so that the measured value MP of the intermediate pressure sensor (72) becomes the target intermediate pressure. Refrigeration equipment for transportation.

3. The lower stage compressor (31) and the upper stage compressor (32) described above have the same volume of refrigerant drawn in per rotation. The rotational speed ratio is obtained by dividing the rotational speed of the lower stage compressor (31) by the rotational speed of the higher stage compressor (32). The rotational speed ratio during the above cooling operation is 0.67 or more and 3.42 or less. A transport refrigeration apparatus according to claim 1 or 2.

4. The system includes a heat exchanger (46) located upstream of the evaporator (34) in the third passage (63), which cools the refrigerant flowing through the third passage (63) by exchanging heat with the refrigerant flowing through the second passage (62). A transport refrigeration apparatus according to claim 1 or 2.

5. A gas-liquid separator (42) separates the refrigerant that has passed through the first passage (61) into gaseous refrigerant and liquid refrigerant, sends the gaseous refrigerant to the second passage (62) and the liquid refrigerant to the third passage (63), The system includes 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). A transport refrigeration apparatus according to claim 1 or 2.

6. The displacement ratio during the above cooling operation is 0.7 or more and 1.93 or less. The transport refrigeration apparatus according to claim 5.

7. The above refrigerant circuit (30) is, A two-stage compression operation is performed by operating both the lower-stage compressor (31) and the higher-stage compressor (32) described above to carry out the refrigeration cycle, The system selectively performs a single-stage compression operation in which one of the above-mentioned low-stage compressor (31) and high-stage compressor (32) is activated while the other is stopped to carry out the refrigeration cycle. A transport refrigeration apparatus according to claim 1 or 2.

8. A transport refrigeration device (10) according to claim 1 or 2, The above-mentioned transport refrigeration device (10) is attached to a container body (2) that forms an internal space (5) for storing cargo. Shipping container.

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