Transportation-use refrigeration device and transportation-use container

Abstract

A transport refrigeration apparatus (10) performs a cooling operation to cool internal air. In a refrigerant circuit (30), a refrigerant having passed through a radiator (33) is distributed to a second passage (62) and a third passage (63). A low-stage compressor (31) sucks a refrigerant having passed through an evaporator (34) in the third passage (63). A high-stage compressor (32) sucks a refrigerant discharged from the low-stage compressor (31) and a refrigerant flowing in the second passage (62). A displacement volume ratio (= low-stage displacement volume / high-stage displacement volume) in the cooling operation is 0.67 or more and 3.42 or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a transport refrigeration apparatus and a transport container.BACKGROUND ART

[0002] Patent Document 1 discloses a transport refrigeration apparatus that cools the inside of a transport container or the like. This transport refrigeration apparatus has a compressor with a first compression stage (the low stage) and a second compression stage (the high stage) and performs a so-called two-stage compression refrigeration cycle. Paragraph

[0022] of Patent Document 1 discloses that each of the first compression stage and the second compression stage may be configured as an independent compressor.CITATION LISTPATENT DOCUMENT

[0003] Patent Document 1: WO2011 / 091014SUMMARY OF THE INVENTIONTECHNICAL PROBLEM

[0004] In the transport refrigeration apparatus of Patent Document 1, if each of the first compression stage (the low stage) and the second compression stage (the high stage) is configured as an independent compressor, the displacement volume of each of the low-stage compressor as the first compression stage and the high-stage compressor as the second compression stage can be adjusted individually. The displacement volume of a compressor is calculated by multiplying the displacement volume of a compressor by the rotational speed of a compressor.

[0005] However, regarding the transport refrigeration apparatus, nothing has been considered about what range the displacement volume of the low-stage compressor and the displacement volume of the high-stage compressor should be controlled within. Accordingly, it may be impossible to operate in an appropriate state the transport refrigeration apparatus that performs the two-stage compression refrigeration cycle.

[0006] An object of the present disclosure is to operate in an appropriate state a transport refrigeration apparatus that performs a two-stage compression refrigeration cycle.SOLUTION TO THE PROBLEM

[0007] A first aspect of the present disclosure is directed to a transport refrigeration apparatus (10) that comprises a refrigerant circuit (30) configured to perform a refrigeration cycle by circulating carbon dioxide as a refrigerant and that is configured to perform a cooling operation to cool internal air of a transport container (1). The refrigerant circuit (30) includes a radiator (33) configured to exchange heat between the refrigerant and outdoor air, a first passage (61) through which all of the refrigerant flowing out of the radiator (33) flows, a second passage (62) through which part of the refrigerant having passed through the first passage (61) flows, a third passage (63) through which a rest of the refrigerant having passed through the first passage (61) flows, an evaporator (34) provided in the third passage (63) and configured to exchange heat between the refrigerant and the internal air, a low-stage compressor (31) configured to suck the refrigerant flowing out of the evaporator (34), and a high-stage compressor (32) configured to suck the refrigerant discharged from the low-stage compressor (31) and the refrigerant flowing in the second passage (62). In this aspect, a volume of the refrigerant sucked per unit time by the low-stage compressor (31) is a low-stage displacement volume, a volume of the refrigerant sucked per the unit time by the high-stage compressor (32) is a high-stage displacement volume, a value obtained by dividing the low-stage displacement volume by the high-stage displacement volume is a displacement volume ratio, and the displacement volume ratio in the cooling operation is 0.67 or more and 3.42 or less.

[0008] In the first aspect, the transport refrigeration apparatus (10) performs the cooling operation. In the cooling operation, the refrigerant circuit (30) performs the refrigeration cycle, and the evaporator (34) cools the internal air. In the refrigerant circuit (30) in the cooling operation, the refrigerant having passed through the first passage (61) is distributed to the second passage (62) and the third passage (63). The low-stage compressor (31) sucks the refrigerant having flowed into the third passage (63) and then having passed through the evaporator (34), then compresses the sucked refrigerant, and then discharges the compressed refrigerant. The high-stage compressor (32) sucks the refrigerant discharged from the low-stage compressor (31) and the refrigerant flowing in the second passage (62), then compresses the sucked refrigerant, and then discharges the compressed refrigerant. The displacement volume ratio in the cooling operation is 0.67 or more and 3.42 or less. Accordingly, in the cooling operation, the transport refrigeration apparatus (10) can be operated in an appropriate state.

[0009] A second aspect of the present disclosure is an embodiment of the first aspect. In the second aspect, the transport refrigeration apparatus further includes: a controller (80) configured to control a rotational speed of the low-stage compressor (31) and a rotational speed of the high-stage compressor (32) individually, wherein the controller (80) sets each of the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) to a value at which the displacement volume ratio in the cooling operation is 0.67 or more and 3.42 or less.

[0010] In the second aspect, the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) individually. In the transport refrigeration apparatus (10), the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) individually, whereby the displacement volume ratio in the cooling operation is 0.67 or more and 3.42 or less.

[0011] A third aspect of the present disclosure is an embodiment of the first aspect. In the third aspect, the container refrigeration apparatus further includes: a controller (80) configured to control a rotational speed of the low-stage compressor (31) and a rotational speed of the high-stage compressor (32) individually, wherein the controller (80) controls the rotational speed of the low-stage compressor (31) based on a physical quantity that correlates to an evaporation temperature of the refrigerant in the evaporator (34), and sets the rotational speed of the high-stage compressor (32) to a value at which the displacement volume ratio is 0.67 or more and 3.42 or less.

[0012] In the third aspect, the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) individually. The controller (80) controls the rotational speed of the low-stage compressor (31) based on a predetermined physical quantity. The controller (80) sets the rotational speed of the high-stage compressor (32) to a value at which the displacement volume ratio is 0.67 or more and 3.42 or less with respect to the rotational speed of the low-stage compressor (31) at the same time.

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

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

[0015] A fifth aspect of the present disclosure is an embodiment of any one of the first to fourth aspects. In the fifth aspect, the transport refrigeration apparatus further includes: a heat exchanger (46) provided upstream of the evaporator (34) in the third passage (63) and configured to cool the refrigerant flowing in the third passage (63) by exchanging heat between the refrigerant flowing in the third passage (63) and the refrigerant flowing in the second passage (62).

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

[0017] A sixth aspect of the present disclosure is an embodiment of any one of the first to fourth aspects. In the sixth aspect, the transport refrigeration apparatus further includes: a gas-liquid separator (42) configured to separate the refrigerant having passed through the first passage (61) into a gas refrigerant and a liquid refrigerant, send the gas refrigerant to the second passage (62), and send the liquid refrigerant to the third passage (63); and a heat exchanger (41) configured to cool the refrigerant flowing in the first passage (61) by exchanging heat between the refrigerant flowing in the first passage (61) and the refrigerant flowing in the second passage (62).

[0018] In the sixth aspect, the refrigerant flowing in 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 received refrigerant into the gas refrigerant and the liquid refrigerant. The gas refrigerant in the gas-liquid separator (42) flows into the second passage (62). The liquid refrigerant in the gas-liquid separator (42) flows into the third passage (63).

[0019] A seventh aspect of the present disclosure is an embodiment of the sixth aspect. In the seventh aspect, the displacement volume ratio in the cooling operation is 0.7 or more and 1.93 or less.

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

[0021] An eighth aspect of the present disclosure is an embodiment of any one of the first to seventh aspects. In the eighth aspect, the refrigerant circuit (30) selectively performs a two-stage compressing operation to operate both the low-stage compressor (31) and the high-stage compressor (32) to perform the refrigeration cycle, and a single-stage compressing operation to operate one of the low-stage compressor (31) or the high-stage compressor (32) and stop the other one of the low-stage compressor (31) or the high-stage compressor (32) to perform the refrigeration cycle.

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

[0023] A ninth aspect of the present disclosure is directed to a transport container (1) including: the transport refrigeration apparatus (10) of any one of the first to eighth aspects; and a container body (2) to which the transport refrigeration apparatus (10) is attached and which forms an internal space (5) for storing cargo.

[0024] In the ninth aspect, the transport refrigeration apparatus (10) and the container body (2) form the transport container (1).BRIEF DESCRIPTION OF THE DRAWINGS

[0025] [FIG. 1] FIG. 1 is a perspective view of a transport refrigeration apparatus of a first embodiment. [FIG. 2] FIG. 2 is a schematic vertical cross-sectional view of the transport refrigeration apparatus and a transport container provided with the transport refrigeration apparatus according to the first embodiment. [FIG. 3] FIG. 3 is a piping diagram of a refrigerant circuit of the transport refrigeration apparatus of the first embodiment. [FIG. 4] FIG. 4 is a Mollier diagram (a pressure-enthalpy diagram) of a refrigeration cycle performed by the refrigerant circuit of the first embodiment. [FIG. 5] FIG. 5 is a block diagram of a configuration of a controller of the transport refrigeration apparatus of the first embodiment. [FIG. 6] FIG. 6 is a flowchart of an operation performed by the controller of the first embodiment. [FIG. 7] FIG. 7 is a graph showing a relationship between the input to a low-stage compressor and a high-stage compressor and a pressure ratio. [FIG. 8] FIG. 8 shows an operating region in which the transport refrigeration apparatus of the first embodiment can operate. [FIG. 9] FIG. 9 is a piping diagram of a refrigerant circuit of a transport refrigeration apparatus of a second embodiment. [FIG. 10] FIG. 10 is a Mollier diagram (a pressure-enthalpy diagram) of a refrigeration cycle performed by the refrigerant circuit of the second embodiment. [FIG. 11] FIG. 11 shows an operating region in which the transport refrigeration apparatus of the second embodiment can operate. DESCRIPTION OF EMBODIMENTS<<First Embodiment>>

[0026] A first embodiment will be described. This embodiment is directed to a transport container (1) provided with a transport refrigeration apparatus (10).-Transport Container-

[0027] As shown in FIG. 1, the transport container (1) is provided with a container body (2) and the transport refrigeration apparatus (10). The transport container (1) is a reefer container which can manage an internal temperature.

[0028] The transport container (1) of this embodiment is used mainly for marine transportation. The transport container (1) is loaded on a ship or the like and then transported. Use of the transport container (1) is not limited to marine transportation. The transport container (1) may be used for land transportation. In this case, the transport container (1) is transported by an automobile such as a truck or by rail.-Container Body-

[0029] As shown in FIG. 2, the container body (2) has a hollow box shape. The container body (2) is horizontally long. One end of the container body (2) in the longitudinal direction is provided with an opening. The opening of the container body (2) is closed by the transport refrigeration apparatus (10). The container body (2) forms an internal space (5) for storing cargo.-Transport Refrigeration Apparatus-

[0030] As shown in FIG. 2, the transport refrigeration apparatus (10) is attached to the opening of the container body (2). The transport refrigeration apparatus (10) has a casing (11), a refrigerant circuit (30), and a controller (80). The transport refrigeration apparatus (10) adjusts a temperature of the air (the internal air) in the internal space (5).<Casing>

[0031] The casing (11) has a division wall (12) and a partition plate (15).

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

[0033] The division wall (12) has an external wall (13) and an internal wall (14). The external wall (13) is located outside the container body (2). The internal wall (14) is located inside the container body (2).

[0034] The external wall (13) closes the opening of the container body (2). The external wall (13) is attached to a peripheral portion of the opening of the container body (2). A lower portion of the external wall (13) bulges toward the inside of the container body (2). The external chamber (23) is formed by the lower portion of the external wall (13).

[0035] The internal wall (14) faces the external wall (13). The internal wall (14) has a shape along the external wall (13). The internal wall (14) is spaced apart from the external wall (13). A thermal insulation material (16) is provided between the internal wall (14) and the external wall (13).

[0036] The partition plate (15) is disposed further inward of the container body (2) than the internal wall (14). The internal flow path (20) is formed between the division wall (12) and the partition plate (15). An air suction port (21) is formed between the upper end of the partition plate (15) and the top panel of the container body (2). An air discharge port (22) is formed between the lower end of the partition plate (15) and the lower end of the division wall (12). The internal flow path (20) extends from the air suction port (21) to the air discharge port (22).<Refrigerant Circuit>

[0037] The refrigerant circuit (30) is a closed circuit filled with a refrigerant. The refrigerant circuit (30) circulates a refrigerant to perform a vapor compression refrigeration cycle. The refrigerant circuit (30) has an outer heat exchanger (33) and an inner heat exchanger (34). The refrigerant circuit (30) will be described in detail later.

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

[0039] The transport refrigeration apparatus (10) has one external fan (26). The external fan (26) is a propeller fan. The external fan (26) is disposed in the external chamber (23). The external fan (26) is disposed inside the outer heat exchanger (33) which has a tubular shape. The external fan (26) sends the external air to the outer heat exchanger (33).<Internal Fan>

[0040] The transport refrigeration apparatus (10) has an internal fan (27). The internal fan (27) is a propeller fan. The internal fan (27) is disposed in the internal flow path (20). The internal fan (27) is disposed above the inner heat exchanger (34). The internal fan (27) sends the internal air to the inner heat exchanger (34).<Electric Component Box>

[0041] As shown in FIG. 1, the transport refrigeration apparatus (10) includes an electric component box (28). The electric component box (28) is disposed in an upper portion of the external chamber (23). The electric component box (28) houses electric components such as an inverter board and a control board.-Cooling Operation of Transport Refrigeration Apparatus-

[0042] The transport refrigeration apparatus (10) performs the cooling operation to cool the internal air. Here, an outline of the cooling operation of the transport refrigeration apparatus (10) will be described.

[0043] In the cooling operation of the transport refrigeration apparatus (10), the refrigerant circuit (30) creates a refrigeration cycle. In the refrigerant circuit (30), the outer heat exchanger (33) functions as a radiator, and the inner heat exchanger (34) functions as an evaporator. In the outer heat exchanger (33), the refrigerant dissipates heat to the external air. In the inner heat exchanger (34), the refrigerant absorbs heat from the internal air to evaporate. The inner heat exchanger (34) cools the internal air.

[0044] The internal air in the container body (2) circulates between the internal space (5) and the internal flow path (20). The internal air in the internal space (5) flows into the internal flow path (20) through the air suction port (21). The internal air flowing in the internal flow path (20) is cooled by the inner heat exchanger (34). The internal air cooled by the inner heat exchanger (34) is supplied to the internal space (5) through the air discharge port (22). In this manner, in the cooling operation of the transport refrigeration apparatus (10), the internal air in the internal space (5) is cooled, and the temperature of the internal space (5) is held at the predetermined target temperature.-Refrigerant Circuit-

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

[0046] The refrigerant circuit (30) has a low-stage compressor (31), a high-stage compressor (32), the outer heat exchanger (33), the inner heat exchanger (34), 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] A discharge pipe of the low-stage compressor (31) is connected to a suction pipe of the high-stage compressor (32). A discharge pipe of the high-stage compressor (32) is connected to one end of the outer heat exchanger (33). The other end of the outer heat exchanger (33) is connected to an inflow port of the gas-liquid separator (42) via the internal heat exchanger (41) and the first motorized valve (51). A gas outflow port of the gas-liquid separator (42) is connected to the suction pipe of the high-stage compressor (32) via the second motorized valve (52) and the internal heat exchanger (41). A liquid outflow port of the gas-liquid separator (42) is connected to one end of the inner heat exchanger (34) via the third motorized valve (53). The other end of the inner heat exchanger (34) is connected to the suction pipe of the low-stage compressor (31).<First Piping, Second Piping, and Third Piping>

[0048] In the refrigerant circuit (30), the piping that connects the other end of the outer heat exchanger (33) and the inflow port of the gas-liquid separator (42) is a first piping (61). The first piping (61) forms a first passage through which all of the refrigerant flowing out of the inner heat exchanger (34) flows. In the first piping (61), the first motorized valve (51) is disposed downstream of the internal heat exchanger (41).

[0049] In the refrigerant circuit (30), the piping that connects the gas outflow port of the gas-liquid separator (42) and the suction pipe of the high-stage compressor (32) is a second piping (62). The second piping (62) forms a second passage through which part of the refrigerant having passed through the first piping (61) flows. In the second piping (62), the second motorized valve (52) is disposed upstream of the internal heat exchanger (41).

[0050] In the refrigerant circuit (30), the piping that connects the liquid outflow port of the gas-liquid separator (42) and the suction pipe of the low-stage compressor (31) is a third piping (63). The third piping (63) forms a third passage through which the rest of the refrigerant having passed through the first piping (61) flows. In the third piping (63), the third motorized valve (53) is disposed upstream of the inner heat exchanger (34).<Low-Stage Compressor and High-Stage Compressor>

[0051] Each of the low-stage compressor (31) and the high-stage compressor (32) is a hermetic scroll compressor. The low-stage compressor (31) and the high-stage compressor (32) are equal in displacement volume. The displacement volume is the volume of fluid sucked per rotation by the compressor.

[0052] Each of the low-stage compressor (31) and the high-stage compressor (32) is not limited to a scroll compressor. Each of the low-stage compressor (31) and the high-stage compressor (32) may be a compressor configured as a positive displacement fluid machine.<Outer Heat Exchanger and Inner Heat Exchanger>

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

[0054] The internal heat exchanger (41) is a heat exchanger that exchanges heat between a refrigerant and a refrigerant. The internal heat exchanger (41) of this embodiment 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 piping (61). The second flow path (41b) of the internal heat exchanger (41) is connected to the second piping (62). The internal heat exchanger (41) exchanges heat between a refrigerant flowing in the first flow path (41a) and a refrigerant flowing in the second flow path (41b).<Gas-Liquid Separator>

[0055] The gas-liquid separator (42) is a container-shape member and separates a refrigerant in the gas-liquid two-phase state flowing from the inflow port into a liquid refrigerant and a gas refrigerant. In the gas-liquid separator (42), the liquid refrigerant accumulates in a lower portion of the gas-liquid separator (42) and flows out from the liquid outflow port at a bottom portion of the gas-liquid separator (42). In the gas-liquid separator (42), the gas refrigerant accumulates in an upper portion of the gas-liquid separator (42) and flows out from the gas outflow port at an upper portion of the gas-liquid separator (42).<Electric Valve>

[0056] Each of the first motorized valve (51), the second motorized valve (52), and the third motorized valve (53) is a so-called electronic expansion valve. The electronic expansion valve is a motorized valve of which the opening degree is variable. The electronic expansion valve has a valve body and a stepper motor that drives the valve body. When the valve body is moved by the stepper motor, the opening degree of the electronic expansion valve changes.<Bypass Pipe and Check Valve>

[0057] The refrigerant circuit (30) has a low-stage bypass pipe (66) and a high-stage bypass pipe (67).

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

[0059] One end of the high-stage bypass pipe (67) is connected to the suction pipe of the high-stage compressor (32), and the other end is connected to the discharge pipe of the high-stage compressor (32). The high-stage bypass pipe (67) is provided with a check valve (67a). The check valve (67a) allows a refrigerant to flow from one end to the other end of the high-stage bypass pipe (67) and blocks a refrigerant from flowing in the reverse direction.<Sensors>

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

[0061] The low-pressure sensor (71) is connected to the suction pipe of the low-stage compressor (31) and measures pressure of a refrigerant sucked 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 pressure of a refrigerant sucked 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 pressure of a refrigerant discharged from the high-stage compressor (32).-Operation of Refrigerant Circuit-

[0062] The refrigerant circuit (30) selectively performs a two-stage compressing operation to perform a two-stage compression refrigeration cycle and a single-stage compressing operation to perform a single-stage compression refrigeration cycle.<Double-Stage Compressing Operation>

[0063] The two-stage compressing operation is performed if the difference between the set value of the internal temperature and the outdoor air temperature is relatively large.

[0064] In the two-stage compressing operation of the refrigerant circuit (30), both the low- stage compressor (31) and the high-stage compressor (32) operate. The low-stage compressor (31) sucks a refrigerant evaporated in the inner heat exchanger (34). The low-stage compressor (31) compresses a sucked refrigerant and discharges a compressed refrigerant. The high-stage compressor (32) sucks a refrigerant having discharged from the low-stage compressor (31) and a refrigerant having passed through the second piping (62). The high-stage compressor (32) compresses a sucked refrigerant and discharges a compressed refrigerant. The refrigerant discharged from the high-stage compressor (32) flows into the outer heat exchanger (33).<Single-Stage Compressing Operation>

[0065] The single-stage compressing operation is performed if the difference between the set value of the internal temperature and the outdoor air temperature is relatively small.

[0066] In the single-stage compressing operation of the refrigerant circuit (30), the low-stage compressor (31) stops and the high-stage compressor (32) operates. The high-stage compressor (32) sucks a refrigerant evaporated in the inner heat exchanger (34) through the low-stage bypass pipe (66). The high-stage compressor (32) compresses a sucked refrigerant and discharges a compressed refrigerant. The refrigerant discharged from the high-stage compressor (32) flows into the outer heat exchanger (33).

[0067] In the single-stage compressing operation of the refrigerant circuit (30), the high-stage compressor (32) may stop and the low-stage compressor (31) may operate. In this case, the refrigerant discharged from the low-stage compressor (31) flows into the outer heat exchanger (33) through the high-stage bypass pipe (67).-Double-Stage Compression Refrigeration Cycle-

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

[0069] The refrigerant in the state at point A turns into the state at point B by being compressed by the low-stage compressor (31). In the refrigerant circuit (30), the refrigerant in the state at point B having discharged from the low-stage compressor (31) and the refrigerant in the state at point J having passed through the second piping (62) merge and turn into the refrigerant in the state at point C. The refrigerant in the state at point C turns into the state at point D by being compressed by the high-stage compressor (32).

[0070] The refrigerant in the state at point D turns into the state at point E by dissipating heat to the external air (the outdoor air) in the outer heat exchanger (33). The refrigerant in the state at point E turns into the state at point F by flowing into the first flow path (41a) of the internal heat exchanger (41) through the first piping (61) and then being cooled by the refrigerant flowing in the second flow path (41b). The refrigerant in the state at point F turns into the state at point G (the gas-liquid two-phase state) by being decompressed by the first motorized valve (51). The refrigerant in the state at point G flows into the gas-liquid separator (42) and is separated into a refrigerant in the state at point H (a saturated liquid refrigerant) and a refrigerant in the state at point I (a saturated gas refrigerant).

[0071] The refrigerant in the state at point H turns into the state at point K by flowing into the third piping (63) from the gas-liquid separator (42) and then being decompressed by the third motorized valve (53). The refrigerant in the state at point K turns into the state at point A by absorbing heat from the internal air in the inner heat exchanger (34) and then being evaporated.

[0072] The refrigerant in the state at point I flows into the second piping (62) from the gas- liquid separator (42). The refrigerant in the state at point I flowing in the second piping (62) turns into the state at point J by flowing into the second flow path (41b) of the internal heat exchanger (41) through the second motorized valve (52) and then absorbing heat from the refrigerant flowing in the first flow path (41a).-Controller-

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

[0074] The controller (80) receives measured values of the high-pressure sensor (73), the intermediate-pressure sensor (72), and the low-pressure sensor (71). The controller (80) receives measured values of the temperature sensors provided in the transport refrigeration apparatus (10). By using the measured value received from the sensors, the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) individually. The controller (80) controls the opening degrees of the first motorized valve (51), the second motorized valve (52), and the third motorized valve (53), individually.<Control of Low-Stage Compressor>

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

[0076] Specifically, the controller (80) controls the rotational speed of the low-stage compressor (31) so that the measured value of the low-pressure sensor (71) becomes the target low pressure. If the measured value of 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 measured value of 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) defines the target low pressure based on a set temperature Ts, a set value of the internal temperature. Specifically, the controller (80) sets a saturation pressure that corresponds to a temperature lower by a predetermined value than the set temperature Ts (Ts-ΔT) as the target low pressure.

[0078] As described above, the low-pressure sensor (71) measures pressure of a refrigerant sucked into the low-stage compressor (31). The pressure of a refrigerant sucked into the low- stage compressor (31) is substantially equal to the pressure of a refrigerant in the inner heat exchanger (34) functioning as an evaporator (the evaporation pressure). The evaporation pressure of a refrigerant is a physical quantity that correlates to the evaporation temperature of a refrigerant. Thus, the controller (80) controls the rotational speed of the low-stage compressor (31) based on the physical quantity that correlates to the evaporation temperature of a refrigerant in the inner heat exchanger (34).<Control of High-Stage Compressor>

[0079] The controller (80) controls the rotational speed of the high-stage compressor (32) based on the measured value of the intermediate-pressure sensor (72). The control operation that the controller (80) performs to control the high-stage compressor (32) will be described with reference to the flowchart in FIG. 6.

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

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

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

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

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

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

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

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

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

[0089] After ending the process of step ST4, step ST6, step ST7, or step ST8, the controller (80) performs the process of step ST9. In the process of step ST9, the controller (80) obtains 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. After ending the process of step ST9, the controller (80) performs the process of step ST10.

[0090] In the process of step ST10, the controller (80) controls the rotational speed of the high-stage compressor (32) based on the measured value of the intermediate-pressure sensor (72) and the target intermediate pressure defined in the process of step ST4, step ST6, step ST7, or step ST8. If the measured value of 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 measured value of 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).<Control of Electric Valves>

[0091] As described above, the controller (80) controls the opening degrees of the first motorized valve (51), the second motorized valve (52), and the third motorized valve (53), individually.

[0092] The controller (80) controls the opening degree of the first motorized valve (51) so that the high pressure HP of the refrigeration cycle becomes the target high pressure in the operating state in which 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 degree of the first motorized valve (51). If the high pressure HP of the refrigeration cycle is lower than the target high pressure, the controller (80) reduces the opening degree of the first motorized valve (51).

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

[0094] The controller (80) adjusts the opening degree of the third motorized valve (53) so that the degree of superheat of a refrigerant at the outlet of the inner heat exchanger (34) becomes a second target degree of superheat. If the degree of superheat of a refrigerant at the outlet of the inner heat exchanger (34) is higher than the second target degree of superheat, the controller (80) increases the opening degree of the third motorized valve (53). If the degree of superheat of a refrigerant at the outlet of the inner heat exchanger (34) is lower than the second target degree of superheat, the controller (80) reduces the opening degree of the third motorized valve (53).-Displacement Volume Ratio-

[0095] In the transport refrigeration apparatus (10) of this embodiment, the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32). In the cooling operation of the transport refrigeration apparatus (10), the displacement volume ratio is 0.7 or more and 1.93 or less. Here, the reason it is desirable that the displacement volume ratio in the cooling operation is 0.7 or more and 1.93 or less will be described.<Identify Refrigeration Cycle>

[0096] Only if the set value of the internal temperature (the set temperature Ts), the temperature of the external air (the outdoor air temperature Ta), and the pressure ratio RP have been specified, the refrigeration cycle performed by the refrigerant circuit (30) is identified. As described above, the pressure ratio RP is defined by Equation 1.

[0097] Specifically, only if the set value of the internal temperature (the set temperature Ts), the temperature of the external air (the outdoor air temperature Ta), and the pressure ratio RP have been specified, point A to point K, which identify the refrigeration cycle shown in FIG. 4, are specified as described below.(High Pressure HP, Intermediate Pressure MP, and Low Pressure LP)

[0098] In the cooling operation, the transport refrigeration apparatus (10) is controlled so that the temperature of the air discharged from the air discharge port (22) becomes the set temperature Ts. The evaporation temperature Te of the refrigerant in the inner heat exchanger (34) becomes a temperature lower by a predetermined value than the set temperature Ts (Ts-ΔT). That is, the low pressure LP of the refrigeration cycle is the saturation pressure that corresponds to the temperature (Ts-ΔT).

[0099] In the cooling operation, the controller (80) of the transport refrigeration apparatus (10) changes ΔT within the range of 5°C to 12°C according to the cooling load of the internal space or the like. Then, in order to identify the refrigeration cycle, ΔT=12°C is employed if the difference between the set temperature Ts and the outdoor air temperature Ta is relatively large, and ΔT=5°C is employed if the difference between the set temperature Ts and the outdoor air temperature Ta is relatively small.

[0100] When the high pressure HP of the refrigeration cycle is higher than the critical pressure of the refrigerant, the cooling performance obtained by the refrigeration cycle becomes higher as the high pressure HP of the refrigeration cycle becomes higher. Accordingly, when the high pressure HP of the refrigeration cycle is higher than the critical pressure of the refrigerant, the high pressure HP is slightly lower than the design upper limit pressure of the refrigerant circuit.

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

[0102] Point A represents a refrigerant sucked into the low-stage compressor (31). The state at point A is substantially equal to the state of a refrigerant at the outlet of the inner heat exchanger (34) functioning as an evaporator. The pressure at point A is the low pressure LP of the refrigeration cycle. Accordingly, assuming that the degree of superheat of a refrigerant at the outlet of the inner heat exchanger (34) is 5°C, the temperature and pressure at point A are specified, and as a result, point A is specified.(Point B)

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

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

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

[0106] Point K represents a refrigerant having passed through the third motorized valve (53). The state at point K is substantially equal to the state of a refrigerant flowing into the inner heat exchanger (34) functioning 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. Accordingly, the pressure and specific enthalpy at point K are specified, and as a result, point K is specified.(Point J)

[0107] Assuming that the degree of superheat of a 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 higher by 10°C than the temperature at point I. The pressure at point J is the intermediate pressure MP of the refrigeration cycle. Accordingly, the temperature and pressure at point J are specified, and as a result, point J is specified.(Point F and Point G)

[0108] Part of a refrigerant having passed through the first piping (61) flows into the second piping (62), and the rest of the refrigerant flows into the third piping (63). Thus, the mass flow rate M1 of a refrigerant in the first piping (61) is equal to the sum of the mass flow rate M2 of a refrigerant in the second piping (62) and the mass flow rate M3 of a refrigerant in the third piping (63) (M1=M2+M3).

[0109] In the first flow path (41a) of the internal heat exchanger (41), the state of a refrigerant changes from point E to point F. The mass flow rate of a refrigerant in the first flow path (41a) is equal to the mass flow rate M1 of a 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 a refrigerant changes from point I to point J. The mass flow rate of a refrigerant in the second flow path (41b) is equal to the mass flow rate M2 of a 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) and the amount of heat absorbed by the refrigerant in the second flow path (41b) match each other. Accordingly, the internal heat exchanger (41) follows Equation 2 below. In Equation 2, hE is the specific enthalpy at point E, hF is the specific enthalpy at point F, hI is the specific enthalpy at point I, and hJ is the specific enthalpy at point J. hJ − hI × M 2 = hE − hF × M 1

[0111] The specific enthalpy hF at point F is equal to the specific enthalpy hG at point G (hF=hG). M2 / M1 is the dryness fraction of a refrigerant at point G. The specific enthalpy hG at point G and the dryness fraction M2 / M1 of a refrigerant at point G correlate to each other. Accordingly, the specific enthalpy hF at point F and the specific enthalpy hG at point G are specified by using Equation 2 and the correlation between the specific enthalpy hG and the dryness fraction M2 / M1 of a refrigerant at point G.

[0112] The pressure at point F is the high pressure HP of the refrigeration cycle. Accordingly, the specific enthalpy and pressure at point F are specified, and as a result, point F is specified. The pressure at point G is the intermediate pressure MP of the refrigeration cycle. Accordingly, the specific enthalpy and pressure at point G are specified, and as a result, point G is specified.(Point C)

[0113] Point C represents a refrigerant sucked into the high-stage compressor (32). The high-stage compressor (32) sucks a refrigerant having discharged from the low-stage compressor (31) (a refrigerant at point B) and a refrigerant having passed through the second piping (62) (a refrigerant at point J). The mass flow rate of a refrigerant discharged from the low-stage compressor (31) is equal to the mass flow rate M3 of a refrigerant in the third piping (63). Thus, the refrigerant at point C follows Equation 3 below. hC × M 2 + M 3 = hB × M 3 + hJ × M 2

[0114] The mass flow rate M1 of a refrigerant in the first piping (61) is the sum of the mass flow rate M2 of a refrigerant in the second piping (62) and the mass flow rate M3 of a refrigerant in the third piping (63) (M1=M2+M3). Accordingly, Equation 3 can be converted into Equation 4. hC = hB × M 3 / M 1 + hJ × M 2 / M 1

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

[0116] Point D represents a refrigerant discharged from the high-stage compressor (32). The pressure at point D is the high pressure HP of the refrigeration cycle. Assuming that the efficiency of the high-stage compressor (32) is "0.7", the specific entropy sD at point D is the value obtained by dividing the specific entropy sC at point C by 0.7 (sD=sC / 0.7). Accordingly, the pressure and specific entropy at point D are specified, and as a result, point D is specified.<Coefficient of Performance>

[0117] The coefficient of performance COP of the refrigeration cycle shown in FIG. 4 is calculated based on Equation 5 below. The specific enthalpy hK at point K is the specific enthalpy of a refrigerant at the inlet of the inner heat exchanger (34) functioning as an evaporator. The specific enthalpy hA at point A is the specific enthalpy of a refrigerant at the outlet of the inner heat exchanger (34) functioning as an evaporator as well as the specific enthalpy of a refrigerant at the inlet of the low-stage compressor (31). The specific enthalpy hB at point B is the specific enthalpy of a refrigerant at the outlet of the low-stage compressor (31). The specific enthalpy hC at point C is the specific enthalpy of a refrigerant at the inlet of the high-stage compressor (32). The specific enthalpy hD at point D is the specific enthalpy of a refrigerant at the outlet of the high-stage compressor (32). COP = hA − hK × M 3 / hB − hA × M 3 + hD − hC × M 1

[0118] Equation 5 can be converted into Equation 6 below. (M3 / M1) is the wetness fraction of a refrigerant at point G. Thus, the coefficient of performance of the refrigeration cycle shown in FIG. 4 is calculated based on Equation 6 below. As described above, (M3 / M1) is the wetness fraction of a refrigerant at point G. COP = hA − hK × M 3 / M 1 / hB − hA × M 3 / M 1 + hD − hC <Optimum Pressure Ratio>

[0119] In the refrigerant circuit (30) of this embodiment, if the set temperature Ts and the outdoor air temperature Ta are constant, the high pressure HP and the low pressure LP of the refrigeration cycle are also substantially constant. On the other hand, in the refrigerant circuit (30) of this embodiment, the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) are controlled individually. Accordingly, in the refrigerant circuit (30) of this embodiment, the intermediate pressure MP of the refrigeration cycle can be adjusted in a state in which the set temperature Ts and the outdoor air temperature Ta are constant.

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

[0121] FIG. 7 shows the input to the low-stage compressor (31) and the input to the high-stage compressor (32), where the pressure ratio RP changes while the high pressure HP and the low pressure LP of the refrigeration cycle are constant. As shown in FIG. 7, the input WL to the low-stage compressor (31) and the input WH to the high-stage compressor (32) change as the pressure ratio RP changes while the high pressure HP and the low pressure LP of the refrigeration cycle are constant. 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. Accordingly, as indicated by the solid line in FIG. 7, the pressure ratio RP includes a pressure ratio 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. Accordingly, in the refrigeration cycle performed by the refrigerant circuit (30) of this embodiment, the pressure ratio RP includes a pressure ratio at which the coefficient of performance COP is highest while the high pressure HP and the 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 "optimum pressure ratio".

[0123] When the high pressure HP and the low pressure LP of the refrigeration cycle are specified, the optimum pressure ratio that corresponds to these pressures is specified. Thus, when the set temperature Ts and the outdoor air temperature Ta are specified, the optimum pressure ratio that corresponds to these temperatures is specified.<Optimum Displacement Volume Ratio>

[0124] The value obtained by dividing the displacement volume VL of the low-stage compressor (31) by the displacement volume VH of the high-stage compressor (32) is defined as the displacement volume ratio RV (RV=VL / VH).

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

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

[0127] When the set temperature Ts, the outdoor air temperature Ta, and the pressure ratio RP are specified, the displacement volume ratio RV that corresponds to these temperatures and the ratio can be calculated. As described above, when the set temperature Ts and the outdoor air temperature Ta are specified, the optimum pressure ratio that corresponds to these temperatures is specified, and also the displacement volume ratio RV that corresponds to the optimum pressure ratio can be specified. The displacement volume ratio RV that corresponds to the optimum pressure ratio is the optimum displacement volume ratio. In this manner, when the set temperature Ts and the outdoor air temperature Ta are specified, the optimum displacement volume ratio that corresponds to these temperatures can be specified.

[0128] The optimum displacement volume ratio is the displacement volume ratio at which the coefficient of performance COP of the refrigeration cycle where the set temperature Ts and the specified outdoor air temperature Ta are specified is highest.<Range of Displacement Volume Ratio>

[0129] The hatched area in FIG. 8 shows the operating region in which the transport refrigeration apparatus (10) of this embodiment can operate. The transport refrigeration apparatus (10) of this embodiment conducts the cooling operation in a situation in which the set temperature Ts and the outdoor air temperature Ta are included in the operating region. In the transport refrigeration apparatus (10) of this embodiment, the high pressure HP of the refrigeration cycle is higher than or equal to the critical pressure of the refrigerant when the outdoor air temperature is 25°C or more, and the high pressure HP of the refrigeration cycle is lower than the critical pressure of the refrigerant when the outdoor air temperature is lower than 25°C.

[0130] FIG. 8 shows the optimum displacement volume ratio of part of the combination of the set temperature Ts and the outdoor air temperature Ta included in the operating region. The optimum displacement volume ratio decreases as the set temperature Ts increases. The optimum displacement volume ratio decreases as the outdoor air temperature Ta decreases in each of the region where the high pressure HP of the refrigeration cycle is higher than or equal to the critical pressure of the refrigerant and the region where the high pressure HP of the refrigeration cycle is lower than the critical pressure of the refrigerant.

[0131] The optimum displacement volume ratio becomes the maximum value "1.93" when the set temperature Ts = -30°C and the outdoor air temperature Ta = 25°C, and the minimum value "0.7" when the set temperature Ts = 10°C and the outdoor air temperature Ta = 30°C. Thus, in the transport refrigeration apparatus (10) of this embodiment, the coefficient of performance COP of the refrigeration cycle performed by the refrigerant circuit (30) can be held high by setting the displacement volume ratio in the cooling operation to 0.7 or more and 1.93 or less.-Feature (1) of First Embodiment-

[0132] In the cooling operation of the transport refrigeration apparatus (10), the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) individually. Accordingly, while the high pressure HP and the low pressure LP of the refrigeration cycle are constant, the intermediate pressure MP of the refrigeration cycle is adjusted.

[0133] In the cooling operation of the transport refrigeration apparatus (10) of this embodiment, the displacement volume ratio is 0.7 or more and 1.93 or less. Thus, in the transport refrigeration apparatus (10) of this embodiment, in the entire part of the operating region of the transport refrigeration apparatus (10) shown in FIG. 8, the intermediate pressure MP of the refrigeration cycle maintains an appropriate value, and the coefficient of performance COP of the refrigeration cycle performed by the refrigerant circuit (30) is held high.-Feature (2) of First Embodiment-

[0134] In the cooling operation of the transport refrigeration apparatus (10), the controller (80) controls the rotational speed of the low-stage compressor (31) based on the measured value of the low-pressure sensor (71), and controls the rotational speed of the high-stage compressor (32) based on the measured value of the intermediate-pressure sensor (72). Thus, in the transport refrigeration apparatus (10) of this embodiment, the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) individually, whereby the low pressure LP and the intermediate pressure MP of the refrigeration cycle are set to appropriate values.-Feature (3) of First Embodiment-

[0135] The low-stage displacement volume is calculated by multiplying "the displacement volume of the low-stage compressor (31)" by "the rotational speed of the low-stage compressor (31)". The high-stage displacement volume 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 volume ratio is the value obtained by dividing the low- stage displacement volume by the high-stage displacement volume. 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 apparatus (10) of this embodiment, the low-stage compressor (31) and the high-stage compressor (32) are equal in displacement volume. Accordingly, in the transport refrigeration apparatus (10) of this embodiment, the displacement volume ratio matches the rotational speed ratio. Thus, in the transport refrigeration apparatus (10) of this embodiment, each of the displacement volume ratio and the rotational speed ratio in the cooling operation is 0.7 or more and 1.93 or less.<<Second Embodiment>>

[0137] A second embodiment will be described. This embodiment is directed to a modified configuration of the transport refrigeration apparatus (10) in the transport container (1) of the first embodiment. The transport refrigeration apparatus (10) of this embodiment is different in the refrigerant circuit (30) and the controller (80) from the transport refrigeration apparatus (10) of the first embodiment.

[0138] The following description of the transport refrigeration apparatus (10) of this embodiment will be focused on differences from the transport refrigeration apparatus (10) of the first embodiment. The following description of the transport refrigeration apparatus (10) of this embodiment basically does not contain the description common to the transport refrigeration apparatus (10) of the first embodiment.-Refrigerant Circuit-

[0139] As shown in FIG. 9, in the refrigerant circuit (30) of this embodiment, the first motorized valve (51) and the gas-liquid separator (42) provided in the refrigerant circuit (30) of the first embodiment are not provided. The refrigerant circuit (30) of this embodiment is provided with an internal heat exchanger (46) connected to the second piping (62) and the third piping (63) instead of the internal heat exchanger (41) of the first embodiment connected to the first piping (61) and the second piping (62).<First Piping, Second Piping, and Third Piping>

[0140] In the refrigerant circuit (30) of this embodiment, the piping that is connected to the other end of the outer heat exchanger (33) is the first piping (61). The other end of the outer heat exchanger (33) is connected with one end of the first piping (61). The first piping (61) forms a first passage through which all of the refrigerant flowing out of the inner heat exchanger (34) flows.

[0141] In the refrigerant circuit (30) of this embodiment, the piping that connects the other end of the first piping 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 part of a refrigerant having passed through the first piping (61) flows. In the second piping (62), the second motorized valve (52) is disposed upstream of the internal heat exchanger (46).

[0142] In the refrigerant circuit (30) of this embodiment, the piping that connects the other end of the first piping (61) 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 rest of the refrigerant having passed through the first piping (61) flows. In the third piping (63), the third motorized valve (53) is disposed upstream of the inner heat exchanger (34).<Internal Heat Exchanger>

[0143] The internal heat exchanger (46) is a heat exchanger that exchanges heat between a refrigerant and a refrigerant. The internal heat exchanger (46) of this embodiment 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 piping (63). The second flow path (46b) of the internal heat exchanger (46) is connected to the second piping (62). The internal heat exchanger (46) exchanges heat between a refrigerant flowing in the first flow path (46a) and a refrigerant flowing in the second flow path (46b).-Operation of Refrigerant Circuit-

[0144] Similarly to the refrigerant circuit (30) of the first embodiment, the refrigerant circuit (30) of this embodiment selectively performs a two-stage compressing operation to perform a two-stage compression refrigeration cycle and a single-stage compressing operation to perform a single-stage compression refrigeration cycle. Similarly to the first embodiment, in the two-stage compressing operation, both the low-stage compressor (31) and the high-stage compressor (32) are operated; and in the single-stage compressing operation, one of the low-stage compressor (31) or the high-stage compressor (32) is operated.-Double-Stage Compression Refrigeration Cycle-

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

[0146] The refrigerant in the state at point A turns into the state at point B by being compressed by the low-stage compressor (31). In the refrigerant circuit (30), the refrigerant in the state at point B having discharged from the low-stage compressor (31) and the refrigerant in the state at point I having passed through the second piping (62) merge and turn into the refrigerant in the state at point C. The refrigerant in the state at point C turns into the state at point D by being compressed by the high-stage compressor (32).

[0147] The refrigerant in the state at point D turns into the state at point E by dissipating heat to the external air (the outdoor air) in the outer heat exchanger (33). Part of a refrigerant in the state at point E flowing in the first piping (61) flows into the second piping (62), and the rest of the refrigerant flows into the third piping (63).

[0148] The refrigerant in the state at point E flowing in the third piping (63) turns into the state at point F by flowing into the first flow path (46a) of the internal heat exchanger (46) and then being cooled by the refrigerant flowing in the second flow path (46b). The refrigerant in the state at point F turns into the state at point G (the gas-liquid two-phase state) by being decompressed by the third motorized valve (53). The refrigerant in the state at point G turns into the state at point A by absorbing heat from the internal air in the inner heat exchanger (34) and then being evaporated.

[0149] The refrigerant in the state at point E flowing in the second piping (62) turns into the state at point I by flowing into the second flow path (46b) of the internal heat exchanger (46) through the second motorized valve (52) and then absorbing heat from the refrigerant flowing in the first flow path (46a).-Controller-

[0150] The controller (80) of this embodiment controls the low-stage compressor (31), the high-stage compressor (32), the second motorized valve (52), and the third motorized valve (53), similarly to the controller (80) of the first embodiment.

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

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

[0153] In the transport refrigeration apparatus (10) of this embodiment, the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32). In the cooling operation of the transport refrigeration apparatus (10), the displacement volume ratio is 0.67 or more and 3.42 or less. Here, the reason it is desirable that the displacement volume ratio in the cooling operation is 0.67 or more and 3.42 or less will be described.<Identify Refrigeration Cycle>

[0154] Only if the set value of the internal temperature (the set temperature Ts), the temperature of the external air (the outdoor air temperature Ta), and the pressure ratio RP have been specified, the refrigeration cycle performed by the refrigerant circuit (30) is identified. As described above, the pressure ratio RP is defined by Equation 1.

[0155] Specifically, only if the set value of the internal temperature (the set temperature Ts), the temperature of the external air (the outdoor air temperature Ta), and the pressure ratio RP have been specified, point A to point I, which identify the refrigeration cycle shown in FIG. 10, are specified as described below.(High Pressure HP, Intermediate Pressure MP, and Low Pressure LP)

[0156] The high pressure HP, the intermediate pressure MP, and the low pressure LP of the refrigeration cycle are specified by conducting the same process as described in the first embodiment.(Point A and Point B)

[0157] Point A represents a refrigerant sucked into the low-stage compressor (31). Point B represents a refrigerant discharged from the low-stage compressor (31). Point A and point B are specified by conducting the same process as described in the first embodiment (the process to specify point A and point B in FIG. 4).(Point E)

[0158] Point E represents a refrigerant at the outlet of the outer heat exchanger (33) functioning as a radiator. The state at point E is substantially equal to the temperature of a refrigerant at the inlet of the first flow path (46a) of the internal heat exchanger (46). Point E is specified by conducting the same process as described in the first embodiment (the process to specify point E in FIG. 4).(Point H)

[0159] Point H represents a refrigerant at the outlet of the second motorized 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. Accordingly, the pressure and specific enthalpy at point H are specified, and as a result, point H is specified.(Point F)

[0160] Point F represents a refrigerant at the outlet of the first flow path (46a) of the internal heat exchanger (46). The state at point F is substantially equal to the state of a refrigerant flowing into the third motorized valve (53). Assuming that the temperature of a refrigerant at the outlet of the first flow path (46a) of the internal heat exchanger (46) is equal to the temperature of a refrigerant at the inlet of the 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. Accordingly, the temperature and pressure at point F are specified, and as a result, point F is specified.(Point I)

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

[0162] The minimum value SH1 of the degree of superheat at point I is 0°C (SH1=0). The maximum value of the temperature at point I is the temperature at point E. Point E represents a refrigerant at the inlet of the first flow path (46a) of the internal heat exchanger (46). Accordingly, the maximum value SH2 of the degree of superheat at point I is determined by (Temperature at Point E)-(Saturation Temperature of Refrigerant at Intermediate Pressure MP). Then, the degree of superheat at point I is set to a value of SH1 or more and SH2 or less.

[0163] When the degree of superheat at point I is specified, the temperature at point I is specified. Accordingly, the pressure and temperature at point I are specified, and as a result, point I is specified.(Point C)

[0164] Part of a refrigerant having passed through the first piping (61) flows into the second piping (62), and the rest of the refrigerant flows into the third piping (63). Thus, the mass flow rate M1 of a refrigerant in the first piping (61) is equal to the sum of the mass flow rate M2 of a refrigerant in the second piping (62) and the mass flow rate M3 of a refrigerant in the third piping (63) (M1=M2+M3).

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

[0166] In the internal heat exchanger (46), the amount of heat dissipated by the refrigerant in the first flow path (46a) and the amount of heat absorbed by the refrigerant in the second flow path (46b) match each other. Accordingly, the internal heat exchanger (46) follows Equation 7 below. In Equation 7, hE is the specific enthalpy at point E, hF is the specific enthalpy at point F, hH is the specific enthalpy at point H, and hI is the specific enthalpy at point I. Also, Equation 7 is converted into Equation 8. hI − hH × M 2 = hE − hF × M 3 M 3 / M 2 = hI − hH / hE − hF

[0167] The refrigerant at point C is the mixture of a refrigerant at point B and a refrigerant at point I. Accordingly, the refrigerant at point C follows Equation 9 below. hB is the specific enthalpy at point B, and hC is the specific enthalpy at point C. Also, Equation 9 is converted into Equation 10. hI × M 2 + hB × M 3 = hC × M 2 + M 3 hI + hB × M 3 / M 2 = hC × 1 + M 3 / M 2

[0168] The specific enthalpy hC at point C is calculated by using Equation 8 and Equation 10. The pressure at point C is the intermediate pressure MP of the refrigeration cycle. Accordingly, the pressure and specific enthalpy at point C are specified, and as a result, point C is specified.<Coefficient of Performance>

[0169] The coefficient of performance COP of the refrigeration cycle shown in FIG. 10 is calculated by Equation 11 below. The specific enthalpy hG at point G is the specific enthalpy of a refrigerant at the inlet of the inner heat exchanger (34) functioning as an evaporator. The specific enthalpy hA at point A is the specific enthalpy of a refrigerant at the outlet of the inner heat exchanger (34) functioning as an evaporator as well as the specific enthalpy of a refrigerant at the inlet of the low-stage compressor (31). The specific enthalpy hB at point B is the specific enthalpy of a refrigerant at the outlet of the low-stage compressor (31). The specific enthalpy hC at point C is the specific enthalpy of a refrigerant at the inlet of the high-stage compressor (32). The specific enthalpy hD at point D is the specific enthalpy of a refrigerant at the outlet of the high-stage compressor (32). COP = hA − hG × M 3 / hB − hA × M 3 + hD − hC × M 2 + M 3

[0170] Equation 11 can be converted into Equation 12 below. (M3 / M2) is calculated by Equation 8. Thus, the coefficient of performance of the refrigeration cycle shown in FIG. 10 is calculated based on Equation 12 below. COP = hA − hG × M 3 / M 2 / hB − hA × M 3 / M 2 + hD − hC × 1 + M 3 / M 2

[0171] As described above, in the process to identify the refrigeration cycle, the degree of superheat at point I is set to a value of SH1 or more and SH2 or less. When the degree of superheat at point I changes, the coefficient of performance COP of the refrigeration cycle changes. Thus, regarding "the degrees of superheat at point I", a plurality of coefficients of performance COP of the refrigeration cycle are calculated, and then the highest one of the plurality of coefficients of performance COP is employed as the coefficient of performance of the refrigeration cycle performed by the refrigerant circuit (30) of this embodiment.<Optimum Pressure Ratio>

[0172] In the refrigeration cycle performed by the refrigerant circuit (30) of this embodiment, similarly to the refrigeration cycle performed by the refrigerant circuit (30) of the first embodiment, the pressure ratio RP includes a pressure ratio at which the coefficient of performance COP is highest while the high pressure HP and the 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 "optimum pressure ratio".

[0173] When the high pressure HP and the low pressure LP of the refrigeration cycle are specified, the optimum pressure ratio that corresponds to these pressures is specified. Thus, when the set temperature Ts and the outdoor air temperature Ta are specified, the optimum pressure ratio that corresponds to these temperatures is specified.<Optimum Displacement Volume Ratio>

[0174] In the transport refrigeration apparatus (10) of this embodiment, similarly to the transport refrigeration apparatus (10) of the first embodiment, when the set temperature Ts and the outdoor air temperature Ta are specified, the optimum displacement volume ratio that corresponds to these temperatures can be specified. The optimum displacement volume ratio is the displacement volume ratio at which the coefficient of performance COP of the refrigeration cycle where the set temperature Ts and the specified outdoor air temperature Ta are specified is highest.<Range of Displacement Volume Ratio>

[0175] The hatched area in FIG. 11 shows the operating region in which the transport refrigeration apparatus (10) of this embodiment can operate. The transport refrigeration apparatus (10) of this embodiment conducts the cooling operation in a situation in which the set temperature Ts and the outdoor air temperature Ta are included in the operating region. In the transport refrigeration apparatus (10) of this embodiment, the high pressure HP of the refrigeration cycle is higher than or equal to the critical pressure of the refrigerant when the outdoor air temperature is 25°C or more, and the high pressure HP of the refrigeration cycle is lower than the critical pressure of the refrigerant when the outdoor air temperature is lower than 25°C.

[0176] FIG. 11 shows the optimum displacement volume ratio of part of the combination of the set temperature Ts and the outdoor air temperature Ta included in the operating region. The optimum displacement volume ratio decreases as the set temperature Ts increases. The optimum displacement volume ratio decreases as the outdoor air temperature Ta decreases in each of the region where the high pressure HP of the refrigeration cycle is higher than or equal to the critical pressure of the refrigerant and the region where the high pressure HP of the refrigeration cycle is lower than the critical pressure of the refrigerant.

[0177] The optimum displacement volume ratio becomes the maximum value "3.42" when the set temperature Ts = -30°C and the outdoor air temperature Ta = 25°C, and the minimum value "0.67" when the set temperature Ts = 30°C and the outdoor air temperature Ta = 30°C. Thus, in the transport refrigeration apparatus (10) of this embodiment, the coefficient of performance COP of the refrigeration cycle performed by the refrigerant circuit (30) can be held high by setting the displacement volume ratio in the cooling operation to 0.67 or more and 3.42 or less.-Feature (1) of Second Embodiment-

[0178] In the cooling operation of the transport refrigeration apparatus (10), the controller (80) controls the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) individually. Accordingly, while the high pressure HP and the low pressure LP of the refrigeration cycle are constant, the intermediate pressure MP of the refrigeration cycle is adjusted.

[0179] In the cooling operation of the transport refrigeration apparatus (10) of this embodiment, the displacement volume ratio is 0.67 or more and 3.42 or less. Thus, in the transport refrigeration apparatus (10) of this embodiment, in the entire part of the operating region of the transport refrigeration apparatus (10) shown in FIG. 11, the intermediate pressure MP of the refrigeration cycle maintains an appropriate value, and the coefficient of performance COP of the refrigeration cycle performed by the refrigerant circuit (30) is held high.-Feature (2) of Second Embodiment-

[0180] In the transport refrigeration apparatus (10) of this embodiment, the low-stage compressor (31) and the high-stage compressor (32) are equal in displacement volume. Accordingly, in the transport refrigeration apparatus (10) of this embodiment, similarly to the transport refrigeration apparatus (10) of the first embodiment, the displacement volume ratio matches the rotational speed ratio. Thus, in the transport refrigeration apparatus (10) of this embodiment, each of the displacement volume ratio and the rotational speed ratio in the cooling operation is 0.67 or more and 3.42 or less.

[0181] While the embodiments and variations thereof have been described above, it will be understood that various changes in form and details may be made without departing from the spirit and scope of the claims. The elements according to the embodiments, the variations thereof, and the other embodiments may be combined and replaced with each other. The ordinal numbers such as "first," "second," "third," . . . in the description and claims are used to distinguish the terms to which these expressions are given, and do not limit the number and order of the terms.INDUSTRIAL APPLICABILITY

[0182] As described above, the present disclosure is useful for a transport refrigeration apparatus and a transport container.DESCRIPTION OF REFERENCE CHARACTERS

[0183] 1Transport Container 2Container Body 5Internal Space 10Transport Refrigeration Apparatus 30Refrigerant Circuit 31Low-Stage Compressor 32High-Stage Compressor 33Outer Heat Exchanger (Radiator) 34Inner Heat Exchanger (Evaporator) 41Internal Heat Exchanger (Heat Exchanger) 42Gas-Liquid Separator 46Internal Heat Exchanger (Heat Exchanger) 61First Piping (First Passage) 62Second Piping (Second Passage) 63Third Piping (Third Passage) 80Controller

Claims

1. A transport refrigeration apparatus (10) that comprises a refrigerant circuit (30) configured to perform a refrigeration cycle by circulating carbon dioxide as a refrigerant and that is configured to perform a cooling operation to cool internal air of a transport container (1), wherein the refrigerant circuit (30) includes a radiator (33) configured to exchange heat between the refrigerant and outdoor air, a first passage (61) through which all of the refrigerant flowing out of the radiator (33) flows, a second passage (62) through which part of the refrigerant having passed through the first passage (61) flows, a third passage (63) through which a rest of the refrigerant having passed through the first passage (61) flows, an evaporator (34) provided in the third passage (63) and configured to exchange heat between the refrigerant and the internal air, a low-stage compressor (31) configured to suck the refrigerant flowing out of the evaporator (34), and a high-stage compressor (32) configured to suck the refrigerant discharged from the low-stage compressor (31) and the refrigerant flowing in the second passage (62), a volume of the refrigerant sucked per unit time by the low-stage compressor (31) is a low-stage displacement volume, a volume of the refrigerant sucked per the unit time by the high-stage compressor (32) is a high-stage displacement volume, a value obtained by dividing the low-stage displacement volume by the high-stage displacement volume is a displacement volume ratio, and the displacement volume ratio in the cooling operation is 0.67 or more and 3.42 or less.

2. The transport refrigeration apparatus of claim 1, further comprising: a controller (80) configured to control a rotational speed of the low-stage compressor (31) and a rotational speed of the high-stage compressor (32) individually, wherein the controller (80) sets each of the rotational speed of the low-stage compressor (31) and the rotational speed of the high-stage compressor (32) to a value at which the displacement volume ratio in the cooling operation is 0.67 or more and 3.42 or less.

3. The transport refrigeration apparatus of claim 1, further comprising: a controller (80) configured to control a rotational speed of the low-stage compressor (31) and a rotational speed of the high-stage compressor (32) individually, wherein the controller (80) controls the rotational speed of the low-stage compressor (31) based on a physical quantity that correlates to an evaporation temperature of the refrigerant in the evaporator (34), and sets the rotational speed of the high-stage compressor (32) to a value at which the displacement volume ratio is 0.67 or more and 3.42 or less.

4. The transport refrigeration apparatus of any one of claims 1 to 3, wherein the low-stage compressor (31) and the high-stage compressor (32) are equal in volume of a refrigerant sucked per rotation, a value obtained by dividing the rotational speed of the low-stage compressor (31) by the rotational speed of the high-stage compressor (32) is a rotational speed ratio, and the rotational speed ratio in the cooling operation is 0.67 or more and 3.42 or less.

5. The transport refrigeration apparatus of any one of claims 1 to 4, further comprising: a heat exchanger (46) provided upstream of the evaporator (34) in the third passage (63) and configured to cool the refrigerant flowing in the third passage (63) by exchanging heat between the refrigerant flowing in the third passage (63) and the refrigerant flowing in the second passage (62).

6. The transport refrigeration apparatus of any one of claims 1 to 4, further comprising: a gas-liquid separator (42) configured to separate the refrigerant having passed through the first passage (61) into a gas refrigerant and a liquid refrigerant, send the gas refrigerant to the second passage (62), and send the liquid refrigerant to the third passage (63); and a heat exchanger (41) configured to cool the refrigerant flowing in the first passage (61) by exchanging heat between the refrigerant flowing in the first passage (61) and the refrigerant flowing in the second passage (62).

7. The transport refrigeration apparatus of claim 6, wherein the displacement volume ratio in the cooling operation is 0.7 or more and 1.93 or less.

8. The transport refrigeration apparatus of any one of claims 1 to 7, wherein the refrigerant circuit (30) selectively performs a two-stage compressing operation to operate both the low-stage compressor (31) and the high-stage compressor (32) to perform the refrigeration cycle, and a single-stage compressing operation to operate one of the low-stage compressor (31) or the high-stage compressor (32) and stop the other one of the low-stage compressor (31) or the high-stage compressor (32) to perform the refrigeration cycle.

9. A transport container comprising: the transport refrigeration apparatus (10) of any one of claims 1 to 8; and a container body (2) to which the transport refrigeration apparatus (10) is attached and which forms an internal space (5) for storing cargo.

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