TWO-STAGE-COMPRESSION REFRIGERATING CYCLE APPARATUS, AND DEVICE AND METHOD FOR CONTROLLING THE APPARATUS

Abstract

 An objective of the present invention is to avoid increases in size and cost of a driver for a higher-stage compressor. In an air conditioner having a two-stage-compression refrigerating cycle, a higher-stage compressor (3b) is driven by a higher-stage driver (22b). The higher-stage driver (22b) controls the rotational speed of the higher-stage compressor (3b) on the basis of a higher-stage rotational speed command (ωb*) given from a compressor control section (21). The compressor control section (21) includes a target setting section (31) which sets a target intermediate pressure in accordance with a predetermined algorithm, a determination section (32) which determines whether or not a motor current in the higher-stage driver (22b) has exceeded a current limit value, and a target changing section (33) which increases the target intermediate pressure if the motor current has exceeded the current limit value.

Description

{Technical Field}

[0001] The present invention relates to a two-stage-compression refrigerating cycle apparatus, and a device and method for controlling the apparatus.

{Background Art}

[0002] A two-stage-compression refrigerating cycle apparatus formed by connecting two compressors in series with each other is known. For example, Patent Literature 1 discloses a two-stage-compression refrigerating cycle apparatus in which a lower-stage compressor, a higher-stage compressor, an outdoor heat exchanger, an outdoor expansion valve, a liquid receiver, an indoor expansion valve and an indoor heat exchanger are successively connected by piping, and in which a gas refrigerant from the liquid receiver is injected into an intermediate portion of the piping connecting the ejection side of the lower-stage compressor and the drawing side of the higher-stage compressor.

[0003] In such a two-stage-compression refrigerating cycle apparatus, an intermediate pressure between a lower-stage compressor and a higher-stage compressor is controlled to be an intermediate pressure at which the total work done by the compressors is minimized, for example, a theoretical intermediate pressure at which the pressure ratio of the lower-stage compressor and the pressure ratio of the higher-stage compressor are equal to each other.

{Citation List}

{Patent Literature}

[0004] 

{PTL 1}
Japanese Unexamined Patent Application, Publication No. 2007-10282

[0005] {Summary of Invention}

[0006] {Technical Problem}

[0007] In the two-stage-compression refrigerating cycle, as disclosed in Patent Literature 1, the amount of refrigerant drawn by the higher-stage compressor is larger than the amount of refrigerant drawn by the lower-stage compressor in the case where the gas refrigerant is injected at the refrigerant drawing side of the higher-stage compressor. In this case, therefore, the amount of work done by the higher-stage compressor is larger than the amount of work done by the lower-stage compressor. Accordingly, the value of the current flowing through the inverter or the like for driving the higher-stage compressor is necessarily larger than that of the current for the lower-stage compressor, and a driver having an allowable current larger than that of a driver for the lower-stage compressor is required for the higher-stage compressor. There is, therefore, a possibility of the driver being increased in size and in manufacturing cost.

[0008] The present invention has been achieved in consideration of these circumstances, and an object of the present invention is to provide a two-stage-compression refrigerating cycle apparatus capable of avoiding increases in size and manufacturing cost of the driver for the higher-stage compressor and a device and method for controlling the apparatus.

{Solution to Problem}

[0009] According to a first aspect of the present invention, there is provided a control device for controlling a two-stage-compression refrigerating cycle apparatus having a compressor including a lower-stage compressor and a higher-stage compressor, a condenser which condenses a refrigerant from the compressor, and an evaporator which evaporates the condensed refrigerant, the refrigerant being injected between the lower-stage compressor and the higher-stage compressor, the control device including a higher-stage drive section which drives the higher-stage compressor, and a control section which controls the higher-stage drive section, the control section including a target setting section which sets a target intermediate pressure in accordance with a predetermined algorithm, a determination section which determines whether or not the value of a current flowing in the higher-stage drive section has exceeded a predetermined current limit value determined from a configuration of the higher-stage drive section, and a target changing section which increases the target intermediate pressure if the value of the current flowing in the higher-stage drive section has exceeded the current limit value.

[0010] In the arrangement according to the first aspect described above, the target setting section sets a target intermediate pressure in accordance with a predetermined algorithm, and the target changing section increases the target intermediate pressure when the determination section determines that the value of a current flowing in the higher-stage drive section has exceeded a predetermined current limit value determined from the configuration of the higher-stage drive section. With the increase in the target intermediate pressure, the amount of injection of the gas refrigerant into the higher-stage compressor, i.e., the dryness, decreases and the amount of work done by the higher-stage compressor is reduced. The current flowing in the higher-stage drive section can thereby be reduced and increases in size and manufacturing cost of the higher-stage drive section can be avoided. The higher-stage compressor is provided on the refrigerant flow downstream side of the lower-stage compressor.

[0011] In the above-described control device, the target changing section may increase the target intermediate pressure until the value of the current flowing in the higher-stage drive section becomes smaller than the current limit value.

[0012] In the arrangement according to the first aspect described above, the target intermediate pressure is increased until the value of the current flowing in the higher-stage drive section becomes smaller than the current limit value. The value of the current flowing in the higher-stage drive section can thus be reduced below the current limit value with reliability.

[0013]  In the control device, the target setting section may set as the target intermediate pressure a theoretical intermediate pressure at which a compression ratio of the lower-stage compressor and a compression ratio of the higher-stage compressor are equal to each other.

[0014] In the arrangement according to the first aspect described above, the target intermediate pressure is set to the theoretical intermediate pressure enabling high-efficiency operation when the value of the current flowing in the higher-stage drive section is smaller than the current limit value, and the higher-stage compressor is controlled on the basis of this target intermediate pressure, thus enabling operation with improved efficiency.

[0015] According to a second aspect of the present invention, there is provided a two-stage-compression refrigerating cycle apparatus including the above-described control device.

[0016] According to a third aspect of the present invention, there is provided a method of controlling a two-stage-compression refrigerating cycle apparatus having a compressor including a lower-stage compressor and a higher-stage compressor, a condenser which condenses a refrigerant from the compressor, an evaporator which evaporates the condensed refrigerant, and a higher-stage drive section which drives the higher-stage compressor, the refrigerant being injected between the lower-stage compressor and the higher-stage compressor, the method including a target setting step of setting a target intermediate pressure in accordance with a predetermined algorithm, a determination step of determining whether or not the value of a current flowing in the higher-stage drive section has exceeded a predetermined current limit value determined from a configuration of the higher-stage drive section, and a target changing step of increasing the target intermediate pressure if the value of the current flowing in the higher-stage drive section has exceeded the current limit value.

{Advantageous Effects of Invention}

[0017] According to the present invention, an effect of avoiding increases in size and manufacturing cost of the driver for the higher-stage compressor is achieved.

{Brief Description of Drawings}

[0018] 

{Fig. 1}
Fig. 1 is a cooling system diagram of an air conditioner according to an embodiment of the present invention.

{Fig. 2}
Fig. 2 is a diagram showing a configuration relating to rotational speed control on a compressor in various functions provided in a control device for the air conditioner according to an embodiment of the present invention.

{Fig. 3}
Fig. 3 is a flowchart showing steps of a process executed by the control device for the air conditioner according to an embodiment of the present invention.

{Description of Embodiments}

[0019] An embodiment of a two-stage-compression refrigerating cycle apparatus, and a device and method for controlling the apparatus according to the present invention will be described with reference to the drawings. The two-stage-compression refrigerating cycle apparatus according to the present invention cools or heats a thermal medium and outputs the thermal medium cooled or heated. Examples of the two-stage-compression refrigerating cycle apparatus include an air conditioner and a turbo refrigerator. The thermal medium may be a gas or a liquid. The two-stage-compression refrigerating cycle apparatus may have only one of the function to cool the thermal medium and the function to heat the thermal medium or have both the two functions. In the following description, an air conditioner is described as an example of the two-stage-compression refrigerating cycle apparatus.

[0020] Fig. 1 is a diagram of a refrigerant system for an air conditioner 10 according to the present embodiment. The air conditioner 10 includes a compressor 3 which compresses a refrigerant, a four-way valve 4 for switching between cooling and heating, an indoor heat exchanger 5 in which indoor air and the refrigerant exchange heat, an outdoor heat exchanger 6 in which outdoor air and the refrigerant exchange heat, and an intercooler 7 which is provided between the indoor heat exchanger 5 and the outdoor heat exchanger 6, and which stores the liquid refrigerant. A first expansion valve 9 is provided in refrigerant piping between the intercooler 7 and the indoor heat exchanger 5, and a second expansion valve 11 is provided in refrigerant piping between the intercooler 7 and the outdoor heat exchanger 6. An accumulator 13 which stores a liquid part of the refrigerant is provided between the compressor 3 and the four-way valve 4 to prevent the refrigerant left ungasified from being drawn in the liquid state into the compressor 3. In the above-described arrangement, the compressor 3, the four-way valve 4, the outdoor heat exchanger 6, the intercooler 7, the second expansion valve 11 and the accumulator 13, for example, are provided in an outdoor unit, while the indoor heat exchanger 5 and the first expansion valve 9 are provided in an indoor unit.

[0021] The compressor 3 is a two-stage compressor including a lower-stage compressor 3a and a higher-stage compressor 3b. For example, the higher-stage compressor 3b has a capacity equal to or higher than about 70% and equal to or lower than 100% of the capacity of the lower-stage compressor 3a. The refrigerant piping between the lower-stage compressor 3a and the higher-stage compressor 3b, in other words, the refrigerant drawing-in side of the higher-stage compressor 3b is connected to an air phase (upper space) in the intercooler 7 by intermediate-pressure refrigerant piping 8.

[0022] The air conditioner 10 includes a pressure sensor (not shown) for measuring the pressure of the refrigerant ejected from the compressor 3 and flowing to the four-way valve 4 (high-level pressure: condensation pressure) and a pressure sensor (not shown) for measuring the pressure of the refrigerant returned from the four-way valve 4 to the compressor 3 (low-level pressure: evaporation pressure).

[0023] During cooling operation of the air conditioner 10 thus arranged, the high-temperature high-pressure refrigerant ejected from the higher-stage compressor 3b is delivered to the outdoor heat exchanger 6 via the four-way valve 4, as indicated by a broken line arrow, and is condensed and liquefied in the outdoor heat exchanger 6 by heat exchange with outside air to become the liquid refrigerant. The refrigerant having become the liquid refrigerant is adjusted to have an intermediate pressure with the second expansion valve 11, and the refrigerant is delivered to the intercooler 7. The intermediate-pressure refrigerant undergoes vapor-liquid separation in the intercooler 7. The gas refrigerant is led to the refrigerant drawing-in side of the higher-stage compressor 3b through the intermediate-pressure refrigerant piping 8, while the liquid refrigerant is stored in the intercooler 7. The liquid refrigerant at the intermediate pressure stored in the intercooler 7 expands adiabatically during passage through the first expansion valve 9, is thereafter delivered to the indoor heat exchanger 5 and evaporates in the indoor heat exchanger 5 by cooling indoor air. The refrigerant having gasified by absorbing heat in the indoor heat exchanger 5 is delivered to the lower-stage compressor 3a in the compressor 3 via the four-way valve 4 and the accumulator 13. The refrigerant compressed by the lower-stage compressor 3a becomes confluent with the gas refrigerant from the intermediate-pressure refrigerant piping 8 to be drawn into the higher-stage compressor 3b. The refrigerant further compressed by the higher-stage compressor 3b is delivered to the four-way valve 4.

[0024] Thus, during cooling operation of the air conditioner 10, the outdoor heat exchanger 6 functions as a condenser and the indoor heat exchanger 5 functions as an evaporator.

[0025] On the other hand, during heating operation of the air conditioner 10, the high-temperature high-pressure refrigerant ejected from the higher-stage compressor 3b is delivered to the indoor heat exchanger 5 via the four-way valve 4, as indicated by a solid line arrow, and is condensed and liquefied in the indoor heat exchanger 5 by releasing heat to indoor air to become the high-pressure low-temperature liquid refrigerant. This liquid refrigerant is adjusted to have an intermediate pressure with the first expansion valve 9, and the refrigerant is delivered to the intercooler 7. The intermediate-pressure refrigerant undergoes vapor-liquid separation in the intercooler 7. The gas refrigerant is led to the refrigerant drawing-in side of the higher-stage compressor 3b through the intermediate-pressure refrigerant piping 8, while the liquid refrigerant is stored in the intercooler 7. The liquid refrigerant at the intermediate pressure stored in the intercooler 7 expands adiabatically during passage through the second expansion valve 11, is thereafter delivered to the outdoor heat exchanger 6 and evaporates in the outdoor heat exchanger 6 by cooling outdoor air. The refrigerant having gasified by absorbing heat in the outdoor heat exchanger 6 is delivered to the lower-stage compressor 3a in the compressor 3 via the four-way valve 4 and the accumulator 13. The refrigerant compressed by the lower-stage compressor 3a becomes confluent with the gas refrigerant from the intermediate-pressure refrigerant piping 8 to be drawn into the higher-stage compressor 3b. The refrigerant further compressed by the higher-stage compressor 3b is delivered to the four-way valve 4.

[0026] Thus, during heating operation of the air conditioner 10, the indoor heat exchanger 5 functions as a condenser and the outdoor heat exchanger 6 functions as an evaporator.

[0027] In the present embodiment, heat exchange with gas is performed in each of the indoor heat exchanger 5 and the outdoor heat exchanger 6. However, the present invention is not limited to this. Heat exchange with a liquid (e.g., water) may alternatively be performed.

[0028] In this air conditioner 10, control of the compressor 3, switching of the four-way valve 4 and control of the openings of the first expansion valve 9 and the second expansion valve 11 are performed by a control device 20 (see Fig. 2). Fig. 2 is a diagram showing a configuration relating to rotational speed control on the compressor 3 in the various functions that the device for controlling the air conditioner 10 has. For example, the control device 20 includes, as its main components, a compressor control section (control section) 21, a lower-stage driver (lower-stage drive section) 22a for driving the lower-stage compressor 3a, and a higher-stage driver (higher-stage drive section) 22b for driving the higher-stage compressor 3b.

[0029] Each of the lower-stage driver 22a and the higher-stage driver 22b includes, for example, an inverter having six switching elements, a gate driver for driving the switching elements constituting the inverter, and a microprocessor which supplies a PWM signal to the gate driver based on the rotational speed command from the compressor control section 21.

[0030] The compressor control section 21 is, for example, a microprocessor. Various functions realized by sections in the compressor control section 21 described below are realized by a CPU reading out to a memory, such as a RAM, a program stored in a recording medium such as a ROM and executing the program. The compressor control section 21 includes, for example, a target setting section 31, a determination section 32, a target changing section 33 and a rotational speed command computation section 34. The target setting section 31 sets a target intermediate pressure in accordance with a predetermined algorithm. For example, the target setting section 31 sets a target intermediate pressure at an intermediate pressure at which the total amount of work done by the compressor 3 is minimized, i.e., an intermediate pressure at which the compression ratio of the lower-stage compressor 3a and the compression ratio of the higher-stage compressor 3b are equal to each other.

[0031] The determination section 32 determines whether or not the value of a predetermined current flowing in the higher-stage driver 22b is equal to or larger than a predetermined current limit value determined from the configuration of the higher-stage driver 22b. The "predetermined current" is, for example, a current supplied to the inverter or a current supplied from the inverter to the compressor motor (hereinafter referred to as "motor current"). A case where the determination is made by using the motor current is described below by way of example for convenience sake. Also, the current limit value is determined, for example, from design values of allowable currents for elements including the inverter, a coil and a smoothing capacitor.

[0032] When the determination section 32 determines that the motor current is equal to or larger than the current limit value, the target changing section 33 increases the target intermediate pressure set by the target setting section 31. For example, the target changing section 33 may increase the target intermediate pressure at a predetermined change rate or change the target intermediate pressure in stepwise. The way of changing is not limited to the above-described examples. Any of various methods of changing the target intermediate pressure can be adopted.

[0033] The rotational speed command computation section 34 generates a lower-stage rotational speed command ωa* to be supplied to the lower-stage driver 22a and a higher-stage rotational speed command ωb* to be supplied to the higher-stage driver 22b.

[0034] More specifically, the rotational speed command computation section 34 generates, when the motor current is equal to or lower than the current limit value, a higher-stage rotational speed command ωb* on the basis of the target intermediate pressure set by the target setting section 31, and generates, when the motor current is higher than the current limit value, a higher-stage rotational speed command ωb* on the basis of the target intermediate pressure changed by the target changing section 33.

[0035] In this case, for example, the rotational speed command computation section 34 may hold a rotational speed command ωb* computation equation including a target intermediate pressure as a parameter and obtain a rotational speed command ωb* by inputting the target intermediate pressure to the computation equation. The rotational speed command computation section 34 may alternatively hold a table in which a target intermediate pressure, and a rotational speed command ωb* are associated with each other and obtain the rotational speed command ωb* corresponding to the target intermediate pressure from this table.

[0036] In such a case, when the target intermediate pressure is increased, the rotational speed of the higher-stage compressor 3b is controlled in the decreasing direction. The reason for this will be described below.

[0037] First, rate of circulating refrigerant Gr1 [kg/h] in the lower-stage compressor 3a is shown by expression (1) below, and rate of circulating refrigerant Gr2 [kg/h] in the higher-stage compressor 3b is shown by expression (2) below.

[0038] {Expression 1}

[0039] In expressions (1) and (2), v1 is the displacement [m³] in the lower-stage compressor 3a; ωa is the rotational speed [rps] of the lower-stage compressor 3a; ρ1 is the refrigerant intake density [kg/m³] in the lower-stage compressor 3a; X is the refrigerant dryness (injection rate); v2 is the displacement [m³] in the higher-stage compressor 3b; ωb is the rotational speed [rps] of the higher-stage compressor 3b; and ρ2 is the refrigerant intake density [kg/m³] in the higher-stage compressor 3b.

[0040] From expressions (1) and (2), the ratio α of the lower-state and higher-stage displacements is shown by expression (3) below.

[0041] {Expression 2}

[0042] Assuming that the intermediate pressure is increased, each of ρ2 and (1 - X) in expression (3) increases, while each of α and (ωa x ρ1) does not change. From this relationship, it can be understood that it is necessary to control the rotational speed of the higher-stage compressor 3b in the reducing direction when the intermediate pressure is increased.

[0043] The rotational speed command computation section 34 generates, as lower-stage rotational speed command ωa*, during cooling operation, a rotational speed command ωa* for the lower-stage compressor 3a such that the low-level pressure (the drawing pressure in the lower-stage compressor 3a) becomes equal to a target low-level pressure determined from a set room temperature, and generates, during heating operation, a rotational speed command ωa* for the lower-stage compressor 3a, for example, such that the high-level pressure (the ejection pressure in the higher-stage compressor 3b) becomes equal to a target high-level pressure determined from the set room temperature. For rotational speed control on the lower-stage compressor, a well-known method can be used as desired.

[0044] The lower-stage rotational speed command ωa* determined by the rotational speed command computation section 34 is supplied to the lower-stage driver 22a, and the higher-stage rotational speed command ωb* is supplied to the higher-stage driver 22b. Each of the lower-stage driver 22a and the higher-stage driver 22b drives the inverter so that the rotational speed of the compressor motor coincides with the supplied lower-stage rotational speed command ωa* or higher-stage rotational speed command ωb*.

[0045] Control of the rotational speed of the compressor 3 executed by the control device 20 for the air conditioner according to the present embodiment will now be described with reference to Fig. 3. Description will be made mainly on control of the higher-stage compressor 3b below while omitting control of the lower-stage compressor 3a.

[0046] First, the theoretical intermediate pressure is set as a target intermediate pressure (step SA1), and rotational speed control on the higher-stage compressor 3b is performed on the basis of the theoretical intermediate pressure (step SA2). Subsequently, determination is made as to whether or not the motor current has exceeded the current limit value (step SA3). If the motor current has not exceeded the predetermined current limit value ("NO" in step SA3), the process returns to step SA2 and rotational speed control on the higher-stage compressor 3b on the basis of the theoretical intermediate pressure is continued. On the other hand, if it is determined in step SA3 that the motor current has exceeded the current limit value ("YES" in step SA3), the target intermediate pressure is changed in the increasing direction, for example, the target intermediate pressure is increased by a predetermined amount from the theoretical intermediate pressure (step SA4), and rotational speed control on the higher-stage compressor 3b is performed on the basis of the changed target intermediate pressure (step SA5). Subsequently, determination is made as to whether or not the motor current has exceeded the current limit value (step SA6). If the motor current has exceeded the current limit value ("YES" in step SA6), the process returns to step SA4 and the target intermediate pressure is further increased. The target intermediate pressure is increased until the motor current becomes equal to or lower than the current limit value. When the motor current becomes equal to or lower than the current limit value ("NO" in step SA6), the process returns to step SA5 and rotational speed control on the higher-stage compressor 3b is performed on the basis of the target intermediate pressure last set.

[0047] In the air conditioner 10 and the control device 20 and method for controlling the air conditioner 10 according to the present embodiment, as described above, the target intermediate pressure is increased when the value of the current flowing in the higher-stage driver 22b exceeds the predetermined current limit value determined from the configuration of the higher-stage driver 22b. Therefore, the amount of work done by the higher-stage compressor 3b can be reduced and the current flowing in the higher-stage driver 22b can be reduced. As a result, the current flowing in the higher-stage driver 22b can be limited within the capacity, and increases in size and manufacturing cost of the apparatus with increase in capacity of the higher-stage driver 22b can be avoided.

[0048] In the present embodiment, the target intermediate pressure is changed in relation to the motor current and the current limit value. However, the control may alternatively be such that the target intermediate pressure is increased when the theoretical power exceeds a limit value set in advance. The ideal power depends on the refrigerant drawing condition of the higher-stage compressor 3b and can be computed from the intermediate pressure and the high-level pressure (the ejection pressure in the higher-stage compressor 3b).

[0049] The target intermediate pressure may be returned to the theoretical intermediate pressure when the motor current becomes equal to or lower than a release current value set in advance while rotational speed control on the basis of the target intermediate pressure set by the target changing section 33 is being maintained. As described above, the target intermediate pressure is reduced or returned to the theoretical intermediate pressure when the load on the compressor 3 is reduced, when the outside air temperature is reduced, when the set indoor temperature is changed or when the operating condition of the higher-stage compressor 3b exits from a severe condition, thus enabling the compressor to operate in an efficient operating range.

[0050] The present invention is not limited to the above-described embodiments. Various modifications of the embodiments can be made within the scope of the appended claims, for example, by combining part or all of the embodiments described above.

{Reference Signs List}

[0051] 

3

Compressor

3a

Lower-stage compressor

3b

Higher-stage compressor

4

Four-way valve

5

Indoor heat exchanger

6

Outdoor heat exchanger

7

Intercooler

8

Intermediate-pressure refrigerant piping

9

First expansion valve

10

Air conditioner

11

Second expansion valve

13

Accumulator

20

Control device

21

Compressor control section

22a

Lower-stage driver

22b

Higher-stage driver

31

Target setting section

32

Determination section

33

Target changing section

34

Rotational speed command computation section

Claims

1. A control device (20) for controlling a two-stage-compression refrigerating cycle apparatus (10) having a compressor (3) including a lower-stage compressor (3a) and a higher-stage compressor (3b), a condenser (5) for condensing a refrigerant from the compressor (3), and an evaporator (6) for evaporating the condensed refrigerant, the refrigerant being injected between the lower-stage compressor (3a) and the higher-stage compressor (3b), the control device comprising:

a higher-stage drive section (22b) for driving the higher-stage compressor (3b); and

a control section (21) configured to control the higher-stage drive section (22b), the control section (21) including:

a target setting section (31) configured to set a target intermediate pressure in accordance with a predetermined algorithm;

a determination section (32) configured to determine whether or not the value of a current flowing in the higher-stage drive section (22b) has exceeded a predetermined current limit value determined from a configuration of the higher-stage drive section (22b); and

a target changing section (33) configured to increase the target intermediate pressure if the value of the current flowing in the higher-stage drive section (22b) has exceeded the current limit value.

2. The control device (20) according to Claim 1, wherein the target changing section (33) is configured to increase the target intermediate pressure until the value of the current flowing in the higher-stage drive section (22b) becomes smaller than the current limit value.

3. The control device (20) according to Claim 1 or 2, wherein the target setting section is configured to set, as the target intermediate pressure, a theoretical intermediate pressure at which a compression ratio of the lower-stage compressor (3a) and a compression ratio of the higher-stage compressor (3b) are equal to each other.

4. A two-stage-compression refrigerating cycle apparatus (10) comprising the control device (20) according to any one of Claims 1 to 3.

5. A method of controlling a two-stage-compression refrigerating cycle apparatus (10) having a compressor (3) including a lower-stage compressor (3a) and a higher-stage compressor (3b), a condenser (5) for condensing a refrigerant from the compressor, an evaporator (6) for evaporating the condensed refrigerant, and a higher-stage drive section (22b) for driving the higher-stage compressor (3b), the refrigerant being injected between the lower-stage compressor (3a) and the higher-stage compressor (3b), the method comprising:

a target setting step (SA1)of setting a target intermediate pressure in accordance with a predetermined algorithm;

a determination step (SA3) of determining whether or not the value of a current flowing in the higher-stage drive section (22b) has exceeded a predetermined current limit value determined from a configuration of the higher-stage drive section (22b); and

a target changing step (SA4) of increasing the target intermediate pressure if the value of the current flowing in the higher-stage drive section (22b) has exceeded the current limit value.

Drawing