Control device, condensing unit, control method, and control program

Abstract

Provided are a control device, a condensing unit, a control method, and a control program with which it is possible to control the middle pressure of a refrigerant circuit. A control device (30) controls a refrigerant circuit which comprises: compressors (11, 12) that are provided in parallel; gas coolers (51, 52) that cool a refrigerant compressed by the compressors (11, 12) to a high pressure equal to or higher than the critical pressure during normal operation; a middle pressure receiver (9) for storing a middle pressure liquid refrigerant cooled by the gas coolers (51, 52); and a solenoid valve SVG and an electronic expansion valve EEVG provided in parallel on the upstream side of the middle pressure receiver (9) and on the downstream side of the gas coolers (51, 52). The control device performs control of changing the solenoid valve SVG from opening to closing upon determination of satisfaction of all conditions that: the high-pressure saturation temperature is equal to or lower than the critical temperature; the evaporation temperature set value is a value at which the inhalation density is equal to or lower than 50% with respect to the set maximum evaporation temperature; the middle pressure measurement value exceeds a target middle pressure; and the electronic expansion valve EEVG has the minimum opening degree.

Description

Technical Field

[0001] The present disclosure relates to a control device, a condensing unit, a control method, and a control program.Background Art

[0002] As a refrigerating apparatus, a refrigerating apparatus including two compressors is known. PTL 1 discloses a refrigerating apparatus that include two compressors and that uses a CO2 refrigerant.Citation ListPatent Literature

[0003] [PTL 1] Japanese Unexamined Patent Application Publication No. 2019-128049Summary of InventionTechnical Problem

[0004] However, in the invention disclosed PTL 1, a solenoid valve installed in parallel with an expansion valve installed at a position downstream of a gas cooler and upstream of a middle pressure receiver for circulation amount processing in a wide area has not been studied. In addition, in a case where the expansion valve and the solenoid valve are installed in parallel, the expansion valve has a variable opening degree and the solenoid valve has a fixed opening degree. Here, when the circulation amount of the refrigerant in a refrigerant circuit is a low circulation amount, the opening degree of the expansion valve has to be reduced since the opening degree of the solenoid valve is not changed. As a result, an increase in middle pressure occurs, but the middle pressure cannot be controlled because the opening degree of the expansion valve cannot be changed.

[0005] The present disclosure has been made in view of such circumstances, and an object of the present disclosure is to provide a control device, a condensing unit, a control method, and a control program that can control a middle pressure of a refrigerant circuit in which a solenoid valve and an expansion valve are installed in parallel.Solution to Problem

[0006] In order to achieve the above-described object, the control device, the condensing unit, the control method, and the control program according to the present disclosure adopt the following means.

[0007] A control device according to the present disclosure is a control device that controls a refrigerant circuit including a plurality of compressors provided in parallel, a gas cooler that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation, a middle pressure receiver that stores a liquid refrigerant at a middle pressure cooled by the gas cooler, and a solenoid valve that has a fixed opening degree and an electronic expansion valve that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of the middle pressure receiver and on a downstream side of the gas cooler, in which the control device performs control of changing the solenoid valve from open to closed when it is determined that the refrigerant circuit satisfies all of the following conditions: a high-pressure saturation temperature of the refrigerant circuit is equal to or lower than a critical temperature of the refrigerant; an evaporation temperature setting value of the refrigerant circuit is a value at which a suction density of the compressors is 50% or lower relative to the suction density at a maximum evaporation temperature setting of the refrigerant circuit; and a middle pressure measured value of the refrigerant circuit exceeds a target middle pressure and the electronic expansion valve has a minimum opening degree.

[0008] A condensing unit according to the present disclosure is a condensing unit including: a plurality of compressors provided in parallel; a gas cooler that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation; a middle pressure receiver that stores a liquid refrigerant at a middle pressure cooled by the gas cooler; a solenoid valve that has a fixed opening degree and an electronic expansion valve that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of the middle pressure receiver and on a downstream side of the gas cooler; and the control device described above.

[0009] A control method according to the present disclosure is a control method of controlling a refrigerant circuit including a plurality of compressors provided in parallel, a gas cooler that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation, and a solenoid valve that has a fixed opening degree and an electronic expansion valve that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of a middle pressure receiver that stores a liquid refrigerant at a middle pressure cooled by the gas cooler and on a downstream side of the gas cooler, the control method including: performing control of changing the solenoid valve from open to closed when it is determined that the refrigerant circuit satisfies all of the following conditions: a high-pressure saturation temperature of the refrigerant circuit is equal to or lower than a critical temperature of the refrigerant; an evaporation temperature setting value of the refrigerant circuit is a value at which a suction density of the compressors is 50% or lower relative to the suction density at a maximum evaporation temperature setting of the refrigerant circuit; and a middle pressure measured value of the refrigerant circuit exceeds a target middle pressure and the electronic expansion valve has a minimum opening degree.

[0010] A control program according to the present disclosure causes a computer to execute the control method described above.Advantageous Effects of Invention

[0011] According to the present disclosure, in a case where an outside air temperature is low and the circulation amount of the refrigerant is small, the middle pressure of the refrigerant circuit can be suppressed to approach the target middle pressure, and a coefficient of performance (COP) can be improved.Brief Description of Drawings

[0012] FIG. 1 is a diagram showing a refrigerant circuit according to some embodiments of the present disclosure. FIG. 2 is a diagram showing an example of a hardware configuration of a control device according to some embodiments of the present disclosure. FIG. 3 is a diagram showing an example of functions of the control device according to some embodiments of the present disclosure. FIG. 4 is a graph showing a correlation between a middle pressure and the COP according to some embodiments of the present disclosure. FIG. 5 is a diagram showing a control flow of the control device according to some embodiments of the present disclosure. FIG. 6 is a diagram showing a control flow of the control device according to some embodiments of the present disclosure. Description of Embodiments

[0013] Hereinafter, embodiments of a control device, a condensing unit, a control method, and a control program according to the present disclosure will be described with reference to the accompanying drawings.

[0014] FIG. 1 is a diagram showing the refrigerant circuit according to some embodiments of the present disclosure.

[0015] FIG. 1 shows a refrigerating apparatus 100 including a condensing unit 1 and a showcase 2. The condensing unit 1 mainly includes compressors 11 and 12, an accumulator 3, oil separators 61 and 62, gas coolers 51 and 52, oil pods 71 and 72, a middle pressure receiver 9, a subcooling coil 10, and a control device 30. The showcase 2 includes an expansion valve 81 and a utilization-side heat exchanger (evaporator) 80. The refrigerant circuit according to the present embodiment is configured to include the compressors 11 and 12, the gas coolers 51 and 52, the expansion valve 81, the utilization-side heat exchanger 80, pipes connecting the compressors 11 and 12, the gas coolers 51 and 52, the expansion valve 81, and the utilization-side heat exchanger 80, and the like.

[0016] The compressors 11 and 12 compress the refrigerant to supply the compressed high-pressure refrigerant to the refrigerant circuit. The compressor 11 is a two-stage compressor having a first-stage compression portion 11a at a first stage and a second-stage compression portion 11b at a second stage. The compressor 12 is a two-stage compressor having a first-stage compression portion 12a at a first stage and a second-stage compression portion 12b at a second stage. In the condensing unit 1, the compressor 11 and the compressor 12 are provided in parallel. As will be described below, the compressor 11 and the compressor 12 are connected to each other such that a refrigerator oil can be moved through the oil pod 71, the oil pod 72, and an oil equalizer pipe 73. For example, CO2 is used as the refrigerant according to the present embodiment. The CO2 refrigerant has a large pressure difference between a high pressure and a low pressure, and a two-stage compression structure is adopted to improve the efficiency of the compressor. In order to improve performance and efficiency, a gas injection mechanism (to be described below) for supplying a refrigerant having an intermediate pressure (middle pressure) to the second-stage compression portions 11b and 12b of the compressors 11 and 12 is adopted. A temperature sensor Tho-D1 is provided on a discharge side of the compressor 11, and a temperature sensor Tho-D2 is provided on a discharge side of the compressor 12.

[0017] A hot gas bypass pipe BP11 is connected to a discharge pipe MP01 on the discharge side of the compressor 11. The hot gas bypass pipe BP11 is provided with a strainer 131, a solenoid valve SVHG1, and a capillary tube 141, and is connected to a return pipe MP13 that is a flow channel of the refrigerant returned from a load side. A flow channel including the hot gas bypass pipe BP11, the strainer 131, the solenoid valve SVHG1, and the capillary tube 141 will be referred to as a hot gas bypass. A flow of the refrigerant in the hot gas bypass is controlled by opening and closing the solenoid valve SVHG1. That is, when the control device 30 opens the solenoid valve SVHG1, the discharge side and a suction side of the compressor 11 communicate with each other, and the refrigerant flows through the hot gas bypass. Similarly, a hot gas bypass pipe BP12 is connected to a discharge pipe MP02 on the discharge side of the compressor 12. The hot gas bypass pipe BP12 is provided with a strainer 132, a solenoid valve SVHG2, and a capillary tube 142 to configure the hot gas bypass, and is connected to the return pipe MP13. A flow of the refrigerant in the hot gas bypass on the compressor 12 side is also controlled by opening and closing of the solenoid valve SVHG2, as on the compressor 11 side. The solenoid valves SVHG1 and SVHG2 are often opened, before the compressors 11 and 12 are activated, to equalize the pressure on the suction side and the discharge side of the compressors 11 and 12, and are often closed after the compressors 11 and 12 are activated. Therefore, during an operation of the condensing unit 1, the refrigerant discharged by the compressors 11 and 12 flows through the discharge pipes MP01 and MP02 without flowing through the hot gas bypass.

[0018] The refrigerant flowing through the discharge pipe MP01 is supplied to the oil separator 61. In the oil separator 61, the refrigerant and the refrigerator oil dissolved in the refrigerant are separated, and the refrigerant flows to a gas pipe MP10 via a check valve CV1. A bottom portion of the oil separator 61 and a side surface of the oil pod 71 communicate with each other through an oil return pipe RP11. The refrigerator oil separated by the oil separator 61 is returned to the oil pod 71 through the oil return pipe RP11. The oil return pipe RP11 is provided with a strainer 121, and the oil return pipe RP11 is divided into two flow channels on a downstream side of the strainer 121. A capillary tube 151 and a solenoid valve SV-OIL1 are provided in a pipe of one flow channel, and a capillary tube 161 is provided in a pipe of the other flow channel. An amount of the refrigerator oil returned to the oil pod 71 is adjusted by opening and closing the solenoid valve SV-OIL1.

[0019] An upper portion of the oil pod 71 and a discharge side of the first-stage compression portion 11a of the compressor 11 communicate with each other through a pipe. The refrigerator oil flows into the oil pod 71 from the communication pipe on the discharge side of the first-stage compression portion 11a during the operation of the compressor 11, separately from the refrigerator oil flowing in from the oil return pipe RP11. A bottom portion of the oil pod 71 communicates with a bottom portion of the compressor 11 through a pipe. The refrigerator oil accumulated below the oil pod 71 is returned to the compressor 11 through a pipe in the bottom portion.

[0020] The oil pod 71 is provided with a level switch 71a. The level switch 71a detects a height of an oil level in the oil pod 71. When the oil level drops to a predetermined height, the level switch 71a outputs a signal indicating the drop of the oil level to the control device 30.

[0021] The same applies to the configuration on the compressor 12 side. The refrigerant discharged by the compressor 12 reaches the oil separator 62 through the discharge pipe MP02, and the refrigerant separated by the oil separator 62 flows to the gas pipe MP10 via a check valve CV2. The separated refrigerator oil passes through a strainer 122, a capillary tube 152, a capillary tube 162, and a valve SV-OIL2 provided in an oil return pipe RP12, and is returned to the oil pod 72. The oil return pipe RP12 branches on an upstream side of the strainer 122, and the branch pipe is provided with a service valve 94. The refrigerator oil flows into the oil pod 72 from a discharge side of the second-stage compression portion 12b of the compressor 12. The refrigerator oil supplied to the oil pod 72 is returned to the compressor 12 through a pipe in the bottom portion. When the level switch 72a provided in the oil pod 72 detects the drop of the oil level in the oil pod 72, the level switch 72a notifies the control device 30 of the drop of the oil level.

[0022] The oil pod 71 and the oil pod 72 are connected to each other through the oil equalizer pipe 73. The oil equalizer pipe 73 is provided to eliminate a bias in an amount of the oil stored in each of the oil pods 71 and 72. One end of the oil equalizer pipe 73 is connected to a position that is higher than the height (height at which the drop of the oil level is detected) at which the level switch 71a is provided on the side surface of the oil pod 71 and that is lower than the connection position of the oil return pipe RP11, and the other end of the oil equalizer pipe 73 is connected to a position that is higher than the height (height at which the level switch 72a is provided) at which the level switch 72a is provided on the side surface of the oil pod 72 and that is lower than the connection position of the oil return pipe RP12. The oil equalizer pipe 73 is provided with a strainer 123, a solenoid valve EV-OIL3, and a strainer 124 in this order from the level switch 71a side. The oil equalizer pipe 73 branches between the level switch 71a and the strainer 123, and the branch pipe is provided with a service valve 93.

[0023] The high-temperature and high-pressure refrigerant discharged by the compressors 11 and 12 is supplied to the gas coolers 51 and 52 provided in parallel, through the gas pipe MP10. The refrigerant supplied to the gas coolers 51 and 52 is condensed by heat exchange with air sent by fans 41 and 42. In a case of the CO2 refrigerant, the CO2 refrigerant is cooled. The gas coolers 51 and 52 cool the refrigerant compressed by the compressors 11 and 12 to a high pressure equal to or higher than a critical pressure during a normal operation that is not abnormal. A temperature sensor Tho-G1, a pressure sensor PSH for high-pressure measurement, and a service valve 92 are provided on an upstream side of the gas coolers 51 and 52 in the gas pipe MP10.

[0024] The refrigerant cooled or condensed by the gas coolers 51 and 52 passes through a strainer 111 provided in a gas-liquid two-phase pipe MP11 on a downstream side of the gas coolers 51 and 52, and is, in a case of the CO2 refrigerant, for example, depressurised to the middle pressure of about 6 Mpa and supplied to the middle pressure receiver 9 by a solenoid valve SVG having a fixed opening degree, that is, an opening-closing valve, and an expansion valve (electronic expansion valve) EEVG having a variable opening degree, the solenoid valve SVG and the expansion valve EEVG being provided in parallel on an inlet side (upstream side) of the middle pressure receiver 9. A temperature sensor Tho-G2 is provided on the downstream side of the gas coolers 51 and 52 in the gas-liquid two-phase pipe MP11, and a temperature sensor Tho-M is provided on the inlet side of the middle pressure receiver 9. In addition, a capillary tube 171 is provided on a downstream side of the solenoid valve SVG.

[0025] The solenoid valve SVG and the expansion valve EEVG having the variable opening degree are provided in parallel for widely processing the circulation amount of the refrigerant. Operation pressures of the refrigerant circuit include a high pressure, a middle pressure, and a low pressure, and the solenoid valve SVG and the expansion valve EEVG perform pressure adjustment of the high pressure and the middle pressure. The solenoid valve SVG having the fixed opening degree is normally opened.

[0026] Liquid bypass pipes BP21 and BP22 for returning the refrigerant after being cooled or condensed to the compressors 11 and 12 are connected to an upstream side of the expansion valve EEVG in the gas-liquid two-phase pipe MP11. The liquid bypass pipe BP21 is provided with an expansion valve EEV-LB1, and the control device 30 controls an opening degree of the expansion valve EEV-LB1 to adjust an amount of the refrigerant flowing into the liquid bypass pipe BP21. Similarly, the liquid bypass pipe BP22 is provided with an expansion valve EEV-LB2, and the control device 30 controls an opening degree of the expansion valve EEV-LB2 to adjust an amount of the refrigerant flowing into the liquid bypass pipe BP22. The liquid bypass pipe BP21 is connected to an injection pipe IP11 to be described later, and the liquid bypass pipe BP22 is connected to an injection pipe IP12. The expansion valve EEV-LB1 is opened by the control device 30 when the temperature on the discharge side of the compressor 11 is equal to or higher than a predetermined temperature. By opening the expansion valve EEV-LB1, the refrigerant that is subjected to the heat exchange in the gas coolers 51 and 52 to have a lower temperature than the discharge side of the compressor 11 is supplied to a middle pressure portion (suction side of the second-stage compression portion 11b) of the compressor 11 through the liquid bypass pipe BP21 and the injection pipe IP11, and an excessive temperature rise on the discharge side of the compressor 11 is suppressed. The same applies to the expansion valve EEV-LB2 and the liquid bypass pipe BP22.

[0027] The middle pressure receiver 9 separates the refrigerant in a gas-liquid two-phase state, which has passed through the solenoid valve SVG and has been expanded by the expansion valve EEVG, into a gas refrigerant that is a gas-phase refrigerant and a liquid refrigerant that is a liquid-phase refrigerant. The middle pressure receiver 9 is provided with a middle pressure receiver level switch 9a that detects whether or not a liquid level of the liquid refrigerant has reached an upper limit. An injection pipe IP10 is connected to an upper portion of the middle pressure receiver 9 to send out the gas refrigerant. The injection pipe IP10 is provided with a pressure sensor PSM1 is provided in a flow channel in which a service valve 95 is provided. In addition, the injection pipe IP10 is provided with a stop valve 96 and a safety valve 97 in the branched flow channel. The gas refrigerant sent out by the injection pipe IP10 is branched into two flow channels (injection pipes IP11 and IP12). The injection pipe IP11 is provided with an expansion valve EEV-INJ1, and the control device 30 controls the pressure of the refrigerant flowing into the injection pipe IP11 by controlling an opening degree of the expansion valve EEV-INJ1. A check valve CV3 is provided on the compressor 11 side of the expansion valve EEV-INJ1, and the gas refrigerant at the middle pressure flowing into the injection pipe IP11 is supplied to the middle pressure portion of the compressor 11. The gas refrigerant at the middle pressure supplied from the injection pipe IP11 is recompressed by the second-stage compression portion 11b. In this manner, the COP of a refrigerating cycle can be improved (gas injection mechanism). A temperature sensor Tho-INJ1 and a pressure sensor PSM21 are provided on a front side of a connection portion with the compressor 11 in the injection pipe IP11.

[0028] The injection pipe IP12 is provided with an expansion valve EEV-INJ2 and the check valve CV4, and the control device 30 controls the pressure of the refrigerant flowing into the injection pipe IP12 by controlling an opening degree of the expansion valve EEV-INJ2. The gas refrigerant at the middle pressure flowing into the injection pipe IP12 is supplied to the middle pressure portion of the compressor 12 and is recompressed. A temperature sensor Tho-INJ2 and a pressure sensor PSM22 are provided on a front side of a connection portion with the compressor 12 in the injection pipe IP12.

[0029] Meanwhile, the liquid refrigerant separated inside the middle pressure receiver 9 flows through a liquid pipe MP12, passes through a strainer 14, and is cooled by the subcooling coil 10. On an upstream side of the subcooling coil 10, a subcooling pipe MP12A is connected to the liquid pipe MP12, and a portion of the liquid refrigerant flowing out from the middle pressure receiver 9 is branched into the subcooling pipe MP12A. The branched liquid refrigerant is depressurized by an expansion valve EEVSC for the subcooling coil to have a low temperature, and the liquid refrigerant (refrigerant supplied to the load side) flowing through the liquid pipe MP12 is cooled in the subcooling coil 10. The refrigerant branched into the subcooling pipe MP12A flows to the accumulator 3 after passing through the subcooling coil 10. The subcooling pipe MP12A is provided with a temperature sensor Tho-SC.

[0030] The refrigerant flowing through the liquid pipe MP12 is cooled by the subcooling coil 10, and then flows out from the condensing unit 1 and is supplied to an external load (showcase 2) through a connection pipe 82. The liquid pipe MP12 is provided with a temperature sensor Tho-R. The showcase 2 includes the expansion valve 81 and the utilization-side heat exchanger 80. The refrigerant supplied from the condensing unit 1 is depressurized by the expansion valve 81, is subjected to the heat exchange in the utilization-side heat exchanger 80, and cools an object. The refrigerant after the heat exchange returns to the condensing unit 1 through the connection pipe 83. In the condensing unit 1, the refrigerant returned from the load side flows to the accumulator 3 through the return pipe MP13. A temperature sensor Tho-S, a pressure sensor PSL for low-pressure measurement, and a service valve 91 are provided on a front side of the accumulator 3 in the return pipe MP13.

[0031] The accumulator 3 separates the refrigerant supplied to the compressors 11 and 12 into gas and liquid. A portion of the gas refrigerant separated by the accumulator 3 passes through a suction pipe MP14 and is supplied to an accumulator 21 attached to the compressor 11. The gas refrigerant from which only the gas phase is further extracted in the accumulator 21 is supplied to the suction side of the first-stage compression portion 11a of the compressor 11. Similarly, the remaining gas refrigerant separated by the accumulator 3 passes through the suction pipe MP15, is supplied to an accumulator 22 attached to the compressor 12, and is supplied to the suction side of the first-stage compression portion 12a of the compressor 12. The condensing unit 1 is provided with a temperature sensor Tho-A that measures an outside temperature.

[0032] The control device 30 is connected to various sensors such as the pressure sensor PSL and the temperature sensor Tho-R, various valves such as the expansion valve EEVG, and devices such as the compressors 11 and 12, acquires a measured value by each sensor, adjusts the rotation speeds of the compressors 11 and 12 and the opening and closing of the expansion valve EEVG, and operates the condensing unit 1.

[0033] FIG. 2 is a diagram showing an example of a hardware configuration of the control device according to some embodiments of the present disclosure.

[0034] As shown in FIG. 2, the control device (controller) 30 is a computer system (computing system), and includes, for example, a central processing unit (CPU: processor) 1100, a secondary storage device (read only memory (ROM), secondary storage: memory) 1200, a main storage device (random access memory (RAM), main memory) 1300, a hard disk drive (HDD) 1400 as a large-capacity storage device, and a communication unit 1500 for connecting to a network or the like. A solid state drive (SSD) may be used as the large-capacity storage device. Each of these units is connected to each other through a bus 1800.

[0035] The CPU 1100 controls the entire control device 30 by, for example, an operating system (OS) stored in the secondary storage device 1200 connected through the bus 1800, and executes various types of processing by executing various programs stored in the secondary storage device 1200. One or a plurality of CPUs 1100 may be provided to implement the processing in cooperation with each other.

[0036] For example, the main storage device 1300 is configured as a writable memory such as a cache memory or an RAM, and is used as a work area for operations such as reading execution programs of the CPU 1100 and writing processing data of the execution programs.

[0037] The secondary storage device 1200 is a non-transitory computer-readable storage medium. The secondary storage device 1200 is, for example, a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, and a semiconductor memory. Examples of the secondary storage device 1200 include an ROM, an HDD, an SSD, and a flash memory. The secondary storage device 1200 stores, for example, an OS for controlling an entire information processing apparatus, such as Windows (registered trademark), iOS (registered trademark), and Android (registered trademark), a basic input / output system (BIOS), various device drivers for hardware operations of peripheral devices, various types of application software, various data and files, and the like. The secondary storage device 1200 stores a program for implementing various types of processing and various data required to implement the various types of processing. A plurality of secondary storage devices 1200 may be provided, and the program or data described above may be divided and stored across each of the secondary storage devices 1200.

[0038] The control device 30 may include an input unit including a keyboard or a mouse, or a display unit including a liquid-crystal display device for displaying data. The control device 30 includes a display unit and a notification unit such as a lamp or a speaker that outputs sound, particularly an alarm sound.

[0039] FIG. 3 is a diagram showing an example of functions of the control device according to some embodiments of the present disclosure.

[0040] As shown in FIG. 3, the control device 30 includes an acquisition unit 31, a determination unit 32, and a valve control unit 33.

[0041] As an example, a series of processing for implementing the functions of the control device 30 are stored in the secondary storage device 1200 (see FIG. 2) in the form of a program, and the CPU (processor) 1100 (see FIG. 2) reads the program to the main storage device 1300 (see FIG. 2) and executes information processing and calculation processing to implement various functions. The program may be in a form in which the program is installed in advance in the secondary storage device 1200, in a form in which the program is provided in a state of being stored in another non-transitory computer-readable storage medium, or in a form in which the program is distributed via wired or wireless communication means. Examples of the non-transitory computer-readable storage medium include a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, and a semiconductor memory.

[0042] The acquisition unit 31 shown in FIG. 3 acquires the measured value from each sensor. For example, the acquisition unit 31 acquires a measured value of a refrigerant pressure on the discharge side (downstream side) of the compressors 11 and 12 from the pressure sensor PSH for high-pressure measurement. The pressure measured by the pressure sensor PSH is a pressure (high-pressure, high pressure) when the pressure of the refrigerant is high in the refrigerant circuit.

[0043] The acquisition unit 31 acquires the evaporation temperature from the temperature sensor Tho-R.

[0044] The acquisition unit 31 acquires a measured value of the refrigerant pressure on the discharge side (downstream side) of the middle pressure receiver 9 from the pressure sensor PSM for middle pressure measurement. The pressure measured by the pressure sensor PSM is a pressure (middle-pressure, middle pressure) when the pressure of the refrigerant is middle in the refrigerant circuit.

[0045] The acquisition unit 31 acquires the opening degree of the expansion valve EEVG and the open / closed state of the solenoid valve SVG.

[0046] The acquisition unit 31 acquires measured values of suction refrigerant pressures (suction pressures) of the compressors 11 and 12 from the pressure sensors PSM21 and PSM22, and acquires measured values of suction refrigerant temperatures (suction temperatures) of the compressors 11 and 12 from the temperature sensors Tho-INJ1 and Tho-INJ2.

[0047] The determination unit 32 performs a determination based on various measured values acquired by the acquisition unit 31.

[0048] Specifically, the determination unit 32 determines whether or not all of the three conditions described below are satisfied in the refrigerant circuit. (Condition 1) A high-pressure saturation temperature of the refrigerant circuit is equal to or lower than a critical temperature of the refrigerant. (Condition 2) An evaporation temperature setting value of the refrigerant circuit is a value at which a suction density of the compressors is 50% or lower relative to the suction density at a maximum evaporation temperature setting of the refrigerant circuit. (Condition 3) A middle pressure measured value of the refrigerant circuit exceeds a target middle pressure, and the electronic expansion valve is at a minimum opening degree.

[0049] (Condition 1) The high-pressure saturation temperature of the refrigerant circuit is equal to or lower than the critical temperature of the refrigerant.

[0050] The control device 30 calculates a pressure saturation temperature of the refrigerant circuit from the high pressure acquired from the pressure sensor PSH. In a case where the refrigerant is, for example, the CO2 refrigerant, the critical temperature of the CO2 refrigerant is about 31°C. It can be said that the refrigerant is in a state of being liquefied in the gas coolers 51 and 52 when the high-pressure saturation temperature is equal to or lower than the critical temperature of the refrigerant. That is, it can be said that the outside air temperature is lower than usual (for example, lower than 18°C). The determination unit 32 determines whether or not the high-pressure saturation temperature of the refrigerant circuit is equal to or lower than the critical temperature of the refrigerant.

[0051] (Condition 2) The evaporation temperature setting value of the refrigerant circuit is a value at which the suction density of the compressors is 50% or lower relative to the suction density at the maximum evaporation temperature setting of the refrigerant circuit.

[0052] A temperature range of the evaporation temperature of the refrigerant circuit is determined to be, for example, -45°C to +5°C. This temperature range is defined as a specified evaporation temperature. A maximum value of the specified evaporation temperature is defined as a maximum evaporation temperature setting (in the above example, +5°C). A user can set any evaporation temperature in accordance with the use, the temperature, or the like of the refrigerating apparatus 100 including the condensing unit 1 within a range of the specified evaporation temperature. The set evaporation temperature is defined as an evaporation temperature setting value.

[0053] The control device 30 obtains the suction density of the compressors 11 and 12 at the maximum evaporation temperature setting. The control device 30 obtains the suction density of the compressors 11 and 12 at the evaporation temperature setting value, and the determination unit 32 determines whether or not the suction density at the evaporation temperature setting value is 50% or lower relative to the suction density at the suction density at the maximum evaporation temperature setting.

[0054] The suction density of the compressors 11 and 12 is a value that serves as a reference for the circulation amount of the refrigerant, and is in a proportional relationship with a target evaporation temperature in the refrigerant circuit. In addition, at the maximum evaporation temperature setting, the circulation amount of the refrigerant is maximized, and the compressors 11 and 12 have a maximum rotation speed. That is, in a case where the suction density is 50% or lower relative to the suction density at the maximum circulation amount of the refrigerant (maximum evaporation temperature setting), it can be said that the target evaporation temperature in the refrigerant circuit is lower than the maximum value of the specified evaporation temperature, and the circulation amount is a low circulation amount that is sufficiently lower than the maximum circulation amount. For example, in a case where a valve flow coefficient of the expansion valve EEVG and a valve flow coefficient of the solenoid valve SVG are the same as each other, the expansion valve EEVG and the solenoid valve SVG are open (fully open) at the maximum evaporation temperature setting. Only the solenoid valve SVG is opened when the suction density is equal to or lower than 50%.

[0055] (Condition 3) The middle pressure measured value of the refrigerant circuit exceeds the target middle pressure, and the electronic expansion valve is at the minimum opening degree.

[0056] In a case where the compressors 11 and 12 are operated, the compressors 11 and 12 are operated at the same rotation speed. In addition, the solenoid valve SVG is opened at the fixed opening degree regardless of the outside air temperature or the circulation amount of the refrigerant. Therefore, the opening degree of the expansion valve EEVG is adjusted in accordance with the outside air temperature or the circulation amount of the refrigerant.

[0057] In a case where the above-described (Condition 1) and (Condition 2) are satisfied, it can be said that the outside air temperature is a low outside air temperature that is lower than normal, and the circulation amount of the refrigerant is a low circulation amount that is lower than the maximum circulation amount. In this case, both the compressors 11 and 12 have a low rotation speed. In a case where a total rotation speed of the two compressors 11 and 12 is, for example, a value close to the maximum rotation speed of one compressor 11 or 12, the opening degree of the solenoid valve SVG having the fixed opening degree is not changed, the liquid refrigerant is preferentially discharged from the gas coolers 51 and 52 to the middle pressure receiver 9, and the opening degree of the expansion valve EEVG is adjusted in the narrowing direction to the minimum opening degree.

[0058] When the opening degree of the expansion valve EEVG is at the minimum opening degree, the middle pressure of the refrigerant circuit increases. As the middle pressure increases, the COP decreases.

[0059] FIG. 4 is a diagram showing a correlation between the middle pressure and the COP according to some embodiments of the present disclosure. In FIG. 4, a vertical axis indicates the COP of the refrigerant circuit, and a horizontal axis indicates the middle pressure of the refrigerant circuit. FIG. 4 shows the correlation between the middle pressure and the COP in a case where the outside air temperature is 5°C, but for example, the same tendency is shown even in a case where the outside air temperature is 15°C.

[0060] As shown in FIG. 4, the correlation between the middle pressure and the COP shows a right shoulder decline, and the COP decreases when the middle pressure is high. On the other hand, the COP is improved when the middle pressure is low, and an optimum COP is specified in a region in which the middle pressure is low.

[0061] In this way, when the expansion valve EEVG is at the minimum opening degree and the middle pressure is a high value, that is, the middle pressure measured value exceeds the target middle pressure, the optimum COP cannot be exhibited.

[0062] The determination unit 32 determines whether or not the middle pressure measured value measured by the pressure sensor PSM1 exceeds the target middle pressure and whether or not the expansion valve EEVG is at the minimum opening degree.

[0063] When the determination unit 32 determines that all of the three conditions are satisfied in the refrigerant circuit, the expansion valve EEVG is at the minimum opening degree and the middle pressure is at a high value in a case where the outside air temperature is a low outside air temperature that is lower than normal and the circulation amount of the refrigerant is a low circulation amount lower than the maximum circulation amount, and the optimum COP cannot be exhibited.

[0064] The valve control unit 33 shown in FIG. 3 controls the valve based on a determination result of the determination unit 32. In the present disclosure, the valve control unit 33 controls the opening and closing of the solenoid valve SVG. When the determination unit 32 determines that all of the conditions are satisfied, the valve control unit 33 changes the solenoid valve SVG from open to closed. When the outside air temperature is low and the circulation amount of the refrigerant is low, the solenoid valve SVG is closed, so that the middle pressure is adjusted only by the expansion valve EEVG. The opening degree of the expansion valve EEVG is adjusted by the valve control unit 33 such that the middle pressure measured value approaches the target middle pressure.

[0065] FIG. 5 is a diagram showing a control flow of the control device according to some embodiments of the present disclosure.

[0066] In step S101, the acquisition unit 31 of the control device 30 acquires the measured value from each sensor.

[0067] In step S102, the determination unit 32 determines whether or not the high-pressure saturation temperature of the refrigerant circuit is equal to or lower than the critical temperature of the refrigerant. In a case where it is determined that the high-pressure saturation temperature is equal to or lower than the critical temperature of the refrigerant (Y in S102), the processing proceeds to step S103. On the other hand, in a case where it is determined that the high-pressure saturation temperature exceeds the critical temperature of the refrigerant (N in S 102), the processing returns to step S101.

[0068] In step S103, the determination unit 32 determines whether or not the evaporation temperature setting value of the refrigerant circuit is a value at which the suction density of the compressor 11 or 12 is 50% or lower relative to the suction density at the maximum evaporation temperature setting of the refrigerant circuit. In a case where it is determined that the evaporation temperature setting value of the refrigerant circuit is a value at which the suction density of the compressor 11 or 12 is 50% or lower relative to the suction density at the maximum evaporation temperature setting of the refrigerant circuit (Y in S103), the processing proceeds to step S104. On the other hand, in a case where it is determined that the evaporation temperature setting value of the refrigerant circuit is a value at which the suction density of the compressor 11 or 12 exceeds 50% of the maximum evaporation temperature setting of the refrigerant circuit (N in S103), the processing returns to step S101.

[0069] In step S104, the determination unit 32 determines whether or not the middle pressure measured value of the refrigerant circuit exceeds the target middle pressure and the electronic expansion valve is at the minimum opening degree. In a case where it is determined that the middle pressure measured value of the refrigerant circuit exceeds the target middle pressure and the electronic expansion valve is at the minimum opening degree (Y in S104), the processing proceeds to step S105. On the other hand, in a case where it is determined that the middle pressure measured value of the refrigerant circuit is equal to or lower than the target middle pressure and / or the electronic expansion valve is not at the minimum opening degree (N in S104), the processing returns to step S101.

[0070] In step S105, the valve control unit 33 changes the solenoid valve SVG from open to closed. The valve control unit 33 performs control of changing the solenoid valve SVG from open to closed when the determination unit 32 determines that all of the conditions of steps S102, S103, and S104 are satisfied. When the outside air temperature is low and the circulation amount of the refrigerant is low, the solenoid valve SVG is changed from open to closed, so that the middle pressure can be adjusted only by the expansion valve EEVG having the variable opening degree. The opening degree of the expansion valve EEVG is adjusted by the valve control unit 33 such that the middle pressure measured value approaches the target middle pressure. As a result, the middle pressure can be kept low, and the COP of the refrigerant circuit can be improved.

[0071] FIG. 6 is a diagram showing a control flow of the control device according to some embodiments of the present disclosure.

[0072] A and B in FIG. 6 correspond to A and B in FIG. 5. After the solenoid valve SVG is closed in step S105 of FIG. 5, the solenoid valve SVG may be further controlled. In this case, the flows of steps S106 and S107 in FIG. 6 are executed.

[0073] When the solenoid valve SVG is closed in step S105 of FIG. 5, the discharge of the liquid refrigerant from the gas cooler 51 and / or 52 to the middle pressure receiver 9 is reduced, so that the liquid refrigerant tends to stay in the gas cooler 51 and / or 52. As a result, an amount of the liquid refrigerant stored in the middle pressure receiver 9 decreases. For example, it can be detected that a large amount of the liquid refrigerant is stored in the gas coolers 51 and / or 52 by the expansion valve EEVSC being fully opened. In a case where the subcooling is not applied to the subcooling coil 10, it can be said that the refrigerant in a gas phase flows in the subcooling coil 10, and the expansion valve EEVSC is fully opened in this case. This is because the solenoid valve SVG and the expansion valve EEVG are too throttled and the amount of the liquid refrigerant stored in the middle pressure receiver 9 is reduced. That is, the liquid refrigerant is stored in the gas cooler 51 and / or 52, and the circulation amount of the refrigerant is reduced. Therefore, in a case where the closed solenoid valve SVG is opened, the liquid refrigerant of the gas cooler 51 and / or 52 can be supplied to the middle pressure receiver 9. It can be said that the fact that a large amount of the liquid refrigerant is stored in the gas coolers 51 and / or 52 means that the amount of the liquid refrigerant stored in the middle pressure receiver 9 is reduced, so that a middle pressure receiver level switch (not shown) that detects whether or not the liquid level of the liquid refrigerant has reached a lower limit may be provided at a predetermined position of the middle pressure receiver 9, and the detection may be performed by detecting that the liquid level has reached the lower limit.

[0074] After the control is performed in step S105 of FIG. 5, the processing proceeds to A in FIG. 6 and proceeds to step S106. In step S106, the determination unit 32 determines whether or not the gas cooler 51 and / or 52 stores the liquid refrigerant. When it is determined that the gas cooler 51 or 52 stores the liquid refrigerant (Y in S106), the processing proceeds to step S107. On the other hand, in a case where it is determined that neither of the gas coolers 51 and 52 stores the liquid refrigerant, the processing returns to step S106.

[0075] In step S107, the valve control unit 33 changes the solenoid valve SVG from closed to open, and the processing proceeds to B in FIG. 5. By opening the solenoid valve SVG, the circulation amount of the refrigerant increases, and the liquid refrigerant stored in the gas cooler 51 and / or 52 is discharged from the gas cooler 51 and / or 52 to the middle pressure receiver 9.

[0076] According to the present embodiment, when the high-pressure saturation temperature of the refrigerant circuit is equal to or lower than the critical temperature of the refrigerant, that is, the outside air temperature is low, and the evaporation temperature setting value of the refrigerant circuit is a value at which the suction density of the compressors is 50% or lower relative to the suction density at the maximum evaporation temperature setting of the refrigerant circuit, that is, the target evaporation temperature (evaporation temperature setting value) of the refrigerant circuit is low and the refrigerant circulation amount is small, the solenoid valve SVG is closed, so that the electronic expansion valve EEVG is prevented from being at the minimum opening degree, and the control of the middle pressure of the refrigerant circuit can be performed by the electronic expansion valve EEVG. Therefore, the middle pressure of the refrigerant circuit can be suppressed to approach the target middle pressure, and thus the COP can be improved.

[0077] It can be said that the liquid accumulation that has occurred in the gas coolers 51 and / or 52 indicates a state where the liquid refrigerant in the middle pressure receiver 9 is reduced, the expansion valve EEVSC of the subcooling coil 10 is fully opened, and the refrigerant flows in a gas phase, that is, the solenoid valve SVG and the electronic expansion valve EEVG provided in parallel on the upstream side of the middle pressure receiver 9 are too narrowed, and the circulation amount of the refrigerant is reduced. According to the present embodiment, by changing the solenoid valve SVG from closed to open, the refrigerant can circulate properly, thereby sustaining operation of the refrigerant circuit.<Supplementary Notes>

[0078] The control device, the condensing unit, the control method, and the control program described in the above-described embodiment are understood, for example, as follows.

[0079] A first aspect of the present disclosure relates to a control device (30) that controls a refrigerant circuit including a plurality of compressors (11, 12) provided in parallel, a gas cooler (51, 52) that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation, a middle pressure receiver (9) that stores a liquid refrigerant at a middle pressure cooled by the gas cooler, and a solenoid valve (SVG) that has a fixed opening degree and an electronic expansion valve (EEVG) that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of the middle pressure receiver and on a downstream side of the gas cooler, in which the control device performs control of changing the solenoid valve from open to closed when it is determined that the refrigerant circuit satisfies all of the following conditions: a high-pressure saturation temperature of the refrigerant circuit is equal to or lower than a critical temperature of the refrigerant; an evaporation temperature setting value of the refrigerant circuit is a value at which a suction density of the compressors is 50% or lower relative to the suction density at a maximum evaporation temperature setting of the refrigerant circuit; and a middle pressure measured value of the refrigerant circuit exceeds a target middle pressure and the electronic expansion valve has a minimum opening degree.

[0080] When the high-pressure saturation temperature of the refrigerant circuit is equal to or lower than the critical temperature of the refrigerant, that is, the outside air temperature is low, and the evaporation temperature setting value of the refrigerant circuit is a value at which the suction density of the compressors is 50% or lower relative to the suction density at the maximum evaporation temperature setting of the refrigerant circuit, that is, the target evaporation temperature (evaporation temperature setting value) of the refrigerant circuit is low and the circulation amount of the refrigerant is small, the solenoid valve is closed, so that the electronic expansion valve is prevented from being at the minimum opening degree, and the control of the middle pressure of the refrigerant circuit can be performed by the electronic expansion valve. Therefore, the middle pressure of the refrigerant circuit can be suppressed to approach the target middle pressure, and thus the COP can be improved.

[0081] A second aspect of the present disclosure relates to the control device according to the first aspect, in which the refrigerant may be a CO2 refrigerant.

[0082] The control device can perform control in accordance with the characteristics of the CO2 refrigerant.

[0083] A third aspect of the present disclosure relates to the control device according to the first or second aspect, in which in a case where it is confirmed that the liquid refrigerant is stored in the gas cooler of the refrigerant circuit when the control of changing the solenoid valve from open to closed is performed, the control device may perform control of changing the solenoid valve from closed to open.

[0084] It can be said that the liquid accumulation that has occurred in the gas cooler indicates a state where the liquid refrigerant in the middle pressure receiver is reduced, a valve of a subcooling section is fully opened, and the refrigerant flows in a gas phase, that is, the solenoid valve and the electronic expansion valve provided in parallel on the upstream side of the middle pressure receiver are too narrowed, and the circulation amount of the refrigerant is reduced. Therefore, by changing the solenoid valve from closed to open, the refrigerant can circulate properly, thereby sustaining operation of the refrigerant circuit.

[0085] A fourth aspect of the present disclosure relates to a condensing unit (1) including: a plurality of compressors provided in parallel; a gas cooler that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation; a middle pressure receiver that stores a liquid refrigerant at a middle pressure cooled by the gas cooler; a solenoid valve that has a fixed opening degree and an electronic expansion valve that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of the middle pressure receiver and on a downstream side of the gas cooler; and the control device according to any one of the first to third aspects.

[0086] A fifth aspect of the present disclosure is a control method of controlling a refrigerant circuit including a plurality of compressors provided in parallel, a gas cooler that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation, and a solenoid valve that has a fixed opening degree and an electronic expansion valve that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of a middle pressure receiver that stores a liquid refrigerant at a middle pressure cooled by the gas cooler and on a downstream side of the gas cooler, the control method including: performing control of changing the solenoid valve from open to closed when it is determined that the refrigerant circuit satisfies all of the following conditions: a high-pressure saturation temperature of the refrigerant circuit is equal to or lower than a critical temperature of the refrigerant; an evaporation temperature setting value of the refrigerant circuit is a value at which a suction density of the compressors is 50% or lower relative to the suction density at a maximum evaporation temperature setting of the refrigerant circuit; and a middle pressure measured value of the refrigerant circuit exceeds a target middle pressure and the electronic expansion valve has a minimum opening degree.

[0087] A sixth aspect of the present disclosure relates to a control program causing a computer to execute the control method according to the fifth aspect.Reference Signs List

[0088] 100: refrigerating apparatus 1: condensing unit 2: showcase 3: accumulator 9: middle pressure receiver 9a: middle pressure receiver level switch 10: subcooling coil 11, 12: compressor 11a, 12a: first-stage compression portion 11b, 12b: second-stage compression portion 21, 22: accumulator 30: control device 31: acquisition unit 32: determination unit 33: valve control unit 41, 42: fan 51, 52: gas cooler 61, 62: oil separator 71, 72: oil pod 71a, 72a: level switch 73: oil equalizer pipe 80: utilization-side heat exchanger 81: expansion valve 82, 83: connection pipe 91, 92, 93, 94, 95: service valve 96: stop valve 97: safety valve MP01, MP02: discharge pipe MP10: gas pipe MP11: gas-liquid two-phase pipe MP12: liquid pipe MP12A: subcooling pipe MP13: return pipe MP14, MP15: suction pipe Tho-D1, Tho-D2, Tho-SC, Tho-S, Tho-G1, Tho-R, Tho-G2, Tho-M, Tho-INJ1, Tho-INJ2, Tho-A: temperature sensor PSL, PSH, PSM1, PSM21, PSM22: pressure sensor CV1, CV2, CV3, CV4: check valve BP11, BP12: hot gas bypass pipe SVHG1, SVHG2: solenoid valve 14, 111, 121, 122, 123, 124, 131, 132: strainer 141, 142, 151, 152, 161, 162, 171: capillary tube RP11, RP12: oil return pipe BP21, BP22: liquid bypass pipe IP10, IP11, IP12: injection pipe SV-OIL1, SV-OIL2, EV-OIL3, SVG: solenoid valve EEV-LB1, EEV-LB2, EEV-INJ1, EEV-INJ2, EEVSC: expansion valve EEVG: expansion valve (electronic expansion valve) 1100: CPU 1200: secondary storage device 1300: main storage device 1500: communication unit 1800: bus

Claims

1. A control device that controls a refrigerant circuit including a plurality of compressors provided in parallel, a gas cooler that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation, a middle pressure receiver that stores a liquid refrigerant at a middle pressure cooled by the gas cooler, and a solenoid valve that has a fixed opening degree and an electronic expansion valve that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of the middle pressure receiver and on a downstream side of the gas cooler, wherein the control device performs control of changing the solenoid valve from open to closed when it is determined that the refrigerant circuit satisfies all of the following conditions: a high-pressure saturation temperature of the refrigerant circuit is equal to or lower than a critical temperature of the refrigerant; an evaporation temperature setting value of the refrigerant circuit is a value at which a suction density of the compressors is 50% or lower relative to the suction density at a maximum evaporation temperature setting of the refrigerant circuit; and a middle pressure measured value of the refrigerant circuit exceeds a target middle pressure and the electronic expansion valve has a minimum opening degree.

2. The control device according to Claim 1, wherein the refrigerant is a CO2 refrigerant.

3. The control device according to Claim 1, wherein in a case where it is confirmed that the liquid refrigerant is stored in the gas cooler of the refrigerant circuit when the control of changing the solenoid valve from open to closed is performed, the control device performs control of changing the solenoid valve from closed to open.

4. A condensing unit comprising: a plurality of compressors provided in parallel; a gas cooler that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation; a middle pressure receiver that stores a liquid refrigerant at a middle pressure cooled by the gas cooler; a solenoid valve that has a fixed opening degree and an electronic expansion valve that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of the middle pressure receiver and on a downstream side of the gas cooler; and the control device according to Claim 1.

5. A control method of controlling a refrigerant circuit including a plurality of compressors provided in parallel, a gas cooler that cools a refrigerant compressed by the compressors to a high pressure equal to or higher than a critical pressure during a normal operation, and a solenoid valve that has a fixed opening degree and an electronic expansion valve that has a variable opening degree, the solenoid valve and the electronic expansion valve being provided in parallel on an upstream side of a middle pressure receiver that stores a liquid refrigerant at a middle pressure cooled by the gas cooler and on a downstream side of the gas cooler, the control method comprising: performing control of changing the solenoid valve from open to closed when it is determined that the refrigerant circuit satisfies all of the following conditions: a high-pressure saturation temperature of the refrigerant circuit is equal to or lower than a critical temperature of the refrigerant; an evaporation temperature setting value of the refrigerant circuit is a value at which a suction density of the compressors is 50% or lower relative to the suction density at a maximum evaporation temperature setting of the refrigerant circuit; and a middle pressure measured value of the refrigerant circuit exceeds a target middle pressure and the electronic expansion valve has a minimum opening degree.

6. A control program causing a computer to execute the control method according to Claim 5.

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