JP3270706B2 - Multi-source refrigeration equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-stage refrigeration system, and more particularly to control of a low-stage refrigeration cycle. In addition, the following description is given about the binary refrigeration apparatus for convenience.

[0002]

2. Description of the Related Art Conventionally, in a refrigerating apparatus, stable operation has been achieved by controlling an expansion apparatus so that the degree of superheat of refrigerant at an evaporator outlet or a compressor suction section becomes a target value. . The same applies to the binary refrigeration system, which is stabilized by controlling the expansion device so that either the dryness of the evaporator outlet in the high and low cycles or the superheat of the refrigerant at the compressor suction section becomes the target value. It is performed that the driving was done.

[0003] For example, the invention described in Japanese Utility Model Laid-Open No. 63-2053 discloses that a condenser of a higher-end refrigeration cycle is connected to a condenser of a higher-end refrigeration cycle by a cascade condenser that exchanges heat. In the two-stage refrigeration apparatus, a proportional control valve is provided as an expander of the high-side refrigeration cycle, and a saturation temperature detector provided in the high-side evaporator and a suction pipe of the high-side compressor are provided. A binary refrigeration system including a controller that controls the opening degree of a proportional control valve based on a detection signal from an intake gas temperature detector and keeps the degree of superheat of the intake gas constant is described.

[0004]

However, when an accumulator or an expansion tank is connected to the evaporator outlet in the low-side refrigeration cycle, the liquid refrigerant is stored in the accumulator and the gas from the expansion tank is not charged. Due to the outflow of the refrigerant, a state occurs in which the degree of superheat of the refrigerant at the outlet of the evaporator is different from the degree of superheat of the refrigerant at the inlet of the compressor.

[0005] Therefore, when the expansion device is controlled only by the degree of superheat of the refrigerant at the outlet of the evaporator, the following problem occurs. For example, when the gas refrigerant flows out of the expansion tank, and the temperature of the gas refrigerant flowing out becomes considerably higher than the temperature of the gas refrigerant at the evaporator outlet, the degree of superheating of the refrigerant at the evaporator outlet is higher than the degree of superheating of the refrigerant at the evaporator outlet. The superheat degree of the refrigerant at the compressor suction portion where the flowing gas refrigerant and the gas refrigerant flowing out of the expansion tank merge is considerably large. In the superheat control of the refrigerant at the outlet of the evaporator, it is not possible to foresee an increase in the superheat of the refrigerant at the compressor suction section, and therefore it is impossible to suppress an increase in the discharge temperature of the compressor due to the increase in the superheat of the refrigerant at the compressor suction section.
Therefore, there is a problem that the protection device of the refrigerating apparatus operates against the rise of the discharge temperature to stop the operation.

If the liquid refrigerant is present in the accumulator, the liquid refrigerant may return due to fluctuations in the liquid level. If the liquid refrigerant returns for a long time, there is a possibility that the liquid is compressed by the compressor and the compressor is damaged. Therefore, it is necessary to perform control so as to shorten the time during which the liquid refrigerant is present in the accumulator. However, the superheat control of the refrigerant at the outlet of the evaporator cannot detect the presence of the liquid refrigerant in the accumulator.Therefore, there is a possibility that the liquid refrigerant will remain in the accumulator for a long time, and in this case, the long-term liquid refrigerant The return causes a problem that the compressor compresses the liquid refrigerant and breaks the compressor.

On the other hand, when the expansion device is controlled only by the degree of superheat of the refrigerant at the suction portion of the compressor, the following problem occurs.
When a liquid refrigerant is present in the accumulator, a saturated gas flows into the compressor suction, so that the degree of superheat becomes zero. Therefore, when controlling the degree of superheat of the refrigerant at the compressor suction portion to a certain value, control is performed so as to increase the degree of superheat by reducing the degree of opening of the expansion device. However, while the liquid refrigerant is in the accumulator, the degree of superheat of the refrigerant in the compressor suction section remains zero, and the control for reducing the opening of the expansion device is performed continuously during that time. Therefore, when a large amount of liquid refrigerant is present in the accumulator, the opening degree of the expansion device is controlled to be too small, and the high pressure of the refrigeration device is high.
The low pressure becomes too low. If the high pressure is too high or the low pressure is too low, there arises a problem that the protection device of the refrigeration system is activated and the operation is stopped.

The present invention has been made to solve such a problem, and provides a multi-source refrigeration apparatus capable of appropriately controlling the degree of superheat of the refrigerant on the low-pressure side of the low-side refrigeration cycle and operating safely and stably. The purpose is to do.

[0009]

A multi-stage refrigeration system according to the present invention comprises a high-stage compressor in which a high-stage compressor, a high-stage condenser, a high-stage expansion device, and a high-stage evaporator are connected in this order. A refrigeration cycle, a low-side compressor, a low-side condenser, a low-side expansion device using an electronic expansion valve, and a low-side evaporator that cools a refrigeration load through a cooling medium are connected in this order. An expansion tank connected to the suction side of the lower-side refrigeration cycle and the lower-side compressor to reduce the pressure at the time of suspension of operation, so that heat can be exchanged between the higher-side evaporator and the lower-side condenser. A cascade condenser that is directly or indirectly combined to connect the high-side refrigeration cycle and the low-side refrigeration cycle, a superheat degree detection device that detects the superheat degree of the refrigerant at the low-stage compressor suction section, Lower refrigeration cycle in the cascade condenser In order to prevent the discharge temperature of the lower compressor from rising due to the superheated refrigerant flowing out of the expansion tank when the lower refrigerant cycle is started from a high refrigeration load after the compressor is cooled. In addition, the superheat degree of the refrigerant in the low-stage compressor suction unit detected by the superheat degree detection device is a predetermined target value such that the high pressure of the low-stage compressor is not too high, and the discharge temperature is not too high. Control means for controlling the electronic expansion valve.

Further, the control means controls the electronic expansion valve by lowering the target value of the degree of superheat of the refrigerant in the suction section of the lower compressor after a predetermined time has elapsed since the start of the lower refrigerating cycle.

When the operating state of the multi-stage refrigeration system changes to a predetermined state after the start of the lower-stage refrigeration cycle, the control means sets the target value of the degree of superheat of the refrigerant in the suction section of the lower-stage compressor. Lower to control electronic expansion valve.

The superheat detecting device also detects the superheat of the refrigerant at the outlet of the lower evaporator, and when the temperature of the cooling medium is lowered and stabilized, the lower evaporator detected by the superheat detecting device. Another control means is provided for controlling the electronic expansion valve so that the degree of superheat of the refrigerant at the outlet of the vessel becomes a predetermined target value which is an optimal value at which the heat transfer efficiency of the lower evaporator is increased.

[0013] Further, a high-side refrigeration cycle in which a high-side compressor, a high-side condenser, a high-side expansion device, and a high-side evaporator are connected in this order; a low-side compressor; When the refrigerant in the two-phase state flows out of the condenser, the lower expansion device using the electronic expansion valve, the lower evaporator that cools the refrigeration load through the cooling medium, An accumulator that separates and prevents liquid refrigerant from flowing into the lower compressor is connected to the lower refrigeration cycle connected in this order and the suction side of the lower compressor to reduce the pressure during operation stop. An expansion tank for lowering, and a high-side evaporator and a low-side condenser are directly or indirectly combined so that heat can be exchanged,
A cascade condenser that connects the high-side refrigeration cycle and the low-side refrigeration cycle, a superheat degree detection device that detects the degree of superheat of the refrigerant at the outlet of the low-side evaporator, When the temperature of the cooling medium is low, in order to quickly avoid a state in which the two-phase refrigerant flows out of the lower evaporator and the liquid refrigerant accumulates in the accumulator, the outlet of the lower evaporator detected by the superheat degree detection device is used. Control means for controlling the electronic expansion valve so that the degree of superheat of the refrigerant becomes a predetermined target value slightly larger than the optimum value at which the heat transfer efficiency of the lower evaporator is increased.

The superheat degree detecting device also detects the superheat degree of the refrigerant at the outlet of the lower evaporator, and the superheat degree of the refrigerant at the outlet part of the lower evaporator detected by the superheat degree detector is low. When the heat transfer efficiency of the lower evaporator becomes a predetermined target value slightly larger than the optimum value, the superheat detection device detects the presence of liquid refrigerant in the accumulator due to the superheat of the refrigerant at the outlet of the lower evaporator. It is determined whether the liquid refrigerant is present, if the superheat degree of the refrigerant at the outlet of the lower evaporator continuously detected by the superheat degree detection device is higher than the optimal value at which the heat transfer efficiency of the lower evaporator is increased. The electronic expansion valve is controlled so as to have a slightly larger target value, and when no liquid refrigerant is present, the degree of superheat of the refrigerant at the outlet of the lower evaporator detected by the superheat detection device is reduced to a lower level. The predetermined target value, which is the optimum value for increasing the heat transfer efficiency of the side evaporator, It provided other control means for controlling the electronic expansion valve so that.

[0015] Further, a high-side refrigeration cycle in which a high-side compressor, a high-side condenser, a high-side expansion device, and a high-side evaporator are connected in this order; a low-side compressor; A condenser, a lower expansion device using an electronic expansion valve, a lower evaporator for cooling a refrigeration load via a cooling medium, and a gas in a two-phase state flowing out of the lower evaporator. An accumulator that performs liquid separation and suppresses the liquid refrigerant from flowing into the lower compressor is connected to the lower refrigeration cycle connected in this order, and to the suction side of the lower compressor, and when the operation is stopped. The expansion tank for lowering the pressure, and the high-side evaporator and the low-side condenser are directly or indirectly combined so that heat can be exchanged, and the high-side refrigeration cycle and the low-side refrigeration cycle are combined. Superheat degree of connected cascade condenser and refrigerant at outlet of lower evaporator A superheat degree detection device that detects the degree of superheat of the refrigerant in the lower-stage compressor suction section, and when the lower-stage refrigeration cycle is in a stable operation, the liquid refrigerant flows into the accumulator due to the superheat degree of the refrigerant in the lower-stage compressor suction section. It is determined whether or not the liquid refrigerant exists, and if the liquid refrigerant is present, the superheat degree of the refrigerant at the outlet of the lower evaporator detected by the superheat degree detection device is greater than the optimal value at which the heat transfer efficiency of the lower evaporator is increased. The electronic expansion valve is controlled so as to have a slightly larger target value, and when no liquid refrigerant is present, the degree of superheat of the refrigerant at the outlet of the lower evaporator detected by the superheat detection device is reduced to a lower level. A control means is provided for controlling the electronic expansion valve so as to reach a predetermined target value which is an optimum value for increasing the heat transfer efficiency of the side evaporator.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows Embodiment 1 of the present invention.
FIG. 4 is a refrigerant circuit diagram of the binary refrigeration apparatus in FIG. In the figure, 1 is a high-end refrigeration cycle, 2 is a low-end refrigeration cycle, 31 is a compressor of a high-end refrigeration cycle, 32 is a compressor of a low-end refrigeration cycle, and 4 is a condensation of a high-end refrigeration cycle. Reference numeral 51 denotes an electronic expansion valve of the high-side refrigeration cycle, 52 denotes an electronic expansion valve of the low-side refrigeration cycle, and 6 denotes an evaporator of the low-side refrigeration cycle.

Reference numeral 7 denotes a cascade condenser, which exchanges heat between the low-pressure two-phase refrigerant of the evaporator 12 of the high-side refrigeration cycle and the high-pressure gas refrigerant of the low-side refrigeration cycle 2, and While the two-phase refrigerant is evaporated and gasified, the high-pressure gas refrigerant of the lower-stage refrigeration cycle 2 is condensed and liquefied.

Reference numeral 8 denotes an accumulator, and the lower evaporator 6
When the refrigerant in the two-phase state flows out of the compressor, gas-liquid separation is performed to suppress the liquid refrigerant from flowing into the lower compressor 32.

Reference numeral 9 denotes an expansion tank connected to the suction side of the lower compressor 32, for preventing the pressure of the refrigerant in the lower refrigeration cycle from exceeding an allowable pressure even if the refrigerant is completely gasified. Things.

Reference numeral 10 denotes a superheat degree detecting device which detects the pressure on the low pressure side of the lower-side refrigeration cycle or the saturation temperature detected by the temperature sensor 13 provided in the lower-side evaporator 6, and the outlet of the lower-side evaporator 6. The temperature of the outlet of the lower evaporator 6 detected by the temperature sensor 14 provided in the
The temperature of the suction section of the lower compressor 32 detected by the temperature sensor 15 provided at the suction section of the lower compressor 32
And the superheat degree of the refrigerant at the outlet of the lower evaporator 6 are detected.

Reference numeral 11 denotes an expansion valve control unit which measures the degree of superheat of the refrigerant at the inlet of the lower compressor 32 and the degree of superheat of the refrigerant at the outlet of the lower evaporator 6 detected by the superheat detector 10. A value is received as a signal, and the degree of superheat is used to control the degree of opening of the electronic expansion valve 52 of the lower refrigeration cycle 2.

R4 is used as the refrigerant of the high-stage refrigeration cycle 1.
04a, R23 is used as a refrigerant for the lower refrigeration cycle 2.

In the condenser 4 of the higher-stage refrigeration cycle 1, the refrigerant is condensed with the cooling water to form a liquid refrigerant, and in the evaporator 6 of the lower-stage refrigeration cycle 2, heat is taken from the brine as the cooling medium to evaporate the refrigerant. It has a structure.

In this refrigerating apparatus, the evaporation temperature of the lower refrigerating cycle 2 is -70 ° C. in a steady operation state due to the characteristics of the refrigerant.
The condensation temperature of the lower refrigeration cycle 2 and the evaporation temperature of the higher refrigeration cycle 1 are about −30 ° C., and the condensation temperature of the higher refrigeration cycle 1 is about 40 ° C. The function of the refrigeration system is as follows:
Brine at a low temperature of about 0 ° C. can be obtained.

First, the starting operation of the binary refrigeration system will be described. In the start-up operation of the refrigeration apparatus, a place having a heat load equal to the outside air temperature, for example, a refrigeration warehouse is cooled by brine and cooled to −70 ° C. Before the start-up, the temperature of the brine is the same as the outside temperature of the freezing warehouse, and the operation is performed such that the brine is first cooled by the refrigerating apparatus, and the freezing warehouse is cooled by the cooled brine. The brine that has cooled the freezing warehouse, after a slight rise in temperature, returns to the freezing device again and is cooled again by the freezing device. Since the difference between the temperature of the freezer warehouse and the outside air temperature is small at the beginning of cooling the freezer warehouse, the heat load is not so large, and the amount of cooling by which the refrigerator cools the brine is larger than the amount of cooling by which the brine cools the freezer warehouse. Become. Accordingly, the temperature of the brine decreases with the operation, and when the temperature reaches about -70 ° C., the heat load of the freezing warehouse and the amount of cooling by which the refrigerating apparatus cools the brine are balanced, and the operation of the refrigerating apparatus becomes stable.

Next, the change in the state of the refrigeration cycle during the start-up operation will be described. First, the state of the lower refrigeration cycle 2 of the binary refrigeration apparatus at the time of stoppage will be described. Since the refrigerant R23 used in the low-stage refrigeration cycle 2 has a low boiling point of -85 ° C. at atmospheric pressure, it is at room temperature.
At 5 ° C., the saturation pressure is 45 kgf / cm 2 abs. Therefore, if the refrigerant is stored in a heat exchanger or accumulator in a state where the liquid refrigerant and the gas refrigerant coexist during shutdown as in a normal refrigeration system, the pressure becomes 45 kgf / cm2abs at an outside air temperature of 25 ° C. This will exceed the allowable pressure of the device of 25 kgf / cm2abs.

Therefore, the lower refrigeration cycle 2 is provided with an expansion tank 9 having a larger volume than the heat exchanger and the accumulator 8, so that even if the refrigerant existing in the lower refrigeration cycle 2 evaporates and is completely gasified, the pressure increases. Should not be high. The size of the expansion tank 9 is about 10 times the internal volume of the low-side refrigeration cycle 2 excluding the expansion tank 9.
It is designed so that it will be about 15kgf / cm2abs at the maximum even at 0 ° C. When such an expansion tank 9 is used, when the outside air temperature is around 25 ° C., which is a normal temperature, the low-side refrigeration cycle 2
Is about 14 kgf / cm2abs. The expansion tank 9
The size of the lower refrigeration cycle 2 excluding the expansion tank 9
Therefore, most of the refrigerant in the lower-stage refrigeration cycle 2 is present in the expansion tank 9 during operation stoppage.

Next, the state of the binary refrigeration apparatus at the time of startup will be described. FIG. 2 shows a pressure change after the start of the binary refrigeration apparatus of the present embodiment. When the refrigeration system is stopped, the pressure in the lower refrigeration cycle 2 is 14 kgf / cm2abs as described above.
In this state, if the low-side refrigeration cycle 2 is started, the high pressure will be 30 kgf / cm2abs or more, exceeding the allowable pressure of the refrigeration system.

Therefore, in the binary refrigeration system, first, the high-stage refrigeration cycle 1 is started, and the cascade condenser 7 serving as the evaporator of the high-stage refrigeration cycle 1 is cooled. When the cascade condenser 7 is cooled, the refrigerant in the lower refrigeration cycle 2 is cooled and liquefied, so that the pressure of the lower refrigeration cycle 2 gradually decreases.

Looking at the movement of the refrigerant in the lower refrigeration cycle 2, the gas refrigerant moves from the expansion tank 9 to the cascade condenser 7, and the moved refrigerant is cooled and liquefied. The pressure of the lower refrigeration cycle 2 decreases and 9
When the pressure becomes about kgf / cm2abs, the low pressure side refrigeration cycle 2 is started because the allowable pressure of the refrigerating apparatus does not exceed the allowable pressure of the refrigerating apparatus even if the high pressure rises when the low pressure side refrigeration cycle 2 is activated.

In the cascade condenser 7 in which the refrigerant is liquefied, the state of the refrigerant at the start of the lower refrigeration cycle 2 is the saturation temperature of the pressure of the lower refrigeration cycle 2, and the pressure of the lower refrigeration cycle 2 is 9 kgf / -34 for cm2abs
° C. The parts other than the cascade condenser 7 of the lower-stage refrigeration cycle 2 including the expansion tank 9 are in a stage where the operation of the lower-stage refrigeration cycle 2 is not started, the outside air temperature around the refrigerating apparatus, that is, the saturation temperature is about 25 ° C. at room temperature. The temperature is much higher than that, and the degree of superheat indicates that the degree of superheat is about 60 ° C. When the compressor is started in this state, the refrigerant at the inlet of the compressor 32 sucks a refrigerant having a pressure higher than a steady suction pressure (about 2 kgf / cm2abs) and a gas refrigerant having a large superheat degree. Immediately after startup, the high pressure rises sharply and the discharge temperature also rises.

Since the low pressure of the lower refrigeration cycle 2 immediately after the start is lower than the pressure of the lower refrigeration cycle 2 immediately before the start, a pressure difference is generated between the refrigerant in the expansion tank 9 and the compressor 32 sucked from the expansion tank 9. The gas refrigerant flows out to the side. Since the temperature of the gas refrigerant in the expansion tank 9 is not the refrigerant circulating in the low-stage refrigeration cycle 2, the temperature of the gas refrigerant is the same as the outside air temperature. On the other hand, since the refrigerant coming out of the evaporator 6 removes heat from the brine and evaporates, it does not reach a temperature higher than the brine temperature. As described above, since the brine temperature decreases with the operation of the refrigeration apparatus, the temperature of the gas refrigerant exiting the evaporator 6 becomes lower than the temperature of the gas refrigerant flowing from the expansion tank 9 to the compressor 32 suction side. Since the gas refrigerant having a temperature higher than the temperature of the gas refrigerant that has exited the evaporator 6 flows from the expansion tank 9 and merges with the suction of the compressor, if the degree of superheat is detected, the degree of superheat of the refrigerant at the outlet of the evaporator 6 is detected. Compressor 32 than
Superheat at inhalation is greater.

After a lapse of time from the start, the temperature of the brine supplied to the evaporator 6 of the lower refrigeration cycle 2 decreases, and the low pressure of the lower refrigeration cycle 2 decreases due to the effect of the decrease in the brine temperature. To go. The gas refrigerant flows out of the expansion tank 9 according to the decrease in the low pressure. Further, since the capacity of the compressor 32 decreases due to the decrease in the low pressure, the high pressure also decreases.

When the time after the start-up further elapses and the brine temperature stops decreasing and stabilizes at -70 ° C., the low pressure of the low-side refrigeration cycle 2 decreases gradually and settles at a certain stable pressure. The outflow amount of the gas refrigerant from the expansion tank 9 decreases as the low pressure gradually decreases, and the outflow amount becomes zero when the low pressure of the low-side refrigeration cycle 2 reaches a stable pressure. Accordingly, the difference between the degree of superheat of the refrigerant at the outlet of the evaporator 6 and the degree of superheat of the refrigerant at the suction of the compressor 32 gradually decreases, and both superheats have the same value when the low pressure is stable. The above is the operating state from the start of the binary refrigeration apparatus to the stable state.

Next, a method of controlling the degree of superheat using the electronic expansion valve 52 from the start of the low-stage refrigeration cycle 2 to the stabilization will be described. FIG. 3 is a flowchart showing a method of controlling the electronic expansion valve 52 from startup to stable operation. First, the electronic expansion valve 52 is fixed at a certain opening in step S10, and the low-side refrigeration cycle 2 is started in step S11.

At this time, when the opening of the electronic expansion valve 52 is smaller than an appropriate value, the low-stage refrigeration cycle 2
Is as follows. Since the opening degree of the expansion valve is small, the low pressure is lower than when the expansion valve opening degree is appropriate. As the low pressure decreases, the capacity of the compressor 32 decreases, so that the high pressure also decreases. Further, the superheat degree of the refrigerant at the outlet of the evaporator 6 increases as the low pressure decreases, so that the superheat degree at the suction of the compressor 32 also increases, thereby increasing the discharge temperature at the outlet of the compressor 32.

On the other hand, when the opening of the electronic expansion valve 52 is larger than an appropriate value, the change of the lower-stage refrigeration cycle is as follows. Since the opening degree of the expansion valve is large, the low pressure rises as compared with the case where the opening degree of the expansion valve is appropriate. As the low pressure increases, the capacity of the compressor 32 increases, so that the high pressure also increases. In addition, the superheat degree of the refrigerant at the outlet of the evaporator 6 decreases with an increase in the low pressure, so that the superheat degree at the suction of the compressor 32 also decreases, and the discharge temperature at the outlet of the compressor 32 also decreases.

As described above, the low-side refrigeration cycle 2 immediately after the start-up has a high pressure and a high discharge temperature.
Therefore, when controlling the opening degree of the electronic expansion valve 52, the opening degree is controlled to be larger than an appropriate value and the superheat degree of the refrigerant at the outlet of the evaporator 6 and the suction part of the compressor 32 is controlled so as to increase the high pressure. The protection device of the refrigeration system is activated and the operation stops. On the other hand, if the opening degree of the electronic expansion valve 52 is controlled to be smaller than an appropriate value and the superheat degree of the refrigerant at the outlet of the evaporator 6 and the suction part of the compressor 32 is controlled to be large, the discharge temperature of the compressor 32 becomes too high. The protection device operates and the operation stops.
Therefore, the opening degree of the electronic expansion valve 52 does not become too high,
Further, it is necessary to control the degree of superheating so that the discharge temperature does not become too high.

The discharge temperature is greatly affected by the degree of superheat of the refrigerant at the suction part of the compressor 32. Considering that the superheat degree is controlled by the superheat degree of the refrigerant at the outlet of the evaporator 6, the superheat degree of the refrigerant at the suction part of the compressor 32 is affected by the outflow of the gas refrigerant from the expansion tank 9. , The degree of superheating of the refrigerant at the outlet of the evaporator 6 is larger than that of the expansion tank 9.
It is necessary to control in consideration of the influence of the outflow of the gas refrigerant from the fuel cell, which makes the control difficult. On the other hand, when the control is performed based on the degree of superheat of the refrigerant at the suction part of the compressor 32, the control of the discharge temperature can be directly performed, so that the control becomes easy. Therefore, the opening degree of the electronic expansion valve 52 immediately after the start is controlled so that the superheat degree of the refrigerant at the suction part of the compressor 32 becomes a target value.

As described above, it is necessary to set the control target value to a degree of superheating at which the high pressure does not become too high and the discharge temperature does not become too high. Outside air temperature is 25
In the case of starting at a temperature of ℃, if the target value of the degree of superheat is controlled at about 50 ° C., it is possible to control the high pressure and the discharge temperature so as not to become too high. As described above, when the outside air temperature is 25 ° C., a gas refrigerant having a superheat degree of 60 ° C. exists other than the cascade condenser 7 immediately before starting. Therefore, immediately after the start, a refrigerant having a superheat degree of about 60 ° C. with a considerably high degree of superheat is sucked into the compressor 32. Here, if 10 ° C., at which the heat transfer efficiency of the evaporator 6 is improved, is selected and controlled as the target value of the degree of superheat, the opening degree of the electronic expansion valve 52 will be greatly increased, and the state in which the high pressure becomes excessively high will occur. Become. On the other hand, if the refrigerant having a superheat degree of about 60 ° C. is sucked as it is, the discharge temperature becomes too high. If the superheat degree is controlled to the target value of about 50 ° C., control can be performed so that the high pressure does not become too high and the discharge temperature does not become too high (step S1).
2 to step S15).

Next, the control in the case where a certain period of time has elapsed after activation will be described. As described above, the brine temperature decreases when a certain period of time elapses after the startup, and the high and low pressures decrease accordingly. As the pressure decreases, the discharge temperature also decreases. Therefore, in the control of the electronic expansion valve 52, when considering a control in which the high pressure does not become too high and the discharge temperature does not become too high, the range of selecting the opening degree of the electronic expansion valve 52 is widened. From the viewpoint of cooling the brine with the refrigeration apparatus, it is desirable to operate the evaporator 6 for cooling the brine so that the efficiency is improved. In the control immediately after the start-up described above, the superheat degree is set to a considerably high value of 50 ° C., so that the heat transfer efficiency of the evaporator 6 is considerably impaired.

In the state where the time after the start-up time has elapsed and the opening degree of the electronic expansion valve 52 has been widened, the superheat degree is controlled to a value smaller than immediately after the start-up, and the efficiency of the evaporator 6 is increased. However, at this stage, although the high and low pressures are lower than immediately after startup, both the high and low pressures are higher than the stable state in which the brine temperature has decreased to -70 ° C. Therefore, if 10 ° C., at which the heat transfer efficiency of the evaporator 6 is improved, is selected and controlled as the target value of the degree of superheat, the state in which the high pressure becomes too high does not change from immediately after the start.

Therefore, the target value of the degree of superheat when a certain time has elapsed after the start is reduced from 50 ° C. to 30 ° C. The switching time of the target value is a time during which the high pressure is reduced so that the high pressure does not become too high even if the superheat degree is controlled to 30 ° C. Change the target value. Since the discharge temperature is higher than the stable state, the superheat degree of the refrigerant at the suction part of the compressor 32 is controlled to be the target value, and the discharge temperature can be controlled directly (steps S17 to S21). ).

Although the method of changing the target value of the degree of superheat with time has been described above, a method of changing the target value with reference to the operating state of the refrigeration system instead of time may be adopted. For example, if the high pressure is reduced by looking at the change in the high pressure of the low-stage refrigeration cycle 2 and the high pressure does not become too high even if the target value of the superheat degree is changed to a small value, the target value of the superheat degree May be changed to be smaller.

When the temperature of the brine approaches -70 ° C. after the start of operation, the change of the low-side refrigeration cycle 2 is changed to a high and low pressure, the discharge temperature is further lowered, and is stabilized at a certain value. At this stage, there is almost no difference between the degree of superheat of the refrigerant at the outlet of the evaporator 6 and the degree of superheat of the refrigerant at the inlet of the compressor 32. Therefore, when controlling the electronic expansion valve 52 using the degree of superheat, May be used.

However, when there is a temperature change of the brine due to a load change from a state close to stable or from the stable state, the low pressure changes under the influence of the temperature change of the brine. At this time, the outflow of the gas refrigerant from the expansion tank 9 or the reverse flow of the gas refrigerant to the expansion tank 9 occurs due to the change in the low pressure, so that the fluctuation of the degree of superheat is larger at the suction of the compressor 32 than at the outlet of the evaporator 6. Become. Therefore, compared to controlling the degree of superheat at the outlet of the evaporator 6 to the target value, the compressor 3
In the case where control is performed so that the degree of superheat in the two suctions becomes the target value, it is necessary to control a value that causes a larger fluctuation to the target value, and control becomes difficult. Therefore, at this stage, it is desirable to control the degree of superheat at the outlet of the evaporator 6 to a target value.

At this stage, even if 10 ° C. at which the heat transfer efficiency of the evaporator 6 is improved is selected as the target value of the degree of superheat, the high pressure does not become too high. Therefore, at this stage, the target value of the degree of superheat is set to 10 ° C. so that the operation efficiency of the refrigeration system is improved.

In the case of starting at an outside air temperature of 25 ° C., the brine temperature approaches −70 ° C. about 120 minutes after the start. Then, 120 minutes after the start, the target value of the superheat is changed to 10 ° C., and control is performed so that the superheat at the evaporator outlet becomes 10 ° C. (steps S22 to S25).

The method of changing the target value of the degree of superheat and the method of controlling which of the degree of superheat of the refrigerant at the suction part of the compressor 32 and the degree of superheat of the refrigerant at the outlet part of the evaporator 6 is controlled by time has been described above. A method may be adopted in which the change is made not by looking at the time but by looking at the operating state of the refrigeration system. For example, by observing the temperature change of the brine, if the temperature stabilizes at about -70 ° C. and the temperature change width per time becomes smaller, the target value of the superheat degree is set to 10
° C and control is performed so that the degree of superheat at the evaporator outlet becomes 10 ° C.

The method of controlling the degree of opening of the electronic expansion valve 52 with respect to the degree of superheating in the above embodiment increases the degree of opening when the current degree of superheating is greater than the target value of the degree of superheating. If the degree is smaller than the target value, the opening is controlled to be small. The change width of the opening at this time may be a fixed value, or may be a value obtained by multiplying a deviation of the current superheat degree from a target value of the superheat degree by a constant.

As another method of controlling the degree of opening of the electronic expansion valve 52 with respect to the degree of superheating in the above-described embodiment, a value of the degree of superheating which will be obtained when the degree of opening is not controlled is estimated.
When the predicted value is larger than the target value of superheat, the opening may be increased, and when the predicted value is smaller than the target value of superheat, the opening may be controlled to be small. The change width of the opening at this time may be a fixed value, or may be a value obtained by multiplying a deviation between the predicted value of the superheat degree and the target value of the superheat degree by a constant.

In the above embodiment, the control is performed by giving a fixed value as the target value of the superheat degree. However, a target zone having a width as the target value of the superheat degree is set, and the control is performed within the zone. It may be.

In the above embodiment, R4 is used as the high-side refrigerant.
04a, the example using R23 as the lower refrigerant is shown, but other refrigerants may be used.

In the above embodiment, the heat exchanger using cooling water is used for the condenser 4 on the higher side and the heat exchanger using brine is used for the evaporator 6 on the lower side. Alternatively, other types of heat exchangers, such as an air-cooled heat exchanger that exchanges heat with air in each heat exchanger, may be used.

Embodiment 2 Hereinafter, another example of the embodiment of the present invention will be described with reference to the drawings. In the first embodiment, the control method at the time of the startup operation when the brine temperature is high is described. In the second embodiment, the control method at the time of the startup operation when the brine temperature is low will be described. Note that the refrigerant circuit shown in FIG. 1 is used as the refrigerant circuit.

When the temperature of the freezing warehouse or the like is adjusted by the freezing device, the operation of the freezing device is stopped when the temperature of the freezing warehouse becomes too low below the set value, and after the stoppage, the heat enters the freezing warehouse. There is an operation method in which the refrigeration apparatus is operated again when the temperature rises and becomes higher than a set value. Before the second operation, the state of the lower refrigeration cycle 2 and the state of the brine are as follows. Since the low-side refrigeration cycle 2 does not protect the outside air, the temperature of the refrigeration cycle 2 becomes the same as the outside air temperature after the operation is stopped. Therefore, the liquid refrigerant evaporates and gasifies, and most of the gasified refrigerant is stored in the large expansion tank 9. On the other hand, since the brine is sufficiently protected from the outside air, there is almost no temperature change after the operation is stopped, and the temperature is almost equal to the temperature of the freezer warehouse at the time of the operation stop. Therefore, when the outside air temperature is 25 ° C., the temperature of the refrigerant is 25 ° C., and when the temperature of the freezing warehouse is −70 ° C., the brine temperature is −70 ° C.

The state after startup of the lower refrigeration cycle 2 when the apparatus is operated in a state where the brine temperature is low is as follows. First, the operation of the high-stage refrigeration cycle 1 is the same as that of the first embodiment until the refrigerant is stored in the cascade condenser 7. Next, the lower-stage refrigeration cycle 2 is started. Here, a point different from the change in the refrigerant state described in Embodiment 1 due to the low brine temperature will be described.

First, since the brine temperature is low, the brine temperature is lower than the low-temperature evaporation temperature of the low-pressure side refrigeration cycle 2 immediately after the start. In this state, evaporation does not occur in the evaporator 6 of the lower refrigeration cycle 2, so the evaporator 6
The entered two-phase refrigerant passes through the evaporator 6 as it is and flows into the accumulator 8. The two-phase refrigerant is gas-liquid separated by the accumulator 8, the saturated liquid is stored in the accumulator 8, and the separated saturated gas flows into the compressor 32 in combination with the superheated gas flowing out of the expansion tank 9.

If the accumulator 8 is not provided, the two-phase refrigerant flowing out of the evaporator 6 is sucked into the compressor 32 as it is, so that the compressor 32 may be damaged by liquid compression or the like. Therefore, the accumulator 8 is necessary for this compressor to protect the compressor 32.

At this stage, the superheat degree becomes 0 because the two-phase refrigerant flows at the outlet of the evaporator 6, and the superheated gas flowing out of the expansion tank 9 into the saturated gas from the accumulator 8 at the suction of the compressor 32. Are superimposed to a certain degree. However, the value of this superheat degree is different from that of the first embodiment because the high and low pressure levels are low and the brine temperature is low.
Is lower than when the brine temperature is high, and is about 20 ° C. immediately after startup, which is a considerably smaller value than in the first embodiment. Also, the discharge temperature of the compressor 32 is lower than that in the first embodiment due to the low high pressure and the small degree of superheat at the suction of the compressor 32.

After a lapse of time after the start-up, the pressure of the entire low-side refrigeration cycle 2 further decreases due to the low brine temperature. When the low-pressure evaporation temperature of the lower refrigeration cycle 2 becomes lower than the brine temperature, evaporation is performed in the evaporator 6, the refrigerant gasifies at the outlet of the evaporator 6, and the degree of superheat becomes greater than 0 ° C. Even if the degree of superheat at the outlet of the evaporator 6 becomes larger than 0 ° C., since the liquid refrigerant exists in the accumulator 8, the saturated gas flows from the accumulator 8 to the compressor 32. If the low-pressure pressure drop is large, the gas refrigerant from the expansion tank 9 flows in a large amount, so that the superheat degree of the refrigerant at the suction part of the compressor 32 becomes a certain degree of superheat, but when the low-pressure pressure change decreases, Since almost no gas refrigerant flows from the expansion tank 9, the degree of superheat of the refrigerant at the suction part of the compressor 32 becomes 0 ° C. in this state. Accordingly, the degree of superheat of the refrigerant at the suction part of the compressor 32 gradually decreases from the start, and becomes almost 0 ° C. when the low pressure starts to stabilize.

When the degree of superheating of the refrigerant at the outlet of the evaporator 6 is increased and the superheated gas is supplied from the evaporator 6 to the accumulator 8, the liquid refrigerant present in the accumulator 8 is evaporated by the superheated gas and gradually decreases. I do. When the liquid refrigerant in the accumulator 8 runs out, the superheated gas supplied from the evaporator 6 flows to the compressor 32 as it is. At this stage, the degree of superheat at the suction of the compressor 32 also becomes larger than 0 ° C., and takes substantially the same value as the degree of superheat at the outlet of the evaporator 6. The above is the state after the start of the low-stage refrigeration cycle 2 when the operation is performed in a state where the brine temperature is low.

Next, a method of controlling the electronic expansion valve 52 when operating at a low brine temperature will be described. FIG.
FIG. 4 is a diagram showing a flowchart of this control method. Immediately after startup, the superheat of the refrigerant at the outlet of the evaporator 6 becomes 0 ° C.,
The evaporator 6 has not been sufficiently evaporated. On the other hand, since the degree of superheat of the refrigerant at the suction part of the compressor 32 is a certain value, it is not possible to determine whether or not the evaporator 6 is sufficiently evaporated just by looking at this value. Therefore, after startup, first, the opening of the electronic expansion valve 52 is controlled so that the degree of superheat of the refrigerant at the outlet of the evaporator 6 becomes the target value.

The target value of the degree of superheat at this stage is determined as follows. At the start-up when the brine temperature is low, the pressure level is lower than in the case of the first embodiment. Therefore, even if the degree of superheat is controlled to be small, there is no danger of a high pressure rise. However, if the liquid refrigerant is present in the accumulator 8, the liquid refrigerant may be accidentally supplied from the accumulator 8 to the compressor 32 due to fluctuations in the liquid level or the like. It is better not to be present. Therefore, the target value of the degree of superheat is set to 20 ° C., which is slightly higher than 10 ° C. at which the heat transfer efficiency in the evaporator 6 is improved, so that the liquid refrigerant in the accumulator 8 decreases quickly.

Next, when the degree of superheat at the outlet of the evaporator 6 can be controlled to 20 ° C., for example, when the degree of superheat at the outlet of the evaporator 6 is at 20 ° C. ± 3 ° C. Check the degree of superheating of the machine 32 suction. At this time, the compressor 3
2 If the degree of superheat of the suction is close to 0 ° C., for example, smaller than 3 ° C., the liquid refrigerant still exists in the accumulator 8,
Continuously, the degree of superheating at the outlet of the evaporator 6 is controlled to 20 ° C.

On the other hand, the degree of superheat of the refrigerant at the suction part of the compressor 32 is zero.
If the temperature is greater than 0 ° C., for example, greater than 3 ° C., it indicates that the liquid refrigerant in the accumulator 8 has run out, and the target value of the degree of superheat is set to 10 ° C. at which the heat transfer efficiency in the evaporator 6 is improved.
Is controlled so that the superheat degree of the refrigerant at the outlet of the evaporator 6 becomes the target value.

Embodiment 3 Hereinafter, another example of the embodiment of the present invention will be described with reference to the drawings. Embodiment 1,
2 describes the control method in the start-up operation of the refrigeration apparatus, but in the third embodiment, the control in the case where there is a change in the lower refrigeration cycle 2 due to a load change or the like during stable operation will be described. FIG. 5 is a flowchart showing a control method in this case. Note that the refrigerant circuit shown in FIG. 1 is used as the refrigerant circuit.

First, load fluctuations and the like are small, and fluctuations in the lower refrigeration cycle 2 are small. Due to fluctuations in the lower refrigeration cycle 2, the outlet of the evaporator 6 does not become two-phase, and the accumulator 8
The control when the liquid refrigerant does not accumulate will be described. In this case, as the fluctuation of the lower refrigeration cycle 2, the pressure and the temperature fluctuate slightly, and the degree of superheat fluctuates accordingly. In this case, control is performed such that the degree of superheat at the outlet of the evaporator 6 becomes a target value. Because, as described above, at this time, the gas refrigerant flows out of the expansion tank 9 or the gas refrigerant flows backward to the expansion tank 9 due to the change in the low pressure.
2 Inhalation is larger. Therefore, compared to controlling the degree of superheat at the outlet of the evaporator 6 to a target value, the compressor 32
In the case of controlling so that the degree of superheat at the suction becomes the target value,
This is because it is necessary to control a value that causes a larger fluctuation to a target value, and control becomes difficult.

On the other hand, the control in the case where the fluctuation of the low-stage refrigeration cycle 2 is large and the fluctuation of the low-stage refrigeration cycle 2 causes the outlet of the evaporator 6 to become two-phase and the liquid refrigerant is stored in the accumulator 8. Will be described. For example,
When the load on the freezing warehouse or the like suddenly decreases, the temperature of the brine that returns from the freezer to the freezing warehouse hardly rises and returns again at a low temperature, and the temperature of the brine is low. , The heat transfer amount at the outlet decreases, the outlet of the evaporator 6 becomes two-phase, and the liquid refrigerant is stored in the accumulator 8. When the liquid refrigerant is stored in the accumulator 8, the superheat degree at the suction of the compressor 32 is smaller than that at the evaporator 6 outlet and becomes almost 0 ° C. even when the superheat degree at which the evaporator 6 outlet becomes gas becomes larger than 0 ° C. .

In such a case, as described above, the liquid refrigerant may be accidentally supplied from the accumulator 8 to the compressor 32 due to the liquid level fluctuation or the like. It is better that no liquid refrigerant is present. Therefore, control is performed using the degree of superheat of the refrigerant at the outlet of the evaporator 6, and the target value of the degree of superheat is set to 20 ° C., which is slightly larger than 10 ° C. at which the heat transfer efficiency in the evaporator 6 is improved, and the liquid refrigerant in the accumulator 8 decreases quickly. To do. When the liquid refrigerant no longer exists in the accumulator 8, the superheat degree of the refrigerant at the suction part of the compressor 32 gradually increases, and takes the same value as the superheat degree of the refrigerant at the outlet part of the evaporator 6. In this case, it is better to control so that the heat transfer efficiency in the evaporator 6 is improved. Therefore, the control is performed using the superheat degree of the refrigerant at the outlet of the evaporator 6 and the target value of the superheat degree is the heat transfer efficiency in the evaporator 6. 10 ° C., which is better. As described above, the determination of the presence or absence of the liquid refrigerant in the accumulator 8 is performed when the superheat degree of the refrigerant in the suction part of the compressor 32 is a value close to 0 ° C., for example, when it is smaller than 3 ° C. If the degree of superheating of the refrigerant at the suction part of the machine 32 is greater than 0 ° C., for example, greater than 3 ° C., it is assumed that the liquid refrigerant in the accumulator 8 has run out.

[0071]

As described above, according to the present invention, when the control unit starts the lower-stage refrigeration cycle from a high-temperature state of the refrigeration load, the refrigerant in the suction unit of the lower-stage compressor detected by the superheat degree detection device. Degree of superheat, the high pressure of the lower compressor does not become too high,
In addition , since the electronic expansion valve is controlled so as to be a predetermined target value at which the discharge temperature does not become too high, it is possible to operate stably. Further, after a predetermined time has elapsed since the start of the low-stage refrigeration cycle, the electronic expansion valve is controlled by lowering the target value of the degree of superheat of the refrigerant in the low-stage compressor suction section.
It can operate stably and can cool the refrigeration load quickly. If the operating state of the multi-stage refrigeration system changes to a predetermined state after the start of the lower-stage refrigeration cycle, the electronic expansion valve is controlled by lowering the target value of the degree of superheat of the refrigerant at the lower-stage compressor suction section. Therefore, stable operation can be performed, and the refrigeration load can be cooled quickly. Further, when the temperature of the cooling medium decreases and becomes stable, the superheat degree of the refrigerant at the outlet of the lower evaporator detected by the superheat degree detection device is an optimum value at which the heat transfer efficiency of the lower evaporator is increased. Since the electronic expansion valve is controlled to have a predetermined target value, efficient and stable operation can be performed. In addition, when the lower-side refrigeration cycle is started when the temperature of the cooling medium is low, the degree of superheat of the refrigerant at the outlet of the lower-side evaporator detected by the superheat degree detection device indicates that the heat transfer efficiency of the lower-side evaporator is lower. Since the electronic expansion valve is controlled so as to be a predetermined target value slightly larger than the increased optimum value, it is possible to suppress accumulation of liquid in the accumulator and liquid compression of the compressor. In addition, when the degree of superheat of the refrigerant at the outlet of the lower evaporator becomes a predetermined target value slightly larger than the optimum value at which the heat transfer efficiency of the lower evaporator is increased, the liquid refrigerant exists in the accumulator. If the liquid refrigerant is present, the degree of superheat of the refrigerant at the outlet of the lower evaporator is a predetermined target value slightly larger than the optimum value at which the heat transfer efficiency of the lower evaporator is increased. When the liquid refrigerant is not present, the degree of superheat of the refrigerant at the outlet of the lower evaporator is a predetermined value that is an optimum value at which the heat transfer efficiency of the lower evaporator is increased. Since other control means for controlling the electronic expansion valve so that the target value is obtained is provided, efficient operation can be performed.
In addition, during the stable operation of the low-stage refrigeration cycle, the superheat degree of the refrigerant at the low-stage compressor suction unit determines whether or not the liquid refrigerant is present in the accumulator. Controlling the electronic expansion valve so that the degree of superheat of the refrigerant at the outlet of the lower evaporator becomes a predetermined target value that is slightly larger than the optimum value at which the heat transfer efficiency of the lower evaporator is increased. Is not present, the superheat degree of the refrigerant at the outlet of the lower evaporator detected by the superheat degree detection device is a predetermined target value which is an optimum value at which the heat transfer efficiency of the lower evaporator is increased. Since the electronic expansion valve is controlled as described above, it is possible to suppress liquid compression of the compressor due to load fluctuation.

[Brief description of the drawings]

FIG. 1 is a refrigerant circuit diagram of a binary refrigeration apparatus according to Embodiments 1 to 3 of the present invention.

FIG. 2 is a diagram showing a pressure change during a start-up operation of the binary refrigeration apparatus of the present invention.

FIG. 3 is a flowchart illustrating a control method during a start-up operation according to Embodiment 1 of the present invention;

FIG. 4 is a flowchart illustrating a control method during start-up operation according to Embodiment 2 of the present invention.

FIG. 5 is a flowchart illustrating a control method during operation according to the second embodiment of the present invention.

[Explanation of symbols]

1 Higher refrigeration cycle, 2 Lower refrigeration cycle, 3
1 Compressor for high-end refrigeration cycle, 32 Compressor for low-end refrigeration cycle, 4 Condenser for high-end refrigeration cycle, 5
DESCRIPTION OF SYMBOLS 1 Electronic expansion valve of high-stage refrigeration cycle, 52 Electronic expansion valve of low-stage refrigeration cycle, 6 Evaporator of low-stage refrigeration cycle, 7 Cascade condenser, 8 accumulator, 9 Expansion tank, 10 Superheat degree detection device, 11 Expansion valve control unit, 12 Evaporator of high side refrigeration cycle.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) F25B 1/00-7/00

Claims (7)

(57) [Claims] 1. A high-side refrigeration cycle in which a high-side compressor, a high-side condenser, a high-side expansion device, and a high-side evaporator are connected in this order, a low-side compressor, and a low-side compressor. A condenser, a lower-stage expansion device using an electronic expansion valve, a lower-stage refrigeration cycle in which a lower-stage evaporator for cooling a refrigeration load via a cooling medium is connected in this order, An expansion tank connected to the suction side, for reducing the pressure at the time of suspension of operation, and directly or indirectly combined so that the high-side evaporator and the low-side condenser can exchange heat, A cascade condenser connecting the higher-stage refrigeration cycle and the lower-stage refrigeration cycle; a superheat degree detection device for detecting the degree of superheat of the refrigerant in the lower-stage compressor suction section; and operation of the higher-stage refrigeration cycle. The lower condenser in the cascade condenser When starting the lower refrigeration cycle from the high temperature state of the refrigeration load after being cooled, the discharge temperature of the lower compressor is suppressed from rising due to the superheated refrigerant flowing out of the expansion tank. Therefore, the degree of superheat of the refrigerant in the low-stage compressor suction section detected by the superheat degree detection device, the high pressure of the low-stage compressor is not too high, and the discharge temperature is not too high in advance Control means for controlling the electronic expansion valve so as to reach a predetermined target value. 2. The control device according to claim 1, wherein after a predetermined time has elapsed since the start of the low-stage refrigeration cycle, the control unit lowers the target value of the degree of superheat of the refrigerant in the low-stage compressor suction section to operate the electronic expansion valve. The multi-stage refrigeration apparatus according to claim 1, wherein the refrigeration apparatus is controlled. 3. When the operating state of the multi-stage refrigeration apparatus changes to a predetermined state after the start of the lower-stage refrigeration cycle, the control means controls the degree of superheat of the refrigerant in the suction unit of the lower-stage compressor. 2. The multi-stage refrigeration system according to claim 1, wherein the electronic expansion valve is controlled by lowering a target value of the electronic expansion valve. 4. The superheat detection device also detects the superheat of the refrigerant at the outlet of the lower evaporator, and when the temperature of the cooling medium decreases and becomes stable, the superheat detection device detects the superheat. Another method of controlling the electronic expansion valve so that the degree of superheat of the refrigerant at the outlet of the lower evaporator is a predetermined target value that is an optimum value at which the heat transfer efficiency of the lower evaporator is increased. The multi-stage refrigeration apparatus according to claim 1, further comprising control means. 5. A high-side refrigeration cycle in which a high-side compressor, a high-side condenser, a high-side expansion device, and a high-side evaporator are connected in this order, a low-side compressor, and a low-side compressor. A condenser, a lower expansion device using an electronic expansion valve, a lower evaporator for cooling a refrigeration load via a cooling medium, and a gas in a two-phase state flowing out of the lower evaporator. An accumulator that performs liquid separation and suppresses a liquid refrigerant from flowing into the lower compressor is connected in this order to the lower refrigeration cycle, and is connected to the suction side of the lower compressor, and when the operation is stopped. An expansion tank for lowering the pressure of the high-side refrigeration cycle and the low-side refrigeration cycle are directly or indirectly combined so that the high-side evaporator and the low-side condenser can exchange heat. A cascade condenser for connecting a side refrigeration cycle, and the low-side evaporator A superheat degree detection device for detecting a degree of superheat of the refrigerant at the outlet portion, and a two-phase refrigerant flowing out of the lower side evaporator when the lower refrigeration cycle starts when the temperature of the cooling medium is low, and the accumulator In order to quickly avoid the state in which the liquid refrigerant accumulates, the superheat degree of the refrigerant at the outlet of the lower evaporator detected by the superheat degree detection device is optimized to increase the heat transfer efficiency of the lower evaporator. Control means for controlling the electronic expansion valve to be a predetermined target value slightly larger than the value,
A multi-component refrigeration system comprising: 6. The superheat degree detecting device also detects the superheat degree of the refrigerant at the outlet of the lower evaporator, and the superheat of the refrigerant at the outlet of the lower evaporator detected by the superheat degree detector. If the degree is a predetermined target value that is slightly larger than the optimum value at which the heat transfer efficiency of the lower evaporator is increased, the superheat degree detecting device detects the degree of superheat of the refrigerant at the outlet of the lower evaporator. It is determined whether or not a liquid refrigerant is present in the accumulator.If the liquid refrigerant is present, the degree of superheat of the refrigerant at the outlet of the lower evaporator continuously detected by the superheat degree detection device is the lower element. The electronic expansion valve is controlled to be a predetermined target value slightly larger than the optimum value at which the heat transfer efficiency of the side evaporator is increased, and when the liquid refrigerant is not present, the low heat detected by the superheat degree detection device is used. The superheat degree of the refrigerant at the outlet of the main evaporator is 6. The multi-stage refrigeration according to claim 5, further comprising another control means for controlling the electronic expansion valve so as to reach a predetermined target value which is an optimum value for increasing the heat transfer efficiency of the low-side evaporator. apparatus. 7. A high-side refrigeration cycle in which a high-side compressor, a high-side condenser, a high-side expansion device, and a high-side evaporator are connected in this order, a low-side compressor, and a low-side compressor. A condenser, a lower expansion device using an electronic expansion valve, a lower evaporator for cooling a refrigeration load via a cooling medium, and a gas in a two-phase state flowing out of the lower evaporator. An accumulator that performs liquid separation and suppresses a liquid refrigerant from flowing into the lower compressor is connected in this order to the lower refrigeration cycle, and is connected to the suction side of the lower compressor, and when the operation is stopped. An expansion tank for lowering the pressure of the high-side refrigeration cycle and the low-side refrigeration cycle are directly or indirectly combined so that the high-side evaporator and the low-side condenser can exchange heat. A cascade condenser for connecting a side refrigeration cycle, and the low-side evaporator A superheat degree detection device that detects a superheat degree of a refrigerant at an outlet part and a superheat degree of a refrigerant at a suction part of the lower compressor; and a lower compressor suction part during a stable operation of the lower refrigeration cycle. It is determined whether the liquid refrigerant is present in the accumulator based on the degree of superheat of the refrigerant.If the liquid refrigerant is present, the degree of superheat of the refrigerant at the outlet of the lower evaporator detected by the superheat degree detection device is determined. The electronic expansion valve is controlled to be a predetermined target value slightly larger than an optimum value at which the heat transfer efficiency of the lower evaporator is increased, and when the liquid refrigerant is not present, the superheat degree detection device The electronic expansion valve is controlled such that the detected degree of superheat of the refrigerant at the outlet of the lower evaporator becomes a predetermined target value which is an optimum value for increasing the heat transfer efficiency of the lower evaporator. Control means, and a multi-component refrigeration system characterized by comprising .