Auto-cascade refrigeration system with adjustable refrigerant components and control method of auto-cascade refrigeration system

Abstract

The invention discloses an auto-cascade refrigeration system with adjustable refrigerant components and a control method thereof, and relates to the technical field of auto-cascade refrigeration systems, the auto-cascade refrigeration system comprises a control module for adjusting mixed refrigerant components in the auto-cascade refrigeration system, and the control module is arranged in the auto-cascade refrigeration system. The control module is arranged in the auto-cascade refrigeration system, and the components of the mixed refrigerant are adjusted through the control module, so that the working components of the mixed refrigerant are automatically matched with the current operation working condition, and the operation energy efficiency of the auto-cascade refrigeration system is kept highest. The condition that the refrigerating capacity and performance are affected when the environment temperature and the evaporator side temperature change is avoided, and the technical problem that along with the change of the environment temperature and the change of the evaporator side temperature, the auto-cascade refrigerating system cannot be under the optimal operation component all the time, and the operation energy efficiency of the auto-cascade refrigerating system is reduced is solved.

Description

Technical Field

[0001] The invention relates to the technical field of auto-cascade refrigeration systems, and in particular to an auto-cascade refrigeration system with adjustable refrigerant components and a control method thereof. Background Art

[0002] The auto-cascade refrigeration system has a simple structure, high reliability, simple operation, economy and energy saving. It uses a single compressor to compress non-azeotropic mixed refrigerants, and can produce low temperatures of -40℃~-160℃, which has attracted widespread attention in the cryogenic industry. A mixture of two or more refrigerants in a certain proportion is called a mixed refrigerant. Depending on whether the boiling points of the mixed components are the same, it is divided into azeotropic mixed refrigerants and non-azeotropic mixed refrigerants. Azeotropic mixed refrigerants have the same boiling points as pure refrigerants and are usually used to improve the performance of refrigeration equipment and improve safety. After the low-boiling point components in the non-azeotropic mixed refrigerant are separated by gas and liquid at a certain temperature, the proportion of low-boiling point components in the gas phase is relatively high. Based on this, the automatic separation of high and low boiling points in the auto-cascade refrigeration system can be achieved.

[0003] The auto-cascade refrigeration system usually requires two or more refrigerants with different boiling points as refrigerants, that is, non-azeotropic mixed refrigerants. In order to ensure the smooth separation of each component, the boiling point difference between the components of the mixed refrigerant must be at least 40°C. They are separated in the gas-liquid separator of the auto-cascade refrigeration system. The separation efficiency determines the performance of the auto-cascade refrigeration system.

[0004] The separation of mixed refrigerants in the cascade refrigeration system relies on the gas-liquid separator to separate refrigerants with different boiling points. The realization of its principle can only be determined by the physical properties of the mixed refrigerant components themselves and the ambient temperature. The efficiency of component separation will have a direct impact on the refrigeration capacity and performance of the system. When the mixed refrigerant used in the cascade refrigeration system is determined, it only targets the optimal ratio under the design conditions, and has the highest energy efficiency under specific operating conditions. During the use of the cascade refrigeration system, with the changes in ambient temperature and the temperature on the evaporator side, the cascade refrigeration system cannot always be under the design conditions, which causes the decline in the operating energy efficiency of the cascade refrigeration system and affects the refrigeration capacity and performance of the cascade refrigeration system. Summary of the invention

[0005] In view of the deficiencies in the above-mentioned background technology, the present invention proposes a cascade refrigeration system with adjustable refrigerant components and a control method thereof, which solves the technical problem that the cascade refrigeration system cannot always be in the designed operating conditions as the ambient temperature and the temperature on the evaporator side change, and the operating energy efficiency of the cascade refrigeration system decreases.

[0006] The technical solution of the present invention is implemented as follows: a self-cascade refrigeration system with adjustable refrigerant components, including a control module for adjusting the mixed refrigerant components in the self-cascade refrigeration system, and the control module is arranged in the self-cascade refrigeration system.

[0007] Preferably, the control module includes a precooling component and a valve component; the self-cascade refrigeration system includes a first compressor, the outlet of the first compressor is connected to the first condenser, the outlet of the first condenser is connected to the first condenser evaporator, the first outlet of the first condenser evaporator is connected to the gas-liquid separator, the second outlet of the first condenser evaporator is connected to the precooling component, the air outlet of the gas-liquid separator is connected to the second condenser evaporator, the liquid outlet of the gas-liquid separator is connected to the liquid inlet of the second condenser evaporator, a second throttling mechanism is provided on the connecting pipeline between the gas-liquid separator and the second condenser evaporator, the valve assembly is arranged in parallel between the liquid outlet of the gas-liquid separator and the second throttling mechanism, the liquid outlet of the second condenser evaporator is connected to the evaporator, the air outlet of the evaporator is connected to the liquid inlet of the second condenser evaporator, and the air outlet of the second condenser evaporator is connected to the inlet of the first compressor.

[0008] Preferably, the precooling assembly includes a second compressor, the outlet of the second compressor is connected to a second condenser, the second condenser is connected to the first condenser-evaporator, and the second outlet of the first condenser-evaporator is connected to the inlet of the second compressor.

[0009] Preferably, a first throttling mechanism is provided between the second condenser and the first condenser evaporator.

[0010] Preferably, the valve assembly includes two solenoid valves and a cut-off pipe, the two solenoid valves are connected via the cut-off pipe, and the two solenoid valves are respectively connected to the liquid outlet of the gas-liquid separator and the second throttling mechanism.

[0011] Preferably, at least two of the valve assemblies are arranged between the liquid outlet of the gas-liquid separator and the second throttling mechanism, and at least two of the valve assemblies are arranged in parallel along the height direction of the gas-liquid separator.

[0012] Preferably, a third throttling mechanism is provided between the liquid outlet of the second condenser evaporator and the evaporator.

[0013] Preferably, the first condenser and the second condenser are both air-cooled fin-tube heat exchangers; the first condenser evaporator and the second condenser evaporator are both shell-and-tube heat exchangers; the first throttling mechanism, the second throttling mechanism and the third throttling mechanism are all electronic expansion valves.

[0014] A control method is applied to the above-mentioned auto-cascade refrigeration system with adjustable refrigerant components. When the temperature in the first condenser and the evaporator caused by the ambient temperature and the working temperature changes, the temperature of the mixed refrigerant in the gas-liquid separator is adjusted by the first throttling mechanism, and then the components of the refrigerant entering the second throttling mechanism and the third throttling mechanism are adjusted; by controlling the opening and closing of the solenoid valve, the sealing and unsealing of the liquid refrigerant in the interception pipeline are achieved, the flow of the liquid refrigerant entering the auto-cascade refrigeration system is adjusted, and then the components of the refrigerant entering the auto-cascade refrigeration system are adjusted.

[0015] Preferably, when the unit of the auto-cascade refrigeration system is shut down, the pre-cooling assembly consisting of the second compressor, the second condenser, the third throttling mechanism and the first condenser evaporator is turned on.

[0016] Beneficial effects of the present invention: The present invention sets a control module in the cascade refrigeration system, and adjusts the mixed refrigerant components through the control module, so that the separation efficiency of the mixed refrigerant components is always kept high, the cascade refrigeration system is always under the design conditions, the operating energy efficiency is maintained at the highest, and the refrigeration capacity and performance are avoided from being affected by changes in ambient temperature and changes in temperature on the evaporator side. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying any creative work.

[0018] Figure 1 It is a schematic diagram of the present invention.

[0019] Figure 2 It is a schematic diagram of the valve assembly of the present invention.

[0020] In the figure: 1 first compressor, 2 first condenser, 3 first condenser-evaporator, 4 gas-liquid separator, 5 second condenser-evaporator, 6 first throttling mechanism, 7 evaporator, 8 second compressor, 9 second condenser, 10 second throttling mechanism, 11 solenoid valve, 12 intercepting pipe, 13 third throttling mechanism. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] Embodiment 1, a self-cascade refrigeration system with adjustable refrigerant composition, such as Figure 1 As shown, it includes a control module for adjusting the mixed refrigerant components in the self-cascade refrigeration system, and the control module is arranged in the self-cascade refrigeration system. A control module is arranged in the self-cascade refrigeration system, and the mixed refrigerant components are adjusted by the control module, so that the separation efficiency of the mixed refrigerant components is always kept high, the self-cascade refrigeration system is always under the design working condition, and the operating energy efficiency is kept at the highest, avoiding the situation that the refrigeration capacity and performance are affected when the ambient temperature changes and the temperature on the evaporator side changes, and solving the technical problem that the self-cascade refrigeration system cannot always be under the design working condition with the change of the ambient temperature and the change of the temperature on the evaporator side, and the operating energy efficiency of the self-cascade refrigeration system decreases.

[0023] As the working fluid in the auto-cascade refrigeration system, the calculation of the thermodynamic performance of non-azeotropic mixed refrigerants is more complicated than that of single-component refrigerants, and the working fluid components and ratios have a great influence on their thermal properties. Since the auto-cascade refrigeration system relies on the different boiling points of the components in the mixed refrigerant to produce high-quality ultra-low temperature cooling, the performance of the auto-cascade refrigeration system using different refrigerant components and ratios will show great differences. The relative content of each component in the mixed refrigerant R134a / R23 has a great influence on the parameters such as the condensing pressure and evaporation pressure of the auto-cascade refrigeration system, as well as the lowest refrigerator center temperature. When the mass fraction of R23 is within a certain fraction range, the performance of the auto-cascade refrigeration system reaches the best. When the mixed refrigerant is charged into the refrigeration system, the components of the refrigerant are fixed, and the online adjustment of the refrigerant components cannot be achieved, and the energy efficiency of the refrigeration system cannot always be maintained at the highest level. During the operation of the auto-cascade refrigeration system, affected by the evaporation temperature and condensation temperature, the refrigerants involved in the operation have low optimal operating efficiency, and there are different optimal operating components. At present, there is still a lack of effective control methods. The present application is to adjust the optimal operating components of the refrigerant so that the refrigerant involved in the operation is kept at the optimal operating energy efficiency.

[0024] Example 2, based on Example 1, a self-cascade refrigeration system with adjustable refrigerant composition, such as Figure 1As shown, the control module includes a precooling component and a valve component; the self-cascade refrigeration system includes a first compressor 1, the outlet of the first compressor 1 is connected to a first condenser 2, the outlet of the first condenser 2 is connected to a first condenser evaporator 3, the first outlet of the first condenser evaporator 3 is connected to a gas-liquid separator 4, the second outlet of the first condenser evaporator 3 is connected to the precooling component, the air outlet of the gas-liquid separator 4 is connected to a second condenser evaporator 5, the liquid outlet of the gas-liquid separator 4 is connected to the liquid inlet of the second condenser evaporator 5, a second throttling mechanism 10 is provided on the connecting pipeline between the gas-liquid separator 4 and the second condenser evaporator 5, the valve component is arranged in parallel between the liquid outlet of the gas-liquid separator 4 and the second throttling mechanism 10, the liquid outlet of the second condenser evaporator 5 is connected to an evaporator 7, the air outlet of the evaporator 7 is connected to the liquid inlet of the second condenser evaporator 5, and the air outlet of the second condenser evaporator 5 is connected to the inlet of the first compressor 1. The temperature of the first condenser evaporator 3 is controlled by the precooling component, and then the temperature of the mixed refrigerant in the gas-liquid separator is adjusted, which is conducive to improving the separation efficiency of the mixed refrigerant; the setting of the valve component is conducive to the adjustment of the refrigerant components in the auto-cascade refrigeration system. The refrigerant used in the auto-cascade refrigeration system is a mixed refrigerant, which can be but not limited to R134a / R23, R600 / R170. The refrigerant used in the precooling component is a pure refrigerant.

[0025] The gaseous mixed refrigerant rich in low-boiling-point components leaves the air outlet of the gas-liquid separator 4 and enters the second condenser evaporator 5. After being condensed into liquid in the second condenser evaporator 5, it passes through the third throttling mechanism 13 along the liquid outlet of the second condenser evaporator 5 to reach the evaporator 7. After evaporating and absorbing heat in the evaporator 7, a gaseous mixed refrigerant is formed. The gaseous mixed refrigerant is mixed with the liquid mixed refrigerant rich in high-boiling-point components from the second throttling mechanism 10 and enters along the liquid inlet of the second condenser evaporator 5. After evaporating and absorbing heat in the second condenser evaporator 5, it enters the air inlet of the first compressor 1.

[0026] Example 3, based on Example 2, a self-cascade refrigeration system with adjustable refrigerant composition, such as Figure 1As shown, the precooling assembly includes a second compressor 8, the outlet of the second compressor 8 is connected to a second condenser 9, the second condenser 9 is connected to the first condenser evaporator 3, and the second outlet of the first condenser evaporator 3 is connected to the inlet of the second compressor 8. The refrigerant used in the precooling assembly is a pure refrigerant. After the motor of the second compressor 8 drives the piston to compress the pure refrigerant, a high-temperature and high-pressure refrigerant gas is formed. The high-temperature and high-pressure refrigerant gas enters the second condenser 9 to dissipate heat and condense into a liquid refrigerant. The liquid refrigerant enters the hot fluid inlet of the first condenser evaporator 3 and is connected. The hot fluid outlet of the first condenser evaporator 3 is connected to the second compressor 8. After the liquid refrigerant passes through the first condenser evaporator 3, it forms a low-temperature and low-pressure refrigerant gas and reaches the inlet of the second compressor 8, thereby forming a cycle. The liquid refrigerant circulates at the hot fluid end of the first condenser evaporator 3 to adjust the temperature of the mixed refrigerant passed by the cold fluid of the first condenser evaporator 3, which is conducive to improving the separation efficiency of the mixed refrigerant. The hot fluid end of the first condenser evaporator 3 is also the condensation end.

[0027] The first condenser evaporator 3 and the second condenser evaporator 5 are each provided with two working fluid flow channels, namely a hot fluid channel and a cold fluid channel. The hot fluid inlet and the hot fluid outlet of the first condenser evaporator 3 are connected through the hot fluid channel, and the cold fluid inlet and the cold fluid outlet are connected through the cold fluid channel; the cold fluid inlet and the cold fluid outlet of the second condenser evaporator 5 are connected through the cold fluid channel, and the hot fluid inlet and the hot fluid outlet are connected through the hot fluid channel; the working fluid used in the system is a binary or higher non-azeotropic mixed refrigerant.

[0028] Embodiment 4, based on embodiment 3, a self-cascade refrigeration system with adjustable refrigerant composition, such as Figure 1 As shown, a first throttling mechanism 6 is provided between the second condenser 9 and the first condenser evaporator 3. The provision of the first throttling mechanism 6 is conducive to controlling the flow rate of the hot fluid end of the first condenser evaporator 3, thereby facilitating the control of the heat exchange in the first condenser evaporator 3.

[0029] Example 5, based on Example 4, a self-cascade refrigeration system with adjustable refrigerant composition, such as Figure 1 and Figure 2 As shown, the valve assembly includes two solenoid valves 11 and a cut-off pipe 12. The two solenoid valves 11 are connected by the cut-off pipe 12, and the two solenoid valves 11 are respectively connected to the liquid outlet of the gas-liquid separator 4 and the second throttling mechanism 10. The space between the two solenoid valves 11 and the cut-off pipe 12 forms a storage space for the liquid refrigerant, which realizes the sealing and unsealing of the liquid refrigerant, and is conducive to adjusting the composition of the mixed refrigerant by adjusting the amount of liquid refrigerant added.

[0030] Example 6, based on Example 5, a self-cascade refrigeration system with adjustable refrigerant composition, such as Figure 1 and Figure 2 As shown, at least two valve assemblies are arranged between the liquid outlet of the gas-liquid separator 4 and the second throttling mechanism 10, and at least two valve assemblies are arranged in parallel along the height direction of the gas-liquid separator 4. At least two valve assemblies are arranged in parallel along the height direction of the gas-liquid separator 4, and the liquid level in the gas-liquid separator 4 can be ensured by opening valve assemblies at different heights in sequence.

[0031] Embodiment 7, based on embodiment 6, a self-cascade refrigeration system with adjustable refrigerant composition, such as Figure 1 As shown, a third throttling mechanism 13 is provided between the liquid outlet of the second condenser evaporator 5 and the evaporator 7. The third throttling mechanism 13 is provided to control the flow rate of the liquid refrigerant passing from the second condenser evaporator 5 into the evaporator 7, so as to control the change of the mixed refrigerant components in the refrigeration system.

[0032] Embodiment 8, based on embodiment 7, a self-cascade refrigeration system with adjustable refrigerant composition, such as Figure 1 As shown, the first condenser 2 and the second condenser 9 are both air-cooled fin-tube heat exchangers; the first condenser evaporator 3 and the second condenser evaporator 5 are both shell-and-tube heat exchangers; the first throttling mechanism 6, the second throttling mechanism 10 and the third throttling mechanism 13 are all electronic expansion valves. The first condenser 2 and the second condenser 9 are both air-cooled fin-tube heat exchangers. By adding fins outside the tubes, the heat transfer area is greatly increased, so that heat can be transferred to the fluid more quickly, thereby achieving efficient heat exchange. Under the same conditions, more heat can be processed, which improves the heat exchange efficiency.

[0033] The first condenser evaporator 3 and the second condenser evaporator 5 are both shell-and-tube heat exchangers. The shell-and-tube heat exchanger has a better heat exchange effect due to direct fluid contact, and has a simple structure and is easy to maintain and overhaul.

[0034] The first throttling mechanism 6, the second throttling mechanism 10 and the third throttling mechanism 13 are all electronic expansion valves, which can achieve high-precision flow control, which is beneficial to the adjustment of the mixed refrigerant components in the auto-cascade refrigeration system.

[0035] When Example 8 is implemented, the high-temperature and high-pressure non-azeotropic mixed refrigerant leaves the outlet of the first compressor 1, enters the first condenser 2 and is condensed into a two-phase fluid, the two-phase fluid enters the first condenser evaporator 3, and after passing through the first condenser evaporator 3, the two-phase fluid enters the gas-liquid separator 4, and the high and low boiling point components of the mixed refrigerant are separated in the gas-liquid separator 4: the gaseous mixed refrigerant rich in low boiling point components leaves the gas phase outlet of the gas-liquid separator 4, enters the hot fluid channel of the second condenser evaporator 5 and is further cooled to become a liquid, and the liquid mixed refrigerant rich in high boiling point components leaves the liquid phase outlet of the gas-liquid separator 4 and passes through the second throttling mechanism 10; the liquid mixed refrigerant with low boiling point components leaves the liquid phase outlet of the second condenser evaporator 5, passes through the third throttling mechanism 13, and flows into the evaporator 7 and is evaporated by the evaporator 7 to form refrigerant vapor, and the refrigerant vapor at the outlet of the evaporator 7 is mixed with the liquid mixed refrigerant rich in high boiling point components flowing out through the second throttling mechanism 10 and enters the cold fluid channel of the second condenser evaporator 5. Absorb heat, cool the hot fluid in the second condenser evaporator 5 and form a gaseous mixed refrigerant at the same time. The gaseous mixed refrigerant returns to the inlet of the first compressor 1 along the gas phase outlet of the second condenser evaporator 5; when the ambient temperature changes and the temperature on the evaporator side changes, start the pre-cooling component composed of the second compressor 8, the second condenser 9, the third throttling mechanism 13 and the first condenser evaporator 3, and control the temperature of the first condenser evaporator 3 through the pre-cooling component. The high-temperature and high-pressure gaseous pure refrigerant generated by the second compressor 8 leaves the outlet of the second compressor 8 and enters the second condenser 9 to condense into a liquid pure refrigerant. The liquid pure refrigerant enters the cold fluid channel on the left side of the first condenser evaporator 3 to absorb heat, and further cools the two-phase fluid in the hot fluid channel on the right side of the condenser evaporator 3, so that the mixed refrigerant in the two-phase fluid is separated more thoroughly. At the same time, the liquid pure refrigerant enters the cold fluid channel on the left side of the first condenser evaporator 3 to absorb heat and becomes a gaseous pure refrigerant and returns to the inlet of the second compressor 8.

[0036] Embodiment 9, based on any one of Embodiments 1 to 8, a control method, such as Figure 1 and Figure 2 As shown, the above-mentioned self-cascade refrigeration system with adjustable refrigerant components is applied. When the temperature in the first condenser 2 and the evaporator 7 caused by the ambient temperature and the working temperature changes, the temperature of the mixed refrigerant in the gas-liquid separator 4 is adjusted by the first throttling mechanism 6, and then the components of the refrigerant entering the second throttling mechanism 10 and the third throttling mechanism 13 are adjusted; by controlling the opening and closing of the solenoid valve 11, the sealing and unsealing of the liquid refrigerant in the intercepting pipe 12 are realized, and the flow rate of the liquid refrigerant entering the self-cascade refrigeration system is adjusted, and then the components of the refrigerant entering the self-cascade refrigeration system are adjusted.

[0037] When implementing Example 9, taking the example of setting three valve assemblies between the liquid outlet of the gas-liquid separator 4 and the second throttling mechanism 10, the three valve assemblies have a total of six solenoid valves S1, S2, S3, S4, S5, and S6. By controlling the opening and closing of the six solenoid valves, the sealing and unsealing of the liquid refrigerant is realized, and the refrigerant components entering the self-cascade refrigeration system circulation are adjusted; when the temperature of the gas-liquid separator decreases and the liquid level inside the separator rises, the solenoid valve S1 and the solenoid valve S2 are opened first, and the solenoid valve S2 and the solenoid valve S1 are closed in sequence after 30 seconds; after 10 minutes of operation, if the liquid level of the gas-liquid separator has no obvious change, the solenoid valve S3 and the solenoid valve S4 are opened, and the solenoid valve S4 and the solenoid valve S3 are closed in sequence after 30 seconds; after 10 minutes of operation, if the liquid level of the gas-liquid separator has no obvious change, the solenoid valve S5 and the solenoid valve S6 are opened, and the solenoid valve S5 and the solenoid valve S6 are closed in sequence after 30 seconds. The liquid level in the gas-liquid separator is ensured by the above steps; when the temperature of the gas-liquid separator rises and the liquid level inside the separator drops, first open the solenoid valve S2 and the solenoid valve S1, and then close the solenoid valve S2 and the solenoid valve S1 in sequence after 30 seconds; after 10 minutes of operation, if there is no obvious change in the liquid level of the gas-liquid separator, open the solenoid valve S4 and the solenoid valve S3, and then close the solenoid valve S4 and the solenoid valve S3 in sequence after 30 seconds; after 10 minutes of operation, if there is no obvious change in the liquid level of the gas-liquid separator, open the solenoid valve S6 and the solenoid valve S5, and then close the solenoid valve S5 and the solenoid valve S6 in sequence after 30 seconds. The liquid level in the gas-liquid separator is ensured by the above steps.

[0038] Embodiment 10, based on embodiment 9, a control method, such as Figure 1 As shown, when the unit of the self-cascade refrigeration system is shut down, the pressure of the self-cascade refrigeration system is controlled by turning on the pre-cooling assembly composed of the second compressor 8, the second condenser 9, the third throttling mechanism 13 and the first condenser evaporator 3 to ensure safety when the unit is shut down.

[0039] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A self-cascade refrigeration system with adjustable refrigerant composition, characterized in that: The invention comprises a control module for adjusting the mixed refrigerant components in the auto-cascade refrigeration system, and the control module is arranged in the auto-cascade refrigeration system.

2. The auto-cascade refrigeration system with adjustable refrigerant composition according to claim 1, characterized in that: The control module comprises a precooling component and a valve component; the self-cascade refrigeration system comprises a first compressor (1); the outlet of the first compressor (1) is connected to a first condenser (2); the outlet of the first condenser (2) is connected to a first condenser evaporator (3); the first outlet of the first condenser evaporator (3) is connected to a gas-liquid separator (4); the second outlet of the first condenser evaporator (3) is connected to the precooling component; the gas outlet of the gas-liquid separator (4) is connected to a second condenser evaporator (5); the liquid outlet of the gas-liquid separator (4) is connected to the second condenser evaporator (5); The liquid inlet of the condenser evaporator (5) is connected, a second throttling mechanism (10) is provided on the connecting pipe between the gas-liquid separator (4) and the second condenser evaporator (5), the valve assembly is arranged in parallel between the liquid outlet of the gas-liquid separator (4) and the second throttling mechanism (10), the liquid outlet of the second condenser evaporator (5) is connected to the evaporator (7), the air outlet of the evaporator (7) is connected to the liquid inlet of the second condenser evaporator (5), and the air outlet of the second condenser evaporator (5) is connected to the inlet of the first compressor (1).

3. The auto-cascade refrigeration system with adjustable refrigerant composition according to claim 2, characterized in that: The precooling assembly comprises a second compressor (8), the outlet of the second compressor (8) is connected to a second condenser (9), the second condenser (9) is connected to the first condenser evaporator (3), and the second outlet of the first condenser evaporator (3) is connected to the inlet of the second compressor (8).

4. The auto-cascade refrigeration system with adjustable refrigerant composition according to claim 3, characterized in that: A first throttling mechanism (6) is provided between the second condenser (9) and the first condenser evaporator (3).

5. The auto-cascade refrigeration system with adjustable refrigerant composition according to claim 4, characterized in that: The valve assembly comprises two solenoid valves (11) and a cut-off pipe (12); the two solenoid valves (11) are connected via the cut-off pipe (12); and the two solenoid valves (11) are respectively connected to the liquid outlet of the gas-liquid separator (4) and the second throttling mechanism (10).

6. The auto-cascade refrigeration system with adjustable refrigerant composition according to claim 5, characterized in that: At least two of the valve assemblies are arranged between the liquid outlet of the gas-liquid separator (4) and the second throttling mechanism (10), and the at least two valve assemblies are arranged in parallel along the height direction of the gas-liquid separator (4).

7. The auto-cascade refrigeration system with adjustable refrigerant composition according to claim 6, characterized in that: A third throttling mechanism (13) is provided between the liquid outlet of the second condenser evaporator (5) and the evaporator (7).

8. The auto-cascade refrigeration system with adjustable refrigerant composition according to claim 7, characterized in that: The first condenser (2) and the second condenser (9) are both air-cooled fin-tube heat exchangers; the first condenser evaporator (3) and the second condenser evaporator (5) are both shell-and-tube heat exchangers; the first throttling mechanism (6), the second throttling mechanism (10) and the third throttling mechanism (13) are all electronic expansion valves.

9. A control method, applied to the auto-cascade refrigeration system with adjustable refrigerant composition according to any one of claims 1 to 8, characterized in that: When the temperature in the first condenser (2) and the evaporator (7) changes due to the ambient temperature and the operating temperature, the temperature of the mixed refrigerant in the gas-liquid separator (4) is adjusted by the first throttling mechanism (6), thereby adjusting the components of the refrigerant entering the second throttling mechanism (10) and the third throttling mechanism (13); by controlling the opening and closing of the solenoid valve (11), the sealing and unsealing of the liquid refrigerant in the intercepting pipe (12) are achieved, the flow rate of the liquid refrigerant entering the cascade refrigeration system is adjusted, thereby adjusting the components of the refrigerant entering the cascade refrigeration system.

10. The control method according to claim 9, characterized in that: When the unit of the auto-cascade refrigeration system is shut down, the pre-cooling assembly consisting of the second compressor (8), the second condenser (9), the third throttling mechanism (13) and the first condenser evaporator (3) is turned on.

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