A control system and control method for a separation-enhanced cascade refrigeration cycle.

Abstract

A control system and method for a separation-enhanced self-cascade refrigeration cycle are disclosed. The system includes a controller connecting sensors, adjustment mechanisms, and a control structure. The method includes inputting preset parameters of the separation-enhanced self-cascade refrigeration cycle system, real-time parameter acquisition by the controller, parameter judgment by the controller, and defining the start-up phase, the low-temperature pull-down phase, and the steady-state operation phase. The system compares the operating time with the set time to distinguish between the start-up phase and the low-temperature pull-down phase; it also compares the evaporator outlet temperature with the set temperature to distinguish between the low-temperature pull-down phase and the steady-state operation phase. In each of the three operating phases, the system achieves efficient operation by adjusting the opening of the electronic expansion valve, the on / off state of the solenoid valve, and the start / stop of the fan and compressor. The control system and method proposed in this invention can protect the safe start-up of the separation-enhanced self-cascade refrigeration cycle system, increase the rate of low-temperature pull-down and the cooling capacity, significantly improve the thermodynamic performance of the separation-enhanced self-cascade refrigeration cycle system, and maintain the system's efficient and stable operation.

Description

Technical Field

[0001] This invention relates to the field of refrigeration and cryogenic technology, specifically to a control system and control method for a separation-enhanced self-cascade refrigeration cycle. Background Technology

[0002] The rapid development of fields such as food freezing, biomedicine, and electronic science has increased the demand for cryogenic refrigeration technology ranging from -40℃ to -90℃. However, traditional single-stage compression refrigeration technology experiences a significant drop in efficiency when achieving temperatures below -40℃. Cascade refrigeration technology is a commonly used refrigeration technique for temperatures between -40℃ and -90℃. Cascade refrigeration cycles include external cascade cycles and self-cascade cycles. Compared to external cascade cycles, self-cascade cycles use a single compressor and a non-azeotropic working fluid. By utilizing the component shift between the working fluids, a gaseous working fluid rich in low-boiling-point components and a liquid working fluid rich in high-boiling-point components are separated. The cryogenic refrigeration cycle is completed by cascading refrigeration between the different refrigerant components.

[0003] Currently, conventional cascade refrigeration systems employ a separator at the condenser outlet to separate the two-phase working fluid into gas and liquid phases, while simultaneously separating the components of the mixed working fluid. However, the component separation efficiency of a single separator is relatively low, hindering the full realization of the advantages of cascade refrigeration systems. When the dryness of the condenser outlet decreases, not only does the flow rate of the separated gas phase decrease, leading to a significant reduction in the flow rate within the evaporator and severely impacting its cooling capacity, but the component separation efficiency also diminishes, severely degrading the performance of the cascade refrigeration cycle. Therefore, to improve the performance of cascade refrigeration systems, it is necessary to enhance the separation efficiency of the gas and liquid phases, and adding an additional separator is a feasible solution. However, current technologies for improving the separation efficiency of cascade refrigeration systems are lacking, along with corresponding control methods. Optimized cycle flow designs are needed to promote the separation of the gas and liquid phases and the separation between components, and appropriate control methods are required to address different operating conditions, such as the high compressor discharge pressure during startup, the slow cooling rate during the low-temperature phase, and the performance degradation under disturbance conditions. Summary of the Invention

[0004] To address the shortcomings and deficiencies of the existing technologies, this invention proposes a control system and method for a separation-enhanced self-cascade refrigeration cycle system applied to cryogenic refrigeration equipment. This control system can control the start-up and shutdown of the compressor and fan, the on / off state of the solenoid valve, and the opening degree of the electronic expansion valve in real time based on the thermodynamic parameters of the separation-enhanced self-cascade system, ensuring the safe and efficient operation of the system. The control method collects data on operating time, compressor outlet pressure, condenser outlet temperature and pressure, second separator inlet temperature and pressure, and evaporator outlet temperature to control the system into different operating stages, including a start-up stage, a cryogenic phase, and a stable operating stage. During the start-up stage, it ensures safe system startup. During the cryogenic phase and stable operating stage, it increases the refrigerant flow rate in the evaporator, increases the concentration of low-boiling-point components in the evaporator, increases the evaporation pressure, reduces the compressor pressure ratio, and lowers the compressor's power consumption, achieving energy savings for the system.

[0005] To achieve the above-mentioned technical objectives, the present invention adopts the following technical solution:

[0006] A control system for a separation-enhanced self-cascade refrigeration cycle is disclosed. The separation-enhanced self-cascade refrigeration cycle includes a second condenser 104 at the gas phase outlet of a first separator 103. After condensation, the gas phase enters a first cascade heat exchanger 105 for subcooling. The liquid phase at the outlet of the first separator 103 is throttled and then enters the first cascade heat exchanger 105 for reheating before entering the second separator 110. The control system includes a first pressure sensor 118 and a first temperature sensor 121 located at the outlet of the first condenser 102, and a second pressure sensor 119 and a second temperature sensor 121 located at the inlet of the second separator 110. Sensor 122, third pressure sensor 120 located at the outlet of compressor 101, and third temperature sensor 123 located at the outlet of evaporator 113; regulating mechanism connected to first electronic expansion valve 106, second electronic expansion valve 107, third electronic expansion valve 112, and fourth electronic expansion valve 114 respectively; control mechanism connected to first solenoid valve 108, second solenoid valve 109, first fan 115, second fan 116, and compressor 101 respectively; the input terminal of controller 117 is connected to all temperature and pressure sensors, and the output terminal is connected to all regulating mechanisms and control structures.

[0007] The control method for a control system applied to a separation-enhanced cascade refrigeration cycle includes:

[0008] Step 1: Input the preset parameters of the separated and efficiency - enhanced auto - cascade refrigeration cycle system: the safety pressure P30 at the compressor outlet, the deviation value Δ3; the set value T30 of the temperature at the evaporator outlet, the deviation value Δ30; the set value q10 of the dryness at the condenser outlet, the deviation value Δ1; the set value q20 of the dryness at the inlet of the second separator, the deviation value Δ2; the starting time t0;

[0009] Step 2: The controller collects parameters in real - time: the running time t of the separated and efficiency - enhanced auto - cascade refrigeration cycle system, and receives the temperature signal T3 from the third temperature sensor at the evaporator outlet;

[0010] Step 3: The controller judges the collected parameters: When t < t0, the separated and efficiency - enhanced auto - cascade refrigeration cycle system enters the startup stage; when t ≥ t0, the separated and efficiency - enhanced auto - cascade refrigeration cycle system enters the low - temperature pulling stage; after the separated and efficiency - enhanced auto - cascade refrigeration cycle system enters the low - temperature pulling stage, the controller receives the temperature signal T3 from the third temperature sensor at the evaporator outlet. When T3 > T30+Δ30, the separated and efficiency - enhanced auto - cascade refrigeration cycle system is in the low - temperature pulling stage; when T3 ≤ T30+Δ30, the separated and efficiency - enhanced auto - cascade refrigeration cycle system is in the stable operation stage;

[0011] Step 4: Startup stage: The controller 117 controls the first fan 115 and the second fan 116 to start. The first electronic expansion valve 106 and the second electronic expansion valve 107 are at full opening, the third electronic expansion valve 112 and the fourth electronic expansion valve 114 are at 50% of full opening, the first solenoid valve 108 is closed, and the second solenoid valve 109 is opened; the controller 117 receives the pressure signal P3 from the third pressure sensor 120 at the compressor outlet; the controller 117 controls the opening degrees of the third electronic expansion valve 112 and the fourth electronic expansion valve 114 to make P3 < P30+Δ3;

[0012] Step 5: Low - temperature pulling stage: The controller 117 controls the first fan 115 and the second fan 116 to start, the first solenoid valve 108 is closed and the second solenoid valve 109 is opened, and the third electronic expansion valve 112 and the fourth electronic expansion valve 114 are at 30% of full opening; the controller 117 calculates the dryness value q1 at the condenser outlet and the first parameter value Z1 according to the pressure signal P1 from the first pressure sensor 118 and the temperature signal T1 from the first temperature sensor 121. The first parameter value Z1 is the concentration of the low - boiling - point component in the refrigerant at the gas - phase outlet of the first separator; the controller 117 calculates the dryness value q2 at the inlet of the second separator and the second parameter value Z2 according to the pressure signal P2 from the second pressure sensor 119 and the temperature signal T2 from the second temperature sensor 122, and at the same time calculates according to 1 - Z1. The second parameter value Z2 is the concentration of the low - boiling - point component in the refrigerant at the gas - phase outlet of the second separator; the controller 117 adjusts the opening degrees of the first electronic expansion valve 106 and the second electronic expansion valve 107 to make Z2 > Z1 and q20 - Δ2 ≤ q2 ≤ q20+Δ2;

[0013] Step 6: Steady-state operation stage: Controller 117 controls the start and stop of the first fan 115 and the second fan 116, controls the start and stop of the compressor 101, and controls the opening and closing of the first solenoid valve 108 and the second solenoid valve 109; Controller 117 calculates the condenser outlet dryness value q1 and the first parameter value Z1 based on the pressure signal P1 of the first pressure sensor 118 and the temperature signal T1 of the first temperature sensor 121; Controller 117 calculates the second separator inlet dryness value q2 and the second parameter value Z2 based on the pressure signal P2 of the second pressure sensor 119 and the temperature signal T2 of the second temperature sensor 122; controls the opening degree of the first electronic expansion valve 106 and the second electronic expansion valve 107, so that q10-Δ1≤q1≤q10+Δ1, q20-Δ2≤q2≤q20+Δ2, and Z2>Z1.

[0014] In step 4, during the startup phase, the controller 117 receives the pressure signal P3 from the third pressure sensor 120 in real time. When P3 > P30 + Δ3, the controller 117 increases the opening of the third electronic expansion valve 112 and the fourth electronic expansion valve 114. When P3 < P30 - Δ3, the controller 117 decreases the opening of the third electronic expansion valve 112 and the fourth electronic expansion valve 114. When P30 - Δ3 ≤ P3 ≤ P30 + Δ3, the controller 117 keeps the opening of the third electronic expansion valve 112 and the fourth electronic expansion valve 114 unchanged.

[0015] In step 5, during the low-temperature phase, when Z2≤Z1 or q2<q20-Δ2, the controller 117 reduces the opening of the first electronic expansion valve 106 and the second electronic expansion valve 107; when q2>q20+Δ2, the controller 117 increases the opening of the first electronic expansion valve 106 and the second electronic expansion valve 107; when Z2>Z1 and q20-Δ2≤q2≤q20+Δ2, the controller 117 keeps the opening of the first electronic expansion valve 106 and the second electronic expansion valve 107 unchanged.

[0016] In step 6, during the stable operation stage, when T3 < T30 - Δ30, the controller 117 controls the compressor 101 to stop running. After that, the controller 117 receives the temperature signal T3 of the third temperature sensor 123 at the evaporator outlet. When T3 > T30 + Δ30, the controller 117 controls the compressor 101 to start. During the stable operation stage, when the external environmental temperature changes and fluctuates, the dryness value q1 at the condenser outlet will change. When q1 < q10 - Δ1, the controller 117 controls the first fan 115 to start, the second fan 116 to close, the first solenoid valve 108 to close, and the second solenoid valve 109 to open. When q1 > q10 + Δ1, the controller 117 controls the first fan 115 to start, the second fan 116 to start, the first solenoid valve 108 to open, and the first solenoid valve 109 to close. When q1 < q10 - Δ1 or q1 > q10 + Δ1, the controller 117 receives the pressure signal P1 of the first pressure sensor 118 and the temperature signal T1 of the first temperature sensor 121, and calculates the dryness q1 at the condenser outlet and the first parameter value Z1. The controller 117 receives the pressure signal P2 of the second pressure sensor 119 and the temperature signal T2 of the second temperature sensor 122, and simultaneously calculates the dryness q2 at the inlet of the second separator and the second parameter value Z2 according to 1 - Z1. When Z2 ≤ Z1 or q2 < q20 - Δ2, the controller 117 reduces the opening degrees of the first electronic expansion valve 106 and the second electronic expansion valve 107. When q2 > q20 + Δ2, the controller 117 increases the opening degrees of the first electronic expansion valve 106 and the second electronic expansion valve 107. When Z2 > Z1 and q20 - Δ2 ≤ q2 ≤ q20 + Δ2, the controller 117 controls the opening degrees of the first electronic expansion valve 106 and the second electronic expansion valve 107 to remain unchanged. When q10 - Δ1 ≤ q1 ≤ q10 + Δ1, the controller 117 controls the opening degrees of the first electronic expansion valve 106 and the second electronic expansion valve 107 to remain unchanged.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] 1. The control system of the present invention can control the start and stop of the compressor and the fan, the on / off of the solenoid valve, and the opening degree of the electronic expansion valve in real time according to the thermodynamic parameter values of the separation-enhanced auto-cascade system, and ensure the safe and efficient operation of the separation-enhanced auto-cascade refrigeration system according to the thermodynamic parameters in real time.

[0019] 2. The present invention takes into account that the exhaust pressure of the compressor is relatively large in the initial stage of starting the auto-cascade refrigeration system. The controller fully opens the electronic expansion valve at the outlet of the first separator, increases the volume of the container in the system, so that the exhaust pressure of the compressor is always lower than the set pressure, and the system can start safely;

[0020] 3. During the low-temperature phase of the system, the controller adjusts the opening of the electronic expansion valve to increase the refrigerant flow rate in the evaporator and the concentration of low-boiling-point components in the evaporator, thereby reducing the compressor's pressure ratio, reducing compressor power consumption, and achieving the goal of system energy saving.

[0021] 4. During the stable operation phase of the system, when the ambient temperature fluctuates, or when the dryness of the condenser outlet decreases or increases, the controller adjusts the opening of the electronic expansion valve to maintain the refrigerant flow in the evaporator and simultaneously increases the concentration of low-boiling-point components in the evaporator to maintain the energy-saving operation of the system. Attached Figure Description

[0022] Figure 1 This is a flowchart of the control method for separation-enhanced self-cascade cooling according to the present invention.

[0023] Figure 2 This is an implementation example of the control system described in this invention, which is applied to separation-enhanced cascade cooling.

[0024] 101. Compressor; 102. First condenser; 103. First separator; 104. Second condenser; 105. First cascade heat exchanger; 106. First electronic expansion valve; 107. Second electronic expansion valve; 108. First solenoid valve; 109. Second solenoid valve; 110. Second separator; 111. Second cascade heat exchanger; 112. Third electronic expansion valve; 113. Evaporator; 114. Fourth electronic expansion valve; 115. First fan; 116. Second fan; 117. Controller; 118. First pressure sensor; 119. Second pressure sensor; 120. Third pressure sensor; 121. First temperature sensor; 122. Second temperature sensor; 123. Third temperature sensor Detailed Implementation

[0025] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and an embodiment. It should be understood that the embodiment described herein is only for explaining the invention and is not intended to limit the invention.

[0027] Implementation Cases

[0028] A control system applied to a separation-enhanced auto-cascade refrigeration cycle. In the separation-enhanced auto-cascade refrigeration cycle, a second condenser 104 is provided at the gas phase outlet of the first separator 103; after condensation, it enters the first cascade heat exchanger 105 for subcooling; the liquid phase outlet of the first separator 103 enters the first cascade heat exchanger 105 for heat regeneration after throttling and then enters the second separator 110; the control system includes a third pressure sensor 120 provided at the outlet of the compressor 101, a first pressure sensor 118 and a first temperature sensor 121 at the outlet of the first condenser 102, a second pressure sensor 119 and a second temperature sensor 122 at the inlet of the second separator 110, and a third temperature sensor 123 at the outlet of the evaporator 113; an adjustment mechanism respectively connected to the first electronic expansion valve 106, the second electronic expansion valve 107, the third electronic expansion valve 112, and the fourth electronic expansion valve 114; a control mechanism respectively connected to the first solenoid valve 108, the second solenoid valve 109, the first fan 115, the second fan 116, and the compressor 101; a controller 117 connecting the sensors and the adjustment mechanism to the control structure.

[0029] As Figure 1 shown, a control method for a control system of a separation-enhanced auto-cascade refrigeration includes:

[0030] Step 1: Input system preset parameters: Input system preset parameters: the safety pressure P30 at the outlet of the compressor 101, the deviation value Δ3; the set temperature value T30 at the outlet of the evaporator 113, the deviation value Δ30; the set dryness value q10 at the outlet of the condenser 102, the deviation value Δ1; the set dryness value q20 at the inlet of the second separator 110, the deviation value Δ2; the start-up time t0.

[0031] Step 2: The controller collects parameters in real time: the system operation time t, and receives the temperature signal T3 from the third temperature sensor 123 at the outlet of the evaporator 113.

[0032] Step 3: The controller judges the collected parameters: When t < t|0, the auto-cascade refrigeration system enters the start-up stage; when t ≥ t0, the auto-cascade refrigeration system enters the low-temperature pulling stage; after the auto-cascade refrigeration system enters the low-temperature pulling stage, the controller 117 receives the temperature signal T3 from the third temperature sensor 123 at the evaporator outlet. When the outlet temperature T3 > T30 + Δ30, the auto-cascade refrigeration system is in the low-temperature pulling stage; when T3 ≤ T30 + Δ30, the auto-cascade refrigeration system is in the stable operation stage.

[0033] Step 4: Start-up Phase: First fan 115 and second fan 116 start; first electronic expansion valve 106 and second electronic expansion valve 107 are fully open; third electronic expansion valve 112 and fourth electronic expansion valve 114 are 50% fully open; first solenoid valve 108 is closed, and second solenoid valve 109 is open; controller 117 receives pressure signal P3 from the third pressure sensor 120 at the compressor outlet; controller 117 controls the opening of the third expansion valve 112 and the fourth electronic expansion valve 114 to make P3... <P30+Δ3;

[0034] Step 5: Low Temperature Stage: First fan 115 and second fan 116 start; first solenoid valve 108 closes and second solenoid valve 109 opens; third electronic expansion valve 112 and fourth electronic expansion valve 113 are at 30% full opening; controller 117 calculates the dryness value q1 and parameter value Z1 at the outlet of condenser 102 based on the pressure signal P1 from first pressure sensor 118 and the temperature signal T1 from first temperature sensor 121; controller calculates the dryness value q2 and parameter value Z2 at the inlet of second gas-liquid separator based on the pressure signal P2 from second pressure sensor 119, the temperature signal T2 from second temperature sensor 122, and 1-Z1; controller adjusts the opening of first electronic expansion valve 106 and second electronic expansion valve 107 to make Z2>Z1 and q20-Δ2≤q2≤q20+Δ2;

[0035] Step 6: Steady-state operation stage: Controller 117 controls the start and stop of the first fan 115 and the second fan 116; controls the start and stop of the compressor 101; controls the opening and closing of the first solenoid valve 108 and the second solenoid valve 109; controller 117 calculates the dryness value q1 and parameter value Z1 of the condenser outlet based on the pressure signal P1 of the first pressure sensor 118 and the temperature signal T1 of the first temperature sensor 121; controller 117 calculates the dryness value q2 and parameter value Z2 of the second gas-liquid separator inlet based on the pressure signal P2 of the second pressure sensor 119, the temperature signal T2 of the second temperature sensor 122, and 1-Z1; controls the opening degree of the first electronic expansion valve 106 and the second electronic expansion valve 107, so that q10-Δ1≤q1≤q10+Δ1, q20-Δ2≤q2≤q20+Δ2, and Z2>Z1.

[0036] Figure 2The diagram shows the setup of the control system for the aforementioned separation-enhanced self-cascade refrigeration cycle in the refrigeration cycle system. In this control system, the input terminal of the controller 117 is connected to the first pressure sensor 118 and the first temperature sensor 121 at the outlet of the condenser 102, the second pressure sensor 119 and the second temperature sensor 122 at the inlet of the second separator 110, the third pressure sensor 120 at the outlet of the compressor 101, and the third temperature sensor 123 at the outlet of the evaporator 113. The output terminal of the controller 117 is connected to the regulating mechanisms of the first electronic expansion valve 106, the second electronic expansion valve 107, the third electronic expansion valve 112, and the fourth electronic expansion valve 114, the control mechanisms of the first solenoid valve 108 and the second solenoid valve 109, the control mechanisms of the first fan 115 and the second fan 116, and the control mechanism of the compressor 101. The compressor 101 outlet safety pressure P30 is 2.8 MPa, with a deviation Δ3 of 0.02 MPa; the evaporator 113 outlet temperature setpoint T30 is -60℃, with a deviation Δ30 of 2℃; the condenser 102 outlet dryness setpoint q10 is 0.5, with a deviation Δ1 of 0.1; the second separator 110 inlet dryness setpoint q20 is 0.5, with a deviation Δ2 of 0.05; the start-up time t0 is 30 min; the controller 117 collects the real-time running time t of the separation-enhanced self-cascade refrigeration cycle system and receives the temperature data from the third temperature sensor 123 at the evaporator outlet. Temperature signal T3; The system judges the collected parameters. When t < 30 min, the separation-enhanced self-cascade refrigeration cycle system enters the start-up stage; when t ≥ 30 min, the separation-enhanced self-cascade refrigeration cycle system enters the low-temperature pull-down stage; after the separation-enhanced self-cascade refrigeration cycle system enters the low-temperature pull-down stage, the controller 117 receives the temperature signal T3 from the third temperature sensor 123 at the evaporator outlet. When the outlet temperature T3 > -58℃, the separation-enhanced self-cascade refrigeration cycle system is in the low-temperature pull-down stage; when T3 ≤ -58℃, the separation-enhanced self-cascade refrigeration cycle system is in the stable operation stage.

[0037] Start-up phase: First fan 115 and second fan 116 start; first electronic expansion valve 106 and second electronic expansion valve 107 are fully open; third electronic expansion valve 112 and fourth electronic expansion valve 114 are 50% fully open; first solenoid valve 108 is closed and second solenoid valve 109 is open; controller 117 receives pressure signal P3 from third pressure sensor 120 at compressor 101 outlet; if P3 > 2.82, controller 117 increases the opening of third electronic expansion valve 112 and fourth electronic expansion valve 114 by 5% each time; if P3 < 2.78, controller 117 decreases the opening of third electronic expansion valve 112 and third electronic expansion valve 114 by 1% each time; if 2.78 ≤ P3 ≤ 2.82, the opening of third electronic expansion valve 112 and third electronic expansion valve 114 remains unchanged.

[0038] During the low-temperature phase: the first fan 115 and the second fan 116 are started; the first solenoid valve 108 is closed and the second solenoid valve 109 is opened; the third electronic expansion valve 112 and the fourth electronic expansion valve 114 are at 30% full opening; the controller 117 calculates the dryness value q1 at the outlet of the condenser 102 and the first parameter value Z1 based on the pressure signal P1 from the first pressure sensor 118 and the first temperature sensor 121. The first parameter value Z1 is the concentration of low-boiling-point components in the refrigerant at the gas phase outlet of the first separator; the controller 117 calculates the dryness value q2 at the inlet of the second gas-liquid separator 110 and the second parameter value Z2 based on the pressure signal P2 from the second pressure sensor 119, the temperature signal T2 from the second temperature sensor 122, and simultaneously based on 1-Z1. The second parameter value Z2 is the concentration of low-boiling-point components in the refrigerant at the gas phase outlet of the second separator; the controller 117 adjusts the opening of the first electronic expansion valve 106 and the second electronic expansion valve 107 to make Z2>Z1 and 0.45≤q2≤0.55;

[0039] Steady-state operation phase: Controller 117 controls the start and stop of the first fan 115 and the second fan 116; controls the start and stop of the compressor 101; controls the opening and closing of the first solenoid valve 108 and the second solenoid valve 109; Controller 117 calculates the dryness value q1 of the condenser outlet and the first parameter value Z1 based on the pressure signal P1 of the first pressure sensor 118 and the first temperature sensor 121; Controller 117 calculates the dryness value q2 of the inlet of the second gas-liquid separator 110 and the second parameter value Z2 based on the pressure signal P2 of the second pressure sensor 119, the temperature signal T2 of the second temperature sensor 122, and 1-Z1; controls the opening degree of the first electronic expansion valve 106 and the second electronic expansion valve 107, so that 0.4≤q1≤0.6, 0.45≤q2≤0.55 and Z2>Z1.

Claims

1. A control method applied to a control system of a separate enhanced auto-cascade refrigeration cycle, characterized in that, The separation and enhancement self cascade refrigeration cycle is provided with a second condenser (104) at the gas phase outlet of a first separator (103); after condensation, it enters a first cascade heat exchanger (105) for supercooling; the liquid phase outlet of the first separator (103) enters the first cascade heat exchanger (105) after throttling, and then enters a second separator (110) after heat recovery; the control system comprises a first pressure sensor (118) and a first temperature sensor (121) arranged at the outlet of a first condenser (102), a second pressure sensor (119) and a second temperature sensor (122) arranged at the inlet of the second separator (110), a third pressure sensor (120) arranged at the outlet of a compressor (101), and a third temperature sensor (123) arranged at the outlet of an evaporator (113); a regulating mechanism connected with a first electronic expansion valve (106), a second electronic expansion valve (107), a third electronic expansion valve (112) and a fourth electronic expansion valve (114) respectively; a control mechanism connected with a first electromagnetic valve (108), a second electromagnetic valve (109), a first fan (115), a second fan (116) and the compressor (101) respectively; and an input end of a controller (117) connected with all the temperature and pressure sensors and an output end connected with all the regulating mechanisms and control structures; The control method comprises: Step 1: inputting preset parameters of the separation and enhancement self cascade refrigeration cycle system: a compressor outlet safety pressure P30 and a deviation value Δ3; an evaporator outlet temperature set value T30 and a deviation value Δ30; a condenser outlet dryness set value q10 and a deviation value Δ1; a second separator inlet dryness set value q20 and a deviation value Δ2; and a starting time t0; Step 2: the controller collects parameters in real time: a separation and enhancement self cascade refrigeration cycle system running time t and a temperature signal T3 of a third temperature sensor at the outlet of the evaporator; Step 3: the controller judges the collected parameters: when t < t0, the separation and enhancement self cascade refrigeration cycle system enters a starting stage; when t ≥ t0, the separation and enhancement self cascade refrigeration cycle system enters a low-temperature drawing stage; after the separation and enhancement self cascade refrigeration cycle system enters the low-temperature drawing stage, the controller receives the temperature signal T3 of the third temperature sensor at the outlet of the evaporator; when T3 > T30 + Δ30, the separation and enhancement self cascade refrigeration cycle system is in the low-temperature drawing stage; and when T3 ≤ T30 + Δ30, the separation and enhancement self cascade refrigeration cycle system is in a stable running stage. Step 4: Start-up phase: the controller (117) controls the first fan (115) and the second fan (116) to start, the first electronic expansion valve (106) and the second electronic expansion valve (107) to be full open, the third electronic expansion valve (112) and the fourth electronic expansion valve (114) to be 50% of full open, the first solenoid valve (108) to be closed, and the second solenoid valve (109) to be opened; the controller (117) receives the pressure signal P3 of the third pressure sensor (120) at the outlet of the compressor; the controller (117) controls the opening of the third electronic expansion valve (112) and the fourth electronic expansion valve (114) so that P3 < P30 + Δ3. Step 5: Low temperature pull-down phase: the controller (117) controls the first fan (115) and the second fan (116) to start, the first solenoid valve (108) to be closed and the second solenoid valve (109) to be opened, and the third electronic expansion valve (112) and the fourth electronic expansion valve (114) to be 30% of full open; the controller (117) calculates the outlet dryness value q1 of the condenser and the first parameter value Z1 according to the pressure signal P1 of the first pressure sensor (118) and the temperature signal T1 of the first temperature sensor (121), and the first parameter value Z1 is the concentration of low-boiling-point components in the gaseous phase outlet refrigerant of the first separator; the controller (117) calculates the inlet dryness value q2 of the second separator and the second parameter value Z2 according to the pressure signal P2 of the second pressure sensor (119) and the temperature signal T2 of the second temperature sensor (122), and the second parameter value Z2 is the concentration of low-boiling-point components in the gaseous phase outlet refrigerant of the second separator; the controller (117) adjusts the opening of the first electronic expansion valve (106) and the second electronic expansion valve (107) so that Z2 > Z1 and q20 - Δ2 ≤ q2 ≤ q20 + Δ2. Step 6: Steady state operation phase: the controller (117) controls the start-stop of the first fan (115) and the second fan (116), the start-stop of the compressor (101), and the on-off of the first solenoid valve (108) and the second solenoid valve (109); the controller (117) calculates the outlet dryness value q1 of the condenser and the first parameter value Z1 according to the pressure signal P1 of the first pressure sensor (118) and the temperature signal T1 of the first temperature sensor (121); the controller (117) calculates the inlet dryness value q2 of the second separator and the second parameter value Z2 according to the pressure signal P2 of the second pressure sensor (119) and the temperature signal T2 of the second temperature sensor (122); the opening of the first electronic expansion valve (106) and the second electronic expansion valve (107) is controlled; so that q10 - Δ1 ≤ q1 ≤ q10 + Δ1, q20 - Δ2 ≤ q2 ≤ q20 + Δ2, Z2 > Z1.

2. The control method according to claim 1, characterized by, In step 4, in the starting stage, the controller (117) receives the pressure signal P3 of the third pressure sensor (120) in real time, when P3>P30+Δ3, the controller (117) increases the opening of the third electronic expansion valve (112) and the fourth electronic expansion valve (114), when P3 3. The control method according to claim 1, characterized by, In step 5, in the pull-down temperature stage, when Z2≤Z1 or q2 4. The control method according to claim 1, characterized by, In step 6, in the stable running stage, when T3T30-Δ30, the controller (117) controls the compressor (101) to stop running, and then the controller (117) receives the temperature signal T3 of the evaporator outlet third temperature sensor (123); when T3T30+Δ30, the controller (117) controls the compressor (101) to start; in the stable running stage, when the external environment temperature changes fluctuates, the condenser outlet dryness value q1 will change; when q1q10-Δ1, the controller (117) controls the first fan (115) to start, the second fan (116) to close, the first electromagnetic valve (108) to close, and the second electromagnetic valve (109) to open; when q1q10+Δ1, the controller (117) controls the first fan (115) to start, the second fan (116) to start, the first electromagnetic valve (108) to open, and the second electromagnetic valve (109) to close; when q1q10+Δ1, the controller (117) receives the pressure signal P1 of the first pressure sensor (118) and the temperature signal T1 of the first temperature sensor (121), and calculates the condenser outlet dryness q1 and the first parameter value Z1; the controller (117) receives the pressure signal P2 of the second pressure sensor (119) and the temperature signal T2 of the second temperature sensor (122), and calculates the second separator inlet dryness q2 and the second parameter value Z2 according to 1-Z1; when Z2Z1 or q2q20-Δ2, the controller (117) reduces the opening of the first electronic expansion valve (106) and the second electronic expansion valve (107); when q2q20+Δ2, the controller (117) increases the opening of the first electronic expansion valve (106) and the second electronic expansion valve (107); when Z2>Z1 and q20-Δ2≤q2≤q20+Δ2, the controller (117) controls the opening of the first electronic expansion valve (106) and the second electronic expansion valve (107) to be unchanged; when q10-Δ1≤q1≤q10+Δ1, the controller (117) controls the opening of the first electronic expansion valve (106) and the second electronic expansion valve (107) to be unchanged.

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