Subcritical carbon dioxide cascade refrigeration system

Abstract

The application relates to the field of refrigeration, in particular to a subcritical carbon dioxide cascade refrigeration system, a high-temperature level liquid storage tank, a high-temperature level condenser and a high-temperature level compressor are sequentially connected to form a circulating loop to constitute a high-temperature level unit; a low-temperature level liquid storage tank, a cold air blower and a low-temperature level compressor are sequentially connected to form a circulating loop to constitute a low-temperature level unit, and the refrigerant in the circulating loop of the high-temperature level unit and the carbon dioxide in the circulating loop of the low-temperature level unit are heat-exchanged through a heat exchanger. The subcritical carbon dioxide cascade refrigeration system can maintain the pressure of the carbon dioxide in the low-temperature level unit within a set pressure range through the control of the opening degree of the high-temperature level electronic expansion valve and the control of the frequency of the high-temperature level compressor; meanwhile, the cold air blower can be accurately defrosted, and the occurrence of defrosting lag, incomplete defrosting and excessive defrosting can be avoided.

Description

Technical Field

[0001] This invention relates to the field of refrigeration, specifically a subcritical carbon dioxide cascade refrigeration system. Background Technology

[0002] During the cooling operation of a carbon dioxide cascade refrigeration system's evaporative cooler, the surface temperature of the coils and fins is below 0°C. Water vapor in the ambient air condenses on the heat exchanger surface, gradually forming a frost layer. As the frost layer accumulates, the cooler's heat transfer efficiency decreases significantly, while airflow resistance increases, severely impacting cooling performance. Therefore, defrosting is necessary. However, during defrosting, the low-temperature carbon dioxide circuit experiences heat transfer via electric heating, causing the carbon dioxide pressure in the closed piping to rise above the pressure tolerance of conventional refrigeration system piping. This poses a serious threat to critical components such as valves, flange seals, and welded joints.

[0003] Currently, the mainstream defrosting method for evaporative air coolers relies on preset fixed defrosting cycles and durations, meaning the defrosting process is automatically or manually triggered after the set cycle is reached. However, the actual operating environment of cold storage facilities varies significantly, and the fixed defrosting strategy has the following drawbacks under these complex conditions: 1. Delayed defrosting: When the frost layer is too thick and the preset cycle has not been reached, delayed defrosting will cause a sharp decrease in the heat transfer coefficient, a significant increase in air resistance, and even blockage of air ducts, resulting in deteriorated refrigeration performance. 2. Incomplete defrosting: If the amount of frost exceeds the processing capacity of the preset defrosting time, residual frost will continue to accumulate. 3. Over-defrosting: Defrostting before the frost layer reaches the critical thickness will waste energy and cause greater temperature fluctuations in the cold storage. It will also cause a greater increase in carbon dioxide temperature and excessive pressure. Therefore, these problems urgently need to be solved. Summary of the Invention

[0004] To avoid and overcome the technical problems existing in the prior art, this invention provides a subcritical carbon dioxide cascade refrigeration system. This invention, through the control of the opening degree of the high-temperature stage electronic expansion valve and the control of the high-temperature stage compressor frequency, ensures that the pressure of carbon dioxide in the low-temperature stage unit is always maintained within the set pressure range; simultaneously, it can precisely defrost the air cooler, avoiding delayed defrosting, incomplete defrosting, and excessive defrosting.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A subcritical carbon dioxide cascade refrigeration system comprises a high-temperature stage unit consisting of a high-temperature stage liquid receiver, a high-temperature stage condenser, and a high-temperature stage compressor connected in sequence; and a low-temperature stage unit consisting of a low-temperature stage liquid receiver, a cooler, and a low-temperature stage compressor connected in sequence. The refrigerant in the high-temperature stage unit's circulation loop and the carbon dioxide in the low-temperature stage unit's circulation loop exchange heat through a heat exchanger.

[0007] The bottom of the air cooler is equipped with a water receiving tray. The drain outlet of the water receiving tray has an annular positioning groove coaxially formed on the outer ring of the opening. A lead wire hole is formed on the positioning groove. The sensing cable of the water immersion sensor passes through the lead wire hole from the outside of the water receiving tray, enters the water receiving tray, circles the positioning groove once, and then leads out from the lead wire hole to the outside of the water receiving tray.

[0008] As a further aspect of the present invention: waterproof sealing treatment is applied to the lead hole, and the sensing cable of the water immersion sensor is bonded and fixed to the positioning groove.

[0009] As a further aspect of the present invention: a high-temperature stage electronic expansion valve and a low-temperature stage electronic expansion valve are respectively installed in the circulation loop of the high-temperature stage unit and the low-temperature stage unit to regulate the flow rate; a pressure sensor is installed at the inlet and outlet of the high-temperature stage compressor, and a pressure sensor is installed at the outlet of the low-temperature stage compressor.

[0010] As a further embodiment of the present invention: at least two sets of water receiving trays are provided, arranged sequentially along the length of the air cooler, and an electric heating module is provided on the surface of the water receiving tray, and a defrosting electric heating system is built into the air cooler.

[0011] As a further embodiment of the present invention, it also includes a PLC controller, which is connected to each pressure sensor, the high-temperature stage compressor, and the high-temperature stage electronic expansion valve, and adjusts the frequency of the high-temperature stage compressor and the opening degree of the high-temperature stage electronic expansion valve according to the pressure value monitored by the pressure sensor.

[0012] During defrosting or shutdown, the carbon dioxide exhaust pressure of the low-temperature stage unit is monitored by the pressure sensor, and the start-up and shutdown of the high-temperature stage compressor and the opening of the high-temperature stage electronic expansion valve are controlled to maintain the carbon dioxide exhaust pressure of the low-temperature stage unit within a safe range.

[0013] During the defrosting stage, the system determines whether defrosting and drainage are complete based on the signal from the water immersion sensor, and controls the end of defrosting accordingly.

[0014] As a further aspect of the present invention: during the defrosting stage, after the defrosting program is started, the defrosting electric heating system in the air cooler and the electric heating module on the water tray are turned on; the total defrosting time and the preset maximum defrosting time are detected in real time; when the total defrosting time exceeds the preset maximum defrosting time, the defrosting is forcibly terminated and an alarm is triggered.

[0015] The system detects the signal from the water immersion sensor in real time to calculate the duration of the continuous waterless state. When the duration of the continuous waterless state exceeds the preset waterless judgment threshold time, it determines that the drainage is complete, shuts down the defrosting electric heating system and electric heating module, and ends the defrosting process.

[0016] As a further aspect of the present invention: during the defrosting or shutdown phase, the high-temperature stage compressor and the low-temperature stage electronic expansion valve are closed, and the high-temperature stage electronic expansion valve is adjusted to the basic opening to balance the suction and discharge pressure of the high-temperature stage unit.

[0017] When the carbon dioxide exhaust pressure of the low-temperature stage unit is higher than the set pressure, and the suction and discharge pressure difference of the high-temperature stage compressor is less than the set pressure difference, the high-temperature stage compressor is started to cool down the low-temperature stage unit.

[0018] When the carbon dioxide exhaust pressure of the low-temperature stage unit is lower than the set pressure, and the suction pressure of the high-temperature stage compressor is lower than the set suction pressure, the high-temperature stage compressor stops, and the high-temperature stage electronic expansion valve is adjusted to the basic opening.

[0019] As a further aspect of the present invention: based on the comparison between the actual rise rate of the carbon dioxide side exhaust pressure of the low-temperature stage unit and the target rise rate, the opening of the high-temperature stage electronic expansion valve is dynamically adjusted within the range of the basic opening and the maximum opening.

[0020] Target rate of increase of carbon dioxide side exhaust pressure in cryogenic stage units for:

[0021] ;

[0022] in, This is the difference between the carbon dioxide exhaust pressure of the cryogenic stage unit and the upper pressure limit.

[0023] The time required for the intake and exhaust pressures of the high-temperature unit to reach equilibrium.

[0024] As a further aspect of the present invention: after the high-temperature compressor starts, the PLC controller performs frequency control on the high-temperature compressor, and the frequency of the high-temperature compressor is:

[0025] ;

[0026] express k The frequency of the high-temperature compressor at that moment;

[0027] in, , , All are proportionality coefficients;

[0028] for k Error term at time t; ;

[0029] for k The suction pressure of the high-temperature compressor at that moment;

[0030] The optimal evaporation pressure for the heat exchanger;

[0031] for k The integral term at time t; ;

[0032] for k－ The integral term at time 1;

[0033] The sampling period;

[0034] This is the differential of the inhalation pressure error;

[0035] ;

[0036] for k－ Error term at time 1.

[0037] As a further aspect of the present invention: after the high-temperature stage compressor starts, the opening degree of the high-temperature stage electronic expansion valve is dynamically adjusted according to the operating parameters. :

[0038] ;

[0039] in, Indicates the opening degree of the high-temperature electronic expansion valve;

[0040] This refers to the current operating frequency of the high-temperature stage compressor.

[0041] This is the maximum operating frequency of the high-temperature stage compressor;

[0042] This refers to the superheat of the high-temperature compressor.

[0043] The target superheat for the high-temperature stage compressor;

[0044] This represents the measured condensation temperature on the condenser side of the heat exchanger.

[0045] This refers to the rated condensing temperature on the condensing side of the heat exchanger.

[0046] The ambient temperature at which the refrigeration system operates;

[0047] This is the basic opening degree for a high-temperature electronic expansion valve;

[0048] , , All are proportionality coefficients;

[0049] When 24℃≤ At ≤38℃:

[0050] ;

[0051] When 38℃ < ,and At ≤60Hz:

[0052] ;

[0053] When 38℃ < ,and >60Hz:

[0054] ;

[0055] when At <24℃:

[0056] ;

[0057] The value range is: 15 / total number of steps of high temperature electronic expansion valve ~ 30 / total number of steps of high temperature electronic expansion valve;

[0058] The value range is: 30 / total number of steps of high temperature electronic expansion valve ~ 60 / total number of steps of high temperature electronic expansion valve.

[0059] Compared with the prior art, the beneficial effects of the present invention are:

[0060] 1. During the defrosting or shutdown phase, this invention controls the start-up and shutdown of the high-temperature stage compressor and the opening of the high-temperature stage electronic expansion valve by monitoring the pressure, ensuring that the carbon dioxide exhaust pressure of the low-temperature stage unit is always maintained within the safe threshold, avoiding system overpressure due to pressure accumulation. At the same time, the water immersion sensor in the water tray, combined with the judgment of defrosting duration and waterless status, enables precise monitoring of the defrosting drainage process. It can monitor the drainage status in real time, avoiding water accumulation and freezing to block the pipes due to insufficient drainage time, and also avoiding excessive heating during the drainage process, which would cause the storage temperature to rise and increase system energy consumption. It can also perform precise defrosting of the air cooler, avoiding defrosting delay, incomplete defrosting, and excessive defrosting.

[0061] 2. The multiple cold water trays in this invention reduce the water flow path and shorten drainage time. The annular positioning groove design of the drain outlet of the water tray, combined with the lead hole for introducing the sensor cable, fixes the sensor cable around the groove, ensuring that the sensor detection area can completely cover the drainage path and improve the reliability of detection. Waterproof sealing and adhesive fixing further improve the reliability of detection, preventing leakage and also preventing the sensor cable from shaking after being impacted by water flow. The electric heating module on the water tray, together with the defrosting electric heating system built into the fan, forms an efficient and reliable defrosting system.

[0062] 3. This invention utilizes multiple pressure sensors in conjunction with a PLC controller and an electronic expansion valve to comprehensively calculate the opening degree of the high-temperature electronic expansion valve based on operating parameters such as compressor frequency, superheat, condensing temperature, and ambient temperature. This ensures that the system always operates within its high-efficiency range, reducing system energy consumption. The PLC control integrates multiple functions such as pressure protection, defrosting management, and capacity adjustment. It automatically switches control modes according to normal cooling, defrosting, and standby operating conditions, enabling the system to respond quickly under different operating conditions.

[0063] 4. This invention uses a water immersion sensor to directly feed back the drainage completion signal, enabling defrosting to end on demand. This avoids insufficient or excessive defrosting, saving energy while ensuring heat exchange efficiency and reducing temperature fluctuations in the storage area. The defrosting duration is precisely matched to actual needs. Simultaneously, using a high-temperature unit as a carbon dioxide pressure maintenance unit during system shutdown enhances system safety, reduces space occupation, and lowers system costs. Attached Figure Description

[0064] Figure 1 This is a schematic diagram of the structure of the present invention.

[0065] Figure 2 This is a schematic diagram of the structure of the cooling fan in this invention.

[0066] Figure 3 This is a schematic diagram of the water receiving tray in this invention.

[0067] In the picture:

[0068] 1. High-temperature stage unit; 11. High-temperature stage liquid storage tank; 12. High-temperature stage condenser;

[0069] 13. High-temperature compressor; 14. High-temperature electronic expansion valve;

[0070] 2. Low-temperature stage unit; 21. Low-temperature stage liquid storage tank; 22. Air cooler;

[0071] 221. Water receiving tray; 222. Drain outlet; 223. Positioning groove;

[0072] 224. Lead wire hole; 225. Water immersion sensor;

[0073] 23. Cryogenic stage compressor; 24. Cryogenic stage electronic expansion valve; 3. Heat exchanger. Detailed Implementation

[0074] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0075] Please see Figures 1-3 In this embodiment of the invention, a subcritical carbon dioxide cascade refrigeration system includes a high-temperature stage unit 1, a low-temperature stage unit 2, and a heat exchanger 3. The high-temperature stage unit 1 consists of a high-temperature stage liquid receiver 11, a high-temperature stage condenser 12, and a high-temperature stage compressor 13 connected sequentially through pipelines to form a closed refrigerant circulation loop. The low-temperature stage unit 2 consists of a low-temperature stage liquid receiver 21, a low-temperature stage compressor 23, and a cooler 22 connected sequentially through pipelines to form a closed carbon dioxide circulation loop. The refrigerant in the high-temperature stage unit 1 and the carbon dioxide in the low-temperature stage unit 2 exchange heat in the heat exchanger 3 to achieve a cascade refrigeration cycle.

[0076] To precisely control system flow, a high-temperature electronic expansion valve 14 is installed in the circulation loop of high-temperature stage unit 1, and a low-temperature electronic expansion valve 24 is installed in the circulation loop of low-temperature stage unit 2. Pressure sensors are installed at both the inlet and outlet of high-temperature stage compressor 13, and a pressure sensor is installed at the outlet of low-temperature stage compressor 23, for real-time monitoring of system pressure status. The system is also equipped with a storage temperature sensor to monitor ambient temperature.

[0077] The system also includes a PLC controller, which is electrically connected to each pressure sensor, the high-temperature compressor 13, and the high-temperature electronic expansion valve 14. Based on the data collected by the pressure sensors, the PLC controller adjusts the operating frequency of the high-temperature compressor 13 and the opening degree of the high-temperature electronic expansion valve 14 in real time, thereby achieving optimized control of the system's operating conditions.

[0078] In the low-temperature stage unit 2, a water collection tray 221 is installed at the bottom of the air cooler 22 to collect condensate or defrost water generated during operation. The water collection tray 221 is provided with a drain outlet 222 for draining accumulated water. To improve the waterproof safety performance of the system, an annular positioning groove 223 is coaxially formed on the outer ring of the drain outlet 222 on the surface of the water collection tray 221, and a lead wire hole 224 is formed on the positioning groove 223.

[0079] The sensing cable of the water immersion sensor 225 passes through the lead hole 224 from the outside of the water receiving tray 221 and enters the inside of the water receiving tray 221. After circling around the positioning groove 223, it passes out again from the same lead hole 224 to the outside of the water receiving tray 221, thus forming a detection loop inside the water receiving tray 221. Once the water level comes into contact with any part of the cable, the water immersion sensor 225 can trigger an alarm signal.

[0080] To improve waterproof sealing performance, the lead hole 224 is waterproofed and sealed, for example, using sealant or a waterproof connector. Simultaneously, the sensing cable of the water immersion sensor 225 is bonded to the positioning groove 223 to prevent cable displacement or loosening. A thin sheet, made of stainless steel or low-temperature resistant engineering plastic, can be placed over the positioning groove 223 and secured with a low-temperature resistant adhesive. The width of the sheet is approximately equal to the width of the groove, and its position avoids the lead hole 224 to prevent the water immersion sensor 225 from moving under the impact of gravity and water flow.

[0081] In this embodiment, at least two sets of water collection trays 221 are provided along the length of the air cooler 22 to accommodate longer air coolers and improve water collection capacity. Each water collection tray 221 is equipped with an electric heating module on its surface, and the air cooler 22 is equipped with a defrosting electric heating system inside. Heating prevents freezing and accelerates defrosting to ensure smooth drainage.

[0082] When the system enters the defrosting stage or is shut down, the pressure maintenance mode is automatically triggered:

[0083] Initialization and standby state: The low-temperature stage electronic expansion valve is closed, and the high-temperature stage electronic expansion valve is opened to a basic opening degree, such as 10%, to balance the suction and discharge pressures when the high-temperature stage compressor is stopped.

[0084] The following two conditions must be met simultaneously for a high-temperature compressor to start:

[0085] The carbon dioxide discharge pressure of the low-temperature stage unit is higher than the set pressure, for example, 3 MPa. This pressure is set according to the system's pressure-bearing capacity. Simultaneously, the suction and discharge pressure difference of the high-temperature stage unit is less than the set value, for example, 0.15 MPa, ensuring a smooth compressor start-up load. When the above start-up conditions are met, the high-temperature stage compressor starts and operates as a pressure maintaining unit.

[0086] The following two conditions must be met simultaneously for a high-temperature compressor to shut down:

[0087] When the carbon dioxide discharge pressure of the low-temperature stage unit is lower than the set pressure, for example, 0.02 MPa, and the suction pressure of the high-temperature stage compressor is lower than the set suction pressure, for example, 0.02 MPa, the high-temperature stage compressor stops, and the high-temperature stage electronic expansion valve is adjusted to the basic opening, for example, 10%, to balance the suction and discharge pressures inside the system and prepare for the next start-up.

[0088] Based on the comparison between the actual rise rate of the carbon dioxide side exhaust pressure of the low-temperature stage unit and the target rise rate, the opening of the high-temperature stage electronic expansion valve 14 is dynamically adjusted within the range of the basic opening and the maximum opening.

[0089] Target rate of increase of carbon dioxide side exhaust pressure in cryogenic stage units for:

[0090] ;

[0091] in, This is the difference between the carbon dioxide exhaust pressure of the cryogenic stage unit and the upper pressure limit.

[0092] The time required for the intake and exhaust pressures of the high-temperature unit to reach equilibrium.

[0093] If the actual rate of pressure change is higher than the target rate of increase If the pressure rises too quickly, increase the opening of the high-temperature electronic expansion valve 14, for example, adjust it to 30%, to accelerate the internal pressure balance of the high-temperature unit and ensure that the compressor can respond and start in a timely manner.

[0094] If the actual rate of pressure change is lower than or equal to the target rate of increase If the opening is maintained, the current opening of the high-temperature electronic expansion valve 14 will be maintained or the opening will be adjusted back to the base opening.

[0095] The opening range of the high-temperature electronic expansion valve 14 can be limited to between 10% and 30% to prevent excessive opening from causing liquid to be drawn into the air during the next start-up of the high-temperature compressor, thus protecting the compressor.

[0096] The operating frequency control of the high-temperature compressor is as follows:

[0097] Initial state: After the compressor starts, it runs at its own starting frequency (e.g., 45Hz), which is between the preset highest frequency (e.g., 90Hz) and the lowest frequency (e.g., 30Hz).

[0098] Based on pressure difference Initial adjustments: Real-time monitoring of the difference between the carbon dioxide exhaust pressure and the upper pressure limit of the cryogenic stage unit. .

[0099] For example, when A decrease indicates an increase in the heat load of the high-temperature stage unit and an increase in the suction superheat. At this time, increase the opening of the high-temperature stage electronic expansion valve and linearly increase the compressor frequency to increase the cooling capacity until... The trend is shifting towards increasing. The refrigerant flow rate is matched to system requirements by controlling the opening of the high-temperature electronic expansion valve.

[0100] The opening degree of the high-temperature electronic expansion valve is calculated using the following formula:

[0101] ;

[0102] in, This indicates the opening degree of the high-temperature electronic expansion valve 14;

[0103] This is the current operating frequency of the high-temperature stage compressor 13;

[0104] This is the maximum operating frequency of the high-temperature compressor 13;

[0105] The superheat of the high-temperature compressor 13;

[0106] The target superheat of the high-temperature stage compressor 13;

[0107] The measured condensing temperature is the condensing temperature on the condensing side of heat exchanger 3.

[0108] This is the rated condensing temperature of the condensing side of heat exchanger 3;

[0109] The ambient temperature at which the refrigeration system operates;

[0110] This is the basic opening degree of the high-temperature electronic expansion valve 14;

[0111] , , All are proportionality coefficients;

[0112] When 24℃≤ At ≤38℃:

[0113] ;

[0114] When 38℃ < ,and At ≤60Hz:

[0115] ;

[0116] When 38℃ < ,and >60Hz:

[0117] ;

[0118] when At <24℃:

[0119] ;

[0120] The value range is: 15 / total number of steps of high temperature electronic expansion valve ~ 30 / total number of steps of high temperature electronic expansion valve;

[0121] The value range is: 30 / total number of steps of high temperature electronic expansion valve ~ 60 / total number of steps of high temperature electronic expansion valve.

[0122] The compressor frequency is controlled using PID control. The frequency control calculation method is as follows:

[0123] The frequency of the high-temperature compressor 13 is controlled by the following calculation:

[0124] ;

[0125] express k The high-temperature compressor operates at a frequency of 13 at this time.

[0126] in, , , All are proportionality coefficients; in this embodiment Set it to 0.5. Set to 0, Set to 0.01;

[0127] for k Error term at time t; ;

[0128] for k Inhalation pressure at any given moment;

[0129] The optimal evaporation pressure;

[0130] for k The integral term at time t; Start-up time =0,

[0131] for k－ The integral term at time 1;

[0132] The sampling period;

[0133] This is the differential of the inhalation pressure error;

[0134]

[0135] for k－ Error term at time 1.

[0136] High-temperature compressor speed n Set to:

[0137] n = 60f(1-s) / p ;

[0138] in, This is the current operating frequency of the high-temperature stage compressor 13;

[0139] s Slippage;

[0140] p This represents the number of pole pairs of the motor.

[0141] This is used to adjust the compressor speed, preventing the high-temperature compressor from rapidly dropping its suction pressure to the starting suction pressure threshold due to excessive speed, thus causing it to shut down before the carbon dioxide system has been sufficiently cooled.

[0142] The defrosting process is coordinated with pressure maintenance, and the control procedure is as follows:

[0143] Initiate defrosting: Triggers the defrosting program, simultaneously activating the defrosting electric heater and the water tray electric heater.

[0144] Safety timer monitoring: Starts timing the total defrosting time and compares it with the preset maximum defrosting time;

[0145] When the total defrosting time exceeds the preset maximum defrosting time, it is determined to be a timeout, and all defrosting operations are immediately and forcibly terminated, the electric heating is turned off, and an alarm is triggered.

[0146] When the total defrosting time is less than or equal to the preset maximum defrosting time, proceed to the next step of drainage status determination.

[0147] When determining the drainage status, the water immersion sensor signal is read in real time, and the time of continuous waterlessness on the water receiving tray is calculated to determine whether the preset waterlessness judgment threshold time has been reached.

[0148] If the water tray remains dry for a period of time that is greater than or equal to the preset water-free threshold time, the defrosting electric heating system and the electric heating module on the water tray will be turned off in sequence, and the defrosting process will end normally.

[0149] If the time that the water tray remains dry is less than the preset waterless judgment threshold time, the safety timed monitoring and drainage status judgment will continue to be performed in a loop until the termination condition is met.

[0150] The basic principles of this application have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in this application are merely examples and not limitations, and should not be considered as essential features of each embodiment of this application. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the application to the necessity of employing the aforementioned specific details for implementation.

[0151] The block diagrams of devices, apparatuses, devices, and systems involved in this application are merely illustrative examples and are not intended to require or imply that they must be connected, arranged, or configured in the manner shown in the block diagrams. As those skilled in the art will recognize, these devices, apparatuses, devices, and systems can be connected, arranged, and configured in any manner. Words such as “comprising,” “including,” “having,” etc., are open-ended terms meaning “including but not limited to,” and are used interchangeably with them. The terms “or” and “and” as used herein refer to the terms “and / or,” and are used interchangeably with them unless the context clearly indicates otherwise. The term “such as” as used herein refers to the phrase “such as but not limited to,” and is used interchangeably with it.

Claims

1. A subcritical carbon dioxide cascade refrigeration system, characterized in that, A high-temperature stage liquid storage tank (11), a high-temperature stage condenser (12), and a high-temperature stage compressor (13) are connected in sequence to form a circulation loop that constitutes a high-temperature stage unit (1); a low-temperature stage liquid storage tank (21), a cooler (22), and a low-temperature stage compressor (23) are connected in sequence to form a circulation loop that constitutes a low-temperature stage unit (2). The refrigerant in the circulation loop of the high-temperature stage unit (1) and the carbon dioxide in the circulation loop of the low-temperature stage unit (2) exchange heat through a heat exchanger (3). The bottom of the air cooler (22) is provided with a water receiving tray (221). The drain outlet (222) of the water receiving tray (221) has an annular positioning groove (223) coaxially opened on the outer ring of the opening. The positioning groove (223) has a lead wire hole (224). The sensing cable of the water immersion sensor (225) passes through the lead wire hole (224) from the outside of the water receiving tray (221) and enters the water receiving tray (221). After passing around the positioning groove (223) once, it is led out from the lead wire hole (224) to the outside of the water receiving tray (221). At least two sets of water receiving trays (221) are provided, arranged sequentially along the length of the air cooler (22). The surface of the water receiving tray (221) is provided with an electric heating module, and the air cooler (22) has a built-in defrosting electric heating system. It also includes a PLC controller, which is connected to each pressure sensor, the high-temperature compressor (13) and the high-temperature electronic expansion valve (14), and adjusts the frequency of the high-temperature compressor (13) and the opening degree of the high-temperature electronic expansion valve (14) according to the pressure value monitored by the pressure sensor. During defrosting or shutdown, the carbon dioxide exhaust pressure of the low-temperature stage unit is monitored by the pressure sensor, and the start-up and shutdown of the high-temperature stage compressor (13) and the opening of the high-temperature stage electronic expansion valve (14) are controlled to maintain the carbon dioxide exhaust pressure of the low-temperature stage unit within a safe range. During the defrosting stage, the defrosting drainage is determined based on the signal from the water immersion sensor (225), and the end of defrosting is controlled accordingly.

2. The subcritical carbon dioxide cascade refrigeration system according to claim 1, characterized in that, Waterproof sealing treatment is applied to the lead hole (224), and the sensing cable of the water immersion sensor (225) is bonded and fixed to the positioning groove (223).

3. The subcritical carbon dioxide cascade refrigeration system according to claim 1, characterized in that, The high-temperature stage unit (1) and the low-temperature stage unit (2) are respectively equipped with a high-temperature stage electronic expansion valve (14) and a low-temperature stage electronic expansion valve (24) to regulate the flow rate; Pressure sensors are installed at both the inlet and outlet of the high-temperature compressor (13), and a pressure sensor is installed at the outlet of the low-temperature compressor (23).

4. The subcritical carbon dioxide cascade refrigeration system according to claim 1, characterized in that, During the defrosting stage, after starting the defrosting program, the defrosting electric heating system in the air cooler (22) and the electric heating module on the water receiving pan (221) are turned on; the total defrosting time and the preset maximum defrosting time are detected in real time. When the total defrosting time exceeds the preset maximum defrosting time, the defrosting is forcibly terminated and an alarm is triggered. The signal of the water immersion sensor (225) is detected in real time to calculate the duration of the continuous waterless state. When the duration of the continuous waterless state exceeds the preset waterless judgment threshold time, the drainage is determined to be completed, the defrosting electric heating system and the electric heating module are turned off, and the defrosting process ends.

5. A subcritical carbon dioxide cascade refrigeration system according to claim 1, characterized in that, During defrosting or shutdown, the high-temperature stage compressor (13) and the low-temperature stage electronic expansion valve (24) are closed, and the high-temperature stage electronic expansion valve (14) is adjusted to the basic opening to balance the suction and discharge pressure of the high-temperature stage unit (1). When the carbon dioxide exhaust pressure of the low-temperature stage unit is higher than the set pressure, and the suction and discharge pressure difference of the high-temperature stage compressor (13) is less than the set pressure difference, the high-temperature stage compressor (13) is started to cool down the low-temperature stage unit (2). When the carbon dioxide exhaust pressure of the low-temperature stage unit is lower than the set pressure, and the suction pressure of the high-temperature stage compressor (13) is lower than the set suction pressure, the high-temperature stage compressor (13) stops, and the high-temperature stage electronic expansion valve (14) is adjusted to the basic opening.

6. The subcritical carbon dioxide cascade refrigeration system according to claim 1, characterized in that, Based on the comparison between the actual rise rate of the carbon dioxide side exhaust pressure of the low-temperature stage unit and the target rise rate, the opening of the high-temperature stage electronic expansion valve (14) is dynamically adjusted within the range of the basic opening and the maximum opening. Target rate of increase of carbon dioxide side exhaust pressure in cryogenic stage units for: in, This is the difference between the carbon dioxide exhaust pressure of the cryogenic stage unit and the upper pressure limit. The time required for the intake and exhaust pressures of the high-temperature unit to reach equilibrium.

7. A subcritical carbon dioxide cascade refrigeration system according to claim 1, characterized in that, When the high-temperature compressor (13) starts, the PLC controller performs frequency control on the high-temperature compressor (13). The frequency of the high-temperature compressor (13) is: ； express k The frequency of the high-temperature stage compressor (13) at that moment; in, , , All are proportionality coefficients; for k Error term at time t; ; for k The suction pressure of the high-temperature compressor (13) at that moment; The optimal evaporation pressure for heat exchanger (3); for k The integral term at time t; ; for k－ The integral term at time 1; The sampling period; This is the differential of the inhalation pressure error; for k－ Error term at time 1.

8. A subcritical carbon dioxide cascade refrigeration system according to claim 1, characterized in that, After the high-temperature stage compressor starts, the opening degree of the high-temperature stage electronic expansion valve (14) is dynamically adjusted according to the operating parameters. : in, Indicates the opening degree of the high-temperature electronic expansion valve (14); The current operating frequency of the high-temperature compressor (13); The maximum operating frequency of the high-temperature compressor (13); The superheat of the high-temperature compressor (13); The target superheat of the high-temperature stage compressor (13); The measured condensation temperature on the condenser side of heat exchanger (3); The rated condensing temperature of the condensing side of the heat exchanger (3); The ambient temperature at which the refrigeration system operates; The basic opening degree of the high-temperature electronic expansion valve (14); , , All are proportionality coefficients; When 24℃≤ At ≤38℃: ； When 38℃ < ,and At ≤60Hz: ； When 38℃ < ,and >60Hz: ； when At <24℃: ； The value range is: 15 / total number of steps of high temperature electronic expansion valve ~ 30 / total number of steps of high temperature electronic expansion valve; The value range is: 30 / total number of steps of high temperature electronic expansion valve ~ 60 / total number of steps of high temperature electronic expansion valve.

Images