CN111023611A - Carbon dioxide refrigerating system capable of cooling in stages and control method thereof - Google Patents
Carbon dioxide refrigerating system capable of cooling in stages and control method thereof Download PDFInfo
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- CN111023611A CN111023611A CN201911291316.XA CN201911291316A CN111023611A CN 111023611 A CN111023611 A CN 111023611A CN 201911291316 A CN201911291316 A CN 201911291316A CN 111023611 A CN111023611 A CN 111023611A
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
- F25B9/002—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the refrigerant
- F25B9/008—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the refrigerant the refrigerant being carbon dioxide
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B41/00—Fluid-circulation arrangements
- F25B41/20—Disposition of valves, e.g. of on-off valves or flow control valves
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B49/00—Arrangement or mounting of control or safety devices
- F25B49/02—Arrangement or mounting of control or safety devices for compression type machines, plants or systems
Abstract
The invention discloses a carbon dioxide refrigeration system capable of cooling in stages and a control method thereof, and relates to the technical field of carbon dioxide refrigeration. The system comprises a low-temperature-level loop and a medium-temperature-level loop, wherein a first electromagnetic valve group is arranged in the low-temperature-level loop and comprises a first electromagnetic valve and a second electromagnetic valve which are connected in parallel, and the drift diameter of the first electromagnetic valve is larger than that of the second electromagnetic valve; and a second electromagnetic valve group is arranged in the medium-temperature stage loop, the second electromagnetic valve group comprises a third electromagnetic valve and a fourth electromagnetic valve which are connected in parallel, and the drift diameter of the third electromagnetic valve is greater than that of the fourth electromagnetic valve. The method comprises the steps that when the system is switched to the refrigeration mode, the electromagnetic valve with the smaller drift diameter in the electromagnetic valve group is opened, and after the pressure difference between the two sides of the electromagnetic valve is reduced to the preset pressure difference, the electromagnetic valve with the larger drift diameter is opened, so that the refrigeration mode switching is completed. The invention can improve the overall energy efficiency of the system, reduce the equipment cost and the operation cost, and solve the problem of impact on pipelines and valves when the system is subjected to temperature change.
Description
Technical Field
The invention relates to the technical field of carbon dioxide refrigeration, in particular to a carbon dioxide refrigeration system capable of cooling in stages and a control method thereof.
Background
Carbon dioxide refrigeration has obvious energy efficiency advantage in a low-temperature section of-25 ℃ to-45 ℃, has excellent safety and environmental protection, and gradually becomes a preferred refrigerant for low-temperature section freezing and refrigeration.
The carbon dioxide refrigeration is widely applied to the field of quick freezing of a refrigeration house or a quick freezer, and the service temperature of the refrigeration house or the quick freezer is very low, generally about minus 30 ℃ to minus 40 ℃. Taking a refrigeration house as an example, a carbon dioxide refrigeration system of the refrigeration house generally comprises a carbon dioxide loop, a carbon dioxide unit and an air cooler, wherein the carbon dioxide loop generally comprises a liquid reservoir, a pipeline and various valves on the pipeline, low-temperature liquid carbon dioxide is stored in the liquid reservoir, and the carbon dioxide loop is communicated with the air cooler. The air cooler is arranged in the refrigerator and comprises an evaporator and a high-speed fan, the evaporator comprises a heat exchange pipe communicated with the carbon dioxide loop, and an air outlet of the high-speed fan faces the evaporator.
When the carbon dioxide return circuit switched on, microthermal liquid carbon dioxide can flow in the heat exchange tube, with the outside air heat transfer of heat exchange tube, makes the air temperature reduce and forms air conditioning, under blowing of high-speed fan, air conditioning can flow to the freezer everywhere, makes the temperature reduction in the freezer. The temperature of the carbon dioxide after heat exchange can be increased, then the carbon dioxide flows back to the liquid storage device, the carbon dioxide is compressed, condensed and throttled by the carbon dioxide unit, so that the carbon dioxide is recovered to be in a low-temperature state, and the carbon dioxide enters the air cooler through the carbon dioxide loop to be reused.
The carbon dioxide refrigerating system applied to the quick-freezing field has the following defects: firstly, because the use temperature of a refrigeration house or a quick freezer is very low, the efficiency of a compressor in a carbon dioxide unit is very low at the moment, and the overall operation efficiency of a refrigeration system is low; secondly, the cooling interval in the quick-freezing field is large, the temperature difference is often more than 30k, and the temperature is required to be quickly cooled and quickly passes through an ice crystal area, so that all parts of the whole carbon dioxide refrigerating system are configured according to the requirement of evaporation temperature of-40 ℃ to-45 ℃, the equipment cost is high, the operation cost and the operation energy efficiency are higher than the actual requirement during operation, and the existing carbon dioxide refrigerating system is unreasonable and uneconomical.
In conclusion, in the prior art, the carbon dioxide refrigeration system applied to the field of quick freezing has the defects of low energy efficiency, high equipment cost and high operation cost.
Disclosure of Invention
In order to solve the problems of the prior art, an object of the present invention is to provide a carbon dioxide refrigeration system with a staged temperature reduction, and an object of the present invention is to provide a method for controlling the carbon dioxide refrigeration system. The invention is applied to the field of quick freezing, can improve the overall energy efficiency of the system, reduce the equipment cost and the operation cost and solve the problem of impact on pipelines and valves when the system is subjected to temperature change under the condition of meeting the requirements of temperature reduction and quick freezing.
The invention relates to a carbon dioxide refrigerating system capable of cooling in stages, which comprises an air cooler, a low-temperature stage module and a medium-temperature stage module; the low-temperature-level module is communicated with the air cooler to form a low-temperature-level loop; the medium-temperature-stage module is communicated with the air cooler to form a medium-temperature-stage loop; the low-temperature-level loop and the medium-temperature-level loop are both used for circulating carbon dioxide, and the temperature of the carbon dioxide in the low-temperature-level loop is lower than that of the carbon dioxide in the medium-temperature-level loop;
the low-temperature-level loop comprises a low-temperature-level inflow section and a low-temperature-level air return section, and a first electromagnetic valve group used for controlling the on-off of the low-temperature-level air return section is arranged in the low-temperature-level air return section; the first electromagnetic valve group comprises a first electromagnetic valve and a second electromagnetic valve which are connected in parallel, and the drift diameter of the first electromagnetic valve is larger than that of the second electromagnetic valve;
the medium-temperature stage loop comprises a medium-temperature stage inflow section and a medium-temperature stage air return section, a second electromagnetic valve group used for controlling the on-off of the medium-temperature stage inflow section is arranged in the medium-temperature stage inflow section, the second electromagnetic valve group comprises a third electromagnetic valve and a fourth electromagnetic valve which are connected in parallel, and the drift diameter of the third electromagnetic valve is larger than that of the fourth electromagnetic valve.
Preferably, the air cooler includes a refrigerant inlet and a refrigerant outlet, and the low-temperature stage inflow section and the medium-temperature stage inflow section are respectively communicated with the refrigerant inlet; the refrigerant outlet is respectively communicated with the low-temperature stage gas return section and the medium-temperature stage gas return section, a first stop valve is arranged at the refrigerant inlet, and a second stop valve is arranged at the refrigerant outlet.
Preferably, the low-temperature stage module comprises a third stop valve, a fourth stop valve, a first filter, a first expansion valve, a first one-way valve, a fifth solenoid valve and a circulation barrel; the circulating barrel is used for storing low-temperature carbon dioxide at the temperature of minus 40 ℃ to minus 30 ℃;
the outlet of the circulating barrel, the third stop valve, the first filter, the fifth electromagnetic valve, the first expansion valve and the first one-way valve are communicated in sequence to form the low-temperature stage inflow section;
the first electromagnetic valve group, the fourth stop valve and the inlet of the circulating barrel are sequentially communicated to form the low-temperature-level air return section.
Preferably, the medium-temperature stage module comprises a fifth stop valve, a second filter, a second expansion valve, a sixth electromagnetic valve, a second one-way valve and a liquid storage device; the liquid storage device is used for storing medium-temperature carbon dioxide with the temperature of-10 ℃ to-5 ℃;
an outlet of the liquid storage device, the fifth stop valve, the second filter, the second electromagnetic valve group and the second expansion valve are communicated in sequence to form the medium-temperature-stage inflow section;
and the sixth electromagnetic valve, the second one-way valve and the inlet of the liquid reservoir are communicated in sequence to form the medium-temperature-stage air return section.
Preferably, the first solenoid valve, the third solenoid valve, the fifth solenoid valve and the sixth solenoid valve are all servo solenoid valves; the second electromagnetic valve and the fourth electromagnetic valve are electric two-way valves.
A control method of a carbon dioxide refrigeration system with staged temperature reduction is applied to the carbon dioxide refrigeration system, and the carbon dioxide refrigeration system comprises a medium-temperature refrigeration mode and a low-temperature refrigeration mode;
in the medium-temperature refrigeration mode, the low-temperature-level loop is closed, the medium-temperature-level loop is opened, and carbon dioxide flows in the medium-temperature-level loop;
in the low-temperature refrigeration mode, the low-temperature-level loop is opened, the medium-temperature-level loop is closed, and carbon dioxide circulates in the low-temperature-level loop;
when the medium-temperature refrigeration mode needs to be switched to the low-temperature refrigeration mode, the medium-temperature stage inflow section and the medium-temperature stage return air section are closed in sequence; then, a second electromagnetic valve is opened firstly, when the pressure difference between the front and the back of the second electromagnetic valve reaches a preset pressure difference, a first electromagnetic valve and a fifth electromagnetic valve are opened, and the carbon dioxide refrigeration system enters a low-temperature refrigeration mode from a medium-temperature refrigeration mode;
when the low-temperature refrigeration mode needs to be switched to the medium-temperature refrigeration mode, the low-temperature-level inflow section and the low-temperature-level return section are closed in sequence; and then, opening a fourth electromagnetic valve, and opening a third electromagnetic valve and a sixth electromagnetic valve when the front-back pressure difference of the fourth electromagnetic valve reaches a preset pressure difference, wherein the carbon dioxide refrigeration system enters a medium-temperature refrigeration mode from a low-temperature refrigeration mode.
Preferably, in the medium-temperature refrigeration mode, medium-temperature carbon dioxide flows out from an outlet of the liquid reservoir, sequentially passes through a fifth stop valve, a second filter, a second electromagnetic valve group, a second expansion valve and a first stop valve, and enters the air cooler to exchange heat with the refrigeration house, so that the temperature of the refrigeration house is reduced; the medium-temperature carbon dioxide after heat exchange sequentially passes through a second stop valve, a sixth electromagnetic valve and a second one-way valve and then flows back to the inlet of the liquid storage device;
in the low-temperature refrigeration mode, low-temperature carbon dioxide flows out from an outlet of the circulating barrel, sequentially passes through a third stop valve, a first filter, a fifth electromagnetic valve, a first expansion valve, a first one-way valve and a first stop valve, and enters an air cooler to exchange heat with a refrigeration house, so that the temperature of the refrigeration house is reduced; the low-temperature carbon dioxide after heat exchange sequentially passes through a second stop valve, a first electromagnetic valve group and a fourth stop valve and then flows back to the inlet of the circulating barrel;
the temperature of the medium-temperature carbon dioxide is-10 ℃ to-5 ℃, and the temperature of the low-temperature carbon dioxide is-40 ℃ to-30 ℃.
Preferably, when the temperature of the refrigeration house is higher than-5 ℃, the carbon dioxide refrigeration system is in a medium-temperature refrigeration mode, and when the temperature of the refrigeration house is-10 ℃ to-5 ℃, the carbon dioxide refrigeration system is switched from the medium-temperature refrigeration mode to a low-temperature refrigeration mode.
Preferably, a temperature sensor is arranged in the refrigeration house to detect the temperature of the refrigeration house, or the temperature of the refrigeration house is controlled by controlling the refrigeration time.
Preferably, said preset pressure difference is less than or equal to 1.5 bar.
The carbon dioxide refrigeration system with the staged temperature reduction and the control method thereof have the advantages that the carbon dioxide refrigeration system is provided with the medium-temperature-level loop and the low-temperature-level loop, the medium-temperature refrigeration mode and the low-temperature refrigeration mode are included, when in quick freezing, the system firstly enters the medium-temperature refrigeration mode, carbon dioxide circulates in the medium-temperature-level loop, and the temperature in a cold storage is reduced through the medium-temperature refrigeration mode. When the temperature in the cold storage is reduced to a certain temperature, the system is switched from a medium-temperature refrigeration mode to a low-temperature refrigeration mode, the temperature in the cold storage is further reduced through the low-temperature refrigeration mode, and quick freezing is realized. Because the equipment cost, the operation cost and the operation energy efficiency of the medium-temperature-level loop are lower than those of the low-temperature-level loop, the carbon dioxide refrigeration system adopts a staged cooling design, so that the equipment cost and the operation cost can be obviously reduced, the overall energy efficiency of the system, particularly the operation energy efficiency of the medium-temperature-level loop, is more reasonable and economic, and the operation energy consumption is reduced.
In addition, the electromagnetic valve group structure is formed by the electromagnetic valves connected in parallel, and the on-off of a loop is controlled by the electromagnetic valve group. When the operation mode needs to be switched, the electromagnetic valve with the smaller drift diameter is opened firstly, so that the carbon dioxide is released slowly from the high-pressure end to the low-pressure end. When the pressure difference between the front and the rear of the electromagnetic valve is reduced to a certain value, the electromagnetic valve with larger drift diameter connected in parallel is opened, so that the loop is completely conducted, and the system finishes mode switching. Therefore, when the system changes the refrigeration mode, the impact on the pipeline and the valve caused by sudden release of the pressure difference between the front and the back of the valve can be effectively avoided, the service life of the pipeline and the valve can be effectively prolonged, and the potential safety hazard is reduced. The concept of the invention can also be applied to a variable-temperature reservoir, and the problem that the pipeline and the valve piece are impacted by huge pressure when the variable-temperature reservoir is switched for use is solved.
Drawings
Fig. 1 is a schematic structural diagram of a carbon dioxide refrigeration system with staged temperature reduction according to the present invention.
Description of reference numerals: 1-air cooler, 2-circulating barrel, 3-liquid reservoir, SVA 1-first stop valve, SVA 2-second stop valve, SVA 3-third stop valve, SVA 4-fourth stop valve, SVA 5-fifth stop valve, E1-first electromagnetic valve, E2-second electromagnetic valve, E3-third electromagnetic valve, E4-fourth electromagnetic valve, E5-fifth electromagnetic valve, E6-sixth electromagnetic valve, REG 1-first expansion valve, REG 2-second expansion valve, SCA 1-first one-way valve, SCA 2-second one-way valve, FIA 1-first filter and FIA 2-second filter.
Detailed Description
As shown in fig. 1, the carbon dioxide refrigeration system with staged cooling provided by the invention comprises an air cooler 1, a low-temperature-stage module and a medium-temperature-stage module, wherein the low-temperature-stage module and the medium-temperature-stage module are both communicated with the air cooler 1 to respectively form a low-temperature-stage loop and a medium-temperature-stage loop. Low-temperature carbon dioxide flows through the low-temperature stage loop, and medium-temperature carbon dioxide flows through the medium-temperature stage loop. When the system operates, the low-temperature-level loop and the medium-temperature-level loop are alternatively conducted to supply low-temperature carbon dioxide or medium-temperature carbon dioxide to the air cooler 1, so that the carbon dioxide refrigeration system enters a low-temperature refrigeration mode or a medium-temperature refrigeration mode. The low-temperature-level loop and the medium-temperature-level loop are respectively communicated with the low-temperature-level carbon dioxide unit and the medium-temperature-level carbon dioxide unit, and the carbon dioxide unit consists of a compressor, a condenser and a throttle pipe which are sequentially communicated and is used for compressing, condensing and throttling the heated carbon dioxide after heat exchange so as to restore the low-temperature state of the carbon dioxide. The low-temperature-level carbon dioxide unit communicated with the low-temperature-level loop is higher in power and energy consumption during operation, and can output carbon dioxide with lower temperature, while the medium-temperature-level carbon dioxide unit communicated with the medium-temperature-level loop is relatively lower in power and energy consumption during operation, and the temperature of the output carbon dioxide is relatively higher.
In the medium-temperature refrigeration mode, medium-temperature carbon dioxide exchanges heat with air in the air cooler 1, the heat exchange capacity is weaker than that of low-temperature carbon dioxide, and the temperature capable of refrigerating is limited. But the medium-temperature loop has lower requirements on pipelines and units and has lower energy consumption during operation.
In the low-temperature refrigeration mode, the low-temperature carbon dioxide exchanges heat with air in the air cooler 1, the heat exchange capacity is stronger, the refrigeration temperature is lower, and correspondingly, the low-temperature requirement on the pipeline, the power requirement on the unit, and the power and energy consumption during operation are higher.
When quick freezing, the system enters a medium-temperature refrigeration mode, carbon dioxide circulates in a medium-temperature stage loop, and the temperature in a refrigerator is reduced through the medium-temperature refrigeration mode. When the temperature in the cold storage is reduced to a certain temperature, the system is switched from a medium-temperature refrigeration mode to a low-temperature refrigeration mode, the temperature in the cold storage is further reduced through the low-temperature refrigeration mode, and quick freezing is realized. Because the equipment cost and the operation cost of the medium-temperature-level loop are lower than those of the low-temperature-level loop, the carbon dioxide refrigeration system can obviously reduce the equipment cost and the operation cost, improve the overall energy efficiency of the system, be more reasonable and economic and reduce the operation energy consumption.
The low-temperature-level loop comprises a low-temperature-level inflow section at the front section of the air cooler 1 and a low-temperature-level air return section at the rear section of the air cooler 1. The low-temperature carbon dioxide flows into the air cooler 1 from the low-temperature stage inflow section, exchanges heat in the air cooler 1, flows out of the air cooler 1, enters the low-temperature stage air return section, flows back through the low-temperature stage air return section, and is cooled and then repeatedly circulated.
The medium temperature stage loop comprises a medium temperature stage inflow section at the front section of the air cooler 1 and a medium temperature stage return air section at the rear section of the air cooler 1. The medium-temperature carbon dioxide enters the air cooler 1 through the medium-temperature stage inflow section, and is subjected to heat exchange and then is subjected to reflux circulation through the medium-temperature stage return air section.
The low-temperature-level air return section is internally provided with a first electromagnetic valve group for controlling the on-off of the low-temperature-level air return section, the first electromagnetic valve group comprises a first electromagnetic valve E1 and a second electromagnetic valve E2 which are connected in parallel, the drift diameter of the first electromagnetic valve E1 is larger than that of the second electromagnetic valve E2, namely, the flow of the first electromagnetic valve E1 is larger than that of the second electromagnetic valve E2, the first electromagnetic valve E1 serves as a main valve of the low-temperature-level air return section, and the second electromagnetic valve E2 serves as a bypass valve. When there is great pressure differential at the both ends of first electromagnetism valves, if directly open the first solenoid valve E1 as the main valve, the pressure at both ends will directly strike first electromagnetism valves and pipeline, and the impact of relapse will shorten the life of valves and pipeline, and then shortens the running life of system to there is great potential safety hazard.
Because the second solenoid valve E2 is disposed at the first solenoid valve E1, when only the second solenoid valve E2 with a smaller drift diameter is opened, the low-temperature stage gas return section is initially conducted, the flow rate per unit time is small, according to the principle of blocking flow, carbon dioxide in the low-temperature stage circuit is slowly released and flows from the high-pressure end to the low-pressure end, and in the flowing process, the pressure difference between the two ends is gradually reduced until the pressures at the two ends tend to be balanced. At this time, the first electromagnetic valve E1 is opened again to make the low-temperature stage loop be completely conducted, so that the problem of shortening the service life of the system caused by pressure difference impact can be solved.
And a second electromagnetic valve group used for controlling the on-off of the medium-temperature stage inflow section is arranged in the medium-temperature stage inflow section, the second electromagnetic valve group comprises a third electromagnetic valve E3 and a fourth electromagnetic valve E4 which are connected in parallel, and the drift diameter of the third electromagnetic valve E3 is larger than that of the fourth electromagnetic valve E4. The third solenoid valve E3 serves as a main valve of the medium temperature stage inflow section, and the fourth solenoid valve E4 serves as a bypass valve. When the medium-temperature stage inflow section is controlled to be conducted, the fourth electromagnetic valve E4 (bypass valve) is opened first, and the third electromagnetic valve E3 (main valve) is opened after the pressures at the two ends are balanced. The principle is the same as that of the first electromagnetic valve group, and the description is omitted here. The structure is also used for solving the problem that the service life of the system is shortened due to the fact that excessive pressure difference directly impacts the valve member and the pipeline.
The electromagnetic valve group structure is formed by the electromagnetic valves connected in parallel, and the on-off of a loop is controlled by the electromagnetic valve group. When the operation mode needs to be switched, the electromagnetic valve with the smaller drift diameter is opened firstly, so that the carbon dioxide is released slowly from the high-pressure end to the low-pressure end. When the pressure difference between the front and the rear of the electromagnetic valve is reduced to a certain value, the electromagnetic valve with larger drift diameter connected in parallel is opened, so that the loop is completely conducted, and the system finishes mode switching. Therefore, when the system changes the refrigeration mode, the impact on the pipeline and the valve caused by sudden release of the pressure difference between the front and the back of the valve can be effectively avoided, the service life of the pipeline and the valve can be effectively prolonged, and the potential safety hazard is reduced. The concept of the invention can also be applied to a variable-temperature reservoir, and the problem that the pipeline and the valve piece are impacted by huge pressure when the variable-temperature reservoir is switched for use is solved.
The air cooler 1 is generally arranged in a refrigerator and comprises an evaporator and a high-speed fan, wherein the evaporator is provided with a heat exchange pipe. The two ends of the heat exchange tube are respectively a refrigerant inlet and a refrigerant outlet, and the air outlet of the high-speed fan faces the heat exchange tube. The refrigerant inlet is respectively communicated with the low-temperature stage inflow section and the medium-temperature stage inflow section, and the refrigerant outlet is respectively communicated with the low-temperature stage outflow section and the medium-temperature stage outflow section. When the medium-temperature-stage loop is conducted, medium-temperature carbon dioxide flows into the heat exchange tube of the air cooler 1 from the medium-temperature-stage loop, air flow blown out by the high-speed fan exchanges heat with the medium-temperature carbon dioxide in the heat exchange tube, the air flow is cooled to form cold air, and the cold air flows to all places of the refrigeration house under the blowing action of the high-speed fan, so that the temperature of the refrigeration house is reduced. And the temperature of the medium-temperature carbon dioxide is increased after heat exchange, and the medium-temperature carbon dioxide is subjected to reflux circulation through a medium-temperature stage gas return section. When the low temperature level return circuit switched on, the heat transfer process of low temperature level return circuit and air-cooler 1 was the same with medium temperature level return circuit, no longer gives details here, and the difference lies in when the heat transfer of low temperature level return circuit, air-cooler 1 exports the air conditioning that the temperature is lower.
In the present embodiment, the first stop valve SVA1 and the second stop valve SVA2 are provided at the refrigerant inlet and the refrigerant outlet, respectively. The first stop valve SVA1 and the second stop valve SVA2 can control the opening and closing of a refrigerant inlet and a refrigerant outlet, the first stop valve SVA1 and the second stop valve SVA2 are generally in an opening state, and the first stop valve SVA1 and the second stop valve SVA2 can be closed when the system needs to be overhauled, so that the system can be well protected during overhauling, and overhauling is facilitated.
The low-temperature stage module specifically comprises a third stop valve SVA3, a fourth stop valve SVA4, a first filter FIA1, a first expansion valve REG1, a first check valve SCA1, a fifth solenoid valve E5 and a circulating barrel 2; the circulating barrel 2 stores low-temperature carbon dioxide with the temperature of minus 40 ℃ to minus 30 ℃.
The outlet of the circulation tub 2 and the refrigerant inlet of the air cooler 1 are communicated through a pipe to form a pipe structure of a low temperature stage inflow section, and the third stop valve SVA3, the first filter FIA1, the fifth solenoid valve E5, the first expansion valve REG1 and the first check valve SCA1 are sequentially disposed on the pipe to form the low temperature stage inflow section.
The refrigerant outlet of the air cooler 1 is communicated with the inlet of the circulating barrel 2 through a pipeline to form a pipeline structure of a low-temperature-level air return section, and the first electromagnetic valve group and the fourth stop valve SVA4 are sequentially arranged on the pipeline to form the low-temperature-level air return section.
The medium-temperature stage module comprises a fifth stop valve SVA5, a second filter FIA2, a second expansion valve REG2, a sixth electromagnetic valve E6, a second one-way valve SCA2 and a liquid reservoir 3; the liquid storage device 3 stores medium-temperature carbon dioxide with the temperature of minus 10 ℃ to minus 5 ℃.
The outlet of the liquid accumulator 3 is communicated with the refrigerant inlet of the air cooler 1 through a pipeline to form a pipeline structure of a medium-temperature-level inflow section, and the fifth stop valve SVA5, the second filter FIA2, the second solenoid valve group and the second expansion valve REG2 are sequentially arranged on the pipeline to form the medium-temperature-level inflow section.
The refrigerant outlet of the air cooler 1 is communicated with the inlet of the liquid storage device 3 through a pipeline to form a pipeline structure of a medium-temperature-level air return section. And a sixth electromagnetic valve E6 and a second one-way valve SCA2 are sequentially arranged on the pipeline to form an intermediate-temperature-stage air return section.
Wherein, first filter FIA1 and second filter FIA2 all are used for filtering out impurity such as welding slag that follows the carbon dioxide flow in the pipeline, prevent that impurity from damaging the subassembly of system.
The first expansion valve REG1 and the second expansion valve REG2 can be manual expansion valves or electronic expansion valves, and have the functions of increasing the supercooling degree of carbon dioxide, further throttling the carbon dioxide, reducing the temperature of the carbon dioxide and improving the system efficiency.
The first check valve SCA1 and the second check valve SCA2 are used for preventing carbon dioxide backflow from causing system damage. In addition, when the system stops running due to unexpected power failure, the electromagnetic valve is in a closed state when power is cut off, and when the temperature and the pressure of carbon dioxide effusion in the system return due to power failure, the second one-way valve SCA2 on the medium-temperature-level air return section can be opened in the forward direction under the action of pressure difference to discharge the pressure in the air cooler 1, so that the air cooler 1, the pipeline and the valve are protected. The potential safety hazard of the temperature and pressure rise of the carbon dioxide effusion in the air cooler 1 and the pipeline to the system when the power failure occurs accidentally or the refrigeration house is in emergency stop is solved.
And the fifth electromagnetic valve E5 is used for controlling the on-off of the low-temperature stage inflow section.
The first electromagnetic valve group is used for controlling the on-off of the low-temperature-level air return section.
And the second electromagnetic valve group is used for controlling the on-off of the medium temperature stage inflow section.
And the sixth electromagnetic valve E6 is used for controlling the on-off of the medium-temperature stage air return section.
The third stop valve SVA3, the fourth stop valve SVA4 and the fifth stop valve SVA5 are generally in an open state and are closed when the system is overhauled, so that the system can be protected when overhauled, and the overhaul is convenient.
In addition, the first electromagnetic valve group and the second electromagnetic valve group can be replaced by ICLX two-step opening electromagnetic valves, but the cost of a single valve is as high as 1-2 ten thousand yuan, the cost is high, the manufacturing cost of the whole set of system is high, the purpose of reducing the equipment cost is not met, and the ICLX two-step opening electromagnetic valves are not adopted.
A single ICM electro valve may also be used, which may be opened in two steps by PLC control. An ICSH regulator valve may also be used, requiring two-step opening by the PLC giving two open signals. The two valves can also realize the inventive concept of two-step opening, but on one hand, the valves have generally higher cost, on the other hand, a PLC needs to be additionally arranged, and the control process is complex, is easy to make mistakes and is not adopted.
Through practical tests, the inventor can well realize the inventive concept that the small electromagnetic valve is opened first and then the large electromagnetic valve is opened to avoid pressure impact in the bypass mode of the small electromagnetic valve, and the valve has the advantages of low price, low cost and convenient control.
The first solenoid valve E1, the third solenoid valve E3, the fifth solenoid valve E5, and the sixth solenoid valve E6 are all servo solenoid valves, and the second solenoid valve E2 and the fourth solenoid valve E4 are electric two-way valves. The servo solenoid valve has excellent controllability and can control the opening angle of the valve according to a command. The electric two-way valve can be opened or closed through electric control, and is low in cost and easy to control. The electric two-way valve with a proper drift diameter is selected as the second electromagnetic valve E2 and the fourth electromagnetic valve E4, so that the technical effect of the invention can be realized, and the cost and the control steps can be saved.
The embodiment also provides a control method based on the carbon dioxide refrigeration system, which is concretely as follows.
The carbon dioxide refrigeration system comprises two refrigeration modes, namely a medium-temperature refrigeration mode and a low-temperature refrigeration mode.
In the medium-temperature refrigeration mode, the first electromagnetic valve group and the fifth electromagnetic valve E5 are closed, the low-temperature-level circuit is closed, the second electromagnetic valve group and the sixth electromagnetic valve E6 are opened, and the medium-temperature-level circuit is opened. The medium-temperature carbon dioxide in the liquid storage device 3 flows out from an outlet of the liquid storage device 3, enters a medium-temperature stage inflow section, then enters the air cooler 1 for heat exchange, performs medium-temperature refrigeration on the refrigeration house, flows back into the liquid storage device 3 through a medium-temperature stage gas return section after the heat exchange is finished, and is cooled by a medium-temperature carbon dioxide unit for recycling.
In the low-temperature refrigeration mode, the second electromagnetic valve group and the sixth electromagnetic valve E6 are closed, the medium-temperature-level circuit is closed, the first electromagnetic valve group and the fifth electromagnetic valve E5 are opened, and the low-temperature-level circuit is opened. The low-temperature carbon dioxide in the circulating barrel 2 flows out from the outlet of the circulating barrel 2, enters the low-temperature-level inflow section, then enters the air cooler 1 for heat exchange, performs low-temperature refrigeration on the refrigeration house, returns to the circulating barrel 2 through the low-temperature-level air return section after the heat exchange is finished, and is recycled after being cooled by the low-temperature carbon dioxide unit.
In the medium-temperature refrigeration mode, the temperature of carbon dioxide circulating in the evaporator of the air cooler 1 is-10 ℃ to-5 ℃. In the low-temperature refrigeration mode, the temperature of carbon dioxide circulating in the evaporator of the air cooler 1 is-40 ℃ to-30 ℃. When the system is switched between modes, the temperature of the carbon dioxide at the front side and the rear side of the electromagnetic valve is greatly different, for example, when the system is switched from a medium-temperature refrigeration mode to a low-temperature refrigeration mode, the temperature of the carbon dioxide at one side of the air cooler 1 is about-10 ℃, and the temperature of the carbon dioxide at one side of the circulating barrel 2 is about-40 ℃. The temperature difference causes pressure difference in the pipelines at the front side and the rear side of the electromagnetic valve, at the moment, the pressure at one side of the air cooler 1 is about 25.5bar (-10 ℃), the pressure at one side of the circulating barrel 2 is about 8.3bar (-40 ℃), and the pressure difference at the two sides of the electromagnetic valve is 17.2 bar. That is, when the mode is switched, the pressure on the medium-temperature carbon dioxide side is higher than the pressure on the low-temperature carbon dioxide side, and at this time, if the electromagnetic valve on the pipeline is directly opened, the pressure difference directly impacts the valve element and the pipeline, so how to avoid the impact of the pressure difference on the valve element and the pipeline during the mode switching is a key technical problem in the mode switching process.
In order to solve the technical problem, the inventor arranges a first electromagnetic valve group on a low-temperature stage air return section, the first electromagnetic valve group is arranged in a two-parallel electromagnetic valve structure, and the drift diameter of a first electromagnetic valve E1 is larger than that of a second electromagnetic valve E2. When the system is switched from the medium-temperature refrigeration mode to the low-temperature refrigeration mode, the pressure on one side of the air cooler 1 is high, the pressure on one side of the circulating barrel 2 is low, and at the moment, the pressure on one side of the air cooler 1 can be released through the low-temperature stage air return section, so that the first electromagnetic valve group is arranged on the low-temperature stage air return section, carbon dioxide is slowly released through the second electromagnetic valve E2 with the small drift diameter, the pressures on two sides gradually tend to be balanced, and impact on a pipeline and a valve piece when the pressure difference is suddenly released is avoided.
When the system is switched from the low-temperature refrigeration mode to the medium-temperature refrigeration mode, the opposite is true, the pressure on one side of the air cooler 1 is lower, and the pressure on one side of the liquid accumulator 3 is higher, so that the pressures on the two sides can be balanced through the medium-temperature stage inflow section. Therefore, the second electromagnetic valve group is arranged on the medium-temperature stage inflow section, carbon dioxide is firstly slowly released through the fourth electromagnetic valve E4 with a smaller drift diameter, and the pressures on the two sides gradually tend to be balanced.
According to the pressure characteristic of the system during mode switching, the first electromagnetic valve group and the second electromagnetic valve group are respectively arranged on the low-temperature stage air return section and the medium-temperature stage inflow section, so that the problem that impact is brought to valves and pipelines due to sudden pressure difference release during mode switching is effectively solved, the service lives of the pipelines and the valves can be effectively prolonged, and potential safety hazards are reduced.
When the system needs to be switched from the medium-temperature refrigeration mode to the low-temperature refrigeration mode, the medium-temperature stage inflow section and the medium-temperature stage return air section are closed in sequence. And then opening a bypass second electromagnetic valve E2 to conduct the small path of the low-temperature stage air return section. Because the drift diameter of the second electromagnetic valve E2 is small, carbon dioxide can be slowly released from a high-pressure end to a low-pressure end, and the pressure cannot suddenly impact the valve and the pipeline; and the pressure intensity on the two sides gradually tends to be balanced, and the pressure intensity difference gradually becomes smaller. When the pressure difference reaches the preset pressure difference, the first solenoid valve E1 and the fifth solenoid valve E5 are opened again, so that the low-temperature-level loop is completely conducted, and the system formally enters a low-temperature refrigeration mode.
When the system needs to be switched from the low-temperature refrigeration mode to the medium-temperature refrigeration mode, the low-temperature stage inflow section and the low-temperature stage return air section are closed in sequence. Then, the bypass fourth electromagnetic valve E4 is firstly opened, so that the small path of the medium-temperature stage inflow section is conducted. Because the drift diameter of the fourth electromagnetic valve E4 is small, carbon dioxide can be slowly discharged from the high-pressure end to the low-pressure end, the pressure can not suddenly impact the valve piece and the pipeline, the pressures on the two sides gradually tend to be balanced, and the pressure difference gradually becomes smaller. When the pressure difference reaches the preset pressure difference, the third electromagnetic valve E3 and the sixth electromagnetic valve E6 are opened, so that the medium-temperature-level loop is completely conducted, and the system formally enters a medium-temperature refrigeration mode.
The temperature of the medium-temperature carbon dioxide is-10 ℃ to-5 ℃, namely, when the temperature of the refrigeration house reaches the range of-10 ℃ to-5 ℃, the medium-temperature refrigeration mode is difficult to continue to provide effective refrigeration effect, if the refrigeration house needs to be further cooled, the low-temperature refrigeration mode needs to be switched, so that the triggering condition for switching the modes of the system can be set as that the temperature of the refrigeration house reaches-10 ℃ to-5 ℃ when the system actually operates. Namely, when the temperature of the refrigeration house is higher than-5 ℃, the system is in a medium-temperature refrigeration mode. In the medium-temperature refrigeration mode, the temperature of the refrigeration house is continuously reduced, when the temperature of the refrigeration house reaches-10 ℃ to-5 ℃, the system is switched to the low-temperature refrigeration mode from the medium-temperature refrigeration mode, low-temperature carbon dioxide at-40 ℃ to-30 ℃ is used for low-temperature refrigeration, so that the temperature of the refrigeration house can be reduced to-40 ℃ to-30 ℃, and quick freezing is realized. The temperature in the cold storage is used as a trigger condition for switching the modes, so that on one hand, the cold storage can provide an effective quick-freezing effect, on the other hand, the energy consumption of the system at a medium temperature level can be reduced, and the overall energy efficiency of the system is improved.
In order to accurately control the timing of the mode switching of the system, the temperature in the refrigerator needs to be monitored. Specifically, a temperature sensor can be arranged in the refrigeration house to detect the temperature in the refrigeration house in real time. Or by controlling the refrigerating time, under a fixed refrigerating mode, the refrigerating capacity of the system in unit time is a fixed numerical range, the total refrigerating capacity can be calculated by calculating the refrigerating time, and then the approximate temperature of the refrigeration house can be calculated according to related parameters such as the volume of the refrigeration house and the like. Or after multiple times of refrigeration, according to experience, operators can know how long the temperature in the cold storage can reach about-5 ℃ in the medium-temperature refrigeration mode. Can be set according to actual requirements.
The predetermined pressure difference is less than or equal to 1.5 bar. The sudden release of the pressure difference will not affect the valve and the pipeline, so as to effectively protect the valve and the pipeline.
For a better understanding and explanation of the invention, an example of the application of the invention will be explained below.
Taking a freezer quick-freezing as an example, the carbon dioxide refrigeration system of the embodiment is arranged in the freezer, and the air cooler 1 is arranged on a top plate of the freezer. The various valves, piping and reservoirs 3 and the recycle drums 2, etc. of the low and medium temperature stage circuits are conventionally arranged in the connection relationship described above.
The initial temperature of a batch of goods is 10-25 c and it is necessary to quickly freeze it to-18 c. The goods are sent into the cold storage, and the sealing door of the cold storage is closed. The carbon dioxide refrigeration system described in this embodiment is started to make the system enter a medium temperature refrigeration mode, at this time, the first electromagnetic valve group and the fifth electromagnetic valve E5 are both closed, the low-temperature-stage inflow section and the low-temperature-stage return air section are both closed, and the low-temperature-stage loop is closed. The second electromagnetic valve group and the sixth electromagnetic valve E6 are both opened, and the medium-temperature stage inflow section and the medium-temperature stage return air section are both opened. The medium temperature carbon dioxide unit works.
In the medium-temperature refrigeration mode, medium-temperature carbon dioxide flows out from an outlet of the liquid reservoir 3, sequentially passes through a fifth stop valve SVA5, a second filter FIA2, a second electromagnetic valve group and a second expansion valve REG2 of the medium-temperature-stage inflow section, enters the air cooler 1 to exchange heat with the refrigeration house, so that the temperature of the refrigeration house is reduced, and the medium-temperature carbon dioxide after heat exchange sequentially passes through a sixth electromagnetic valve E6 and a second one-way valve SCA2 and then flows back to the liquid reservoir 3. And the medium-temperature carbon dioxide unit cools the carbon dioxide after heat exchange to restore the low temperature, and then the process is repeated to circularly flow.
Under the continuous refrigeration of the medium-temperature refrigeration system, the temperature of the refrigeration house and goods in the refrigeration house is continuously reduced, the temperature in the refrigeration house is reduced to minus 10 ℃ to minus 5 ℃, and the system is switched from medium-temperature control to low-temperature control.
At this time, the second electromagnetic valve group is closed first, so that the medium temperature stage inflow section is closed, and then the sixth electromagnetic valve E6 is closed, so that the medium temperature stage return section is closed. Then the second solenoid valve E2 is opened, carbon dioxide is slowly released from the high pressure end to the low pressure end through the second solenoid valve E2, and the pressures on both sides gradually approach equilibrium. When the pressure difference between the front and the rear of the second electromagnetic valve E2 reaches about 1.5bar, the first electromagnetic valve E1 and the fifth electromagnetic valve E5 are opened, the low-temperature stage loop is conducted, and the system is switched from the medium-temperature refrigeration mode to the low-temperature refrigeration mode.
In the low-temperature refrigeration mode, low-temperature carbon dioxide flows out from an outlet of the circulating barrel 2, sequentially passes through a third stop valve SVA3, a first filter FIA1, a fifth electromagnetic valve E5, a first expansion valve REG1 and a first one-way valve SCA1, enters the air cooler 1, and exchanges heat with a refrigeration house, so that the temperature of the refrigeration house is reduced; the low-temperature carbon dioxide after heat exchange sequentially flows back to the inlet of the circulating barrel 2 through the first electromagnetic valve bank and the fourth stop valve SVA4
Under the continuous refrigeration action of the low-temperature refrigeration mode, the temperature in the cold storage is continuously reduced, and after the goods are quickly frozen to the designated temperature, the quick freezing of the goods is completed. When the next batch of goods is quickly frozen, the system needs to be switched from a low-temperature refrigeration mode to a medium-temperature refrigeration mode.
At this time, the fifth solenoid valve E5 and the first solenoid valve group are closed in sequence, so that the low-temperature stage circuit is closed. The fourth solenoid valve E4 is opened again, carbon dioxide is slowly released from the high pressure end to the low pressure end through the fourth solenoid valve E4, and the pressures on the two sides gradually approach equilibrium. And when the pressure difference between the front and the back of the fourth electromagnetic valve E4 reaches 1.5bar, opening the third electromagnetic valve E3 and the sixth electromagnetic valve E6, and enabling the carbon dioxide refrigeration system to enter a medium-temperature refrigeration mode from a low-temperature refrigeration mode. And repeating the medium-temperature refrigeration process.
The carbon dioxide refrigeration system adopts the staged cooling design of the low-temperature-level loop and the medium-temperature-level loop, so that the equipment cost and the operation cost can be obviously reduced, the overall energy efficiency of the system, particularly the operation energy efficiency of the medium-temperature level, is more reasonable and economic, and the operation energy consumption is reduced. And the first electromagnetic valve group and the second electromagnetic valve group which are in parallel connection are respectively arranged at the low-temperature stage air return section and the medium-temperature stage inflow section, so that the problem that the valve and the pipeline are impacted by the pressure difference during mode switching is solved.
In the description of the present application, it is to be understood that the orientation or positional relationship indicated by the directional terms such as "front, rear, upper, lower, left, right", "lateral, vertical, horizontal" and "top, bottom", etc., are generally based on the orientation or positional relationship shown in the drawings, and are used for convenience of description and simplicity of description only, and in the case of not making a reverse explanation, these directional terms do not indicate and imply that the device or element being referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore, should not be considered as limiting the scope of the present application.
Spatially relative terms, such as "above … …," "above … …," "above … …," "above," and the like, may be used herein for ease of description to describe one device or feature's spatial relationship to another device or feature as illustrated in the figures, and it is to be understood that spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device in the figures is turned over, devices described as "above" or "on" other devices or configurations would then be oriented "below" or "under" the other devices or configurations. Thus, the exemplary term "above … …" can include both orientations of "above … …" and "below … …". The device may be otherwise variously oriented (rotated 90 or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
It should be noted that the terms "first", "second", and the like are used to define the components, and are only used for convenience of distinguishing the corresponding components, and the terms have no special meanings unless otherwise stated, and therefore, the scope of protection of the present application is not to be construed as being limited.
It will be apparent to those skilled in the art that various other changes and modifications may be made in the above-described embodiments and concepts and all such changes and modifications are intended to be within the scope of the appended claims.
Claims (10)
1. A carbon dioxide refrigerating system capable of cooling in stages is characterized by comprising an air cooler, a low-temperature stage module and a medium-temperature stage module; the low-temperature-level module is communicated with the air cooler to form a low-temperature-level loop; the medium-temperature-stage module is communicated with the air cooler to form a medium-temperature-stage loop; the low-temperature-level loop and the medium-temperature-level loop are both used for circulating carbon dioxide, and the temperature of the carbon dioxide in the low-temperature-level loop is lower than that of the carbon dioxide in the medium-temperature-level loop;
the low-temperature-level loop comprises a low-temperature-level inflow section and a low-temperature-level air return section, and a first electromagnetic valve group used for controlling the on-off of the low-temperature-level air return section is arranged in the low-temperature-level air return section; the first electromagnetic valve group comprises a first electromagnetic valve and a second electromagnetic valve which are connected in parallel, and the drift diameter of the first electromagnetic valve is larger than that of the second electromagnetic valve;
the medium-temperature stage loop comprises a medium-temperature stage inflow section and a medium-temperature stage air return section, a second electromagnetic valve group used for controlling the on-off of the medium-temperature stage inflow section is arranged in the medium-temperature stage inflow section, the second electromagnetic valve group comprises a third electromagnetic valve and a fourth electromagnetic valve which are connected in parallel, and the drift diameter of the third electromagnetic valve is larger than that of the fourth electromagnetic valve.
2. The staged cooling carbon dioxide refrigeration system according to claim 1, wherein the air cooler comprises a refrigerant inlet and a refrigerant outlet, and the low-temperature stage inflow section and the medium-temperature stage inflow section are respectively communicated with the refrigerant inlet; the refrigerant outlet is respectively communicated with the low-temperature stage gas return section and the medium-temperature stage gas return section, a first stop valve is arranged at the refrigerant inlet, and a second stop valve is arranged at the refrigerant outlet.
3. The carbon dioxide refrigeration system of claim 2, wherein the low temperature stage module comprises a third stop valve, a fourth stop valve, a first filter, a first expansion valve, a first one-way valve, a fifth solenoid valve, and a recycle bin; the circulating barrel is used for storing low-temperature carbon dioxide at the temperature of minus 40 ℃ to minus 30 ℃;
the outlet of the circulating barrel, the third stop valve, the first filter, the fifth electromagnetic valve, the first expansion valve and the first one-way valve are communicated in sequence to form the low-temperature stage inflow section;
the first electromagnetic valve group, the fourth stop valve and the inlet of the circulating barrel are sequentially communicated to form the low-temperature-level air return section.
4. The carbon dioxide refrigeration system with staged temperature reduction according to claim 3, wherein the medium-temperature stage module comprises a fifth stop valve, a second filter, a second expansion valve, a sixth solenoid valve, a second one-way valve and a liquid reservoir; the liquid storage device is used for storing medium-temperature carbon dioxide with the temperature of-10 ℃ to-5 ℃;
an outlet of the liquid storage device, the fifth stop valve, the second filter, the second electromagnetic valve group and the second expansion valve are communicated in sequence to form the medium-temperature-stage inflow section;
and the sixth electromagnetic valve, the second one-way valve and the inlet of the liquid reservoir are communicated in sequence to form the medium-temperature-stage air return section.
5. The carbon dioxide refrigeration system with staged temperature reduction according to claim 4, wherein the first solenoid valve, the third solenoid valve, the fifth solenoid valve and the sixth solenoid valve are all servo solenoid valves; the second electromagnetic valve and the fourth electromagnetic valve are electric two-way valves.
6. A control method of a carbon dioxide refrigeration system with staged temperature reduction is applied to the carbon dioxide refrigeration system as claimed in any one of claims 1 to 5, and is characterized in that the carbon dioxide refrigeration system comprises a medium-temperature refrigeration mode and a low-temperature refrigeration mode;
in the medium-temperature refrigeration mode, the low-temperature-level loop is closed, the medium-temperature-level loop is opened, and carbon dioxide flows in the medium-temperature-level loop;
in the low-temperature refrigeration mode, the low-temperature-level loop is opened, the medium-temperature-level loop is closed, and carbon dioxide circulates in the low-temperature-level loop;
when the medium-temperature refrigeration mode needs to be switched to the low-temperature refrigeration mode, the medium-temperature stage inflow section and the medium-temperature stage return air section are closed in sequence; then, a second electromagnetic valve is opened firstly, when the pressure difference between the front and the back of the second electromagnetic valve reaches a preset pressure difference, a first electromagnetic valve and a fifth electromagnetic valve are opened, and the carbon dioxide refrigeration system enters a low-temperature refrigeration mode from a medium-temperature refrigeration mode;
when the low-temperature refrigeration mode needs to be switched to the medium-temperature refrigeration mode, the low-temperature-level inflow section and the low-temperature-level return section are closed in sequence; and then, opening a fourth electromagnetic valve, and opening a third electromagnetic valve and a sixth electromagnetic valve when the front-back pressure difference of the fourth electromagnetic valve reaches a preset pressure difference, wherein the carbon dioxide refrigeration system enters a medium-temperature refrigeration mode from a low-temperature refrigeration mode.
7. The control method according to claim 6, wherein in the medium-temperature refrigeration mode, medium-temperature carbon dioxide flows out from an outlet of the liquid reservoir, sequentially passes through a fifth stop valve, a second filter, a second electromagnetic valve group, a second expansion valve and a first stop valve, and enters an air cooler to exchange heat with the refrigeration house, so that the temperature of the refrigeration house is reduced; the medium-temperature carbon dioxide after heat exchange sequentially passes through a second stop valve, a sixth electromagnetic valve and a second one-way valve and then flows back to the inlet of the liquid storage device;
in the low-temperature refrigeration mode, low-temperature carbon dioxide flows out from an outlet of the circulating barrel, sequentially passes through a third stop valve, a first filter, a fifth electromagnetic valve, a first expansion valve, a first one-way valve and a first stop valve, and enters an air cooler to exchange heat with a refrigeration house, so that the temperature of the refrigeration house is reduced; the low-temperature carbon dioxide after heat exchange sequentially passes through a second stop valve, a first electromagnetic valve group and a fourth stop valve and then flows back to the inlet of the circulating barrel;
the temperature of the medium-temperature carbon dioxide is-10 ℃ to-5 ℃, and the temperature of the low-temperature carbon dioxide is-40 ℃ to-30 ℃.
8. The control method according to claim 7, wherein the carbon dioxide refrigeration system is in a medium-temperature refrigeration mode when the temperature of the refrigeration house is higher than-5 ℃, and is switched from the medium-temperature refrigeration mode to a low-temperature refrigeration mode when the temperature of the refrigeration house is between-10 ℃ and-5 ℃.
9. The control method according to claim 8, wherein a temperature sensor is provided in the refrigerator to detect a refrigerator temperature, or the refrigerator temperature is controlled by controlling a refrigerating time.
10. Control method according to claim 6, characterized in that said preset pressure difference is less than or equal to 1.5 bar.
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