Carbon dioxide refrigeration system with maintenance unit and control method thereof

Abstract

The present application relates to the technical field of refrigeration system, provide a kind of carbon dioxide refrigeration system with maintaining unit and control method thereof, the carbon dioxide refrigeration system with maintaining unit includes liquid storage tank, refrigeration unit and maintaining unit, wherein, the liquid storage tank is connected with the refrigeration unit circulation, for the refrigeration unit liquid supply;The maintaining unit is connected with the liquid storage tank circulation, for maintaining the pressure in the liquid storage tank, and for predicting the change trend of future liquid level in the liquid storage tank.The present application compared with prior art carbon dioxide refrigeration system, can estimate future liquid level in the liquid storage tank in advance, and combined with fuzzy self-tuning PID algorithm, the maintaining unit can be kept in optimum condition, effectively avoid the energy waste caused by the sudden increase of liquid in the liquid storage tank, while the maintaining unit still keeps higher frequency operation;Simultaneously effectively avoid the situation of unstable refrigeration system caused by the sudden decrease of liquid in the liquid storage tank.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of refrigeration systems, and in particular to a carbon dioxide refrigeration system with a maintenance unit and a control method thereof. BACKGROUND

[0002] Carbon dioxide, a natural working medium, has excellent environmental protection, refrigeration, safety and chemical stability, and is low in price and easy to obtain, and can replace HFCs and other working media as an environmentally friendly refrigerant.

[0003] During normal operation of the carbon dioxide refrigeration system, the pressure in the liquid storage tank and the refrigeration system is relatively low, especially in the carbon dioxide subcritical state refrigeration system, the exhaust pressure of the compressor does not exceed 5.0 MPa.

[0004] However, if the carbon dioxide refrigeration system suddenly stops, the temperature of the liquid storage tank and the refrigeration system will continue to rise, resulting in an increase in the saturation pressure of the system, which requires a higher pressure-bearing capacity of the carbon dioxide refrigeration system. In view of the continuous change of pressure when the refrigeration system is accidentally stopped, a maintenance unit is generally added to keep the pressure in the liquid storage tank within a certain range. However, the existing maintenance unit can only start working when the system is stopped, resulting in poor stability of the refrigeration system pressure. SUMMARY

[0005] The present application provides a carbon dioxide refrigeration system with a maintenance unit and a control method thereof to solve the problem that the existing maintenance unit can only start working when the system is stopped, resulting in poor stability of the system pressure.

[0006] To solve the above problems, the first aspect of the present application provides a carbon dioxide refrigeration system with a maintenance unit, comprising:

[0007] a liquid storage tank;

[0008] a refrigeration unit, the liquid storage tank and the refrigeration unit are connected in circulation, for supplying liquid to the refrigeration unit;

[0009] a maintenance unit, the maintenance unit and the liquid storage tank are connected in circulation, for maintaining the pressure in the liquid storage tank, and for predicting the future change trend of the liquid level in the liquid storage tank.

[0010] The carbon dioxide refrigeration system with a maintenance unit provided by the present application comprises a heat exchanger, a first pipeline and a second pipeline;

[0011] The first pipeline and the second pipeline are respectively connected to the heat exchanger, and the second pipeline is connected in circulation to the liquid storage tank.

[0012] The carbon dioxide refrigeration system with a maintenance unit provided by the application comprises a first pipeline, wherein the first pipeline comprises a maintenance compressor, a condenser and a maintenance throttle valve;

[0013] The first pipeline is connected with the first outlet of the heat exchanger and the first inlet of the heat exchanger, and the maintenance compressor, the condenser and the maintenance throttle valve are sequentially arranged on the first pipeline along the flow direction of the cooling working medium.

[0014] The carbon dioxide refrigeration system with a maintenance unit provided by the application comprises a second pipeline, wherein the second pipeline comprises a working medium pump and a maintenance flow regulating valve;

[0015] The second pipeline is connected with the second outlet of the heat exchanger and the second inlet of the heat exchanger, the maintenance flow regulating valve is connected to the second pipeline and located at the inlet of the liquid storage tank, and the working medium pump is connected to the second pipeline and located at the outlet of the liquid storage tank.

[0016] The carbon dioxide refrigeration system with a maintenance unit provided by the application comprises a liquid storage tank, wherein a pressure sensor and a liquid level sensor are arranged on the liquid storage tank;

[0017] The pressure sensor is used for monitoring the pressure in the liquid storage tank, and the liquid level sensor is used for monitoring the liquid level in the liquid storage tank.

[0018] The carbon dioxide refrigeration system with a maintenance unit provided by the application comprises a refrigeration unit, wherein the refrigeration unit comprises a refrigeration flow regulating valve, an evaporator, a refrigeration compressor, a cooler and a refrigeration throttle valve along the circulation path of the refrigeration working medium.

[0019] The refrigeration flow regulating valve and the refrigeration throttle valve are respectively connected to the liquid storage tank through pipelines.

[0020] The carbon dioxide refrigeration system with a maintenance unit provided by the application comprises a plurality of refrigeration modules, and the plurality of refrigeration modules are connected in parallel and then connected in series with the liquid storage tank.

[0021] The carbon dioxide refrigeration system with a maintenance unit provided by the application comprises a refrigeration module, wherein the refrigeration module comprises a refrigeration flow regulating valve, an evaporator, a refrigeration compressor, a cooler and a refrigeration throttle valve along the circulation path of the refrigeration working medium.

[0022] The refrigeration flow regulating valve is connected to the liquid storage tank through a first stop valve, and the refrigeration throttle valve is connected to the liquid storage tank through a second stop valve.

[0023] To solve the above problems, the second aspect of the application provides a control method of a carbon dioxide refrigeration system with a maintenance unit, comprising the following steps:

[0024] Step S10: the carbon dioxide refrigeration system with a maintenance unit comprises a liquid storage tank, a refrigeration unit and a maintenance unit, and the carbon dioxide refrigeration system with a maintenance unit has a preset upper limit operating pressure and a lower limit operating pressure;

[0025] Step S20: a maintenance flow regulating valve is arranged on the circulating loop of the maintenance unit, and the operating pressure of the carbon dioxide refrigeration system with a maintenance unit is judged;

[0026] If the operating pressure of the carbon dioxide refrigeration system with a maintenance unit is higher than the upper limit operating pressure, the maintenance flow regulating valve is fully opened, and the operating frequency of the maintenance compressor is adjusted to the maximum operating frequency at the same time;

[0027] If the operating pressure of the carbon dioxide refrigeration system with a maintenance unit is lower than the lower limit operating pressure, the maintenance flow regulating valve is adjusted to the minimum flow, and the operating frequency of the maintenance compressor is adjusted to intermittent start or direct stop at the same time.

[0028] According to the control method of the carbon dioxide refrigeration system with a maintenance unit provided by the present application, when the operating pressure of the carbon dioxide refrigeration system with a maintenance unit is judged, the liquid level condition in the liquid storage tank is judged, which comprises the following steps:

[0029] Step S21: if the liquid level condition in the liquid storage tank is within the set range value, the difference between the total inflow of the carbon dioxide working medium in the liquid storage tank and the total outflow of the carbon dioxide working medium in each refrigeration component is calculated, and the operating frequency of the maintenance compressor and the opening degree of the maintenance flow regulating valve are determined;

[0030] Step S22: if the liquid level condition in the liquid storage tank is lower than the set range value, the maintenance flow regulating valve is fully opened, and the operating frequency of the maintenance compressor is adjusted to the maximum operating frequency;

[0031] Step S23: if the liquid level condition in the liquid storage tank is higher than the set range value, the maintenance flow regulating valve is adjusted to the minimum flow, and the operating frequency of the maintenance compressor is adjusted to the minimum operating frequency or stopped.

[0032] The carbon dioxide refrigeration system with a maintenance unit provided by the present application can estimate the future liquid level condition in the liquid storage tank in advance compared with the existing carbon dioxide refrigeration system, and can keep the maintenance unit in the best working condition by combining the fuzzy self-tuning PID algorithm, effectively avoiding the energy waste caused by the sudden increase of the liquid in the liquid storage tank while the maintenance unit still runs at a high frequency; at the same time, it effectively avoids the unstable situation of the refrigeration system caused by the sudden decrease of the liquid in the liquid storage tank, so that the liquid level in the liquid storage tank is always in a relatively stable state, so as to improve the operating stability of the entire carbon dioxide refrigeration system.

[0033] In addition, the control method of the carbon dioxide refrigeration system with a maintenance unit provided by the present application can predict the future liquid level of the carbon dioxide refrigeration system with a maintenance unit in advance by calculating the difference between the total inflow of the carbon dioxide working medium in the liquid storage tank and the total outflow of the carbon dioxide working medium in each refrigeration component, and combining the actual liquid level value in the liquid storage tank, so as to adjust the opening and closing of the maintenance flow regulating valve and the operating frequency of the maintenance compressor, keep the pressure of the carbon dioxide refrigeration system with a maintenance unit in a balanced state, and ensure the stable operation of the carbon dioxide refrigeration system with a maintenance unit.

[0034] In addition, the control method of the carbon dioxide refrigeration system with a maintenance unit provided by the present application can predict the future liquid level of the carbon dioxide refrigeration system with a maintenance unit in advance by calculating the difference between the total inflow of the carbon dioxide working medium in the liquid storage tank and the total outflow of the carbon dioxide working medium in each refrigeration component, and combining the actual liquid level value in the liquid storage tank, so as to adjust the opening and closing of the maintenance flow regulating valve and the operating frequency of the maintenance compressor, keep the pressure of the carbon dioxide refrigeration system with a maintenance unit in a balanced state, and ensure the stable operation of the carbon dioxide refrigeration system with a maintenance unit. BRIEF DESCRIPTION OF DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort.

[0036] Figure 1 is a system block diagram of one embodiment of the carbon dioxide refrigeration system with a maintenance unit provided by the present application;

[0037] Figure 2 is a system block diagram of another embodiment of the carbon dioxide refrigeration system with a maintenance unit provided by the present application;

[0038] Figure 3 is one of the flowcharts of the control method of the carbon dioxide refrigeration system with a maintenance unit provided by the present application;

[0039] Figure 4 is the second flowchart of the control method of the carbon dioxide refrigeration system with a maintenance unit provided by the present application;

[0040] Figure 5 is the liquid level control flowchart of the liquid storage tank in the carbon dioxide refrigeration system with a maintenance unit provided by the present application;

[0041] REFERENCE NUMERALS:

[0042] 10: liquid storage tank; 20: refrigeration unit; 21: refrigeration compressor;

[0043] 22: cooler; 23: refrigeration throttle valve; 24: refrigeration flow regulating valve;

[0044] 25: evaporator; 26: first stop valve; 27: second stop valve;

[0045] 30: maintenance unit; 31: maintenance compressor; 32: condenser;

[0046] 33: maintenance throttle valve; 34: heat exchanger; 35: working medium pump;

[0047] 36: maintenance flow regulating valve; 37: first pipeline; 38: second pipeline. DETAILED DESCRIPTION

[0048] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the present application will be described clearly and completely below in conjunction with the drawings in the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort belong to the scope of protection of the present application.

[0049] In the description of the present application, it should be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "linking" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integral connection; it can be mechanical connection, or electrical connection; it can be direct connection, or indirect connection through intermediate medium, or internal connection of two elements. "First", "second", "third", "fourth" do not represent any sequence relationship, but only distinguish for the convenience of description. For those of ordinary skill in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances. "Current" is the time when a certain action is performed, and multiple currents appear in the text, which are recorded in real time as time elapses.

[0050] The embodiments of the present application will be described below in conjunction with Figures 1 to 5 The embodiments of the present application will be described below in conjunction with

[0051] Carbon dioxide (CO2) is a natural refrigerant, which has the characteristics of high density and low viscosity, small flow loss, good heat transfer effect, and can make up for the disadvantage of low circulation by strengthening the heat transfer effect. Therefore, carbon dioxide as an environmentally friendly refrigerant is gradually promoted and applied by more and more enterprises.

[0052] However, the critical pressure of carbon dioxide is high (7.38 MPa), so the working pressure of the transcritical carbon dioxide refrigeration cycle is much higher than that of the traditional subcritical two-phase refrigeration cycle, about 6-8 times the pressure of the traditional refrigerant CFC or HCFC system. Therefore, a corresponding maintenance unit needs to be configured for the carbon dioxide refrigeration unit to maintain or relieve the pressure in the carbon dioxide refrigeration system. When the carbon dioxide refrigeration system is unexpectedly powered off or disconnected, the maintenance unit starts to work to keep the pressure in the carbon dioxide refrigeration system within a reasonable range.

[0053] For details, please refer to Figure 1 The first aspect of the present application provides a carbon dioxide refrigeration system with a maintenance unit, comprising a liquid storage tank 10, a refrigeration unit 20 and a maintenance unit 30.

[0054] The liquid storage tank 10 is connected in circulation with the refrigeration unit 20 for supplying liquid to the refrigeration unit 20; the maintenance unit 30 is connected in circulation with the liquid storage tank 10 for maintaining the pressure in the liquid storage tank 10 and predicting the future change trend of the liquid level in the liquid storage tank 10.

[0055] It can be understood that, compared with the existing carbon dioxide refrigeration system, the present application can predict the future change trend of the liquid level in the liquid storage tank 10, so as to adjust the liquid level in the liquid storage tank 10 according to the change trend of the liquid level, effectively ensure the height of the liquid level in the liquid storage tank 10, provide continuous carbon dioxide liquid supply for the refrigeration unit 20, and keep the liquid level height in the liquid storage tank 10 in a relatively stable state, thereby improving the stability of the refrigeration system operation.

[0056] The maintenance unit 30 comprises a heat exchanger 34, a first pipeline and a second pipeline, the first pipeline and the second pipeline are connected in circulation with the heat exchanger 34 respectively, and cooperate to maintain the pressure in the liquid storage tank 10 below the upper limit operating pressure set by the maintenance unit 30.

[0057] Specifically, the first pipeline comprises a maintenance compressor 31, a condenser 32 and a maintenance throttle valve 33, a first outlet (not shown in the figure) of the heat exchanger 34 and a first inlet (not shown in the figure) of the heat exchanger 34 are connected with a first pipeline 37, and the maintenance compressor 31, the condenser 32 and the maintenance throttle valve 33 are arranged in sequence on the first pipeline 37 along the flow direction of the cooling medium.

[0058] The low-temperature and low-pressure carbon dioxide gas after heat exchange in the heat exchanger 34 is sucked into the maintaining compressor 31 to be compressed into high-temperature and high-pressure carbon dioxide gas, and the high-temperature and high-pressure carbon dioxide gas becomes high-pressure and high-temperature carbon dioxide liquid after entering the condenser 32, and the high-pressure and high-temperature carbon dioxide liquid becomes low-temperature liquid mixture after throttling in the maintaining throttle valve 33 to enter the heat exchanger 34 to cool the carbon dioxide liquid in the second pipeline circulating in the liquid tank 10 to reduce the temperature of the carbon dioxide cooling working medium flowing into the liquid tank 10, so that the pressure in the liquid tank 10 is lower than the set upper limit operating pressure.

[0059] More specifically, the maintaining throttle valve 33 controls the flow of the carbon dioxide cooling working medium by changing the throttling section or the throttling length. The condenser 32 converts the high-temperature and high-pressure carbon dioxide gas or vapor compressed by the maintaining compressor 31 into liquid, and transfers the heat in the condenser 32 to the air near the condenser 32 in the conversion process to release heat and cool down.

[0060] Specifically, the second pipeline includes a working medium pump 35 and a maintaining flow regulating valve 36, and the second pipeline 38 is connected between the second outlet of the heat exchanger 34 (not shown in the figure) and the second inlet of the heat exchanger 34 (not shown in the figure).

[0061] The working medium pump 35 is arranged on the second pipeline 38 connected between the second outlet of the heat exchanger 34 and the liquid tank 10, and can deliver the cooling working medium (i.e. the carbon dioxide cooling working medium) after heat exchange in the heat exchanger 34 to the liquid tank 10.

[0062] The maintaining flow regulating valve 36 is arranged on the second pipeline 38 connected between the liquid tank 10 and the second inlet of the heat exchanger 34, and adjusts the opening of the maintaining flow regulating valve 36 according to the pressure in the liquid tank 10, so that the pressure in the liquid tank 10 can be maintained within a stable range; or adjusts the opening of the maintaining flow regulating valve 36 according to the liquid level in the liquid tank 10, so that the liquid level in the liquid tank 10 is in a relatively balanced state.

[0063] More specifically, a pressure sensor (not shown in the figure) is installed on the liquid tank 10 to monitor the pressure in the liquid tank 10. If the pressure in the liquid tank 10 rises and the rising pressure is greater than the highest pressure of the system (which can also be understood as the maximum pressure value that the tank body of the liquid tank 10 can withstand. This value can be measured in advance, so it can be set as a pressure threshold in the system), when the pressure in the liquid tank 10 approaches or is greater than the threshold, the maintaining flow regulating valve 36 is fully opened to quickly deliver the carbon dioxide cooling working medium in the liquid tank 10, and at the same time, the operating power of the maintaining compressor 31 is increased to reduce the pressure in the liquid tank 10.

[0064] In addition, a liquid level sensor (not shown in the figure) is arranged in the liquid storage tank 10, by which the liquid level in the liquid storage tank 10 can be clearly understood; at the same time, a first flow meter (not shown in the figure) is installed on the inlet pipeline of the liquid storage tank 10, for measuring the total flow of the cooling medium flowing into the liquid storage tank 10, and a second flow meter (not shown in the figure) is installed on the inlet pipeline of the refrigerating unit 20, for measuring the total flow of the cooling medium flowing out of the refrigerating unit 20, and the controller calculates the flow difference measured by the first flow meter and the second flow meter, and the controller can adjust the operating frequency of the compressor 31 and the opening degree of the flow regulating valve 36 in combination with the size of the difference and the liquid level in the liquid storage tank.

[0065] As shown in Figure 5 , the controller collects the total flow Q i of the cooling medium flowing into the liquid storage tank 10, the total flow Q o of the cooling medium flowing out of the liquid storage tank 10, and the current liquid level L of the liquid storage tank 10.

[0066] In combination with the collected values, the values of E(k) and EC(k) are calculated according to the following formula (1-1):

[0067] E(k) = set(k) - {∫[Q i (k) - Q o (k)]dt + L(k)}

[0068]

[0069] Table 1 Fuzzy Value Reference Table

[0070]

[0071] In combination with the above Table 1, the fuzzy set of E(k) and EC(k) is as shown in the following (1-2):

[0072] [E min E max ]→[-6 6]

[0073] [EC min EC max ]→[-6 6] (1-2)

[0074] According to the above fuzzy values, the three parameter values of the PID controller are as shown in the following formula (1-3):

[0075]

[0076]

[0077]

[0078] The three parameters of the PID controller are obtained according to the calculation of the above formula, and then the operating frequency of the compressor 31 and the opening degree of the flow regulating valve 36 are maintained, so that the liquid level in the liquid storage tank 10 is maintained in a relatively stable range, and the refrigeration unit 20 can be provided with continuous carbon dioxide liquid supply, so as to improve the stability of the refrigeration system.

[0079] The PID controller is a feedback loop component, and the controller compares the collected data with the reference value, and then uses the difference to calculate a new input value, so that the data of the system can reach or remain near the reference value, so that the system is more accurate and stable.

[0080] The embodiment can adjust the maintenance unit 30 according to the real-time flow of the carbon dioxide refrigeration system, so as to realize dynamic adjustment and fine adjustment of the refrigeration unit 20, ensure that the pressure matches during the operation of the refrigeration unit 20, and improve the operation stability of the refrigeration unit 20.

[0081] Please refer to Figure 1 , specifically, the refrigeration unit 20 includes a refrigeration flow regulating valve 24, an evaporator 25, a refrigeration compressor 21, a cooler 22 and a refrigeration throttling valve 23 along the circulation path of the refrigeration working medium, and the carbon dioxide cooling working medium in the liquid storage tank 10 enters the refrigeration unit 20 through the refrigeration flow regulating valve 24, and flows out of the refrigeration unit 20 through the refrigeration throttling valve 23, and enters the liquid storage tank 10, forming a closed circulation loop.

[0082] The low-temperature condensing working medium exchanges heat with the outside air through the evaporator 25, and the gasification absorbs heat to achieve the effect of refrigeration. The refrigeration compressor 21 sucks in low-temperature and low-pressure refrigerant gas from the suction pipe, and after being compressed by the motor operation to drive the piston, high-temperature and high-pressure refrigerant gas is discharged to the exhaust pipe to provide power for the refrigeration cycle.

[0083] It should be noted that the refrigeration unit 20 can be a small CO2 refrigeration system of the prior art, and its specific working principle will not be described here, and it can also be other carbon dioxide refrigeration systems.

[0084] Please refer to Figure 2 In some embodiments, the refrigeration unit 20 includes a plurality of groups of refrigeration modules, and the plurality of groups of refrigeration modules are arranged in parallel with each other and then arranged in series with the liquid storage tank 10, so that the liquid storage tank 10 can supply liquid to the plurality of groups of refrigeration modules arranged in parallel.

[0085] Wherein, each refrigeration module is equivalent to the refrigeration unit 20 in the above embodiment, it can be understood that the refrigeration unit 20 can be a group, and the refrigeration unit 20 can also be a plurality of groups in parallel, and the specific setting is set according to the use condition.

[0086] When the refrigeration unit 20 is a plurality of groups in parallel, it is equivalent to include the first refrigeration unit N1, the second refrigeration unit N2, and the Nth refrigeration unit Nn, and the total flow of the cooling working medium flowing out of the refrigeration unit 20 is the sum of the flow of the cooling working medium flowing out of each group of refrigeration units.

[0087] Wherein, when measuring the total flow of the cooling working medium flowing into the refrigeration unit 20 in a plurality of groups in parallel, the sum can be obtained, that is, a second flow meter (not shown in the figure) is installed on each refrigeration module in parallel, and the total flow of the cooling working medium flowing out of the refrigeration unit 20 is equivalent to the sum of the flows of the refrigeration modules N1, N2, N3,..., and Nn; or a total flow meter is arranged on the dry road of the plurality of parallel refrigeration modules, and is used to measure the total flow of the cooling working medium flowing into the refrigeration unit 20.

[0088] In addition, it should be noted that the flow change of a single refrigeration unit 20 has no certain connection with the total flow. That is, under normal circumstances, the flow of some units in the plurality of groups of refrigeration units 20 in parallel may increase, and the flow of some units may decrease, so the total flow is equivalent to remain unchanged, and the change trend of the future liquid level in the liquid storage tank 10 is predicted according to the result of the total flow.

[0089] In addition, the refrigeration unit 20 includes a dry road pipeline and a branch pipeline, wherein the refrigeration flow regulating valve 24, the evaporator 25, the refrigeration compressor 21, the cooler 22 and the refrigeration throttling valve 23 are all located on the branch pipeline.

[0090] The first stop valve 26 and the second stop valve 27 are arranged on the dry road pipeline, and the dry road pipeline of each group of refrigeration units 20 is provided with a stop valve, and the stop valve can realize the pipeline on-off between the liquid storage tank 10 and the refrigeration unit 20.

[0091] The first stop valve 26 and the second stop valve 27 on any dry road pipeline are closed, and any group of refrigeration units 20 is stopped. The stop valve belongs to a forced sealing type valve, which can instantaneously cut off the flow.

[0092] It can be understood that the main role of maintaining the unit 30 in the application is to maintain the pressure in the liquid storage tank 10 at the take-off pressure of the safety valve. When the refrigeration unit 20 does not need to carry out refrigeration and liquid supply, the refrigeration unit 20 is in a dormant state, that is, the refrigeration compressor 21 and the cooler 22 are both in a closed state. The carbon dioxide cooling working medium in the liquid storage tank 10 will still continuously exchange heat with the environment, so that the pressure in the liquid storage tank 10 will continuously rise.

[0093] When the total flow rate Q of the cooling medium flowing into the liquid storage tank 10 is measured and calculated c Close to or greater than the total flow rate Q of the cooling medium flowing out of the refrigeration unit 20 o When the temperature drops to 0, it indicates that the refrigeration unit 20 is about to stop working or has stopped working, or some of the refrigeration modules in the refrigeration unit 20 have stopped working, which is equivalent to the pressure in the liquid storage tank 10 gradually increasing. At this time, it is necessary to adjust the opening of the maintenance flow control valve 36, which is equivalent to opening the maintenance unit 30 to cool the carbon dioxide in the liquid storage tank 10. When the pressure drops to the lower limit set value, the maintenance flow control valve 36 is closed, which is equivalent to shutting down the maintenance unit 30.

[0094] See Figures 3 to 5 A second aspect of the present invention provides a method for controlling a carbon dioxide refrigeration system having a maintenance unit, comprising the following steps:

[0095] Step S10: The carbon dioxide refrigeration system with a maintenance unit includes a liquid storage tank 10, a refrigeration unit 20, and a maintenance unit 30, and the carbon dioxide refrigeration system with the maintenance unit has a preset upper limit operating pressure and a preset lower limit operating pressure;

[0096] Step S20: A flow regulating valve 36 is provided on the circulation loop of the maintenance unit 30 to determine whether the operating pressure of the carbon dioxide refrigeration system of the maintenance unit is sufficient;

[0097] If the operating pressure of the CO2 refrigeration system with the maintenance unit is higher than the upper operating pressure limit (i.e., the maximum operating pressure), the maintenance flow regulating valve 36 is fully opened, and at the same time, the operating frequency of the maintenance compressor 31 is adjusted to the maximum operating frequency;

[0098] If the operating pressure of the carbon dioxide refrigeration system with the maintenance unit is lower than the lower limit operating pressure (i.e., the minimum set pressure), the maintenance flow regulating valve 36 is adjusted to the minimum flow of the incompletely closed state, and at the same time, the operating frequency of the maintenance compressor 31 is adjusted to intermittent start or direct shutdown;

[0099] Step S21: Based on the above steps, the future liquid level in the liquid storage tank 10 is estimated, and the flow of the refrigeration system is controlled in combination with the PID controller;

[0100] If the liquid level in the liquid storage tank 10 is within the set range, the difference between the total inflow of the carbon dioxide working medium in the liquid storage tank 10 and the total outflow of the carbon dioxide working medium in each refrigeration component is calculated, and the difference is compared with the actual liquid level value in the liquid storage tank 10 to determine whether to maintain the operating power or operating frequency of the compressor 31 and the opening of the flow control valve 36;

[0101] Step S22: On the basis of the above steps, the liquid level in the liquid tank 10 is judged, and if the liquid level in the liquid tank 10 is lower than the set range value, the maintaining flow regulating valve 36 is kept fully open, and at the same time, the operating frequency of the maintaining compressor 31 is adjusted to the maximum operating frequency;

[0102] Step S23: On the basis of the above steps, the liquid level in the liquid tank 10 is judged, and if the liquid level in the liquid tank 10 is higher than the set range value, the maintaining flow regulating valve 36 is adjusted to the minimum flow that is not fully closed, and at the same time, the operating frequency of the maintaining compressor 31 is adjusted to the lowest operating frequency for safe operation or the maintaining compressor 31 is directly stopped;

[0103] The carbon dioxide liquid in the liquid tank 10 is throttled and flow-distributed by the carbon dioxide liquid throttling valve according to the required cooling capacity and the required evaporation temperature of each carbon dioxide refrigeration system with a maintaining unit, so as to complete the carbon dioxide refrigeration cycle;

[0104] If the carbon dioxide refrigeration system with a maintaining unit needs to be stopped, the first stop valve can be switched to realize the on-off.

[0105] According to the above-described control method, the liquid level in the liquid tank 10 is taken as an example for illustration, for example, calculation is performed according to the following formula (1-4):

[0106] set(k) = a x (0.8-0.3) + 0.3 (1-4)

[0107] When the stability requirement of the refrigeration system is low, 0.1 < a < 0.5 can be selected;

[0108] When the stability requirement of the refrigeration system is high, 0.5 < a < 0.9 can be selected;

[0109] The value of set(k) obtained by calculation is brought into the above formula (1-1), and the opening degree of the maintaining flow regulating valve 36 and the control data of the operating frequency of the maintaining compressor 31 are calculated according to the algorithm process of the PID controller.

[0110] The control method of the carbon dioxide refrigeration system with a maintaining unit provided by the application can predict the future liquid level of the carbon dioxide refrigeration system with a maintaining unit by calculating the difference between the total inflow of the carbon dioxide working medium in the liquid tank 10 and the total outflow of the carbon dioxide working medium in each refrigeration component, and combining the actual liquid level value in the liquid tank 10, adjust the opening degree of the maintaining flow regulating valve 36 and the operating frequency of the maintaining compressor 31 according to the predicted liquid level trend, keep the pressure of the carbon dioxide refrigeration system with a maintaining unit in a balanced state, and ensure the stable operation of the carbon dioxide refrigeration system with a maintaining unit.

[0111] And, the control method of the carbon dioxide refrigeration system with a maintaining unit provided by the present application can supply liquid to multiple small carbon dioxide refrigeration assemblies in parallel through a single liquid storage tank 10, and keep the pressure in the liquid storage tank 10 at a low level when unexpected power failure, system failure or short-term no cooling demand occurs, thereby effectively keeping the pressure in the carbon dioxide refrigeration system with a maintaining unit within a safe pressure range, and improving the safety of the carbon dioxide refrigeration system with a maintaining unit and reducing the design difficulty of the carbon dioxide refrigeration system with a maintaining unit.

[0112] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A control method for a carbon dioxide refrigeration system having a maintenance unit, characterized in that: The carbon dioxide refrigeration system with a maintenance unit includes a liquid storage tank, a refrigeration unit, and a maintenance unit. The liquid storage tank is cyclically connected to the refrigeration unit for supplying liquid to the refrigeration unit. The maintenance unit is cyclically connected to the liquid storage tank for maintaining the pressure in the liquid storage tank and for predicting future changes in the liquid level in the liquid storage tank. The control method comprises the following steps: Step S10: The carbon dioxide refrigeration system with a maintenance unit includes a liquid storage tank, a refrigeration unit, and a maintenance unit, and the carbon dioxide refrigeration system with a maintenance unit has a preset upper limit operating pressure and a preset lower limit operating pressure; Step S20: A flow regulating valve is provided on the circulation loop of the maintenance unit to determine the operating pressure of the carbon dioxide refrigeration system of the maintenance unit; If the operating pressure of the carbon dioxide refrigeration system having the maintenance unit is higher than the upper limit operating pressure, the maintenance flow regulating valve is fully opened, and at the same time the operating frequency of the maintenance unit is adjusted to the maximum operating frequency; If the operating pressure of the carbon dioxide refrigeration system with the maintenance unit is lower than the lower limit operating pressure, the maintenance flow regulating valve is adjusted to the minimum flow, and at the same time the operating frequency of the maintenance unit is adjusted to intermittent start or direct shutdown.

2. The control method of the carbon dioxide refrigeration system with a maintenance unit according to claim 1, characterized in that: The maintenance unit includes a heat exchanger, a first pipeline and a second pipeline; The first pipeline and the second pipeline are respectively connected to the heat exchanger in a circulation manner, and the second pipeline is also connected to the liquid storage tank in a circulation manner.

3. The control method of the carbon dioxide refrigeration system with a maintenance unit according to claim 2, characterized in that: The first pipeline includes a maintenance compressor, a condenser and a maintenance throttle valve; A first pipeline is connected between the first outlet of the heat exchanger and the first inlet of the heat exchanger, and the maintenance compressor, the condenser and the maintenance throttle valve are sequentially arranged on the first pipeline along the flow direction of the cooling medium.

4. The control method of the carbon dioxide refrigeration system with a maintenance unit according to claim 2, characterized in that: The second pipeline includes a working fluid pump and a flow maintaining regulating valve; A second pipe is connected between the second outlet of the heat exchanger and the second inlet of the heat exchanger, the flow maintaining regulating valve is connected to the second pipe and is located at the inlet of the liquid storage tank, and the working fluid pump is connected to the second pipe and is located at the outlet of the liquid storage tank.

5. The control method of a carbon dioxide refrigeration system with a maintenance unit according to claim 3 or 4, characterized in that: The liquid storage tank is equipped with a pressure sensor and a liquid level sensor; The pressure sensor is used to monitor the pressure in the liquid storage tank, and the liquid level sensor is used to monitor the liquid level in the liquid storage tank.

6. The control method of a carbon dioxide refrigeration system having a maintenance unit according to claim 1, characterized in that: The refrigeration unit includes a refrigeration flow regulating valve, an evaporator, a refrigeration compressor, a cooler and a refrigeration throttle valve in sequence along the circulation path of the refrigerant; Wherein, the refrigeration flow regulating valve and the refrigeration throttle valve are respectively connected to the liquid storage tank through pipelines.

7. The control method of a carbon dioxide refrigeration system having a maintenance unit according to claim 1, characterized in that: The refrigeration unit includes a plurality of refrigeration modules, which are connected in parallel and then connected in series with the liquid storage tank.

8. The control method of a carbon dioxide refrigeration system having a maintenance unit according to claim 7, characterized in that: The refrigeration module includes a refrigeration flow regulating valve, an evaporator, a refrigeration compressor, a cooler and a refrigeration throttle valve in sequence along the circulation path of the refrigerant; Wherein, the refrigeration flow regulating valve is connected to the liquid storage tank through a first stop valve, and the refrigeration throttle valve is connected to the liquid storage tank through a second stop valve.

9. The control method of a carbon dioxide refrigeration system having a maintenance unit according to claim 2, characterized in that: When determining the operating pressure of the carbon dioxide refrigeration system having the maintenance unit, determining the liquid level in the liquid storage tank includes the following steps: Step S21: If the liquid level in the liquid storage tank is within a set range, the difference between the total inflow of the carbon dioxide working medium in the liquid storage tank and the total outflow of the carbon dioxide working medium in each refrigeration component is calculated to determine the operating frequency of the maintenance compressor and the opening of the maintenance flow control valve; Step S22: If the liquid level in the liquid storage tank is lower than the set range value, the flow control valve is fully opened and the operating frequency of the compressor is adjusted to the maximum operating frequency; Step S23: If the liquid level in the liquid storage tank is higher than the set range value, the flow control valve is adjusted to the minimum flow, and the operating frequency of the compressor is adjusted to the lowest operating frequency or shut down.

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