A method for identifying and controlling oil return in connection with uneven distribution of a multi-evaporator system for cold storage

Abstract

The present application relates to cold storage refrigeration control technical field, provide a kind of cold storage with multiple evaporative system liquid distribution uneven identification and oil return associated control method, by screening liquid supply reasonable, heat transfer effective, stable evaporation branch of stable control, with its statistical average of real-time parameter constructs dynamic virtual reference branch as adaptive diagnosis reference;Determine the liquid supply state in conjunction with the comparison of target branch and the reference, and determine the oil return state by locking the time sequence correlation of branch state change and compressor oil level change;On this basis, the double-dimension determination of liquid supply and oil return is formed to distinguish local liquid supply abnormality, oil return lag abnormality and liquid supply and oil return coupling abnormality;And for coupling abnormality, different control modes are executed according to the order of abnormality occurrence to infer cause and effect.The method realizes the intelligent closed loop of full-condition adaptive diagnosis, root cause accurate positioning and collaborative control, significantly improves the reliability, energy efficiency and operation safety of the system.

Description

Technical Field

[0001] This invention relates to the field of cold storage refrigeration control technology, specifically to a method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage. Background Technology

[0002] Cold storage refrigeration systems typically include compressors, condensers, throttling devices, and evaporators. For medium to large-sized cold storage facilities or multi-zone cold storage facilities, multi-unit evaporator systems are often used to meet the refrigeration needs of different zones and load areas. These systems generally consist of a compressor module supplying refrigerant to multiple evaporator branches through a common liquid line. Each evaporator branch is equipped with an electronic expansion valve and an evaporator. The evaporated refrigerant then returns to the compressor module through a common return gas line.

[0003] During the operation of a multi-unit evaporator system, the load status, pipe length, local resistance, evaporator heat exchange capacity, and electronic expansion valve operation status of each evaporator branch may differ, resulting in inconsistent refrigerant distribution among multiple evaporator branches. Some evaporator branches may experience insufficient liquid supply, manifested as increased return gas superheat and decreased heat exchange capacity; while some evaporator branches may experience excessive liquid supply, manifested as decreased return gas superheat, insufficient utilization of liquid refrigerant, and even increased risk of liquid return.

[0004] Meanwhile, the compressor oil in the cold storage refrigeration system needs to circulate with the refrigerant and return to the compressor module via the return gas pipe. If the liquid supply in a certain evaporator branch is abnormal, it may cause changes in the refrigerant flow rate in the evaporator or return gas pipe of that branch, thereby affecting the oil carrying capacity and causing lubricating oil to remain in the evaporator or return gas pipe, resulting in a drop in the oil level of the compressor module. Conversely, if the compressor module has experienced oil return lag, it may also cause fluctuations in the system's operating status, causing abnormal liquid supply in some evaporator branches.

[0005] Existing multi-unit evaporation systems in cold storage are typically controlled based on the return gas superheat, evaporation pressure, or electronic expansion valve opening of a single evaporation branch, or by implementing oil return protection independently based on the compressor oil level. While these methods can maintain system operation to some extent, they lack effective means to identify and coordinate the correlation between uneven liquid distribution among multiple branches, abnormal liquid supply in branches, and abnormal oil return from the compressor module.

[0006] Therefore, it is necessary to provide a method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage, which can construct a dynamic benchmark based on the operating status of multiple evaporation branches and combine it with the oil level changes of the compressor module to identify abnormal liquid supply, abnormal oil return lag, and abnormal coupling between the two. Summary of the Invention

[0007] To address these issues, this invention provides a method for identifying uneven liquid distribution and coordinating oil return control in a multi-unit evaporation system for cold storage. This method overcomes the problems in existing technologies, such as the reliance on fixed thresholds making it difficult to adapt to dynamic operating conditions of multiple branches, the difficulty in distinguishing between abnormal valve opening and actual liquid distribution, and the disconnect between liquid supply control and oil return protection, which prevents coordinated control based on the sequence of abnormalities.

[0008] To achieve the above objectives, the present invention provides a method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage. The multi-unit evaporation system for cold storage includes a compressor module and several parallel evaporation branches connected by a common liquid pipe and a common gas return pipe. Each evaporation branch includes an electronic expansion valve and an evaporator. The method for identifying uneven liquid distribution and controlling oil return includes: The return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each evaporation branch are obtained, as well as the oil level change of the compressor module are obtained. Stable evaporation branches are selected based on the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each evaporation branch to construct a virtual benchmark branch. The liquid supply status of the target evaporation branch is determined based on the comparison results between the target evaporation branch and the virtual reference branch; The associated oil return status is determined based on the time relationship between the change in the target evaporator branch status and the change in the compressor module oil level. Based on the liquid supply status and the associated oil return status of the target evaporation branch, determine whether the corresponding target evaporation branch is abnormal and the cause of the abnormality. The cause of the abnormality includes local liquid supply abnormality, oil return lag abnormality, and liquid supply and oil return coupling abnormality. In response to an abnormality in the liquid supply and oil return coupling, the joint control mode is determined based on the sequential changes in the liquid supply state and the associated oil return state of the target evaporation branch. The joint control modes include a fluid supply priority compensation followed by oil return confirmation mode, an oil return priority recovery mode, and a system-level rebalancing mode.

[0009] As a preferred technical solution for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage, if the return gas superheat and the heat exchange temperature difference of a single evaporation branch are both within the corresponding standard range, and the fluctuation range of the electronic expansion valve opening within a preset time window is less than the preset fluctuation value, then the evaporation branch is determined as a stable evaporation branch.

[0010] As a preferred technical solution for identifying uneven liquid distribution and controlling oil return in multi-unit evaporation systems for cold storage, the reference return superheat, reference heat exchange temperature difference, and reference electronic expansion valve opening of the virtual reference branch are determined based on the average values ​​of the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each stable evaporation branch, so as to construct the virtual reference branch.

[0011] As a preferred technical solution for identifying uneven liquid distribution and controlling oil return in multi-unit evaporation systems used in cold storage, the liquid supply status is determined based on the return gas superheat and heat exchange temperature difference of the target evaporation branch before determining the liquid supply status, including: If both the return gas superheat and the heat exchange temperature difference are higher than the corresponding reference range, it is initially judged to be a state of insufficient liquid supply. If both the return gas superheat and the heat exchange temperature difference are below the corresponding reference range, it is initially judged to be an oversupply of liquid. If both the return gas superheat and the heat exchange temperature difference are within the corresponding reference range, it is initially determined that the liquid supply is in a normal state.

[0012] As a preferred technical solution for identifying uneven liquid distribution and controlling oil return in multi-unit evaporation systems used in cold storage, the liquid supply status is determined based on the return gas superheat and heat exchange temperature difference of the target evaporation branch before determining the liquid supply status, including: If the return gas superheat and the heat exchange temperature difference contradict each other relative to the corresponding reference range, the liquid supply status is determined by combining the opening degree of the electronic expansion valve, wherein: If the opening degree of the electronic expansion valve is not lower than the reference range of the electronic expansion valve opening degree, then the liquid supply state is determined to be an insufficient liquid supply state. If the opening degree of the electronic expansion valve is lower than the reference range of the electronic expansion valve opening degree, the liquid supply state is determined to be an over-supply state.

[0013] As a preferred technical solution for the identification and oil return control method of uneven liquid distribution in multi-unit evaporation system for cold storage, if the oil level of the compressor module drops and does not recover within the preset recovery time window after the target evaporation branch is put into operation, the opening degree of the electronic expansion valve changes, or the liquid supply status changes, it is determined that the target evaporation branch is in the associated oil return lag state. Conversely, if the target evaporation path is in a normal state of associated oil return, it is determined that the target evaporation path is in a normal state of associated oil return.

[0014] As a preferred technical solution for the method of identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage, based on the determination result that the liquid supply status and / or the associated oil return status of the target evaporation branch are abnormal, the cause of the abnormality of the corresponding target evaporation branch is determined, including: In response to the fact that only the liquid supply state is a state of excessive liquid supply or insufficient liquid supply, the cause of the abnormality is determined to be a local liquid supply abnormality. In response to the fact that only the associated return oil state is an associated return oil lag state, the cause of the abnormality is determined to be an abnormal return oil lag. In response to both the fluid supply status and the associated oil return status being abnormal, the cause of the abnormality is determined to be an abnormal coupling of fluid supply and oil return.

[0015] As a preferred technical solution for identifying uneven liquid distribution and coordinating oil return control in multi-unit evaporation systems used in cold storage, in response to abnormalities in the liquid supply and oil return coupling, the abnormal times of the liquid supply status and the associated abnormal times of the oil return status of the target evaporation branch are determined to establish the coordinating control mode, wherein: In response to the abnormal liquid supply status occurring earlier than the abnormal return oil status, it is determined that the uneven liquid distribution induces the abnormal return oil status, and the joint control mode is switched to the liquid supply priority compensation followed by return oil confirmation mode, and the opening of the electronic expansion valve of the target evaporation branch is adjusted and the associated return oil status is reconfirmed. In response to the abnormal return oil status occurring earlier than the abnormal supply fluid status, the system determines that the return oil lag induces the supply fluid disturbance and switches the control mode to the return oil priority recovery mode, and adjusts the operating status of the compressor module and limits the increase in the opening of the electronic expansion valve. In response to abnormal liquid supply conditions, the system synchronizes with abnormal oil return conditions, identifies system-level liquid separation and oil return coupling disturbances, switches the joint control mode to system-level rebalancing mode, and re-screens stable evaporation branches and updates virtual reference branches.

[0016] As a preferred technical solution for identifying uneven liquid distribution and controlling oil return in multi-unit evaporation systems used in cold storage, the control mode is switched to a local liquid supply compensation mode or an oil return priority recovery mode in response to local liquid supply anomalies or oil return lag anomalies.

[0017] Compared with existing technologies, the beneficial effects of this invention are that it provides a method for identifying uneven liquid distribution and coordinating oil return control in a multi-unit evaporation system for cold storage. Its core lies in constructing a virtual benchmark branch based on the average state of stable branches as an adaptive reference system, and performing dual-dimensional diagnosis and coordinated control through time-series correlation analysis of oil level changes in the compressor module. This fundamentally solves the three major technical bottlenecks of traditional methods: poor adaptability of fixed thresholds, confusion between liquid supply and oil return faults, and blind intervention exacerbating system fluctuations when coupling anomalies occur. First, this method abandons fixed alarm thresholds and selects stable evaporation branches with reasonable liquid supply, effective heat exchange, and stable control from the operating branches in real time. The statistical average value of its parameters forms a dynamic virtual benchmark, which changes synchronously with the storage temperature, load, and compressor frequency, ensuring that the diagnostic criteria always match the current operating conditions and significantly reducing false alarms and missed alarms. Second, this method locks the branch status changes and compressor oil level changes in the same time window for correlation analysis, forming a dual diagnosis combining the liquid supply status and the associated oil return status. This allows for accurate differentiation between local liquid supply anomalies limited to the branch itself, oil return lag anomalies hidden in the oil circuit system, and coupling anomalies of liquid supply and oil return that exacerbate each other, avoiding misjudgments. Finally, for the most complex coupled anomalies, this method further infers the causal chain by determining the order of occurrence of the two anomalies. Based on this, it selects a joint control mode of liquid supply priority, oil return priority, or system-level rebalancing, ensuring that every intervention targets the root cause and preventing secondary disasters caused by reverse regulation. This achieves a complete intelligent closed loop from monitoring, diagnosis to control, significantly improving the system's operating efficiency and reliability. In particular, traditional methods often use fixed superheat or temperature difference thresholds to judge uneven liquid distribution. However, under wide operating conditions such as cold storage temperature increases from ambient to low temperatures and sudden changes in cargo load during inbound and outbound operations, fixed thresholds inevitably fail. This invention creatively introduces a virtual reference branch, which is dynamically constructed from the average values ​​of healthy branch parameters selected under the current operating conditions. The diagnostic criteria migrate in real time with the system's operating status, ensuring that the definition of normal always matches the current storage temperature, frequency, and load level. This eliminates the interference of operating condition drift on diagnostic accuracy and fundamentally solves the problem of inaccurate anomaly identification under varying operating conditions in cold storage. In particular, this invention physically integrates the fluid supply status and the oil return status. By establishing a diagnostic matrix of the fluid supply status and the associated oil return status, anomalies are precisely decoupled into three types. Changes in branch fluid supply parameters and compressor oil level changes are analyzed on the same time axis: fluid supply anomalies alone correspond to local fluid supply anomalies within the branch; oil level anomalies alone, but the fluid supply parameters are pseudo-normalized, correspond to hidden oil return lag anomalies; and both are abnormal, which is determined to be a fluid supply and oil return coupling anomaly. This fine classification provides a solid foundation for subsequent targeted measures and avoids blind intervention. In particular, when dealing with coupling anomalies, this invention infers the root cause based on the order in which the abnormality in the fluid supply state and the associated abnormality in the return oil state occur, and matches three targeted joint control modes: if the fluid supply is abnormal first, it means that uneven fluid distribution is the culprit, so the fluid supply priority compensation followed by the return oil confirmation mode is adopted for regulation; if the return oil is abnormal first, it means that the oil circuit problem is the source, so the return oil priority recovery mode is entered and the valve action is restricted to prevent secondary liquid hammer; if the two are simultaneous, it indicates that there is a system-level disturbance, and the system-level rebalancing mode needs to be entered to stabilize the system as a whole. This decision based on timing logic ensures that each intervention is a precise adjustment oriented towards the root cause, fundamentally improving the robustness and safety of the system under complex faults. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the workflow of the method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of the multi-unit evaporation system used in an embodiment of the present invention. Detailed Implementation

[0019] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0020] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0021] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "inner", "outer", etc., which indicate directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0022] Furthermore, it should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0023] Please see Figure 1 and Figure 2The figures shown are, respectively, a flowchart of the method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage according to an embodiment of the present invention, and a structural schematic diagram of the multi-unit evaporation system according to an embodiment of the present invention. Figure 2 It shows N parallel evaporation branches and one virtual reference branch.

[0024] In a multi-unit evaporator system, multiple evaporator branches share a single compressor module, a common liquid line, and a common return gas line. Due to differences in load, pipe resistance, electronic expansion valve opening, and evaporator heat exchange status among the branches, it is easy for one branch to have excessive or insufficient refrigerant, resulting in uneven liquid distribution. Simultaneously, abnormal liquid supply to a branch can affect the return gas flow rate and refrigerant oil carrying capacity, further impacting the compressor module's oil return. Therefore, this method first screens stable evaporator branches and constructs a virtual baseline branch from these stable branches (the virtual baseline branch is not an actual existing branch, but a dynamic reference object formed by the average state of multiple stable branches, to avoid the problem of fixed thresholds being unsuitable for different storage temperatures, loads, ambient temperatures, and compressor frequencies). Then, the target evaporator branch is compared with the virtual baseline branch. It should be understood that the return gas superheat reflects whether the refrigerant at the evaporator outlet is too dry or too wet; the heat exchange temperature difference reflects the actual heat exchange effect of the evaporator; the opening of the electronic expansion valve reflects the control action of the liquid supply to this branch; and the change in the compressor module oil level reflects the return oil status under the combined action of multiple branches. In implementation, the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening can determine the liquid supply status of the target evaporator branch to determine whether the refrigerant in the target evaporator branch is too much / too little / normal. The time relationship between the change in the status of the target branch and the change in the compressor module oil level can determine whether there is a related return oil lag in the target evaporator branch. Then, by combining the current performance of these two situations, it can be determined whether the target evaporator branch is abnormal, the cause of the abnormality, and how to control it. In implementation, temperature sensors are installed on the outer wall of the evaporator outlet return pipe of each evaporator branch (i.e., the outer surface of the branch return pipe after the evaporator refrigerant outlet and before the return pipe enters the common return pipe), and pressure sensors are installed at the branch evaporator outlet return pipe. The return gas superheat is calculated as: temperature at the evaporator outlet return pipe of a certain evaporator branch - saturated evaporation temperature calculated from the corresponding evaporation pressure of that evaporator branch. Temperature sensors are installed on the return and outlet sides of the evaporator's fan (avoiding defrosting heating pipes, dead air corners at the outlet, and areas prone to thermal disturbance at the doorway). The device obtains the evaporator inlet and outlet air temperatures, and the temperature difference is equal to the evaporator inlet air temperature minus the evaporator outlet air temperature. The opening of the electronic expansion valve is directly read by the electronic expansion valve controller. The oil level change of the compressor module can be obtained through the compressor oil level sensor, oil separator oil level sensor, compressor crankcase electronic oil level switch (preferred), or a combination of high and low oil level switches. The oil level is usually set at the compressor crankcase, oil separator, or oil balance pipe. The change is usually represented as a graded state, including normal oil level, low oil level, severely low oil level, and oil level decreasing trend.

[0025] This invention provides a method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage, comprising: Step S1: Obtain the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each evaporation branch, as well as the oil level change of the compressor module; Step S2: Based on the return gas superheat, heat exchange temperature difference and electronic expansion valve opening of each evaporation branch, a stable evaporation branch is selected to construct a virtual reference branch. Specifically, if the return gas superheat and the heat exchange temperature difference of a single evaporation branch are both within the corresponding standard range, and the fluctuation range of the electronic expansion valve opening within the preset time window is less than the preset fluctuation value, then the evaporation branch is determined as a stable evaporation branch.

[0026] It is understandable that if the return gas superheat and the heat exchange temperature difference of a single evaporation branch are both within the corresponding standard range, it means that the refrigerant evaporation state is stable, the evaporator heat exchange state is stable, and the control action is also stable. If the return gas superheat is too high or too low, it means that the evaporation branch may be short of liquid or have too much liquid. Frequent fluctuations of the electronic expansion valve indicate that the branch is being adjusted or is oscillating. In practice, due to the slow temperature change in cold storage, the effect of a single adjustment of the expansion valve on the return gas superheat may take several minutes to manifest. Therefore, the preset window time is usually 15 to 30 minutes, and the opening fluctuation of the electronic expansion valve is usually less than 5% of the full opening (when using step control, it is set to less than 20 to 40 steps); the standard range for return gas superheat is usually 6K to 12K; and the standard range for heat exchange temperature difference is usually 1.5K to 4K. Understandably, during the initial cooling phase or after defrosting, the evaporator in a cold storage unit experiences extremely high and drastic heat load fluctuations, making incomplete refrigerant evaporation highly likely. The superheat measurement on the branch return pipe is closer to the compressor than the evaporator outlet; therefore, the lower limit should be set slightly higher than the universal value (5K) to compensate for the delay and accuracy error in pipe wall temperature measurement, ensuring sufficient superheat of the return gas before it enters the common pipe. In multi-split systems, a brief period of liquid carryover in a branch can cause a sudden drop in the superheat of that branch. If this branch is mistakenly selected as the virtual reference, the reference superheat will be too low. Therefore, the setting... The 6K lower limit can exclude critically stable branches that frequently touch the liquid slugging boundary. In a low-temperature cold storage with a temperature of -25℃, the evaporation temperature may be as low as -35℃. If the superheat exceeds 12K, a large section of useless superheat will appear in the downstream section of the evaporator, and the coefficient of performance will drop sharply. At the same time, the abnormally high return gas temperature of this branch will affect the overall superheat of the common return gas pipe and interfere with the suction cooling of the compressor. In addition, excessively high superheat usually means that the liquid supply of this branch is significantly insufficient, its refrigerant mass flow rate is low, and the return gas flow rate is insufficient, which itself will lead to difficulties in oil return in this branch. It is understandable that the heat exchange temperature difference directly reflects the actual heat exchange intensity of the evaporator to the air inside the cold storage. In a cold storage, the evaporator fan continuously circulates the air inside the storage. When the storage temperature stabilizes near the set value, the temperature difference between the inlet and outlet air should remain within a small range. If the temperature difference is extremely small (<1.5K), the following situations may exist: severe frost buildup on the evaporator, blocked air ducts, sharply reduced airflow, extremely poor heat exchange, excessive liquid supply, evaporator coil temperature close to the storage temperature with no driving force for heat exchange, or residual water still evaporating on the coils after the defrosting process of the evaporator fan has just ended. The heat absorption presents a false low temperature difference, and the sensor is located in a dead corner of the air outlet or is interfered with by the residual heat of the heating tube; during the stable storage temperature period, if the heat exchange temperature difference of a certain branch is unusually large (>4K), it means that the local heat load borne by that branch is abnormally high (such as large loss of cold at the door, heat generation of goods entering the storage), the liquid supply of that branch is seriously insufficient, the evaporator coil temperature is too high, and the heat exchange is maintained by increasing the temperature difference; however, during the temperature pull-up stage, the temperature difference of all branches may be large, at this time, stable branches should not be selected or the upper limit should be dynamically relaxed.

[0027] Specifically, the reference return gas superheat, reference heat exchange temperature difference, and reference electronic expansion valve opening of the virtual reference branch are determined based on the average values ​​of the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each stable evaporation branch, so as to construct the virtual reference branch.

[0028] It is understandable that the operating conditions of cold storage will change with the temperature of the storage, the load of goods, the ambient temperature and the frequency of the compressor. Therefore, setting a fixed benchmark value cannot accurately suit the current situation. Thus, the average value of the stable branch is used to form a reference benchmark that changes dynamically with the current operating conditions. In practice, when the number of stable evaporation branches is not less than two, a virtual reference branch is constructed based on the average parameter value of each stable evaporation branch; when the number of stable evaporation branches is less than two, the virtual reference branch of the previous cycle is used as the current virtual reference branch.

[0029] Step S3: Determine the liquid supply status of the target evaporation branch based on the comparison result between the target evaporation branch and the virtual reference branch; it can be understood that the target evaporation branch is an unstable evaporation branch. Specifically, before determining the liquid supply status, the liquid supply status is determined based on the return gas superheat and heat exchange temperature difference of the target evaporation branch, including: If both the return gas superheat and the heat exchange temperature difference are higher than the corresponding reference range, or if one of them is higher than the corresponding reference range and the other is within the corresponding reference range, then the initial judgment is that the refrigerant supply is insufficient. This can be understood as the refrigerant quantity being insufficient to fully wet the evaporator, the superheated zone in the later stage being too long, and the superheat being increased; the average coil temperature of the evaporator being too high, the heat exchange driving force on the air side being large, and the temperature difference being widened; both parameters point to the reason that the refrigerant is insufficient. If both the return gas superheat and the heat exchange temperature difference are below the corresponding reference range, or if one of them is below the corresponding reference range while the other is within the corresponding reference range, then it is initially judged to be an over-supply of refrigerant. It can be understood that when there is an over-supply of refrigerant, the evaporator outlet is still liquid or close to saturation, and the superheat is extremely low. The evaporation temperature is raised to be close to the storage temperature, the heat transfer temperature difference decreases, and at the same time, the two-phase region occupies most of the coil but the heat transfer coefficient does not increase significantly. The total heat transfer does not increase, and the temperature difference narrows instead. Both parameters point to the reason of excessive refrigerant. If the return gas superheat and the heat exchange temperature difference are both within the corresponding reference range, it is initially judged that the liquid supply is in normal condition; at this time, the liquid supply and heat load matching degree of the target branch are consistent with the stable branch group, the evaporator outlet superheat state and the air side heat exchange performance are no different from the reference, and the branch is at the system average level. If the return gas superheat and heat exchange temperature difference contradict their corresponding reference ranges (i.e., one is higher than the reference range while the other is lower), the liquid supply status cannot be directly determined through initial judgment. Therefore, the opening degree of the electronic expansion valve must be introduced for arbitration: if the electronic expansion valve opening degree is higher than the reference range, it is determined to be an insufficient liquid supply state; if the electronic expansion valve opening degree is lower than the reference range, it is determined to be an excessive liquid supply state; if the electronic expansion valve opening degree is within the reference range, it is temporarily classified as a normal liquid supply state but marked as pending observation. It is understandable that (1) when the evaporator is severely frosted, the frost layer blocks the heat exchange on the air side, resulting in a sharp reduction in air volume. Although the liquid supply is normal or even low, the air cannot effectively pass over the coil to absorb heat. The amount of refrigerant evaporating in the evaporator is very small, and most of the coils are in an ineffective state. Therefore, a small amount of refrigerant evaporates rapidly in the later stage, resulting in the phenomenon that the return gas superheat is higher than the return gas superheat reference range, but the heat exchange temperature difference is lower than the heat exchange temperature difference reference range. The same phenomenon will also occur if the air duct of the air cooler is blocked or the fan fails, causing the air-side heat exchange to fail. (2) When the local heat load of a single branch is abnormally large (such as hot air intrusion at the door or newly delivered hot goods), a large amount of heat rushes in, causing the refrigerant to evaporate violently. The evaporator exchanges heat efficiently throughout the process, and the outlet is still close to saturation with extremely low superheat. At the same time, because the heat load is large, the air cools down significantly after passing through the evaporator, resulting in a high heat exchange temperature difference. In this state, the heat exchange is normal or even efficient (heat exchange is normal). The temperature difference is higher than the heat exchange temperature difference reference range, but the liquid supply may already be insufficient relative to the heat load (the return gas superheat is lower than the return gas superheat reference range). In addition, when the refrigerant distribution in the evaporator is unevenly shifted forward, some channels of the distributor after the expansion valve are blocked, resulting in local concentrated liquid supply and local liquid shortage in the coil. The concentrated liquid supply area results in a low outlet superheat, while the air side passes through the entire evaporator, and the average heat exchange temperature difference is larger due to the high coil temperature in the liquid shortage area. It should be understood that when the superheat and heat exchange temperature difference are contradictory, the opening of the electronic expansion valve can directly reflect the current liquid supply intention (that is, the expansion valve opening reflects the controller's judgment and adjustment direction on the liquid supply status). At this time: a large valve opening indicates that the controller is trying to increase the liquid supply to suppress the superheat, which means that the system has a tendency to insufficient liquid supply; a small valve opening indicates that the controller is limiting the liquid supply to prevent the superheat from decreasing further, which means that the system has a tendency to over-supply liquid.

[0030] In implementation, the reference range is determined by the corresponding virtual reference value. The standard deviations of the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each stable evaporation branch are determined and recorded as the standard deviations of the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of the virtual reference branch. The reference range is the corresponding standard deviation of the virtual reference value ±1 to 3 times. In one implementation, the reference range of return gas superheat is the corresponding standard deviation of the reference return gas superheat -1 to 3 times to the corresponding standard deviation of the reference return gas superheat +1 to 3 times. The reference ranges of heat exchange temperature difference and electronic expansion valve opening are determined in the same way as the reference range of return gas superheat, and will not be repeated here.

[0031] Step S4: Determine the associated oil return status based on the time relationship between the change in the target evaporator branch status and the change in the compressor module oil level; Specifically, if the oil level of the compressor module drops within the associated time window after the target evaporation branch is put into operation, the opening degree of the electronic expansion valve changes, or the liquid supply status changes, and fails to recover within the preset recovery time, then the target evaporation branch is determined to be in the associated oil return lag state. Conversely, it indicates that the target evaporation path is in a normal state of associated oil return; It is understandable that compressor oil level is a module-level parameter and does not belong to a specific branch. Therefore, it is not possible to determine the abnormal oil return of a branch solely based on the oil level. Instead, the correlation between the branch and oil return is established by the time relationship between the target branch events (including the target evaporator branch being put into operation, changes in the opening of the electronic expansion valve, or changes in the liquid supply status) and the oil level changes. These events will change the refrigerant flow rate and oil carrying capacity of the return gas in that branch. Specifically: when the branch is put into operation or the opening is increased, the refrigerant flow rate increases and the return gas velocity rises, thus enhancing the oil carrying capacity of that branch; when the opening is reduced or the liquid supply is decreased, the refrigerant flow rate decreases and the return gas velocity drops, thus weakening the oil carrying capacity of that branch; when the liquid supply status changes abnormally (such as excessive liquid supply causing intermittent liquid carryover), the two-phase flow return gas will significantly change the oil return pattern. It should be understood that short-term oil discharge from the compressor is a normal phenomenon (for example, when the load suddenly increases, more oil is discharged from the compressor with the high-speed gas flow). In a healthy system, the discharged oil will be quickly brought back to the compressor through various return gas lines, and the oil level will dynamically stabilize. If the compressor module oil level drops but does not recover within a certain period of time after the event, it indicates that more oil is discharged and less oil is returned, indicating that there is a persistent bottleneck in oil return. Since the occurrence time of this bottleneck is closely related to the state changes of the target branch, it can be determined that the current oil return capacity of the branch is insufficient, causing the oil that should return to the compressor to remain in the branch or the common return gas line; that is, the oil remains on the evaporator branch side, lagging behind the normal cycle of returning to the compressor. In implementation, the associated time window must allow the oil to complete at least one full cycle; otherwise, if the oil is still en route, it will be considered a delay, causing a false alarm. However, the longer the compressor runs without oil, the greater the risk of wear on the bearings and scroll / screw. 10 minutes is a compromise between safety and waiting for self-healing. Therefore, the associated time window is usually 5 to 15 minutes, preferably 10 minutes (for defrosting recovery conditions, the associated time window can be extended to 10 to 20 minutes). The preset recovery time is usually 5 to 10 minutes. For small units (short pipelines, small charging volume) with fast oil circulation, the recovery time is 5 to 7 minutes. For large parallel systems (short pipelines, small charging volume), the recovery time is 5 to 7 minutes. (For long oil pipelines with large oil storage capacity) oil circulation is slow, and the recovery time is set at 8-10 minutes. If the oil level drops by more than 5%-10% or there is a continuous low oil level signal, it is considered to be in a state of lagging oil return. In one implementation, if a target branch event occurs in a target evaporation branch, the oil level is continuously monitored within the associated time window. If the oil level does not drop, it is determined to be in a normal state of associated oil return. If the oil level drops, a timer is triggered to record the drop time and start the countdown of the preset recovery time. If the oil level recovers within the countdown, it indicates that the current dynamic oil discharge is normal and is determined to be in a normal state of associated oil return. If the countdown ends and the oil level still has not recovered, it indicates that the oil return is continuously unsmooth and is determined to be in a state of lagging oil return.

[0032] Step S5: determining whether the corresponding target evaporation branch is abnormal and the cause of the abnormality according to the liquid supply state and the associated oil return state of the target evaporation branch, wherein the abnormality causes include local liquid supply abnormality, oil return lag abnormality and liquid supply-oil return coupling abnormality; In implementation, the abnormal liquid supply states include an insufficient liquid supply state and an excessive liquid supply state, and the abnormal associated oil return state is an associated oil return lag state; Specifically, determining the abnormality cause of the corresponding target evaporation branch according to the determination result that the liquid supply state and / or the associated oil return state of the target evaporation branch is abnormal comprises: In response to that only the liquid supply state is the excessive liquid supply state or the insufficient liquid supply state, determining the abnormality cause thereof as local liquid supply abnormality; it can be understood that in a refrigeration system, when insufficient or excessive liquid supply occurs in one branch, if the return air flow rate of the branch changes but is still within the tolerable range of the oil return capacity of the system, or the amount of oil circulated in the branch itself is small, it is not enough to cause a significant drop in the oil level of the compressor within the associated time window, and the compressor oil level remains normal, indicating that the overall oil circulation balance of the system has not been broken temporarily, that is, the problem only occurs in the liquid supply control or distribution link of the target branch itself; therefore, the root cause of the problem lies in the branch, and the valves, sensors and pipeline resistance of the branch should be checked first, and the system oil return strategy should not be adjusted easily; therefore, the coordinated control mode is switched to the local liquid supply compensation mode, and the control mode is determined according to the specific abnormal liquid supply type.

[0033] Specifically, based on the determination result that the target evaporation branch is in the insufficient liquid supply state, determining the abnormal liquid supply type of the target evaporation branch according to the comparison result between the target evaporation branch and the virtual reference branch comprises: combining the comparison result that the opening degree of the electronic expansion valve is lower than the reference range of the opening degree of the electronic expansion valve, determining that the insufficient liquid supply is caused by insufficient valve opening; it can be understood that the target evaporation branch has shown liquid shortage, and the valve opening degree is smaller than that of the reference branch, indicating that the insufficient liquid supply is probably caused by insufficient opening degree; at this time, the electronic expansion valve of the branch should be gradually opened, with each step ≤5% and an interval ≥3 minutes, so that the return air superheat and the heat transfer temperature difference fall back to the reference range. If the associated oil return lag state is triggered during the adjustment process, the opening should be stopped immediately, the abnormality is upgraded to liquid supply-oil return coupling abnormality, and then handled by coordinated control decision; Based on the comparison results showing that the electronic expansion valve opening is not lower than the reference range, the problem is identified as insufficient liquid supply due to valve inefficiency. It's understandable that the target evaporation branch is short of liquid, but the electronic expansion valve opening is not small, indicating that it's not simply a matter of insufficient control opening. Possible causes include: valve core jamming, pre-valve filter blockage, excessive branch pipeline resistance, poor branch liquid distribution, or normal valve opening feedback but insufficient actual flow. In this case, further valve opening should be stopped (it's ineffective and may cause liquid slugging), and the current opening should be maintained to trigger the insufficient valve efficiency alarm and prompt on-site inspection. Inspection items include whether the electronic expansion valve body is jammed, whether the pre-valve filter is blocked, and whether there is abnormal resistance in the branch distributor or pipeline. If the oil level starts to drop, the problem is also upgraded to coupling abnormality.

[0034] Specifically, based on the determination that the target evaporation branch is in an over-supply state, the abnormal liquid supply type of the target evaporation branch is determined according to the comparison result between the target evaporation branch and the virtual reference branch, including: Based on the comparison results showing that the opening of the electronic expansion valve is higher than that of the reference electronic expansion valve, it is determined that the valve opening is too large and the liquid supply is excessive. It is understandable that the target branch has already shown excessive liquid supply and the valve opening is larger than that of the reference branch, indicating that this excessive state is likely caused by the valve opening being too large. At this time, the electronic expansion valve of this branch should be gradually closed, with each step ≤5% and an interval ≥3 minutes, so that the return gas superheat and heat exchange temperature difference rise to the reference range. Closing the valve will reduce the return gas flow rate, and the compressor module oil level needs to be closely monitored. If the oil level drops after adjustment and does not recover, it means that the previous high flow rate of this branch has been helping the return oil. At this time, the closure should be stopped and upgraded to an abnormal coupling of liquid supply and return oil. Based on the comparison results of the electronic expansion valve opening not exceeding the reference electronic expansion valve opening, it was determined to be a non-valve opening-dominated type of refrigerant oversupply. It is understandable that the target branch exhibits oversupply despite a relatively small valve opening, indicating that the oversupply is not directly caused by an excessively large opening. Possible causes include: the actual load on this branch is too low, evaporator frosting or insufficient airflow, the electronic expansion valve not closing tightly, refrigerant misflow in the branch, or temperature or pressure sensor drift. In this situation, the valve should not be blindly closed (the superheat is already very low; closing it may cause the evaporation temperature to drop too low, resulting in a sudden decrease in cooling capacity). Maintain the current opening and trigger a non-valve opening-dominated oversupply alarm, prompting checks on whether the room's heat load is extremely low (constant temperature), whether the evaporator is severely frosted, whether the outlet air temperature sensor is drifting, and whether the electronic expansion valve is not closing tightly. If continued oversupply leads to a risk of liquid carryover in the return gas, defrosting can be forced or the cooling demand of this branch can be temporarily reduced. During implementation, if the return gas superheat, heat exchange temperature difference, and valve opening of the target branch all return to the virtual reference range and remain stable for more than 10 minutes, the correction is successful and the branch exits the local liquid supply abnormality state. The branch is then re-included in the stable branch candidate.

[0035] In response to the fact that only the associated oil return state is associated oil return lag state, the cause of the abnormality is determined to be oil return lag abnormality; this indicates that the liquid supply system is trying to maintain normal operation but can no longer compensate for the oil return defect (the actual refrigerant mass flow rate of this branch is low (partial blockage of the pipeline), but the superheat is forcibly stabilized by opening the expansion valve to compensate, creating the illusion of normal liquid supply, but the actual return gas flow rate is still insufficient to carry oil; local structure of the evaporator or return gas pipe causes oil accumulation (at the riser pipe); the compressor's short-term oil discharge volume is abnormally increased, exceeding the normal oil return capacity); at this time, the normal liquid supply is a pseudo-balance forcibly compensated by the controller. The essence of the problem is that there is a fault in the oil return path or the compressor's oil discharge state. Therefore, the electronic expansion valve should not be adjusted (breaking the pseudo-balance and causing superheat out of control). Instead, the return gas pipe design, evaporator oil return bend, compressor operating conditions, and oil separation efficiency should be checked first. In response to the fact that both the fluid supply status and the associated oil return status are abnormal, the cause of the abnormality is determined to be an abnormal coupling of fluid supply and oil return; this indicates that the abnormal fluid supply and abnormal oil return may have a causal relationship or be jointly affected by system disturbances. In response to the fluid supply status and the associated oil return status being normal respectively, it is determined that there is no abnormality.

[0036] Step S6: In response to an abnormality in the liquid supply and oil return coupling, a joint control mode is determined based on the sequential changes in the liquid supply state and the associated oil return state of the target evaporation branch, wherein: In response to an abnormal liquid supply condition occurring earlier than an abnormal oil return condition, when the liquid supply to a certain branch is consistently low, the refrigerant mass flow rate in the evaporator is insufficient, and the airflow velocity in the return pipe is lower than the minimum shear velocity carried by oil droplets, the refrigerant oil in that branch's evaporator and the bottom of the return pipe will stagnate, accumulate, and form an oil sump instead of being carried back to the compressor. This oil accumulation further occupies the internal flow area of ​​the evaporator, reducing the effective heat transfer coefficient and making the liquid supply to that branch even more insufficient, leading to a further increase in superheat. In this case, the control mode should be switched to a liquid supply priority compensation followed by an oil return confirmation mode. Specific measures include: 1. Opening the electronic expansion valve of the target branch, using the reference opening of the virtual reference branch as a reference, increasing the opening by 10%–20% to bring the return superheat back to the reference range and simultaneously increase the heat exchange temperature difference. 1. Adjust the electronic expansion valve opening of the target evaporator branch at intervals of ≥3 minutes to prevent over-adjustment and reconfirm the associated oil return status; 2. After the liquid supply status returns to normal, enter a 15-30 minute oil return confirmation observation period, closely monitor whether the oil level of the compressor module rises to the normal graded state. If the oil level recovers, the diagnosis is correct, maintain the current parameters and continue monitoring. If the oil level still does not recover, it indicates that the liquid supply compensation is in place but there is still physical blockage in the oil return path (such as oil accumulation in the evaporator not being completely carried back, or the oil return bend failing), triggering an alarm that the oil return does not recover with the liquid supply, and switch to manual inspection; 3. If the return gas superheat drops below the safety lower limit (<2K) during the liquid supply compensation process, immediately stop opening the valve and prioritize liquid slugging prevention, and record it as this branch cannot solve the oil return problem through liquid supply compensation; If the abnormal oil return status occurs earlier than the abnormal liquid supply status, it indicates that the problem first occurred in the oil return path or the compressor's oil discharge status. This causes some oil to fail to return to the compressor normally, instead randomly entering one or more evaporation branches. The oil clumps entering the evaporator flow with the liquid refrigerant, forming an irregular liquid-oil two-phase slug flow within the evaporator. When the oil clumps pass through, heat exchange deteriorates and superheat increases sharply. After the oil clumps are pushed away, the superheat drops sharply, causing the electronic expansion valve control to experience severe overshoot and reverse adjustment, with the liquid supply status rapidly oscillating between excess and insufficient. At this time, blindly opening the electronic expansion valve may worsen the oil return problem. Therefore, oil return should be restored first, and the control mode should be switched to oil return priority restoration mode. Specific measures include: 1. Prioritize oil return restoration and prohibit blindly opening the electronic expansion valve (which increases refrigerant flow, seemingly increasing the return gas velocity, but actually dilutes the oil concentration and reduces the return gas superheat, potentially sending more liquid refrigerant into the compressor and causing liquid slugging). 1. Adjust the compressor's operating status to force oil return. If using a variable frequency compressor, briefly increase the operating frequency (increase by 10-15Hz for 3-5 minutes) to increase the overall refrigerant circulation speed of the system and forcefully carry the accumulated oil back. If the system is equipped with an active oil return solenoid valve, trigger the forced oil return cycle. If multiple compressors are connected in parallel, some compressors can be shut down briefly to increase the load on the operating compressors, reduce the suction pressure, and create a higher flow rate. 2. During the oil return recovery cycle, temporarily switch the electronic expansion valve control mode of the target branch to a fixed opening mode and lock it at the reference opening to prevent ineffective chasing adjustment of the valve due to false signals of overheating. 3. After the compressor performs oil discharge or oil return, enter a 10-20 minute observation period. If the oil level rises to normal and the liquid supply status of the target branch stabilizes, the diagnosis is correct, and the automatic control of the expansion valve is gradually restored. If the oil level still does not recover, there may be hardware faults such as decreased oil separator efficiency or mechanical blockage of the oil return pipeline, requiring manual intervention. The response to the abnormal liquid supply status occurring simultaneously with the abnormal oil return status indicates that a common system-level disturbance source simultaneously impacted both the liquid supply and oil return subsystems. A typical disturbance source is a drastic fluctuation in discharge pressure: for the liquid supply side, a sudden change in discharge pressure instantaneously alters the pressure difference across all electronic expansion valves. Due to varying resistances in the upstream pipelines of each branch, the arrival time and attenuation amplitude of the pressure waves differ, leading to inconsistent liquid supply disturbances in each branch and momentary disorder in liquid distribution. For the oil return side, a sudden change in discharge pressure alters the refrigerant circulation velocity, instantly discharging a large amount of oil from the compressor or significantly reducing the oil-carrying capacity of the return gas, resulting in a sudden change in oil level. The system-level liquid distribution and oil return coupled disturbance was identified, and the joint control mode was switched to the system-level rebalancing mode. Specific measures included: 1. Suspending local... 1. Intervention measures include suspending all automatic correction actions of electronic expansion valves in all branches, restoring them to their pre-disturbance opening or default safe opening, and suspending active oil return actions (such as compressor frequency increase) to avoid additional disturbances when the system is unstable; 2. Check the operation of the condenser fan, the flow rate of the water cooling system, and whether non-condensable gases have entered, and wait for the pressure fluctuations to naturally decay or accelerate stabilization through optimization of PID parameters on the condenser side; 3. After the exhaust pressure stabilizes, repeat step S2, i.e., re-select stable evaporation branches under the current new operating conditions, construct new virtual benchmark branches, re-evaluate the liquid supply status and associated oil return status of all branches with the new benchmark, and gradually restore the closed-loop control of the electronic expansion valves of each branch, with a single-branch interval of ≥5 minutes, to prevent simultaneous actions from triggering a new round of coupling; In practice, if the time difference between the occurrence of the two anomalies is less than the synchronization time window (1 min to 5 min), they are considered to be synchronized; otherwise, they are judged to be asynchronous, and the joint control mode is determined according to the order of occurrence.

[0037] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

[0038] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage, wherein the multi-unit evaporation system for cold storage includes a compressor module and several parallel evaporation branches connected by a common liquid pipe and a common gas return pipe, and each evaporation branch includes an electronic expansion valve and an evaporator; Its features are, The method for identifying uneven liquid distribution and controlling oil return includes: The return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each evaporation branch are obtained, as well as the oil level change of the compressor module are obtained. Stable evaporation branches are selected based on the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each evaporation branch to construct a virtual benchmark branch. If the return gas superheat and the heat exchange temperature difference of a single evaporation branch are both within the corresponding standard range, and the fluctuation of the electronic expansion valve opening within the preset time window is less than the preset fluctuation value, then the evaporation branch is determined as a stable evaporation branch. Furthermore, the reference return gas superheat, reference heat exchange temperature difference, and reference electronic expansion valve opening of the virtual reference branch are determined based on the average values ​​of the return gas superheat, heat exchange temperature difference, and electronic expansion valve opening of each stable evaporation branch, so as to construct the virtual reference branch. The liquid supply status of the target evaporation branch is determined based on the comparison results between the target evaporation branch and the virtual reference branch; The associated oil return status is determined based on the time relationship between the change in the target evaporator branch status and the change in the compressor module oil level. Based on the liquid supply status and the associated oil return status of the target evaporation branch, determine whether the corresponding target evaporation branch is abnormal and the cause of the abnormality. The cause of the abnormality includes local liquid supply abnormality, oil return lag abnormality, and liquid supply and oil return coupling abnormality. In response to an abnormality in the liquid supply and oil return coupling, the joint control mode is determined based on the sequential changes in the liquid supply state and the associated oil return state of the target evaporation branch. The joint control modes include a fluid supply priority compensation followed by oil return confirmation mode, an oil return priority recovery mode, and a system-level rebalancing mode.

2. The method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage according to claim 1, characterized in that, Before determining the liquid supply status, the liquid supply status is determined based on the return gas superheat and heat exchange temperature difference of the target evaporation branch, including: If both the return gas superheat and the heat exchange temperature difference are higher than the corresponding reference range, it is initially judged to be a state of insufficient liquid supply. If both the return gas superheat and the heat exchange temperature difference are below the corresponding reference range, it is initially judged to be an oversupply of liquid. If both the return gas superheat and the heat exchange temperature difference are within the corresponding reference range, it is initially determined that the liquid supply is in a normal state.

3. The method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage according to claim 2, characterized in that, Before determining the liquid supply status, the liquid supply status is determined based on the return gas superheat and heat exchange temperature difference of the target evaporation branch, including: If the return gas superheat and the heat exchange temperature difference contradict each other relative to the corresponding reference range, the liquid supply status is determined by combining the opening degree of the electronic expansion valve, wherein: If the opening degree of the electronic expansion valve is not lower than the reference range of the electronic expansion valve opening degree, then the liquid supply state is determined to be an insufficient liquid supply state. If the opening degree of the electronic expansion valve is lower than the reference range of the electronic expansion valve opening degree, the liquid supply state is determined to be an over-supply state.

4. The method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage according to claim 1, characterized in that, If the oil level of the compressor module drops and fails to recover within the preset recovery time window after the target evaporation branch is put into operation, the opening degree of the electronic expansion valve changes, or the liquid supply status changes, then the target evaporation branch is determined to be in a state of associated oil return lag. Conversely, if the target evaporation path is in a normal state of associated oil return, it is determined that the target evaporation path is in a normal state of associated oil return.

5. The method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage according to claim 1, characterized in that, Based on the determination that the liquid supply status and / or associated oil return status of the target evaporation branch are abnormal, the cause of the abnormality in the corresponding target evaporation branch is determined, including: In response to the fact that only the liquid supply state is a state of excessive liquid supply or insufficient liquid supply, the cause of the abnormality is determined to be a local liquid supply abnormality. In response to the fact that only the associated return oil state is an associated return oil lag state, the cause of the abnormality is determined to be an abnormal return oil lag. In response to both the fluid supply status and the associated oil return status being abnormal, the cause of the abnormality is determined to be an abnormal coupling of fluid supply and oil return.

6. The method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage according to claim 5, characterized in that, In response to an anomaly in the liquid supply and oil return coupling, the abnormal times of the liquid supply and associated oil return states in the target evaporation branch are determined to establish the joint control mode, wherein: In response to the abnormal liquid supply status occurring earlier than the abnormal return oil status, it is determined that the uneven liquid distribution induces the abnormal return oil status, and the joint control mode is switched to the liquid supply priority compensation followed by return oil confirmation mode, and the opening of the electronic expansion valve of the target evaporation branch is adjusted and the associated return oil status is reconfirmed. In response to the abnormal return oil status occurring earlier than the abnormal supply fluid status, the system determines that the return oil lag induces the supply fluid disturbance and switches the control mode to the return oil priority recovery mode, and adjusts the operating status of the compressor module and limits the increase in the opening of the electronic expansion valve. In response to abnormal liquid supply conditions, the system synchronizes with abnormal oil return conditions, identifies system-level liquid separation and oil return coupling disturbances, switches the joint control mode to system-level rebalancing mode, and re-screens stable evaporation branches and updates virtual reference branches.

7. The method for identifying uneven liquid distribution and controlling oil return in a multi-unit evaporation system for cold storage according to claim 5, characterized in that, In response to local fluid supply anomalies or oil return lag anomalies, the joint control mode will be switched to local fluid supply compensation mode or oil return priority recovery mode.

Images