All-year low-temperature refrigeration variable frequency screw unit and control method thereof

Abstract

The present application relates to the technical field of industrial refrigeration equipment control, and particularly relates to a kind of all-year low-temperature refrigeration variable frequency screw unit and its control method.The present application provides a kind of control method of all-year low-temperature refrigeration variable frequency screw unit, including compressor capacity control step, compressor frequency control step, water temperature high limit frequency step and winter start machine preheating step;Compressor capacity control step and compressor frequency control step work cooperatively, when unit is loaded, first adjust compressor capacity to maximum, then frequency is promoted;When unit is unloaded, first unload compressor frequency to minimum, then capacity is unloaded.The scheme has the advantages of improving the adjustment efficiency and operation stability of compressor under dynamic load, avoiding surge or shutdown.

Description

Technical Field

[0001] This invention relates to the field of industrial refrigeration equipment control technology, and in particular to a year-round low-temperature refrigeration variable frequency screw chiller and its control method. Background Technology

[0002] In the field of industrial refrigeration, variable frequency screw chillers are widely used in processes requiring year-round low-temperature refrigeration due to their high energy efficiency and wide adjustment range, such as chemical plants, food freezing facilities, or special environmental laboratories. These applications typically require the units to maintain stable and efficient operation even under conditions of significant fluctuations in external ambient temperature and internal heat load. However, it is precisely this stringent requirement for uninterrupted operation throughout the year that poses a significant challenge to the control strategies of existing variable frequency screw chillers.

[0003] Currently, the control logic of many units is relatively simple, and the regulation of capacity and frequency often fails to achieve optimal coordination. A common practice is to perform linear regulation based on a single parameter, which easily leads to sluggish system response or oscillations under dynamic loads. Especially during loading, blindly increasing the frequency without prioritizing sufficient intake can cause a significant decrease in compressor efficiency under partial load; conversely, during unloading, simultaneously reducing both frequency and capacity can cause the system operating conditions to rapidly deviate from the design point, triggering surge or shutdown. Furthermore, existing control schemes generally lack adaptability to extreme operating conditions.

[0004] Existing chiller units often perform poorly when started up or operated under conditions outside of their design specifications, such as when the chilled water temperature is extremely high during initial startup in summer, or when providing cooling for special processes in severe winter. High inlet water temperatures cause a sharp rise in evaporation pressure, easily triggering compressor overload alarms and forcing the unit to start and stop frequently, failing to smoothly transition to the target low temperature. In low-temperature winter environments, excessively low cooling water temperatures prevent the system from establishing the high-low pressure difference necessary for normal operation, similarly leading to difficulties in starting the unit, low operating efficiency, and even alarm shutdowns, severely impacting production continuity.

[0005] While some existing technologies offer adjustment functions targeting water temperature or seasonality, most are relatively isolated and passive, lacking a comprehensive control scheme that deeply integrates multiple control dimensions such as compressor capacity, operating frequency, water temperature limits, and low-temperature preheating, and proactively anticipates changes. Therefore, there is an urgent need in this field for a more intelligent and adaptive control method to ensure that low-temperature refrigeration variable frequency screw chillers can achieve stable, reliable, and efficient optimal operation under various complex operating conditions throughout the year.

[0006] To address the aforementioned issues, existing technologies urgently need improvement. Summary of the Invention

[0007] To address the aforementioned problems, the present invention aims to provide a year-round low-temperature refrigeration variable frequency screw compressor unit and its control method, which has the advantages of improving the compressor's regulation efficiency and operational stability under dynamic loads, and avoiding surge or shutdown.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] This application provides a control method for a year-round low-temperature refrigeration variable frequency screw chiller unit. The technical solution is as follows: it includes a compressor capacity control step, a compressor frequency control step, a water temperature high limit frequency step, and a winter start-up preheating step. The compressor capacity control step and the compressor frequency control step work together. When the unit is loaded, the compressor capacity is adjusted to the maximum first, and then the frequency is increased. When the unit is unloaded, the compressor frequency is unloaded to the minimum first, and then the capacity is unloaded.

[0010] Furthermore, this application also proposes that the compressor capacity control steps specifically include:

[0011] Before starting the unit, set the compressor capacity to the lowest setting;

[0012] After startup, it enters temperature control mode:

[0013] When in the loading zone, the control loading valve operates at a preset loading time cycle until the capacity reaches 100%;

[0014] When in the hold zone, maintain the current capacity;

[0015] When in the unloading zone, after the compressor frequency is unloaded to the minimum frequency, the control unloading valve operates according to the capacity demand calculated by the PID, with a preset unloading time cycle, until the capacity reaches the target value.

[0016] Furthermore, this application also proposes that the compressor frequency control steps specifically include:

[0017] When the unit starts up, the compressor is controlled to run at the starting frequency for a period of time before being increased to the minimum operating frequency;

[0018] Once the compressor capacity is loaded to 100%, the system adjusts the frequency based on the set temperature and the actual outlet water temperature by combining PID calculation with a time cycle mode.

[0019] When in the loading or unloading zone, the frequency converter is controlled to gradually increase or decrease the frequency at a fixed time period until the target frequency calculated by the PID is reached.

[0020] Furthermore, this application also proposes that the high water temperature limiting frequency step specifically includes:

[0021] Multiple chilled water inlet temperature ranges are preset, and a maximum compressor operating frequency limit is set for each range;

[0022] When the unit is running, it detects the current chilled water inlet temperature and, based on the temperature range it falls within, limits the compressor frequency adjustment range to between the lowest frequency and the corresponding highest limit frequency.

[0023] Furthermore, this application also proposes that the chilled water inlet temperature range includes at least a first range (zone a), a second range (zone b), a third range (zone c), and a fourth range (zone d), with the corresponding maximum limiting frequencies decreasing sequentially.

[0024] Furthermore, this application also proposes the following correspondence between temperature ranges and maximum limiting frequencies:

[0025] When the chilled water inlet temperature is ≤10℃, the maximum limiting frequency for the first interval is 70Hz.

[0026] When 10℃ < chilled water inlet temperature ≤ 15℃, corresponding to the second interval, the maximum limiting frequency is 60HZ;

[0027] When 15℃ < chilled water inlet temperature ≤ 20℃, corresponding to the third interval, the maximum limiting frequency is 50Hz;

[0028] When 20℃ < chilled water inlet temperature ≤ 30℃, corresponding to the fourth interval, the maximum limiting frequency is 40Hz.

[0029] Furthermore, this application also proposes that the winter start-up and preheating steps specifically include:

[0030] A bypass pipeline with an electric valve is installed between the cooling water inlet and outlet of the condenser of the unit;

[0031] When the machine is turned on, the cooling water inlet temperature is detected. If it is lower than the set target temperature, the electric valve is controlled to open at different speeds according to the difference between the current temperature and the target temperature, so that part of the cooling water is short-circuited and flows back to increase the condensation temperature.

[0032] When the cooling water inlet temperature reaches or exceeds the set target temperature, the electric control valve gradually closes.

[0033] Furthermore, this application proposes to set multiple non-overlapping temperature condition thresholds based on the difference between the cooling water inlet temperature and the set target temperature. When the temperature falls into different threshold ranges, the electric valve is controlled to open at different speeds, and the lower the temperature, the faster the opening speed.

[0034] Furthermore, this application also proposes setting the target temperature to 30°C and including the following control conditions:

[0035] When the cooling water inlet temperature is in the first low temperature range (28℃>T≥20℃), the electric control valve opens at the first speed.

[0036] When the cooling water inlet temperature is in the second low temperature range (20℃>T≥10℃), the electric valve is controlled to open at a second speed, which is greater than the first speed.

[0037] When the cooling water inlet temperature is in the third low temperature range (T<10℃), the electric control valve opens at the third speed, which is greater than the second speed.

[0038] When the cooling water inlet temperature is in the fourth low temperature range (30℃>T≥28℃), the electric valve maintains its current opening degree.

[0039] Valve closing condition: When the unit's cooling water inlet temperature is detected to be ≥30℃, the electric valve gradually reduces its opening.

[0040] Furthermore, this application also proposes a year-round low-temperature refrigeration variable frequency screw chiller unit, characterized in that it includes a variable frequency screw compressor, a condenser, a throttling valve and an evaporator connected by pipelines to form a refrigerant circulation loop;

[0041] The variable frequency screw compressor is equipped with a capacity regulating device, which includes an unloading valve and a loading valve.

[0042] It also includes a frequency converter connected to the variable frequency screw compressor for adjusting the operating frequency of the compressor;

[0043] A system bypass pipeline is connected between the cooling water inlet and outlet of the condenser. An electric bypass valve is installed on the system bypass pipeline to adjust the bypass water flow according to the cooling water temperature in order to maintain stable system operation.

[0044] It also includes a temperature sensor for detecting the temperature of the cooling water inlet, and a system controller;

[0045] The system controller is electrically connected to the unloading valve, loading valve, frequency converter, electric bypass valve and temperature sensor respectively, and is configured to execute the above control method.

[0046] As can be seen from the above, the variable frequency screw chiller unit and its control method provided in this application for year-round low temperature refrigeration effectively optimizes the operating efficiency under dynamic load by coordinating the adjustment sequence of compressor capacity and frequency. When loading, the capacity is maximized first and then the frequency is increased, and when unloading, the frequency is reduced first and then the capacity is reduced. This improves the adjustment efficiency and operating stability of the compressor under dynamic load and avoids surge or shutdown. Attached Figure Description

[0047] Figure 1This application provides a schematic diagram showing the correspondence between the chilled water inlet temperature range and the compressor's maximum operating frequency.

[0048] Figure 2 This application provides a control flowchart for a year-round low-temperature refrigeration variable frequency screw chiller unit.

[0049] Figure 3 This is a schematic diagram of the year-round low-temperature refrigeration variable frequency screw chiller unit provided in Embodiment 2 of this application. Detailed Implementation

[0050] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0051] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0052] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more, unless explicitly defined otherwise.

[0053] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., 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.

[0054] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature. Example 1

[0055] like Figure 2 As shown, this embodiment proposes a control method for a year-round low-temperature refrigeration variable frequency screw chiller unit, including a compressor capacity control step, a compressor frequency control step, a high water temperature limiting frequency step, and a winter start-up preheating step. The compressor capacity control step and the compressor frequency control step work together. When the unit is loaded, the compressor capacity is preferentially adjusted to the maximum before the frequency is increased. When the unit is unloaded, the compressor frequency is preferentially unloaded to the minimum before the capacity is unloaded.

[0056] The compressor capacity control step refers to the process of changing the effective working volume of the compressor by adjusting the opening of the capacity regulating valve. This can be achieved by using a solenoid valve to control the position of the slide valve, ensuring sufficient suction volume is established during the initial loading phase. The compressor frequency control step refers to the process of adjusting the motor speed through a frequency converter. This can be achieved by using a PID controller linked with the frequency converter, finely adjusting the cooling capacity output after maximizing capacity. The high-temperature water inlet frequency limit step refers to the process of dynamically setting the upper limit of the frequency based on the chilled water inlet temperature. This can be achieved by using a multi-temperature range segmented frequency limiting method, preventing excessive evaporation pressure due to high-temperature inlet water. The winter start-up preheating step refers to the process of adjusting the cooling water temperature through the bypass pipeline. This can be achieved by using an electric valve to control the bypass flow rate, ensuring sufficient condensing pressure is established in low-temperature environments. Traditional solutions simultaneously adjust capacity and frequency during loading, resulting in volumetric efficiency loss. This solution uses staged control to ensure the compressor always operates in the high-efficiency range.

[0057] In the specific scheme, the compressor capacity control steps include: setting the compressor capacity to the minimum before the unit starts; after starting, entering the temperature control mode; when in the loading zone, controlling the loading valve to work at a preset loading time cycle until the capacity reaches 100%; when in the holding zone, maintaining the current capacity; and when in the unloading zone, waiting for the compressor frequency to unload to the minimum frequency, and then controlling the unloading valve to work at a preset unloading time cycle according to the capacity requirement calculated by PID until the capacity reaches the target value.

[0058] The loading valve is the actuator that controls the increase of compressor capacity. It can be implemented using a solenoid valve or an electric regulating valve. By periodically opening the loading valve, the effective volume of the compressor's working chamber is gradually increased. The unloading valve is the actuator that controls the decrease of compressor capacity. It can be implemented using a proportional valve or a servo valve. The unloading valve's operating cycle is dynamically adjusted by a PID controller to match the actual capacity demand. The preset loading time cycle is the fixed interval between two adjacent capacity adjustment operations during loading. For example, it could be a setting where the unloading valve operates for 0.5 seconds every 3 seconds, used to avoid pressure fluctuations caused by sudden capacity changes. The preset unloading time cycle is the variable interval between two adjacent capacity adjustment operations during unloading. For example, it can be dynamically adjusted between 3 and 20 seconds based on the PID output, used to balance response speed and system stability. The minimum frequency is the frequency value corresponding to the minimum allowable operating speed of the compressor. For example, it can be set to 25Hz to 30Hz to ensure that the basic circulation pressure is maintained during unloading.

[0059] Specifically, during the unit startup phase, the compressor capacity is forcibly limited to its minimum to avoid excessive starting current or mechanical shock. Once in temperature control mode, when the system is detected to be in the loading zone, the loading valve gradually opens at fixed time intervals, for example, increasing capacity by 10% every 10 seconds, until full capacity is reached. At this point, the compressor has maximum suction capacity, providing a stable pressure base for subsequent frequency increases. When the system is in the holding zone, both the loading and unloading valves remain closed to avoid unnecessary capacity adjustments that could lead to energy loss. When the system enters the unloading zone, the compressor frequency is first reduced to a preset minimum frequency, such as 25Hz, to ensure that the suction pressure remains within a safe range during unloading. Subsequently, the unloading valve gradually reduces the compressor's working volume based on the capacity requirement calculated in real-time by the PID controller, for example, decreasing capacity by 5% every 15 seconds, until the target capacity value is reached.

[0060] Compared to existing technologies, traditional control methods typically employ simultaneous capacity and frequency regulation, such as reducing both frequency and capacity during unloading, which can easily lead to a sudden drop in suction pressure and surge. This solution eliminates the risk of sudden pressure changes by mandating that frequency reduction be completed before capacity adjustment during unloading. Existing technologies often use single-time-cycle or continuous adjustment methods for capacity regulation, such as continuously opening the valve until the target capacity is reached, which can easily lead to over-adjustment or oscillation. This solution uses a fixed time cycle to achieve rapid capacity increase during the loading phase, and combines PID dynamic adjustment of the time cycle during the unloading phase, ensuring both loading efficiency and improving unloading accuracy. Through the above technical solutions, this application solves the problem of compressor efficiency decline after frequency increase due to insufficient capacity during loading. Prioritizing loading to full capacity ensures the effectiveness of subsequent frequency regulation; during unloading, sequential control of frequency reduction followed by capacity unloading avoids surge caused by sudden pressure drops; and the combination of a fixed cycle during loading and a dynamic cycle during unloading improves regulation speed while ensuring system stability.

[0061] In the specific scheme, the compressor capacity has an upper limit of 100% and a lower limit of 25%, using stepless adjustment from 25% to 100%. Capacity adjustment is achieved using two capacity regulating valves: an unloading valve and a loading valve. The adjustment method employs time-cycle control. The system collects the compressor's current feedback capacity from 0% to 100% via MODBUS RTU communication. When the unit receives a start-up command, before starting, the compressor capacity is first set to the minimum of 25%, i.e., the unloading valve operates, at which point the compressor capacity is 25%. Then, the variable frequency compressor is started, and after stabilizing for 30 seconds, the unloading valve stops operating, at which point the start-up is complete. Entering the temperature control mode, when the user's demand temperature is in the loading zone, the compressor loads the valve for 1 second every 2 seconds. The loading phase is divided into four stages: 45%, 65%, 85%, and 100%, with a 15-second pause at each stage to prevent excessively rapid capacity loading, which could cause excessive fluctuations in the evaporator liquid level and unstable opening of the throttling electronic expansion valve. Typically, maintaining the electronic expansion valve opening at 70% ± 5% is optimal. Ultimately, the load is increased to 100% capacity. (Note: To ensure maximum air intake during unit loading, the capacity is prioritized to 100% before frequency adjustment. Adjusting the frequency only when the air intake is at its maximum ensures maximum operating efficiency and energy efficiency.) When the unit temperature is in the holding zone, neither the compressor loading nor unloading valves operate, maintaining the current capacity. When the unit temperature is in the unloading zone, the capacity is not immediately unloaded; the original capacity of 100% is maintained. The compressor capacity is only unloaded when the compressor frequency is reduced to the minimum of 25Hz. The unloaded capacity is adjusted using PID calculation based on the set temperature and actual outlet water temperature. For example, if the PID calculation indicates a control capacity requirement of 50%, the unloading valve operates for 0.5 seconds every 3 seconds until the compressor feedback capacity is 50±1%, at which point the unloading valve stops operating.

[0062] Furthermore, the compressor frequency control steps specifically include: when the unit starts, controlling the compressor to run at the starting frequency for a period of time and then increasing to the minimum operating frequency; after the compressor capacity is loaded to 100%, adjusting the frequency based on the set temperature and the actual outlet water temperature through a combination of PID calculation and time cycle mode; when in the loading or unloading zone, controlling the frequency converter to gradually increase or decrease the frequency at a fixed time cycle until the target frequency is reached.

[0063] The starting frequency refers to the initial operating frequency of the compressor when it starts up. This can be achieved using a preset value, such as 25Hz, to avoid mechanical shock caused by direct high-frequency starting. The minimum operating frequency refers to the minimum speed at which the compressor maintains stable operation. This can be achieved using a set value, such as 20Hz, to ensure that the compressor maintains effective lubrication and thermal balance under low load. The combination of PID calculation and time-cycle mode refers to superimposing the real-time adjustment signal output by the proportional-integral-derivative algorithm with a periodic time-stepping mechanism. This can be achieved by adjusting the frequency every 30 seconds to balance dynamic response speed and system stability. Fixed-time-cycle gradual adjustment refers to performing frequency increase / decrease operations at constant time intervals. This can be achieved by adjusting the frequency by 1Hz every 5 seconds to transform abrupt frequency changes into gradual adjustments.

[0064] Specifically, the compressor frequency has an upper limit of 70Hz and a lower limit of 25Hz. The system exchanges data with the frequency converter via MODBUS RTU communication. During the unit startup phase, the compressor initially runs at a 20Hz startup frequency for 10 seconds. Once the oil temperature reaches the lubrication requirement, it automatically switches to the minimum operating frequency of 25Hz. When the capacity is loaded to 100%, the system collects the deviation between the outlet water temperature and the set temperature in real time. The target frequency adjustment is calculated using a PID algorithm, and a periodic correction signal triggered every 30 seconds is added to this, all acting on the frequency command generation module of the frequency converter. When the system is detected to be in the loading zone, the frequency converter gradually increases the frequency by 1Hz every 5 seconds; when in the unloading zone, the frequency gradually decreases by the same increment until the target frequency value calculated by the PID is reached. For example: If in the loading zone, the PID calculates a control frequency requirement of 60Hz, while the actual frequency is 30Hz, then the frequency is loaded. The inverter increases the frequency by 1Hz every 5 seconds until the feedback frequency is 60±1Hz. If in the holding zone, the current frequency is maintained without loading or unloading. If in the unloading zone, the PID calculates a control frequency requirement of 40Hz, while the actual frequency is 60Hz, then the frequency is unloaded. The inverter decreases the frequency by 1Hz every 5 seconds until the feedback frequency is 40±1Hz. If still in the unloading zone, the frequency is eventually unloaded to the minimum of 25Hz. When the frequency is unloaded to the minimum of 25Hz, if further unloading is needed, the compressor capacity is unloaded to 75%, then 50%, and so on. When the capacity is unloaded to 50%, the compressor frequency is increased to 40Hz. This is to ensure sufficient compressor intake and oil return, ensuring stable unit operation. If still in the unloading zone, the unit waits 2 minutes before shutting down the compressor, awaiting the next startup. When the difference between the feedback frequency and the control frequency exceeds 5 Hz and lasts for more than 5 minutes during operation, it indicates that the frequency converter is malfunctioning, and the unit will shut down for protection and issue a warning.

[0065] Compared to existing technologies, traditional methods typically employ single PID control or fixed frequency steps, which can easily lead to frequency surge in the compressor under dynamic loads. This solution, through a phased start-up strategy and a dual-mode control mechanism, achieves a smooth frequency transition while ensuring mechanical safety, and simultaneously avoids high-frequency oscillations by utilizing time period constraints. The lack of timing coordination between frequency regulation and capacity loading in existing technologies is resolved in this solution through a coordinated mechanism that starts frequency regulation only after the capacity is fully loaded. Through the above technical solutions, this application effectively coordinates the timing relationship between compressor frequency and capacity regulation, maintaining stable operating conditions under dynamic load changes. The phased start-up strategy avoids mechanical damage caused by insufficient lubrication in low-temperature environments, and the dual-mode control mechanism eliminates system oscillations caused by high-frequency regulation while ensuring temperature control accuracy. The linkage control of fixed-time-period step regulation and capacity loading status successfully prevents surge caused by frequency abrupt changes, ensuring the compressor always operates within its high-efficiency operating range.

[0066] Furthermore, the high-frequency limit step of this solution includes presetting multiple chilled water inlet temperature ranges and setting a maximum operating frequency limit for the compressor for each range. When the unit is running, the current chilled water inlet temperature is detected and the compressor frequency adjustment range is limited to between the minimum frequency and the corresponding maximum limit frequency according to the temperature range in which it is located.

[0067] The chilled water inlet temperature range refers to dividing a continuous chilled water inlet temperature range into multiple sub-ranges with clearly defined boundaries. This can be achieved by using temperature sensors to collect data in real time and combining this with a range division algorithm, discretizing different temperature conditions into independent control levels. The maximum operating frequency limit refers to the upper limit of the compressor's operating frequency set for each temperature range. This can be configured through a mapping table built into the controller to prevent the compressor from overloading due to excessive frequency at high temperatures. Specifically, after the chilled water inlet temperature is divided into multiple ranges, each range corresponds to a different maximum frequency limit. For example, when the chilled water inlet temperature is detected to be in a high-temperature range, the maximum frequency limit is set to a lower value. In this case, the compressor can only adjust its operating frequency between the lowest frequency and the upper limit. This stepped constraint mechanism ensures that the compressor will not overload due to insufficient suction volume caused by excessive frequency at high temperatures, while allowing a higher frequency upper limit at low temperatures to maintain refrigeration efficiency. By matching the temperature range and frequency limit in real time, the compressor's operating range is always dynamically limited within a safe range, avoiding the rigidity of adjustment caused by a single frequency threshold and achieving non-linear matching of temperature and frequency. This solution, through multi-range division and stepped frequency limiting, can actively adjust the upper frequency limit in different temperature ranges, preventing high-temperature overload and avoiding energy efficiency losses caused by excessive frequency limiting under low-temperature conditions. Through the above technical solution, this application solves the problem of compressor overload or unstable operation caused by excessively high chilled water inlet temperature, while achieving adaptive frequency adjustment under different temperature conditions. Under high-temperature conditions, lowering the upper frequency limit effectively suppresses the risk of compressor overload; under low-temperature conditions, appropriately relaxing the frequency limit improves refrigeration efficiency. Therefore, the compressor can maintain a safe and efficient operating state under different inlet water temperatures.

[0068] In the specific design, the chilled water inlet temperature range includes at least a first range, a second range, a third range, and a fourth range, with the corresponding maximum frequency limits decreasing sequentially. The chilled water inlet temperature range refers to dividing the chilled water temperature into four consecutive temperature ranges, each corresponding to a different upper limit for the compressor's operating frequency. This can be achieved by using a temperature sensor to monitor the inlet water temperature in real time and using a controller to determine the appropriate range. The maximum frequency limit refers to the highest frequency value that the compressor is allowed to operate at for each temperature range. This can be dynamically adjusted using a preset parameter table built into the controller; the higher the temperature range, the lower the upper limit of the frequency.

[0069] like Figure 1In the detailed scheme shown, the correspondence between the chilled water inlet temperature range and the compressor's maximum operating frequency is as follows: when the chilled water inlet temperature is less than or equal to 10℃, it corresponds to the first range (zone a), with a maximum limiting frequency of 70Hz; when the chilled water inlet temperature is between 10℃ and 15℃, it corresponds to the second range (zone b), with a maximum limiting frequency of 60Hz; when the chilled water inlet temperature is between 15℃ and 20℃, it corresponds to the third range (zone c), with a maximum limiting frequency of 50Hz; and when the chilled water inlet temperature is between 20℃ and 30℃, it corresponds to the fourth range (zone d), with a maximum limiting frequency of 40Hz. The chilled water inlet temperature range refers to multiple operating condition zones divided according to temperature ranges. This can be achieved by using temperature sensors for real-time monitoring combined with a threshold comparator to characterize the changing trend of system pressure difference under different temperature conditions. The maximum limiting frequency refers to the upper limit frequency value allowed for compressor operation within different temperature ranges. This can be achieved through the frequency limiting module built into the inverter to prevent the evaporation pressure from exceeding the compressor's capacity. The correspondence between temperature range and frequency refers to a pre-established step-by-step mapping rule, which can be implemented using a lookup table in a programmable controller to dynamically adjust the frequency constraint range.

[0070] Specifically, under low-temperature conditions where the chilled water inlet temperature is less than or equal to 10°C, the heat exchange temperature difference between the refrigerant and chilled water in the evaporator is relatively large. In this case, the compressor is allowed to operate at a high frequency of 70Hz, utilizing the sufficient pressure difference to improve refrigeration efficiency. When the temperature rises to the range of 10°C to 5°C, the evaporation pressure begins to increase, limiting the maximum frequency to 60Hz to maintain cooling capacity while preventing the suction pressure from exceeding the compressor's design value. When the temperature further rises to 5°C to 20°C, the evaporator heat exchange efficiency decreases, leading to a smaller pressure difference. The 50Hz frequency limit prevents the motor current from exceeding its rated value. In the high-temperature range of 20°C to 30°C, the system pressure difference decreases significantly. A strict frequency limit of 40Hz suppresses abnormal increases in evaporation pressure, preventing high-pressure protection shutdown. Through the above technical solution, this application effectively solves the compressor overload problem caused by excessively high chilled water inlet temperature, while optimizing operating efficiency and system stability in different temperature ranges. In the high-temperature range, the frequency limit was reduced to avoid protection shutdown caused by a sudden increase in evaporation pressure. In the low-temperature range, the cooling capacity was enhanced by increasing the upper limit of the frequency, thus achieving adaptive adjustment across the entire operating range.

[0071] In winter, some production processes still require cooling. Since the cooling water tank is located outdoors, when the outdoor ambient temperature is very low, the cooling water temperature also approaches the ambient temperature. If the cooling water inlet temperature cannot be raised to the rated temperature of 28-30℃ within a short time after startup, the unit may fail to establish high and low pressure, leading to system instability or alarm shutdown. Prolonged operation may also damage the unit and result in low energy efficiency. Therefore, this solution installs a bypass pipeline with an electric valve between the cooling water inlet and outlet of the unit's condenser. Upon startup, the cooling water inlet temperature is monitored. If it is lower than the set target temperature, the electric valve is opened at different rates based on the difference between the current and target temperatures, allowing some cooling water to short-circuit and flow back to increase the condensing temperature. When the cooling water inlet temperature reaches or exceeds the set target temperature, the electric valve is gradually closed.

[0072] The bypass pipeline refers to the circulation channel connecting the cooling water inlet and outlet of the condenser. It can be implemented using a metal pipe with a flange connection. Its function is to create a short-circuit path for cooling water circulation, reducing the water flow through the condenser. The electric valve is a valve equipped with an electric actuator, specifically a ball valve driven by a stepper motor. Its function is to dynamically adjust the opening of the bypass pipeline according to the control signal, achieving precise control of the cooling water flow. Cooling water inlet temperature detection refers to the real-time acquisition of temperature data at the cooling water inlet using a temperature sensor, specifically a PT100 platinum resistance thermometer. Its function is to provide real-time feedback signals for preheating control. The target temperature setting is the minimum cooling water temperature threshold required to maintain normal compressor startup, specifically set to 30℃. Its function is to serve as the reference parameter for the electric valve's operation.

[0073] Specifically, when starting the unit in low-temperature winter conditions, the cooling water inlet temperature may be lower than the minimum threshold required for normal system operation. When the cooling water temperature is detected to be lower than the set target temperature, the electric valve of the bypass pipeline opens at different speeds based on the difference between the actual and target temperatures. When the temperature difference is large, the electric valve opens rapidly, allowing more cooling water to flow directly back to the inlet, reducing the amount of cooling water flowing through the condenser, thereby increasing the pressure and temperature inside the condenser. As the condensing temperature rises to the set target value, the electric valve gradually closes, restoring normal water circulation. During this process, the coordinated operation of the bypass pipeline and the electric valve enables dynamic switching of the cooling water circulation path, and the graded speed control strategy ensures preheating efficiency under different temperature difference conditions. Through the above technical solution, this application effectively solves the problem of compressor start-up difficulties caused by excessively low cooling water temperatures in low-temperature winter environments. By dynamically adjusting the bypass pipeline flow rate, the condensing temperature is quickly increased to the normal operating range, ensuring that the compressor can smoothly establish the high and low pressure differences required for operation under low-temperature conditions. This control method avoids frequent start-up and shutdown failures caused by insufficient preheating in traditional solutions, improving the start-up success rate and operational reliability of the unit in frigid environments.

[0074] Furthermore, in the winter start-up preheating mode, this solution sets multiple non-overlapping temperature condition thresholds based on the difference between the cooling water inlet temperature and the set target temperature. When the temperature falls into different threshold ranges, the electric valve is controlled to open at different speeds, with the opening speed increasing as the temperature decreases. The temperature condition thresholds divide the continuous cooling water temperature into multiple clearly defined intervals, which can be achieved by using preset temperature ranges. For example, the temperature difference can be divided into three intervals, each corresponding to a different control strategy. This feature allows the system to respond in stages based on the degree to which the actual temperature deviates from the target value, thereby accurately matching preheating requirements. Differential opening speed control refers to setting different action rates for the electric valve within different temperature ranges, which can be achieved by adjusting the pulse frequency or duty cycle of the electric valve's stepper motor. This feature ensures that under extreme low-temperature conditions, the condensing pressure is rapidly increased through faster bypass reflux, while a slower speed is used when approaching the target temperature to avoid over-adjustment.

[0075] Specifically, this solution proposes setting the target temperature at 30℃ and includes the following control conditions:

[0076] When the cooling water inlet temperature is in the first low temperature range (28℃>T≥20℃), the electric valve is controlled to open at the first speed; specifically, the electric valve can gradually increase its opening by 1% every 6 seconds.

[0077] When the cooling water inlet temperature is in the second low temperature range (20℃>T≥10℃), the electric valve is controlled to open at a second speed, which is greater than the first speed; specifically, the electric valve can gradually increase its opening by 4% every 6 seconds.

[0078] When the cooling water inlet temperature is in the third low temperature range (T < 10℃), the electric valve is controlled to open at the third speed, which is greater than the second speed; specifically, the electric valve can gradually increase its opening by 6% every 6 seconds.

[0079] When the cooling water inlet temperature is in the fourth low temperature range (30℃>T≥28℃), the electric valve maintains its current opening degree, neither increasing nor decreasing it.

[0080] Valve closing condition: When the unit's cooling water inlet temperature is detected to be ≥30℃, the electric valve gradually decreases its opening at a rate of 1% every 3 seconds.

[0081] The target temperature of 30℃ refers to the minimum condensing temperature threshold required to maintain the compressor's normal operation. This can be achieved by using a temperature sensor to monitor the cooling water inlet temperature in real time and comparing it with the set value. This temperature threshold ensures the system can establish the necessary high and low pressure differential. Temperature range division refers to dividing the cooling water inlet temperature into multiple continuous and non-overlapping ranges. This can be achieved by using preset temperature thresholds to trigger different control logics. This division method allows the system to implement differentiated control based on the degree to which the actual temperature deviates from the target value. Graded valve opening speed control refers to setting incremental valve opening rates for different temperature ranges. This can be achieved by using a variable frequency motor to drive the electric valve and configuring multiple speed parameters. This speed gradient design ensures faster bypass flow adjustment as the temperature drops further. Specifically, when the cooling water inlet temperature is lower than the target temperature, a portion of the cooling water is short-circuited and recirculated through the bypass pipeline to increase the condensing temperature. When the temperature is detected to be in the first low-temperature range, the electric valve opens at a lower initial speed to avoid over-adjustment when the temperature approaches the target value. When the temperature is in the second low-temperature range, a higher second speed is used to accelerate the increase of the condensing temperature. When the temperature is in the third low-temperature range, the fastest third speed is used to quickly establish the pressure conditions required for system operation. When the cooling water inlet temperature is in the fourth low-temperature range, the electric valve maintains its current opening. When the unit's cooling water inlet temperature is detected to be higher than the set target temperature, the electric valve gradually closes.

[0082] This solution establishes a correlation between temperature ranges and opening speeds, automatically matching valve action speed to the degree of temperature deviation. It enables rapid establishment of condensing pressure at extremely low temperatures and transitions to fine-tuning as the target temperature approaches, balancing start-up efficiency and system stability. Through this technical solution, this application can rapidly raise the cooling water temperature to the minimum threshold required for compressor start-up in low-temperature winter environments, avoiding start-up failures due to insufficient condensing pressure. The graded speed control effectively balances the conflict between preheating speed and system stability, shortening start-up preparation time while preventing mechanical wear caused by frequent valve operations. The above scheme ensures stable operation of the unit in both summer and winter. In summer, when the temperature exceeds 40°C and the user's chilled water inlet temperature exceeds 20°C, the maximum frequency corresponding to the chilled water inlet temperature is limited to prevent high pressure exceeding 16.8 Bar due to high evaporation pressure, which could cause the unit to alarm and shut down. In winter, when the ambient temperature is below 10°C and the user's chilled water inlet temperature exceeds 20°C, the maximum frequency corresponding to the chilled water inlet temperature is limited simultaneously, and the cooling water return is controlled by the bypass electric valve, thus preventing three faults: excessively high evaporation pressure, excessively low condensation pressure, and excessively low high-low pressure difference. Example 2

[0083] like Figure 3 As shown, this embodiment relates to a year-round low-temperature refrigeration variable frequency screw chiller unit, characterized in that it includes a variable frequency screw compressor 1, a condenser 4, a throttling valve 6 and an evaporator 7 connected by pipelines, forming a refrigerant circulation loop;

[0084] The variable frequency screw compressor 1 is equipped with a capacity adjustment device, which includes an unloading valve 2 and a loading valve 3.

[0085] It also includes a frequency converter connected to the variable frequency screw compressor 1 for adjusting the operating frequency of the compressor;

[0086] A system bypass pipeline is connected between the cooling water inlet and outlet of the condenser 4. An electric bypass valve 5 is installed on the system bypass pipeline to adjust the bypass water flow according to the cooling water temperature in order to maintain stable system operation.

[0087] It also includes a temperature sensor for detecting the temperature of the cooling water inlet, and a system controller;

[0088] The system controller is electrically connected to the unloading valve 2, the loading valve 3, the frequency converter, the electric bypass valve 5, and the temperature sensor, respectively.

[0089] The core refrigeration cycle of this low-temperature refrigeration variable frequency screw chiller unit is composed of the following components connected in sequence through pipelines: variable frequency screw compressor 1, condenser 4, expansion valve 6, and evaporator 7.

[0090] The specific workflow is as follows: The low-temperature, low-pressure gaseous refrigerant from the evaporator 7 is compressed into a high-temperature, high-pressure gaseous refrigerant by the variable frequency screw compressor 1; the high-temperature, high-pressure gaseous refrigerant enters the condenser 4 and exchanges heat with the cooling water flowing through it, condensing into a high-pressure, medium-temperature liquid refrigerant; this liquid refrigerant is then throttled and depressurized by the expansion valve 6, becoming a low-temperature, low-pressure two-phase refrigerant; finally, this two-phase refrigerant absorbs heat from the chilled water flowing through it in the evaporator 7, evaporates into a low-temperature, low-pressure gaseous refrigerant, and re-enters the compressor, completing a complete refrigeration cycle.

[0091] To achieve precise, stable, and efficient control of the unit under different operating conditions, this system integrates the following key control components:

[0092] Capacity regulating device: directly acts on the variable frequency screw compressor 1, including unloading valve 2 and loading valve 3, for stepless adjustment of the compressor's output capacity.

[0093] Inverter: Connected to the variable frequency screw compressor 1, used to adjust its operating frequency.

[0094] System bypass pipeline and electric bypass valve 5: This bypass pipeline connects between the cooling water inlet and outlet of the condenser 4, and the electric bypass valve 5 installed on it is used to adjust the bypass water flow according to the cooling water temperature. Its core function is to quickly increase the condensing temperature and system pressure difference by short-circuiting part of the cooling water under operating conditions such as low temperatures in winter, ensuring stable start-up and operation of the unit.

[0095] Temperature sensor: Used to detect key temperature parameters of the system in real time, especially the cooling water inlet temperature, and to provide feedback to the control logic.

[0096] System controller: As the control center of the entire unit, it is electrically connected to all the valves, frequency converters and sensors mentioned above, and is specifically configured to execute the control method detailed in Example 1, thereby realizing advanced functions such as coordinated control of compressor capacity and frequency, high water temperature limiting frequency and winter start-up preheating.

[0097] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0098] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A control method for a year-round low-temperature refrigeration variable frequency screw chiller unit, characterized in that, The method includes: Compressor capacity control steps, compressor frequency control steps, high water temperature frequency limit steps, and winter start-up preheating steps; The compressor capacity control step works in conjunction with the compressor frequency control step. When the unit is loaded, the compressor capacity is adjusted to the maximum first, and then the frequency is increased. When the unit is unloaded, the compressor frequency is unloaded to the minimum first, and then the capacity is unloaded. The compressor capacity control steps specifically include: Before starting the unit, set the compressor capacity to the lowest setting; After startup, it enters temperature control mode; When in the loading zone, the control loading valve operates at a preset loading time cycle until the capacity reaches 100%; When in the hold zone, maintain the current capacity; When in the unloading zone, after the compressor frequency is unloaded to the minimum frequency, the unloading valve is controlled to work according to the capacity demand calculated by PID, with a preset unloading time cycle, until the capacity reaches the target value. The compressor frequency control steps specifically include: When the unit starts up, the compressor is controlled to run at the starting frequency for a period of time before being increased to the minimum operating frequency; Once the compressor capacity is loaded to 100%, the system adjusts the frequency based on the set temperature and the actual outlet water temperature by combining PID calculation with a time cycle mode. When in the loading or unloading zone, the frequency converter is controlled to gradually increase or decrease the frequency at a fixed time period until the target frequency calculated by the PID is reached. The high water temperature limiting frequency step specifically includes: Multiple chilled water inlet temperature ranges are preset, and a maximum compressor operating frequency limit is set for each range; When the unit is running, it detects the current chilled water inlet temperature and, based on the temperature range it falls within, limits the compressor frequency adjustment range to between the lowest frequency and the corresponding highest limit frequency. The winter start-up and preheating steps specifically include: A bypass pipeline with an electric valve is installed between the cooling water inlet and outlet of the condenser of the unit; When the machine is turned on, the cooling water inlet temperature is detected. If it is lower than the set target temperature, the electric valve is controlled to open at different speeds according to the difference between the current temperature and the target temperature, so that part of the cooling water is short-circuited and flows back to increase the condensation temperature. When the cooling water inlet temperature reaches or exceeds the set target temperature, the electric valve is controlled to gradually close.

2. The control method for a year-round low-temperature refrigeration variable frequency screw chiller unit according to claim 1, characterized in that, The chilled water inlet temperature range includes at least a first range, a second range, a third range, and a fourth range, with the corresponding maximum limiting frequencies decreasing sequentially.

3. The control method for a year-round low-temperature refrigeration variable frequency screw chiller unit according to claim 2, characterized in that, The correspondence between the temperature range and the maximum limiting frequency is as follows: When the chilled water inlet temperature is ≤10℃, the maximum frequency limit is 70Hz for the first interval. When 10℃ < chilled water inlet temperature ≤ 15℃, the corresponding second interval has a maximum frequency limit of 60Hz; When 15℃ < chilled water inlet temperature ≤ 20℃, the maximum frequency limit is 50Hz, corresponding to the third interval. When 20℃ < chilled water inlet temperature ≤ 30℃, corresponding to the fourth interval, the maximum limiting frequency is 40Hz.

4. The control method for a year-round low-temperature refrigeration variable frequency screw chiller unit according to claim 1, characterized in that, In the winter start-up preheating step, multiple non-overlapping temperature condition thresholds are set based on the difference between the cooling water inlet temperature and the set target temperature. When the temperature falls into different threshold ranges, the electric valve is controlled to open at different speeds, and the lower the temperature, the faster the opening speed.

5. The control method for a year-round low-temperature refrigeration variable frequency screw chiller unit according to claim 4, characterized in that, The target temperature is set at 30°C, and includes the following control conditions: When the cooling water inlet temperature is in the first low temperature range of 28℃ > T ≥ 20℃, the electric control valve opens at the first speed. When the cooling water inlet temperature is in the second low temperature range of 20℃ > T ≥ 10℃, the electric valve is controlled to open at a second speed, which is greater than the first speed. When the cooling water inlet temperature is in the third low temperature range T < 10℃, the electric valve is controlled to open at a third speed, which is greater than the second speed; When the cooling water inlet temperature is in the fourth low temperature range of 30℃ > T ≥ 28℃, the electric valve maintains its current opening degree. Closure condition: When the unit's cooling water inlet temperature is detected to be ≥30℃, the electric valve gradually reduces its opening.

6. A variable frequency screw chiller unit for year-round low-temperature refrigeration, characterized in that, The system includes a variable frequency screw compressor (1), a condenser (4), a throttle valve (6), and an evaporator (7) connected by pipelines, forming a refrigerant circulation loop; The variable frequency screw compressor (1) is equipped with a capacity adjustment device, which includes an unloading valve (2) and a loading valve (3); It also includes a frequency converter connected to the variable frequency screw compressor (1) for adjusting the operating frequency of the compressor; A system bypass pipeline is connected between the cooling water inlet and outlet of the condenser (4). An electric bypass valve (5) is installed on the system bypass pipeline to adjust the bypass water volume according to the cooling water temperature in order to maintain stable system operation. It also includes a temperature sensor for detecting the temperature of the cooling water inlet, and a system controller; The system controller is electrically connected to the unloading valve (2), loading valve (3), frequency converter, electric bypass valve (5) and temperature sensor respectively, and is configured to execute the control method of a year-round low temperature refrigeration variable frequency screw chiller as described in any one of claims 1 to 5.

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