Condensing pressure reference control method and device for ammonia absorption type refrigerating system

Abstract

The invention discloses a condensation pressure reference control method and device for an ammonia absorption type refrigeration system. The method comprises the following steps: establishing an absolute physical reference, namely a target condensing pressure, determined by ammonia physical properties, wherein the value is uniquely determined at a given condensing temperature; the condensing pressure is detected in real time; and with approaching as a target, working condition parameters of the ammonia purification device are adjusted, so that the purity of ammonia vapor entering the evaporator is close to 100%. The device is an electronic control or self-operated machine. According to the method, the condensing pressure is increased to be an active physical reference based on the double-rigidity constraint of fixed ammonia charge and determined condensing pressure, so that the system automatically approaches to a theoretical optimal state in a wide temperature range, the rectification fault risks such as tower flooding and tower flushing are remarkably reduced, the energy efficiency is improved, and the method is suitable for industrial waste heat recovery, refrigeration houses and other scenes.

Description

Technical Field

[0001] This invention relates to the field of absorption refrigeration system control technology, specifically to a control method and device that uses condensation pressure as a physical reference and achieves self-adaptive purity control by adjusting the reflux ratio of the distillation column. Background Technology

[0002] Ammonia absorption refrigeration systems are driven by thermal energy and can utilize low-grade waste heat. They offer a wide refrigeration temperature range and are widely used in cold storage, petroleum refining, and other chemical processes. Unlike lithium bromide-water systems, ammonia-water systems face a fundamental technical challenge: because the standard boiling points of ammonia (boiling point -33℃) and water (boiling point 100℃) differ by only 133.4℃, the vapor produced during the process inevitably contains water vapor. The residual water content in the distillate determines the evaporation / absorption pressure. Excessive water content can lead to a decrease in the system's COP and even cause serious malfunctions such as evaporator icing and distillation column flooding (flooding).

[0003] In existing technologies, ammonia vapor purification is mainly achieved through distillation columns. A distillation column is a highly coupled, multivariable system; its separation efficiency is influenced by numerous factors, including cooling water temperature, reflux ratio, feed flow rate, feed concentration, number of trays, and originating temperature, making its control extremely complex. Research indicates... [1-3] The performance of the distillation column directly affects the purity of ammonia vapor and the overall efficiency of the system, and there exists an optimal reflux ratio and an optimal generation temperature that maximize the system's COP. However, existing research has only revealed the influence of various parameters on system performance, but has not provided a control method that can adaptively adjust system parameters in actual variable operating conditions. In actual operation, changes in factors such as heat source temperature and cooling water temperature can cause the system to deviate from the design conditions, preventing the distillation column from always operating in its optimal state, leading to failures such as flooding, and seriously affecting system efficiency and stability.

[0004] Therefore, providing a control method for an ammonia absorption refrigeration system based on physical property parameters as the absolute physical benchmark, enabling the system to adapt to changes in heat source and environment, fundamentally ensuring ammonia purity and avoiding distillation failures, has significant theoretical and engineering value. Summary of the Invention

[0005] This invention provides a method and apparatus for controlling the condensing pressure reference in an ammonia absorption refrigeration system, aiming to solve the problems of difficult multivariable coupling control of distillation columns, easy occurrence of flooding failure, and large impact of operating condition fluctuations on system efficiency in the prior art.

[0006] 3.1 Physical basis of the invention

[0007] The physical basis of this invention lies in two rigid constraints:

[0008] • Constraint 1: The amount of ammonia injected is fixed. The amount of ammonia injected at one time during system installation cannot be changed during operation. This is a rigid constraint at the material conservation level, which allows adjusting the operating conditions of the purification unit to affect the system state.

[0009] Constraint 2: The condensation pressure is determined by the cooling water temperature. At a given condensation temperature, the saturated vapor pressure of pure ammonia is uniquely determined. When water is mixed into ammonia vapor, the partial pressure of ammonia decreases, leading to a drop in condensation pressure. Therefore, the condensation pressure directly reflects the purity of the ammonia vapor.

[0010] Under these two constraints, this invention proposes using condensation pressure as the absolute physical reference: when condensation pressure Below the benchmark value When the ammonia vapor contains water, it indicates that the ammonia vapor is not pure enough; when rebounded to When the purity reaches the required level, it indicates that the purity has met the standard.

[0011] 3.2 Design-Operation Unified Control Concept

[0012] This invention follows the core concept of "design based on benchmarks, adaptive operation." During the system design phase, the optimal distillation column reflux ratio is determined based on target operating conditions (e.g., condensation temperature 40℃, evaporation temperature -15℃) to maximize system performance. And obtain the corresponding condensing pressure reference value under this operating condition. (Uniquely determined by the physical properties of ammonia). This benchmark value solidifies the optimization results throughout the design phase.

[0013] During system operation, regardless of changes in external conditions such as heat source temperature, cooling water temperature, and evaporation temperature (refrigerant temperature), the controller always maintains a constant state. With the sole control objective, the reflux ratio of the distillation column is adjusted in real time to control the condensation pressure. Approaching Since changes in evaporation temperature are ultimately reflected in condensation pressure through system coupling, this control logic naturally includes adaptive adjustment for evaporation temperature fluctuations. This process essentially allows the operating system to automatically return to its optimal design conditions, fundamentally eliminating the disconnect between "design parameters" and "operating parameters" in traditional refrigeration systems.

[0014] 3.3 Technical Solution

[0015] This invention includes two aspects: control method and control device.

[0016] The control method includes the following steps:

[0017] 1. Determine the absolute physical reference determined by the physical properties of ammonia—the target condensation pressure. This value is uniquely determined at a given condensation temperature;

[0018] 2. Real-time monitoring of the absolute pressure inside the condenser. ;

[0019] 3. With Approaching To control the target, the reflux ratio of the distillation column was adjusted so that the purity of the ammonia vapor entering the evaporator was close to 100%.

[0020] The control device includes:

[0021] • Pressure detection unit: Located at the condenser, used to monitor the absolute pressure inside the condenser in real time. ;

[0022] • Reference unit: Provides the target condensation pressure The corresponding reference signal;

[0023] • Comparison unit: Connected to the pressure detection unit and the reference unit, it receives the pressure signal and the reference signal, and outputs the pressure deviation signal;

[0024] • Execution unit: Connected to the comparator unit, it adjusts the reflux ratio of the distillation column based on the pressure deviation signal, so that... Approaching .

[0025] The execution unit can take any of the following forms:

[0026] • Electronic control method: The electric regulating valve is installed on the cooling water supply line of the reflux condenser and the cooling water flow rate is adjusted by the PID controller, thereby indirectly changing the reflux ratio;

[0027] • Variable frequency reflux pump: The reflux flow rate can be directly changed by adjusting the speed of the reflux pump;

[0028] • Self-operated mechanical type: The self-operated differential pressure driven valve is installed on the reflux liquid line of the distillation column. It is connected to the condenser pressure tap and the column top pressure tap via pressure guide pipes. The valve core is displaced using the system's own differential pressure, directly regulating the reflux liquid flow rate without requiring an external power source.

[0029] 3.4 Theoretical Derivation and Feasibility Analysis

[0030] To further clarify the mechanism by which this invention ensures the stability of the distillation column, the basic theory of the distillation process is introduced for derivation.

[0031] Minimum number of theoretical plates given separation requirements It can be estimated using the Fenske equation:

[0032]

[0033] in The mole fraction of ammonia vapor at the top of the column (i.e., purity) ), The molar fraction of ammonia in the dilute solution at the bottom of the tower. This represents the average relative volatility of the ammonia-water system between the top and bottom of the column.

[0034] Theoretical plates required for actual distillation columns With minimum theoretical plate number Reflux ratio and minimum reflux ratio The relationships between them can be described using Gilliland correlations. A commonly used simplified form is:

[0035]

[0036] From the above theoretical model, we can see that:

[0037] • Ammonia purity at the top of the tower It is a decision Key variables;

[0038] • When condensing pressure Stable to the benchmark value At that time, it means the actual ammonia purity at the top of the tower Stabilizing near the target value provides a constant separation target for the distillation process;

[0039] • Based on this constant objective, the reflux ratio is adjusted in real time through feedback control. This keeps it near the optimal value, avoiding gas-liquid phase load imbalance caused by large fluctuations in the reflux ratio, thereby significantly reducing the risk of flooding.

[0040] • Because the control system ensures stability and With optimal optimization, the distillation column does not need to reserve excessive safety margins to cope with a wide range of operating condition fluctuations during the design phase, and has the potential to adopt fewer trays and reduce the size of the equipment.

[0041] 3.5 Beneficial Effects

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. Unified physical benchmark control: For the first time, the condensation pressure is used as the basis for control. It provides a unified control paradigm for ammonia absorption refrigeration systems based on absolute physical standards and is compatible with distillation columns.

[0044] 2. Reliable control based on dual rigid constraints: Fully utilize the fixed ammonia charge and the condensation pressure to determine two rigid constraints, unify material conservation and thermodynamic laws into the physical basis of closed-loop control, so that the control logic has natural theoretical completeness.

[0045] 3. Design-Operation Unified Adaptive Control: Following the core concept of "design sets the benchmark, operation is adaptive", the condensing pressure corresponding to the optimal operating condition is established as the benchmark during the design phase, and the system is automatically adjusted based on this during the operation phase, so that the system always automatically returns to the optimal state at the time of design.

[0046] 4. Effectively reduces the risk of distillation failures: Adjusting the reflux ratio based on the condensing pressure helps maintain a stable vapor-liquid load within the distillation column. Theoretical analysis shows that when the condensing pressure is stable, the composition at the top of the column is stable, and the fluctuation of the reflux ratio decreases, thereby significantly reducing the probability of flooding, overflow, and other liquid overflow failures.

[0047] 5. Optimize the distillation column design process: Using condensation pressure as a single benchmark and combining distillation design theories such as the Fenske equation, a more optimized reflux ratio can be adopted while meeting separation requirements. This allows the distillation column design to break free from excessive reliance on multi-parameter empirical formulas and has the potential to use fewer trays and reduce equipment volume.

[0048] 6. Wide temperature range adaptability: The method of this invention can work effectively within a wide evaporation temperature range, and is uniformly applicable to various ammonia absorption refrigeration scenarios such as conventional cold storage, industrial cryogenics, and air conditioning temperature zones.

[0049] 7. Data as a Service Capability: The device of this invention has real-time online monitoring and data output functions, and can continuously record key operating parameters such as ammonia purity, condensation pressure, and system energy efficiency, forming a traceable operating log, providing a true and reliable basis for carbon trading, green building certification, and energy-saving subsidies.

[0050] 8. Feasibility of retrofitting existing units: The control device of this invention can be installed on existing ammonia absorption units to achieve high-efficiency and intelligent retrofitting of existing units, thereby improving the reliability and efficiency of existing equipment. Attached Figure Description

[0051] Figure 1 is a schematic diagram of the structure of the electronically controlled condensing pressure reference controller of the present invention (the installation position of the self-operated differential pressure drive valve is also shown in the figure).

[0052] Figure 2. Flowchart of the control method of the present invention.

[0053] Figure 3. Reference analysis diagram of condensing pressure in ammonia absorption system (relationship between condensing pressure and ammonia purity). Detailed Implementation

[0054] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments are for illustrative purposes only and do not constitute a limitation on the scope of protection.

[0055] Example 1: Electronically Controlled Condensing Pressure Reference Controller

[0056] like Figure 1 As shown, the controller includes a pressure sensor 200, a PID controller 300, and an electric actuator 400. The pressure sensor 200 is installed at the condenser 100 to detect the condensing pressure in real time. PID controller 300 receives Signal and preset reference (Determined based on actual condensation temperature), calculate the deviation and output a control signal. The electric regulating valve 500 is installed on the cooling water supply line of the reflux condenser, integrating an electric actuator. It is connected to the PID controller 300 via wires, adjusting the valve opening according to the control signal to change the cooling water flow rate, thereby regulating the reflux ratio of the distillation column. Approaching .

[0057] Example 2: Self-operated differential pressure driven valve solution

[0058] As another way to implement the actuator, a self-operated differential pressure driven valve can be used. For example... Figure 1 As shown, this valve is installed on the outlet pipe of the reflux condenser. The valve has an internal pressure chamber isolated from the main flow channel, connected to the condenser pressure tap and the distillation column top pressure tap via external pressure guide pipes. An internal spring is installed, with its preload corresponding to the pressure difference between the condenser and the column top under full-load design conditions. The valve's reflux passage opening automatically changes with the valve core displacement.

[0059] When the purity of ammonia decreases, it leads to condensation pressure. When the pressure decreases, the pressure difference between the condenser and the top of the column changes. Under the action of the pressure difference, the valve automatically adjusts the opening of the reflux channel, directly changing the reflux liquid flow rate, thereby adjusting the reflux ratio, improving the purification degree of the distillation column, and causing the condensation pressure to rise back to the reference value.

[0060] This solution requires no external power supply or electronic controller; it utilizes the system's own differential pressure to achieve purely mechanical adaptive control.

[0061] Example 3: Variable Frequency Return Pump Solution

[0062] Another implementation method for the execution unit is to use a variable frequency reflux pump. The reflux pump is installed on the reflux liquid line, and its speed is controlled by a frequency converter. The controller outputs an analog signal to the frequency converter based on the condensing pressure deviation to adjust the pump speed, thereby directly changing the reflux liquid flow rate (reflux ratio). This solution has a fast response and high control accuracy, and is suitable for applications with high control quality requirements.

[0063] Example 4: Control Method

[0064] As shown in Figure 2, the control method of the present invention includes the following steps:

[0065] 1. During system installation and commissioning, determine the target condensing pressure based on the design condensing temperature. (For example, 1.555 MPa is used when the condensation temperature is 40℃, but the actual condensation temperature during operation shall prevail.)

[0066] 2. Real-time monitoring of condensing pressure ;

[0067] 3. Calculate the pressure deviation. ;

[0068] 4. According to Calculate the adjustment amount according to the PI control law;

[0069] 5. Adjust the reflux ratio of the distillation column (via cooling water valve, reflux pump, or self-regulating valve);

[0070] 6. Repeat steps (2)–(5) to make It stabilized near the benchmark value;

[0071] Example 5: Thermodynamic Analysis of Condensation Pressure Reference

[0072] like Figure 3 As shown, Figure 3 The condensation pressure in the ammonia absorption system was demonstrated. With ammonia vapor purity M and condensation temperature The thermodynamic relationship between them is shown in the figure. The saturated pressure curves of pure ammonia at different condensation temperatures, and the pressure-purity relationship curves when ammonia vapor is mixed with different proportions of water, are plotted. The gray horizontal dashed line represents the condensation pressure reference. This value is uniquely determined by the saturation pressure of pure ammonia at the condensation temperature and serves as the absolute physical reference for the system. The red area represents the region of insufficient ammonia purity (excessive moisture content). The figure shows that, under the control of the condensation pressure reference, the operating point is always within the high-purity safe zone.

[0073] Example 6: Wide Temperature Range Adaptability Verification

[0074] References [4]Studies have shown that lithium nitrate-ammonia absorption refrigeration systems can operate stably within an operating temperature range of 75-85°C, with evaporation temperatures as low as -20°C. The study also indicates that an increase in condensation temperature leads to a significant decrease in the system's COP, demonstrating the crucial role of precise control of condensation pressure in maintaining efficient system operation. The condensation pressure reference controller of this invention is designed to address this issue. By real-time detection of the condensation pressure and comparison with a reference value, it automatically adjusts system operating parameters, effectively suppressing the adverse effects of condensation temperature fluctuations on system performance.

[0075] Example 7: Upgrading of Existing Systems

[0076] For existing ammonia absorption chiller units, the condensing pressure reference controller of this invention can be installed. In specific implementation, the original distillation column and reflux pump can be retained, an electric regulating valve can be installed on the reflux pipeline, and the controller can be connected to the original control system. The valve reference opening under full-load design conditions is determined through on-site commissioning, and the controller adjusts the valve opening according to the condensing pressure deviation, thereby realizing the high-efficiency and intelligent transformation of the existing unit.

[0077] Example 8: Real-time Online Data Service

[0078] By connecting the controller of this invention to a cloud monitoring platform via a communication interface, key operating parameters such as ammonia purity, condensation pressure, and system energy efficiency can be continuously recorded. This data can provide reliable evidence for carbon trading and green building certification, enabling a business model upgrade from "equipment sales" to "data services."

[0079] Design Basis Explanation

[0080] The target condensation pressure described in this invention The specific condensing pressure value is determined by the physical properties of ammonia and can be obtained from the ammonia saturation pressure table based on the condensing temperature. According to the national standard GB50072-2021, the design condensing temperature of an ammonia refrigeration system should not exceed 40℃. Within this standard's design operating range, the condensing pressure reference is uniquely determined by the ammonia saturation pressure corresponding to the actual condensing temperature. Those skilled in the art can select an appropriate condensing temperature and corresponding reference pressure based on the actual cooling water conditions and system design operating conditions.

[0081] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0082] References

[0083] 1. Kong Dingfeng, Liu Jianhua, Wang Jin, et al. Numerical simulation and experiment of a single-stage ammonia absorption chiller distillation column [J]. Chemical Industry and Engineering Progress, 2010, 29(10): 1825-1832.

[0084] 2. Kong Dingfeng, Liu Jianhua. Performance simulation and experiment of ammonia absorption refrigeration system [J]. Modern Chemical Industry, 2013, 33(12):118-123.

[0085] 3. Dai Yongqing, Geng Huibin, Cai Xiaorong. A review of the recent development of ammonia absorption chillers in Japan [J]. Refrigeration Technology, 2000(4):1-5.

[0086] 4. Xu Shangxin, Gao Hongtao. Performance of lithium nitrate-ammonia absorption ice-making system for fishing vessels [J]. Journal of Dalian Maritime University, 2025, 51(2): 58-65.

Claims

1. A method for controlling the condensing pressure reference in an ammonia absorption refrigeration system, characterized in that, Includes the following steps: (1) Determine the absolute physical reference determined by the physical properties of ammonia—the target condensation pressure. The target condensation pressure has a unique value at a given condensation temperature; (2) Real-time monitoring of the absolute pressure inside the condenser ; (3) With the aforementioned condensation pressure Approaching To achieve the target, the operating parameters of the ammonia purification unit were adjusted so that the purity of the ammonia vapor entering the evaporator was close to 100%.

2. The method according to claim 1, characterized in that, The target condensation pressure The condensation pressure is determined by the saturated vapor pressure of pure ammonia at the condensation temperature. According to national standard GB50072-2021, the design condensation temperature of an ammonia refrigeration system should not exceed 40℃. Within this standard's design operating range, the target condensation pressure... It is uniquely determined by the ammonia saturation pressure corresponding to the actual condensation temperature.

3. The method according to claim 1, characterized in that, The reflux ratio of the distillation column is adjusted to regulate the reflux flow rate or the cooling capacity of the reflux condenser.

4. A condensing pressure reference controller implementing the method of any one of claims 1-3, characterized in that, include: (a) A pressure detection unit, installed at the condenser, is used to detect the absolute pressure inside the condenser in real time. ; (b) Reference unit, providing the target condensation pressure The corresponding reference signal; (c) A comparison unit, connected to the pressure detection unit and the reference unit, receives the pressure signal and the reference signal, and outputs a pressure deviation signal; (d) An execution unit, connected to the comparison unit, adjusts the operating parameters of the ammonia purification unit according to the pressure deviation signal, so that... Approaching .

5. The controller according to claim 4, characterized in that, The execution unit is any one of the following: • An electric regulating valve is installed on the cooling water supply line of the reflux condenser. It adjusts the cooling water flow rate through a PID controller, thereby indirectly changing the reflux ratio. • Variable frequency pump, used to adjust the speed of the return pump, directly changing the return flow rate; • A self-operated differential pressure driven valve is installed on the reflux liquid line of the distillation column. It is connected to the pressure tapping point of the condenser and the pressure tapping point at the top of the distillation column through pressure guide pipes. The valve core is displaced by overcoming the spring force under the action of differential pressure, and directly adjusts the reflux liquid flow rate. The spring preload corresponds to the pressure difference between the condenser and the top of the column under full load design conditions.

6. The controller according to claim 4, characterized in that, It also includes a purity monitoring unit, which has a built-in ammonia-water solution thermophysical property database and monitors the purity based on real-time condensation pressure. The purity of ammonia vapor is calculated based on the condensation temperature. When the calculated purity is lower than a preset threshold, an alarm or auxiliary purification device is triggered.

7. An ammonia absorption refrigeration system comprising the controller according to any one of claims 4-6, characterized in that, The system uses condensation pressure as the absolute physical reference and automatically adjusts the reflux ratio of the distillation column to ensure that the purity of ammonia entering the evaporator is always close to 100%.

8. The system according to claim 7, characterized in that, The system is a single-stage, two-stage, or multi-stage ammonia absorption refrigeration system, or it is a modified existing unit with the controller added.

9. The system according to claim 7, characterized in that, The system can maintain constant ammonia purity through adaptive adjustment over a wide evaporation temperature range, ensuring that the evaporator does not freeze and the distillation column does not flood.

10. The system according to claim 7, characterized in that, It also includes a real-time online monitoring and data output unit, which outputs data based on the real-time condensing pressure. The system calculates the current ammonia vapor purity and system energy efficiency (COP) based on the condensation temperature, and continuously outputs the data through a communication interface to form a traceable operational data record, providing real data support for carbon trading and green building certification.

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