Throttle Valve Control Method and System for Chiller Based on Fuzzy Control Algorithm

Abstract

The present invention discloses a chiller throttle valve control method and system based on fuzzy control algorithm, which controls the throttle valve opening by detecting multiple variables, and can control the throttle valve according to the current condenser liquid level change and the evaporator heat exchange temperature difference change at the same time, so that the throttle valve can be adjusted according to the operating conditions of the system, and the evaporator heat exchange temperature difference is introduced as the detection quantity. When the evaporator refrigerant supply is insufficient and the heat exchange temperature difference increases, the throttle valve opening can be adjusted in time, and the evaporator heat exchange temperature difference can be reduced while ensuring the condenser liquid level as much as possible. The heat exchange efficiency of the evaporator can be improved, thereby improving the energy efficiency of the chiller system and extending the service life of the unit.

Description

Technical Field

[0001] The present invention relates to the technical field of refrigeration and air conditioning, and particularly relates to a control method and system for a throttle valve of a chiller based on a fuzzy control algorithm. Background Art

[0002] During the operation of existing refrigeration and air conditioning chillers, the liquid level of the condenser is generally detected in real time through a condenser liquid level sensor. A target liquid level of the condenser is set, and the opening of the throttle valve is adjusted based on this target. The throttle valve is a continuously adjustable valve such as an electronic expansion valve or a ball valve, and an actuator is provided on the valve to adjust the valve opening; only adjusting the throttle valve opening with the condenser liquid level as the control target will cause the following problems in the actual operation process: The conventional control method for the throttle valve of a chiller uses PID control with the set condenser liquid level as the control target. The detected variable is single and cannot reflect the influence of multiple variables on the throttle valve control. When the refrigerant supply to the evaporator is insufficient, if the condenser still maintains the original liquid level target unchanged, the heat exchange effect of the evaporator will deteriorate, the heat exchange temperature difference will increase, resulting in a decrease in the overall energy efficiency of the unit. At the same time, it will also cause the liquid level of the evaporator to be too low, resulting in too low evaporation pressure and failures such as frozen pipes, and the unit cannot operate normally. Summary of the Invention

[0003] Technical Objective: Aiming at the deficiency that the existing refrigeration and air conditioning chillers only control the opening of the throttle valve through the condenser liquid level and the detected variable is single, the present invention discloses a control method and system for a throttle valve of a chiller based on a fuzzy control algorithm, which performs throttle valve opening control through multiple parameter detections and uses a fuzzy control algorithm to maintain the operation of the unit when the refrigerant is insufficient.

[0004] Technical Solution: To achieve the above technical objective, the present invention adopts the following technical solution:

[0005] A control method for a throttle valve of a chiller based on a fuzzy control algorithm includes the steps of:

[0006] S01. Select the liquid level deviation dL, deviation change rate dLc between the currently detected liquid level of the condenser of the chiller and the set reference liquid level, and the current evaporator heat exchange temperature difference value dT as the input parameters of the fuzzy control system, and the throttle valve action opening dV as the output parameter of the fuzzy control system; wherein, dL c is the ratio of the difference between the current liquid level deviation and the previous liquid level deviation to the detection time interval;

[0007] S02. Then perform fuzzy definition of the input parameters and output parameters:

[0008] The liquid level deviation range is taken as [dL_min, dL_max], the universe of discourse of the fuzzy subset is taken as [NL_min, NL_max], the fuzzy linguistic variable of the liquid level deviation is defined into five levels: {negative large, negative small, zero, positive small, positive large}, and is correspondingly expressed as {NB, NS, ZO, PS, PB}. The value NL of dL in the universe of discourse is as follows:

[0009] The range of the liquid level deviation change rate is [dLc_min, dLc_max], with the unit of s -1 , the universe of discourse of the fuzzy subset is taken as [NLc_min, NLc_max], the fuzzy linguistic variable of the liquid level deviation change rate is defined into five levels: {negative large, negative small, zero, positive small, positive large}, and is correspondingly expressed as {NB, NS, ZO, PS, PB}. The value of dLc in the universe of discourse is:

[0010] The range of the heat exchange temperature difference of the evaporator is [dT_min, dT_max], with the unit of °C. The universe of discourse of the fuzzy subset is [NT_min, NT_max]. The fuzzy linguistic variable of the heat exchange temperature difference is defined into 4 levels, taken as {zero, positive small, positive medium, positive large}, and is correspondingly expressed as {ZO, PS, PM, PB}; the value of dT in the universe of discourse is:

[0011] The range of the throttle valve actuation opening is [dV_min, dV_max], the universe of discourse of the fuzzy subset is [NU_min, NU_max]. The fuzzy linguistic variable of the throttle valve actuation opening is defined into 7 levels, taken as {negative large, negative medium, negative small, zero, positive small, positive medium, positive large}, and is correspondingly expressed as {NB, NM, NS, ZO, PS, PM, PB}. The actual throttle valve actuation opening dV is:

[0012] S03. Fix the fuzzy linguistic variable of one of the three input parameters, and give the output values corresponding to the changed fuzzy linguistic variables of the other two transfer parameters to establish a fuzzy rule control table;

[0013] S04. Defuzzify the fuzzy quantity output by the fuzzy control system, and calculate the corresponding throttle valve actuation opening accordingly.

[0014] Preferably, the liquid level deviation adopts the percentage deviation corresponding to the liquid level height, and the throttle valve actuation opening adopts the percentage opening of the valve.

[0015] Preferably, the membership degrees of the elements within the fuzzy subsets of the input parameters and output parameters are one of triangular, double-S-shaped, or Gaussian-shaped.

[0016] The present invention also provides a throttle valve control system for a water chiller based on a fuzzy control algorithm, which uses the above-mentioned water chiller throttle valve control method and includes a compressor, a condenser, an evaporator, a throttle valve, and a controller. The compressor, condenser, and evaporator are connected in sequence to form a refrigeration circuit. The throttle valve is arranged between the condenser and the evaporator to control the refrigerant flow from the condenser to the evaporator through the throttle valve. An evaporation pressure sensor for detecting the pressure of the gaseous refrigerant in the evaporator and a chilled water outlet temperature sensor arranged at the chilled water outlet of the evaporator are provided in the evaporator. A liquid level gauge for detecting the liquid level of the condenser is provided in the condenser. The throttle valve, evaporation pressure sensor, chilled water outlet temperature sensor, and liquid level gauge are all electrically connected to the controller. The controller receives signals for calculation and outputs an analog signal to control the opening degree of the throttle valve.

[0017] Beneficial effects: The throttle valve control method and system for a water chiller based on a fuzzy control algorithm provided by the present invention have the following beneficial effects:

[0018] 1. The present invention detects multiple variables for throttle valve opening control, and can simultaneously control the throttle valve according to the current change in the condenser liquid level and the change in the heat exchange temperature difference of the evaporator, so as to adjust the throttle valve according to the operating conditions of the system.

[0019] 2. The present invention introduces the heat exchange temperature difference of the evaporator as a detection quantity. When the refrigerant supply to the evaporator is insufficient and the heat exchange temperature difference increases, the opening degree of the throttle valve can be adjusted in a timely manner, and the heat exchange temperature difference of the evaporator can be reduced while trying to ensure the condenser liquid level. The heat exchange efficiency of the evaporator can be improved, and thus the energy efficiency of the water chiller system can be improved.

[0020] 3. When the refrigerant supply to the evaporator of the control method of the present invention is insufficient, the pressure of the evaporator is ensured to be normal, the unit can operate normally, and the service life of the unit is extended.

[0021] 4. The present invention converts both the liquid level deviation and the opening degree of the throttle valve into percentages for calculation, which can meet the requirements of different models and specifications of units, is not affected by actual values, and is convenient for calculation and control. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art.

[0023] Figure 1 It is the structure diagram of the water chiller system of the present invention;

[0024] Figure 2Schematic diagram of the triangular membership function corresponding to the output parameter NU of the present invention;

[0025] Among them, 1 - compressor, 2 - condenser, 3 - evaporator, 4 - throttle valve, 5 - controller, 6 - evaporation pressure sensor, 7 - chilled water outlet temperature sensor, 8 - liquid level gauge. Specific embodiments

[0026] The present invention will be more clearly and completely described below by way of a preferred embodiment in conjunction with the drawings, but the present invention is not limited thereto within the scope of the described embodiments.

[0027] As Figure 1 shown, a throttle valve control system for a water chiller based on a fuzzy control algorithm disclosed by the present invention includes a compressor 1, a condenser 2, an evaporator 3, a throttle valve 4, and a controller 5. The compressor 1, the condenser 2, and the evaporator 3 are sequentially connected to form a refrigeration circuit. The throttle valve 4 is arranged between the condenser 2 and the evaporator 3 to control the refrigerant flow rate from the condenser 2 to the evaporator 3 through the throttle valve 4. An evaporation pressure sensor 6 for detecting the pressure of the gaseous refrigerant in the evaporator and a chilled water outlet temperature sensor 7 arranged at the chilled water outlet of the evaporator are provided in the evaporator 3. A liquid level gauge 8 for detecting the liquid level of the condenser 2 is provided in the condenser 2. The throttle valve 4, the evaporation pressure sensor 6, the chilled water outlet temperature sensor 7, and the liquid level gauge 8 are all electrically connected to the controller 5. The controller 5 receives signals for calculation and outputs an analog signal to control the opening of the throttle valve 4

[0028] The present invention discloses a throttle valve control method for a water chiller based on a fuzzy control algorithm, including the steps of:

[0029] S01. Select the liquid level deviation dL between the currently detected liquid level of the condenser of the water chiller and the set reference liquid level, the deviation change rate dLc, and the current evaporator heat transfer temperature difference value dT as the input parameters of the fuzzy control system, and the throttle valve action opening dV as the output parameter of the fuzzy control system. Among them, dL = L - Lmb, where L is the actual liquid level of the condenser; Lmb is the target liquid level of the condenser, and dL c is the ratio of the difference between the current liquid level deviation and the previous liquid level deviation to the detection time; dT = TLo - Te; TLo is the chilled water outlet temperature, and Te is the saturation temperature corresponding to the evaporation pressure Pe. In order to adapt to the control requirements of different water chillers, the liquid level deviation of the present invention uses the percentage deviation corresponding to the liquid level height, and the throttle valve action opening uses the percentage opening of the valve, which is not affected by the liquid level detection range of the liquid level gauge and the opening amplitude of the throttle valve.

[0030] S02. Then, perform fuzzy definition of the input parameters and output parameters:

[0031] The liquid level deviation range is taken as [dL_min, dL_max], the universe of discourse of the fuzzy subset is taken as [NL_min, NL_max], the fuzzy linguistic variable of the liquid level deviation is defined into five levels: {negative large, negative small, zero, positive small, positive large}, and is correspondingly expressed as {NB, NS, ZO, PS, PB}. The value NL of dL on the universe of discourse is as follows:

[0032] The range of the liquid level deviation change rate is [dLc_min, dLc_max], with the unit of s. -1 , the universe of discourse of the fuzzy subset is taken as [NLc_min, NLc_max], the fuzzy linguistic variable of the liquid level deviation change rate is defined into five levels: {negative large, negative small, zero, positive small, positive large}, and is correspondingly expressed as {NB, NS, ZO, PS, PB}. The value of dLc on the universe of discourse is:

[0033] The range of the heat transfer temperature difference of the evaporator is [dT_min, dT_max], with the unit of °C. The universe of discourse of the fuzzy subset is [NT_min, NT_max]. The fuzzy linguistic variable of the heat transfer temperature difference is defined into 4, which are taken as {zero, positive small, positive medium, positive large}, and are correspondingly expressed as {ZO, PS, PM, PB}; the value of dT on the universe of discourse is:

[0034] The range of the action opening of the throttle valve is [dV_min, dV_max], the universe of discourse of the fuzzy subset is [NU_min, NU_max]. The fuzzy linguistic variable of the action opening of the throttle valve is defined into 7, which are taken as {negative large, negative medium, negative small, zero, positive small, positive medium, positive large}, and are correspondingly expressed as {NB, NM, NS, ZO, PS, PM, PB}. The actual action opening dV of the throttle valve is:

[0035] The membership degree of the elements in the fuzzy subsets of the input parameters and output parameters is one of triangular, double S-shaped or Gaussian-shaped.

[0036] S03. Fix the fuzzy linguistic variable of one of the three input parameters, and give the output values corresponding to the changed fuzzy linguistic variables of the other two transfer parameters to establish a fuzzy rule control table;

[0037] In the implementation of the present invention, the fixed heat transfer temperature difference element NT is adopted, and the other two variables NL and NLc are changed, and the corresponding output value NU is used to formulate rules, totaling 4×5×5 = 100 rules.

[0038] When the heat transfer temperature difference NT is ZO, the output NU is as follows in the table:

[0039]

[0040] When the heat transfer temperature difference NT is PS, the output NU is as follows in the table:

[0041]

[0042] When the heat transfer temperature difference NT is PM, the output NU is as follows in the table:

[0043]

[0044] When the heat transfer temperature difference NT is PB, the output NU is as follows in the table:

[0045]

[0046] The calculation method of the fuzzy control relationship is as follows:

[0047]

[0048] S04. Defuzzify the fuzzy quantity output by the fuzzy control system, and calculate the corresponding throttle valve opening based on this. The defuzzification method can adopt the maximum membership degree method, the centroid method, the median method, etc. The present invention adopts the centroid method.

[0049]

[0050] Where a and b are the upper and lower limits of the universe of discourse of the fuzzy subset corresponding to the output quantity; x is the value of each variable in the subset; μ(x) is the membership degree of each variable value.

[0051] The following uses a specific embodiment to illustrate the application of the present invention.

[0052] First, define the value range, universe of discourse, and membership function of each variable:

[0053] The value range of the liquid level deviation dL is [-30, 30], and the universe of discourse is [-3, 3]. Take the triangular membership function, and the variable assignment table is as follows:

[0054]

[0055] The value range of the liquid level deviation change rate dLc is [-5, 5], and the universe of discourse is [-3, 3]. Take the triangular membership function, and the variable assignment table is as follows:

[0056]

[0057] The value range of the heat transfer temperature difference dT is [0, 5], and the universe of discourse is [0, 6]. Take the triangular membership function, and the variable assignment table is as follows:

[0058]

[0059] The throttling valve adjustment opening dV range is taken as [-3, 3], and the universe of discourse is taken as [-3, 3]. Taking the triangular membership function, the variable assignment table is as follows:

[0060]

[0061] The output parameter NU, and an example of its triangular membership function is as Figure 2 shown:

[0062] According to the input membership function and the established fuzzy control rules, program to calculate the fuzzy control relationship and the fuzzy decision-making under the corresponding input conditions: When the liquid level deviation is -15, corresponding to NL = -1.5, that is, the liquid level is low; the change rate is -2, corresponding to NLc = -1.2, that is, the liquid level is still decreasing; the heat exchange temperature difference of the evaporator is 1°C, corresponding to NT = 1.2. At this time, the heat exchange temperature difference is basically normal, and the throttling valve is normally closed. After the fuzzy decision-making is defuzzified, the output result NU = -0.319, corresponding to the throttling valve closing 0.319%.

[0063] When the liquid level deviation is -15, corresponding to NL = -1.5, that is, the liquid level is low; the change rate is -2, corresponding to NLc = -1.2, that is, the liquid level is still decreasing; the heat exchange temperature difference of the evaporator is 5, corresponding to NT = 6. At this time, the heat exchange temperature difference is too large. In this case, although the liquid level is low, in order to ensure sufficient liquid supply to the evaporator, the throttling valve cannot be closed. After the fuzzy decision-making is defuzzified, the output result NU = 1.54, corresponding to the throttling valve still needs to be opened 1.54% to maintain the operation of the chiller and ensure the system energy efficiency.

[0064] The above is only the preferred embodiment of the present invention. It should be pointed out that: for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A control method for the throttle valve of a chiller based on a fuzzy control algorithm, characterized in that, Including the steps: S01. Select the liquid level deviation dL, deviation change rate dLc between the currently detected liquid level and the set reference liquid level of the condenser of the chiller, and the current heat exchange temperature difference value dT of the evaporator as the input parameters of the fuzzy control system, and the throttle valve opening dV as the output parameter of the fuzzy control system; where dLc is the ratio of the difference between the current liquid level deviation and the previous liquid level deviation to the detection time interval; S02. Then, perform fuzzy definition of the input parameters and output parameters: The liquid level deviation range is taken as [dL_min, dL_max], the universe of discourse of the fuzzy subset is taken as [NL_min, NL_max], and the fuzzy linguistic variable of the liquid level deviation is defined as five levels: {negative large, negative small, zero, positive small, positive large}, which are correspondingly expressed as {NB, NS, ZO, PS, PB}. The value NL of dL in the universe of discourse is as follows: The range of the liquid level deviation change rate is [dLc_min, dLc_max], with the unit of s -1 , the universe of discourse of the fuzzy subset is taken as [NLc_min, NLc_max], and the fuzzy linguistic variable of the liquid level deviation change rate is defined into five levels: {negative large, negative small, zero, positive small, positive large}, which are correspondingly represented as {NB, NS, ZO, PS, PB}. The value of dLc in the universe of discourse is as follows: The heat transfer temperature difference range of the evaporator is [dT_min, dT_max], with the unit of °C. The universe of discourse of the fuzzy subset is [NT_min, NT_max]. The fuzzy linguistic variables of the heat transfer temperature difference are defined as 4, taken as {zero, positive small, positive medium, positive large}, corresponding to {ZO, PS, PM, PB}; the values of dT in the universe of discourse are: The throttle valve opening range is [dV_min, dV_max], the domain of the fuzzy subset is [NU_min, NU_max], and the fuzzy language variables of the throttle valve opening are defined as 7, taken as {negative large, negative medium, negative small, zero, positive small, positive medium, positive large}, corresponding to {NB, NM, NS, ZO, PS, PM, PB}, and the actual throttle valve opening dV is: S03. Fix the fuzzy language variable of one of the three input parameters, give the output values corresponding to the changed fuzzy language variables of the other two input parameters, and establish a fuzzy rule control table; S04. Defuzzify the fuzzy quantity inferred and output according to the fuzzy rule control table, and calculate the corresponding throttle valve opening based on this.

2. The throttling valve control method for a chiller based on a fuzzy control algorithm according to claim 1, characterized in that, The liquid level deviation uses the percentage deviation corresponding to the liquid level height, and the throttle valve opening uses the percentage opening of the valve.

3. The throttle valve control method of the chiller based on the fuzzy control algorithm according to claim 1, characterized in that The membership degrees of the elements in the fuzzy subsets of the input parameters and output parameters are one of triangular, double S-shaped or Gaussian-shaped.

4. A throttle valve control system for a chiller based on a fuzzy control algorithm, characterized in that, Using the throttle valve control method for a chiller according to any one of claims 1-3, including a compressor (1), a condenser (2), an evaporator (3), a throttle valve (4) and a controller (5), the compressor (1), the condenser (2) and the evaporator (3) are connected in sequence to form a refrigeration circuit, the throttle valve (4) is arranged between the condenser (2) and the evaporator (3), and the refrigerant flow from the condenser (2) to the evaporator (3) is controlled by the throttle valve (4); an evaporation pressure sensor (6) for detecting the pressure of the gaseous refrigerant in the evaporator and a chilled water outlet temperature sensor (7) arranged at the chilled water outlet of the evaporator are provided in the evaporator (3), a liquid level gauge (8) for detecting the liquid level of the condenser (2) is provided in the condenser (2), the throttle valve (4), the evaporation pressure sensor (6), the chilled water outlet temperature sensor (7) and the liquid level gauge (8) are all electrically connected to the controller (5), and the controller (5) receives signals for calculation and outputs an analog signal to control the opening of the throttle valve (4).

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