A method for controlling superheat temperature of a centrifugal compressor in an MVR evaporation system

Abstract

The present invention discloses a method for controlling the superheat temperature of a centrifugal compressor in an MVR evaporation system. This method utilizes PLC automated control technology to track temperature changes in real time, adjust the pumping flow of a variable-frequency water pump based on the outlet steam temperature deviation and the rate of change of the deviation, select characteristic states of corresponding conditions from a characteristic modal library, and determine the frequency setpoint of the inverter based on the characteristic states. This frequency setpoint is transmitted to the inverter's input channel by an analog-to-digital conversion module. The method tracks the deviation between the outlet steam temperature and the target temperature in real time, compensates for the variable-frequency water pump's frequency increase or decrease, and automatically adjusts the inverter frequency, thereby changing the pumping flow of the variable-frequency water pump and reducing the superheated steam temperature to a slightly superheated state close to the desired saturated steam temperature. This control method achieves automatic control of the superheat temperature and exhibits excellent robustness and adaptability.

Description

Technical Field

[0001] The present invention relates to a superheat temperature control method, in particular to a superheat temperature control method for a centrifugal compressor in an MVR evaporation system, and belongs to the technical field of compressor control. Background Art

[0002] Mechanical vapor recompression (MVR) evaporation system varies according to the process, but the main equipment includes preheater, evaporator, separator, steam compressor, temperature control device and circulation pump, etc. Figure 1 As shown. The secondary steam generated by the evaporator enters the separator to remove the entrained droplets, and then enters the centrifugal compressor for adiabatic compression. The enthalpy of the compressed steam increases, generating superheated steam. If the superheated steam heat exchanger is used directly, the heat exchange area needs to be larger. The direct use of superheated steam for process heat exchange affects the output of the equipment, thereby affecting the production output. According to actual experience, the superheat degree increases by 10%. The heat exchanger's heat transfer area needs to be increased by 1%. Therefore, a temperature control device is required to convert the superheated steam into saturated steam. This steam, serving as a heat source, enters the evaporator shell side for isobaric condensation, with condensate discharged. This cycle repeats, continuously discharging condensate to concentrate or crystallize the raw liquid. Therefore, converting superheated steam into saturated steam is an essential step in the MVR evaporation system. If the spraying is insufficient (insufficient spraying), the compressed steam temperature will be too high, preferentially occupying the heat exchanger area for heat exchange with the feed liquid, turning it into saturated steam. This reduces evaporation capacity and reduces heat exchange efficiency. This requires more heat exchange area, which means higher design parameters and manufacturing costs for the heat exchanger equipment. Furthermore, excessively high superheated steam creates a temperature gradient across the heat exchange surface, generating thermal stress, which can damage the equipment and increase maintenance frequency, downtime, and costs. Excessive spraying (excessive spraying) results in unsaturated steam before entering the heat exchanger, reducing the heat transferred to the feed liquid, disrupting the thermal balance of the evaporation system and, in severe cases, affecting system stability. Therefore, precise control of the compressed steam temperature directly affects the heat exchange efficiency of the evaporator and the evaporation efficiency and energy-saving effect of the entire evaporation system.

[0003] At present, most manufacturers still achieve steam cooling by manually controlling the water spray volume of the water pump. Although some have adopted automated adjustment solutions, the automatic adjustment effect is not ideal. There is still insufficient or excessive cooling, which reduces the output of the evaporation system. In serious cases, it may even damage the equipment or increase manufacturing costs. Its operation still requires frequent manual intervention and the control effect is poor. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a superheat temperature control method for a centrifugal compressor in an MVR evaporation system, which can automatically, quickly and stage by stage control the atomizing water spray volume of a desuperheating water pump to achieve superheated steam cooling.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A method for controlling the superheat temperature of a centrifugal compressor in an MVR evaporation system, characterized by comprising the following steps:

[0007] S1. Obtain the pressure measurement values ​​of the outlet pipe of the centrifugal compressor in the MVR evaporation system and the temperature measurement value after water spray cooling ;

[0008] S2. According to the pressure measurement value of the outlet pipe Calculate the corresponding saturated steam temperature of the outlet pipe , set the superheat value for saturated steam , then the target temperature value of the outlet pipe saturation temperature is ;

[0009] S3, according to the outlet superheated steam enthalpy , outlet saturated steam enthalpy and desuperheating water enthalpy , combined with the evaporation mass flow rate of the MVR system under the design conditions Determine the mass flow rate of cooling water , and according to the mass flow rate of cooling water Select a variable frequency water pump with appropriate flow range;

[0010] S4. Set the frequency setting value of the variable frequency water pump according to the working characteristic curve of the variable frequency water pump. The flow rate of the variable frequency water pump increases as the frequency of the variable frequency water pump increases.

[0011] S5. In the MVR evaporation system, the variable frequency water pump is allowed to start only after the centrifugal steam compressor is working. When the outlet pipe temperature measurement value When the frequency conversion water pump is allowed to start, the frequency conversion water pump receives the start command and the frequency conversion water pump closes and works; if , the variable frequency water pump stops working; within 5 seconds after the start command is issued, it is determined whether the variable frequency water pump operation signal is received. If there is an operation signal, it means that the variable frequency water pump is working normally, otherwise it is determined that the cooling device is faulty;

[0012] S6. According to the actual working conditions, the adjustment parameters are determined by the evaporation temperature, cooling water temperature and the response speed of the variable frequency water pump. , adjustment parameters It is also the quantitative factor of the controlled object;

[0013] S7. Compare the temperature measurement values ​​after water spray cooling and target temperature value , find the error and error rate , then formulate the error and error rate The control rule matrix , input matrix and control rule matrix Combined with the output corresponding control quantity , Directly act on the controlled object variable frequency water pump;

[0014] S8, input matrix For any error and error rate The change of the frequency converter corresponds to the quantitative relationship of the given value, which is determined by the quantitative factor control, For different errors and error rate Regarding the adjustment speed of the frequency converter to deal with different overshoots, ,in, Indicates different errors and error rate The proportional factor under When it is an even number, , and relation, When is an odd number, and relation;

[0015] S9, Error of the controlled object and error rate The control rules can be summarized as "if...then" statements, ,Each rule can be expressed as a control relation ,Right now:

[0016]

[0017] Representation error and error rate With output The mapping relationship between them; thus confirming the control matrix ,Right now:

[0018]

[0019] For any given input Both can calculate the control quantity ,Right now:

[0020] ；

[0021] S10. Selecting a characteristic state that meets the conditions from the frequency characteristic mode library based on the calculated steam temperature error and error rate, determining a frequency setting value of the inverter based on the characteristic state, and transmitting the frequency setting value to the input channel of the inverter through the analog-to-digital conversion module;

[0022] S11. Based on the current frequency measurement value of the variable frequency water pump and the frequency setting value of the inverter corrected by the characteristic state, the frequency increase value or frequency decrease value of the variable frequency water pump is compensated in real time, thereby controlling the atomizing water spray volume of the desuperheating water pump, thereby adjusting the outlet steam temperature to the slightly superheated target temperature value of the saturation temperature.

[0023] Furthermore, the saturated steam temperature in step S2 is The calculation process is: pre-collect multiple groups of outlet pipe pressure measurement values And obtain the pressure measurement value of each group of outlet pipes by looking up the table The corresponding saturated steam temperature , use Excel data fitting method to get saturated steam temperature Pressure measurement value of the outlet pipe Functional relationship , the pressure measurement value of any outlet pipe can be calculated through this functional relationship The corresponding saturated steam temperature .

[0024] Furthermore, in step S3, the steam temperature and pressure of the outlet pipe before and after desuperheating are input into the steam characteristic software to obtain the outlet superheated steam enthalpy value. , outlet saturated steam enthalpy and desuperheating water enthalpy , combined with the steam mass flow of the MVR system under the design conditions , calculate the mass flow rate of cooling water , and by Determine the selection of variable water pump.

[0025] Furthermore, in step S7, the error , error rate , the actual allowable error range of the MVR evaporation system .

[0026] Compared with the prior art, the present invention has the following advantages and effects:

[0027] 1. The control method of the present invention establishes a characteristic modal library of input and output variables, and tracks the output state corresponding to the input state in the characteristic modal library in real time. It does not require precise mathematical modeling and is easy to implement online. While ensuring that water spraying does not excessively reduce the temperature, it improves the stability of the unit, thus achieving system temperature regulation and enabling automatic control.

[0028] 2. The control method of the present invention can set the quantization factor and proportional factor according to the MVR evaporation system with different evaporation capacity and evaporation temperature conditions. This control method will not cause the control effect to deteriorate due to changes in object characteristics caused by load changes, thereby enhancing the robustness and adaptability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a process schematic diagram of the MVR evaporation system of the present invention.

[0030] Figure 2 It is a flow chart of determining the outlet steam target temperature value of the present invention.

[0031] Figure 3 It is a PLC control logic diagram of the variable frequency water pump of the present invention.

[0032] Figure 4 It is a flow chart of the temperature control automatic adjustment closed-loop control of the present invention. DETAILED DESCRIPTION

[0033] In order to elaborate on the technical solutions adopted by the present invention to achieve the predetermined technical purpose, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments, and the technical means or technical features in the embodiments of the present invention can be replaced without creative work. The present invention will be described in detail below with reference to the drawings and in conjunction with the embodiments.

[0034] A method for controlling the superheat temperature of a centrifugal compressor in an MVR evaporation system according to the present invention comprises the following steps:

[0035] S1. Obtain the pressure measurement values ​​of the outlet pipe of the centrifugal compressor in the MVR evaporation system and the temperature measurement value after water spray cooling .

[0036] like Figure 2 As shown, a pressure sensor is installed in the outlet pipe of the centrifugal compressor in the MVR evaporation system, and the pressure value of the outlet pipe measured by the sensor is After A / D conversion, it is transmitted to PLC.

[0037] S2. According to the pressure measurement value of the outlet pipe Calculate the corresponding saturated steam temperature of the outlet pipe , set the superheat value for saturated steam , then the target temperature value of the outlet pipe saturation temperature is .

[0038] Pre-collect pressure measurements of multiple groups of outlet pipes And obtain the pressure measurement value of each group of outlet pipes by looking up the table The corresponding saturated steam temperature , use Excel data fitting method to get saturated steam temperature Pressure measurement value of the outlet pipe Functional relationship , the pressure measurement value of any outlet pipe can be calculated through this functional relationship The corresponding saturated steam temperature . And it is implemented in the form of algorithm programming in PLC.

[0039] In actual steam transmission, energy loss is inevitable. Based on factors such as the actual evaporation system pipeline length and ambient temperature, a certain superheat value is set for saturated steam. , general value , then the target setting value of the actual outlet saturation temperature is .

[0040] S3, according to the outlet superheated steam enthalpy , outlet saturated steam enthalpy and desuperheating water enthalpy , combined with the evaporation mass flow rate of the MVR system under the design conditions Determine the mass flow rate of cooling water , and according to the mass flow rate of cooling water Select a variable frequency water pump with an appropriate flow range.

[0041] According to the principle of heat conservation, the heat absorbed by cooling water is equal to the heat released when superheated steam is reduced to saturated steam, which is used to calculate the flow rate of cooling water. The evaporation capacity of the MVR system under actual working conditions, that is, the mass flow rate of water vapor is , according to the steam temperature and pipeline pressure before and after desuperheating, the outlet superheated steam enthalpy value is obtained by querying the water vapor characteristic software , outlet saturated steam enthalpy and desuperheating water enthalpy , calculate the mass flow rate of cooling water .

[0042] S4. According to the working characteristic curve of the variable frequency water pump, the frequency setting value of the variable frequency water pump is set. The flow rate of the variable frequency water pump increases as the frequency of the variable frequency water pump increases.

[0043] S5, such as Figure 3 As shown in the figure, in the MVR evaporation system, the variable frequency water pump is allowed to start only after the centrifugal steam compressor is working. When the frequency conversion water pump is allowed to start, the frequency conversion water pump receives the start command and the frequency conversion water pump closes and works; if , the variable frequency water pump stops working; within 5 seconds after the start command is issued, it is determined whether the variable frequency water pump operation signal is received. If there is an operation signal, it means that the variable frequency water pump is working normally, otherwise it is determined that the cooling device is faulty.

[0044] S6. According to the actual working conditions, the adjustment parameters are determined by the evaporation temperature, cooling water temperature and the response speed of the variable frequency water pump. , adjustment parameters It is also the quantitative factor of the controlled object. Due to the complexity of the actual working conditions, the adjustment parameters Affected by evaporation temperature, cooling water temperature and variable frequency water pump response speed, adjustment parameters The fixed value is usually an empirical value.

[0045] S7. Compare the temperature measurement values ​​after water spray cooling and target temperature value , find the error and error rate , then formulate the error and error rate The control rule matrix , input matrix and control rule matrix Combined with the output corresponding control quantity , Directly act on the controlled object, the variable frequency water pump.

[0046] error , error rate , the actual allowable error range of the MVR evaporation system .

[0047] S8, input matrix For any error and error rate The change of the frequency converter corresponds to the quantitative relationship of the given value, which is determined by the quantitative factor control, For different errors and error rate Regarding the adjustment speed of the frequency converter to deal with different overshoots, ,in, Indicates different errors and error rate The proportional factor under When it is an even number, , and relation, When is an odd number, and relation.

[0048] S9, Error of the controlled object and error rate The control rules can be summarized as "if...then" statements, ,Each rule can be expressed as a control relation ,Right now:

[0049]

[0050] Representation error and error rate With output The mapping relationship between them; thus confirming the control matrix ,Right now:

[0051]

[0052] For any given input Both can calculate the control quantity :

[0053] .

[0054] Variable frequency water pump automatic adjustment optimization algorithm process: Comparison of temperature measurement values ​​after water spray cooling (i.e. superheated steam temperature) and target set value ,error , error rate , the actual allowable error range of the MVR evaporation system According to the error The error classification is determined by the quantitative accuracy level, that is, dimension.

[0055] Here we take the 6-dimensional error and error rate as an example. The error can be defined as:

[0056]

[0057] The corresponding error rate can be defined as:

[0058]

[0059] Then the error of the controlled object and error rate The control rules can be summarized as 36 "if...then" statements, each of which can be expressed as a control relationship , indicates whether the corresponding frequency-controlled water pump is working under different errors and error rates, 1 means working, 0 means not working. That is:

[0060]

[0061] Input Matrix The quantitative relationship is determined by the quantitative factor Control, indicating that for different errors and error rate Regarding the adjustment speed of the frequency converter to deal with different overshoots, ;in, Indicates different errors and error rate The scaling factor under , , , .

[0062] For any given input The control quantity can be output through the following PLC programming , It can be expressed as:

[0063]

[0064]

[0065] .

[0066] S10. Selecting a characteristic state that meets the conditions from the frequency characteristic mode library based on the calculated steam temperature error and error rate, determining a frequency setting value of the inverter based on the characteristic state, and transmitting the frequency setting value to the input channel of the inverter through the analog-to-digital conversion module;

[0067] S11. Based on the current frequency measurement value of the variable frequency water pump and the frequency setting value of the inverter corrected by the characteristic state, the frequency increase value or frequency decrease value of the variable frequency water pump is compensated in real time, thereby controlling the atomizing water spray volume of the desuperheating water pump, thereby adjusting the outlet steam temperature to the slightly superheated target temperature value of the saturation temperature.

[0068] like Figure 4As shown, the above control method is programmed through PLC to track temperature changes in real time, adjust the pumping flow of the variable frequency water pump according to the outlet steam temperature deviation and the deviation change rate, and select the characteristic state of the corresponding condition from the characteristic mode library. The frequency setting value of the inverter is determined by the characteristic state and transmitted to the input channel of the inverter by the analog-to-digital conversion module. The deviation between the outlet steam temperature and the target temperature is tracked in real time, the frequency increase or decrease value of the variable frequency water pump is compensated, and the frequency of the inverter is automatically adjusted, thereby changing the pumping flow of the variable frequency water pump to achieve a slightly superheated state where the superheated steam temperature is reduced to a state close to the required saturated steam (generally close to the saturated steam temperature). ).

[0069] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the present profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical content disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent replacement and improvement of the above embodiments made according to the technical essence of the present invention, within the spirit and principles of the present invention, without departing from the content of the technical solution of the present invention, shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for controlling the superheat temperature of a centrifugal compressor in an MVR evaporation system, characterized in that The following steps are involved: S1. Obtain the pressure measurement values ​​of the outlet pipe of the centrifugal compressor in the MVR evaporation system and the temperature measurement value after water spray cooling ; S2. According to the pressure measurement value of the outlet pipe Calculate the corresponding saturated steam temperature of the outlet pipe , set the superheat value for saturated steam , then the target temperature value of the outlet pipe saturation temperature is ; S3, according to the outlet superheated steam enthalpy , outlet saturated steam enthalpy and desuperheating water enthalpy , combined with the evaporation mass flow rate of the MVR system under the design conditions Determine the mass flow rate of cooling water , and according to the mass flow rate of cooling water Select a variable frequency water pump with appropriate flow range; S4. Set the frequency setting value of the variable frequency water pump according to the working characteristic curve of the variable frequency water pump. The flow rate of the variable frequency water pump increases as the frequency of the variable frequency water pump increases. S5. In the MVR evaporation system, the variable frequency water pump is allowed to start only after the centrifugal steam compressor is working. When the outlet pipe temperature measurement value When the frequency conversion water pump is allowed to start, the frequency conversion water pump receives the start command and the frequency conversion water pump closes and works; if , the variable frequency water pump stops working; within 5 seconds after the start command is issued, it is determined whether the variable frequency water pump operation signal is received. If there is an operation signal, it means that the variable frequency water pump is working normally, otherwise it is determined that the cooling device is faulty; S6. According to the actual working conditions, the adjustment parameters are determined by the evaporation temperature, cooling water temperature and the response speed of the variable frequency water pump. , adjustment parameters It is also the quantitative factor of the controlled object; S7. Compare the temperature measurement values ​​after water spray cooling and target temperature value , find the error and error rate , then formulate the error and error rate The control rule matrix , input matrix and control rule matrix Combined with the output corresponding control quantity , Directly act on the controlled object variable frequency water pump; S8, input matrix For any error and error rate The change of the frequency converter corresponds to the quantitative relationship of the given value, which is determined by the quantitative factor control, For different errors and error rate Regarding the adjustment speed of the frequency converter to deal with different overshoots, ,in, Indicates different errors and error rate The proportional factor under When it is an even number, , and relation, When is an odd number, and relation; S9, Error of the controlled object and error rate The control rules can be summarized as "if...then" statements, ,Each rule can be expressed as a control relation ,Right now: Representation error and error rate With output The mapping relationship between them; thus confirming the control matrix ,Right now: For any given input Both can calculate the control quantity ,Right now: ； S10. Selecting a characteristic state that meets the conditions from the frequency characteristic mode library based on the calculated steam temperature error and error rate, determining a frequency setting value of the inverter based on the characteristic state, and transmitting the frequency setting value to the input channel of the inverter through the analog-to-digital conversion module; S11. Based on the current frequency measurement value of the variable frequency water pump and the frequency setting value of the inverter corrected by the characteristic state, the frequency increase value or frequency decrease value of the variable frequency water pump is compensated in real time, thereby controlling the atomizing water spray volume of the desuperheating water pump, thereby adjusting the outlet steam temperature to the slightly superheated target temperature value of the saturation temperature.

2. The method for controlling the superheat temperature of a centrifugal compressor in an MVR evaporation system according to claim 1, wherein: The saturated steam temperature in step S2 The calculation process is: pre-collect multiple groups of outlet pipe pressure measurement values And obtain the pressure measurement value of each group of outlet pipes by looking up the table The corresponding saturated steam temperature , use Excel data fitting method to get saturated steam temperature Pressure measurement value of the outlet pipe Functional relationship , the pressure measurement value of any outlet pipe can be calculated through this functional relationship The corresponding saturated steam temperature .

3. The method for controlling the superheat temperature of a centrifugal compressor in an MVR evaporation system according to claim 1, wherein: In step S3, the steam temperature and pressure of the outlet pipe before and after desuperheating are respectively input into the steam characteristic software to obtain the outlet superheated steam enthalpy value. , outlet saturated steam enthalpy and desuperheating water enthalpy , combined with the steam mass flow of the MVR system under the design conditions , calculate the mass flow rate of cooling water , and by Determine the selection of variable water pump.

4. The method for controlling the superheat temperature of a centrifugal compressor in an MVR evaporation system according to claim 1, wherein: In step S7, the error , error rate , the actual allowable error range of the MVR evaporation system .

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