[align=left] 1. Introduction Shell-and-tube heat exchangers, characterized by their large heat transfer area, compact structure, and excellent heat transfer effect, are widely used in chemical enterprises for heating, cooling, and waste heat recovery in production processes. The glyphosate synthesis section of Nantong Jiangshan Agricultural Chemical Company uses a shell-and-tube heat exchanger to recover heat from the shift gas to increase the temperature of the hot water entering the saturation tower, thus providing the necessary conditions for the synthesis reaction. The circulating water pump of the heat exchanger is driven by a three-phase asynchronous motor. Previously, without speed regulation, the inlet and outlet water flow rates of the heat exchanger were fixed. Due to the large fluctuations in the temperature of the recovered shift gas, the temperature at the outlet of the heat exchanger was difficult to control within the specified range, which had a certain impact on the production quality of the synthesis section, mainly reflected in excessive deviations in the effective glyphosate content. Based on the above reasons, while ensuring the production process requirements, it was decided to use a general-purpose frequency converter to control the speed of the circulating water pump motor. The frequency converter operates under the program control of a microcontroller, and its control signal is given by the microcontroller after algorithm calculation based on the combined gas temperature and outlet water temperature. This article focuses on analyzing the structure and working principle of the control system in conjunction with the specific application of this variable frequency speed control system. Figure 1: Schematic diagram of the working principle of a shell-and-tube heat exchanger. 2. Working principle of a shell-and-tube heat exchanger As shown in Figure 1, the circulating gas is the shell-side fluid, with a high inlet temperature and a low outlet temperature; water is the tube-side fluid, with a low inlet temperature and a high outlet temperature. The corresponding temperature values ​​are shown in the figure. The speed of the circulating water pump motor directly affects the inlet and outlet water flow rates and the outlet temperature of the heat exchanger. 3. Hardware composition of the microcontroller-based variable frequency speed control system The main task of variable frequency speed control is to transform the process requirement for the outlet water temperature of the heat exchanger into the speed control of the water pump motor. When the temperature of the circulating gas inlet changes, the control system can promptly sample the temperature value, analyze and process it, and then send a control signal to the frequency converter. The frequency converter then controls the water pump motor to change its speed, thereby increasing or decreasing the inlet water flow rate to achieve the purpose of controlling the outlet water temperature. Therefore, we designed a microcontroller-based variable frequency speed control system (as shown in Figure 2). This system mainly consists of a microcontroller, analog I/O units, and a frequency converter. The microcontroller programs the frequency converter, which then drives the water pump motor to operate at varying speeds. Figure 2 shows the circuit diagram of the variable frequency speed control system. The functions and wiring methods of each main chip and the frequency converter are described below with reference to the circuit diagram. 3.1 Microcontroller AT89C51 The AT89C51 microcontroller from Atmel is a very cost-effective microcontroller. It is not only fully compatible with the 8051 microcontroller in terms of instructions and pins, but its on-chip 4K program memory is made using flash technology, which allows users to electrically erase and rewrite it online. The control program of this control system is stored in this program memory. The microcontroller obtains various temperature values ​​from the ADC0809 chip, then performs calculations and processing using the PI algorithm. The processing results and control data are sent to the DAC0832 through the P0 port, and the operation of the DAC0832 is controlled by the P2.7 and WR pins. 3.2 Analog-to-Digital Converter Chip ADC0809 The ADC0809 is a commonly used 8-bit successive approximation A/D converter. Its 8 analog input channels can be connected to various temperature sensors to receive temperature values ​​and convert them into digital values. After the analog conversion of any channel is completed, an interrupt signal is sent to the microcontroller through the EOC pin. After responding to the interrupt request, the microcontroller can read the digital value for analysis and processing. 3.3 Digital-to-Analog Converter Chip DAC0832 The DAC0832 is an 8-bit single-chip D/A converter. In this control system, it is responsible for converting the motor speed control data output by the microcontroller into continuously changing voltage values, thereby realizing the control of the inverter output. By shorting the CS and XFER pins of the DAC0832 and shorting the WR1 and WR2 pins, the DAC0832 operates in unipolar single-buffer mode, simplifying hardware wiring and program design. The VRef reference voltage is obtained by voltage division from -12V, and then VRef = -10V can be achieved through a precision adjustable resistor. Therefore, the final output voltage Vout of the DAC0832 is 0~10V, which also serves as the analog signal input of the frequency converter. 3.4 I/O Interface Chip 8255 Considering that the microcontroller's P1 port is already used as the system's button input, the frequency converter's abnormal alarm signal outputs FA and FB, and overload warning signal outputs OLW are connected to the microcontroller through the 8255 expansion I/O interface chip. The 8255 has three 8-bit parallel I/O interfaces; the PA port is mainly used here. Through the PA port, the microcontroller can read the operating status signals of the frequency converter. When an inverter overload or other abnormal alarm signal occurs, the microcontroller can react promptly and control the entire system to stop working through the program. Since the inverter's OLW terminal has an open-circuit output of 24V, an optocoupler is required to use it as the input signal for the 8255. 3.5 Inverter The inverter uses the IF-15K model from the Samco-I series by Sanken, Japan. The inverter receives a 0-10V analog signal from the DAC0832 and uses this analog signal to control the inverter's frequency conversion output, thereby ultimately controlling the water pump motor speed. This inverter is set to use the analog signal input at the VRF terminal as the control signal to determine the inverter's output frequency. To prevent motor overload or short circuit, the thermal relay contact FR1 is connected in series in the ES terminal circuit. The FR terminal is connected to the common terminals DCM1 and DCM2 to ensure the motor always rotates in the correct direction. In addition, the overload alarm signal output terminal OLW and the abnormal alarm signal output terminals FA and FB are connected to the I/O ports of the 8255, which facilitates the system program to monitor the working status of the inverter. During the wiring process of the inverter, the following two points should be paid special attention to: (1) The R, S, T terminals and the U, V, W terminals must not be reversed; (2) When the inverter stops working, if the wiring needs to be modified, it must be done after the "charge" light on the inverter operation panel is turned off. In addition, a 1kb EEPROM chip 24C08 is also set in the control system to store the initialization parameters of the control system and the relevant process parameters of the heat exchanger. The five buttons K1 to K5 are connected to P1.3 to P1.7 of the AT89C51, respectively, as start, stop and function selection buttons. The watchdog timer chip DS1232 is used to prevent the control system from crashing due to interference and other reasons. 4. Variable frequency speed control system program design 4.1 Microcontroller assembly language program design (1) Main program The main program functions to initialize the microcontroller, set the heat exchanger process parameters, set the A/D conversion interrupt, perform PI algorithm calculation and processing, and perform D/A conversion. Here is the main control code (the PI algorithm part is omitted). main: mov sp,#60h ; Set stack pointer mov r2,#08h ; Initial value of 8-channel A/D conversion count mov r0,#30h ; Address of A/D conversion result buffer unit mov r1,#40h ; Address of heat exchanger process parameter buffer unit setb it1 ; Pulse trigger mode setb ex1 ; Enable external interrupt setb ea ; Enable interrupt mov dptr,#0fef8h ; Indicate the address of adc0809 stat: movx @dptr,a ; Start A/D conversion here: jnb f0,here ; Wait for interrupt clr f0 ; Clear interrupt flag djnz r2,stat ; Loop, continue if not finished lcall pical ; Call pi algorithm operation and data processing subroutine lcall dtoa ; Call d/A conversion subroutine······ (2) d/A conversion subroutine dtoa: push dph push dpl ; Protect the original content of the address pointer dptr mov dptr, #7fffh ; dac0832 address mov a,40h ; Extract the data to be converted movx @dptr,a ; Start d/a conversion pop dpl pop dph ; Restore the address pointer dptr ret ; Subroutine return (3) A/D conversion interrupt service routine This interrupt service routine is used to process the result after A/D conversion in a timely manner, refresh the contents of the A/D conversion result buffer, change the new A/D conversion channel, and establish a connection with the main program through the set interrupt flag. movx a,@dptr ; Read the A/D conversion result mov @r0,a ; Store the data inc dptr ; Update the conversion channel inc r0 ; Update the temporary storage unit setb f0 ; Set the interrupt flag reti ; Interrupt return 4.2 Inverter function instruction code In this control system, all function instruction codes of the inverter are set through the panel. The following lists the main function instruction codes and their setting values. cd000=1; Monitor displays frequency. cd001=2; Operation controlled by external signal. cd002=3; Setting via analog signal input from VRF terminal (0-10V). cd005=380; Reference frequency voltage is 380V. cd006=50; Reference frequency is 50Hz. cd007=60; Upper limit frequency is 60Hz. cd008=10; Lower limit frequency is 10Hz. cd054=0; Offset frequency is 0Hz. cd055=50; Gain frequency is 50Hz. cd070=1; ES terminal normally inputs on signal. 5. Conclusion Actual operation of the circulating water pump motor in the shell-and-tube heat exchanger after adopting variable frequency speed control has proven that the outlet water temperature of the heat exchanger can remain stable. This reduces the glyphosate content deviation in the synthesis section from 3% to 0.5%, increasing revenue per ton of product by approximately 5%, resulting in an additional profit of about 120,000 yuan annually. Furthermore, when the heat exchanger temperature is too high or too low, the control system can promptly alarm and shut down the water pump motor to prevent quality accidents. Therefore, with the increasing application of general-purpose frequency converters in industrial production, especially in the technological transformation of traditional equipment and the development of corresponding control devices, not only can investment be reduced and results achieved quickly, but the entire production equipment can also be made safer and more reliable.