With the development of the economy, we use advanced science and technology to reasonably allocate heat; we also allow the available thermal energy to be fully utilized. We should provide better heating services to more users, which is the primary task of heating enterprises. We introduce microcomputer monitoring and automatic control into the heating system and shift from manual to automatic regulation of the heating system, which can meet the heating demand in the new situation.
We have composed the components of the programmable and inverter pressure transmitter into an intelligent control system for heat exchange stations, and the temperature transmitter can be operated in the same way. Today, we focus on the system composition of heat exchange stations and the design of automatic control systems. We have commissioned the control system in several power plants, and the users are more satisfied. The system is highly automated, remotely monitored, safe, reliable, and easy to service and maintain.
1. Overview of heat exchange stations
The heat exchange station is mainly used to generate primary high-temperature water through the heat exchanger equipment. When the user’s secondary network of low-temperature return water is closed, it exchanges heat. It then uses the circulation pump equipment to overcome the resistance formed by the secondary network and then transmits the hot water to the indoor heat sink.
2. Control of heat exchange station systems
2.1 Main controlled equipment for control purposes
Control purpose of the heat exchange station system.
- We control the circulating pump of the regulating valve, the charge pump, the heat exchanger, the temperature control valve, and the system pipeline.
- We adjust the operation mode of each application’s working condition, and we will provide a stable water supply temperature to the end in a very economic situation.
- We also improve the level of automation of the system to increase efficiency and reduce the management effort.
2.2 Design of the electronic control system
2.2.1 We have to write the I/O address assignment table according to the equipment operation flow and the electrical components to be controlled and then arrange the PLC interface.
2.2.2 While our touch screen communicates with the PLCCOM1 port, the print board communicates through the PLCCOM2 port.
2.2.3 Program design.
① Control processes
After the PLC is powered on and running, the program is first initialized. Then, we set the control parameters such as secondary side supply and return water pressure and temperature. The primary side produces water pressure and temperature, heat exchanger discharge temperature, and overtemperature via the touch screen. When we hit the automatic position, we then start the control. We sample the pressure and temperature values when the program is periodically scanned, and PID controlled. When we compare them with the set values after the internal calculation of the program, we make PID adjustments according to the results. The calculated values give switching and analog signals to control the pump start/stop and frequency regulation of the inverter.
② Programmable design
The automatic control system is composed of PLC, frequency converter, pressure transmitter, temperature transmitter, liquid level transmitter, and other main electrical components. The charge pump is controlled by the return water pressure to start and stop; the circulation pump is operated by frequency conversion and the frequency is adjusted by the supply and return water pressure difference; the condensate pump is operated by the water level of the condensate tank. Charging pump control principle: the current signal is transmitted through the return water pressure transmitter to the PLC. The feedback value is compared with the set value through the PLC operation. When the pressure reaches the set lower limit value, the PLC gives a signal to automatically start the charging pump; when the pressure reaches the set upper limit value, the PLC gives a signal to automatically stop the charging pump; at the same time, when the water level of the charging water tank is lower than the protection value, the charging pump is forced to stop.
③Circulation pump control principle
We through the return water pressure transmitter and water supply pressure transmitter respectively, the current signal to the PLC, through the PLC operation, the feedback of the supply and return water pressure difference value and the set pressure difference value for comparison, when lower than the set value, the PLC output voltage signal to the inverter, circulation pump frequency linearly increasing, speed up; when higher than the set value, the PLC output voltage signal to the inverter, circulation pump frequency linearly decreasing, speed reduction; when close to the set value, circulation pump constant speed operation.
④Principle of condensate pump control
The magnetic level gauge is used in the condensate tank to transmit the current signal to the PLC, and the feedback value is compared with the set value by the PLC operation. When the tank’s water level reaches the set upper limit, the PLC will automatically signal to start the condensate pump command; when the water level comes to the set lower limit, the PLC will signal to stop the condensate pump command.
The program below is a temperature and pressure acquisition program, which first transfers the analog data collected in AIW0 (channel 1) to the VW140 and obtains the temperature or pressure value by calculation; the program in the diagram also detects if the transmitter is damaged, and if it is damaged a transmitter damage service command will be issued.
The control system of the heat exchange station adopts PLC control technology, frequency conversion debugging technology, and electronic signal technology, which realizes the automatic control of pressure and temperature of the heat exchange station, we not only save the equipment cost but also improve the automation degree of the equipment so that the equipment runs reliably and stably. We want to communicate via Ethernet so that the central control room receives various signals. We can also set parameters via the touch screen and start and stop with one key to achieve remote monitoring functions.