"I. original system analysis
There are two lithium bromide units in the air conditioning system of Zhuzhou Cemented Carbide plant, including two chilled water pumps with a power of 200kW, which are started by self coupling step-down. The operator shall open one or two according to the load. The maximum output flow of a single water pump is 600m3 / h and 1200m3 / h when the system is fully open. The operating outlet pressure of the existing water pump is 4.5-9kg, the minimum flow demand is 200m3 / h, and the maximum flow demand is 950m3 / h. The reason for the large pressure fluctuation is that the flow of the chilled water pump cannot be adjusted according to the load change. Once the chilled water volume in the later stage section is reduced, the outlet pressure of the water pump will rise sharply. The chilled water circulates rapidly in the pipeline and returns without sufficient heat exchange, resulting in low return water temperature. Both the water pump and the main engine consume energy in vain.
After the 1# unit is started, the 1# flow must meet the minimum flow value of 200 m3 / h. If the user flow does not reach the minimum flow value, the unit will be shut down for protection. The personnel on duty must open the bypass valve in advance to circulate part of the chilled water in the unit to ensure the flow of more than 200 m3 / h in the unit. 2# so it is.
2、 Modification control requirements:
The automatic control is completed in three stages. Take 1# air conditioning unit as an example, and the details are as follows:
Ø the flow at the cold water outlet of the unit shall not be less than 200 m3 / h;
Ø when the output pressure of one pump cannot meet the requirements, the system prompts to start another pump manually;
Ø select 1# or 2# pump as variable frequency operation pump.
Ø when the demand flow is 0 ~ 300 m3 / h, the 1# pump operates at a constant frequency to ensure the total flow output of 300 m3 / h. the bypass regulating valve is opened and PID regulation is made to ensure the output of the flow required by the user;
Ø when the demand flow is 300 m3 / H ~ 600 m3 / h, the 1# pump operates with frequency conversion and PID regulation, and the bypass valve is fully closed;
Ø when the demand flow is 600 m3 / H ~ 900 m3 / h, the 1# pump operates at a constant frequency and outputs a flow of 300 m3 / h; 2# pump operates at power frequency and outputs 600 m3 / h flow; Open the bypass regulating valve and make PID regulation to ensure 6kg output pressure;
Ø when the demand flow is 900 m3 / H ~ 1200 m3 / h, the 1# pump operates with frequency conversion and PID regulation, the 2# pump operates at power frequency and the bypass valve is fully closed.
The frequency conversion system selects any water pump to adopt the frequency conversion control mode through the selection switch on the control cabinet, and other water pumps also adopt the original control mode. The unit selected as the frequency conversion water pump is used as the regulating water pump of the whole system to automatically regulate the chilled water flow. In the automatic control mode, PLC is used to collect the pressure signal of the main pipe, send the frequency setting signal of the frequency converter according to the PID operation, and automatically adjust the speed of the motor (water pump) to change the chilled water pressure to realize automatic control. The output flow is also collected and sent to PLC for PID calculation, and then 4 ~ 20mA current signal is output to control the bypass valve to achieve the effect of constant current.
At the same time, PLC uploads the field parameters, inlet and outlet pressure value, frequency converter frequency, current, motor speed, etc. to the man-machine interface in the control room for parameter display. In the control room, the given pressure value, lower limit flow value, PID parameters and variable frequency pump can be selected and started through man-machine interface. The remote realization of various operations greatly facilitates the system control. Control mode this scheme sets interlocking with the original power frequency system on the basis of retaining the original power frequency system to ensure the safety of the system.
3、 Device configuration
(1) Friendly man-machine interface: because the system needs to set the upper and lower limits of inlet pressure, outlet pressure, outlet flow and total pressure of each unit, PID parameters, display all pressure and flow values, and record alarm information. Eview's mt506lv man-machine interface is adopted. The interface edits six basic screens: main screen, control screen, parameter setting, fault record, pressure curve and help. The main screen displays the collected pressure, flow value, operation frequency and current of the frequency converter, as well as the current state and fault display of the system, so that the user can see it at a glance.
（2）PLC：
PLC is the brain of the equipment. It selects EC20 series PLC, analog input module and analog output module, a new product of Emerson Network Energy Co., Ltd. EC20 series PLC is a high-performance general PLC with a memory instruction capacity of 8K; Typical basic instruction execution speed: 0.09 ~ 0.42 μ S typical application instruction is 5 ~ 280 μ S supports high-speed input up to 50KHz and high-speed output up to 80kHz; It has rich interrupt functions, including 8 input interrupts, 3 timing, 6 high-speed counting, and supports the industrial standard Modbus communication network; There are 243 instructions in 20 categories, including floating-point operation, PID, high-speed I / O and communication. It has power down detection and backup battery holding. It can expand multiple modules. The expansion modules include digital, analog and temperature modules. EC20 adopts window software with friendly interface and supports a variety of programming methods: ladder diagram, instruction list and sequential function diagram. It is convenient for monitoring and debugging, and the program can be modified online.
(3) PLC configuration
The main module adopts ec20-2012bra, 20 point input and 12 point relay output. The analog input module adopts 8-channel ec20-8ad, and the analog output adopts 4-channel ec20-4da.
(4) Input / output device configuration
The input equipment has a "manual / automatic" selection switch, which selects the variable frequency pump "1# / 2#", the system "start" and "stop", and also collects the power frequency and variable frequency contactor signals for interlocking and status display.
Output equipment includes 1# pump variable frequency operation, 2# pump variable frequency operation, fault and reset relay.
(5) Analog I / O device configuration
Ec20-8ad is used to collect header pressure, bypass valve position feedback, 1# unit outlet pressure, 2# unit outlet pressure, 1# pump outlet flow and 2# pump outlet flow; Use ec20-4da output current signal to set ev2000-4t2000p frequency, and the other circuit outputs 4 ~ 20mA current signal to bypass valve.
4、 Working principle
The main circuit diagram is as follows:
The system adopts industrial frequency conversion and mutual standby. In addition, it can flexibly select whether the motor requiring frequency conversion is 1# or 2# completed by KM3 and km4.
5、 Key points of procedure
COM0 of EC20 communicates with eview man-machine interface through Modbus protocol, and COM1 communication port communicates with Emerson EV2000 frequency converter through free protocol. PLC software settings:
The baud rate, data bit, stop bit and validity bit in the free port protocol of COM1 must be the same as those set in the frequency converter before communication. EC20 always detects the frequency, current and fault of the frequency converter, and then reflects them on the man-machine interface.
The system needs constant pressure and constant current, so it is made into double PID to control the frequency converter and bypass valve. The controlled pressure and flow are set on eview man-machine interface.
During the on-site commissioning, it was found that the on-site load would suddenly change at 5:00 p.m. every day. Because the flow of 500m3 / h load suddenly changed into dozens of flow, the sudden change time was only two or three seconds, while the action speed of the bypass valve was very slow, and it took two minutes for a stroke of 38mm, which would cause the protection shutdown of the air conditioning unit. In order to solve this problem, The real-time clock timing opening bypass valve is adopted, that is, an opening is given to the bypass valve before 5 o'clock every day. When the set time arrives, the system resumes PID regulation. The time period setting of load sudden change and the opening setting screen of bypass valve are as follows:
6、 Conclusion
The system has been completed and officially put into use. Because Emerson's EC20 series PLC is used for control, it has stable performance, reliable operation, compact system structure, energy saving and easy maintenance. The frequency of the frequency converter is about 40Hz, the pressure is stable, and the energy saving rate reaches 48%. Therefore, it not only greatly improves the technical content and automation level of products, but also greatly improves the economic efficiency of enterprises, which is of great promotion value., Read the full text, technical section
The application requirements of PLC users are divided into two aspects: one is non-standard customization, the other is networking
Differences between servo system and frequency converter
Honeywell releases a new PLC that supports the industrial Internet of things
It is urgent to prevent hackers from invading the industrial control system
The future of China's numerical control technology can not be underestimated“
Our other product:
