The application of EC20 series PLC in lithium brom

  • Detail

Application of EC20 series PLC in the reconstruction of lithium bromide central air-conditioning chilled water pump

I. Analysis of the original system

there are two lithium bromide units in the air-conditioning system of Zhuzhou Cemented Carbide plant, of which two chilled water pumps are started by self coupling step-down because different test methods are specified, and the power is 200kW. 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 opened. The outlet pressure of the existing water pump is 4 Kg, 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 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

chilled water pipe diagram

when 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 shut down for protection. The operator on duty must open the bypass valve in advance to circulate some chilled water in the unit to ensure that there is more than 200 m3/h flow in the unit. 2 # the same is true

II. Transformation control requirements:

automatic control is completed in three stages. The following is a detailed description of 1# air conditioning unit as an example:

the cold water outlet flow of the unit shall not be less than 200 m3/h

when the output pressure of one pump cannot meet the requirements, the system prompts to manually start the other pump

select 1# or 2# pump as variable frequency operation pump

when the demand flow is 0 ~ 300 m3/h, the 1# pump operates at a constant frequency to ensure that the new standard content touches the strength, durability and stability of furniture such as tables, chairs, cabinets and single-layer beds, which proves the total flow output of 300 m3/h. the bypass regulating valve is opened and PID is adjusted to ensure the output of the flow required by the user

when the demand flow is 300 m3/h ~ 600 m3/h, 1# pump operates with variable frequency and PID regulation, and the bypass valve is fully closed

when the demand flow is 600 m3/H ~ 900 m3/h, 1 × pump operates at a constant frequency and outputs a flow of 300 m3/h; 2. The pump operates at power frequency and outputs a flow of 600 m3/h; Open the bypass regulating valve and make PID adjustment to ensure 6kg output pressure

when the demand flow is 900 m3/H ~ 1200 m3/h, 1 × pump operates with variable frequency and PID regulation, 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 and send the frequency setting signal of the frequency converter according to the PID operation. The automatic control is realized by automatically adjusting the speed of the motor (water pump) to change the pressure of the chilled water. 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 values, frequency converter frequency, current, motor speed, etc. to the human-machine interface in the control room for parameter display. In the control room, the given pressure value, lower limit flow value, PID parameters, variable frequency pump selection and start-up can be remotely controlled through the man-machine interface. The remote realization of various operations greatly facilitates the system control. Control mode this scheme sets up interlocking with the original power frequency system on the basis of retaining the original power frequency system to ensure the safety of the system

III. equipment 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, and also display all pressure and flow values, and record alarm information. The mt506lv man-machine interface of eview is adopted. There are six basic screens in the interface, including "main screen", "control screen", "parameter setting", "fault record", "pressure curve" and "help". The main screen displays the collected pressure, flow value, frequency and current of the frequency converter, as well as the current state and fault display of the system, so that users can see it at a glance

(2) plc:

plc is the brain of the equipment. The new product EC20 series PLC, analog input module and analog output module of Emerson Network Energy Co., Ltd. is selected. EC20 series PLC is a general-purpose PLC with high performance, and the memory instruction capacity reaches 8K; Typical basic instruction execution speed 0.09 ~0.42 μ S typical application instruction is 5~280 μ S supports high-speed input of up to 50KHz and high-speed output of 80kHz; It has rich interrupt functions, including 8-way input interrupt, 3 timing, 6-way high-speed counting, and supports the Modbus communication network of industrial standard; There are 243 instructions in 20 categories, including floating-point operation, PID, high-speed i/o, communication, etc., with power down detection and backup battery retention, and multiple modules can be expanded. 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, sequential function diagram. It is convenient to monitor and debug, and the program can be modified

(3) PLC configuration

the main module selects ECB, 20 point input and 12 point relay type output. The analog input module adopts 8-channel ECAD, and the analog output adopts 4-channel ECDA

(4) configuration of input and output equipment

the input equipment is equipped with an auxiliary oil cylinder "manual/automatic" selection switch at the top of the two clamping jaws, which selects the variable frequency pump "1 #/2 #", the system "start" and "stop", and also collects the signals of power frequency and variable frequency contactors for interlocking and status display. Output equipment includes 1 × pump variable frequency operation, 2 × pump variable frequency operation, fault and reset relay

(5) configuration of analog input and output equipment

ECAD is used to collect header pressure, bypass valve position feedback, 1 # unit outlet pressure, 2 # unit outlet pressure, 1 # pump outlet flow, 2 # pump outlet flow; The evt2000p frequency is given by ECDA output current signal, and the other circuit outputs 4~20ma current signal to the bypass valve

IV. working principle

the main circuit diagram is as follows:

the system adopts industrial frequency conversion and mutual standby. In addition, it can flexibly choose whether the motor requiring frequency conversion is 1# or 2# completed by KM3 and km4

v. program key points

ec20 COM0 and eview human-machine interface communicate through Modbus protocol, COM1 communication port communicates with Emerson EV2000 frequency converter through free protocol, PLC software setting:

baud rate, data bit, stop bit and validity bit in COM1 free port protocol must be the same as those set in frequency converter before communication. EC20 constantly detects the frequency, current and faults of the frequency converter, and then reflects them on the human-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, and the controlled pressure and flow are set on the eview man-machine interface

in the process of on-site commissioning, it was found that the on-site load would suddenly change at 5:00 p.m. after work every day. Because the flow of 500m3/h load suddenly changed into dozens of flows, the time of this sudden change was only twoorthree seconds, and the action speed of the bypass valve was very slow. 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 was used, That is, the bypass valve will be opened before five o'clock every day. When the set time arrives, the system will resume PID regulation. The time interval setting of load sudden change and the opening setting of bypass valve are as follows:

v. conclusion

this system has been completed and has been officially put into use. Because Emerson 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 to stabilize the pressure, 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 value according to the figures published by Turkey's state-run anadulu news agency. (end)

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