There are two lithium bromide units in the air conditioning system of Zhuzhou Carbide Factory, of which two chilled water pumps have a power of 200KW and are started by auto-coupling step-down. The operator opens 1 or 2 units according to the load. The maximum output flow of a single pump is 600m3/h, and 1200m3/h when the system is fully open. The current pump outlet pressure is 4.5-9kg, the minimum flow requirement is 200m3/h, and the maximum flow requirement is 950m3/h. The reason for the large pressure fluctuation is that the flow rate of the chilled water pump cannot be adjusted according to the load change. Once the amount of chilled water in the post-stage section decreases, the pump outlet pressure will rise sharply. The rapid circulation of chilled water in the pipeline returns water without sufficient heat exchange, resulting in a low return water temperature. Both the pump and the host consume energy in vain.
When the 1# unit starts, the 1# flow must meet the minimum flow value of 200 m3/h. If the user flow rate does not reach the minimum flow value, the unit will be shut down for protection. The on-duty personnel must open the bypass valve in advance to make part of the chilled water Circulate in the unit to ensure that there is more than 200 m3/h flow in the unit. The same is true for 2#.
2. Transformation control requirements:The automatic control is completed in three stages. The following takes the 1# air conditioning unit as an example to explain in detail as follows:
Ø The cold water outlet flow of the unit should not be less than 200 m3/h;
Ø When the output pressure of one pump cannot meet the requirements, the system prompts to manually start the other pump;
Ø Choose 1# or 2# pump as frequency conversion running pump.
Ø When the required flow rate is 0～300 m3/h, the 1# pump runs at a fixed frequency to ensure a total flow output of 300 m3/h. The bypass adjustment valve is opened and PID adjustment is performed 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 is operated with frequency conversion and PID adjustment, and the bypass valve is fully closed;
Ø When the demand flow is 600 m3/h～900 m3/h, 1# pump runs at fixed frequency and outputs 300 m3/h flow; 2# pump runs at industrial frequency and outputs 600 m3/h flow; bypass regulating valve Open and do PID adjustment to ensure 6kg output pressure;
Ø When the demand flow is 900 m3/h～1200 m3/h, the 1# pump is operated with frequency conversion and PID adjustment is made, the 2# pump is operated with 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 the other water pumps also use the original control mode. The unit selected as the variable frequency water pump is used as the regulating pump of the entire system to automatically adjust the chilled water flow. In the automatic control mode, the PLC is used to collect the pressure signal of the main pipe and send out the frequency given signal of the inverter according to the PID operation, and automatically adjust the speed of the motor (water pump) to change the chilled water pressure to achieve automatic control. The output flow is also collected and sent to the PLC for PID calculation, and then a current signal of 4 to 20 mA is output to control the bypass valve to achieve the effect of constant current.
At the same time, the PLC uploads on-site parameters, inlet and outlet pressure values, inverter 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, variable frequency pump selection and remote control can be controlled through the human-machine interface. Various operations are realized remotely, which greatly facilitates system control. Control method This scheme sets up a chain between the original power frequency system and the original power frequency system to ensure the safety of the system.
Third, the configuration of the device(1) Friendly man-machine interface: Since the system needs to set the upper and lower limits of the inlet pressure, outlet pressure, outlet flow, and total pressure of each unit, PID parameters are set, and all pressures and flows The value is displayed and the alarm information is recorded. Using Eview's MT506LV man-machine interface. The interface has edited six basic screens of "Main Screen", "Control Screen", "Parameter Setting", "Fault Record", "Pressure Curve" and "Help". On the main screen, the collected pressure, flow value, running frequency and current of the inverter are displayed, and the current status and fault display of the system are also displayed, allowing users to see at a glance.
(2) PLC:
The PLC is the brain of the device. The new product EC20 series PLC and analog input module and analog output module of Emerson Network Energy Co., Ltd. are selected. The EC20 series PLC is a high-performance general-purpose PLC with a memory command capacity of 8k; typical basic command execution Typical application instructions for speeds from 0.09 to 0.42 μS are 5 to 280 μS. High-speed input up to 50 kHz and high-speed output at 80 kHz are supported; it has rich interrupt functions, 8 input interrupts, 3 timers, 6 high-speed counts, and supports industry 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 retention. It can be expanded with multiple modules. The expansion modules are digital, analog, and temperature. Type module. The programming of EC20 adopts friendly interface window software, supports multiple programming methods: ladder diagram, instruction list, sequence function diagram, convenient monitoring and debugging, and can modify the program online.
(3) PLC configuration
The main module uses EC20-2012BRA, 20 points input and 12 points relay type output. The analog input module uses 8-channel EC20-8AD, and the analog output uses 4-channel EC20-4DA.
(4) Input and output device configuration
The input device has a "manual/automatic" selector switch, selects the frequency conversion pump "1#/2#", the system "starts" and "stops", and also collects the power frequency and frequency conversion contactor signals for interlocking and status display.
The output device has 1# pump frequency conversion operation, 2# pump frequency conversion operation, fault and reset relay.
(5) Analog input and output device configuration
EC20-8AD was used to collect main pipe pressure, bypass valve position feedback, 1# unit outlet pressure, 2# unit outlet pressure, 1# pump outlet flow, 2# pump outlet flow; use the EC20-4DA output current signal to give EV2000- 4T2000P frequency, the other way outputs 4~20mA current signal to the bypass valve.
four. working principle The main circuit diagram is as follows:
The system adopts industrial frequency conversion and mutual preparation. In addition, it can flexibly choose whether the motor to be converted is 1# or 2# by KM3 and KM4.
Fives. Program highlightsEC20 COM0 communicates with EVIEW man-machine interface through MODBUS protocol, COM1 communication port communicates with Emerson's EV2000 inverter freely, and PLC software settings:
The baud rate, data bit, stop bit and check bit in the free port protocol of COM1 must be the same as those set in the inverter to communicate. EC20 always detects the frequency, current, and fault of the inverter and reflects it on the man-machine interface.
The system needs constant pressure and constant current, so the dual PID is used to control the inverter and the bypass valve, and the controlled pressure and flow are set on the EVIEW man-machine interface.
During the on-site commissioning process, it was found that the on-site load would suddenly change when it was off work at 5 pm every day. Due to the sudden change of the load of 500m3/h into a few tens of flows, the time of this mutation was only two or three seconds, and the bypass valve The speed of the action is very slow, a stroke of 38mm takes 2 minutes, which will cause the air conditioning unit to shut down. To solve this problem, a real-time clock is used to open the bypass valve regularly, that is, it is bypassed before five o'clock every day. A valve opening, when the set time is reached, the system resumes PID adjustment. The screen for setting the period of sudden load change and the opening of the bypass valve are as follows:
This system has been completed and has been officially put into use. Due to the use of Emerson's EC20 series PLC for control, the performance is stable, the operation is reliable, the system structure is compact, energy consumption is saved, and maintenance is easy. The frequency of the frequency converter stabilizes the pressure at around 40HZ, and the energy saving rate reaches 48%. Therefore, it not only greatly improves the technical content and automation level of the product, but also greatly improves the economic efficiency of the enterprise, which is very popular.
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