With the rapid development of China's economy, domestic employers are increasingly demanding the comprehensive quality of employment-employed students. In combination with the actual situation of the school, in order to improve students' brains and hands-on ability, students can better connect theory with practice to meet the needs of social development. Systematic transformation of our school's cold storage and central air conditioning integrated teaching machine. The modified system mainly adopts PLC and touch screen automatic control technology, which adds monitoring function, has the advantages of good safety, high reliability, high accuracy, energy saving, etc., which greatly reduces switch damage, human error operation or large starting current. The resulting failure reduces the cost of internship, and achieves the teaching effect of optimizing the refrigeration and electrician, electromechanical professional training operation, and improves the learning efficiency and comprehensive quality of students.
Refrigeration system
The system of the cold storage and central air conditioning integrated teaching machine is mainly composed of four basic components: a refrigeration compressor, a condenser, an evaporator, and a throttle expansion valve. The working cycle is as follows: First, the low-pressure gaseous Freon is pressurized by the compressor into the condenser and gradually condensed into a high-pressure liquid. During the condensation process, Freon releases a large amount of heat, which is absorbed by the cooling water in the condenser and sent to an outdoor cooling tower, which is finally released into the atmosphere. The high pressure liquid Freon in the condenser then vaporizes as it flows through the throttling device before the evaporator, forming a gas-liquid mixture into the evaporator. The refrigerant is continuously vaporized in the evaporator and absorbs heat from the cold storage to a lower temperature. Finally, the vaporized Freon turned into a low-pressure gas in the evaporator, re-entering the compressor, and so on.
The following adjustment elements are also used in the device: one-way check valve, solenoid valve, thermal expansion valve, temperature relay, oil pressure difference controller, high and low voltage controller.
These adjustment elements play a role in the device as follows: The solenoid valve mainly stores the refrigerant liquid in the reservoir and the condenser during shutdown to prevent liquid shock and leakage, wherein the solenoid valve is closed off to prevent the evaporator from being drained. The rupture phenomenon; on the one hand, the thermal expansion valve can reduce the pressure of the refrigerant, on the other hand, through the temperature sensing package, the change of the superheat degree of the refrigerant at the outlet of the evaporator is sensed, and the opening degree is automatically adjusted to make the refrigerant flow rate and the heat load of the evaporator Matching; the temperature controller senses the temperature of the warehouse to control the opening and closing of the No. 1 compressor, and the No. 2 compressor is controlled to start and stop by the suction pressure. Under the condition of large cooling load, the two compressors work simultaneously to increase the cooling capacity. That is, the system realizes energy regulation by controlling the number of compressor running units; the high and low voltage controllers play a safety protection function, and automatically cut off the power when the system high pressure is too high or the low pressure is too low; the oil pressure difference controller is lower than the oil pressure setting When fixed, the oil pump is automatically activated to supply oil to the compressor crankcase to ensure lubrication and cooling of moving parts during compressor operation.
2.PLC
2.1 I/O wiring diagram
The PLC uses Mitsubishi's FX1N-40MR programmable controller. The programmable controller is suitable for harsh environments in the field, has strong anti-interference ability, convenient programming and easy to use; the extended function is also very strong. It has 24 channels of switching input and 16 channels of output, high precision, and short command processing time. The average time between failures is 300,000 hours, which can meet the system control requirements. According to the process flow, the input and output wiring diagram of the programmable controller is shown in Figure 3. The solenoid valve coils YV1 to YV4, the AC contactors KM0 to KM5, and the alarm HA in the control circuit are all ～220V voltage.
2.2 Software Programming
According to the control flow of the cold storage and central air-conditioning integrated teaching machine, the working process is as follows: start-up, the special auxiliary relay M8002 pulse sets the initial state S0, and transmits the initial data to the D0-D7 transmitter. Press the start button X1 state to go to S20, the system power supply AC contactor KM0 is closed, and the mode is selected by X2 and X3.
X2 is the central air conditioning mode selection, the state goes to S21, X3 is the cold storage mode selection, and the state goes to S23. State S21 central air-conditioning fan AC contactor is closed and self-locking, delay to state to S22, state S22 is connected to the ventilation pipe damper solenoid valve, and the damper is opened to supply air to the air-conditioned room. The delay to state goes to S23, which first drives the cooling tower fan contactor to close and self-lock. After 10 seconds delay, the cold storage fan, the suction solenoid valve and the exhaust solenoid valve coil are driven. The compressor is started again after a delay of 10 seconds. However, compressor No. 2 is controlled by a temperature switch. Work only when the cold load is large. Finally, for 10 seconds, the liquid discharge solenoid valve coil is activated to allow the refrigerant to enter the low pressure side, and the system enters a normal operating state. When shutting down, press the stop button X0 state to go to S24, the closing sequence is reversed to the startup sequence.
The program has multiple protections: (1) In the event of an accident or an emergency during start-up or operation, simply press the emergency stop button X20 to cut off the entire output component and stop the output. To restart, the entire system must be powered down for proper operation.
(2) When the cooling fan, cooling tower fan, water pump, I compressor, II compressor, etc. are overloaded, the PLC drives the HA alarm and drives the motors to stop working. (3) When the water temperature, storage temperature, exhaust temperature, high pressure, low pressure, oil pressure, etc. of the warehouse reach the set value, the corresponding input contacts X4, X5, X6, X7, X10, X11, X12 are closed, and the auxiliary relay is driven. When M100 is closed, the PLC closes all outputs and stops working.
3.
Conclusion
The improved control system makes up for the shortage of discrete components, contact points, error-prone, and high failure rate of the contact relay control system. Through the experimental teaching of the cold storage and central air-conditioning integrated teaching machine, students can learn the working principle of the cold storage, master the operation process of the central air-conditioning, and further improve the students' ability to comprehensively use the PLC control technology, which stimulates the students. Learning interest and reducing the cost of the school's investment, the program has been well received by the school's teachers and students. The author believes that the control device should have a certain promotion value in similar schools.
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