1 Introduction
The refrigeration unit control system in this project is mainly used to cool heat-generating equipment in the printing industry and other industrial sites to ensure their normal operation. This refrigeration system uses circulating chilled water to cool the UV lamps of the printing press, automatically adjusting the chilled water flow, automatically scheduling the number of refrigeration units in operation, and controlling the water pumps via frequency converters. The project utilizes a Siemens S7-300 series PLC, Siemens touchscreens, and frequency converters to design the refrigeration unit's automatic control system.
2. Principle of centralized control of refrigeration units
2.1 Process Principle
The water circulation process of the refrigeration system is shown in Figure 1. Ten refrigeration units are centrally located, one of which has a power of 30kW, while the other nine are identical refrigeration units. A new water tank was added for heat exchange of the circulating water. The water in the tank can be replenished through an external water supply pipe to maintain the normal water level. The tank also has a drain pipe for wastewater discharge. All the water cooled by the refrigeration units is supplied by the water in the tank, and the cooled water is returned to the tank for storage after cooling. The water flow branches of the ten refrigeration units are connected in parallel and exchange water with the water tank through a main water flow pipe. Therefore, the water in the tank is chilled water at a certain temperature. Next, a main water supply pipe, controlled by a variable frequency pump, draws the chilled water from the tank and circulates it through branch water pipes to cool the UV lamps of our eight printing presses. The cooled water from each branch is then returned to the water tank through the main water pipe. This completes the water circulation process flow diagram of the refrigeration unit. Clearly, this involves the so-called outlet water (cold water output) and outlet water temperature; return water (hot water return) and return water temperature. To detect their temperatures, we install a temperature sensor at both the outlet and return water points to detect the outlet water temperature and return water temperature. Variable frequency water pumps can be used to control the water flow rate and water pressure in the pipes.
2.2 Electrical Control Principle
The main physical parameters regulated and controlled by the refrigeration system are the temperature of the circulating water and the flow rate of the water pipes. The circulating water temperature is controlled primarily by operating the number of refrigeration units to reach the set temperature. The water flow rate and pressure in the water pipes are controlled by the frequency converter of the water pumps, based on the number of printing presses being operated. The circulating water temperature and flow rate control process is shown below.
Water temperature control process: During the water temperature control process of the 10 chiller units, we manually divided them into four levels, as shown in Figure 2. In the chiller unit control system, we first set a suitable outlet water temperature, and then compared the difference between the return water temperature and the outlet water temperature detected by the sensor to determine which level of chiller unit to start.
Of the 10 refrigeration units, one is a high-power, 30kW imported American unit. This unit can provide most of the required cooling capacity, therefore it must be running in every mode. The other nine units are auxiliary, lower-power, domestically produced units, which can be selectively operated to meet the cooling needs. Furthermore, even if one or more of these nine units malfunction, other working units can be selected to ensure the system continues cooling. However, if the 30kW American-imported unit malfunctions, the entire system must be shut down. Because of this malfunction, the required cooling capacity cannot be achieved, even if the other nine units are running at full capacity.
The water pump frequency conversion control process for water pipe flow: In this process, we estimate the required chilled water flow rate based on the number of printing presses that are running to meet the cooling needs of each printing press. Generally, if all printing presses are running, our water pump operates at a base frequency of 50Hz because the required chilled water flow rate is relatively high in this case. If only some printing presses are running, the required chilled water flow rate is relatively low, so we consider using frequency conversion (below the base frequency of 50Hz) for the water pump. We set different frequencies for different numbers of printing presses running. Clearly, in frequency conversion (below base frequency) control mode, we can significantly save energy, thus achieving our initial energy-saving goal.
Figure 1. Flow chart of water circulation in the chiller unit
Figure 2 Cooling control process
Figure 3. Simplified diagram of the refrigeration unit control system
Figure 4 MPI network diagram
3PLC control system
3.1 Control Network Structure
The system hardware mainly consists of one Siemens CPU313C module, one SM321 input module, two SM322 output modules, two CPU226 modules, two touchscreens (TP170 and MP277), and dozens of relays (Schneider and Omron). Due to the decentralized nature of the entire control system and for better ease of operation, MPI network control is implemented. The entire MPI network consists of four Siemens S7-300 modules, two S7-200 modules, and two touchscreens. This allows for free control of the refrigeration units inside the ceiling of the printing production line workshop and monitoring of the entire control system's status. See Figures 3 and 4.
3.2 Programming
The flowchart of the control system PLC program design is shown in Figure 5.
The main steps include: ① Program initialization: clearing intermediate registers. ② The PLC receives instruction signals from the touchscreen and sets the circulating water temperature of the chiller unit. ③ The PLC determines the variable frequency or mains frequency operating status based on the number of printing presses in operation. ④ The PLC controls the start and stop of the chiller unit based on the chilled water and return water temperatures in the circulating water circuit. ⑤ Mechanical fault alarm.
When the PLC fails to operate, the original manual operation mode of the refrigeration unit is restored. The refrigeration unit is manually started according to the number of UV lamps activated, ensuring a flow of chilled water in the refrigeration unit's circulating water circuit. In the UV lamp cooling water circulating water circuit, all UV lamp inlet solenoid valves are opened, and the water pumps operate directly without frequency converter control, providing cooling water to the UV lamps to ensure safe production and improve system reliability.
Figure 5 PLC Programming Flowchart
6. Touchscreen main interface diagram
4 Interface Design
Figure 6 shows the main interface of the touchscreen. This interface vividly displays the control process of the entire system, accurately showing the operating status of the 10 refrigeration units and the printing press. It includes buttons for automatic and manual switching, a parameter setting area, and a user management area including alarm queries and password settings for user permissions. The entire interface is simple, clear, and easy to operate on-site.
On the main touchscreen interface of the control cabinet (as shown in Figure 6), click the manual button, enter the system password, and you can enter the manual interface as shown in Figure 7.
In the manual interface, you can further access the start and stop operation interfaces of each refrigeration unit, as well as start and stop the water pump, and monitor the status of the printing press production line.
The alarm query sub-interface is shown in Figure 8. ① During system operation, if a partial system fault occurs (such as a chiller fault, water pump fault, etc.), the audible and visual alarm on the control cabinet will sound. You can view the alarm fault type by pressing the alarm query button on the main interface of the touch screen (Figure 7), as shown in Figure 9, and perform corresponding fault handling. After handling, turn any reset alarm button on the control cabinet and operating cabinet to the right to clear the audible and visual alarm. After the fault is cleared, ensure that the reset alarm buttons on both the control cabinet and operating cabinet are turned to the left. ② When the frequency converter malfunctions, after repair, you must press the reset button on the control cabinet to reset it; otherwise, the fault display will remain. ③ Abnormal system shutdown: In the event of a sudden power outage or other abnormal shutdown, after the system restarts, carefully check the status of each button on the touch screen and the water pump speed value, and confirm that the cold water switch next to the currently operating printing press is turned on.
Figure 7 Touchscreen Manual Interface
Figure 8 Alarm query interface
5. Conclusion
The control system has been running continuously for over a year. It is stable, safe, reliable, energy-efficient, highly automated, easy to operate, and intuitive. It reduces the labor intensity of workers and fully leverages the advantages of PLC, such as fast computing speed, powerful functions, simple programming, easy program modification, fewer wiring connections, low failure rate, easy maintenance, flexible use, and strong anti-interference ability, as well as the user-friendly human-machine interface of the touch screen. It has achieved satisfactory results for users and is worthy of promotion.
