1. Introduction Juhua Group's ammonia synthesis plant mainly produces ammonia carbonate, urea, methanol, and liquid ammonia. Each product's process requires a large amount of refrigeration capacity. The ammonia synthesis plant utilizes gaseous and liquid ammonia for energy conversion, supplying this refrigeration capacity through refrigeration ice machines. Considering the overall energy-saving renovation plan and refrigeration capacity requirements of the ammonia synthesis plant, and combining the requirements for increased production, energy saving, and upgrades in the refrigeration sector, technical upgrades were undertaken to address the problems of high losses, low output, low operating efficiency, and overly complex electrical equipment associated with the existing reciprocating compressors. The original BTD-ICC type reciprocating ice machine used relay control, which resulted in complex and cumbersome control circuit wiring, a high failure rate, numerous mechanical transmission components, cumbersome operation, frequent malfunctions, and inconvenient maintenance. Therefore, the ammonia synthesis plant decided to replace the original reciprocating ice machine with a screw ice machine with a refrigeration capacity of 1 million kcal/hour and a power consumption of 450 kW. PLC control is adopted in the electrical control circuit. PLCs offer advantages such as high reliability, strong anti-interference capability, variable control programs with good flexibility, simple programming, ease of use, comprehensive functions, convenient expansion, flexible combination, small size, and light weight. This design has achieved the expected results in practice. 2. Process Flow Introduction The working process of the refrigeration ice machine is based on the physical conversion: (pressure × volume) / temperature = constant (i.e., P1V1/T1 = P2V2/T2), which converts gaseous ammonia into liquid ammonia. Therefore, the pressure and temperature of the gaseous ammonia are important parameters for process control. In production, gaseous ammonia with a pressure below 2 kg/m³ enters the ammonia separator at the inlet through the system's main gaseous ammonia pipe, separating liquid ammonia droplets. The gaseous ammonia, after filtering out the liquid ammonia droplets, flows through the system pipe into the compressor unit's suction filter. Small impurities entrained in the gaseous ammonia are then filtered out through the filter screen (the suction filter is equipped with a thermometer to indicate the suction temperature and a suction pressure gauge connected to a shut-off valve to indicate the suction pressure). Clean ammonia gas enters the screw compressor for compression and pressurization (i.e., the gas pressure increases from 0.3 MPa to 1.57 MPa), and the compressed gas is discharged through the exhaust port. During compressor operation, an oil pump injects approximately 0.5% to 1% of the volumetric flow rate of lubricating oil into the compressor. This lubricating oil serves to cool, seal, and lubricate. The injection pressure of the oil must be greater than the pressure of the ammonia gas inside the compressor to ensure smooth oil injection; the oil-gas pressure difference detection point here is an important parameter. This lubricating oil is discharged with the exhaust gas into an oil separator for oil separation. The oil separator is equipped with a safety valve, which releases excess pressure within the separator. After this, the system is divided into a gas path process and an oil path process. From the perspective of the gas flow process: Ammonia gas separated by oil enters the condenser at a temperature of 60-70℃ and a pressure of 1.35-1.40MPa, where it condenses into liquid ammonia. The liquid ammonia then enters the liquid ammonia collector. From the perspective of the oil flow process: The oil separated in the oil separator passes through the oil cooler. The cooled oil then passes through the check valve (which allows only unidirectional flow) and enters the coarse oil filter. After filtering out large particles such as iron filings, the oil flows to the inlet of the fuel injection pump. The pump pressurizes the oil, and the oil then passes through the fine oil filter for further filtration before flowing back to the fuel injection manifold and into the compressor. The oil pump is connected to an auxiliary valve to regulate the pump pressure. The fine oil filter is connected to a pressure gauge (normally, the pressure value should be low, ≤0.07MPa; a high pressure value indicates that the filter screen is clogged and needs to be cleaned). The basic process flow diagram is shown in Figure 1: [ALIGN=CENTER] Figure 1 Basic Process Flow Diagram[/ALIGN] Because the front and rear bearings of the compressor host generate heat during long-term operation, they need to be lubricated with oil for cooling. To address this, two additional thin oil pumps are installed to draw oil from the oil tank, pass it through an oil filter and oil cooler, and then spray it onto the bearing bush. When the pressure is sufficient, one pump supplies oil while the other serves as a backup; when the pump pressure is insufficient, both pumps are activated simultaneously to supply oil. The required oil pressure injected into the bearing bush is generally 0.15 MPa. 3. PLC Control A Programmable Logic Controller (PLC) is an industrial control product based on a microprocessor, integrating computer technology and automatic control technology. It was developed based on hard-wired logic control technology and computer technology. A PLC is typically considered a device composed of equivalent relays, timers, counters, and other components. Unlike relay control, which requires connecting many actual hardware relays, a PLC consists of "soft relays," avoiding maintenance due to component wear and a series of complex wiring tasks. (1) Main features: high reliability and strong anti-interference ability; variable control program with good flexibility; simple programming and easy to use; complete functions; convenient expansion and flexible combination; reduced workload of control system design and construction; small size and light weight, which is a product unique to "mechatronics". From the perspective of electrical instrumentation, the centralized control interface can be used to flexibly utilize PLC control, fieldbus control system (FCS) or distributed system (DCS) to realize the display and control of process parameters. In terms of the scale of this renovation, investment price and process control points, we use programmable controller to realize the display of electrical indicators and tripping and alarm. (2) PLC selection: PLC selection is mainly based on the required functions and capacity, and considers the convenience of maintenance, the universality of spare parts, whether it is easy to expand, and whether there are special functional requirements. Through comparison, we selected the Mitsubishi FXON series of micro programmable controllers. The FXON series is a micro programmable controller that integrates many functions into an ultra-small housing. Compared with the F1/F2 series, the FXON series has an installation area of ​​only 41% and a volume of only 37% of the F1/F2 series, and is equipped with convenient functions such as analog potentiometers and RUN/STOP switches in the controller. Through the connection of expansion units, expansion modules and basic connections, the number of input and output points can be freely selected. The FX ON series inherits the fixed combination and flexibility of the original series. (3) Design of PLC control system According to the conditions and control requirements proposed by the process, the specific design idea is as follows: The screw ice machine has 1 circulating oil pump. When running, the oil pressure is adjusted by the auxiliary line valve. There are 2 thin oil pumps. When the oil pressure is normal, 1 is running and 1 is on standby and can be automatically switched. When the oil pressure is low, the 2 thin oil pumps start at the same time; when the oil pressure difference is too low, the machine trips after a delay of 6 seconds. In addition, high exhaust temperature, high oil temperature, high north bearing temperature, high south bearing temperature, high exhaust pressure, and high oil filter pressure difference will all cause the machine to trip. However, in the thin oil station, the oil pressure is low, the oil-gas pressure difference is low, the DC power supply is lost, and the circulating oil pump is overloaded. When the No. 1 and No. 2 thin oil pumps are overloaded, they do not trip, but only issue an alarm signal. To achieve the above functions, dozens of intermediate relays are needed, and the wiring is very complex, maintenance is extremely difficult, and reliability is poor. However, with PLC, the wiring is much simpler, and reliability is greatly improved. Its ladder diagram is shown in Figure 2. [ALIGN=CENTER] Figure 2 PLC Control Ladder Diagram[/ALIGN] 4 Conclusion Replacing piston ice machines with screw ice machines can save 11.5 tons/year of refrigeration lubricating oil per unit. Replacing traditional fixed-program relay-contactor systems with small PLC industrial computers...