Abstract This paper introduces a screw compressor control system consisting of a PLC and monitoring instruments. This system is an upgrade and modification of the screw compressor control system based on practical application. The results show that the system operates reasonably, provides convenient and intuitive real-time data recording and display, and operates more safely and stably.
Keywords : Hollysys LM series PLC; screw compressor control system
The Application of Hollysys LM PLC in the Control System of Screw Rod Compressor
Fan Zhansong
(Beijing Hollysys Automation & Drive Ltd.,Co, 100176)
Abstract : This paper describes the control system of screw rod compressor which composed by the PLC and instrumentation. Based on the real situation of equipment, the control system of screw rod compressor has been reformed. By using that, changing the system operation become more fast and reasonable. Real-time date is recorded more convenient, and the system becomes more safe and stable.
Key Words : Hollysys LM PLC, Control System of Screw Rod Compressor
1 Introduction
Screw compressors are widely used in industries such as mining, chemical, power, metallurgy, construction, machinery, and refrigeration. They offer advantages such as high reliability and wide applicability, and have gradually replaced other types of compressors. Statistics show that screw compressors account for over 80% of all positive displacement compressor sales. In the future, the market share of screw compressors will continue to expand, especially oil-free screw compressors, which will experience even faster growth.
2. Basic Working Principle of Screw Compressors
Screw compressors are a type of positive displacement compressor. Inside the cylinder of a screw compressor is a pair of meshing helical rotors, each with several concave teeth, rotating in opposite directions. The clearance between the rotors and the casing is only 5-10 micrometers. The main rotor, also called the male rotor or convex rotor, is driven by an engine or electric motor, with the majority being driven by an electric motor. The other rotor, also called the female rotor or concave rotor, is driven by the main rotor through an oil film formed by oil injection, or by synchronous gears at the ends of the main rotor and the concave rotor.
When the grooves of the screw rotor pass through the intake port, they fill with gas. As the rotor rotates, the grooves are sealed by the casing wall, forming a compression chamber. After the grooves are sealed, lubricating oil is sprayed into the compression chamber, providing sealing, cooling, and lubrication. As the rotor rotates and compresses the oil-gas mixture (i.e., the mixture of lubricant and air), the compressor chamber volume decreases, compressing the oil-gas mixture towards the exhaust port. When the compression chamber passes through the exhaust port, the oil-gas mixture is discharged from the compressor, completing one intake, compression, and exhaust cycle. The working cycle of a screw compressor can be divided into three processes: intake, compression, and exhaust. As the rotor rotates, each pair of meshing teeth successively completes the same working cycle.
3 Hardware Design of Screw Compressor Control System
Early screw compressor control systems had instruments installed in field instrument boxes, lacking remote control and relying on manual operation on-site. The compressor's start-up and shutdown were controlled via field pressure switches, differential pressure switches, and temperature switches in conjunction with the electrical cabinet. Temperature, pressure, and differential pressure switches, along with electrical contactors and time relays, worked together to provide automatic protection interlocks, ensuring stable and safe compressor operation. This control method had significant drawbacks: operators lacked detailed information about the compressor's operating status, and the instruments exhibited poor stability and low accuracy.
To address these shortcomings, the hardware of this control system consists of a PLC, monitoring instruments, input devices, output devices, and a touchscreen. Remote instruments are used for field data acquisition, replacing the original pressure switches, differential pressure switches, temperature switches, and field thermometers with pressure transmitters and RTDs to achieve remote data transmission and improve measurement accuracy. The control section uses a PLC and HMI, leveraging the PLC's powerful logic control and the host computer's recording and analysis functions. The user-friendly and intuitive interface enables centralized data display and operation, enhances the unit's safety interlock protection functions, and improves operational control performance. This ensures both centralized safe operation and allows operators to have detailed information about the unit's operating status. A schematic diagram of the screw compressor control system hardware structure is shown in Figure 1.
Figure 1. Schematic diagram of the hardware structure of the screw compressor control system
The programmable controller uses Hollysys' LM 3109. This module's I/O includes 24 channels of 24VDC input and 16 channels of relay output, expandable up to 7 expansion modules. It has 120KB of user program storage space and 6KB of power-off retention space; password protection and a real-time clock; unlimited timer/counter outputs; 340 basic instructions and 47 extended instructions; communication interfaces include one RS232 and one RS485; supporting proprietary protocols (RS232 only), MODBUD RTU protocol, and free protocols. A touchscreen is used for displaying operating status, data, and alarms, as well as parameter setting and remote control.
4. Software Design of Screw Compressor Control System
The load regulation of the screw compressor is achieved by six solenoid valves. Each solenoid valve can be opened at 25%, 50%, 75%, and 100%. When the compressor is loaded, the valve with the shortest operating time opens first, followed by valves at 25%, 50%, 75%, and 100% capacity. When the compressor is unloaded, the valve with the longest operating time unloads first, followed by valves at 100%, 75%, 50%, and 25% capacity. It is important to note that at the 25% capacity level, the compressor is shut down first, and the 25% solenoid valve closes after a certain delay. The main program flowchart of the screw compressor control system is shown in Figure 4, and the loading and unloading flowcharts are shown in Figure 5.
Figure 4. Flowchart of the main program of the screw compressor control system
Figure 5. Flowchart of loading and unloading of screw compressor
5. Conclusion
This article introduces a screw compressor control system based on Hollysys LM series PLCs. Using this control system improves the compressor's operating condition. In particular, the use of a touchscreen allows for modification of the compressor's energy level and operating status, making the system more stable and safer.
References
[1] Hollysys LM Series PLC Hardware Manual
[2] Dong Tianlu, Screw Refrigeration Units and Their Applications; Machinery Industry Press
About the author: Fan Zhansong, Technical Support Engineer, Marketing Department, Technical Support Group, Beijing Hollysys Automation Drive Technology Co., Ltd.
