Abstract: Air-cooled screw chillers are one type of cooling source in air conditioning systems. Their stability directly affects the cooling effect of the air conditioning system. This paper describes a control system for air-cooled screw chillers based on Hollysys LM series PLCs, which enables seamless integration of the chiller and PLC, resulting in fast dynamic response.
Keywords : Hollysys LM series PLC; air-cooled screw chiller control system
1 Introduction
A central air conditioning system consists of a cold/heat source system and an air conditioning system. The refrigeration system provides the necessary cooling capacity to the air conditioning system to offset the cooling load of the indoor environment. The refrigeration system is a crucial part of the central air conditioning system; its type, operating mode, and structural form directly affect the economy, efficiency, and rationality of the central air conditioning system during operation. Screw chillers are named for their screw compressors. The working principle of a screw chiller is as follows: the gaseous refrigerant from the evaporator is adiabatically compressed by the compressor, becoming a high-temperature, high-pressure state; the compressed gaseous refrigerant is then cooled and condensed at equal pressure in the condenser; after condensation, it becomes a liquid refrigerant, which expands to low pressure through a throttling valve, becoming a gas-liquid mixture; the low-temperature, low-pressure liquid refrigerant absorbs heat from the cooled substance in the evaporator, becoming a gaseous refrigerant again; the gaseous refrigerant then re-enters the compressor through pipes, starting a new cycle. These are the four processes of the refrigeration cycle, and also the main working principle of the screw chiller. Screw chillers can be divided into water-cooled screw chillers and air-cooled screw chillers according to their heat dissipation methods.
The air-cooled screw chiller control system based on Hollysys LM series PLCs has been widely used due to its high reliability, fast processing speed, and convenient logic modification. The control system handles the on/off control of all electric valves in the refrigeration process, including data acquisition, processing, calculation, and real-time output of switch signals, transmitting the relevant data to the touchscreen. The air-cooled screw chiller control system works in conjunction with the air conditioning system to achieve temperature regulation.
2. Design of Control System for Air-Cooled Screw Chiller Unit
The control system of an air-cooled screw chiller unit generally consists of a control cabinet with a PLC as its core, actuators, a touch screen, and primary and secondary peripheral components, as shown in Figure 1. The control cabinet of the air-cooled screw chiller unit is equipped with a PLC, phase sequence protector, trip relay, thermal relay, etc. Each input and output channel has a corresponding indicator light, allowing for monitoring of the operation of a specific channel, and the system also has online testing capabilities.
Due to the extremely high reliability requirements of the system—namely, ensuring accurate shutdown of the compressor turbine unit in any possible emergency—the air-cooled screw chiller unit control system utilizes Hollysys' high-performance LM series PLCs. The LM3109 is available in 24DI and 16DO versions, with each PLC including a CPU module and an AI module. The CPU module handles all input signal logic processing and output signal logic control, achieving temperature control both in hardware and software. A schematic diagram of the hardware structure of the air-cooled screw chiller unit control system is shown in Figure 2, and the PLC hardware configuration is shown in Table 1.
Figure 1. Composition of the control system of an air-cooled screw chiller unit
Figure 2. Schematic diagram of the hardware structure of the control system of the air-cooled screw chiller unit.
Table 1. PLC hardware configuration for the air-cooled screw chiller control system.
Serial Number | signal type | Input/output signal description |
1 | DI | Phase sequence protection |
2 | DI | Chilled water flow switch |
3 | DI | Chilled water pump overload |
4 | DI | #1 Hydraulic Protection |
5 | DI | #1 Internal Overheat Protection |
6 | DI | #1 High Voltage Protection |
7 | DI | #1 Low voltage protection |
8 | DI | #1 Internal Overload Protection |
9 | DI | #1 Condenser fan malfunction |
10 | DI | #2 Hydraulic Protection |
11 | DI | #2 Internal Overheat Protection |
12 | DI | #2 High Voltage Protection |
13 | DI | #2 Low voltage protection |
14 | DI | #2 Internal Overload Protection |
15 | DI | #2 condenser fan malfunction |
16 | DI | Firefighting coordination |
17 | DI | Fault Reset |
18 | DO | #1 Main Contactor |
19 | DO | #1 machine △ contactor |
20 | DO | #1 machine Y main contactor |
twenty one | DO | 25% capacity regulating valve for Unit #1 |
twenty two | DO | 50% capacity regulating valve for Unit #1 |
twenty three | DO | 75% capacity regulating valve for Unit #1 |
twenty four | DO | #2 Main Contactor |
25 | DO | #2 machine △ contactor |
26 | DO | #2 Machine Y Main Contactor |
27 | DO | 25% capacity regulating valve for Unit #2 |
28 | DO | 50% capacity regulating valve for Unit #2 |
29 | DO | #2 Unit 75% Capacity Control Valve |
30 | DO | Chilled water pump operation |
31 | DO | #1 condenser fan in operation |
32 | DO | #2 condenser fan in operation |
34 | AI | Inlet water temperature |
35 | AI | water outlet temperature |
3. Features of the control system for air-cooled screw chillers
The control system for air-cooled screw chillers based on Hollysys LM series PLCs has the following characteristics.
(1) Accuracy and reliability
Accuracy and reliability are crucial for the control system of air-cooled screw chillers. Hollysys LM series PLCs, equipped with high-performance industrial-grade processors, boast nanosecond-level processing speeds and large-capacity memory, enabling accurate and rapid execution of the screw chiller's protection logic program. This significantly enhances the system's high reliability through hardware improvements. It ensures that the entire unit can reliably and correctly implement the screw chiller's tripping function under any circumstances. Issues such as compressor "failure to trip" or "false tripping," which were previously a major concern in such systems, are now adequately and appropriately addressed during the program design, technically guaranteeing the system's high reliability.
(2) The system has strong functions
Because Hollysys LM series PLCs are characterized by their small size, high integration, fast processing speed, and large logic control capacity, the entire program, in addition to conventional functions such as status alarms and shutdown, can also perform online testing. The purpose of the online testing function is to check the status of peripheral components such as pressure switches and solenoid valves during normal unit operation, thereby improving the overall safety factor of the system.
The addition of a first-outage cause memory function for the air-cooled compressor helps operators identify the primary cause of a shutdown immediately, avoiding many previous shortcomings. For example, previous systems displayed multiple alarm indicators on the panel, making it difficult to pinpoint the primary cause; or, with rapidly changing conditions and indicator lights going out, the primary cause remained elusive. This function also provides verification for the incident recording system (SOE). This function can be implemented through programming, requiring no additional hardware and relying entirely on programming skills. When no shutdown conditions occur, this function does not need to execute, thus not consuming system monitoring time and maximizing the system's monitoring and shutdown capabilities.
References
1. Xu Desheng. Refrigeration and Air Conditioning [M]. Shanghai Jiaotong University Press.
2 Xing Ziwen. Screw Compressors [M]. Machinery Industry Press.
3 Fang Chengyuan. Factory Electrical Control Technology [M]. Machinery Industry Press.
4. Gu Zhansong. Programmable Controller Principles and Applications [M]. National Defense Industry Press.
About the author: Chen Shen, Technical Support Group, Marketing Department (Nanjing Office), Beijing Hollysys Automation Drive Technology Co., Ltd.
