With my country's economic development, the number of intelligent buildings in cities has increased significantly, and most of these buildings use central air conditioning to provide a comfortable office or living environment. However, the high energy consumption of central air conditioning systems is also restricting their development. Therefore, energy saving and high efficiency are important issues for central air conditioning systems. The following section uses the design of a central air conditioning system in an office building as an example to introduce the system's design in terms of energy saving and high efficiency.
1 System Composition
1.1 Composition of a Central Air Conditioning System
The central air conditioning system mainly consists of a cold and heat source, a chilled water system, a cooling water system, a cooling tower, and air conditioning terminals. Unlike typical central air conditioning systems, this system uses water-cooled chillers for its cold source and municipal steam for heat exchange via heat exchangers to increase the temperature of the circulating water. Two 130kW compressor chillers provide the cold source for cooling; two heat exchangers increase the circulating water temperature for heating. This configuration achieves good energy efficiency. The air conditioning terminals use both fresh air handling units and fan coil units. The fresh air handling units primarily ensure the quality of fresh indoor air and control the temperature and humidity of the supply air; the fan coil units provide cooling and heating to the room through heat exchange.
1.2 Composition of the Control System
Currently, the main control methods for central air conditioning systems include relay control, programmable logic controller (PLC) control, and direct digital controller (DDC) control. A more advanced method utilizes Building Automation Systems (BAS) to monitor and integrate building equipment such as central air conditioning systems. Relay control systems have been gradually phased out due to their high failure rate, system complexity, and high power consumption. Traditional central air conditioning control methods use DDC control, connecting various temperature and humidity detection points and control points to multiple DDCs for multi-point monitoring. However, due to the large number of floors in modern intelligent buildings, with multiple central air conditioning units located on different floors and temperature and humidity detection points distributed across various rooms, DDC control suffers from drawbacks such as complex wiring, inconvenient construction, resource waste, and low system real-time performance and reliability. PLC control has a lower integration level than DDC, can be freely programmed, is inexpensive, operates reliably, has strong anti-interference capabilities, and is convenient to use and maintain. These advantages have led to its widespread application.
The central air conditioning system's field equipment includes a Siemens S7-200CPU226 PLC as the main controller; two EM223 digital input/output modules (32DI/32DO and 8DI/8DO respectively); one EM2318AI analog input module; one EM2324AQ analog output module; one EM321RTD RTD input module providing two analog inputs; and an MP277 touchscreen as the host computer. The host computer is responsible for monitoring and controlling the entire system's operation, recording various parameters in real time, and saving them to a real-time database. The system structure is shown in Figure 1.
Figure 1. Structure diagram of the central air conditioning system
2. System Applications and Functions
2.1 Application and Function of Chiller Units
The chiller unit provides the cooling source for the entire system. After passing through the chiller unit, the chilled water circulation system lowers the temperature of the circulating water. Then, it is supplied to the air conditioning terminals via chilled water pumps and collectors. Since the development of chiller units has matured, their internal working principle will not be discussed in this article. To meet different cooling capacity requirements, based on the relatively mature chiller unit technology, precise group control is implemented for the number of chiller units and their cooling capacity to achieve constant room temperature and balanced power consumption. Compared to a single-chiller central air conditioning system, group control offers more cooling capacity redundancy and more energy-efficient operating strategies, meeting the varying cooling needs of the building complex at different times.
2.2 Selection characteristics and functions of the control system
The control system consists of an S7-200 series PLC and HMI devices. In terms of selection, Siemens PLCs are chosen for their high stability, and since large redundancy is not required for central air conditioning group control, a Siemens S7-200 series PLC is selected for the control section. The Siemens EM231 module collects on-site temperature and flow rate data to calculate whether the current system cooling capacity is sufficient. The cooling capacity is adjusted by regulating the speed of the chilled water pumps. Since the chiller units of the central air conditioning system can automatically adjust their own workload based on the outlet and return water temperatures, this type of control is handled by the chiller units themselves and is not interfered with by the group control PLC.
When the cooling demand of the building complex is high, the group control PLC calculates the required cooling capacity based on differential pressure signals and flow rates, checking if it exceeds 80% of the rated cooling capacity of the current chiller unit . If it does, it compares the chilled water outlet and return temperatures to see if they exceed permissible set values (e.g., chilled water outlet temperature greater than 7℃, return temperature greater than 12℃). If they do, and the first group of chillers is already operating at 80% of its rated capacity, an additional chiller unit is added to the system. During system operation, if the calculated cooling capacity is less than 80% of the chiller unit's rated cooling capacity (minus one), and both the outlet and return water temperatures are below their set values, a chiller unit is removed from the system.
In summary, the main function of the S7-200 PLC in the control system is to calculate the system's cooling capacity to determine whether to add or remove chillers. Using this type of control system, combined with multiple low-power chiller units, forms a group control system. This makes it easier to meet the cooling capacity requirements of the building complex and ensures that the required cooling capacity balances with the consumed cooling capacity. This improves the comfort of the central air conditioning system and prevents energy waste.
Since the system control components described in this article are all located on the ground floor (-1 floor) of the building complex, a touchscreen (MP277-10 inch) was chosen as the primary monitoring and control device, as it is more suitable for the current environment. In addition to basic control and monitoring functions, this touchscreen can also meet the needs of system reports and data statistics, and it conforms to industry standards.
2.3 Chilled Water Circulation System
The chilled water circulation system functions as a transfer mechanism for cooling or heating within the entire system. It uses three 37kW variable frequency motors as the power source. During the cooling process, the chilled water pumps deliver chilled water from the chiller units to the building terminals for heat exchange. The same process occurs during heating.
The difference lies in the fact that the three chilled water pumps in this system are driven by a single MM440 frequency converter. This borrows the method used in constant pressure water supply systems where one frequency converter drives multiple motors. Only one pump is always in frequency conversion speed control mode, effectively reducing hardware costs. Furthermore, the switching between power frequency and frequency conversion for the pumps is handled by a group control PLC.
2.4 Cooling water circulation system
During the operation of the chiller unit, the chilled water return temperature has already participated in heat exchange at the air conditioning terminal. During this heat exchange, the water temperature rises. The chiller unit then cools the chilled water through heat exchange. The heat generated during this cooling process is released by the cooling water circulation system. The cooling water circulation system carries the heat generated during the chiller unit's heat exchange into the cooling tower, where it is released. In this example, the cooling water circulation system consists of two cooling water circulation pumps and two cooling towers. All motors are individually controlled by the chiller unit's internal program and are not interfered with by the group control system.
2.5 Heat exchanger section
This system uses municipal steam supplied through heat exchangers to heat the water. Heat is then transferred to the building terminals via a chilled water circulation system. Due to ongoing national regulations on steam metering for heating, flow meters and adjustable proportional valves are used at the municipal steam supply end for control. Flow rate is statistically analyzed and adjustable. This municipal steam regulation, combined with frequency converter regulation of the chilled water pumps in the heating circulation system, prevents energy waste. All of the above control functions are handled by a group control PLC.
During the heating process, the group control PLC determines the number of pumps to be engaged and the degree of opening of the municipal proportional valve by using the difference between the return water temperature and the outlet water temperature. The above control is achieved through PID control.
3. Summary of the necessity of system functions
In the implementation of this system, based on the continuous maturation of the technology of chiller units in the current central air conditioning industry, the purpose is to optimize the distribution and energy management of cooling and heating in building complexes. Multiple low-power (relative to the total cooling capacity) chiller units are used as distributed cold sources for the system, and then group control is used to achieve a wider range of adjustable cooling capacity and improved energy utilization efficiency.
