Against this backdrop, automated control technology has been listed as one of the basic requirements for building design. As a typical example of large public buildings in transportation infrastructure, the HVAC system in airport terminal buildings is enormous and complex, with numerous equipment operation control points. The application of automated control systems has greatly improved the rationality of indoor environmental parameters, enhanced the safety of operation between systems, and increased the economic efficiency of each system, which is of great significance for rationally reducing the energy consumption of building equipment.
1. Project Overview
This project is located in Yichang City, Hubei Province. It involves the construction of a new two-and-a-half-story, front-row terminal building, which is a sub-project of the terminal building renovation and expansion project. The building area is 41,674.67 m2. It has two underground floors and two floors above ground (partial mezzanine), with a total building height of 23.60 m. Its functions include domestic check-in, security check, waiting area, arrival area, baggage claim, transportation transfer, and equipment rooms.
2. Principles of Automatic Control Systems
The primary task of the HVAC automatic control system in this project is to ensure that the indoor air design parameters of each functional area of the terminal building meet the design requirements, guarantee a relatively comfortable thermal and humidity environment, and ensure that all equipment in the heating, ventilation, and air conditioning systems operates efficiently and reliably, thereby reducing the labor intensity of maintenance and management. Secondly, through interlocking control between systems, it provides optimized operation and energy consumption control schemes for the heating, ventilation, and air conditioning systems, provides equipment operation information, conducts energy-saving management, and serves as a basis for system equipment management decisions.
3. Scope of Automatic Control System
The HVAC automation system in this project centrally manages the operation of various systems within the building, including the cooling source system, distribution system, air conditioning system, and ventilation system. The system's monitoring and control functions include parameter detection, parameter and equipment status display, automatic adjustment and control, automatic switching of operating conditions, equipment interlocking and automatic protection, energy metering, and central monitoring and management.
4. Monitoring of the cold source system
This project utilizes electric refrigeration as its cooling source, selecting a combination of water-cooled centrifugal variable frequency chillers and water-cooled screw variable frequency chillers. An operational strategy is developed based on the terminal building's load characteristics, the refrigeration station's installed capacity, and the number of chillers. Actual cooling load is measured, and the chiller capacity is adjusted accordingly based on the operational strategy to achieve variable frequency control; the number of chillers starting and stopping is also adjusted to achieve group control. The cooling source components are integrated into a monitoring system via a gateway to control the chillers and monitor and control temperature, pressure, and water flow status.
5. Monitoring of the transmission and distribution system
The distribution system plays a crucial role in the process of transporting cold sources from the station to the terminal. Maintaining energy balance between the load side and the chiller side is the primary task of controlling the air conditioning distribution system.
5.1 Start-up and shutdown sequence
The chiller unit, chilled water pump, and cooling water system are electrically interlocked and automatically start and stop. The start-up sequence is: electric water valve - cooling water pump - chilled water pump - cooling tower fan - chiller unit. When the system is shut down, the sequence is reversed.
5.2 Chilled Water System
The chilled water system in this project adopts a variable flow rate approach on both the main pump side and the load side. The speed of the chilled water circulation pump is adjusted according to the differential pressure signal at the end of the most unfavorable loop of the air conditioning water system, and the flow rate is adjusted according to the actual needs of the user side. An electric differential pressure bypass valve is installed between the water loop manifolds, and the system is designed according to the minimum allowable flow rate of the evaporator of the largest chiller unit.
5.3 Cooling Water System
Adjust the operating conditions of the cooling water according to the outdoor climate conditions and cooling load to keep the cooling water temperature within the design range.
6. Monitoring of air conditioning terminal systems
6.1 All-air system
For large, open spaces within the terminal building, such as check-in areas, security checkpoints, and waiting areas, temperature and humidity sensors are installed in the air-conditioned areas. The average temperature and humidity at each measurement point, or the temperature and humidity data at the most unfavorable point, are used as control parameters to determine the air supply status parameters of the all-air system, ensuring that the temperature and humidity in the air-conditioned areas remain within a comfortable range throughout the year. Based on measured data from the air-conditioned areas and combined with PID calculation results, the two-way electric regulating valve installed on the water side of the air-water heat exchanger is controlled to ensure that the cooling capacity of the all-air system is equivalent to the required cooling load, reducing energy waste. The all-air system's electrically controlled drum nozzles, spherical nozzles, and swirl nozzles automatically switch between winter and summer states according to the operating conditions.
When the combined air conditioning unit is in automatic mode, the direct digital controller installed in the air conditioning room follows a pre-programmed software program to meet the automatic control and operation sequence of the all-air system. It can control the start and stop of the system according to a predetermined schedule, or control the changes in start and stop times according to the actual flight dynamic information of the terminal.
The system centrally manages several modular air conditioning units within the terminal building, displaying the start/stop status, air supply parameters, and air/water valve status of each unit, and automatically calculating the operating time of each unit. When a unit stops due to excessive filter pressure differential, motor overload, or other reasons, a fault alarm is triggered by the central control unit to provide protection.
6.2 Fresh Air System
The monitoring functions of the terminal's fresh air handling units include the manual/automatic switching status of the fans, filter clogging alarms, and fresh air temperature. Fresh air is controlled based on the supply air temperature. The supply air temperature is compared with the set value to the DDC (Distributed Control Unit), and after PID calculation, the control valve on the water side of the air-water heat exchanger is activated to ensure that the fresh air outlet temperature deviation remains within the design tolerance range, thus reducing energy waste.
The system centrally manages several fresh air handling units within the terminal building, displaying the start/stop status, supply air temperature, and air/water valve status of each unit, and provides fault alarm protection functions.
6.3 Fan Coil System
The fan coil unit adopts a two-pipe system with dual-position control, relying on an indoor temperature controller and an electrically operated two-way (on/off adjustable) water valve. The control method is set to both local and centralized modes. Based on the measured data signal from the indoor temperature controller, the indoor temperature is regulated by adjusting the high, medium, and low speeds of the fan and regulating the on/off state of the water valve.
6.4 Multi-condition operation monitoring
The air conditioning system operates using energy-saving control based on year-round operating conditions. Different air handling schemes are employed to meet indoor design requirements based on indoor parameter requirements and changes in outdoor air parameters. Changes in outdoor air conditions alter the heat transfer of the building envelope and the air supply status, both of which affect the air quality in the air-conditioned rooms. The automatic control system addresses regulation within operating ranges through analog control loops and transitions between operating conditions through logic control loops, thus satisfying the multi-condition operation requirements of the all-air system throughout the year.
The air conditioning system in the terminal building of this project is mainly comfort air conditioning, with a relatively large allowable fluctuation range for indoor air parameters. The automatic control system monitors the humidity of fresh air and supply and return air in real time, and readjusts the setpoint of return air temperature based on the fresh air temperature. During the air conditioning season, it operates with the minimum fresh air ratio to facilitate energy saving; during the transition season, it operates with 100% fresh air, so that the indoor state point changes with the outdoor air state, which can maximize energy saving and improve indoor air quality and comfort.
7. Monitoring of the ventilation system
The ventilation system normally functions to remove indoor pollutants and improve the indoor air quality. When the terminal's ventilation system is in building automation mode, the start and stop of the ventilation fans can be controlled according to a predetermined schedule, or the start and stop times can be adjusted based on actual flight information. It also monitors the operating status of the ventilation fans and issues fault alarms. In the event of a fire, the automatic fire alarm system will forcibly control the operation of the ventilation fans.
8 Conclusion
This paper presents an application design for the automated control of HVAC systems based on actual engineering cases. It provides strong support for achieving the indoor air design parameters of the terminal building and ensuring the safe and reliable operation of the systems. While improving the rationality of HVAC system operation, it saves labor costs, minimizes energy consumption, reduces operating costs, and generates certain economic benefits.
