Definition and working principle of air handling units

I. What is an air handling unit ?

An air handling unit (AHU, also called an air conditioning unit) is the core indoor air handling equipment in a heating, ventilation, and air conditioning (HVAC) system, essentially acting as a "centralized processing center" for indoor air in a building. It does not produce cooling or heating itself; it must work in conjunction with chillers, boilers, outdoor heat pumps, and other cold and heat source equipment. Its primary responsibility is to comprehensively process indoor return air and outdoor fresh air through processes such as filtration, temperature control, humidity control, ventilation, and purification, delivering compliant air to the indoor environment. This ensures that the cleanliness, temperature, humidity, and fresh air volume of the indoor air meet usage standards. AHUs are widely used in various building scenarios, including residences, office buildings, shopping malls, factories, and hospitals.

Unlike ordinary air purifiers that can only purify indoor air, air handling units have more comprehensive functions, which can realize integrated treatment such as air circulation, temperature and humidity control, fresh air replacement, and noise reduction. They are the core equipment for large-scale and systematic regulation of indoor air environment.

II. Core Components

All functions of the air handling unit are realized through internal modular functional sections. The core components have a uniform and regular structure, and mainly include the following key modules:

• Fan section : The core of the equipment's power, mostly centrifugal fans, are responsible for drawing indoor return air and outdoor fresh air, driving the air to circulate continuously in the unit and indoor duct system, and determining the system's air volume and air pressure.
• Filtration section : The core of air purification, divided into pre-filter, medium-efficiency filter and high-efficiency filter, which can intercept particulate matter such as hair, dust, PM2.5, pollen, and bacteria in stages. Some high-end models are equipped with activated carbon modules, which can adsorb odors, formaldehyde, TVOC and other harmful gases.
• Heat exchange coil section : The core of temperature and humidity regulation, divided into evaporator coil and heating coil. Cold water flowing through the coil can cool the air and remove moisture (dehumidification), while hot water or electric heating elements can heat the air, achieving precise air temperature control.
• Humidification/Dehumidification Section : Adapted to special environmental needs, it humidifies the air through spraying, steaming, etc. in dry seasons, and condenses water vapor in humid environments with the help of cooling coils to reduce air humidity and maintain indoor humidity balance.
• Fresh air mixing section : Outdoor fresh air and indoor return air can be mixed as needed. While ensuring fresh indoor air, the temperature of the indoor return air is used to reduce heating and cooling energy consumption and improve the energy efficiency of the equipment.
• Auxiliary function section : includes silencing section, air valve control component, automatic control module, etc., which can reduce equipment operating noise, adjust air volume in the air duct, and automatically adapt to the operating status in conjunction with the temperature control system.
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III. Core Working Principle

The air handling unit follows a closed-loop cycle logic of "air intake - treatment - air supply - recirculation", and operates fully automatically. The specific workflow is as follows:

1. Air extraction and mixing : After the equipment is started, the fan starts to run. On the one hand, it extracts indoor polluted air through the return air duct, and on the other hand, it introduces a certain amount of outdoor fresh air according to the system settings. The two types of air are fully mixed in the mixing section to avoid a sudden increase in energy consumption caused by directly introducing a large amount of fresh air.

2. Multi-stage purification and filtration : The mixed air first enters the filtration section, passing through the pre-filter and medium-efficiency filter to intercept large particles and dust, and then through the high-efficiency filter to refine and purify the air, adsorbing fine particles and harmful gases, thoroughly removing pollutants from the air and improving air cleanliness.

3. Precise temperature and humidity control : Purified air flows through the heat exchange coil, completing heat exchange according to the set indoor temperature. In summer, cold water is introduced into the coil, cooling the air as it passes through, while water vapor in the air condenses and is released, achieving cooling and dehumidification. In winter, hot water is introduced into the coil or the electric heating module is activated to raise the air temperature. In dry weather, the humidification module can supplement the air humidity, ensuring that the air temperature and humidity reach a comfortable standard.

4. Stable air supply and delivery : Clean, temperature- and humidity-controlled air that has been treated to meet standards is supplied with stable air pressure by a fan and evenly delivered to each room through a whole-house duct system to complete the replacement and renewal of indoor air.

5. Recirculation and Renewal : After being consumed by human activities and the environment, indoor air is returned to the air handling unit through the return air duct , repeating the above process to form a continuous closed loop circulation. At the same time, fresh air is constantly replenished to ensure that the indoor air is always fresh and the environment is stable and comfortable.

IV. Core Work Characteristics

1. Passive heat exchange and active air supply: It does not generate heat or cold itself, but only transfers energy through heat exchange and relies on a fan to achieve air circulation. It operates stably and has controllable energy consumption.

2. Integrated and multifunctional: It integrates fresh air, purification, temperature control, humidity control and noise reduction, which can optimize indoor air quality in all aspects;

3. Energy-saving and efficient: By circulating fresh air and return air in a balanced ratio, it reduces the loss of cooling and heating energy, making it more energy-efficient than independent air conditioners and fresh air equipment.