Air handling unit (AHU): The air handling unit (AHU) is a centralized air handling system, which originates from a forced hot air heating and ventilation system with centralized equipment and distribution of heated air through air ducts.  The basic centralized system is an all-air single-zone system, generally including fan, heater, cooler and filter components.  AHU here refers to a primary air return system. Its basic working process is: after fresh air from outside is mixed with a part of indoor air return, harmful substances such as dust, smoke, black smoke and organic particles in the air are filtered through a filter.
Clean air is sent to cooler or heater for cooling or heating through fan to make people feel comfortable and suitable, and then sent to room.  According to the changes of winter and summer seasons, the air conditioning process is different for typical centralized air treatment systems.
Equipment for adjusting indoor air temperature, humidity and cleanliness.  There are air heaters, air coolers, air humidifiers, air filters for purifying air, air mixing boxes for adjusting fresh air and return air, and mufflers for reducing noise of fans to meet the requirements of heat and moisture treatment.  Air handling units are equipped with ventilators.  According to the requirements of annual air conditioning, the unit can be equipped with an automatic regulation system connected with cold and heat sources.
The fresh air unit mainly deals with the state point of outdoor fresh air, while the air handling unit mainly deals with the state of indoor circulating air.  Compared with fan coil plus fresh air system and unit air conditioner, it has the advantages of large air volume, high air quality, energy saving, etc., and is especially suitable for large space and adult flow systems such as shopping malls, exhibition halls, airports, etc.
A good air treatment unit should have the characteristics of less occupied space, multiple functions, low noise, low energy consumption, beautiful appearance, convenient installation and maintenance, etc.  However, due to its many functional segments and complex structure, to achieve a comprehensive consideration, designers and construction units are required to compare materials, manufacturing processes, structural characteristics, selection and calculation in many ways, so as to achieve a more satisfactory effect.