Usually located in a dedicated computer room or ceiling environment. Air handling unit (AHU) is a centralized air handling system, which originated from the forced hot air heating and ventilation system with centralized equipment, and distributed heated air through air ducts. The basic centralized system is an all-air single-zone system, which generally includes fans, heaters, coolers and filters. AHU here refers to the primary return air system, and its basic working process is: after the fresh air sent from the outdoor is mixed with some indoor return air, harmful substances such as dust, smoke, black smoke and organic particles in the air are filtered out by the filter.