An AHU is a metal box with fans, cooling and heating elements, valves, filters, etc. It is connected with air ducts for the air distribution, conveying it into the whole building and then extracting it back to the AHU itself. Filters are essential in the AHU since they remove contaminants from air, mainly solid particles like dust, pollen, and bacteria. Filters can be made with different materials: the most used are fibres such as cotton.
In an air conditioning system, also fire dampers are present: they are installed in the air ducts through fire walls and have the function of preventing the spreading of flames, smoke and hot gases through ducts.