FLAT DUCT CONVERTER: A transformer element that can be placed between the start and end points of an air-conditioning system, for connecting a flat duct and an air-conditioning pipe with a round cross-section. By using air-conditioning ducts with a rectangular cross-section and the corresponding fittings, the system can be more easily covered/hidden. The equivalent " It has a smaller "thickness" size compared to the elements of a "traditional" circular cross-section pipe system, so it can also be placed in narrow places (e.g. above false ceilings, behind floor beams) without reducing (narrowing) the useful cross-section. Restrictions used in the ventilation system can result in higher pressure loss and noise from higher air flow.