Constant flow dampers for circular duct, which facilitates balancing of ventilation systems. Damper suitable for rectangular duct mounting. Those dampers maintains a set flow constant at varying pressures, caused by connection and disconnection of system parts, clogging of filters and ducts, wind effects, window opening etc. SKC is an automatic damper independent of external energy sources. The damper is operated by an opposed opening force from a spring on the blade.Housing and damper blade are made of galvanized steel and tightness joint from rubber.