In air conditioning systems , which need flow volume of parts or all of the ducts for ventilation system are constant at working ,to guarantee the systems operate well . As that reason we developed DF series constant flow damper to control the flow volume which circulate in the duct precisely for projects. The device can be widely used at hospital, lab, purify workshop and so on. 3,Characteristic and classify (1)Foundation type DF constant flow damper are mechanical system powered controllers for constant volume systems which is used in air duct system need constant volume and also used as controllers to control the supply and return air constant for the systems. Operating temperature is 10 to 80 , differential pressure drop range from 50 Pa to 1000 Pa.. No need external power supply , air flow is no irrespective with systems change of stated the pressure and other characteristics. It can be used in supply air and extract air system. (2)According various demands there are 3 kinds damper frame to choose which are galvanized steel , steel with surface powder-coated and stainless steel. (3)There are round and rectangle types according appearance