[EN 12792:2003, 185]:device having the purpose of maintaining a flow rate across it at a required constant value when the pressure differential between high and low pressure sides vary within the limits for which the equipment is designed. There are different types of flow rate control devices such as:mechanical constant flow rate controllerself actuating and deriving its energy from the air stream to maintain the constant flow rate function.mechanical variable flow rate controllerself actuating and deriving its energy from the air stream to maintain the constant flow rate function and having facilities for resetting the required value depending on an external signal.pneumatic, electric etc. flow rate controllerderiving the energy for maintaining the constant flow rate function from an external source. It can be either of the constant or variable type.system powered flow rate controllerderiving its energy from the dynamic pressure in the air stream to maintain its constant flow rate function and can be either a constant or variable type.