An apparatus and a method for controlling the velocity of a particulate material conveyed by a gas flow under negative pressure in a duct said device comprising means for separating the flow into first and second streams. The first stream has a relatively high particulate material content and the second stream has a relatively low particulate material content. The flow of the second stream is restricted for example by a flow restricting or constricting orifice or valve. The duct diameter may increase incrementally along its length.