Powder flow control valves.

摘要

The disclosure relates to a powder flow control valve comprising a nozzle (33) formed from a material such as high density polythene having an upper frusto-conical section (33a) which converges towards the lower end of the nozzle and terminates in a cylindrical section (33b). The nozzle is encircled by a plenum chamber (24) to which sources of vacuum and air pressure are selectively connectable through a pneumatic control system (14). When vacuum is connected to the plenum chamber, the resulting pressure drop across the wall of the porous nozzle causes powder flowing through the nozzle to consolidate in the nozzle and terminate the flow. When air pressure is applied to the plenum chamber, the resulting pressure rise across the boundary surface accelerates powder flow through the nozzle. The control means is arranged to establish air pressure in the plenum chamber to initiate opening of the valve and flow through the valve and, after a pre-determined time, to terminate the air pressure supply and allow powder to flow under gravity through the nozzle before flow is terminated when a pre-determined quantity of powder has been delivered by the nozzle by applying vacuum to the plenum chamber.