A low pressure feeder includes a rotor containing at least one pocket having a single access opening to it, the rotor rotatable about an axis and enclosed in a housing having an inlet port vertically above an outlet port. A sealing structure is disposed in a large clearance volume between the rotor and the housing, the sealing structure comprising a single shoe having an opening communicating with the inlet port, and one or more pressure compensators for biasing the shoe into engagement with the rotor. The pressure compensators are responsive to the process pressure at the feeder high pressure outlet port. A gas blast structure is provided for supplying a blast of gas to the pocket to effect discharge of material from the pocket when it is in communication with the outlet port. A vent is provided immediately before the inlet port in the direction of rotation of the roller, the access opening being dimensioned with respect to the vent and inlet port so that it can communicate with both during a portion of path of rotation thereof.