Abstract: A gassing apparatus including a composite gas flow screen having a plurality of respective screen elements, each with at least one respective flow opening therein, the cross-sectional areas of the respective flow openings in respective elements decreasing in a downstream direction with respect to gas flow and at least one other screen element having no flow opening. A nozzle preferably produces a relatively high velocity central gas stream with surrounding gas streams of decreasing velocity as they are spaced further away from the central gas stream. Methods and apparatus are described.