1,021,987. Gas-filters. P. G. VICARD. Nov. 23, 1964 [Nov. 25, 1963; March 5, 1964], No. 47551/64. Heading BIT. In a gas-filter comprising a plurality of openended filter sleeves 8, 9 which each extend from an inlet chamber 4a or 4b to a common dustcollecting hopper 10, gas flows from inlet 6 down and through one set of sleeves 8 and hence into hopper 10 and up into sleeves 9, this order being abruptly reversed periodically by means 7, causing elements 8 (for example) to contract under the resulting reduced pressure wave, breaking up any solids deposited thereon. Filtered gas which has passed outwards through sleeves 8, 9 leaves at 13. The through passage of the sleeves may be annular in section by virtue of solid cores 15, and helical flow may be imparted by vanes 14 as gas enters the sleeves. To increase the "through velocity" of the gas in sleeves 8, 9 a recirculation line 17, 18, 19 may be connected as shown or inlet 17 may connect to the inlet chamber 4a or 4b which is currently cut off from inlet 6 by hinged shutter 7. An alternative embodiment, Figs. 4-6 (not shown), comprises radially separated inlet chambers each having its own inlet valve and leading to one sleeve element, one valve out of the six being open at any one time, so that the element on 'through flow" carries the whole gas input to the others. A rotary distributer could replace the valves.