Filters

Claim 1

IN A FILTER UNIT OF THE TYPE HAVING AN ENCLOSING STRUCTURE FOR A FILTER BED OF GRANULAR MATERIAL SUPPORTED ON A FORAMINOUS PLATFORM TO PROVIDE A CHAMBER THEREUNDER AND WHICH IS CLEANED BY A FLUID SYSTEM FOR PRODUCING A BACK FLOW OF FLUID FROM THE CHAMBER UPWARDLY THROUGH THE FILTER BED, THE COMBINATION WITH SUCH A FILTER UNIT OF AN ARRANGEMENT TO AVOID CHANNELING OF THE WASH FLUID AT HIGH VELOCITY IN LOCALIZED STREAMS AND LATERAL DISPLACEMENT OF THE GRANULAR MATERIAL OF THE FILTER BED DURING CLEANING COMPRISING IMPERVIOUS PARTITION WALLS EXTENDING UPWARDLY FROM THE FORAMINOUS PLATFORM THROUGH AND ABOVE THE FILTER BED TO DIVIDE THE FILTER BED INTO SEPARATE ADJACENT CELLS AND PROVIDE AN EXPANSION CHAMBER AT THE TOP OF EACH CELL FOR EXPANSION OF GRANULAR MATERIAL, A PERFORATED COVER OVERLYING THE TOP OF AND SEALED TO THE PARTITION WALLS OF EACH CELL TO RESTRICT THE FLOW OF WASH FLUID THEREFROM, AND THE COVERS FOR THE PLURALITY OF CELLS EACH HAVING PERFORATIONS OF A NUMBER AND CROSS-SECTIONAL AREA TO PRODUCE A PREDETERMINED RESISTANCE TO FLOW FROM EACH OF THE CELLS AT A PARTICULAR DESIRED RATE OF FLOW WHEREBY TO PRODUCE A UNIFORM FLOW FROM THE CHAMBER THROUGH ALL OF THE CELLS.