leading from the outlet chamber 33 each filter cartridge 40 as is best illustrated in fig4 and 5 more particularly comprises a filter element 41 having an elongate hollow cylindrical configuration with an internal passageway 42 at the top end of the filter element 41 is a top mounting plate 50 at the opposite bottom end of the filter element is a bottom end plate 60 as illustrated more particularly in fig7 the filter element 41 has a generally circular cross sectional shape and has a diameter less than the diameter of opening 31 in dividing wall 30 so as to allow the filter element to be removably positioned therein the filter element 41 further includes a generally rigid inner perforated tubular wall 43 and a coaxial outer perforated tubular wall 44 the tubular walls 43 and 44 define respective inner and outer cylindrical surfaces of the filter element and define an annular space therebetween in the preferred embodiment the perforated tubular walls 43 and 44 are made of expanded metal with welded overlapping edges in the annular space between the inner and outer perforated tubular walls 43 and 44 is positioned a filter medium 45 which preferably comprises a sheet of porous filter paper which is impermeable to particulate matter but permeable to air and gases as illustrated the filter medium 45 is pleated in accordion fashion to provide a large surface area at the upper and lower regions of the filter medium 45 potting compound 46 is used to prevent air leaks around the edges thereof the top mounting plate 50 is preferably a rigid metal.