The use of filter bags is a simple and effective way of filtration. The filter media in the form of woven mono and multi-filament or needle felt fibres, manufactured into standard shapes and sizes of bags, are fitted with a top seal arrangement for location and sealing into a suitable vessel. In view of potentially high differential pressures, which can develop across the bag, it is logical to provide the housing with a support basket manufactured from perforated or woven wire mesh to retain the bag during operation. Filtration occurs on the inner face of the bag by impingement, inertial impact and diffusion. Thus all contaminants are collected in the bag, simplifying disposal of the bag, and contaminants, on change-out.