Abstract: An air filtration unit, includes a filter cartridge (4) of the filtration bag (6) type, the bags (6) being arranged vertically and held extended by appropriate fixing elements (16, 17), between two support structures (12, 13). The upper ends (15) of the bags (6) are fixed to an upper support structure (13), in the form of a perforated sheet and the lower ends thereof (14) are fixed to a lower support structure (12), in the form of a grille. According to the invention, the fixing elements (16, 17), for the filtration bags (6), are embodied to permit the tensioning of the bags (6) at the upper end (15) thereof. The invention further relates to a particular method for assembly of a filtration bag within a unit as above, a tool for tensioning bags, embodied to be suitable for the unit.