A filtering unit (10) has four filter holders (20) arranged in a rectangle around a blower (36). Each filter holder holds up to two filter elements (11). The blower draws air through the filter elements (11) and outputs filtered air through a vent (40). Refillable filter frames (16) are provided to allow...http://www.google.fr/patents/US4749390?utm_source=gb-gplus-shareBrevet US4749390 - Four-sided air filter
