Composite filter set for gas flow ducts

Claim 1

A composite filter set for use in existing walls and especially in duct systems of ventilating installations, comprising: a. a first filter element comprising a plurality of first unsupported prismatic filter pockets each having a given opening length and into which trapezoidal reinforcing inserts are welded and a first square head frame holding the pockets at their open ends in substantially the same plane with the closed ends projecting outwardly in the same direction, b. a plurality of second filter elements each having the same construction as the first filter element and each including at least one second pocket having half the opening length dimension as said first filter pockets and a second square head frame having one-fourth the area of said first square head frame, and c. means joining at least two second frames around the periphery of the first frame in substantially the same plane and with the filter pockets projecting in the same direction and in a manner to form a gas-tight seal therebetween and to closely fit within the cross section of the duct.