Abstract: The present invention relates to a filter unit having a sheet of filtering material mounted on a first frame and a second frame received within and below the first frame. The second frame is configured such that the filtering material is tightly held between the first and second frames and the filtering material is stretched by the second frame. In accordance with one of the aspects of the present invention, the second frame is provided with ear members. The ear members are sized such that the ears penetrate the filtering material and hold the first frame tightly between the ear members. The ear members reinforce the first frame by the second frame, through a mechanical interconnection of the frames and also, prevent slippage of the filtering material.