Negative Air Machines 3-Ply Ring Pre Filter - 24" x 24" x 1" - Case Of 24SKU: NC2424R Description 24"x 24" x 1" 3 ply Ring Filter for Negative Air Machine Case of 24 Ring panels are used for second stage filtration. The wire reinforcement border helps the filters to fit snugly, disallowing air bypass. These filters can also withstand moderate amounts of moisture without losing strength, making them ideal for high humidity or variable temperature environments. This filter will fit the stage 2 slot in the following machines: