The middle two fans are hooked on the outer rail, and the left and right sides are locked on the inner rail. The sealing material is installed between two hook-shaped fans to block the gap. From top to bottom, the first and fourth fans are lock fans (the directions are symmetrical, that is, the two fans are in opposite directions), and the second and third fans are hook fans (the directions are symmetrical, that is, the two fans are in opposite directions).