The evaporator sleeve, sometimes called a humidifier pad, media filter, or water filter, absorbs water from the pan within the humidifier. The sleeve fits over the drum assembly, which slowly spins and picks up water to circulate as humidity into the air. The sleeve should be replaced at least once a year to ensure optimal performance. This sleeve is also available as part of the GeneralAire 81 tune up kit, which provides everything necessary to make your humidifier like new.