These use a desiccant material, typically Zeolite, which absorbs water vapour in much the same way as silica gel (a packet of which you would often find in a pair of new shoes). A fan draws air into the dehumidifier and passes it through a section of a slowly rotating wheel which holds the desiccant. The material draws the moisture out and dries the air which then exits the unit. The moisture is then extracted from the desiccant; this is done by heating a different portion of the wheel not being used to dry the air. The water then evaporates from the desiccant, is passed over a cold plate where it condenses, runs off and is collected in a reservoir or a permanent feed.