All cool rooms and freezers use evaporators or coolers to help regulate temperature inside the room.
When people and equipment enter and exit a room they let varying amounts of ambient air into the space which needs to be brought back down to the regulated temperature. This air generally contains a significantly higher level of water vapour than the room. The water vapour or moisture in the air is naturally drawn to the coldest element in the room which just happens to be the fin and tube surfaces within the evaporator.
