Screw Plug Immersion Heaters consist of tubular elements welded or brazed into a threaded screw plug which can then be inserted into a threaded opening in a tank wall or through a mating full or half coupling. Screw plug immersion heaters consist of tubular elements in a threaded hex plug. They screw directly through threaded openings in tank walls to heat liquids, viscous fluids, forced air and gases by direct contact.