STANDARD SCREW PLUG IMMERSION HEATERS - …- 3 phase industrial immersion heater with screw plug for oil ,Watt Density for water heaters is max 45 watts/inch & 15 watt/ sq inch for oil heaters Each heater shall be having 2 Nos entries of 3/4”ET each. Each Screw Plug would be brass.Screw Plug Immersion Heaters - tempco.comScrew Plug Immersion Heaters 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..