Flange and screw plug immersion heaters are used in applications that require the heater to be immersed in the substance to be heated. They are equipped with pipe threads, flanges or other mounting hardware. Flange immersion heaters are equipped with a flange that is installed by bolting the heater to a matching flange that is welded to the vessel wall. Screw plug immersion heaters are equipped with a pipe fitting which screws into a threaded well. With both types of heaters, caution must be taken in evaluating the desired heat transfer rate to the heated substance. High-watt density immersion heaters should not be used with extremely viscous materials, explosives, or substances that are not well circulated because the risk of fire or scorching increases as fluid viscosity and heater-watt density increase. Common applications for flange and screw plug immersion heaters include the heating of gases or vapors, clean water, process waters, high-purity waters, lightweight oils and degreasing solutions, heavy weight oils, medium weight oils, mild corrosive solutions, severe corrosive solutions, caustic solutions, and liquid paraffin.