In industry, immersion heaters are installed in a tank, drum or other vessel to heat the contents, which may be oil, water, solvents or gas. This is a highly efficient form of energy use, since immersion heaters come in direct contact with the material to be heated. They come in different forms. For example, screw-plug heaters are designed to fit into a threaded receptacle in industrial tanks or pipes. Cartridge heaters fit where standard immersion heaters won’t, and drum & tank immersion heaters attach to the side of an open tank, drum or barrel to heat their liquid contents. Circulation heaters heat the content as it flows past the heating elements, making them well suited to water heating, protection against freezing, heat transfer oil heating, and other uses.