Explosion proof heaters are constructed with housings that can withstand explosions, sparking and flames during the heating of materials. Most devices include a protective sheath that covers the heating element. There are several basic types of explosion proof heaters. Cartridge heaters are cylindrical devices that are generally inserted into a heated substance. Circulation heaters are used to heat moving or circulating fluids. Duct heaters are used to heat moving gas streams. Immersion heaters are used in applications that require the heater to be immersed in the substance being heated. For example, over-the-side heaters are immersion heaters that hang over the side of a tank of heated material. Both radiant and infrared heaters use a reflective shield to direct radiant heat onto a heated surface. Common types of infrared heaters include metal-sheathed tubular heaters, quartz tubes and lamps, gas-fired catalytic heaters, flat faced panels and ceramic emitters.