A direct-fired heater used in processing petroleum. The basic feature of these heaters is that the fluid (liquid, vapor, or a mixture of both phases) is heated in carbon- or alloy-steel tubes, which in turn receive their heat from hot products of combustion. The fuel may be liquid, such as heavy fuel oil, or gas, such as natural gas or cracked refinery gas. Burners of the steam, gas, or mechanically atomizing type may be used for liquid fuels, and burners of the simpler ring-type, spider, or center-nozzle type for gas fuels. Combination burners designed to handle both fuels are in wide use.