The direct-fired thermal oxidizer is mainly used for thermal oxidation process. A process stream is introduced into a firing box through or near the burner and enough residence time is provided to get the desired destruction removal efficiency of the VOC'S. Also called afterburners, these systems are the least capital intensive, but when applied incorrectly, the operating costs can be devastating because there is no form of heat recovery. These are best applied where there is a very high concentration of VOC'S to act as the fuel source for complete combustion at the targeted operating temperature.

Direct-fired vaporizer is a vaporizer in which heat furnished by a flame is directly applied to some form of heat exchange surface in contact with the liquid LP-gas to be vaporized. The direct-fired vaporizers use the propane itself as fuel for the direct flame. A vaporizer in which heat furnished by a flame is directly applied to some form of heat exchange surface in contact with the liquefied petroleum gas to be vaporized.