Apparatus for the diacritic cracking of heavy hydrocarbon feeds includes a combustor for burning a fuel, to provide the hot combustion products to the reactor. The combustor is water cooled and includes gas filming with an appropriate gas such as N.sub.2 or CO.sub.2 to avoid coking in the combustor by preventing impingement of the fuel and combustion products on the combustor walls. The reactor is of reduced cross-section, also preferably utilizing gas filming to avoid coking on the reactor walls. Feed stock injectors are provided adjacent the reactor inlet. Adjacent the outlet end of the reactor is a quench section designed for the rapid injection of a quench fluid to reduce the stream temperature below that at which further cracking will take place, with the quench injectors also being gas filmed to avoid the coking thereof. Thereafter a tubular heat exchanger is provided to remove additional heat from the primary flow stream from the reactor for recovery purposes, the heat exchanger again using gas filming to minimize the build-up of coking and tar build-up, particularly encouraged by the rapidly decreasing temperatures. The gaseous products are then further quenched and cooled for processing by conventional means.