fans radiant coils are usually hung in a single plane down the center of the fire box they can be nested in a single plane or placed parallel in a staggered double row tube arrangement heat transfer from the burners to the radiant tubes occurs largely by radiation hence the term u201c radiant section u201d where the hydrocarbons are heated to from about 1 400 f to about 1 550 f and thereby subjected to cracking with consequent coke formation the initially empty radiant coil is therefore a fired tubular chemical reactor hydrocarbon feed to the furnace is preheated to from about 900 f to about 1 000 f in the convection section by convectional heating from the flue gas from the radiant section steam dilution of the feed in the convection section and the like after preheating the feed is ready for entry into the radiant section the cracked gaseous hydrocarbons leaving the radiant section are rapidly reduced in temperature to prevent destruction of the cracking pattern cooling of the cracked gases before further processing of same downstream in the olefin production plant recovers a large amount of energy as high pressure steam for re use in the furnace and or olefin plant this is often accomplished with the use of a transfer line exchanger or tle processing of furnace product downstream of the tle although it can vary from plant to plant typically employs an oil quench of the cooler but still hot cracked furnace effluent thereafter the cracked hydrocarbon stream typically is subjected to primary fractionation to remove heavy liquids followed by compression of uncondensed hydrocarbons and acid gas and water removal therefrom various desired products are then individually separated e g ethylene propylene a mixture of hydrocarbons having four carbon atoms per molecule fuel oil.