The furnace heats the crude oil to 370°C before it is pumped into the bottom of the distillation column.
The oil is pumped through pipes which line the inside of the furnace and is heated by large gas burners. The hot oil stays inside the pipes and, naturally, never goes anywhere near the flames.
When it passes into the bottom of the distillation column at this temperature, most of the hydrocarbons boil off. They pass up the distillation column as gases where they are cooled, condensed and collected.