Process and draft control system for use in cracking a heavy hydrocarbon feedstock in a pyrolysis furnace

A process for cracking a heavy hydrocarbon feedstock in a furnace having a convection section and a radiant section, said radiant section having radiant section burners which provide hot flue gas in said furnace, said process comprising:

• (a) heating said heavy hydrocarbon feedstock in said convection section of said furnace to form a heated heavy hydrocarbon feedstock;
• (b) mixing said heated heavy hydrocarbon feedstock with a primary dilution steam stream to form a mixture stream;
• (c) heating said mixture stream in said convection section of said furnace to form a hot mixture stream, said hot mixture stream having a vapor phase and a liquid phase;
• (d) separating said vapor phase from said liquid phase;
• (e) cracking said vapor phase in said radiant section of said furnace to produce an effluent containing olefins;
• wherein said furnace further has draft and said draft is continuously measured and periodically adjusted to control the temperature of at least one of said hot mixture stream and said vapor phase.