The main function of a stack is to induce the flow of combustion air into, and flue gas out of, the fired heater, and to produce negative pressure throughout the heater. Positive pressure within a heater can cause outward flow of hot gases which can overheat and corrode the steel structures, and be a personnel hazard.
The other function of a stack is to safely discharge hot flue gas at a point significantly above grade. High discharge avoids hazards caused by the hot combustion gases.
The minimum recommended stack height for personnel protection is 20 feet higher than the highest operating platform within a 100-foot radius.
Also, note a higher stack may be required to satisfy local requirements for emission dispersion. Sections 130 and 260 discuss general emissions and regulations, but you will need to check for specific local restrictions on dispersion. You can also contact the Environmental Engineering Division of ETD for assistance.