Fired heaters — an open flame next to a hydrocarbon inventory

A fired heater is where a plant puts heat in that no exchanger can supply — above roughly 400 °C, and up to 900 °C or more in a cracking furnace.

It is also the only place on most plants where an open flame and a hydrocarbon inventory are deliberately kept a few centimetres apart, separated by a tube wall. Nearly everything below follows from that.

How one is arranged

Section	How heat gets in	Share of duty	Watch out for
Convection section	Hot flue gas flowing over finned tubes	Roughly 25 – 40%	Fins foul with soot, and fouling quietly cuts efficiency
Shock or shield rows	The first bare rows, seeing both radiant and convective heat	Part of the convection duty	Hardest duty in the heater — bare tubes taking radiant heat
Bridgewall	The transition between radiant and convection	—	Bridgewall temperature is a key operating indicator
Radiant section	Direct radiation from the flame and hot refractory	Roughly 60 – 75%	Flame impingement on tubes, and local hot spots
Burners	Where fuel and air mix and burn	—	Flame shape, stability and clearance from the tubes
Stack	Carries flue gas away and creates natural draught	—	Stack temperature shows how much heat is being thrown away

Follow the process fluid, not the flue gas — they run in opposite directions. The fluid enters cold at the top of the convection section and leaves hot from the radiant section at the bottom, while the flue gas travels the other way, giving up heat as it goes.

Follow the process fluid and the flue gas separately, because they travel in opposite directions:

• The fluid enters cold, at the top of the convection section, and works downward.
• It leaves hot, from the radiant section at the bottom.
• The flue gas goes the other way — hottest at the burners, giving up heat as it rises, and leaving through the stack.

That counter-current arrangement is what makes the heater efficient. The coldest fluid meets the coolest flue gas, and the hottest fluid meets the flame.

Roughly 60 to 75% of the duty is absorbed in the radiant section, by direct radiation from the flame and the glowing refractory. The convection section picks up much of the rest from hot flue gas over finned tubes.

Passes

The fluid is split into parallel passes through the heater, because one continuous tube would give an impossible pressure drop.

That split introduces the problem that defines fired heater operation:

Draught and excess air

Draught is the slight negative pressure inside the firebox that pulls combustion air in and flue gas out.

• Natural draught — the stack does it, because hot gas is less dense than ambient air.
• Forced draught — a fan pushes air in.
• Induced draught — a fan pulls flue gas out.
• Balanced draught — both, which gives the most control.

Excess air is the air supplied above what the fuel strictly needs. Some is essential — mixing is never perfect, and without it some fuel leaves unburnt. But every extra kilogram of air is heated to flue gas temperature and thrown away up the stack.

That is the trade-off an oxygen analyser in the flue gas exists to manage: typically a few percent O₂, enough for complete combustion without paying to heat the atmosphere. Together with stack temperature it is the main efficiency indicator on the heater.

Hazards

Hazard	Usual cause	Protection	Note
Furnace explosion on light-off	Unburnt fuel accumulating in the firebox before ignition	Mandatory purge cycle before any attempt to light	Skipping the purge to save time has destroyed heaters and killed people
Tube rupture	Overheating from low flow, coking or flame impingement	Low flow trip, tube metal temperature monitoring, snuffing steam	Releases hydrocarbon directly into a firebox at full temperature
Loss of process flow	Pump trip or a blocked pass while firing continues	Low flow trip on every pass, tripping the fuel	Per pass, not just total — one blocked pass overheats on its own
Coking inside the tubes	Film temperature too high, or residence time too long	Velocity control, steam injection, planned decoking	Coke insulates, so the tube runs hotter and cokes faster still
Flame impingement	Burner misadjusted, damaged tip, or wrong fuel composition	Routine visual checks through sight ports	Produces a local hot spot that no bulk measurement will see
Loss of flame	Fuel pressure swing, or moisture in the fuel gas	Flame scanners tripping the fuel valves	Fuel entering an unlit hot firebox is the explosion case again
Positive firebox pressure	Draught control failure or a blocked stack	Draught measurement and control, induced draught fan trip logic	Pushes flame and hot gas out of openings at operator level
Fuel gas carryover	Liquid in the fuel gas reaching the burners	Fuel gas knock-out drum with level protection	Liquid slugs cause sudden flare-ups and flame instability

A fired heater is the only place on most plants where an open flame and a hydrocarbon inventory are deliberately kept a few centimetres apart, separated by a tube wall. Nearly every protection listed here exists because of that.

Two deserve expanding.

Light-off

The burner management system (BMS) enforces that sequence — purge, pilot, main fuel, with flame proven at each step — and trips the fuel on loss of flame, low flow, high firebox pressure or low fuel pressure. It is a safety instrumented system in its own right, sized to a SIL like any other, as covered in safety instrumented systems.

Note which way the fuel valves fail: closed. That is the worked example in fail position — unburnt fuel into a hot firebox is an explosion, while starving the burners is merely a trip.

Tube rupture

A ruptured tube releases the process fluid straight into a firebox already at full temperature. The responses are snuffing steam to smother the box, isolation, and depressuring.

Prevention is better: tube metal temperature monitoring, flow protection per pass, controlling coking, and routine visual checks through sight ports for flame impingement — a burner playing directly onto a tube, which creates a hot spot no bulk measurement will ever see.

Coking, and why it accelerates

Heavy hydrocarbon at high film temperature lays down coke on the inside of the tube.

Coke is a good insulator. So the tube wall must run hotter to push the same heat through it — which raises the film temperature, which lays down coke faster. The process feeds itself.

Countermeasures are velocity (keeping the film moving), steam injection to reduce residence time and partial pressure, and planned decoking — steam-air decoking or mechanical pigging — on a cycle. Rising tube metal temperature at constant duty is the signal that the cycle is due.

Where the other disciplines meet it

• Layout puts heaters upwind of the process, because they are a permanent ignition source — see plant layout.
• Materials — radiant tubes run hot enough to need alloy, which is why ASTM A335 P11 and P22 exist.
• Civil carries substantial structural steel, the stack, and a refractory-lined casing.
• Instrumentation provides the BMS, flame scanners, oxygen analysis, draught and tube metal temperatures.

What to take away

• Process fluid and flue gas run counter-current. Fluid in cold at the top, out hot at the bottom.
• Most of the duty is radiant. The convection section recovers the rest.
• Flow must be balanced per pass — a single restricted pass overheats while totals look normal.
• The firebox stays slightly negative, or flame comes out where people stand.
• Excess air buys complete combustion and costs efficiency. The O₂ analyser finds the balance.
• Purge before light-off, every time. This one has killed people.
• Coking is self-accelerating, because coke insulates.