Furnaces and fired heaters

In short

A fired heater (process furnace) burns fuel in a refractory-lined box and transfers the heat of combustion to a process fluid flowing inside tubes — mainly by radiation in the firebox and by convection in the bank above it. It is the largest single energy consumer and the biggest fuel bill in a refinery, petrochemical plant or fertiliser complex, and also the most common source of fire and tube-failure incidents. In Indian universities it is examined in Heat Transfer and Process Equipment Design (radiant flux, Lobo–Evans, draft, efficiency by the stack-loss method, tube skin temperature); in GATE it appears as efficiency, duty and radiation questions. Employers who test it in placement interviews include IOCL, BPCL, HPCL, Reliance, Nayara, ONGC, GAIL, IFFCO, Thermax, Technip Energies, L&T Technology Services, Fluor, Toyo and Bharat Heavy Electricals.

Explained

What a fired heater is, and how it differs from a boiler

• Fired heater / process furnace: fuel is burnt in an open firebox; the process fluid (crude, naphtha, gas oil, thermic fluid, reformer feed) passes once through tubes and is heated or partly vaporised. No steam drum, no circulation.
• Boiler: the same combustion side, but the duty is to generate steam; in India boilers above the Boilers Act, 2025 threshold (the Act replaced the Boilers Act, 1923 from 1 May 2025) are statutory equipment under the Indian Boiler Regulations, 2026 (in force from October 2026, superseding the 1950 Regulations), requiring IBR certification and periodic inspection. Process fired-heater coils are normally outside IBR and designed to API 530 with piping to ASME B31.3 — a point interviewers like to check.
• Fired reboiler, hot-oil heater, thermic-fluid heater, steam superheater, reformer and cracker furnace are all fired heaters with different coil service.
• Typical split of duty: radiant section 50–70 %, convection section the rest; radiation dominates because firebox gas is at 900–1,300 °C and radiation varies as T⁴.

Parts and terminology

• Firebox (radiant section): refractory-lined chamber with burners; tubes line the walls (vertical cylindrical) or walls and roof (cabin).
• Burners: mix fuel and air and stabilise the flame. Gas, oil (atomised with steam), or dual-fuel. Types: raw-gas, premix, staged-air / staged-fuel ultra-low-NOx, flue-gas-recirculation.
• Shield / shock tubes: the first 2–3 bare rows above the firebox; they see direct radiation, so they are never finned.
• Convection section: extended-surface tubes (studs for oil firing, serrated fins for clean gas firing) that recover heat from the flue gas.
• Bridgewall (arch) temperature: flue-gas temperature leaving the radiant section, normally 750–900 °C. It is the key operating indicator — a rising bridgewall temperature at fixed firing means the radiant coil is absorbing less than it should (internal coking or external fouling of the radiant tubes, or a changed flame pattern/excess air); a fouled convection bank shows up instead as a rising stack temperature.
• Stack / breeching / damper: creates and controls draft. Stack temperature fixes the largest efficiency loss.
• Refractory: ceramic-fibre modules or castable; soaking pit / hearth, peep doors, explosion (relief) doors, snuffing-steam connections, decoking connections, header boxes.
• Crossover piping links radiant and convection coils; manifolds and flow-control valves split the feed into passes (2, 4, 6, 8) to limit pressure drop.

Classification

• Vertical cylindrical: cheapest per unit duty, small plot, good for 3–30 MW; cabin/box with horizontal tubes: better flux uniformity and easier decoking, used for large crude and vacuum heaters.
• Steam-methane reformer (top-fired): catalyst-filled tubes, extremely high tube metal temperature (~900 °C), HP-Nb/micro-alloy centrifugally cast tubes.

Draft and the air–flue-gas path

• Draft is the small negative pressure that pulls air in and pushes flue gas out. Caused by the density difference between hot flue gas in the stack and cold ambient air (chimney effect).
• A natural-draft heater is kept negative everywhere (typically −2.5 mm WC at the arch, −10 to −12 mm WC at the burner floor) so hot gas cannot leak out of peep doors. Positive pressure in the box is a safety hazard.
• Tramp (leak) air entering through cracks and open doors raises stack O₂ without helping combustion — it looks like excess air on the analyser but cools the box.
• Forced draft (FD fan) is needed when an air preheater (APH) is added, because the APH adds pressure drop; induced draft (ID fan) pulls flue gas; balanced draft uses both and keeps the firebox slightly negative.
• Draft losses: burner/air register, convection bank (several mm WC), damper, stack friction and exit velocity head.

Combustion, excess air and emissions

• Excess air is needed to complete combustion; too little gives CO, smoke and afterburning in the convection section, too much wastes fuel as stack sensible heat.
• Practical targets: gas firing 10–15 % excess air (2–3 % O₂ dry); oil firing 15–25 % (3–4.5 % O₂); natural draft needs more than forced draft.
• Rule of thumb: every ~15 % of unnecessary excess air costs roughly 1 percentage point of efficiency at a 300 °C stack; every 20 °C reduction in stack temperature gains roughly 1 point.
• NOx forms thermally at flame temperature and from fuel-bound nitrogen; controlled by staged-air/staged-fuel low-NOx burners, flue-gas recirculation, steam injection, and post-combustion SCR/SNCR (API 536 covers these).
• SO₂ comes from fuel sulphur; it also fixes the acid dew point, which limits how cold the APH metal and stack may run (typically keep cold-end metal above ~130–150 °C for sulphur-bearing fuel oil).
• Indian regulation: emission and stack-height standards for petroleum refineries and furnaces are notified under the Environment (Protection) Act, 1986 and its rules, enforced by CPCB and the State Pollution Control Boards through consent to operate and continuous emission monitoring; limit values differ by fuel and vintage, so the plant's own consent conditions govern. Energy performance is regulated by BEE under the Energy Conservation Act, 2001 (amended 2022) through the PAT scheme, where refinery fired-heater efficiency is a major lever. Workplace safety is now under the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025 (it replaced the Factories Act, 1948); refineries additionally follow OISD standards and PESO rules for fuel handling.

Thermal design method (what is derived in exams)

1. Process-side duty: Q = ṁ·cp·ΔT, or ṁ·Δh when there is vaporisation (crude heaters are usually two-phase at outlet, with a specified percent vaporisation).
2. Fix the heat-flux density: choose average radiant flux from service — crude 30–38 kW/m², vacuum/coker 25–32 kW/m² (coking-limited), hot oil 30–40 kW/m², reformers much higher.
3. Radiant area and tube count: A = Q_radiant / flux; then number of tubes = A / (π·D_o·L_eff).
4. Radiant heat transfer — Lobo–Evans: treats the firebox as a gas-filled enclosure exchanging with a "cold plane"; uses cold-plane area A_cp, tube-bank efficiency factor α, and overall exchange factor F from Hottel's charts. Convection inside the box is added as a small term (~7 W/m²K on the tube).
5. Firebox heat balance gives the bridgewall temperature by trial: heat released = heat absorbed by radiant coil + losses + enthalpy of gas leaving at T_g.
6. Volumetric heat release check: roughly 80–200 kW/m³ of firebox; too high means flame impingement.
7. Convection section: sized by LMTD and an overall U on extended surface, counter-current, row by row; mass flux of flue gas 1.5–3.5 kg/m²·s.
8. Hydraulics: pressure drop per pass, mass velocity 1,000–2,000 kg/m²·s for liquid coils, kept high enough to protect the film and avoid coking.
9. Tube metal (skin) temperature by API 530, then tube thickness from the allowable stress at that temperature (elastic design for the convection section, rupture/creep design for the radiant section with a design life, usually 100,000 h).
10. Materials: A106 Gr B / A192 (low temperature), 1.25Cr-0.5Mo, 2.25Cr-1Mo, 9Cr-1Mo (P9) for radiant crude/vacuum service, TP321H / TP347H austenitic and cast HP-modified alloys for very hot service.

Efficiency, operation and common problems

• Efficiency (LHV basis) = useful duty / fuel heat input = 100 − stack loss − radiation (casing) loss − unburnt loss. Radiation loss 1.5–2.5 %.
• Typical: old natural-draft heater 70–80 %; modern heater with a good convection bank 88–92 %; with air preheat up to ~93 %. Always state whether LHV or HHV — HHV efficiency is 8–10 points lower for gas.
• Monitoring instruments: stack O₂ and CO analysers with O₂ trim, draft gauges, bridgewall and stack thermocouples, tube-skin thermocouples, infrared pyrometer surveys of tubes, pass flow and outlet temperatures.
• Coking: thermal cracking of hydrocarbon on the inner wall forms a low-conductivity layer; skin temperature climbs, pressure drop rises. Cured by steam-air decoking, steam spalling, or mechanical pigging.
• Other failures: flame impingement, tube sagging and creep bulging, oxidation/scaling, vanadium–sodium ash corrosion from fuel oil, cold-end sulphuric-acid corrosion in APH and stack, carburisation, refractory spalling, afterburning (combustion continuing in the convection bank from poor air supply — detected by convection-outlet temperature exceeding the bridgewall temperature trend).
• Safety and interlocks (BMS): firebox purge of at least five volume changes before light-up, pilot and main-flame scanners, low/high fuel-gas pressure trip, low process-flow trip, snuffing steam, explosion doors. Guided by API 556 and NFPA 86/85; emergency shutdown must cut fuel, not air.

Key points

• Fired heater — refractory box where burnt fuel heats a process fluid flowing once-through inside tubes.
• Radiant section — 50–70 % of duty, heat transfer ∝ (T_g⁴ − T_w⁴); governs tube metal temperature.
• Convection section — recovers lower-grade heat using finned or studded tubes; fixes stack temperature and therefore efficiency.
• Shield tubes — bare rows protecting finned tubes from direct radiation.
• Bridgewall temperature — flue-gas temperature at the radiant exit, 750–900 °C; a key health indicator.
• Average radiant flux — 25–40 kW/m² for most hydrocarbon service; coking service is flux-limited.
• Draft — negative pressure from the chimney effect; keep the whole box negative, arch about −2.5 mm WC.
• Excess air — 10–15 % for gas, 15–25 % for oil; more air = more stack loss, less = CO and soot.
• Stack loss — the largest loss; cut it with a bigger convection bank, an air preheater, or lower excess air.
• API 560 — design standard for fired heaters for general refinery service; API 530 — tube thickness and skin temperature; API 535 burners; API 536 NOx control; API 573 inspection.
• IBR — Indian Boiler Regulations, 2026 (which superseded the 1950 Regulations in October 2026) under the Boilers Act, 2025 (successor to the 1923 Act), apply to boilers, not normally to process heater coils (B31.3 piping).
• Tube skin temperature = bulk fluid + film ΔT + coke ΔT + metal ΔT; design adds margin for fouling and flux maldistribution.
• Creep/rupture design — radiant tubes are sized for a 100,000 h rupture life, not elastic stress.
• Double-fired heater — radiation on both sides of a tube row, flattest flux profile, used for vacuum and coker heaters.
• Decoking — steam-air burn or spalling to remove the internal coke layer that drives skin temperature up.
• O₂ trim control — automatic excess-air control from a stack O₂ (and CO) analyser; the cheapest efficiency gain.
• PAT scheme — BEE's energy-saving mandate under the Energy Conservation Act, 2001 (amended 2022); refinery heater efficiency is a designated lever.

Formulas and examples

Example 1 — Duty, fuel and the value of 3 efficiency points (crude heater). Crude: 200 t/h from 230 °C to 360 °C, cp = 2.6 kJ/kg·K (no vaporisation assumed).

• Q = 200,000 × 2.6 × 130 = 67.6 × 10⁶ kJ/h = 67.6/3.6 = 18.78 MW.
• At η = 88 % (LHV) on fuel gas of LHV 48,000 kJ/kg: fuel heat = 67.6/0.88 = 76.82 × 10⁶ kJ/h → ṁ_f = 76.82 × 10⁶/48,000 = 1,600 kg/h.
• Raise η to 91 %: ṁ_f = (67.6/0.91) × 10⁶/48,000 = 1,548 kg/h → saving 52 kg/h, i.e. 416 t/yr at 8,000 h.
• At an indicative ₹45/kg for refinery fuel gas, that is about ₹1.87 crore a year — which is why O₂ trim and convection-bank cleaning pay back in months. (Fuel price varies; use the plant's internal transfer price.)

Example 2 — Efficiency by the stack-loss method. Fuel: refinery fuel gas, LHV 48,000 kJ/kg, stoichiometric air 17.2 kg/kg. Stack 320 °C, air 30 °C, measured dry O₂ = 3.5 %.

• EA ≈ 3.5/(21 − 3.5) × 100 = 20 % → air supplied = 1.20 × 17.2 = 20.6 kg/kg fuel.
• Flue gas = 20.6 + 1 = 21.6 kg/kg fuel; take cp_g = 1.15 kJ/kg·K.
• Stack loss = 21.6 × 1.15 × (320 − 30)/48,000 × 100 = 7,203/48,000 × 100 = 15.0 %.
• With radiation loss 1.5 % and no unburnt loss: η ≈ 83.5 % (LHV).
• Now tune to 10 % excess air (O₂ ≈ 1.9 %): air = 18.92, flue gas = 19.92 → loss = 19.92 × 1.15 × 290/48,000 × 100 = 13.8 % → η ≈ 84.7 %, a gain of 1.2 points from air control alone.

Example 3 — Radiant area and tube count. Radiant share 60 % of the 18.78 MW duty of Example 1 → Q_R = 11.27 MW. Design average flux 32 kW/m²; tubes 141.3 mm OD, effective length 6 m.

• Radiant area = 11,270/32 = 352 m².
• Area per tube = π × 0.1413 × 6 = 2.66 m² → N = 352/2.66 = 132.2 → 133 tubes (round up, then split into equal passes: 4 passes × 34 tubes = 136 tubes).
• Check flux with 136 tubes: 11,270/(136 × 2.66) = 31.2 kW/m², safely inside the limit.

Example 4 — Tube skin temperature and thickness (API 530 outline). Outside flux q_o = 32,000 W/m², tube 141.3 mm OD × 8 mm wall → D_i = 125.3 mm. Inside film h_i = 1,200 W/m²K, bulk fluid 360 °C, k_metal (9Cr-1Mo) ≈ 25 W/m·K, clean tube.

• Inside-referred flux q_i = 32,000 × 141.3/125.3 = 36,086 W/m².
• Film ΔT = 36,086/1,200 = 30.1 °C → inside wall 390.1 °C.
• Metal ΔT = 32,000 × 0.07065 × ln(141.3/125.3)/25 = 32,000 × 0.07065 × 0.1202/25 = 10.9 °C → outside metal 401 °C.
• Add 25 °C for coke and flux-maldistribution margin → design metal temperature ≈ 426 °C.
• Thickness at 40 bar (4 MPa) with S ≈ 100 MPa at that temperature: t = 4 × 141.3/(2 × 100 + 4) = 565.2/204 = 2.77 mm; plus 3.2 mm corrosion allowance = 5.97 mm → select the standard 8 mm wall. (Radiant tubes are then rechecked for 100,000 h rupture life.)

Example 5 — Natural stack draft. Stack 30 m tall, average flue-gas temperature 300 °C (573 K), ambient 30 °C (303 K).

• ρ_air = 353/303 = 1.165 kg/m³; ρ_gas ≈ 353/573 = 0.616 kg/m³.
• Δp = (1.165 − 0.616) × 9.81 × 30 = 0.549 × 9.81 × 30 = 161.6 Pa = 16.5 mm WC available.
• If convection bank + damper + stack friction + exit velocity head consume 14 mm WC, about 2.5 mm WC remains as arch draft — exactly the design target. Fouling the convection bank eats this margin and pushes the firebox positive.

Where it is used

• Refinery CDU/VDU, delayed coker, hydrotreater and reformer heaters — IOCL Panipat/Paradip, BPCL Kochi/Bina, HPCL Visakh, Reliance Jamnagar, Nayara Vadinar; a single crude heater can fire 3–6 t/h of fuel.
• Petrochemicals and fertilisers — ethylene cracker furnaces (Reliance, HMEL) and steam-methane reformers in ammonia/hydrogen plants (IFFCO, RCF, Chambal, GNFC).
• Hot-oil and thermic-fluid heaters — used across textile processing, paper, edible oil, pharma and plywood plants; Thermax, Forbes Marshall and Isgec are major Indian suppliers.
• Job roles — process design engineer (heater duty, flux, hydraulics), thermal/mechanical design engineer (API 560/530 datasheets), furnace operations engineer, energy/PAT engineer, inspection and reliability engineer (tube IR surveys, remaining-life assessment), EPC proposals engineer at Technip Energies, Toyo, L&T, Fluor and Petrofac.
• Energy audits — BEE-accredited audits of heaters under PAT, where O₂ trim, APH retrofits, burner upgrades and insulation surveys are the standard recommendations.
• Captive power and incineration — fired superheaters, waste-gas and incinerator-heat recovery heaters in chemical complexes.

Interview questions

• Q: Why is radiation the dominant mode in the firebox? A: Flue gas sits at 900–1,300 °C, several hundred kelvin above the tube wall, and radiant exchange scales as T⁴, so it swamps convection; the CO₂ and H₂O in the gas are strong band emitters.
• Q: What is bridgewall temperature and why is it monitored? A: The flue-gas temperature leaving the radiant section, typically 750–900 °C; a rise at the same firing rate signals coked or fouled radiant tubes, or a changed flame pattern/excess air — a fouled convection bank, being downstream, raises the stack temperature instead.
• Q: Why are shield tubes left bare? A: They face direct firebox radiation; fins there would overheat and burn away, and local flux would exceed the tube's allowable.
• Q: How do you decide the design heat flux? A: From the service — coking tendency of the fluid, film temperature, mass velocity and run length; coker and vacuum service use 25–32 kW/m², clean hot oil 30–40 kW/m².
• Q: What happens if excess air is too low, and how would you know? A: Incomplete combustion: CO and smoke rise, flames lengthen and lick the tubes, and heat releases late in the convection bank (afterburning); stack CO analyser and a smoky, hazy box confirm it.
• Q: Why must the firebox be kept at negative pressure? A: So that flames and hot flue gas cannot blow out of peep doors, header boxes and burner plenums — a personnel-burn and fire hazard.
• Q: Is a fired heater covered by IBR? A: Normally no — IBR under the Boilers Act, 2025 covers boilers and steam piping; process heater coils are designed to API 530 with ASME B31.3 piping, though a fired steam generator or superheater in the same plant will be IBR equipment.
• Q: What is tube skin temperature made of? A: Bulk fluid temperature plus the inside film ΔT, plus ΔT across any coke layer, plus the conduction ΔT through the metal wall, plus design margin for flux maldistribution.
• Q: How does coking raise skin temperature even if flux is unchanged? A: Coke has very low thermal conductivity, so the same flux needs a much larger ΔT across it, which lifts the metal temperature and accelerates creep.
• Q: Why limit how cold the stack or air-preheater metal runs? A: Below the sulphuric-acid dew point, SO₃ and water condense and cause severe cold-end corrosion; metal is usually kept above about 130–150 °C for sulphur-bearing fuel.
• Q: Which gives a bigger efficiency gain — reducing excess air by 5 % or dropping stack temperature by 20 °C? A: Roughly comparable (each about 0.3–1 point depending on fuel and temperature), but excess-air trimming is far cheaper and faster to implement.
• Q: First actions on a high tube-skin alarm in one pass? A: Check that pass's flow and outlet temperature, balance the pass flows upward, reduce firing on nearby burners, look for flame impingement, and plan for decoking — never just reset the alarm.

Common mistakes

• Quoting efficiency without stating LHV or HHV; the two differ by 8–10 points for gas, and comparisons across vendors become meaningless.
• Using total coil area instead of radiant area when computing average radiant flux, or forgetting to use effective tube length.
• Sizing radiant tubes on elastic allowable stress only — radiant service must be checked for creep-rupture life.
• Treating stack O₂ as proof of good combustion when the real cause is tramp air leaking into a cracked box.
• Mixing units in draft work: 1 mm WC = 9.81 Pa, and draft is a few mm WC, not a few kPa.
• Ignoring the percent vaporisation at a crude-heater outlet, so duty is computed with cp·ΔT alone and comes out far too low.
• Assuming more burners or more firing fixes low outlet temperature; if the convection bank is fouled, firing harder only raises the stack temperature, burns fuel and overheats tubes.

Related topics

Heat transfer: conduction, convection and radiation · Heat exchangers · Boiling, condensation and evaporators · Fuels and combustion calculations · Energy conservation, audits and the PAT scheme · Process equipment design and codes (API, ASME B31.3, IBR).