Fired Heater & Furnace Theory
What is a Fired Heater?
A fired heater (also called a process furnace or direct-fired heater) is equipment that transfers heat from combustion gases to a process fluid flowing through tubes. Unlike steam or hot oil systems, fired heaters generate heat directly through fuel combustion, making them essential for high-temperature applications.
Fired heaters are widely used in refineries, petrochemical plants, and process industries for crude oil heating, reforming reactions, thermal cracking, and heat recovery applications.
Furnace Components
Radiant Section
- • Contains burners and radiant tubes
- • Heat transfer by radiation (60-70%)
- • Highest temperature zone
- • Refractory-lined walls
Convection Section
- • Located above radiant section
- • Heat transfer by convection (30-40%)
- • Lower temperature zone
- • May include economizer, air preheater
Burners
- • Natural draft or forced draft
- • Floor-fired, wall-fired, or terrace-fired
- • Fuel: gas, liquid, or dual fuel
- • Low NOₓ designs available
Stack
- • Discharges flue gases to atmosphere
- • Provides draft for natural draft heaters
- • Height based on dispersion requirements
- • May include dampers for draft control
Radiant Heat Transfer
The radiant section operates based on thermal radiation, where heat is transferred from hot gases and refractory walls to process tubes without requiring a physical medium.
Stefan-Boltzmann Law
The rate of radiant heat transfer follows the Stefan-Boltzmann law:
Qrad = σ × A × F × ε × (T₁⁴ - T₂⁴)
where:
• σ = Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴)
• A = tube surface area (m²)
• F = view factor (geometric factor)
• ε = emissivity of surfaces
• T₁, T₂ = absolute temperatures of hot gas and tube (K)
The fourth power dependence on temperature means radiant heat transfer is extremely sensitive to temperature - doubling the temperature increases heat transfer by 16 times. This is why fired heaters can achieve very high heat flux rates.
Heat Duty Calculation
The required heat duty is determined from the process fluid energy balance:
For sensible heating:
Q = ṁ × Cp × (Tout - Tin)
For vaporization:
Q = ṁ × [Cpliq × ΔT + λ + Cpvap × ΔTsuperheat]
Thermal Efficiency
Thermal efficiency is the ratio of heat absorbed by the process to the heat released from fuel combustion:
η = (Heat Absorbed / Heat Released) × 100%
η = Qprocess / (ṁfuel × LHV) × 100%
Heat Losses
- Stack Loss (largest loss, 60-80% of total): Heat carried away by flue gases. Depends on flue gas temperature and excess air.
- Radiation Loss (10-20%): Heat radiated from furnace casing to surroundings. Higher for smaller heaters (larger surface/volume ratio).
- Incomplete Combustion (if any): Unburned fuel (CO, soot). Minimized with proper excess air.
- Other Losses: Opening losses, ash sensible heat (for solid fuels).
Typical Efficiencies (LHV basis):
- Old heaters without air preheaters: 70-75%
- Modern heaters: 80-85%
- Heaters with air preheaters: 88-92%
- Ultra-efficient designs: 93-95%
Fuel Combustion
Lower vs Higher Heating Value
Fuel heating values can be expressed on two bases:
- Lower Heating Value (LHV): Excludes heat of condensation of water vapor formed during combustion. Water leaves as vapor.
- Higher Heating Value (HHV): Includes latent heat of water vapor condensation. Assumes water condenses to liquid.
For fired heaters, LHV is used because flue gas temperature is always above water dew point (~60°C), so water remains as vapor and its latent heat is not recovered.
HHV = LHV + 2.442 × wH₂O
where wH₂O = mass of water formed per unit fuel (kg/kg)
Excess Air
Excess air is air supplied beyond the stoichiometric requirement to ensure complete combustion:
Excess Air (%) = [(Actual Air - Stoich. Air) / Stoich. Air] × 100
- Too low (<5%): Incomplete combustion, CO formation, soot, safety hazard
- Optimal (10-20%): Complete combustion with minimal stack loss
- Too high (>30%): Excess cold air increases stack losses and reduces efficiency
Flue Gas Calculations
Flue gas composition and flow rate are calculated from combustion stoichiometry:
Example: Natural Gas (CH₄) Combustion
CH₄ + 2O₂ → CO₂ + 2H₂O
Stoichiometric air requirement:
Air = 2 × (O₂/0.21) = 9.52 Nm³ air/Nm³ CH₄
With 15% excess air:
Actual air = 9.52 × 1.15 = 10.95 Nm³/Nm³
Flue gas temperature depends on heat balance, excess air, and heat recovery equipment. Typical values range from 150°C (with economizer) to 350°C (without).
Efficiency Optimization Strategies
| Strategy | Efficiency Gain | Implementation |
|---|---|---|
| Air Preheater | 5-10% | Preheat combustion air with flue gas |
| Economizer | 3-5% | Preheat feedstock or generate steam |
| Optimize Excess Air | 2-5% | Maintain 10-15%, use O₂ trim control |
| Improve Insulation | 1-3% | Repair/upgrade refractory and casing |
| Low NOₓ Burners | 0-2% | Reduce emissions while maintaining efficiency |
| Stack Damper Control | 1-2% | Optimize draft, minimize air infiltration |
Common Applications
Refining
- • Crude oil heating (300-400°C)
- • Vacuum unit heaters
- • Visbreaker heaters
- • Coker heaters
Petrochemicals
- • Steam cracking (800-900°C)
- • Reformer furnaces
- • Hydrogen plants
- • Ammonia synthesis
Chemical Processing
- • Hot oil systems
- • Thermal fluid heaters
- • Process reboilers
- • Reactor feed heating
Power & Utilities
- • Boiler feedwater heating
- • Thermal oxidizers
- • Waste heat recovery
- • Cogeneration systems
Design Considerations
Important: The calculator provides preliminary estimates using simplified models. Detailed design requires:
- Detailed zone-by-zone heat transfer calculations
- Tube wall temperature analysis to prevent coking or thermal stress
- Computational Fluid Dynamics (CFD) for burner placement and flame pattern
- Mechanical design per API 560 or ASME standards
- Process safety analysis (emergency shutdown, pressure relief)
- Emissions compliance (NOₓ, SOₓ, CO, particulates)
References
- API Standard 560, "Fired Heaters for General Refinery Service", 5th Edition, American Petroleum Institute (2016)
- Baukal, C.E., "The John Zink Hamworthy Combustion Handbook", 2nd Edition, CRC Press (2013)
- Ganapathy, V., "Industrial Boilers and Heat Recovery Steam Generators: Design, Applications, and Calculations", Marcel Dekker (2003)
- Hottel, H.C., Sarofim, A.F., "Radiative Transfer", McGraw-Hill (1967)
- Kern, D.Q., "Process Heat Transfer", McGraw-Hill (1950)
- Jones, J.C., "Combustion Science: Principles and Practice", Elsevier (2020)