1. Overview & Heater Types
A fired heater (also called a process furnace) transfers heat from combustion of fuel gas to a process fluid flowing through tubes. In midstream gas processing, fired heaters serve multiple roles: inlet gas heating, reboiler duty for amine and glycol regenerators, condensate stabilizer reboilers, and hot oil heating systems.
1.1 Heater Types
| Type | Configuration | Duty Range | Application |
|---|---|---|---|
| Cabin (Box) | Rectangular firebox, tubes along walls | 5–200 MMBTU/hr | General process heating, reboilers |
| Cylindrical (Vertical) | Circular firebox, tubes around wall | 1–50 MMBTU/hr | Gas processing, wellhead heaters |
| Cabin (Box) with bridge wall | Divided firebox for multi-zone | 50–500 MMBTU/hr | Refinery crude and vacuum heaters |
1.2 Major Components
- Radiant section (firebox): Contains burners and radiant tubes. Heat transfer is primarily by radiation.
- Convection section: Flue gas passes over tube banks before exiting to stack. Heat transfer is by convection.
- Stack: Chimney providing natural draft or containing induced-draft fan.
- Burners: Fuel combustion devices — natural draft, forced draft, or premix types.
- Tube coils: Process fluid tubes — radiant coil (in firebox) and convection/shield tubes.
2. Radiant Section Design
The radiant section absorbs 60-75% of total heat duty. Heat transfer is dominated by radiation from the flame and hot flue gases to the tube surfaces lining the firebox walls.
2.1 Lobo-Evans Method
The industry-standard approach for radiant section design. It relates firebox heat transfer to gas temperature, tube wall temperature, and geometry through an exchange factor:
σ = Stefan-Boltzmann constant (0.1713 × 10⁻⁸ BTU/hr-ft²-°R⁴)
α = tube absorptivity (~0.9 for oxidized steel)
Acp = cold plane area of tube bank
F = exchange factor (geometry + gas emissivity)
Tg, Tw = gas and wall temperatures (°R)
2.2 Heat Flux Limits
API 560 specifies maximum average radiant flux to prevent tube damage:
| Service | Max Avg Flux (BTU/hr-ft²) | Max Peak Flux |
|---|---|---|
| Gas heating | 10,000 | ~1.5× average |
| Oil heating | 10,000 | ~1.5× average |
| Boiling / vaporizing | 8,000 | ~1.5× average |
| Steam / water | 12,000 | ~1.5× average |
3. Convection Section Design
The convection section recovers additional heat from the flue gas after it leaves the radiant section. Hot flue gas at the bridgewall temperature (typically 1400-1800°F) flows across multiple rows of tubes before exiting to the stack at 350-500°F.
3.1 Heat Transfer
U = overall heat transfer coefficient (3-6 BTU/hr-ft²-°F for bare tubes)
A = total outside surface area of convection tubes
LMTD = log-mean temperature difference
3.2 Shield Tubes
The first 1-2 rows of convection tubes (shield tubes or shock tubes) see direct radiation from the firebox and experience higher heat flux. These tubes are typically bare (unfinned) and use higher alloy materials.
3.3 Extended Surface
Downstream convection tubes commonly use finned surfaces to increase the effective area for gas-side heat transfer. Typical fin configurations include studded tubes, serrated fins, and solid fins. Extended surface can increase effective area by 5-10× versus bare tube.
4. Tube Design per API 530
API 530 provides the methodology for calculating heater tube thickness based on operating conditions, material properties at temperature, and design life considerations.
4.1 Tube Wall Thickness
P = design pressure (psig)
Do = tube outside diameter (inches)
Sa = allowable stress at design metal temperature (psi)
CA = corrosion allowance (typically 0.050")
4.2 Tube Wall Temperature
Tfluid = bulk process fluid temperature
q = local heat flux (BTU/hr-ft²)
hi = inside film coefficient (BTU/hr-ft²-°F)
t = tube wall thickness (ft)
k = tube thermal conductivity (BTU/hr-ft-°F)
4.3 Material Selection
| Material | Max Service Temp | Typical Application |
|---|---|---|
| Carbon Steel (SA-106 Gr B) | 800°F | General service, low-temp gas heating |
| 5Cr-½Mo (SA-213 T5) | 1050°F | Moderate temperature, sulfur service |
| 9Cr-1Mo (SA-213 T9) | 1100°F | High temperature, sulfur resistance |
| 304 SS (SA-213 TP304) | 1500°F | High temperature, corrosive service |
| 316 SS (SA-213 TP316) | 1500°F | High temperature, chloride resistance |
5. Thermal Efficiency & Fuel Consumption
Thermal efficiency is the ratio of heat absorbed by the process fluid to the total heat released by fuel combustion. Losses include stack gas sensible heat, casing radiation, and unburned fuel.
ηgross = efficiency based on higher heating value (HHV)
ηnet = efficiency based on lower heating value (LHV)
5.1 Major Heat Losses
| Loss Component | Typical % | How to Minimize |
|---|---|---|
| Stack gas sensible heat | 5–25% | Lower stack temp, reduce excess air |
| Casing radiation/convection | 1.5–2% | Better insulation |
| Unburned fuel | 0.5% | Proper burner maintenance |
| Moisture in fuel (HHV vs LHV) | 8–10% | Inherent — use LHV for true comparison |
References
- API 560 — Fired Heaters for General Refinery Service
- API 530 — Calculation of Heater-Tube Thickness in Petroleum Refineries
- GPSA, Chapter 8 (Heat Exchangers and Heaters)
- ASME Section II Part D — Material Properties
- Lobo & Evans — Heat Transfer in the Radiant Section of Petroleum Heaters