Tube Coil as the Primary Heat Transfer Surface
A fired heater tube coil is the continuous tubular assembly through which the process fluid flows as it is heated by combustion gases within a fired heater. The tube coil is the primary heat transfer surface of the fired heater, absorbing thermal energy from flame radiation (radiant section) and hot flue gas convection (convection section).
The fired heater is a direct-fired heating device in which the process fluid flows inside the tubes while fuel combustion occurs external to the tubes. The tube coil is the critical pressure-containing component of the fired heater and is designed and manufactured per API 560 (Fired Heaters for General Refinery Service), ASME B31.3 (Process Piping), and GB/T 16507 (Water-Tube Boilers) for domestic applications.
Fired Heater Sections – Coil Configuration by Location
Radiant Section Coil
The radiant section (firebox) is where the process fluid receives the majority of its heat duty through direct flame radiation. Tubes in the radiant section are exposed to the highest heat flux and tube metal temperatures.
Tube arrangement: Vertical (helical or serpentine) or horizontal (single or multi-row)
Tube support: Radiant tubes are supported by tube hangers, guide supports, or refractory anchors
Heat flux: Radiant heat transfer predominates; tube metal temperature is typically 50°C–100°C above the process fluid outlet temperature
Common configurations:
Helical coil: Tubes spiraling around the firebox circumference; common in cylindrical heaters
Serpentine coil: Tubes arranged in multiple horizontal or vertical passes with return bends connecting adjacent tubes
Single-pass coil: Each tube runs from inlet to outlet without intermediate bends
Multi-pass coil: Process fluid makes multiple passes through the radiant section via return bends
Convection Section Coil
The convection section is located above the radiant section, where heat transfer occurs primarily by forced convection from hot flue gases. Tubes in the convection section operate at lower heat flux and tube metal temperatures.
Tube arrangement: Horizontal or vertical rows with staggered or in-line tube pitch
Extended surface: Convection tubes are often provided with fins or studs to increase heat transfer area
Shield tubes: The first 2–3 rows of convection tubes exposed to radiant heat from the firebox are typically bare (unfinned) to prevent fin damage from high-temperature exposure
Common tube types: Bare tube, finned tube, studded tube (pin tube)
Transition / Crossover Piping
Crossover piping connects the convection section outlet to the radiant section inlet (or vice versa), typically located outside the heater casing. Crossover piping must accommodate thermal expansion between sections.
Coil Design Parameters – Per API 560 / ASME B31.3
| Parameter | Range | Standard / Note |
|---|---|---|
| Tube OD | 50.8mm – 273.1mm (2" – 10.75") | Per API 560 Table 4 |
| Tube wall thickness | 3.0mm – 25.0mm | Per API 530 (tube thickness calculation) |
| Tube length (per straight section) | 3.0m – 15.0m | Limited by transport and handling |
| Design pressure | Up to 35 MPa | Per ASME B31.3 / API 560 |
| Design temperature (tube metal) | Up to 590°C | Material dependent; Cr-Mo for > 425°C |
| Corrosion allowance | Per API 560 7.1.2 | Minimum per purchaser specification |
| Tube pitch (center-to-center) | 2× tube OD (standard short radius) | Per API 560 Table 4 |
| Maximum allowable heat flux | Per API 560 / radiant section design | Determines tube metal temperature |
Material Selection – Tube Coil Materials (Per API 560 Table 5)
Carbon Steels
| Grade | Spec | Max Temp | Application |
|---|---|---|---|
| SA-106 Gr.B / SA-192 | ASTM A106 / A192 | ~425°C | General refinery service, moderate temperature |
| SA-210 Gr.A1 | ASTM A210 | ~425°C | Boiler and heater tube service |
| 20# | GB 9948 | ~425°C | Domestic standard for petroleum cracking service |
Chromium-Molybdenum Alloy Steels
| Grade | Composition | Max Temp | Application |
|---|---|---|---|
| SA-209 T1a | 0.5Cr-0.5Mo | ~540°C | Moderate elevated temperature |
| SA-213 T11 | 1.25Cr-0.5Mo | ~590°C | Refinery heater tubes; moderate creep resistance |
| SA-213 T22 | 2.25Cr-1Mo | ~590°C | Hydrocracker, reformer, high-temp/high-pressure |
| SA-213 T5 | 5Cr-0.5Mo | ~590°C | High-temperature corrosive service |
| SA-213 T9 | 9Cr-1Mo | ~590°C | High-temperature erosive service |
| SA-213 T91 | 9Cr-1Mo-V | ~590°C | Advanced high-temperature creep service |
Austenitic Stainless Steels
| Grade | Max Temp | Application |
|---|---|---|
| TP304H / TP321H | ~600°C | Elevated temperature, corrosion resistance, creep resistance |
| TP316L | ~450°C | Chloride-containing service (moderate) |
| TP347H | ~600°C | High-temperature creep resistance; stabilized grade |
| SUS310S | ~700°C | Extreme high-temperature oxidation resistance |
Nickel Alloys
| Grade | Max Temp | Application |
|---|---|---|
| Alloy 800H / 800HT | ~600°C | High-temperature corrosive service; sour gas |
Coil Configuration – Tube Supports and Guides
Per API 560, tube supports are required to maintain tube alignment and prevent sagging or vibration:
Radiant section: Tube hangers (for vertical tubes), guide supports (for horizontal tubes), or refractory anchors
Convection section: Support plates or tube sheets with drilled holes to accept tubes
Support spacing: Determined by tube size, material, and operating temperature – typically 1.5m – 3.0m
Tube guides: Allow axial thermal expansion while restraining lateral movement
Thermal Expansion Management
Fired heater coils experience significant thermal expansion due to temperature differentials between start-up and operating conditions
Expansion is accommodated through:
Return bends: U-shaped fittings that absorb differential expansion between adjacent tubes
Expansion loops: Piping configurations that absorb axial expansion
Guided supports: Allow tubes to slide axially while maintaining alignment
Floating manifolds: Headers that move with the coil to absorb expansion
Extended Surface – Fins and Studs (Convection Section)
To increase heat transfer in the convection section, tubes are often provided with extended surface:
| Extended Surface Type | Description | Application |
|---|---|---|
| Finned tube | Helical fins welded or extruded onto the tube OD | Convection section; increases surface area by 5–10× |
| Studded tube (pin tube) | Studs (pins) welded to tube surface | Fouling service; allows soot blowing without fin damage |
| Serrated fin | Slit or serrated fin for higher heat transfer coefficient | High-efficiency convection service |
Typical fin/stud parameters:
Fin height: 12mm – 25mm
Fin pitch: 4 – 12 fins per inch (FPI)
Fin material: Carbon steel, stainless steel, or alloy steel
Stud material: Typically same as tube material or compatible alloy
Return Bends – Coil Connections
Return bends connect adjacent tubes in multi-pass coils, reversing flow direction by 180°:
Location: Inside the firebox (radiant section) or inside a header box (insulated compartment outside the firebox)
Design pressure and temperature: Return bends inside firebox – same as connecting tubes; inside header box – same design pressure, fluid temperature +30°C
Thickness: At least the same thickness as the connecting tubes
Fittings: Longitudinally welded fittings shall not be used
Standards: SH/T 3065 (Petrochemical Tubular Heater Return Bends), ASME B16.9
Return bend center-to-center dimensions (per API 560 Table 4):
| Tube OD (mm) | Header Center-to-Center (mm) |
|---|---|
| 60.3 | 101.6 |
| 73.0 | 127.0 |
| 88.9 | 152.4 |
| 101.6 | 177.8 |
| 114.3 | 203.2 |
| 127.0 | 228.6 |
| 141.3 | 254.0 |
| 152.4 | 279.4 |
| 168.3 | 304.8 |
| 193.7 | 355.6 |
| 219.1 | 406.4 |
| 273.1 | 508.0 |
Headers and Manifolds
Headers (inlet and outlet manifolds) distribute process fluid to and from the tube coil:
Plug headers: Removable plugs for tube access; used for clean service
Cover-plate headers: Bolted cover for full tube access; used for fouling service
Pipe manifold headers: Welded construction; used for high-pressure service
Design pressure: Same as connecting tubes
Design temperature: Fluid temperature at that location +30°C for header box installation
Fabrication Process – Tube Coil Assembly
Tube preparation: Tubes cut to length; ends beveled per ASME B16.25 for butt welding
Return bend attachment: Return bends welded to tube ends (GTAW root + SMAW fill) per ASME Section IX
Fin/stud welding: Fins or studs welded to convection tubes (high-frequency welding or resistance welding)
Coil assembly: Tubes assembled into rows and secured to supports
Header attachment: Headers welded to coil inlet/outlet
PWHT: Required for Cr-Mo alloys and thick-wall carbon steel sections (per ASME B31.3 / API 560)
Hydrostatic testing: 1.5 × design pressure (per API 560 / ASME B31.3)
Refractory/insulation application: Applied to casing and tube supports (field-installed)
Inspection and Testing (Per API 560 / ASME B31.3)
Non-Destructive Examination (NDE)
| Examination | Method | Scope | Acceptance Criteria |
|---|---|---|---|
| Tube surface | PT or MT | 100% of welds and bends | Per ASME / API 560 |
| Butt welds | RT or UT | 100% (per API 560 / ASME B31.3) | Per ASME Section VIII / B31.3 |
| Return bend welds | PT + RT | 100% | Per SH/T 3065 / ASME B16.9 |
| Fin/stud welds | Visual + pull test | Sample per batch | No cracks; pull strength ≥ specified |
| Dimensional | Template / CMM | Per coil drawing | Per ASME B16.9 / SH/T 3065 |
| PMI | XRF spectrometer | 100% of material lots | Matches material certificate |
Hydrostatic Test
Test pressure: 1.5 × design pressure (per API 560 / ASME B31.3)
Test medium: Clean water with corrosion inhibitor
Hold time: ≥ 30 minutes
Acceptance: Zero pressure drop; no visible leakage at any weld or fitting
Pneumatic Leak Test (if specified)
Test pressure: 0.6 MPa air or nitrogen
Leak detection: Soap bubble or helium mass spectrometry
Acceptance: No bubbles; leakage rate ≤ 1×10⁻⁵ Pa·m³/s
Application Profiles – Coil Configurations by Heater Type
Cylindrical Heater (Vertical)
Coil type: Helical (spiral) radiant coil + horizontal convection coil
Tube arrangement: Vertical tubes in concentric circles; return bends at top and bottom
Application: Small to medium refinery heaters; reboilers; process heaters
Box Heater (Horizontal)
Coil type: Serpentine radiant coil (horizontal or vertical passes) + horizontal convection coil
Tube arrangement: Tubes in multiple rows; return bends in header boxes outside firebox
Application: Large refinery heaters; crude heaters; vacuum heaters
Double-Fired / Multi-cell Heater
Coil type: Dual radiant sections sharing a common convection section
Tube arrangement: Tubes fired from both sides
Application: High-capacity refinery heaters; reformer furnaces
Specialty Heaters
Reformer furnace: Catalyst-filled tubes (vertical) with U-bends; high-temperature alloy materials
Ethylene cracking furnace: High-alloy tubes with specialized coil geometry
Hydrogen production furnace: Reformer tubes with catalyst; high-pressure hydrogen service
Documentation per Shipment
Material test certificates (EN 10204 3.1 or 3.2) – tubes, fittings, headers, fins/studs
API 560 / ASME B31.3 datasheet
Dimensional inspection report (tube OD, wall thickness, pitch, bend radius, center-to-center)
NDE reports – PT/MT/RT/UT/PMI as applicable
Hydrostatic test report (with pressure and time records)
Pneumatic leak test report (if performed)
Weld procedure specification (WPS) and procedure qualification record (PQR)
PWHT chart (time-temperature recording) – if performed
Coil as-built drawing – with tube count, pass configuration, header dimensions
Fin/stud welding procedure and pull test records (if finned/studded)
Selection Checklist – Fired Heater Tube Coil
Heater type – Cylindrical / Box / Double-fired / Reformer / Cracking
Coil section – Radiant / Convection / Both
Tube OD and wall thickness – Per API 560 / ASME B31.3
Tube material grade – CS / Cr-Mo / SS / Alloy – per API 560 Table 5
Design pressure and temperature – Per API 560 / ASME B31.3
Coil configuration – Serpentine / Helical / Single-pass / Multi-pass
Number of passes – Determines return bend quantity and layout
Extended surface required – Bare / Finned / Studded – specify fin/stud parameters
Return bend type – Short radius (2× tube OD) / Long radius (3× tube OD)
Header type – Plug / Cover-plate / Pipe manifold
Corrosion allowance – Per API 560 7.1.2
PWHT requirement – Based on material and thickness
NDE requirement – Per API 560 or project specification
Cleaning method – Steam-air decoking / Mechanical pigging – affects coil design
Design Limitation Statement – Fired Heater Tube Coil
The fired heater tube coil is subject to the following limitations:
It is rated only for the design pressure and temperature specified on the datasheet; operation outside these limits voids the design
Tube metal temperature must not exceed the material's maximum allowable service temperature at the specified design pressure
Longitudinally welded fittings (return bends) shall not be used per API 560 8.3.4
Coil must be designed for thermal expansion; inadequate expansion accommodation leads to tube distortion, support damage, or weld failure
Not suitable for fluids with high solids content (> 2% by weight) without erosion-resistant material selection or increased wall thickness
Finned/studded tubes in convection sections require soot blowing or cleaning to maintain performance; fouling increases pressure drop and reduces heat transfer
Coil replacement requires careful match to existing heater geometry (tube pitch, pass configuration, header dimensions) – field modifications may require re-rating
