Fired Heater Tube Coil with Design Temp Up to 590°C and Design Pressure Up to 35

Product Specification

 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

Material Selection – Tube Coil Materials (Per API 560 Table 5)

Carbon Steels

Chromium-Molybdenum Alloy Steels

Austenitic Stainless Steels

Nickel Alloys

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:

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):

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

1. Tube preparation: Tubes cut to length; ends beveled per ASME B16.25 for butt welding

2. Return bend attachment: Return bends welded to tube ends (GTAW root + SMAW fill) per ASME Section IX

3. Fin/stud welding: Fins or studs welded to convection tubes (high-frequency welding or resistance welding)

4. Coil assembly: Tubes assembled into rows and secured to supports

5. Header attachment: Headers welded to coil inlet/outlet

6. PWHT: Required for Cr-Mo alloys and thick-wall carbon steel sections (per ASME B31.3 / API 560)

7. Hydrostatic testing: 1.5 × design pressure (per API 560 / ASME B31.3)

8. Refractory/insulation application: Applied to casing and tube supports (field-installed)

Inspection and Testing (Per API 560 / ASME B31.3)

Non-Destructive Examination (NDE)

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

1. Heater type – Cylindrical / Box / Double-fired / Reformer / Cracking

2. Coil section – Radiant / Convection / Both

3. Tube OD and wall thickness – Per API 560 / ASME B31.3

4. Tube material grade – CS / Cr-Mo / SS / Alloy – per API 560 Table 5

5. Design pressure and temperature – Per API 560 / ASME B31.3

6. Coil configuration – Serpentine / Helical / Single-pass / Multi-pass

7. Number of passes – Determines return bend quantity and layout

8. Extended surface required – Bare / Finned / Studded – specify fin/stud parameters

9. Return bend type – Short radius (2× tube OD) / Long radius (3× tube OD)

10. Header type – Plug / Cover-plate / Pipe manifold

11. Corrosion allowance – Per API 560 7.1.2

12. PWHT requirement – Based on material and thickness

13. NDE requirement – Per API 560 or project specification

14. 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