API 661 Finned Tube Bundle for Air-Cooled Heat Exchangers – Forced & Induced Draft Configurations for Refinery and Petrochemical Service
Yuhong Group Co., Ltd. engineers and supplies complete finned tube bundles for air-cooled heat exchangers (ACHEs) under API Standard 661, ASME Section VIII Division 1, and ISO 13706. Our product portfolio includes shell and tube heat exchangers, pressure vessels, U-tubes, finned tubes, tubesheets, baffles, and heads. All products are fabricated under ASME U-Stamp certification and comply with international classification society requirements.
Product Overview
The finned tube bundle is the core heat transfer element of an air-cooled heat exchanger. Ambient air is forced or induced across the bundle to remove heat from process fluid flowing inside the tubes. Fins are applied to the tube outside surface to compensate for the low convective heat transfer coefficient of air--typically 10-60 W/m²*K--compared to liquid or condensing fluids inside the tubes.
This bundle is designed and manufactured per API 661 (Air-Cooled Heat Exchangers for General Refinery Service), with thermal design following the API 661 framework. Design temperature ranges from -40°C to +450°C, with design pressure up to 10.0 MPa. The bundle is applicable to both forced-draft and induced-draft configurations, with horizontal tube arrangements as the standard configuration.
Key Components and Material Specifications
| Component |
Material / Standard |
Key Specifications |
| Base Tubes |
ASME SA179 seamless cold-drawn low-carbon steel |
OD: 16-63 mm; wall thickness per ASME SA179 minimum-wall requirements |
| Base Tubes (alternative) |
ASTM A106 Gr.B / A213 TP304/316/321 / Duplex 2205 / Monel 400 / Inconel 625 |
Per ASME SA / ASTM standards |
| Fins |
Aluminum 1060 (Al1060) |
Fin height: 8-16 mm; fin pitch: 7-11.5 FPI; fin thickness: ~0.4 mm |
| Fin Bonding Types |
Embedded (G-type), extruded, L-foot, welded |
Max service temperature: embedded 400°C (750°F); welded >400°C |
| Tube Sheets |
Carbon steel / alloy steel |
ASME Sec. VIII Div. 1 compliant |
| Headers / Manifolds |
Carbon steel (coated) or stainless steel |
Designed per API 661; plug-type or cover-plate configuration |
| Structural Frame |
Hot-dip galvanized or stainless steel |
Supports wind, seismic, and operational loads |
ASME SA179 Base Tube Properties
Tensile ≥325 MPa, Yield ≥180 MPa, Elongation ≥35%, Hardness ≤72 HRB.
Al1060 Fin Properties
Tensile ≥90 MPa, Yield ≥35 MPa, Elongation ≥25%, Thermal conductivity ~237 W/m*K, Max continuous operating temperature 285°C (545°F).
Testing and Quality Assurance
All finned tube bundles undergo inspection and testing in accordance with ASME Code requirements and API 661 guidelines:
| Test Type |
Standard / Method |
Acceptance Criteria |
| Hydrostatic Test |
ASME Sec. VIII Div. 1 |
1.5 * design pressure; no leakage |
| Eddy Current Testing (ECT) |
ASME SA179 / ASTM A179 |
Detects surface and subsurface defects |
| Radiography (RT) |
ASME Sec. VIII |
Critical weld examination |
| Dye Penetrant (PT) / Magnetic Particle (MT) |
ASME Sec. V |
Surface defect detection |
| Dimensional Inspection |
Per design drawings |
Tube alignment, fin spacing, tube OD, fin height, fin pitch |
| Flattening & Flaring Tests |
ASME SA179 |
Verifies ductility and workability |
| Pull-Out Resistance |
In-house per ASME guidelines |
≥70 N (fin-to-tube bond integrity) |
Applications
API 661 finned tube bundles are deployed across the petroleum, petrochemical, and natural gas industries:
- Crude Distillation Units (CDU) - Cooling of distillates, fuel oil residue, and side distillate streams
- Hydrotreating Units (HDS, VGO HDT) - Cooling of reactor effluent containing H₂S and NH₄HS
- Hydrocracking Units - Cooling of high-pressure post-reactor streams (sour service)
- Fluid Catalytic Cracking (FCC) Units - Cooling of gas and heavy fractions, condensation of light hydrocarbons
- Reforming Units (CCR, semi-regenerative) - Cooling of stabilized reformate and hydrogen-rich gas
- Gas Fractionation Units (GFU) - Cooling of ethane, propane, and butane with partial condensation
- Compressor Stations - Cooling of compressed gas (operating pressure up to 50 bar)
- LNG Plants - Pre-cooling of natural gas prior to deep dehydration
- Power Generation - Cooling of turbine and process streams
Technical Advantages and Customization
Thermal Performance - Parameter-Driven
Air-side heat transfer coefficients are calculated per the Briggs-Young fin-tube correlation, with LMTD correction for cross-flow applied per TEMA. Typical air face velocity ranges from 1.5 to 4.0 m/s, selected based on fan power limits and bundle geometry.
Material Selection Flexibility
Tube materials can be specified from carbon steel (SA179, SA106B) through stainless steels (TP304, TP316L, TP321), duplex (2205), and nickel alloys (Monel 400, Inconel 625) to match process corrosivity and temperature requirements.
Bundle Configuration Options
- Single-pass or multi-pass (1 to 4 passes) tube side arrangements
- Forced draft or induced draft fan configurations
- Plug-type or cover-plate header designs
- Horizontal, A-frame, or V-frame bundle orientations
Corrosion Protection
- Minimum corrosion allowance: 1.5 mm (1/16 in) unless otherwise specified
- Structural supports: hot-dip galvanized or epoxy-coated carbon steel
- Fin ends protected with zinc or aluminum metallized coating per API 661
Sour Service Compliance
For hydrotreating and hydrocracking reactor effluent air coolers, materials comply with NACE MR0175 / ISO 15156 (SSCC/HIC).
Frequently Asked Questions
Q: What is the maximum design temperature for embedded G-type finned tubes?
A: Embedded (G-type) fins can operate continuously up to 400°C (750°F). Extruded fins are rated to 300°C (570°F), and L-footed fins to 130°C (270°F).
Q: What hydrostatic test pressure is applied to API 661 finned tube bundles?
A: Hydrostatic testing is performed at 1.5 * design pressure for both tubes and headers, with no leakage permitted.
Q: What NDT methods are typically specified for API 661 bundles?
A: Radiography (RT) for critical welds, dye penetrant (PT) or magnetic particle (MT) for surface defects, and eddy current testing (ECT) for tube integrity.
Q: What is the difference between forced draft and induced draft configurations?
A: Forced draft places fans beneath the bundle for easier maintenance access. Induced draft positions fans above the bundle, reducing hot-air recirculation risk and protecting the bundle from direct sunlight.
Q: Can the bundle be designed for sour service (wet H₂S)?
A: Yes. Materials for sour service are selected per NACE MR0175 / ISO 15156, with appropriate hardness control and resistance to sulfide stress corrosion cracking (SSCC) and hydrogen-induced cracking (HIC).
Q: What is the typical fin pitch range for air-cooled heat exchanger bundles?
A: Fin pitch typically ranges from 7 to 11.5 fins per inch (FPI), selected based on thermal duty, air-side pressure drop, and fouling propensity.
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