A finned tube bundle is the primary heat transfer component of an air-cooled heat exchanger (ACHE). It consists of multiple rows of tubes with external fins attached to the outside surface, secured at each end by tube sheets, and connected to inlet and outlet header boxes. The bundle is installed within a structural bay, where ambient air is forced or drawn across the finned surfaces by axial or centrifugal fans, removing heat from the process fluid flowing inside the tubes.
This product is designed and manufactured per API 661 (Air-Cooled Heat Exchangers for General Refinery Service), ISO 13706, and TEMA Class R or B where applicable. The bundle is supplied as a complete assembly, including tubes, fins, tube sheets, header boxes, side frames, and lifting lugs, ready for installation into an existing or new ACHE bay.
The finned tube bundle is configured according to the ACHE type and site requirements:
Horizontal bundle: Tubes are horizontal; air flows vertically upward (forced draft) or downward (induced draft). Used in general process cooling, compressor intercoolers, and hydrocarbon condensers.
A-frame bundle: Two flat bundles inclined in an inverted V-shape (apex at top); fans below blow air upward through both sloped surfaces. Used in air-cooled steam condensers (ACC) and large petrochemical units.
V-frame bundle: Two flat bundles inclined in a V-shape (apex at bottom); fans above pull air upward. Used in steam condensing with induced draft for freeze protection.
Air flow arrangement:
Forced draft: Fans below the bundle push air across the tubes. Easier maintenance access; lower installed cost.
Induced draft: Fans above the bundle pull air across the tubes. Better air distribution; less recirculation; protects bundle from rain and snow.
Tubes
Tube OD (base): 15.88 mm – 50.8 mm (5/8" to 2")
Tube wall thickness: 1.2 mm – 5.0 mm (seamless or welded, per ASTM or equivalent)
Tube length (between tube sheets): 2.0 m – 12.0 m (per API 661 standard)
Tube pitch (center-to-center):
Transverse (across air flow): 1.5 * to 2.5 * tube OD
Longitudinal (along air flow): 1.5 * to 2.5 * tube OD
Tube arrangement: Staggered (triangular) or in-line (square). Staggered provides higher air-side heat transfer; in-line allows easier mechanical cleaning.
Tube Materials (Per Service Conditions)
| Material | Specification | Temperature Range | Chloride Limit | Application |
|---|---|---|---|---|
| Carbon steel SA-179 / SA-106 Gr.B | ASTM A179 / A106 | -20°C to +425°C | Not applicable | Water, oil, clean hydrocarbons, steam |
| Stainless 304L / 316L | ASTM A213 TP304L/316L | -196°C to +600°C | ≤ 200 ppm | Corrosive fluids, clean steam, food/pharma |
| Duplex 2205 | ASTM A789 S32205 | -40°C to +280°C | ≤ 300 ppm at 80°C | Chloride-containing hydrocarbons, offshore |
| Titanium Gr.2 (TA2) | ASTM B338 | -40°C to +230°C | Up to 20,000 ppm | Seawater, brine, high-chloride cooling |
| Copper-nickel C70600 | ASTM B111 | -40°C to +200°C | < 500 ppm | Marine cooling, brackish water |
Fins – Attachment Types and Parameters
The following fin-to-tube attachment methods are available, selected based on operating temperature, corrosion environment, and thermal cycling:
| Fin Type | Attachment Method | Max Operating Temp | Fin Materials | Typical Application |
|---|---|---|---|---|
| L-foot (tension wound) | L-shaped foot wound under tension | ≤ 150°C | Aluminum 1100, Copper C1100 | Low-temp gas cooling, intercoolers |
| KL-foot (knurled L) | Knurled tube surface + mechanically interlocked foot | ≤ 320°C | Aluminum 1100 | Medium-temp process gas, aftercoolers |
| LL-foot (overlapped L) | Overlapped foot fully covers tube OD | ≤ 177°C | Aluminum 1100, Copper C1100 | Coastal/marine atmosphere, corrosion protection |
| Extruded (bimetallic) | Outer aluminum sleeve extruded into fins | ≤ 230°C | Aluminum 1100 (outer) | Offshore, marine, high-corrosion environments |
| High-frequency welded | Fin strip resistance-welded to tube (metallurgical bond) | ≤ 450°C | Carbon steel, stainless steel | High-temp gas cooling, waste heat recovery |
| Bare (no fin) | — | Up to 600°C | As tube material | High-temp clean gas, steam, low fouling |
Fin Geometry (Typical per API 661)
| Parameter | L-foot / KL / LL | Extruded | High-frequency welded |
|---|---|---|---|
| Fin height (above tube OD) | 8 mm – 16 mm | 8 mm – 16 mm | 8 mm – 19 mm |
| Fin pitch (fins per inch – FPI) | 7 – 11.5 | 7 – 11 | 2 – 7 |
| Fin thickness | 0.25 mm – 0.5 mm | Integral (0.4 – 0.8 mm equivalent) | 0.8 mm – 3.2 mm |
| Surface area ratio (extended / bare) | 10 – 23 | 10 – 20 | 6 – 15 |
Fin Bond Resistance Consideration
For tension-wound fin types (L-foot, KL-foot, LL-foot), the mechanical bond between fin and tube introduces thermal contact resistance. Design compensation is required:
For tube-side temperatures above 120°C, a bond conductance value of 2,000 – 5,000 W/m²·K is typically applied for new bundles.
In-service bundles may have reduced bond conductance due to thermal cycling; additional surface area of 3% – 30% is often included in the design (depending on operating temperature and bond type).
Extruded and high-frequency welded fins have no bond resistance – the fin is integral or metallurgically bonded, providing full thermal contact.
Tube Sheets
Location: Front (inlet) and rear (outlet) ends of the bundle
Thickness: 15 mm – 80 mm, per ASME VIII-1 / TEMA RCB-4.3
Material: Carbon steel, stainless, or clad steel – matched to tube-side fluid and pressure
Hole diameter tolerance: H11 per ISO 286 (e.g., Ø25.2 mm +0.13/0)
Hole finish: Ra ≤ 1.6 μm for expanded joints; Ra ≤ 3.2 μm for welded joints
Tube retention: Tubes are expanded, welded, or combined (weld + expand) into tube sheets
Header Boxes (Inlet / Outlet Manifolds)
Types per API 661:
Plug header (threaded or bolted plugs) – for tube access; low to medium pressure
Cover-plate header – full bolted cover for complete tube access; used in fouling service
Pipe manifold header – welded construction; high pressure, no tube access
Construction: Rectangular or round cross-section; flanged connections per ASME B16.5 / B16.47
Pass partitions: Internal baffles to configure single, two, or four tube-side passes
Design pressure rating: 0.1 – 10.0 MPa (custom higher available)
Drain and vent connections: NPT threaded or socket-weld fittings at low/high points
Side Frames and Lifting Lugs
Structural frame: Steel channel or I-beam, designed to support bundle weight during lifting, transport, and operation
Lifting lugs: Welded to side frames or tube sheets – rated at 2* bundle weight (safety factor)
Support points: Located to minimise tube sag and maintain bundle levelness within ±3 mm over the entire length
| Parameter | Typical Range | Notes |
|---|---|---|
| Air face velocity (approach) | 1.5 – 4.0 m/s | Selected based on fan power and noise limits |
| Air mass velocity (through bundle) | 2 – 6 kg/m²·s | Based on free area between tubes and fins |
| Air-side convective HTC (bare tube equivalent) | 30 – 120 W/m²·K | Depends on fin geometry, velocity, and material |
| Fin efficiency | 40% – 90% | Calculated per standard methods (Kern / ESDU) |
| Overall HTC (air-to-tube, referenced to bare area) | 15 – 60 W/m²·K | Typical for ACHE finned bundles |
| Air-side pressure drop (across bundle) | 100 – 300 Pa | Per API 661 design limit; fan selected accordingly |
| Design ambient temperature | -40°C to +55°C | Based on site meteorological data |
| Bundle face area per unit | 10 – 200 m² | Single bay; larger duties use multiple bays |
Tube preparation: Tubes cut to length, ends beveled (if welded), OD and wall thickness verified.
Fin application: Fins attached to tubes per specified type (winding, extruding, or welding) – process parameters recorded (tension, temperature, welding current, rolling pressure).
Tube sheet drilling: CNC drilling of tube sheets to H11 tolerance; holes deburred and cleaned.
Tube insertion: Tubes inserted through front and rear tube sheets.
Tube-to-tubesheet joining: Expanded (hydraulic or roller), welded, or combined – per joint type. Pull-out test performed on first article.
Header box attachment: Headers welded or bolted to tube sheets; pass partitions installed.
Side frame assembly: Structural supports and lifting lugs attached.
Hydrostatic testing: Complete bundle tested at 1.3 * tube-side design pressure per ASME VIII-1 UG-99.
Painting and coating: External surfaces (carbon steel frames and headers) sandblasted to Sa2.5, primed with zinc-rich epoxy, and finished with polyurethane topcoat – minimum dry film thickness 250 μm.
Dimensional Inspection
| Item | Tolerance | Method |
|---|---|---|
| Tube OD | ±0.11 mm | Micrometer (sample) |
| Fin height | ±0.5 mm | Caliper / template |
| Fin pitch | ±0.2 mm per 100 mm length | Template / optical |
| Bundle overall length | ±1.5 mm | Tape measure / laser |
| Tube sheet hole pattern | Per drawing | CMM / optical comparator |
| Header flange orientation | Per ASME B16.5 | Template / protractor |
Non-Destructive Examination (NDE)
| Examination | Method | Scope | Acceptance |
|---|---|---|---|
| Tube-to-tubesheet welds | PT (liquid penetrant) | 100% (if welded) | No cracks, porosity ≤ 0.8 mm |
| Fin welds (HF-welded) | Visual + PT (sample) | 100% visual; 5% PT | No cracks, no loose fins |
| Header welds | RT or UT | Per ASME VIII-1 UW-51 / UW-52 | Per code |
| Tube internal condition | Borescope | 5% of tubes (random) | No scratches > 0.1 mm, no blockages |
Hydrostatic Test (Tube Side)
Test pressure: 1.3 * design pressure * (S at test temp / S at design temp) per ASME VIII-1 UG-99
Test medium: Clean water with corrosion inhibitor (chloride ≤ 50 ppm for stainless steel)
Hold time: ≥ 30 minutes
Acceptance: Zero pressure drop; no visible leakage from tube joints, headers, or plugs
Pneumatic Leak Test (If Specified)
Test pressure: 0.6 MPa air or nitrogen
Leak detection: Soap bubble application on all joints and welds
Acceptance: No bubbles; leakage rate ≤ 1*10⁻⁵ Pa·m³/s per equivalent orifice method
Compressor Intercooler / Aftercooler
Tube-side fluid: Compressed air or natural gas (after compression)
Tube-side temp: Inlet 120°C – 180°C, outlet 40°C – 60°C
Pressure: 0.5 – 10.0 MPa
Tube material: Carbon steel (air) / 304L (natural gas with CO₂/H₂O)
Fin type: L-foot (≤ 150°C) or KL-foot (150–180°C)
Fin geometry: OD 25.4 mm * 2.0 mm wall; fin height 12.5 mm; 10 FPI
Bundle rows: 4 – 6 rows
Air face velocity: 2.5 – 3.5 m/s
Air-side ΔP: ≤ 200 Pa
Refinery Overhead Condenser
Tube-side fluid: Hydrocarbon vapor + steam (distillation overhead)
Tube-side temp: Inlet 100°C – 150°C (condensing), outlet 40°C – 60°C
Pressure: 0.1 – 0.5 MPa (vacuum to near-atmospheric)
Tube material: Carbon steel (non-corrosive) or 316L (sour service)
Fin type: L-foot or KL-foot (Al 1100) – for H₂S service, extruded with 316L tube
Fin geometry: OD 25.4 mm * 2.0 mm; fin height 12.5 mm; 10 FPI
Bundle rows: 4 rows; sloped 1:50 toward outlet for liquid drainage
Natural Gas Cooler (After Dehydration or Compression)
Tube-side fluid: Treated natural gas
Tube-side temp: Inlet 60°C – 100°C, outlet 30°C – 45°C
Pressure: 4.0 – 10.0 MPa
Tube material: 316L or Duplex 2205 (if chlorides present)
Fin type: Extruded (Al 1100 outer) – for offshore/coastal corrosion protection
Fin geometry: OD 19.05 mm * 2.0 mm; fin height 12.5 mm; 10 FPI
Bundle rows: 2 – 3 rows (low pressure drop)
Air face velocity: 2.0 – 2.5 m/s
Air-side ΔP: ≤ 150 Pa
Air-Cooled Steam Condenser (ACC) – Power Plant
Tube-side fluid: Steam (turbine exhaust) condensing to water, vacuum condition
Tube-side temp: Inlet 60°C – 100°C, outlet 35°C – 45°C
Pressure: Vacuum (10 – 50 kPa absolute) – external pressure design per ASME VIII-1 UG-28
Tube material: Carbon steel (galvanized or coated) or stainless steel (if ammonia/chlorides)
Fin type: High-frequency welded (metallurgical bond) – hot-dip galvanized carbon steel fins
Tube geometry: Elliptical or flat-oval tubes (reduce air-side ΔP) with plate fins or helically welded fins
Bundle orientation: A-frame or V-frame (inclined 60° to horizontal)
Air flow: Induced draft (fans above)
Special features: Bundles sloped 1:80 toward condensate collection header; freeze protection (steam tracing or electric heating for winter operation below 0°C)
Air-side fouling (dust, pollen, chemical deposits) increases pressure drop and reduces heat transfer. Cleaning is recommended when:
Air-side ΔP exceeds design ΔP by 30% (per API 661)
Air-side ΔP increase reaches 50 Pa above start-of-run (whichever occurs first)
Cleaning methods:
Water wash: Low-pressure spray (≤ 1.0 MPa) for water-soluble deposits
Air blow: Compressed air (≤ 0.6 MPa) for dry loose dust
Chemical cleaning: Alkaline or acid-based detergent (approved for aluminum fins; avoid high-alkali solutions)
Tube-side cleaning: Not mechanically feasible in a fixed bundle (tubes are not individually removable). Chemical cleaning only, or use of on-line pigging systems (for larger-diameter tubes with smooth bends).
Material test certificates (EN 10204 3.1 or 3.2) – tubes, fins, tube sheets, headers, structural steel
API 661 completed datasheet (or ISO 13706 datasheet)
TEMA datasheet (Class R/B – if applicable)
ASME U-stamp data report (if applicable)
Dimensional inspection report (bundle length, tube sheet hole layout, fin geometry, tube OD)
Fin attachment process parameters (winding tension, extrusion pressure, welding current)
Hydrostatic test report (with pressure chart recording)
Pneumatic leak test report (if performed)
NDE reports – PT, UT, RT as performed
Weld procedure specification (WPS) and qualification record (PQR) – for tube-to-tubesheet and header welds
Tube bundle as-built drawing – with tube count, pass configuration, header dimensions, and lifting lug locations
Painting/coating specification and thickness verification report
Cleaning and maintenance instructions (if requested)
ACHE type – Forced draft / Induced draft / A-frame / V-frame / Horizontal
Bundle orientation – Horizontal / Sloped (angle)
Tube material and size (OD, wall thickness)
Fin type – L-foot / KL / LL / Extruded / HF-welded / Bare
Fin material and geometry (height, pitch/FPI, thickness)
Tube-side design pressure and temperature
Tube-side fluid composition (corrosivity, chlorides, H₂S)
Tube-side allowable pressure drop
Air inlet temperature (summer design)
Air face velocity or total air flow
Air-side allowable pressure drop (typically 100 – 300 Pa)
Header type – Plug / Cover-plate / Pipe manifold
Number of tube-side passes – Single / Two / Four
Tube count and tube layout (per existing or new bay)
Corrosion protection requirements (coating, galvanizing, or special fin/tube materials)
Freeze protection (for ACC service) – steam tracing, electric heating, or recirculation provisions
Lifting and transport constraints (maximum weight and dimensions)
Applicable standards – API 661 / ISO 13706 / TEMA / Customer specification
The finned tube bundle is subject to the following inherent limitations:
It is designed only for the specified tube-side pressure and temperature; operation outside these limits voids the design.
Air-side performance is dependent on ambient air temperature and fouling; cooling capacity decreases with increasing ambient temperature.
Tube-side cleaning is not mechanically feasible in a fixed bundle – chemical cleaning or on-line pigging is required for fouling services.
Air-side fouling increases pressure drop and reduces heat transfer; regular cleaning schedules must be maintained.
For finned bundles operating in freezing conditions, proper freeze protection (sloping, drainage, and heating) must be provided for condensing services.
Maximum fin tip temperature must not exceed the material-specific limits (e.g., L-foot ≤ 150°C; extruded ≤ 230°C; HF-welded ≤ 450°C).
For thermal cycling service (> 500 cycles/year), tension-wound fin types (L, KL, LL) are not recommended – use extruded or HF-welded fins.
The bundle must be installed on a level support structure; excessive misalignment causes tube sheet distortion and joint leakage.
For very high tube-side pressure (> 10.0 MPa) or extreme vacuum, consult for custom header and tube sheet design.
