Overview – Air Cooler Tube Bundle as the Heat Transfer Core
An air cooler tube bundle is the heat transfer core assembly of an air-cooled heat exchanger (ACHE). It consists of rows of bare or finned tubes, fixed at each end by tube sheets, and connected to inlet and outlet header boxes. The bundle is installed within a structural bay, where axial or centrifugal fans force or draw ambient air across the external tube surface to cool 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. It is supplied as a replacement bundle, spare bundle, or complete assembly for a new ACHE bay.
Air Cooler Types – Bundle Orientation and Fan Configuration
The tube bundle is the same core assembly across ACHE types; orientation, support structure, and air flow direction vary by configuration.
Forced Draft
Fans located below the bundle, pushing air upward through the tube rows
Air distribution is uniform due to the fan plenum below
Fan and drive components are accessible at ground or platform level
Bundle orientation: Horizontal
Typical application: General refinery process cooling, compressor intercoolers
Induced Draft
Fans located above the bundle, pulling air upward through the tube rows
Plenum chamber above the bundle ensures uniform suction
Fan and drive components require an elevated access platform
Bundle orientation: Horizontal
Typical application: Steam condensers, high-temperature services, large refinery units
Advantage: Better air distribution, less recirculation, bundle protected from precipitation
A-Frame
Two flat tube bundles arranged in an inverted V-shape (apex at top)
Fans located below the apex, blowing air upward through both sloped bundles
Bundle orientation: Sloped, typically 30° to 60° from horizontal
Typical application: Air-cooled steam condensers (ACC) in power plants
Advantage: Reduces plot area for large bay areas; condensate drains by gravity along sloped tubes
V-Frame
Two flat tube bundles arranged in a V-shape (apex at bottom)
Fans located above the apex, pulling air upward through both sloped bundles
Bundle orientation: Sloped, typically 30° to 60° from horizontal
Typical application: Steam condensers where induced draft is preferred for freeze protection
Advantage: Footprint reduction similar to A-frame; condensate drains to a bottom collector
Horizontal Single Bundle
Single flat tube bundle installed horizontally
Fans below (forced draft) or above (induced draft)
Bundle orientation: Horizontal, 0° inclination
Typical application: General process cooling, gas cooling, hydrocarbon condensing
Advantage: Simplest construction; easiest bundle replacement and maintenance access
Bundle Construction – Core Components
Tubes
| Parameter | Range | Note |
|---|---|---|
| Tube OD (base) | 15.88 mm – 50.8 mm | Smaller OD for higher pressure; larger OD for fouling service |
| Tube wall thickness | 1.2 mm – 5.0 mm | Seamless or welded |
| Tube length (between tube sheets) | 2.0 m – 12.0 m | Per API 661 standard |
| Transverse tube pitch | 1.5 × – 2.5 × tube OD | Across air flow direction |
| Longitudinal tube pitch | 1.5 × – 2.5 × tube OD | Along air flow direction |
| Tube arrangement | Staggered (triangular) or in-line (square) | Staggered provides higher air-side heat transfer; in-line allows easier cleaning |
Tube Sheets
| Parameter | Range / Value | Note |
|---|---|---|
| Thickness | 15 mm – 80 mm | Per ASME VIII-1 / TEMA RCB-4.3 |
| Hole diameter tolerance | H11 per ISO 286 | Example: Ø25.2 mm +0.13/0 |
| Hole finish | Ra ≤ 1.6 μm (expanded) / Ra ≤ 3.2 μm (welded) | Finer finish required for expansion |
| Material | CS / SS / Clad | Matched to tube-side fluid corrosivity and pressure |
| Tube retention | Expanded / Welded / Combined | Per joint type selection |
Header Boxes (Inlet / Outlet Manifolds)
| Header Type | Tube Access | Pressure Capacity | Typical Service |
|---|---|---|---|
| Plug header (threaded) | Limited, individual plug removal | Up to 2.0 MPa | Low-pressure clean service |
| Plug header (bolted) | Easy, bolted plug removal | Up to 5.0 MPa | Medium-pressure mildly fouling service |
| Cover-plate header | Full access, entire cover removed | Up to 5.0 MPa | Fouling service requiring mechanical cleaning |
| Pipe manifold header | No tube access, welded construction | Up to 10.0 MPa | High-pressure gas or liquid, no cleaning required |
Header box construction:
Rectangular or round cross-section
Flanged connections per ASME B16.5 or B16.47
Pass partitions for single, two, or four tube-side passes
Drain and vent connections at low and high points
Side Frames and Lifting Lugs
Structural frame: Steel channel or I-beam
Lifting lugs: Welded to side frames or tube sheets, rated at 2× bundle weight
Support points located to minimise tube sag and maintain levelness within ±3 mm over bundle length
Fin Types – Selection by Temperature and Environment
| Fin Type | Attachment Method | Max Operating Temp | Fin Material | Typical Application |
|---|---|---|---|---|
| L-foot (tension wound) | L-shaped foot wound under tension | ≤ 150°C | Aluminum 1100 | Low-temperature gas cooling, intercoolers |
| KL-foot (knurled L) | Knurled tube surface with mechanically interlocked foot | ≤ 320°C | Aluminum 1100 | Medium-temperature process gas, aftercoolers |
| LL-foot (overlapped L) | Overlapped foot fully covers tube OD | ≤ 177°C | Aluminum 1100, Copper C1100 | Coastal and marine atmosphere |
| 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 | ≤ 450°C | Carbon steel, stainless steel | High-temperature gas cooling, waste heat recovery |
| Bare (no fin) | Not applicable | Up to 600°C | As tube material | High-temperature clean gas, steam, low fouling |
Fin Geometry (Typical)
| Parameter | L-foot / KL / LL | Extruded | High-frequency welded |
|---|---|---|---|
| Fin height | 8 mm – 16 mm | 8 mm – 16 mm | 8 mm – 19 mm |
| Fin pitch (FPI) | 7 – 11.5 | 7 – 11 | 2 – 7 |
| Fin thickness | 0.25 mm – 0.5 mm | Integral | 0.8 mm – 3.2 mm |
| Surface area ratio (extended / bare) | 10 – 23 | 10 – 20 | 6 – 15 |
Fin Bond Resistance
For tension-wound fin types (L-foot, KL-foot, LL-foot), the mechanical bond between fin and tube introduces thermal contact resistance:
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, as the fin is integral or metallurgically bonded.
Air-Side Thermal and Hydraulic Parameters
| Parameter | Typical Range | Note |
|---|---|---|
| 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 |
| Overall HTC (air-to-tube, bare area reference) | 15 – 60 W/m²·K | Typical for ACHE finned bundles |
| Air-side pressure drop (across bundle) | 100 – 300 Pa | Per API 661 design limit |
| 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 |
Material Selection – Tubes, Tube Sheets, and Headers
Tube Materials
| Material | Specification | Temperature Range | Chloride Limit | Application |
|---|---|---|---|---|
| Carbon steel | SA-179 / SA-106 Gr.B | -20°C to +425°C | Not applicable | Water, oil, clean hydrocarbons, steam |
| Stainless 304L / 316L | ASTM A213 | -196°C to +600°C | ≤ 200 ppm | Corrosive fluids, clean steam, food and pharmaceutical |
| Duplex 2205 | ASTM A789 | -40°C to +280°C | ≤ 300 ppm at 80°C | Chloride-containing hydrocarbons, offshore |
| Titanium Gr.2 | 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 |
Tube Sheet and Header Materials
Carbon steel SA-516 Gr.70 for non-corrosive tube-side service
Stainless clad over carbon steel for corrosive service with high pressure; cladding thickness minimum 3 mm per ASME VIII-1 UW-13
Solid stainless 304L / 316L for fully corrosive service
Solid duplex or titanium for severe chloride service or offshore duty
Header box material is typically matched to tube sheet material to avoid galvanic corrosion at bolted or welded joints. For carbon steel headers in corrosive service, internal coating (epoxy or phenolic) is applied per API 661.
Fabrication Process – Key Controls
Tube Sheet Drilling
Drill bit size controlled to H11 tolerance per ISO 286
Hole finish: Ra ≤ 1.6 μm for expanded joints; Ra ≤ 3.2 μm for welded joints
Ligament deviation ≤ ±0.2 mm from drawing
All holes deburred and cleaned prior to tube insertion
Tube Expansion
Hydraulic expansion pressure: 160 – 220 MPa; hold time 5 – 8 seconds
Expansion depth: Minimum 1.5 × tube OD or 2 × tube wall thickness into tube sheet
Post-expansion gauge check: 100% of tubes accept a gauge ball 0.5 mm smaller than tube ID
Pull-out strength verification: First-article test per bundle, target ≥ 20 MPa for carbon steel and ≥ 25 MPa for stainless steel
Tube-to-Tubesheet Welding (If Specified)
GTAW (TIG) process, automatic or manual
Fillet leg height 1.5 – 2.0 mm for seal weld
Full penetration weld root penetration ≥ 0.5 mm into tube wall
100% liquid penetrant testing per ASME VIII-1 UW-51
Weld procedure qualified per ASME Section IX
Header Box Fabrication
Welded steel construction with ASME B16.5 or B16.47 flanged connections
Header dimensions matched to the tube sheet hole pattern
Pass partitions welded to header interior, material and welding procedure matched to header material
Drain and vent connections: NPT threaded or socket-weld fittings
Bundle Assembly and Lifting
Tubes inserted and expanded or welded into tube sheets
Baffles or tube supports installed at specified spacing
Side frames and lifting lugs attached
Lifting test performed at 1.1 × bundle weight if specified
Inspection and Testing Per Bundle
Dimensional Inspection
| Item | Tolerance | Method |
|---|---|---|
| Tube OD | ±0.11 mm | Micrometer (sample) |
| Fin height | ±0.5 mm | Caliper or template |
| Fin pitch | ±0.2 mm per 100 mm length | Template or optical |
| Bundle overall length | ±1.5 mm | Tape measure or laser |
| Tube sheet hole pattern | Per drawing | CMM or optical comparator |
| Header flange orientation | Per ASME B16.5 | Template or protractor |
Non-Destructive Examination
| Examination | Method | Scope | Acceptance |
|---|---|---|---|
| Tube-to-tubesheet welds | PT (liquid penetrant) | 100% if welded | No cracks, porosity ≤ 0.8 mm |
| Fin welds (high-frequency welded) | Visual and PT (sample) | 100% visual; 5% PT | No cracks, no loose fins |
| Header welds | RT or UT | Per ASME VIII-1 UW-51 or 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 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
Application Profiles – By Service Type
Compressor Intercooler and Aftercooler
Tube-side fluid: Compressed air or natural gas
Tube-side temperature: Inlet 120°C – 180°C, outlet 40°C – 60°C
Tube-side pressure: 0.5 – 10.0 MPa
Tube material: Carbon steel for air; 304L for natural gas containing CO₂ or water
Fin type: L-foot if tube-side temperature ≤ 150°C; KL-foot if 150°C – 180°C
Fin geometry: OD 25.4 mm × 2.0 mm wall; fin height 12.5 mm; 10 FPI
Bundle rows: 4 – 6
Air face velocity: 2.5 – 3.5 m/s
Air-side pressure drop: ≤ 200 Pa
Refinery Overhead Condenser
Tube-side fluid: Hydrocarbon vapour with steam
Tube-side temperature: Inlet 100°C – 150°C condensing, outlet 40°C – 60°C
Tube-side pressure: 0.1 – 0.5 MPa
Tube material: Carbon steel for non-corrosive service; 316L for sour service
Fin type: L-foot or KL-foot for standard service; extruded with 316L tube for H₂S service
Fin geometry: OD 25.4 mm × 2.0 mm; fin height 12.5 mm; 10 FPI
Bundle rows: 4
Special feature: Bundle sloped 1:50 toward outlet header for liquid drainage
Natural Gas Cooler After Dehydration or Compression
Tube-side fluid: Treated natural gas
Tube-side temperature: Inlet 60°C – 100°C, outlet 30°C – 45°C
Tube-side pressure: 4.0 – 10.0 MPa
Tube material: 316L or Duplex 2205 if chlorides are present
Fin type: Extruded with aluminum 1100 outer for offshore and coastal corrosion protection
Fin geometry: OD 19.05 mm × 2.0 mm; fin height 12.5 mm; 10 FPI
Bundle rows: 2 – 3
Air face velocity: 2.0 – 2.5 m/s
Air-side pressure drop: ≤ 150 Pa
Air-Cooled Steam Condenser (ACC) for Power Plants
Tube-side fluid: Steam condensing to water under vacuum
Tube-side temperature: Inlet 60°C – 100°C, outlet 35°C – 45°C
Tube-side pressure: Vacuum, 10 – 50 kPa absolute
Tube material: Carbon steel galvanised or coated; stainless steel if steam contains ammonia or chlorides
Fin type: High-frequency welded with metallurgical bond; hot-dip galvanised carbon steel fins
Tube geometry: Elliptical or flat-oval tubes with plate fins or helically welded fins
Bundle orientation: A-frame or V-frame, inclined 60° to horizontal
Air flow: Induced draft
Special feature: Bundles sloped 1:80 toward condensate collection header; freeze protection via steam tracing or electric heating for winter operation below 0°C
Cleaning and Maintenance
Air-side fouling from dust, pollen, or chemical deposits increases pressure drop and reduces heat transfer. Cleaning is recommended when:
Air-side pressure drop exceeds design pressure drop by 30%
Air-side pressure drop 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 aluminium fins; avoid high-alkali solutions
Tube-side cleaning is not mechanically feasible in a fixed bundle. Chemical cleaning or on-line pigging is required.
Documentation Per Shipment
Material test certificates per EN 10204 3.1 or 3.2 for tubes, fins, tube sheets, headers, and structural steel
API 661 completed datasheet or ISO 13706 datasheet
TEMA datasheet if applicable
ASME U-stamp data report if applicable
Dimensional inspection report including bundle length, tube sheet hole layout, fin geometry, and tube OD
Fin attachment process parameters including winding tension, extrusion pressure, and welding current
Hydrostatic test report with pressure chart recording
Pneumatic leak test report if performed
NDE reports for PT, UT, and RT as performed
Weld procedure specification and procedure qualification record for tube-to-tubesheet and header welds
Tube bundle as-built drawing with tube count, pass configuration, header dimensions, and lifting lug locations
Painting and coating specification with thickness verification report
Selection Checklist – Air Cooler Tube Bundle
ACHE type: Forced draft, induced draft, A-frame, V-frame, or horizontal
Bundle orientation: Horizontal or sloped with angle
Tube material and size (OD and wall thickness)
Fin type: L-foot, KL, LL, extruded, high-frequency welded, or bare
Fin material and geometry (height, pitch or FPI, thickness)
Tube-side design pressure and temperature
Tube-side fluid composition including corrosivity, chlorides, and H₂S
Tube-side allowable pressure drop
Air inlet temperature for summer design
Air face velocity or total air flow
Air-side allowable pressure drop, typically 100 – 300 Pa
Header type: Plug, cover-plate, or pipe manifold
Number of tube-side passes: Single, two, or four
Tube count and layout per existing or new bay
Corrosion protection requirements including coating, galvanising, or special fin and tube materials
Freeze protection for ACC service including steam tracing, electric heating, or recirculation
Lifting and transport constraints including maximum weight and dimensions
Applicable standards: API 661, ISO 13706, TEMA, or customer specification
Design Limitation Statement – Air Cooler Tube Bundle
The air cooler tube bundle is subject to the following limitations:
It is designed only for the specified tube-side pressure and temperature. Operation outside these limits voids the design.
Air-side performance depends on ambient air temperature and fouling. Cooling capacity decreases as ambient temperature increases.
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 condensing services in freezing conditions, freeze protection through sloping, drainage, and heating must be provided.
Maximum fin tip temperature must not exceed material-specific limits: L-foot ≤ 150°C, extruded ≤ 230°C, high-frequency welded ≤ 450°C.
For thermal cycling service above 500 cycles per year, tension-wound fin types such as L-foot, KL-foot, and LL-foot are not recommended. Use extruded or high-frequency welded fins.
The bundle must be installed on a level support structure. Excessive misalignment causes tube sheet distortion and joint leakage.
For tube-side pressure above 10.0 MPa or extreme vacuum, consult for custom header and tube sheet design.
