Air Cooler Tube Bundle | Forced / Induced Draft | A-Frame / V-Frame / Horizontal | Plug / Cover-Plate / Pipe Header | Tube Materials: CS / SS / Ti / Duplex | Temp -40°C to +450°C | Pressure Up to 10.0 MPa | Per API 661 / ISO 13706
Overview – Air Cooler Tube Bundle as Independent Replacement Unit
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, secured by tube sheets at each end, with header boxes (inlet and outlet manifolds) attached to distribute process fluid through the tubes. The bundle is installed within a structural bay, with axial or centrifugal fans forcing or drawing ambient air across the external tube surface.
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 as a new bundle for a new ACHE bay.
ACHE Types – Fan Configuration and Bundle Orientation
The following air cooler types define the bundle's installation configuration and fan arrangement. The bundle itself is the same core assembly, but orientation, support structure, and air flow direction vary.
Forced Draft ACHE
Fans are located below the tube bundle, pushing air upward through the bundle
Air distribution: Uniform air flow across the bundle face due to fan plenum
Maintenance: Fan and drive components accessible at ground or platform level
Bundle orientation: Horizontal (tubes horizontal, air flows vertically upward)
Typical application: General refinery process cooling, compressor intercoolers
Advantage: Lower installed cost, easier fan maintenance
Limitation: Hot air recirculation risk if wind walls are not installed; fan heat adds to inlet air temperature
Induced Draft ACHE
Fans are located above the tube bundle, pulling air upward through the bundle
Air distribution: Plenum chamber above bundle ensures uniform suction
Maintenance: Fan and drive components at elevated platform – requires access platform
Bundle orientation: Horizontal (tubes horizontal, air flows vertically upward)
Typical application: Steam condensers (ACC), high-temperature services, large refinery units
Advantage: Better air distribution; less recirculation; protects bundle from precipitation
Limitation: Higher installed cost; fan access at elevation; larger fan stack height
A-Frame ACHE
Bundle consists of two flat tube bundles arranged in an inverted V-shape (apex at top)
Fans are 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, large petrochemical units
Advantage: Reduces footprint for large bay areas; provides condensate drainage by gravity (sloped tubes)
Limitation: More complex support structure; larger plenum volume required
V-Frame ACHE
Bundle consists of two flat tube bundles arranged in a V-shape (apex at bottom)
Fans are 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: Same footprint reduction as A-frame; condensate drains to bottom collector
Limitation: Fan and drive at elevation; more complex header piping
Horizontal ACHE (Single Bundle per Bay)
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
Limitation: Requires larger plot area for large duties (more bays required)
Bundle Construction – Core Components
Tubes
Tube types: Bare tube (low fouling, low-temperature gas) or finned tube (for low air-side HTC)
Tube OD (bare / base): 15.88mm – 50.8mm (5/8" to 2")
Tube wall thickness: 1.2mm – 5.0mm (seamless or welded)
Tube length (between tube sheets): 2.0m – 12.0m per API 661 standard
Tube pitch (center-to-center):
Transverse (across air flow): 1.5× – 2.5× tube OD
Longitudinal (along air flow): 1.5× – 2.5× tube OD
Tube arrangement: Staggered (triangular) or in-line (square) – staggered provides higher air-side HTC; in-line allows easier cleaning.
Tube Sheets
Location: Front (inlet) and rear (outlet) ends of the bundle
Thickness: 15mm – 80mm (selected per tube-side pressure and tube-to-tubesheet joint load)
Hole diameter tolerance: H11 per ISO 286 (e.g., Ø25.2mm +0.13/0)
Hole finish: Ra ≤ 1.6μm for expanded joints; Ra ≤ 3.2μm for welded-only joints
Material: Carbon steel, clad stainless, or solid stainless – matched to tube-side fluid corrosivity and operating pressure
Tube retention: Tubes are expanded (hydraulic or roller), welded (seal weld), or combined (weld + expand) into tube sheets
Header Boxes (Inlet / Outlet Manifolds)
Type selection (per API 661):
Plug header – removable threaded or bolted plugs for tube access; suitable for clean service and low pressure
Cover-plate header – bolted cover plate for full tube access; recommended for fouling service where tube cleaning is required
Pipe manifold header – welded pipe connections; used for high pressure or where tube access is not required
Header box construction:
Rectangular or round cross-section
Flanged connections per ASME B16.5 or B16.47 (welding neck or slip-on)
Pass partitions (internal baffles) to configure tube-side flow path (single, two, or four passes)
Design pressure rating: Matched to tube-side design pressure (0.1 – 10.0 MPa)
Drain and vent connections: Threaded NPT or socket-weld connections at low/high points
Side Frames and Supports
Structural frame: Steel channel or I-beam construction to support bundle weight during lifting and operation
Lifting lugs: Welded to side frames or tube sheets – rated for bundle weight x 2.0 safety factor
Support points: Located to minimize tube sag and maintain bundle levelness within ±3mm over bundle length
Fin Types – For Air-Cooler Tube Bundles
The following fin attachment types are available for air cooler tube bundles. Selection is based on tube-side temperature, atmospheric corrosivity, and thermal cycling.
| Fin Type | Max Temp | Fin Material | Application |
|---|---|---|---|
| L-foot (tension wound) | ≤ 150°C | Al 1100 | Low-temperature gas cooling, intercoolers |
| KL-foot (knurled L) | ≤ 320°C | Al 1100 | Medium-temp process gas, aftercoolers |
| LL-foot (overlapped L) | ≤ 177°C | Al 1100 or Cu | Corrosive atmosphere, coastal service |
| Extruded (bimetallic) | ≤ 230°C | Al 1100 outer | Marine, offshore, high-corrosion atmosphere |
| High-frequency welded | ≤ 450°C | CS or SS | High-temp gas cooling, waste heat recovery |
| Bare tube (no fin) | Up to 600°C | As tube material | High-temp clean gas, steam, low fouling |
Header Configuration – Impact on Bundle Replacement
When ordering a replacement bundle, the header configuration must match the existing bay piping and tube access requirements.
| Header Type | Tube Access | Pressure Capacity | Typical Service |
|---|---|---|---|
| Plug header (threaded plugs) | Limited – individual plug removal; plug can be difficult to remove in service | Up to 2.0 MPa (standard) / up to 5.0 MPa with bolted plug | Low-pressure clean service, non-corrosive |
| Plug header (bolted plugs) | Easy – bolted plug removal for tube cleaning | Up to 5.0 MPa | Medium-pressure clean or mildly fouling service |
| Cover-plate header | Full access – entire cover removed for mechanical cleaning of all tubes | Up to 5.0 MPa | Fouling service, where tube cleaning is scheduled |
| Pipe manifold header | No tube access – welded construction | Up to 10.0 MPa | High-pressure gas or liquid service, no cleaning required |
Tube-Side Pass Configuration
The number of tube-side passes is determined by tube-side allowable pressure drop and required heat transfer coefficient:
Single pass – lowest tube-side ΔP; suitable for high-flow, low-pressure-drop services
Two pass – moderate ΔP; provides higher tube-side velocity for better HTC
Four pass – highest ΔP; used when tube-side HTC limits overall performance and pressure drop is available
Pass partitions within the header boxes direct flow from inlet to outlet. The number of tube rows is also affected by pass arrangement:
For single-pass: tube rows = total tubes ÷ number of tubes per row (even distribution)
For multiple passes: pass partitions split the tube bundle into equal groups
Material Selection – Tube, Tube Sheet, and Header
Tube Material Options (Per API 661)
| Material | Temperature Range | Chloride Limit | Application |
|---|---|---|---|
| Carbon steel SA-179 / 106 Gr.B | -20°C to +425°C | N/A | Water, oil, clean hydrocarbons |
| Stainless 304L / 316L | -196°C to +600°C | ≤ 200ppm | Corrosive process fluids, clean steam |
| Duplex 2205 | -40°C to +280°C | ≤ 300ppm at 80°C | Chloride-containing fluids, offshore |
| Titanium Gr.2 | -40°C to +230°C | Up to 20,000ppm | Seawater cooling, brine service |
| Copper-nickel C70600 | -40°C to +200°C | < 500ppm | Marine cooling water, brackish water |
Tube Sheet and Header Material Options
Carbon steel SA-516 Gr.70 – non-corrosive tube-side service
Stainless clad (316L or 304L over CS) – corrosive service with high pressure; cladding thickness ≥ 3mm per ASME VIII-1 UW-13
Solid stainless 304L / 316L – fully corrosive service, no carbon steel backing
Solid duplex / 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 Parameters – Process Control
Tube Sheet Drilling
Drill bit size: Controlled to H11 tolerance per ISO 286
Hole finish: Ra ≤ 1.6μm for expanded joints; Ra ≤ 3.2μm for welded joints
Ligament (web between holes) deviation: ≤ ±0.2mm from drawing
Hole cleaning: Debris removed and holes inspected for burrs and chips prior to tube insertion
Tube Expansion (Hydraulic)
Expansion pressure: 160 – 220 MPa (hydraulic) or roller expansion (mechanical) with calibrated roller
Expansion depth: Minimum 1.5× tube OD or 2× tube wall thickness into tube sheet
Expansion hold time: 5 – 8 seconds (hydraulic)
Post-expansion gauge check: 100% of tubes accept a gauge ball 0.5mm smaller than tube ID
Pull-out strength verification: First-article pull-out test per bundle – target ≥ 20 MPa (CS) / ≥ 25 MPa (SS)
Tube-to-Tubesheet Welding (if specified)
Welding process: GTAW (TIG) – automatic or manual
Weld type: Fillet weld (seal) or full penetration weld (for toxic/high-pressure service)
Fillet leg height: 1.5mm – 2.0mm for seal weld
Full penetration: Weld root penetration ≥ 0.5mm into tube wall
Inspection: 100% liquid penetrant (PT) per ASME VIII-1 UW-51 – acceptance: no cracks or porosity
Weld procedure: Qualified per ASME Section IX with PQR and WPS
Header Box Fabrication
Header box (inlet/outlet) – welded steel construction with ASME B16.5 flanged connections
Header box dimensions matched to bundle tube sheet pattern (tube count and layout)
Pass partitions (internal baffles) welded to header box 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 from one end and expanded/welded into tube sheets
Baffles or tube supports (if required) installed at specified spacing
Side frames and lifting lugs attached to support bundle weight
Bundle lifting test (if required): Lift bundle at 1.1× bundle weight – verified by manufacturer prior to shipment
Operational Parameters – Air Cooler Tube Bundle Rating
| Parameter | Range | Notes |
|---|---|---|
| Tube-side design pressure | Vacuum (–0.1 MPa) to 10.0 MPa | Higher pressure per special design |
| Tube-side design temperature | -40°C to +450°C | Material dependent; higher with bare tube / high-temp alloy |
| Air-side inlet temperature (ambient) | -40°C to +55°C | API 661 design basis |
| Air face velocity (approach) | 1.5 – 4.0 m/s | Selected based on fan power and noise limits |
| Air-side pressure drop | 100 – 300 Pa | Across bundle; fan selected to overcome this |
| Tube-side fluid velocity (liquid) | 1.5 – 4.5 m/s | Erosion velocity limit per TEMA / API 661 |
| Tube-side fluid velocity (gas) | 10 – 30 m/s | Limited by pressure drop and vibration |
| Bundle face area per unit | 10 – 200 m² | Standard per bay; larger duties use multiple bays |
Leak Testing and Hydrostatic Testing
Tube Bundle (Before Header Installation)
Tube-to-tubesheet joints: Hydrostatic test at 1.3 × tube-side design pressure (per ASME VIII-1 UG-99) – hold 30 minutes, zero pressure drop, no visible leakage
After test: Tube side drained and dried; tube holes inspected for weepage
Header Box (Assembled to Bundle)
Final hydrostatic test (complete bundle including headers): Test pressure per UG-99 (tube side only – shell side is atmospheric)
Test medium: Clean water (potable or demineralized) – with corrosion inhibitor for carbon steel
Test duration: 30 minutes minimum
Acceptance: Zero pressure drop; no visible leakage from tube joints, header welds, flanged connections, or plugs
Pneumatic Leak Test (if specified)
Used for hydrogen service, toxic fluids, or where water residue is unacceptable
Test pressure: 0.6 MPa air or nitrogen
Leak detection: Soap bubble application on all joints and welds
Acceptance: Leakage rate ≤ 1×10⁻⁵ Pa·m³/s per ASME Appendix VI equivalent orifice method
Documentation per Shipment – Replacement Bundle
Material test certificates (EN 10204 3.1 or 3.2) – tubes, tube sheets, headers, flanges, and structural steel
API 661 completed datasheet (or ISO 13706 datasheet)
ASME U-stamp data report (if applicable)
TEMA datasheet (Class R or B – if applicable)
Dimensional inspection report (bundle length, tube sheet hole layout, tube OD, fin height/pitch if finned)
Hydrostatic test report (with pressure chart recording)
Pneumatic leak test report (if performed)
NDE reports (PT / UT / VT as performed)
Weld procedure specification (WPS) and procedure qualification record (PQR) – for tube-to-tubesheet welds and header welds
Tube bundle as-built drawing – with tube count, tube pass arrangement, header dimensions
Painting/coating specification (if applied) – epoxy/phenolic coating thickness and batch test certificate
Selection Checklist – Replacement Air Cooler Tube Bundle
When ordering a replacement bundle, provide the following to ensure correct fit and performance:
Original bundle drawing number (if available)
API 661 bay number and bundle position (top, middle, bottom if multi-bay)
Tube count, tube OD, wall thickness, and length (between tube sheets)
Tube pitch (transverse and longitudinal) and tube arrangement (staggered or in-line)
Fin type (if finned) – L-foot / KL / LL / Extruded / HF-welded / Bare tube
Fin specification (height, pitch/fins per inch, material)
Header type – Plug (threaded or bolted) / Cover-plate / Pipe manifold
Header connection size and rating – flange rating (ASME Class 150 / 300 / 600) or weld connection
Tube-side pass configuration – single / two / four pass (with pass partition drawing if available)
Tube sheet material and thickness (if exact match required)
Design pressure and design temperature (tube side)
Fluid composition – to verify material compatibility
Required corrosion allowance (tube wall and tube sheet face)
Painting/coating specification – if atmospheric corrosion protection required
Lifting lug rating and location (to match existing lifting equipment)
Inspection Points for Receiving (Buyer Side)
Upon delivery of a replacement bundle, verify the following prior to installation:
Bundle overall length (between tube sheets) – within ±1.5mm of drawing
Tube sheet hole pattern – tube count and pass partition configuration (match existing header box)
Tube OD, fin height/pitch (if finned) – within tolerance per drawing
Header box flange orientation and bolt hole alignment (match existing nozzles)
Tube-to-tubesheet joint visual check – no signs of cracking or weld defects
Hydrostatic test records – verify pressure and hold time
Lifting lugs – visual check for correct size and weld integrity
Painting/coating – verify thickness (dry film thickness per specification), no bare spots or pinholes
Bundle packaging – wrapped or crated to prevent damage during transport and storage
Storage and Preservation Guidelines
If the bundle is not installed immediately upon receipt:
Store indoors or under weatherproof cover – protect from rain, snow, and direct sunlight
Store in horizontal position – on supports at manufacturer-specified lifting points to prevent tube sag
For carbon steel bundles: Apply temporary corrosion inhibitor coating (VCI paper, oil film, or desiccant) – per manufacturer's recommendation
Tube ends and header openings: Sealed with plastic caps or flanged covers to prevent debris ingress
Inspection prior to installation: Remove caps, inspect tube ends and header interior for moisture or debris
Design Limitation Statement – Air Cooler Tube Bundle
The air cooler tube bundle as a standalone product is subject to the following limitations:
It must be installed in a bay with matching geometry (tube sheet hole pattern, header box configuration, and bundle envelope dimensions)
It is not rated for internal shell-side pressure (air side is atmospheric – no internal pressure containment)
It is not suitable for tube-side solids content > 2% by weight (erosion and tube wall thinning risk)
It cannot be used with fluids that are incompatible with tube material (validate with material selection table)
For very high tube-side pressure (> 10.0 MPa), consult for custom header design (pipe manifold with full penetration welds and heavy-wall tube sheets)
