API 660 Design Air Cooler Tube Bundle with High Heat Transfer Efficiency and

Product Specification

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.

Header Configuration – Impact on Bundle Replacement

When ordering a replacement bundle, the header configuration must match the existing bay piping and tube access requirements.

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)

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

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:

1. Original bundle drawing number (if available)

2. API 661 bay number and bundle position (top, middle, bottom if multi-bay)

3. Tube count, tube OD, wall thickness, and length (between tube sheets)

4. Tube pitch (transverse and longitudinal) and tube arrangement (staggered or in-line)

5. Fin type (if finned) – L-foot / KL / LL / Extruded / HF-welded / Bare tube

6. Fin specification (height, pitch/fins per inch, material)

7. Header type – Plug (threaded or bolted) / Cover-plate / Pipe manifold

8. Header connection size and rating – flange rating (ASME Class 150 / 300 / 600) or weld connection

9. Tube-side pass configuration – single / two / four pass (with pass partition drawing if available)

10. Tube sheet material and thickness (if exact match required)

11. Design pressure and design temperature (tube side)

12. Fluid composition – to verify material compatibility

13. Required corrosion allowance (tube wall and tube sheet face)

14. Painting/coating specification – if atmospheric corrosion protection required

15. 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)