Tube Bundle as the Primary Heat Transfer Assembly
A tube bundle is the core heat transfer assembly of a shell and tube heat exchanger. It consists of tubes, tube sheets, baffles or support plates, tie rods, spacers, and associated components. The bundle separates tube-side fluid from shell-side fluid and provides the heat transfer surface between the two streams.
A tube bundle can be supplied as a replacement bundle, spare bundle, or complete assembly for new heat exchangers. It is designed and manufactured per TEMA Class C, B, or R, ASME Section VIII Division 1 or Division 2, and GB/T 151 for domestic applications.
Tube Bundle Types – Structural Classification
Fixed Tube Sheet Bundle (TEMA L / M / N)
Tubes are fixed at both ends into stationary tube sheets. The tube sheets are welded or bolted to the shell. Thermal expansion is accommodated by an expansion joint on the shell when the temperature differential between tubes and shell exceeds the allowable limit for the material combination.
Typical application: clean shell-side fluids, moderate temperature differential, non-fouling service.
U-Tube Bundle (TEMA U)
Tubes are bent into a U-shape, and both ends are fixed to a single tube sheet. Each tube expands independently, so no shell expansion joint is required for large temperature differentials.
Typical application: high thermal differential, high-pressure gas service, hydrogen service, thermal shock conditions.
Limitation: individual tubes cannot be mechanically cleaned from the inside; chemical cleaning is required.
Floating Head Bundle (TEMA P / S / T / W)
One tube sheet is fixed, and the other is free to move with thermal expansion. The rear head is removable, allowing the bundle to be extracted for mechanical cleaning.
Typical application: severe fouling service, large temperature differential, high shell-side fouling factor.
Core Components – Specifications
Tubes
| Parameter | Range | Notes |
|---|---|---|
| Tube OD | 12.7 mm – 50.8 mm | Smaller OD for higher pressure; larger OD for fouling service |
| Tube wall thickness | 0.9 mm – 5.0 mm | Selected per ASME VIII-1 UG-27 |
| Tube length | 1.5 m – 12.0 m | Standard lengths include 2.44 m, 4.88 m, 6.10 m, 9.75 m |
| Tube pitch | 1.25 × – 2.0 × tube OD | Triangular pitch for higher heat transfer; square pitch for mechanical cleaning |
| Tube arrangement | Triangular (30°/60°) / Square (90°/45°) | Triangular gives higher surface area per shell cross-section |
Tube Sheets
| Parameter | Range / Value | Notes |
|---|---|---|
| Thickness | 15 mm – 350 mm | Per ASME VIII-1 Appendix AA / TEMA RCB-4.3 |
| Hole diameter tolerance | H11 per ISO 286 | Example: Ø25.2 mm +0.13/0 |
| Hole finish | Ra ≤ 1.6 μm for expanded joints; Ra ≤ 3.2 μm for welded joints | Finer finish required for expansion |
| Ligament width | Minimum 0.8 × hole diameter | Per ASME VIII-1 Appendix A |
| Material | CS / SS / Duplex / Ti / Alloy / Clad | Matched to tube-side and shell-side fluids |
Baffles and Support Plates
| Parameter | Range / Value | Notes |
|---|---|---|
| Baffle type | Segmental / Double segmental / Helical / Full support | Segmental cut typically 20% – 35% |
| Baffle spacing | 0.2 × – 1.0 × shell ID | Based on unsupported tube length and cross-flow velocity |
| Maximum unsupported tube length | CS: ≤ 36 × tube OD; SS: ≤ 30 × tube OD | Per TEMA RCB-4.2 |
| Baffle thickness | 3.0 mm – 16 mm | Based on pressure differential and support span |
| Tie rod diameter | M8 – M24 or equivalent | Minimum 4 tie rods per bundle |
Material Selection – Tubes and Tube Sheets
Tube Materials
| Material | Specification | Temperature Range | Chloride Limit | Application |
|---|---|---|---|---|
| Carbon steel | SA-179 / SA-106 Gr.B / 20# | -20°C to +425°C | Not applicable | Water, oil, steam, clean hydrocarbons |
| Stainless 304L / 304 | ASTM A213 TP304L | -196°C to +600°C | ≤ 200 ppm | Mildly corrosive fluids, clean steam, food grade |
| Stainless 316L / 316 | ASTM A213 TP316L | -196°C to +500°C | ≤ 200 ppm | Organic acids, salt solutions, chloride-containing hydrocarbons |
| Duplex 2205 | ASTM A789 S32205 | -40°C to +280°C | ≤ 300 ppm at 80°C | Offshore, seawater, CO₂/H₂S service |
| Titanium Gr.2 | ASTM B338 | -40°C to +230°C | Up to 20,000 ppm | Seawater, brine, high-chloride environments |
| Copper-nickel C70600 | ASTM B111 | -40°C to +200°C | < 500 ppm | Marine cooling water, brackish water |
| Alloy 625 / 825 / 800H | ASTM B163 / B444 | -196°C to +600°C | Up to 500 ppm | High-temperature corrosive service, sour gas |
Tube Sheet Materials
Carbon steel SA-516 Gr.70 for non-corrosive tube-side fluids
Stainless clad over carbon steel for corrosive service with high pressure; cladding thickness minimum 3 mm
Solid stainless 304L / 316L for fully corrosive service
Duplex or titanium for offshore and seawater service
Alloy overlay such as Inconel 625 for severe sour or chloride service
Tube-to-Tubesheet Joint Types
| Joint Type | Method | Pull-Out Strength | Applicable Service |
|---|---|---|---|
| Expanded only | Hydraulic expansion at 160–220 MPa, hold 5–8 s | ≥ 20 MPa for CS; ≥ 25 MPa for SS | Non-toxic, non-cyclic, clean fluids |
| Welded only | GTAW fillet weld, leg 1.5–2.0 mm | Mechanical retention from weld only | High-pressure gas, hydrogen service |
| Weld + expand | Seal weld plus hydraulic expansion at 160–200 MPa | ≥ 25 MPa | Cyclic thermal service, high pressure, toxic fluids |
| Full penetration weld | GTAW full penetration | ≥ 30 MPa | Extreme pressure, hydrogen, high safety factor |
Fabrication Process – Key Controls
Tube Sheet Drilling
Drill bit size controlled to H11 tolerance per ISO 286
Ligament deviation ≤ ±0.2 mm from drawing
Hole surface finish Ra ≤ 1.6 μm for expanded joints
All holes deburred on both faces and cleaned before 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 test on first article per bundle
Tube-to-Tubesheet Welding
GTAW 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
Post-Weld Heat Treatment
Required when carbon steel tube sheet thickness exceeds 38 mm
Required when service contains wet H₂S per NACE MR0175 / ISO 15156
Typical PWHT cycle: 620°C ± 10°C, hold 1 hour per 25 mm thickness, minimum 1 hour
Controlled cooling rate ≤ 50°C/hour to 400°C
Inspection and Testing Per Bundle
Dimensional Check
| Item | Tolerance | Method |
|---|---|---|
| Tube OD | ±0.11 mm per ASTM B730 | Micrometer |
| Tube wall thickness | ±10% of nominal | Ultrasonic gauge |
| Bundle length | ±1.5 mm per TEMA RCB-8 | Tape measure or laser |
| Baffle spacing | ±1.5 mm | Caliper or template |
| Hole ligament deviation | ≤ ±0.2 mm | Optical comparator |
| U-bend radius | R ≥ 2 × tube OD | Template gauge |
Non-Destructive Examination
Tube-to-tubesheet welds: 100% liquid penetrant testing per ASME VIII-1 UW-51
Tube sheet cladding: 100% ultrasonic testing for bond integrity per ASME VIII-1 UW-13
Tube internal surface: Borescope inspection on random sample; 100% of tubes pass gauge ball
Finned tubes, if applicable: Visual inspection for loose or missing fins
Pressure Testing
Hydrostatic test: 1.3 × design pressure per ASME VIII-1 UG-99
Hold time: minimum 30 minutes
Acceptance: zero pressure drop and no visible leakage
Pneumatic leak test if specified: 0.6 MPa air or nitrogen; soap bubble detection
Leakage rate acceptance: ≤ 1×10⁻⁵ Pa·m³/s per equivalent orifice method
Documentation Per Shipment
Material test certificates per EN 10204 3.1 or 3.2
ASME U-stamp data report if applicable
TEMA datasheet completed per design
Dimensional inspection report
Hydrostatic test report with pressure recording chart
NDE reports for PT, UT, RT, or VT as performed
Weld procedure specification and procedure qualification record
Tube bundle as-built drawing
Tube pull-out test report for first article
Packing list with bundle shipping dimensions and weight
Selection Checklist – Tube Bundle
TEMA type – Fixed tube sheet / U-tube / Floating head
Shell inside diameter and tube sheet diameter
Tube OD, wall thickness, and length
Tube pitch and arrangement – Triangular / Square
Tube material and tube sheet material
Tube-to-tubesheet joint type – Expanded / Welded / Combined
Design pressure for tube side and shell side
Design temperature for tube side and shell side
Fluid composition for both sides – to verify material compatibility
Corrosion allowance for tubes and tube sheet
Baffle type and spacing – based on allowable pressure drop and vibration criteria
NDE and testing requirements per project specification
PWHT requirement based on material and service
Quantity – replacement bundle, spare bundle, or new equipment
Design Limitation Statement – Tube Bundle
The tube bundle is subject to the following limitations:
It must match the shell geometry, tube sheet dimensions, and flange pattern of the existing heat exchanger.
It is not rated for internal pressure until assembled with the shell, channels, and flanges.
Tube-side solids content above 2% by weight is not recommended without upstream filtration, due to erosion and tube blockage risk.
Expanded-only joints are not recommended for thermal cycling above 500 cycles per year or for toxic service.
U-tube bundles cannot be mechanically cleaned inside the tubes; chemical cleaning is required.
Fixed tube sheet bundles may require a shell expansion joint when the temperature differential exceeds the allowable limit.
For vacuum service below 1 kPa absolute, additional stiffening or support may be required per ASME VIII-1 UG-29.
Hydrotest water for austenitic stainless steel must have chloride content ≤ 50 ppm to prevent stress corrosion cracking.
