Tubesheets, Baffles and Tube Bundles: How the Core of a Heat Exchanger Is Built

Quick Answer

A tube bundle is the tubes plus everything that holds them: tubesheets, baffles or support plates, tie rods and spacers, and often sealing strips and an impingement plate. TEMA sets the key geometry: tube pitch of at least 1.25 × tube OD; a 30° triangular layout fits about 15% more tubes than a square layout at the same pitch, while square layouts with cleaning lanes of at least 6.4 mm (¼ in.) allow mechanical shell-side cleaning; segmental baffles are cut at 20–35% of shell diameter, 25% being the common starting point, and spaced no closer than one-fifth of the shell diameter or 51 mm, with the maximum set by the unsupported tube span and a flow-induced vibration check. Baffle holes are only 0.4 to 0.8 mm larger than the tube. Tubes are fixed into the tubesheet by expansion, by welding, or both; welded joints are qualified on a mock-up under ASME Section IX, and all joints are normally leak tested before the hydrostatic test. Most bundle failures — leaking joints, tube fretting at baffles, erosion at the inlet, bypassing — trace back to one of these few numbers or to the fabrication sequence.

The plates in this photograph are drilled baffles for a heat exchanger order in production. Look at the hole pattern: thousands of holes in a precise grid, and a blank band running across it. That band is the pass-partition lane. In a multi-pass exchanger the channel's pass partition plate seats against the tubesheet along that line, so no tubes can sit there — and every baffle has to carry the same lane in exactly the same place.

Each of these holes is only a fraction of a millimetre larger than the tube that will pass through it. Each tube passes through every baffle in the stack and ends in two tubesheets — or, in a U-tube unit, in one. If the pattern on one plate is misplaced by one hole pitch, or drilled with a slightly wandering drill, the tubes will not go through, or they will go through under load and fret against the plate edges for the rest of the exchanger's life.

The bundle is where the heat transfer happens, where the two fluids are kept apart, and where most heat exchanger failures start. This guide covers how it is designed and built.

Anatomy of a Tube Bundle

• Tubes — straight or U-bent, plain or enhanced, in the material chosen for the tube-side and shell-side fluids. See our guide to heat exchanger tube materials.
• Tubesheets — thick drilled plates or forgings into which the tube ends are fixed. One for a U-tube bundle; two for fixed-tubesheet and floating-head bundles.
• Baffles — drilled plates that support the tubes and direct the shell-side flow across them.
• Support plates — plates that only support the tubes, for example at U-bends or in kettle reboilers, where cross-flow is not the aim.
• Tie rods and spacers — rods screwed into the stationary tubesheet with spacer tubes between baffles, fixing the baffle positions and making the cage rigid.
• Sealing strips and dummy tubes — devices that close bypass lanes between the bundle and the shell, or along pass lanes.
• Impingement plate or rods — protection for the tubes opposite the shell-side inlet nozzle.

How these parts are arranged depends on the TEMA construction type — fixed tubesheet, U-tube or floating head. Our guide to TEMA types and configurations explains the options.

Tubesheets

What they do

The tubesheet separates the two fluids, anchors every tube, and — depending on construction — acts as a pressure boundary for one or both sides. In a fixed-tubesheet exchanger it also takes the load from the difference in thermal expansion between the shell and the tubes, which is why fixed-tubesheet units with large temperature differences need an expansion joint in the shell.

How thickness is set

Tubesheet thickness is calculated, not chosen. ASME Section VIII Division 1 Part UHX gives the rules for U-tube, fixed and floating tubesheets; TEMA gives its own method and minimums. The drivers are the pressure on each side, the tube layout (which determines how much the drilled pattern weakens the plate), the type of connection to the shell and channel, and — for fixed tubesheets — the differential thermal expansion. Thick tubesheets are common because the drilled pattern removes a large share of the metal.

Materials

• Forgings — SA-105 or SA-266 carbon steel, SA-350 LF2 Class 1 for low temperature, SA-182 alloy and stainless grades. Forgings are preferred for thick tubesheets because properties are more uniform through the thickness.
• Plate — SA-516 and equivalents for thinner tubesheets.
• Clad tubesheets — a carbon steel backing with a corrosion-resistant layer on the face that sees the corrosive fluid, explosion-bonded or weld-overlaid. This puts the expensive alloy only where it is needed. See our guide to weld overlay and clad construction.

Tube and tubesheet materials have to be compatible: for welding, for thermal expansion, and against galvanic corrosion at the joint.

Drilling

Tubesheets are drilled on CNC deep-hole drilling machines. What is controlled:

• Hole diameter — to TEMA's standard fit, or a special close fit where the joint design requires it
• Hole position — to the layout drawing, measured from the tubesheet centre
• Drill drift — how far the drill wanders through the thickness. On thick tubesheets, drift decides whether the tube lines up with the baffles
• Ligament — the metal left between adjacent holes. Drift and oversize holes thin the ligaments, which weakens the tubesheet and the joints
• Surface finish in the hole — important for expanded joints

For expanded joints, tube holes are normally machined with grooves — usually two — so that the expanded tube flows into them and the joint gains pull-out strength and leak resistance.

Connection to shell and channel

• Integral (welded) on both sides — fixed-tubesheet construction; the tubesheet is welded to shell and channel
• Gasketed on both sides — removable bundles; the tubesheet is clamped between shell and channel flanges
• Extended as a flange — the tubesheet itself extends to form a bolted flange

Tube Layout and Pitch

Layout angle

At the same pitch, a triangular layout places about 15% more tubes in a given area than a square one, because each tube occupies 0.866 p² instead of p². That is more surface in the same shell. The price is that the lanes between tubes are not straight, so the shell side cannot be cleaned mechanically.

Pitch

TEMA requires a pitch of at least 1.25 × tube OD. For square layouts where mechanical cleaning is specified, TEMA also requires continuous cleaning lanes of at least 6.4 mm (¼ in.). Tighter pitch packs more tubes but thins the tubesheet ligaments and raises shell-side pressure drop.

Pass lanes and the outer tube limit

In multi-pass exchangers, rows of tubes are left out where pass partition plates seat — the blank bands visible in the baffles above. The outer tube limit is the diameter beyond which no tube may sit; between it and the shell is the bundle-to-shell clearance, which depends on construction:

• Smallest in fixed-tubesheet and U-tube units
• Larger in split-ring floating-head units (TEMA S)
• Largest in pull-through floating-head units (TEMA T), where the floating head has to pass through the shell

The larger the clearance, the more shell-side fluid bypasses the tubes. Sealing strips — longitudinal strips fixed to the baffles in the bypass gap — push that flow back into the bundle. Leaving them out is a common reason a new exchanger underperforms its data sheet.

Baffles and Support Plates

Baffle types

• Single segmental — a disc with one segment removed. The most common type, giving good cross-flow and heat transfer at moderate pressure drop.
• Double segmental — alternating central and outer segments. Roughly halves shell-side velocity and pressure drop for the same spacing; used where pressure drop is limited.
• Triple segmental and no-tubes-in-window (NTIW) — for very low pressure drop and vibration-sensitive designs. NTIW removes tubes from the baffle windows so every tube is supported by every baffle.
• Rod baffles — grids of rods instead of plates, giving longitudinal flow with very low pressure drop and good vibration resistance.
• Disc-and-doughnut — alternating discs and rings.
• Helical baffles — inclined segments forming a helical flow path; low pressure drop and reduced dead zones.

Baffle cut

The cut is the height of the removed segment as a percentage of shell inside diameter — commonly 20% to 35%, with 25% as the usual starting point. Too small a cut creates high velocity and pressure drop through the window; too large a cut lets flow short-circuit past the tubes, leaving dead zones.

Cut orientation

• Horizontal cut (flow up and down across the bundle) — common for single-phase liquids
• Vertical cut (flow side to side) — preferred for condensing and boiling on the shell side, for gases with condensate, and for shell-side fluids carrying solids, because liquid and solids can drain along the bottom rather than collecting behind each baffle

Baffles in condensing or draining services usually have small notches at the bottom to let liquid drain when the unit is emptied.

Baffle spacing

• Minimum: TEMA sets one-fifth of the shell inside diameter or 51 mm (2 in.), whichever is greater. Closer spacing gives high velocity and pressure drop with little gain.
• Maximum: set by the maximum unsupported tube span in TEMA's tables, which depends on tube diameter and material. With single segmental baffles, tubes in the baffle window are supported only by every second baffle, so their unsupported span is twice the baffle spacing.
• Vibration: the spacing must also pass a flow-induced vibration check — especially near the inlet and outlet nozzles and at U-bends, where spans are longest and velocities highest. A design that meets the TEMA span limit can still vibrate.

Hole and edge clearances

Baffle holes are only slightly larger than the tube — TEMA sets 0.8 mm (1/32 in.) over the tube OD for shorter spans and larger tubes, and 0.4 mm (1/64 in.) where spans are longer. Loose holes let tubes vibrate and fret against the baffle edge until they wear through. Clearance between the baffle and the shell is tabulated by TEMA by shell diameter; every millimetre beyond it is bypass leakage.

Baffle thickness

Thickness is taken from TEMA tables by shell diameter and the unsupported tube length between baffles. Thin baffles in a vibrating bundle act as knife edges against the tubes.

Tie Rods, Spacers, Sealing Devices and Impingement Protection

Tie rods and spacers. TEMA tabulates the minimum number and diameter of tie rods by shell diameter. They are screwed into the stationary tubesheet, carry spacer tubes cut to the baffle spacing, and are locked with nuts at the last baffle.

Sealing strips and dummy tubes. Fitted in the bundle-to-shell bypass and in pass-partition lanes running in the direction of cross-flow, so that shell-side fluid cannot bypass the tubes.

Impingement protection. Fluid entering through the shell-side inlet nozzle strikes the first rows of tubes directly. TEMA requires an impingement plate or equivalent protection where the inlet momentum exceeds set limits, and for vapours, two-phase flow and erosive fluids. An impingement plate that is too close to the nozzle simply moves the erosion to the shell. Alternatives include impingement rods and vapour belts, which distribute the inlet flow around the bundle.

Tube-to-Tubesheet Joints

The joint has two jobs: hold the tube against the pressure and thermal loads trying to push it out or pull it out, and keep the two fluids apart. There are three basic types.

Expanded joints

The tube is expanded radially into the tubesheet hole until it is plastically deformed against the hole wall and into the grooves.

• Roller expansion — rotating rolls inside the tube. Simple and widely used; the amount of expansion is controlled by torque.
• Hydraulic expansion — high-pressure fluid inside the tube between seals. Uniform, controllable expansion over a long length; preferred for thick tubesheets and for materials sensitive to rolling.

Expansion is controlled as a wall reduction — the percentage by which the tube wall is thinned — set by the qualified procedure for the tube and tubesheet materials. Too little and the joint leaks or pulls out; too much and the tube is work-hardened, thinned and can crack. The expanded length stops short of the shell-side face of the tubesheet so the tube is not bulged beyond it.

Welded joints

• Seal welds — a small fillet or fusion weld at the tube end that provides leak tightness, while the expansion carries the load
• Strength welds — a weld designed to carry the full joint load on its own, often with a light expansion afterwards to close the crevice between tube and hole

ASME Section VIII Division 1 recognises full-strength and partial-strength welded joints, and its non-mandatory rules for tube-to-tubesheet joints cover joint strength. Tube-to-tubesheet welds are usually made by automatic orbital TIG welding.

Qualification and testing

• Mock-up qualification. Welded tube-to-tubesheet joints are normally qualified on a mock-up — a block of the tubesheet material with tubes welded into it — under ASME Section IX. The mock-up is sectioned and examined. ASME Division 1 also accepts a conventional groove-weld procedure qualification, but owner specifications for critical service commonly require the mock-up. See our guide to WPS, PQR and welder qualification.
• Sequence. Whether tubes are welded before or after final expansion is part of the qualified procedure. Welding first is common, because expanding first can trap moisture and contaminants in the crevice and cause porosity in the weld.
• Leak testing. Before the hydrostatic test, joints are usually leak tested on their own — commonly with low-pressure air and a soap solution applied to the tube ends, or with helium leak testing where the fluid is toxic or the specification is strict.
• Heat treatment. Post-weld heat treatment relaxes expanded joints, so the sequence is arranged so that heat treatment comes before final expansion.

Building the Bundle

The typical sequence in the shop:

1. Stationary tubesheet set up vertically or horizontally on a bundle assembly frame
2. Tie rods screwed in, baffles and spacers threaded on in order, last baffle locked
3. Sealing strips and impingement plate fitted
4. Tubes inserted one by one through the tubesheet and every baffle, usually by hand with a protective nose cone
5. Second tubesheet fitted (fixed or floating designs), or U-bends positioned and supported
6. Tube ends trimmed to the specified projection, cleaned and degreased
7. Tube-to-tubesheet joints welded and/or expanded to the qualified procedure
8. Joint leak test, then insertion into the shell — after the bundle clearance check through the shell

U-tube bundles

U-tubes are bent before insertion. The outer wall of the bend thins and the inner wall thickens, so the tube is ordered with enough wall that the bent portion still meets the required thickness; TEMA gives a formula for it. Small-radius bends in the inner rows are the most severe. Depending on material and service, U-bends may need heat treatment after bending — for example to reduce stress corrosion risk in some stainless steel services — and individual U-tubes are often hydrostatically tested after bending. The trade-off is well known: U-tube bundles handle thermal expansion freely, but the inside of the bends cannot be cleaned mechanically. See our guide to heat exchanger cleaning and maintenance.

Common Specification Mistakes

1. Triangular pitch on a fouling shell-side fluid. The shell side cannot be cleaned mechanically, and performance falls year after year.

   Prevention: Use square or rotated square pitch with 6.4 mm minimum cleaning lanes where the shell-side fluid fouls and mechanical cleaning is needed.

2. Baffle spacing set by heat transfer alone. The design meets the duty but vibrates, and tubes fail at the baffles within months.

   Prevention: Require a flow-induced vibration check covering inlet and outlet regions and U-bends, not only the TEMA span limit.

3. No sealing strips on a floating-head bundle. Large bypass clearance lets shell-side flow avoid the tubes.

   Prevention: Specify sealing strips sized to the bundle-to-shell clearance of the construction type.

4. Horizontal baffle cut in condensing or solids-bearing shell-side service. Condensate or solids collect behind each baffle.

   Prevention: Use vertical cut with drain notches for condensing, two-phase and solids-bearing shell-side fluids.

5. No impingement protection, or a plate too close to the nozzle. The first tube rows erode, or the erosion moves to the shell.

   Prevention: Check inlet momentum against TEMA's limits, and design the plate position and nozzle escape area, or use impingement rods or a vapour belt.

6. Expansion procedure not qualified. Joints are under- or over-expanded.

   Prevention: Qualify the expansion — and the weld, where used — on a mock-up with the actual tube and tubesheet materials, and record torque or pressure during production.

7. Heat treatment after expansion. Joints relax and leak.

   Prevention: Arrange the sequence so heat treatment precedes final expansion.

8. Tube-joint leak test skipped. Leaks show up at hydrotest, after the unit has been filled.

   Prevention: Include a joint leak test as a hold point in the inspection and test plan.

9. Replacement bundle built from an old drawing without a site survey. Shell distortion, corrosion or nozzle changes since the original build mean the new bundle does not fit.

   Prevention: Survey the shell — inside diameter at several points, straightness, nozzle positions, flange faces — before releasing a replacement bundle.

10. Tube-to-tubesheet materials chosen separately. Galvanic corrosion or differential expansion attacks the joint.

    Prevention: Select tube and tubesheet materials together, and use clad tubesheets where the tubesheet must match a high-alloy tube.

Supply from Kasko Makine

Heat exchangers and bundles

• Complete tubular heat exchangers: U-tube, floating-head, fixed tube plate and tube-in-tube designs, predominantly to TEMA Class R
• Replacement tube bundles built to original drawings or to a new thermal design, after a shell survey
• Retubing of existing bundles

Bundle components

• Tubesheets in forged and plate carbon steel, low-temperature, alloy and austenitic grades, and clad tubesheets
• Baffles, support plates, tie rods and spacers
• Sealing strips, impingement plates and impingement rods
• Tubes in carbon steel, stainless, duplex, nickel alloys, titanium and copper alloys

Joint qualification and testing

• Tube-to-tubesheet mock-ups qualified to ASME Section IX
• Roller and hydraulic expansion to qualified procedures
• Joint leak testing by air or helium, followed by hydrostatic testing

Engineering support

Send the TEMA data sheet or the existing bundle drawing, with the shell-side and tube-side fluids and the operating history if the bundle is a replacement. We will review the layout, baffle arrangement and joint design, flag vibration or bypass risks, and quote the bundle or the complete exchanger.

Certification

Material certificates to EN 10204 3.1 or 3.2, drilling and dimensional reports, mock-up qualification records, expansion and welding records, leak test and hydrostatic test certificates.

Send your data sheet or bundle drawing for a construction review and quotation within five working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Heat Exchanger Fabrication Series

• Heat Exchanger Manufacturing Process: Step by Step
• Shell and Tube Heat Exchangers: TEMA Types and Configurations
• Heat Exchanger Tube Materials: Selection Guide
• Heat Exchanger Cleaning and Maintenance

Frequently Asked Questions

Q: What is the minimum tube pitch in a shell-and-tube heat exchanger?
A: TEMA requires a tube pitch of at least 1.25 times the tube outside diameter. Where a square layout is used so the shell side can be cleaned mechanically, TEMA also requires continuous cleaning lanes of at least 6.4 mm (¼ in.). Tighter pitch fits more tubes but thins the tubesheet ligaments and increases shell-side pressure drop.

Q: Triangular or square tube pitch — which should I use?
A: At the same pitch, a 30° triangular layout fits about 15% more tubes than a square layout, giving more surface in the same shell, but its lanes are not straight so the shell side can only be cleaned chemically. Square or rotated square layouts with cleaning lanes allow mechanical shell-side cleaning and are the choice for fouling shell-side fluids.

Q: What is a typical baffle cut and baffle spacing?
A: Segmental baffles are commonly cut at 20% to 35% of the shell inside diameter, with 25% as the usual starting point. TEMA sets the minimum spacing at one-fifth of the shell inside diameter or 51 mm, whichever is greater, and the maximum by the allowable unsupported tube span. The spacing should also pass a flow-induced vibration check.

Q: Why do heat exchanger tubes fail at the baffles?
A: Usually from flow-induced vibration: tubes vibrating in oversized baffle holes rub against the baffle edges until they wear through, a mechanism called fretting. The causes are excessive unsupported spans, high shell-side velocity near inlet nozzles or in U-bends, loose baffle holes and thin baffles. Correct spacing, a vibration analysis and TEMA hole clearances prevent it.

Q: What is the difference between expanded and welded tube-to-tubesheet joints?
A: An expanded joint is made by plastically expanding the tube into the tubesheet hole and its grooves, by rolling or hydraulically, and relies on contact pressure for strength and tightness. A welded joint adds a seal weld for leak tightness or a strength weld that carries the full joint load. Critical services commonly combine welding with expansion.

Q: How are tube-to-tubesheet welds qualified?
A: Tube-to-tubesheet welding procedures are normally qualified under ASME Section IX on a mock-up that reproduces the actual tube, tubesheet material, hole geometry and welding process, which is then sectioned and examined to confirm penetration and soundness. Division 1 also accepts a conventional groove-weld qualification, but critical-service specifications commonly require the mock-up. Production joints are then made to the qualified procedure and leak tested before the hydrostatic test.

Q: Why are sealing strips used in a tube bundle?
A: Between the outermost tubes and the shell there is a clearance that shell-side fluid can flow through without crossing the tubes. The gap is smallest in fixed-tubesheet and U-tube bundles and largest in pull-through floating-head bundles. Sealing strips fixed to the baffles block this bypass and push the flow back into the bundle, restoring the heat transfer the design assumed.