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U-Tube Bundle Design
Everything about how a U-bundle performs is decided by geometry set before fabrication: how tight the bends are, how closely the tubes are packed, how many passes there are and how far the tubes span unsupported.
A U-tube bundle is a set of compromises resolved together rather than in sequence. A tighter bend radius packs more surface into the shell but thins the tube wall. Closer baffle spacing raises the coefficient but costs pressure drop and shortens the unsupported span. More passes raise velocity and multiply pressure drop. The rating software resolves the thermal side; the mechanical checks decide whether the answer survives.
Bend radius, tube pitch, pass count and baffle spacing are set before fabrication and cannot be changed once the tubes are bent. Each is a trade between surface, velocity, pressure drop and mechanical life: a tighter bend packs in more tubes but thins the wall, more passes buy velocity at the cost of pressure drop, and closer baffles raise the coefficient while shortening the span that governs vibration.
The three pages in this section take the bend radius, the pass arrangement and the baffle layout in turn. Each explains what the choice buys, what it costs and the point at which the rating should send you to a different shell rather than a tighter bundle.
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Tube Bundle Design Decisions
Every page in this section deals with one bundle design choice, what it trades off and how to word the specification so the result is right.
Call and talk it through with an engineer: 1-805-484-2992.
U-Bend Radius & Bending
Minimum bend radius, thinning on the outside of the bend, work hardening and residual stress, and the cases where stress relief becomes mandatory.
Tube Passes & Flow
Why a U-bundle is always even-pass, how pass partition plates work, the velocity window for fouling and erosion, and how fast pressure drop climbs.
Baffles & Tube Supports
Segmental and double-segmental baffle cut and spacing, rod baffles, and the unsupported span that decides flow-induced vibration.
Have This Ready When You Call
Start with both fluids and their flow rates, inlet temperatures and required outlet temperatures. Then the operating and design pressure on each side, the pressure drop you can allow, any code, alloy, hygienic or documentation requirement, and the available space and orientation.
After that, the practical facts that alter a bundle design: expected turndown, known fouling, the cleaning method and the room available to pull the bundle. If you do not know one of them, tell us which rather than guessing, and the sizing will rest on a stated assumption.
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Bundles fail far more often than shells. With a sound shell, a drop-in replacement bundle is the cheaper choice. Send the shell inside diameter, tube length, tube count, tube diameter, pass arrangement and tubesheet or flange dimensions, with a photograph of the tubesheet face, and we hold the original fit.
When the shell has corroded, the nozzles no longer fit the piping or the duty has moved on, the better answer is a complete unit rated to the new conditions. We will tell you which applies.
Talk It Through
Each quote we send is written by an engineer and states its basis. If you are unsure which bundle detail the duty calls for, send the data you have and we will tell you what is missing and why it matters.
Call and talk it through with an engineer: 1-805-484-2992, 8am to 5pm, or email Sales@HeatX.com.
Bundle Geometry Trade-Offs
TEMA allows a centerline radius as tight as 1.5 times the tube outside diameter, and anything near that limit must be checked against the wall thinning the pressure code allows. Only the innermost row sits at that radius; each row outward bends more gently, so the check is made on the inner row, and the heavier guage it may need is applied either to those rows alone or to the whole bundle, since the outside diameter and the tubesheet holes stay the same. Ask for the inner-row radius on the drawing. If it is at the minimum and the alloy is duplex or titanium, expect a stress relief operation or a change of tube size.
Baffle spacing is set by the shell-side velocity you want, bounded above by the maximum unsupported span TEMA allows for the tube size and material and below by pressure drop. Closer baffles raise crossflow velocity and the coefficient, but pressure drop climbs roughly with the square of velocity, so halving the spacing can cost nearly four times the shell-side loss for a modest thermal gain. The common error is quoting a generous shell-side allowable and then finding the plant pump cannot deliver it. Give the pressure drop you can actually spare and the spacing falls out of the rating.
Yes; the last baffle rarely sits at the start of the bend, so the U-bend region is the longest unsupported span in the bundle and the first place flow-induced vibration shows up. A bend support, usually a slotted plate or tie bars across the bend region, limits that span. A shell inlet nozzle near the bends makes it worse, because the entering stream hits the outer rows at full velocity; an impingement plate protects those rows but does not stiffen the bends. Specify both when the shell-side fluid is steam or gas.
