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Heat Exchangers2026-08-1412 min read

Heat Exchanger Mechanical Design

Mechanical design turns a rated bundle into pressure parts: tubesheets, shell, channels, nozzles, and an expansion scheme that matches the metal temperatures.

mechanical designTEMAtubesheetASMEIS 2825expansion joint

Once thermal rating has a diameter, length, tube count, and nozzle sizes, mechanical design sizes the pressure boundary and the internals that TEMA governs. The named pressure code (ASME Section VIII Division 1 or IS 2825, when the enquiry specifies them) sets allowable stress, joint efficiency, opening reinforcement, and hydro test. TEMA is a mechanical design standard for shell-and-tube construction — class, minimum thicknesses, spans, tie rods — not a product certification and not a substitute for the pressure code.

TEMA practice on the drawing

The GA should carry TEMA type letters and class (R, B, or C) when the purchaser wants TEMA construction. Class R is the heavy refinery set (corrosion allowance floors, span limits, nozzle-load rules). Class B is the usual chemical-plant set. Class C is the lightest TEMA construction. Omitting class does not default to R; it defaults to whatever the bidder can defend. Ohm applies TEMA rules when they are named on the enquiry — we do not stamp a TEMA certificate, because TEMA does not operate that way.

Tubesheets

Tubesheets are designed for the worst combination of shell-side and tube-side design pressures the datasheet allows (including differential cases). Thickness, cladding or weld overlay, groove for pass partitions (required in some TEMA classes), and tube-to-tubesheet joint type (strength weld, expand, or both) are specification items. Fixed tubesheets are welded to the shell; a U-tube unit has one tubesheet. Ligament efficiency, pass-partition groove, and gasket seating at the channel all compete for the same piece of plate — changing pass count after the tubesheet is cut is a new tubesheet.

Shells, channels, and flanges

Shell and channel cylinders are sized for internal (and vacuum, if specified) pressure, corrosion allowance, and mill undertolerance. Girth flanges follow the specified flange standard and rating — matching existing piping is a purchaser input, not a designer preference. Pass-partition plates in the channel must be gasketed or welded as specified; a leaking partition turns a two-pass unit into a poorly distributed one-pass unit. Kettle shells add a vapour space and weir; diameter is a process as well as a pressure calculation.

Nozzles, reinforcements, and loads

Nozzle necks and reinforcement are calculated to the pressure code. TEMA Class R additionally discusses external nozzle loads; if the piping stress analysis will apply large moments, put those loads on the datasheet or they will not be in the calculation. Impingement protection is a mechanical attachment with thermal consequences. Saddles, lifting lugs, and anchor bolts belong on the GA with loads — site-fabricated saddles after arrival are how nozzles go out of alignment.

Expansion: joint, U-bend, or purchaser-specified floating head

Differential metal temperature between tubes and shell produces axial load in a fixed-tubesheet exchanger. Options Ohm manufactures: accept the load if it is within allowable; or fit a shell expansion joint (bellows) designed for the displacement and cycle life on the datasheet. U-tube construction avoids a second tubesheet and lets the bends flex; the bundle can be pulled. Floating-head rear ends (TEMA S/T/W) are an industry/TEMA option for expansion plus bundle withdrawal; Ohm does not manufacture floating-head units. If the purchaser spec requires a floating head, that must be explicit so scope is clear.

ℹ

An expansion joint is a specified component with a cycle life, not a generic 'bellows'. Give metal temperatures (or enough process data to calculate them) and expected start-up/shutdown frequency.

Named codes — when specified

  • TEMA: mechanical practice for S&T geometry and class — design standard, not a certification.
  • ASME VIII-1: pressure-part design, materials, PWHT, NDE extent via joint efficiency, hydro — when named. Code stamping, if required, must be stated on the RFQ; it is not implied by 'design to ASME'.
  • IS 2825: Indian unfired pressure vessel code when the enquiry names it instead of or in addition to ASME.
  • ASME IX: welding procedures and welder qualification when the construction code requires it.
  • Purchaser specs (API 660, owner standards): applied when attached — they are not a shop holding or a default.

RFQ for mechanical design

Send design pressures and temperatures both sides, MDMT if applicable, corrosion allowance, MOC line-by-line (shell, channel, tubesheet, tubes, bolting, gaskets), TEMA type/class, expansion requirement, nozzle sizes/ratings/loads if known, wind/seismic if the vessel is large, and whether TPI and an ITP with hold points are required. Attach the purchaser specification. Use /rfq?product=heat-exchangers. Fabrication is at Vadodara with MTC traceability, dimensional and visual inspection, NDT as the ITP names, and hydrostatic testing of pressure parts.

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