TEMA Classes and the 3-A Symbol

Using TEMA and 3-A on a sanitary order

What a TEMA class buys

Specifying TEMA class C on a sanitary shell-and-tube exchanger tells the fabricator to use TEMA's standard tolerances on tube hole diameter and pitch, baffle cut and spacing, shell-to-baffle clearance, and tie rod count; the class C minimum thicknesses and a corrosion allowance appropriate to commercial service; and the standard nomenclature so the drawing reads the same to any engineer. It also brings in TEMA's guidance on flow-induced vibration and on nozzle loads, which matters on long sanitary bundles with thin-wall polished tube.

Class B raises the minimums and tightens the corrosion allowance for chemical service, and class R goes further for refinery duty. Neither is needed for a food or pharmaceutical exchanger unless the buyer's corporate standard says so; the extra metal does nothing for cleanability and adds weight and cost. Class C is the sensible default and is what most sanitary units are designed to.

• Class C: food, dairy, beverage, pharma water, CIP supply
• Class B: where the buyer's engineering standard requires it across the plant
• Class R: rarely justified on sanitary duty

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Reading a TEMA type

The three letters describe front head, shell and rear head. A BEM is a bonnet front head, single-pass shell and fixed tubesheet rear: the simplest and most common sanitary construction, fully welded on the product side with no internal gaskets. A BEU has the same front head and shell with a U-tube bundle, which absorbs thermal expansion and can be pulled for inspection. An AEW or BEW adds a removable channel and an externally sealed floating tubesheet. For sanitary duty the fixed-tubesheet and U-tube types dominate, because floating-head designs put a gasket inside the shell where it cannot be inspected or cleaned in place.

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Where TEMA stops on sanitary duty

TEMA is silent on everything that makes an exchanger sanitary. It does not specify surface finish, does not require the unit to drain, does not address sanitary fittings, elastomer compliance or product-side weld grinding, and does not cover plate, tube-in-tube or double-tubesheet arrangements beyond noting that they exist. A specification that reads TEMA class C and nothing more will produce a well-made industrial exchanger with a mill-finish tube side and NPT nozzles. The sanitary requirements have to be added explicitly, and the fabricator applies them on top of the TEMA mechanical design.

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The 3-A symbol on the nameplate

The symbol is earned by the fabricator, not by the unit. A shop applies to 3-A SSI for authorization to build to a specific standard (12-07 for tubular exchangers), submits its design, passes a Third Party Verification by a certified evaluator, and receives an authorization number. Units built to that design may then carry the symbol with that number on the nameplate. The authorization is time-limited and renewed by re-evaluation, and the register of current authorizations is public, so a buyer or inspector can check the number on any plate.

What the symbol tells the inspector is that the product side was designed to the standard and that the shop's practice has been checked. It does not certify the individual unit's finish readings; those are in the finish report. It does not cover pressure; that is the U-stamp beside it. And it does not transfer: a unit modified in the field or rebuilt by a shop without authorization loses the right to the symbol.

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Specifying both

Write TEMA class C with the type designation you expect (BEM or BEU for most sanitary units), then the sanitary standard and the symbol requirement as separate lines, then the pressure code. Three lines, no overlap, no gap. Call and talk it through with an engineer: 1-805-484-2992