Electropolished U-Tube
Heat Exchangers

With a profilometer on the accessible product-contact surfaces, recorded on a finish certificate that names the designation, the locations measured and the readings. Tube interiors are verified on the tubing supplier's certificate, because a stylus cannot be run down a bundle after assembly; the tubesheet face, channel and shell interior are measured directly. A surface that cannot be reached is not reported as meeting the designation, and the certificate says so. If your qualification protocol expects a reading from every wetted surface, agree at inquiry which surfaces will be measured and which will be certified by material record, because that is what a validation engineer will ask about and it cannot be fixed after the unit is closed up.

Yes, by a small but real amount, because it removes metal rather then redistributing it. On straight tubing the removal is uniform and is already allowed for in the tubing wall tolerance. The place to think about it is the U-bend, where the outside of the bend has thinned during forming and any further removal comes off an already reduced wall. The design wall is therefore specified before finishing, with the bend thinning and the polishing allowance both deducted, and the tube gauge is chosen so the minimum wall after both still meets the pressure calculation. Ask for the minimum bend wall on the drawing rather than the nominal tube gauge.

Before, as electropolished tubing from the mill, since the bore of a bent tube cannot be processed afterward. The straight legs keep their certified finish and chromium-enriched surface through bending. In the bend the surface is strained, the outside stretches and the inside compresses, and while the electropolished layer is still there it can no longer be measured or certified. So the bend remains the surface the specification has to accept as unverified, whichever route is taken. Pre-polished tubing is still the better route, because it gives the bend the best starting surface available. Bending after polishing also requires clean tooling and a lubricant that passivation will remove.

Normal CIP with caustic and nitric or phosphoric acid does not; those chemistries clean the surface and reinforce the passive layer. What removes it is pickling, meaning a nitric-hydrofluoric bath used to strip heat tint, and repeated aggressive derouging. If a loop rouges and is derouged often, each cycle takes a little of the chromium-enriched layer with the oxide, and after enough cycles the surface behaves like mechanical polish. Chloride-bearing sanitizers are a separate issue: they pit the surface rather than strip it. If a derouging program is planned, tell us at inquiry so the finish specification accounts for it.

Not measurably. The roughness differences involved are far below anything that affects the heat transfer coefficient, and the wall thinning is too small to matter. Where it helps is in keeping the clean condition longer: a smoother, more passive surface holds less soil and rouge, so the fouling allowance in the design is reached more slowly and the unit stays nearer its clean rating. That is a maintenance benefit, not a sizing one, and we rate an electropolished unit with the same fouling factors as a mechanically polished one. If a supplier offers a smaller exchanger on the strength of the finish, ask what fouling factor was used, because that is where the surface went.