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Quotes - Engineering - SalesSurface finish
Why the bore finish matters in a tubular heater
In a tube-in-tube unit the product touches hundreds of feet of tube bore plus every return bend and ferrule, and the finish of that surface is specified as Ra in microinches. 32 Ra µin (0.8 µm) is the common dairy and food product-contact finish; 20 Ra and 15 Ra are used for pharmaceutical duty, and ASME BPE gives the designations, SF1–SF3 for mechanically polished surfaces and SF4, 15 µin Ra maximum and electropolished, as the finest. Unlike a plate or a tank wall, the bore of a long tube cannot be reached with a polishing head once the unit is built, so the finish is bought with the tubing and protected through fabrication.
Finish matters because rough metal holds soil. Protein, sugar and starch films anchor in scratches and grain boundaries, build faster on a rough wall at heating temperatures, and take longer to strip in CIP; bacteria lodge in the same defects. A smoother bore runs longer between cleans on products that brown or stick. Ra is an average, though, and one defect outweighs it: a weld with suck-back, a pit, or a gap at a ferrule holds more soil than 10 µin of average roughness across the rest of the tube.
So a finish specification covers the tubing, the welds, the bends and the treatment after fabrication.
Product tubes are 316L sanitary tubing to ASTM A270 with a specified bore Ra; electropolish and passivation are added as the duty requires, and the annulus stays mill finish. The Material & Construction page compares the grades.
A long product tube is finished at the mill, not afterward
Specify the bore finish on the tubing, 32 Ra µin for most food duty and 15–20 Ra for pharma, and protect it through every weld and bend; electropolish removes 0.0005–0.001 in. and roughly halves Ra.
What the product asks of the surface
Viscous and sticky products are where finish pays: dairy desserts, cheese sauce, liquid egg, caramel and starch-thickened fillings form a burned-on layer on the hottest passes, and a finer bore delays it and lets the caustic step lift it sooner. Aseptic heating at 250–285°F asks for more, because sterility depends on a product surface with no crevice that a cleaning or sterilizing step cannot reach. Thin, clean liquids such as juice and water gain far less from a finer finish.
Electropolishing removes 0.0005–0.001 in. of metal, takes the peaks off first so Ra drops by roughly half, and leaves a surface richer in chromium than iron, which strengthens the passive film. It does not fill a pit, erase a deep scratch or change the chloride guidance for 316L. On corrugated tubes, where a polishing head cannot follow the profile, it is the practical way to bring the troughs and crests to one finish.
How the finish survives fabrication
Product welds are orbital autogenous butt welds made with inert backing gas, fully penetrated and smooth, with concavity and discoloration held within the stated acceptance limits and no suck-back beyond them; borescope inspection and a weld record are supplied where specified, to ASME BPE acceptance criteria when the order calls for it. Return bends are pre-polished sanitary fittings rather than tube bent in the shop, because bending roughens the outer radius and can wrinkle the inner one.
After assembly the product side is cleaned to ASTM A380 and passivated to ASTM A967 with citric or nitric acid. Passivation removes free iron left by tooling and handling; it does not polish, and it is repeated after any later mechanical work. The annulus is left mill finish, since polishing a utility surface buys nothing, and the outside of the unit is finished to a separate cosmetic specification.
What to put in a finish specification
A finish requirement written as a single Ra figure leaves the welds, bends and treatment open. These lines close them.
- Bore Ra in µin, maximum or average, and where it is measured
- Electropolished or mechanically polished, or the ASME BPE SF designation
- Weld acceptance standard, borescope record and percentage of welds inspected
- Passivation standard and acid: ASTM A967, citric or nitric
- Whether the annulus is also product contact, as on a regeneration section
- Documents: Ra readings, MTRs, weld map, passivation certificate as specified
Call and talk it through with an engineer: 1-805-484-2992
Finish questions for tubular units
For most cooked sauces, yes; 32 Ra µin is the common food product-contact finish and runs long production days when the CIP program suits the soil. Where a sauce carries starch or cheese and burns on in the last heating passes, a 20 Ra or electropolished bore on those sections extends the run. Weld quality on the product side matters more than the next step down in Ra.
Somewhat at the surface, not enough to change the grade. The chromium-rich film from electropolishing resists the start of pitting better than a mechanically polished bore, but the chloride guidance for 316L still applies, and a hot salted product past about 3% salt is reviewed for duplex or 6-moly whatever the finish. Treat electropolish as a cleanability choice.
Usually the tube lengths and fittings are electropolished before final assembly, then the welds are made with full purge and the finished product side is cleaned and passivated. Electropolishing a completed long section needs an internal cathode drawn through the bore and is done only where the specification demands it. We state on the quote which sequence the finish requirement implies.
Three ways together: the polished tubing arrives with mill readings for its heat, a profilometer measures the accessible bore at each end and on the bends, and a borescope records the welds where the order specifies it. A reading taken only on the outside of a fitting says nothing about the product side, so ask for bore readings by location in the documents.
Only when it carries product. On a regeneration section raw product runs in the annulus against pasteurized product in the inner tube, so both surfaces are product contact and both take the specified finish and passivation. On a plain hot water, glycol or steam annulus a mill finish is correct, and polishing it adds cost with no return.
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