The wall is the smallest resistance in the sum

Heat crossing a tube wall passes five resistances in series: the film on the hot side, the deposit on it, the wall itself, the deposit on the cold side, and the film there. Only one of the five is steel. The resistance of the wall is its thickness divided by its conductivity, and the numbers are small. A 2.0 mm wall of 304 at 16.2 W/m.K works out at 0.00012 m²K/W; the same wall in carbon steel, which the 904L datasheet puts at roughly three times the conductivity of any austenitic, is about 0.00005. Doubling the wall from 1.2 to 2.4 mm adds only a fraction of that again. Fouling allowances specified on the two sides routinely sit between 0.0002 and 0.0005 m²K/W each — general practice rather than a mill figure — so a single fouling film can cost the duty more than the entire alloy wall does. The practical reading is that the wall is sized for pressure and corrosion life; its thermal price is real, but it is the smallest line in the sum.

What each alloy costs in conductivity

The conductivity ladder is worth knowing because it bounds the argument. 310S publishes 13.8 W/m.K at 100 °C rising to 18.7 at 500 °C — conductivity improving with temperature, which surprises buyers who assume the opposite. 904L sits at 12 W/m.K at 20 °C, 825 at 11.3, and 625 lists 8.5 kcal/(hr.m.°C) at 21 °C climbing to 19.6 by 871 °C, roughly a 10-to-23 W/m.K band across the working range. The spread from the leanest austenitic to the fullest nickel alloy is therefore a factor of about two. That is not nothing; on a long service run it shows up as a slightly hotter tube wall for the same duty. But it is second-order beside the chemistry differences that decide whether the tube survives its first shutdown, which is why the A213 boiler tube programme — 12.7 to 114.3 mm outside diameter, 0.8 to 15 mm wall, grades T2 through T92 — is sold by grade and condition, never by a conductivity figure.

Fouling is a materials decision

Most of what a heat exchanger loses to fouling, it loses to corrosion products and scale, and both are grade decisions. 304/304L pipe carries clean water duty; the 316/316L page adds 2 to 3 per cent of molybdenum for chloride service; 904L names sulphuric and phosphoric acid, flue gas desulphurisation and seawater on its application list; 825 is built for oxidising and reducing media at once and is titanium-stabilised so it can be welded without sensitising. Where the medium defeats most grades, C-276 is one of the few materials that withstands wet chlorine gas, hypochlorite and chlorine dioxide. Velocity is the other half of the argument: at high water speeds erosion-corrosion strips the protective film, and that is where the duplexes earn their place — S32760 is published with PREN 40 and explicit resistance to pitting, crevice and erosion corrosion in seawater at elevated temperature, while the titanium programme corrodes at under 0.005 mm a year in seawater, about a twentieth of the 316L rate.

Temperature reorders the ladder

Above roughly 400 °C the ordering changes, because oxidation and creep join the argument. 310S is rated for continuous service to 2000 °F and lists heat exchangers and furnace parts among its applications, with 24 to 26 per cent chromium doing the scaling work; the 310S pipe page carries the same grades into tube form. Stabilised grades follow: the 347 tube page recommends the grade for intermittent heating between 427 and 899 °C, where columbium and tantalum hold carbon away from the grain boundaries, and 347H carries the same logic at 515 MPa tensile with a columbium floor of eight times carbon for the creep end. 321 does the same job with titanium. In the ferritic boiler range, P22 and P91 cover water-wall and superheater duty, and above them 625 keeps useful strength from cryogenic to 1093 °C while 718 holds creep-rupture strength to 700 °C. One ceiling is easy to miss: 2507, so strong at room temperature, should stay below 316 °C, where extended exposure costs it both toughness and corrosion resistance.

Where strength takes metal out of the wall

Conductivity punishes added wall; strength rewards removing it. The 2205 page puts the yield at about twice that of the austenitic grades, which lets a designer thin the wall against the same pressure, and the 2205 pipe page shows the numbers: 550 MPa yield against 800 MPa tensile at 7.8 g/cm³. The 2507 page states the consequence plainly — a light gauge of 2507 reaches the design strength of a thicker nickel alloy, and the resulting weight saving reduces the cost of the whole fabrication. Less metal in the pressure boundary is also less metal in the thermal path, so the strength argument and the heat transfer argument pull the same way. The S31803 programme makes the substitution practical at scale: 6 to 1174 mm outside diameter, 1 to 80 mm wall and lengths to 12,000 mm, with S32205 alongside it for welded work.

Condition, finish and the tests behind the tube

A tube starts fouling at its surface, so the delivery condition belongs on the order. The duplex listing spells the options out — 2B, pickled, polished, brushed, sandblasted, BA and EP — with electropolish the smoothest surface a tube leaves the mill with. The 347H page quotes annealed-and-pickled, polished and cold-drawn delivery conditions, and its test list of flattening, flaring, visual and chemical checks is the mechanical half of acceptance. The A213 listing adds the volumetric half: eddy current testing, hydrostatic testing or ultrasonic examination, plastic caps on both ends, and single or double random lengths up to a maximum of 25,000 mm. Where the bundle is built by a fabricator rather than a tube shop, the welding, cutting and surface treatment around the tubesheets are priced on the same enquiry rather than assumed.

What the enquiry needs

Six lines settle most of it. State the media on both sides with chloride content and pH, because that picks the rungs from 304 through 904L, 825 and C-276 to titanium. State design and operating temperature, which moves the choice to 321, 347H, 310S or the nickel grades and flags the 316 °C duplex ceiling. Give the fouling allowance and the design velocity, so erosion is argued before corrosion arrives. Fix the tube as outside diameter against wall — 12.7 to 114.3 mm by 0.8 to 15 mm covers most bundles — rather than as a schedule. Ask for the finish, the volumetric test and the certificate in one line. And say whether the fabrication — bends, tubesheet drilling and support sections from the pipe and tube range — is to be quoted with the tubes. A bundle enquiry carrying those lines is answerable in days; one carrying only a grade and a tonnage becomes a corrosion argument after delivery.