Cold-drawn seamless steel tubes for heat exchangers

The role of the tube in a heat exchanger

In shell & tube heat exchangers designed according to TEMA and ASME VIII Div.1, the tube is the element that ensures heat transfer between the tube-side fluid and the shell-side fluid. Maximising the overall heat transfer coefficient (U) requires thin walls, tight dimensional tolerances and smooth internal surfaces: requirements met by cold-drawn seamless tubes.

The cold-drawing process

After hot rolling, the tube undergoes cold drawing over a mandrel or plug, reducing its diameter and wall thickness. The process provides:

• very tight dimensional tolerances on outside diameter (OD) and wall thickness (WT);
• improved surface finish and low internal roughness;
• controlled work hardening, subsequently removed by stress-relief annealing or final normalising;
• excellent concentricity, useful for tube-to-tubesheet expansion.

Main standards and grades

• ASTM A179 / ASME SA-179: low-carbon steel for heat exchangers and condensers, with excellent ductility and suitability for U-bending.
• ASTM A192 / SA-192: tubes for high-pressure boilers.
• ASTM A213 (T5, T9, T11, T22): ferritic Cr-Mo alloy steels for superheaters and high-temperature service.
• ASTM A334 Gr.1 and Gr.6: low-temperature service.
• EN 10216-2 (P235GH, P265GH, 16Mo3, 13CrMo4-5, 10CrMo9-10): pressure tubes with elevated-temperature properties, required by the PED Directive.
• EN 10305-1: cold-drawn precision tubes for mechanical and hydraulic applications.

Tolerances and finishes

The outside diameter is generally supplied in inches or millimetres (e.g. 19,05 mm, 25,4 mm), with wall thickness expressed in BWG or mm and OD tolerances of ±0,10 mm for the most restrictive supplies. Lengths may be fixed or cut to size to optimise the bundle layout. For U-bend tubes, the minimum bend radius and stress-relief heat treatment at the bend are specified.

Non-destructive inspection and testing

To ensure the absence of leaks and through-wall defects, the following are performed:

• eddy current testing (ET) or flux leakage testing for surface and subsurface defects;
• hydraulic testing or, alternatively, full ET inspection;
• flattening and flaring tests;
• dimensional and straightness inspection;
• heat and product chemical analysis, tensile and hardness tests.

Corrosion and material selection

For aggressive fluids (seawater, chlorides, acids), austenitic stainless steels (A269 TP316L), duplex steels or copper-nickel alloys are considered; for steam and demineralised water, carbon steels remain the most economical choice. Contact us for a supply with an EN 10204 3.1 or 3.2 certificate.