Thin-Walled Seamless C68700 Condenser Tube
A condenser tube is a pressure boundary with a very small cross-section of metal doing all the work. Thin walls raise heat transfer and cut weight, but they also shrink the corrosion allowance available over a thirty year plant life. C68700 in the form of thin-walled seamless round tube is the answer that the power and desalination industries settled on: an inhibited aluminum brass, drawn to a closely controlled wall, supplied annealed and dimensionally straight so it can be rolled into a tube sheet without splitting.
The same alloy is also produced as plate, foil and wire, and it appears in many stock lists under several trade descriptions. For condenser service the relevant product is always the seamless round tube, and the parameters that matter are wall thickness, grain size, straightness and cleanliness of the bore.
Seamless Versus Welded Construction
Seamless tube is made by piercing and hot extruding a billet and then cold drawing to final size, which leaves the tube with no longitudinal seam. A welded tube is formed from strip and joined along its length. Both routes can produce sound C68700 tube, and both are covered by recognised specifications, but the requirements of condenser service favour the seamless product for three reasons.
No weld seam means no fusion line, no weld bead and no heat affected zone to act as a preferential site for corrosion or cracking.
Wall thickness and concentricity can be held more tightly over long lengths, which matters when the wall may be only a fraction of a millimetre thicker than the minimum allowed after expansion.
Seamless tube accepts repeated roller expansion into grooved tube sheet holes without the risk of opening a seam.
Where space or cost pressure is severe, welded tube can still be specified provided the governing standard permits it for the application and the inspection regime is agreed. On critical seawater condensers, seamless tube remains the default.
Size Window and Wall Thickness Selection
Condenser tubes are supplied in a standard family of outside diameters and wall thicknesses, and the selection is a trade between heat transfer, pressure rating, expansion behaviour and corrosion allowance. The figures below describe the range that dominates power station and desalination condensers; the exact size should be taken from the size tables of the governing specification and from the thermal design of the plant.
| Parameter | Typical condenser service range | Main design driver |
|---|---|---|
| Outside diameter | About 12 mm to 32 mm | Surface area per unit length and tube sheet layout |
| Wall thickness | About 0.7 mm to 1.6 mm | Corrosion allowance, pressure, and expansion strength |
| Supplied length | Straight lengths to around 12 m; U-bend to drawing | Condenser length and handling limits on site |
| Temper | Annealed, or fine grain annealed | Formability, expansion and ammonia resistance |
A thinner wall raises the overall heat transfer coefficient because the metal path is shorter, and it also reduces tube weight and cost. The limits are practical. The wall must survive roller expansion into the tube sheet without falling below the minimum permitted thickness, it must resist the differential pressure between water and steam side, and it must retain enough metal to reach the next planned retubing outage.
Grain Size, Annealing and Residual Stress
Grain size is a controlled characteristic in condenser tube, not a by-product of the mill route. A fully recrystallised, fine and uniform grain structure gives predictable elongation during expansion and a surface film that forms evenly along the bore. Coarse or mixed grain structure behaves the opposite way: expansion becomes erratic and cracking risk rises at the tube ends.
Annealing also controls residual stress. Ammonia and its compounds, which are present in some condensers and in many closed circuits, cause stress corrosion cracking in stressed copper alloys. Tube that has been properly annealed and stress relieved is far less exposed to that failure mode, which is why hardened or heavily drawn tube is not accepted for these circuits.
Testing, Tolerances and Delivery Condition
Thin walls leave little margin, so inspection matters more than it does on heavy wall pipe. A typical delivery regime for C68700 condenser tube includes:
Eddy current examination of the full length to detect wall defects and discontinuities.
Dimensional inspection of outside diameter, wall thickness and concentricity.
Hydrostatic or pneumatic pressure testing where required by the governing specification.
Visual inspection of bore and outside surface for scratches, dents and embedded foreign matter.
Straightness control so that tube can be pushed through support plates on site without forcing.
Tubes are normally supplied with ends capped and the bore protected for transport. Any bore contamination that dries in place becomes a nucleation site for localised attack once the condenser is commissioned, so clean handling and clean storage are part of the material specification in practice.
Installation: Expanding, Support Spacing and Vibration
Roller expansion fixes the tube into the tube sheet by cold working the wall outward into the hole. Thin walls expand easily, which is an advantage, but over expansion thins the wall and under expansion leaves a leak path. Expansion should therefore be controlled by procedure and checked by measurement rather than judged by feel. Grooved holes and appropriate expansion length improve the joint and reduce the risk of tube pull out.
In service, thin tubes fail from flow induced vibration more often than from uniform corrosion. Support plate spacing, baffle arrangement and the avoidance of gaps that let water jet through are the design measures that protect the tube. A tube that is free to vibrate will fret at support plates, wear through and leak, however good the alloy is. Careful handling during installation, with the tube protected from impact and from scoring at the tube sheet, completes the picture.
FAQ
Q: What is C68700 thin-walled seamless condenser tube used for?
It is used as the heat transfer surface in seawater cooled condensers, evaporators, distillers and heat exchangers, particularly where cooling water velocity is high and the water carries salt or suspended solids.
Q: Why choose seamless rather than welded tube?
A seamless tube has no weld seam, so there is no fusion line or heat affected zone to corrode preferentially, and it can be expanded into a tube sheet repeatedly without risk of opening a seam. It also allows tighter control of wall thickness and concentricity.
Q: How thin can the wall be?
The wall is set by the design pressure, the corrosion allowance needed to reach the next retubing outage, and the minimum thickness permitted after roller expansion. Thinner walls transfer heat better, so the correct answer is the thinnest wall that still satisfies those three limits.
Q: Why is grain size specified on the order?
Grain size controls how the tube behaves during expansion and how evenly the protective film forms. A fine, uniform, recrystallised grain structure gives predictable forming behaviour, while coarse or mixed grain structure increases the risk of cracking at the tube ends.
Q: Is eddy current testing always required?
For thin-walled condenser tube it is normal practice and is called for by the relevant product specifications. The full length examination detects wall defects, dents and discontinuities that dimensional checks cannot reveal.
Q: What causes early failure of thin-walled tubes in service?
Flow induced vibration and fretting at support plates are the most common causes, followed by inlet end impingement and poor expansion practice. Correct support spacing and careful installation usually matter more than any change of alloy.
