Welded and Seamless Tube in Copper Alloy Heat Exchangers
C68700 aluminium brass heat exchanger tube is produced in two ways. Seamless tube is made from a cast and hot worked billet that is cold drawn to size, and its quality is governed by ASTM B111/B111M, the standard specification for seamless copper and copper-alloy condenser tubes and ferrule stock. Welded tube is formed from strip that is rolled into a cylinder, joined by a continuous longitudinal weld and then annealed and drawn or sized, and welded heat exchanger tube is covered by its own product specification for welded copper and copper-alloy tube rather than by B111.
That distinction matters commercially and technically. Buyers who need a welded product should state the welded tube specification on the order, so that the weld quality, the testing regime and the acceptance limits are the ones written for welded tube instead of being read across from a seamless standard. This article compares the two routes and explains where each one belongs.
Common Grades of Copper Alloy Heat Exchanger Tube
| Grade | UNS number | Principal alloying elements | Typical application |
|---|---|---|---|
| Admiralty brass | C44300 | Copper, zinc, tin, arsenic inhibitor | Marine condensers, power plants, clean seawater |
| Aluminium brass | C68700 | Copper, aluminium, zinc, arsenic inhibitor | Seawater condensers and heat exchangers |
| 90-10 copper-nickel | C70600 | Copper with about 10 % nickel and iron | Desalination plants, shipbuilding, higher velocity water |
| 70-30 copper-nickel | C71500 | Copper with about 30 % nickel and iron | High pressure condensers, offshore platforms |
| 80-20 copper-nickel | C71000 | Copper with about 20 % nickel | Feedwater heaters and evaporators |
Among these, aluminium brass remains the most common welded or seamless choice for seawater cooled units because it balances corrosion resistance against thermal conductivity and cost. Copper-nickel grades are specified when the water is polluted, when velocity is high or when higher temperatures are involved.
Tempers and Supply Conditions
| Condition | Description | Suitability |
|---|---|---|
| Annealed (soft) | Fully recrystallised after the final drawing or weld anneal | U-bending, tube sheet expansion, general heat exchanger duty |
| Light drawn | Moderate cold work after annealing | Straight tubes needing additional stiffness |
| Hard drawn | Maximum strength and hardness | High pressure straight tube applications |
| As welded and annealed | Welded strip, weld normalised then sized | Long length bundles in low and medium pressure service |
In the annealed condition, ASTM B111 requires a minimum tensile strength of 345 MPa and a minimum yield strength of 125 MPa for C68700, which is the basis on which most seawater condenser tube is ordered. The anneal after cold working or welding is what removes residual stress, restores ductility and, for welded tube, normalises the weld zone so that its corrosion behaviour approaches that of the parent metal.
Welded versus Seamless Tube
Seamless tube is the default for critical condensers because it contains no longitudinal weld and therefore no weld-related discontinuities. It tolerates higher pressures and is universally accepted by utility specifications, and its dimensional control after cold drawing is excellent. Its disadvantages are cost and the limitation on the maximum length that can be produced and shipped.
Welded tube is made from strip, which makes it cheaper and allows very long coil or straight lengths with fewer joints in the bundle. Modern weld quality is high, and after the weld anneal the corrosion resistance of the seam is close to that of the parent strip. Its limitations are that the weld remains a potential site for preferential attack if the anneal is incomplete, that the pressure capability is lower than a comparable seamless tube, and that some end users simply do not permit welded tube in condensers regardless of quality.
A practical selection rule is to use seamless tube where the exchanger is critical, where pressure or water velocity is high, or where the specification is written for a power utility. Welded tube is appropriate for lower pressure heat exchangers, for large bundles where long lengths reduce joints and erection time, and for applications where the buyer's specification explicitly permits it. In every case the purchase order should state which product specification applies, the weld quality requirement, the testing regime and whether the weld must be positioned away from the zones of highest erosion.
Testing, Inspection and Installation
Both products are examined by chemical analysis, tensile and hardness testing, dimensional and surface inspection and hydrostatic pressure testing. Welded tube additionally requires verification of the weld, typically by eddy current examination of the weld zone and by flattening and reverse flattening tests that strain the seam and reveal poor fusion. Eddy current examination of the full length is specified for both seamless and welded tube when the duty demands it, with the sensitivity set by a drilled reference hole or equivalent artificial defect agreed with the buyer.
During installation the tube is expanded into the tube sheet with controlled force, baffles are checked for correct spacing and tubes are kept free of scratches and of ammonia-bearing marks, since any of these will initiate failures in seawater service. Tube ends should stay capped until erection and the bundle should be flushed before commissioning to remove debris that would otherwise settle and cause under-deposit corrosion.
FAQ
Q: Is ASTM B111 a specification for welded tube?
No, ASTM B111 covers seamless copper and copper-alloy condenser tube and ferrule stock; welded heat exchanger tube is ordered against the welded tube specification.
Q: What temper is used for C68700 heat exchanger tube?
The annealed condition with a minimum tensile strength of 345 MPa and a minimum yield strength of 125 MPa is the normal choice for U-bend and expanded tube bundles.
Q: Does a welded aluminium brass tube corrode at the weld?
Not if the weld is properly formed and fully annealed; an incomplete anneal leaves a stressed, compositionally disturbed seam that can corrode preferentially.
Q: How does welded tube compare with seamless tube in cost and length?
Welded tube is generally cheaper and can be supplied in longer lengths, which reduces the number of joints in a large bundle, but it is limited in pressure capability.
Q: Which copper alloy should be used for polluted seawater?
Copper-nickel grades such as C70600 and C71500 are preferred for polluted or higher velocity seawater, where aluminium brass would suffer impingement or selective attack.
Q: What testing proves the quality of a welded tube bundle?
Hydrostatic pressure testing, full length eddy current examination and flattening or reverse flattening tests of the weld, supported by chemical and mechanical test certificates for the heat.
