C68700 Brass Pipe in Seawater Coolers and Heat Exchangers
UNS C68700, aluminium brass, is one of the small group of copper alloys whose properties suit seawater cooling service particularly well. A copper matrix of 76.0-79.0 % is alloyed with about 2 % aluminium and a controlled arsenic addition, producing a material that combines high thermal conductivity with a tough, self-repairing surface film. In a seawater cooler the same alloy may appear in two roles: as the heat transfer tube inside the bundle, and as the pipework, headers and fittings that carry cooling water to and from the exchanger.
Its selection is almost always driven by water quality. Fresh water circuits are usually served by carbon steel, stainless steel or copper; it is when chlorides are present, when the water is aerated and moving, and when corrosion rather than pressure determines the design that aluminium brass becomes competitive.
Grade Equivalents and Composition
The alloy is designated differently in each standard system, and the ranges are not identical, so the designation alone is not enough to confirm that two products are interchangeable.
| System | Grade | Nominal composition | Notes |
|---|---|---|---|
| ASTM B111 (UNS) | C68700 | Cu 76.0-79.0 %, Al 1.8-2.5 %, As 0.02-0.06 %, Zn balance | Seamless condenser tube and ferrule stock |
| DIN 1785 / EN | CuZn20Al2 (2.0460) / CW702R | Cu 76.0-79.0 %, Al 1.8-2.3 % | Lower aluminium top limit in the German grade |
| BS 2871 Part 3 | CZ110 | Cu 76.0-78.0 %, Al 1.8-2.3 %, As 0.02-0.06 % | Tubes for heat exchangers |
| JIS H3300 | C6870 | Cu 76.0-79.0 %, Al 1.8-2.5 %, As 0.02-0.06 % | Copper and copper alloy tubes |
| GB/T 5231 | HAl77-2 | Cu 76.0-79.0 %, Al 1.8-2.5 %, As 0.02-0.06 % | Chinese wrought copper alloy designation |
Iron, lead, tin and antimony are held to low residual limits because they disturb the protective film or embrittle the structure. Any supplier comparison should therefore be made against a full chemical analysis rather than against the grade name.
Sizes, Wall Thickness and Product Forms
Aluminium brass is produced as seamless tube in outside diameters from a few millimetres up to several hundred millimetres and in wall thicknesses from under a millimetre to heavy pipe sections, in round, square or rectangular form according to the application. Lengths are normally supplied in the range of one to twelve metres, or cut to the exchanger drawing.
| Parameter | Typical range | Selection driver |
|---|---|---|
| Outside diameter | From about 6 mm to several hundred mm | Bundle layout, flow area, header connection |
| Wall thickness | 0.2 mm to over 100 mm depending on form | Pressure rating plus corrosion allowance |
| Length | 1 m to 12 m, or cut to drawing | Tube sheet span, handling and transport |
| Temper | Annealed, light drawn or hard drawn | Bending and rolling requirements |
| Surface | Bright, polished, oiled or as drawn | Corrosion initiation risk in seawater |
For heat transfer tube the wall is selected from a corrosion allowance rather than from pressure alone, because the failure mode in a seawater cooler is normally wall thinning and pitting rather than bursting. For the connecting pipework, pressure rating and support spacing dominate, and heavier wall is chosen where the water carries sand or where flow is turbulent at bends.
Service Limits in Seawater
Aluminium brass thrives in clean, aerated seawater moving at a moderate velocity, and is notably resistant to impingement attack compared with plain brass. The following limits reflect normal good practice rather than a guarantee, because water chemistry and operating pattern both matter.
Velocity: keep flow within the moderate range usual for aluminium brass; excessive velocity or repeated air bubbles attack the inlet end of tubes.
Temperature: condenser service is a low temperature duty; sustained operation well above normal cooling water temperatures shortens tube life.
Water quality: avoid high sulphide content, heavy suspended sand and ammonia-bearing streams.
Stagnation: standing, poorly aerated water prevents the protective film from repairing itself and leads to pitting.
Cleaning: use inhibited chemicals, soft brushes or water jets only, never strong mineral acids.
Fouling performance is a further advantage. Copper released at the wetted surface limits the settlement of marine organisms, so aluminium brass tube bundles foul more slowly than titanium or stainless steel equivalents and lose less heat transfer between cleanings.
Installation, Handling and Inspection
Tube is rolled into the tube sheet with controlled expansion; over-rolling leaves residual tensile stress and encourages stress corrosion cracking, so the expansion torque or pressure must be specified by the exchanger designer rather than left to the fitter. Baffle spacing and support plate positions are checked so that tubes cannot vibrate and fret against each other, since fretting in a vibrating bundle is a common cause of early failure. When aluminium brass pipework is joined to other materials, insulating spools or compatible fittings prevent galvanic attack, particularly where stainless steel is involved.
Handling rules are simple and often ignored. Keep the tubes dry and ventilated in storage, never mark them with ammonia-containing or graphite pens, and protect the ends with caps until installation. Incoming inspection normally includes chemical analysis, tensile and hardness testing, dimensional and surface examination, grain size, hydrostatic testing, expansion and flattening tests and, when specified, eddy current examination of the full length. For pipework, additional checks cover wall thickness measurement at the ends and ultrasonic or eddy current verification where the specification requires it.
FAQ
Q: Is C68700 pipe suitable for seawater?
Yes, it is a standard alloy for seawater coolers and condensers, provided the water is reasonably clean and the flow velocity is kept within the moderate range the alloy is designed for.
Q: What is HAl77-2?
It is the Chinese wrought copper alloy designation for the 77 % copper, 2 % aluminium brass with an arsenic addition, corresponding to UNS C68700.
Q: How thick should the wall of an aluminium brass cooler tube be?
The wall is set by the required corrosion allowance for the water chemistry and expected life, then checked against the design pressure rather than the other way round.
Q: Can aluminium brass tube be used in fresh water circuits?
It can, but it is normally reserved for brackish and seawater duty because cheaper materials such as copper or stainless steel perform well in clean fresh water.
Q: Why must over-rolling of tubes into the tube sheet be avoided?
Excessive expansion leaves the tube under residual tensile stress, which promotes stress corrosion cracking in service and shortens bundle life.
Q: How should aluminium brass coolers be cleaned?
Use inhibited chemical cleaners, soft brushes or water jetting, and avoid strong mineral acids and ammonia-bearing solutions, which attack the alloy or cause cracking.
