The Question Behind the Question
There is no single best material for heat exchanger tubes. The right answer is the least expensive material that will survive both process streams for the design life of the plant, and that depends on five variables acting together: the chemistry of the tube-side and shell-side fluids, the operating temperature and pressure, the flow velocity, the required heat-transfer duty, and the mechanical arrangement of the exchanger.
In practice the selection process narrows the field quickly. Copper and copper alloys are used for water-cooled dut anywhere the temperature is moderate and the water is compatible with copper, because they conduct heat better than any other common tube material and are easily expanded into tubesheets. Carbon steel takes over for hot oil, steam and higher-pressure service. Stainless steels and titanium are specified when the process is aggressive or the temperature high, accepting a large penalty in thermal conductivity.
Thermal Conductivity by Material Family
| Tube material | Typical thermal conductivity at 20 °C, W/(m·K) | Usual application field |
|---|---|---|
| C12200 deoxidised copper | 339 | Fresh water, refrigeration, HVAC, moderate temperatures |
| C44300 admiralty brass | 109 | Clean sea water, brackish water, condensers |
| C68700 aluminium brass | approximately 100 | Polluted or high-velocity sea water |
| C70600 copper-nickel 90/10 | 45 | Sea water, sand-laden water, marine coolers |
| C71500 copper-nickel 70/30 | 29 | High-velocity sea water, severe duty |
| Carbon steel, ASTM A179 / A192 | approximately 50 | Steam, oil, high pressure, moderate temperature |
| Titanium Grade 2 | approximately 22 | Sea water, chlorinated water, chemical streams |
| Austenitic stainless 304 / 316 | approximately 16 | Food, chemical, high-temperature, clean streams |
The table explains why copper alloys dominate water-cooled duty: a copper tube transfers heat roughly twenty times faster than a stainless steel tube of the same geometry, so a much smaller surface area, fewer tubes and a smaller shell are needed for the same duty.
Corrosion, Velocity and Temperature Limits
Conductivity alone does not decide the material. Each family has limits that a designer must respect.
Copper and admiralty brass give good service in clean fresh and sea water. They are sensitive to ammonia and ammonium compounds, to sulphides, and to erosion at high velocity, and they should not be used where the water side is heavily polluted without a protective film-management programme.
Copper-nickel alloys tolerate higher water velocities than brass and resist sand erosion, biofouling and polluted sea water much better. They are the standard answer for marine coolers and for once-through sea water systems in which the water quality is unpredictable.
Carbon steel is strong, cheap and well understood, but requires water treatment and protective measures on the water side; it is normally chosen for steam and oil service where the tube is not wetted by an aggressive aqueous stream.
Stainless steel resists many chemical streams and high temperatures but is vulnerable to chloride pitting and stress-corrosion cracking in hot chloride-bearing water, which is precisely the duty for which copper alloys are chosen.
Titanium offers excellent sea water resistance and very high velocity tolerance, with good resistance to chlorination, at a high material cost and with low conductivity.
Matching the Material to the Duty
| Duty | First choice | Alternative |
|---|---|---|
| Fresh cooling water, low velocity | C12200 copper | Carbon steel with treatment |
| Clean sea water, moderate velocity | C44300 admiralty brass | C70600 copper-nickel |
| Polluted or high-velocity sea water | C70600 copper-nickel | C71500 or titanium |
| Chlorinated sea water, very high velocity | Titanium | C71500 copper-nickel |
| Steam and condensate at high pressure | ASTM A179 carbon steel | Stainless steel |
| Aggressive chemical stream | Stainless steel or titanium | Nickel alloys |
The governing standards follow the material: ASTM B111 for copper-alloy condenser and heat exchanger tubes, ASTM A179 and A192 for carbon steel, ASTM A213 for alloy steel, ASTM A268 and A789 for ferritic and duplex stainless steel, and ASTM B338 for titanium tube. European and Japanese equivalents, such as EN 12451 and JIS H3300, cover the copper-alloy tube range.
Design and Fabrication Notes
Once a family is selected, three details decide whether it performs. Wall thickness must cover the corrosion and erosion allowance, not only the pressure requirement. The tube-to-tubesheet joint must be matched to the material: copper alloys are usually rolled, steels may be welded, and titanium requires careful joint preparation. Tube supports and baffle spacing must keep the highest local velocity below the level at which the chosen alloy starts to erode.
Water-side cleanliness is the final factor. Fouling resistance usually dominates the overall heat-transfer coefficient after a few months in service, so the material with theoretically higher conductivity will not deliver its advantage unless strainers, filters and cleaning routines keep the tube surfaces clean.
FAQ
Q: What is the best material for heat exchanger tubes?
Copper and copper alloys for most water-cooled duty, because they combine the highest thermal conductivity with good corrosion resistance and easy rolling into tubesheets. Carbon steel, stainless steel or titanium become the better answer as temperature, pressure or corrosivity rises.
Q: Are copper alloys better than stainless steel for heat exchangers?
For water-cooled duty, usually yes. Copper alloys conduct heat far better and resist the chloride-bearing waters that cause pitting and stress-corrosion cracking in stainless steel. Stainless steel is preferred for high-temperature, clean or strongly chemical streams.
Q: Which copper alloy should be used for sea water?
Admiralty brass such as C44300 for clean sea water at moderate velocity, aluminium brass C68700 for polluted water, and copper-nickel C70600 or C71500 for higher velocities, sand-laden water and unpredictable water quality.
Q: When is titanium the correct choice?
When the water is chlorinated or polluted, when velocity is very high, or when a very long interval between retubings is required. The penalty is low thermal conductivity and a high purchase price, which must be justified by the reduced maintenance.
Q: How is tube velocity taken into account?
Each family has an upper velocity limit above which impingement and erosion begin. Admiralty brass limits are the lowest of the copper alloys, copper-nickel and titanium tolerate progressively more, and the design must keep inlet velocities and baffle-gap velocities below the limit for the selected material.
Q: What standards cover heat exchanger tubes?
ASTM B111 and its ASME equivalent SB111 for copper-alloy condenser tubes, ASTM A179 and A192 for carbon steel, ASTM A213 for alloy steel, ASTM A268, A789 and A790 for stainless and duplex grades, and ASTM B338 for titanium.
