The Short Answer: Why Conductivity Drives Heat Exchanger Design
The core job of heat exchange equipment is to move heat from one medium to another through a separating wall. The tube wall is the only path available, so the rate at which heat crosses it sets the size, the surface area and the pumping cost of the whole unit. CuNi 90/10 white copper tube, standardised as UNS C70600, is a copper-nickel alloy with roughly 10% nickel plus small iron and manganese additions. It combines a conductivity that is high for a seawater-resistant alloy with the corrosion behaviour that marine and desalination duty demands.
Two properties therefore have to be read together. Conductivity determines how much heat a given area of tube wall can carry per degree of temperature difference. Corrosion and fouling resistance determine how long that area keeps working before the tube wall thins or the surface becomes insulated by deposits. CuNi 90/10 is specified precisely because it performs well on both counts rather than optimising only one.
How Heat Moves Through a CuNi 90/10 Tube Wall
Heat transfer through any tube wall combines a convective film on the hot side, conduction through the metal, and a convective film on the cold side. Metal conduction is usually the smallest of the three resistances, which is why alloying for corrosion resistance is affordable: even a moderate conductivity keeps the wall resistance low compared with the two boundary layers.
| Tube material | Typical thermal conductivity at room temperature | Seawater corrosion performance |
|---|---|---|
| CuNi 90/10, UNS C70600 | About 45 W/(m·K) | Good, widely used for seawater tube |
| Pure copper, UNS C11000 | About 390 W/(m·K) | Poor in flowing seawater |
| Stainless steel, type 316 | About 16 W/(m·K) | Good, but sensitive to chloride pitting and crevice attack |
The comparison shows the design trade-off clearly. Pure copper conducts far better but does not survive seawater service. Type 316 stainless steel resists seawater better in some conditions but conducts only about a third as well as CuNi 90/10, so it needs more surface area or a larger temperature difference for the same duty. CuNi 90/10 sits between the two and removes the need to compensate for a low-conductivity wall. CuNi 90/10 tube is normally supplied to ASTM B111 for condenser and heat exchanger service, with seamless pipe and tube also covered by ASTM B466 and the equivalent Japanese designation C7060 under JIS H3300.
Faster Heat Transfer in Desalination and Evaporation Duty
In a multi-stage flash or multi-effect desalination plant, seawater is heated to promote evaporation and the vapour is then condensed on the cold side. Both steps rely on heat crossing tube walls quickly. High conductivity lets the heating stage bring the brine up to its target temperature in less time and with a smaller temperature difference between the heating medium and the brine.
Shorter heating cycle - the same steam or hot-water heat source raises the brine to the target temperature faster, which shortens the residence time per stage.
Higher heat transfer per square metre - more heat crosses each unit of tube surface, so a given duty can be met with less tube area or fewer stages.
Better condensation - on the condensing side the same conductivity removes the latent heat quickly, keeping the vapour-to-condensate path stable.
Because the alloy also resists the aggressive, aerated, chloride-rich seawater of a desalination plant, the conductivity advantage does not decay as it would on a plain copper or mild steel tube that corrodes or scales over time.
Operational Stability: Marine Engine Cooling and Local Overheating
Conductivity also governs how evenly temperature is distributed. If heat cannot leave a local area quickly, the wall above that area overheats while the surrounding metal stays cooler. That temperature spread stresses tube-to-tubesheet joints, drives uneven thermal expansion and, in the worst case, allows the coolant to boil locally and form a vapour pocket that stops heat transfer entirely.
In a ship engine cooling circuit, large amounts of heat leave the engine in the coolant and are transferred to seawater in a shell-and-tube heat exchanger. CuNi 90/10 tube moves that heat away from the wall fast enough to keep the coolant inside its design temperature band and to prevent local vaporisation. The result is stable cooling capacity, fewer unplanned shutdowns caused by temperature excursions, and a longer working life for the exchanger and the engine it serves. The same reasoning applies to lubricating oil coolers, fresh water generators and central cooling systems on board.
Lower Energy Cost in Continuous Duty
Every degree of extra temperature difference a heat exchanger needs must be paid for in energy at the heat source. A tube wall with higher conductivity reaches a given duty with a smaller driving temperature difference, so less fuel or steam input is needed for the same process result. In continuous marine and offshore duty that saving accumulates.
A practical example is crude oil preheating on an offshore platform, where crude is warmed before separation and the heat is recovered from a hotter process stream. A tube bundle with better conductivity recovers more of that otherwise wasted heat, so the fired heater has less work to do. The saving shows up twice: reduced fuel consumption on the platform and reduced flue gas and carbon dioxide output for the same production rate. Over the multi-decade life of a platform or desalination train, that difference in energy input is one of the largest cost items an operator can influence at the specification stage.
Specifying CuNi 90/10 Tube
Three points are worth fixing at the enquiry stage. First, state the tube standard rather than only the alloy, since wall thickness, temper and test requirements differ between specifications. Second, confirm the water chemistry: chloride content, flow velocity and the presence of suspended solids decide whether 90/10 or the more highly alloyed 70/30 grade is appropriate for the tube and whether inlet end protection is needed. Third, state the required cleanliness and eddy-current test acceptance level, because a perfect alloy with a defective wall will not deliver the conductivity the design assumes.
Frequently Asked Questions
Q: What is CuNi 90/10 white copper tube?
It is a copper-nickel alloy containing about 10% nickel with small iron and manganese additions, designated UNS C70600. The alloy is called white copper because its higher nickel content gives it a silvery colour and a much better seawater corrosion resistance than plain copper.
Q: What is the thermal conductivity of CuNi 90/10?
Roughly 45 W/(m·K) at room temperature. That is lower than pure copper, which is about 390 W/(m·K), but several times higher than type 316 stainless steel at about 16 W/(m·K), which is what makes the alloy effective in a seawater-resistant heat exchanger.
Q: Why not simply use pure copper tube for better conductivity?
Because pure copper does not survive flowing seawater. It corrodes and erodes quickly, so the conductivity advantage disappears as the wall thins. CuNi 90/10 gives up some conductivity in exchange for a tube that lasts for decades in the same duty.
Q: Which standards cover CuNi 90/10 heat exchanger tube?
ASTM B111 covers seamless copper-nickel condenser and heat exchanger tube in C70600, and ASTM B466 covers seamless copper-nickel pipe and tube. The corresponding Japanese designation is C7060 under JIS H3300. The order should always name the standard as well as the alloy.
Q: How does higher conductivity reduce operating cost?
It lowers the temperature difference needed to reach a target heat duty, so the system needs less steam or fuel input. In continuous duty, such as crude oil preheating offshore, that smaller energy input compounds over the operating life of the equipment.
Q: What should be checked before ordering CuNi 90/10 tube?
Confirm the water chemistry and flow velocity to choose between 90/10 and 70/30, state the applicable tube standard and wall thickness, and agree the eddy-current test and cleanliness requirements, since these control whether the delivered wall performs as designed.
