What Thermal Conductivity Measures
Thermal conductivity, expressed in watts per metre kelvin, W/(m·K), states how much heat crosses a unit thickness of a material for a given temperature difference. It is the single most important material property in the wall-thickness term of any heat-transfer calculation, because it sets how much of the total resistance to heat flow comes from the metal itself rather than from the two fluid films.
For the wrought brasses the property falls in a band of roughly 109 to 170 W/(m·K) at room temperature. That places the family well above carbon steel and stainless steel and below unalloyed copper. The spread within the band is not random: it is driven almost entirely by how much zinc and how many other elements have been added to the copper lattice.
Typical Thermal Conductivity Values by Grade
| Grade | Nominal composition | Thermal conductivity at 20 °C, W/(m·K) |
|---|---|---|
| C11000 ETP copper | 99.90 % Cu | 388 |
| C23000 red brass | 85 % Cu, 15 % Zn | 159 |
| C26000 cartridge brass | 70 % Cu, 30 % Zn | 121 |
| C36000 free-cutting brass | 61.5 % Cu, 35.5 % Zn, 3 % Pb | 115 |
| C44300 admiralty brass | 71 % Cu, 28 % Zn, 1 % Sn | 109 |
| C70600 copper-nickel 90/10 | Cu, 10 % Ni, 1.3 % Fe | 45 |
The values apply to wrought material in the annealed condition at approximately 20 °C. They are the figures normally used for preliminary design; where the duty is critical, the mill certificate for the actual heat should be used together with the temperature correction appropriate to the service range.
What Controls the Value
Zinc content. Zinc in solid solution scatters the electrons that carry heat in copper. Every additional per cent of zinc lowers thermal conductivity, which is why 85/15 red brass is roughly 30 % more conductive than 70/30 cartridge brass.
Tin, arsenic and aluminium. These elements are added for corrosion resistance and strength, and each of them reduces conductivity further. Admiralty brass, with 1 % tin and a trace of arsenic, sits at the bottom of the wrought brass band.
Lead. Lead is added to free-cutting brasses for machinability. Its effect on conductivity is modest at the 2-3 % levels used in practice.
Cold work. Cold reduction changes mechanical properties and electrical resistance markedly, but the effect on thermal conductivity is small over the strain levels encountered in commercial tempers.
Temperature. Over the normal service range of copper alloys, roughly -50 to 200 °C, temperature has only a modest effect and the room-temperature value is adequate for most engineering estimates.
Electrical Conductivity as a Fast Cross-Check
Heat and electric charge are both carried mainly by free electrons, so the two properties move together in the same alloy family. A wrought brass such as C26000 with about 28 % IACS electrical conductivity has a thermal conductivity close to 121 W/(m·K). C44300, at roughly 26 % IACS, falls to about 109 W/(m·K). Impure, oxidised or heavily worked material loses both properties together.
This relationship is useful in purchasing. When a data sheet quotes a thermal conductivity that is inconsistent with the electrical conductivity given on the same page, one of the two figures has been copied incorrectly and the material should not be accepted for a thermally critical application until the mill certificate is re-checked against the governing standard.
Where the Property Decides the Design
Heat exchanger and condenser tubes. Copper alloys are used precisely because their conductivity keeps the wall resistance small relative to the water-side film, allowing compact shells and fewer tubes.
Radiators and heat spreaders. Here high conductivity spreads heat laterally so that a small fin area can be used; red brass and cartridge brass are common choices.
HVAC and refrigeration. Tube and valve bodies in brass benefit from rapid, uniform heat transfer and from the strength that zinc adds.
Electrical connectors and busbar. Conductivity and current-carrying capacity must be balanced against the strength and wear resistance that brass provides.
Practical Selection Notes
Conductivity should be treated as one property in a set, not as the objective on its own. A grade chosen purely for its conductivity figure may prove too soft for the loads involved or too prone to dezincification in the service water. Typical practice is to select the alloy first on corrosion, strength and formability grounds, and then to check that the thermal conductivity still meets the heat-transfer duty with an acceptable surface area.
Two further points are worth remembering. First, surface fouling and scale usually dominate heat transfer once an exchanger is in service, so a small difference in metal conductivity between two candidate alloys rarely justifies a change in design. Second, because brasses differ from copper by a factor of three in conductivity, replacing copper with brass for a thermally critical component is a real engineering decision rather than a simple material substitution.
FAQ
Q: What is the thermal conductivity of brass?
For wrought brasses the value normally lies between about 109 and 170 W/(m·K) at room temperature. The precise figure depends on composition: high-copper red brasses are at the upper end and the tin-bearing admiralty brasses at the lower end.
Q: Which brass has the highest thermal conductivity?
Within the brass family, the high-copper, low-zinc grades conduct heat best. C23000 red brass at 85 % copper is around 159 W/(m·K), noticeably better than 70/30 cartridge brass at about 121 W/(m·K).
Q: Does zinc reduce thermal conductivity in brass?
Yes. Zinc dissolved in the copper lattice scatters conduction electrons, so each added per cent of zinc lowers thermal conductivity as well as electrical conductivity.
Q: How does brass compare with copper for heat transfer?
ETP copper at around 388 W/(m·K) is more than three times as conductive as cartridge brass. Brass is nevertheless preferred for many components because it is stronger, machines better and costs less, which often matters more than the conductivity difference.
Q: How do I convert imperial conductivity units to W/(m·K)?
Multiply a value in Btu·ft/(ft²·h·°F) by 1.7307 to obtain W/(m·K). The check is simple: 121 W/(m·K) is the same as about 70 Btu·ft/(ft²·h·°F), the figure usually quoted for 70/30 cartridge brass.
Q: Is brass thermal conductivity affected by cold working?
Only slightly. Cold reduction raises strength and electrical resistance but changes thermal conductivity by a small amount at commercial temper levels, so the annealed value is normally used in design calculations.
