Thermal Conductivity of Copper

In metals the same free electrons carry both heat and charge, so the best electrical conductors are the best thermal conductors: the Wiedemann-Franz law makes the ratio k/(σT) nearly universal. Silver leads, copper follows at about 402 W/(m·K), and gold and aluminium trail. Copper wins on cost per unit of conductivity, which is why it is in every heat exchanger tube, cold plate and bus bar.

Alloying wrecks it. Brass (70/30) is about 120 W/(m·K), bronzes are lower, and cupronickel condenser tube is nearer 30: a third of the alloying additions can cost three quarters of the conductivity, because solute atoms scatter electrons. Even oxygen content matters, which is why high-conductivity applications specify C10100 or C11000. Use 402 for clean copper near room temperature and derate for the actual alloy. Note too that in most real heat exchangers the tube wall is not the resistance that matters: the water-side and air-side films and any fouling layer dominate, so upgrading tube metal often changes the overall U by a percent or two.