Acounter-flow heat exchange consists of two concentric copper tubes with wall thick- nesses t = 0.5 mm and lengths 1.5 m, and diameters 15 mm and 21 mm.. the hot water flow through the inner tube and the counterflowing cold water flow in the annulus between the inner and outer tubes are both equal to 2 litres per hour, and their input temperatures are 80 °c and 20 °c, respectively. calculate their output temperatures when thermal equilibrium has been established. assume the outer cylinder is well insu- lated. the nusselt number when there is a constant heat transfer rate and the water flow is laminar is equal to 4.36. the thermal conductivities of water and copper are 0.58 and 400 w m'k, respectively. the specific heat of water is 4180 j kg k.