This application models the temperature dynamics of a counter-current double pipe heat exchanger. Three partial differential equations describe - heat balances across the tube-side and shell-side liquids - and a heat balance across the tube wall (taking into account the heat flow from the shell- and tube-side liquids, and conduction along the length of the tube. The equations are solved numerically, and the temperature profiles are plotted. The heat exchanger is assumed to be perfectly insulated. Densities, specific heat capacities, heat transfer coefficients and thermal conductivities are assumed to be constant.