It is proposed to improve the energy utilisation of a steel works by transferring the heat from the gases leaving the blast furnace at 600 °C to those entering the furnace at 50 °C (before the heat exchanger is fitted). The minimum temperature of the flue gases is limited to 150 °C to avoid condensation of sulfurous acid in the pipework at exit pressure of 1 bar.

Draw a simple schematic diagram of the heat exchanger you would design, showing the hot and cold gas streams. Explain with the aid of T-s diagrams, why a counterflow heat exchanger is the most efficient. If the minimum temperature difference between the hot and cold streams is 10 °C, calculate the minimum loss of exergy for both types of heat exchanger, based on deadstate conditions of 1 bar and 20 °C.
[72.33 kJ/kg; 45.09 kJ/kg]