A heat exchanger is constructed so that hot flue gases at 700K flow inside a 2.5 cm ID copper tube with 1.6 mm wall thickness. A 5 cm ID tube is placed around the copper tube. In the annular space, high-pressure water at 300 K flows at the rate of 1.5 kg/s and the total heat fransfer is 15 kW. Estimate the length of the heat exchanger for a gas flow rate of 0.8 kg/s. You may ignore the tube wall resistance and the flue gas properties are the same as those of air at 700K.