Outdoor air (cp = 1.005 kJ/kg·°C) is to be preheated by hot exhaust gases in a cross-flow heat exchanger before it enters the furnace. Air enters the heat exchanger at 101 kPa and 30°C at a rate of 0.5 m3/s. The combustion gases (cp = 1.10 kJ/kg ˆ™ °C) enter at 350°C at a rate of 0.85 kg/s and leave at 260°C. Determine the rate of heat transfer to the air and the rate of exergy destruction in the heat exchanger.

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Air 101 kPa 30 C 0.5 mls Exhaust gases 0.85 kg/s 260 C FIGURE P8-81