Question 10 in the morning session of the 2014 Chemistry exam
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There is a heat exchanger with good thermal insulation; the inlet and outlet temperatures of the hot fluid are 423 K and 308 K respectively, with a flow rate of 2.5 kg/min and a constant-pressure heat capacity of 4.36 kJ/(kg·K). The inlet and outlet temperatures of the cold fluid are 298 K and 383 K respectively, with a constant-pressure heat capacity of 4.69 kJ/(kg·K). The ambient temperature is 298 K. What is the efficiency (%) value? (A) 71.32 (B) 56.61 (C) 67.73 (D) 80.60 Answer: (C) The efficiency should be the useful energy output divided by the useful energy input. The solution to this problem uses the difference in useful energy of the cold fluid divided by the difference in useful energy of the hot fluid; I think this approach is incorrect. I hope experts can provide some guidance!
Cold fluid: 3.14*4.69*（383-298-298ln（383/298）） Hot fluid: 2.5*4.36*（423-298-ln（423/298））-2.5*4.36*（308-298-298ln（308-298））