2016 Case Upper 5
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There is a heat exchanger with good thermal insulation. The inlet and outlet temperatures of the hot fluid are 150°C and 35°C respectively, with a flow rate of 25 kg/min; its constant-pressure heat capacity is 4.36 kJ/(kg·K). The inlet and outlet temperatures of the cold fluid are 0°C and 110°C respectively, with a constant-pressure heat capacity of 4.69 kJ/(kg·K). The ambient temperature is 0°C. What value can the efficiency be reduced to? There is a heat exchanger with good thermal insulation. The inlet and outlet temperatures of the hot fluid are 150 and 35 degrees respectively, with a flow rate of 25 kg/min; its constant-pressure heat capacity is 4.36 kJ/(kg·K). The inlet and outlet temperatures of the cold fluid are 25 and 110 degrees respectively, with a constant-pressure heat capacity of 4.69 kJ/(kg·K). The ambient temperature is 25 degrees. Ask about lost work? Does effective energy reduction equal lost work? ?
Definition of lost work: WL = T0ΔS – Q (where Q is the heat exchanged between the system and the environment at temperature T0)； ΔS—entropy change of the system). Since the system is well insulated, Q≈0; heat exchange occurs only between the hot and cold fluids, that is: WL=T0ΔS. This is my opinion; I’m not sure if it’s correct.