Chapter_7

# Chapter_7 - 7.23 A rigid tank contains an ideal gas that is...

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7.23 A rigid tank contains an ideal gas that is being stirred by a paddle wheel. The temperature of the gas remains constant as a result of heat transfer out. The entropy change of the gas is to be determined. Assumptions The gas in the tank is given to be an ideal gas. Analysis The temperature and the specific volume of the gas remain constant during this process. Therefore, the initial and the final states of the gas are the same. Then s 2 = s 1 since entropy is a property. Therefore, 0 sys S 7.25 Heat is transferred directly from an energy-source reservoir to an energy-sink. The entropy change of the two reservoirs is to be calculated and it is to be determined if the increase of entropy principle is satisfied. Assumptions The reservoirs operate steadily. Analysis The entropy change of the source and sink is given by kJ/K 0.0833 K 600 kJ 100 K 1200 kJ 100 L L H H T Q T Q S Since the entropy of everything involved in this process has increased, this transfer of heat is possible . 7.29 A reversible heat pump with specified reservoir temperatures is considered. The entropy change of two reservoirs is to be calculated and it is to be determined if this heat pump satisfies the increase in entropy principle. Assumptions The heat pump operates steadily. Analysis Since the heat pump is completely reversible, the combination of the coefficient of performance expression, first Law, and thermodynamic temperature scale gives 73 . 26 ) K 294 /( ) K 283 ( 1 1 / 1 1 COP rev HP, H L T T The power required to drive this heat pump, according to the coefficient of performance, is then kW 741 . 3 26.73 kW 100 COP rev HP, in net, H Q W According to the first law, the rate at which heat is removed from the low-temperature energy reservoir is kW 26 . 96 kW 741 . 3 kW 100 in net, W Q Q H L The rate at which the entropy of the high temperature reservoir changes, according to the definition of the entropy, is kW/K 0.340 K 294 kW 100 H H H T Q S IDEAL GAS 40 C Hea t 20 H 100 L Q net W

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and that of the low-temperature reservoir is kW/K 0.340 K 283 kW 26 . 96 L L L T Q S The net rate of entropy change of everything in this system is kW/K 0 340 . 0 340 . 0 total L H S S S as it must be since the heat pump is completely reversible. 7.35E R-134a is expanded in a turbine during which the entropy remains constant. The enthalpy difference is to be determined. Analysis The initial state is superheated vapor and thus EES) (from R Btu/lbm 23281 . 0 Btu/lbm 95 . 129 F 175 psia 250 1 1 1 1 s h T P The entropy is constant during the process. The final state is also superheated vapor and the enthalpy at this state is EES) (from Btu/lbm 95 . 106 R Btu/lbm 23281 . 0 F
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Chapter_7 - 7.23 A rigid tank contains an ideal gas that is...

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