entropy rate balance for control volumes

3. EX6.7: An inventor claims to have developed a device requiring no energy transfer by work or heat transfer, yet able to produce hot and cold streams of air from a single stream of air at an intermediate temperature. The inventor provides steady-state test data indicating that when air enters at a temperature of 70°F and a pressure of 5.1 atm, separate streams of air exit at temperatures of 0 and 175°F, respectively, and each at a pressure of 1 atm. Sixty percent of the mass entering the device exits at the lower temperature. Evaluate the inventor's claim, employing the ideal gas model for air and ignoring changes in the kinetic and potential energies of the streams from inlet to exit.

4. EX6.8: Components of a heat pump for supplying heated air to a dwelling are shown in the schematic below. At steady state, Refrigerant 22 enters the compressor at -5°C, 3.5 bar and is compressed adiabatically to 75°C, 14 bar. From the compressor, the refrigerant passes through the condenser, where it condenses to liquid at 28°C, 14 bar. The refrigerant then expands through a throttling valve to 3.5 bar. The states of the refrigerant are shown on the accompanying T–s diagram. Return air from the dwelling enters the condenser at 20°C, 1 bar with a volumetric flow rate of 0.42m3/s and exits at 50°C with a negligible change in pressure. Using the ideal gas model for the air and neglecting kinetic and potential energy effects, (a) determine the rates of entropy production, in kW/K, for control volumes enclosing the condenser, compressor, and expansion valve, respectively. (b) Discuss the sources of irreversibility in the components considered in part (a).

8. 8

10. EX6.9: Air undergoes an isentropic process from p1=1 atm, T1=540 oR, to a final state where the temperature is T2=1160 oR. Employing the ideal gas model, determine the final pressure , in atm. Solve using (a) data from Table A-22E, (b) a constant specific heat ratio k evaluated at the mean temperature, 850°R from Table A-20E.

11. EX6.10: A rigid, well-insulated tank is filled initially with 5 kg of air at a pressure of 5 bar and a temperature of 500 K. A leak develops, and air slowly escapes until the pressure of the air remaining in the tank is 1 bar. Employing the ideal gas model, determine the amount of mass remaining in the tank and its temperature.

13. EX 6.11: A steam turbine operates at steady state with inlet conditions of p1=5 bar, T1=320 oC. Steam leaves the turbine at a pressure of 1 bar. There is no significant heat transfer between the turbine and its surroundings, and kinetic and potential energy changes between inlet and exit are negligible. If the isentropic turbine efficiency is 75%, determine the work developed per unit mass of steam flowing through the turbine, in kJ/kg.

14. S2s=s1=7.5308 kJ/kg

15. EX6.12: A turbine operating at steady state receives air at a pressure of p1=3.0 bar and a temperature of T1=390 K. Air exits the turbine at a pressure of p2=1.0 bar. The work developed is measured as 74 kJ per kg of air flowing through the turbine. The turbine operates adiabatically, and changes in kinetic and potential energy between inlet and exit can be neglected. Using the ideal gas model for air, determine the isentropic turbine efficiency.

18. 6.134 Air enters the compressor of a gas turbine power plant operating at steady state at 290 K, 100 kPa and exits at 420 K, 330 kPa. Stray heat transfer and kinetic and potential energy effects are negligible. Using the ideal gas model for air, determine the isentropic compressor efficiency.

22. 6.153 Refrigerant 134a enters a compressor operating at steady state at 1.8 bar, -10°C with a volumetric flow rate of 2.4x10-2 m3/sec. The refrigerant is compressed to a pressure of 9 bar in an internally reversible process according to p?1.04=constant. Neglecting kinetic and potential energy effects, determine (a) the power required, in kW. (b)  the rate of heat transfer, in kW.