12 07 18t

Pagina 1

ENERGY CONVERSION A 2017-18 Name/Lastname:__________________________________________ Code n.___________ Exam 12/07/2018 Exercise 1 (10 points) Consider a hypothetical natural gas-fired steam Rankine cycle characterized by:  mass flow rate of the steam to the high-pressure turbine: 30 kg/s;  evaporation temperature: 250°C;  maximum temperature of the steam to the high-pressure turbine: 550°C;  evaporation enthalpy at 250°C: 1700 kJ/kg;  mean specific heat (at constant pressure) in the superheated region: 2.5 kJ/(kg K);  deaerator pressure: 4 bar;  water temperature increase in the high-pressure feedwater preheater: 30°C;  air temperature at combustion chamber inlet: 200°C. Moreover, the cycle employs the reheating option and it adopts only one high -pressure feedwater preheater, which utilizes bled steam from the outlet of the high-pressure turbine. Please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. sketch the plant layout and the cycle on the Ts diagram (2 points); iii. sketch the heat transfer diagram of the high-pressure feedwater preheater and determine the mass flow rate of the bled steam (3 point); iv. estimate the mass flow rate of the natural gas and of the exhaust gases at the stack (2 points); v. compute the second-law efficiency loss due to the heat transfer in the air preheater heat exchanger (2 points). Exercise 2 (12 points) Consider the design of a cogeneration power plant employing a natural gas-fired gas turbine and a heat recovery steam generator. The steam is delivered to a nearby industry and returned via a closed loop; in particular, the steam is generated at saturated conditions and 20 bar, while returned at 70°C and 5 bar. The gas turbine, an Ansaldo Energia AE64.3A, in simple cycle…