2019 Past Paper EC by HT

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ENERGY CONVERSION A 2018-19 Note: you can keep this text for self -evaluation at the end of the exam; in case , though, you withdraw dur- ing the exam, you shall return it. Exam 13/06/2019 Exercise 1 (10 points) A fire-tube industrial boiler produces 3.6 t/h of saturated steam at 1 MPa. The boiler is not equipped with any economizer to reduce the investment cost. The fraction of 40% of this steam is utilized on the industrial site at 1 MPa and returned at 0.2 MPa and 70 °C. Moreover, the remaining fraction of 60% of the steam is throttled to 0.5 MPa, utilized on the site and returned at the same conditions as the other stream. Please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. sketch the process flow diagram as well as the main thermodynamic points on the Ts diagram (2 points); iii. determine the mass flow rate of the fuel (3 point); iv. compute the wasted mechanical power due to the steam throttling and explain how this loss could be conceptually eliminated (2 points); v. determine the boiler efficiency and explain how it could be improved (2 points). Exercise 2 (12 points) An industrial process provides a hot gas stream at 650 °C that must be cooled down by 200 K before entering a heat recovery steam generator. The thermal power from the stream is exploited in an experimental power plant based on an open-loop Brayton gas cycle. This cycle does not exploit any other hot source than the hot gas stream. The cycle has a net electric power output of 1 MWe, is characterized by a compression ratio of 4, and includes a recuperator. Moreover, the compressor is equipped with inlet guide vanes and is mounted on the same shaft as the expander and the 50-Hz electric generator. During the off-design operation, a control valve…