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Politecnico di Milano Energy Conversion Technologies prof. Emanuele Martelli - Academic Year 2023-24 page 1 of 2 Written Exam of 26 January 2024 - Problems Time: 1:10 hours for 5 CFU exam 1) Exam is open book, but computers and cell phones are NOT allowed. Talking with colleagues and/or copying will lead to the immediate cancellation of the exam. 2) Answer clearly ONLY to the questions posed by the problem sets. Even if correct, additional considerations and/or calculations will NOT be considered. Energy Conversion Technologies Problem 1 (24 points) A boiler, consisting of a combustor, a heat exchanger (flue gas – steam), an air preheater and a stack, features the following design operating parameters: - Evaporation of 15 t/h of saturated steam at 10 bar, from saturated liquid to saturated steam - Minimum temperature difference between the combustion products and the evaporating water: 50 °C - Flue gas temperature at the stack: 100 °C - Fuel: methane (CH4) - Ambient air temperature: 10 °C - Air composition: 21% O2, 79% N2 (molar mass = 28.84 kg/kmol) - Excess air: 30% - Specific heat capacity cp of air: 1.02 kJ/(kg K) - Specific heat capacity cp of flue gas: 1.10 kJ/(kg K) - Heat losses from the boiler walls = 1 % of the fuel LHV Given the above: 1) Draw a simple block diagram of the boiler with the evaporator and the air preheater and draw a qualitative T-Q diagram (indicating only the known temperatures and the heat exchanged in the evaporator). (5 points) 2) Calculate the boiler efficiency. (5 points) 3) Calculate the effectiveness of the air preheater. (4 points) 4) Calculate the molar basis composition of the flue gases (4 points) After some months of operation, the air preheater is subject to fouling, causing its overall heat transfer coefficient ‘U’ to drop by…