Exam 2018 09 05 solution

Pagina 1

Department of Energy Politecnico di Milano Author M. Rocco Pag.1 of 12 Date 10/09/2018 Milan, 28th August 2018 Exam – Advanced Thermodynamic and Thermoeconomics Session of 05-09-2018 Exercise 1. (7pt) da 21/07/2015 A vapor-compression refrigeration system is designed to cool down a mass flow rate of a fluid for an industrial process. The cycle works with ammonia and it is characterized by the following transformations: • 1-2: vapor enters the compressor (CMP) as saturated vapor; • 2-3: vapor condensates in the condenser (COND) and exits as saturated liquid. The heat is rejected to the environment at 25 °C and 1 bar (reference conditions); • 3-4: liquid goes through an isenthalpic process in a lamination valve (LAM); • 4-1: the multi -phase flow evaporates in to the evaporator (EVA) and exits as saturated vapor . Evaporations process cool down a liquid mass flow rate from 15°C to 7°C; Ammonia T p h s ex °C MPa kJ/kg kJ/kg-K kJ/kg 0 25 0.1013 1689.8 7.0732 0.0 1 -10.0 0.2908 1593.9 6.2284 2 141.7 1.1670 1924.8 6.5699 3 30.0 1.1670 484.9 1.9596 4 -10.0 0.2908 484.9 2.0142 With reference to the properties collected in the table, it is required to: a. Draw the scheme of the system, with all the components, highlighting directions of work, heat and bulk flow streams. Compute the exergy value for each stream. b. Define boundaries of the whol e system and apply the energy and the exergy balances. Derive the analytical expressions for the COP and the functional exergy efficiency ηex,f of the system. Which are the differences among the two performance indicators? c. Apply the exergy balance to all the components of the system, deriving the analytical expressions for the exergy destructions (specific to the mass flow of working fluid). d. Compute numerical results for questions b…