Second law analysis of a commercial absorption refrigeration system that utilizes ammonia and water pair as working fluid
Larissa Costa Sobral1; Josegil Jorge Pereira de Araujo1; Iana Mota Araujo1; Larissa Menezes Santos1
1 Universidade Federal de Sergipe
doi:10.20906/CPS/CILAMCE2017-1079
Resumo
One alternative for the Vapor-Compression Refrigeration System (VCRS) is the Absorption Refrigeration System (ARS). Unlike VCRS, which is started by electrical energy, ARS starts with thermal energy that comes from different sources like fuel combustion, solar heating, thermal waste, etc. ARS application can be justified in places where electrical energy cost is high, remote areas that have no electrical energy nearby and cogeneration system. Comparing with VCRS, ARS efficiency is smaller so exergy analysis is recommended; it will show where the biggest irreversibilities are located. One ARS very commonly used is the ROBUR's chiller, model ACF60, with a refrigeration capacity of 60.000 Btu/h (5 TR) that works with the ammonia water mix as working fluid and thermal energy coming from gas fuel combustion. This system can be applied for air condition (commercial and residential) and as industrial refrigeration processes, the latter being the analysis object of this study. For the second-law analysis the system was divided in 15 control volumes and 18 state points, using the following considerations: 1) system operates in steady state; 2) Kinetic and potential energy effects are negligible; 3) There are no pressure loss through the pipes; 4) Expansion valves are isenthalpic and 5) the flux of matter will be considered unidimensional. The following are determined: exergy transfer rates, entropy production rates, exergy destruction rates. Exergetic efficiency is determined on each control volume. The environment temperature, 25 °C, was considered the temperature of the dead state. The model was implemented in the EES (Engineering Equation Solver) software, which uses the Newton-Rapson method to find the solution for the equation systems. In this study it was possible to determinate the second-law efficiency for the ROBUR commercial refrigeration system and which components presented the highest irreversibilities.
Palavras-chave: Modelling; Simulation; Refrigeration; Absorption; Exergy