The thermodynamic design and rating of the heat exchangers for an Aqua-Ammonia absorption-desorption heating and refrigeration cycle

Abstract

In today’s day and age the search for more reliable, sustainable, energy efficient systems is a constant. Over the last three decades large emphasis has been placed on vapour compression cycles and making them as energy efficient as possible. To increase or decrease the temperature of a controlled volume requires large amounts of shaft work [kW], however by developing and conducting research into old technologies it could be possible to obtain a sustainable alternative heating/cooling system. Aqua-ammonia absorption-desorption cycles are able to operate on renewable energy, as this process removes the energy hungry compressor from the refrigeration cycle and replaces it with a generator, absorber, rectifier, and regenerative heat exchanger. Absorption-desorption cycles are the earliest form of refrigeration cycle, with the earliest dating back to 1824. Pure ammonia refrigerant is one of only few alternative refrigerants with zero ODP (Ozone Depletion Potential) and zero GWP (Global Warming Potential) accepted by all governments, ASHRAE, UNEP, International Institution of Refrigeration, and almost all Institutes of Refrigeration worldwide (ASHRAE, 1994). In view of that, an investigation is required into alternative heating/cooling cycles, which can be adapted and optimised to suit the limitations and requirements of alternative energy sources. Contained within this dissertation are the thermodynamic and mechanical designs required for the heat exchangers of an aqua-ammonia absorption-desorption cycle. The study includes an extensive theoretical background and literature review, which led to the further investigation of the thermophysical properties of aqua-ammonia refrigerant. Furthermore, the oeuvre includes the verification and validation of the thermodynamic design and rating model for seven heat exchangers, which are required to fully optimise the coefficient of performance of the aqua-ammonia absorption-desorption cycle. The software package MS Excel was used to code the preliminary thermodynamic design model, after which the software package EES was utilised to verify that the preliminary thermodynamic design model had no mathematical errors. Validation of the thermodynamic design models came in the form of predicted overall heat transfer coefficient values versus typically expected overall heat transfer coefficient ranges. Where the typical Uc ranges have similar or near identical heat exchanger configuration to that of the thermodynamic design models in this dissertation. The mechanical design isn’t directly related to the main focus of this study, but does form an integral part of the overall design of aqua-ammonia heat exchangers and is included in the scope of work. The mechanical design includes the necessary first principle considerations to ensure the safe operation of the heating and refrigeration package unit