Design of a heat recovery unit using exhaust gases for energy savings in an absorption air conditioning unit

Abstract

Abstract Optimum utilisation of the available energy is vital to eliminate waste heat and address other environmental issues. This study is concerned with the design of a heat exchanger for the optimum use of loss heat from flue gases. The heat content in exhaust gases can be applied in the absorption chiller to preheat a dilute solution (lithium bromide) before entering the high-temperature generator. The exploitation of the energy from a glass furnace outlet flue gas temperature of 400 °C will reduce the rate consumption of natural gas. The thermodynamic properties of flue gases were determined based on chemical composition analysis measured by the gas analyser. Moreover, the solution at the inlets and outlets of the Li-Br absorption chiller unit components was estimated using heat and mass balance equations. The heat recovery unit was designed to extract heat from flue gases to preheat, directly or indirectly, the Li-Br solution before entering the high-pressure generator using water as an intermediate solution. The heat exchanger design specification was determined for both cold fluids (Li-Br and H2O) at different tube diameters using copper and steel materials. Furthermore, the theoretical design analysis was executed using a FORTRAN program. It was found that the compact direct heat exchanger showed the best performance with respect to the overall heat transfer area, while the shell-and-tube heat exchanger was favourable because of its simple design and manufacturing. The results showed that the minimum outlet flue gas temperature was 200 °C to keep the glass furnace operating as a natural draught system. The feasibility study was carried out by the CONSUKORRA Company (Hitachi agent in Egypt) based on the current study data. The proposed feasibility study showed that the annual fuel savings was about 23%, while the annual cost saving was 18%. Moreover, the coefficient of performance of absorption unit will be enhanced by 34% after installing the heat recovery unit.