Exergoeconomic analysis of staggered tube cross-flow heat recovery unit incorporated into industrial air-compressor for process water heating

Abstract

Abstract Energy management through waste heat recovery can result in significant savings in energy costs in industries. In this study, the heat recovery potential of hot air from the heat exchangers of two industrial air-compressors was assessed for process heating at a cocoa processing company, using a novel staggered tube cross-flow heat exchanger (STC-HX) with multi-tube passes. Initial assessment showed that, hot water with temperatures between 42 and 70 °C could be achieved with the cross-flow air to water heat exchanger. A suitable heat recovery unit (HRU) was selected with appropriate geometrical configuration and used in heat transfer analysis with MATLAB. The results indicated maximum heat recovery rate of 158 kW, capable of producing warm water at an average temperature of 56 °C, and at flow rate of 40 L per minute. This paper has established that, the maximum heat recovery rate was obtained when the temperature difference between the outlets of the hot and cold fluids of the STC-HX was minimum. The heat recovery rate corresponds to about 54.5% of the total 163 kW power consumption of the two compressors and their blowers. This implies energy savings of 710 MWh per year for compressor runtime of 8000 h, annually. The effectiveness of the HRU was determined to be 57%. An economic assessment of the feasibility of installing the HRU on the plant indicated a simple payback period of 8 months and a net present value of US$ 177,500 for a ten-year period. The analysis further showed that the implementation of the heat recovery system yields a benefit-to-cost ratio of 2.5 with an annual energy cost savings of US$ 33,800 for the company.