Waste heat recovery in commercial gas-fired tumble dryers

Thermal drying is energy intensive due to the unavoidable thermodynamic constraint of supplying sufficient energy to evaporate the water (hfg > 2000 kJ kg−1). Textile dryers only use about half (∼55%) of the total input energy for drying. To reduce the wasted energy, exhaust gas recirculation and waste heat recovery using a recuperative heat exchanger are investigated numerically and experimentally. The two methods aim to recover energy from the exhaust stream typically at T < 55 °C for most of the drying period. Experiments are conducted on a commercial gas-fired tumble dryer. A theoretical model is developed to simulate and optimize the drying process. Recirculating the hot humid exhaust gases improves heat and mass transfer in the drum; however, the high humidity ratio of the recirculated air is not amenable to enhancing evaporation. An optimal recirculation ratio of 51% is found to reduce the specific moisture extraction ratio and drying time by 9% and 164 s, respectively. To avoid the rise in humidity associated with recirculation, a compact plate-fin heat exchanger is used to recuperatively preheat the air entering the combustor. Energy savings of ∼15% and reduction in drying time by 216 s over the baseline case are estimated. The predicted data demonstrate good agreement with experiments.