Integration of desiccant wheels and high-temperature heat pumps with milk spray dryers

Abstract

• Integrating desiccant wheels (DW) with heat pumps for improving dryer heat recovery. • DW reduces evaporator size and variations in drying capacity and energy efficiency. • DW integrated heat pumps expedient for applications with highly humid ambient air. Heat pump technologies are promising to decarbonise industrial drying processes by improving energy efficiency and shifting the energy source from fossil fuels to renewable-sourced electricity. While desiccant materials have proved effective in improving drying efficiency for dryers at temperatures < 120 °C, this paper focuses on the performance of desiccant wheel assisted high temperature heat pump (HTHP) technology to heat the dryer inlet air up to 150 °C using heat recovered from the dryer exhaust. Thermodynamic models of heat pump cycles were established to evaluate the performance of all the HTHP components. A one-dimensional finite difference model was used to simulate the dehumidification of exhaust air through the evaporator of the HTHP. The performance of the desiccant wheel was numerically simulated considering only the gas-side heat and mass transfer resistance. When compared with configurations of a simple HTHP, the desiccant wheel assisted HTHP requires a much smaller evaporator size (50–60 % less than the simple HTHP), similar to the HTHP with a heat recuperator. Its improvement in overall system coefficient of performance (COP), exergy efficiency and the drying capacity when applied to a spray dryer is marginal when compared to the model accuracy. However, comparisons between two cases with different ambient conditions (15 °C and 60 % RH vs 25 °C and 80 % RH) show that such improvement by the desiccant wheel assisted HTHP develops with increasing humidity in the incoming drying air.