Industrial drying processes are energy-intensive and account for a significant portion of energy consumption in the paper, food, and wood processing industries. As the goal of achieving net-zero emissions by 2050 gains importance, electrifying these processes with heat pumps emerges as a promising, innovative solution. This study investigates the optimal configuration of heat pumps, the selection of working fluids, and the sizing of components for a non-isenthalpic 530 kWth drying cycle that includes evaporator bypass and recirculation of the humid air.

Two heat pump configurations were analysed: a single-stage heat pump and a cascade heat pump. Both cycles were simulated and optimised using zeotropic mixtures of natural refrigerants, with molar compositions adjusted to minimise total entropy production. The compressors and heat exchangers were modelled and sized to facilitate a component-level comparison between the cycles.

The results indicate that using zeotropic mixtures improves the performance of both heat pump configurations. However, it is not always necessary to operate in transcritical mode for optimal performance. A single-stage heat pump cycle using a zeotropic mixture of 96 mol% isobutane and 4 mol% propylene exhibits the best performance, achieving a coefficient of performance (COP) of 2.54. In comparison, the best-performing cascade heat pump cycle has a COP that is 4.3% higher than that of the best single-stage heat pump cycle. However, due to its 91% larger total heat transfer area and the requirement for two compressors, the capital costs of the cascade system are significantly higher than those of the single-stage system.

In conclusion, the best option is a single-stage heat pump equipped with an internal heat exchanger, using a zeotropic mixture of 96 mol% isobutane and 4 mol% propylene as the working fluid. This heat pump cycle comprises three brazed plate heat exchangers with a total heat transfer area of 134 m², two finned-tube heat exchangers, and a two-stage centrifugal compressor. The compressor features impellers with outer diameters of 61.9 mm for the first stage and 60.1 mm for the second stage.