Design of new heat pump dryer system: A case study in drying characteristics of kelp knots

The heat pump dryer system (HPDS) attracts much attention for its high energy saving and excellent environmental performance. In this paper, a new set of double evaporator and normally closed loop HPDS (DENC-HPDS) with dehumidifying capacity of 3500 g/h and condensation temperature of 60 °C was designed. Drying experiments of kelp knots with DENC-HPDS under the conditions of drying temperature of 35, 40, 45 and 50 °C, wind speed of 0.5, 1.0, 1.5 and 2.0 m/s, loading density of 8.29, 12.29, 22.29 and 32.29 kg/m3, tray spacing of 50, 100, 150 and 200 mm were carried out, and the drying characteristics and mathematical drying models of the DENC-HPDS were investigated and analyzed. The designed DENC-HPDS solves the problem of heat and moisture imbalance and pollution of traditional HPDS effectively with short start-up time compared with traditional HPDS. The drying experiments shows that drying time generally decreases with the increase of drying temperature, wind speed and tray spacing. The optimal drying conditions are drying temperature of 45 °C, wind speed of 1.5 m/s, loading density of 22.29 kg/m3 and tray spacing of 150 mm with the highest mean specific moisture extraction rate of 1.63kg/kWh. In ten drying models, the Verma et al. model has the best fitness to the experimental data and is more suitable to describe the heat pump drying process of kelp knots in this paper. Under the experimental conditions, the calculated effective moisture diffusion coefficients are in the range of 3.4687 × 10−9–11.3189 × 10−9m2/s and the average drying activation energy is 21.02 kJ/mol.