Hybrid Heat Pipe Screw Dryer: A Novel, Continuous and Highly Energy-Efficient Drying Technology

This work investigated a novel type of screw conveyor dryer (SCD) equipped with a passive heating provided by an annular heat pipe, termed “Heat Pipe Screw Dryer” (HPSD). The device was designed as a compact and continuous drying equipment with an energy efficient operation as its core principle. The heat pipe was firstly characterised, resulting in the lowest thermal resistance of 5.6 x 10^-3 °C/W at certain conditions: 200°C setting temperature, 11% filling ratio and in (laid flat) horizontal orientation. These conditions therefore, were employed in the drying of a ceramic raw material slurry (initial moisture content: 33 wt. %, wet basis). Moisture reduction of the material increased with increasing temperature and residence time. Approximately 46% moisture reduction (corresponding to 18 wt. % final moisture content) was achieved at 200°C and 260s conditions with the resulting energy efficiency of 2.04 kg/kWh, which places the HPSD as more energy efficient than conventional dryers e.g. fluidised beds, rotary and spray dryers. In addition, thermal analyses showed that the HPSD performed better than a current steam jacketed-SCD, by ~35% higher in the overall heat transfer coefficients. These were attributed to the excellence of the annular heat pipe as a thermal conductor as well as the negation of any pumping requirement of heating fluid. On the other hand, solids buildup (fouling) also increased with moisture reduction, from 3 to 62% for 10 to 46% moisture reductions, respectively. Applying a non-stick “Xylan” coating on the screw conveyor resulted in a ~10% fouling remediation. From experimental data, it was concluded that the fouling could be improved significantly if the entire inner barrel was coated. Reducing the barrel clearance and periodic wash-ups were among other fouling remediation strategies. The HPSD as a multifunctional technology, combining both drying and heating, is a worthy technology to be further investigated, developed and applied, creating a more energy-efficient drying processing.