Energy Efficiency in Industrial Drying (HPT TCP Annex 59)

Drying is one of the most dominant industrial preservation processes for innumerable products to this day. Industrialization has optimized drying processes, which are conducted under varying, but controlled conditions. Every product requires a specific drying process, which is determined by the type of dryer, the quality of the final product, the extended product shelf life, additional processing, ease of handling, sanitation, cost/energy effectiveness, and investment.

The basic principle of drying has not changed in thousands of years, and the most common dryer type is based on convective drying. In drying technology, the term convective drying is used to indicate when heat and mass transfer are due to the temperature and pressure gradients, respectively, between the drying agent (DA) and the drying product. However, other drying agents like superheated steam are also used in convective dryers, because of improved thermal properties and/or drying potential. Industrial convective drying plants are mainly operated by burning fossil energy sources and product waste. The moisture extracted from the material to be dried is released into the environment in pure gaseous form or with a drying medium. This exhaust air contains high amounts of energy, which is currently only partially used by heat recovery. Modern industrial drying processes are either an open loop system using heated ambient air or a closed loop system with recirculation of the drying air. Heat Pumps are the ideal solution for closed-loop drying. They can utilize a heat source at low temperatures (at the evaporator) and supply a heat sink at a higher temperature (condenser). This combined heat and cool load is a perfect fit for heat pumps resulting in: - Higher efficiency -> Recovery and use of the energy of the water vapor through condensation as it exits the air from the drying system, thus increasing efficiency. - Better controllability -> Air flow, temperature and humidity can be precisely controlled with compression heat pumps in drying systems. - Product quality -> Lower drying temperatures result in higher product quality. In addition, flavor preservation can be improved by using an inert atmosphere in recirculating air operation.