Investigation of Drying Under Impingement Flow: Dry, Moist Air and Superheated Steam

Industrial convective drying is energy-intensive, and its specific energy consumption (SEC) directly determines its greenhouse gas emissions. Recirculation of the exhaust drying agent and superheated steam drying (SSD) reduce the SEC with little investment, but their benefit in impingement dryers depends on fan power and on finite heat and mass transfer. This study presents a system-level model of an electrically heated dryer with impingement flow, comprising a mixer, heater, fan, dryer and recirculation. Coupled mass and energy balances and impingement transfer correlations are solved for drying agents from ambient air to superheated steam at up to 350 °C. Three evaluation quantities are introduced: the evaporation potential, the evaporation potential fraction and the normalized drying power. Drying agents are compared at constant volume flow, mass flow and Reynolds number and, as a new case, hydraulic power. With recirculation, superheated steam at about 105 °C to 200 °C yields the highest normalized drying power in all cases (up to 0.92 for a pressure loss coefficient of 10). Only at high pressure losses and constant mass flow does the optimum shift to moist air. Higher vapor fractions reduce the drying rate, except at constant mass flow above an inversion temperature of about 240 °C.