Convective Drying with Time-Varying Heat Input: Simulation Results

Abstract

Intermittent drying aims to match the heat input rate to the drying kinetics of the material so as to avoid thermal degradation of heat-sensitive products in particular. This paper presents results of a liquid diffusion model to examine the effect of varying the rates of heat input by convection heat transfer. This is accomplished by varying the drying air velocity, varying the air temperature as well as its relative humidity over different periods of time in a sequential manner. One of the outcomes of this work is guidelines for use of a heat pump to dehumidify the drying air. While most heat pump dryers are designed to operate continuously, our results show that it is not necessary to use heat pump continuously over the entire drying period. This option saves running costs by reducing use of electrical power in the drying cycle. Furthermore, it is possible to save capital costs by utilizing a smaller heat pump for a given dry product output. Alternatively, a given heat pump system can be used to service two or more drying chambers that may dry the same or different products by simply switching the dehumidified and heated air from one chamber to the other sequentially. When the heat pump air is switched off, unsaturated ambient air maybe used to accomplish rest of the drying. It is shown that using heat pump air over only a part of the drying cycle does not increase the drying time appreciably.

Keywords:

• Heat sensitive materials
• Liquid diffusion
• Heat pump drying
• Multi-chamber dryers
• Energy savings
• Variable velocity
• Variable temperature
• Variable humidity

Acknowledgment

The authors acknowledge with gratitude a grant awarded by the National University of Singapore (RP: R-265-000-074-112) under which this work was performed.