ABSTRACT
Convective drying involves simultaneous exchange of moisture and heat between the material undergoing drying and the flowing moist air. The magnitude of the exchange depends on the driving forces provided by the temperature and vapour pressure gradients that are established inside the material and at the air boundary layer over the surface. Moist air is the most suitable drying agent because it has acceptable properties and is nearly cost free and readily available worldwide. Thus, knowledge of the moist air properties at each state point of drying is essential in the calculations of water removal, energy utilisation and dryer capacity. Psychrometric charts, equations and software libraries are applied to determine the moist air properties that are fundamental in design, analysis and evaluation of energy, moisture removal and capacity of heat pump dryers. The continuous phase in the majority of convective drying processes is a mixture of air and water vapour. The state points and properties of the mixture change as it flows through the components of the heat pump drying processes. The changes must be determined at the inlet and outlet of the heat exchangers and the drying chamber to establish the state points in each process and for calculations of energy use and water removal during drying.
