Law, Mujumdar - Energy Savings in Drying of FVFFigure 7.1. Factors that cause low energy efficiency in conventional drying and the re-
spective drying techniques and drying strategy to improve the energy efficiency7.2.2. Energy loss resultant from discharging of exhaust gas
In conventional convective drying, hot air is typically generated by heating the at-
mospheric air at room temperature to a desirable temperature. After passing through
the drying material in drying chamber, it becomes humid and still hot. Thereafter the
humid hot air is discharged into the atmosphere. Toward the end of drying, the exhaust
gas is typically not humid and its temperature is typically a few degrees lower than the
inlet hot air. There is a huge loss of energy occurs when enthalpy in the exhaust air is
vented without partial recycle or heat recovery. This is even more significant if the flow
rate of the exhaust air is high.
In this regard, waste heat recovery has been proposed to increase the energy effi-
ciency of a drying system. A heat exchanger can be installed to transfer the heat in the
exhaust air to the inlet prior to the heating of inlet air. This in turn saves the energy cost
for heating the inlet air to desirable temperature. In addition, the waste heat can be
stored in a phase change material and the heat is then transferred to the inlet air imme-
diately or when the heat source is not available for the case of sun drying.
7.2.3. Lack of heat insulation
Poor insulation in dryers causes appreciable loss of energy to the environment. In
this regard, drying chamber and air duct or piping can be insulated to minimize energy
loss to the environment.
7.2.4. Improper drying strategy due to lack of understanding of drying
science
Drying generally involves removal of surface moisture as well as internal moisture
unless the drying material is rigid and contains only surface moisture. The removal of