Abstract:
In this research project the tea withering process, which accounts for the bulk of
electricity consumption in tea manufacturing was investigated. The main objective of
the project was to develop a proper flow control strategy for the withering process to
conserve energy. The other objective was to devise a method to select a suitable axial
flow fan to optimize energy utilization. With the present design of withering troughs,
process parameters such as temperature and relative humidity could not be precisely
controlled. Due to this, empirical approach involving multiple tests was used to derive
the relationship between the drying rate and other major parameters. Air flow rate,
temperature and relative humidity of inlet air, pressure inside the trough, and energy
consumption of the fans were the measured parameters. Several fans with different
make and different operating conditions were studied during the experimental testing.
The results of the study reveal that during the first few hours (around 4 hours) the
drying rate was dependent upon both the flow rate and the wet and dry bulb
temperature difference. But in the latter hours, drying rate is mainly dependant on wet
and dry bulb temperature difference. Therefore, it was concluded that best flow
schedule is that the fan runs at full speed at the first 4-5 hours and then the speed is
lowered in such a way that the pressure inside the trough does not go below 75 Pa, the
recommended minimum pressure. When this flow schedule was coupled with other
factors, such as the quality of tea deteriorating due to long withers, uneven withering,
and variation of crop, it was revealed that conventional fans, which are usually overdesigned
with higher pressure and flow rate capacities, when operated with variable
speed drives with the developed flow schedule, can be more effective than the fans
recently introduced to the industry. Further research work on thick bed drying is
needed to confirm the results. It is recommended to construct a test rig which can
regulate the parameters such as flow rate, humidity and temperature and can record
the test parameters real time, and hence derive an accurate drying rate equation for tea
withering process.
