| Summary: | <p>This thesis is concerned with the technology of heat pump assisted drying of
specialty crops, the benefits and potential of the process in the agriculture sector. The
purpose of using a heat pump for drying of specialty crops is to dry them at lower
temperature than would otherwise be possible using conventional drying techniques.
Low temperature drying of specialty crops reduces the risk of loss in nutrient content
and damage to physical properties which are important aspects considering their high
commercial value. Heat pump drying is potentially more energy efficient since it is
possible to recover latent heat from the humid dryer exhaust air.</p>
<p>In this research, cabinet and prototype continuous bed heat pump dryers were
studied. Simulation models for the heat pump dryers were developed to predict the
performance of the dryer, i.e. the drying rate of the material and psychrometric
conditions of the air, and also to determine the refrigerant mass flow rate and
corresponding temperatures at the condenser and evaporator coils of the heat pump
system, based on the psychrometirc conditions of process air inside the drying
chamber. The accuracy of the predicted results is later verified with the experimental
results.</p>
<p>Chopped alfalfa was dried in a cabinet dryer in batches and by emulating the
continuous bed drying using two household dehumidifiers. The reason for using
alfalfa instead of specialty crops like ginseng, herbs, echinacea, feverfew, etc. was that
the material and its drying properties were readily available. Also the structure of
alfalfa leaves and stems is similar to that of many herbs and specialty crops.</p>
<p>Results showed that alfalfa was dried from an initial moisture content of 70%
(wb) to a final moisture content of 10% (wb ). It was noticed that batch drying took
about 4.5 h while continuous bed drying took 4 h to dry the material. The initial
weight of alfalfa in each tray was 400 g. The average air velocity inside the dryer was
0.36 m/s. Low temperatures (30-45°C) for safe drying of specialty crops were
achieved experimentally. Specific moisture extraction rate was maximum when
relative humidity stayed above 40%. The household type dehumidifiers used in this
study were about 50% more efficient in recovering the latent heat from the dryer
exhaust compared to the conventional dryers. It was concluded that continuous bed
drying is potentially a better option than batch drying because high process air
humidity ratios at the entrance of the evaporator and constant moisture extraction rate
and specific moisture extraction rate values can be maintained.</p>
<p>Simulation results for a prototype heat pump continuous bed dryer system
suggested that the change in dryer inlet temperatures of the process air has an
insignificant effect on drying of the material. Therefore, based on the results, it was
concluded that the dryer inlet air temperature could be kept as low as 30°C, if required,
to maintain product quality. The material was dried to a safe limit of 10% moisture
content. The material mass flow rate was only 3-4.5 kg/h, indicating that it might be
advisable to use the heat pump dryer in combination with some other technique.</p>
<p>****Disk with program codes was included with the original thesis****</p>
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