in Africa due to the farmers’ low incomes [26]. The mixed-mode dryer, therefore,
is more acceptable because of its lower capital requirement of US $151.79 compared
to the indirect-mode with a requirement of US $220.29.
The changes in the drying costs with dryer dimensions are shown in Figs. 6 and
7 for the mixed and indirect-mode dryers, respectively. Each curve was obtained by
holding two of the three dryer dimensions (L, W, B) at their optimum values whilst
varying the third dimension from its minimum value to its maximum value. In computer
optimization, each dryer dimension is converted to a coded value which is set
at 1.0, 0 or +1.0 representing the minimum value, middle value or maximum value,
respectively, for that dimension. The optimization then makes all the possible combinations
of the dryer dimensions at different coded values and finds the combination
with the minimum drying cost. In both dryers, the drying cost is most sensitive to
grain depth, decreasing by about 60% for a grain depth range from 0.025 m (1.0)
to 0.1 m (+1.0).