## 3 Results and Discussion
### Operating Conditions and Energy Performance
Overall, the energy performance indices obtained for the pneumatic dryer suggest that the equipment was properly designed. For pneumatic dryers, dimensions have a direct implication on their energy performance [61] and one of the most critical dimension is the length of the drying duct, that must be long enough to allow sufficient time for the particles to dry during the transport [49]. Despite being correctly dimensioned, the equipment was not as efficient as the large-scale pneumatic dryer reported by [PERSON] et al. [62]. In addition, it was not as efficient as the medium-scale pneumatic dryers reported by [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [54]. This is explained by the much smaller capacity of the studied pneumatic dryer, designed specifically to be used by small-size enterprises. In small-scale dryers, heat losses are always higher because of their inevitably unfavourable surface-to-volume ratio [63]. Figure 4 shows the uniformity of the air across the perforated screen of the fixed-bed dryer and Figure 5 shows that despite the turning and moving of the cassava grits during drying, moisture \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Dryer Type**} & **Heat Input Rate** & **Water Evaporation** & **Specific Heat** & **Energy** \\  & **Rate (kg h\({}^{-1}\))** & **Consumption (k) kg\({}^{-1}\))** & **Efficiency (\%)** \\ \hline Fixed-bed & 54.0 \({}^{\text{a}}\)\(\pm\) 3.2 & 32.5 \({}^{\text{a}}\)\(\pm\) 2.2 & 6005.4 \({}^{\text{a}}\)\(\pm\) 339.1 & 41.3 \({}^{\text{a}}\)\(\pm\) 2.4 \\ Pneumatic & 29.9 \({}^{\text{b}}\)\(\pm\) 2.5 & 25.6 \({}^{\text{b}}\)\(\pm\) 4.8 & 4316.5 \({}^{\text{b}}\)\(\pm\) 690.3 & 58.7 \({}^{\text{b}}\)\(\pm\) 8.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Energy performance indices of a fixed-bed dryer and of a  
For the fixed-bed dryer, several studies have shown ways to improve the uniformity of the moisture content of the dried product [16, 19, 65]. Most of them focused on adding air-guides under the perforated screen to promote a better airflow distribution. While some studies report a high degree of success, the proposed solutions cannot be easily replicated to other dryers. Therefore, another possible way to improve the uniformity would be to add a mechanical device to constantly move and turn the solid during drying. This device could even be designed in a way that enhances the contact between the product and the hot air. For the pneumatic dryer, uniformity of the dried product can be improved by reducing the fluctuation in the temperature of the drying air and minimizing the variation in the feeding rate of wet solid. The first can be achieved by employing a more responsive temperature control system. In addition to the variation over time shown in Figure 6, Table 2 also shows a discrepancy between drying air temperature and the temperature set at the dryer thermostat, attributed to a low response time of the control system. Furthermore, [PERSON], [PERSON] and [PERSON] [53] suggest placing the temperature sensor at the dryer outlet instead of at the drying inlet, as this provides better control of the drying air temperature and makes it account for variations on the moisture content of the solid being loaded to the dryer. Moreover, variations in the wet solid feeding rate can be reduced with a better design of the feeder. Additional improvements can also be achieved by coupling the temperature control unit to the feeding system, in a way that when the temperature is above the target, the wet solid feeding rate would increase to compensate for it. Conversely, when the temperature is below the target, the feeding rate would decrease to compensate for it. [PERSON] et al. [66] observed that in pneumatic dryers a control system that can also control the feeding rate not only improves the uniformity of the dried product moisture content but also the energy performance of the dryer.