## 1 Introduction
In this dryer wet solids are constantly loaded at the feeder and dried solids are constantly unloaded at the outlet [12]. This kind of equipment also uses a burner to produce heat, and a fan to generate forced airflow. However, on pneumatic dryers, drying occurs at the drying duct while the solids are being pneumatically transported. At the end of the drying duct, one or more cyclones are installed for \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**Fixed-bed**} & \multicolumn{2}{c}{**Pneumatic**} \\ \hline Drying air temperature & ++ & [20] & ++++ + + & [22] \\ Solid temperature & ++ + & [20] & ++ & [22] \\ Dried product uniformity & + & [17] & ++ & [23] \\ Capital costs & ++ & [24] & ++++ & [25] \\ Maintenance cost & ++ & [17] & ++ & [25] \\ Versatility & ++++ + & [24] & ++++ & [26] \\ Simplicity & ++++ & [27] & ++++ + & [23] \\ Energy efficiency & + & [24] & ++++ + & [28] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Major characteristics of fixed-bed dryer and pneumatic dryers, where plus signs denote levels, ranging from one plus sign (+) to five plus signs (++++ +). separating the solids from the drying air. Like fixed-bed dryers, pneumatic dryers use hot air to supply the heat for moisture evaporation and to transport it away from the solid being dried [11]. However, at the pneumatic dryers, air velocity in the drying duct is high enough to make the solid suspend and be conveyed pneumatically [9]. This improves greatly the contact between solid and hot air, enhancing heat and mass transfer and consequently increasing the water evaporation rate. The higher evaporation rate, and the consequent short drying time, allows using elevated air temperatures. This results in superior levels of energy efficiency [7], without overheating the solid, or jeopardizing product quality [29]. Drying is an energy-intense operation and the evaluation of the energy performance of a drying system is a fundamental step towards its improvement [30]. Significant improvements in the energy performance of a dryer can be achieved by simply evaluating it, and subsequently making the necessary amendments [31]. To evaluate the energy performance of a dryer various indices can be used, but energy efficiency and specific heat consumption are the most useful ones [30]. The energy performance of a dryer is an important aspect to be considered when determining the best choice of drying equipment. One reason is that it has a direct implication on cost. However, the type of dryer and its operating conditions are also important because they have a direct implication on the product quality [32], and product quality determines consumers' acceptance or rejection [33]. Consumers judge the quality of food mostly by looking at its appearance [34] and among the various properties of appearance, colour is the one with by far greater influence in their decision [35]. This is particularly true for dried products [36], and for dried cassava products the critical colour parameter is whiteness [5]. While the quality of the final product is an important aspect to be considered when determining which is the best choice of drying equipment, the cost is another, as it has a direct implication on profits [37]. However, costs and product quality are often competing factors [38] and, therefore, it is fundamental to find a favourable combination between them [37]. The objective of this study was, therefore, to evaluate and compare the energy performance, product quality and costs of the two most common dryer types used for cassava drying by small-size enterprises in Africa. The evaluation provided guidelines on how to improve those dryers and the comparison determined which of the equipment is best suited for the small-scale operation, a question that cassava processors, policymakers and researchers working on rural development are frequently confronted with.