## 1 Introduction  
Cassava (_Manihot esculent_ Crantz) is a perennial root crop native to South America [1] but is nowadays cultivated throughout the humid tropics [2]. The roots are rich in starch and are the main source of calories for many people living in sub-Saharan Africa [3]. However, two days after being harvested, the cassava roots become unsuitable for human consumption and consequently, post-harvest losses are high [4]. To extend cassava shelf-life, the most common practice is to process the roots into flour; a dried product that can be used later as the basis for many dishes [2]. To process cassava into flour, the roots are peeled, grated, pressed, pulverized, dried and milled [4]. Large-scale cassava processing equipment is nowadays efficient because it has been subject to development for a long time [5]. In contrast, small-scale cassava equipment, of a size suitable for small-size enterprises, is in the early stages of development [6] and have variable levels of efficiency [7]. Cassava processing enterprises in Africa are mostly of small size [3]. A few large cassava processing industries, using state-of-the-art dryers, operate in West and South Africa. However, they produce cassava starch for industrial use and do not compete with the small-size enterprise producing cassava flour for domestic consumption [2]. For the small-size enterprise, processing equipment should have low capital and operating costs, should be easy to build and should be simple to operate [8]. Fixed-bed dryers and pneumatic dryers, also known as flash dryer, have those characteristics and therefore are the main types of dryers used by small-size enterprises in Africa to produce cassava flour [2]. Pneumatic dryers are more efficient than fixed-bed dryers when drying solids that can be transported pneumatically [9] but, for cassava processing by small-size enterprises in Africa, fixed-bed dryers are more common because of its greater versatility, simplicity and lower capital cost [10]. However, there has been no study determining which of those two types of dryers is the best choice of equipment for this operation. Table 1 shows the main differences and similarities between fixed-bed dryers and pneumatic dryers. Fixed-bed dryers, also known as flatbed dryer or bin dryer, are batch operated [11], thus the wet solid is loaded to the dryer and kept until it reaches the desired moisture level [12]. This kind of dryer usually consists of simple containers with a perforated screen in the middle, as a false floor, where the solid to be dried is placed [13]. Fixed-bed dryers use hot air as a drying medium and thus is a convection dryer. The heated air is forced into a plenum chamber below the perforated screen, passing through the orifices and then through the solid [14]. In this kind of dryer, heat is usually generated by a burner, fuelled by kerosene, diesel or gas. Forced airflow is produced by a fan, powered by an electric motor or a combustion engine [15]. While fixed-bed dryers have the advantage of being simple in design, construction and operation [12], its shape and configuration inevitably create an uneven air distribution that results in non-uniform drying [16]. Several solutions have been tested to improve this [17], but the most common practice is still to manually move the solid being dried at regular intervals [18], a practice that increases the work drudgery and jeopardizes equipment energy performance [19]. In fixed-bed dryers, as the solid becomes drier the temperature of the air at the outlet increases, and its relative humidity drops. This allows for air recirculation, and this energy-saving method has been successfully applied in this kind of equipment [20]. However, for cassava drying, because the material is rich in starch, air recirculation must be approached with caution. If a proper heat recovery unit is not used, dust explosion is likely to occur [21]. Like fixed-bed dryers, pneumatic dryers are also a convection dryer but continuously operated.