4. Conclusion
This study presented a critical analysis of the dryers used in sub-Saharan Africa for tropical agricultural product drying based on an investigation and survey on the ground in Togo, Benin and Burkina Faso. To characterize the special range of dryers surveyed in this study, first a review of the characteristic criteria given by existing literature for dryer classification was carried out. The parameters
obtained by this review for dryers used in industrialized countries were not all adequate or sufficient for characterizing and especially for differentiating between the dryers used in the field. The work provided some other new criteria. This made it possible to take into account the geographical area, the types of users, their level of organization, the availability of materials, the availability and cost of energy, the available space in the field, the advantages and disadvantages expressed by users, the targeted markets and the ratio of investment to evaporative flow. All the criteria, from the literature and from this study, were grouped in five classes: dryer specifications, specifications of the products, energy and mass balance aspects, specifications of the surrounding environment and lastly, economic aspects. All these criteria enabled us to characterize the existing and long-standing inventoried dryers. The results of the survey showed that the dryers were different from those proposed in the literature. Most of the dryers found in the field and also studied beforehand were indirect solar dryers, but they did not satisfy the users because of their lowcapacity and over-long drying time. Only a small variety of dryer types was actually used. This contrasted with the large number of dryers presented in existing literature, mainly dryers with solar collectors for such climatic zones. The numberof uses of a given dryer was not correlated tothe number of its corresponding research studies. All the dryers were grouped in 10 representative types (classes). The dryers used in the field were mostly traditional dryers exposed to the sun, direct and indirect solar dryers, very few hybrid dryers, and two kinds of gas dryers. The most widespread dryers, and those renewed by the users themselves, were traditionalsystemsusingdirectexposuretothesunandtheAtestagas dryers without solar energy.