Fig. 2 illustrates 3D surface plots of energy efficiency for the recirculation ratio RR = 70%. The two independent variables were varied while the other two variables were kept at fixed levels as hold values denoted in Table 3. The results shown in Fig. 2 indicate that the energy efficiency of drying process is improved by decreasing the drying air temperature (Fig. 2(a), (b) and (f)) and the drying air flow rate (Fig. 2(c), (d) and (f)). These trends are in agreement with results published by Toneli et al. [9] for spray drying of green banana biomass.