Static pressure drop in a fixed bed of root crop chips during drying as affected by airflow rate and moisture content

Among the crops tested, cassava chips provided the greatest resistance to airflow at all moisture levels. The average value of 26.02 mm H2O/m depth (0.31 in H2O/ft depth) in cassava was followed by sweet potato and gabi chips, which were about 7.5 percent and 9.4 percent lesser than that of cassava, respectively. The effect is attributed to greater starch content in cassava (81.25 percent) than in sweet potato (74.26 percent) and gabi (77.26 percent, 13 percent M.C.) The curves of the pressure drop versus airflow on log-log paper were visible as two straight line segments. The left straight line corresponded to laminar flow, airflows lower than 11.00 m3/min/m2 and the right straight line corresponded to transitional flow, airflows higher than that. At airflows below 8.07 m3/min/m2 a change in moisture did not significantly influence the pressure drop, but above that, pressure drop increased rapidly with increasing moisture content. Pressure drops were generally higher in high-moisture chips than in low-moisture chips. Relationships were found to be normally linear for sweet potato and gabi and logarithmically linear for cassava. Porosity also increased with moisture, but despite the presence of more void fraction, pressure drop remained high. This was primarily because inter-spaces in the bed were lined with starch granules and water molecules, thus suppressing the flow of air. When dried, chips decrease in volume and weight and projected smooth surfaces, thereby causing less resistance to airflow. Among the empirical equations that have been evaluated, the Ramsin and Ergun equations fitted very well with the experimental data. The coefficient of determination, r2, were 0.92-0.99; all values have significant effect. But the Haque equation was the simplest and most practical method of estimating pressure drop below 63 percent moisture level. Root crop chips have lower pressure drop compared to grains such as grain sorghum, rough rice, shelled corn and soybeans at about 11-13 percent moisture levels.