## 4 Discussion
### Flow Rate  
As can be seen in Figures 6 and 7, the weight loss of the samples observed in Tube 4 is greater compared to all other tubes. In Table 5, where the air flow through the tubes was highlighted, it can be observed that the average air flow through Tube 4 was always higher compared to all other treatments. The height of the tube had a major role in the air flow rate. Tubes 4 and 3 have similar values of flow rate, and the slightly increased flow rate was due to the black color of Tube 4, which led to a higher solar radiation absorption factor. In fact, the dark blue to black color has an absorption factor between 80% and 90%, while the grey to dark grey color has between 40% and 50% [36]. Considering this difference in the absorption factor, it seems that height plays a more crucial role in the flow rate as this percentage difference in the absorption factor is transposed in only 10-15% in the overall temperature difference and, as a result, in the air flow rate. Tubes 1 and 2, which have a lower height, result in a much lower air flow rate compared to Tube 3 and Tube 4. Tube 1, which was covered with a black plastic polyethylene sheet and a 10 cm polyurethane layer, had the poorest performance. Even though the absorption factor of the black color (80-90%) is much higher than the absorption factor of the aluminum (15-30%) [36], the much lower thermal resistance of the aluminum tube compared to the insulated PVC tube [37] led to an increased air flow in Tube 2 since the temperature rise was higher. It seems that the insulation played a negative role in Tube 1. The initial rationale was to apply the insulation to minimize the variation of the temperature inside the tube and achieve a more stable drying process for more time throughout the day. This was achieved in general, with temperature differences being higher than expected during the less warm hours but not as high as expected during the warmer hours. The insulation applied in Tube 1 did not have a positive impact on the temperature rise phenomena. On the contrary, it did not allow the temperature to rise inside Tube 1 at the level that was observed for the rest of the tubes. It was proven that it blocked in a higher grade the heat flux from the environment to Tube 1 compared to the other tubes. According to Table 6, the difference in the air flow rate presents a more standard variation between Tube 4 and Tube 3 (4-14%) and between Tube 4 and Tube 2 (43-48%) compared to the difference between Tube 4 and Tube 1 where the difference was about 80-100%. The wider difference occurs during the period of 04-06/07. This difference had to do with the fact that Tube 2 was insulated, so it presented a less intense temperature fluctuation compared to the other tubes, so during warmer days, when the flow in the rest of the tubes was sufficiently increased, that did not happen in the case of Tube 1. The differences between Tube 4 and 3 are lower than the absorption percentage of black and gray colors. That means that height plays a more crucial role in the phenomenon and not the color. When comparing the difference between Tubes 2 and 4, the difference was close to 50%. This difference was slightly lower than one of the absorption factors, which means that the high thermal conductivity of the aluminum plays a role in increasing the temperature difference between the inlet and outlet. When comparing the measured air flow with the calculated, a decreased value of air flow occurs. This was due to losses that might occur inside the tubes and flow phenomena at the area of the tube's inlet. It seems that in Tubes 1 and 2, where the height is 2.0 m, the differences between the actual values of the flow and the calculated ones were not as wide and the average difference for the examined days and hours ranges between 0.7% and 1.2% for Tube 1 and Tube 2. In the case of the higher tubes, the difference between the calculated and actual flow was between 1.0% and 1.7%. That means that the losses were higher in these tubes because of friction phenomena and flow losses.