## 4 Discussion  
### Drying Performance  
Time is an important factor for the efficiency and the economic viability of the drying industry, so results from the first 24 h of the drying procedure are mainly discussed and analysed below. All the tubes had a better drying efficiency in comparison with the drying box for the potato samples. Tube 4 had the greater drying efficiency in the first 24 h of potato drying in terms of weight loss for all replications. Tube 3 was second in potato drying efficiency, and together with Tube 4, they were made from the same material and had the same dimensions, with the only difference being that Tube 4 was painted black. Being black and exposed to direct sunlight, Tube 4 had achieved higher temperatures than the other three tubes, which most likely increased the air flow rate inside the tube by the solar chimney effect. However, statistical analysis (Supplementary Material) showed for all replications that there was no statistical significance between the drying efficiency of Tubes 3 and 4 after 24 h of potato drying. Furthermore, Tube 1 and 2 had almost the same efficiency, with Tube 2 being slightly better at drying the potato samples. Probably, the low vertical height of the tube above the ground was not enough to create a strong solar chimney effect, which was verified by low air velocities inside Tubes 1 and 2 (see Section 4.2). Similar results were observed for the banana samples in terms of tube drying efficiency, with Tube 4 having the best results. For all the tubes at the first 24 h of drying, the best results were obtained during Replication 4. The statistical analysis (supplementary material) showed that there was not a significant difference in the percentages of weight loss of the banana samples in all tubes of Replication 4. However, the efficiency of the drying box was statistically significantly lower in comparison with the tube's efficiency in the same replication. Moreover, Tubes 3 and 4 had a better efficiency in drying the banana and the potato samples (first 24 h) as in all replications. There was a statistically significant difference with the lower efficiency of the drying box. At the end of the drying process, all the tubes drying efficiency were closed for the same replication. The best results for potato drying were obtained at the end of Replication 3 and for banana drying from Replication 1. The only difference between these replications, except the environmental conditions, was the duration of each replication. Specifically, Replication 1 was the only one that had a duration of 3 days in contrast with the other replications, which their duration was 2 days. This drying duration (2-3 d) is comparable to other alternative drying methods, such as solar drying, where the drying duration varies between 1 and 3 days depending on the type of solar drier [35]. Weather conditions and especially relative humidity indicated a significant role in the drying performance. During the first 24 h of Replication 4, the daily average relative humidity was between 48.32% and 55.47% (Table 4), which was the lowest among all replications. The low relative humidity of the environment affected the drying efficiency of all tubes that reached an average weight loss for potatoes between 72.45% and 81.05% and for the bananas between 56.98% and 58.05% in the first 24 h of drying for Replication 4. Moreover, the wind speed outside the greenhouse construction probably affected the \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Period** & **Tube 1** & **Tube 2** & **Tube 3** \\ \hline
19/06/2021–22/06/2021 & 100\% & 43\% & 10\% \\
26/06/2021–28/06/2021 & 99\% & 45\% & 13\% \\
28/06/2021–30/06/2021 & 81\% & 46\% & 4\% \\
04/07/2021–06/07/2021 & 94\% & 48\% & 15\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Air flow rate percentage difference between Tube 4 and each of the rest of the tubes. drying efficiency in combination with relative humidity.