# Drying Characteristics of Chicken Manure Under a Variable Temperature Process
## 3 Results and Discussion
### Drying Characteristics at Different Air Velocities  
The drying time at different air velocities of 0.6 m/s, 0.9 m/s, 1.2 m/s, 1.5 m/s, and 1.8 m/s is shown in Figure 6. In this group, the drying temperature in the second stage was fixed at 55 \({}^{\circ}\)C, and the moisture content node was set at 45%. The results showed that drying time decreased as air velocity increased. During the early drying stage, when the drying temperature was relatively low, the time required to reduce the moisture content of chicken manure to below 45% was 26.7 h, 22.8 h, 20.7 h, 19.5 h, and 19 h, respectively. This trend occurs because an increase in airflow over the surface of the chicken manure reduces the thickness of the boundary layer, thereby decreasing the water vapor impedance transfer from the manure surface to the air. This process facilitates water evaporation from the chicken manure, effectively increasing the surface evaporation rate. However, as shown in Figure 6, when the air velocity increased to 1.2 m/s, the rate of dehydration improvement decreased significantly. This can be attributed to the fact that once the air velocity reaches a certain threshold, the airflow becomes sufficient to remove the evaporating water from the manure surface. At this point, the rate of water diffusion within the manure becomes the primary factor influencing dehydration during the low-temperature drying stage. In the second drying stage, as the remaining water in the chicken manure primarily exists as bound water, the drying temperature becomes the dominant factor affecting dehydration. Therefore, the influence of air velocity on drying time weakens beyond 1.2 m/s. The results indicated no significant difference in drying time among the groups with air velocities of 1.2 m/s, 1.5 m/s, and 1.8 m/s (\(p>0.05\)). The findings regarding the effect of air velocity on the dehydration rate of chicken manure in this study are consistent with the results reported by [PERSON] et al. [10]. Similar conclusions were also drawn by [PERSON] [24] and [PERSON] et al. [25]. Figure 5: Effect of drying temperature in the second stage on total nitrogen loss of the variable temperature process. Notes: The same letters (a) are not significant (\(p>0.05\)). Figure 6: Effect of air velocity on drying time of the variable temperature process. Notes: Different letters (a, b, c) are significant (\(p<0.05\)). The same letters are not significant. Figure 7 shows the energy consumption of each group under different air velocities. The results indicate that higher air velocities correspond to increased fan energy consumption. When the air velocity reached 1.8 m/s, energy consumption was at its highest. This outcome can be attributed to the fact that the fan used in the experiment is a variable frequency fan, where fan power consumption is directly proportional to air velocity. Additionally, since there is no significant difference in drying time during the second drying stage across different air velocities, the energy consumption of the resistance wire remains essentially constant. As a result, higher air velocities led to higher total energy consumption. This conclusion was consistent with [PERSON]'s study on the relationship between wind speed and energy consumption in a sludge drying process [20]. Although chicken manure and sludge are different, they are both porous materials. Consequently, their drying characteristics are similar. As shown in Figure 8, the total nitrogen loss decreased with increasing air velocity. During the early stage of drying, the temperature was relatively low, and the drying time was prolonged. Under these conditions, an increase in air velocity accelerated the dehydration rate, which in turn inhibited the degradation of uric acid and the generation of NH\({}_{4}\)-N. In the second drying stage, the drying time is relatively short, and the degradation of uric acid and generation of NH\({}_{4}\)-N is negligible [26]. Nitrogen loss increased with the increase in the thickness of the manure layer and the decrease in the air velocity.