# Experimental Study on Drying Characteristics of Alfalfa Hay Bales Using Hot Air Convection
## 3 Results and Discussion
### Analysis of Drying Characteristics for Different Bales and Optimization Proposal  
Based on the experimental results, when the hot air temperature is around 70 \({}^{\circ}\)C and the air velocity is around 2 m/s, with an initial moisture content of about 25% (w.b. ), the average moisture content of two bales weighing 19 kg and 20 kg, as well as one bale weighing 40 kg, can be reduced to 17% (w.b.) within 1.5 h. The drying efficiency is 26.67 kg/h of the original grass. For two bales weighing 25 kg each, the average moisture content can be reduced to 17% (w.b.) within 2 h, with a drying efficiency of 25 kg/h of the original grass. For two bales weighing 27 kg each, the average moisture content can be reduced to 17% (w.b.) within 4 h, with a drying efficiency of 13.5 kg/h of the original grass. Under the aforementioned drying conditions, two bales weighing 40 kg each cannot be dried effectively. When drying three bales weighing 33 kg each, with the middle bale being an already-dried bale, the upper and lower bales dry very slowly, almost at a standstill. From the experimental data, it can be seen that the drying process of forage is essentially a process of moisture movement, transfer, and evaporation. In this process, only two factors affect drying: the first is the driving force that generates moisture movement, transfer, and evaporation; the second is the resistance encountered by moisture during its movement, transfer, and evaporation. The interaction of these two factors affects the drying process. Therefore, it is necessary to consider increasing the driving force by raising the air pressure and temperature, as well as reducing resistance by adjusting the amount of forage dried per batch, bale density, and the position of the end face, where hot air is blown into the forage. It is also important to reduce the unevenness of the driving force and resistance to avoid short-circuiting in mass transfer. Previous studies have made attempts with regard both aspects. For example, the oven-drying study by [PERSON] [17], which increased the driving force (temperature above 70 \({}^{\circ}\)C), significantly shortened the drying time. [PERSON]'s [6] research used a disc mower to crush stems and reduce density, thereby lowering the resistance to accelerate drying. Meanwhile, [PERSON] [26] controlled the relative humidity and recirculating airflow to reduce the retention of humid air, enhance the moisture gradient, and improve drying efficiency and uniformity. Based on the principles derived from the existing experimental data above, as well as the literature review, potential solutions for the following two schemes can be proposed:  
Scheme 1 (see Figure 9a): Increase the driving force for moisture movement, transfer, and evaporation by raising the fan's air pressure to ensure that the drying air penetrates through the bales. Enhance the moisture gradient between the forage and the external environment; as the gradient increases, the driving force becomes stronger. Corresponding measures include raising the temperature of the drying air, reducing the humidity of the drying air, or adding a suction device. The structure can be designed to enclose the bales on all sides, with the upper and lower end faces serving as the air inlet and outlet, respectively. One side is the air inlet, where a blower introduces hot air, preferably after passing through a chemical desiccant to dehumidify the air before blowing it into the bales. The other side is the air outlet, where another set of exhaust fans quickly removes the moist air blown out. Scheme 2 (see Figure 9b): Reduce the resistance encountered by moisture during its movement, transfer, and evaporation by decreasing the density of the bales. The advantage of this method is that the drying efficiency will increase, meaning that more forage can be dried in the same amount of time. However, the disadvantage is that the deformation of the bales will be more significant, increasing the space required for transportation and storage after drying.