The hot air drying of a combination dryer is achieved by heating air to dry materials. Hot air drying is the process of heating the hot air generated by combustion through a heat exchanger and sending it into a drying chamber. After the hot air comes into contact with the material, it takes away the moisture on the surface of the material, thus achieving the purpose of drying. Hot air drying can quickly and uniformly evaporate the moisture on the surface of materials, improving drying efficiency.