The air deflector is made of 1Cr18Ni9Ti heat-resistant steel. It is fixed on the inner wall of the furnace through several shelf bars. The resistance band is wrapped inside. The heat emitted by the resistance band is thermally circulated through the hot air circulation system to make the temperature in the furnace uniform. The ventilation device is made according to the volume of the furnace to ensure that the number of hot air cycles is more than 40 times / minute.