There are many varieties and series of hot-air furnace, and they are divided into coal, oil and gas furnace based on the type of fuel. The medium output by the hot-air furnace can be fully heat exchanged or highly purified through the heat exchanger. The clean hot air can be used for drying materials that are not allowed to be contaminated, or can be used for drying heat-sensitive materials with lower temperatures. Such as milk powder, pharmaceuticals, synthetic resins, fine chemicals, etc.