In the distillation column of air separation, the air is usually pre separated in the lower column, and then distilled into oxygen rich liquid air and pure nitrogen, and then further distillation in the upper column to obtain oxygen and nitrogen products. As a condensing evaporator connecting the upper and lower columns, the pressure nitrogen of the lower column is used to heat the liquid oxygen of the upper column. When the liquid oxygen is evaporated, the gas nitrogen is condensed. The two-stage distillation column can make oxygen and nitrogen products obtain higher extraction rate and higher purity. The working principle of single-stage distillation column is similar to that of the lower column of two-stage distillation column. Oxygen enriched liquid air is at the bottom of the column, and pure nitrogen gas can be obtained at the top of the column. The single-stage distillation column can't separate the air perfectly, so it is seldom used in practice. However, because of its simple structure, it is sometimes used in small air separation unit for producing single product.