In order to liquefy the air, different deep refrigeration cycle devices can be used, mainly based on the Linde cycle and the Crowd cycle. The former is cooled by throttling expansion; in addition to the throttling expansion, the latter also has an isentropic expansion in the expander. When the gas is isentropically expanded, the temperature is reduced more than the throttling expansion, and a part of the compression work can be recovered, so it is economical than the throttling expansion. Other various improved deep freezing cycles include a double pressure throttling cycle, an ammonia precooling throttling cycle, and a stepwise overlapping cycle.

In the various cycles of the deep freezing method, the typical process is to first filter the air in the filter to remove impurities such as dust into the compressor, and then remove the air at a low temperature, such as water vapor and carbon dioxide, through the molecular sieve purifier. The purified air is cooled by the product nitrogen and oxygen in the first heat exchanger. After the first heat exchanger is taken out, the air is divided into two paths: one road is cooled by the second heat exchanger, and then depressurized by the throttle valve; the other is stepped down by the expander. The air temperature after expansion of both channels is reduced to about 103K, and enters the bottom of the lower tower of the two-stage distillation column.