Why can air become liquid

The air separation cold box is filled with insulation material, generally pearlite, and the pearlite particles are filled with air. After the operation of the air separation unit, the temperature of the insulation material and the air will decrease, and the volume will decrease. At this time, if the cold box is well sealed, negative pressure will be formed in the cold box, and the cold box may be compressed under the effect of internal and external pressure difference. If the cold box is not well sealed, the moisture outside the cold box will enter the cold box, making the insulation material damp, and the cold loss of the unit will be greatly increased. In order to prevent the cold box from being compressed and the thermal insulation material from becoming damp, dry nitrogen or crude nitrogen is usually filled into the cold box after the unit is running to keep the cold box at slightly positive pressure.

What is the principle of the expansion air in the internal compression process entering the lower tower?