The compressed air, cooled to about 15 to 20 Deg.C free of moisture and carbon dioxide will enter the Cold Box (15). It initially passes through a Heat Exchanger No.1 (16); the incoming air will be cooled by the outgoing Oxygen and Nitrogen. The air will be cooled to around -100 Deg.C. In this Heat Exchanger. This can be single or divided two parts in series.
The air will then be into two streams. The main air stream will enter Expansion Engine (14) at 40 - 45 Kgs./Cm2 and will be expanded to 5 Kgs./Cm2 and -150 to 160 Deg.C the rest of the air will pass through Heat Exchanger No. 2 (17) to be cooled to about -160 Deg.C. by the outgoing Oxygen and Nitrogen. This air will then be expanded by an Expansion Valve V3 to form liquid air. Both the air streams will now enter bottom portion of the Lower Column (19). Operating pressure of the column is around 45 kg/cm2 under normal operating conditions.
As the air enters the Lower Column, after the Expansion Engine and after Expansion Engine valve V3, a part of this air condenses into liquid and falls at the bottom of the column. This liquid is about 40% Oxygen and 60% Nitrogen and is usually called the "Rich Liquid" and as Nitrogen is more volatile it rises to top of the lower column where it gets cold from the condenser and become liquefied. This liquid nearly free of oxygen collected in the (Pockets in the condenser) trap. As this liquid poor in oxygen is called poor liquid.