A part of this cold air then flows through an expansion machine and the balance through the 2nd heat exchanger. The ratio of the two air streams is controlled by an expansion valve, RI.
Both these streams of air then unite in the lower pressure column where it partially liquefies.
The liquid air (rich air) then passes through the expansion valve R2 to the upper column which is at a lower pressure than the lower column. Similarly the liquid nitrogen (poor liquid) travels from the lower column to the upper column through an expansion valve R3 where the separation of oxygen and nitrogen occurs. Nitrogen being more volatile passes out as a gas from the top of the column and this waste nitrogen flows through both the heat exchangers cooling the in-coming air. Similarly product oxygen is also passed through the two heat exchangers to cool the in-coming air and then to the filling manifold via a liquid pump. If a small amount of air is vented out from the upper column, higher purity nitrogen can also be obtained from this plant. R4 Valve is provided in order to fasten cooling during start-up.