What role does the expansion heat exchanger play and where should it be placed?

Answer: On the one hand, the expansion heat exchanger is to reduce the temperature of the air after expansion by utilizing the cooling amount of the nitrogen returned from the upper tower, so as to reduce the superheat of the air after expansion into the upper tower; on the other hand, it plays a role in distributing the cooling capacity of the upper and lower towers. Through it, the cooling amount of dirty nitrogen from the outlet supercooler to the switch-type heat exchanger (or the cooler) is partly returned to the upper tower, which improves the rectification of the upper tower.
The location of the expansion heat exchanger can be placed in front of or behind the expansion machine, which is determined by the process design. The large air separation equipment in our country is often set in front of the machine, using part of the dirt nitrogen from the supercooler to cool the air in front of the expansion machine, so that the temperature behind the expansion machine is not too high, solve the problem of superheat of air entering the tower. The disadvantage of this setting is that the temperature drop before the expansion machine will lead to the problem of reduction of unit refrigerating capacity (unit enthalpy drop) of the expansion machine.
Some of the large air separation plants introduced also put the expansion heat exchanger behind the machine. This arrangement is suitable for the situation that the length of the plate unit of the switching heat exchanger increases, the circulation temperature increases, and the temperature in front of the expansion machine increases. The increase of the temperature before the machine can increase the refrigerating capacity per unit of the expansion machine and reduce the expansion air volume entering the upper tower; and the heat exchanger after the machine can reduce the superheat of the expansion air entering the upper tower, both of them are beneficial to the rectification of the upper tower, and the disadvantage is that the back pressure after expansion increases slightly. But because the resistance of Plate-fin Heat Exchanger is not big, it has little influence on enthalpy drop of expansion machine.
Due to the setting of expansion heat exchanger, the moisture content of the air in the lower tower is reduced, which can improve the purity of the liquid space and also benefit the rectification of the tower.