Deep cooling air separation equipment

According to a DEAR air separation user, during the operation of a set of air separation plant that has been in operation for 5 years, it is found that under the condition that the inlet guide vane does not change, the expansion air volume of the expander gradually decreases, and the rotation speed of the expander gradually decreases, the outlet pressure of the booster end of the expander rises from 0.76MPa to 0.8MPa, the inlet temperature rises from - 110 ° C to - 95 ° C, and the working condition of the chilled water pump is poor.

What are the functions of the upper and lower towers of the air separation equipment?