The air then enters into cascade an Evaporating Cooler (5) where it gets cooled to about 20 Deg.C. This unit is optional. The cooler is a cubical vessel where, there is pipe coil and is inter connected. The coils are half submerged in water in the vessel Dry Nitrogen will be bubbled through this water to become wet gas. As the water vaporizes, it requires latent heat which is absorbed from water itself. So, water gets cooled. Thus, air inside the pipe coil will get cooled. Compressed air, cooled in evaporation cooler will enter into a Moisture Separator (4 & 8).
Moisture condensed as water will be separated and drained once in an hour. It is important to drain moisture from the bottom of the Oil Absorber (9) at regular intervals and also change the Alumina every 6 to 12 months. After this the air will pass through an additional cooler called Chilling Unit (7).
After this the air will pass through Oil Adsorber. (9) Packed with Alumina balls. Here the Oil Vapor carried over from Air Compressor will be removed. If this oil vapor is not removed sufficiently, due to spent carbon or due to high temperature of process air, the oil vapor will damage the Molecular Sieves. To obtain a long life of Molecular Sieve ensure the Alumina is well maintained.
The air then enters one of the Molecular Sieve vessels (11). The moisture and carbon dioxide in the air will be removed in this drier. If they are not removed before entry to Cold Box, they will form Ice and dry Ice which will choke the Heat Exchanger Tubes and other equipments. There are two driers. One will be (on line with the process air) in operation for around 10 hours and the other will be under regeneration. Regeneration is done by heating and cooling with not-going Nitrogen. An electric regeneration gas heater (12) is used for regeneration. For further details, refer separate chapter on Molecular Sieve Driers.
The dry air is again filtered in a Dust Filter (13) before entry to Cold Box to avoid any dust entry to Cold Box. In some plants the air is further cooled through special coils provided in the Chilling Unit Tank (6), which is called an equalizing coil as it equalizes the temperature after the Molecular Sieve drier before Air enters the Cold Box.