The process air, necessary for sulphur combustion and following SO₃ production, after filtration and compression, has to be cooled to condensate the water vapour content.

Air cooling occurs in two steps, first in a water-cooler and then in a glycol-cooler, reaching the temperature and relevant dew point of about +5 °C.

After cooling, the air enters a drying system, on allumina beds, where it is dried up to the final dew point of - 80 °C.

This dried process air is partly fed to the furnace for sulfur combustion and partly to the SO₂/SO₃ converter. A small quantity is used for the final SO₃ dilution.