Air-drying calls for large quantities of extremely dry, oil-free pressurized air. Moist ambient air is initially compressed by oil-free compressors then passed through an air-drying unit, where the compressed air is cooled to condense and to separate a large portion of the present water vapor.
This dry air is then passed through descants absorber which retains all but a few p.p.m. of the remaining moisture. The discharged air has a dew point of approximately -40 ºC.
The procedure used for air drying is similar to the one used for nitrogen drying. However, discharge pressure is limited to approximately 150-200 psig.
If a time lag is expected between the drying and final commissioning, the system can be purged with a pressure blanket of nitrogen in order to inhibit the oxidization of the pipe surface, which also allows for hydrocarbons to be introduced into an inert atmosphere.
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