Sweating on Line: High vs Low Pressure

[cyclone]

Why sweating on line occures when Air flow from high pressure(6.5kg/cm2g) to Low pressure(0.1kg/cm2g)?
Is this because of air expansion?
Then Why sweating not occur if Air flows from high pressur(6.5 kg/cm2g) to atmosphere?

 

[FredGarvin]

You need to describe the situation better. From the sounds of it, you are experiencing condensation on the outside of a pipe simply because of the expansion and resultant cooling of the media. If the cooling is below that day's dew point, you'll get condensation.

What is your situation exactly?