Compressed Air Dew Point

[LG63]

I’ve been researching old threads on after coolers, dryers, and moisture separators and I’ve seen reference to dew point in terms of the temperature at which the moisture will start to condense but didn’t see any info on how dew point changes with pressure. I did a few google searches and found that the dew point increases with increasing pressure but couldn’t find a simple chart or calculation to determine the dew point at a given pressure. I’m not looking for precise number, just a general idea. For example if the ambient air is at 50% humidity, what will the approx dew point be for that air compressed to 100 psi? Maybe there’s more variables involved, but it seems like if I know my ambient humidity (or dew point) and the pressure, I should be able to calculate the new dew point.

 

[lametec]

http://www.dew-point.com/equivalents.asp

 

[LG63]

Thanks! Who would have thunk it.... dew-point.com!

 

[Alan Douglas]

So you're almost guaranteed to have condensation if the ambient dew point is higher than about 25F. I wouldn't have guessed that.

 

[LG63]

So you're almost guaranteed to have condensation if the ambient dew point is higher than about 25F. I wouldn't have guessed that.

I was surprised too. Today with the ambient dew point ar 42F, the dew point at 100 psi is 103F so condensation could even start in the aftercooler. This seems like a good thing, the more condensation that can take place in the aftercooler and tank, then the less saturated the air will be as it exits the tank.

 

[2oolhound]

I'm not sure where you're going with this but I've always assumed that the process of compressing air and the heat caused by friction of the compressor moving parts is what heated the air in the tank. (how many times have you burnt yourself on your compressor). As long as the air inside is hotter than the ambient temp outside there will be condensation inside the tank. If you have a good air cooler in front of the tank along with an air dryer and are near the equator you may see condensation on the outside of the tank. I'm sure greater pressure increases the moisture because there is more volume of air (with moisture content) inside the tank than at lower pressures.

 

[lametec]

I'm not sure where you're going with this but I've always assumed that the process of compressing air and the heat caused by friction of the compressor moving parts is what heated the air in the tank.

The heat is mostly from the act of compressing the air. When you compress a gas (whether there's friction or not) it gets warmer. Similarly, when you decompress a gas, it gets colder. This is how your AC and fridge work.

 

[rlitman]

The heat is mostly from the act of compressing the air. When you compress a gas (whether there's friction or not) it gets warmer. Similarly, when you decompress a gas, it gets colder. This is how your AC and fridge work.

Not quite. They work on a phase change, but yes, refrigeration can be done without a phase change. A Stirling engine is one thing that comes to mind that uses these properties well.

 

[lametec]

True, I was referring to the compressing/decompressing part, which is the same. Refrigeration units just use a different gas, that turns to liquid when compressed.

 

[Handoogies]

Pressure dew point and ambient dew point are different things. The dew point ratings on compressed air dryers deal with pressure dew points. If you live in a colder climate eschew the refrigerated style dryers and op for a desiccant dryer, which have standard pressure dew points of -40F. So unless you live in Siberia you shouldn't have any condensation in your lines.

 

[gavinator]

Bringing one back from the dead, eh?

 

[popbehert]

I wouldn't have guessed that.