Water Droplets in Compressed Air

[KenRad]

I have a production room that uses compressed air for blowoff of parts. Since this is a critical operation, I've installed a membrane dryer, and 1 micron filtration. The air is dried and filtered, then regulated from 100 psig down to about 30 psig, with the resulting pressure dewpoint around -60F (verified by instrument). The air system is kept pressurized at all times.

I'm ocassionally getting reports of water droplets on the parts being blown off with the air, although I can't imagine how this would be possible. I have not been around to witness it when the reports come in. My only thought is that it could be some other contamination, like flux from copper joints. Does anyone have any ideas on what else can be checked?

---KenRad

 

[Compositepro]

It is possibly water. Moist air can get past driers during periods of high flow. Moisture will condense in cool sections of pipe and collect in low spots where it may be very slow to re-evaporate, so it accumulates over time until it comes out as droplets in the airflow. I have often seen water in instrument airlines that are supposed to be very dry.

Piping systems for compressed air should be designed with the assumption that there is water in the lines. Air tanks should be placed near users that periodically use high flow in order to level the flow through the drier. If you have users that sometimes use air at a rate greater than the dryer capacity, you will get water in your pipes.

 

[MintJulep]

It's possible that while blowing the nozzle temperature drops below the dewpoint of the air in the room, then when you stop blowing some water condenses inside the nozzle.

 

[KenRad]

Compositepro - I installed a calibrated dewpoint meter and watched it over the course of a day, and the dewpoint never rose to over -45F. Of course they're not seeing droplets everyday, so I should probably trend for several days.

MintJulep - I hadn't thought of that. I'll have to see how they are using the air nozzles, and whether or not they cool to the room dewpoint.

Thanks!

---KenRad

 

[quark]

-45F ADP may be a problem. Suppose, 100psig (7.8bara)air is at 20C, you can calculate the final temperature when the air is expanded to 30psig (3.04bara) as below.

T1/T2 = (P1/P2)^(1-k)/k

So, T2 = 293/((7.8/3.04)^{(0.4/1.4)=223K or 49C. This is 56.2F. So, air is cooling down below the ADP. With -60f ADP, you will have few problems.}

 

[KiwiMace]

Quark's temps above are missing the negative signs, but the logic seems solid. Your nozzle is cooling down below the dewpoint temperature of your compressed air. Dry, but not dry enough.

 

[willard3]

If the vapor pressure of the air is higher outside the pipe than inside (which it is in your case), water will migrate through fittings and etc. to the inside of the pipe.