Water Mist in Compressed Air Blowgun

[KenRad]

I'm stumped about a problem that my production folks are having with my compressed air.

This particular production line uses compressed air blowguns to blowoff glass lenses. When they then look at the lens through a magnifying glass, they see small water droplets randomly spread over the lens surface.

I immediately told them that this could not be so, for two reasons. First, all of the compressed air, generated at about 110 psig, goes through refrigerated dryers. The pressure dewpoint is lowered to about 40F, as shown by a calibrated Vaisala dewpoint meter. We get alarms if the dewpoint rises to 60F. We have had no dewpoint problems for months. Second, the blowoff guns are served by a regulator, which drops pressure down to 20 psig. This further suppresses the dewpoint, to something like 20F. None of the compressed air lines pass through any room that is less than 70F, so we are not cooling the air and causing condensation in the lines.

I looked at the lenses, and they were right in what they were seeing. What am I missing?

---KenRad

 

[BigInch]

Assuming the droplets are not from the compressed air itself, are the lenses separated from the room environment?

Are the lenses being cooled by the compressed air such that humidity from the room can condense on their surfaces?

http://virtualpipeline.spaces.msn.com

 

[zdas04]

Also, the dry air will pick up some humidity from the room air that it is flowing through.

David

 

[djack77494]

For instrument air systems, we'd typically dry our air to a dewpoint of -40 degrees (F or C). This is a prudent approach, since as the air is condensed, it is less able (on a pound to pound basis) to hold moisture. Later, at its point of use, there is a sudden pressure reduction which results in significant cooling of the air. I think it's common for moisture droplets to form under these conditions, even though a bit further downstream, after good mixing with ambient air, the conditions do not favor liquid water.
Doug

 

[Montemayor]

KenRad:

The "Dewpoint" you are registering is the saturated condition of the air AFTER it leaves the vapor-liquid separator that must be used with the refrigeration type of "dryer". This type of operation does not "dry" the air in the conventional sense. It merely depresses the dewpoint by using refrigeration and pressure to condense out a majority of the water content in the air. The air is still saturated at the conditions of the refrigeration unit's air outlet out of the separator.

The problem often experienced with this type of low capital cost "dryer" is that the vapor-liquid separator, for some reason or other, either gets full of liquid condensate or its separation efficiency falls off. The end result is mechanical entrainment of water droplets - albeit microscopically small. When you resort to mechanical means to separate a liquid from a vapor you are going to experience some degree of entrainment - however small. This comes with the territory. I suspect, from your detailed description, that this is what has happened. I would never apply this type of cheap air dryer if the product lenses are to be of high quality. I would opt for an adsorption type of dryer - but that doesn't fix your present problem.

If your application is critical (as I also suspect), then I would resort to inserting a refrigeration adsorbent cartridge (usually filled with silica gel or activated alumina) on the compressed air line that is used to furnish the air blowguns. This will easily furnish super-dry (approximately -50 ^oF dewpoint) compressed air for at least while you find the fault in your vapor-liquid separator and repair it. You can buy these cartridges at your friendly refrigeration equipment store or get it from a repairman. If it were me and the value of your glass lenses was high, I would use the cartrige ALL the time - just to ensure good air quality at the point of application.

 

[KenRad]

Montemayor,

I understand what you're saying. The reason that I was skeptical about the presence of liquid droplets is that after the air exits the refrigerated heat exchanger, it passes through an air-to-air heat exchanger, which both pre-cools the incoming air, and warms up the air coming from the dryer. So the air leaving the compressor room is warmed up to ambient temperature, and is no longer saturated. I've always assumed that whatever small droplets were still present would vaporize along the way in the piping network. This process would then be assisted by the pressure drop of the regulator, which would drop the dewpoint and %RH even further.

I've used a lot of membrane-type dryers for instrument air applications (-40 dewpoint), and might try one here.

Thanks for the help!

---KenRad

 

[zdas04]

KenRad,
None of the processes are instantaneous. When we use terms like "dew point" and "relative humidity" we are talking about steady-state conditions. It can take minutes for a drop of water to be assimilated into a super-heated gas stream. Often in the kind of flow you're talking about there just isn't enough time.

If the result is critical, I think I'd go with Montmayor's cartridge drier.

David

 

[unclesyd]

We had a situation when using a OSHA safety nozzles in cleaning/inspection station. We were using instrument air, supposedly very dry, to supply the sir guns. It turned out that the particular nozzle we were using aspirated a lot of local air to remove the high air pressure hazard. The ambient air at this location due to process reasons had high humidity to help prevent static electricity. The combination of wet air being aspirated and cooling caused our problem. We had to move this operation into a room with conditioned, dryer, air.

 

[BronYrAur]

Montemayor,

Would that be a refrigeration liquid line or suction line filter? I assume suction?

 

[ClicketyClack]

Are they definitely water droplets and not oil droplets?
Is your dewpoint sensor working properly?
Are you located in the Northern Hemisphere? It has been very hot lately and more moisture is in the air than in the previous months.
Has anything changed lately in the system?

A 40 degree pressure dewpoint at 110 PSI equates to about -10 degree dewpoint at atmospheric pressure. If everything is working properly I would be surprised to see water condensing.

 

[itsmoked]

Page 1055

http://www.grainger.com/Grainger/wwg/catalogPDF.shtml

I used one of these canister filters to dry my otherwise wet air in front of a paint gun. It solved some huge painting problems for ~150 bucks. Most have a sight window and colored media that changes color with moisture content.

Highly recommended. Try one on one line and see if it solves the problem.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rmw]

I tried to search for a previous thread but the search feature on the site is down right now so I hope you will search for it when it comes back up, but there was an in-depth discussion of a similar problem and after I posted an experience in a similar situation where I related that once at a large paper mill in the humid deep south after installing a -40F dewpoint dryer at the paper mill, the paper machine superintendant about 1/2 mile away from the power plant complained about their compressed air being wet.

No way said the utilities area engineer who had just spent big bucks on a project to purchase and install the dryer (I think it was a combination refrigerated/dessicant).

The problem ended up being numerous air leaks in the long piping between the utilities area and the end user. The air line passed through several paper areas before it ended up at the last user on the line. At every leak along the way, as air leaked out moisture diffused in.

I just related the story and the solution and others contributed to the thread with information and links that substantiated the science behing it and the result (and the ultimate fix which was doing a leak survey and fixing lots of leaks).

rmw