Shop Air Supply Cleaning 3

[NickE]

Well now that we finally have close to what we think is a root cause, I figured that I'd aske the compression pros.

Our plant makes thin stamped parts, our customer has alerted us to a problem with residue on the surface of the parts. This residue seems to be compressor oil. Our shop air system is fed with Atlas Copco compressors I think they are screw type. (standard shop air ~100to 120psi)

There is a robot that uses air blowoffs as part of its handling. What are the different levels of fitering I can reccommend to my boss as ways to prevent air from reaching the robot?

(Currently I have some minor experience in asssuring I have clean dry air for the lab. I have had sucess with wilkerson using a 5micron dryer/filter followed by a .5micron oil coalescing filter.)

Nick E.
metallurgist/trouble shooter....

 

[Montemayor]

Nick:

First of all, if you value the quality of the stamped parts you are producing and selling, then I would strongly advise you to use an adsorption system downstream of the screw compressor discharge. I'm presuming the screw compressor you're using is OIL-FLOODED and, if so, this should have alerted everyone to what would happen if the air were applied on the parts. If you want quality parts you must employ quality air.

I would have assumed you were using dry air as a "blowoff" and, as such, also an adsorption dryer. This would automatically remove the water and the oil in the compressed air coming from the compressor. However, if that's not the case, you can install an oil sorber - a fixed bed of either activated alumina or another adsorbent such as activated carbon. I favor the use of activated alumina in these cases because I suspect your operation is not continuous, around the clock and the quantity of air used is relatively low in total quantity. Using activated alumina, you can visually "see" the trapped and adsorbed oil on the adsorbent and this allows you to monitor the depletion of the adsorbent and change it out before the oil "breaks through" - a term meaning that the oil in the air passes through the entire bed due primarily to the bed's loss of sorptive ability. You can also use activated carbon, but it's impossible to detect the oil's progress in the carbon.

You would normally use a batch vessel (or cannister, if your amount of air is really low) and manually change the adsorbent before it depletes. The time it takes to deplete is dependent on your usage of air. Find the root cause is not difficult. Many times you can prove the oil contamination by blowing the air into a pad made of tissue paper. The oil stains are obvious when the contamination is heavy.

Hope this experience helps. I've solved this problem many times by using adsorbers with either activated carbon or activated alumina. I know I can produce food-grade quality air with this operation.

 

[zdas04]

Most oil-flooded screws have coalescing filters on the outlet stream. I've found that the key to minimizing (not eliminating) the oil in the gas is to make certain that the gas temperature out of the screw is 200-210F. If you're getting a lot of oil downstream it is usually because the gas is too cool to cook water vapor from the oil. The contaminated oil doessn't aggregate as well in the coalescor and you get more oil downstream--an added expense to replace and an extra load on downstream devices.

Once you get the screw working perfectly, then the media in the filter that Montemayor recommends will last longer and work better.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[quark]

Good comments above. I will have a second thought if I have to use alumina for both drying and oil removal. Oil quickly deteriorates alumina and it is better to remove oil before the dryer. We in Pharma, generally use baffle type separators before the air receiver and then dryers come in line. All upward mains will have drain points. This arrangement can reduce load on the dryers. 5 micron, 1 micron and 0.01 micron filters are used in line (or 0.01 terminally). Domnick Hunter guarantees 0.01ppm oil content after their 0.01 micron filters. Alternately, you can go with Parker or Zanders.

 

[NickE]

Stars round!

 

[crjones]

If you have a flooded screw compressor, the oil carryover is at the vapor pressure of the lubricant. A PAO lubricant has 1/12 the vapor pressure of a mineral oil. So you can significantly reduce the challenge to any downstream system.

Atlas-Copco makes both flooded and dry screw compressors. If you have the later the problem is inlet air or a seal failure on the inlet to the first stage.

 

[rmw]

You may want to consider an oil-less instrument type air compressor for the parts blowing robot operation.

But remember the old adage. Clean dry instrument air is an oxymoron.

rmw