Oil in compressed air

You may place a specialized in-line separator/filter downstream at the linear point at which you want your measurement to be taken. The separator will remove moisture including oil. The oil will float on the surface. With isolation valves installed both upstream and downstream, you will be able to remove the filter bowl and suction out the oil using a metered syringe giving you an inexpensive way to approximate the oil content downstream. If your amount of downstream oil is in the ppm, you will need a mass spectroscopy sensor, gas chromatograph or some other hydro-carbon analyzer. The first method would cost $100, the second $100,000. I am not aware of a midrange solution.

I think you have already figured it out, as long as you know whether the ppm is measured in mass or volume. If it is in mass, then you may need to know there are .0024 slugs of air per cubic foot, at standard temp and pressure. (Assuming you are not doing aero calcs, etc. you can consider .0024 slugs equivalent to .0768 pounds.)

By volume, you'd have 500 x .0000005 = 0.00025 CFM of oil.

That helps. I am the Maintenance Mechanic where I work, second shift, I take care of the air system to recommending filtration and compressor upgrades. The guy who does the Paint Dept. maintenance is harping about getting around a cup of oil in the SATA sytem(fresh air breathing only not for the paint system) at a couple of the 6 paint booths every two to three weeks. Says it leaves fisheyes in the paint.He has been campaining for oiless compressors for each booth, run around 10K per booth. I tell him the amount of oil is minor and it would be better to do a filter, coalasing filter, desicant unit at each booth. Now that I can utilize a means to determine oil content I can figure filtration requirements. I have been trying to get one more filter for each of our two comps that would take it down to 0.01 micron. that and the setup I'm leaning towards ought to clean up the oil situation.

Thanks for the assist.

Following my previous reply, I see you are looking at filtration as a solution. This is probably the best way to do it because you will need filtration anyway. An oil-free compressor does not delivery breathable air without proper filtration.

Or should I say proper 'purification' - because in the traditional acceptance of the word a 'filter' does not remove toxic contaminants - particularly carbon monoxide (CO) and carbon dioxide (CO2) and water vapour - so therefore you will need something more sophisticated than a 'filter' - ie, a purifier.

Your purifier will therefore need a stage to remove water vapour (a dryer) a stage to remove oil vapour (charcoal filter) as stage to remove CO (catalyst) and a stage to remove CO2 (molecular sive or sodalime). Each of these stages get saturated depending on the amount of contamination and usage - thus requiring regular element (cartridge) replacement.

The frequency of replacement should not be based on standard manufacturers literature (where ideal conditions are assumed) but in response to a written statement from the manufacturer (supplier) based on your 'worse' case conditions.

Inevitably this will predict a very short interval (that you will not want to believe) and a cost (that you will not want to incur) - thus under-sizing the purifier - or running it long after's its 'end-of-life'. Guesswork is not allowed by law, so you will need to monitor each stage for efficiency.

For this you will need to implement an air quality testing program for each foreseeable contaminant - from which the frequency of testing is governed by the 'worse' conditions. From which you might find it cheaper to install an on-line monitor (one for each foreseeablec contaminant rememer). Testing is expensive.

In other words you have to balance the cost of testing against the cost of purifying.

You will find the sums stack up in favour of a purifier because testing is a costly exercise. Please remember that 'testing' does not provide protection in the interval between tests. Only the purifier does that.

Taken to a logical conclusion, you need to find a purifier that has a proven extended life in your worse conditions. For this you will need to look at those that can fully reactivate themselves - preferably those that have in-built self monitoring.

Alternatively you can save a lot of money by pretending not to foresee contaminants and the risk in the first place. For this you must not do any research.

Look on the internet for combinations of compressed - air - breathing - purifiers, and you will get 100's of hits.

If you are in the UK one useful site is (buried deep in google) www.willpower-ltd.co.uk

I hope this helps.

Previous replies point you in the right direction but you need to query the figure of 0.5 ppm. Where does that come from?, and what is it related to. Is it mass or volume, oil mist or oil vapour, or is it liquid. And at what point of the system was this measured. Was it at the compressor outlet, at the mainline filter outlet, or somewhere down the system. And at what pressure and temperature?
All these matter if you want to pin down the problem - or more to the point if you want to find a solution. But whatever you intend, your numbers if taken at face value tells us something that doesn't add up.
If 0.5 ppm refers to volume then 1,000,000 ft3 of air will deposit 0.5 ft3 of oil - which in terms of a cup-full (say 1/2 pint) is 1/100 ft3. Thus your compressor has only to deliver 10,000 ft3 of air, and that means 20 minutes running - assuming that every drop of oil ends up in the cup. Which of course it doesn't. So where is the oil going? In the paint??.
Alternatively, for a different approach (see reply by Blink) - By volume, you'd have 500 x .0000005 = 0.00025 CFM of oil entering the system.
Air quality is only part of your problem. At 1/2 pint in 20 mins you will need a gallon of oil every 5 hours.
You need to pin down the 0.5 ppm. Because in terms of air quality, oil as a liquid or mist is expressed on a density basis - eg, lbs/ft3 or g/m3 or in terms of very clean air - mg/m3. If oil is expressed as ppm then it usually refers to vapour/gas - which in you case means you would have an empty cup. ie, no liquid at all.
If you want to get technical then refer to ISO 8573. There are 10 parts to it.

I hope this helps.

PS. My special interest is in compressed air breathing purifiers.

Sorry about not being clear. My mistake. As it is my system has filtration after the compressor with a rating of stopping everything down to 1 micron/0.05ppm manufactures rating. The refrigerated dryer then pulls off alot of the water vapour. The oil used is a PAO. At the booths we use a sintered bronze filter,micro fine element filter(0.01micron[mfg rating]),charcoal triple filter for 'fresh air' breathing. I was looking to fiqure a means of determining volume of oil getting past my 1 micron filter in something I could understand instead of ppm. If I had a formula to go by I can get an idea of what additional filtration I need. Currently I am looking at adding a 0.01 micron mainline filter after the dryer and a desicant filter at the booths for the paint air. We only use 5 micron and coalasing filters for the paint supply air.

A 1 um filter and a fridge dryer in this day and age gives good quality compressed air for general purpose use. From here you need to improve on this for special applications. It would help to know what type of compressors you have.

Whether you need a desiccant filter/dryer for paint spraying is doubtful, but if you do you are right to look at 0.01 um high efficiency filters because these are essential to protect desiccant dryers - and the charcoal filters. Your original 0.5ppm of oil suggests they will only last a few hours - how often do you change them?

You also need to make sure that poor/spoiled paint finish is due to oil contamination. You might find it is caused by sub-micron particles of PTFE so popular with silicon spray cans nowadays (which might be on the workpiece to start with).

A very simple DIY test for oil mist contamination (used in breathing air circles) can be carried out using gas detector tubes which change colour due to the amount of oil present. You pass a small sample of air throught the tube and read off the colour against the scale. It will probably be in mg/m3. This is a quantitive measurement that has recognised meaning in most circles. Compressors, filters, dryers, purifiers, laboratories, health and safety professionals, standard, lawyers etc.

You can buy a DIY kit to carry out this test. Do it at the point of use (where you intend to install the filter) and any where else you need to know the quality. You can get tubes for numerous gases if you wish.

By all means put a filter at source (downstream of the existing one) to prevent more particles entering the system, but bear in mind that it will do nothing to remove existing water, oil and dirt. You will be better served by a local filter for immediate effect - new hoses etc - to prove it works.

Please note that no toxic gases will be removed. Your PAO oil migh be resistant to heat and production of CO, but not entirely, and certain not against toxic additives that will be given off when compressed. You need ultra dry desiccant, charcoal, molecular sieves and catalyst purifiers to remove these.

Also a filter rated at 0.01 um might have impressive performance figures but the real problem lies in the 0.3 um range where the efficiencey is much less. These paricles pass through most filter mediums - to penetrate deep into the lungs. However, the overall efficiency will be well within practical and legal limits. This is the best you can do.

So to sum up it seems you need a high performance coalescing filter (with a reliable auto-drain) for the paint spray air. And another to protect the charcoal filters in the breathing air, if not on the same circuit. And if you really want to ensure compliance with all breathing air quality legislation you will need proper purifier(s) at point of use.

Please come back if anything is not clear.

Very clear and highly informative. While we have sintered bronze, microfine(I think 0.01 micron) then charcoal and the paint maintenance. guy has put filter/coalescing prior to this at each booth he is still concerned. He is using a 5 um filter then coalescing filter prior to the paint pump, supply air. he is of the opinion that we need desiccant after the dryer but I'm looking at doing a desiccant at at the inlet side of the paint pump/supply air. As for the breathing air this is a monitored system with a CO sensor that is calibrated to go off before 10ppm. So I think we are covered there. I like the idea though of the detector tubes. I will look into it.

This is our system: compressor to 1 micron filter(changed quarterly whether it needs it or not) to refrigerated dryer to 5 micron filter to coalescing filter(for paint air).

to 5 micron filter to coalescing filter to sintered bronze to microfine filter to charcoal filter to CO monitor for breathing air. The breathing air system is a SATA unit.

I would like to do the desiccant filters because I think they have airborne oil particles getting into their paint. If I do the desiccant I can get them to change that portion of their paint system so they don't get airborne contaminants.

We need to clarify what is meant by desiccant filter.

This usually means a device to remove water vapour. So unless you have problems with condensation, you will not see any improvement if you fit one. However, it would be worth checking the ambient temperatures around you spray booth.

Normally water vapour condenses in the compressed air as a mist, even with a good fridge dryer system. But it is possible that condensation from the compressed air takes place on the product being painted because it is cold to start with, or perhaps the product gets cooled by the airspray where the condensation comes from the surrounding room air if it has high humidity.

If it is not condensation, then a desiccant filter will not be much use. In fact desiccants give off dust. (And so does charcoal). The dust is held back by filter pads in the vessel that holds the desiccant. Any dust in the air to start with will be stopped only by virtue of these filter pads. These filter pads, although they could be microfine, are not intended to act as filters for incoming particles - only water vapour. They do remove dust - but will soon block up if there is a lot. This will be an expensive way of filtering the air. Normally, if used as a dryer, they saturate with water vapour pretty quickly so they are changed long before blockage arises.

For airborne particles (aerosols) you need a high efficiency microfibre coalescing filter - as fine as you can get - with a dp gauge to show when it is blocked, and a drain to dump condensate. There shouldn't be any water (liquid) if your fridge dryer is working and is suitable for your ambient temperatures - but there might be oil if you have a carryover of 0.5 ppm. And that's were we came in.

Try the filter first.

PS. Fit a dp gauge to your mainline filter. Then change it on pressure drop instead of quarterly.

Good point on the desicant filter. I knew it but spaced it trying to appease the paint maint guy. The mainline coalasing filter has a gauge to tell me when it is time for a change out. It never gets there as I change it sooner than needed. It is one of those deals where right now the company doesn't want to spend a couple of thousand dollars on two extra filters(they went hog wild on more buildings and machines) so the oil carry over from the filter I use right now has them asking what is going on. I tell them we need the 0.01 micron filters to catch as much as is feasible. But they will not spend the money so I tell them "Well I just replaced the filter a couple of months ago, so I'll toss in another one." That makes them happy for awhile, they spend $265 X 2 filters, three times a year. But it looks like I'll be able to get my extra filters come the new fiscal. It gets to be a head scratcher after awhile. Spend a couple of million on new equipment that needs air to work, either primary or secondary, but don't spend a couple of thousand to feed them clean air. And trying to get high pressure(>110psi) out of compressors designed as 110psi operating range units. At least we should be also getting a hp unit ordered in May. I have our air salesman working with me but to no avail right now. They think thier multi million dollar machines are more important than my 25k air comps. till they go off-line.

While you are waiting for the fiscal year, a couple of things to consider first: If you have modern coalescing filters then there really is no need to change your elements any sooner than necessary.

Coalescing elements are self draining regarding oil and water, and thet run on indefinitely, until they block up with solids. The pressure drop across the filter then increases until it becomes uneconomic to force the air through them. By this I mean the pressure loss costs money to overcome because it is a waste of power.

But until then, the filters work well on liquid droplets so changing them doesn't improve matters. One thing to think about though: finer filters block quicker so if yours are not blocking, then it suggests they are not fine enough. As elements are usually interchangeable, try a finer version. If they block (much sooner) you know you are on the right track because they block up with what the previous elements did not stop. And make sure your auto-drains are working otherwise the condensate gets re-entrained.

We are still not clear on what was meant by 0.5 ppm of oil. Have you managed to pin that down yet.

Are your compressor running cool, and are your after-coolers and fridge dryers working at low temperature. As a rule-of-thumb an increase of 11°C doubles the oil and water carry-over.

One final point. Coalescing filters operate best when thelr design flow is not exceeded. The filters might have been matched to the compressors and thus are the 'right' size - but do you turn your air on and off very often?? If you do, and you use, ball valves, you get an instantaneous blast of air miles in excess of the design flow, and that picks up condensed oil and water from the surface of the filter element in extremely fine mist - and that goes everywhere down the pipes.

Good luck.

The 0.5ppm is the manufacture's rating of how well the filter does. I have two 75hp rotary screw compressors, the rotors are in an oil bath, with a seperator element, aftercooler, coalasing filter and refrigerated cooler, then it goes into a reciever tank with a float drain and manual drain. I check all of the drains daily. The compressor has a coalasing unit with auto drain and manual, the coalasing unit has a float drain and manual drain, the dryer has an auto drain. So I'm campaining for a 0.01 micron coalasing filter to put after the reciever tank. Everthing is actually somewhat oversized for the compressors so we do have reserve capacity. Generally one compressor runs constantly and the other one spools up as demand rises.

You are quite right to campaign for a high performance microfibre coalescing oil filter. (they are made from borosilicate microfibre papers).

Put it downstream of both your existing filter and dryer. Because the oil carryover from your compressor will be what comes through the high efficiency filter inside the compressor oil seprator. These are high efficiency anyway, so any oil passing them will be in an extremely fine oil mist of submicron size. This mist will pass through any filter that is not of the microfibre coalescing type.

This mist will zap the carbon unless you remove it first. Make sure your carbon filters are protected this way.

One last point don't become mesmerized by claims of 0.01 or 0.001 um filtering size. For compex technical reasons these small particles are easier to remove than larger ones. You need to ask your supplier how well the filter does at 0.3 um.

You will then have a state-of-the-art system - and make sure your compressors run cool.

But unfortunately you might have to weight a while for any entrained oil (and water) in the system to 'dry' out before there is a marked improvement.

Do you have any ideas for cooling paint booths simply and cheaply?

the only 'quick thought' on cooling paint booths is with a ambient heat exchanger. Not greatly efficent as far as how low the temp will be but it would be an improvement. otherwise a thought comes to mind is a water cooled version. It would cost a bit, I would not hazard a quess, but it would work. The problem might lie in the increase of humidity.

Phone a local compressor dealer and ask them. They have charts which can give show how much for 0.5 ppm in terms of ounces /100 cfm. If they can't help you phone Shawn Homer at Sullair Corp, Mich. City, IN and they can help you (or call a local Sullair compressor distributor) I can help but all my stuff s in storge as moving. I can only tell you that you are still getting plenty of oil carrover down stream as 0.5 ppm considered to be only General Purpose Air quality. There are a lot of factors involved such as type of oil, air temperatures, rotary screw or reciprocating piston,etc. If oil is a problem (is to most manufactures) then consider installing a High Grade Oil Coalescing Filter either centrally in the main line or smaller filters on branch lines closest to the application that needs protection. A 0.5 ppm filter/mist eliminator most often is considered to only being a first stage filter and thus for cleaner air a second stage or third stage should be added 0.1 ppm or 0.01 ppm. It is advisable to have an air dryer as well which i am assuming you already have ?? Contact me if need more info. airboss2@comcast.net