Compressed Air Question

Dew point depends on relative humidity too.

...and the relationship may be viewed on a psychrometric chart.

Guest is right. Look here, http://www.weatherquestions.com/What_is_dewpoint_temperature.htm, if water in your air is unacceptable, Google, air dryers. I would think that your supplier would be able to make a recommendation based on your needs.

The guest is back ,

In all our machines, that uses the compressed air for various purposes (mist lubrication/ guide ways cleaning, tool locking,...)

There is a

a) Filter

b) De-humidifier

and (c) lubricator - pumping in tiny mists of lubricating oils- more like vapours

We never bother about dew points under the RH >90% to <20% as the seasons fluctuate with the official dew points too (which we have to monitor for the equipment painting)

Good point - call it what you will. It seems to me that as air is compressed its ability to contain water is decreased but also the temperature rises which allows the hot compressed air to hold more water.

The compressor people tell me that the air leaving a two stage compressor at 320 deg F and 100 psi will be "saturated". As this air is cooled at constant pressure water is released - condenses. Our process likes hot air - higher volume flow rate, etc. so we do not want to cool the air to dry it.

So the question remains - What is the dew point of the air at atmospheric pressure?

The air that you start compressing has a dewpoint that differs from a day to day, climate parameter.

This air, when compressed will lose water.

When that water remains somewhere, like in a tank, it means, the air coming out of the system will be dryer in the condition below.

When on the contrary, the air is heated enough, so that there is no condensation in the system, but all the moisture joins the stream as vapor particles, your moisture content at 1 atm and at the same temperature will be "exactly" the same as the air you are starting with to compress. This is a start.

I don't see how the air can be saturated, at your process temperature without adding water.

<...Saturated compressed air at 320 deg. F is called steam....>

Quite.

I do hope this was meant as a joke because you will confuse a lot of people.

Steam is the vapour product of water and is a pure substance containing just water in its gaseous form.

As it says on the tin saturated compressed air is air that has been compressed and is carrying the max possible quantity of water. The actual quantity surprised me as I had to do a quick run with pure nitrogen and water but it should be close enough as my psychometric charts dont go to high enough temps. The water load is ~70% of the total flow. However although most of the stream is 'steam' the stream itself is not steam, does not behave as steam and will have different physical properties eg if you cool it to 300°F some of the water will condense leaving air which is now saturated at 300°F

In any case looking up in Crane tech Paper 410 I find that the boiling point of water at 100 psi (absolute) is 327.8°F and 100 psi (gage) 338.1°F so even in a pure water environment you would not generate steam at the conditions given just very hot water

I respectfully disagree totally.

Do the test with a cylinder of air at that pressure and heat it to 320 degrees.

There will be no steam at all coming out.

If you can produce steam from air with a compressor you need to get it patented immediately.

Steam is water vapor and is produced from water, heated up to vapor phase.

To help the OT it is important to know where in the process the temperature is applied. And by the way: as soon as there is water deposit in a compressor tank, the air is saturated at these process conditions.

But of course – the dew point temperature is the temperature at which the relative humidity is 100 percent and condensation begins.

But that's not the point, nor the question. If air at 100% relative humidity at 100 psi is expanded to atmospheric pressure what will the dew point be – or if you prefer what will the relative humidity be at, say, 60 deg F?

As some of the water will have condensed and will still be in contact with this "air", the answer is either 60degF or 100%, depending on which preferential question is being answered.

Sorry but I've been reading the posts and none has answered the question which I'll repeat in a slightly different way here:

What is the dew point of 320 degF 115 psia 100% relative humidity air when the pressure is reduced to 14.7 psia?

OR

What is the relative humidity of 320 degF 115 psia 100% relative humidity air when the pressure is reduced to 14.7 psia and the temperature lowered to 60 degF?

<...What is the dew point of 320 degF 115 psia 100% relative humidity air when the pressure is reduced to 14.7 psia?...>

At what temperature?

<...What is the relative humidity of 320 degF 115 psia 100% relative humidity air when the pressure is reduced to 14.7 psia and the temperature lowered to 60 degF?...>

100%. Some of the water in the hotter air, which is called steam at these figures as has been correctly pointed out earlier, will have condensed and will be in contact with this cooler "air".

Forgive me, we've had some bad threads on here lately. Here is some info on relative humidity, http://en.wikipedia.org/wiki/Relative_humidity

Here is a dew point calculator: http://www.decatur.de/javascript/dew/index.html

These may help.

From what I can see, when the relative humidity is 100%, the dew point is whatever the temperature is. Temp 80= Dew point 80, Temp 60=dew point 60 and so on....until you reach freezing of course

one of the formulae in the wiki reference has a pressure component - decatur reference is a nice calculator but lacks the pressure component.

When ambient air is compressed the temperature rises and no water condenses however when the pressure remains constant and the temperature decreases back to ambient water condenses. This tells me that as air is compressed at constant temperature the relative humidity increases - the dew point temperature increases.

It would seem the reverse is also true - that as air is decompressed at constant temperature the relative humidity decreases - the dew point temperature decreases.

Remember that the dew point temperature is the temperature at which the relative humidity is 100%.

<...as air is compressed at constant temperature the relative humidity increases - the dew point temperature increases...>

The dew point temperature will be the same as the air temperature when the RH reaches 100%.

<...as air is decompressed at constant temperature the relative humidity decreases - the dew point temperature decreases....>

That's only true if all the water has boiled.

I think you may have been thrown off by my earlier dew point post.

Dew point is the temperature at which water vapor will condense into water and is directly related to relative humidity.

At 100% relative humidity, temperature and dew point will be the same. The lower the relative humidity, the lower the dew point.

If you are maintaining compressed air at 320 degrees, water will remain as vapor and will not condense.

When you compress the surrounding air, you are increasing the relative humidity of that air. When you release it under pressure, it will be more saturated than the air going in, and therefore the dew point of the released air will be higher, resulting in condensation occuring at a higher temperature than the ambient air that you started with.

Pressure, in this case doesn't really come into play, except the fact that you are raising relative humidity when compressing ambient air.

Thank you for your attention.

Everything you present is true with some condition(s)

When you compress the surrounding air, you are increasing the relative humidity of that air. True at constant temperature but as temperature increases, heat of compression, the RH decreases.

When you release it under pressure, it will be more saturated than the air going in, and therefore the dew point of the released air will be higher, resulting in condensation occuring at a higher temperature than the ambient air that you started with. I think you mean when you cool it under pressure.

Pressure, in this case doesn't really come into play, except the fact that you are raising relative humidity when compressing ambient air. At constant temperature. But now the pressure and temperature go back to ambient.

It just now occurs to me that a two stage compressor with an intercooler will condense water from the air thus when the air returns to ambient T & P the RH and dew point will be lower than the ambient air that was compressed. Maybe I won't worry so much about how much lower.

You've lost me. If none of these posts have come close to answering your question. It is beyond my scope, I'll stop wasting your time.

Hang in there though, the answer you're looking for, could pop up at any time.

You have just answered your own question. Once water has condensed out but is still in contact with the air, the R_H level will always be 100% because there is liquid water present. It's totally irrelevant what the air temperature may be because even if the R_H level is as low at 1%, when the air comes into contact with liquid water, the water will evaporate until the R_H level reached 100%.

What is our problem really? Do you want to produce hot dry air for your process or something? If that is the case, then pass it through a heated dessicant bed. The dessicants will remove the moisture from the air while the heating will dry the dessicants to keep them in dry working condition.

I hope this finally answers your question.

Once water has condensed out but is still in contact with the air, the R_H level will always be 100%. OK but what I do not want is water to condense. The process is in a heated building SO if the dew point at 15 psia is 50 deg F, for example, I'm fine since the building will always be over 50 deg. F and the expanded air will never be less than 50 deg F.

I've already answered your question. If you don't want water to condense out of the air, then dry it. If you want to keep the air hot, then dry it by passing it through a heated dessicant bed. If your RH is 100% at 320^o F, what do you think will happen the moment it cools down? The excess water will condense out, that's what. This is a fact of life: nothing anyone says or does will change it. Certainly arguing over semantics wouldn't.

I have already given you a practical solution to your problem. Now the onus is on you to find out how best to implement it.

The dew point temperature at atmospheric pressure varies with weather conditions and can be measured with a wet bulb thermometer.

We are planning to install a process that will use compressed air at 100 psi and 320 deg F. The compressor supplier indicates that the air leaving the compressor at this pressure and temperature will be water saturated. QUESTION - what is the dew point temperature at atmospheric pressure?

Better: Compressor supplier indicated that water saturated air could be a problem.

QUESTION-How can I dry it out?

Why not add an airdryer then reheat the air to what you desire. No condensation and preheated air to your desired temp.

Realist, are you asking about the dew point under pressure to design an in-house fix for your air system (feed hot dry air vs. hot wet air to your equipment) or just getting details for documentation of your new system?

Several posters have given descriptions of dew point variables, but I think Kramarat's is the clearest, I give him a GA for that. On the practical side, DVader1000's suggestion of using a desiccant bed is a good one, as it is to my knowledge, the only method that can dry air without a pressure or a temperature drop. Desiccant canister dryers have been used for years air brake systems on trucks. They work by absorbing water into the media. When it is saturated it is automatically purged via a timer or sensor. to use this kind or system you will either have to have an air tank down stream from the dryer that is heated (tank walls make good radiators) or have parallel dryers that alternate cycles so that your air supply in not interrupted. Hope this helps a little bit.

Realist try this web site for your answers i think it is what you are looking for, as their is no correct answer that can be given for your question as it is not giving any constants to work with eg what is the relative humidity at atmopheric pressure this will effect the dew point temperature

http://www.kaeser.com.au/Online_Services/Toolbox/Condensate/default.asp#0

Thanks for the link.

I'll explain again - 320 deg F air at 100 psig, according to the link provided, is at 100% relative humidity. Now this air is expanded to 0 psig, what would the dew point be? OR if you prefer, what would the RH be at some temperature, say 60 deg F. This is independent of the atmospheric conditions of the surrounding area.

To answer the first preferential question: clouds of steam will have been generated during this expansion, so the RH will still be 100%.

To answer the second preferential question: the RH of the same air at 100psig and 60degF will still be 100%, as much of the water vapour originally in it will have condensed to the liquid phase.

Hi All

I have lurking for about 2 years now.I have done this exercise before.

compressed air at ambient room temperature will still be saturated with water,to drop the water the temp needs to be reduced by a further 15 C which will remove 90+ percent of water .this applies I think up to around 50c(correct me if wrong) the remainder can be removed depending on volume and cost.Low volume can be 30 to 70 cfm and I would use a receiver then pre filter before an air dryer and an after filer on its outlet then carbon filter further down the line nearer the heating source.A desiccant filer is more mid volume(also for low volume but costly and prone to lots of mis adventures with tunneling and solidification of the salts) etc,etc,etc

Drying the air first is the easy part,reheating is to depending on volume of air usage ie cfm is low(under 30),I have reheated air by using heating elements similar to you home oven in a secondary receiver where air velocity is low but pressure was still at 100 psi.

I have had systems from 10 to 1150 cfm compressors(recip ,vane and screw) made the air but where breathing /dental/laser and laboratory air was needed in small volumes at the end source so I need to know the end use volumes before going on

the temp straight out of a reciprocating air compressor outlet manifold is already near the high temp you require as you know

hope this helps...Tim

Two things as I am not sure I understand what you are driving at

1 Are you sure the water ex compressor is saturated. Even if the suction air was >90% humidity your exit temp is very high and I would have thought that the impact of higher temp increasing the water content of the air would overcome the effect of increasing pressure reducing the water content

2 Are you sure you want the dewpoint at amospheric pressure because this will depend on the ambient temp and relative humidity for your location. Psychometric charts will tell you this for various relative humidities - you find the Temp and RH point on the chart for your starting point and then read off the dew point that corresponds to that initial condition (ie based on the starting temp and RH how much water is held in the air.

If the air is saturated exiting the compressor then the dewpoint at 100 psi (pls confirm this is 100 psig which I have assumed and not 100 psia) will be 320°F

Otherwise you will need to find the water content at the suction condition (temp and RH) from charts and then use this water content and find a dewpoint based on 100 psig and the water content calculated.

Using driers at high temp is a problem as there is more equipment that could burn you if touched.

I work in oil/gas industry and have had to specify instrument air systems that operate at 100psig and have a -40°C dewpoint. These use 3 stage compressors, with intercooling and liquid knock out at the insterstages. The outlet gas is also cooled to ambient to knock out more liquid. This gas then goes to a drier. There are two driers one on line one being regenerated (the one being regenerated uses a slip stream of the dry air which canbe heated which reduces the amount of air you need)

You dont want to cool as the hot gas suits your process better.

The driers would work I am sure at higher temps but if you dont cool to knock out some of the liquid you massively increase the load on your driers making them much bigger than they need to be. One of the other posters made the comment that 90% is knocked out cooling to ambient and that seems reasonable so not cooling would make the driers 10 times the size

Hope this helps

I need to ask a question before giving an answer on this one.

Are you heating the air on the outputside of the compressor? (like a plasctic welding process?

Typically one would install a refrigerated air dryer between the compressor and the desired output. you can get a better idea at the following link. http://www.ecompressedair.com/air-dryers.aspx

Not heating air other than the heat of compression. Don't want to cool either - cool air has less volume at constant pressure.

<...cool air has less volume at constant pressure....>

It will be wetter, though.

QUESTION - what is the dew point temperature at atmospheric pressure?--dew point at 1 bar is usually more than 15c(14 minimum) lower than ambient temp eg 32 c then reduce temp by 15 c ie 17 c

We are planning to install a process that will use compressed air at 100 psi and 320 deg ..do you want an answer to this.........or do you want an answer to this ..what is the dew point temperature at atmospheric pressure?????

if it the last one see if you have google in the top right hand side of your computer screen(ps make sure your computer is turned on first)

Is your supplier angling for an additional air dryer sale?

Will this water content affect the downstream process?

If not, why do you care? If so, there are several suggestions in this thread on how to deal with it.

The RH - DP - T - P relationships have been set out in other posts. I suggest the pertinent questions for you are the ones I've posed.

If you're still unconvinced, perhaps you should employ an HVAC or compressor consultant.

You got it - THANKS for your interest, attention, and help. (I eventually came to a similar conclusion several posts ago.)

Yep. The compressor specialist's bullshine was 100% saturated.

with your back to the wind in the Norther Hemisphere, low pressure is on your left.
it seems to me it's an over complication. You either want dry air or you don't .
If it's at 100% humidy at any point or time then it's too damn wet.
Del

Stop simplifying things! I'm done with this one, just wanted to see you're input.

BTW, I noticed that, when I get out of the shower and I don't towel off, my clothes get wet. Is there a formula to prevent this, without actually having to dry off? UGH!

Yep. Heat the towel to 100psig and 320degF.....

But..... I don't want to use a towel.....and I don't want to air dry....and I don't want to drip dry....I Just want to be dry...formula please? Nevermind

you must think clean dry thoughts....mmm
I have an easier time thinkung dirty thoughts.
Del

Me too! I googled wet kitty. I was scared to use the other word for kitty, don't want to get in trouble before the weekend even gets here!

Simple!

Remove the water at source and shower at the dry cleaners.

Obviously

Duh.... Why didn't I think of that? Thanks

All I have to do is bring some salve to treat the chemical burns and I'll be good to go!

Oh no you rae behind the times they are much more gentle now - no chemical burns or anything nasty like that. Ok so they cause cancer but you dont notice that til it kills you and it does get you nice and clean and dry!

Good news!

Ps. I had to post something, I just noticed my last post was #666.