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Negative Dew Point Of Compressed Air


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#1 pawan

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Posted 06 March 2010 - 12:27 AM

Compressed air used for instruments is generally at 100 psig. Now suppose RH is 70% Dry bulb is 30 C. Now if I use any dessicant dryer the supplier says suppose his system is giving a dew point of -40 C - what does it mean.

My doubt or confusion is how do I calculate the water content for a compressed air at 100 psig with a dew point of -40 C?

The doubt is because at sub zero temperature the vapor pressure of water is zero ideally. so how to calculate absolute water content in the compressed air.

Nos given above are just some assumptions nearer to practical conditions. I need to know the calculation method for identifying water kg/kg of dry air in compressed gas once it leaves dessicant dryer. I know the method if I use cooling/chilling but in those cases dew point is not in sub zero zone.

Thanks

#2 Zauberberg

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Posted 06 March 2010 - 02:01 AM

Dew point of -40C refers to atmospheric pressure. At 100 psig, that would be equivalent to approx -19C. Drying the air from 80% humidity @ 30C to -40C dew point reduces the water content from 24.1 to 0.1 g/m3 absolute humidity. See attached worksheet for dew point calculations. Similar results can be obtained by using air saturation charts.

Attached Files



#3 pawan

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Posted 06 March 2010 - 09:31 AM

Thanks Zauberberg for your reply & Excel sheet. Its really useful.
BUt my meaning of calculation was to understand the underlying fundamentals. Why this is -40 C dew point. What is the significance of negative dew point?

Also anyone on practical experience for how much low I can go in terms of dew point. Is it possible to go say -100 C dew point or not? What is the practical limit? What are the systems to achieve that.

Edited by pawan, 06 March 2010 - 09:33 AM.


#4 Zauberberg

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Posted 06 March 2010 - 10:27 AM

By using Mol Sieves, dew point can go as low as < -100C. For most practical applications this is more than sufficient. It is hard to find equipment that can detect moisture contents below 0.1ppm.

Ambient conditions determine the level of air dehydration. If you are based in hot, tropical region, Activated Alumina is more than enough (dew point -40C). On the other hand, if you are based in Kazakhstan or Siberian fields, you'll probably need Mol Sieves equipped with heating regeneration system in order to properly drive the moisture out of the desiccant.

Attached is the link to one file that shows various performance characteristics of different adsorbents: http://www.axens.net..._adsorbents.pdf
As you can see, Mol Sieves are capable of removing more moisture at lower relative humidities of feed stream but, on the other hand, they require more energy for desorption process exactly because of higher moisture holding capabilities at lower partial pressures of water. GPSA Databook can also provide you with quality information regarding this subject.

#5 AZIZ_MN

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Posted 07 March 2010 - 01:24 AM

Hi, pawan

I measured the dewpoints of line and other equipments several times during a pre-commissioning of our plant and for nitrogen, I obtained a maximum of -60 oC dew point and the relative moisture is 10 - 15 ppm. The hydrogen dew point at a hydrogen dryer outlet was < -65 oC using a dewpoint meter and -60 oC with a draegger tube. Other than that, we have an on-line dewpoint meter for instrument air. The dew point measurement requires special skill and passions to get accurate readings, but in experience, I haven't obtained a reading of -100 oC dew point in pratice.

Thanks, best regards.

#6 ankur2061

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Posted 07 March 2010 - 01:32 AM

Pawan,

There is an old post about water dew point where I have also attached a spreadsheet for dew point & humidity (water content calculations). Check this out:

http://www.cheresour...833

Hope this helps.

Regards,
Ankur.

#7 MrShorty

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Posted 08 March 2010 - 10:52 AM

Pawan:

In many ways, I think you've almost answered your own question right here. You are making the assumptions that the sublimation pressure of ice is exactly 0. In many applications, this may be an adequate assumption, but it is not true.

I don't know all of the inner workings of Zauberberg's spreadsheet, but, comparing the partial pressure of water that it calculates to the sublimation pressure of ice from DIPPR and they are essentially the same.

I would suggest that the first step to understanding these dew points you are talking about is to drop the assumption that the sublimation pressure of water is 0. Once you look up the sublimation curve for water, I expect this will start to make more sense.

#8 DrDewLittle

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Posted 14 August 2010 - 02:11 PM

The water content of any gas is determined by the partial water vapor pressure.
Goff–Gratch equations can be used to convert between dew point temperature to partial water vapor pressure. See Wikipedia for the equation, they also have links to free programs for the conversion.

Edited by DrDewLittle, 14 August 2010 - 02:12 PM.





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