instrument air dewpoint temp 1

[pmover]

anyone know of a simple technique/process to determine instrument air dewpoint temp - confirming existing instrumentation. i'm thinking something like a litmus paper test or otherwise. some oil from compressor was carried over and suspect degradation of desicant. hence, the desire to check dewpoint temp and compare with existing instrumentation. there are filters upstream of air dryers. p~100 psig, t~70 degF.

thanks!
-pmover

 

[Montemayor]

pmover:

The way I've always handled this problem is that I've used a very common and inexpensive dewpoint tester that works with Acetone and Dry Ice particles.

The tester is available for under $100 (I believe) and you put the Acetone in a cup and drop Dry Ice chunks into it, gradually reducing the temperature while visually watching a bleed stream of your sample air impinge on the highly polished surface of the cup. When you witness the appearance of the first drop of dew formed on the cup's surface, you have identified the dewpoint and you note the temperature of the Acetone bath.

With the dew point measured at atmospheric pressure, you refer to a table of dew points versus water content and you have the lbs of H2O/cu.ft.

I've described the process in a very limited and short description, but I'm sure you can get full instructions and explanations from the dew point cup manufacturer. The Internet (Google?) will probably be a good source for suppliers of this type of equipment. I've operated air liquefaction plants as well as CO2 and other industrial gas drying operations on nothing more than this level of sophistication with marked success and repeatability in the past.

I hope this experience helps out.


Art Montemayor
Spring, TX

 

[m777182]

A simple technique works fine if you do not require a very low dewpoint temperature. With a dewpoint of the instrument air at -50C and a reliable measurement you need a high grade instrument and, what is more painfull, a reference gas with known dewpoint. Even materials like teflon or SS 316 desorb surface moisture very slowly.
m777182

 

[pmover]

Art/m777182,

a simple, practical technique is what was desired and received! Thanks!

since i'm located in AK interior, i'll just wait for one of those cold winter days (been around zero lately), and i'll use small marbles and the acetone. i suppose your next question is with the artic air, there is minimal moisture. well, the compressor intake is in bldg interior, where the moisture content is higher.

Thanks!
-pmover

 

[Montemayor]

pmover:

Let me add a little twist that I've done in the past to read a dewpoint down to -90 oF:

If you're in Alaska (North Slope?) at present during the winter, I know that you must have a thorough safety coverage for fires within your facilities. The pain in the backside (or trade-off) in the "simple" and inexpensive dew cup method that I recommended is the ability to have a ready and convenient supply of small, Dry Ice pieces to cool down your acetone. When you don't have ready access to Dry Ice (as is often the case for operations outside the industrial gas production field) I have resorted to using a CO2 cylinder and expanding the liquid content by inverting the cylinder. The adiabatic free expansion of the H.P. liquid CO2 will give you a mixture of Dry Ice "snow" and flash vapor - both at -109 oF. I used to use a common filter paper to catch the Dry Ice snow and use it as the coolant for the acetone.

Now, here's a more practical twist to the method:

I know for a fact that you must have a bountiful supply of CO2 fire extinguishers in your living and operating facilities. Now, bear in mind, that I'm not advocating you resort to using your CO2 fire extinguishers as a Dry Ice source without discretion. But we know that your facilities must be ever-ready for the unexpected interior fire - especially since residual cigarette smokers still survive and persist in smoking in bed. So, what I'd do is contact the person (or persons) in charge of maintaining and servicing the CO2 fire extinguishers and explain your need to them. I'm sure they have spare extinguishers that they always keep charged and ready to go at any instant and they would be glad to cooperate by furnishing you with a spare one that they also keep charged. I prefer a CO2 extinguisher because it is smaller and easier to handle and it comes complete with a syphon tube in the cylinder valve that allows for adiabatic expansion without having to turn the cylinder upside down. It also comes with the customary "horn" that greatly reduces the velocity of the flashing vapors. You can also use something like a pillow case to capture the "snow" product. If you resort to this method, you'll find it convenient and soon learn by experience just how much liquid CO2 to expand to run your dew point cup tester effectively and with relative accuracy. I've done this countless times with repeated success. The worst inaccuracy that I remember registering using this method was around 10 oF. In other words, I registered -80 oF while an on-line dewpoint meter read -70 oF. And there was doubt about the accuracy of the automatic on-line meter because it hadn't been calibrated in a long time. I know from experience that I can easily obtain an accuracy within 5 oF using the dewpoint cup method - if I use diligent care and patience in visually observing the cup's surface.

I hope this is of some help to you.

(By the way, your product air will probably be saturated prior to being dryed - even if you use external, cold atmospheric air. The reason is that you are compressing and cooling in the air compressor and if you have a 2-stage compressor producing approx. 100-150 psig, you may not get any moisture condensed after the 1st stage intercooler but you probably will get moisture condensation after the 2nd stage aftercooler - depending on the dewpoint of the outside air)

Since you are so close to the North Pole, have a Merry Christmas with Santa and all the Elves.

Art Montemayor
Spring, TX

 

[25362]

Similarly "chilled mirrors" are used for continuous on-line, or off-line spot-check, measuring of the compressed air dew point, optically, or using high-frequency sound, among others. I believe chilling is achieved by Peltier effects.

Googling around one can find a variety of instrument offers. Two examples (among many others) claiming accuracies of +/-0.1^oC, and even lower repeatabilities:

http://www.fluidenergy.com/hygro.html#chi

http://www.vaisala.com/page.asp?Section=21230&Loc=238

 

[pmover]

Art,

Thanks!
all i was after is a crude, but effective means of determining/confirming dewpoint temperature of inst air. based on reading your previous responses, i suspected your practical, yet ingenious, skills would be of use. i will provide the plant personnel these practical solutions.
btw, location is AK interior - coal-fired power plant. The city of North Pole is nearby, and yes, Santa and his Elves are busy. I saw them this morning preparing the sled and feeding the reindeer. Looks like a bountiful season!

Thanks again!
-pmover

 

[pmover]

Art & 25362,

been awhile, but . . .

will be getting a mirror (polished surface) and set outdoors. outdoor air temp varys, but lately been around minus 30°F. route some "dry" inst air (< 1 cfm) to outdoors and will direct inst air onto mirror surface. if condensation/freezing takes place, dewpoint temp of inst air is higher than ambient temp. if not, then dewpoint temp is lower than ambient temp. crude, but effective . . .

-pmover