7 replies to this topic

[Enginerdo]

Hello All,

I'm having trouble figuring out the water content (mass/mole fraction) contained in the outlet stream of an instrument air dryer.

The conditions at the outlet of the instrument air dryer are as follows:
Dew Point = -60°F (-51°C)
Pressure = 790 psig (5447 kPag)
Temperature = 115°F (46°C)

If there's any insight that you can provide, it would be greatly appreciated.

Thanks,

Enginerdo

Edited by Enginerdo, 17 October 2011 - 10:37 AM.

[ankur2061]

Enginerdo,

Why don't you have a look at the spreadsheet- "Psychro.xls" at the link below. From your data:

The Dry Bulb Temperature is 115 deg F

Use the goal seek function of Excel by providing the Dew Point in the output and manipulating the Wet Bulb Temperature. The output provides the water vapor flow rate for a given air flow rate

The link:

http://www.cheresour...6833#entry26833

Regards,
Ankur.

[breizh]

I've added an other resource ;
http://www.uigi.com/WebPsycH.html

Hope this helps too.
Breizh

[ramlalithravi]

@Ankur,
It's really a great stuff, surely anyone will get an idea about this chapter (Psychrometric) if goes throguh ur spreadsheet.

Further i need to ask something related to humidity.
hope i can clarrify more about this topic.Partial pressure of water in air at amb. Temperature / Partial pressure of water in air at saturated condition at amb. Temperature, is the Humidity of air as we all know. But my question is, which is discomfort to living condition either at high %RH or at low %RH. If so what is the reason behind this?

Which is actually not related to the original topic of this discussion but it will give some ideas about humidity .

[ankur2061]

ramesh,

My knowledge about HVAC is quite limited. But climate control is a widely discussed topic on the internet and you might want to have a look at some of the links provided below:

http://www.engineeri...dity-d_895.html

http://en.wikipedia....lative_humidity

Hope this helps.

Regards,
Ankur.

[Enginerdo]

Ankur / Breizh, thank you for your guidance.

[kkala]

A look at http://en.wikipedia....Thermal_comfort would be also useful for those interested in what ambient conditions are pleasant or disturbing to people.
Seeing that psychrometry does not provide data for temperatures lower than 0 oC, I have tried to calculate humidity by considering equilibrium water vapor pressure over ice, as indicated in the attached "VPw.xls". Guidelines were taken from http://www.cheresour...6833#entry26833, as well as http://www.its.calte...als/ice/ice.htm. Resulting humidity 0.022 gr H2O / kgr dry air if dew point of -51 oC is considered at atmospheric pressure, or roughly 0.0004 gr H2O / kgr dry air if dew poit of -51 oC is considered at 54.47 barg. Comments on the calculation would be appreciated, since I have never so far tried to understand what these "dew point -40 oC" etc really mean.

Editing note, 21 Jan 2012: Correct values for 0.022 and 0.0004 gr / kgr were transferred from attached "VPw.xls".

Attached Files

•  VPw.xls   20KB   66 downloads

Edited by kkala, 21 January 2012 - 02:05 PM.

[GS81Process]

Psychrometric charts down to -25C are available:

http://www.engineeri...llier-d_27.html

I would consider air with a dew point of -51C for all intents and purposes to be "dry air".

When it is colder than -25C during the winter time here in Canada, we are desperately trying to humidify the air in our homes (after it is heated) to avoid our skin from flaking off. It becomes drier than the driest desert. Air has essentially no water carrying capacity at these conditions. Having said that, much appreciation to KKala for taking the efforts to estimate the maximum water content based on solid/vapour equilibrium.

Edited by GS81Process, 18 May 2012 - 10:40 AM.