Dew point change with increase in pressure and decrease in temperature 1

[vroma]

Hi everyone,

Hope I am posting in the right forum. I had a question about dew point and, since this is not my field, I am somewhat lost. I am trying to find out what happens to the dew point of a saturated gas under vacuum when the temperature drops by 20 deg F and the pressure increases to 5 psig. Any guidance on this issue would be much appreciated.

Thank you,
vroma

 

[Compositepro]

Water will condense to form fog.

 

[vroma]

Ok, I guess I should clarify my question. I wanted to see if someone could point me in the right direction in terms of calculating the actual dew point reduction number.

Thanks.

 

[IRstuff]

You can start here:

http://en.wikipedia.org/wiki/Psychrometrics

TTFN

FAQ731-376

 

[ChasBean1]

I think for your problem pressure is irrelevant. For example if you start at 70°F saturated at 10 psia your dew point is 70°F. If you drop to 50°F and increase the pressure to 20 psia (or ~5 psig) you'll still be saturated and your dew point will be 50°F.

 

[Zesti]

"a saturated gas under vacuum"

Sounds like a contradiction in terms to me...

"when the temperature drops by 20 deg F and the pressure increases to 5 psig"

A rise in pressure is usually accompanied by a rise in temperature instead of a drop in temperature.

If in your system the temperature drops while pressure is raised, then extra gas has been added to the system, otherwise this can not be true.

 

[vroma]

Well, for further context, this is gas collected from a landfill. The gas is being drawn by wells under negative pressure and that gas in the landfill is saturated. It is then sent to a chiller system that reduces the gas temperature and, then, the pressure is increased through blowers. ChasBean1's answer is what I expected. That is, that a reduction in the temperature of a saturated gas would be equivalent to a reduction in the dew point. However, I was not sure whether or not the change in pressure would still make this true.

 

[Compositepro]

Compressing a mixture of gasses increases the partial pressure of each component. So increasing the pressure does, indeed, increase the dew point.

 

[vroma]

Any suggestions on how to calculate the new dew point (that is, at the lower temperature but higher pressure)?

 

[CH5OH]

download a dewpoint calculator

http://www.phymetrix.com/Software.htm

 

[walz]

vroma ...

your question is about dew point, and the dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point.

Further to this, i believe that your question is about air (moist air)...

there is a relation to find out the dew-point....
If your gas at the lower tempeature is also saturated... (which i think it would be) ... the dew point will be the same ... cuz at saturation the dew point is equal to the temperature itself.

If your gas at lower temperature is not saturated, then you need one more parameter from any of these to calculate dew-point.
Relative humidity, degree of saturation, enthalpy, humidity ratio, specific volume, density, vapour pressure ...etc.

Pressure does changes the solubility of gas.... but for each pressure there is a different data sets for each of these parameters. sp increase in pressure isnt sufficient enough to calculate the new dew point. I'll tell you how to calucalte this if you tell me what are the conditions of gas before and after. And what gas is this

 

[walz]

also... visit the following... which may be very useful for you

http://en.wikipedia.org/wiki/Dew_point

 

[ChasBean1]

Dew point may not be your paramter of interest. A saturated solution of temperature A reduced to temperature B produces a dew point temperature at B equal to the dry bulb at B, but you may be looking for real air moisture removal(?)