×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Dew point change with increase in pressure and decrease in temperature

Dew point change with increase in pressure and decrease in temperature

Dew point change with increase in pressure and decrease in temperature

(OP)
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

RE: Dew point change with increase in pressure and decrease in temperature

Water will condense to form fog.

RE: Dew point change with increase in pressure and decrease in temperature

(OP)
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.

RE: Dew point change with increase in pressure and decrease in temperature

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.
 

RE: Dew point change with increase in pressure and decrease in temperature


"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.
 

RE: Dew point change with increase in pressure and decrease in temperature

(OP)
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.

RE: Dew point change with increase in pressure and decrease in temperature

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

RE: Dew point change with increase in pressure and decrease in temperature

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

RE: Dew point change with increase in pressure and decrease in temperature

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



 

RE: Dew point change with increase in pressure and decrease in temperature

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

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

 

RE: Dew point change with increase in pressure and decrease in temperature

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(?)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources