×
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

water steam pressure at ambient temp

water steam pressure at ambient temp

water steam pressure at ambient temp

(OP)
Talking about psichrometry
Given T in Kelvin or absolute temp
For Pws [presure water steam]
Is this formula valid ?
for Water air from 0 to 200 C degree
ln Pws= a/t + b+c*t+d*t^2+e*t^3+f*lnT

for the followings values

a      -5800.2206
b      -5.5162256
c      -0.048640239
d      4.18E-05
e      -1.45E-08
f       6.5459673

I used it for 100 C dgree and give
p mbar      1014.217979 mbar , that is a normal atmospheric pressure

My concern is if it is valid for the air ambient temperature

I wan to calc the Relative humidity by reading the DryBulb and WETbulb  temperature.

I have 2 data loggers and  I modify one of them to read wet bulb temperature, so I can charge the data from it to a spread sheet and get the HR %  .

maybe it is not a conventional way of doing , but is it what I have and will have  for ever. (lack of money to buy a HR datalogger)

Hope you can give me any advice.

PS I have the spread sheet done.



Pardal

RE: water steam pressure at ambient temp

I get 10 times lower values with your formula but i think this is a question of units. If I compare your values to my values calculated with the formula from International Critical values I get a 0,9% deviation at zero C and 0,2% deviation at 100 C.However I do not claim that my formula is better-both are good enough for technical purposes.I keep some other polynomial expression to calculate the Pws, but at the moment I cannot remember where they are. By the way if you are interested in a small VBa code to calculate air-steam properties I can send it to you. Contact me at dixie.m@email.si.
M777182

RE: water steam pressure at ambient temp

your idea should work

RE: water steam pressure at ambient temp

It may give you only one part of the equation i.e saturation vapor pressure. Now you have to find out the actual vapor pressure based on DBT and WBT.

Check this link,

http://www.uswcl.ars.ag.gov/exper/relhumeq.htm

Regards,

Eng-Tips.com : Solving your problems before you get them.

RE: water steam pressure at ambient temp

One approximation taken from Holmans book on heat transfer is:

Tdb = dry bulb temp = 40C
Twb = wet bulb temp = 25C

From steam tables
sat press at Tdb = Pgd = 73.75mbar
sat press at Twb = Pgw = 31.66mbar

Atmos press = Pa

Partial press of vapour = Pv
                        = Pgw - ((Pa-Pgw)(Tdb-Twb)/(1537.778-Twb)) = 21.927mbar

Relative humidity = Pv*100/Pgd = 29.73%

Specific humidity = 0.622*Pv/(Pa-Pv) = 0.01376 kgH2O/kg dry air

athomas236

RE: water steam pressure at ambient temp

(OP)
Dears all people, thanks for your tip.

Pardal

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