Solving for Wetbulb Temperature with Excel
Solving for Wetbulb Temperature with Excel
(OP)
I am relatively new to working with VBA although I am comfortable in my Excel abilities. I am trying to solve a system of equations for the wetbulb temperature(twb) given the dry bulb temperature (tdb), relative humidity (relh), and atmospheric pressure (Patm). The equations are as follows:
Wsat = 0.622(Pws/(Patm-Pws))
W = 0.622(Pw/(Patm-Pw))
Pw = relh*Pws
Pws = Exp(c8/twb+c9+c10*twb+c11xtwb^2+c12*twb^3+c13*log(twb))
W = ((1093-0.556*twb)*Wsat-0.240(tdb-twb))/(1093+0.444*tdb-twb)
I know that it is within Excel's power to solve for twb but at this point in my experience I cannot get the programming to work. Obviously the whole mess can be written in one big equation with the unknown on both sides of the equation but how do I get Excel to solve for it? Any assistance would be greatly appreciated.
Wsat = 0.622(Pws/(Patm-Pws))
W = 0.622(Pw/(Patm-Pw))
Pw = relh*Pws
Pws = Exp(c8/twb+c9+c10*twb+c11xtwb^2+c12*twb^3+c13*log(twb))
W = ((1093-0.556*twb)*Wsat-0.240(tdb-twb))/(1093+0.444*tdb-twb)
I know that it is within Excel's power to solve for twb but at this point in my experience I cannot get the programming to work. Obviously the whole mess can be written in one big equation with the unknown on both sides of the equation but how do I get Excel to solve for it? Any assistance would be greatly appreciated.





RE: Solving for Wetbulb Temperature with Excel
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Solving for Wetbulb Temperature with Excel
Function RelativeHumidity(WetBulbTemp, DryBulbTemp, AtmosPress)
Rem Calculates the relative humidity from
Rem Wet bulb temperature WetBulbTemp deg C
Rem Dry Bulb temperature DryBulbTemp deg C
Rem Atmospheric pressure AtmosPress kPa
Rem This equation is taken from BS 4485, part 2, 1988, Appendix D
RelativeHumidity = 100# * (AirSatPress(WetBulbTemp) - 1000# * AtmosPress * (0.000666 * (DryBulbTemp - WetBulbTemp))) / AirSatPress(DryBulbTemp)
End Function
Function AirSatPress(AirTemp)
Rem Calculates the saturation vapour pressure of water (Pa) from temperature deg C
Rem This equation is taken from BS 4485, part 2, 1988, Appendix D
AirSatPress = 10 ^ (-2948.997118 / (AirTemp + 273.15) - 2.1836674 * Log(AirTemp + 273.15) - 0.000150474 * 10 ^ (-0.0303738468 * (AirTemp - 0.01)) + 0.00042873 * 10 ^ (4.76955 * (1 - 273.16 / (AirTemp + 273.15))) + 25.83220018)
End Function
RE: Solving for Wetbulb Temperature with Excel
RE: Solving for Wetbulb Temperature with Excel
Could you tell us how to do that.
RE: Solving for Wetbulb Temperature with Excel
Cheers,
Joerd
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Solving for Wetbulb Temperature with Excel
RE: Solving for Wetbulb Temperature with Excel
http://www.4wx.com/wxcalc/rh.php
RE: Solving for Wetbulb Temperature with Excel
someone's already done nearly everything (busy person that someone). the trouble these days is getting the question and the solution together, when someoneelse asks the same question.
RE: Solving for Wetbulb Temperature with Excel
RE: Solving for Wetbulb Temperature with Excel
Given any two moist air properties at any given condition you can look up on a psychrometric chart, or calculate all of the other properties for that condition.
While wet bulb temp is relatively easy to measure with a sling psychrometer or other similar device it is a royal pain to measure WB for use in an automatic control system because you need a wet bulb.
To get a wet bulb you need water. And not just any water, you need nice clean distilled water, otherwise your wick gets crudded up with salt and mineral deposits over time.
Much easier to use a capacitive sensor that provides an RH number.
RE: Solving for Wetbulb Temperature with Excel
It may be that I'm interested in recording / analysing the performance of a cooling tower. This is very much related to wet bulb temperature, but I tend to measure dry bulb temperature an relative humidity instead, for the reasons stated by MintJulep - it's therefore useful if I can accurately (reasonably...) convert to wet bulb temperature.
Brian
RE: Solving for Wetbulb Temperature with Excel
RE: Solving for Wetbulb Temperature with Excel
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Solving for Wetbulb Temperature with Excel
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Solving for Wetbulb Temperature with Excel
"Read it again. He wants wet bulb temp. He has RH and dry bulb temp."
So, what are you going to do with wet bulb temp besides compute humidity? Nothing. It's not good for anything. It's an abstract measurement that's been calibrated to use with dry bulb to compute humidity. That's all it's good for.
RE: Solving for Wetbulb Temperature with Excel
"Cooling tower selection and performance is based on water flow rate, water inlet temperature, water outlet temperature and ambient wet bulb temperature."
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Solving for Wetbulb Temperature with Excel
RE: Solving for Wetbulb Temperature with Excel
I think coatings inspectors get excited about wet bulb temps too.
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: Solving for Wetbulb Temperature with Excel
RE: Solving for Wetbulb Temperature with Excel
truer (sp?) words.... I give you a star. And if it were possible, a star to "someone" as well!
Regards,
chichuck