Solving for Wetbulb Temperature with Excel 5

[navynuke]

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.

 

[electricpete]

This sounds like a job for goal seek or solver. Are you familiar with those tools?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[athomas236]

I would use the two functions below and iterate until the wet bulb temperature I guessed gave the required relative humidity

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

 

[danberry]

Excel will solve an equation that references itself. Try it

 

[athomas236]

danberry
Could you tell us how to do that.

 

[joerd]

You'll have to go to Tools/Options/Calculation and then check Iterations. This has been around since Lotus 123.

Cheers,
Joerd

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[LCruiser]

I'm not exactly sure *why* you would want to calculate wet bulb temperature...

 

[aviat]

Someone has already done that at:

http://www.4wx.com/wxcalc/rh.php

 

[rb1957]

great post aviat ! ...

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.

 

[LCruiser]

I'm still curious why you would want to calculate wet bulb. Wet bulb is raw data used to calculate humidity, not the other way around.

 

[MintJulep]

LCruiser,

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.

 

[briand2]

LCruiser,

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

 

[LCruiser]

You want humidity, right? You are trying to convert from humidity to wet bulb and back to humidity, correct? Is this a smoke and mirrors project?

 

[electricpete]

Read it again. He wants wet bulb temp. He has RH and dry bulb temp.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Somewhere someone who computed RH may have known wetbulb tmep, but they didn't provide it to him. Same as my weatherman, tells me RH and [dry bulb] temperature.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[LCruiser]

electricpete said:

"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.

 

[electricpete]

http://www.advantageengineering.com/fyi/273/advantageFYI273.php

"Cooling tower selection and performance is based on water flow rate, water inlet temperature, water outlet temperature and ambient wet bulb temperature."



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[LCruiser]

Imagine that. How primitive.

;-)

 

[JoeTank]

LCruiser,
I think coatings inspectors get excited about wet bulb temps too.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[LCruiser]

Interesting. I would think dew point would be more appropriate.