Calculate Wet Bulb based on changing dry bulb.
Calculate Wet Bulb based on changing dry bulb.
(OP)
Manual J publishes design dry and wet bulb data for each city.
So, for instance: Phoenix, AZ would be 107 db, and 71 wb for outdoor air.
However, I want to calculate what the wet bulb would be if I took the published conditions and changed the dry bulb to indoor air temperature of, for instance: 75F.
Using a Psychometric chart this is very simple. You just look left along a horizontal line until you hit the new dry bulb temperature, and you have your wet bulb.
Does anyone have a formula that would do this so I can do it in Excel?
So, for instance: Phoenix, AZ would be 107 db, and 71 wb for outdoor air.
However, I want to calculate what the wet bulb would be if I took the published conditions and changed the dry bulb to indoor air temperature of, for instance: 75F.
Using a Psychometric chart this is very simple. You just look left along a horizontal line until you hit the new dry bulb temperature, and you have your wet bulb.
Does anyone have a formula that would do this so I can do it in Excel?





RE: Calculate Wet Bulb based on changing dry bulb.
RE: Calculate Wet Bulb based on changing dry bulb.
Pswb = saturation vapor pressure at Twb, Pa
Patm = atm pressure, in Pa
Pv = vapor pressure, in Pa
Twb = wet bulb temp, in K
T = dry bulb temp, in K
Pswb - Pv = B1(Twb - T)
where B1 = {1006.9254(Pswb - Patm)(1 + 0.15577[Pv/Patm])}/(0.62194 hfg1) ; [for 255.38 <= T <= 533.16]
gets rewritten as
Twb = (Pswb - Pv)/B1 + T
That's it?
RE: Calculate Wet Bulb based on changing dry bulb.
RE: Calculate Wet Bulb based on changing dry bulb.
I don't know the numbers of the equations in your handbook, which is probably up to date.
Per my handbook, given db, wb, and pressure, you can solve directly for the rest of the properties.
Given db and dew point temp. and pressure, you can directly for all EXCEPT wet bulb. It says reqs. trial & error solution or numerical sol.
Given db, RH and pressure, you can directly for all EXCEPT wet bulb. It says reqs. trial & error solution or numerical sol.
However, notice the equation for W, humidity ratio
W=0.62198x(pw/(p-pw). (in 1977 vers, it is eq. 23)
In my handbook, it also lists another version
W=((1093-.556t*)x(Ws*)-.24x(t-t*))/(1093+.444t-t*) (eq. 35)
where t* = wb temp
t = db temp
Ws* = hum ratio @ saturation.
Re-arrange the equation to solve for t*, using the other values calculated.
t*=(Wx(1093+.444t)+.24t-1093Ws*)/(.24+W-.556Ws*)
Now, given db, dew point, press., you can solve for t*.
The same if you are given db, RH and pressure.
Let me know how it works.
RE: Calculate Wet Bulb based on changing dry bulb.
The first one is not possible, the second is a cooling coil calculation.
RE: Calculate Wet Bulb based on changing dry bulb.
I'm trying to take 107F (dry bulb) 72 wet bulb air and
cool it to 75F dry bulb, and determine what the new wet bulb would be.
RE: Calculate Wet Bulb based on changing dry bulb.
RE: Calculate Wet Bulb based on changing dry bulb.
It's very easy with a chart.
How do you do it with formulas. I'm still working on that.
RE: Calculate Wet Bulb based on changing dry bulb.
I2I
RE: Calculate Wet Bulb based on changing dry bulb.
RE: Calculate Wet Bulb based on changing dry bulb.
RE: Calculate Wet Bulb based on changing dry bulb.
http://biomet.ucdavis.edu/conversions/CalHum.htm
RE: Calculate Wet Bulb based on changing dry bulb.
Trashcanman has a good idea:
t*=(Wx(1093+.444t)+.24t-1093Ws*)/(.24+W-.556Ws*)
the problem with this is that given Db, Td (dewpoint) and Patm you cannot solve for t* since Ws* must be calculated using:
Ws* = 0.62198(Pws/P-Pws)
where Pws is Pws(t*).
As you can see, this still leaves me stuck since I have to calculate t* using t* :D
I'd greatly appreciate a solution to this problem since I am creating my own psychrometric calculator.
Thanks,
-Vince
RE: Calculate Wet Bulb based on changing dry bulb.
RE: Calculate Wet Bulb based on changing dry bulb.
Plus, I'm making mine cross-platform (Win, Mac, Linux) :D
RE: Calculate Wet Bulb based on changing dry bulb.
We both are in the same boat. I could do the calculation for a given DBT and WBT but calculating psychrometric properties, when DBT and RH (or DPT) are given, gives you circular reference error in excel due to the problem you mentioned. However, try checking the iterative option from Tools--->Options--->Calculation Tab. I tried this but without result. You can discuss your problem in forum770: -Engineering spreadsheets. I will let you know if I come up with something, meanwhile.
RE: Calculate Wet Bulb based on changing dry bulb.
If you want to use the application I'm making, watch this page:
http://www.v2cstudios.com/en/psychrometree.html
I'll be launching the BETA (currently it's in Alpha) as soon as I figure this Wb story out.
RE: Calculate Wet Bulb based on changing dry bulb.
The 1st set is easy. Given t, t*, and p (or altitude)
2nd set is a little more tricky:
Given t (dry bulb), p (or altitude), and td (dew point):
Calculate all but t* using formulas 5,6,22,23,23,25,28,32
p (total pressure)= 0.1450377*101.304*exp(-alt/26216) Alt in ft above sea level, p in psia.
Now, assume wet bulb, t*. As a 1st guess say dry bulb-5 Deg.
with that, calculate Pws*
with that, calculate Ws*
with that, calculate W
with that, calculate Pw=p*W/(0.62198+W)
with that, calculate alpha = ln(Pw)
with that, calculate td' (dew point)
with that, calculate error = given td-td'
Use goal seek to quickly zoom in on t* (change t* so that td-td' is approx 0)
The 3rd set (given t, rel hum, and p (or altitude) is done in a similar fashion.
RE: Calculate Wet Bulb based on changing dry bulb.
RE: Calculate Wet Bulb based on changing dry bulb.
Thanks a lot for the explanation!
RE: Calculate Wet Bulb based on changing dry bulb.
RE: Calculate Wet Bulb based on changing dry bulb.
I got the whole thing working :D
If you want to try out the BETA of my program go to:
http://www.v2cstudios.com/en/psychrometree.html
Read my blog page for more info.
RE: Calculate Wet Bulb based on changing dry bulb.