Calculate Wet Bulb based on changing dry bulb.

[rifter1]

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?

 

[trashcanman]

That is fine IF your psyc chart is for Phoenix elevation (1117' as per ASHRAE). Use the formulas in Chapter 5 of handbook of fundamentals.

 

[rifter1]

Ah. OK looks easy...
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?

 

[rifter1]

I know I'm missing something simple. In order to find Pswb I need to know Twb. But I don't know Twb, that's what I'm trying to solve for...

 

[trashcanman]

Well, I only have the 1977 Fund. handbook with me (I am in Iraq). It has the so-called Goff formulas. One is for sat. press. over ice and one for sat. press over liquid water. I used the one over liq. water. Anyway, it yields Pws. There are a couple of other formulas as well. The formulas do not say Pws is at the wet bulb temp.

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.

 

[ChrisConley]

Are you trying to extrapolate what the outdoor wetbulb temperature would be at 75F, or trying to determine what the new wetbult would be if you cooled the outdoor air from 107 to 75?

The first one is not possible, the second is a cooling coil calculation.

 

[rifter1]

(B)

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.

 

[trashcanman]

Assuming sensible cooling only, on a psych chart, that would be about 75db/62wb, about 49% RH

 

[rifter1]

Precisely.
It's very easy with a chart.
How do you do it with formulas. I'm still working on that.

 

[insult2injury]

Most thermo books cover all the necessary equations.

I2I

 

[trashcanman]

Look at my 08/27 post again. As per my handbook, given db, dew point and pressure, or db, RH & pressure, you can compute wb. The later handbook should have even better formulas.

 

[rifter1]

Yes I still have that on my desktop I haven't had time to look at the solution yet, believe it or not. After 12 hours a day I draw the line. I think in autumn or winter I will have time. I'll get to it. I'll let you know.

 

[gepman]

Use the Excel spreadsheet located at

http://biomet.ucdavis.edu/conversions/CalHum.htm

 

[Wizeman]

I know this thread is a month old, but it's never too late

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*

I'd greatly appreciate a solution to this problem since I am creating my own psychrometric calculator.

Thanks,

-Vince

 

[gepman]

If my previous post doesn't satisfy you with the free spreadsheet then you can buy what you are trying to develop for about $35 at

http://www.parkssoft.com/ezair.html.

I downloaded the demo and it looks like it works fine.

 

[Wizeman]

Thanks for the link, but I'd rather make my own, that way I will understand how it all works together.

Plus, I'm making mine cross-platform (Win, Mac, Linux)

 

[quark]

Wizeman,

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. I will let you know if I come up with something, meanwhile.

 

[Wizeman]

Ok, well in my case the only thing I can't calculate is Wb. But I will keep looking too.

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.

 

[trashcanman]

Ok. I have solved the infamous ASHRAE psychrometric card trick, using the psychro formulas in the Handbook of Fundamentals. I did it with EXCEL spreadsheet.

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.

 

[gepman]

trashcanman has shown that it is an iterative process (this is discussed in ASHRAE). That is why some of the psychrometric Excel calculators that I mentioned basically use lookup tables and then interpolate between the values on the lookup tables. I am not sure if iterative multiple equations actually improves the understanding of the process which wizeman indicated he wanted to do in a previous post. I think that the greatest understanding comes from looking at the chart.