# Formula (not chart) using Wetbulb to get Enthalpy?

• 08-23-2008, 08:55 AM
Hewhois
Formula (not chart) using Wetbulb to get Enthalpy?
Greetings!

I am looking for a formula (not a chart) that uses Wetbulb to get Enthalpy values.

I am trying to make an excel mobile spreadsheet for my PDA that will calculate BTUH using Wetbulb and CFM as the variables. Currently, I have to pull out a chart to convert the WetBulb into Enthalpy first, for the following equation: BTUH = 4.5*CFM*Delta H

I would like to be able to leave the chart in the van and just type the Wetbulb into the PDA and have it calculate the enthalpy and have that input directly into the BTUH equation.

Thanks!
• 08-23-2008, 09:53 AM
lwarren
Why not just take the values from a table and put them into excel and then curve fit the data to get a formula from excel. Here is a link to a table:
http://www.energyrightpartners.com/p...alpy_Chart.pdf

I found these formulas, but I do not know how accurate they are;

10 REM ENTHALPY CALCULATION
20 REM Assumes standard atmospheric pressure (14.7 psi), see line 110
30 REM Dry-bulb temperature in degrees F (TEMP)
40 REM Relative humidity in percentage (RH)
50 REM Enthalpy in BTU/LB OF DRY AIR (H)
60 T = TEMP + 459.67
70 N = LN( T ) : REM Natural Logrithm
80 L = -10440.4 / T - 11.29465 - 0.02702235 * T + 1.289036E-005 * T ^ 2 - 2.478068E-009 * T ^ 3 + 6.545967 * N
90 S = LN-1( L ) : REM Inverse Natural Logrithm
100 P = RH / 100 * S
110 W = 0.62198 * P / ( 14.7 - P )
120 H = 0.24 * TEMP + W * ( 1061 + 0.444 * TEMP )

Or -

for DB in Deg F and RH (0 to 1.0)
A = .007468 * DB^2 - .4344 * DB + 11.1769
B = .2372 * DB + .1230
Enth = A * RH + B
It gives a result that is accurate to within .7% over the range
40 F <= DB <= 80 F
0 <= RH <= 1.0
• 08-23-2008, 10:31 AM
Hewhois
Quote:

Originally Posted by lwarren
Why not just take the values from a table and put them into excel and then curve fit the data to get a formula from excel. Here is a link to a table:
http://www.energyrightpartners.com/p...alpy_Chart.pdf

I am trying to avoid having to do all that data entry, lol. I will do it though if all else fails.

They obviously came up with those numbers in the chart somehow or other. IMO, using the source formula would be a better approach than reverse engineering from the results.

My thanks for the link though. It looks exactly like the one in my van. It's nice to have an electronic version as well.

Quote:

I found these formulas, but I do not know how accurate they are;....
They don't use WetBulb temps though, they use DB and RH.
• 08-23-2008, 11:06 AM
Andy Schoen
One cannot curve fit an equation for enthalpy based on wet bulb temperature alone. You need another input. Dry bulb temperature would be my choice.

You could also use a DB and RH correlation like that posted above.
• 08-23-2008, 12:00 PM
Hewhois
Guess I will just use the "vlookup" function in Excell and create an array using the chart.