Excel Formula for Calculating Air's Enthalpy from dry bulb temperature and relative humidity.

[Engineer Joe]

This isn't actually a question. I worked for a while to come up with a feasible formula working between 0 and 100°F to provide an enthalpy value based on dry bulb temperature and relative humidity. It uses a polynomial model to match Psat to temperature in order to avoid referencing a table (I wanted it to be able to work by a simple math equation...no steps.) You can see the polynomial in it's own bracket with several terms raised to the 4,3,2,1st order. That's the approximation model. It's >99% accurate between 0 and 100°F. I'm posting here so that if someone googles the key terms, maybe this will pop up for them. I've googled this several times before with no results that were any good. So, future HVAC/excel users, here ya go!

h = 0.24*OAT+(0.6219)*(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH)/(14.7-(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH))*(1061.2+0.444*OAT)

Entalpy: h is in btu/lb(dry air)
OAT: Temperature in °F
RH: Relative Humidity as a percentage. (e.g. 40% RH would just be 40, not 0.40)

Obviously, plugging this into excel, you would replace references to OAT and RH with cell references.

Have a nice day! And yes, I'm aware I didn't need to use 15 sig figs. i went into the trend in the chart and just started clicking the decimals button until i got tired and then copy/pasted.

 

[milkeshp004]

This isn't actually a question. I worked for a while to come up with a feasible formula working between 0 and 100°F to provide an enthalpy value based on dry bulb temperature and relative humidity. It uses a polynomial model to match Psat to temperature in order to avoid referencing a table (I wanted it to be able to work by a simple math equation...no steps.) You can see the polynomial in it's own bracket with several terms raised to the 4,3,2,1st order. That's the approximation model. It's >99% accurate between 0 and 100°F. I'm posting here so that if someone googles the key terms, maybe this will pop up for them. I've googled this several times before with no results that were any good. So, future HVAC/excel users, here ya go!

h = 0.24*OAT+(0.6219)*(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH)/(14.7-(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH))*(1061.2+0.444*OAT)

Entalpy: h is in btu/lb(dry air)
OAT: Temperature in °F
RH: Relative Humidity as a percentage. (e.g. 40% RH would just be 40, not 0.40)

Obviously, plugging this into excel, you would replace references to OAT and RH with cell references.

Have a nice day! And yes, I'm aware I didn't need to use 15 sig figs. i went into the trend in the chart and just started clicking the decimals button until i got tired and then copy/pasted.

Dude, Thanks a million ! :*

 

[mcb777]

Terrific work Engineer Joe! This is very helpful. Any chance of adding one for Metric (SI) in °C and kJ/kg? It would be very much appreciated. Thanks and regards.

 

[gmadero]

Terrific work Engineer Joe! This is very helpful. Any chance of adding one for Metric (SI) in °C and kJ/kg? It would be very much appreciated. Thanks and regards.

Try this one
=0.24*OAT+(597.3+0.441*OAT)*0.622*(10^(8.10765-1750.29/(OAT+235))*RH/100)/(760-(10^(8.10765-1750.29/(OAT+235))*RH/100))

Substitute OAT and RH as Engineer Joe stated

 

[GibbsENG]

This isn't actually a question. I worked for a while to come up with a feasible formula working between 0 and 100°F to provide an enthalpy value based on dry bulb temperature and relative humidity. It uses a polynomial model to match Psat to temperature in order to avoid referencing a table (I wanted it to be able to work by a simple math equation...no steps.) You can see the polynomial in it's own bracket with several terms raised to the 4,3,2,1st order. That's the approximation model. It's >99% accurate between 0 and 100°F. I'm posting here so that if someone googles the key terms, maybe this will pop up for them. I've googled this several times before with no results that were any good. So, future HVAC/excel users, here ya go!

h = 0.24*OAT+(0.6219)*(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH)/(14.7-(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH))*(1061.2+0.444*OAT)

Entalpy: h is in btu/lb(dry air)
OAT: Temperature in °F
RH: Relative Humidity as a percentage. (e.g. 40% RH would just be 40, not 0.40)

Obviously, plugging this into excel, you would replace references to OAT and RH with cell references.

Have a nice day! And yes, I'm aware I didn't need to use 15 sig figs. i went into the trend in the chart and just started clicking the decimals button until i got tired and then copy/pasted.

Have you by chance reversed the formula to produce %RH from h and T?

 

[BEBurgdorf]

This isn't actually a question. I worked for a while to come up with a feasible formula working between 0 and 100°F to provide an enthalpy value based on dry bulb temperature and relative humidity. It uses a polynomial model to match Psat to temperature in order to avoid referencing a table (I wanted it to be able to work by a simple math equation...no steps.) You can see the polynomial in it's own bracket with several terms raised to the 4,3,2,1st order. That's the approximation model. It's >99% accurate between 0 and 100°F. I'm posting here so that if someone googles the key terms, maybe this will pop up for them. I've googled this several times before with no results that were any good. So, future HVAC/excel users, here ya go!

h = 0.24*OAT+(0.6219)*(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH)/(14.7-(0.01*(0.000000007401234*OAT^4 - 0.000000493526794*OAT^3 + 0.000071281097208*OAT^2 - 0.000489806163078*OAT + 0.039762055806989)*RH))*(1061.2+0.444*OAT)

Entalpy: h is in btu/lb(dry air)
OAT: Temperature in °F
RH: Relative Humidity as a percentage. (e.g. 40% RH would just be 40, not 0.40)

Obviously, plugging this into excel, you would replace references to OAT and RH with cell references.

Have a nice day! And yes, I'm aware I didn't need to use 15 sig figs. i went into the trend in the chart and just started clicking the decimals button until i got tired and then copy/pasted.

Thanks, this is extremely useful. New to this so apologize if I ask something that seems obvious.

You state "Enthalpy: h is in btu/lb(dry air)" but this seems to return total Enthalpy. Dry air Enthalpy is an east calc (DBT*0.24), and doesn't require humidity, right?

Also trying to get an Enthalpy Calc varying for elevation. Is this at 1 Atmosphere?

 

[Dave Patton]

It would be helpful if participants showed the Excel version that they use in their profile and
provided an example or examples with expected results.

I am not an engineer. My comment does not add anything to the challenge or the math.

With Excel 365, one can use Lambda to build a new function.
With the above information, I created a function with the parameters OAT and RH.

LAMBDA.xlsm
	A	B	C
4	OAT	RH
5	82	40	120.0317
7a

Cell Formulas
Range	Formula
C5	C5	=Enthalpy(A5,B5)

Function is named Enthalpy and
Value =LAMBDA(OAT,RH,0.24*OAT+(597.3+0.441*OAT)*0.622*(10^(8.10765-1750.29/(OAT+235))*RH/100)/(760-(10^(8.10765-1750.29/(OAT+235))*RH/100)))