Relative humidity and humidity ratio question. 2

[jasno999]

I am trying to set up a program on excel to look at air conditioning loads and what not. In doing this I would liek to use equations and data fro mcharts as much as possiable so that I do nto need to reference the psychrometric chart - I want to keep the model I am building dynamic and easy to use.

So my quetion is this. I have a portion of mixed air that is entering my evaporator.

My split is 80% return air and 20% outdoor air.

My outdoor air is 103F and 100%RH

My return air is 80F and 48%RH

I know how to find the mixed air temperature:

Tma = (103 X .2)+(80 X .8)= 84.6F

I also know that I can use the humidity as measured in grains/lb to find humidity:

H(gr/lb)= (75gr/lb X .8) + (325gr/lb X .2) = 125gr/lb

This all makes sense cause if I draw a line on the psyh chart between the points and then use the 80% mark away from the outdoor point I get the point that corresponds with what I jsut found.


HERE IS THE REAL QUESTION:

Is there any way to look at the humidity measured in grains/lb and use an equation and maybe a chart of values to convert it to a Relative Humidity Ratio measured in % ?

 

[insult2injury]

Okay, so I just curve-fit the saturation curve up to 200F

[h in BTU/lbm, T in degrees F]

A linear fit with a maximum deviation of 0.073%:
h = 1062.9855 + 0.42175562*T

Better equation fit with a maximum deviation of 0.04%:
h = (1127984.6 + 937.80641*T)^(0.5)

Best equation fit with maximum deviation of 0.02%:
h = 1059.8474 + 0.63214839*T^(0.92927487)

Note that the coefficients of the linear equation with increased range vary only slightly from my previous post. In my opinion, you shouldn't really see a noticeable difference by using any of the 4 correlations I posted to calculate the saturation enthalpy.

I2I

 

[Yorkman]

I2I,
When you say I just curve-fit the saturation curve up to 200F. How did you do tha? Is this done on a spread sheet? Or did you manually crunch the numbers? Just curious and impressed.


I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[insult2injury]

Generally done by a least-squares method which can be done by hand but is most efficiently done by software. I've had to program the method into several software applications such as fitting fan curves, etc... All you really need to know is the form of the equation you want to map the data to. A spreadsheet could probably be made to do it. I used a software package called TableCurve2D to find those. It tries throusands of equation forms simultaneously and then ranks them based on R^2 values. There is a 3D version as well.

I2I

 

[Yorkman]

Well thank you very much for the reveal, and the equation(s). I'll keep those handy when I'm teaching that psychrometrics class to my 5th year a/c apprentices this fall. It will give me some wiggle room when the topic of enthalpy and what to do when the charts range runs out;

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[KiwiMace]

Following through after a few days away from the machine, I see that perhaps my response was a bit too precise for your needs, but I guess still nice to know where the numbers come from.

I have tried curve-fitting the saturation curve before and found that it only works for short segments with polynomial fits - I wouldn't rely too heavily on these for extrapolation. The appropriate equations are fairly well documented anyway.

Excel will fit curves to data with the Trendline function.