Psychrometric calculation errors about psychrometry HOT 9 OPEN

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from psychrometry.

Good day Prof.

Thanks for the feedback. I do admit that I wish they bothered to name the absolute humidity something a little more discerning as the terminology gets lost, even on myself. Thus from here I will refer to them as such:
The value indicating the absolute humidity in [kg moisture/kd dry air] will be 'apparent specific humidity'
and the value indicating the fraction of the total massflow consistsing of water vapor will be 'vapor fraction'.
I will also use g/kg to avoid decimals when stating the values but calculations should, of course, be done in the correct units.

-According to the Barenburg charts, (I do not have the ASHRAE charts, but I'm sure they are the same), at 35 C, 100kPa, saturated air should have an apparent specific humidity of 35.4 [g/kg da]. See Image 1. The vapor fraction is calculated as: Vapor fraction = (app. spec. hum.)/(app. spec. hum. + 1 kg da). In this case it comes to 34.2 [g vapor mass/kg total mixture mass].
-I can also calculate this myself using various equations provided in literature and I find the value for the apparent specific humidity to be 37.1 [g/kg da]. This is fairly close already. See Image 2. Subsequently the vapor fraction is 37.1/(1000+37.1) = 35.7 [g vapor mass/kg total mixture mass].
-Your applet is unable to produce a saturated air stream when I specify the 100% humidity condition. It does some calculations, unknown to me, and then reduces the humidity to approx. 96 % (sometimes less). See Image 3 & 4.
From Image 4 you can also see that it predicts the vapor fraction to be 37.091345 [g vapor mass/kg total mass]. But as you can see from the two samples above, this is NOT actually the vapor fraction. In reality this is the apparent specific humidity value. The true vapor fraction has already been shown above, but to drive it home let's reverse the calculation to get the "apparent specific humidity" value from this "vapor fraction": (vapor fraction)/(1000-vapor fraction) = apparent specific humidity = 38.52 [g/kg da], which is even higher than my calculated values.

Now I don't doubt that your Psychrometric charts may have some increased accuracy over mine, but coupling the applet's inability to calculate the saturated conditions and the fact that I have fed it several points for which it produces a "vapor fraction" that exactly matches what I expected for the apparent specific humidity, I have concluded that it is not functioning as intended.

I hope to have informed you sufficiently and look forward to your response. We all learn at different times and places and perhaps you have some information I am missing. I am not too proud to be taught.
Kind Regards.
Ir. Austin Labuschagne

Image 1:

Image 2:

Image 3:

Image 4:

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Open again for review.

from psychrometry.

Dear @Scorpiomom69

I upload the Template again with some changes. Could you download the new version and make some checks again?

Thank you very much for your help.

Best regards,

Nicolas

from psychrometry.

I'll need to take a deeper look also in the best Thermodynamic models to use on these cases.

from psychrometry.

Good day Nicolas

Thanks for taking another look!
I also checked and it seems to be working now, except for the mass-frac value in the simulation:

The value being displayed on the left is the 'absolute humdity' but you should be displaying the water mass fraction (vapor fraction). To display the right value use: (abs hum)/(abs hum+1) = vapor fraction [In this case 35.73g]

from psychrometry.

from psychrometry.

Hi Prof. I am doing well. How are you?

Indeed the calculations now work but they are going in the wrong direction. I.E. you are using x=f(y) but is should be y=f(x). See:

I have added you on WA and sent you a message. I'm not sure if you wanted me to post this last issue there or here?

Kind Regards

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