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Old January 22nd, 2018, 07:54 AM   #1
saultgeorge
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Air Enthalpy calcs

Morning, folks. I hope I get some views on this and maybe a reply. I'm reaching out to the best of the best here.
I'm writing logic for some Air Handling Units (AHU's) for a customer that has contracted us to do so. I'm using RS5k v. 20.04
Anyway, to be brief, I need to perform an enthalpy calculation on outside air temp and humidity percent, which are the only 2 values I get into the logic.
If you'll note the 2 links, the equation gives you the result in kJ/kg, which I don't know how to relate to the temp. I am using the one from wiki without steam tables. I have absolutely no problem using the equation in the logic and doing the math, I just don't understand how to use the result. The reason for this routine is that an AHU can use outside air to help with cooling without using the chilled water at certain outside temps. But if the humidity is too high, even though the outside temp is low, say 50, then it would do no good to bring in that air. There will probably be some kind of a comparison I will have to make. I have emailed the engineers that do this and that sent me the links, but no reply so far. Any enlightenment is more than I have now. Thanks in advance, folks. Happy Monday.

https://sciencing.com/calculate-enth...r-6118694.html

https://www.wikihow.com/Calculate-th...y-of-Moist-Air
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Old January 22nd, 2018, 12:05 PM   #2
James Mcquade
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It's been too many years since I hade to deal with thermodynamics.

this maybe a stupid question on my part, if you have a humidity limit (not to exceed this value), why not program that into your logic as the determining factor?

just a question and as I said, to many years ago for this calculation but I still have my books.

james
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Old January 22nd, 2018, 12:55 PM   #3
Tom Jenkins
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I guess I'm not sure what the question is. I assume that the AHU is going to be used with tempering air, and the enthalpy calculation is part of that. Here is one explanation of tempering air: https://www.garagejournal.com/forum/...hp/t-3860.html Some searching by you will yield more results. You might also look at the ASHRAE site.

A couple of notes. First, do some checking on the formula's accuracy over the range of operation you are expecting. These look like empirical formulas, and they may or may not be accurate for your conditions. A check calculation with known accurate results provided by your client would be in order.

Second, note that the moisture content portion of the equation is the humidity ratio, kg/kg. This is not equal to relative humidity (%RH) but is a function of %RH and temperature.
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Old January 24th, 2018, 12:11 PM   #4
saultgeorge
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Thanks Tom and James. Yes, I have received even more equations from their engineers. They use an on-line type calculator (similar to a mortgage calculator) to do their math. A comment to Tom's reply, that sounds like a good idea, using RH to say yea or nay to using the outside air economizer, but the specs call for enthalpy calculation of the outside air and humidity. I'll keep at it. Really, thanks so much for taking the time to help.
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Old January 24th, 2018, 01:51 PM   #5
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https://www.vaisala.com/sites/defaul...210973EN-F.pdf


This pdf shows how to calculate various air properties with temperature and RH, etc. I have used calculations from this pdf to calculate the dewpoint give drybulb T and RH inside a P2k plc. I believe that chapter 6 is enthalpy. I assume the mechanical guys are saying to use the dampers to select the lowest enthalpy air to use to (freshaire or recirc air).
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Old January 25th, 2018, 05:34 PM   #6
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Have a look at the functions on the website of oscat.de they are used to make the calcs in a simple way.
and yes it is enthalpy , however the moist will change the enthalpy as it takes more energy to cool moist air, and if the water condenses it even takes more energy.
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Old January 26th, 2018, 03:48 PM   #7
saultgeorge
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Thank you g.mccormick and shooter. I was given that PDF from Vaisala and those are the equations I'm using to compute the enthalpy in my logic. I then compare outside air enthalpy to return air enthalpy. If outside is more, then no economizer dampers open and no free cooling. Thanks to all for the replies and research and your time.
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Old January 27th, 2018, 10:55 AM   #8
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Quote:
Originally Posted by saultgeorge View Post
Thank you g.mccormick and shooter. I was given that PDF from Vaisala and those are the equations I'm using to compute the enthalpy in my logic. I then compare outside air enthalpy to return air enthalpy. If outside is more, then no economizer dampers open and no free cooling. Thanks to all for the replies and research and your time.

You're welcome.
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