R717 Pressure - Temperature Conversion.

[Synyster]

I'm trying to help a friend with a small project in which we are wanting to take R717 pressure and convert it to temperature, in a building automation system. My background is not one of refrigeration unfortunately, but I intend on writing code in the automation system to accomplish this.

I understand how the P/T chart works, but as far as developing a formula that I can interpret to code is where I am having issues.

I tried to import a P/T chart into Excel and plot out a graph and capture the formula, but my Excel/Math skills aren't quite where they need to be.

Anyone have any good suggestions on figuring out a formula for this?

Thanks,

[Brian_UK]

That's why people pay big bucks to the people who can do that.

If it was really easy think how cheap it could be.

[Josip]

Hi, Synyster :)

welcome to RE forums .... maybe you should start here with your first post to introduce yourself to us ....
http://www.refrigeration-engineer.com/forums/forumdisplay.php?72-New-to-RE

anyway never too late to do that ;)



I'm trying to help a friend with a small project in which we are wanting to take R717 pressure and convert it to temperature, in a building automation system. My background is not one of refrigeration unfortunately, but I intend on writing code in the automation system to accomplish this.

I understand how the P/T chart works, but as far as developing a formula that I can interpret to code is where I am having issues.

I tried to import a P/T chart into Excel and plot out a graph and capture the formula, but my Excel/Math skills aren't quite where they need to be.

Anyone have any good suggestions on figuring out a formula for this?

Thanks,



Try to search RE forums .... searching window is right above .... or use link below ...

http://www.refrigeration-engineer.com/forums/showthread.php?4278-PH-Chart-and-tables&p=62168#post62168


then, you can try with CoolPack .... to see how it works ... it is free and very useful
http://www.en.ipu.dk/Indhold/refrigeration-and-energy-technology/coolpack.aspx

and this one .... with formula to make your own program ... but, not cover full property range for NH3 (Tmin=-77,66*C, Tcritical=132,35*C)
http://profmaster.blogspot.hr/2009/08/ammonia-vapor-pressure-temperature.html


In my opinion the best is to buy something (ready program) what is proved good ... like refprop from .... http://www.nist.gov/srd/nist23.cfm .... and incorporate into your program ....

anyhow, good luck and let us know about your outcome ...


Hope this will be of some help to you.

Best regards, Josip :)

[nike123]

http://www.nt.ntnu.no/users/skoge/bok/mer/flash_english_edition_2009

[cduque]

I'm trying to help a friend with a small project in which we are wanting to take R717 pressure and convert it to temperature, in a building automation system. My background is not one of refrigeration unfortunately, but I intend on writing code in the automation system to accomplish this.

I understand how the P/T chart works, but as far as developing a formula that I can interpret to code is where I am having issues.

I tried to import a P/T chart into Excel and plot out a graph and capture the formula, but my Excel/Math skills aren't quite where they need to be.

Anyone have any good suggestions on figuring out a formula for this?

Thanks,

Hi Synyster,
I went through that problem many years ago and first I used the reverse engineering you suggest. The problem is that the polynomial approach is valid only for specific and limited areas of the ph graph.
So I've been using another approach that proved good enough for me that is a 1986 study from the Department of Biotechnology of the Massey University - New Zealand, from A.C. Cleland:
Psat[Pa]=EXP(22,11874+(-2233,8226/(Tsat[ºC]+244,2)))
Tsat[ºC]=2233,8226/(LN(Psat[Pa])-22,11874)-244,2
This formulaes are established for SI units, so if you pretend other units you have to adapt it.
They are considered valid in the range -60ºC to +60ºC with an error not greater than 0.25%.
Hope it's useful for you.
CDuque

PS: note that 1 bar = 100.000 Pa
1 bar ~= 14.5 psi
1 psi = 6894.76 Pa

[Synyster]

Thanks guys.

Please forgive my manners...

I'm in the HVAC industry, primarily HVAC controls. I have more of a computer-based background and typically deal with programming, almost exclusively. I know the basic concept of refrigeration.....mainly the mechanical/component side of it, but have little knowledge of refrigerants and their properties.

I found this site by following the Google trail of a guy, years back, who had a similar project but was doing conversions with R-22.

As far as I knew, it would be just as simple as finding a formula, coding it out and being done......it appears thats not the case :)

cduque - I tried your 2nd formula for Tsat[ºC] but my end result is WAY off. It has to be an error with my interpretation. I'm getting like -482ºC for 344 kPa.

[cduque]

Sorry, it's missing a minus at the start, try with: Tsat[ºC]= -2233,8226/(LN(Psat[Pa])-22,11874)-244,2

[sterl]

If this is not SI units:

Try http://web.ornl.gov/info/reports/1977/3445605557732.pdf

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