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  1. #1
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    R717 Pressure - Temperature Conversion.



    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,



  2. #2
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    Re: R717 Pressure - Temperature Conversion.

    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.
    Brian - Newton Abbot, Devon, UK
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  3. #3
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    Re: R717 Pressure - Temperature Conversion.

    Hi, Synyster

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

    anyway never too late to do that


    Quote Originally Posted by Synyster View Post
    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.co...2168#post62168


    then, you can try with CoolPack .... to see how it works ... it is free and very useful
    http://www.en.ipu.dk/Indhold/refrige.../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/2009/0...mperature.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
    Last edited by Josip; 07-08-2016 at 09:56 AM. Reason: typing error

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
    No job is as important as to jeopardize the safety of you or those that you work with.

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    Re: R717 Pressure - Temperature Conversion.

    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

  5. #5
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    Re: R717 Pressure - Temperature Conversion.

    Quote Originally Posted by Synyster View Post
    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[C]+244,2)))
    Tsat[C]=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 -60C to +60C 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
    Last edited by cduque; 08-08-2016 at 11:04 AM. Reason: addicional information

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    Re: R717 Pressure - Temperature Conversion.

    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[C] but my end result is WAY off. It has to be an error with my interpretation. I'm getting like -482C for 344 kPa.

  7. #7
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    Re: R717 Pressure - Temperature Conversion.

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


  8. #8
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    Re: R717 Pressure - Temperature Conversion.

    If this is not SI units:

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

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