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  1. #1
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    Mar 2016
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    Pressure = Function of (T)

    Dear Friends,
    By some application such as refrigerant slider [photo at below] we can obtain pressure of refrigerant when we have temperature of it (and vice versa) in saturation area.
    Also same as this, In drive of electronic expansion valves we have same as this matter and drive by measuring suction pressure can realize evp. Temperature and finally obtain superheat.

    Definitely these applications and devices use a formula and function such as :
    Pressure = Function of (T).

    Suppose I want to program a SIMENS MINI PLC that measure pressure of suction and suction temp. then by using a function can understand evp. Temperature and then obtain superheat to open or close electronic expansion valve.

    Definitely these devices or softwares do not use table, They use a formula to obtain temperature or pressure on saturation area.

    I need this formula or function for different refrigerants. Have you ever seen these formulas on any sources ?


    Sincerely yours.
    Last edited by A.Mortezania; 12-05-2018 at 06:25 PM.

  2. #2
    Join Date
    Dec 2007
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    Re: Pressure = Function of (T)

    I doubt anyone has a formula or algorithm to pass on that would be deadly accurate.
    The ones I know of have a PT entered for that type of refrigerant, which Danfoss have made pretty easy for us to memory accurate than ever before.
    If someone did work it out, I would expect they would keep it to themselves.
    Why not just buy a Danfoss or whatever brand controller?

  3. #3
    Brian_UK's Avatar
    Brian_UK is offline Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
    Join Date
    Mar 2001
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    Re: Pressure = Function of (T)

    This is a question often asked over the years by people who want program PLCs etc without having to pay the big bucks for the data handling.

    As yet I haven't found anyone who has done it.

    One old thread found by searching this forum gives a formula which may be good enough for you.

    You could, of course, purchase NISTs Refprop for $325 and have the work done for you.

    Download the user guide and you may begin to understand the complexities of the subject.
    Last edited by Brian_UK; 12-05-2018 at 11:18 PM.
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  4. #4
    Join Date
    Feb 2015
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    Re: Pressure = Function of (T)

    Different refrigerant p/T dependencies you can find in internet (mostly they are presented in tables) or you can download software like Solkane that presents quite long list of refrigerants where you can check it graphs.

    What you said about programming - it's not worth. The market is quite full of controllers. Like CAREL controllers, got own algorithms between refrigeration circuit and electronic expansion valve openings where you set SH value and so on.

  5. #5
    Join Date
    Mar 2012
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    Re: Pressure = Function of (T)

    Just a note on the RefProp option, try check out what ILK Dresden has available in the form of ASEREP.
    It's for free, and derived from the RefProp.
    - But I guess it does not make the programming much easier, though.
    What we mostly use in Refrigerant Slider is just the simple Antoine equation, which requires three constants, that we have derived from RefProp (licensed, of course)
    When I say "mostly" it's, as some of you may have noticed, because for the transcritical range in CO2 we had to work a bit differently.
    But again, try check out the Aserep stuff:
    Last edited by deulengame; 18-05-2018 at 11:39 AM.

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