Temperature to Pressure Formulas for R22,R12 R717 etc

[Kostass]

Hello Everybody,

I am Looking For formulas to convert Temperature to Pressure and opposite for Refrigerants R22,R12,R717,R134a etc.

Cheers Kostass.

[BigJon3475]

Might be easier to just print off the charts from googling each ref. and see for yourself. The blends I don't believe you will have a "mathematical equation" although I could be wrong.

[BigJon3475]

http://www.theweatherprediction.com/basic/equations/


I found this but I'm not really 100% on it as some of this is over my head...

" LN(Es/6.11) = (L/Rv )(1/273 - 1/T)
Es = Saturation vapor pressure
L = Latent heat of vaporization = 2.453 × 10^6 J/kg
Rv = Gas constant for moist air = 461 J/kg
T = Temperature in Kelvins

Interpretation: This equation gives the relationship between saturation vapor pressure and the temperature in Kelvins. This equation is used also to calculate relative humidity and other moisture variables.

Example problem: What is the saturation vapor pressure when the temperature is 30° C?

Answer: Convert temperature to Kelvins, 30° C = 303 K
LN(Es/6.11) = (2.453×10^6 J/kg/461 J/kg)(1/273 - 1/303)
LN(Es/6.11) = (5,321.041215)(0.003663004 - 0.00330033)
LN (Es/6.11) = 1.929801333
Es/6.11 = e^1.929801333
Es = (e^1.929801333)(6.11) = 42.1 mb"

[Pooh]

BigJon
you are right in saying pressure temperature is not a simple equation, I have been looking at programming a PLC to control superheat via a stepper motor driven valve, the mechanics are easy its just the algebraic equations required to work out the pressure temperature relationship thats making my head hurt. Luckily a freind of mine already has done the work so I am aquiring the sub routine from him and just writing the rest of the program.

Ian

[Kostass]

BigJon
you are right in saying pressure temperature is not a simple equation, I have been looking at programming a PLC to control superheat via a stepper motor driven valve, the mechanics are easy its just the algebraic equations required to work out the pressure temperature relationship thats making my head hurt. Luckily a freind of mine already has done the work so I am aquiring the sub routine from him and just writing the rest of the program.

Ian

And I programming a plc and i wont to build a routine that converts temperature to pressure any help that would be gratefull.
Thang you my friends.

[coolkid]

Kostass,

why don't you just buy a comparator????

[Kostass]

Kostass,

why don't you just buy a comparator????

Because that it will help me at programming generally. Its a program that i wont to do with this convertion. If i use the chart to convert the values i will right a lot lines of code.

[lana]

Hi there,

Get the refrigerant data from the manufacturer. At the beginning of their catalog they write all the equations. Try BOC, DuPont, ICI, ....

I got some of them. There you can find the equations with all the constants you need to solve them.

Cheers

[Kostass]

Thank you guys for your help.

[BigJon3475]

Thank you guys for your help.

Lana is the man take his advice I was just doing some research as I probably should have kept my mouth shut since I didn't know the exact answer it just seemed very complicated at least...and at most wasn't possible.

[squil]

Yes that's the kind of information manufacturers can usually give...

[US Iceman]

If you are trying to put this into a PLC the math coprocessor may choke on the equation of state listed for the various refrigerants.

A simple method is to curve fit the vapor-pressure curve of the desired refrigerant. I have found if you use an exponential fit to the 6th order you will get a very good fit for the data. The temperature to pressure comes out very nice, however, the pressure to temperature gets a little questionable. If you don't mind some error tolernace, you should be OK.

[AJS]

Hi Mr Iceman,

I am trying to do the same thing as discussed in this thread. Come up with an equation (curve fit is fine) to convert pressure to temperature for R-410A refrigerant.

I know I saw an equation somewhere that I think you posted but there are a lot of threads on here and I can't seem to find it again. I plotted the chart in excel and used a 6th order polynomial but this didn't work... seems excel equations are not accurate after the 2nd order.

Do you have an equation based on pressure that would work to convert to temperature for use in a PLC? Any help is greatly appreciated! Thanks!AJS