R22 Temp To Pressure Formula

[SteinarN]

Is this formula derived from Excel, Iceman?

[US Iceman]

Is this formula derived from Excel, Iceman?

Hi,

No it was generated with the EES program Josip mentioned in one of his posts. There will be some minor deviations in the data since this is very hard to curve fit.

To see if it meets the requirements the equation can be plugged into Excel and copied down a column. Put the pressures in a column to feed the equation and then check the generated numbers (temperature) to the pressures in a PT chart.

This should be within 0.5 degree (in F scale) or less as an approximate average. At the low end (around -25°F and below) the deviation jumps up to about 1-2°.

It's not exact, but it should be close enough.

[Josip]

Hi, US Iceman

Well it seems everyone is insisting on doing this the hard way. Try this...


.....Edit: this applies to R-22 only!!

.... seems it is in our nature

.... then again convert it to *C

Thanks US Iceman, (that sausage is too loooong) I will stick with charts ...faster and seems even more accurate.....


Best regards, Josip

[lwarren]

Wow, I never figured it would be this hard to get this formula. I would certainly never have even got close to figuring this out.

If your formula is as close as you say UsIceman, that will be more than close enough for me.

Thanks everyone for all of your help.

[US Iceman]

You can get something comparable to this using Excel also. I just happen to use EES more often and can get it done faster.

I could probably fine tune the equation for better accuracy if the range were not so large. There are other ways of doing this also, but this was the fastest, since I'm working for free!

[frank]

Looks like a simple Polynomial equation to 5 places

[lwarren]

US Iceman-

Well I am sure it is something I am doing wrong, but I cannot get your formula to come out right. I have even entered it in several online formula calculators and it does not come out right there either.

[jahangir.ahmad]

mmmmmmmmmmmm nice
thanks a lot josip nice websites

[US Iceman]

US Iceman-

Well I am sure it is something I am doing wrong, but I cannot get your formula to come out right. I have even entered it in several online formula calculators and it does not come out right there either.

I checked it before it was posted.

If you copied the equation into a cell in Excel an adjacent cell would be the pressure in psia. For example, copy the equation into cell A4.

=-81.3681513+3.20065359*A3-0.0381813098*A3^2+0.000296408154*A3^3-0.00000129284314*A3^4+0.00000000290447694*A3^5-2.61193736E-12*A3^6

A3 would be the input pressure in psia. The value calculated in cell A4 would be the saturation temperature for that pressure listed in cell A3.

If the equation is not working then you are either using gauge pressure or the decimal point was in the wrong position in interpreting the engineering notion used in the earlier post.

Copy the equation above and try it again.

[frank]

I checked it before it was posted.

=((-81.3681513)+(3.20065359*A3)-(0.0381813098*A3^2)+(0.000296408154*A3^3)-(0.00000129284314*A3^4)+(0.00000000290447694*A3^5)-(2.61193736E-12*A3^6))

A3 would be the input pressure in psia. The value calculated in cell A4 would be the saturation temperature for that pressure listed in cell A3.

Copy the equation above and try it again.

It just needed some brackets adding US Iceman. Works fine now. Straight forward Polynomial

[US Iceman]

Ooops.

Well, that's another possibility also frank. I did not think about operator precedent.

In Excel the equation worked just fine as posted, but the precedence of the math operations (addition, subtraction, etc.) could have been over ridden and caused some real issues also.

It is always fun trying to find these sort of errors in Excel. They will absolutely drive you nuts. However, one way of watching this is in the formula bar in Excel. When you place the cursor in the formula bar, the brackets change to different colors to show the matching bracket location.

[frank]

When you look in the 7th segment (2.61193736E-12*A3^6) you can note that your eqation has an Error-12 but when you copy and paste the formula into Excel (or OpenOffice Calc which I've just started using) the error disappears

Don't ask me to explain that one though

Anyone else use OpenOffice?

[US Iceman]

E-12 is simply engineering notation. The negative 12 simply means the decimal point should move 12 places to the left. In effect the number becomes very small.

If the notion had shown E12, the decimal point moves 12 places to the right. Therefore the number gets very large.

I suspect this is what the Doh! was all about, but thought I should mention this for others who might not be familiar with this.

[lwarren]

Thanks guys for your time and help, and special thanks to you US Iceman for producing the formula.

I will give it another try.

[US Iceman]

How can I send you the invoice?








Just kidding.

[frank]

I suspect this is what the Doh! was all about, but thought I should mention this for others who might not be familiar with this.

No my friend, this was because I clicked on the reply button twice and double posted.

Thanks for the explanation of the E-12 though - as they say, you learn something everyday.

[lwarren]

Ok, let me ask some more stupid questions because I am still not getting this to work right. Your first posted equation is like this:

Temp=-8.13681513E+01+3.20065359E+00*Press-3.81813098E-02*Press^2+2.96408154E-04*Press^3-1.29284314E-06*Press^4+2.90447694E-09*Press^5-2.61193736E-12*Press^6

Your second posted equation starts out like this:

Temp=-81.3681513E The decimal has moved one place to the right. I tried it with both with unsucessful results, but which one is correct?

Keep in mind I am trying to put this in a custom application that is java based, not excel.

[US Iceman]

-8.13681513E+01 = -81.3681513

3.20065359E+00 = 3.20065359

3.81813098E-02 = 0.0381813098

2.96408154E-04 = 0.000296408154

etc, etc.

If you are doing this like above, then the simple answer may be Java can't do math equations like this, or, something else is wrong with how the equation is being formatted in the code.

This works in Excel, it works in Visual basic or any other code I have worked in, but I can't comment about Java. Sorry.

[lwarren]

Fair enough, just wanted to make sure I had all of the entries correct.

If anyone is up on java here is the formula so far:

double a = -81.3681513;
double b = 3.20065359;
double c = 0.0381813098;
double d = 2;
double e = 0.000296408154;
double f = 3;
double g = 0.00000129284314;
double h = 4;
double i = 0.00000000290447694;
double j = 5;
double k = 0.000000000000261193736;
double l = 6;
double aa = (b*A3);
double bb = (a+aa);
double cc = (c*A3);
double dd = Math.pow(cc,d);
double ee = (bb-dd);
double ff = (e*A3);
double gg = Math.pow(ff,f);
double hh = (ee+gg);
double ii = (g*A3);
double jj = Math.pow(ii,h);
double kk = (hh-jj);
double ll = (i*A3);
double mm = Math.pow(ll,j);
double nn = (kk+mm);
double oo = (k*A3);
double pp = Math.pow(oo,l);
double qq = (nn-pp);

getOut().setValue(qq);

[lwarren]

Well through the help of someone much smarter than me, It is finally working in my program. It is very accurate from 0- 300 psia,and psig.

Thanks to all who contributed here.

[shooter]

my plc has two types of tables inside
1 for linear and one for 10 points.
and yes i did use the formulas and checked them against the ashrae books.
the formula of iceman is the one described in ashrae.
because calculating is difficult in a plc and specially when converting from one type to the other now i am using tables again. is faster as the calculation and as we only use a small portion of the big graph better precision as the formula.
i do have all these formulas in SI not in us sorry

[SteinarN]

I have figured out a formula to convert absolute bar to degree Kelvin for R404 a. It is mostly within 0,5C accuracy between -47C and +18C. It is probably possible to get it even more accurate and extend the scale by adding several more + or - (P upx x x.x)

P is the pressure in absolute Bars. The result is degree Kelvin.
The formula looks like this:

Log(P;5)x32+226,2+(P up1.8x0.58)-(P up3x0.0187)

[TXiceman]

ASHRAE published the Equations of State for refrigerants. It is a lot easier to look there rather than doing curve fitting to data.\ unless you like the math and computer exercise.

Ken

[SteinarN]

I couldn't find the ASHRAE publication. I didn't find a curve fitting function in my open office calc either.

However after I realised I had to start with a log function it's actually quite easy to extend and adjust the formula.

I have time to spare and I sometimes enjoy ponder on different mathematical challenges. And this time I did learn something in the end

[TXiceman]

I searched and it may no longer be produced. I have an old copy somewhere....but can't locate it. Try contacting the refrigerant vendors as they did have this data available at one time as well.

Ken

[shooter]

I do have all the formulas in my computer for most of the refrogerants.

[SteinarN]

shooter, could you share the R404A formula with me?