Flash gas calculation software

[nike123]

I found this (http://www.irc.wisc.edu/?/file&id=64) handy software for calculation of flash gas ratio.

And this is continuation on discussion about software in this thread (http://www.refrigeration-engineer.com/forums/newreply.php?do=newreply&p=135173).





http://www.refrigeration-engineer.com/forums/picture.php?albumid=117&pictureid=704


As you can see the % volume is 100% but the mass is only 53%.
Something is wrong here. It does not really compute.


To me it is still not something unusual because that 100% could be easy 99,99% and software rounded it to 100%.
Volume of gas is many time greater than volume of liquid so it could be that only 0,01% of total volume is liquid with 53% of mass ratio.

Specific volume of R134a gas at 2psi is 1,16070m^3/kg and R134a liquid specific volume at 2psi is 0,00066m^3/kg.
Ratio is 1/1758,636 and by this, it makes perfect sense to me that it could be 99,99 volume of gas and only 0,01 volume of liquid while mass of gas is 53%.

I made some calculations and I wish that someone confirm that this is OK calculation.

If we have 530 gr. of gas and 470 grams of liquid that give us 0,615171m^3 of gas and 0,00040601286 m^3 of liquid.

0,00040601286/0,615171=0,00066 volume ratio of liquid

0,00066*100=0,06% percentage ratio of liquid

100%-0,06%=99,94% percentage ratio of flash gas

How is software came to mass ratio is something I don't know. Is this obtained from P-h Chart?





I can explain why this happens in EES if you wish and it may be an intersting topic but its not what I seek at the moment.
If I am able to folow you, (I dont have faculty education) I will be glad to see why is that the case.

[Chef]

If I am able to folow you, (I dont have faculty education) I will be glad to see why is that the case.

Well it seems you dont need the faculty part as your reply is very detailed. I will need some time to absorb them but they are fine.

I will post a response tomorrow as at the moment the sails need changing and that takes a while but later when I get in and can sit down in the calm, hopefully! I will research this.

Chef

[Chef]

If we have 530 gr. of gas and 470 grams of liquid that give us 0,615171m^3 of gas and 0,00040601286 m^3 of liquid.

0,00040601286/0,615171=0,00066 volume ratio of liquid

0,00066*100=0,06% percentage ratio of liquid

100%-0,06%=99,94% percentage ratio of flash gas

How is software came to mass ratio is something I don't know. Is this obtained from P-h Chart?
.

When you first look at it and see 53% by mass and then see 100% by volume the first instinct is to say “that cannot be right” and ignore the results.

But your analysis is correct and it is a truncation of the real number because there are not enough decimal places in the result.

I wrote the program in EES to check it out and got a value of 0.0486% ratio of liquid which would be the same as your 0.06%. Your value is slightly higher because you used the 53% of the mixture is gas and in fact this number is also truncated and rounded up.

The value of gas as a percentage should read 99.9514. If only the programmers had added a couple of decimal places it would have saved many hours of yours and my time analysing it. Still I now have a new program and it was an interesting learning curve.:)

The way EES works is to compile all of the equations that have been entered and to solve them simultaneously using an initial guess and then iterate the solution to get closer and closer to the answer. On a fairly simple program like the Flash Gas it can solve it easily and has no residual error on any of the variables.

When you give it a more complex problem with say 4000 or 5000 equations it tries to manipulate the values of each variable to come up with a solution. Sometimes it gives you what appears to be a good answer and one you might expect so you go along with it but to solve the problem it may use some strange values. For instance it may choose the quality X as -1.93E27 to allow the other variables to converge. All seems well but unless you check each and every variable for a reasonable value it can give false results. It does not know that X can only be between 0 and 1 and relies on the writer to include code to check this and force it be in the range, then it works, its just one example.

I spent months using the results of a program that gave excellent results until was pushed near the limits of the two phase zone and it went a bit wonky, after much time the problem was solved but now I am much more wary of the results of any program.

But it is a truly wonderful program and I suggest you try it. You can download a demo version from FChart to try it out – it is well worth the effort to learn it.

Chef

[nike123]

OK, thanks for your input and confirmation of my results.

I will try EES.

[nambkdn]

Thanhk you. This software is well.