REL

28-11-2003, 08:27 AM

I am having a bit of trouble with the PH diagrams. I posted information on another forum, however, interest level is really low on such things there. I am working from the RSES manual 2 and Dossat's book.

I thought I had a pretty good handle on it, but I hit a road block when I started using Dossat's. I am really trying to self improve and just started off with the book like a green newbie page one.

Here is a copy of what I have posted and any help is appreciated. If you do not mind keeping your thoughts in IP instead of SI, I also would appreciate that as I do not yet have a handle on the conversions, working on that too :)

R134a

100 degree saturation temp, outlet of the condenser is point A with no subcooling.

This gives you

p = 138.83 psia

T = 100 F

h = 44.23 Btu/lbm

s = 0.89848 Btu/lbm Degree F

v = 0.339 cf/lbm

!

S is absolute entropy

s is specific entropy (lowercase)

sm specific per unit mass

Now ASHRAE list r134a @ 100 degrees at sf (f is subscript for liquid) .09142 and sg (g is subscript as well) at .21868 both are specific and subtraction results in 0.12726

Since sm = (1-X)(sf)+(x)(sg)

Where X is percent quality

So now I have to solve for X not sm since they gave me sm, my freakin brain hurts!

s = 0.89848 = (1-X)(.09142)+(x)(.21868)

Ok, Back at the book a bit indicates there is perhaps an error in the book, but my confidence is NOT high about that so I will explain.

If you change the temperature to 40 with 15% quality you have:

sm = (1-X)(sf)+(x)(sg)

sm = (1 - 0.15)(.05359)+(0.15)(0.22172)

sm = 0.788 Btu/lbm degrees F

To me, that states the entropy of R134a entering the evaporator with an 85/15 mix has an entropy of 0.788Btu's per pound of mass.

If that thinking is correct, then No number can replace X and come out with the result of the first problem as point A on the PH chart is right at the point of 100% liquid, with no subcooling.

I thought I had a pretty good handle on it, but I hit a road block when I started using Dossat's. I am really trying to self improve and just started off with the book like a green newbie page one.

Here is a copy of what I have posted and any help is appreciated. If you do not mind keeping your thoughts in IP instead of SI, I also would appreciate that as I do not yet have a handle on the conversions, working on that too :)

R134a

100 degree saturation temp, outlet of the condenser is point A with no subcooling.

This gives you

p = 138.83 psia

T = 100 F

h = 44.23 Btu/lbm

s = 0.89848 Btu/lbm Degree F

v = 0.339 cf/lbm

!

S is absolute entropy

s is specific entropy (lowercase)

sm specific per unit mass

Now ASHRAE list r134a @ 100 degrees at sf (f is subscript for liquid) .09142 and sg (g is subscript as well) at .21868 both are specific and subtraction results in 0.12726

Since sm = (1-X)(sf)+(x)(sg)

Where X is percent quality

So now I have to solve for X not sm since they gave me sm, my freakin brain hurts!

s = 0.89848 = (1-X)(.09142)+(x)(.21868)

Ok, Back at the book a bit indicates there is perhaps an error in the book, but my confidence is NOT high about that so I will explain.

If you change the temperature to 40 with 15% quality you have:

sm = (1-X)(sf)+(x)(sg)

sm = (1 - 0.15)(.05359)+(0.15)(0.22172)

sm = 0.788 Btu/lbm degrees F

To me, that states the entropy of R134a entering the evaporator with an 85/15 mix has an entropy of 0.788Btu's per pound of mass.

If that thinking is correct, then No number can replace X and come out with the result of the first problem as point A on the PH chart is right at the point of 100% liquid, with no subcooling.