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Wednesday, March 29, 2006
HW #2 - P#2 - R-134a Table Fundamentals
Please post any questions about this problem as comments on this posting.
On 1 a at 30C, 200Kpa is more preasure than the saturation preasure I think that means in is a compressed liquid. But I can not find a table for 200Kpa or .200MPa where am I going wrong
On b 130C abd 270.3Kpa it is at saturation but no additional information is given on the quality how do I get the V, U, and Q.
On at 80 PSI the H at 78 is less than the sat lig H at 80 psi. I think this means it is a compessed liquid. So we need to do an interpilation. 78 doesn't show up on 100 psi table so I interpilated to find the T for 78 H at 100 PSI and found it to be 6.338. I thought that I needed to use interpilation to find the sat temp and preasure for 78 H but this gives a temp and preasure that is quite small compared to the the awnser that was given. Where am I going wrong or is my whole reasoning not on the right track.
On b there is x given so I believe that this means that there is a saturated mixture. So the awnser should be between the sat vap and sat liq values for H. H liq =80.634 and H vap = 169.07 but the awnser is 69.92 that was given. How is this possible.
OK, there is a flaw in this problem. This became clear when I tried to answer the last two comments.
Here is what we need to do: ADD 63.768 Btu/lbm to each H^ value given in the problem statement.
This difficulty is caused by a difference in reference state. We will discuss reference states in some detail in class on Wed. For now, just take my word for it.
I just corrected the HW online to reflect this change and I will send email to my class and to Prof Castner and the TA's.
I am sorry this happened, but it painfully illustrates the importance of reference states !
graham: You don't need to interpolate because this is a sat vap at a friendly T that is in the tables ! Just read the values out of the R-134 saturation temperature table.
Does the quality number only correspond to saturated mixtures? I assumed that 100% could be all vapor or superheated vapor, and 0% could be all liquid or subcooled liquid. Is this wrong?
In part c, is it safe to assume its a subcooled liquid? and if so...how do I go about solving for this one..the subcooled liquid pressures aren't small enough and the saturated pressure table gives a temperature of -29.5 degrees. I'm not sure where to begin.
jo @ 4:10 PM You don't have to assume it is a subcooled liquid. You can SHOW that it is by looking up Tsat at 70 psia. I think you'll find that Tsat > 10 degF. From this you conclude that it is a subcooled liquid.
Tsat = -29.5 degF at 10 psia, but the P in this problem is 70 psia. I think you switched the T and the P !
Interpolate between Hsatliq at 10 degF and Psat@10degF AND the H at 10 degF and the lowest P in the subcooled liquid tables.
14 comments:
Hi Lan, I just posted this anonymous comment.
On 1 a at 30C, 200Kpa is more preasure than the saturation preasure I think that means in is a compressed liquid. But I can not find a table for 200Kpa or .200MPa where am I going wrong
On b 130C abd 270.3Kpa it is at saturation but no additional information is given on the quality how do I get the V, U, and Q.
I think the 2nd comment here belongs on Hw#2, P#1. I will copy it there and then answer it.
On at 80 PSI the H at 78 is less than the sat lig H at 80 psi. I think this means it is a compessed liquid. So we need to do an interpilation. 78 doesn't show up on 100 psi table so I interpilated to find the T for 78 H at 100 PSI and found it to be 6.338. I thought that I needed to use interpilation to find the sat temp and preasure for 78 H but this gives a temp and preasure that is quite small compared to the the awnser that was given. Where am I going wrong or is my whole reasoning not on the right track.
On b there is x given so I believe that this means that there is a saturated mixture. So the awnser should be between the sat vap and sat liq values for H. H liq =80.634 and H vap = 169.07 but the awnser is 69.92 that was given. How is this possible.
OK, there is a flaw in this problem. This became clear when I tried to answer the last two comments.
Here is what we need to do: ADD 63.768 Btu/lbm to each H^ value given in the problem statement.
This difficulty is caused by a difference in reference state. We will discuss reference states in some detail in class on Wed. For now, just take my word for it.
I just corrected the HW online to reflect this change and I will send email to my class and to Prof Castner and the TA's.
I am sorry this happened, but it painfully illustrates the importance of reference states !
Your reasoning is on the right track. The problem has some issues that I just explained.
Sorry :(
For part c, are we looking at a similar situation as #1 part b, where we don't have enough information to calculate h?
Answer to 10:14 PM:
No. In 1b, the T & P corresponded to the saturation T & P, so both liquid and vapor COULD be present.
In 2c, the T & P do NOT correspond to the saturation conditions. Only one phase is present. We will learn more about why this is true on Wed.
graham:
You don't need to interpolate because this is a sat vap at a friendly T that is in the tables ! Just read the values out of the R-134 saturation temperature table.
Does the quality number only correspond to saturated mixtures? I assumed that 100% could be all vapor or superheated vapor, and 0% could be all liquid or subcooled liquid. Is this wrong?
Katie 7:58 PM:
Your assumption about x = 1 is not correct.
Sat'd Liq: x = 0
Sat'd Vapor: x = 1
Sat'd Mixtures: 0 < x < 1
Subcooled Liquid: x = undefined
Superheated Vapors: x = undefined
x = 1 means sat'd vapor.
In part c, is it safe to assume its a subcooled liquid? and if so...how do I go about solving for this one..the subcooled liquid pressures aren't small enough and the saturated pressure table gives a temperature of -29.5 degrees. I'm not sure where to begin.
jo @ 4:10 PM
You don't have to assume it is a subcooled liquid. You can SHOW that it is by looking up Tsat at 70 psia. I think you'll find that Tsat > 10 degF. From this you conclude that it is a subcooled liquid.
Tsat = -29.5 degF at 10 psia, but the P in this problem is 70 psia. I think you switched the T and the P !
Interpolate between Hsatliq at 10 degF and Psat@10degF AND the H at 10 degF and the lowest P in the subcooled liquid tables.
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