Subcooling at TEV vs Superheat at Compressor relation in Steady State
:off topic:Warning! This post title may sound more appealing than it really is! Some of you will find this post boring, disappointing, useless and hard to swallow. So if you are not Geeks, Nerds, or simply love the subject, you should leave!:rolleyes:
I only posted it because I’m a stubborn mule that sometimes does not follow wife’s recommendations.
This is an effort to take this subjet off US_Iceman post on “How to design refrigeration systems” which is not boring and worth reading, you can see here: http://www.refrigeration-engineer.co...ad.php?t=14013
If you think on expansion devices in general I suppose most will agree that if you subcool the liquid at the entrance of the expansion device, the compressor superheat gets lowered.
What happens with TEV?
Short version for transient conditions (not the main point of this post):
It is clear to most I suppose that during transient conditions the superheat at the expansion valve gets lower because what you really did was produce less flash gas and more refrigerant in liquid state entered the evap so the point of evaporation of the last drop should be nearer the bulb of the TEV than before. This means that superheat went down the valve opened in reaction but meanwhile the superheat at the compressor did go down.
Unfortunately I have to do some simplifying hypothesis because my wife demanded I leave partial derivatives and Taylor series expansion of two variables out of this! Therefore this is not a proof!
SYMPLIFYING HYPOTHESIS:
We should all agree that suction pressure goes up because heat exchange inside the evaporator was improved, because liquid refrigerant has much higher heat transfer coefficients than refrigerant vapour. So the suction pressure must go up.
This should also increase the mass flow of refrigerant you can clearly check this from any compressor manufacturer data.
I’ll take R717 as an example but this will be true for every refrigerant that has a dew point curve with positive slope when you draw a P-h diagram. This means that the dew point curve goes right and up when enthalpy increases (to the right).
Also I need to split the effect of the expansion valve and the suction line pressure drop so all changes in temperature are caused by the superheat of the expansion valve (suction line perfectly insulated) all pressure drop occurs between the bulb and the compressor suction.
Also that all changes are small, for if they were large it means other variables I’ll suppose constant are moving, like thermal load, etc.
http://www.refrigeration-engineer.co...&pictureid=212
Drawing from Refrigeration Utilities version 2,84 Department of Energy Engineering, Technical University of Denmark (great program TKS DTU!).
IN STEADY STATE THEN:
the superheat in point 3 is Temperature(3)-Saturation Temperature(3) i.e. horizontal line that projects to point X. Superheat at the new situation is Temperature(C)-Saturation Temperature(C) that projects to point Y.
From the drawing you can see that if the DewPoint line is tilted enough to the right the line Y-C is shorter to X-3.
With numbers:
I’m sure I don’t need to point this out but the calculations are:
Known (1)
Pressure(2)<-Pressure(1)
Temperature(2)=Saturation Temperature(1)+10c
Calculate Enthalpy(2) given Pressure(2) and Temperature(2)
Enthalpy(3)<-Enthalpy(2)
Pressure(3)=Pressure(2)-Pressure drop in suction line
Calculate Temperature(3) given Pressure(3) and Enthalpy(3)
Calculate Saturation Temperature at Pressure(3)
Subtract the last to numbers above.
The expansion valve will try to keep a 10 kelvin superheat. The pressure drop in the suction line is 0,1 bar, the saturated temperature at the evaporator outlet pressure is -20 centigrade. The evaporator saturation temperature goes up 1 kelvin.
I’ll take NIST Refprop standard database 23 version 8 numbers here. They will differ from other sources. Also I’ll use all decimals of course this is nonsense but math of very close numbers won’t be a problem.
Temp. Pressure Density Enthalpy Entropy
(°C) (bar) (kg/m³) (kJ/kg) (kJ/kg-K)
(vapor) (vapor) (vapor)
1 -20.000 1.9008 1.6033 1580.8 6.3757 (*)
2 -10.000 1.9008 1.5317 1604.7 6.4680
3 -10.430 1.8008 1.4510 1604.7 6.4937
X -21.211 1.8008 1.5240 1579.2 6.3944 (*)
Superheat 3 10.781
A -19.000 1.9867 1.6711 1582.2 6.3604 (*)
B -9.000 1.9867 1.5965 1606.1 6.4528
C -9.45 1.8867 1.5163 1606.1 6.477
Y -20.167 1.8867 1.5921 1580.6 6.3783 (*)
Superheat C 10.717
Now you see why I used all the decimals.
10.717<10.781 the original point was that if subcooling makes suction pressure go up then superheat goes down (you don’t have to tell me you can’t possibly measure this difference in the field).
The only trick here is that I used the same pressure difference in both cases and in the second case the pressure should go up if the mass flow went up. But if you cant measure this superheat I can assure you the difference in mass flow is much less. So the conclusion still stands.
CONCLUSION
Though not formally proved, at least acknowledge that If subcooling goes up in general superheat goes down if the refrigerant is well behaved. (Most refrigerants are).
In the field you cannot measure this difference. (This doesn’t mean it IS there).
Didn’t I tell you this was boring? You should see the real proof and for different refrigerants and try to calculate in what range this is valid, I can’t think of a better torture for a student, he’ll change careers immediately!
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
WOW! Someone has too much time on their hands. You should be helping your wife...:p
I will have to start reading this now. I'll be back....
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
Just a small question before I will read this tomorrow (just went sleeping, my wife already called where I was now again, same story everywhere allover the world with our wives) : what's a transient condition?
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
One that vanishes with time. The TEV senses low superheat then closes until it senses high superheat then opens, ok transient is all this process that ends with enough time.
Steady state is the oposite this means everything reached equilibrium again.
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
Quote:
Originally Posted by
GXMPLX
I only posted it because I’m a stubborn mule that sometimes does not follow wife’s recommendations...
........my wife demanded :eek: I leave partial derivatives and Taylor series expansion of two variables out of this!
Very interesting concept ...I'm going to read it again (4th time) to let my other brain cell absord some more.
Seems like your wife is a sensible person ;) .....by the way does she wear the trousers in your house? :D
Only a free spirit can soar through the mystery of imagination.
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
Quote:
Originally Posted by
US Iceman
WOW! Someone has too much time on their hands. You should be helping your wife...:p
I will have to start reading this now. I'll be back....
Man, you don't know her! I must!
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
GXMPLX, never dug that far in the log/P, interesting stuff for me, I'm sure others like BESC5240 likes this kind of topics.
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
Quote:
Originally Posted by
nevgee
Very interesting concept ...I'm going to read it again (4th time) to let my other brain cell absord some more.
Seems like your wife is a sensible person ;) .....by the way does she wear the trousers in your house? :D
Only a free spirit can soar through the mystery of imagination.
The probem in refrigeration systems is that not always what you think that should remain constant does. You may get systems allways in TRANSIENT CONDITIONS (changing constantly of state).
Also different variables like pressure and temperature may produce the same effects.
Some statements that are true with on refrigerant may not be with another.
I find this program from the DTU ideal to play with because the refrigerant is the blood of the system and test statements one usually says. You don't need math knowlege to use it. If you know some math you should also play with EES (Engineering Equation Solver) from www.fchart.com the program on which CoolPack and Refrigeration Utilities were made.
The problem is of course get the right information. This has improved with compressor manufacturers that give you ARI 540 data, but not from TEV manufactures.
Anyway subcooling is a hairy subject, many people think many different things about it, like refrigerants and flooded evaporators, and most of them are right in some condition, right US_Iceman?
Anyway don't tell NIST I used so many digits in the calcs or they (like my wife) will want to hang me from where it hurts! ... not that they can!
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
That's why the equilibrium diagram as mentioned in the other interesting thread now going on on this forum remains important for me. Although some don't see why you have to learn this.
Perhaps because you don't find free software to do this :D
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
Quote:
Originally Posted by GXMPLX
Anyway subcooling is a hairy subject, many people think many different things about it, like refrigerants and flooded evaporators, and most of them are right in some condition, right US_Iceman?
Subcooling and superheating are indeed hairy subjects as you say. That's why I started the design thread. For system analysis these two state points (operating conditions on a Mollier diagram) are absolutely critical to know to solve problems. And... how these affect compressor ratings is equally mysterious.
Quote:
Originally Posted by GXMPLX
Anyway don't tell NIST I used so many digits in the calcs or they (like my wife) will want to hang me from where it hurts! ... not that they can!
I'm calling NIST and telling them you did this.:p:D
Quote:
Originally Posted by GXMPLX
The problem in refrigeration systems is that not always what you think that should remain constant does. You may get systems always in TRANSIENT CONDITIONS (changing constantly of state).
Now there is a true statement.... One of the major issues with designing a refrigeration system is knowing when to make different assumptions on how something works or reacts. Assuming the system ALWAYS works in steady state conditions is one major way of making mistakes.
EES is a fabulous program. Expensive, but very good!
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
Quote:
Originally Posted by Peter_1
That's why the equilibrium diagram as mentioned in the other interesting thread now going on on this forum remains important for me. Although some don't see why you have to learn this.
Perhaps because you don't find free software to do this :D
Peter, I know you have limited time to spend on a long thread but perhaps you could start a thread for explaining the equilibrium chart?
I'll be honest and admit I have never done this before on paper. I tend to do it in my head because it's faster.
I'm sure some others would find this very useful.
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
Quote:
Originally Posted by
nevgee
Very interesting concept ...I'm going to read it again (4th time) to let my other brain cell absord some more.
Seems like your wife is a sensible person ;) .....by the way does she wear the trousers in your house? :D
Only a free spirit can soar through the mystery of imagination.
My brain cells are deep POE fried!
She does wear trousers, mine are too big, won't fit her!
She can beat anyone but doesn't so sensible? ... maybe yes!
Re: Subcooling at TEV vs Superheat at Compressor relation in Steady State
Quote:
Originally Posted by
Peter_1
GXMPLX, never dug that far in the log/P, interesting stuff for me, I'm sure others like BESC5240 likes this kind of topics.
P-h or log/P like you call it is the way to study how the system behaves.
The beauty of it is that you may do everything qualitatively (no numbers) and let you study What-if scenarios.
I used numbers only because it would be shorter than explain it in words.
I think every field engineer should know how to use these diagrams and attach them to any statement they make because it is the only way to assure it is correct. With programs like RefUtil from DTU this is not a problem.
The real problem in refrigeration systems is that many causes have the same effect and many variables acting at the same time. These diagrams let you manage this problem.
Quote:
Originally Posted by
Peter_1
That's why the equilibrium diagram as mentioned in the other interesting thread now going on on this forum remains important for me. Although some don't see why you have to learn this.
Perhaps because you don't find free software to do this
The equilibrium diagrams are a must for an application engineers that have to study how external variables like ambient or room temperature act on the system.
I consider room temperature an external variable because it depends on control settings, load and product type which are external.
They are also the correct way to estimate year round operating costs for a given demand.
