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How to assign thermodynamic properties after splitting two phase into gas and liquid
Looking at a simple TXV system. Lets assume after the TXV we seperate the gas down one pipe and the liquid down another. The question is how do we assign the correct thermodynamic properties to each fluid when it gets to end of its pipe.
The gas we want to do work on like heat it or compress it but before we can see where it ends up and predict its temperature and pressure we need to know from where we started? So do we move the gas out of the middle of the bell on a PH diagram to its edge and similarly do we move the liquid back to the saturated liquid line to gets its new properties?
Simply jumping out of the bell interior means that the enthalpy and entropy both make huge changes without any process being done. Hmmm
Your thoughts welcome
Chef
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
Chef
Looking at a simple TXV system. Lets assume after the TXV we seperate the gas down one pipe and the liquid down another. The question is how do we assign the correct thermodynamic properties to each fluid when it gets to end of its pipe.
The gas we want to do work on like heat it or compress it but before we can see where it ends up and predict its temperature and pressure we need to know from where we started? So do we move the gas out of the middle of the bell on a PH diagram to its edge and similarly do we move the liquid back to the saturated liquid line to gets its new properties?
Simply jumping out of the bell interior means that the enthalpy and entropy both make huge changes without any process being done. Hmmm
Your thoughts welcome
Chef
Gents, to save time & make ease of reading, Jump to; # 103
Just drop the txv & sounds like flooded plate heat exchangers with low pressure vessel, chilled water system, if you plot that on a chart it will come back as you described,
(other than that I am lost for what your trying to acheive)
The liquid remains in the heat exchangers, with some in the vessel, then the boiled of vapour is piped from the top of the vessel back to compressor
You can not get much work out of the vapour compared to the liquid, transfered to vapour
What are we trying to acheive ? :confused:
R's chillerman
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
i posed that question to "ask jeeves.com"(other similar sites are available)and the reply was-if u chop out the tev then each seperated pipe is going to p*ss out refrigerant.
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
install monkey
i posed that question to "ask jeeves.com"(other similar sites are available)and the reply was-if u chop out the tev then each seperated pipe is going to p*ss out refrigerant.
I dont know for sure, you can not believe everything you hear on the net, (as tex found out) just asked the mrs though & she has confirmed "you would loose the gas & some oil"
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
(other than that I am lost for what your trying to acheive)
R's chillerman
So I split the gas and liquid but what h and s values are then given to each seperate flow. In a simple example lets have the evap at 1 bar and x=0.3
The gas/liquid is at h=231 and s=1.13 but now rip away the gas and suddenly it has a value of h=384 and an s=1.75 but we have not done anything except seperate the flows and we see a huge change in properties?
This means the gas/liquid together has one set of properties but when seperated they both have differant set of properties. What are the rules for reassigning the properties to each of the seperate streams.
Simply I want to know where to plot the 2 seperate streams on a PH diagram.
Chef
install monkey - seems Jeeves is way up to speed on this topic eh!
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
Chef
So I split the gas and liquid but what h and s values are then given to each seperate flow. In a simple example lets have the evap at 1 bar and x=0.3
The gas/liquid is at h=231 and s=1.13 but now rip away the gas and suddenly it has a value of h=384 and an s=1.75 but we have not done anything except seperate the flows and we see a huge change in properties?
This means the gas/liquid together has one set of properties but when seperated they both have differant set of properties. What are the rules for reassigning the properties to each of the seperate streams.
Simply I want to know where to plot the 2 seperate streams on a PH diagram.
Chef
chef
I am struggling here with what you mean & what your trying to achieve
As far as plotting the cycle on a chart, your vapour is to the right, your saturated vapour is in the middle & the liquid is to the left
but I cant see how this can be plotted without combining temperatures with pressure or why you would want to do this
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
A system needs to be balanced and an equilibrium from the energy transfering into an evaporator needs to balance out with the energy leaving the condensor, if you remove all vapour, what are we doing pumping neat liquid round a system (& as you mentioned) no work is being done
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Energy can niether be created or destroyed just transfered from one form to another, with this in mind and my previous posts combined
The wheel has been created and the rules of thermodynamics are set in stone
My question is what are we trying to acheive ? (the impossible)
I dont understand :confused:
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Come on chef
not many are like me & dont care what others think !
am not at this level, not at design level even
and do not see what benefit it is
this could be here a while as 60+ views already & nobody has a clue
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Hi Chef, good to see you are looking into our little problem.
As far as enthalpy goes, if the vapor is just that vapour (full seperation) then the enthalpy will be on the saturation point of the curve. But how is entropy effected.???????
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
Come on chef
not many are like me & dont care what others think !
am not at this level, not at design level even
and do not see what benefit it is
this could be here a while as 60+ views already & nobody has a clue
The question is not based upon what you would expect!
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Well chef
I could repeat all of what I have now been told & pretend I fully understand
But I have to work through some of it
I'll let you know I have been put on the right track though :)
utilizing the flash gas potential for generating flow through an evaporator
gaining maximum velocity through the evap coil which makes it 1 to 1 efficient
r's chillerman
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
well chef
i could repeat all of what i have now been told & pretend i fully understand
but i have to work through some of it
i'll let you know i have been put on the right track though :)
utilizing the flash gas potential for generating flow through an evaporator
gaining maximum velocity through the evap coil which makes it 1 to 1 efficient
r's chillerman
---------no--------
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Well Mr Chef
I shall thank you for bringing this to my attention
I have and will continue to gain from this
As I said earlier I am not at this level
But will be following the progress & eagerly await the outcome
To see what can be acheived at such a level, I am yet to fully understand
And thankyou Mad for setting me straight, I can sleep now today
Good night Gents
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
mad fridgie
Hi Chef, good to see you are looking into our little problem.
As far as enthalpy goes, if the vapor is just that vapour (full seperation) then the enthalpy will be on the saturation point of the curve. But how is entropy effected.???????
Mad
Maybe 4:00 here, I'm not sleeping till I have exhausted my understanding or lack of :)
Trying to understand here full seperation
And by this I mean on the chart the point of seperation I can see
With (as you said) enthalpy following the curve of saturation/vapour
enthalpy tails off at the point of the opposite curve saturation/liquid
So (correct me) this point where enthalpy tails of is the point of entropy
The point of diss-order prior to order and the point at which you need to achieve full seperation
If this is correct, how do you physically get to this point to acheive seperation in the first place ?
And how would this be put to use ?
What sort of system (design) if possible to get to this point & what would this acheive ?
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Firstly read the first question, there is no mention of an evaporator.
So without thinking any further than what I say next
We have an open flash economiser vessel, liquid in, liquid out vapour out. what are the properties of all. Are there any strange things happening???
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
mad fridgie
We have an open flash economiser vessel, liquid in, liquid out vapour out. what are the properties of all. Are there any strange things happening???
There is no stage of saturation
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
There is no stage of saturation
????????????????????
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
There is no stage of saturation
No there must be (surely) a point of saturation, just this is somehow minimal as you have found a way for full seperation
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
No there must be (surely) a point of saturation, just this is somehow minimal as you have found a way for full seperation
Yes seperation is an issue, but not the issue in hand (it must happen in an open flash vessel anyway) We are onlt talking about internal energy, (law of conservation)
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
mad fridgie
????????????????????
There is stage of saturation, the point of entropy, where diss order becomes order once more , the point at which you achieve full seperation ??
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
I am following I am just trying to work out where we are tranfering the energy from and to - to enable enough to achieve this full seperation
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
high pressre liquid in (from cond) reduce pressure, what happens?
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
The liquid entering the flash econimiser has a specific heat/energy & at the point of seperation you are transfering the heat/energy from the liquid into the vapour, in effect subcooling the liquid
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
The liquid entering the flash econimiser has a specific heat/energy & at the point of seperation you are transfering the heat/energy from the liquid into the vapour, in effect subcooling the liquid
Not quite, apart from liquid head there is no sub cooling, and you are converting some liquid into vapour to keep the energy balanced. What then are actual properties of each exit stream. This is the question in hand?
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
The liquid entering the flash econimiser has a specific heat/energy by flashing of some lquid into vapour, you lower the pressure/temperature of the remaining liquid transfering the heat/energy to the vapour, subcooling the remaining liquid...
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
The liquid entering the flash econimiser has a specific heat/energy by flashing of some lquid into vapour, you lower the pressure/temperature of the remaining liquid transfering the heat/energy to the vapour, subcooling the remaining liquid...
You are cooling the liquid by vapourizing, the pressure has dropped, so the liquid is at saturation, hence is not sub cooled.
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
mad fridgie
We have an open flash economiser vessel, liquid in, liquid out vapour out. what are the properties of all. Are there any strange things happening???
Liquid in - saturated liquid
liquid out - liquid only
vapour out - vapour only
neither are superheated or subcooled
& therefore
to go back to the question from Chef - #5
They would both be plotted on a ph chart along their respective lines at the point of saturation
???????????
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
chillerman2006
Liquid in - saturated liquid
liquid out - liquid only
vapour out - vapour only
neither are superheated or subcooled
& therefore
to go back to the question from Chef - #5
They would both be plotted on a ph chart along their respective lines at the point of saturation
???????????
Yes one would think so?????, unless someone can show something else??? and this is all chef was asking
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Thanks Mad
;)
I understand what you mean at last
Talk about - I beat around every bush first & need spoon feeding
Thanks Chef, if you had not have posted that question, there would still be that gap in my knowledge
;)
(But scrolling back up - no-one else came along and attempted it did they)
Back on track
So what you's were looking for was not me butting in, but someone on par with yourselves, who maybe could have come up with that something else that may or may not be there ?
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
no butting in is good, if you do not ask, then you will not get an answer. No one knows everything,
" the more I know, more I know, I know nothing"
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
mad fridgie
no butting in is good, if you do not ask, then you will not get an answer. No one knows everything,
" the more I know, more I know, I know nothing"
Yeah, thanks Mate
Got a new signature too now
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
The methodology for solution will very much depend on the two-phase model you select. Flow regime for two-phase flow can be rather interesting & the theory is far from settled.
With different flow regimes, comes different levels of phase interaction.
So, pick a suitable model, or models, predict the flow regime & determine the appropriate properties.
I have a few very useful references to hand. If you pm me, with your current e-mail address, I'll get these onto you.
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Chillerman, Mad is right and the whole question is just about how to get the properties of the split streams. If I read the previous posts correctly it seems it should be possible to simply move the 2 streams to their respective lines on the bubble but does that meet the no work and no heat input criteria is met. So I propose this:- (which is way too complicated for a Sunday morning I should add)
As no work or heat has been applied to either of the separated streams we can write the state function as
dS=dQ-dW
and the process as dh=dQ-dW+pdv+vdp
As dQ the heat input is zero and dW the work done is zero the dS must be zero
So x*S4-S3+S5-x*S5 = 0
And as both p the pressure and v the volume are constant dh can be set to zero so there is no gain or loss of enthalpy
So m*(h4-h3)-m*(1-x)*(h4-h5) = 0
Where h3 is the enthalpy at a quality of x
And h4 is the enthalpy at the gas line
And h5 is the enthalpy at the liquid line
Also S3, S4 and S5 are the entropy at the same locations.
This boils down to the fact if these state functions are equal to zero then we can indeed split the gas out and position directly on the gas line whilst the liquid must then be plotted on the liquid line with the caveat that both streams must have their respective masses carried with them and new processes can then be started.
But I am still mystified why the gas can have its entropy changed without work or heat input?
Chef
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
desA
The methodology for solution will very much depend on the two-phase model you select. Flow regime for two-phase flow can be rather interesting & the theory is far from settled.
With different flow regimes, comes different levels of phase interaction.
So, pick a suitable model, or models, predict the flow regime & determine the appropriate properties.
I have a few very useful references to hand. If you pm me, with your current e-mail address, I'll get these onto you.
Thanks for the offer of the 2 phase papers but we really need to get to the pure gas and liquid states and the 2 phase episode is already past. There seems to be very little referance to this problem where ever I have looked.
Chef
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
Chef
Chillerman, Mad is right and the whole question is just about how to get the properties of the split streams. If I read the previous posts correctly it seems it should be possible to simply move the 2 streams to their respective lines on the bubble but does that meet the no work and no heat input criteria is met. So I propose this:- (which is way too complicated for a Sunday morning I should add)
As no work or heat has been applied to either of the separated streams we can write the state function as
dS=dQ-dW
and the process as dh=dQ-dW+pdv+vdp
As dQ the heat input is zero and dW the work done is zero the dS must be zero
So x*S4-S3+S5-x*S5 = 0
And as both p the pressure and v the volume are constant dh can be set to zero so there is no gain or loss of enthalpy
So m*(h4-h3)-m*(1-x)*(h4-h5) = 0
Where h3 is the enthalpy at a quality of x
And h4 is the enthalpy at the gas line
And h5 is the enthalpy at the liquid line
Also S3, S4 and S5 are the entropy at the same locations.
This boils down to the fact if these state functions are equal to zero then we can indeed split the gas out and position directly on the gas line whilst the liquid must then be plotted on the liquid line with the caveat that both streams must have their respective masses carried with them and new processes can then be started.
But I am still mystified why the gas can have its entropy changed without work or heat input?
Chef
Afternoon Chef
Thanks for such a detailed answer to work with,
(This is going to take me all day to get my head around as I have to reference all the letter symbols first, before i can attempt to work out what you mean.)
Morning MF
But I will, you's have me hooked now.
side tracking slightly away from the original reason for posting as i always look at the final acheivement
the research that you's are doing is for the design of 'A'
the problem i have is where they (liquid/vapour) end up and thus system balanceing
the component that does the system work has a limited ability
with this in mind
the more work it does removing the vapour from 'A'
the less work it can do in the usual way
ideally the liquid will be subcooled before it leaves 'A' moving it away from the bell
but it appears to me that this will reduce overall system ability to cool
and there is a fine line of how much vapour can be removed from 'A' without this reduction
I know you's must have already considered this so,
Just asking, can enough vapour be removed from the liquid to increase cooling without loosing cooling ability in the conventional way ?
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Chef is in Fiji (Lucky bar stewerd), would I be working on refrigeration fomulures, or sucking dowm a Pina Colarda?
Good on chef for making the wrong choice.
CM, this not the process only the very beginning
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Quote:
Originally Posted by
mad fridgie
Chef is in Fiji (Lucky bar stewerd), would I be working on refrigeration fomulures, or sucking dowm a Pina Colarda?
Good on chef for making the wrong choice.
CM, this not the process only the very beginning
Bar Steward, :confused:
When your hooked on info/solutions - yes the right choice!
All I could think of last night during dinner - when most would be thinking of serviceing:)
Oh well I will have to wait until your in a position to safely reveal
Am even more entreagued now - as i was thinking it could only be in one position on a conventional system, obviously much more to it,
Hope it all goes well next month :)
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
Your were right Mad and so we went to the sun deck and had a couple of Fiji Golds and the watched the world cup final and saw Brazil win 3 - 2. Exemplorary. Good input.
So the last test seems to be that h5-h3-x(h5+h4)+2xh3 must tend to zero to make
dh=d(S+pv) true.
But for this to happen it makes it an isochoric process and that then means that the gas portion at the outlet of the TXV already has the properties of the gas at the gas line. It is therefore only the liquid that holds the core thermodynamic properties. ie it does not matter what you do with the gas as it cannot affect the liquid except for normalising the mass if one removes it. Interesting!
How to calculate this on the fly and use it is a distant goal at the moment. But without it how can we go to stage 2.
Knickers, we are going back to the sun deck for some more Fiji Gold.
Chef
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Re: How to assign thermodynamic properties after splitting two phase into gas and liq
World cup???? Brazil???? Soccer???? (surely not that far behind the times????????)
Golds! yummmmmmmmmmmmm!
So it would seem that all we need to know at this point, is the % by mass of the lquid and vapour streams at the relevent saturation points?