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how to increase the portion of liquid after TXV
Usually, after the thermostatic expansion valve, the R134a will change from single liquid state into two phase.
Firstly, I want to know: which should be more? liquid or vapor? after TXV. In my opinion, liquid should be much more than vapor. But I found the portion of liquid is very small (18%) after TXV, using rotor meter for flow rate measurement.
I heard about there is more vapor than liquid after using commercial TXV, because people want that there is only vapor after evaporator. In this case, compressor could work under dry and good condition.
Is it true?
If that is true, what should I do in order to increase the portion of the liquid?
Thanks a lot!
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Re: how to increase the portion of liquid after TXV
In order to liquid to evaporate (change state) it need to absorb heat. If more heat is not available right after TXV valve, there is no possibility to be more gas in liquid gas mixture.
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Re: how to increase the portion of liquid after TXV
Hi Frank721,
The ratio of gas to liquid just after the expansion valve will change depending on the temperature of the liquid feeding the expansion valve and the evaporation temperature. There is no 'correct' ratio for all conditions.
The liquid refrigerant being fed to the valve will need to be cooled to the exaporating temperature before any useful cooling can be done.
In a system evaporating at -5c and with a liquid line temperature of 55c the refrigerant will need to be cooled by 60c, in another system still evaporating at -5c and with a liquid line temperature of 15c the refrigerant will only need to be cooled by 20c, this will produce less gas at the expansion valve outlet than the 60c system despite the same evaporation temperature.
The function of the expansion valve is to ensure that only superheated vapour exits the evaporaor. The proportion of liquid gradualy getting less and less towards the evaporator outlet with the last bit of evaporator only superheating the vapour to provide a safety margin of a few degrees (5c)
Why do you think you need to increase the portion of liquid in your systems evaporator?
A good indication of your evaporators performance would be to measure the superheat at its outlet.
Jon
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Re: how to increase the portion of liquid after TXV
If you use an accumalator you can then lower your superheat(increase liquid) excess liquid collects in the accumalator and vapour is pulled back to the compressor from the top of the accumalator. The increase in liquid in turn increases heat transfer but is still restricted by the amount of heat you can draw across the coil/air flow.
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Re: how to increase the portion of liquid after TXV
Thanks a lot for your reply. I agree with you. I think there is no additional heat around my TXV. That is exactly what confused me so much.
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
monkey spanners
Hi Frank721,
The ratio of gas to liquid just after the expansion valve will change depending on the temperature of the liquid feeding the expansion valve and the evaporation temperature. There is no 'correct' ratio for all conditions.
The liquid refrigerant being fed to the valve will need to be cooled to the exaporating temperature before any useful cooling can be done.
In a system evaporating at -5c and with a liquid line temperature of 55c the refrigerant will need to be cooled by 60c, in another system still evaporating at -5c and with a liquid line temperature of 15c the refrigerant will only need to be cooled by 20c, this will produce less gas at the expansion valve outlet than the 60c system despite the same evaporation temperature.
The function of the expansion valve is to ensure that only superheated vapour exits the evaporaor. The proportion of liquid gradualy getting less and less towards the evaporator outlet with the last bit of evaporator only superheating the vapour to provide a safety margin of a few degrees (5c)
Why do you think you need to increase the portion of liquid in your systems evaporator?
A good indication of your evaporators performance would be to measure the superheat at its outlet.
Jon
Thank you very much. I want more liquid in my separator, because I want get more liquid evaporate and absorb more heat from my evaporator.
Do you know any method of increasing the portion of liquid after TXV?
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
chillerman2006
If you use an accumalator you can then lower your superheat(increase liquid) excess liquid collects in the accumalator and vapour is pulled back to the compressor from the top of the accumalator. The increase in liquid in turn increases heat transfer but is still restricted by the amount of heat you can draw across the coil/air flow.
Thank you for your reply. I also used an accumulator in my system. Currently, I have already drop the superheat to the lowest point, but I still want more liquid.
So do you have any suggestion on increasing the liquid portion after TXV?:)
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Re: how to increase the portion of liquid after TXV
I have been following this thread and wonder what the original poster is doing or thinking...
First, we have this comment.
Quote:
Originally Posted by frank721
what should I do in order to increase the portion of the liquid?
If this is a theoretical discussion on refrigerants or components you might get a different answer than if this is directed to the operation of a refrigeration system.
In an attempt to understand what is being asked an RE member asked this...
Quote:
Originally Posted by monkey spanners
Why do you think you need to increase the portion of liquid in your systems evaporator?
Then later we have, these...
Quote:
Originally Posted by frank721
I want more liquid in my separator, because I want get more liquid evaporate and absorb more heat from my evaporator.
Quote:
Originally Posted by frank721
So do you have any suggestion on increasing the liquid portion after TXV?
There is a limit on the amount of liquid you can allow in the evaporator based on the type of system and components you have.
Why do you need more liquid in the separator?
I have read these posts three times and still do not understand what the issue is.
Trying to adjust TXV's for lower superheat (more chance of liquid after TXV bulb) is very risky and should not be attempted.
frank721, can you explain why you think you need more liquid refrigerant in the evaporator?
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Re: how to increase the portion of liquid after TXV
As your requireent is not very clear but still i would suggest.
1. Increase the refrigerant qty to get more liquid at the entry of Evaporator.
2. Increase the condenser size as much as you can.
But if you expain the exact requirement then idea can be much better.
As above two idea should be implemented with accumulators at eva exit otherwise your compressor will suffer severily.
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Re: how to increase the portion of liquid after TXV
Evaporator liquid ratio can be safely increased by using two suction/liquid heat exchangers in series and mounting the TXV bulb on the suction line between them.
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
US Iceman
I have been following this thread and wonder what the original poster is doing or thinking...
First, we have this comment.
If this is a theoretical discussion on refrigerants or components you might get a different answer than if this is directed to the operation of a refrigeration system.
In an attempt to understand what is being asked an RE member asked this...
Then later we have, these...
There is a limit on the amount of liquid you can allow in the evaporator based on the type of system and components you have.
Why do you need more liquid in the separator?
I have read these posts three times and still do not understand what the issue is.
Trying to adjust TXV's for lower superheat (more chance of liquid after TXV bulb) is very risky and should not be attempted.
frank721, can you explain why you think you need more liquid refrigerant in the evaporator?
Firstly, sorry for my late reply and thank you so much. I feel so warm that you are trying to help me to solve this problem.
In a word, the reason i want more liquid in evaporator is i am doing liquid cooling with refrigerant system. So the more liquid in evaporator, the more latent heat i could use. Now, I hope you could undertand my question.
Fortunately, i did try all of your suggestions. It works now.:)
Thanks again!
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
amitsaxena
As your requireent is not very clear but still i would suggest.
1. Increase the refrigerant qty to get more liquid at the entry of Evaporator.
2. Increase the condenser size as much as you can.
But if you expain the exact requirement then idea can be much better.
As above two idea should be implemented with accumulators at eva exit otherwise your compressor will suffer severily.
Thanks a lot for your good suggestion. It works now.:)
By the way, what is the normal inlet pressure of compressor with R134a? Is it 1 bar too low?
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
Gary
Evaporator liquid ratio can be safely increased by using two suction/liquid heat exchangers in series and mounting the TXV bulb on the suction line between them.
Thanks a lot for your reply. Could you give me some more information about two suction/liquid heat exchangers? I am not quite clear.:)
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Re: how to increase the portion of liquid after TXV
Welcome again,
As you ask for pressure for 134a, Actually it solely depend upon your operating conditions as you can change the same as per you requirement by positioing the TXV bulb. Also as per your data 1 bar (-26°C) hence its operating in LBP zone which normally happens in the case of REFRIGERATORS.
Thanks
Amit
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Re: how to increase the portion of liquid after TXV
What does the compressor think about all of your liquid coming back to the suction?
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
amitsaxena
Welcome again,
As you ask for pressure for 134a, Actually it solely depend upon your operating conditions as you can change the same as per you requirement by positioing the TXV bulb. Also as per your data 1 bar (-26°C) hence its operating in LBP zone which normally happens in the case of REFRIGERATORS.
Thanks
Amit
Thanks for your quick reply. one more question, if i want to increase my inlet pressure of compressor, what could i do?:-)
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
NoNickName
What does the compressor think about all of your liquid coming back to the suction?
do you mean the liquid after evaporator? it will go back to the accumulator firstly, then evaporate and flow into compressor.:)
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Re: how to increase the portion of liquid after TXV
Dear There are two possible ways of increasing the Suction pressure
1. In the present change the TXV, go for more orifice dia
2. Increase the dis. pressure with the help of
a) Cond. size can be reduce
b) Increase the spring load in present TXV
c) Reduce the Air flow rate across the cond
d) Locate the present TXV in the Colder region.
Thanks
Amit
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Re: how to increase the portion of liquid after TXV
The quantity of refrigerant in liquid and vapour form are dependant on the load on evaporator.
Ideally more vapor is desirable otherwise liquid(un evaporated ) may enter compressor and as liquid cannot be compressed the compressor may breakdown.
Quote:
Originally Posted by
frank721
Usually, after the thermostatic expansion valve, the R134a will change from single liquid state into two phase.
Firstly, I want to know: which should be more? liquid or vapor? after TXV. In my opinion, liquid should be much more than vapor. But I found the portion of liquid is very small (18%) after TXV, using rotor meter for flow rate measurement.
I heard about there is more vapor than liquid after using commercial TXV, because people want that there is only vapor after evaporator. In this case, compressor could work under dry and good condition.
Is it true?
If that is true, what should I do in order to increase the portion of the liquid?
Thanks a lot!
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Re: how to increase the portion of liquid after TXV
Dear Mr. Sridhar,
I agree that the quantity of ref. in liquid and vapour form are dependant on the load on evaporator, but if i am not wrong then the major factor which controls the Quality(quantity of ref. in liquid and vapour form) is your system designing and balacing as If you have designed your system for a given conditions then in that condition the quality is not going to change much.
Correct me if i am wrong.
Thanks
Amit
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
frank721
do you mean the liquid after evaporator? it will go back to the accumulator firstly, then evaporate and flow into compressor.:)
Really? Amazing.
So you end up with a flooded evaporator and a frozen accumulator.
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
frank721
Thanks a lot for your reply. Could you give me some more information about two suction/liquid heat exchangers? I am not quite clear.:)
With the TXV bulb mounted between the heat exchangers, normal superheat can be maintained at the bulb (and at the compressor) while the superheat at the evaporator outlet is very low. In effect, the suction side of the second heat exchanger becomes an extension of the evaporator.
On the liquid side, going through two heat exchangers maximizes the liquid subcooling which minimizes flashing at the evaporator inlet.
Both of these factors add up to maximum liquid ratio in the evaporator without endangering the compressor.
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Re: how to increase the portion of liquid after TXV
To obtain a larger quantity of liquid after metering device... in one word...
SUBCOOLING.
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Re: how to increase the portion of liquid after TXV
I agree with US Iceman's earlier post that there is too little information and too much speculation and assumptions. More system information is required, application, compressor super heat, suct/HP pressures, evap air on/ off suction temp at evap.
And I agree with Gary and 750 valve, subcooling helps, but there again can create problems, with TEV over performing and high compressor superheat. Below system normal subcooling every two degrees of subcooling improves TEV performance by 1 %. The TEV super heat should be retested.
Are the condensers clean , has system got air in it, the possibilities are endless.
More info required.
magoo thinking out loud again.
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by magoo
magoo thinking out loud again.
You say that like it's something bad.;):D
If we worked on systems the way we answer questions, nothing would get fixed.
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Re: how to increase the portion of liquid after TXV
subcooling has another issue of superheated refrigerant at suction valve of compressor; particularly with R22. One degree drop in liquid temp will raise the suction vapor temp by about 4 degrees . Hence improving the liquid flow at TEX outlet has a balancing effect on compressor capacity ; comp capacity could drop in such an event . Subcooling can reduce flashing in TEX valve & improve system performance.
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
D.D.KORANNE
subcooling has another issue of superheated refrigerant at suction valve of compressor; particularly with R22. One degree drop in liquid temp will raise the suction vapor temp by about 4 degrees. Hence improving the liquid flow at TEX outlet has a balancing effect on compressor capacity ; comp capacity could drop in such an event.
This depends entirely upon how the subcooling is acheived. I assume you are talking about suction/liquid heat exchangers?
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Re: how to increase the portion of liquid after TXV
Thank you guys! I love this forum so much and discussing with you on refrigeration system.:)
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
Gary
This depends entirely upon how the subcooling is acheived. I assume you are talking about suction/liquid heat exchangers?
I agree with Gary, thats a pretty generalised statement you would have to be talking suc/liq HX but even then compensations can be made to offset the increased suction temp
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
750 Valve
To obtain a larger quantity of liquid after metering device... in one word...
SUBCOOLING.
do u mean that replace the TXV with a manual metering valve?
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Re: how to increase the portion of liquid after TXV
Hi US Iceman, thanks for comment, had a laugh.
But I get frustrated locally here in NZ when young techs give me half the facts, and a thousand questions.
My main gripe is the servive dispatchers that send the young guys to a call and comments like "sounds like it short of gas " or similar stupid comments, so the techs arrive with pre-conceived plans in their heads and totally miss the real problem.
Hence the saying thinking out loud, usually extends to say tell everything that is going on with system.
They refer to me as the "grumpy old sod ".
magoo
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by Magoo
...the techs arrive with pre-conceived plans in their heads and totally miss the real problem.
That is the biggest problem with training people. They want to know the answer before they understand the question!
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Re: how to increase the portion of liquid after TXV
When teaching, I emphasize the "Joe Friday" approach. That is "The facts, just all of the facts"
You cannot properly trouble shoot anything without them, so please include them when posting as this makes it easier to help.
Besides, "Psychic Network" is closed. :D:D
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
Gary
This depends entirely upon how the subcooling is acheived. I assume you are talking about suction/liquid heat exchangers?
Yes, ideally heat should be rejected out of system for sub-cooling to be effectively utilized for efficiency gain. Besides the original question about flash gas after TEX is to do with SST & DST difference , larger the difference more flash gas & viceversa . The flash gas generation is common in TEX valves.
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Re: how to increase the portion of liquid after TXV
Well this is less information...
Need more to provide some good advice as said of other member.. The easy way to do it is to move the bulb to 3 or 4 oclock... Got more liquid then but the problem is when lower the superheat is liquid special when start up the comp and we got heavy flow into compressor cause the tvx slow operations... Well just speculations from yours side i guess... More info...
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Re: how to increase the portion of liquid after TXV
Forgett to add that... To "tweak" the bulb or tvx need tank to carry the difference statement or have to add more "soup" :-).
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Re: how to increase the portion of liquid after TXV
Quote:
Originally Posted by
swecool
Forgett to add that... To "tweak" the bulb or tvx need tank to carry the difference statement or have to add more "soup" :-).
Thanks a lot for your reply. what is the meaning of soup?
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Re: how to increase the portion of liquid after TXV
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Re: how to increase the portion of liquid after TXV
Gary I like the double heat exchanger.
I know adding one heat exchanger will lower the suction pressure what does the double do?
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Re: how to increase the portion of liquid after TXV
EH? Firstly suction to liquid line heat exchangers are just a slosh pot to ensure there is no liquid in the suction. Install a proper seperate subcooler and have it working correctly will not effect the compressor capacity at all. The expansion valve capacity will increase. And oh yeah insulate your liquid line!!!