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    Smile 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.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

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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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    Smile Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by monkey spanners View Post
    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 View Post
    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?
    If all else fails, ask for help.


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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 View Post
    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 View Post
    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 View Post
    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 (-26C) 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 View Post
    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 (-26C) 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 View Post
    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 View Post
    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 View Post
    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 View Post
    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.
    Last edited by Gary; 12-05-2009 at 03:25 PM.

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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.
    ...and she said "give it to me you big fridgie"

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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.

    If we worked on systems the way we answer questions, nothing would get fixed.
    If all else fails, ask for help.


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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 View Post
    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 View Post
    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
    ...and she said "give it to me you big fridgie"

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    Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by 750 Valve View Post
    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!
    If all else fails, ask for help.


  33. #33
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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.

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    Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by Gary View Post
    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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    Smile Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by swecool View Post
    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

    Great post

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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!!!
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    Re: how to increase the portion of liquid after TXV

    750 valve got the temp rite. If you'll excuse the pun. In order to limit flash gas and increase the percentage of liquid in the mixture after the TX valve you will need to subcool the liquid entering the TX valve first. The energy penalty will be paid at the compressor, unless you are using a scroll with PHE and vapour injection or screw with side port as this will not increase your mass flow into the compressor, being injected after the inlet and will only increase the mass flow leaving the compressor. Using Liquid to suction heat exchangers dont help a great deal for 2 reasons. You need a large quantity of suction vapour to cool liquid and the increase in suction temperature is an energy penalty at the compressor.In other words what you gain at one point you lose at the other. I regard to the VI swcroll or screw, you will not gain a great deal from the evaporator in cooling effect percentage wise, but you will gain by needing a smaller compressor to do the same job.

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    Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by HallsEngineer View Post
    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!!!
    Additional sub-cooling will increase the enthalpy difference across the evaporator & generally lower the evolved refrigerant mass-flow. This will affect the compressor throughput & efficiency.

    Be careful not to pull the entry vapour fraction (x) into the evap too far outside the 0.2 < x < 0.3 startup operating window.

    Danfoss have a good technical paper about deep sub-cooling.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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    Re: how to increase the portion of liquid after TXV

    Firstly sub-cooling has nothing to do with the compressor, the compressor no's nothing, it is only interested in the inlet pressure and temperature and the outlet pressure and the ambient if you want to go to the finer details.
    Sub-cooling effects the rating of the TXV and the performance of the evaporator.
    We will focus on the evap.
    At the inlet of the evap (after TXV) you have a mixture of liquid and vapor, as a percentage of mass, this is simpley based upon liquid inlet temp into TXV and pressure at evap inlet.
    What is of more interest is the mixture by volume. Under normal working conditions (no special liquid sub cooling) you will always have a much higher % of vapor.
    It is this vapor which causes the pressure drop through the evap and reduces the amount of liquid in contact with the wall of the tubes.
    These two effects reduce the overall performance of the evaporator.
    In amny case the pressure drop across the evap can be quite high (thus the need for external equalised valves) So the actual refrigerant temp at the inlet is somewhat higher than the theoratical outlet temp. To absorb the required amount out of the evap, the outlet pressure must be somewhat lower than expected. By Sub-cooling your liquid, you reduce vapor content both mass and volume, reducing the pressure drop, increasing the wetted surface area, and rising the evap outlet pressure, for the same amount of energy absorbed.
    Re; suction/liquid heat exchangers, if the TXV bulb is fitted after one these heat exchanger, it could be said that the refrigerant leaveing the evap is wet, thus getting full use of the evap, verses an evap which has to incur superheat. Vapor is a very poor heat transfer medium. Again this will reduce the pressure drop through the final stages of the evap.
    You how ever need to carefull that you do not over sub-cool (close maximum) as you will end up with no vapor and are likely to flood the evap and no turbulance to aid heat transfer.

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    Re: how to increase the portion of liquid after TXV

    Nice review, MF.

    In this type of system, you should also take care of the high-side pressure drops. Actually, these can often end up substantially higher than the low-side pressure drops. Sometimes, the evap pressure drop component, as a % of the overall system pressure-drop, is actually pretty small - when the distributor & feeder lines are factored into it.

    To get maximum performance from the system, the whole thing must really be well thought through. Trying to optimise one part alone (local optimisation) can move the system off its optimum (global optimisation).

    There is no silver bullet...
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    Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by mad fridgie View Post
    ....
    What is of more interest is the mixture by volume. Under normal working conditions (no special liquid sub cooling) you will always have a much higher % of vapor.
    It is this vapor which causes the pressure drop through the evap and reduces the amount of liquid in contact with the wall of the tubes.
    These two effects reduce the overall performance of the evaporator.....
    MF, this is an explanation I will translate in Dutch to give to my students
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by mad fridgie View Post
    ....
    Re; suction/liquid heat exchangers, if the TXV bulb is fitted after one these heat exchanger, it could be said that the refrigerant leaveing the evap is wet, thus getting full use of the evap, verses an evap which has to incur superheat. Vapor is a very poor heat transfer medium. Again this will reduce the pressure drop through the final stages of the evap......
    So, what you're saying - and I agree if I understand you correctly - is...install the bulb of the TEV after the HE to improve evaporator efficiency.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: how to increase the portion of liquid after TXV

    The bulb is always installed after the HE, or am I wrong?

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    Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by NoNickName View Post
    The bulb is always installed after the HE, or am I wrong?
    In almost any schoolbook and every brochure of HE's, and also some times on RE, bulb must be installed before the HE. Like you can find in the Danfoss brochures.
    I never agreed with this but who am I.

    Same remark for the bulb of a TEV, placing after or before the external equalization?
    I say after it and Danfoss insists before the equalization line.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: how to increase the portion of liquid after TXV

    Ah, sorry, in this case HE is the IHE, not the evaporator.
    Putting the bulb after the IHE will result in high SSH, while doing your way, surely the titration in the evaporator is richer in liquid.
    I think I may agree with you.

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    Re: how to increase the portion of liquid after TXV

    Quote Originally Posted by Peter_1 View Post
    Same remark for the bulb of a TEV, placing after or before the external equalization?
    I say after it and Danfoss insists before the equalization line.
    Alco also advises TXV bulb be placed between evap discharge & TXV equalization line.

    Perhaps this could be a good place to discuss the logic, or otherwise, of this choice?

    A thought is that, perhaps the original designers consider the equalization line to create a temperature effect on the suction line & that it would be best to set bulb before this disturbance?

    The problem with this arrangement in very compact installations, is that the bulb then often lands up fairly close to the evap discharge header, which has large thermal mass. This is not good.

    I'd be very interested in reasoning through this matter.
    Last edited by desA; 09-01-2010 at 03:16 PM.
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