how to increase the portion of liquid after TXV

[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!

[nike123]

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.

[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

[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.

[frank721]

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.

[frank721]

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?

[frank721]

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?:)

[US Iceman]

I have been following this thread and wonder what the original poster is doing or thinking...

First, we have this comment.



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...



Why do you think you need to increase the portion of liquid in your systems evaporator?


Then later we have, these...



I want more liquid in my separator, because I want get more liquid evaporate and absorb more heat from my evaporator.




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?

[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.

[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.

[frank721]

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!

[frank721]

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?

[frank721]

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.:)

[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

[NoNickName]

What does the compressor think about all of your liquid coming back to the suction?

[frank721]

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?:-)

[frank721]

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.:)

[amitsaxena]

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

[Sridhar1312]

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.



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!

[amitsaxena]

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

[NoNickName]

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.

[Gary]

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.

[750 Valve]

To obtain a larger quantity of liquid after metering device... in one word...


SUBCOOLING.

[Magoo]

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.

[US Iceman]

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.

[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 . Subcooling can reduce flashing in TEX valve & improve system performance.

[Gary]

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?

[frank721]

Thank you guys! I love this forum so much and discussing with you on refrigeration system.:)

[750 Valve]

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

[frank721]

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?

[Magoo]

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

[US Iceman]

...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!

[icehouse]

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

[D.D.KORANNE]

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.

[swecool]

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...

[swecool]

Forgett to add that... To "tweak" the bulb or tvx need tank to carry the difference statement or have to add more "soup" :-).

[frank721]

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?

[shaman]

Great post

[freeze53]

Gary I like the double heat exchanger.
I know adding one heat exchanger will lower the suction pressure what does the double do?

[HallsEngineer]

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!!!

[oldesky]

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.

[desA]

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.

[mad fridgie]

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.

[desA]

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... :)

[Peter_1]

....
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 :cool::cool:

[Peter_1]

....
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.

[NoNickName]

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

[Peter_1]

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.

[NoNickName]

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.

[desA]

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.

[Gary]

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.

If the seals inside the TEV leak, then a small amount of liquid will flow through the equalizer line to the suction line. If the bulb then senses this liquid it will close the TEV and starve the coil.

This was a common problem back in the good old days, but is now rare with the closer manufacturing tolerances and new seal materials.

[monkey spanners]

I think the idea with putting the txv bulb before the equalizing line is that if the valve lets liquid flow down the equalizer due to a fault it won't shut the valve down, personally having seen a system getting a lot of liquid back due to this fault i wonder if it would have been better for the bulb to have been after the equalizer and with a bit of luck the customer would have noticed poor system performance.

Something else i have been wondering about is equalizing line size, maybe i'll start another thread for it.

Edit: just noticed Gary's post explaining what i was but betterer

[mad fridgie]

Firstly bulb and equalising position, I think gary covered this well, the older valves used to "weep" so if equalizer line was fitted before the bulb, the TXV would close prematurely.
If I have choice, I install, the heat exchanger between evap outlet and TXV bulb. Your choice is normally finacially driven not purely engineering driven. Is it cheaper to have 15% more evap coil/surface area (to allow for superheating) or cheaper to have a vapor/liquid heat exchanger.
If we look at pumped industrial refrig systems, why do they overfeed by so much? Simply it is to ensure that the evap is fully wetted and that the suction has no superheat (thus less suction line pressure drop)
By increasing liquid sub-cooling and removing the need for ssuperheating in an evap, your are reducing the temperature difference between the product and refrigerant for a specific load and surface area, this will increase the system efficiency. (by raising the system SST)

[desA]

If the seals inside the TEV leak, then a small amount of liquid will flow through the equalizer line to the suction line. If the bulb then senses this liquid it will close the TEV and starve the coil.

This was a common problem back in the good old days, but is now rare with the closer manufacturing tolerances and new seal materials.

Thanks Gary. That makes the logic very clear.

The thing is, wouldn't it be more safe for the system if the TXV shut-down mechanism was allowed to happen? It would protect the compressor.

Interesting...

With an adjustable TXV, the weep could be accommodated via a TXV setting offset, could it not?

[Gary]

Thanks Gary. That makes the logic very clear.

The thing is, wouldn't it be more safe for the system if the TXV shut-down mechanism was allowed to happen? It would protect the compressor.

Interesting...

With an adjustable TXV, the weep could be accommodated via a TXV setting offset, could it not?

The theory is that the weep is not enough to flood the compressor, so it is better to place the bulb before the equalizer.

If the weep were predictable and steady, then it could be accomodated... but it is not predictable nor steady.

[HallsEngineer]

We have all dropped off the end of the world, Seriously. Subcooling is good up to a point. If you get too much your system will suffer. The entire system has to be sized and designed to be used with a "true" subcooler i.e. economiser or as an addition to a high stage evap. For best results he needs to work out the best pressure diff for the valves he has or buy an electronic valve with settings for superheat. For best and simplest results use a VFD on your condenser fans to maintain a steady and optimal head pressure and lag your liquid line.

[mad fridgie]

We have all dropped off the end of the world, Seriously. Subcooling is good up to a point. If you get too much your system will suffer. The entire system has to be sized and designed to be used with a "true" subcooler i.e. economiser or as an addition to a high stage evap. For best results he needs to work out the best pressure diff for the valves he has or buy an electronic valve with settings for superheat. For best and simplest results use a VFD on your condenser fans to maintain a steady and optimal head pressure and lag your liquid line.
Why will your system suffer, if industrial and on a pump system with high or low side flow float, more sub-cooling the better, no negative effects, I think you are misunderstand the difference between pressure ratio (needed to size valves). On a DX system again sub-cooling (not reduced pressure ratio) all positive unless your liquid gets close to SST, at which gas the liquid/vapour volume is so low that you get very little turbulance within the evap (reduced heat transfer properties)
When designing with sub-cooling (economiser or what ever) you allow for this in equipment selection.
On your pump systems why do you think you overfeed, as far as the evap/valve, the liquid is already highly sub-cooled. The overfeed is to ensure fully wetted surfaces.

[HallsEngineer]

No pumped systems are not sub-cooled as the gas is cooled to saturation and then artificially pumped up to around 2 or 3 bars, not massive amouts of subcooling. Then the liquid is fed through a metering device, not to be confused with expansion device and then returned to the drum having taken in a small amount of heat energy around a 1/4 of that of a DX system. The flooded coils are generally sized to accomodate this. DX is a different kettle of fish completely. For example if i take a 134a system and pull the liquid off the reciever and put it through a true subcooler bigger than the system it'self then thats not effiecient. May as well use a seconadry refrigerant. there has to be an energy trade between subcooling and plant cost.
The other point is startup. you take a system sized for 30'c subcooling through an economiser, and at start up you are boned majorly as the system cannot achieve the required subcooling instantly. HP will cause the system to limit and possibly not be able to achieve temp. The system has to be sized to do most if not all of the duty and then bring in the subcooler to boost capacity as the cooler nears setpoint. This is due to the gas in the flute or cylinder being at a slightly higher pressure than suction. Also as you system pulls down the load on your subcooler is increased as the flow rate through your expansion device is higher. This is why economiser ports on compressors are sized to limit the flow of gas.
True subcooling is good but up to a point. best results - lag your liquid line! why do the A/C boys do it?

[mad fridgie]

again shows limited knowledge, liquid sub-cooling occurs on a pump system prior to entering your pot (drum) commonly called "makeup". the float is the expansion device! On the pump side the liquid is sub cooled as the pressure is being elevated above the pressure in the pot. the valves at the coils being auto or manual are pressure reducers. so in them selves are indeed expansion devices.
How and when an econimiser is best used is more to do with the application and is working pressures.
It would seem that you only work on the very large equipment, but there a lot more refrigeration than just this big stuff. The average commercial refrigeration does not have economiser, VSDs, PLC, Variable VI, Infinate unloading and all the other wonderful benefits of large system.
Now insulating your liquid line is only good if the liquid temp is below the ambient temp in which the liquid is flowing, more approiate if a water cooled systems, but as the most common form of condensing is air cooled insulating the liquid line in many (not all) case has a negative effect.
the reason the AC boys insulated there liquid lines is that the expansion device(s) is in the outside unit, the actual liquid is cold and is at low side pressure plus pressure drops.

[US Iceman]

No pumped systems are not sub-cooled as the gas is cooled to saturation and then artificially pumped up to around 2 or 3 bars, not massive amounts of subcooling.


Not quite right. If you look at a PH diagram, by increasing the pressure of the liquid with a pump you are effectively increasing the subcooling. Most people think of subcooling as only a colder liquid. Strictly speaking this is true, until you plot the pump pressure addition on a PH diagram. The higher the pump pressure above saturation the greater the subcooling, and... the liquid temperature doesn't change at all.;)

[HallsEngineer]

I know this. I work on industrial systems all the time the term expansion means the refrigerant boils itself in order to cool it's self. This does not happen in a liquid overfeed system the liquid entering the evap doesn't mystically lower its temperature lower than it already is! The subcooling as you call it has no effect on the refrigerant at all. It purely ensures better transfer of the liquid. It is not useful subcooling. Usefull subcooling means the liquid does not have to expend as much latent energy to cool it's self to the correct saturation temp. That is useful. A liquid overfeed system purely has throtling valves to meter the flow and balence the system and not over tax your pump. A ratio of 4:1 or close as should be maintained. HP float systems are the bane of the world, but people like them because they are simple and easy to work. Like two stage systems because they are more complicated doesn't mean they are worse. The AC boys lag their liquid line to make heat transfer to the liquid minimal thats the point im trying to make. I know the expansion device is in the outdoor but it is not always (VRV)! I have worked on many small commercial systems also. As soon as you put any refrigerant into any vessel where there is a mixture of gas and liquid it is at that saturation temp. Whether this be a HP reciever or LP reciever. So as the liquid exits your LP vessel there is NO subcooling. If you have subcooling on your reciever, purge it you have air in there. and the float is not the item in question its the metering near the coil. Don't say i have limited knowledge of pumped overfeed systems HP or LP control. And actually most commercial systems now have VSD on condenser fans as they are peanuts for systems that size. Yes you won't always have an economiser but you wont get a lot of subcooling out of an air cooled condenser without detremental effects on the compressor side. You would have to run the head a lot higher than ambient taking more power. And are we discussing SUBCOOLING or not as in liquid leaving my aircooled condenser cooler than the head pressure, which should be only a little higher than dry bulb anyway.
As time goes on and people finally realise how much fridge costs to run they will change their attitude.

[mad fridgie]

Expansion means filling a large void from a small void. in refrigeration terms this means turning a liquid to vapour by means of changing pressure "its boiling point".
In a liquid over feed system, where does the LP liquid come from, you have guessed from expanding HP liquid. This HP liquid is sub-cooled by what ever method. It then goes through an expansion device of some for. I totally agree about sub-cooled liquid coming out of a high side liquid reciever "it is not" it gives the impression of being sub-cooled because the liquid temp is compared against compressor discharge pressure. (I had a thread about this)
A thottling valve is a expansion device, if not then the pressurerised subcooled liquid (out of the pump) would just act like a heat transfer fluid. You have to have apressure drop to bring the liquid pressure back to an evaporating pressure thus latent exchange can happen. You pump pressure is there to ensure that the liquid does not boil down the pipe during transportation.
You overfeed your coils to ensure that the internals of the evap are totally wetted ensuring optium heat transfer co-efficients and to ensure that the suction has no superheated vapour, so reducing piping pressure drop.
Go to your local butchers shop, see if he has a VSD on his 3HP unit, or his little display counter. I think not.
If we look at VRV this liquid line is more likely to be insulated to aid the heating cycle (when keeping energy "in the system" is more important.
Again i will agree with you on when people realise the cost of running refrigeration. This comes down to one simply issue (as far as a compressor, the power user in most cases) "compression ratio" we need to keep SCT low, SST high, then manage all the other problems

[US Iceman]

There are two forms of subcooling. One where the temperature is reduced (the only everyone thinks of) and increasing the pressure above saturation. Both help to prevent flash gas.

Colder liquid helps to increase the net refrigeration effect. Pumping the liquid does not increase this, it just allows you overcome pressure losses (friction or static) so that 100% liquid is delivered to the expansion device, just like subcooling where the temperature is reduced.

You can have similar problems with either form. If the liquid is colder than the evaporating temperature the liquid will not boil until it warms up to the evaporating temperature.

If the liquid pressure is too high on a pumped system the hand expansion valve has to reduce the pressure down to the evaporating pressure before the liquid will boil. This is why you see liquid overfeed evaporators experience something called brining. The liquid does not boil. The same thing occurs if the pumped liquid is too cold also.

[oldesky]

The idea stems from liquid being able to bleed down the equalizing line from the Tx valve, affecting the temperature of the suction line at the bulb and therefore destabilising superheat. Honeywell (Flica) valves say that their passage is sealed so the bulb can be mounted after the equalising line.

[Aik]

Firstly, I want to know: which should be more? liquid or vapor? after TXV. In my opinion, liquid should be much more than vapor.
Thanks a lot!
As example check this:
http://www.sporlanonline.com/20-10.htm
Figure 1 shows the answer...see weight, because mass flow in system is constant.

[AlwaysLearning]

After I install a new TXV or re build one I always adjust it after my temp is reached, turn the adjuster CCW to lower superheat and CW to increase, more liquid lower superheat