Liquid subcooling real or not

[mad fridgie]

If you look at standand ratings of compressors they always state a reasonable amount of liquid subcooling, where does this sub cooling occur?
Is this sub cooled liquid in the reciever?
Trick question!!!!!!!!

[nike123]

Not always!
For example, Copeland sometimes states standard ratings at 0K subcooling.

Why is important where it occurs?

[mad fridgie]

I will simplify the question, do you have sub cooled liquid in a reciever

[nike123]

I will simplify the question, do you have sub cooled liquid in a reciever

That was discussed before.
Try search function!

[US Iceman]

The compressor manufacturers use a standard rating basis for comparison of performance values between various manufacturers. Just because they state a specific value of subcooling does not mean it will be present during operation. To provide subcooling something has to be done; it doesn't just appear magically. Something has to provide it.

[nike123]

http://www.refrigeration-engineer.co...ht=gonefishing
http://www.refrigeration-engineer.co...t=11717&page=2
http://hvac-talk.com/vbb/showthread.php?t=56495&page=2
http://www.hvacprotech.com/viewtopic.php?f=98&t=5724

[GHAZ]

hi madfridgie the sub cooling occures after the condensor i,e say 10c but it does,nt matter how much subcooling you get because when the liquid is in the reciever it is same temperature and pressure as before it entered the condensor, so if you have 3c or 10c subcooling it dos,nt matter as long as you have some subcooling to condense vapour to liquid, as to your question the reciever doesnt have subcooled liquid.

[mad fridgie]

I will get to the point, I see all the time poorly sized TEVs, asked the engineers how they have sized, Simply they use design condensing pressure and liquid temperature, would seem right, (correct for comp selection) I believe (as well as testing) that in many cases the liquid does not actually have much sub-cooling, but is actually a much reduced pressure compared to the condensing pressure. As a expansion device is primiarily a fuction of pressure drop, incorrect sizing is occuring, which of course causes other downstream problems

[mad fridgie]

Sub cooling real or not?

[Gary]

I will get to the point, I see all the time poorly sized TEVs, asked the engineers how they have sized, Simply they use design condensing pressure and liquid temperature, would seem right, (correct for comp selection) I believe (as well as testing) that in many cases the liquid does not actually have much sub-cooling, but is actually a much reduced pressure compared to the condensing pressure. As a expansion device is primiarily a fuction of pressure drop, incorrect sizing is occuring, which of course causes other downstream problems

I'm confused. Is your point that TEV's are poorly sized?... or that liquid lines are undersized/restricted?

[mad fridgie]

Yes expansion device (TEV) are incorrectly sized

[Gary]

The way you are explaining it sounds more like undersized/restricted liquid lines.

[mad fridgie]

The actual problems we encounter is the pressure drop through the de-superheaters, condensers and reciever.

[desA]

@ mad_fridgie

A suggestion, if I may, to clarify the situation & focus all our thought processes. I think your discussion is an excellent topic...

Sketch a system from condenser, through to TXV inlet. Along the way, include known (estimated) component pressure drops.

We can then use thermodynamics to explain exactly the state of the refrigeration at each point along that line. By the time we reach the TXV, we will know if the state point represents a sub-cooled liquid, or otherwise.

I can plug in the exact thermodynamics, if you provide the system sketch.

[mad fridgie]

The actual problems we encounter is the pressure drop through the de-superheaters, condensers and reciever.

This gives the impression, that the liquid is subcooled, because the actual temp is somewhat lower than the SCT (comp), when infact there is very little subcooling (additional flash gas down the liquid line as gary as stated) So when it comes to the correct selection of TEVs, allowance should be made for these pressure drops with little subcooling, rather than choosing SCT and subcooled liquid for sizing purpose. The down side is either lower SST (and increase superheat if enough load) or driving up the SCT (condenser control) to balance the system, either one reduces performance and efficiency

[mad fridgie]

This gives the impression, that the liquid is subcooled, because the actual temp is somewhat lower than the SCT (comp), when infact there is very little subcooling (additional flash gas down the liquid line as gary as stated) So when it comes to the correct selection of TEVs, allowance should be made for these pressure drops with little subcooling, rather than choosing SCT and subcooled liquid for sizing purpose. The down side is either lower SST (and increase superheat if enough load) or driving up the SCT (condenser control) to balance the system, either one reduces performance and efficiency

Downside of not choosing the correct TEV

[Magoo]

Mad fridgie,
The majority of suppliers in NZ offer c/u that average 5 to 7 'C natural sub-cooling. As in condensing Temp 35 'C liquid out at between 28 > 30 'C, at design conditions.
The problems start with liquid line, and suction line sizing.
Vertical separation and length of liquid line and bends, produces pressure losses, so at TEV the pressure of liquid is reduced, TEV's are designed/ selected on delta P across vav.
Reduced P/D effects TEV performance.
Next is suction line size and length of. Pressure drop again.
Try installing a site-glass before the TEV and fit a schrader vav on suction at evaporator, all will become clearer.
magoo

[mad fridgie]

This is not a specific problem, but a generic problem, with small to medium stand alone units. Magoo you are correct about the ratings, apart from how can you have 5-7C subcooling in a reciever, what you actually have is a liquid at a pressure between 5-7C below that of SCT. I have test rigs with NZ brands all showing similar results. (liquid line on test rig 1.8m)
You have sort of clarified my point, your starting point for liquid line pressure drops starts at the condensing unit outlet, which is rated at 35C and 5-7C subcooling, when in fact the liquid pressure would be equal to that of a SCT of 28-30C, This of course an be corrected by TEV selection

[desA]

Mad fridgie,
The majority of suppliers in NZ offer c/u that average 5 to 7 'C natural sub-cooling. As in condensing Temp 35 'C liquid out at between 28 > 30 'C, at design conditions.

I'd imagine that these would be tube-in-tube coils.

This level of sub-cooling is not available for plate condensers, where 0.5K<SC<4K.

[mad fridgie]

I'd imagine that these would be tube-in-tube coils.

This level of sub-cooling is not available for plate condensers, where 0.5K<SC<4K.

These are actually air cooled condensers, (does not get to hot over here)

[desA]

^ A long, tubular-type condenser will give reasonable level of sub-cooling. This does seem to be a nice feature.

[Gary]

apart from how can you have 5-7C subcooling in a reciever

Imagine a half full container of refrigerant lying on a block of ice with a heating pad on top.

You have superheated vapor at the top, subcooled liquid at the bottom, and saturation at the liquid/vapor interface.

Yes, you can have subcooling in a receiver... even though there is vapor on top.

[mad fridgie]

Imagine a half full container of refrigerant lying on a block of ice with a heating pad on top.

You have superheated vapor at the top, subcooled liquid at the bottom, and saturation at the liquid/vapor interface.

Yes, you can have subcooling in a receiver... even though there is vapor on top.

I may not have as much experience or seen the whole world, I can say that I have never seen a reciever with a block of ice at the bottom and a heating pad at the top? Have seen very large and high vertical recievers with large static heads, so yes you are right, the liquid at the bottom would be sub cooled and the vapour at the very top would be superheated, but this is a function of stratification, I very much doubt in a small to medium system that this would occur (flow, turbelence, breaking of boundry layers) but on the other hand there could be a layer of oil between the liquid and the vapour, thus acting as an insulator and reducing the chance of bubble entrapment thus reducing the the chance of condensation, which would allow for some level of subcooling dependent of the static head and natural and forced flow convection currents within the body of the fluid.

[nike123]

I am sure that IR camera would reveal the true.
Anyone have IR camera to show us picture of receiver and liquid line?
Something like this:
http://www.fluke-ti25.com/Image-Gallery_ep_48.html

[desA]

^ Can this determine the STATE of the fluid at any point in the system i.e. Temperature + pressure => state. Only from this can you determine if the liquid is, per definition, subcooled...

[nike123]

^ Can this determine the STATE of the fluid at any point in the system i.e. Temperature + pressure => state. Only from this can you determine if the liquid is, per definition, subcooled...

Surely not! But it would be interesting to see and analyze that image in software.
I presume that pressure in receiver is same in all points. Am I wrong?

[TRASH101]

This discussion confuses me

The facts are

1) If liquid exists then it is sub cooled
2) If vapour exists then it is super heated

The question should be is why they both exist in the reciever?

Stratification?

Convection?

Expansion(flashing @ reciever inlet)?

What else?

[mad fridgie]

So I think we can say that it is "possible" to have sum level of sub-cooling, how much practically depends upon many factors.
So let apply this to a system
We are doing a medium temp cool room (nothing to special) We purchased one of those packaged condensing units, all the bits and piecies inside, all we have do is connect the liquids and suction lines (3meters long pd has been allowed for).
The evap by pure miricle, is exact fit to
the unit. All we have do is size the TEV(TEX) to pair the 2 together.
The unit is rated 30Kw @ Ambient 32C, SCT 45C SST-4C, 7C liquid subcooling, 10C Comp inlet superheat, Reciever sized for pumpdown, so is not grossly oversized, Fan speed control on cond fans
How would you size the TEV? (no cheating by using EEV) Method?

[nike123]

So I think we can say that it is "possible" to have sum level of sub-cooling, how much practically depends upon many factors.
So let apply this to a system
We are doing a medium temp cool room (nothing to special) We purchased one of those packaged condensing units, all the bits and piecies inside, all we have do is connect the liquids and suction lines (3meters long pd has been allowed for).
The evap by pure miricle, is exact fit to
the unit. All we have do is size the TEV(TEX) to pair the 2 together.
The unit is rated 30Kw @ Ambient 32C, SCT 45C SST-4C, 7C liquid subcooling, 10C Comp inlet superheat, Reciever sized for pumpdown, so is not grossly oversized, Fan speed control on cond fans
How would you size the TEV? (no cheating by using EEV) Method?

What refrigerant? What evaporator distributor (or what pressure drop at distributor), if any? What pressure drop at liquid line?

Me?
I would use CoolCat from Danfoss!

[TRASH101]

All we have do is size the TEV(TEX) to pair the 2 together.
The unit is rated 30Kw @ Ambient 32C, SCT 45C SST-4C, 7C liquid subcooling, 10C Comp inlet superheat, Reciever sized for pumpdown, so is not grossly oversized, Fan speed control on cond fans
How would you size the TEV? (no cheating by using EEV) Method?

To choose the TEV you must match it to the evaporator spec. for your application/load characteristics. If you don't get good superheat feed back from the evaporator then the system will never work at its optimum. Then make the condensor/comp. side match what you've got.

[mad fridgie]

To clarify this not specific, but is fundemental, when choosing your tev, (all other issues being equal) by manufacture software or of a chart,
is the liquid entering the TEV at a pressure of 45C and subcooled by 7C or is the liquid pressure at 38C and has minimal subcooling. Remember that a TEV is just a hole, and flow through a hole is basically a function pressure differantial (all other issues being equal)

[mad fridgie]

To clarify this not specific, but is fundemental, when choosing your tev, (all other issues being equal) by manufacture software or of a chart,
is the liquid entering the TEV at a pressure of 45C and subcooled by 7C or is the liquid pressure at 38C and has minimal subcooling. Remember that a TEV is just a hole, and flow through a hole is basically a function pressure differantial (all other issues being equal)

We are asuming that a single valve/orifice can not do the same job. If the mass flow was equal on both cases then the rest of the system would also be equal (flash gas would be the same)

[US Iceman]

Remember that a TEV is just a hole, and flow through a hole is basically a function pressure differantial (all other issues being equal)

Bingo. There you go. A TXV orifice is a variable sized orifice. The actual size is controlled by the pin carrier when it pushes up into the orifice trying to control the mass flow for a given pressure differential. For that given pressure differential and refrigerant with a fixed evaporator pressure you can vary the valve capacity by the amount of subcooling present at the valve inlet.

Anything that cause the liquid enthalpy or quality to change will result in a a loss or gain or valve capacity.

Liquid exists in two normal states: subcooled or saturated. If the refrigerant is vapor it has two normal states; saturated or superheated. Liquid does not exist as superheated. That is what happens to liquid after it has boiled off and heated beyond the saturation temperature.

[mad fridgie]

Bingo. There you go. A TXV orifice is a variable sized orifice. The actual size is controlled by the pin carrier when it pushes up into the orifice trying to control the mass flow for a given pressure differential. For that given pressure differential and refrigerant with a fixed evaporator pressure you can vary the valve capacity by the amount of subcooling present at the valve inlet.

Anything that cause the liquid enthalpy or quality to change will result in a a loss or gain or valve capacity.

Liquid exists in two normal states: subcooled or saturated. If the refrigerant is vapor it has two normal states; saturated or superheated. Liquid does not exist as superheated. That is what happens to liquid after it has boiled off and heated beyond the saturation temperature.

Agree, so if your enthalpy was the same (as stated) how would you choose the valve sizing "a" higher liquid pressure with sub cooling "b" lower liquid pressure no subcooling (remember that subcooled liquid has little or no effect on mass flow at a specific pressure drop.
For the purpose of this thread think of your TEV as fixed orifice (of course there is adjustment, but it is limited why else do you have head pressure control)

[US Iceman]

If the liquid is colder than the saturation temperature it is subcooled. The colder liquid has a lower enthalpy therefore the net refrigeration effect increases, so the capacity also increases, while the mass flow remains constant.

The main consideration is differential pressure. The valve (orifice) capacity changes faster with a changing differential pressure than it does with various liquid enthalpies.

[mad fridgie]

If the liquid is colder than the saturation temperature it is subcooled. The colder liquid has a lower enthalpy therefore the net refrigeration effect increases, so the capacity also increases, while the mass flow remains constant.

The main consideration is differential pressure. The valve (orifice) capacity changes faster with a changing differential pressure than it does with various liquid enthalpies.

Going back to the original question liquid subcooling real or not, I have undertaken testing of many units and determined that the inlet pressure to TEVs (short pipe runs) is somewhat lower than the compressor outlet pressure, thus the liquid is not actually subcooled, (give or take) The conclusion is that many engineers are incorrectly sizing the TEVs, as they are using the SCT pressure as the input pressure to the TEV selection with a level of subcooling, These systems are balancing, but are less efficient than they could be, if the correct TEV was selected.
I think the manufactures of the package condensings units should indicated outlet liquid pressure, along with all the usual.

[Gary]

Going back to the original question liquid subcooling real or not, I have undertaken testing of many units and determined that the inlet pressure to TEVs (short pipe runs) is somewhat lower than the compressor outlet pressure, thus the liquid is not actually subcooled, (give or take) The conclusion is that many engineers are incorrectly sizing the TEVs, as they are using the SCT pressure as the input pressure to the TEV selection with a level of subcooling, These systems are balancing, but are less efficient than they could be, if the correct TEV was selected.
I think the manufactures of the package condensings units should indicated outlet liquid pressure, along with all the usual.

I suppose the reason I find this thread confusing is that:

a. I have never heard of sizing a TEV in the way that you describe.

and...

b. SCT and subcooling are determined by the pressure at the receiver/condenser outlet, not the compressor discharge pressure.

[mad fridgie]

I suppose the reason I find this thread confusing is that:

a. I have never heard of sizing a TEV in the way that you describe.

and...

b. SCT and subcooling are determined by the pressure at the receiver/condenser outlet, not the compressor discharge pressure.

The thread has move in a slightly different direction than I expected?
a. I do believe that many are sizing TEVs incorrectly, for the reason that are using the wrong pressure and subcooling figures.
b SCT is the same as the comp discharge pressure (in my info anyway, in stated different in the states then i apologise) subcooling I donot believe this really exists in a small reciever (it may happen over the liquid line, or via other devices)
If you were to re-evaluate the sizing of the TEV you may find that you can increase system efficiency, only by a little, but if you look at how many millions of these small systems are about, the global saving would be massive