Sub cooled r22 still flashing?

[Drew]

Hey all,

I have a commercial system running on r22 low temp ( -32 c sst ) . The liquid line at the sight glass is sub cooled by 8 degrees C. The sight glass still shows as if it's short of refrigerant, full of bubbles. The sight glass is on a vertical pipe . As the liquid line is subcooled why would it show to be short?

[Rob White]

.

Could it be short?

With subcooled liquid you will have the liquid present
but if there is not enough to meet demand it will bubble in the sight glass.

Rob

.

[The Viking]

Another question,

What is your liquid line temperature compared to the ambient temperature where the sight glass is located?

:cool:

.

[sterl]

What is the elevation difference between the liquid level in your receiver and this sight glass?

If you are trying to lift refrigerant more than 10-meters, you need more than 8-deg of subcooling COMPARED TO THE PRESSURE AT THE RECEIVER because that is the upstream point of saturation, in most cases....

[mad fridgie]

Good posts by all
I would say you need to read tbe pressure at the point you are measuring
Plus it is likly you temp measurement is being influenced by the ambient

[Drew]

Thanks all,
The sight glass is 1m above receiver which is running at 35% full. I took my pressure and temp readings within 200mm of sight glass. How can I be short if I'm sub cooled at all? Where would you plot that point on an enthalpy chart? Can a liquid line be short and sub cooled?

[Tesla]

Hi Drew
I have had many chillers show bubbles with 10K subcooling and they were fully charged. A few more things which effect this is the load, velocity of refrigerant and the sight glass is usually slightly larger than the pipe before and after. So you have a lot to study to understand and check out cavitation and a triple point diagram just for measure.

[RANGER1]

Just to add,you also have to verify how accurate your gauge & thermometer are as well.

[Drew]

Yes, I should check how accurate my gear is.

I thought the whole idea about obtaining subcooled refrigerant was to ensure the texv got a nice charge of liquid and to gain some capacity.All texv capacity charts are based on liquid flow? If the bubbles are refrigerant vapor then the refrigerant can't be sub cooled? Its starting to flash? What's the point of having a sight glass that doesn't show full when the liquids subcooled? I understand sight glasses on some systems ,which are critically charged, will show short, but these systems are not subcooled?

[TRASH101]

Yup your thoughts are right pretty much right Drew but are the bubbles vapor or none condensibles? is there a restriction prior to the sight glass? are the ambient conditions ruled out? Are you getting steady flow? Is your equipment responding quick enough?

You measuring after the sight glass for temp and pressure?

[Coorsman777]

Is the system large enough to have reciever pressurization? I have had on supermarket systems where there is not enough pressure in the receiver when the hold back valve is holding back and the liquid is being pulled out of the receiver rather than pushed. It causes a lazy flow of refrigerant in the liquid line.

[RANGER1]

Yes, I should check how accurate my gear is.

I thought the whole idea about obtaining subcooled refrigerant was to ensure the texv got a nice charge of liquid and to gain some capacity.All texv capacity charts are based on liquid flow? If the bubbles are refrigerant vapor then the refrigerant can't be sub cooled? Its starting to flash? What's the point of having a sight glass that doesn't show full when the liquids subcooled? I understand sight glasses on some systems ,which are critically charged, will show short, but these systems are not subcooled?

You could always add a few kg of refrigerant & see what happens at sight glass!
Surely if low on refrigerant superheat is all over the place.

[Drew]

The superheats at the four low temp evaps are high, which lead me to check sight glass and then sub cooling. So all indications are short of refrigerant except for the sub cooling? My hp is low to try and manage the compression ratio of the mycom compressors (30c) for R22. Therefore -32csst 30csct. Currently the receiver is 35% full but I have seen in more full ,still with a sight glass showing bubbles. If their were non condensibles wouldn't my hp be higher? The sight glass is just after the drier , but I have got the drier in bypass mode so it wouldn't be influencing the readings.

[Magoo]

I agree with Ranger1, high velocities in liquid line can be deceptive, the flashing liquid can give a false sub cooling indication after condenser and receiver. Add more refrigerant and monitor result. Check the obvious of temperature drop across filter drier, the temp of liquid leaving condenser etc..

[Drew]

If the liquid is flashing how can it be subcooled? It must be in the 2 phase zone on the enthalpy chart? Or is it the fact that evaporating refrigerant requires energy which it removes from the liquid refrigerant lowering its temp giving me this false reading? It would still be flashing and shouldnt be sub cooled though?If it wasn't for the compression ratio issues I would raise the hp and see if that improves things? sadly I cannot get sub cooling readings closer to the evaps. The sight glass is the only spot on the liquid line where I can take a pressure reading. I ve got an engineer coming to inspect the site on Tuesday and I've got to explain the high superheats and flashing sight glass and he's bound to think me stupid and say that it's short of refrigerant !:mad: I'll show him the sub cooling reading and let him explain it to me

[RANGER1]

At the end of the day we can theorize about it all we like, but everything except subcooling says its probably short.
Sometimes you have to try things to eliminate problems (also to prevent ourselves "looking stupid").
The only thing these days is the expense of R22 to experiment.

Again forgetting the subcooling, the liquid reciever dip tube might be a little off the bottom, so a good level in reciever may also be needed. Have also seen horizontal liquid recievers sloping away from dip tube end.

Possibly a check could be made if you turned a few units off to, so refrigerant level rises in reciever.
Then monitor sight glass, subcooling, superheats etc, re-analyse with hopefully new results.


Could it be the liquid line size is to small if above does not improve things?

[Tayters]

I've seen it mentioned on this site that as the refrigerant enters the sight glass the pipe gets a bit bigger because of the sight glass housing causing a bit of a lower pressure which causes the bubbles.
If all your cases are reaching temperature with correct superheats then you must have enough refrigerant in the system.

If your system was short of charge then you should have the cases on the end of the run running at a higher temperature and superheat.




If the liquid is flashing how can it be subcooled? It must be in the 2 phase zone on the enthalpy chart? Or is it the fact that evaporating refrigerant requires energy which it removes from the liquid refrigerant lowering its temp giving me this false reading? It would still be flashing and shouldnt be sub cooled though?If it wasn't for the compression ratio issues I would raise the hp and see if that improves things? sadly I cannot get sub cooling readings closer to the evaps. The sight glass is the only spot on the liquid line where I can take a pressure reading. I ve got an engineer coming to inspect the site on Tuesday and I've got to explain the high superheats and flashing sight glass and he's bound to think me stupid and say that it's short of refrigerant !:mad: I'll show him the sub cooling reading and let him explain it to me

[Drew]

Seems silly to manufacture sight glasses that show short when one of their functions is to give you an idea when the charge is good ,how many systems are over charged because of this? Ha ha.

[RANGER1]

Seems silly to manufacture sight glasses that so short when one of their functions is to give you an idea when the charge is good ,how many systems are over charged because of this? Ha ha.

Not yours, yet anyway!

[jdunc2301]

At the end of the day we can theorize about it all we like, but everything except subcooling says its probably short.
Sometimes you have to try things to eliminate problems (also to prevent ourselves "looking stupid").
The only thing these days is the expense of R22 to experiment.

Again forgetting the subcooling, the liquid reciever dip tube might be a little off the bottom, so a good level in reciever may also be needed. Have also seen horizontal liquid recievers sloping away from dip tube end.

Possibly a check could be made if you turned a few units off to, so refrigerant level rises in reciever.
Then monitor sight glass, subcooling, superheats etc, re-analyse with hopefully new results.


Could it be the liquid line size is to small if above does not improve things?

Sometimes you just have to go with your gut feeling, high SH on all EVP and all signs pointing towards charge issues, its worth it to cover your backside and show you are trying to cover all the bases.

[THE DUDE]

One thing that comes to my mind....Is the high side pressure stable? When a Condenser Fan turns on the pressure in the Receiver/Liquid Line will drop, when the pressure drops the Liquid will boil to its new Condensing temp.
Ive noticed that Electronics can be slow to Update data, so I usualy verify data with Gauges
I also change my Gauge heads often.
Best of luck, I am intrested to see what you find out.

[Drew]

The hp is controlled by a direct on line fan via a hp switch which gives slight variation in the hp, but the system runs on a cooling tower which moderates the change.the system shows short regardless of fan on or off.
The engineer came to site today,added 130kg R22 (rather him than me at that cost hey Ranger1)raised the hp to 40C ( compression ratio?) and monitored system. The sight glass is still showing short with about the same amount of sub cooling.i haven't checked the evap superheats yet. With the raised hp and more refrigerant I expect them to be lower. This is ,of course, only if we were short to start. I would have preferred to do one improvement at a time,either raise hp or add refrigerant, to see which improves the superheats the most.

Neither of the improvements have filled the sight glass. I don't see much improvement in fact.

[Coorsman777]

Is this a new or recently modified system? If not has the system worked properly in the past?

[TRASH101]

If it's still showing loads of bubbles and your sub cooling is as you say then with r22 you got yourself non condensibles or such the like. The increase of condensing temperature is only going to increase your evaporating temperature and, likely, the energy consumption of the compressors. Just seems odd that if it is non condesibles that they are present at the sightglass as it should only be fed by liquid from the reciever. Still sounds like its flashing to me.

Is there anything else, barring the filter/ drier, between the reciever and sightglass? How long is the liquid line?

Incidentally what are the superheat measurements?

[Drew]

The engineer is convinced the flashing has improved since adding the 130kg, but it looks similar to me. He might want to add a third cylinder .the superheats vary across the 4 coils from 16 to 20 degrees. I think it's the style of sight glass, ( quite deep) still can't understand how we can get flash gas at all if we are sub cooled. I'm realitivly new on the site, so this could have been like this forever.

[chillerman2006]

Hi Drew

is the liquid line fed from the bottom of the reciever, have had debri [bits of plate] restricting flow through reciever outlet causing similar issue .... another possibility, is it just R22 in there or could it have been topped with another gas by mistake, also dont trust liquid level indicators, seeing is believing

R's chillerman

[Magoo]

superheats appear high, indicating short of refrigerant at evaps.

[RANGER1]

Drew what is your liquid reciever level after the top up. If I understood correctly it was approx 35%.

Did you try turning off a few units & pumping them down to increase level in liquid reciever, to see if it improves things?

Do you have symtoms of rooms not getting down to tempreture, or are they OK?

[TRASH101]

Superheats sound way too high, is the compressor roasting? If you are 100% sure the liquid line is being fed with 100% liquid from the reciever at all times then there must be something wrong with the liquid/ liquid line. can you measure pressure an temp at reciever outlet and compare with the readings at the sight glass? you might also want to check line sizes for capacities of evaps and comps.

You can have 8 sub cooled r22 and bubbles if its flashing or if the bubbles aren't r22

[Drew]

Room seems happier. Super heats have improved. Suction is up to -27 C hp at 42C. Will be checking to see whether this is ok. The sight glass on the receiver shows between 40 and 45% . Sucooling 8c. Adding the refrigerant has helped, BUT I still have a flashing sight glass. Maybe not as much though.
Raising the hp back to what might have been its original design pressure could have influenced texv capacity. The assumed need to keep the hp down to protect the compression ratio and the relative lack of refrigerant could have created high superheats . The system is happier with these improvements , but still not understanding the sub cooled thing with saturated vapor in the sight glass?

If we take a refrigerant cylinder half full of refrigerant liquid it would be saturated. Even if we cooled it it would stay saturated. It would not become sub cooled because the temp would drop with the pressure. Liquid can only become sub cooled if their is no vapor present? So how in this #+*^% sight glass can I have sub cooled liquid and saturated gas?:rolleyes:

[TRASH101]

A different gas lol

[Drew]

Would explain it!

[Drew]

What is the state of the refrigerant in a receiver, shouldn't it be sub cooled? but it is also in the presence of saturated vapor. Again we have saturated vapor with sub cooled liquid. What am I missing?

[Rob White]

The system is happier with these improvements , but still not understanding the sub cooled thing with saturated vapor in the sight glass?

If we take a refrigerant cylinder half full of refrigerant liquid it would be saturated. Even if we cooled it it would stay saturated. It would not become sub cooled because the temp would drop with the pressure. Liquid can only become sub cooled if their is no vapor present? So how in this #+*^% sight glass can I have sub cooled liquid and saturated gas?:rolleyes:

In principle you are correct.

Liquid that is subcooled is not in contact with the vapour.

The trouble with subcooling is it is not as straight forward as that,
even at degree level there is debate about how subcooled liquid flows
through a system.

I can explain it in laymans terms and it works for me, ask me to go deeper
though and my brain freezes up because it does not work on pure science.

So my understanding of liquid is.

Subcoold liquid can be in contact with vapour if it is flowing.
The second it becomes stationary it is saturated and the reason
is because the flowing liquid has temperature levels in it. As liquid
flows through a pipe it flows in thermoclimes? (thermocline?), basically
it flows at different temps through the pipe and therefore it can still be
classified as subcooled because the liquid is below the saturation temp.

I think it is something to do with a bloke called Reynolds (Reynolds number)
look him up on google and he will confuse you with the science of fluid mechanics. :o

http://en.wikipedia.org/wiki/Reynolds_number

It is a lot more complicated than that but that works for me.

Regards

Rob

.

[Drew]

Thanks rob, I'll look into that . I'm happy to see an explanation! A receiver would be similar then? Sub cooled refrigerant flowing through a vessel in the presence of saturated vapor?
I've always understood that in the absence of a sight glass in a liquid line we can charge the system by the sub cooling to ensure a good liquid charge ? Is this not the case or is there a degree of sub cooling we need to obtain to ensure a liquid charge? On r22 the enthalpy chart shows a definite line and no glide?

[RANGER1]

I thought reynolds number is mainly used for heat transfer in heat exchangers.
The turbulent flow is necassay for good heat tranfer, refrigerant, water etc.

We will try to find out best way to explain, but I believe gas & subcooled liquid can exist together, as it happens all the time in an "operating system".

ie in the bottom of a condensor, gas space in liquid reciever.
The gas is always present in these & vapuor will continue to condense as it is colder.

You could put a half full bottle in freezer & liuid would be cold, then hook up to a system, increase pressure in bottle so it then would be subcooled with a gas space on top.

If in your case drawing subcooled liquid from a reciever which may have a low level, so you get subcooled liquid & gas.

[RANGER1]

Drew to plot your system on a P/E diagram I think you would find the following, all which were obvious.
Forget about sight glass in liquid line for the time being.

Before adding refrigerant

high suction superheat
higher discharge temperature
lower mass flow through compressor
poor room temps

After adding refrigerant

lower superheat
lower discharge temp, or same as before due to raising discharge pressure
higher suction pressure
more mass flow of refrigerant, so more refrigeration effect

If all superheats settle down then all is well.
If not add more refrigerant or check liquid line sizing first.

Subcooled can exist with saturated vapour, even in a liquid line, but it can't really
be shown on P/E diagram, only the above conditions show & info from previous post.
Maybe you expect it to show up on the P/E chart on the subcooled side of chart, but
it cannot be plotted to my knowledge ,subcooled or not.

If you can supply compressor model, speed, lenth of liquid line & new conditions a liquid line size could be checked for right size.

Other measurements could also be taken for analysis
condensor liquid drain temp
liquid reciever temp on bottom
liquid leaving reciever
several liquid line temps but also just before TX valves.

If you had a sight glass level indication on liquid reciever I would use it in conjuction with
liquid line sight glass.

If you look at Danfoss link it shows subcooled liquid in presence of vapour PAGE 11

http://www.danfoss.com/NR/rdonlyres/F35695FE-F4A2-40B9-847F-E81E20A5EEA8/0/PF00F202.pdf

[Drew]

Thanks for the effort everyone. Good danfoss article Ranger1.

[Rob White]

Thanks for the effort everyone. Good danfoss article Ranger1.

And to your signature

The more I learn the less I know......

Welcome to my world.

:o

Rob

.

[moideen]

please recheck your liquid line size.down load pipe sizing program.it may help you.

http://www.saravel.com/tools.php?id=18 (http://www.saravel.com/tools.php?id=18)

:cool:

[Drew]

Maybe we need to split the liquid line on the enthalpy chart ? One line going into the sub cooled region and another stopping in the 2 phase area?

[Magoo]

Drew,
you are confusing yourself to be polite. Check system balance charts in the book you have not read yet. Sorry for being blunt.
magoo

[RANGER1]

Maybe we need to split the liquid line on the enthalpy chart ? One line going into the sub cooled region and another stopping in the 2 phase area?


Also another way could be ...___...__....__...___

So hows he system performing now its settled down?

[Drew]

The system is balanced at - 27 c sst which is better than the original -32 c sst. Clearly the added refrigerant has helped and the system is better fo it. Confusion is my middle name Magoo, as you can see by my signature, but to understand how we can have flash gas in a sub cooled liquid line is fundamental to understanding refrigeration . Of course I could walk away and say its all ok, which it is , which I have, is easy, but I am curious ,as you can see from the above, how this can happen? It changes the way I've been charging systems without sight glasses as I've always looked for a certain amount of sub cooling to ensure we have a full charge. So it's not really about this specific site, it's a fundamental question. And If you could provide me with the book I haven't read yet Ill give it a go.

Very criptic advice Ranger1.

[The MG Pony]

You can not rely on a sight glass as a means of ensuring full charge and sub cooling is not a sure shot way either, you must look at the system as a whole: super heat, discharge temp, and sub cooling.

Super heat, we normally want to see 20 degrees for cap tube systems, 10 degrees for TXV, Discharge temp will be set by the oil and refrigerant type cooler the better for the compressor valves with in reason. Sub cooling at the condenser as technically once we hit the receiver there can technically be no such thing!

There will all ways be bubbles in the sight glass most so small we do not see them usually by the time it hits the txv it will be a solid column of liquid (Which where we'd want our sight glas any ways! just sadly not so practical)

The sight glass serves as a quick check of moisture and to small extent gas charge but your T/P relationships will tell you fare more then the sight glass ever will!

So on your system what is the discharge temp measured from within 6 inches of the compressor, what is the SH at the compressor suction side within 6" (SH is what determines to some extent compressor loading and discharge temp we need enough to keep it cool but not so much as to over load it) This is what will tell you if the gas charge is good!

[Drew]

So are you saying that with a 16 c compressor superheat ( long runs ,old insulation ) -27C c sst, 38 c act, 8 c liquid line sub cooling, (not sure of comp discharge temp and not sure that it's that relevant) AND the flashing sight glass the system is as good as it'll be considering the room runs at -20 c ?
Strange how something as simple as bubbles in a sight glass could attract such varied approaches.

[RANGER1]

Drew just to confuse you even more, there is a method by charging to theoretical compressor discharge temperature, system pressures, compressor amps etc etc as well!
Discharge temperature is also important as it is influenced predominantly by compressor suction superheat.
Different refrigerants have different temps at same conditions.
Ammonia needs water cooled heads & oil coolers on recip compressors.
If it gets to hot it could break down oil & compressor valves break.


There are a number of ways to to the same thing, at the same time you can use several of them at once to confirm whats happening in system.

[The MG Pony]

Well if the insulation lagging is old there is your first thing to attend to, get that right, then start again!

On a TXV I personally would tune it till I saw about 10K super heat at the compressor (This ensures the evap is well flooded as it should be, after ensuring the pipe lagging was as sound as I could get it.

All ways start simple, is the evap clean? Are the fans running well? it all starts there, as your suction line pressure and density ultimately control mass flow and compressor loading, not enough we get an under performing system, too much we cook the compressor.

If the lagging on the suction line is poor then we are getting to much super heat where we do not want it, this increases discharge temps, lowers compressor loading, and reduces mass flow.

Start with the simple but not so obvious things. 16K is good on a cap tube system, not so good for a TXV system!

So check the pipe lagging then ensure all evaps are running correctly and clean. check the TXV's sense bulb, is it making good contact and mounted at the top of the pipe? The truest saying I have ever heard is: The devil is in the details! non are too small.

Once the evaps and TXV's are dealt with then we can look on the condensing side of the equation.

The discharge temp can help tell us how our charge and system are behaving, to dense suction gass, or to much super heat raises our discharge, it will all so tell us the ruff quality of our oil in the system tell us if we are burning it or damaging the valves, it is often over looked as your statement shows us.

Discharge temp is another valuable measure that tells us allot about what is going on inside the compressor

[RANGER1]

Well if the insulation lagging is old there is your first thing to attend to, get that right, then start again!

On a TXV I personally would tune it till I saw about 10K super heat at the compressor (This ensures the evap is well flooded as it should be, after ensuring the pipe lagging was as sound as I could get it.

All ways start simple, is the evap clean? Are the fans running well? it all starts there, as your suction line pressure and density ultimately control mass flow and compressor loading, not enough we get an under performing system, too much we cook the compressor.

If the lagging on the suction line is poor then we are getting to much super heat where we do not want it, this increases discharge temps, lowers compressor loading, and reduces mass flow.

Start with the simple but not so obvious things. 16K is good on a cap tube system, not so good for a TXV system!

So check the pipe lagging then ensure all evaps are running correctly and clean. check the TXV's sense bulb, is it making good contact and mounted at the top of the pipe? The truest saying I have ever heard is: The devil is in the details! non are too small.

Once the evaps and TXV's are dealt with then we can look on the condensing side of the equation.

The discharge temp can help tell us how our charge and system are behaving, to dense suction gass, or to much super heat raises our discharge, it will all so tell us the ruff quality of our oil in the system tell us if we are burning it or damaging the valves, it is often over looked as your statement shows us.

Discharge temp is another valuable measure that tells us allot about what is going on inside the compressor

Mounting TEV bulb on top of suction line is to general & I do not 100% agree with.

It all depends on size of pipe, where you place it.

[The MG Pony]

that is indeed correct between 3am to 9pm on the pipe would be a more percice one I supose

[RANGER1]

10154Drew tis picture may satisfy you in regards to sub cooled liquid & liquid/vapour on P/E diagram

[Gary]

There is no concept in refrigeration that confuses more people than the static gas law which says you can't have superheat in the presence of liquid nor subcooling in the presence of vapor. The key word here is static, meaning non-energized. As soon as energy is applied, it all changes. It is only true when the system is off.

In a functioning system, it is entirely possible to have superheated vapor bubbles surrounded by subcooled liquid and subcooled liquid droplets surrounded by superheated vapor... and this is in fact common in portions of the liquid and suction lines.

Velocity carries the bubbles and droplets far past the points where you might expect them to disappear. That's why you need substantial amounts of subcooling to clear a sight glass and substantial amounts of superheat to avoid flooding the compressor. How much you need depends on the velocity.

It varies, but essentially you need 10-15F SC (5.5-8.5K SC) to clear the sightglass... and sometimes a little more. If any of the TXV's are flooding the coil, it should be fixed before you try to clear the sight glass.

[Gary]

You might find this thread helpful:

http://www.refrigeration-engineer.com/forums/showthread.php?19701-Refrigeration-101

[Drew]

Thanks Ranger1 and Gary. I like the sound of a static gas law as opposed to the other type.
ill see further what the inter web has to say.