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bernard
22-09-2006, 06:53 PM
Hi


I have a recently installed scroll pack and condenser on the roof of a supermarket.

The reciever is full but when I look in the sight glass I see the liquid in the bottom of the 1 3/8 liquid line trickling past.The drier is good,Head pressure 13 bar suction 2.7 bar.If I increase the head pressure the sight glass clears for a minute then returns to a trickle.Is this just lack of demand on pack or ????.



Regards Bernard

Peter_1
22-09-2006, 07:18 PM
No, only a lack of subcooled liquid, that's all.
When it passes through the sight glass, due to the velocity of the liquid, you have a small pressure reduction which forms flash gas.
Be carefull if the reciever is full while running: you propably won't have enough storage capacity in the reciever when all the units will shut down.

Peter_1
22-09-2006, 08:03 PM
Bernard, what's +/- the capacity of the system?
Just to recalculate once the size.

I think I gave you wrong information: i thought that you meant with trickle that you were seeing bubbles in the liquid and that they disappeared temporarily when increasing pressure.

But if I re-read your first message now, you only see a bottom of liquid passing through the sight-glass.

Mostly you see this with an oversized condenser, low outside temperature and a rather big demand for cooling.
I always explains this to my son as there is much more refrigerant pumped by the compressors from the evaporators then there's feed toward the evaporators.
The driving force to the coolers is not big enough.

So you need to increase pressure and/or increase subcooling.

But you only can gave partial gaseous refrigerant in your 100% departing liquid if the liquid is just at the edge of being not subcooled (just at the saturation line) and there's a very slight pressure increase (or temperature increase which isn't the case here) .

But, at low ambient as you have now, or/and during low load conditions, refrigerant velocity is low because enthalpy is increased, so pressure decrease will also be low during cold ambient.

Something must be wrong with my thinking.

My son just told me a similar phenomena: when load is low during cold and very cold ambient, you see liquid flowing through a vertical mounted sight glass and many times, you see that it fills just halfway.
Why just there and why +/- halfway the sightglass?
Why not lower or higher somewhere in the liquid line.

But it intrigues me also that I haven't THE answer for your question. So I will follow this post very close.

bernard
22-09-2006, 08:40 PM
Yes Peter I have suspected the condenser is oversized,I will find out the capacity of the equipment,I have no experience with sizing.

I took the head pressure up to 15 bar,but this did not clear it so its back at 13 bar,I can forsee problems over the winter with this site unless I can cure it.


Regards Bernard

chemi-cool
22-09-2006, 08:46 PM
I'd go with Peter way of thinking,

sound as the components are not all in the right size,

do you have any way to check it out?

Chemi:)

bernard
22-09-2006, 08:52 PM
Hi Chemi

I will take all the plate details of the eqipment and post them here.

Regards Bernard

monkey spanners
22-09-2006, 09:33 PM
Has the sight glass ever been full? If not is it possible that the reciever is piped up with the inlet as the outlet and hence no dip tube?
How much refrigerant is charged into it compared to the design charge?
Maybe what my old boss used to call overcondensing, too low a liquid line pressure/temperature, or am i showing my ignorance again;)

Cheers Jon

ps my old boss also used to say i belonged up a tree on the newbury bypass:mad:

Mark
22-09-2006, 10:08 PM
Is the reciever sized so system charge can be pumped down?.

US Iceman
22-09-2006, 10:40 PM
Bernard,

If I might ask... How do you know the reciever is full?

Mark
22-09-2006, 10:56 PM
Bit of a grey area,sometimes reciever data plates can be a misleading way of determining total charge.

NoNickName
23-09-2006, 10:09 AM
No, the charge here is done depending on subcooling. Same size of unit can have a difference of +/-10% depending on manufacturing tolerances.
I ripped the balance off the testing department. They have to look at manometers and thermometers, not at Kgs.

Mark
23-09-2006, 10:34 AM
This thread opens another question.

Determining an existing Pack install refrigerant charge.

Without reclaiming and weighing:D

Andy
23-09-2006, 11:18 AM
Hi Bernard:)

I have seen this too:D It happens during low load situation. In theroy it should happen when you have a cold receiver, where the charge wants to stay in the coldest spot. But I have seen in on housed acoustic packs with the receiver mounted on the pack.
From what I understand it's a good example of lanier (non turbulent) flow. This is due to the velocity of the refrigerant in the piping (the pipe is much too large for the duty at that point).

Hope this helps

Kind Regards Andy:)

NoNickName
23-09-2006, 02:49 PM
This thread opens another question.

Determining an existing Pack install refrigerant charge.

Without reclaiming and weighing:D

One simply can not

bernard
24-09-2006, 05:11 PM
Hi All


Sorry for the delay on my response as I,m on call at moment,I must also point out that there is no problem with case temps.

To answer a few question before I go out again:( The reciever has two bullseyes lower and upper part of recievers to indicate level.

Reciever is piped correctly as I also suspected this.

Hi Andy this lanier flow you mention,could this happen if the defrost schedule was incorretly spaced so at certain times of the day demand was low.

I was on site at 4am on another job and noticed the sight glass was full,I have the makes and models of all equipment Is it worth posting or not.


Regards Bernard

US Iceman
24-09-2006, 05:56 PM
HI Bernard,

Thanks for the feedback. Since the receiver has bulls-eyes and they indicate the receiver is full (as best as you can see), this would indicate the system has been pumped down, or at least partially.

This in itself brings up an issue as to whether the receiver has sufficient volume to accomodate a full pump down and still have at least 20% volume available as a gas space in the receiver.

On the other hand, if the receiver is showing full (at least to the top bulls-eye) the small trickle of liquid flowing through the main liquid line site glass would indicate a very small demand in relation to the maximum flow capacity of the pipe.

This would produce a condition known as laminar flow, and as Andy mentioned this type of flow is non-turbulent. In laminar flow the liquid (in this example) is essentially moving very slowly, so I would expect some areas of the pipe to not fill completely.

The low load condition which could cause this can be attributed to low demand (very little capacity required).

This could be from either the case temperatures being satisfied with only a very small load being present, or a majority of the cases being in defrost simultaneously.

As you said, the case temperatures are OK which leads me to believe the condition you are seeing is due to low load (or defrosts occuring together) at that precise time.

If the receiver is full and the system must operate during cold weather, what type of discharge pressure control does the system have?

The reason I ask is... If the condenser has controls to flood the condenser for cold weather operation, will the receiver or refrigerant charge be large enough?

bernard
24-09-2006, 06:47 PM
Hi Thanks for your last post:)

Head pressure is controlled with a Danfoss EKC 531B Capacity Controller via pressure tranducers.


Regards Bernard

markh
24-09-2006, 09:59 PM
im new to this posting lark, but being a commissioning engineer i would look at a couple of things and take these points into consideration.
firstly, if the plant is 1 or 2 floors above the cases/coldrooms you may have a suction pressure drop ,if so drop the target suction pressure down,increasing duty on the pack.Secondly ,check the stage delays on the pack.
Also, check the transducers are callibrated correctly as i have had a lot of them 1/2 to 1 bar out.

markh
24-09-2006, 10:14 PM
sorry feel like a complete donut as i didnt see the second page! defrost settings are critical but i will say that certain supermarkets will demand the site defrosts in islands so there aint a lot you can do if its one of them!

Peter_1
25-09-2006, 03:23 PM
This would produce a condition known as laminar flow, and as Andy mentioned this type of flow is non-turbulent. In laminar flow the liquid (in this example) is essentially moving very slowly, so I would expect some areas of the pipe to not fill completely.

If 100% pure liquid is departing from the vessel, why you see then 2 inches further in the liquid line at a sudden gasseous refrigerant?

I understand we can have a laminar flow but how is this 2-state refrigerant then formed?

I said previously, this is something I saw so many times and I never had THE answer for it.
As soon as you block the condensor with a paper, it disappears and sometimes, it comes back even with the paper in place in front of the condensor.

US Iceman
25-09-2006, 04:27 PM
Hi Peter,



I understand we can have a laminar flow but how is this 2-state refrigerant then formed?


My thought was the pipe is not flowing full, so the clear area of the site glass was simply a lack of liquid refrigerant flowing. In other words, there is not enough refrigerant flowing to fill the pipe.

I have seen this exact thing before many times also and it always seemed to coincide with low load operation, or other conditions which simulated low load conditions.

Placing a piece of paper over the condenser would increase the discharge pressure, so the pressure differential across the TXV would increase. The valve temporarily gains capacity so more refrigerant would flow, thus maybe filling the pipe more.

I seem to remember this is also seen with air-cooled condensers in the winter time if no head pressure controls are used.

With very low head pressure there is not a sufficient pressure differential to get the TXV to open enough. Even with the compressor running I believe the same low site glass flow was seen then.

That's my best guess right now.

bernard
25-09-2006, 08:08 PM
Hi

So in low load conditions does this mean that the cases in demand are not being supplied with a full bore of liquid.

Regards Bernard

DANIEL ARYEE
25-09-2006, 08:38 PM
Hi


I have a recently installed scroll pack and condenser on the roof of a supermarket.

The reciever is full but when I look in the sight glass I see the liquid in the bottom of the 1 3/8 liquid line trickling past.The drier is good,Head pressure 13 bar suction 2.7 bar.If I increase the head pressure the sight glass clears for a minute then returns to a trickle.Is this just lack of demand on pack or ????.



Regards Bernard
HI MY NAME IS DANNY. I WILL LIKE TO KNOW THE KIND OF REFRIGERANT YOU HAVE IN THE SYSTEM. I MAY BE ABLE TO HELP.

bernard
25-09-2006, 08:46 PM
Hello Danny

Its refrigerant 404a

Regards Bernard

US Iceman
25-09-2006, 09:10 PM
So in low load conditions does this mean that the cases in demand are not being supplied with a full bore of liquid.


Hi Bernard,

If I can turn this around some, I think the site glass appears to have this condition when the demand is low. If the demand is lower then you would have less refrigerant flow through the liquid line (which is now much larger than it needs to be for the reduced demand).

If the refrigerant mass flow is reduced, then the volume flow would also be reduced, hence the lower level in the site glass.

There may be sufficient refrigerant to feed the lower demand requirements (fewer cases/coils in operation).

I've also seen this on some systems where the discharge pressure is too low to feed liquid to the TXV's. There may be enough load to keep the compressor running on the LP switch. However, the discharge pressure could be just low enough to only allow a minor flow of refrigerant. This would eventually show up as a loss of temperature control for the space.

If the TXV's are only partially open due to low differential pressure they will not be feeding a lot of liquid. In this case, the liquid would back up into the receiver as if the system were pumped down.

These are the only times I have seen this type of incident occur.

What I don't know is the operating conditions of the system when you noticed this. If the day was much cooler, the discharge pressure control setpoint may have been too low. In effect, this could starve the coils due to the lower differential pressure available to obtain valve capacity.

If the system has multiple evaporators, were a majority of the coils off on temperature? This could then say the demand was lower with smaller or less coils in operation.

Does that make sense?

bernard
25-09-2006, 10:30 PM
Yes the was ambient was lower than normal due to high winds.How high do you think I can take the discharge set-point as 14 bar is higher than any other packs I have.

I was chatting to the in store tech who does the case cleans he said all the coils he washes are only iced up in the pack 2 or three passes.I,ve had that problem before and increasing the head pressure cures it.

I,ve been told in the past I look to deep into things,But the way I see it is a problem will find you sooner than later so its best to find it before it finds you.At the moment its a case he who shouts loudess gets served first.

I was there yesterday the reciever float was at the top of the upper buulseye,the sight glass was full/large bubble.I think I will recover some refrigerant,the recievers hold 115 litrs.

There an on-going problem with the monitoring system so accsess to to graph for evap and suction temps is out.

Thanks Bernard

US Iceman
26-09-2006, 02:49 AM
I was chatting to the in store tech who does the case cleans he said all the coils he washes are only iced up in the pack 2 or three passes.I,ve had that problem before and increasing the head pressure cures it.


What changes when the head pressure is increased? Obviously, the pressure increases so I suppose this affects something. It could be the higher pressure differential for the TXV.

It's been almost 20 years since I have done any real service work so I'm sort of rusty. But I seem to think a TXV with continued low head pressure will cause an inordinate amount of frost to form on the first few coil passes. With low head pressure the valve will not open enough, so the little bit of refrigerant that flows through the valve is all evaporated on these first few passes. Hence the heavier frost patterns. Once all of the liquid is evaporated you only have gas which just superheats.

I'm not familiar with controller you said was installed. Is it just a fan cycling switch of sorts?

The other thing could be the TXV's. I really like the balanced port valves as they react very good to low head pressure operation if they are selected correctly.

Are the valves that are installed balanced port valves? If they are not, and you only have fan cycling control for head pressure this could be part of the problem.

I think this is a good discussion and would like to see some input from others about their experiences with this.



But the way I see it is a problem will find you sooner than later so its best to find it before it finds you.


I call that pro-active. Or another way to phrase it is... do something before it bites you.

Peter_1
26-09-2006, 08:12 PM
What changes when the head pressure is increased? Obviously, the pressure increases so I suppose this affects something. It could be the higher pressure differential for the TX.

What also changes is the following (I think): the receiver is full of liquid and also the steel of the receiver has the same temperature. So we're in a saturated state and there's not so much movement in the liquid and there are also no big changes in load.

If we at a sudden increase pressure by stopping fans, restricting the air flow over the condenser, then we're going temporarily straight up in the log/P diagram, so making mechanical a bigger subcooling.

This increased subcooling will remain the same and slowly reduce till all the now warmer entering liquid in the receiver has mixed with the remaining colder liquid and the steel of the receiver has risen to this new 'mix-temperature' at a higher equilibrium temperature.

So the gaseous phase disappears due to a sudden increase in subcooling. (it's in fact not a real subcooling, USIceman mentioned some weeks ago a special term for this phenomena)

US Iceman
26-09-2006, 09:12 PM
This increased subcooling will stay and slowly reduce till all the warmer entering liquid in the receiver has mixed with the remaining colder liquid and the steel of the receiver has risen to this new 'mix-temperature' at a higher equilibrium temperature.


WOW! Thats really a very good translation from Dutch to English Peter. I'm envious of you.:D

I read the discussion you and Dan had on the SI vs IP units for the posted threads and keep forgetting that you have to translate twice for some of us (SI vs. IP & English to Dutch).

You do such a good job on the language translation it is hard to remember (at least for me) that you have two steps.:o

Dan
27-09-2006, 12:32 AM
My son just told me a similar phenomena: when load is low during cold and very cold ambient, you see liquid flowing through a vertical mounted sight glass and many times, you see that it fills just halfway.
Why just there and why +/- halfway the sightglass?
Why not lower or higher somewhere in the liquid line.

I have seen precisely what your son describes many times on Hussmann Superplus racks. I have also seen what was origianlly described in horizontal sightglasses... we call it a "lazy sightglass."

To be honest, the 1/2 full sightglass in the vertical line still mystifies me. How does the liquid ever get above the sightglass I ask myself.

But besides the mysterious aspects, we are seeing a saturated liquid. A bubbling sightglass is a high velocity saturated liquid and a "rivering" or "lazy" sightglass is a low velocity saturated liquid.

I wish it were the only mystery confronting me in this wonderful trade. :)

Dan
27-09-2006, 12:44 AM
So the gaseous phase disappears due to a sudden increase in subcooling. (it's in fact not a real subcooling, USIceman mentioned some weeks ago a special term for this phenomena)

By all definitions of subcooling it is real. I think Iceman used the word "unfavorable" versus "favorable". When we speak of physical properties such qualifiers are unnecessary and confusing. Increasing pressure on a liquid without any other changes, causes the liquid to be subcooled. Period.

US Iceman
27-09-2006, 01:36 AM
To be honest I don't remember what I called it. Too many things to store on the hardrive in my head (at least that is how I prefer to see it:rolleyes: ).

Subcooling is normally easy as most people think of it in terms of the liquid being cooled at a constant pressure. Since the temperature does change in this instance the term subcooling is easily understood.
(temperature dependent)

On the other hand, if the liquid pressure is increased and the temperature does not change the liquid is still subcooled. (pressure dependent)

This is the type of subcooling (pressure dependent) that sometimes confuses people since the term subcooling is usually related to a temperature change. I think is what Peter is describing as "not real subcooling" from one of my earlier posts. It is indeed subcooling, NOT just the type of subcooling people usually think of first.

This can be as confusing as superheat (sometimes it can be considered useful, other times it can be considered non-useful). It depends on how the superheat affects the Net Refrigeration Effect per pound (or kg) and the specific volume of the refrigerant for the compressor performance.

In the example of superheat these qualifiers are indeed the correct ones to use. In attempting to describe the different types of subcooling I may have created some mis-leading terminology.

I believe if we think of subcooling in terms of temperature or pressure dependent it may be more clearly understood.

Dan
27-09-2006, 03:07 AM
Ah... that's what you called it. Not Favorable/non-favorable, but Useful and non-useful. Pressure dependent or temperature dependent are good qualifiers as well.

But let the definition stand by itself:

Superheat is the difference between the temperature of a gas above the boiling temperature of that gas at a common pressure.

Subcooling is the difference between the temperature of a liquid below the condensation temperature of that liquid at a common pressure.

Qualifiers are fine as long as they add context to a term. But the definitions of subcooling and superheat must stand alone. Each represents a relationship of temperature and pressure to a fluid. And there are some confusing and misleading definitions abounding. For example, in Websters:

"Superheat: To heat a liquid above its boiling point without converting it into vapor." The truth is that such a liquid is subcooled. That is, of course until you take the cap off the radiator.:)

Whether a superheat or subcooled status is useful or not, or detrimental or beneficial, is purely a contextual qualification. Gravity is bad when I am dropping from an airplane without a parachute, but good when it draws me back to earth when I jump. But as a force it does not require qualification.

Superheat and subcooling are simply relative states that have no implications toward good or bad, enthalpy, or net refrigerating effect.

3 degrees superheat is wonderful at the outlet of an evaporator, but frightening at the inlet of a compressor.

30 degrees subcooling sounds like a good thing, but it would be better to reduce it to a minimum by lowering the pressure on the liquid.

It's the old tech writer in me that causes me to be such a pain in the ass. I only argue such fine points with those whom I have the highest regard. :)

US Iceman
27-09-2006, 04:05 PM
Hi Dan,

I certainly don't find any fault with your basic definitions of subcooling or superheat. Those are what we have used for a long time and were probably taught in our formative years.

The definition from Websters is totally misleading.:(

The main issue I think is how to describe the different methods of obtaining subcooling. Peter has said this before, but I will repeat again.

On a P/H chart when the state point is in the liquid region an increase in pressure will create subcooling. That is absolutely true and based on good theory.

The problem lies in the fact that the actual liquid temperature has not changed, which is what the definition of subcooling presents, i.e., a temperature change.

We have in fact created a riddle which can be misunderstood.

I believe the problem arises when we forget to use the term "saturated" when dealing with pressures and temperatures. In effect, what we call temperature is not entirely correct. As with most things we work with there are two of everything.;)

Temperature is another variant of where qualifiers should be used, so I'll add to the already stirred pot.:D

Actual temperature or saturation temperature.

The actual temperature of course describes the exact temperature that can be measured with a thermometer.

Saturation temperature on the other hand can be found by the use of a P/T chart or P/H diagram. It however can also be found when the liquid/vapor co-exist in equilibrium. At this point, the saturation temperature becomes the actual temperature.:D

My basis of thought on all of was developed during my days doing service and later reinforced when performing system designs.

At any point in a refrigeration system we have pressures and temperatures co-existing. Always.

In finding problems we need to understand how these interact and their relative effect and impact on component performance (TXV's, compressors, condensers, piping, etc.).

Most people who have not worked on screw compressors have never heard the term discharge superheat. Most however, know what evaporator superheat is.

"Discharge" and "evaporator" are qualifiers for where the superheat is being determined.

"Useful" superheat occurs in the evaporator as it contributes to the Net Refrigeration Effect.

"Non-useful" superheat is just that. It is just adding heat to the vapor which provides no benefit to the system performance. In fact, this "type" of superheat lowers the performance of the compressor by increasing the specific volume of the vapor, which in turn reduces the mass flow circulated.

So... What I think this can be summarised as... we need to be careful when describing this pressure/ temperature relationship of the refrigerants.

The qualifiers we use to describe the various state points in a refrigeration system are useful in describing the actual operation.

They also help to find the problems after the system is installed, or help to prevent the problems from being created during the initial system design.



It's the old tech writer in me that causes me to be such a pain in the ass.


:D :D

It's the old service man in me that makes me like this!:p

Dan
11-10-2006, 04:04 AM
So... What I think this can be summarised as... we need to be careful when describing this pressure/ temperature relationship of the refrigerants.

Yes, Mr. old service man.

We should simply use pressure temperature relationships.

And we should record where we took each measurements.

And we should pretty much let these measurements stand proudly for analyisis alone. Neither useful, profitable, good, bad, indifferent, etc.

Proud and solid measurements whose stripes are in pressure and temperature relationships... hopefully in the proper units of measurements to be understood by the technicians on call.

I have not weighed my position lightly. Subcooling and superheat must stand alone as pressure-temperature relationships. We can add the qualifiers only after purity of measurement. There is no such thing as false subcooling because non-condesnibles might exist... there is no such thing as reverse superheat when the pressure is measured in the wrong place.

There is no "useful" subcooling, or other adjectives to be applied to superheat.

Subcooling and Superheat measurements must be without adjectives, opinions, or desires.

They can only come after seeing an accurate pressure-temperature relationship with a known fluid.

This has to be the firm ground we stand upon when discussing refrigeration.

US Iceman
13-10-2006, 10:19 PM
Hi Dan,

I just saw your reply. I have not been ignoring you.:)

It is not my desire to beat a dead horse, but I'll wrap up my comments with the following statements.

Some of the measurements and methods of explanations I have used in this discussion are my attempt to show some of the strange findings that can be discovered during service or design processes.

All of these are founded on the refrigerant properties and/or the way the refrigerant interacts with the system in operation (or by design or installation).

Any adjectives used are solely meant as a poor attempt (on my part) to describe some of these unusual behaviours and the strange ways they might affect the system operation.

In closing up with this I'll add some final thoughts... Most of the stuff we have been taught is correct, but I will go and remain on record by saying, there are some strange things that can happen in a refrigeration system that can be difficult to explain without reverting to various means.

All in all, this has been a very good discussion.;)

Dan
13-10-2006, 11:59 PM
All in all, this has been a very good discussion.

I agree. And please do not think that I do not respect your concern for "non-useful" subcooling or "none-useful" superheat. Once we stand on the firm rock of accurate measurements, your qualifications are essential in further analysis and discussion.

And yes, there are strange things that can happen in a refrigeration system that can be difficult to explain without reverting to various means. My example of non-condensibles causing subcooling is a wonderful instance of a phenomenom that I have difficulty explaining even to myself, let alone to others. :)

US Iceman
14-10-2006, 01:10 AM
Once we stand on the firm rock of accurate measurements...


This is absolutely essential, otherwise it's just and argument for the sake of arguing when trying to discuss this stuff.

Some of this is very close to the following...

Is a peanut a vegetable or a nut to a squirrel? The squirrel might say, who cares?:D

Dan
14-10-2006, 02:32 AM
Is a peanut a vegetable or a nut to a squirrel? The squirrel might say, who cares?

Precisely. Because we call it a nut and use it as a nut in our mixed nut cans, no wonder squirrels really prefer grade A corn.

Dan
14-10-2006, 02:33 AM
I promise. I am stopping now. This was a fun discussion though!

taz24
14-10-2006, 09:26 PM
And yes, there are strange things that can happen in a refrigeration system that can be difficult to explain without reverting to various means. My example of non-condensibles causing subcooling is a wonderful instance of a phenomenom that I have difficulty explaining even to myself, let alone to others. :)[/quote]


I'll give you strange things that happen in a fridge system.
Liquid in a receiver is subcooled. The vapour therefore cannot exist above it - but it does.
Liquid in a eceiver is subcooled and yet it is in contact with vapour;)

Cheers taz

Peter_1
14-10-2006, 11:55 PM
Liquid in a receiver is subcooled. The vapour therefore cannot exist above it - but it does.
Liquid in a eceiver is subcooled and yet it is in contact with vapour;)

??Liquid in a receiver is mostly in a saturated condition, reason why gas and liquid can be seen together in a receiver. This seems very logic for me.
In the log/P diagram, gas and vapour can be found together at same temperature and same pressure, so...???

Andy P
15-10-2006, 04:19 PM
We should simply use pressure temperature relationships.

And we should record where we took each measurements.


This thread is in danger of becoming a loop - Dan made the point very well a few posts ago that the readings you take can be at best meaningless and at worst misleading if you don't know where they came from. Don't forget that some changes in refrigeration systems happen pretty much instantly - eg pressure increases - whereas others are time dependent - eg heat transfer through liquid.

You might have the appearance of subcooled liquid in a receiver along with gas if either
a) the liquid at the surface is at the saturation temperature but the lower level liquid is colder and the temperature has not had time to equalise, or
b) if there is air in the system - hence the talk in earlier posts about non-condensibles and "apparent" subcooling

I was told years ago that the most useful bit of kit for a service tech would be a pair of the X-ray specs you used to see advertised in Marvel Comics (did anyone ever buy a pair???) In the absence of these you need to make do with a combination of a vivid imagination and Sherlock Holmes' famous saying "Eliminate the impossible, and whatever remains, however improbable, is the truth." Don't be fooled into failing to differentiate between the truly impossible and the norm in refrigeration - good old fashioned "misleading".

A bucket of ammonia is a good example: if you take a standard metal bucket half full of liquid ammonia and sit it in the open air the liquid will not boil off rapidly, but will sit quietly in the bucket looking like water. Why? If you measure the temperature of the liquid in the bucket it will be about -50 oC (-58 oF). Why? Ammonia is supposed to boil at -33 oC at atmospheric pressure....

cheers
Andy P

taz24
15-10-2006, 07:03 PM
??Liquid in a receiver is mostly in a saturated condition, reason why gas and liquid can be seen together in a receiver. This seems very logic for me.
In the log/P diagram, gas and vapour can be found together at same temperature and same pressure, so...???

Yes same pressure and temp = saturated state.
But liquid in the receiver is subcooled. Liquid at a lower temp than the condensing temp is subcooled.
Liquid coming out of the condenser is subcooled, liquid in the liquid line is subcooled so liquid in the receiver is subcooled.

Cheers taz.

US Iceman
15-10-2006, 11:30 PM
Here is a nice pearl of wisdom to remember when working on refrigeration systems. It ties in nicely with the comments of Andy P. Text courtesy of Wikipedia...

Occam's razor (also spelled Ockham's razor) is a principle attributed to the 14th-century (http://en.wikipedia.org/wiki/14th_century) English logician (http://en.wikipedia.org/wiki/Logician) and Franciscan (http://en.wikipedia.org/wiki/Franciscan) friar William of Ockham (http://en.wikipedia.org/wiki/William_of_Ockham) (Guilhelmi Ockam and Guillermi de Ockam in Latin (http://en.wikipedia.org/wiki/Latin) [1] (http://www.hds.harvard.edu/library/collections/rare_books/incunabula_list.html)). Originally a tenet of the reductionist (http://en.wikipedia.org/wiki/Reductionism) philosophy of nominalism (http://en.wikipedia.org/wiki/Nominalism), it is more often taken today as a heuristic (http://en.wikipedia.org/wiki/Heuristic) maxim (http://en.wikipedia.org/wiki/Maxim) that advises economy, parsimony (http://en.wikipedia.org/wiki/Parsimony), or simplicity in scientific theories (http://en.wikipedia.org/wiki/Theory#Science).
Occam's razor states that the explanation of any phenomenon (http://en.wikipedia.org/wiki/Phenomenon) should make as few assumptions as possible, eliminating, or "shaving off," those that make no difference in the observable predictions of the explanatory hypothesis (http://en.wikipedia.org/wiki/Hypothesis) or theory (http://en.wikipedia.org/wiki/Theory). In short, when given two equally valid explanations for a phenomenon, one should embrace the less complicated formulation. The principle is often expressed in Latin as the lex parsimoniae (law of succinctness):
<DL><DD>entia non sunt multiplicanda praeter necessitatem, </DD></DL>which translates to:
<DL><DD>entities should not be multiplied beyond necessity. </DD></DL>(That is, the fewer assumptions an explanation of a phenomenon depends on, the better it is.)
Furthermore, when multiple competing theories have equal predictive powers (http://en.wikipedia.org/wiki/Predictive_power), the principle recommends selecting those that introduce the fewest assumptions and postulate the fewest hypothetical entities. It is in this sense that Occam's razor is usually understood.



I was told years ago that the most useful bit of kit for a service tech would be a pair of the X-ray specs you used to see advertised in Marvel Comics (did anyone ever buy a pair???)


I never bought a pair, but still remember seeing the advertisement.:)

Another good visualization technique is to imagine one molecule of refrigerant flowing through the system and the changes in pressure and temperature that can occur.

winfred.dela
16-10-2006, 04:34 AM
. . .
Another good visualization technique is to imagine one molecule of refrigerant flowing through the system and the changes in pressure and temperature that can occur.


Thanks US Iceman, :)
It's nice piece of advise to simplify and understand it easily.

Been following this thread and it is becoming kind of like the thread Liquid Receiver:

http://www.refrigeration-engineer.com/forums/showthread.php?t=4365&highlight=liquid+receiver

US Iceman
16-10-2006, 05:30 PM
Yes, it seems like we have been having guite the go on the receiver subject lately. It's amazing how much fun you can have on such a simple vessel.:D

taz24
16-10-2006, 11:17 PM
Iv'e seen a 7kg dumpy used as an extra receiver once. It was working well, the two valve on top one for liquid in and the one with the dip tube for liquid out.

Cheers taz.

US Iceman
17-10-2006, 01:03 AM
Iv'e seen a 7kg dumpy used as an extra receiver once. It was working well, the two valve on top one for liquid in and the one with the dip tube for liquid out.


Now don't take this the wrong way, but I would be a much happier camper if this sort of practice was not advertised or posted.

Before you know it, someone else will start to do this since they saw it posted on the RE site and the next thing you know... more people are doing it.:(

It's hard enough to get people to do it right without giving them information on how to do it improperly.:o

taz24
19-10-2006, 01:20 PM
It's hard enough to get people to do it right without giving them information on how to do it improperly.:o[/quote]

Ok point taken:)

I will pay at little more attention to what I post.

Cheers taz.

Andy P
19-10-2006, 09:36 PM
Yes same pressure and temp = saturated state.
But liquid in the receiver is subcooled. Liquid at a lower temp than the condensing temp is subcooled.
Liquid coming out of the condenser is subcooled, liquid in the liquid line is subcooled so liquid in the receiver is subcooled.

Cheers taz.


Maybe I am too gullible Taz, but I have no problem believing that a condenser can produce a flow of subcooled liquid and at the same time there can be a flow of subcooled liquid out the bottom of a receiver while there is gas in the top of the receiver. However I am certain that the temperature at the liquid surface in the receiver will be at saturation temperature. It seems to me perfectly feasible in a dynamic system with flow through the receiver to have a few degrees temperature gradient across the depth of the liquid. If the plant stops and there is no flow then the temperature in the receiver will gradually equalise and some of the gas will condense onto the surface of the liquid as the temperature falls. This will cause a drop in receiver pressure until the system stabilises. This can be quite dangerous because it is possible to overfill the receiver by "syphoning liquid into it".

No answer to my questions about the bucket of ammonia?

cheers

Andy P

Andy
21-10-2006, 12:06 PM
A bucket of ammonia is a good example: if you take a standard metal bucket half full of liquid ammonia and sit it in the open air the liquid will not boil off rapidly, but will sit quietly in the bucket looking like water. Why? If you measure the temperature of the liquid in the bucket it will be about -50 oC (-58 oF). Why? Ammonia is supposed to boil at -33 oC at atmospheric pressure....

cheers
Andy P

I have seen this also with R22:) must be not enough surface area to start the boiling process, that's a guess:) when you look into the bucket there is always a cloud of gas hanging around, this is also stopping the boiling, cold vapour falls trapping in the bucket, stopping any more boiling. I suppose it's also down to latent heat capacity, a large thermal mass with a high heat capacity surrounded by a mass of air with a poorer heat capacity

Kind Regards Andy:)

taz24
21-10-2006, 02:34 PM
I have seen this also with R22:) must be not enough surface area to start the boiling process, that's a guess:)
Kind Regards Andy:)


I agree with you I have experienced a simlar problem with R22. I have known it to go into a state of dormancy or hibination (not very technical but a good description) when being reclaimed and the pressure is reduced to 0barg. Allowing the system to warm up and equalize will sort the anomily but it is an unusual situation.

Cheers taz.

Andy
21-10-2006, 09:00 PM
I agree with you I have experienced a simlar problem with R22. I have known it to go into a state of dormancy or hibination (not very technical but a good description) when being reclaimed and the pressure is reduced to 0barg. Allowing the system to warm up and equalize will sort the anomily but it is an unusual situation.

Cheers taz.

I've seen R22 in a bucket, just sitting there as a liquid:)
was a while ago, I suspect no one would normally use a bucket to recover R22:D
The R22 was trapped in the bottom of an Insulated LPR vessel and drained out when a vavle was being replaced.

Kind Regards Andy:)

Dan
22-10-2006, 02:40 AM
In short, when given two equally valid explanations for a phenomenon, one should embrace the less complicated formulation. The principle is often expressed in Latin as the lex parsimoniae (law of succinctness):


My first smile is that quoting in two languages disobeys the rule. My second smile is regarding the two equally valid explanations, and the embracing of the less complicated formulation.

This thread..or whatever you call these things... "embraces" what I consider the least explained, understood,or misunderstood phenomenom in our industry.

The properties of saturated liquids and vapors.

There is the razor, Mr. Iceman.

Can a vessel hold a subcooled liquid and also have vapor in it? I can pump air into a tank of R22 and achieve subcooling. Is this reason enough to redefine subcooling? Or is it better to examine possible causes of subcooling.

Where is the firmament?

Occam's razor reminds me of Solomon's decision. I have always seen these things as somebody attempting to tell us we should use our own minds and make correct decisions instead of turning to authority.

Dan
22-10-2006, 02:55 AM
It is my preference to refer to subcooling and superheat as simple pressure/temperature relationships, assuming there are no foreign compounds or vapors present.

My reason is simple. I can use measurements that are beyond parameters I consider normal to consider the existence of non-condensibles, or mixed refrigerants.

Pure measurements and published properties must be our standard. Qualifications and amendments are the wonders we can debate. Andy's apparent aberration of the saturation temperature of a refrigerant can be explained, once we debate the details.

This has been a worthy source of debate regarding bubbles. :)

winfred.dela
22-10-2006, 09:53 AM
I have seen this also with R22 :) must be not enough surface area to start the boiling process, that's a guess :)


Will surface tension or thermodynamic theory of capillarity a factor in this phenomenon?

I hope a physicist will come across this thread/post and shed light to this phenomenon. :)

taz24
22-10-2006, 05:48 PM
I hope a physicist will come across this thread/post and shed light to this phenomenon. :)


Nah!! no need for Physicst-ist-tist or what ever:p

We need a big enough bucket so we can all look in:)


Cheers taz.

winfred.dela
23-10-2006, 01:14 AM
Nah!! no need for Physicst-ist-tist or what ever:p

We need a big enough bucket so we can all look in:)


I just don't know your learning method but for me i've always enjoyed reading things i do not understand.

If you wanted to understand what am trying to say, please verify the link below:

http://www.refrigeration-engineer.com/forums/showpost.php?p=40029&postcount=105

Maybe better, try to understand the whole thread: Liquid Receiver. :rolleyes:

Cheers
winfred.dela :)

Andy P
24-10-2006, 05:23 PM
OK - time for some answers. Hopefully this will not get too heavy. The answer to the questions about buckets lies in Dalton's law of partial pressures. According to Dalton (and who am I to argue!) each gas in a mixture behaves as if the other gases were not there: pretty antisocial if you ask me! The total pressure of the mixture of gases is made up of a component of pressure for each of the gases. If it were a 50/50 mixture each gas would behave as if it were the only thing filling the volume. In the bucket of ammonia (or R-22) there is a layer above the liquid which is a mixture of refrigerant gas and air, and so (provided there's not a strong breeze blowing) the total content of the bucket can be thought of as a sealed system. The ammonia behaves as if the air wasn't there and so has a partial pressure lower than atmospheric pressure. Hence it boils at a temperature lower than -33C. The high latent heat of ammonia means that you can get lots of "subcooling" by this effect - typically the temperature of -50C that I mentioned before. This is why the R-22 in Andy's bucket is sulking - although the temperature dip is probably not as impressive as ammonia because the R-22 latent heat is much less.

Understanding this is key to knowing how a ncg purger works: the liquid coming out the condenser seems to be subcooled because the temperature is lower than the sat. temp at the total pressure ought to be. However if there is air present then the air provides a portion of the total pressure and the refrigerant will be at a temperature equal to saturation for its "proportion" of the total pressure - because it is behaving as if the air wasn't there (ie condensing at a lower temperature). The killer here is that in order to get the condensing temperature high enough to reject heat to atmosphere when there is air in the system the compressor has to work harder because it has to provide the total pressure, not just the refrigerant proportion of it.

The difference between total pressure and refrigerant temperature in the case where air is present is called "apparent" subcooling because it looks like subcooling, but as far as the refrigerant is concerned it is condensing at the temperature equivalent of its share of the pressure ie it is not really subcooled.

check http://en.wikipedia.org/wiki/Dalton's_law for more info - and let me know if any of this makes sense!

cheers

Andy P

winfred.dela
29-10-2006, 01:50 PM
OK - time for some answers. Hopefully this will not get too heavy.



A little. . . heavy. :confused:

Need to read more re Dalton's Law

Thanks for the explanation. :)

everton
29-10-2006, 02:02 PM
I'm new to this site and wanted to start a new thread but could not find where to do it. I recently defrosted a fridge freezer by leaving the door open on the freezer but leaving the fridge on. This was left for over a day and i eventually turned the fridge and freezer off all together. Now neither the fridge or freezer work and it smell awful. Is this refrigerant leaking ? I'm a bit converned as the whole kitchen smells now. Any help would be really appreciated.

taz24
29-10-2006, 08:04 PM
I'm new to this site and wanted to start a new thread but could not find where to do it. I recently defrosted a fridge freezer by leaving the door open on the freezer but leaving the fridge on. This was left for over a day and i eventually turned the fridge and freezer off all together. Now neither the fridge or freezer work and it smell awful. Is this refrigerant leaking ? I'm a bit converned as the whole kitchen smells now. Any help would be really appreciated.

To start a new thread just go to the topic you want and then just to the top left of the border is a box with the start new icon..

Cheers taz.

taz24
29-10-2006, 08:18 PM
This was left for over a day and i eventually turned the fridge and freezer off all together. Now neither the fridge or freezer work and it smell awful. Is this refrigerant leaking ? I'm a bit converned as the whole kitchen smells now. Any help would be really appreciated.

Fridges fail for lots of reasons and one symptom they show before they fail is the symptom of being iced up. Then when you defrost the thing and switch on, it does not recover and fails.
Most fridge freezers are one system, just one colder that the other and to defrost one part you must turn it off at the plug and wait 24 hours.
Some are two seperate systems joined together to form one fridge-freezer, turning one of these off will not effect the other.
Your system sounds like the first. If it is smelling and not getting cold then there is somthing wrong with it.
With it switched on put your ear to it and listern if it is running. If it is and it is not cold you have a serious fault that may cost ££££s to sort out. If it is quiet it could be the stat / controler, which is easily repaird.
How old is it. Repairing domestics can be half the price of a new one.

Cheers taz.

Bones
06-11-2006, 10:13 AM
404a is a zeotrope, have you thought that what you are seeing is the glide through the site glass in its superheated state?

taz24
06-11-2006, 10:57 PM
404a is a zeotrope, have you thought that what you are seeing is the glide through the site glass in its superheated state?

Superheated state in the liquid line?
Are you trying to start another fight?:)

Cheers taz.

mr cool
12-11-2006, 05:49 PM
i have had problems with liquid sitting in condensers before now, but have also seen this before when engineers are convinced that pack is over condensing but is actually short on charge (reciever level indicator stuck showing 40%, faulty indicator) is there any heat left on liquid line or is it at ambient?

del boy
22-11-2006, 07:08 AM
hello you may have allready solved this problem if not i remember in the late seventies we had similar problems with liquid sitting in the receiver due to pressure through condenser and condenser back up valve this was over come buy fitting an ord 15% diff valve between the discharge pipe and top of the reciever so if liquid pressure dropped 15% below discharge the ord would open and push the liquid out and down to the expansion valves may be old hat now regards del boy.

Mark
23-11-2006, 07:30 PM
Hi del boy

Yes there are still a few ORI,ORD sites out there.Common set up for cool/hot gas defrost.
Will energy saving prove that gas defrost will not return??.

Kind regards. Mark

Capt Steve
28-11-2006, 06:55 PM
If you have an oversized condenser, you will have problems in low ambient conditions. Have you looked at piping the condenser as a split condenser with a splitting valve, and only using the section for low ambient temperatures?

Electrocoolman
29-11-2006, 03:33 AM
I recently defrosted a fridge freezer by leaving the door open on the freezer but leaving the fridge on. This was left for over a day and i eventually turned the fridge and freezer off all together. Now neither the fridge or freezer work and it smell awful. Is this refrigerant leaking ? I'm a bit converned as the whole kitchen smells now. Any help would be really appreciated.


Everton,
If compressor not running smell could very well be from PTC starter overheating / breaking down....these pong alot.
What make/model of compressor....Danfoss?

Better start your reply as a new thread...and not a post on this thread otherwise it will get lost!

johnyfreon
03-12-2006, 11:12 PM
I was taught that due to the saturated conditions (vapor liquid interface) inside the receiver all subcooling is lost, where the heat or the the pressure drop that takes the subcooling away comes from?

DeB
11-12-2006, 08:52 AM
I have read everyones contribution and found Peter,Andy and Icemans interpretations all helpful. I think johny***** has part of the answer also. I'll give you guys out there my theory and will hope you will give your feedback. Firstly, if we assume no subcooling after the receiver,we can assume as the saturated liquid travels down the liquid line it is picking up losses causing flash gas. If the liquid line has too much slope. IE where the slope is greater than the slope required to balance out the gravitational force and pressure drop,to prevent flash gas.This causes vapour to backflow along the top of the liquid, causing the river effect.On high ambient days the liquid receiver is warmer and the pressure is higher. The liquid PD needs to be higher to produce flash gas combined with a possible higher temperature loss in the liquid line between receiver and TXV. in effect causing little or no flash gas. On colder ambients the receiver is cooler combined with less PD required to produce flash gas and overall temperature change from receiver to TXV is less, therefore less PD compensation and more gas flashing. Combined with liquid line slope we get a river effect. As I said ,only a theory I would be very interested in all of your feedback wether you agree or think I just forgot to take my medication.

Andy
11-12-2006, 12:09 PM
Hi DEB:)
think about the air temperature around the liquid line as it travels to the expansion valve. In the summer the ambient around the liq line is 21 deg c and above. Condensing temperature is above this so the liq line looses heat to the ambient and becomes subcooled:)
In the winter if the head pressure is allowed to float the plant could be delivering liquid at a condensing temperature at 18 deg c or lower:) the ambient temperature in the shops will usually be 21 deg c or around that, so the liq line picks up heat causing the liquid to flash, leaving poor quality liquid to feed the expansion valves:eek:

Kind Regards Andy:)

Dowadudda
12-12-2006, 05:22 PM
That would be my question. How did we find the receiver full? If that receiver is full and that receiver is piped correctly, that liquid should be at the sight glass. What is your subcooling measurment outlet right off the condenser prior to it reaching receiver? The raising of pressure, and your witnessing full column of liquid in for a period of time is telling me, your receiver is not full. The higher pressure is actually forcing the liquid sitting on the bottom of the receiver out. I am saying right now given the thoughts, you don't have a enough gas in there.

Dowadudda
12-12-2006, 05:27 PM
I was taught that due to the saturated conditions (vapor liquid interface) inside the receiver all subcooling is lost, where the heat or the the pressure drop that takes the subcooling away comes from?


That is false. Any field subcooling measurement on the liquid line would tell you that is an incorrect falsehood.

The fact there does exist a liquid to vapor interface, allows for the fact that all liquid beneath maintains for the most part the subcooling it got leaving the condenser.

DeB
13-12-2006, 12:01 AM
Andy, your point is valid. I was thinking in terms of pipes in roof space temperatures of 50C+when ambients are 40C+.Your summer temp of 21C+ is closer to our daytime winter temperatures.Maybe the answer lies in a combination of factors rather than one specific factor.An interesting fact from the debate is our different operating ambients.I would assume that we in much of OZ are generally more experienced in fixing hot weather problems while you guys are more experienced in fixing cold weather problems.Although I think the cold weather problems associated with keeping good refrigerating temperatures can often be more complex.I suppose fore you Iceman it depends if your working in Washington State in Winter or Palm Springs in the middle of summer! Re Dowadudda s assumption. My argument would be the receiver liquid is at saturation and as sub cooled liquid from the condenser enters the heat exchange will cause some of the vapour to condense in the receiver bringing the sub cooled liquid back to saturation temperature.

US Iceman
13-12-2006, 01:58 AM
This thread is certainly getting some mileage out of it!:D

To go back to a basic concept, it is important to recognize the liquid line line temperature versus the surrounding ambient temperature.

If the ambient temperature is lower than the liquid line temperature, the heat flows from the liquid line to the ambient.

Conversely, if the ambient temperature is higher than the liquid line temeprature, the heat flows into the liquid line with all of the problems that entails.

This also applies to receivers. Heat gain or heat loss is the key point to remember.

It's just not about the location in a specific country or time of year. That's only part of the issue.

Consider this... If a liquid line is installed vertically on the outside of the building (to get the liquid to the TXV's mounted above) you have several issues; static pressure loss due to the lifting of liquid, friction loss due to the pipe & fittings/valves, and possible solar heat gain on the liquid line itself. In this case you would need a fair amount of subcooling to overcome these issues.

If not, then you will have problems with flash gas in the liquid line.

On the other hand, if the liquid line was installed vertically inside of the cooler/freezer this helps to cool off the liquid line. The cold air blowing around the liquid line certainly helps. It might also be interesting to use some finned tube similar to baseboard radiators (as part of the liquid line) to enhance the heat transfer from the cold air to the warm liquid line.

The trick would be to estimate the amount of subcooling so you don't gain too much TXV capacity.

Dowadudda
13-12-2006, 03:47 PM
What is the definition of subcooling? Subcooling is the measure of Liquid Refrigerant 1 degree or more below saturation. So if your theory of what goes on in the recevier is true, then how can I have any liquid?

The question is answered in a very complex way, to which I am not nearly smart enough. I have heard it explained and still don;t quite get it. As a mechanic, I have to know that most of the integrity of my subcooled liquid stays during it's time in the recevier.

US Iceman
13-12-2006, 04:18 PM
Subcooling in the condenser is a questionale practice in my opinion. Unless, the condenser is sized accordingly to provide additional heat transfer surface for the liquid without the discharge pressure increasing.

If you are using the condenser for subcooling you could be loosing condensing capacity. The cost of the higher discharge pressure is more than the benefit of the subcooling.

Having said that, if the receiver is at a temperature greater than the air temperature, which it normally is during the summer with air-cooled condensers you don't have a problem.

Andy
13-12-2006, 10:47 PM
What is the definition of subcooling? Subcooling is the measure of Liquid Refrigerant 1 degree or more below saturation. So if your theory of what goes on in the recevier is true, then how can I have any liquid?

The question is answered in a very complex way, to which I am not nearly smart enough. I have heard it explained and still don;t quite get it. As a mechanic, I have to know that most of the integrity of my subcooled liquid stays during it's time in the recevier.

Hi Dowadudda:)

if the temperature (measured by a thermometer) of the liquid is anything measureable below the pressure read off you gauges converted to temperature then you have subcooling.

I have measured liquid off receiver tempeartures and found the liquid both subcooled and saturated.

What happens is the liquid flows into the receiver and as it moves into a bigger area it expands back to saturated liquid:) but you can still get subcooled liquid out of the receiver as the fans cycle on the condenser. Fans off and the pressure rises and the liquid is still at the temperature it entered the receiver so it becomes subcooled (because the liquid in the receiver is cooler than the pressure on the gauges) fans on and the gauge pressure drops and the liquid is saturated again.

Hope this helps:)

Kind Regards Andy:)

DeB
14-12-2006, 03:03 AM
I think its also useful to remember that the PD in the condenser will cause a different pressure reading in the receiver depending on where the pressure is read and also the level is rising and falling due to demand. This causes slight increase and decreases in pressure as the volume of vapour above the liquid changes. All very nice but can someone give a viable alternative answer to the original question of what causes the river in the sight glass if they believe my theory/answer is not correct.

Rtic
15-12-2006, 12:37 AM
Maybe you should consider changing the Orifice to the next size. This should decrease the head pressure and all your problems will be solved (fingers crossed!).

M Nasir khokhar
15-12-2006, 11:16 AM
dear, if the temprature of displays and cold room are ok, so the compressors will be also ways run on demand of tempratures, so first check the temprature of dispalys and cold rooms, 2nd the ambeanit temprature shall be also check

Andy P
17-12-2006, 11:17 PM
You've got to remember that pressure changes happen quickly, but temperature change takes time - this can cause subcooling if the pressure is increased without adding heat. You also need to bear in mind that you are thinking of a flow process - I doubt that the liquid travelling at several metres per second is in the pipe long enough to pick up/reject significant amounts of heat. Put it another way: if you were designing a heat exchanger it wouldn't look like that!

Final thought on the subject (for tonight anyway) - if you feed subcooled liquid into a receiver with a liquid/vapour interface then the gas above the liquid surface will tend to condense onto the surface of the liquid. Turning gas to liquid warms up the liquid that it is condensing on, and also reduces the pressure, so the two parameters that originally indicated subcoling tend to converge. Eventually there will be no more subcooling: the liquid has been warmed and the pressure has been reduced - both by the effect of condensation - until there is no more heat transfer.

This process can be very dangerous in dealing with hp receivers, or when decanting liquid into a recovery cylinder. It can be possible to overfill the vessel, even if the gas inlet pipe is set partway down the vessel because the condensing of gas onto subcooled liquid draws the liquid up above the inlet pipe. The space above the gas inlet pipe is called "ullage" and the designer allowed for it deliberately so that the vessel would not overfill - but there's nothing he can do to prevent condensation filling the vessel right up. If this happens then there could be a serious accident.

cheers

Andy P

Dan
21-12-2006, 02:50 AM
IE where the slope is greater than the slope required to balance out the gravitational force and pressure drop,to prevent flash gas.This causes vapour to backflow along the top of the liquid, causing the river effect.

Deb, much of what you say has merit. The text quoted above actually has or at least had been a component of sizing return lines in ASHRAE. It was referred to as "sewer flow" and many sizing and piping recommendations were based on it. The flaw in this theory is that once you have a trap, you cannot have true sewer flow. The recommendation for a receiver to condenser equalizing line or vent line has been proposed as a solution for vapor return in trapped liquid return lines... applying plumbing ventilation principles to the closed refrigration system.

Many instances of seeing what you refer to as river effect has been called a lazy sightglass by technicians I have worked with. I see it as a "non-turbulent" flashing sightglass. I have seen a sightglass that appears only half full in a vertical rise, yet somehow liquid is rising above it (or travelling as vapor and condensing and going into the header to feed the branches. I imagine if the line were smaller and the velocities higher, it would be a bubbling sightglass. It's a mystery my mind still struggles to unravle and explain. But this is how I see it.

When vapor and liquid are present in the same place, you have a saturated surface of refrigerant - both liquid and vapor and a very slightly subcooled lower area of liquid refrigerant that maintains its subcooling by virtue of liquid head pressure, and a very slightly superheated refrigerant vapor that has less heat exchange the farther away it is from the surface as a result of vapor pressure losses.

Most of your other observations are spot on, in my opinion. :)

Dowadudda
22-12-2006, 03:58 AM
Hi Dowadudda:)

if the temperature (measured by a thermometer) of the liquid is anything measureable below the pressure read off you gauges converted to temperature then you have subcooling.

I have measured liquid off receiver tempeartures and found the liquid both subcooled and saturated.

What happens is the liquid flows into the receiver and as it moves into a bigger area it expands back to saturated liquid:) but you can still get subcooled liquid out of the receiver as the fans cycle on the condenser. Fans off and the pressure rises and the liquid is still at the temperature it entered the receiver so it becomes subcooled (because the liquid in the receiver is cooler than the pressure on the gauges) fans on and the gauge pressure drops and the liquid is saturated again.

Hope this helps:)

Kind Regards Andy:)

I think your wrong. Plain and simple. As I said. I have been given the very specific reasons why your wrong explained to me a few times. And if your able to understand the explanation, even for a moment, you soon forget. I don't remember the explanation good enough.

Measure subcooling on any liquid manifold. How can it go from saturation, to subcooled leaving a receiver. There is no heat exchange.

Dowadudda
22-12-2006, 04:00 AM
condenser does 3 things. And that is all it does.

1. Desuperheat.

2. Heat Rejection to Condense. "latent heat of Condensation"

3. Subcooling.

Dowadudda
22-12-2006, 04:05 AM
A receiver has indeed a line it. At any given time there is a Liquid to Vapor interface. Of course. There has to be. Liquid can't just sit there with out it. But that is the line we usually determine when we torch a receiver to find it's level.

I am telling everyone reading this. Go into work tommorow, and on your first rack call of the day, just see for your self and do a subcooling measurement on both liquid manifold, and drop leg. And then you all will see for your self.

Dowadudda
22-12-2006, 04:12 AM
and obviously, if undercharged, or a pressure drop, a pressure drop can mean up a riser, or across a filter, then you will see flash gas. I say that to not have anyone mistake what I am saying.

How is it I got a full column of liquid where ever I know there is one and can measure the amount of subcooling?

Then just ask yourself. Just ask your self, "hmm, what is the defintion of subcooling?" And then it makes complete sense.

On top of it. I aint an engineer. I am a refrigeration mechanic. I don't hold a Doctorate of engineering. I could not be that smart on my best days.

It is what it is. I don;t question it. I just know it's suppose to be that. And then it proves itself when you go measure it. It proves the mystery. It solves the mystery actually.

Peter_1
22-12-2006, 05:09 AM
I think your wrong. Plain and simple. As I said. I have been given the very specific reasons why your wrong explained to me a few times. And if your able to understand the explanation, even for a moment, you soon forget. I don't remember the explanation good enough.
.

Dowadude, this becomes a tone - at least for me - which becomes dangerous in this forum.

Something else, your answering/quoting your own posts, edit them instead.;)

US Iceman
22-12-2006, 05:28 AM
condenser does 3 things. And that is all it does.

1. Desuperheat.

2. Heat Rejection to Condense. "latent heat of Condensation"

3. Subcooling.


I will give you the first two; heat rejection for desuperheating (sensible heat removal) & heat rejection for condensation (latent heat removal). Those are simply basic principles for condenser operation.

The last one (condenser subcooling) I take issue with, but let me state my view point.

An air-cooled condenser in the summer should not have liquid backed-up in it to provide subcooling, unless one of two things was incorporated in the design.

If the condenser is providing subcooling, it has to have a larger surface area (more capacity) to provide the subcooling. The condenser ratings typically are for condensing only. Using the condensing surface to subcool liquid detracts from available condensation surface and increase the compressor energy input (if the condenser surface did not include additional capacity for the subcooling).
Or, the condenser is actually two separate coils circuits stacked on top of one another. The lower coil is for subcooling (and has no impact on condensing capacity), while the top coil is for condensing only. In this case the outlet of the condenser coil is the inlet to the subcooling coil.In the summer, unless these two criteria are provided in the design of the condenser, subcooling comes at the expense of potentially higher discharge pressure.

Sure you get subcooling, but it costs you something in compressor energy. That's bad!

Any subcooling coming from the condenser also has the potential to be partially lost if the liquid flows into a flow thru receiver (the one with a dip tube in it for the king valve). The subcooled liquid flows into the receiver and has the potential to warm up, or cool down (depending on the ambient air temperature) or other interactions as Andy P described.

A surge type receiver (where the liquid flows directly to the TXV's and only backs up into the receiver when loads are low) allows the subcooled liquid to flow directly to the TXV's, so the subcooling is maintained (if the liquid line is insulated to prevent heat gain).

Now on the other hand, if the air-cooled condenser is operating in the winter with flooded head pressure controls installed, you will get a lot of subcooling with no increase in discharge pressure.

The lower air dry bulb temperature increases the temperature difference across the coil and increases the condenser capacity, even with the liquid stacked up in the condenser coil.

You don't have to have a temperature change to have subcooling either, but this reverts back to another discussion already held.

It's OK to disagree. Some of us moderators also disagree with one another sometimes.

Dowadudda
23-12-2006, 05:33 AM
tell me how a txv is rated?

US Iceman
23-12-2006, 05:50 AM
The short explanation is a specific mass flow of refrigerant flows across an orifice (of a specific diameter) at a specific pressure differential.

Let's say the orifice diameter is fixed at some size to fix the process variables.

If the orifice diameter is fixed, the mass flow across the orifice can be changed by changing the pressure difference across the orifice.

Since the evaporator pressure fixes one part of the pressure differential, the other component of the pressure differential is liquid feed pressure.

Liquid feed pressure minus the evaporator pressure is the differential the orifice sees in operation.

The resulting mass determines the capacity of the valve at that pressure differential. By using the refrigerant properties, the manufacturers calculate the valve capacity.

One of the refrigerant properties is the vapor enthalpy, which is fixed by the evaporating pressure and temperature.

The other process variable is the liquid enthalpy. If colder liquid is supplied to the valve, the valve capacity increases.

So, when you look at the TXV ratings you see them rated on pressure differential (usually in the form of saturation temperatures at the evaporating condition) and liquid feed temperature.

Increase the pressure differential and the valve capacity increases. Lower the differential and the valve capacity decreases.

The similar event occurs with liquid feed temperature except if the liquid is subcooled the valve capacity increases. If the liquid temperature is higher than the basis of the valve rating, the valve capacity decreases.

How's that...:cool:

Peter_1
23-12-2006, 03:29 PM
If the condenser is providing subcooling, it has to have a larger surface area (more capacity) to provide the subcooling. The condenser ratings typically are for condensing only. Using the condensing surface to subcool liquid detracts from available condensation surface and increase the compressor energy input (if the condenser surface did not include additional capacity for the subcooling).

You can see this physically if the condenser has a build-in subcooler: the bends on a standard condenser are all soldered in such a way that the refrigerant is flowing downwards (or horizontally). This way all the condensed liquid is flowing to the bottom collector.

A condenser with a build-in subcooler is constructed in a different way: the bends are also facing downwards where the liquid refrigerant is flowing to the bottom of the condenser. But at the bottom of the condenser which is the lowest liquid collecting point, the bends are going up again. So the pure liquid is pushed up again one or 2 rows and is then subcooled and flows then in the liquid collector.

As USIceman said, most condensers doesn't have a subcooler (lower price) and the subcooling is lost indeed with a 'normal 'receiver.

The explanation of bottom fed receiver where the liquid is going direct to the TEV and the receiver acts as a buffer, I never saw it this way.

If yo want to be sure of subcooling, the flow from the receiver must go back to the outside condenser to a separate coil and flow back via an insulated line to the evaporator.

Peter_1
23-12-2006, 03:37 PM
I have seen a sightglass that appears only half full in a vertical rise, yet somehow liquid is rising above it (or travelling as vapor and condensing and going into the header to feed the branches. I imagine if the line were smaller and the velocities higher, it would be a bubbling sightglass. It's a mystery my mind still struggles to unravle and explain. But this is how I see it.


Exactly the same phenomena I described in post 3 of this thread. This is something I also never could explain Dan.
I'm also struggling with an explanation for this strange behavior of a sight glass.

johnyfreon
23-12-2006, 09:57 PM
There is not absolute true when it comes to liquid refrigerant interacting with vapors inside a receiver, it all depends on the particular system dynamics, I agree that the bottom layer of refrigerant will be at a different pressure and temperature than the upper layers that comes in direct contact with the refrigerants vapors, there is a constant exchange of energy between liquid and vapors at their point of interface, I always wonder if when the TXV opens wide the decrease on pressure and increase on flow will somehow reduce the pressure at the bottom of the receiver, the opposite could happens when the TXV closes and liquid level rises inside the receiver, I suspect the interface point will be at higher pressure than the bottom or any other part of the receiver, I sort of picture the liquid pushing against the vapors increasing the pressure, reducing the volumes and utterly condensing.

Abby Normal
27-12-2006, 10:20 PM
I don't buy the interface being the highest pressure.

DeB
29-12-2006, 04:15 AM
Just a little note re Andy P and a bucket of liquid refrigerant.When the bucket is filled the resulting liquid and vapour layer cools the air causing the air/refrigerant vapour pressure/temperature to stratify in the top of the bucket with a corresponding drop in pressure/temperature. This decrease in "surface" pressure causes more refrigerant to evaporate and consequently cools the liquid further until the heat flow reaches some temporary (theoretical) balance point. My theory anyway.Please feel free to shoot it down!

Andy P
29-12-2006, 03:42 PM
Dear DeB,

You're digging pretty far back into the thread! You are pretty much right except that of course the total pressure at the surface of the liquid cannot change - it is atmospheric pressure. What changes is the composition of the air/refrigerant mixture there, and hence the partial pressure of the refrigerant. However, as more refrigerant boils off the % of refrigerant in the mix above the liquid goes up, so the pressure balance point you refer to is reached from below, not above (ie the refrigerant partial pressure rises and the liquid temperature falls until they meet at the balance point).

cheers

Andy P

johnyfreon
30-12-2006, 12:23 AM
I don't buy the interface being the highest pressure.

somehow I knew you were going to point that out:D

shooter
31-12-2006, 02:35 PM
the receiver is too hot.
Just wrap it in with insulation and it will solve two problems.
1. the gas you see in the liquid line
2. the expansion valve will work proper again.
On our ships we do not insulate the liquid line inside the cool rooms this to subcool the liquid before the valve.
We do not panic when seeing gas in the sight glass as we know it will be correct in the rooms.

little yapper
05-01-2007, 12:50 PM
is it a horizontal recievor?
if so is it piped up correctly.the dip tube on the recievor has to be in the liquid .this sounds a bit basic but i have come across it once.other wise it's head pressure.