Andy Schoen's Receiver Subcooling Article

[DTLarca]

This matter of subcooling and receivers is a good example of the nonsense that pervades our industry and probably because historically those who write the books and design the courses just do not properly know what they are talking about. I know because I have scrutinised nearly all the books and have presented some of the key courses myself.

Andy Shoen is correct - and I want to make more salient some of the key principles. But I will get a little more technical in the article version in a few months time. I just want to give at least Mad Fridgie something to think about since he has shown a little skepticism toward Andy's arguments.

Here's the article in question...

http://www.sporlanonline.com/February%2010%20Cold%20WAR.pdf

I'm just going to start off with the main and very basic points - in case no one is interested anyway.

Consider water in a pot on the stove. It has a vapour pressure always. When that vapour pressure equals atmospheric pressure the water boils. The water boils at 100°C at atmospheric pressure. At atmospheric pressure it cannot properly boil at any temperature above 100°C. This is to say that when adding heat the water rises and rises but eventually reaches a certain temperature above which it cannot go - this highest temperature is a function of the prevailing pressure.

What is important to notice here is that I am talking about things as they happen when we are ADDING heat. Not removing heat. ADDING heat. We generally expect that no matter how fast we add the heat the water will not rise above 100°C - this is a reasonable expectation to have when thinking of how things are within the confines of refrigeration systems.

Because the vapour at 100°C is leaving the water body the vapour just above is also at 100°C. This vapour rising above the water will have a pretty constant density.

Now here is a key point overlooked relevant to scenarios where we are going in the opposite direction - its oversight is the source of confusion - which is a principle not brought into salience by Andy.

Consider a rigid container with 1 bar of nitrogen gas at 20°C where the weight of gas is 1kg. I can double the pressure of the gas by either halving the volume of the cylinder slowly allowing the gas to cool thus also maintaining a gas temperature of 20°C. I can also double the pressure of the gas by raising its thermodynamic (absolute) temperature - say 290K to 580K. But I can also double the pressure of the gas by simply adding another 1kg Nitrogen or by adding 1bar of any other gas. If I added another 1kg of Nitrogen and thus another 1 bar of nitrogen then I would have doubled the density of the gas. So I can have double the pressure of the same gas while maintaining the same low temperature - just that the density of the gas has increased (doubled in this case). This is very important to picture mentally.

When we are removing heat from a liquid vapour mixture at a constant pressure, as happens to say R22 in the condenser, the liquid and vapour temperature is by no means prevented from dropping below the saturation temperature. Who says just because when you are adding heat the liquid cannot reach a temperature above saturation that when you are removing heat the temperature cannot drop below saturation? It just tends not to in the early stages of the condenser. Hell, we regularly see 4K subcool with vapour presence in liquid lines - even slow liquid lines where there is minimal vapour carry-over occurring on account of sweeping.

All that happens then - toward the end of the condenser - is that we have a cooler but higher density vapour above the subcooled liquid where it simply is the case that both the liquid and the vapour are subcooled for the existing pressure. The higher pressure in the condenser/receiver is being caused by the hotter, higher pressure, lower density gas coming in the from the compressor and also that at saturated condensing temperature where all the condensing is taking place. But the vapour with the liquid toward the end of the condenser and in the receiver is equal in temperature with the liquid - its density is just greater having been compressed by the higher temperature, less dense, hotter vapour entering the condenser. Any heat that tries to get to the liquid at the end of the condenser from the hot gas at the beginning of the condenser is quicker removed along its way by the cooler heat exchanger walls than it is conducted by the separating liquid vapour bridging beginning to end.

[mad fridgie]

In reverse, i have not stated that sub-cooling does not occur in a condensor (different to my skeptism in a reciever), I also except that you can get bubble entrapment in a sub cooled liquid (temp and time limited).
I also except that you can have sub cooled liquid in a vessel (more than just its static head), this due to stratification (changes of density) heavy at the bottom, light at the top.
To have flow you must have a pressure drop, or flow will not occur.
Thus your reciever will be at lower pressure than your compressor discharger pressure or even SCT depending i which point this measured.
So lets look at the diagram.
1: The sub cooled liquid is injected directly into the vapour. Example on a cooling tower do we just have fan blowing over the surface of the water to achieve our cooling effect or do we spray the water. We spray the water to give optimum heat transfer between the water and the air.
The bigger the temp gradient the bigger the flow of energy.
What about pressure, i hear you say, it will drop. come to this later.
The liquid leaves the pipe and free falls until it hits the surface of the liquid within the reciever. The statement clearly says that the interface between the vapour and liquid will be at the higher temperture.
Depending upon how much liquid/vapour is the reciever, how far the sub cooled liquid falls, will determine how much of the boundry surface is broken, and how much recoil is rejected back upwards into the vapour and how big the waves and ripples are. basically is the reciever in a mixed state or not.
After hitting the boundry layer, will it sink, like a lead shot straight down or does it disperse slightly, moving in a downward direction, this being the case then (forgeting the vapour at this stage) the sub cooled liquid must be in direct contact with warm liquid and heat transfer will occur, warming the sub cooled, coolling the saturated.
rate flow decreases as density become closer, increasing thermal length. Are we not now producing a internal current within the vessel aid mixing further.
Let look at the practical application.
What the purpose of the reciever, in simple terms to allow for varying mass flows of refrigerant (no need to go into this area) or as storage vessel for pump down.
We will use pump down as it will show best effect (oversized reciever in running conditions)
The main object if possible is to keep a liquid seal on the reciever outlet (yes or no), the object is not to keep the reciever full of liquid. (yes or no)
Basically you charge the system until you have achieved this liquid seal (when the sight glass is full, I want to make a note that a sight glass is an aid to charging not the be all and end all)
This being the case, by volume the reciever does not have a great deal of liquid refrigerant in it.
So all the effects above will take effect, and no/little actual sub cooling.

[mad fridgie]

HOW TO MAKE THE THEORY FIT.
looking at the drawing, If the liquid feed was fitted with a perforated tube which went close to the bottom of the reciever and was surround another tube with open ends towords the top and the bottom, then your sub cooled liquid would remain at the bottom, the vapour could escape from the top , no mixing/boundry breaking would occur, you would then have genuine sub cooled liquid leaving the reciever. (temp, time, and sized based)

[DTLarca]

After hitting the boundry layer, will it sink, like a lead shot straight down or does it disperse slightly, moving in a downward direction, this being the case then (forgeting the vapour at this stage) the sub cooled liquid must be in direct contact with warm liquid and heat transfer will occur, warming the sub cooled, coolling the saturated.


The liquid arriving at the receiver from the condenser is subcooled to the same temperature as the equally subcooled liquid in the receiver.

The vapour arriving in the receiver is not really arriving - it is hanging around there above the liquid in the receiver and along the last runs of the condenser including the drain leg and this vapour in the area is supercooled. It wants to condense but it does not -it just a lot more dense than we would normally expect.

[mad fridgie]

The liquid arriving at the receiver from the condenser is subcooled to the same temperature as the equally subcooled liquid in the receiver.

The vapour arriving in the receiver is not really arriving - it is hanging around there above the liquid in the receiver and along the last runs of the condenser including the drain leg and this vapour in the area is supercooled. It wants to condense but it does not -it just a lot more dense than we would normally expect.
The articles clearly states that the liquid at the boundry layer is at saturation point, super cooling is an effect , and i have covered this in the "time temp base"
The vapour can actually travel up and down the drain leg or equalisation leg if it has one, it does not just hang around

[mad fridgie]

Per article

It is simple: we will have 110°F at the liquid and vapor interface, but the refrigerant liquid immediately below the
interface will be at 100°F. The 110°F saturation temperature will only be found at the interface, and with the vapor
above the interface.

[DTLarca]

The articles clearly states that the liquid at the boundry layer is at saturation point, super cooling is an effect , and i have covered this in the "time temp base"
The vapour can actually travel up and down the drain leg or equalisation leg if it has one, it does not just hang around

Up and down is hanging around - as in if the condenser fans speed up or the condenser fans slow down or the TEV opens up or the TEV closes down - during these sorts of events the supercooled vapour will be drifting up and down the drain line tending to equalise pressure differences between the receiver and the condenser - I call this hanging around because it is not flowing through - it can't get past the liquid seal.

Here's a video of supercooled water.
http://www.youtube.com/watch?v=fSPzMva9_CE

[mad fridgie]

The article does not bring to the attention pressure drop, most liquid sub cooling is actually pressure drop. hence not actual sub cooling

[DTLarca]

Per article

It is simple: we will have 110°F at the liquid and vapor interface, but the refrigerant liquid immediately below the
interface will be at 100°F. The 110°F saturation temperature will only be found at the interface, and with the vapor
above the interface.


In simplistic thinking - as Andy says in his article - he is assuming a simplistic model to impart the fundamentals.

In reality the liquid arrives at the receiver already subcooled and on the way there through the last leg passes supercooled, not saturated, vapour.

[DTLarca]

The article does not bring to the attention pressure drop, most liquid sub cooling is actually pressure drop. hence not actual sub cooling

This is only if there is a pressure drop between where you measure pressure and where you measure temperature.

The article assumes negligible pressure drop as is typically the case in low horse power commercial fridge condensing units.

[mad fridgie]

Great video, also called solid rain in Canada

[mad fridgie]

This is only if there is a pressure drop between where you measure pressure and where you measure temperature.

The article assumes negligible pressure drop as is typically the case in low horse power commercial fridge condensing units.
There lies practical reality. when is it sub cooled and when is not.
Having $100,000s of test rigs and measuring many (not all by a long way) units the actual amount of liquid sub cooling leaving a reciever is very small, in comparrision what folks like to believe is sub cooling.
It is not as simple as saying you or I are right or wrong. A bumble bee can not fly, but no one has told the bee that!

[mad fridgie]

Just one point, if the vapour does not get past the liquid seal, how do you see bubbles in the sight glass, and if the the liquid is highly sub cooled, how could bubble form if there is no pressure drop.

[DTLarca]

Just one point, if the vapour does not get past the liquid seal, how do you see bubbles in the sight glass, and if the the liquid is highly sub cooled, how could bubble form if there is no pressure drop.

These are the results of constant but slight variations in receiver and liquid line pressure - both static and dynamic.

Ask yourself also why could you possibly have vapour in the receiver when only subcooled liquid is falling into there via the condenser drain where there is a liquid seal leaving the condenser in that drain leg giving positively subcooled liquid to the receiver.

The vapour in the receiver is supercooled - it is quasi-stable - it is not saturated vapour and it is not saturated liquid - it is in between - it is denser than a saturated vapour would be but it is no where enough dense to be a liquid.

[mad fridgie]

Knowing that you are not going to ask a question without first knowing the answer, why is it the vapour remains supercooled, when all around it is shock energy and seed particles

[mad fridgie]

These are the results of constant but slight variations in receiver and liquid line pressure - both static and dynamic.

Ask yourself also why could you possibly have vapour in the receiver when only subcooled liquid is falling into there via the condenser drain where there is a liquid seal leaving the condenser in that drain leg giving positively subcooled liquid to the receiver.

The vapour in the receiver is supercooled - it is quasi-stable - it is not saturated vapour and it is not saturated liquid - it is in between - it is denser than a saturated vapour would be but it is no where enough dense to be a liquid.
Correct, so the article is then incorrect, and the article is the reference point, hence my points are valid, moving the goal posts does not count.

[DTLarca]

Correct, so the article is then incorrect, and the article is the reference point, hence my points are valid, moving the goal posts does not count.

I'm missing something - I'm using the article as a foundation of agreeables and going further. What in particular do you say you do not agree with the article?

[DTLarca]

Knowing that you are not going to ask a question without first knowing the answer, why is it the vapour remains supercooled, when all around it is shock energy and seed particles

This is something to think about :D

It's almost like the vapour in the receiver wants to be a fog :D

The vapour makes up for its lower temperature, for the given pressure, without condensing, by instead holding more molecules in suspension - that is the greater density.

[mad fridgie]

I'm missing something - I'm using the article as a foundation of agreeables and going further. What in particular do you say you do not agree with the article?
Hi Marc,
"

It is simple: we will have 110°F at the liquid and vapor interface, but the refrigerant liquid immediately below the
interface will be at 100°F. The 110°F saturation temperature will only be found at the interface, and with the vapor
above the interface."

This clearly indicates that the vapour is at saturation and is not supercooled, would you not agree.
Thus this being the case, then the rest argument is incorrect.

[mad fridgie]

This is something to think about :D

It's almost like the vapour in the receiver wants to be a fog :D

The vapour makes up for its lower temperature, for the given pressure, without condensing, by instead holding more molecules in suspension - that is the greater density.
This question was pure curiosity! no counter arguments on this one

[DTLarca]

Hi Marc,
"

It is simple: we will have 110°F at the liquid and vapor interface, but the refrigerant liquid immediately below the
interface will be at 100°F. The 110°F saturation temperature will only be found at the interface, and with the vapor
above the interface."

This clearly indicates that the vapour is at saturation and is not supercooled, would you not agree.
Thus this being the case, then the rest argument is incorrect.


I agree with that scenario where it can be that the surface of the liquid is saturated on account of taking in heat from a recently arrived warmer saturated or even still superheated vapour but it coincides with load or liquid mass flow transients - it does not make sense when we are imagining a stable system.

I do not believe it can possibly be the normal state of things. It has to be the normal state of things for the vapour above the liquid to be supercooled.

How can the receiver receive subcooled liquid if in a continuous steady load state subcooled liquid is traveling turbulently through a mass of warmer saturated vapour? If the liquid can subcool then how come not the vapour?

[mad fridgie]

I agree with that scenario where it can be that the surface of the liquid is saturated on account of taking in heat from a recently arrived warmer saturated or even still superheated vapour but it coincides with load or liquid mass flow transients - it does not make sense when we are imagining a stable system.

I do not believe it can possibly be the normal state of things. It has to be the normal state of things for the vapour above the liquid to be supercooled.

How can the receiver receive subcooled liquid if in a continuous steady load state subcooled liquid is traveling turbulently through a mass of warmer saturated vapour? If the liquid can subcool then how come not the vapour?
Now we are getting somewhere.
It then begs the question , how much sub cooling do we actually get out of a free draining condensor (i am presuming that your arguments are correct) (I have not completed testing because is not important for what i do), but I must include true and tested observations, into the argument. That little genuine sub cooling exists from a reciever ( I have covered when I would expect it)
Are actually introducing superheat/saturated vapour into the reciever, which is being carried soley by velocity with insufficient thermal legth to either condense or supercool with may some level of sub cooling, or are we just supply liquid close to saturation.

[DTLarca]

how much sub cooling do we actually get out of a free draining condensor (i am presuming that your arguments are correct)

I've seen 4K

But more interesting, when I think about it, is the 6K subcool seen on a non free draining condenser/receiver circuit - that implies a massive 6K supercooling.

[mad fridgie]

I've seen 4K

But more interesting, when I think about it, is the 6K subcool seen on a non free draining condenser/receiver circuit - that implies a massive 6K supercooling.
Have these systems been in series cond to reciever or a type of parrarlel (like a level tank) where the level goes up and down but not full flow.

[DTLarca]

Have these systems been in series cond to reciever or a type of parrarlel (like a level tank) where the level goes up and down but not full flow.

Series style - not vented to the condenser inlet nor of the Plus Two configuration.

[mad fridgie]

I do think that this article should be re written, clearly indicating correct temperatures (removing doubt and counter acting my arguments, which would be true as stated at the moment) It should also include a pressure drop between the discharge and liquid line, to indicate true measurement of sub cooling (which practically is my biggest bone of contention). Technical information should be correct or stated that it is not, but has been written to aid understanding.
The term "supercooling" should be used, with an ananolgy of cloads are supercooled water vapour, something that most of us understand.

[mad fridgie]

The next question, should be?
Why do we need know what the "actual liquid sub cooling" is leaving the reciever?

[lawrence1]

stay tuned for round 2

[mad fridgie]

stay tuned for round 2
According to my score card, that was my round, even though he did have the technical moves, dancing and jabbing, he just could see the large upper cut coming, even though I had try to throw it once during the middle of the round, that floored him at the end of the round, he managed to get himself up, or was it the end of the round that saved him.
Marc only picks fight he is sure he can win, he does have technical skill to bash most around the ring, you can only beat him with cunning and pure practical brute force.
Mad F, does not mind loosing a fight, as long as he can learn from the fight, he already knows in this game there is not a single master, many weight catagories, and fighting styles. Nothing like a good "scrap" to keep us fight fit

[desA]

Some useful academic links.

http://www.wlv.com/products/thermal-management-databooks.html

Collier G.C., Thome J.R., Convective boiling and condensation, 3 ed, Oxford Science Publications, 1994.

[desA]

The article does not bring to the attention pressure drop, most liquid sub cooling is actually pressure drop. hence not actual sub cooling

No, not necessarily the case - at all.

The condenser (non receiver type) is capable of producing sub-cooled condensate in the range of 4-5K (plate), or down to 8.5K (tube-in-tube), or more, if required - dependent on system mass charge.

[DTLarca]

Not much chance of adding more comments just yet but I do have a moment enough to post two images taken recently measuring subcool (3.5K) on a water cooled pack system with a receiver. The pressure and temperature readings are at the same location -that is on the liquid line leaving the receiver.

[monkey spanners]

Monkey at work measuring condenser u bends.

http://www.youtube.com/watch?v=UUpwH-NHkuY


Jon :)

[mad fridgie]

No, not necessarily the case - at all.

The condenser (non receiver type) is capable of producing sub-cooled condensate in the range of 4-5K (plate), or down to 8.5K (tube-in-tube), or more, if required - dependent on system mass charge.
As stated earlier, i have no problem with sub cooling and when or where it is produced, my issue is that you need to measure correctly liquid pressure and liquid temperature.
The nature of our industry, does allow for a wide range figures, rules and practical application.
As i am most familar with air cooled condensing units, this where I base my observations, my observations indicated pressure drops between comp discharge and liquid pressure out of the reciever.
On a shell tube, your pressure drop would be minimal (in comparrrision)
The point of this thread was around the article, and the article from Marcs point of veiw was incorrect, as proved, But his theory of course is correct, when it can happen.
Your examples are not free draining,
Like most things in life, finding the balance between academic excellence and practical application.
How many times (even on this forum) have we seen engineers give discharge pressure and liquid line temps and give some massive level of liquid sub cooling. " Measure you liquid pressure and your liquid temperature"

[DTLarca]

Mad F, sorry, texting from blackberry, but have you never heard of Berno
ull's therum? Liquid and gas pendulums :)

[nike123]

http://www.tutorvista.com/content/physics/physics-iii/solids-and-fluids/bernoullis-theorem-animation.php

[Quality]

Monkey at work measuring condenser u bends.

http://www.youtube.com/watch?v=UUpwH-NHkuY


Jon :)
Now thats what I call subcooling

ps DTlarca needs a new manifold those digi refco are carp

Testo all the way

[mad fridgie]

Mad F, sorry, texting from blackberry, but have you never heard of Berno
ull's therum? Liquid and gas pendulums :)
Yes i do understand the theory of a venturi, also how flow is attracted to a surface, can not remember the name of the principle, begins with "C", eastern european sceintist.

[mad fridgie]

Monkey at work measuring condenser u bends.

http://www.youtube.com/watch?v=UUpwH-NHkuY


Jon :)
Hi MS, what refrigerant, and what was your pressure (a bit hard to see) It looked like you were measuring liquid pressure, is that correct.

[monkey spanners]

Hi MS, what refrigerant, and what was your pressure (a bit hard to see) It looked like you were measuring liquid pressure, is that correct.

Hi, its R437A or MO49 plus, looks to be about 220psi, the gauge is on the reciever as there is no compressor high side service valve. Its only a 1/2 hp ish tecumseh unit.

Jon :)

[mad fridgie]

Hi, its R437A or MO49 plus, looks to be about 220psi, the gauge is on the reciever as there is no compressor high side service valve. Its only a 1/2 hp ish tecumseh unit.

Jon :)
Hi Jon,

thanks, no data on that refrigerant, you are reading at the correct place for liquid sub cooling.
It would be intersting to see the temperature across the height of the reciever

[DTLarca]

Yes i do understand the theory of a venturi, also how flow is attracted to a surface, can not remember the name of the principle, begins with "C", eastern european sceintist.

Not sure how this will come out. I can hardly see the text on my blackberry screen.

But, my question then, how would you describe the efficiency losses, in nature, when considering liquid pendulums?

[monkey spanners]

Hi Jon,

thanks, no data on that refrigerant, you are reading at the correct place for liquid sub cooling.
It would be intersting to see the temperature across the height of the reciever


Its an R12 drop in, very similar to R417A but with some extra propane or something! I believe the pipe clamp thermometer is on the liquid line after the reciever. A few months ago now, job to remember :o Was just playing about and thought it would be interesting to see the temperature measurments and put on my youtube channel :)

[mad fridgie]

Not sure how this will come out. I can hardly see the text on my blackberry screen.

But, my question then, how would you describe the efficiency losses, in nature, when considering liquid pendulums?
I am a simpleton, please explain as if i was an idiot, because I am!
Slightly of subject, why is at Christmas, they have to show the Wizard of OZ? struggling with whos law this belongs to?
It is present 1.29pm xmas day, under the influence of russian based working fluid!

[desA]

Mad F, sorry, texting from blackberry, but have you never heard of Berno
ull's therum? Liquid and gas pendulums :)

Marc, you will need to expound further on this. Please be advised that the Bernoulli equation is applied to flow along a streamline only, by definition & for a single-phase fluid.

It cannot be used across a streamline, for instance.

[desA]

Yes i do understand the theory of a venturi, also how flow is attracted to a surface, can not remember the name of the principle, begins with "C", eastern european sceintist.

Coanda effect.

[mad fridgie]

Coanda effect.
Can always rely on old desA to know his facts, good onya!
For the next round, against marc, can I have desA in my corner, braun and brains, must be a winning combination:D

[desA]

LOL... You are most welcome... :D

[desA]

http://i52.tinypic.com/14bkinp.png

Perhaps it's time to begin defining the problem a little more clearly?

Incoming fluid properties? Pressure, temp, refrigerant.
Receiver vented, or not?
Heat-transfer to/from receiver, or adiabatic?
Receiver internal pressure?

Actually, working through my process simulator, I suspect that Andy's temperature explanation at the fluid interface is not correct at all.

[mad fridgie]

http://i52.tinypic.com/14bkinp.png

Perhaps it's time to begin defining the problem a little more clearly?

Incoming fluid properties? Pressure, temp, refrigerant.
Receiver vented, or not?
Heat-transfer to/from receiver, or adiabatic?
Receiver internal pressure?

Actually, working through my process simulator, I suspect that Andy's temperature explanation at the fluid interface is not correct at all.
As a rule,but not limited to, non vented is the most common.
Andy figures are incorrect, this is has been agreed by Marc, but I also believe he has amore than valid point about the super cooling.
A do see vertical and horizontal recievers acting slightly different (practical) and how these could be effected by external forces, so I would for this excersise call it adiabatic.
You pick a refrigerant (R134a would be OK as this one you are familar with, and has no glide, so no deviations for other reasons)

[DTLarca]

Perhaps it's time to begin defining the problem a little more clearly?

Patients - one thing at a time - things are made more clear by a slow process of eliminating those blury edges stunting our progress here.


Marc, you will need to expound further on this. Please be advised that the Bernoulli equation is applied to flow along a streamline only, by definition & for a single-phase fluid.

You'll have to wait a few days till I get back home to the comfort of my own desk.

However - let me put it this way for starters. Are you sure you cannot see any similarility in the principles of physics between a pendulum and, say, an orifice plate - in either air or water?

[desA]

LOL... I tend to make my simulated theoretical pendulums display chaos - even though the gurus say it can't be done...

[DTLarca]

LOL... I tend to make my simulated theoretical pendulums display chaos - even though the gurus say it can't be done...

Come on Des. Because the pendulum is a paradigmic icon - since back with even Galileo - but still especially central to the structure of our current scientific paradigm it is one of the first tools of description and analysis used in any field of physics including chaos theory. Did you read James Gleick?

The pendulum is a prime tool used in the demonstrations of chaos theory.

Watch this video to the end

http://www.youtube.com/watch?v=Qe5Enm96MFQ

Are you sure you are not just stalling on answering my last question :)

[desA]

Come on Des. Because the pendulum is a paradigmic icon - since back with even Galileo - but still especially central to the structure of our current scientific paradigm it is one of the first tools of description and analysis used in any field of physics including chaos theory. Did you read James Gleick?

The pendulum is a prime tool used in the demonstrations of chaos theory.

Watch this video to the end

http://www.youtube.com/watch?v=Qe5Enm96MFQ

Are you sure you are not just stalling on answering my last question :)

You are confusing the classical 2D pendulum, with a 3D pendulum. Chaos theory requires an additional degree of freedom over that which a 2D pendulum can provide... however... it is a trivial matter to introduce chaos into the 2D classical nonlinear pendulum system.

James Gleick's book is rather ancient, by now... :D

You should attempt to further qualify your assertion of the link between the simple 2D pendulum & whatever you are attempting to prove. The proverbial ball is now in your court... Let your infinite wisdom proceed to shine... :)

Personally, though, I would prefer you to set the process conditions for your receiver problem, so that detailed process modeling & simulation can proceed. I suspect that we'll be able to provide a few insights into your earlier dilemma.

[DTLarca]

You are confusing the classical 2D pendulum, with a 3D pendulum.

It doesn't really matter - how many components the pendulum has - does it? :)

The fact is there actually is no chaos in any of them - as the Gleick example illustrates - you will always find, from some perspective, a pattern in the chaos - chaos is only apparent. Put it this way - pendulums - all types - obey the laws we use to describe how nature goes - they cannot go any other way.


Chaos theory requires an additional degree of freedom over that which a 2D pendulum can provide... however... it is a trivial matter to introduce chaos into the 2D classical nonlinear pendulum system.

Which was demonstrated in the video I linked to :)


James Gleick's book is rather ancient, by now... :D

But not refuted :)


You should attempt to further qualify your assertion of the link between the simple 2D pendulum & whatever you are attempting to prove. The proverbial ball is now in your court... Let your infinite wisdom proceed to shine... :)

Yes - I would like to discuss gas and liquid pendulums (Newton and Bernoulli) - especially liquid to eliminate a concern that Mad Fridgie has.


Personally, though, I would prefer you to set the process conditions for your receiver problem, so that detailed process modeling & simulation can proceed. I suspect that we'll be able to provide a few insights into your earlier dilemma.

We know we cannot achieve subcool in say a shell and tube condenser on, for instance, a water cooled centrif which has no liquid capturing tube at the bottom of the condenser still exposed to the condenser water - we have instead a low side float valve or even a high side fload valve.

We are talking serpentine condenser coils per the article being examined here.

Certainly 4K Subcool tends to be quite the normal thing on a serpentine condenser with receiver and site glass. 6K is achievable at full design load. Although with electronic expansion devices the subcool remains pretty constant through all load conditions because they do not have the capacity/superheat curve that TEV's do.

[desA]

It doesn't really matter - how many components the pendulum has - does it? :)

The fact is there actually is no chaos in any of them - as the Gleick example illustrates - you will always find, from some perspective, a pattern in the chaos - chaos is only apparent. Put it this way - pendulums - all types - obey the laws we use to describe how nature goes - they cannot go any other way.


Would your pendulum be of the linear, or non-linear variety?

The observed effects of chaos are purely because the viewers have missed an important component in correctly visualising the complete description of the motion. The so-called chaos effects all suffer from the same fundamental mistake. In fact, the answers to chaos theory is incredibly trivial, if only the mathemagicians would bother to correctly review their knowledge of functions. I'll elaborate more on this aspect in a later paper.

So now, would you like to proceed to draw the analogy between pendula & your refrigeration system? :)

[DTLarca]

Would your pendulum be of the linear, or non-linear variety?

Both, Des, they both can only have apparent chaos - to be truly chaotic they would have to occur outside of the current natural descriptions of how our mechanical surroundings go - that is they would have to defy the universal laws of physics.

If a computer can simulate a non-linear pendulum then it would be demonstrating predictive powers which could not be if there was chaos. Where something can be predicted - simulated - then there will also be patterns.

Let me quote another source to back up my argument:

Quote: "The observed effects of chaos are purely because the viewers have missed an important component in correctly visualising the complete description of the motion. The so-called chaos effects all suffer from the same fundamental mistake.


So now, would you like to proceed to draw the analogy between pendula & your refrigeration system? :)

Indeed, I will, firstly, are you sure you do not see pendulum principles in an orifice plate?

[desA]

Indeed, I will, firstly, are you sure you do not see pendulum principles in an orifice plate?

Please proceed to enlighten us... :)

[research]

If one believes that you can put subcooled liquid into a receiver, and take subcooled liquid out of the receiver.
This happens quite often. it happens when the receiver is liquid full.

On a receiver that has gas /liquid mixture in it. If there is subcooled liquid entering the receiver, by definition the saturated liquid must be warmer... ( subcooled is colder than saturation. )
Therefore for this to be true we would have to believe that it is possible to put cold refrigerant into the receiver, take cold refrigerant out, yet maintain the receiver at a higher temperature (saturated), but we add no heat. hmmmm...

One interesting thing that I used to do when I taught refrigeration, was throttle a ball valve until I had bubbles in the sight glass. (saturated) I would cover the sight glass and have students fasten thermisters and gauges to the liquid line and measure the subcooling. Typical measured values would be 4 to 5 degrees subcooling.


Yes I believe that you can measure subcooling coming out of a saturated receiver. I believe that this is a very common measurement.
I dont however believe that this is true.

[DTLarca]

If one believes that you can put subcooled liquid into a receiver, and take subcooled liquid out of the receiver.
This happens quite often. it happens when the receiver is liquid full.

I like your Avatar - a PVT chart :)

So you are saying that a sight glass - especially when located after a filter drier - that is located on a vertical rise up from the dip-tube from a receiver - must always flash?

[Gary]

I like to use the example of a half full refrigerant container on a block of ice with a heating pad on top. There is superheated vapor on top, subcooled liquid on the bottom and saturation at the vapor/liquid interface. It is entirely possible to have superheat, subcooling and saturation all within the same container.

[DTLarca]

I like to use the example of a half full refrigerant container on a block of ice with a heating pad on top. There is superheated vapor on top, subcooled liquid on the bottom and saturation at the vapor/liquid interface. It is entirely possible to have superheat, subcooling and saturation all within the same container.

Indeed, if the middle of the same container is at the triple point of the substance in question, which is located around the bottom left hand third of research's avatar, which is a PVT diagram of a substance that contracts on freezing (not water) then we could have a superheated vapour at the top, then going downward it could/would be saturated vapour, saturated liquid, subcooled liquid and finally "ice" or solid then finally a subcooled solid.

[DTLarca]

The PVT chart in research's avatar even depicts how ice has a vapour pressure - otherwise we could not have humidities of any degree whenever the atmosphere and ground were below freezing :)

This is depicted along the bottom 3rd of the PVT diagram.

[research]

If the process was adiabatic with pressure drop.

Yes the glass would bubble.


However, The copper pipe rejects heat quickly.
and it takes very little subcooling to overcome the small pressure drop of a drier, and a few feet of rise.

With R134a, for a pressure loss of 2 psig, we would need less than 1 degree subcooling.
For R410A a loss of 2 psig would need less than 1/2 °F subcooling.

This is all that is required to keep the sight glass from bubbling.

There can also be a 'small' amount of subcooling in the receiver as heat is lost through the bottom of the receiver to ambient and there is a small static head. This is small but can still overcome small pressure drops.


There are countless applications with a drier sight glass located at the receiver that work just fine with clear glasses.

However there are also some with receivers that must be almost liquid full before the glass will clear.

[research]

Even a simple ***** tank sitting in the sun can have superheated vapor on top, saturated in the middle, and subcooled on the bottom.
Momma nature has no problems with that.

However this system is static.
Heat is added to the top. vapor is superheated.
Liquid vapor iterface is saturated,
liquid on the bottom can be cooler.. subcooled.
no problems.

If you were to pump the subcooled liquid out of the bottom of the tank and back into the top. and have the subcooled liquid travel through the superheated gas, then the liquid vapor interface and remove any sources of heat transfer, other than the liquid being pumped.
I think that the temperature would end up the same everywhere. saturated.

On a working system, I believe that there can be "small" amounts of subcooling in the bottom of a receiver, as there is a static increase in pressure, and the tank is warmer than ambient. However must emphasize small.

I have been in this industry 40 years, and have changed my belief system on receivers several times.
I guess that is why I love this industry so much.

Isn't it amazing that the simplest piece of equipment on a refrigeration system, (it is basically just a pregnant pipe) can be such a difficult thing to understand. And that leaders in the industry can have such different opinions on the subject.

[mad fridgie]

Regardless if you have small or large amonts of sub cooling, it comes down to how you measure. My argument comes from those who measure discharge pressure and liquid temperature to give sub cooling. You must measure pressure and temp at the same point.

[research]

Regardless if you have small or large amonts of sub cooling, it comes down to how you measure. My argument comes from those who measure discharge pressure and liquid temperature to give sub cooling. You must measure pressure and temp at the same point.

Absolutely....

100% agreement...

[DTLarca]

If the process was adiabatic with pressure drop.

Yes the glass would bubble.


However, The copper pipe rejects heat quickly.
and it takes very little subcooling to overcome the small pressure drop of a drier, and a few feet of rise.

What are the numbers? :)

Do you agree or disagree that even with between 2 and 3K measured subcool we can see vapour carry-over in a sight glass - especially with receiver-less systems and especially with liquid line velocities above 0.5m/s?

[research]

What are the numbers? :)

Do you agree or disagree that even with between 2 and 3K measured subcool we can see vapour carry-over in a sight glass - especially with receiver-less systems and especially with liquid line velocities above 0.5m/s?

I agree that there can be flashing vapor is subcooled liquid. However this is a function of time. Vapor bubbles transfer heat poorly, and once created take time to collapse.
The proof is simple. Cavitating propellers are simply creating flash gas by the reduction of pressure.
This link clearly shows flash gas in subcooled liquid.
wikipedea under cavitation
it wont let me post a url. so i have removed the link

Submarines can cavitate propellers at great depths. often subcooled more than 150°F .


The dynamics of receivers is very complex, and is rarely static.

I do not agree with the theory presented in Andy Shoens article.

[desA]

On a receiver that has gas /liquid mixture in it. If there is subcooled liquid entering the receiver, by definition the saturated liquid must be warmer... ( subcooled is colder than saturation. )
Therefore for this to be true we would have to believe that it is possible to put cold refrigerant into the receiver, take cold refrigerant out, yet maintain the receiver at a higher temperature (saturated), but we add no heat. hmmmm...


That is the essence of it. For some part of the system to be at higher temp than the surroundings, heat-transfer will take place. Practically, the heat-transfer between atmosphere & vessel should be understood.

The way in which the system boundary is selected, will be important i.e. the macro-picture is important. These decisions are pretty much forced on you, if a process simulator is used. Here we install in/out conditions onto a receiver vessel - can include a pre-valve as well - then study how the system reacts. The flash processes in the receiver are also included.

This is why I suggested that the exercise be reduced to a simulation & the results be interrogated. All of the many possible scenarios can be shown.

[desA]

http://i55.tinypic.com/20gig6x.png

I have extended the model of the sub-cooled liquid feeding through a restriction, then into the receiver (flash vessel) - with vapour & liquid take-offs.

If someone would like to firm up the simulation scenarious, I'd be happy to proceed with a range of simulations, which could prove very informative & put a number of myths to bed.

Propose :
R-134a ; Tc,sat = 65'C ; SC=3K ; T,liq,in = 62'C @ 1.89 MPa; adiabatic (insulated) receiver.

We will then gradually increase the valve restriction & observe what occurs in streams 2, 3, & 4.

[desA]

http://i51.tinypic.com/2ldw2kl.png

Varying dP across valve. Temp & vapour fraction at valve exit.

At present simulated as unhindered vapour feed off top of receiver - doesn't affect action across the valve/restriction. We can discuss further cases after this e.g. high-pressure balance pre-condenser.

So, depending on where the system is measured, the receiver exit could show deep sub-cooling.

[research]

So, depending on where the system is measured, the receiver exit could show deep sub-cooling.[/QUOTE]

[research]

http://i51.tinypic.com/2ldw2kl.png


So, depending on where the system is measured, the receiver exit could show deep sub-cooling.

This is a very interesting statement. As mad fridgie has said earlier, it depends on where you measure your values.
The graph presented clearly shows a vapor mass fraction (shown in pink) if there is vapor and liquid present at the same temperature and pressure. If there is liquid and gas together, by definition the mixture is saturated.

Suppose we measure the pressure before the throttling valve and the temperature after. Could we theoretically determine that the mixture is subcooled? Yes we could, however, this would be incorrect.
The properties of a substance must be measured at a single point to determine the state of the substance.


The true conditions in a receiver are very complex. Most simulations attempt to model the receiver as a STATIC entity. This is seldom true.
The flow through a operating receiver is very seldom static.
Example. The TX valve or metering device on a system typically will open and close according to conditions it sees at the evaporator outlet. It does not communicate to the receiver when it will open or close. Therefore the flow at the outlet of a receiver will change in response to the opening and closing of the metering device. (tx valve).

The flow INTO a receiver is a function of how fast the gas pumped by the compressor is being condensed and drained into the receiver. Therefore at any given time, the flow rate into and out of the receiver are not the same.
This has many implications on the dynamics in the receiver.

If the discharge pressure in a system is not stable (ie windy day) The pressure can slowly rise and than fall. Interesting things happen.
(I have seen many systems where the discharge temperature will hunt 25psig (250 to 275) and then back again over 5 minute intervals.)

When the discharge pressure drops, the receiver pressure drops and the liquid line pressure drops. With a drop in pressure all saturated liquid must boil. (again definition)

It is not uncommon to see flashes of bubbles in the liquid line at this time. What causes the bubbles is that the temperature of the liquid must also drop. (cannot have liquid at a temperature above its boiling temperature)
The bubbles are cause by the liquid self cooling because of the pressure drop.

When the pressure in the system starts to rise, it rises in the receiver, liquid line etc. We are changing the conditions.

The TEMPERATURE of the liquid in the receiver will not change without the addition of heat. So as the pressure increases the saturation temperature increases, the actual temperature stays the same, and the liquid is subcooled.
<THIS IS POSSIBLE . This can be measured. BUT This is not a STATIC system.

The explanation by andy shoen is of a static system. One where a cold liquid enters a warmer tank. The cold liquid leaves, but the surface of the liquid in the tank stays warm. AND, there is no external energy added to the system.

My current position is that andys explanation is incorrect.

[DTLarca]

If you are not measuring the pressure at the same frictional and elevational point as you are measuring your temperature then you are NOT measuring subcool. If you measured your pressure somewhere frictionally and statically different from where you measure your temperature then to derive subcool you will have do some compensatory calculation (liquid pendulum).

[research]

In my lab, I have set up a test jig to test this principal.
i took an air conditioner i was testing and installed a sporlan ORI (open on rise of inlet) head pressure control, a Sporlan ORD4 (open on rise of differential) discharge bypass valve. and a receiver. I put multiple sightglasses on the system to see the refrigerant at various points of interest

R407C air conditioning system with a flooded head pressure control.


The system is a Ten ton roof top air conditioning system. I tapped into the liquid line after the condenser, and piped the liquid line to a receiver mounted external to the unit. The piping from the condenser to the receiver is through a Sporlan ORI 10, I installed a sight glass immediately at the outlet of the ORI . To control saturated conditions entering the receiver, i installed an ORD4-30 with a ball valve to shut it off, between the discharge line and the condensate line after the ORI sight glass.
After the ORD connection, into the condensate line I installed another sight glass so I could see the refigerant entering the receiver.
At the outlet of the receiver there is another sight glass installed.
Flow
Compressor....discharge line....TEE1.....into condenser.. turn to liquid... into subcooler circuit..... out of condenser..... into liquid line...into ORI 10....into sightglass1....TEE2.....into sightglass2....into receiver......receiver outlet.....sightglass 3. liquid line... TX valve ...Distributor.... evaporator.... suction line...compressor.

Bypass gas ... Discharge line TEE1.... Ball valve ...ORD4-30... into TEE2.

This is set up exactly like a low ambient air conditioning system, except that I left the subcooling circuit in the condenser (the condenser has subcooling circuits integral to the coil), to demonstrate the issues with subcooling before receivers.

The receiver is a Westermyer receiver 6" diameter, and 60 inches long.

The system was evacuated and charged with refrigerant to a clear sight glass leaving the receiver. At this point the receiver was "torched" and an operating level was determined and marked on the side. This is done to ensure that the system was not overcharged and operating with a liquid full receiver.

Normal operation
Sight glass 1

[research]

Normal operation With low ambient...Ord operating
Sight glass 1 (ORI) Sight glass 2 (into receiver) Sight glass 3 (Out of receiver)
Clear bubbly clear
If I shut off the ORD valve
CLEAR CLEAR Bubbly

If I adjusted increased the ambient in the test chamber. ORI open.... ORD closed

CLEAR CLEAR Bubbles... clear
OSCILLATING in 1 to 3 minute hunts.

These tests were repeatable and consistent. The affect on the system when the liquid bubbles were oscillating was that the TXvalve was also hunting as it tried to accommodate the changing head pressure and the affects of the bubbles.

When subcooled liquid entered into the receiver, the outlet sightglass would bubble and then clear and then bubble and then clear and then bubble and then ..... etc.....etc.....etc....

The ONLY way to stop this, was to add refrigerant until the liquid receiver was full.. (90%+)
Then Sightglass 1 was clear, sight glass 2 was clear, sightglass 3 was clear.

[sterl]

That was some Christmas Eve you boys had!

And you can preserve subcooling coming from a condenser, all the way to the point of expansion....Tube in Tube arrangement at Through Type receiver will come close, but not for more than about 10-deg K of SC and even then not with a free draining arrangement. Surge Type receiver will let cold liquid go right by it and still maintain liquid pressures....It is one way to utilize condensers on a big circuit during the winter months. Flood the smallest condenser in reverse flow (or even one section of a split) and your liquid delivery temp at the expansion device will be withing 5-deg of the ambient. Maybe not good for a TXV but frequently works very well with motor driven expansion valves, etc.

[research]

Yep..
If you want subcooling and you want a receiver, then you either need a receiver then a subcooler.... OR
you need a subcooler and a surge receiver.
Surge receivers work great when properly applied.

[desA]

Thanks very much 'research' for a fascinating experiment. :)

I'm most interested in the thermal oscillations in the system dynamics. Could you please elaborate a little further on the TXV control strategy used? Standard, MOP, cross charge etc.

[research]

The test unit is located in our environmental test chamber. The ambient temperature is controlled to a very stable 95°F. The evaporator air is ducted from another environmental room controlled to a temperature of 80°F/67WB. as per ARI standard 340/360. The static across the unit is controlled and the supply air temperature is ducted through parabolic nozzles to measure air flow. As per the standard. We are typically measuring capacity and EER of our systems.
All conditions through the air conditioning system are controlled and are stable.

When running a test, we monitor pressure and temperatures with a data logging system. there are 40 temperature channels and twenty analog channels for pressure, power, and process monitoring. All channels are fed into a computer with using LABVIEW to analyze the data.

Without the receiver, the saturated pressures and subcool/superheat temperatures are extremely stable. Flat line on the graphs. We are using a standard TXV with a 100psi MOP. Typical settings superheat= 12°F. Subcooling = 10°F.
(these can be changed depending on the tests we are doing) but stable once adjusted.

When our company uses a flow-through receiver, we always size the condensate line for sewer flow and free drain down to the receiver as per recommendations in ASHRAE REFRIGERATION HANDBOOK. All subcooling circuits are piped after the receiver. These systems are extremely stable and reliable.


To test the affects of subcooling BEFORE the receiver, we put a receiver on a 10 ton rooftop unit with integral subcooling circuits in the condenser. The receiver was mounted on the back wall of the environment chamber. The line entering the receiver was liquid line size, and the receiver was not vented.

When the receiver was added we were just interested in the affects of subcooled liquid entering a flow through receiver.

The environmental room conditions were stable but unfortunately, as this was not a specific test, the data acquisition system was not connected. (it takes a couple of days to set up)

When the ORI/ORD were active, the system was stable. Constant Head pressure, constant suction pressure.

The ORI was adjusted to a low pressure (IE OPEN) and the ORD bypass was shut off.

This allowed subcooled liquid to enter the receiver. This causes a periodic change in the head pressure.

Approximately 10-15psi change in head pressure (275sig=>265psig=>275) The pressure would drop fairly quickly then rise slowly over a 2 to 3 minute period. The TX valve would get bubbles, then would correct by opening, then would get clear liquid, overfeed then close. The suction pressure fluctuated 2 to 5 psig with the head pressure. Although not in sync. The conditions in the environment room were stable, and airflows were constant.

The fluctuations in head pressure were thought to be a function of the dynamics of the subcooled liquid entering the receiver only.

A change in the receiver saturation temperature of 2°F will cause a change in the receiver pressure of (115°=274.9psig 113°=267.5psig delta2°=7.4psig.)

The system was watched and the following is our belief of what was happening.
It was felt that when the liquid line bubbled, the txv would open, thus increasing flow in the liquid line, reducing pressure slightly, thus making more bubbles. When the valve finally opened enough to stabilize the evaporator superheat, the valve would close. This reduced liquid line flow, reduced bubbles, sightglass would clear. Flow from the receiver would be reduced. As the receiver was not vented, flow INTO the receiver would then be reduced as well. This increases subcooling in condenser. Subcooled liquid then enters the receiver. Receiver temperature drops several degrees. High side pressure drops everywhere by 7 -10 psig. Saturated liquid in the liquid line drops pressure therefore flashes. TX valve opens to compensate. Cycle repeats over and over.

This is what I believe is happening. This phenomenon was repeated over and over again. We demonstrated this affect to several of our designers and service people. The unit was then removed to make room for real testing.

[mad fridgie]

"research" very good info. It would have been interesting to compare the R407 with say R22, and how glide (or lack of it) would have influence your TEV control, would your hunting swings be as large (diff in presuure) and would the time of each cycle be quicker (my thought) or slower. What is also interesting is how do you size your expansion device. Many suppliers are now suggesting over sizing TEVs for the R4*** based refrigerants, this suggests that they seem to expect some level of vapour entrapment in the liquid feed to the TEV. (normally oversized TEVs cause hunting) One evap manufacturer says he is seeing systems that are correctly sized,(good theoretical balance of equipment) actually running lower SST and higher superheats, indicating that the evaps are actually being starved. (he had been called in as the systems did not seem to perform as indicated, and the evap design had been called into question)

[desA]

Excellent review. Thank you.

Flash dynamics versus TXV response characteristic. Would be interesting to see how a 'tuned' EEV would perform.

[research]

On our units, the TX valve are selected using manufacturers extended capacity tables. we've had good luck so far.


We evaluated Electronic expansion valves (EEV) last year. We found that they can be dialed in when conditions are stable and give extremely good performance. However we were using them on the Danfoss VSH 088 variable speed scroll compressor and could not find a suitable PID setting to stablilize the EEV over the wide capacity range of the compressor. (4 to 12 tons) We tried two valves, one valve, and then we tried an off the shelf, old fashioned dual ported TX valve.
We found that overall the old fashioned standard TX valve gave better overall performance.
At a specific capacity we could tune the EEV to give better performance. But found that it would hunt when the capacity changed. This was felt to be not the fault of the TX valve but of the PID control loop not adapting to the variable loading.

The standard old TX valve gave equivalent or better performance over the entire range of capacity.
We now uses standard dual ported TX valves on all our variable compressor applications and have had very good success with them. They are also easier to understand for the average mechanic and therefor we have less service issues.

note, this was for variable speed compressors, when we use the Digital scroll, the electronic expansion valve is required.

[desA]

We found that overall the old fashioned standard TX valve gave better overall performance.
At a specific capacity we could tune the EEV to give better performance. But found that it would hunt when the capacity changed. This was felt to be not the fault of the TX valve but of the PID control loop not adapting to the variable loading.

Agreed. PID loops have definite limits, unless they are well understood - especially in tracking control problems.

[DTLarca]

Here's an example of the sort of flow through receiver Calvin uses. The subcool cyclic pattern occurs here too unless you have both stable condenser capacity control (inverters with clever PID algorithms) and stable TEV/EEV operation. Of course the larger the receiver volume in relation to the system capacity the better.

6492

[research]

DTLarca

That receiver looks like a pumped refrigerant assembly... It also look like a quality job.
Who sells that??

[Tradewinds]

Yeah. That looks like a Liquid Pressure Amplification (LPA) set up.


6513

I believe in recent years, it's starting to be used a lot more on bigger systems in the UK. It allows the head pressure to be reduced considerably to improve the energy efficiency of the system, but uses the pump to maintain the liquid pressure to the expansion device.

Rule of thumb - For every 1k we lower SCT = 3% decrease in energy consumption.

Apparently, by reducing the condensing temperatures by 15-20'c can see energy savings of 30-40%.

http://www.acr-news.com/news/news.asp?id=1507

[DTLarca]

Research, Calvin Bekker, originally from Durban South Africa but now here in the UK, owns the patents. His website is...

http://www.hysave.com/

Even with head pressures floated lower - EEV's tend not to need the pressure boost given by the pump so the pump is generally not needed for close coupled systems so long as the drainage from condenser to EEV is cleverly thought-out. But for remote evaporators either at the same level as the receiver or higher than the receiver you need a liquid delivery system to ensure a solid bore of liquid is delivered to the expansion device inlet at all times. None the less stable EEV and Condenser capacity is still required though less so than without the delivery system. The Assembly in the image is simply called a Liquid Delivery System or LDS.

[research]

Thanks

looks good.

[Peter_1]

Regardless if you have small or large amonts of sub cooling, it comes down to how you measure. My argument comes from those who measure discharge pressure and liquid temperature to give sub cooling. You must measure pressure and temp at the same point.

Was searching for something on RE and stumbled on this post of MF within this thread some posts back.
Sorry to re-open it a little bit but I think my experiences an/or visions can throw (perhaps) some light on this issue.
Or make it even more confusing.

You're right MF when you say that you must measure pressure and temperature at the same point but you have also to take in account the measuring fault of your thermometer.

Same remark for DTLArca when measuring SC with - sorry - the rubbish Refco digital manifold (we have 3 of those and rarely use those any longer)
The temperature measured on the outside of a pipe - even a copper pipe - is never on the same actual temperature of the liquid flowing in that pipe.

When we take F-gas exams, we even monitor if the candidates are aware of this measuring error.

If you measure right after the TEV or on the first bend of an evaporator, then you should measure exactly Te (given no Dp on the evaporator)
Then, if you measure the bends on a condenser (not the beginning an not the end of course and neglect Dp) - like in the video of Monkey Spanners - then you should measure exactly your condensing temperature.

You will find out you never will measure the correct Te and Tc and you will notice differences of 2 to 4K.
Let this now be the amount of measured SC on the outside of a copper tube.

This is for me the measuring error, partly due to thermal resistance of the copper and partly due to a measuring error of the instrument and partly due to the temperature probe.

Therefore, processes which needs very accurate temperature measurements always have immersed probes and many times in a bend where there's a good contact with the flowing liquid.

PS: the small digital meter from Monkey Spanners is a cheap infrared meter which works very well on black surfaces. When you need to measure Stainless steel , color it first with a black marker. I have the opportunity in evening classes to buy all these things/gadgets and test them while been paid for it. And find solutions for it.
We also found out something where you can optimize pressure testing with nitrogen and where I never saw this method in books. Will make a separate post for this.