Variable RPM compressor/cap tube issue

[Chef]

Maybe I may respond to post by inserting blue text within your quote.

I think that you are confused on the technical definitions of x (vapour fraction).

Thankyou for your kind interpretation of my understanding of X - the vapour fraction.

If you do want to refer to 'x' then it may be wiser to keep this to the LP portion of the graph - this is the traditional place in which it is used in relation to the vapour compression cycle.

Well, X occurs in the condenser and the evaporator and always has I beleive, in fact in seems to occur everywhere within the bell (inside the saturation line). I take note of your desire to more precisely position the quoted X and propose XC and XE for condenser and evap respectively, XCT will be the value somewhere along the cap tube.

The reason for this is that the cycle is defined on full saturated liquid at point 3.

Not sure that is an absolute! There are instances when point 3 (h3) is inside the saturation curve, for instance in a classic freezer. You might like to give some references for the cycle being strictly 'saturation'. Otherwise to me its a diagram that descibes what is happening and can have h3 wherever it may be.


There should be NO vapour fraction going into a cap tube, or TXV - none...

I hope you are absolutely sure about that! There seems to be some (many in fact) references that show vapour entering the cap tube is part of the whole process especially when operating in freezer temperature ranges (all else being normal (average) world conditions). In fact Supco report on a design that actually introduced vapour into the cap tube entrance.

"In the early 1960s, a manufacturer of window air conditioners took advantage of bubbles entering a cap
tube by placing a small heater around the strainer before the cap tube inlet. The thermostat was in
reality a rheostat that controlled the intensity of heat to the strainer—which in turn regulated the
amount of bubbles entering the cap tube. The first stage of heat to the strainer was to reduce the
subcooled liquid temperature (increasing the bubble length). The second stage was to create a boiling

action, in various heat intensities, to decrease the overall efficiency of the evaporator."
From

THEORY OF THE CAP TUBE AS A
REFRIGERANT CONTROL

By Henry Ehrens, Sr Engineer - Sealed Unit Parts Co., Inc

There are many other referances that you may wish to discover.


If there is, then the design needs to be altered - the cap tube is not meant as a total system imbalance corrector.

You might like to clarify that comment and provide some eveidence that it is not the total system imbalance corrector, preferably something published rather than conjecture.

If you do want to refer to the vapour fraction in some other place, then it may be wiser to state clearly 'x=0 @ HP' or something similar, to avoid confusion.

Chef

[desA]

Many thanks...

[Chef]

The heater you mentioned will, in all likelihood, not be found on the refrigerator under review. This is merely a flow control mechanism. The key words here are - measure, control. This is a pure red-herring to try & bolster your argument.

Of course it wont be found on the refrigerator I mentioned - its a quote from the 1960's. It is a reference related to your catagoric statement that NO VAPOUR ENTERS THE CAP TUBE, purely that and not, I reckon, a red herring, its a quotable and reputable source.

The cap tube you are using seems to be a cut-to length version. Perhaps I'm mis-reading things.

I am pretty sure it is cut to a length - yes.

I would refer you to any thermodynamics text book, which discusses the terminology used in vapour compression cycles. This may make it a little less difficult to follow your logic, in future. (I am saying this because of the general confusion already existing in this thread.) There is no need for vapour entry into the cap tube, I'd bet.

But what if there is - and I have good evidence to suggest it does. The whole point of a discussion is to actually look at what the other person is saying and see if it is valid - not to just throw it away because it does not meet your ideas. Try to see where this is going and just for a small amount of time look seriously at it.

Uncontrolled vapour entry into a cap tube will present difficulties.

No mention of uncontrolled anywhere. Fine statement but no reasoning behind it.

Based on the data you have submitted thus far, I'd suggest that the evap & condenser are not totally suited to the compressor under the situations presented.

I have already stated the system works like a charm and is performing fantistically, really good cold produced, excellent efficiency, top notch ... What else can I say except it works perfect.


This will explain some of the variation you see in SH & SC across the range of operation.

I have tried to explain that this is normal in a cap tube system - you can only have perfect conditions for a cap tube at a single point of conditions and elsewhere it will exhibit strange but definable characteristics.

There is no magic, nor mystery to all of this - it's straightforward engineering & system balancing.

Correct - but who is going to determine the system balance and rules governing it. By addage or engineering. I choose engineering.

Why dont you have a read of this
http://www.refrigeration-engineer.co...ad.php?t=16920
and concentrate on the limiting phase of the system.
You may find it gives you the information you need.

The solution is well within grasp and at the moment I am working on 4 system and 3 of the 4 exhibit gas/liquid at the tube entrance.

Chef

[mad fridgie]

Hi,
X=0 is at 100 liquid, at cap inlet, (it shows very little sub-cooling, also no allowance has been made for pressure drop through the heat exchangers.
A cap system is critical charge and self balances and at different conditions the varying amount of refrigerant mass are held within different parts of the system.
The mass transfer can be charted by chefs figures
At low Te mass is in the evap, hence little SH, and little mass in the condensor so no liquid seal is apparent in the consensor so no sub-cooling (vapour and liquid mix)
The opposite at higher evaps.
Using a P/E diagram on its own to diagnose in efficient compressor not possible.
As far as the current problem it seems to me that we do not have reliable constant (excludes comp displacement).
It would be nice for calculation if the sucton return had a much higher super heat (from this we could calculate actual mass flow) at present it seems that we could have a vapor/liqud mix (by what % you can not know)

[mad fridgie]

One point I did not bring up was that in many cases the cap is in contact with suction, this is to ensure ad excess liquid in the suction is boiled of and it reduces the pressure drop across the cap (reducing liquid to vapour change)

[Chef]

Hi,
X=0 is at 100 liquid, at cap inlet, (it shows very little sub-cooling, also no allowance has been made for pressure drop through the heat exchangers.
A cap system is critical charge and self balances and at different conditions the varying amount of refrigerant mass are held within different parts of the system.

Precisely and these constitute part of the control process.

The mass transfer can be charted by chefs figures
At low Te mass is in the evap, hence little SH, and little mass in the condensor so no liquid seal is apparent in the consensor so no sub-cooling (vapour and liquid mix)

This is the point and your conclusion is the same as we have. No subcooling and so it will be a vapour/liquid mix entering the tube. It then enters the next pahse of its control, gas entrainment to increase the pressure drop.

The opposite at higher evaps.
Using a P/E diagram on its own to diagnose in efficient compressor not possible.

I think this is just one of Gary's jokes, but as he says he does not use PH diagrams it may be rather a cruel one.

As far as the current problem it seems to me that we do not have reliable constant (excludes comp displacement).
It would be nice for calculation if the sucton return had a much higher super heat (from this we could calculate actual mass flow) at present it seems that we could have a vapor/liqud mix (by what % you can not know)

We running simulations on it to get the various fractions and so far they agree well with the test data. Hopefully soon we will have a way to determine the various % vapour/liquid at different parts of the system and begin to develop an analytical tool.

The 3 systems we are working on all exhibit gas entrainment and we have a way to actually see it for comparison to the predictions.

Chef

[desA]

A cap system is critical charge and self balances and at different conditions the varying amount of refrigerant mass are held within different parts of the system.

At low Te mass is in the evap, hence little SH, and little mass in the condensor so no liquid seal is apparent in the consensor so no sub-cooling (vapour and liquid mix)

MF, does it sometimes occur, in cap systems, that the charge holdup occurs in the opposite way ie. the condenser bleeds off over the course of part of the cycle?

I've seen this in some TXV systems & put it down to orifice size & system balances. In other words, is it a given to expect that a system will always move to empty the evap & fill up the condenser?

[Gary]

One point I did not bring up was that in many cases the cap is in contact with suction, this is to ensure ad excess liquid in the suction is boiled of and it reduces the pressure drop across the cap (reducing liquid to vapour change)

I suspect the zero SC and SH system Chef keeps referring to has just such a suction/liquid HX arrangement... and this changes everything.

[Gary]

Using a P/E diagram on its own to diagnose in efficient compressor not possible.

Using a P/E diagram on its own to diagnose anything is questionable.

There is a time for calculations (before the system is built) and a time for measurements (after the system is built). But those who calculate want to keep on calculating.

If you are a hammer, everything looks like a nail.

[Gary]

Imagine that your fridge is not functioning properly. You call a service company and the tech walks in the door saying, "Hmmm... I wonder what's wrong with the cap tube sizing?"

Does this seem like an insane approach to anyone but me?

We can't see the system from here, we can only see the OP's description of it. Where is the detailed description of the system? Pics/diagrams would be nice.

Where is the step-by-step process of elimination by which we have narrowed the problem(s) down to the cap tube?

Or maybe its just me who is insane.

[mad fridgie]

From my experience (many years ago) when producing cap systems, a single method was not used,
Simply you calculated (theory) cap size and charge (this would normally bring you within a reasonable starting point) and is needed, then was tuned, by the practical test method.
I suspect simulators now a days are a lot better than simple steady state calcs that we used in the past.
Cap systems are really designed for fixed load systems. (think of you old fridge with freezer plate in side and skin condensor. fairly constant.)
As long as it worked OK at max design ambient, we never worried about, performance at lower ambients. At the lower ambients the the need for nett refrigeration effect is reduced, so Te normally dropped, with little concern about efficiency. The only concern was liquid flood back.

[desA]

I suspect simulators now a days are a lot better than simple steady state calcs that we used in the past.

The trouble is - they're not.

A simulator generally gives a static snapshot of what is a thermodynamically-stable system i.e. one that has been stable & unchanging for a long, long time (infinite time). Coolpack, for instance works this way. In this sense, all they do is to automate the hand calculations that any good fridgie could do, anyway.

Things like dynamic response, mass charge & so forth, would be found in some academic research codes, but, they are not common in the public domain.

I agree with the folks who refer to the use of simulators to get through the initial design stage & then hands-on tuning to settle the real system. This is the strength of it. Sometimes, referring back to the simulator to assist analysis of the real operational data can be useful, but, in the end, where the rubber hits the road is whether the unit actually works as per original design, or not.

In many cases, if the original design is too tight, then real performance can suffer. On the other hand, if the spec is too loose, then system stability may suffer, as well as system costs. It's a balancing act all the way.

[Chef]

Imagine that your fridge is not functioning properly. You call a service company and the tech walks in the door saying, "Hmmm... I wonder what's wrong with the cap tube sizing?"

Does this seem like an insane approach to anyone but me?

We can't see the system from here, we can only see the OP's description of it. Where is the detailed description of the system? Pics/diagrams would be nice.

Where is the step-by-step process of elimination by which we have narrowed the problem(s) down to the cap tube?

Or maybe its just me who is insane.

Completely correct, no one is going to visit a running system and say oooh looks like the tubes wrong. They will look at leaks, blocked condensers etc.

But the original post was handed over to Aaron and the aim is to try and help him sort out his problem.

He has had built, by a subcontractor, a unit to operate in some very unforgiving conditions and it seems the decision on the cap tube was made by using a freely availble progam. It is a new system and all new components.

Now his problem is poor cooling and well...... you have all seen the data he has posted.

As its a new design and a cap tube system (and using a software program freely available) one might say:-

1. Is the condenser big enough - well at quoted size maybe but a fan would help as you suggested.
2. Is the evap Ok well it seems small but it is fan assisted so that makes a big differance to its perfromance. So that seems OK
3. Is the tube OK - well at its stated length and diameter it is really short.
4. The compressor is new so that should be OK in performing to specs. Is it sized right - only have to look at the heat load and the quoted cooling effect to see its in the ball park.
5. Is the charge OK - well cant see any data on that yet to suspect it but its always a suspect

So first conclusion is - wait a minute - I think the cap tubes wrong.

Its a differant scenario to the one you have pointed out but it is Aarons scanario.

Chef

[Gary]

Assuming the system does not have a suction/liquid heat exchanger to add SH, he is currently flooding the compressor.

He needs to remove refrigerant until the compressor inlet SH is about 20F/11K and then take a full set of pressure and temperature readings, so we can see how it runs without a flooded compressor.

[desA]

What would the impact be of flooding the compressor, in this case?

Catastrophic, or mainly a power-consumption issue?

[Gary]

What would the impact be of flooding the compressor, in this case?

Catastrophic, or mainly a power-consumption issue?

It could be catastrophic. I would not rule out damaged compressor at this point.

[Gary]

Completely correct, no one is going to visit a running system and say oooh looks like the tubes wrong. They will look at leaks, blocked condensers etc.

But the original post was handed over to Aaron and the aim is to try and help him sort out his problem.

He has had built, by a subcontractor, a unit to operate in some very unforgiving conditions and it seems the decision on the cap tube was made by using a freely availble progam. It is a new system and all new components.

Now his problem is poor cooling and well...... you have all seen the data he has posted.

As its a new design and a cap tube system (and using a software program freely available) one might say:-

1. Is the condenser big enough - well at quoted size maybe but a fan would help as you suggested.
2. Is the evap Ok well it seems small but it is fan assisted so that makes a big differance to its perfromance. So that seems OK
3. Is the tube OK - well at its stated length and diameter it is really short.
4. The compressor is new so that should be OK in performing to specs. Is it sized right - only have to look at the heat load and the quoted cooling effect to see its in the ball park.
5. Is the charge OK - well cant see any data on that yet to suspect it but its always a suspect

So first conclusion is - wait a minute - I think the cap tubes wrong.

Its a differant scenario to the one you have pointed out but it is Aarons scanario.

Chef

To take this step-by-step:

We don't have enough info for dT's, but since dT must be less than TD and TD's are low, we know that evap airflow is not the problem, nor is condenser airflow... although these conclusions may change when the system is actually handling a decent load.

The subcooling is not excessive (liquid not backing up into the condenser).

Next on the checklist would be low superheat. We must assume that the system has no form of suction/liquid heat exchange, therefore 1'F SH is much too low and is flooding the compressor. Refrigerant must be removed in order to raise superheat at the compressor inlet.

The troubleshooting procedure must halt at this point until the excess refrigerant is removed. Once this is done we need a whole new set of temperature and pressure measurements and (this is the part that everyone hates) we then need to start the troubleshooting procedure all over again, taking it from the top and checking the airflows.

[icecube51]

why not try to put a sightglass just before the captube and see what happens.
if the glass is just "a point" full, take notes and put them in a PTchart.
thats wat i would do.

Ice

[Chef]

why not try to put a sightglass just before the captube and see what happens.
if the glass is just "a point" full, take notes and put them in a PTchart.
thats wat i would do.

Ice

Ice, yes its a great idea.

Another system we are working on (not the Aaron system) has a sight glass just before the cap entrance. As the unit gets colder and colder so the outlet side of the sight glass shows less and less of the hole covered by liquid. At the end of the pulldown maybe only the bottom 5% is covered by liquid and rest is gas entrainment. Needless to say the system is not performing well but what a totally excellent tool to see what is happening.

The main drawback is that a service engineer could be fooled into charging until it is solid liquid which would be a disaster, instead of putting in a weighed charge. Usful for R&D work and commissioning of a new build though.

Although you see the glass shows a liquid/gas mix there is no way yet to get an actual value of X from it.

Chef

[desA]

Another system we are working on (not the Aaron system) has a sight glass just before the cap entrance. As the unit gets colder and colder so the outlet side of the sight glass shows less and less of the hole covered by liquid. At the end of the pulldown maybe only the bottom 5% is covered by liquid and rest is gas entrainment. Needless to say the system is not performing well but what a totally excellent tool to see what is happening.

This does not sound right at all. It would surely be extremely difficult to ask a cap tube to manage this?

Is this flashing situation occurring at the same time as the SH~0K situation?

[mad fridgie]

This does not sound right at all. It would surely be extremely difficult to ask a cap tube to manage this?

Is this flashing situation occurring at the same time as the SH~0K situation?

At this point refrigerant mass is held up in the evap, and very little in the condensor hence low SH and Liquid sub cooling.

[desA]

At this point refrigerant mass is held up in the evap, and very little in the condensor hence low SH and Liquid sub cooling.

Ok, but, is this a safe operating situation for the compressor? Would it not perhaps be useful to have a receiver on the hp side, to retain some of the liquid during the off-times?

I'd imagine a vertical riser outlet of evap could help a little, to keep some of the liquid out of the compressor inlet.

Seems a little precarious.

[Chef]

This does not sound right at all. It would surely be extremely difficult to ask a cap tube to manage this?

Is this flashing situation occurring at the same time as the SH~0K situation?

This is another system, not anything to do with previous posts except as an example to Ice's sight glass proposal. But as it is a general discussion and this system does need sorting we can have this as the sight glass system.

The SH is somewhere in the region of 3 to 5K but has not been accurately determined on the sight glass system. No measurable SC and suspected as being SC=0.

Its a complex situation where we see the exit to the tube is sonic so it has a maximum flow rate and is limiting the through put. As a consequence the suction pressure falls well below its 'normal' and the mass flow goes way down. So as you say it is not normal. Once the tube oulet goes sonic no lowering of the suction pressure will increase the flow rate - its just the laws of sonic flow.

The question is, now the cap tube has entrained gas at the inlet the gas velocities at the outlet are much larger than if it had more liquid in the inlet, so the outlet becomes sonic and limiting. So is it the high XC entering the tube or the sonic limitation that is causing the problem. Its a bit of a chicken and egg situation.

Normally the first step would be increase the tube diameter and select a new appropriate length. This should stop the sonic limitation and allow a new set of measurements to be taken.

Chef

[Gary]

This is another system, not anything to do with previous posts except as an example to Ice's sight glass proposal. But as it is a general discussion and this system does need sorting we can have this as the sight glass system.

The SH is somewhere in the region of 3 to 5K but has not been accurately determined on the sight glass system. No measurable SC and suspected as being SC=0.

Its a complex situation where we see the exit to the tube is sonic so it has a maximum flow rate and is limiting the through put. As a consequence the suction pressure falls well below its 'normal' and the mass flow goes way down. So as you say it is not normal. Once the tube oulet goes sonic no lowering of the suction pressure will increase the flow rate - its just the laws of sonic flow.

The question is, now the cap tube has entrained gas at the inlet the gas velocities at the outlet are much larger than if it had more liquid in the inlet, so the outlet becomes sonic and limiting. So is it the high XC entering the tube or the sonic limitation that is causing the problem. Its a bit of a chicken and egg situation.

Normally the first step would be increase the tube diameter and select a new appropriate length. This should stop the sonic limitation and allow a new set of measurements to be taken.

Chef

Does the sight glass system have a fixed speed compressor or variable speed?

[Gary]

This is another system, not anything to do with previous posts except as an example to Ice's sight glass proposal. But as it is a general discussion and this system does need sorting we can have this as the sight glass system.

The SH is somewhere in the region of 3 to 5K but has not been accurately determined on the sight glass system. No measurable SC and suspected as being SC=0.

Adding refrigerant would give you some subcooling, but would probably drop the superheat down... unless the heat load were increased (more airflow) to add SH, in which case it should balance out.

Or... the cap tube could be a little more restrictive, assuming you are designing for the current evap load and cond load.

[desA]

Or... the cap tube could be a little more restrictive, assuming you are designing for the current evap load and cond load.

Judging from Chef's comments above, it seems like there is already a sonic choke on the current cap tube. Can't get much more restrictive than that... unless you can limit the vapour entry into the orifice.

Seems like a new orifice selection is due.

[Gary]

I have no idea what a sonic choke is.

[desA]

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

This should explain the concept of a sonic choke.

[Chef]

Adding refrigerant would give you some subcooling, but would probably drop the superheat down... unless the heat load were increased (more airflow) to add SH, in which case it should balance out.

Or... the cap tube could be a little more restrictive, assuming you are designing for the current evap load and cond load.

A sonic limitation is where the local velocity in the pipe, usually the end of the cap tube, is at the speed of sound in the medium. The gas cannot go faster than the speed of sound unless it is in a specially designed nozzle. Also if the suction pressure is reduced it has no influence on the flow, it is effectively at its maximum throughput.

So with sudden stop in flow I think adding gas may not help - it may put a little more in the condensor but it is not going to cure this problem.

The cap tube is already too restrictive and so needs to be made of a larger diameter and a new appropriate length.

We did try changing the gas charge a little and no effect, I suspect the sonic limitation is starving the compressor and so reducing flow into the condenser and now we have a classic scenario for the XC to be around 0.3 or even 0.4 - very strange indeed.

Chef

[Gary]

A sonic limitation is where the local velocity in the pipe, usually the end of the cap tube, is at the speed of sound in the medium. The gas cannot go faster than the speed of sound unless it is in a specially designed nozzle. Also if the suction pressure is reduced it has no influence on the flow, it is effectively at its maximum throughput.

So with sudden stop in flow I think adding gas may not help - it may put a little more in the condensor but it is not going to cure this problem.

The cap tube is already too restrictive and so needs to be made of a larger diameter and a new appropriate length.

We did try changing the gas charge a little and no effect, I suspect the sonic limitation is starving the compressor and so reducing flow into the condenser and now we have a classic scenario for the XC to be around 0.3 or even 0.4 - very strange indeed.

Chef

Apparently there is enough refrigerant getting through to handle the heat load and end up with a few degrees of superheat at the coil outlet, so how restrictive can it be?

If you are reducing the suction pressure by speeding up the compressor, then you might expect increased flow. If you are reducing the suction pressure by reducing the heat load, then the flow would be reduced.

[mad fridgie]

A cap is not really a control device, as itself does not change, so you can not compare to any form of modulating valve. It is the inlet and outlet conditions that determine the caps performance not the other way around.
If you use a reciever you are unable to flood the condensor (without loosing lquid seal)
In this case sizing and charge needs to be dertermine by max operating conditions (when less than this performance and efficiency go out of the window.)
The unit is over sized as the OP wants a 30% run time.
The cap needs to be larger diameter and longer, in contact with suction.
An accumulator can fitted if an over charge is required. (always a safe option) as far RD goes I would install one (this could save the compressor)
Particular with danfoss compressors.

[Gary]

The key to any experiment is to control all of the variables except the one being tested.

But here we are discussing one component in isolation as if the rest of the system had no effect on it.

[desA]

The cap tube is already too restrictive and so needs to be made of a larger diameter and a new appropriate length.

We did try changing the gas charge a little and no effect, I suspect the sonic limitation is starving the compressor and so reducing flow into the condenser and now we have a classic scenario for the XC to be around 0.3 or even 0.4 - very strange indeed.

I wouldn't be surprised to find the system stabilising at solutions outside of the traditional refrigeration operating window.

Each major item in the system is inherently non-linear in its response - even the piping. The system can definitely stabilise at such local solutions. The problem is how to kick it off there, & back to a vapour compression cycle. (I say 'kick', because sometimes it will not willingly flip over to the next solution - it needs coercion.)

I would suggest that the condenser be boosted in your experiment. Take a few fans & blast the thing with cold air - observe. If this helps, then your condenser is undersized for the climatic conditions in your hot, humid, wet part of the world.

Add an exit lift from the condenser, to force a liquid seal at exit.

Get those in place, then begin backing off on other things. Mass charge either low, or high, can play incredible tricks on you.

[Chef]

Apparently there is enough refrigerant getting through to handle the heat load and end up with a few degrees of superheat at the coil outlet, so how restrictive can it be?

If you are reducing the suction pressure by speeding up the compressor, then you might expect increased flow. If you are reducing the suction pressure by reducing the heat load, then the flow would be reduced.

No the system is running poorly and that is why we are looking at it. It should be producing at least 30 to 40 percent more cold. The main reason we beleive is that as the XC is around 0.3 or maybe even 0.4 we are only getting about 60 percent of the cooling possible if we had condensed all the gas.

Also the flow rate is down as the tube is sonic so we have less Kj/Kg of cooling and less Kg/s passing around the system - so combining these it gives a really poor performance.


The suction pressure is not being reduced by RPM change or heat load, the suction pressure is being reduced because the tube has gone sonic at the end.

Chef

[Chef]

A cap is not really a control device, as itself does not change, so you can not compare to any form of modulating valve. It is the inlet and outlet conditions that determine the caps performance not the other way around.

Thankyou Mad, this is exactly the case and it the conditions in the condenser and evap that change to meet a system balance. The main item is obviously pressure and comes SC and then comes x=0.1 or 0.2 etc. The evap has much less control because it is limited to pressure only.

If you use a reciever you are unable to flood the condensor (without loosing lquid seal)
In this case sizing and charge needs to be dertermine by max operating conditions (when less than this performance and efficiency go out of the window.)
The unit is over sized as the OP wants a 30% run time.
The cap needs to be larger diameter and longer, in contact with suction.
An accumulator can fitted if an over charge is required. (always a safe option) as far RD goes I would install one (this could save the compressor)

Most systems have a little accumulator on the outlet of the evap that must help (production units), but for and new designs and cut it try it stuff an accumulator is a good idea, cant hurt but it might save.
Particular with danfoss compressors.

The point about the tube is well meritted and after all it is only a tube. It will pass whatever you throw at it and how fast it passes it depends on the pressures.

Chef

[Gary]

No the system is running poorly and that is why we are looking at it. It should be producing at least 30 to 40 percent more cold. The main reason we beleive is that as the XC is around 0.3 or maybe even 0.4 we are only getting about 60 percent of the cooling possible if we had condensed all the gas.

Also the flow rate is down as the tube is sonic so we have less Kj/Kg of cooling and less Kg/s passing around the system - so combining these it gives a really poor performance.


The suction pressure is not being reduced by RPM change or heat load, the suction pressure is being reduced because the tube has gone sonic at the end.

Chef

Compressor manufacturers require at least 8.5K SH at the compressor inlet. You have said that your system has 3-5K SH. You have also said that the compressor was starved. Which is it? Is the SH borderline low (3-5K) or is the SH high (starving the compressor)?

Or perhaps you are talking about coil outlet SH and there is heat being added between the coil outlet and the compressor inlet? For diagnostic purposes, we should be talking about the compressor inlet SH, not the coil outlet SH.

You seem obsessed with the "sonic limitation" theory. I have seen no evidence of any such limitation.

[Gary]

The suction pressure is not being reduced by RPM change or heat load, the suction pressure is being reduced because the tube has gone sonic at the end.

Any reduction in suction pressure gives you a reduction in mass flow, the single exception being increased compressor RPM... yet you seem to be expecting the mass flow to increase as suction pressure is reduced?

[Chef]

Compressor manufacturers require at least 8.5K SH at the compressor inlet. You have said that your system has 3-5K SH. You have also said that the compressor was starved. Which is it? Is the SH borderline low (3-5K) or is the SH high (starving the compressor)?

I mentioned we think the SH is around 3-5K but not sure as accurate measurements were unavailable. I understand your point that if the compressor is starved then the SH should be higher and worth getting better measurements. Its a new build from a hotch potch of parts and so I dont think they ever thought about what the manufacturers would like in terms of SH.

Or perhaps you are talking about coil outlet SH and there is heat being added between the coil outlet and the compressor inlet? For diagnostic purposes, we should be talking about the compressor inlet SH, not the coil outlet SH.

Before the suction receiver.

You seem obsessed with the "sonic limitation" theory. I have seen no evidence of any such limitation.

Not obsessed with it but as its happening and you dont accept it it is in an attempt to get you too realise it could be a phenomena and then offer some solid advice.

If you google 'choked flow in capillary tubes' you will get pages of references alluding to this, both from fridge companies and research institutes.

Chef

[Chef]

yet you seem to be expecting the mass flow to increase as suction pressure is reduced?

No I dont expect that and did not say it.

If the tube has gone sonic then it cant pass any more refrigerant, the compressor suction requirement is higher than the tubes flowrate and so the suction pressure falls till equilibrium is reached.

Its part of the characteristics of choked flow (sonic) and the references via google in the previous post explain it fully

Chef

[Gary]

If refrigerant was added until the subcooling was about 15F/8.5K and the superheat was then high, you might be able to convince me that the flow is restricted, sonic or otherwise. With zero SC, there is not enough refrigerant there to restrict.

[mad fridgie]

If refrigerant was added until the subcooling was about 15F/8.5K and the superheat was then high, you might be able to convince me that the flow is restricted, sonic or otherwise. With zero SC, there is not enough refrigerant there to restrict.

The ambient is 50C, SCT is 55C then max possible SC could only be 5C (unless an external force is used)
Refrigeration in mathmatical terms is a " circular reference" there is not a start or finish point.
So at some point you start with your best guess, to get things started. (we do this without even thinking)
In this case best guess seems to be rather out, thus we are unable to resolve the problem. Regardless of where you start.

[Gary]

The ambient is 50C, SCT is 55C then max possible SC could only be 5C (unless an external force is used)
Refrigeration in mathmatical terms is a " circular reference" there is not a start or finish point.
So at some point you start with your best guess, to get things started. (we do this without even thinking)
In this case best guess seems to be rather out, thus we are unable to resolve the problem. Regardless of where you start.

Good point... but we are talking about two different systems. You are talking about Aaron's system and I am talking about Chef's system.

Chef has not told us what the temps/pressures are for his system. But he knows that the cap tube is sonically limited.

[mad fridgie]

I have no knowledge of sonic chocking, but have come across damage done to valve seats, from sonic explosions (may have used the wrong term there), so presume that the 2 are the same/similar thing

[desA]

If refrigerant was added until the subcooling was about 15F/8.5K and the superheat was then high, you might be able to convince me that the flow is restricted, sonic or otherwise. With zero SC, there is not enough refrigerant there to restrict.

Chef's situation is far worse than this.

1. Under-performing condenser;
2. Condensation process incomplete, still 30-40% vapour remaining;
3. Vapour & condensate exit condenser, move to cap tube;
4. Vapour component moves at high velocity through cap tube;
5. Vapour component velocity reaches sonic speed (local speed of sound), cap tube chokes;
6. Once cap tube chokes, no further increase in mass flow will occur;
7. Compressor keeps pulling - lowering suction pressure until system equilibrium is met.

In the end, the system settles with a condenser not completely condensing; a locked fixed choked mass flow;
lower than expected suction pressure; under-performing system.

This is not the vapour compression cycle as most RHVAC personnel would know. It is an intermediate system.

The question then should be:
How to move Chef's system towards the recognised vapour compression cycle?


Ok, gentlemen?

[Chef]

Good point... but we are talking about two different systems. You are talking about Aaron's system and I am talking about Chef's system.

Chef has not told us what the temps/pressures are for his system. But he knows that the cap tube is sonically limited.

Actually there are 3 systems, my system which I posted early 3 operating conditions.

Aarons system and a third system which is called the sight glass system.

It is the sight Glass system that has a sonic tube. The Dx on this is 150psig and the Sx is 7psig, ambient is 32 to 33 and fan cooled condenser, SC=0 and SH not too sure but some.

Here is nice quote from an excellent tutorial on cap tubes.

Critical charge is a definite amount of refrigerant that is put into the refrigeration system so that in the eventuality of all of it accumulating in the evaporator, it will just fill the evaporator up to its brim and never overflow from the evaporator to compressor. The flooding of the evaporator is also a transient phenomenon, it cannot continue indefinitely. The system has to take some corrective action. Since the capillary tube feeds more refrigerant from the condenser, the liquid seal at the condenser exit breaks and some vapour enters the capillary tube. The vapour has a very small density compared to the liquid; as a result the mass flow rate through the capillary tube decreases drastically.
http://www.nptel.iitm.ac.in/courses/...cture%2024.pdf
This the link to full tutorial and is very good reference.

It also discusses choked flow (sonic flow)

Gary you mention 'With zero SC, there is not enough refrigerant there to restrict.'

Well as the above quote suggest any time gas entrainment happens the mass flow rate decreases drastically. This is a pretty key point and once x=0.1 or x=0.2 at the inlet to cap is used in a diagnosis things start to be more apparent.

Chef

[desA]

If you really want to beef up your condensing capacity on the research unit, install an inline de-superheater (water, or air-cooled) & see what happens.

[mad fridgie]

Well, that is what I thought I said, (ealier thread) perhaps thats why I am not in the business of writing tutorials, explains it very well!

[Gary]

Actually there are 3 systems, my system which I posted early 3 operating conditions.

Aarons system and a third system which is called the sight glass system.

It is the sight Glass system that has a sonic tube. The Dx on this is 150psig and the Sx is 7psig, ambient is 32 to 33 and fan cooled condenser, SC=0 and SH not too sure but some.

Here is nice quote from an excellent tutorial on cap tubes.

Critical charge is a definite amount of refrigerant that is put into the refrigeration system so that in the eventuality of all of it accumulating in the evaporator, it will just fill the evaporator up to its brim and never overflow from the evaporator to compressor. The flooding of the evaporator is also a transient phenomenon, it cannot continue indefinitely. The system has to take some corrective action. Since the capillary tube feeds more refrigerant from the condenser, the liquid seal at the condenser exit breaks and some vapour enters the capillary tube. The vapour has a very small density compared to the liquid; as a result the mass flow rate through the capillary tube decreases drastically.
http://www.nptel.iitm.ac.in/courses/...cture%2024.pdf
This the link to full tutorial and is very good reference.

It also discusses choked flow (sonic flow)

Gary you mention 'With zero SC, there is not enough refrigerant there to restrict.'

Well as the above quote suggest any time gas entrainment happens the mass flow rate decreases drastically. This is a pretty key point and once x=0.1 or x=0.2 at the inlet to cap is used in a diagnosis things start to be more apparent.

Chef

Shall I assume R134a?

The quote says exactly what I have been saying. The system has stopped performing because it has reached its charge limit. If you add refrigerant it will have a different charge limit and will drop to a lower heat load/higher ambient combination before it once again has zero subcooling and stops performing.

[Chef]

Chef's situation is far worse than this.

1. Under-performing condenser;
2. Condensation process incomplete, still 30-40% vapour remaining;
3. Vapour & condensate exit condenser, move to cap tube;
4. Vapour component moves at high velocity through cap tube;
5. Vapour component velocity reaches sonic speed (local speed of sound), cap tube chokes;
6. Once cap tube chokes, no further increase in mass flow will occur;
7. Compressor keeps pulling - lowering suction pressure until system equilibrium is met.

In the end, the system settles with a condenser not completely condensing; a locked fixed choked mass flow;
lower than expected suction pressure; under-performing system.

This is not the vapour compression cycle as most RHVAC personnel would know. It is an intermediate system.

The question then should be:
How to move Chef's system towards the recognised vapour compression cycle?


Ok, gentlemen?

Well that is the very best synopsis I have seen and is the situation. Thankyou DesA.

As you say RHVAC'ers dont normally see these sort of conditions and the complexity of mixed gas and liquid in the entire cycle causes many problems.

It is not actually my system - this is the sight glass system and is for another person so just trying to help him out.

Now you have posted the real scenario we may get to move forward.

Chef

[Gary]

By adjusting the charge, we adjust the heat load/ambient temp combination which causes zero SC at the cap tube inlet.

When the cap tube inlet SC is zero, the SH should be almost low enough to flood the compressor... but not quite.

If the SH is high at the zero SC condition, then the cap tube is too restrictive. If it is low, the cap tube is not restrictive enough.