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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
desA
Is this correct ie. taken at those temps (was a little confused by your reference).
Thoughts:
If that is the case, then I feel that the condenser is running out of capacity (large SC), with the evap slightly over-capacity (large SH), at the 0'C operating condition. It seems to have a system imbalance.
I'd not be at all happy with SH=0K heading towards a compressor, unless the line on the way gets hot & evaporates the droplets.
Very likely Chef's system has a suction/liquid HX... which changes everything by adding both SC and SH.
As far as we know Aaron's system does not have a suction/liquid HX.
Why he (or his contractor) would charge the system to the point where it is flooding the compressor (1'F SH) is anybody's guess.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Gary
The rules apply for both TXV and cap tube systems.
.
I thought you did mention that the rules apply for both TXV and cap tube systems?
The point of posting the 3 conditions of a system (not Aaron's) was to demonstrate that a cap tube system operates with varying SH and SC and that it cannot be used to diagnose the systems condition directly.
I dont think its useful for the forum to use TXV based analysis on a cap tube system - much better to use cap tube based analysis. Some of it has been documented but much of it has not. On the basis that they are very very different this may be an opportunity to move forward and start to develop some rules and guidelines.
DesA kindly demonstrated this with a possible analysis of a condenser running out of capacity, a slightly oversize evap and system imbalance. Well this is a production unit with thousands of units worldwide and the system functions perfectly, the results shown are exactly correct for a cap tube system.
Also as it is critically charged running with very low SH is tyipical when the tube inlet is right on the saturation line, no SC and X=0.
Control and balance are more determined from the excess pressure in the condenser which could be caused, for one, by a high heat load. This may show a SC value of between 5.5K to 8.5K which Gary suggests would be his aim point, when in fact it represents a cap tube that is too long - it would force excessive head pressure, refrigerant backed into the condenser, starve the evap and have a lower than optimum COP.
Another example may be low TD's, here balance is achieved by suction throttling the compressor, lowering the flow rate, allowing gas and liquid to pass through the tube (with an x=0.4 perhaps), less gas is condensed and so the TD shows up as low. This indicates a short cap tube. By TXV rules it seems to indicate an inefficient compressor.
The thread should really concentrate on the cap tube issues.
Chef
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Re: Variable RPM compressor/cap tube issue
Think of a TXV as a variable capacity cap tube - the variation driven from a temp signal. If you took the bulb off the evap line & stuck it in a fluid at preset reference temp, you could tune the 'variable cap tube'.
The rules of RHVAC then should apply. If not, then why not?
Actually, you could perform the same function with an AEV, orifice or even fine-tuned hand valve. They are all restriction devices. There is no black art in this.
Why did the 'large manufacturer' get it wrong? Simple. Their systems were designed in a country far from the current environment in which they currently operate, or, ... they messed up... :)
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1 Attachment(s)
Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Gary
I have no idea what your point is. Where did I say I could tell if all is well or not from those few numbers? I didn't. Nor did I say that all rules apply the same for TXV and cap tubes. The specific rules we were discussing were low TD's coupled with low superheat being an indicator of inefficient/undersized compressor.
Oh and last:
I was challenging you to plot an inefficient compressor on a PH diagram thereby enlightening us. If you don't want to do this just say so.
Nicely sidestepped. Not sure if it is rhetoric or semantics though.
The specific rule we are discussing then is low TD's and low SH. Well as mentioned in the above post it is TXV rules that suggest an ineffcient compressor and cap tube rules that suggest it is a very short tube. My point again is we should be using and building on the right set of rules.
Oh and last.
I did indeed suggest you plot an x=0.4 on a PH diagram and was also just challenging you, so here is the plot I was alluding to.
Attachment 3197
Hopefully you will see the h2-h3 is greatly reduced and hence the TD's will fall.
Presumably you'll do the gentlemanly thing and post your inefficient compressor PH diagram.
Chef
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
desA
Why did the 'large manufacturer' get it wrong? Simple. Their systems were designed in a country far from the current environment in which they currently operate, or, ... they messed up... :)
Quote:
Originally Posted by
desA
Why don't you test it for yourself, instead of slagging off a perfectly sound company?
Build a small prototype & test if your theory is correct, or not.
Then tell us the results of your experiment.
So which one of these 2 quotes do you stand by?
Chef
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Re: Variable RPM compressor/cap tube issue
Both, actually.
The first quote is a typical occurrence for foreign companies manufacturing for a market with a local climate different to their own. It happens in the automotive heat-transfer industry all the time. It is the principal reason for CKD pack substitutions with local parts.
The second is that you punted the 'name-dropping' scenario of 'big company is perfect'. I don't know their name, nor do I wish to. No big company is perfect - they are as good as their weakest designer. Your results prove exactly this.
Better to keep the discussion technical - not personal, eh... :)
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Chef
The specific rule we are discussing then is low TD's and low SH. Well as mentioned in the above post it is TXV rules that suggest an ineffcient compressor and cap tube rules that suggest it is a very short tube.
TXV rules would suggest an inefficient compressor or TXV stuck wide open (overfeed).
Cap tube rules would suggest the same, the equivalent for TXV stuck wide open being the short cap tube (overfeed).
IOW, inefficient compressor or overfeed, be it TXV or cap tube.
Quote:
Originally Posted by
Chef
Oh and last.
I did indeed suggest you plot an x=0.4 on a PH diagram and was also just challenging you, so here is the plot I was alluding to.
Attachment 3197
Hopefully you will see the h2-h3 is greatly reduced and hence the TD's will fall.
Presumably you'll do the gentlemanly thing and post your inefficient compressor PH diagram.
Chef
PH diagrams are your thing, not mine. I don't use them.
I declined your challenge and apparently you have declined mine. Fair enough.
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Re: Variable RPM compressor/cap tube issue
http://www.refrigeration-engineer.co...7&d=1259551642
I don't want to be rude, but this diagram is plainly incorrect.
x=0 must, by definition, sit on the liquid saturation line - if you are referring to the LP line. If you are referring x=0 to the HP line, then your condenser is critically sized & any x>0 implies an undersized condenser... :)
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
desA
http://www.refrigeration-engineer.co...7&d=1259551642
I don't want to be rude, but this diagram is plainly incorrect.
x=0 must, by definition, sit on the liquid saturation line - if you are referring to the LP line. If you are referring x=0 to the HP line, then your condenser is critically sized & any x>0 implies an undersized condenser... :)
Plainly incorrect or misread and misunderstood?
Previously we have been talking about the conditions at the entrance to the cap tube ie at point h(3) on the PH diagram. The arrows point to 3 cycles on the PH diagram for different X values at the h(3) point.
As you can see the red line touches the saturation curve in the top left hand side. Maybe I should have been totally correct and said "This curve shown in red is the PH cycle for a cap tube system with an entry to the tube of x=0 ie at the h(3) point" but it did not fit on the diagram. I just figured you had been following the debate more closely, sorry for that.
Quote:
Originally Posted by
desA
If you are referring x=0 to the HP line, then your condenser is critically sized & any x>0 implies an undersized condenser...
Again your quoting some dogma from TXV conditions and this just does not apply to cap tube systems - and the condenser is not critically sized nor is it undersized its just perfect. Whats imperfect is the cap tube length for the system and the PH diagram shows how it chooses to balance itself out even when the tube is wrong. By having entrained gas in the flow to make up more pressure drop. It is not an ideal situation nor one that should be designed for but it happens and it is used as a tool to evalute the correctness of the tube, not the condenser.
Chef
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Re: Variable RPM compressor/cap tube issue
I think that you are confused on the technical definitions of x (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. The reason for this is that the cycle is defined on full saturated liquid at point 3.
There should be no vapour fraction going into a cap tube, or TXV... If there is, then the design needs to be altered - the cap tube is not meant as a total system imbalance corrector.
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.
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Re: Variable RPM compressor/cap tube issue
Maybe I may respond to post by inserting blue text within your quote.
Quote:
Originally Posted by
desA
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
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Re: Variable RPM compressor/cap tube issue
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
desA
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
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Re: Variable RPM compressor/cap tube issue
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)
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Re: Variable RPM compressor/cap tube issue
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)
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
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.
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
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
mad fridgie
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?
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
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)
I suspect the zero SC and SH system Chef keeps referring to has just such a suction/liquid HX arrangement... and this changes everything.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
mad fridgie
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.
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Re: Variable RPM compressor/cap tube issue
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.
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Re: Variable RPM compressor/cap tube issue
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.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
mad fridgie
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.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
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.
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:-
- Is the condenser big enough - well at quoted size maybe but a fan would help as you suggested.
- 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
- Is the tube OK - well at its stated length and diameter it is really short.
- 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.
- 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
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Re: Variable RPM compressor/cap tube issue
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.
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Re: Variable RPM compressor/cap tube issue
What would the impact be of flooding the compressor, in this case?
Catastrophic, or mainly a power-consumption issue?
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
desA
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.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Chef
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:-
- Is the condenser big enough - well at quoted size maybe but a fan would help as you suggested.
- 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
- Is the tube OK - well at its stated length and diameter it is really short.
- 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.
- 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.
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Re: Variable RPM compressor/cap tube issue
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
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
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
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
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Chef
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?
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
desA
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.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
mad fridgie
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.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
desA
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
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Chef
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?
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Chef
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.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Gary
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.
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Re: Variable RPM compressor/cap tube issue
I have no idea what a sonic choke is.
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Re: Variable RPM compressor/cap tube issue
http://en.wikipedia.org/wiki/Choked_flow
This should explain the concept of a sonic choke.
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Gary
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
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Re: Variable RPM compressor/cap tube issue
Quote:
Originally Posted by
Chef
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.