Results 51 to 100 of 115
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30-11-2009, 09:26 AM #51
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30-11-2009, 09:39 AM #52
Re: Variable RPM compressor/cap tube issue
Many thanks...
Last edited by desA; 30-11-2009 at 10:14 AM. Reason: What else can be said... bypass required...
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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30-11-2009, 10:41 AM #53
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30-11-2009, 10:41 AM #54
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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30-11-2009, 11:02 AM #55
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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30-11-2009, 11:27 AM #56
Re: Variable RPM compressor/cap tube issue
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.
ChefLast edited by Chef; 30-11-2009 at 11:30 AM.
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30-11-2009, 11:40 AM #57
Re: Variable RPM compressor/cap tube issue
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?Last edited by desA; 30-11-2009 at 11:42 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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30-11-2009, 04:55 PM #58
Re: Variable RPM compressor/cap tube issue
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30-11-2009, 05:13 PM #59
Re: Variable RPM compressor/cap tube issue
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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30-11-2009, 06:41 PM #60
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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30-11-2009, 08:37 PM #61
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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30-11-2009, 11:07 PM #62
Re: Variable RPM compressor/cap tube issue
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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01-12-2009, 11:46 AM #63
Re: Variable RPM compressor/cap tube issue
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
Its a differant scenario to the one you have pointed out but it is Aarons scanario.
Chef
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01-12-2009, 03:27 PM #64
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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01-12-2009, 03:34 PM #65
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?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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01-12-2009, 03:51 PM #66
Re: Variable RPM compressor/cap tube issue
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01-12-2009, 04:10 PM #67
Re: Variable RPM compressor/cap tube issue
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.Last edited by Gary; 01-12-2009 at 04:17 PM.
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01-12-2009, 07:44 PM #68
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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02-12-2009, 02:22 AM #69
Re: Variable RPM compressor/cap tube issue
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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02-12-2009, 03:35 AM #70
Re: Variable RPM compressor/cap tube issue
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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02-12-2009, 03:55 AM #71
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02-12-2009, 04:05 AM #72
Re: Variable RPM compressor/cap tube issue
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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02-12-2009, 04:18 AM #73
Re: Variable RPM compressor/cap tube issue
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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02-12-2009, 05:20 AM #74
Re: Variable RPM compressor/cap tube issue
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02-12-2009, 05:35 AM #75
Re: Variable RPM compressor/cap tube issue
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.Last edited by Gary; 02-12-2009 at 05:40 AM.
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02-12-2009, 05:53 AM #76
Re: Variable RPM compressor/cap tube issue
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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02-12-2009, 06:08 AM #77
Re: Variable RPM compressor/cap tube issue
I have no idea what a sonic choke is.
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02-12-2009, 06:46 AM #78
Re: Variable RPM compressor/cap tube issue
http://en.wikipedia.org/wiki/Choked_flow
This should explain the concept of a sonic choke.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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02-12-2009, 06:49 AM #79
Re: Variable RPM compressor/cap tube issue
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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02-12-2009, 06:56 AM #80
Re: Variable RPM compressor/cap tube issue
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.
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02-12-2009, 07:28 AM #81
Re: Variable RPM compressor/cap tube issue
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.
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02-12-2009, 08:02 AM #82
Re: Variable RPM compressor/cap tube issue
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.
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02-12-2009, 08:51 AM #83
Re: Variable RPM compressor/cap tube issue
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.Last edited by desA; 02-12-2009 at 08:56 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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02-12-2009, 09:09 AM #84
Re: Variable RPM compressor/cap tube issue
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
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02-12-2009, 09:30 AM #85
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02-12-2009, 02:55 PM #86
Re: Variable RPM compressor/cap tube issue
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.Last edited by Gary; 02-12-2009 at 03:21 PM.
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02-12-2009, 03:31 PM #87
Re: Variable RPM compressor/cap tube issue
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03-12-2009, 01:05 AM #88
Re: Variable RPM compressor/cap tube issue
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
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03-12-2009, 01:15 AM #89
Re: Variable RPM compressor/cap tube issue
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
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03-12-2009, 01:43 AM #90
Re: Variable RPM compressor/cap tube issue
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.
Last edited by Gary; 03-12-2009 at 01:48 AM.
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03-12-2009, 02:31 AM #91
Re: Variable RPM compressor/cap tube issue
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.
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03-12-2009, 02:42 AM #92
Re: Variable RPM compressor/cap tube issue
Last edited by Gary; 03-12-2009 at 02:44 AM.
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03-12-2009, 03:55 AM #93
Re: Variable RPM compressor/cap tube issue
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
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03-12-2009, 03:56 AM #94
Re: Variable RPM compressor/cap tube issue
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?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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03-12-2009, 04:07 AM #95
Re: Variable RPM compressor/cap tube issue
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
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03-12-2009, 04:12 AM #96
Re: Variable RPM compressor/cap tube issue
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.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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03-12-2009, 04:21 AM #97
Re: Variable RPM compressor/cap tube issue
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!
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03-12-2009, 04:44 AM #98
Re: Variable RPM compressor/cap tube issue
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.Last edited by Gary; 03-12-2009 at 04:49 AM.
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03-12-2009, 04:46 AM #99
Re: Variable RPM compressor/cap tube issue
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
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03-12-2009, 05:02 AM #100
Re: Variable RPM compressor/cap tube issue
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.
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