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  1. #101
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    Re: Variable RPM compressor/cap tube issue



    Quote Originally Posted by desA View Post
    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?
    What you have described here is what happens when there is no subcooling.



  2. #102
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Gary View Post
    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.
    So a critically charged system that has the evap full of charge to its limit needs more gas? I think not.

    DesA has put it in a nutshell and there is much more to consider than just adding charge. It is not a charge issue at all but cap tube and evaps and condensers not balanced to perform correctly.

    I said earlier we added charge and it made difference.

    Chef

  3. #103
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Chef View Post
    So a critically charged system that has the evap full of charge to its limit needs more gas? I think not.
    How do you know the evap is full to its limit?

  4. #104
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Chef View Post
    ...there is much more to consider than just adding charge. It is not a charge issue at all but cap tube and evaps and condensers not balanced to perform correctly.
    It would be helpful if the condenser, evaporator & compressor operating curves could be set up, with the appropriate balance curve for the cap tube superimposed & evap air pull-down curve.

    We could start with what is known about the size of each part, operating conditions - design & measured, & slowly progress these towards some useful system insights.

    This is a lovely challenge - a hybrid between design & commissioning (set-up).

    We could tackle all 3 systems - give each one a name e.g. Chef1, Chef2, Aaron_k - each with its design, physical & measured parameters. I think we'd all learn a great deal from the process.
    Last edited by desA; 03-12-2009 at 06:44 AM.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
    Rarefied Technologies ( SE Asia )

  5. #105
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by desA View Post
    It would be helpful if the condenser, evaporator & compressor operating curves could be set up, with the appropriate balance curve for the cap tube superimposed & evap air pull-down curve.

    We could start with what is known about the size of each part, operating conditions - design & measured, & slowly progress these towards some useful system insights.

    This is a lovely challenge - a hybrid between design & commissioning (set-up).

    We could tackle all 3 systems - give each one a name e.g. Chef1, Chef2, Aaron_k - each with its design, physical & measured parameters. I think we'd all learn a great deal from the process.
    Okay... let's start with the full evaporator at zero SC.

    If we increase the airflow through the evap it can hold more liquid because it is boiling it off faster. Conversely, if the airflow is reduced the evap will hold less liquid.

    On the other side of the system, if the airflow through the condenser is reduced the pressure will be elevated, pushing more liquid through the cap tube, thereby hastening the moment where there will be zero SC. IOW, the system will reach critical charge at a lower ambient temp.

    It is absolutely essential that the airflow through both coils is checked before making any judgements concerning the charge or the cap tube.
    Last edited by Gary; 03-12-2009 at 09:00 AM.

  6. #106
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Gary View Post
    Okay... let's start with the full evaporator at zero SC.

    If we increase the airflow through the evap it can hold more liquid because it is boiling it off faster. Conversely, if the airflow is reduced the evap will hold less liquid.

    On the other side of the system, if the airflow through the condenser is reduced the pressure will be elevated, pushing more liquid through the cap tube, thereby hastening the moment where there will be zero SC. IOW, the system will reach critical charge at a lower ambient temp.

    It is absolutely essential that the airflow through both coils is checked before making any judgements concerning the charge or the cap tube.
    Hi Gary, I think you have this wrong (or maybe just worded wrong) With more airflow you will hold less liquid, this because the liquid is boiling so the Liquid to vapor % changes, how ever the mass flow through the evap is going to increase, the reverse happen with less air flow.
    The very first stage is just a steady state start point
    "best guess" Which I would use max ambient for condenseing, and just above chilled chamber design temp.
    Using these we can then use manufactor data, to balance the refrige system Comp, evap and cond.
    We should then have a set of conditions to which we are able to select a cap with. This should bring us within +/- 15% of perfect. I would then start the system, slowly adding refrigerant (ensuring that Ct is close to design) ignore Te during pull down. When close to design take measurements, make adjustment with charge or cap length. When at this point is correct (over a time cycle). We then run the system out design conditions, and see if the changes within the process are within acceptable limits. At this stage we are not optomising the system in any form, we are just protecting the equipment. If optimum is required then you do not use a cap.
    Forgot to mention speed, this is done in the last process

  7. #107
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Gary View Post
    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.
    When the SC is zero and the SH almost zero - perfect - your at the point of where the next phase begins.

    All the charge is in the evap, it continues to cool and so the pressure slowly falls.

    The SC is still zero but now as the Sx falls the mass flow rate falls. Less gas enters the condensor.

    But the pressure it enters at is only just enough to condense some of the gas and some is left to pass into the tube. It is how almost all freezer systems work that are equiped with a cap tube.

    So the only way to restict the flowrate is to have some gas and some liquid - not all liquid in the tube.

    If we can somehow convince you this is a real world situation and not all systems can have or are designed to have 5.5 to 8K SC then it will be day of days. Here is a nice graph which shows how the tube flow decreases with reducing SC and then after it hits 0 it goes into the quality mode. It starts at x=0 when SC=0 and then steadily increases as the mass flow falls.

    Of course the SC will always measure zero even when the quality has positve values. Maybe thats why no-one has seen it from measurements.

    SC and X.GIF

  8. #108
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    Re: Variable RPM compressor/cap tube issue

    Mad and DesA
    If we are to progress forward we need to get Gary to accept these concepts. Not every system on the planet has to have loads of SC!

    You are both on the right track and see it as it is and it will be a great discussion.

    However.

    Tommorrow I leave for uncharted waters where they have no internet or any sort of life at all. I shall be there for a month and may be able to get a signal on the odd occasion.

    I am sorry but I will not be able to continue in this lively discussion - bit of a bugger really as was just getting to the good stuff.

    Maybe we can start it all rolling again in January?

    Duty calls I'm afraid.

    Chef

    PS Best wishes for Christmas and the New Year

  9. #109
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    Re: Variable RPM compressor/cap tube issue

    Chef, Mad, DesA, Gary,

    I sort of feel bad to have kicked the hornets nest (inadvertently) but I think everyone appreciates the discussion; I certainly do.

    Some news and updates on my system. Chef is correct - this is a new machine, so its actual design is what is in question. I have contacted Richard Farr, the originator of the R&R Supply simulation software used by our contractor. He's likely the most experienced refrigeration engineer in the world (he's 91 and still active). You can read about him here:

    www dot appliancemagazine dot com/editorial.php?article=967&zone=1&first=

    (sorry - I'm still not allowed to post URL's - fill in the dots)

    He has re-run the analysis for our system and determined that as expected the cap tube length was way too short. The new value is close to what other published literature would suggest. We're in the process of fitting the new cap tube for testing.

    I mentioned to him that the Danfoss cap tube program was used to pick the cap tube length. He replied that the published data in the ASHRAE fundamentals is very accurate - there's another source for everyone to refer to.

    By the time Chef returns I will have the results of the changes we've made.

    Thanks again for your passion for this topic. I will update as new data presents itself. Happy holidays to everyone.

    Aaron_K

  10. #110
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    Re: Variable RPM compressor/cap tube issue

    Chef, you take care on your travels. We'll have lots to discuss when you return in the new year. Excellent topic.

    We can all work on Aaron_k's system in your absence & apply the findings to your system on your return.

    I suspect that each type of condenser system will have its maximum sustainable SC. For instance, plate-type condensers seem to prefer SC less than 4K. Some run comfortably at 1-2K. A lot will have to do with the liquid seal, at the condenser discharge, as MF has mentioned previously (not sure if it was this thread).
    Last edited by desA; 03-12-2009 at 05:09 PM.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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  11. #111
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Chef View Post
    If we can somehow convince you this is a real world situation and not all systems can have or are designed to have 5.5 to 8K SC then it will be day of days.
    I have never said that all systems under all conditions should have 5.5-8.5K SC. This is obviously not the case.

    Our entire disagreement centers around your insistence that your system is sonically challenged. This may or may not be the case and you have presented zero evidence to demonstrate that it is so.

    You say that the cap tube cannot increase its flow because it is sonically limited. I say that given increased SC at its inlet the flow would indeed increase.
    Last edited by Gary; 03-12-2009 at 06:20 PM.

  12. #112
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by mad fridgie View Post
    Hi Gary, I think you have this wrong (or maybe just worded wrong) With more airflow you will hold less liquid, this because the liquid is boiling so the Liquid to vapor % changes, how ever the mass flow through the evap is going to increase, the reverse happen with less air flow.
    Apparently I have explained this badly. My entire point was that we cannot determine if the charge is right or the cap tube is right without first determining that the airflow is right.

    And just because the SC has dropped to zero does not prove that the evap is full. For that we need to know the compressor inlet superheat.
    Last edited by Gary; 03-12-2009 at 06:51 PM.

  13. #113
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    Re: Variable RPM compressor/cap tube issue

    Hi Aaron, there nothing like a hornets nest to keep the mind simulated, if we all agreed all the time it would be a bit boring!
    Good luck with your trails.
    Chef I hope you have smooth waters.
    Des and gary clash horns later

  14. #114
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    Re: Variable RPM compressor/cap tube issue

    Hello everyone,

    I have an update for you on my predicament. We increased the cap tube length and reduced the condenser size, while increasing charge to match the maximum compressor rated charge size. I ended up finding **** Farr, the founder of R&R Supply (no one has heard of them). He's 91, and been a refrigeration engineer for a long long time... anyway....

    Our system is now able to operate for over 3X the original duration while running at the lowest compressor speed (2000 rpm) - the original system wouldn't even cool if run below 2800 rpm, and used 3X the power. We're really thrilled with the results. I'm literally within 5% of where I need to be to prove out the design, and we think it can be achieved with a little more adjustment to the software.

    So it turns out that the published cap tube lengths are pretty close to what we needed. The charge size is also vital, as we have our vendor telling us that 40% less refrigerant is the way to go, but the resulting lack of subcooling ends up causing the system to consume 50% more power.... we have one of each configuration running side by side, with the data to prove it. And Danfoss is OK with our over all system volume to prevent compressor damage at lower operating ambients.

    So now we can keep 60 pounds of material cold (5C) while running in a 52C ambient for 24 hours on one car battery, off the grid... pretty nice.

    I want to say thank you very sincerely for everyone's help. We couldn't have really understood the cause and effect without your passionate discussion.

    All the best,

    aaron_K

  15. #115
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    Re: Variable RPM compressor/cap tube issue

    aaron_K - it great to hear the system is up and running as you hoped it would and performing to spec. For the record it would be nice to know what your final choice of condenser size was and the dimensions of the cap tube.

    Chef

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