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  1. #1
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    Question about de-superheat, condensing and sub-cooling.

    Okay guys we have a discussion going on one person says that the condensing unit is broken into thirds the first for de-superheating the second condensing and third for sub cooling. Another says it is 25% 50% and 25%. I say it depends on the current conditions and efficiency of the condenser so the percentage will vary less coil given up to de superheat and sub cooling for older less efficient units than newer more efficient units. The cooler the indoor and or outdoor temps the less coil will be required for de-superheat and sub cooling.

    What say you?

  2. #2
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    Quote Originally Posted by classical View Post
    Okay guys we have a discussion going on one person says that the condensing unit is broken into thirds the first for de-superheating the second condensing and third for sub cooling. Another says it is 25% 50% and 25%. I say it depends on the current conditions and efficiency of the condenser so the percentage will vary less coil given up to de superheat and sub cooling for older less efficient units than newer more efficient units. The cooler the indoor and or outdoor temps the less coil will be required for de-superheat and sub cooling.

    What say you?
    From a refrigeration guys views...i agree with you..

    I am seeing much much larger condensers than older units.

    Example a 2 door true reach in cooler. For years had a large 2 fan evaporator paired with a 1/2 hp small single fan condensing unit.
    It ran very warm and put out alot of heat.

    Now in the present..the same 2 door has what i see to be nearly the same evap with a completely different condensing unit.
    The compressor is tiny but the condenser coil is Huge!!
    Liquid line temps are almost room temperature and only the first 3 rows are warm.

    So i think they are experimenting with capacity by getting very high subcooling and reducing compressor horsepower.
    I could be wrong but just a thought.
    The first time i saw this i instantly thought i had a refrigeration issue...but i waited...and it pulled down to temp very quickly

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  3. #3
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    As Scott alluded to, the age of the unit will have a lot to do with what's going on inside of the condenser. First, to answer your question, then we can discuss it. The ratios are probably more like 15%, 70%, 15%, but that is just a guess.

    When I first got out in the field, I played around quite a bit checking and proving, or disproving, what I was taught in school. For purposes of this discussion, we'll talk about package units, right from the factory. As you would think they were properly charged; compared to splits that could be charged willy nilly.

    What I found, especially with BDP, is that they ran much higher subcooling than I had ever heard anyone speak about. Twenty, 25, and even approaching 30*F was not the least bit uncommon. As a newbie in the field, I asked several people about this, and never got much of an answer. Because most techs in the field really didn't have the thirst for knowledge that I had.

    Long story short, the high subcooling was to get the capacity rating. And Freon was cheap. So was energy back then. So you could stack up the Freons and let them reject the heat.

    Nowadays, manufacturers are cutting costs where ever they can.

    But let's get back to your original question. If you really want the exact answer, it is easy to determine yourself. Just take your hand and feel the return bends, the saturation will remain the same temp after desuperheating and before subcooling. Should be able to get a real good idea of the percentages you are looking for. I used to do it all the time on small refrigeration, you can get a lot of information for troubleshooting from just doing that.

    And if you want to be more precise, or make a science project out of it, just get some thermocouple wire and wire tie it to the return bends, your digital thermometer will tell you exactly what you want to know.

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  5. #4
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    KindaSorta I agree w/ BBeerme & his percentage ratios. I STRONGLY agree w/ his "touchy/feely" thing of the U bends approach, to helping w/ troubleshooting and thinking and imagining and fixing. I do that on damn near every system that I work on! IT'S NORMAL!!!

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  7. #5
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    Quote Originally Posted by BBeerme View Post
    The ratios are probably more like 15%, 70%, 15%, but that is just a guess.
    That's closer to what I read in an RSES book which allowed 80% for condensing...

  8. #6
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    I am sure that RSES taught a 10-80-10 ratio as a design rule of thumb.
    Touching the return bends when accessible is a good practice.

  9. #7
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    I think the "thirds" idea sort of snuck in due to the identifying of "three areas" of the condenser for training purposes. To say that you have three areas of 33 1/3% is not correct.

    A thermal image of the coils is very revealing, in that you can clearly see the liquid area that is being subcooled, and how much it extends upward. It is VERY obvious in an MC coil.
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  10. #8
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    Thread Starter
    As you may know I am now teaching HVAC/R at a trade college. The refrigeration encyclopedia generically talks about 10% in one chapter and then an undetermined % in another.

    When I teach basic refrigeration and the refrigeration cycle/diagram I list saturated liquid and saturated vapor at 90% of the coils so the students have a reference point to work with.

    Additionally I further explain in the field it is going to vary according to the efficiency of the system and the temperatures at the time. Basically I try to impress the need to take proper temperatures and other tactile impressions of the equipment to determine system operation.

    Another instructor is trying to say that the condenser is hard and fast broken into three distinct even sections.

    Just wanted other opinions and pretty much ya'll seem to agree with me.

  11. #9
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    There may be three 'distinct' sections, but those sections will vary in ratio system to system, and under varying loads, as you said.

    The OCD in me likes to bust the chops of those who live by hard fast and unchanging rules. Your personality may be different.


    Quote Originally Posted by classical View Post
    As you may know I am now teaching HVAC/R at a trade college. The refrigeration encyclopedia generically talks about 10% in one chapter and then an undetermined % in another.

    When I teach basic refrigeration and the refrigeration cycle/diagram I list saturated liquid and saturated vapor at 90% of the coils so the students have a reference point to work with.

    Additionally I further explain in the field it is going to vary according to the efficiency of the system and the temperatures at the time. Basically I try to impress the need to take proper temperatures and other tactile impressions of the equipment to determine system operation.

    Another instructor is trying to say that the condenser is hard and fast broken into three distinct even sections.

    Just wanted other opinions and pretty much ya'll seem to agree with me.

  12. #10
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    Traditionally, the 25-50-25 would be about right - subject to operating conditions. However, these ratios are different today with oversized condensers and undersized compressors - That's how they get the kw/ ton down - efficiency up. The problem is with microchannel condensers. Its a critical charge because there's no room for subcooling. Your capacity to subcool is extremely limited. I've had good success using a receiver when I couldn't get a fin/tube condenser. I hope this add to the discussion in s positive way.

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  13. #11
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    Welcome bobby. Sounds like you know some stuff. Keep making good contributions and apply for professional.

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  14. #12
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    Additionally, the operating conditions (OAT in particular) will DEFINITELY have an effect on the ratios used in the condenser (for de - superheating, condensing & subcooling).

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  15. #13
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    Thank you for the compliment. What do u mean by "apply for professional"? I'm new to the site and love tech talk. I was a service engineer for Carrier for many years. And did a lot of training and consults (field and office). I still train and conduit currently. None of us knows or has seen everything; but I'm a strong believer in that in the abundance of counselors - there's wisdom. Just glad to put my 2 cents in .

    Sent from my SM-G935T using Tapatalk

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