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  1. #1
    Join Date
    Apr 2001
    Posts
    5

    Confused Charge by sub cooling?

    I'm a beginner at this, though I have a degree and the liscense, I am getting the "hands on experience" on my own with no instructor.
    I changed out a 3/4 hp roof top condensing unit that was on a walk in cooler last week. It was an R12 unit, and I replaced it with an R22 unit of the same type. I was having a problem overcharging it, as I was trying to charge by reading the evap. temp on the low side guage. A friend of mine said the best way to charge this unit is "10 degrees subcooling". What does he mean, and exactly what are the steps you take to do this? Is it the best way to charge, and if not, what other methods are there?

  2. #2
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579


    The first thing you need is a solid understanding of superheat and subcooling. Then you need to learn how to charge by watching BOTH the superheat and subcooling. There are some here who will try to teach you that you charge by subcooling for one type of metering device and by superheat when you have another type of metering device.

    It is always best to watch both the superheat and subcooling when charging any system.

    For an understanding of superheat and subcooling go here:

    http://hvac-talk.com/vbb/showthread.php?threadid=33829

    Norm

  3. #3
    Join Date
    Mar 2004
    Location
    Columbus Ohio
    Posts
    328
    Ussually walk in coolers/freezers have expansion valves
    which will maintain a fairly consistant superheat when
    overcharged, I agree subcooling is an excellant indication
    of refrigerant charge and 10f is a good target. Of course
    things change if its cold outside and a headmaster or fan
    cycling is controlling head pres.
    I also agree with NormChris that you cant look at
    subcooling alone, sightglass,comp amps,pressures/temps,
    superheat,etc should all be checked

  4. #4
    Join Date
    Apr 2004
    Posts
    7,680
    I hope you changed the indoor unit as well. Especially the TXV.

  5. #5
    Join Date
    Sep 2003
    Location
    Atlanta, GA
    Posts
    128
    If the new condensing unit has a receiver, forget subcooling. The condenser will not hold any liquid to subcool.

  6. #6
    Join Date
    Jan 2003
    Location
    North of Boston, MA
    Posts
    270

    Low Charge vs Overcharge and subcooling

    We have a difference of opinion at school. We agree that a low charge makes superheat readings higher and overcharge lowers superheat. What happens to subcoling? I say that a low charge increases subcooling (vapor condenses more quickly and the liquid has more time to reach ambient temperature). Anyone disagree?
    hockey
    "You miss 100% of the shots you don't take"--Wayne Gretzky

  7. #7
    Join Date
    Jun 2004
    Location
    Maryland's Eastern Shore
    Posts
    892
    Nope, not quite... Low Charge=High Superheat, Low Subcooling... Over Charge=Low Superheat, High Subcooling...

  8. #8
    Join Date
    May 2004
    Location
    Los Angeles, CA
    Posts
    1,560

    Back to the books my friend..........

    Dear Hockey,

    Regarding your post:

    “We have a difference of opinion at school. We agree that a low charge makes superheat readings higher and overcharge lowers superheat. What happens to subcoling? I say that a low charge increases subcooling (vapor condenses more quickly and the liquid has more time to reach ambient temperature). Anyone disagree?”

    Yes, I do. Like Cde72 said above, when a system is undercharged, it will have low subcooling and high superheat, and when a system is overcharges, it will have high subcooling and will have a tendency to have a lower superheat based upon whether it has a TXV or capillary tube metering device.

    I’m not sure who “we” are or what school you attend, but unfortunately the “we” of the school need to hit the books again………..

    Respectfully Submitted,
    John J. Dalton


  9. #9
    Join Date
    Jan 2003
    Location
    North of Boston, MA
    Posts
    270

    Thanks for the Replies

    CDE72 & J Dalton, Thanks for the replies! I know I can always count on reading solicited responses AND some "extra" replies on this site. It's a great resource.
    hockey
    "You miss 100% of the shots you don't take"--Wayne Gretzky

  10. #10
    Join Date
    Nov 2004
    Location
    up in the hizzy
    Posts
    1,430
    There is one thing you should always take into cosideration while reading sub-cooling or superheat and that is the boiling point of the substance,as obvious as that sounds is often overlooked by the new generation.without it you dont have an starting point.

  11. #11
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579


    Go here and read all about it:

    http://hvac-talk.com/vbb/showthread.php?threadid=49522

    Norm

  12. #12
    Join Date
    Jun 2001
    Location
    San Diego, CA
    Posts
    1,149
    Gpurves, tell us

    Did you change out that TXV?? What was your final conclusion regarding superheat and subcool, and how is it running now?

  13. #13
    Join Date
    Feb 2005
    Posts
    2

    The general rule for charging systems. If the system has a TXV you must charge to the sub cooling, and set the SH with an adjustable txv.
    If you have a cap tube then you charge to Superheat.
    10 to 15 deg of Subcooling will get you in range. An adjustable txv is very important.

    I'm a lab tech for carrier corp, and i do this stuff day in and day out.

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