Superheat & Subcooling - An Explanation
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  1. #1
    Join Date
    Jul 2000
    Location
    Ottawa, Ont. Canada
    Posts
    1,729
    A request was made by Simpleman for me to find an old post:

    Originally posted by simpleman
    Is there a way to go back in the archives and bring up
    a thread from 6-11-02?

    It was a reply from fixitman.It was the best explanation of
    sh/sc that I ever herd before.

    I sure wish I could bring it up to put back into my archives,I've lost it some how.

    Thank you!
    Yes, I do have the post, here it is:

    Originally posted by fixitman
    The superheat and subcooling tell you where your refrigerant is in the system, and how well you're using your coils, assuming clean coils (you can't always assume that!). You are boiling liquid/vapor in one of them to make a gas, and condensing gaseous refrigerant in the other one to make a liquid.

    The superheat tells you, relatively, how much of the coil is in the gaseous phase, after all the refrigerant has boiled. Less cooling takes place in that portion of the coil, and the temperature of the gaseous refrigerant rises pretty quickly. So you get a measurable difference between that temperature and the temperature of the boiling refrigerant.

    If the superheat is high, you are not maximizing the use of the coil. Maybe the charge is low, maybe the coil is too big, maybe, maybe, maybe. But you have an idea how much of the coil is delivering cooling. If superheat's too low, you're in danger of sending liquid to the compressor.

    Subcooling tells you, relatively, how much of the coil you are actively using to make liquid from gas. You'd like to use most of the coil to do actual condensing of the gas into liquid. Once all the gas is condensed and turns into liquid, the temperature begins to drop pretty quickly.

    A big liquid temperature drop means that there is a lot of the coil just holding liquid. That leaves less of the coil to condense the gas, and you effectively reduce the size of the coil. That's what techs mean when they say that a lot of liquid is “stacking up” in the condenser. So there's less effective area of coil for heat exchange, the compressor has to work harder, raising the gas to a higher temperature, trying to get it to condense in fewer inches of the fin tubes.

    On the other hand, you don't want near-zero subcooling. If the subcooling is near zero, you are sending gas bubbles to the metering device, and that reduces the system's ability to cool.

    Using these ideas, you can tell a great deal about the health of the system, and, eventually, ensure that it is correctly charged. Too much refrigerant increases subcooling and makes the compressor work too hard. Not enough refrigerant decreases the subcooling, and increases the superheat, too, because there's not enough refrigerant and the evaporator coil starves.

    Other things also affect the subcooling and superheat -- refrigerant flow restrictions, air movement, air temperature, humidity, and other things. Subcooling and superheat don't tell you everything, but they are incredibly useful diagnostic tools. Along with other things you learn about the system, they help give a complete picture of the system's health.

    don
    don sleeth - HVAC-Talk Founder
    HVAC Computer Systems
    Heat Load Calculation Software

  2. #2
    Join Date
    May 2004
    Posts
    475
    Yes, excellent explanation of SH+SC.

    Where should these readings be taken from ?

    Would I be correct in saying at the exit of each coil ?
    (ei; SH at the evap exit, SC at the cond exit)

    (I've read here that SH is taken a few (2"-3") from suc valve, but haven't read exactly where SC is measured)

  3. #3
    Join Date
    Dec 2002
    Posts
    9,871
    Supertheat- at the txv bulb

    Subcooling- liquid line at the txv

  4. #4
    Join Date
    May 2004
    Posts
    475
    Thermistor and insulation or infrared temp-gun at point blank range suffice ?

  5. #5
    Originally posted by will 2
    infrared temp-gun at point blank range suffice ?
    Junk for this application. You need something pretty reliable. I only use infrareds for head temps or things like that on a system. Clamp type thermometer would be best, insulated bead type is my next choice.

  6. #6
    Originally posted by will 2
    Thermistor and insulation or infrared temp-gun at point blank range suffice ?
    Now you've gone and done it .... you've opened up Pandora's Box!!!


    I have taken my Fluke 52 and set it up right alongside my infrared and received very very close readings.

    Safe to say, on the fly readings can be safe using an infrared. IN MY OPINION.


    When I want to know dead on what my temp is, I bring in the Fluke.
    When I am diagnosing a system and dont have a clue where it's at ... I grab the infrared.

    Now if anybody has a problem with that .... then just write Fluke and ask them why they make infrared thermometers and what their purpose is in this industry!

    But tools are like tools. Not everyone who owns one or has access to one can make it do what the next guy can make his do.


    So when I take my measurement of a line and say this is the temp ... and the next guy comes along and tries out his insturement and he disagrees with me ..... so what's that supposed to mean?
    I'm wrong and he's right? Or visaversa?


    What's important is that ... by the end of the day you dont get called back to the same job for the same issue! Period.
    And the number of to do's you get accomplished during the day will of course vary from tech to tech ... but overall what managers are looking at is how many return visits they have to send you back on cause you messed up.


    Whether you carry a Fluke 52 or a Fluke 16 and use the beaded probes to take your readings with .... it's all about getting it as close as you possibly can to dead on.


    The next time some guy thinks his poop dont stink ... I'll just ask to watch him do a sealed system repair on a critical charged unit.
    And I'll wait to see him clean out his vacuum pump BEFORE using it. Then I'll linger until he goes to weigh in the charge and watch as he heats up his hoses to drive the charge from the high side into the low side.

    Oh ... and he will have changed hoses on his gae set from the usual length to the short ones ... if it's real small system.
    And he will have removed the valve cores first and replaced them jus prior to the last vacuuming.

    Ys see.... I can be persnickety too. I can have standards no one else lives up to also.
    I can pull things out of the book of rules/proceedures that no one wants to practice or follow.


    Like I said a minute ago ... we are attempting to get as close to dead on as ce can.

    Lack of experience is a definate hinderance.
    Also lack of the right tools/equipment.
    Also being pressed for time.
    And last but not least is a total disregard for following the rules.

    I didnt make the rules. I just follow them, as close as I can.
    And when I dont know what the rules are, I can come here and learn.
    And when there is a disagreement over what is the best method to use for a given proceedure ... go out and try them all. Do it yourself. Jump in with both feet.

    And then you will develope a feel for who you can trust on certain areas of advice here on this forum.


    There are many here that no matter what they say, unless of course they said something in jest ... their words can be taken for the truth.
    And then again .. there are some here who have their own little areas of expertice ... and within them .. they are fine. Just dont ask for their advice OUTSIDE of the parameters of their given areas.


  7. #7
    Join Date
    Feb 2004
    Location
    Midwest
    Posts
    9,932
    Geez, I hate to agree with R12

    But I've got a clamp on and have double checked it against my infrared, and they're within a degree (on the suction line.) I wouldn't trust it on a discharge line unless it was over 1/2" in dia.

  8. #8
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579
    Each IR thermometer is calibrated to be accurate at a specific surface emmissivity. The emmissivity of the surface you are measuring the temperature of must be close to that of the IR thermometer in order to be accurate. If not then people who don't understand how they work simply blame the instrument when it is the ignorance of the user to blame.

    SURFACE EMMISSIVITY

    Surfaces can be dark or light, dull or bright, smooth or rough, flat or curved. Depending upon the surface you are measuring some of the IR is absorbed and some reflected. This accounts for differences in readings. These factors when accounted for are boiled down to a coefficient called "emmissivity". Look carefully on your instrument and you will see what yours is rated for.

    A list of common emmissivity factors for a variety of surfaces is available from the instrument company as well as in many physics books.

    Don't blame an IR thermometer for doing exactly what it was designed to do and do accurately. Learn how to use and apply it properly for your application.

    Norm

  9. #9
    Join Date
    Apr 2002
    Location
    Dallas TX
    Posts
    2,216
    I never trust the IR. I did back in the day but it was only correct some of the time and I was always double checking myself once I learned it was not correct so then I was like "aww f*** it!".

  10. #10
    Join Date
    Dec 2002
    Posts
    9,871
    I use my Fluke52 and a hi dollar IF. Have checked them against each other and they are within 1 degree.

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