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Thread: Theory Fundamentals

  1. #21
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    And yes for your question is like BB said above but also the efficiency of the condenser and the efficiency of the medium to remove the heat from the refrigerant at a certain flow rate. So there are lots of variables to get the actual working condensing pressure.
    Last edited by Robber; 03-20-2016 at 04:56 PM.
    There is only one truly right way to do something, but there are thousands of wrong ways to varying degrees to do it.
    So the question is: If you don't do it right, then how wrong is it going to be???

  2. #22
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    Quote Originally Posted by szw21 View Post
    The condensing pressure should be such that the condensing temperature should be above the current ambient temperature in order to be able to reject the heat collected at the evap and the heat of compression. By how much depends on the design of the system.

    As far as an exercise in reading pressure enthalpy charts it should not really matter what refrigerant is used - however I found it hard to find a pressure enthalpy chart for r12 in imperial units in any of my books or online. Most of the ones I saw online were for SI units (KJ/Kg). I finally found a hard to read one in the RSES SAM. The exact value will depends on your entry point into the saturation curve which is dictated by the amount of flash gas once the refrigerant exists the expansion device. very crudely the amount I get is about 55 Btu/lb (because the chart I have is really hard to read) based on a 70% liquid/30% gas at the input to the evap.
    The p/h charts (AKA, Mollier chart) are useful to plot cycle diagrams to better explain what's going on within the system, but to get the numbers for calculations go to the thermodynamic properties tables like this one from National Refrigerants (See p.13): http://www.refrigerants.com/catalog.pdf

  3. #23
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    I think there two different angles that most not be confused, first is the reason why the pressure increases in a closed refrigerant loop and that's driven by the need to compress the refrigerant vapors to a pressure/temperature higher than the condensing medium, now the second part is the heat transfer process that takes place between refrigerant and air,water etc. the factors governing that event are 1-Delta T between refrigerant and condensing medium. 2-Area of the condenser or heat exchange and 3-The heat transfer coefficient of the material of which the heat exchanger is made. The timeless formula is Q=U X A X LMTD. There used to be lots of discussions here on the subject some went on for month or years.

  4. #24
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    Quote Originally Posted by icemeister View Post
    The p/h charts (AKA, Mollier chart) are useful to plot cycle diagrams to better explain what's going on within the system, but to get the numbers for calculations go to the thermodynamic properties tables like this one from National Refrigerants (See p.13): http://www.refrigerants.com/catalog.pdf
    Why took you so long to chime in? and where is Andy when you need him?

  5. #25
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    Quote Originally Posted by icemeister View Post
    The p/h charts (AKA, Mollier chart) are useful to plot cycle diagrams to better explain what's going on within the system, but to get the numbers for calculations go to the thermodynamic properties tables like this one from National Refrigerants (See p.13): http://www.refrigerants.com/catalog.pdf
    I completely misunderstood the original question - I thought he was asking about the heat content gain of the refrigerant through the evap just prior to reaching saturation at condensation.

    Now I looked at the charts you linked and while I see liquid and vapor enthapies I don't see where the enthalpy changes based on degrees of saturation is shown. These are generally marked on the P-H chart. If you want a value somewhere in-between how do you do it. Do you just perform a linear (first order) interpolation or it is more complex than that.

  6. #26
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    Quote Originally Posted by szw21 View Post
    I completely misunderstood the original question - I thought he was asking about the heat content gain of the refrigerant through the evap just prior to reaching saturation at condensation.

    Now I looked at the charts you linked and while I see liquid and vapor enthapies I don't see where the enthalpy changes based on degrees of saturation is shown. These are generally marked on the P-H chart. If you want a value somewhere in-between how do you do it. Do you just perform a linear (first order) interpolation or it is more complex than that.
    There's no need to calculate the enthalpy entering the evaporator by analyzing the refrigerant quality (liquid/vapor %) because it's equal to the enthalpy of the liquid leaving the condenser as shown by the vertical line straight down to the evaporator inlet on the diagram.


  7. #27
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    Quote Originally Posted by valdelocc View Post
    Why took you so long to chime in? and where is Andy when you need him?
    Andy hasn't been around for a quite some time. He is missed by many here.

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  9. #28
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    Quote Originally Posted by icemeister View Post
    There's no need to calculate the enthalpy entering the evaporator by analyzing the refrigerant quality (liquid/vapor %) because it's equal to the enthalpy of the liquid leaving the condenser as shown by the vertical line straight down to the evaporator inlet on the diagram.

    Doesn't the line on the left actually encroach further left into the sub-cooling area depending on the amount of sub-cooling?

  10. #29
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    Quote Originally Posted by szw21 View Post
    Doesn't the line on the left actually encroach further left into the sub-cooling area depending on the amount of sub-cooling?
    Not in the question stated in this thread. It stated the refrigerant is saturated leaving the condenser. If it were subcooled, then yes, the line would extend to the left of the saturation curve. The same could be said of the conditions at the outlet of the evaporator.

  11. #30
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    Quote Originally Posted by icemeister View Post
    Not in the question stated in this thread. It stated the refrigerant is saturated leaving the condenser. If it were subcooled, then yes, the line would extend to the left of the saturation curve. The same could be said of the conditions at the outlet of the evaporator.
    My question in post 25 was more general in nature and not referring to the OP's question.

  12. #31
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    OK, but I don't understand why you would need to know that value. I don't recall offhand how that would be calculated either.

  13. #32
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    Quote Originally Posted by icemeister View Post
    OK, but I don't understand why you would need to know that value. I don't recall offhand how that would be calculated either.
    Oh just curious

  14. #33
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    To the OP, My thinking was obviously off-base.So congrats to you and your classmate for thinking/asking a good Q and shame on me for not recognizing that.. My thinking was from a view point that the Q asked does not exist as "normal'"in the field. If running across a unit that is working under/near those given conditions it is our responsibly to correct that "wrongness" and quickly.
    I look forward to your next Q.

  15. #34
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    I'd like too also thank the OP from coaxing Ice out of his preferred hideaway and guiding us
    into these voyages into the secrete parts of the mysteries of phase change I think is something
    some of us need just to keep us honest not to mention better informed.

    Some here use PE diagrams to analyze system performance rather than staring at the gauges.
    That's an engineering approach that could be a benefit.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  16. #35
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    Quote Originally Posted by icemeister View Post
    Andy hasn't been around for a quite some time. He is missed by many here.
    I've been known to lurk once in a while.
    If the only tool you have is a hammer, you tend to see every problem as a nail. – Abraham Maslow

  17. #36
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    Quote Originally Posted by Andy Schoen View Post
    I've been known to lurk once in a while.
    and it's damn good to see and here from you again Andy

  18. #37
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    Quote Originally Posted by Andy Schoen View Post
    I've been known to lurk once in a while.
    Long time no see old friend.

    You're in Dallas now?

  19. #38
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    Hey look it's Andy!!
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  20. #39
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    Welcome back, Andy.

    I still miss the "Cold War" emails....
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist
    Member, IAEI

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  21. #40
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    Quote Originally Posted by icemeister View Post
    Long time no see old friend.

    You're in Dallas now?
    That's a fact. Been in Dallas for over 2 years. As migrants to Texas like to say: I wasn't born here, but I got here as fast as I could!

    I'm currently working for Sanhua International. Long story...

    I can't seem to get RSES to switch my chapter from Detroit to Cowtown (Fort Worth). Jim Malone who runs the Cowtown chapter is a good guy. RSES will hold their annual conference in Fort Worth Sep 28 - Oct 1. Jim has me slated to do an educational session.

    Anyone attending the conference, I'm available for chauffeur and tour guide duties.

    Andy
    If the only tool you have is a hammer, you tend to see every problem as a nail. – Abraham Maslow

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