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Thread: Theory Question

  1. #1

    Theory Question on Freon

    (3rd paragraph contains the question)
    I hope this is in the right forum, I didn't see a differen't one would apply. I'll start by saying I'm not an HVAC technician, nor do I pretend to be one. I have family in the business, and curiosity get's the better of me, so I'm learning just for fun.

    In my attempts to learn, I have read numerous websites, looked at charts, and talked to people in the business. I understand the electronics side of HVAC, it's the refrigerant side that leaves me a little confused at times.

    So, I drew up a diagram, in hopes of helping myself better understand. What I would like from the group, is to look at my diagram, and tell me if I'm dead wrong, or close. Feel free to correct anything about the diagram.







  2. #2
    Join Date
    Jan 2004
    Posts
    108

    1) When the pressure of a saturated (or slightly subcooled) liquid suddenly decreases, some (not all) of the liquid flashes into vapor. You should be able to calculate how much flashes from a refrigerant pressure-enthalpy chart.

    2) So I'm a nitpicker, but check spelling of losses vs loses.

    3) Nice diagram.

  3. #3
    Originally posted by engineerguy

    1) When the pressure of a saturated (or slightly subcooled) liquid suddenly decreases, some (not all) of the liquid flashes into vapor. You should be able to calculate how much flashes from a refrigerant pressure-enthalpy chart.

    2) So I'm a nitpicker, but check spelling of losses vs loses.

    3) Nice diagram.
    Thanks for the comments. I fixed the typo. I understand what you are saying in your #1 comment. Just like if you turned a bottle of Freon upside down and opened the valve 1 foot from a wall, you wouldn't get a pure stream of liquid. You'd get a lot of liquid, and a lot of vapor because of the instant "flash" state change.

    I'm surprised there weren't more problems with the chart. This was my first attempt. Oh, I updated the picture, so anyone who views it as of this post, won't see the typo.


    [Edited by briandugas on 10-25-2006 at 02:14 PM]

  4. #4
    Join Date
    Apr 2004
    Posts
    7,680
    I wouldnt know how to fix your srawing, but here it is briefly...

    the low pressure vapor enters the compressor in a superheated state. Generally about 10-20 degrees of superheat. In the compressor, the gas is superheated further as the pressure is increased. The motor heat and heat of compression are added. You're superheat will increase by somewhere near 80 degrees. (there are variables, so I'm hitting close).

    Now yo have a hot, high pressure vapor. The condenser can quickly remove that sensible heat from the vapor. Since the saturation temperature is above the outdoor ambient, the refrigerant contiues to give off heat until it reaches that saturation temperature. At that point the vapor changes state to a liquid. Once the refrigerant is all liquid, heat removed will subcool the liquid. You can never cool the refrigerant to a temperature below ambient (unless there is a probelm).

    The idea of having subcooling is because the small liquid line will have a pressure drop between the condenser and the evaporator. The temperature of the liquid generally will not change, but since the pressure does, the saturation temperature does also. The subcooling prevents flash gas.

    The solid column of liquid enters the metering device and the pressure is dropped. The temperature is also dropped. The cooler refrigerant absorbs the heat of the air passing through the coil. As heat is added, the refrigerant is at saturation temp and changes to a vapor throughout the evaporator. Once the refrigerant is all vapor, it will increase in temperature (superheat). A piston has no control over superheat, it is sized to offer a pressure drop over a range which is dependant on the high side pressure going in. It doesnt care whats happening in the evaporator. An expansion valve on the otherhand monitors the evaporator pressure and refrigerant temperature and controlls the flow of refrigerant going in to provide the correct superheat coming out.

    If there is no, or little superheat returning to the compressor, it's possible to have liquid return. Liquid can wash out the oil. You also know you cannot compress a liquid and the internal pressures are more like hydraulics and will destroy the compressor. Too much superheat, on the other hand, will not be able to keep the compressor motor cool. Eventually, this will result in a overheated and failed motor. It's safe to say that most compressor failures are refrigerant related. Even electrical failures more often than not have a refrigerant related root cause.

    And you start all over again.

    Overall you did it pretty well.

  5. #5
    Thanks docholiday for the extensive explanation. It's all making sense now. Sometimes a picture is worth a thousand words. The picture will stay on my site so it can forever be shown in this thread. If I had this picture to start with, then comprehending the purpose and usage of freon would have been a lot easier to understand. Even though my picture doesn't account for 100% of what's going on, I think it gets the point across (from what I can tell via feedback here).

    Brian

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