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  1. #1
    Join Date
    Jul 2006
    Posts
    24

    Confused Simple air con question......

    So, assuming that at a certain temperature the refrigerant turns from liquid to gas and vice versa, and that high temperature = high pressure & low temperature = low pressure;

    In a simple Air Conditioning Split System (compressor, evaporator, condenser, expansion valve), how can there be low pressure/low temperature liquid in one part, and high pressure/high temperature liquid in another part?

    If it's liquid then it must be low temperature & therefore a low pressure??

    If the refrigerant 'boils' at a certain temperature then why isn't it always low temperature/low pressure liquid and high temperature/high pressure gas?

    How can there be cool gas after the evaporator? If it is cool and at a low temp & pressure how can it have 'boiled' (evaporated) into a gaseous state?

    I've searched for a diagram that shows example temps & pressures in different parts of the system but can't find one.

    I'm confused.......help!

  2. #2
    Join Date
    Nov 2010
    Location
    N. Idaho
    Posts
    586
    Because its not water.

    I can't explain but I know what the refrigerant does. I'll sit and listen, we'll both learn something!
    "I've got my Gas Mechanics license"
    "Yea, well my 16yr old daughter has her drivers license, doesn't mean she knows how to drive"

  3. #3
    Join Date
    Oct 2004
    Location
    Central WA
    Posts
    1,311
    You are missing a lot. The boiling/condensing (saturation) temperature changes with the pressure. Water will always boil at 212 degrees, right? You can pour the coals on, but the water will never rise above 212 degrees. Unless you put a lid on it causing it to pressurize and boil at a higher temp. But if you drive it up the mountain where atmospheric pressure is lower it can't even get to 212.

    That principle of absorbing heat during the change of state is the whole basis for refrigeration. That 212 degree water will never get any hotter as it boils off, but it keeps sucking up that heat. That heat (latent heat) is rejected as it condenses back into water.

    Someone else will chime in soon enough with the gas laws, etc.

  4. #4
    Join Date
    Nov 2009
    Location
    Richmond, working under tarps
    Posts
    460
    because low temp does not equal low pressure. as you state they do.......saturated states.

    also temps and pressures are relative,,,, not what you perceive to be hot or cold.

    IE liquid in a tube can feel HOT to you, but can be in a subcooled state.

    if you look at PE diagrams for relative refrigerants, including water you will understand better.

  5. #5
    Join Date
    Oct 2008
    Location
    mid-Tennessee
    Posts
    582
    Two things about pressure and it's effects:

    1. Pressure goes up...temperature goes up. Pressure goes down, temperture goes down. Reverse those two dynamics and they still affect each other. So, temperature goes up, pressure goes up. Temperature goes down, pressure goes down. Pressure and temperature are in unison. One affects the other.

    Forget liquid for a minute and consider just vapor. A vapor expands with increased temperature, which means the pressure it exerts in an enclosed space will increase as a result of it's expansion. If you put a pressure gage on it, you'll see a notable rise in pressure. Take the same vapor and cool it (remove heat), the volume of vapor contracts.

    EXAMPLE: Take a two liter plastic coke bottle of just air at room temperature. Screw its lid on tight and put it in the freezer. Come back 30 minutes later and that bottle will have partially collapsed due to a drop in it's internal pressure due to the extreme temperature drop.

    Go the other way now, but with an empty one gallon milk jug with a snap-on lid. Take the lid off and put the plastic jug in the freezer for about ten minutes. You still have the lid in your hand, so just wet it under the faucet. After ten minutes, open the freezer, slap that lid on and set the jug onto the kitchen counter. As the air in the jug warms up, guess what happens...

    Now you should realize that pressure in an enclosed container will noticably rise or fall with temperature. That's an example of the pressure/temperature relationship. AGAIN - remember that the roles can be reversed so that a increase or decrease in PRESSURE will equally result in a increase or decrease in temperature.

    So the next question is what affect would that pressure change have on a liquid contained with it? On a much grander scale than those two examples, the pressure affects the liquid's boiling point.

    2. When pressure goes up, boiling point of a liquid goes up. Pressure goes down, boiling goes down.

    In Denver CO, water boils at 196 degrees, while those living at sea level gotta wait for 212 degrees. Why? Because there's less atmospheric pressure at a mile high in Denver.

    Have you ever removed a radiator cap on a car's radiator after driving it awhile? I DON'T RECOMMEND doing that because it's under pressure from the heat of expansion. Is the radiator fluid boiling in there? NO! Thanks to that pressure it had built up, the boiling point has increased so that the fluid DOESN'T boil.

    However, if you were to be so careless as to remove that cap and release the pressure, that sudden drop in the boiling point will cause the radiator fluid to rapidly boil - thus it will blast out of there like a volcano! You decreased the pressure and thereby decreased the boiling point.

    APPLY THESE TWO BASIC LAWS OF PHYSICS on a much grander scale now when you think of a refrigerant. In the examples above, the most extreme example was the mere 10 to 12 psi that the radiator cap was containing under pressure. However, a typical refrigeration cycle manipulates a liquid (refrigerant) at a change in pressure in the hundreds of PSI range.

    I hope this helps.

    Your next critical levels of understanding a refrigeration cycle is each step in that refrigeration cycle - AND - its role in harnessing heat (latent & sensible). The "system's" attraction and expulsion of heat is it's entire purpose. The refrigeration system is specifically engineered to run in a harmonious balance:
    -Just the right size compressor displacement.
    -Just the right amout of refrigerant.
    -Just the right amount of coil surface area
    -Just the right amount of coil surface area, etc.
    ALL for an anticipated and calculated amount of heat load for that system.

    Keep asking questions. It'll all come together for you.

  6. #6
    Join Date
    Jul 2006
    Posts
    24
    Great replies guys - thanks a lot, just read up on it again with that added gem of knowledge (that I knew but had forgotten - honest!) about the evaporation & condensing points being different on the cooling side (due to the low refrigerant pressure) than on the heating side (due to the high refrigerant pressure).

    And now it all falls into place! Once again, great advice and very well presented, highly appreciated.

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