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  1. #14
    Join Date
    May 2003
    Location
    Sarasota. Fl
    Posts
    1,276
    Yes, the vapor pushes the liquid. But if the volume taken up by the liquid in the system is constant (with ambient temp), which should be the case more or less, since the liquid line is supposed to be completely in liquid form, and then there is a short section of the condensing coil which is liquid. Then the volume of the rest of the system where the refrigerant is in gaseous form is also constant. That means that the average density of the gaseous refrigerant is also constant. The only thing that could change with (ambient) temperature is that the density upstream of the compressor could become lower and the density downstream of the compressor then becomes higher. This in turn would mean that the compressor has a changing pressure ratio, which I find hard to believe for a positive displacement pump.

    Bottomline: I still think that the decreasing superheat with increasing ambinet condensor temp is caused by the increase in pressure (not density, remember this is a closed system) that in turn increases the saturation temp. So it's the increasing saturation temp that is causing the superheat to decrease, not the lowering of the gas temp.

    This is an intersting discussion!

    Sam [/B][/QUOTE]



    The funny thing is......

    I bet you dont even own a screwdriver.


    It is the job of thinking people not to be on the side of the executioners.

    ~Albert Camus

  2. #15
    Join Date
    Jul 2005
    Posts
    6
    Originally posted by miami mike

    The funny thing is......

    I bet you dont even own a screwdriver.


    ??? What's that supposed to mean?

    BTW, thank you to all the other posters that posted useful information!

    Sam

  3. #16
    Join Date
    Sep 2002
    Posts
    3,708
    Originally posted by jacob perkins
    You might find this interesting,but I find it confusing and apt to give me a headache.However,I will try respond.

    First,I will just state some facts that you seem to have missed.You stated something about liquid and vapor levels being constant.I dont know what you meant,but I can say with certainty that liquid and vapors are not constant but a dynamic in the system.The only constants I can think of are the weight of the refrigerant charge and the volume of all the interconnected componants.

    let me tell you a little story...and since the purpose of mechanical refrigeration is cooling,our story begins in the evaporator:

    At start up the heat load on the evaporator is high.Our evaporator will require alot of liquid refrigerant from the condenser ,to boil and absorb heat from the air passing over the coil.Our saturation temperture and superheat will be high,and our vapor will be very dense.This heavy suction vapor will require more work from our compressor.With an increased amp draw the compressor will send this vapor on to the condenser,where pressure will be increased to send more liquid back to where it is needed--the highly loaded evaporator.We will have less liquid in our condenser at this time,because conditions of the evaporator.

    Now later that very same day...when the room temperture has dropped,the heat load on the evaporator is less.With lower evaporator heat load,our suction pressure,superheat,and vapor density levels have dropped.
    The vapor enters the compressor,and very little electromechanical work is required to send it on to the condenser.This light vapor causes a lower highside pressure and more liquid being stored in the condenser circuit,and less being sent to the evaporator.


    The moral of the story is this:The evaporator heat load and the liquid requirementof the evaporator are factors influencing the density of suction vapors.And suction vapors will influence the work of the compressor,highside pressure,and level of liquid storage in the condenser.


    Or in other words,the way it was taught to me:
    "the evaporator is the Boss and all other componants do what it says via its bully messenger,the vapor density.


    And...it's saturday nite!

    This post here should be put in the archives.
    Thanks Jacob for your contribution.That was great!


  4. #17
    Join Date
    Jul 2005
    Posts
    6
    Originally posted by jacob perkins
    ...
    The only constants I can think of are the weight of the refrigerant charge and the volume of all the interconnected componants.

    ...

    The moral of the story is this:The evaporator heat load and the liquid requirementof the evaporator are factors influencing the density of suction vapors.And suction vapors will influence the work of the compressor,highside pressure,and level of liquid storage in the condenser.


    Or in other words,the way it was taught to me:
    "the evaporator is the Boss and all other componants do what it says via its bully messenger,the vapor density.


    And...it's saturday nite!
    Thanks for this elaborate but illustrative example! It did help clarify a lot of things and think I understand the basis of this effect now.
    What I meant by the constants is exactly what you mentioned: the total volume and the weight of the refrigerant charge. This means that the only way the vapor density can change is by having more or less refrigerant in liquid form in the system. What you just made clear to me in the above is that this is dependent on the heat load on the system.

    Thanks a lot!

    Sam

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