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  1. #1
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    Pressure Increase Generated by the Compressor



    Hi there!

    Please could someone help me with the following query on compressor's operation...

    Is the pressure increase from the input to the output of the compressor a constant or does
    it reduce with lower input pressure ?

    i.e.
    Given an input pressure to the compressor of 1 bar, producing an output pressure of 2 bar (1 bar increase)
    Then, given an input pressure to the compressor of 0.5 bar, would this produce an output pressure of more or less than 1.5 bar (1 bar increase) ?

    I am just trying to look at the compressor's performance in isolation, without the TXV valve's effects on pressure...


    Many thanks in advance



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    Re: Pressure Increase Generated by the Compressor

    I hardly know where to start. You have a very poor grasp of how a refrigeration system works.

    It's not about pressures, it's about temperatures. The purpose of reading the gauges is to determine the saturation temperatures, not the pressures.

    The compressor moves the refrigerant vapor.

    The TXV regulates the flow of liquid refrigerant.

    The pressures are primarily effected by the temperature and volume of air moving through the coils.
    Last edited by Gary; 31-01-2008 at 06:53 AM.

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    Re: Pressure Increase Generated by the Compressor

    Hi I find your questions quite bizzare are you involved in the industry by your questions I think not

    Norm
    My wife used to say you never listen to a word I say at least I think that what she said

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    Re: Pressure Increase Generated by the Compressor

    Hi!
    Stephen, I guess you should read this book:
    ww
    w.mediafire.com/?60gxbot2nmg

    Especially chapter #1
    Regards!
    Last edited by narkom; 31-01-2008 at 06:12 PM.

  5. #5
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    Re: Pressure Increase Generated by the Compressor

    I'm still trying to get an answer to what sort of "Engineer" our friend Stephen is.

    The questions that appear seem to come from someone who is moving up from Meccano.
    Brian - Newton Abbot, Devon, UK
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    Re: Pressure Increase Generated by the Compressor

    I apologise if my questions seem strange, my background is in electrical engineering and was trying to relate (refrigerant flow to electrical current flow) and (pressure changes to voltage changes) in my minds eye and now realise that I have been approaching the system with the wrong preconceptions.

    I was just trying to grasp the fundamental properties of each component in isolation before
    looking at the overall system.

    Thank you for your advice
    best regards
    Stephen.t

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    Re: Pressure Increase Generated by the Compressor

    Quote Originally Posted by stephen.t View Post
    I apologise if my questions seem strange, my background is in electrical engineering and was trying to relate (refrigerant flow to electrical current flow) and (pressure changes to voltage changes) in my minds eye and now realise that I have been approaching the system with the wrong preconceptions.

    I was just trying to grasp the fundamental properties of each component in isolation before
    looking at the overall system.

    Thank you for your advice
    best regards
    Stephen.t
    This being a refrigeration engineering forum, you might want to change your profile to read "electrical engineer" in order to avoid confusion.

    Possibly, given some thought we may be able to draw some valid correlations to a DC system.
    Last edited by Gary; 01-02-2008 at 01:49 PM.

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    Re: Pressure Increase Generated by the Compressor

    Quote Originally Posted by Gary View Post
    Possibly, given some thought we may be able to draw some valid correlations to a DC system.
    Hmmmm... to start, this being a loop system, mass refrigerant flow (wattage) is equal at all points in the loop and a restriction (resistance) at any point reduces the mass flow/wattage throughout the loop.

    The TXV (aka TEV) is just such a restriction/resistance. The mass of refrigerant being pumped by the compressor (generator) is automatically reduced to match the TXV, the vapor being more/less dense upon entering the suction side.

    To complicate matters, our system has a device which transfers [heat] energy to the loop (evaporator) and a device which transfers [heat] energy from the loop (condenser).

    The evaporator has been sized to transfer limited [heat] energy to the loop so as not to overload the compressor, yet it must transfer sufficient [heat] energy to cool the product. The amount of transfer varies between these limitations, as does the low side pressure.

    The condenser has been sized to transfer limited [heat] energy from the loop so as to maintain minimum pressure (voltage) to feed the evaporator, yet it must transfer sufficient [heat] energy to avoid maximum pressure, again overloading the compressor. The amount of transfer varies between these limitations, as does the high side pressure.

    The compressor adds only a limited portion of the energy to our circuit. The main source of energy input is in fact the evaporator. You might want to think of the evaporator/compressor combination as the generator.

    Feel free to jump right in here, folks. I'm outside my area of expertise... LOL
    Last edited by Gary; 01-02-2008 at 04:37 PM.

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    Re: Pressure Increase Generated by the Compressor

    To continue the comparison:

    In designing a system, we must first determine the maximum amount of heat energy that needs to be transferred from the product to our system and size the evaporator accordingly.

    We then size the compressor to handle this maximum energy transfer [input] without overloading.

    We then size the condenser to shed sufficient heat energy [output] from the system under maximum conditions to avoid overloading the compressor.

    We size the TXV to vary the resistance according to the [variable] evaporator heat load.

    The low side pressure/voltage varies according to the heat energy input and the high side pressure/voltage varies according to the heat energy output.
    Last edited by Gary; 01-02-2008 at 03:37 PM.

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    Re: Pressure Increase Generated by the Compressor

    I think possibly looking at a ph chart may help this fellow and seeing the entropy lines that the compression stroke will follow at any given inlet temp and the resultant discharge conditions, this may help to see visually the conditions that can and will arise from different variables like superheat and suction pressure.
    ...and she said "give it to me you big fridgie"

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    Re: Pressure Increase Generated by the Compressor

    Quote Originally Posted by 750 Valve View Post
    I think possibly looking at a ph chart may help this fellow and seeing the entropy lines that the compression stroke will follow at any given inlet temp and the resultant discharge conditions, this may help to see visually the conditions that can and will arise from different variables like superheat and suction pressure.
    I doubt that an Electrical Engineer will be able to understand a PH chart and the variables that can arise from differing operating envelopes.
    Good point though, as the Mollier chart is generally an under used tool in our trade.
    I'm back on the Pale

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    Smile Re: Pressure Increase Generated by the Compressor

    I would like to say a big thank-you Gary your answer has really helped my understanding of the system operation and further the design process. I was quite lost before I read your explanation.

    I have looked at a ph chart as 750 Valve suggested and I can see how this how this relates to your explanation.

    Thanks again
    Best regards
    Stephen

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    Re: Pressure Increase Generated by the Compressor

    Quote Originally Posted by stephen.t View Post
    I would like to say a big thank-you Gary your answer has really helped my understanding of the system operation and further the design process. I was quite lost before I read your explanation.

    I have looked at a ph chart as 750 Valve suggested and I can see how this relates to your explanation.

    Thanks again
    Best regards
    Stephen
    Glad I could help.

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