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  1. #1
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    Newbie Question: QL vs. QH efficiency



    Greetings,

    This question might be somewhat elementary, but anyways here it goes:

    If we look at COP(refrigeration) it tells us the ratio of Work in compared to energy drawn from cold side. However I wonder what is the energy drawn from cold side vs. the total thermal energy of system. Since the work put in to the refrigeration cycle is converted to mechanical work and heat and others. Therefore not all work put in to the cycle is converted to heat.

    So, what are typical QL(energy drawn from cold side) vs. the total heat energy produced by system, neglecting all the mechanical loss, electrical loss, etc.


    Thanks in advance,
    - Tinska



  2. #2
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    Re: Newbie Question: QL vs. QH efficiency

    Welcome to the forum. Is there another name for this energy ratio you are looking for?

  3. #3
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    Re: Newbie Question: QL vs. QH efficiency

    Quote Originally Posted by tinska.h View Post
    Greetings,

    This question might be somewhat elementary, but anyways here it goes:

    If we look at COP(refrigeration) it tells us the ratio of Work in compared to energy drawn from cold side. However I wonder what is the energy drawn from cold side vs. the total thermal energy of system. Since the work put in to the refrigeration cycle is converted to mechanical work and heat and others. Therefore not all work put in to the cycle is converted to heat.

    So, what are typical QL(energy drawn from cold side) vs. the total heat energy produced by system, neglecting all the mechanical loss, electrical loss, etc.


    Thanks in advance,
    - Tinska

    I think in real terms the energy in, comprises of the laten and sensible heat
    that is absorbed over the evaporator and suction line.

    The amount of heat gained in the sensible zone is small but real.

    I have just plotted a R404a system with an exaggerated superheat of 40 degsC
    And for ever 10K increase of superheat the energy imparted was approx 10 kj/kg.

    Then there is the Work Done (WD) over the compressor the software I'm using
    has a compressor Isentropic efficiency of 1.0. No compressor is perfect so there
    will always be losses, but specific information must be gained from compressor manufacturers.

    Rob

    .
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

  4. #4
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    Re: Newbie Question: QL vs. QH efficiency

    For a simple one on one: One evaporator, one compressor: Heat Absorbed is the enthalpy of the refrigerant vapor as it leaves the evaporator, minus the enthalpy of the liquid upstream of that evaporator's expansion device....nominally warm liquid. Any superheat developed by the evaporator is portion of the heat absorbed. Total refrigeration effect is that enthalpy difference times the mass flow of the refrigerant.

    Heat rejected is enthalpy of the refrigerant as it leaves the compressor: and is nominally the sum of the enthalpy of the vapor entering the compressor plus the power input to the compressor (motor Hp actual) expressed as heat, divided by the mass flow of the compressor.

    Liquid injection cooling, economizer cycles and to a lesser extent, liquid-suction heat exchange and similar will change both the power requirement and the mass flow and make the calculations more involved.

    At the simplest level: KWrej=KWabsorbed + KW input as power

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