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Thread: Log p/h diagram

  1. #1
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    Log p/h diagram



    CAn someone verify this for me please:

    When you install the liquid receiver on the roof besides the condensor and the liquid is going straight down to the evaporator - let's say 20 m (65ft) - with a liquid temperature of 30°C and the refrigerant R134a gives me 0.8416 dm³/kg or 0.0008416 m³/kg or a density of 1/0.0008416 = 1188 kg/m³ (74.16 lb/ft³)

    This gives me a pressure of 1188 kg/m³x 9.81 N/kg x 20 m = 233127 N/m² or 2.33 bar (33.79 psi) gain in 'mechanical subcooling'

    When I draw my upper horizontal condensing line in my Log p/h diagam, going from right to left while condensing and I take 5 K SC, then I draw the line with this mechanical SC straight up starting from the end point of the upper line.
    Is this correct?
    Last edited by Peter_1; 08-01-2007 at 07:47 AM. Reason: changed vertical in horizontal, mistake of me


    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  2. #2
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    Re: Log p/h diagram

    I understand your using a Moliere enthalpy pressure diagram then:
    "When I draw my upper vertical (horizontal ) condensing line in my Log p/h diagam, going from right to left while condensing and I take 5 K SC, then I draw the line with this mechanical SC straight up starting from the end point of the upper line."

    Your increasing absolute pressure of a liquid (hopefully not part vapour/liquid mix) and of its delivery to the evaporator... therefore all your pressures there will be higher... the opposite problem of A/C evaporators higher than their condenser units using rich liquid/vapour lines where the problem is of concern due to the ratio changing. What needs thought is the difference of weight of the refrigerant in the suction line (a relative vacuum) going to the compressor relative to the liquid line - presenting an additional load possibly affecting your calcs. on compression ratio etc. Also watch for accumulations of liquid in the evap. at end of temperature cycle - though I assume the compressor is controlled by Low Pressure cut out.
    Saludos

  3. #3
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    Re: Log p/h diagram

    Hi Peter,

    Sounds like you are working on a class project.

    Using the R-134a liquid density you provided {1188 kg/m³ (74.16 lb/ft³)} and my calculations I ended up with 33.47 psi ( about 2.31 bar) for the static pressure due to the 20 m (65ft) elevation difference.

    So that is close enough.

    As far as plotting this on a log P/H chart you would have to make some assumptions first.

    The liquid is all condensed coming out of the condenser, right? So this location would be at the condensing pressure on the saturated liquid curve on the left.

    Now, if there is any subcooling from the condenser, the line would extend to the left (at the condensing pressure) for the amount of subcooling generated in the condenser. If the condenser subcooling is zero, then the line stops at the saturated liquid curve.

    At this point the liquid flows through the liquid line down. During this, the liquid is loosing some small pressure due to friction loss in the pipe. At the same time, static head is increasing. The static head is increasing faster than the friction loss.

    A simple way to look at this is; at condensing pressure the liquid is saturated, so if the pressure increases due to static head, the line is drawn straight up from the condensing pressure at the saturated liquid curve.

    As the liquid line pressure begins to decrease, the line is drawn straight down to the pressure entering the TXV.

    In essence, the height of the liquid line drawn on the chart will be higher than the condensing pressure (due to the static head).

    I know I'm going to catch some flak for this, but I would NOT consider this mechanical subcooling.

    In my view, mechanical subcooling is done by a separate refrigeration system, which reduces the liquid temperature. (this is reducing the temperature at a constant pressure)

    The form of subcooling you are using is strictly due to an increase in pressure at a constant temperature.

    Pressure moves up and down. Temperature moves left and right.

    Does that make sense Peter? It's sometimes hard to describe this in words...

    Quote Originally Posted by MRcoolingMAGIC
    Did you add gravity gaining higher velocity pressure....20m
    I think it's safe to say this can be disregarded as the pressure increase due to acceleration (velocity head) is miniscule.

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    Re: Log p/h diagram

    Opps, just thought of something else.

    The liquid enthalpy does not change. It's constant for the process you describe.

    Therefore you cannot include a credit for subcooling on the TXV's. The only thing you can use is the higher liquid feed pressure to the TXV inlet, which will increase the dP across the valve.

    In effect, the valve capacity will increase due to the higher inlet pressure (greater differential pressure (dP) across the valve).

    The actual liquid temeprature does not change, so you can't include any higher subcooling as a credit for valve capacity. Only the higher dP.

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    Re: Log p/h diagram

    US Iceman, its indeed for a class project.
    I'm a little bit diasapointed in myself if I see how deep basic things are already in my mind. The years I think.

    And some things can't simply be found in schoolbooks.

    It's indeed not an enthalpy gain you get, it prevents just flashgas in the liquid line due to the static head.
    Just the bigger DP over the valve and teh flas gas prevention technique was something I wanted to explain the boys.

    There's almost no velocity in the liquid line. .
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Log p/h diagram

    Hi Peter,

    Quote Originally Posted by Peter 1
    It's indeed not an enthalpy gain you get, it prevents just flashgas in the liquid line due to the static head.
    Just the bigger DP over the valve and the flas gas prevention technique was something I wanted to explain the boys.
    This is one of those fine-points you almost have to discover by homework and personal review. I continue to believe the use of a Moller chart is worth the effort due to the valuable information that can be learned from it's use. It some ways, using a Mollier chart is like using a slide rule. It's almost a lost art.

    Quote Originally Posted by Peter 1
    And some things can't simply be found in schoolbooks.
    Which takes us back to our conversation with Josip.

    I hope I was able to explain that OK for you.

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