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  1. #1
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    how does temp differ with out pressure



    Ever since i started doing this job I'e been told that pressure and temperature go hand in hand, pressure goes up so does temp and same thing for loosing heat. I am going to tafe now and im getting taught something differant. There can only be two pressures in the system at any given time regardless of the temp.

    For argument sake we will say the system is running on R134a, give 10K subcooling and 10K superheat leaving evap and 12K superheat leaving comp. LP 241kpa and HP 977kpa

    Therefore our condensing temp is 42*C and evap would be 4*C
    dischage line is 42*C + 12K = 54*C BUT pressure is STILL 977kpa. why wouldnt it be 1370kpa???
    AGAIN
    liquid line is 42*C - 10K = 32*C while pressure remains at 977kpa???

    On the other side
    suction line is 4*C + 10K = 14*C / 241kpa instead of 379kpa

    discharge 54*C 977kpa
    condenser 42*C 977kpa
    liquid line 32*C 977kpa
    evaporator 4*C 241kpa
    suction 14*C 241kpa

    I can't see how that works out... can anyone explain it better then my teacher?



  2. #2
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    Re: how does temp differ with out pressure

    Quote Originally Posted by jakabus View Post
    Ever since i started doing this job I'e been told that pressure and temperature go hand in hand, pressure goes up so does temp and same thing for loosing heat. I am going to tafe now and im getting taught something differant. There can only be two pressures in the system at any given time regardless of the temp.

    For argument sake we will say the system is running on R134a, give 10K subcooling and 10K superheat leaving evap and 12K superheat leaving comp. LP 241kpa and HP 977kpa

    Therefore our condensing temp is 42*C and evap would be 4*C
    dischage line is 42*C + 12K = 54*C BUT pressure is STILL 977kpa. why wouldnt it be 1370kpa???
    AGAIN
    liquid line is 42*C - 10K = 32*C while pressure remains at 977kpa???

    On the other side
    suction line is 4*C + 10K = 14*C / 241kpa instead of 379kpa

    discharge 54*C 977kpa
    condenser 42*C 977kpa
    liquid line 32*C 977kpa
    evaporator 4*C 241kpa
    suction 14*C 241kpa

    I can't see how that works out... can anyone explain it better then my teacher?
    Saturated temperature varies with pressure and visa verse.

    Subcool is how much cooler the liquid is than saturated and superheated is how far warmer the vapour is than saturated.

    It is not easy to have subcooling in the presence of vapour and it is not easy to have superheat in the presence of liquid but both are possible under particular conditions in close proximity to saturated temperature.

  3. #3
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    Re: how does temp differ with out pressure

    Air or non condensibles in a system can screw up the pressure temperature theory.

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    Re: how does temp differ with out pressure

    Quote Originally Posted by jakabus View Post
    Ever since i started doing this job I'e been told that pressure and temperature go hand in hand, pressure goes up so does temp and same thing for loosing heat. I am going to tafe now and im getting taught something differant. There can only be two pressures in the system at any given time regardless of the temp.

    For argument sake we will say the system is running on R134a, give 10K subcooling and 10K superheat leaving evap and 12K superheat leaving comp. LP 241kpa and HP 977kpa

    Therefore our condensing temp is 42*C and evap would be 4*C
    dischage line is 42*C + 12K = 54*C BUT pressure is STILL 977kpa. why wouldnt it be 1370kpa???
    AGAIN
    liquid line is 42*C - 10K = 32*C while pressure remains at 977kpa???

    On the other side
    suction line is 4*C + 10K = 14*C / 241kpa instead of 379kpa

    discharge 54*C 977kpa
    condenser 42*C 977kpa
    liquid line 32*C 977kpa
    evaporator 4*C 241kpa
    suction 14*C 241kpa

    I can't see how that works out... can anyone explain it better then my teacher?

    I think you are confusing temperature and the boiling temp (or condensing temp) known as saturation temperature.

    Pressure and temp are related but the temp in question is the saturation temp (boiling or condensing).

    So if a liquid is colder than the condensing temp then the liquid must be sub cooled.

    If the vapour is hotter than the evaporation temp then it must be superheated.

    All the best

    coolrunnings

    .

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    Re: how does temp differ with out pressure

    Good you question this as a young tech

    I will try to explain it to you with water.
    Have you ever seen a log p/h?
    This is the log p/h of water
    The same exists for all refrigerants and it has +/- the same size, only the numbers differ.

    Then you know that the bubble in that diagram is the transition from liquid (left side of the bubble) to gas (right side of the bubble) of a refrigerant, water in this diagram.

    If you look at the right vertical axis side and you take 1 bar absolute or atmospheric pressure, then you will notice on the left side of the bubble 100C (red numbers)
    Water start to boil at 100C and has a heat content of 417 kJ/kg of water (horizontal lower axis)
    You further add heat to the water and the water always boils more and further until you have added 2257 kJ/kg water. But.. the temperature remains the same.
    Then, after adding 2257 kJ/kg heat, all the water is vaporized at the same pressure of 1 bar and same temperature of 100C. But you changed the water from liquid state to vapor state. So changing state happens always at the same pressure and temperature but at a different heat content.

    To understand your problem: we go back to the left side of the bubble where the water of 100C just came from 99.9999C and started to boil.
    We cool the water down now along the 1 bar pressure line, so going horizontal to the left. The energy content decreases (the water becomes colder) but we don't have a phase change state now. So we subcool the water from 100C to 95C, 90C always further down but at the same atmospheric pressure of 1 bar. The relation between temperature and pressure is gone now, same pressure, different temperature. It's gone because you don't have a phase change now.
    So, the relation of the temperature with the pressure is only valid within the bubble. Or it is only valid when the liquid changes from 100% liquid to 100% vapor. Reducing heat content of a 100% liquid will subcool the liquid and once all vaporized, you then superheat the vapor when further adding heat.
    Or compared with water: water always boils at 100C when pressure is at 1 bar and water boils at (+/-) 85 C on the Mont Blanc where pressure is +/- 0.6 bar and it boils in an old household steamer at 120C when pressure is at 2 bar in the casserole.


    Same for refrigerants, relation is only valid when the refrigerant is on its boiling phase, at the beginning of it till the end and where only the heat content of the refrigerant changes.

    Hopes this clarified a little bit your big question marks in your head.
    Last edited by Peter_1; 29-11-2010 at 08:33 PM.
    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: how does temp differ with out pressure

    So you've got the 2 pressures in your system. At those pressures the refrigerant will be at the point of turning from liquid to gas (evaporator) or gas to liquid (condensor). Those are called the saturation points amongst other names.
    Anything above or below this will enter into superheated gas or subcooled liquid territory.

    In your example:

    Therefore our condensing temp is 42*C and evap would be 4*C
    dischage line is 42*C + 12K = 54*C BUT pressure is STILL 977kpa. why wouldnt it be 1370kpa???
    Because the pressure in the high side is 977kPa, R134a condenses at 42*C. Leaving the compressor it's a gas superheated 12K

    AGAIN
    liquid line is 42*C - 10K = 32*C while pressure remains at 977kpa???
    Same again. R134a condenses at 42*C then gets cooled 10K. The saturation temp remains the same.
    It's pressure wouldn't get lower. It stays the same and the liquid becomes subcooled.

    On the other side
    suction line is 4*C + 10K = 14*C / 241kpa instead of 379kpa
    R134a is boiling at 4*C. In the evaporator it's changing state (latent heat) at 4*C. Heat it until it's 14*C and it's now superheated gas by 10K.


    Like Peter says you can boil water at home at 100*C. Give it more heat and it still boils at 100*C. Depends on the pressure and in your kitchen it wont alter so neither will the boiling (or condensing) point.
    Pour 10*C water out the tap, it's subcooled by 90K at atmospheric pressure. There's an example of a liquid and the same pressure but a different temperature.

    Hope that helps,

    Andy.
    Health and safety first..........unless I'm in a hurry.

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    Re: how does temp differ with out pressure

    thanks guys, you all helpped out heaps... wish i looked at an enthalpy chart before i asked the question but >.> I know know, cheers!

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    Re: how does temp differ with out pressure

    Hi jakadus.
    so what was the problem and how did you resolve it. magoo

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    Re: how does temp differ with out pressure

    Quote Originally Posted by Tayters View Post
    So you've got the 2 pressures in your system. At those pressures the refrigerant will be at the point of turning from liquid to gas (evaporator) or gas to liquid (condensor). Those are called the saturation points amongst other names.
    Anything above or below this will enter into superheated gas or subcooled liquid territory.

    In your example:

    Therefore our condensing temp is 42*C and evap would be 4*C
    dischage line is 42*C + 12K = 54*C BUT pressure is STILL 977kpa. why wouldnt it be 1370kpa???
    Because the pressure in the high side is 977kPa, R134a condenses at 42*C. Leaving the compressor it's a gas superheated 12K

    AGAIN
    liquid line is 42*C - 10K = 32*C while pressure remains at 977kpa???
    Same again. R134a condenses at 42*C then gets cooled 10K. The saturation temp remains the same.
    It's pressure wouldn't get lower. It stays the same and the liquid becomes subcooled.

    On the other side
    suction line is 4*C + 10K = 14*C / 241kpa instead of 379kpa
    R134a is boiling at 4*C. In the evaporator it's changing state (latent heat) at 4*C. Heat it until it's 14*C and it's now superheated gas by 10K.


    Like Peter says you can boil water at home at 100*C. Give it more heat and it still boils at 100*C. Depends on the pressure and in your kitchen it wont alter so neither will the boiling (or condensing) point.
    Pour 10*C water out the tap, it's subcooled by 90K at atmospheric pressure. There's an example of a liquid and the same pressure but a different temperature.

    Hope that helps,

    Andy.
    Very well put.

  10. #10
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    Re: how does temp differ with out pressure

    Spank your hairy crutch!
    Health and safety first..........unless I'm in a hurry.

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    Re: how does temp differ with out pressure

    Fairy fluff, carry your bag for you someday sir!

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    Re: how does temp differ with out pressure

    Hi Magoo,

    After peter was talking about an anthelpy chart for water I realised my error. At first i didn't even consider the state of the substance (refrigerant or water) and a PT chart reads in the saturation temp. it doesn't cover all three states.

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    Re: how does temp differ with out pressure

    Great lesson chaps! thanks!!

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