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Pressure, Temperature and Fluid phase change

  1. May 1, 2015 #1
    Some please clear my confusion here, when you compress a fluid the temperature increases. Since the temperature increases (fluid gets warmer), why is the fluid condensing from gas to fluid?

    when temperature increases, isn't that the fluid will evaporate? This has been bugging for awhile. Please help!
  2. jcsd
  3. May 1, 2015 #2


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    Perhaps you should stipulate, or re-examine the conditions under which the fluid is condensing as it is being compressed.
    For example, I have never had had the chance to observe air turning into a liquid as it is being compressed to pump up a car tire.

    In other words, where on a phase diagram are you referring.

    An example PT phase diagram for water:
    If we start with the gaseous phase of water somewhere in between temperatures Ttp and Tcr, any compression of suitable magnitude will eventually cause a phase change from gaseous to liquid. Below Ttp, a compression will turn the gas into a solid ( and then a liquid with more compression ). Above Tcr, any amount of compression does not cause a phase change, but the fluid changes into what is called a supercritical fluid above a pressure of Pcr.

    Of course, I am using a constant temperature for the fluid throughout, assuming that the heat of compression is not being retained by the fluid to cause a temperature increase.

    If the fluid is insulated so no heat can flow out, the temperature will, as you say, increase during compression. It may be that the fluid may cross a phase change boundary during the process, or it may not. A calculation would be in order to determine the state of the fluid.
  4. May 3, 2015 #3
    Maybe I should put it in this way. When you are compressing gas into liquid in a processing equipment (eg. compressor), the pressure and temperature increases due to PV/T.

    My confusion is, if temperature increases, why is the compressed gas turning into liquid (condensing) and not evaporating/vaporizing?
  5. May 4, 2015 #4
    You are thinking about a phase in the traditional sense. A liquid is defined by density, so is a gas. This definition comes from statistical mechanics and is one reason why we do not have formulas for liquids that work as nicely for gasses (PV = rho RT is not applicable, for example, for liquids). Temperature is due to multiple modes of energy in an ensemble of molecules.

    You have translational (mostly dealt
    with in kinetic theory), vibrational, electrical, rotational, and sometimes magnetic modes of energy that comprise temperature we perceive.

    A cool example is ice (nice pun??). Ice has 17 different forms according to Wikipedia. These forms are defined by their microscopic crystal structure and are affect by temperature, pressure and a few other variables.
  6. May 5, 2015 #5
    If the vapor is being compressed adiabatically, you need to consider the slope of the adiabatic temperature-pressure relationship compared to the slope of the saturation temperature vs saturation pressure relationship. If the slopes are such that the adiabatic temperature-pressure graph intersects the saturation temperature vs saturation pressure relationship as the pressure on the vapor is increased, then even though the gas temperature is increasing along the adiabat, condensation will occur.

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