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So after reading about this topic i wanted to share what i learned and i wanted you guys to tell me if what i am writting is correct:

A pressure vessel has propane inside and it is pressurized. The liquid is at atmospheric temperature meaning that it is in equilibrium with the outside temperature. The pressure inside the pressure vessel will change according with ambient temperature.

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If I open the valve of the pressure vessel, the gas will come out and the pressure inside the vessel will decrease and the liquid propane inside the vessel will immediately star to boil so it can re-establish the equilibrium. When boiling the liquid goes into the gaseous state and to do this it requires energy in the form of heat (latent heat of vaporization).

Since the liquid has the same temperature that the outside it cannot take heat from the outside and it will need to take this energy from itself, decreasing the temperature of the liquid.

Now there is a ΔT between the liquid and the outside temperature, so the liquid will steal heat from the outside (sensible heat) so it can incease his temeprature in order to have enough energy to go to the gaseous state.

If the flow of gas that is coming out of the vessel is constant the pressure and the temperature of the liquid and of the gas will initially decrease a little but it will stabilize, because the heat that is being taken from the outside is enough to keep the liquid boiling without decreasing more of his temperature and pressure.

If the flow of gas that is coming out of the vessel increases the outside heat may not be enough to maintain the flow of gas required, so the temperature of the liquid will start to decrease (auto refrigeration) (heat absorbed by the liquid < heat used by liquid to go to the gaseous state due to the high gas demand)

If the temperature of the liquid keeps decreasing so will the pressure and if it decreases below the atmospheric boiling point, the liquid will stop boiling and the gas will stop to come out (negative pressure inside the pressure vessel).

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  • $\begingroup$ Keep in mind the Andselisk's note. $\endgroup$
    – Poutnik
    Commented May 30, 2020 at 21:07
  • $\begingroup$ @ Mathew Mahindaratne what is said in that question is correct? $\endgroup$
    – pedro vaz
    Commented May 30, 2020 at 22:49
  • $\begingroup$ This is a shorter attempt than your previous post, but it still rambles a little and as @andselisk commented, this is not a proof-reading service. One comment: the following is not logically correct: "Since the liquid has the same temperature that the outside it cannot take heat from the outside.". Separate statements that describe properties or states of the system (adiabatic) from those that describe changes in the state. Heat transfer is a change. A particular temperature or the inability to exchange heat are descriptions of states. $\endgroup$
    – Buck Thorn ♦
    Commented May 31, 2020 at 0:42
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    $\begingroup$ What do you think will happen if you let propane escape (at whatever rate), but the propane is insulated thermally from the outside temperature? $\endgroup$
    – Chet Miller
    Commented May 31, 2020 at 20:39
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    $\begingroup$ Sounds pretty correct. Quantitatively the entropy per unit mass of the combination of liquid and vapor remaining in the vessel will remain constant. $\endgroup$
    – Chet Miller
    Commented Jun 1, 2020 at 2:05

1 Answer 1

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All is about 3 basic principles.

  1. Liquid evaporation consumes thermal energy, taken from liquid itself or surrounding. Vapour condensation releases this energy.
  2. Liquid saturated vapour pressure (about) exponentially raises with temperature.
  3. If saturated vapour pressure exceeds external pressure, liquid boils. If vapour pressure exceeds the saturated vapour pressure, vapour condensates.
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