Discussion on the heat absorption that occurs when compressed gas is depressurized from a compressed gas cylinder

The results vary depending on the phase state of the medium contained in the gas cylinder. If a liquid substance is to vaporize, it must absorb heat from the environment in order to do so. Therefore, the pipe wall feels cold, and even frosting occurs, as can be deduced from the latent heat of vaporization. If it is compressed gas (gaseous), releasing it under reduced pressure will definitely release heat. Derived from P=nkT. ———————————— Regarding those who have doubts about P=nkT, my explanation is as follows: this is another way of writing the ideal gas law, where n represents the number of molecules per unit volume, and k is Boltzmann’s constant, given by k=R/Na=1.38*POWER(10, -23) J/K. Additionally, since I’ve seen some people use the formula PV=nRT for derivations, it’s a bit difficult to apply this formula directly here! On the contrary, it is easy to get confused by it, as there are too many variables. This post was last edited by tfx511024 on 2009-4-11 13:38.]

Both nitrogen and hydrogen absorb heat when under reduced pressure, which can be proven using the law of conservation of energy

Whether P*V=n*R*T results in heat absorption or heat release depends on the extent of changes in pressure and volume; haha, if the product of PV increases, then the temperature will rise.

They all belong to the category of pressure-reducing expansion throttling, so the temperature should decrease in each case.

Hydrogen also absorbs heat when its pressure is reduced. That person is wrong; a high pipe temperature could be caused by other reasons, and it’s necessary to investigate carefully.

It can be derived from the ideal gas law that pressure reduction is endothermic. It can also be derived from the law of conservation of energy: as the volume expands, the gas does work on its surroundings, resulting in a decrease in internal energy.

Thank you all for your active participation. Perhaps you didn’t read the title carefully; I’m referring to compressed gas, so there’s no issue of checking its state as mentioned on the second floor – it’s in gaseous form. In fact, the critical temperature of nitrogen is around -195°C, while that of hydrogen is -239.97°C. Therefore, it’s very difficult to maintain these temperatures in ordinary compressed gas cylinders, which is why they are called compressed gas cylinders. The equation P*V=n*R*T makes it hard to understand this phenomenon, as V increases when pressure decreases; the equation PM=pRT also isn’t easy to explain. I wonder if there are any experts who can clarify this, but once again, thank you all for your active participation!

It can be explained based on the principle of throttling expansion!

To the moderator of Floor 2: I still have some doubts. Your formula is correct; for more details, see that the ideal gas law is generally expressed as PV=nRT or PV=NKT. Here, P represents pressure, V represents volume, and T represents temperature in Kelvin units. n is the amount of substance, R is the universal gas constant, with a value of 8.3145 J/mol·K. N represents the number of molecules, and K is Boltzmann’s constant, with a value of 1.38066 x 10^-23 J/K, or 8.617385 x 10^-5 eV/K. However, when the pressure of a compressed gas is reduced, the number of molecules per unit volume, N/V, should decrease. Since the pressure P also decreases, how do you determine that the temperature T also decreases?

For details, you can refer to the university’s course on «General Physics»

Rising temperature? Is it related to the activity of microscopic molecules?

I heard that the temperature rises after H2 pressure is reduced. I don’t know what’s going on either.

I have also encountered this situation; how can it be explained?