Law of expansion for super heated steam

[Soumalya]

Ideal gas behavior of super heated steam

Why is super heated steam said to exhibit ideal gas behavior?

 

[Chestermiller]

Why is super heated steam said to exhibit ideal gas behavior?

It exhibits approximately ideal gas behavior if the pressure is low enough.

Chet

 

[Philip Wood]

Chestermiller: I prefer to say "if the density is low enough". I'm not sure that high pressure due to high temperature would make a gas less like an ideal gas.

There may be another point here…Saturated vapours don't appear to obey Boyle's law at all. If we decrease their container volume, the vapour condenses, so their are fewer molecules in the 'gas' and the pressure fails to rise. But with superheated steam, there's no chance of it condensing, so the number of 'gas' molecules doesn't change, and Boyle's law is obeyed well.

 

[Chestermiller]

Chestermiller: I prefer to say "if the density is low enough". I'm not sure that high pressure due to high temperature would make a gas less like an ideal gas.

Thanks Philip.

Most thermo books have graphs of compressibility factor z as a function of reduced temperature and reduced pressure. This pretty much tells most of the story. One need only identify combined ranges of T and P for which z is close to 1.0. Of course, in the limit of low pressures, z approaches 1.0 irrespective of the temperature.

Chet

 

[Soumalya]

Chestermiller: I prefer to say "if the density is low enough". I'm not sure that high pressure due to high temperature would make a gas less like an ideal gas.

There may be another point here…Saturated vapours don't appear to obey Boyle's law at all. If we decrease their container volume, the vapour condenses, so their are fewer molecules in the 'gas' and the pressure fails to rise. But with superheated steam, there's no chance of it condensing, so the number of 'gas' molecules doesn't change, and Boyle's law is obeyed well.

Agreed!

From the T-V plot of steam at superheated regions the ideal gas behavior is most accurate with increasing specific volumes at low pressures obviously.Also as pressure is increased a significantly higher rise in specific volumes would mean ideal gas approximation to be suitable.This indicates gases would approach ideal behavior at very low densities.

 

[Soumalya]

Thanks Philip.

Most thermo books have graphs of compressibility factor z as a function of reduced temperature and reduced pressure. This pretty much tells most of the story. One need only identify combined ranges of T and P for which z is close to 1.0. Of course, in the limit of low pressures, z approaches 1.0 irrespective of the temperature.

Chet

Thanks Chestermiller

 

[Chestermiller]

Agreed!

From the T-V plot of steam at superheated regions the ideal gas behavior is most accurate with increasing specific volumes at low pressures obviously.Also as pressure is increased a significantly higher rise in specific volumes would mean ideal gas approximation to be suitable.This indicates gases would approach ideal behavior at very low densities.

I see what you're saying. You're saying that the limit of low densities covers more territory in P-T space than just low pressure in identifying a broad region where the ideal gas law will apply.

Chet

 

[Soumalya]

I see what you're saying. You're saying that the limit of low densities covers more territory in P-T space than just low pressure in identifying a broad region where the ideal gas law will apply.

Chet

Exactly Chet!