Throttling of saturated steam 1

[Gvcr]

Dear all,

I would like know the logical reasoning for temperature reduction during throttling process of saturated and superheated steam eventhough its enthalpy, meaning total energy of the system is constant? What is the science behind it. Thank you.

 

[Latexman]

H = U + PV.

In real gases, PV changes.

If PV increases, with H constant, then U must decrease as a result of the fluid doing work on its surroundings. This produces a decrease in temperature.

This, and more, should be in your College Thermo textbook(s).

Good Luck,
Latexman

 

[Gvcr]

Thanks for your reply sir, you mean to say, the reduction of pressure to be seen as expansion of gas, that work done causes lesser internal energy. Right sir?.

Most of the throttling videos on YouTube, showing workdone as zero, have no idea why it is done so, causing much more confusion to it.

 

[Snickster]

Joules Law states that for real gases temperature is dependent on pressure even though no work is done and enthalpy remains constant (internal energy also remains constant and so does PV term although in state 2 pressure is lower and volume is larger) in a throttling process such as across a pinched valve. See thermodynamics books of fundamentals. This change in temperature with pressure at constant enthalpy is more noticeable at higher pressures.

 

[pierreick]

Hi,
Probably good to read:

https://www.thermal-engineering.org/what-is-throttling-process-isenthalpic-process-definition/

Pierre

 

[Latexman]

Thanks for your reply sir, you mean to say, the reduction of pressure to be seen as expansion of gas, that work done causes lesser internal energy. Right sir?.

Most of the throttling videos on YouTube, showing work done as zero, have no idea why it is done so, causing much more confusion to it.

Right. A real gas usually does positive internal work during a throttling expansion, and its temperature decreases. No external work is done.

I didn't see the videos, so cannot comment. Have you looked at Wikipedia?

Not a criticism; just advice. Your choice of words is a bit sloppy, and I think it would help you if you were more exact/precise. I.e. be true to the exact definitions of the thermodynamic terms, and I think you will get through your confusion faster and easier. Enthalpy is total internal energy plus pressure times volume. H = U + PV. PV is referred to as pressure energy. "Total energy of the system" is somewhat right, but it lacks precision, and, therefore, limits the conversation and the understanding. Again, this is advice; not criticism.

Good Luck,
Latexman