Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Aragorn992 on 14/10/2004 00:48:49

Title: Temperature / Pressure Relationship
Post by: Aragorn992 on 14/10/2004 00:48:49

I have a question (and it probably sounds really stupid),

A closed vessel containing pure O2 has a tempreature of 20 degrees (C) and a pressure of 3000 psi. If it is heated to 800 degrees (C) what is the change in pressure and how do you work it out?

My Dad wants to know this for where he works, hes a welder / engineer by trade and apparently they're creating some kind of vessel thats going to do this.

Title: Re: Temperature / Pressure Relationship
Post by: qpan on 14/10/2004 08:19:41

The pressure will be 10,986 psi by the relationship pv=nrt as the volume remains constant. Pressure is directly proportional to temperature when volume is constant.

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

Title: Re: Temperature / Pressure Relationship
Post by: gsmollin on 14/10/2004 22:29:19

Do you know that oxygen remains an ideal gas at 11,000 psi?

Also, aragorn tell your dad that the O2 will be about 10 times as reactive under those conditions as it is at 20 C and 3000 psi. The vessel in question must not begin reacting with the O2, or a serious accident could occur.

Title: Re: Temperature / Pressure Relationship
Post by: qpan on 15/10/2004 22:11:00

Jeebus - i don't normally work in psi - just did the conversion and realised 11 000 psi = 758bar!

In that case oxygen will deviate from the standard gas model (probably quite significantly too!). However, the temperature of 1073 K is quite high, which will pull it closer to the ideal gas model.

The real gas equations are like the ideal gas equations but with an extra correction term. There's more info here:
http://itl.chem.ufl.edu/2045/lectures/lec_e.html
Show this to your dad- he may be able to use it. If not, i'll gladly do the calculation (should only (hopefully) take 10 minutes for me).

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

Title: Re: Temperature / Pressure Relationship
Post by: gsmollin on 16/10/2004 01:43:50

Running through the calculator at the end of the section, I got 15725 psi. That sounds more like the right answer, remembering "thermo", although I did not verify the derivation. Interestingly, O2 has already deviated from an ideal gas at 3000 psi. I still remember how non-ideal steam is at that pressure.

Title: Re: Temperature / Pressure Relationship
Post by: qpan on 16/10/2004 11:04:28

Bugger- didn't realise there was a calculator at the end - was preparing to work it out by hand, but didn't have the constants handy!

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe