h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: The Cool Joule-Thomson Effect - A1066259
Author: Hell - U171578

Dunno, might be good. Some spots might be a bit confusing. But in general I think it's good to go.

HELL

Hell, you may be right. The only mistake I found was:
"Apart from the deoforant..."

thx, done.

Anything else? Was it easy to follow, or too techy?

HELL

"Some might have noticed that a soft-dink..." --> soft-drink (also available in cans! )

"Joule started studying science in 1834 when he went to Cambridge in 1834..." -- we only need the year once!


Other than that, looks good!

didn't get it: cans - link?

other than that: thx, corrections done.

H

There's still a 'soft-dink' in there...

Any chance of quatifying the examples? For instance, I've noticed the effect with deodorant cans (so it must be a difference of at least several degrees), but I've never noticed it with fizzy drinks.

The next time you pop open a soft-drink, watch carefully for some visible vapor - that would be water vapor condensing in the air which has suddenly been cooled to below the dew point thanks to the J-T effect!

Hell - great entry! Man o man, you took me right back to my thermodynamics courses in college with that nice diff. eq. in that footnote! Ahh, college.

- JD

drrrrink! AHA!

fixed

HELL

Hi HELL,

I only skimmed over it so far. I'll study it later.

The first thing that jumped into my head was adiabatic expansion ( and isothermal expansion and fractional distillation). I Googled for adiabatic, and found lots of experiments, but not a definition.
Is it the same (or related)?

Awu.

Adiabatic expansion is of course related, it is a special case of expansion where there is no exchange of heat with the outside. In a reversible process (like for an ideal gas), it means that the internal energy is constant, and hence the temperature also doesn't change.

So, the topic is related, yes. But I think it is out of the scope of this Entry...

HELL

T sub 1 seems to come out of nowhere. Is it representing a particular inversion temperature for the relevant gas?

Hi Hell

I'm just wondering why high mountains like Mt. Fuji most of the times have a cloud surrounding their summits, and why aircraft produce contrails. Is that because of the Joule-Thomson effect (because the wind has to speed up and thus cools down below the dew point), is because of Bernoulli's principle, or do they both have an effect there?


Bossel

Hi HELL,

thanks. I wasn't suggesting expanding (adiabatically or isothermally - ha ha) the scope of your entry. It was just that, a flood of school physics memories came back, and as I started to read your entry I thought I might finally understand

The entry is good stuff, but just re-whetted my appetite for going off on a tangent and finally coming to grips with the topic.

Awu.

I don't want to start a dry academic discussion off here, but I can't help feeling a bit uneasy about this. Are you *sure* that adiabatic effects aren't significant in this? There *is* an exchange of energy with the surroundings: all the mechanical work done in change of volume either results in a loss or net gain of internal energy of the gas. It's just that there is no exchange of *heat* with the surroundings: the expansion is not isothermal. Adiabatic expansion has to occur rapidly, which is exactly what happens when you press the button on an aerosol can. A gas which expands adiabatically will cool internally, which means that it will chill any solid or liquid it comes into contact with.

FM, you have a good point there. I was hoping to be able to explain this stuff without going into too dull details, but I think there's no way around this. I'll have to add a few things to make it clearer... Hang on! And thanks!

HELL

PS: Just three tiny notes: I didn't say adaiabatic effects do not play a role, I just thought it would be out of this Entry's scope. And two: I didn't say *energy exchange* I said *heat exchange*. Anyways. I think I need to make this passage clearer. Oh, and three: Dunno if the releasing of the compressed deodorant aerosol is adiabatic, it's almost like releasing a gas into a vacuum. I'm not sure if it would feel cooler if you used compressed argon. Well... I'll come up with something. In the meantime, thanks a lot.

OK, I'm removing this from PR, there's no way around some more complicated thermodynamics.

I'll re-write this and put it up for PR later on.

Thanks for all your help.

HELL

It's not so much a case of biting off more than one can chew: it's whether one can chew it in a manageable mouthful. If you can cram adiabatic effects into this and still make it a reasonable size, then that would be no mean feat.

yeah, exactly. but I want to do this with the Entry not in PR.

Thanks for nudging me into the right direction, FM!

HELL

Lets assume I’m in the right place!!!!

OK, Joule – Thomson with the known variables of pressure and volume.

How do I address this problem: - Saturated vapour flowing at a pressure of P1 and a mass flow rate of M (dot) and a temperature of T1 passes through a choked (throttling) orifice.

Constant enthalpy is assumed and knowing the downstream pressure, P2 how do I get back to the downstream temperature, T2?

Is being saturated causing a difference? What if the fluid was say a dry gas, what happens then?

Kind regards,
Fog Jones

You know, it's been so long since I did thermodynamics and psychrometry type stuff, that this question has vexed me for hours today. So, I did a little looking up in some old books that needed dusting off anyway. First, I'm assuming you're talking about water in air for your saturated vapor system (this may be completely wrong). If that's the case, I've jotted down some thoughts that I hope will help, but bear in mind that this is from a guy who's not studied this sort of stuff in ten years (so he, too, may be completely wrong).

1. If you have a non-ideal gas (such as, saturated water vapor in air), then the J-T effect would be significant and should be accounted for. I'm not sure how to apply it to this case (I've forgotten nearly all of my physical chemistry it seems), but it is logical that since you are already at the saturation temperature, any further temperature drop will cause some of the vapor to condense. The energy from the J-T effect would go into the phase change and loss of some of the vapor to condensate, and under adiabatic conditions all of this energy would go towards condensation and the vapor would remain at saturation temperature (wet bulb temperature it's also called), which would not change. Of course, if the gas is dry none of this would occur - but that's not to say that J-T effect should or shouldn't be ignored. As Hell says in this article, determining when to use/ignore J-T effect is more dependent on how ideal the gas behaves. It's just much more complicated when you're not talking about a dry gas.

2. I'm assuming that the T2 you are after is the dry bulb temperature since the wet bulb temperature should not change at saturation and adiabatic conditions. If it's adiabatic, then the only heat loss will be through the enthalpy lost to the water vapor that condenses. It seems like this could be found using various psychrometric charts like the one I'm looking at in Perry's Chemical Engineer's Handbook, (pp. 12-4 thru 12-10, 6th ed - what! there's even an example there, but for a slightly different direction of water evaporation into the air rather than condensation. Careful reading of those charts combined with some unit changes and mass balancing of condensed water (usually expressed as grains per pound of dry air, at least here in the confusing ol' US of A) should lead you to the downstream dry bulb temperature T2 eventually.

3. Psychrometric charts are typically developed for a very specific pressure (usually around atmospheric pressure at sea level), so some corrections must be applied to use them at different pressures - they won't work well at all if you have a significant pressure drop. If that is the case, then reading charts is off and you will have to develop your own expression to solve the problem. This might be the best way to go anyhow, if this is for a chemical engineering course of some sort.

I'm a lot of help, aren't I?

- JD