[TheForge] Building a propane forge

[John Husvar]

Bruce Freeman wrote:
> Mikey,
>  
> I object to the term "scolder."  I am not scolding you.  I am merely
> correcting a misstatement you made.  I don't question your practical
> knowledge of your subject otherwise.
>  
> I don't object in the least to the use of the term "vapor."  What you
> don't seem to realize it that it is a synonym for "gas."  There is no
> difference.  I am aware that some folks choose to use "vapor" to mean a
> gas evaporating from a liquid.  However, the distinction is no more
> meaningful than the term "permanent gas," which applies to nitrogen,
> oxygen, and many other gases.  Those gases can be, and frequently are,
> condensed into liquids.  If we reevaporate them, are they then not
> "vapors"?   Such distinctions are meaningless.
>  
> Now, the term "gas" was coined some centuries ago from the term "chaos"
> (or its Latin precursor, no doubt).  It described a phase of matter in
> which there was no interaction between the molecules (as there is in
> solids and liquids).  Strictly speaking there is some interaction
> between gas molecules, but it is of a much lower magnitude than in
> solids and liquids and doesn't become important until the gas density
> has increased substantially, usually by compression to hundreds or
> thousands of psi. 
>  
> What you are suggesting is a phase of matter ("vapor") in which there
> is some stronger interaction between molecules than for a gas, but which
> is not a liquid.  As I mentioned before, there is such a phase: 
> supercritical liquid.  However, this phase is well characterized. 
> According to the weldingsupply.net (the first website I found that
> listed it), the critical point of propane is about 200 F and 600 psig. 
> For the propane to be supercritical, you have to equal or exceed both of
> those values.  You are not going to exceed them with propane passively
> drawn from a normal cylinder.  (To exceed the critical point, you'd have
> to  tap the liquid propane  from the bottom of the cylinder and pump it
> at high pressure into a heated device.  I could easily do this in the
> lab but don't try it at home.)
>  
> So I reiterate:  There is no separate "vapor" phase.  Vapor = gas.  Gas
> = vapor.
> 
> I do not doubt that it takes signifcant energy to disperse propane in
> air, but this is simply a reflection of the gas density, not any sort of
> clumping, as you describe it.  In my limited experience, the pressure
> from a propane cylinder provides plenty of energy for this dispersion. 
> 
> 
> I have not the experience with propane to know the cause of the pulsing
> you describe, but I would speculate that it is equivalent to the
> "bumping" of hot water, known to all Chem 1A students:  If you heat a
> beaker of water to boiling, superheating sometimes occurs at the bottom
> of the container, and "bumping"occurs.  "Bumpint is the spontaneous
> conversion of a small amount of superheated water to gas, accompanied
> with a "bump" sound.  (It is dangerous because it is violent and
> splatters hot water.)  
>  
> Bumping can occur in any liquid heated from below the surface.  In the
> case of propane, bumping could carry liquid into the valve and
> regulator.  This would cause an effect similar to that seen when working
> with a Bernzomatic type hand-held torch bottle, when you accidently tip
> the bottle too far.  (I.e,  Liquid spits through the orifice, either
> extinguishing the flame, or cutting air aspiration to near zero,
> resulting in a large, typically yellow, candle-like flame.)
> 
> If my speculation is correct, there would be a relationship between
> "freezing up" and pulsing , in that "freezing up" (a misnomer, since the
> only freezing will be of water vapor on the outside of the tank) is when
> the propane drops below its boiling point at the pressure of the
> cylinder.  That is the temperature (range) at which bumping could occur.
>  
> 
> (Note to the scientifically bewildered:  The boiling point of any
> liquid increases with increasing pressure.  That's why water boils at a
> lower temperature in Denver than in NYC.  I could explain this further
> if anyone is really interested...)  
>  
> Bruce
> NJ

Well, darn. I just posted a kind of explanation my propane suppliers 
gave me that parallels Mikey's. They equated it with gasoline through a 
carburetor, saying it doesn't really become a molecular gas, but remains 
in larger particles they called vapor.

Gasoline through a carburetor never becomes a gas before it burns. It 
remains a liquid, but in very finely divided particles. That condition 
is often called vaporous, not gaseous. Hmmmm.

Well, one thing for sure: We'll all be having "the vapors" over this for 
a while. :)


-- 
Life is not a journey to the grave with the intention of
arriving safely in one pretty and well-preserved piece.
One should rather skid in broadside, thoroughly used up,
totally worn out, and loudly proclaiming -- "WOW!  WHAT A RIDE!"