Re: thinkin of you guys...

Robin Arnold <arny@xxxxxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:

Keep in mind that it's not really gas inside the propane cynlinders
but a flammable liquid similar to petrol. The main difference being
the boiling point is lower, but both have to either boil or evaporate
to go into a gas which both require lots of heat from somewhere.

Umm. Dropping the pressure will let the propane turn back into gas,
surely?

Dropping the pressure just lowers the boiling point of the liquid
from room temperature to minus something centigrade. Once the
boiling point of the liquid is lowered, it still has to boil
though.

Well, yeah, obviously, but it's got heat energy in it, hasn't it? If
there's `plenty' of specific heat in the liquid and the latent heat of
vapourisation is `low enough', you'll boil off a lot of it pretty
quickly.

When the pressure is dropped, gas boils off the liquid
instantly at first, but that quickly lowers the temperature of the
liquid to it's boiling point.

What are the specific heat figures, then?

Imagine putting a bowl of boiling water into an oven at 200C, it
still takes a while to turn into gas (OK water takes an unusually
high amount of energy to turn into gas, but you get the idea).

OK a better example is taking the valve off a pressure cooker full
of water in an oven.

Not at all - because the specific heats of water are completely
different.

An even more exact example is taking the valve off a pressure cooker
full of petrol, in an oven, but I don't suggest trying that at home
;-)

If
you open the valve on a propane cylinder, loads of gas will come out
at first, until the bulk of the liquid in the cylinder has cooled down
to its boiling point and then gas will only come off as it slowly
simmers from the warmth of the surroundings.

Oh yes, but the idea is to get the rest of 'em to overheat and go `bang'
quicker than otherwise.

The problem is that even if it only takes a second for the liquid
to boil into gas, if the reaction takes a whole second instead of
a millisecond, you'd get a big rolling fireball, not a bang.

Well, yes, but that is what the rig was set up to do, wasn't it? I was
talking about a way to get it to do what was intended, not how to get it
to cause a large explosion. That'd be different.

You've mentioned stuff that I think really is on the right track,
but I feel abit self conscious about talking about what I think
really would make a big bang.

I've read up on fuel-air explosives recently, and it seems that they're
a bugger to get right. It's actually quite hard to make a *reliable*
big bang that way - unless you're in a position to meter exactly the
right amount of fuel into a measured volume of oxidant, sort of thing.
Which is what you do with an oxy-acetylene bomb filled from yer welding
bottles.

Anyway, even the `proper' factory made fuel-air bombs are unreliable.
There's not a lot of chance of clownish `terrorists' of the sort we've
seen lately getting it right.

Rowland.

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