Propane EFI - part duece

[Franc Buxton]

> >
		(...stuff deleted)
> >  I wrote:
> >       (I believe 100 deg C is what is called the 'critical temperature' for
> >propane, above which it will _not_ remain liquid under any
> >circumstances, and
> >fairly suddenly begins to obey the ideal gas laws, resulting in a
> >_huge_ rise
> >in pressure - enough to rupture the tank if it's not _very_ well
> >protected
> 
	And Greg Hermann wrote:

> You are right that there is a "critical" pressure, and about right about
> where it is for propane--BUT-- at the critical pressure, it simply turns to
> gas. NO huge pressure rise once it is there. Not even any change in volume.
> 
> Nor is there any latent heat of vaporization at the critical point.
> 
> The thing that breaks tanks is the thermal expansion of the liquid in tanks
> that are filled too full of liquid.
> 
> Greg (worked with design of propane and ammonia refrigeration systems)
> 

	Hmm - I will always bow to a greater experience, Greg, and I mean
no offence, but please could you (or anyone) explain where these calculations
go wrong:

	1 mole of a gas (6.02 * 10^23 molecules according to Avogadro)
occupies 22.4 litres at Standard Temperature and Pressure (1 bar and either
20 deg.C or 0 deg.C, I can't remember)

	1 mole of a gas weighs its molecular weight in grams, so for propane:

	C3H8 = (3*12) + (8*1) = 44g

	22.4 litres is very close to the volume of a 10Kg propane cylinder
(Density of liquid propane = approx 0.5 Kg/L, plus 20% = 24 litres), so we can
estimate that 1 mole (44 grams) of propane gas will occupy that cylinder at
approx. 1 bar pressure. But there are 10 _kilograms_ of propane in there, so
according to the gas laws, the pressure will be:

	10000/44 = 227 bar (!). (Plus a bit for temp. difference from
Standard Temp.)

	That seems like rather a lot to me, in a cylinder tested and certified
to I think somewhere around about 30 bar. Having said all that, I don't have
any info about the vapour pressure of liquid propane at 90 deg.C to make a
comparison, but even a fraction of 227 bar sounds pretty dodgy! The system
temperature would have to be controlled significantly above maximum ambient
(which let's say would be 50 deg. C, just to make sure), and would need to 
be _failsafe_, i.e. proof against failure of temperature control, etc.

	Am I barking up the wrong tree, or is this just slightly scary?

	Just an enquiring mind - I have never done the above calculation
before, merely accepted what I had been told...

	Regards again, 
		Franc.

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