Propane EFI - part duece

[Gerald Pelnar]

> Date: Wed, 05 Jul 2000 15:34:32 +0100
> From: Franc Buxton <fab at dcs.warwick.ac.uk>
> Subject: Re: Propane EFI - part duece
> 
> > >
> 		(...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.)

The problem with the calulation is the condensation of the
gas.
Once the gas becomes saturated and condenses, it no longer
maintains ideal gas properties, It changes to follow
properties
of saturation. You mention maintaining propane at 50 deg C.
At this temp vapor pressure is about 230 psig. (16.6 Bar?).

> 
> 	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?

Yep and Yep.
I personally find the prospect of heating propane very
distasteful.  :)
Which brings me to my question. I have read and re-read
this thread.
I'm a little confused. I thought the idea was liquid
propane injection.
If this is true, why the concern with critical temperature.
Critical 
temperature is the point at which any increase in pressure
of a
gas will not cause condensation. Hmmm... no condensation,
no 
liquid to inject. I know what ever I missed has got to be
simple,
but you're going to have to help me out.

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

thanks,
Gerald Pelnar
wd0fyf at mpks.net
http://www.mpks.net/homepage/wd0fyf/
McPherson, KS

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