Propane EFI - part duece

[Greg Hermann]

>> 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.
>
Franc--

While what you have said follows the gas laws properly, things get HIGHLY
non-linear around the critical point for any given liquid-gas system. Which
is another way of saying that the gas laws won't tell you much about what
you have. At the critical point, the specific volume of the liquid and the
gas ARE identical!

The place where most of the work with super critical fluids has been done
is in super critical steam power generating plants. The TRICKIEST part of
said plants is boiler water level control. This type of plant doesn't even
HAVE a steam drum, per se!

I don't have the books right where I can pick them up right now, but later
on I will try to find the time to post the specific volume --and other--
data for propane at its critical point. For now, suffice to say that the
critical pressure for propane is nowhere NEAR 227 bars! The source book
will be "Thermodynamic Properties of Refrigerants", as published by ASHRAE.
Propane is a _VERY_ good refrigerant, thermodynamically speaking. The only
real problem with it is its high flammability. It is the refrigerant of
choice in most refineries--where its flammability is not of particular
concern, since everything in the place burns anyway!

Regards, Greg


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