thepotter on wed 26 may 04
Thank you again for your responses.
Is it correct to assume:
1. Propane is liquid and gas under pressure
2. It requires energy to convert the liquid to gas
3. The less gas, the less pressure and the lower the temp.
4. As the propane is used, it is colder because it requires (more?)energy
to convert to gas but also as it goes down, you are using the gas
faster than it can convert from the liquid which causes the sputtering and
sometimes stalling.
5. This can be corrected by immersing the container in a bath of warm
water.
The water will equallize the temperature on the inside and outside and
facilitate
the conversion from liquid to gas w/o icing up and stalling.
I hope that this is correct. It certainly makes more sense anyway.
Please correct me if it is not. Thanks again.
Raku in RI
Steve Slatin on wed 26 may 04
This is one of those conceptual things I like to stay out of. (I once got in a flame war
of several week's duration with a total stranger over how refrigerators work.)
The molecules we call gasses are, at standard temperature and pressure, gasses.
(Duh!) It takes energy to force them together hard enough for them to become
liquids. Consequently, when you fill a propane is gasified, it gets hot. (Energy is neither
created nor destroyed, so the energy from the pump shows up as heat in the tank.)
The reason why you don't see much heat energy transfer when you fill a tank from a
bigger tank is the gas is compressed and in liquid form on both sides of the exchange,
hence there is little thermal effect.
As the liquified gas sits in your truck or your backyard or whatever, it cools or heats
up to ambient temperature. When you open the valve, molecules jump out into
a gasified state, releasing that stored up pressure-energy. (If ignited the gas also
releases THAT energy, which is a separate energy transaction.) The remaining volume
of liquid (compressed) gas in the tank suffers a corresponding loss in heat-energy
-- again, energy is neither created nor destroyed -- equal to the BTU energy of the
lost pressure.
So it's not the quantity of the gas that controls the pressure or the temperature, but
the quantity released from liquified to gaseous state that controls the BTU drop. The
reason why potters often have to get really big containers for their propane is they use it so quickly that the temperature loss is sufficient to freeze what remains -- the extra LPG in a big container is just there to absorb the heat loss so the remainder doesn't freeze.
thepotter wrote:
Thank you again for your responses.
Is it correct to assume:
1. Propane is liquid and gas under pressure
2. It requires energy to convert the liquid to gas
3. The less gas, the less pressure and the lower the temp.
-- Steve Slatin -- Entry-level potter, journeyman loafer, master obfuscator
No website, no sales room, no scheduled hours
All talk, no action
Sequim, Washington, USA
48.0937°N, 123.1465°W or thereabouts
---------------------------------
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Bruce Girrell on wed 26 may 04
> Is it correct to assume:
> 1. Propane is liquid and gas under pressure
Yes, when confined as it is in a propane tank.
> 2. It requires energy to convert the liquid to gas
Yes, it requires heat energy.
> 3. The less gas, the less pressure and the lower the temp.
You cannot assume that. I could have a 100 lb capacity tank that has only 5
lb of propane remaining. If the tank has stabilized at 70 deg F, the
internal pressure will still be 109 psi, just like a full tank that has
stabilized at 70 deg F.
Where the remaining quantity of gas makes a difference regarding temperature
is that with a small amount of liquid remaining in the bottle, the
temperature of the liquid will drop very rapidly as gas is withdrawn. The
liquid propane is the primary heat source for conversion of liquid propane
to gaseous propane.
> 4. As the propane is used, it is colder because it requires (more?)energy
> to convert to gas
The amount of energy required to convert liquid propane to gas depends only
on the amount of gas being withdrawn. If you stop withdrawing gas the liquid
will stop evaporating and therefore requires no more energy.
> but also as it goes down, you are using the gas
> faster than it can convert from the liquid which causes the sputtering and
> sometimes stalling.
A regulator can supply the desired (setpoint) pressure only if the pressure
on the upstream (tank) side is greater than the setpoint, or downstream
pressure. When the temperature of the liquid propane falls low enough, the
vapor pressure of the liquid will fall below the set point of the regulator.
The propane runs short of available energy to convert liquid to gas, so the
pressure in the tank drops below the setpoint and the regulator can no
longer supply gas at the setpoint pressure. The result is that the flame
begins to die.
> 5. This can be corrected by immersing the container in a bath of warm
> water.
It doesn't even have to be warm. Any liquid water will do. Warm water simply
has more calories available and it will accomplish the job quicker. The heat
from the water is conducted through the metal of the tank and warms the
liquid propane, which can then evaporate.
> The water will equallize the temperature on the inside and outside and
> facilitate the conversion from liquid to gas w/o icing up and stalling.
The water doesn't necessarily equalize the temperature on the inside and
outside, but it certainly does supply sufficient heat to keep the liquid
propane evaporating.
Bruce Girrell
in (finally!!!!!) sunny northern Michigan
where the lilacs are in bloom
Trying not to be too technical, but still accurate.
thepotter on thu 27 may 04
thank you to Bruce and Steve for the patient explanation of propane.
Obviously, I missed that day of Chemistry class and I appreciate your
efforts. Thanks. Raku in RI.
Glenn Allenspach on fri 28 may 04
Seems more or less correct.
Regarding Point #5, you
can also solve your stalling
by using a larger propane bottle,
or by connecting two or more bottles
together using pigtails and tees.
Your friendly dealer of propane
and propane accessories could
set you up.
Then, you don't have to mess
with hot water in cold weather!
On the verge of "Up North"
Glenn Allenspach
East Bethel, MN
GlennAllenspach@aol.com
651.779.8470
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