thepotter on tue 25 may 04
While doing a raku yesterday, we noticed that as the propane was used, the
line of condensation got lower but also colder. Does anyone know the
relationship between pressure , condensation, and temperature?
thank you
raku in RI
Laurie Kneppel on tue 25 may 04
Hi,
I don't know the relationship, but I do know that the liquid propane
gets colder as the tank empties out during a firing. My dad (who works
for the gas company) explained it to me once, but I can't remember the
details. As it gets colder, the pressure drops. All i know is that I
have to either spray the tank with water to warm it back up or stick it
in a big plastic tub full of water to get the pressure back up to
finish the firing when the tank is close to empty. Full tanks don't
seem to do that before I finish a raku firing. Only drawback to
spraying the tank is that during the winter the tank tends to freeze
itself to the sidewalk when it gets wet so I can't put it away right
away. And I just spray it with the garden hose, you don't need to
use hot water as long as the tap water is warmer than the tank. Be
careful not to get your pressure regulator all wet.
laurie
Sacramento, CA
On May 25, 2004, at 6:11 AM, thepotter wrote:
> Does anyone know the
> relationship between pressure , condensation, and temperature?
> thank you
> raku in RI
Craig Dunn Clark on tue 25 may 04
Hi, I'll give it a shot but may be off a bit on specifics. To begin,
think of the gas, in this case propane, in the rigid cyclinder as a SYSTEM.
It is the relationship of that system to the rigid container which will
determine the temperature of the gas. As such, temperature and pressure are
directly proportional (I think)
Next, visualize the rigid cylinder being filled with gas. A simplistic
image is that when the gas system is first introduced into the rigid
cylinder there is a good amount of space between the molecules of the gas.
Because of this there is less molecular vibration contact, therefore a lower
temperature.(I'm shaky on specifics of this one....)
As the rigid tank is gradually filled the space between the molecules of
the gas system becomes increasingly small. Because of this there is more
heat being produced due to the molecular vibration. This illustrates that as
the pressure of the gas system goes up the temperature of the gas system
also increases. This is in relationship to the rigid cyclinder that contains
the gas system. The converse of this explanation is also true. Just reverse
the explanation.
It is important to note that the fixed volume of the container is a
critical part of understanding the why's and wherefores. If there is a
larger fixed volume container and a larger volume of the gas, then the time
it takes to "bleed it down" is longer and the temp does not drop to "freeze
up" point until more gas has been used.
Keeping in mind the idea of a larger fixed volume giving one more
available gas and a longer period of time before the pressure in the tank
drops off enough to freeze up it is important to have as large a fixed
container as possible for your gas.
The typical, BBQ style tanks won't provide enough volume of gas to fire
that many times before you need to replace them. The taller, I'm not sure
how many gallon they are....they stand a little over 4ft tall, tanks are
preferable for multiple fireings. If you are doing, or considering, high
fire they are indespensable.
A solution to the problem, outside of the water bath already described
by others, is to manifold your smaller tanks together if that is all you
have or can afford. Remember, volume is the key. The greater a storage
capacity for your gas system that you have the longer and higher you will be
able to fire.
Hope this helps
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "thepotter"
To:
Sent: Tuesday, May 25, 2004 8:11 AM
Subject: raku/propane
> While doing a raku yesterday, we noticed that as the propane was used, the
> line of condensation got lower but also colder. Does anyone know the
> relationship between pressure , condensation, and temperature?
> thank you
> raku in RI
>
>
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Bruce Girrell on tue 25 may 04
> While doing a raku yesterday, we noticed that as the propane was used, the
> line of condensation got lower but also colder. Does anyone know the
> relationship between pressure , condensation, and temperature?
I don't have enough information at my fingertips to do all the necessary
conversions, but the basic idea is this: At a temperature of 70 deg F,
propane has a vapor pressure of 109 psi. In other words, if you have a
sealed container of propane that has stabilized at a temperature of 70 deg
F, the pressure inside the bottle will be 109 psi. At this pressure the
liquid phase and the vapor phase are in equilibrium. If you then open the
valve and begin to withdraw propane gas from the container, the pressure
inside the container is lowered and some of the liquid remaining in the
container will evaporate to reestablish equilibrium between the vapor phase
and the liquid phase.
Evaporation doesn't come for free, though, It requires 109 calories/gram
(sorry about mixing units) to convert the liquid into gas. A calorie is the
amount of heat required to change the temperature of one gram of water one
degree Celsius (at a specific temperature). Now, propane isn't water so we
need a conversion factor. If I am reading my web pages properly, it requires
only 0.62 calories to change the temperature of propane liquid by one degree
Celsius. This sounds about right to me, so let's use it.
If we lower the temperature of one gram of propane by one degree Celsius we
would be able to free up 0.62 calories of heat to help the evaporation
process. So to convert that one gram of liquid propane liquid to gas,
requiring 109 calories, we will need to lower the temperature of 109/0.62 or
176 grams of propane liquid by one degree. Equivalently, we can say that we
will lower the temperature of 1760 grams of liquid propane by 0.1 degrees. A
100 lb tank contains about 45 kg (45000 grams), so evaporating a gram of
liquid propane will lower the tank temperature by 0.004 deg C (176/45000).
As you can imagine, it doesn't take long to use up the gas produced by the
evaporation of a single gram of propane liquid. Based on my usage of
propane, I estimate my rate of usage at 50 grams of liquid propane per
minute. In one minute I would lower the temperature of my tank by about
50*.004 or 0.2 deg C.
Remember, though, that this is for a full 100 lb cylinder. What happens when
the liquid level is only 10 lbs? The primary source of heat for the
evaporation of the liquid still comes from the liquid itself. So now we
would be dropping the tank temperature by 2 degrees C (7 deg F) per minute!
That is why small propane cylinders or almost empty ones freeze up so
quickly.
Some heat from the air outside the tank is conducted through the metal of
the cylinder and helps to heat the liquid. Because liquid propane conducts
heat better than gas, the metal is cooled more rapidly by the liquid. Any
water in the atmosphere then tends to condense on the exterior of the
cylinder providing, at first, a good indication of the liquid level. As the
temperature drops, though, the condensation turns to frost, then a nice
thick sheath of ice. The ice slows down the transfer of heat from the air to
the tank and the propane liquid bears the burden of providing heat for the
evaporation process to continue.
As the temperature inside the tank continues to drop, so does the vapor
pressure of the propane liquid. When the propane gets cold enough it will
not evaporate as fast as you are withdrawing the gas from the tank. All you
have to do, though, is warm the tank - supply heat for the evaporation
process - and you are back in business. The liquid evaporates, the pressure
rises, and the burners roar.
Bruce "the roar of the burners, the smell of the pots" Girrell
Jocelyn McAuley on tue 25 may 04
Isn't this pervert: pv=nrt? Playing around with the Ideal Gas Law may be
fun at this point since you have a real life example of this principle
on your hands (well, not an ideal gas, but will work none the less).
P= pressure of a gas
V= its volume
T= its temerature (in Kelvin)
This is talked about pretty thoroughly in the archives.
Have fun
Jocelyn
Laurie Kneppel wrote:
> Hi,
> I don't know the relationship, but I do know that the liquid propane
--
Jocelyn McAuley
http://www.ceramicism.com
Ivor and Olive Lewis on wed 26 may 04
<and temperature?>>
Yes PV=3DRT where R is the Gas Constant and T is in Kelvin's when V
=3DVolume and P =3D pressure.But you need to be able to measure everythin=
g
!
To explain the effect you describe. Evaporating LPG takes energy. This
energy comes from the LPG and from the environment. If the environment
cannot supply heat as fast as it is taken form the LPG, then the LPG
gets cold. When the temperature falls below 0 =BA C (32=BA F or ~273
Kelvins) atmospheric water vapour condenses on the cool part of the
cylinder wall. Taking gas form the cylinder causes the fluid level to
fall, so the frosty band becomes smaller and retreats to the base of
the cylinder.
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
thepotter on wed 26 may 04
thanks so much for your responses and help. I am beginning to understand
the hyroglyphics. Thanks. Raku in RI.
Glenn Allenspach on wed 26 may 04
Raku in RI:
What you saw was the principal of refrigeration in action.
The relationship between pressure and temperature is,
as pressure in creases, so does temperature,
and as pressure decreases, so does temperature.
When firing from a propane tank, you are removing
vapor from the tank. When you remove the vapor faster than
the tank can regenerate vapor, the temperature
inside the tank drops.
When the temperature inside the tank drops, so does the
surface temperature of the tank, and when
the surface temperature drops below the
dewpoint of the surrounding air, you
get condensation on the surface of the tank.
Continue this long enough, and the condensation
may become frost or ice.
Eventually, the self-refrigeration can cause the tank to "bog down,"
or to fail to produce enough volume of vapor to continue
firing at the earlier established rate.
Hope this helps,
On the verge of "Up North"
Glenn Allenspach
East Bethel, MN
GlennAllenspach@aol.com
651.779.8470
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