Ask A Scientist© General Science Archive

 >    name         Mike
 >    status       other
 >    age          40s

 >    Question -   I am comparing fuel gasses, with the intent of issuing a
 > corporate policy statement precluding the use of accetylene for "cutting"
 > purposes by our (mostly) untrained work crews.  We now believe propane,
 > propylene, or "MAPP" gass (a proprietary mixture of Methyl Acetylene,
 > Propane, and  Propadiene), or other comercially developed proprietary
fuels
 > based largely on propane.
 >
 >A concern:  We have been warned a cylinder of propane, or similar
 >"alternative fuel gas," will be susceptible to evaporative cooling, to the
 >point that in cold weather, the supply of fuel gas vapor will drop off,
 >making the fuel impractical.  Our crews DO work outdoors, even in winter
 >weather running down to about 10 degrees F.
 >
 >My questions:
 >1.  I have looked in vain for any reference that will describe (table or
 >graphically) the relationship between temperature and vapor pressure for
 >these various gasses.  At what temperature does the cylinder vapor
 >pressure drop so low as to be a problem?  (< 5psi)
 >
 >2.  How can I compare the rate at which these fuels will cool their source
 >cylinder, if they are each vented at a rate typical of a cutting torch use.
 >
 >Can you direct me to a publication that might provide this info?  [Or, is
 >this all hogwash pitched by the disgruntled accetylene salesman?]

Well, there's a grain of truth to your concerns, but I don't think it's
going to be amajor problem.  I suspect your acetylene salesman is thinking
about butane or propane/butane mixtures.  Butane liquefies at atmospheric
pressure at about 33 degrees F, which makes it impractical for use at low
temperatures.

I don't have information on the vapor pressures of the MAPP gases or on
their heats of vaporization and heat capacities.  Those numbers are what
you'd need to calculate the cooling effect of drawing off the fuel gas.  You
might ba able to get some of the numbers from the Matheson Gas Data Book,
and others from published thermochemical tables.  You would need to find a
chemical engineer to get the data and run the rather involved calculations.


However, I don't think that evaporative cooling will be too much of a
concern for cutting use;  cutting torches use the fuel gas only
intermittently, to heat up the metal until the oxygen jet can take over to
do the actual cutting.  Propane boils at about -44 degrees F;  as long as
the tank is a few degrees warmer than this, you should have enough pressure
to work.  The other constituents of MAPP boil at slightly higer
temperatures:  methylacetylene at -10 degrees F, and propadiene at -30
degrees F.  These are all below your practical cutoff of 0 F.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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Unfortunately for you, it is not hog wash, but a fundamental principle of
thermodynamics. The ideal gas law will dictate pressure, temperature, and
volumes dependant on one another.  Although the gases you use may not be
completely ideal, the ideal gas law gives a very close rough estimate.  The
ideal gas law is PV=nRT where P is pressure, V is volume, n is the number of
moles of gas (i.e. the amount of gas present), R is the universal gas
constant, and T is the temperature.  Because you have a combination of
gasses, n is dependant on how much of each different gas you have.  Without
getting into specific numbers, lets look at the relationship between P, V,
and T.  Assuming no gasses leave the cylinder (i.e. it's sitting in the
truck waiting to be used) and the temperature goes down, either the pressure
or the volume (or both) have to go down as well.  Because it is a steel
cylinder, let us assume that the volume is constant over the temperature
range.  So, if the temperature goes down, the pressure must go down in order
for the two sides of the equation to still be equal.  Likewise, if the
temperature rises, the pressure will rise, so watch out on those hot days as
well.  As you begin to use the cylinder you decrease the amount of gas in
the cylinder (i.e. n goes down).  Once again as you change one side of the
equation, you must change the other to maintain equality.  At a constant
temperature and volume, if you decrease n, you decrease P.  As you
mentioned, as you empty the cylinder, there is a cooling effect that also
tips the balance of the equation that will need to be compensated by a
change in P.
With the proper properties of the gases and the known flow rates of the
gases, you can get a good idea what is the minimum temperature you can use
the cylinders.  I would somewhat trust the cylinder salesman with the
information he gives you because his company has been doing this for a while
(but always question authority ;-)).  I would imagine they have an engineer
back at the company headquarters that good give you good calculations on
what you want to know.  They are the most likely source of information on
the gas properties.  Otherwise, you might can use a CRC Chemistry Handbook
for reference.  You may be able to find it in the reference section of your
local library.
I hope this helped.
Dr. C. Murphy

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