On Aug 22, 9:16 pm, Paul wrote:
Jürgen Exner wrote:
wrote:

Calculating the BTU output of a torch, or single hole orifice.

Why this complicated approach instead of just a simple empirical test:
Weigh the cylinder, burn the torch for let's say 15 minutes, weigh the
cylinder again, and multiply by 4 to get the hourly consumption.
Multiply by 12,874kWh/kg (the heating value of Propane), convert into your
BTU, and you are all set.

jue

Yes, heating values for various gases are readily available from tables.
The problem I see with any method you use is the assumption that you
have 100% propane, you don't, unless you're buying 'research grade' or
'chemical grade' propane. Common 'propane', as supplied for home
heating, can contain a significant amount of butane depending on source,
season, and location. There will also be a possibly significant amount
of the other light hydrocarbons both lighter and heavier than propane.
Propane as purchased in small cylinders for torches for sure has other
gases in it to some degree or another, but it could be purer than that
sold for heating.

If your calculated values need to be exact an analysis of the fuel in
question is needed. Then knowing the fractions and BTU content of the
various components the overall BTU content can be calculated. This may
or may not be overkill for your application.

As an aside fuel gases vary widely in composition and to compare blends
to insure comparable performance in appliances something called the
Wobbe Index or WI of the blend in question is determined. The Wobbe
Index of a particular gas is the BTU content divided by the square root
of the specific gravity. Blends with similar WI's will behave the same
at a common pressure and flow regardless of chemical composition.

There are two common methods to determine WI, the simplest is the
calorimeter, a device that measures the output of a thermocouple
suspended above a flame of the fuel in question supplied at a precisely
controlled pressure and temperature. Most calorimeters also have a cell
to measure the fuel's specific gravity to supply the other variable in
the equation. The other method is via gas chromatography to determine
the actual makeup of the fuel in question. The fuel's overall BTU
content and specific gravity is then calculated using the component
gases' BTU values and specific gravities supplied as constants. Both
these methods can be done 'online' more or less continuously.

The above is probably more than you want to know but you might find it
interesting. For more on the subject:

http://en.wikipedia.org/wiki/Wobbe_index

Regards
Paul
--
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It's a Linux world....well, it oughta be.
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Well, Yes if you are trying to get exact figures there are always real
world wrinkles in any calculation. If though, you are building a
burner or other device and know that you need something to base your
design on you can use this calculator to estimate the requirements
needed to achieve the necessary btu with given fuel, pressure, and use
it to determine your hole size and air intake size needed...

Obviously any calculator will assume perfect conditions, i.e. pure
propane, i.e. wood gas = CO (actually is a mixture of CO, H2, various
other chemicals, etc... ) but the point is that just like a tape
measure, or a building plan this gives you something to start from.

It's kind of hard to do an empirical test, when you haven't yet built
your burner. How many prototypes do you want to go through before you
get it right.... ???

It is common practice to model an assembly on computer before actually
putting together a project. This increases production and reduces
development cost.

"Weigh the cylinder, burn the torch for let's say 15 minutes, weigh
the
cylinder again, and multiply by 4 to get the hourly consumption.
Multiply by 12,874kWh/kg (the heating value of Propane), convert into your
BTU, and you are all set."

That sure sounds like a lot of steps compared to simply entering some
figures into a calculator and having it spit out a close number...
though it would be a great way to verify your build, it certainly cant
be done until you have finished your first prototype. Not to mention
the waste of fuel and time compared to using the calculator.

Another thing is that your figures will not be 100 percent accurate
either, because of the mix of propane that you have, errors in timing,
weighing errors and other factors that include human error, this
method is probably not intrinsically any more accurate than estimating
it with a handy calculator...

Best regards,

Chris Frostic
www.frostic.com