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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