thanks chris for your advice. you too jon.
a marine batery would be too big for the device i would like to build.
i want to build a portable device, no bigger than a nerf football. i
thought making a series of 4 C batteries would allow for the ideal
size, but i got tripped up with my equations.
so there is a line voltage restriction. and that restriction is what
ever we can fit into a small set of rechargeable batteries. 4D ? 6D? a
special order battery? (is there any place to do this?)
no fan is needed. a person will blow into one end of the football,
through a heated chamber at the end and want to be able to brown a
piece of paper on the other side, but not ignite it. the paper would be
held very close to the end of the heat chamber. room temperature air.
could be fluctuating, as long as its hot enough to dry out paper. does
that satisfy your questions jon about the air temperature?
so chris, is a nichrome wire a heating element? any idea where i can
get a wire that draws low current but puts out the most heat? would
soldering the wire's together work? i've never welded something so
small.
thanks again for the dialogue, i hope i answered your questions well.
max
Hi, Max. Nichrome wire (also called resistance wire) is the stuff used
in electric heating elements for space heaters, toasters, hair dryers,
etc. The nickel-chromium alloy has a reasonable bulk resistivity, as
well as being resistant to oxidation at high temperatures. Also, the
bulk resistivity is somewhat constant over temperature range (usually
+/-10 to 20% from cool to red hot). These qualities make it suitable
for heating applications. The least expensive source for nichrome wire
is discarded toasters, space heaters, hair dryers, and power resistors.
Nichrome wire can also be purchased from a number of sources.
Since nichrome wire glows red hot when heated, solder isn't a good
idea. It will melt. Also, nichrome wire doesn't "wet". Your solder
will just sit on top of the wire like a blob of glue, and will open up
the first time its heated. Resistance wire is typically welded to a
hi-temp electrical connector in order to get a good connection (even
"gas-tight" electrical connections suffer when subjected to extreme
thermal expansion/contraction cycles).
Before you get too far here, I think you should do some science. Look
at the thermal mass of air, determine what kind of power source you
require to heat two liters of air to exactly what temperature (paper
eventually burns at 451 degrees, but you'll need air much hotter than
that to cause instantaneous browning of a sheet of paper). Also
remember that exhaled breath can be assumed to have 100% humidity,
which will drastically increase the heating requirement. Experiment by
following Mr. Kirwan's advice, and set up a heating element with a
bench power supply and try to get what you want. You'll probably go
through several iterations on your heater setup. This will give you a
handle on your power requirements. You'll have to look at finding a
method to ensure the safety of the person who's blowing through the
football (i.e. not burning their lips off).
Once you're there (these are not insurmountable problems), you can then
take a look at your power source. Just as a WAG, you're going to need
at least several hundred watts to your heaters for those five seconds
or so. There is no way 4 or 6 "C" or "D" batteries can provide that,
much less rechargeables. If I absolutely had to take a stab at doing
this job in this way, and money was no object, I might consider a bank
of Polapulse batteries (Polaroid P100). Each one can reliably provide
5 amps at 6V for at least 12 seconds. If you need only 300 watts
(almost certainly you'll require a lot more than that), you should be
able to fit 10 of these in easily. They're small and can provide high
currents for limited times, but they're fairly expensive (and you would
have to replace the bank of batteries after every other use). But it's
theoretically possible you might be able to fit the whole thing into a
spheroid the size of a nerf football. If you even consider this route,
be ABSOLUTELY sure you thermally insulate the batteries as well as your
volunteer's lips from heat (the MSDS shows possibility of hazardous
gases if they burn, and you're looking at a high probability of
inhaling here), or have your volunteer quarterback stop the clock by
hurling the football out of bounds immediately after use.
Polaroid data sheet on P100 batteries:
http://www.polaroid.com/global/prin...4488338439&bmUID=1107857297463&bmLocale=en_US
MSDS on Polapulse batteries:
http://www.polaroid.com/service/msds/m_0658.pdf
Summary: Hot idea, but severe practical problems with power
requirements. Safety issues that cannot be ignored. Cost a major
constraint. Seriously consider either abandoning project or
drastically revising specifications (Mr. Kirwan's butane idea above
looks to me to be a far more practical way to accomplish this).
Good luck, and be careful
Chris