Re: science project help: batteries and heating element
From: Jonathan Kirwan (jkirwan_at_easystreet.com)
Date: 02/08/05
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Date: Tue, 08 Feb 2005 08:29:14 GMT
On 7 Feb 2005 23:27:44 -0800, maxslomoff@aol.com wrote:
>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.
Oh, my. Sounds like a gag or a magic trick. Blowing into the end of an
innocuous device that makes it look as though you are just blowing air and
instead you are toasting some paper.
By the way, paper is fairly dry and your breath will probably be a lot moister.
What I'd now like to know is what does it take to get paper to brown and what
does it take to hit the "flash point" before, during and after browning. My
hunch is that there isn't a lot of margin to work with, safely. One advantage
that comes to mind here is that breathing out means that the air won't have as
much oxygen in it for the side of the paper on the receiving end of the flow.
That may help the flash-point margin some. Then there is the fair moisture
content and possibly mucous particles and bacteria, etc. You plan to keep this
clean?
A calculating approach to take is to think this way:
* Less than 2 liters of heated air blowing across a specified area of paper,
must be sufficient to brown it, as desired.
* Time for those 2 liters (or less) to be blown will be about ... ? ... 6 or 7
seconds. Volumetric air flow is thus (assuming a typical volume of about 0.8
liters) something like 150 cc/s.
* How large of an area of paper must be browned?
* How far away will the paper be held? (The further the air has to travel after
being heated, the more you'll lose in turbulence and mixing before it reaches
the paper.)
* Can the air flow be focused through the use of some kind of mechanical
"lensing" to a known distance ahead? (I don't know.)
* What is the rate at which heat will diffuse away through convection outward
from the heated spot through the paper and also to the air behind the paper?
* What temperature is required in the air used in order to compensate for the
losses and still heat the paper at the required rate?
But I don't think all that is needed. Forget theory and just go empirical.
Probably the easiest way for you to find out what works will be to arrange some
nichrome wire to cover the near-exit aperture of your tubing, set up an AC
desktop-type powered DC supply that will allow you to set the current used to
heat the nichrome, and then start running some tests with different current
settings and trying out your abilities to heat the paper adequately. One you
figure out the necessary heating, you can write down the current setting and the
voltage used across the nichrome and the product of these will tell you the
wattage. That will then tell you what kind of battery arrangement you'll need
and how long it will last.
Another possibility might be to consider using a halogen flash lamp with a
parabolic or (more likely) a spherical reflector. You might be able to brown
paper in a single flash pulse from one. Forget the air flow, entirely.
Another possibility might be to use myriad tiny ceramic capillaries (perhaps
only 1 cm long, each) stacked into a honeycombed grid. A simple chemical flame
(butane, for example) would jet from a tiny tube behind the grid, ignited by a
piezo-igniter, and would be immediately diffused into an even heat by the
capillaries, so that when the flow exited it would be very uniform without hot
spots in it. Chemical energy in hydrocarbons is very dense and probably a lot
better than some battery would be. However, I'm not entirely sure how to
arrange this with using a breath blown through to automatically adjust the tiny
flame to correspond with the flow rate -- some kind of venturi?
Jon
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