I am building a compact air heater as a hobby project where the air is
blown between fine fins of several aluminum heat sinks. The
temperature needs to be raised from ~0C to ~37C and needs to handle
air flows of up to ~100 L/min. I did an envelope calculation taking
into account heat capacity of dry air, and it appears that I would
need ~100W, 300W to be safe, for that.
So I want to run a nichrome wire in a spiral pattern along the surface
of the aluminum heat sinks. But wouldn't that cause a short? Can I get
insulated nichrome which would conduct heat, but not conduct
electricity? Any suggestions?
For very little cash outlay, you can go buy a hair dryer that has the
fan and a (presumably) safe heating element, all in one compact case.
I'd guess the fans in those are capable of providing your desired 100
L/min airflow.
If you INSIST on using a heatsink, why not just mount two or three
cheap power semiconductors to it and use them to generate the heat?
This is done in some crystal ovens. Or get some of the power
resistors designed to mount to heat spreaders. Caddock and others
make them in cases similar to power semiconductors, or you can find
ones in aluminum cases with a flat side and mounting holes that bolt
easily to a heatsink. Another alternative would be something like a
hot water tank heater of the sort that's a metal tube bent in a tight
"U" shape with terminals at one end. The tube is already insulated
from the heating element. They are typically rated a couple kilowatts
or more, but that's in water for cooling; if you control the power so
the case doesn't get too hot you should be OK. You're still left with
the problem of getting it to make good thermal contact with the heat
sink. -- But as others have pointed out, you can make nichrome wire
hot enough that it will have no trouble getting rid of the heat you
want just into the forced airstream, as is done in a hair dryer (or,
alternatively, a heat gun; Harbor Freight has one on sale at the
moment for under $10).
Or, go get one or two tubular ceramic-core 200W resistors and arrange
a fan to blow through their open centers. Since they are designed to
dissipate 200 watts in free air, you should be able to use just one to
get the effect you want, keeping it a bit cooler by blowing air
through it.
In short, don't mess with trying to use nichrome wire directly; use a
heating source someone else has already done for you. It will be
cheaper and much safer in the long run. (Though there are still
plenty of ways you can make it un-safe, of course.)
Cheers,
Tom