On Thu, 24 Mar 2005 08:19:50 GMT, Gunner
wrote:

A young entrepreneur friend of mine makes spear guns, spears, etc. He
has been getting his spears heat treated at a commercial treater, but
the costs are eating him up.

Here is the requirements... a stack of 6 foot long spear blanks has to
be kept at 925F for 2 hours. They are about .30 in diameter.

Pretty simple.

Now the problem...making an oven 6+ feet long that will reach that
temp fairly uniformly over the entire length.

Propane is probably the cheapest way, or even city gas.


We were thinking of a 7' length of 6" Sched 40, slid into a 7' lenght
of 12" Sched 40, using either ribs welded to the outside of the 6"
pipe to keep it centered, or something similar.

Now the big issue...how to make a burner(s) that will uniformly heat
the inner pipe.

I recall seeing a set up many years ago, that somehow managed to get a
spiral of flame down the inside of the outer pipe that actually
wrapped around the inner pipe. It was fascinating to see. But we
would be happy to get any sort of unifiorm flame...G

Anyone with any hints, links ideas on how to accomplish this task?

I do have a 350' roll of .065 x .125 nickle flat resistance
ribbon..but think it would be a bitch to make this thing run on
electricity, and expensive to operate.

Help!

Gunner, actually starting a thread on-topic G

Electric would be a lot easier to control and to get even temp.
Electricity is indeed more expensive energy, but perhaps not
prohibitively so here.

I figure 3 inches of insulation, perhaps Kaowool for the first hot
inch and glass wool for the balance, would have an R factor of about
10. That's based on 2 inches of glass wool being R 6.7 according to
the package.

From that, I figure a bit less than 500 watts would hold your oven
at 950F, using mean diameter of 9" (6 inner, 12 outer) and 7 ft
length.

If you had a kilowatt of heater, it would take about 7 minutes to heat
up the inner pipe to 950F. (I used 1/4 wall thickness as a guess)
The mass of the spear blanks would add to the heatup time, of course.

I don't know what electricity costs in LA, but at 10 cents/ KWH, your
talking well under $1 worth of juice to heat up a load to 950F and
hold it there for 2 hours. You might want to make the heater 2 KW
because it's easy to throttle electric heat. A 220-volt 20-amp solid
state relay (good for 5 KW) costs about $25 brand new. PID
controllers are on EBay for $100 or less most of the time, if you
don't have a barrel of them already. You might segment your heater,
maybe two 500 watt end sections and a 1000 watt middle section, each
with its own controller and thermocouple, for most even temperature
along the length. You probably already have a bunch of type K
thermocouples. (Type K is usually red-yellow, standard connectors are
yellow) If you don't, TC wire is cheap and couples are easy to
make.

Your nichrome ribbon is about .067859 ohms per foot. I would run it
at about 20 amps max, which would give it a temp of about 1400F at
full current. You can always throttle. That's about 80 feet of wire
and about 2200 watts of heat at 110 volts.