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

Lathe Dementia. Recognized as one of the major sub-strains of the
all-consuming virus, Packratitis. Usual symptoms easily recognized
and normally is contracted for life. Can be very contagious.
michael

1. You need to keep the flame away from the work? It'll cost you in
gas. Just lining the big pipe with Kaowool or refractory cement and
running the flame inside may work better (but the surface will be more
fragile).

2. Getting the flame to spiral should be easy, but getting even heat
over that length from one flame would be a bitch.

3. I would be way tempted to have N burners and N controllers, all run
from one master scheduler. Experiment with how much length will give
you even heat to figure out N. Use one temperature sensor per burner,
placed where that burner's flame has the most effect. Not only can you
insure even heat, but you may even be able to play tricks with
non-uniform heating along the length.

4. Make sure that he estimates his costs (including lost revenue and
lost family time) to build the furnace. Then he should calculate the
payments he'd make on a 4-year loan to borrow that much money. Then he
should compare the payments to what he's paying now for heat treating.

4a. Then he should build the damn thing anyway!

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com