Heat treating furnace design help needed

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

He
but
to
lenght
a
Gunner,
Find a temperature controlled pottery kiln. Certainly over here in the UK the bigger they are, the cheaper they are second hand. I bought a medium sized one a few years back to use for lost wax burn out, but actually have used it more for heat treatment.
AWEM
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Or try dropping in and having a chat with a local foundry. They may be happy to put a couple of dozen spears into their larger heat treat oven along with their own similar heat cycle jobs for a carton of beer for their friday arvo bar-b-q.
Your mate may have to wait a week or two to get the right heat cycle, but they would fit in right beside all the other things they carry in with a forklift. And if the time frame was not an object, they would probably give a good price.
Hope this helps, Pete
teyeu

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
GAWD Gunner, Ya forget yer med's.. starting an on topic post! <vbg>
OK OK So here's my $0.02. I'm with Andrew on this, I actually have the same thing, a lost wax burnout oven that I use for heat treaing... and yes mine is a lot smaller than what you are looking at but it IMHO a good sized kiln is what you want... It may cost more to run but will be eayser to keep the temp controlled... In truth if you didn't mind 'batching' them even a small oven like mine would do the trick.. it's about 1 cubic foot and cost me $200.
Good luck..
Dave

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Gunner wrote:

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
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:
Another thought:
Since you have a 6" dia pipe as a "muffle", why not forget about the nichrome wire and just use several of your cartridge heaters? Make little clamps to transfer heat from them to the inner pipe, use enough of 'em to comprise 2 or 3 KW and control the temp as previously mentioned. Make as many "zones" as you can scrounge controllers to drive.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@NOSPAMgoldengate.net says...

The high temperature (up to 3000F) electric muffle furnaces I'm familiar with are constructed much like this, except the elements aren't attached to the muffle. A heavily insulated cavity encloses both the elements and the center section of the muffle. Think of the muffle as being inside a furnace with its ends poking out.
Ned Simmons
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Do a search of the live steam sites, I seem to recall seeing several designs for propane burners for railroad boilers that were relatively long with a consistent flame along their length. Think barbeque burner technology, where the fuel and air are mixed, then released by a tube with slots or holes along the length. I don't remember what there was for for baffles, or how much the design of the surrounding boiler affected the burner design, but it's a place to start.
--Glenn Lyford
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Gunner wrote:

Unless there is a compelling reason to go with gas, go electric. No way you can run a gas heater and control the temps to the level you want as easy or as cheap as electricity.
This is for heat treating aluminum right?
A single coil type element on a controller, and a motorized impeller inside the furnace (motor outside, impeller inside, think convection oven) to keep the air moving and , thusly, the heat even throughout the interior.
Works for the heat treating furnace at work.
Cheers Trevor Jones
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 24 Mar 2005 22:16:07 -0700, Trevor Jones

Heat treating a stainless steel spear gun spear.
Gunner
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Gunner wrote:

Yup. It occurred to me after, that it could be a precip hardening stainless. We use a fair bit of 17/4 PH for bushings and harden it in the same oven we use for treating some biggish sheet aluminum parts.
Cheers Trevor Jones
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.