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  #51  
Old 01-19-2010, 01:15 PM
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I really like the cleanout on moly's but not for cleaning it out. It's a great place to stuff an Exact torch into the furnace when you are bringing the pot up to temp and Moly's are really bad for heat stratification from top to bottom. I measured the furnace in Eugene which had a 34" and it was 1100F at the lip while it was 500 F at the foot. Mine gives the same readings and that is very hard on the pot to be kind about it. The cleanout offers the only way to balance the temps short of dropping elements way below glass line.
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  #52  
Old 01-20-2010, 07:51 PM
Marcel Braun Marcel Braun is offline
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elements below glass line?

Why is this such a taboo in design?

Is it because they would shock the pot from being too close/ reguire too much cubic footage to be far enough away?

Conversely.... the area above the pot is necessary for some reason and to have the extra space (for the elements) completely above the glass line is less total than the extra girth required to be next to the pot plus the required space above the glass line?

Last edited by Marcel Braun; 01-21-2010 at 12:44 PM. Reason: Clarify third sentence
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  #53  
Old 01-21-2010, 06:27 AM
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The current approach with moly's has been to keep the chamber as tight as possible so the pots almost hit the walls. Passing the elements down towards the floor makes that space too small. The elements should ideally have about two inches clearance from the pot.

I have read your third sentence three times and do not understand it at all.

The area above glass line is pretty much dictated by the size of the door opening. That casting needs some structure above the opening or it will collapse.

This is really only a problem on the 28 inch and 34 inch sized furnaces. It has been worked around with the heating of the foot of the pot. My 28 inch has these problems and I just go slowly. I have never lost a pot in a moly furnace.
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  #54  
Old 01-21-2010, 12:48 PM
Marcel Braun Marcel Braun is offline
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OK thanks Pete!

Yeah that third sentence is a tad unclear.... thats what I get for trying to keep it short. See if you can get it now.....

IF one could figure out a way to get the elements down next to the pot without going too excessive on overall cubic footage would there be any other advantage? Better glass? Faster charge?
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  #55  
Old 01-21-2010, 05:00 PM
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I would think you would get a better melt because it melted more evenly. I thought not wrecking your crucible was more than enough justification. It is not easy to do it though, or it would be done. It would cost more to run.
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  #56  
Old 01-22-2010, 12:02 PM
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Kurt Walrath Kurt Walrath is offline
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Marcel
I think you have the right idea IMHO. I saw a moly furnace over 10 years ago that had all the elements hanging behind the pot at the back of the furnace.
the crown was higher at the front where the gathering port was and angled
down to the back where the elements hung. the pot was changed thru the
front like a conventional gas furnace.

you can get the chamber size much smaller with this configuration than
the way most are currently built (ie stadlemelters )

steves design seems to be optimized for easiest and lowest cost to build,
not for smallest chamber size / best economy.

eventually someone will build a moly unit that addresses these issues,
chamber size, pot replacement, heat loss through door.

I wish I could afford to experiment with it...
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  #57  
Old 01-22-2010, 01:04 PM
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I do not subscribe to the notion of putting all the elements along the back, nor would I think that having them hanging at differing heights to be a good idea either. The goal is to heat the crucible evenly.
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  #58  
Old 01-22-2010, 07:35 PM
Marcel Braun Marcel Braun is offline
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Quote:
Originally Posted by Kurt Walrath View Post
Marcel
I think you have the right idea IMHO. I saw a moly furnace over 10 years ago that had all the elements hanging behind the pot at the back of the furnace.
the crown was higher at the front where the gathering port was and angled
down to the back where the elements hung. the pot was changed thru the
front like a conventional gas furnace.

you can get the chamber size much smaller with this configuration than
the way most are currently built (ie stadlemelters )

steves design seems to be optimized for easiest and lowest cost to build,
not for smallest chamber size / best economy.

eventually someone will build a moly unit that addresses these issues,
chamber size, pot replacement, heat loss through door.

I wish I could afford to experiment with it...

Well I MIGHT have an opportunity to give it a shot in the future. My thinking is to cast cubbies in the liner walls to give the elements clearance to the pot and avoid some of the problems of dropping through a steeply arched crown. It would be a very complex casting though...for both the crown and the walls.
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  #59  
Old 01-22-2010, 09:52 PM
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Marcel.... While I have not built a moly furnace I have thought that the elements could be put in indentations in the wall.

When I built my gas furnace I tried to address the minimizing volume issue by making a throat on one side of my lid which is two part castable. The opening is vertical so there is less chance of debris falling into the glass. The back half of the lid is only about 4 inches above the edge of the crucible. The door is like the old Denver doors that lift straight up. I know that many don't like the Denver style door because it can't be opened half way. It does have advantages.. one is the area to the sides of the door can be better insulated and it can be made to have a pretty tight fit. When I change the crucible which means removing all the insulation and cast pieces [ much harder as I get older] I make a gasket of zircon impregnated fiber gasket to seal the door .

The idea of basically making a cup or tub to hold the glass if the crucible fails seems like a good idea. Of course if the clean out could be attached with seamless integrety to the tub itself and the tub was large enough to hold a total blowout then you wouldn't have to worry about glass seeping through joints. You would have keep the bottoms of your moly elements from hanging low enough to be wetted by the rising glass level from a blow out.
I'm not sure if I have been that clear about how my lid is constructed but could get a photo sometime if any one is interested.
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  #60  
Old 01-23-2010, 02:06 AM
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Quote:
Originally Posted by Marcel Braun View Post
My thinking is to cast cubbies in the liner walls to give the elements clearance to the pot .
Marcel,
this is how fred metz at spiral arts builds his moly glories...looks a bit like
a buttress to protect the elements, not really complex at all.

the interior walls of my furnaces have always been machined 2600 ifb.
quicker to build than a form for a casting... and they insulate
really well. I replace my pots before they blow out ( so far anyways and
my furnace gets shut off and on every time I go to a show)

I like the sound of your idea. hope you are able to try it out.
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  #61  
Old 01-06-2011, 10:42 PM
Lawrence Duckworth Lawrence Duckworth is offline
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what would you folks think of a design that seperates the furnace body from the floor. this design could use off-the-shelf 3" thick castings.

anyone have any idea how hot the backside of the 3" thick casting is in normal operating use?

is 2.5 inches enough clearance between the pot and wall castings for moly tubes?

Last edited by Lawrence Duckworth; 03-03-2014 at 06:41 AM.
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  #62  
Old 01-09-2011, 01:29 AM
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What happens when your furnace top is stuck to the furnace floor with glass?
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  #63  
Old 01-09-2011, 07:01 AM
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I would not put a channel into the furnace wall. I don't know of a castable I would use to withstand the 2800F that a moly element has on the surface temp. Further, since a moly element changes shape some when it heats up. I think it could break it.

The jelly roll problem that exists in Steve's furnaces comes from having a cold joint at the floor. The floor aea needs to be impermeable so that a full pot dump can be accommodated in that area leading to the cleanout hole. Getting away from the whole notion of lifting off the crown to change the pot is going to be part of what Charlie and I do. Bashing your crown around is just not a good idea if you want any longevity at all.

You might use those off the shelf castings but completely close the joint up with rammable plastics coming up about 6 inches.
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  #64  
Old 01-10-2011, 11:42 AM
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Quote:
Originally Posted by Jordan Kube View Post
What happens when your furnace top is stuck to the furnace floor with glass?
i was thinking as long as the parting line is above full pool of a pot crash it wouldn't be a problem.

_______________
I can see lifting the entire top/body of the furnace as a solid fixture as a way of limiting the risk of damage to tubes and such. however, I’m not so sure it’ll be that big a deal for me to swap out a pot with the existing Stadelman design.....l00ks likeits dooable

btw..I’m not familiar with the “Jelly Roll” or cold joint problem?? Also, what are the issues with the crown?
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  #65  
Old 01-10-2011, 04:41 PM
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I will go to great extremes to avoid moving a furnace crown.
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Old 01-10-2011, 06:38 PM
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Agreed.

How is everything going to be contained in the upper chamber when you lift it up? What's keeping the castings and insulation from falling out the bottom?
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Old 01-10-2011, 07:29 PM
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I should make it clear that I really like my furnace that Steve built me. Sometimes its easy to bash without having a good overview. It has made my life a lot easier. Thanks Steve.
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  #68  
Old 01-11-2011, 08:06 PM
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Quote:
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Agreed.

How is everything going to be contained in the upper chamber when you lift it up? What's keeping the castings and insulation from falling out the bottom?
Jordon, the base of the body would have to be supported just under the wall castings. The deal breaker here would be if the heat on the backside of the casting is to much for a half inch metal plate. But,…what if the casting and the metal ring failed at the same rate…..
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  #69  
Old 01-11-2011, 08:41 PM
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Steel "fails" at 800F which is not very hot. Those were the numbers we always used in the fire service to determine whether a steel beam was no longer viable. Also keep in mind that at those temperatures as it gets hotter, it expands almost ten percent, which is huge. A car on fire under an overpass can destroy the overpass if the I-beams are not insulated from the heat. If there is pre or post tensioned concrete in a heated situation, where the steel cable in the concrete can heat to that level, the steel fails and then so does the concrete because it's no longer in tension.

The cold face of a three inch refractory casting is going to be a lot hotter than 800F.
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Old 01-13-2011, 06:43 PM
Lawrence Duckworth Lawrence Duckworth is offline
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so how HOT are these furnace when swaping out pots?

....will your front loader be available in a kit?
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  #71  
Old 01-13-2011, 10:45 PM
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I get it cold enough that I can handle everything. When it's first coming apart I use Kevlar mitts.Then I go away for a while. Patience is a virtue. If it needs air chiseling, I want it down around 140F.

Charlie and I are sitting down this weekend bashing on design issues. It's way too early to think about kits. Right now we're working on a prototype for the 24 inch pot that has the elements going down below pot line by about five inches. I want to see the lower half of the furnace getting some heat.
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  #72  
Old 01-18-2011, 02:41 PM
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Hey Pete,


Go back and look at those pictures from Norway. They had the moly elements running all the way down the sides of the pots. Then again there was like 12 of them in there if I remember correctly.

http://talk.craftweb.com/attachment....8&d=1263959333
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  #73  
Old 01-18-2011, 03:43 PM
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Going all the way to the bottom will create an element that is very as in (VERY) hard to ship as well as a lot more expensive. I don't think the furnace needs 12 elements based on my experience. Six is quite adequate for a 300 lb pot. We don't actually know the capacity of the Norwegian pot in that picture. Going down five inches should do it in my mind since the pot curves in towards the center after five inches. Getting the pot off the furnace floor and on to a pedestal will really help as well. Having the pot right on the floor really creates a cold spot for the pot which in turn creates stress.
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Old 01-19-2011, 10:05 PM
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Are the heaters and crown so fragile that moving or rolling the furnace with the heaters installed a bad idea? Will that lill bit of vibration break ‘em?


I fear I’ve screwed up before even getting started. I may not have the overall headroom to pull the heaters out from the top unless I roll the furnace out into the open shop....[i have an 8 foot ceiling behind the 7 foot hood aprin.]

Last edited by Lawrence Duckworth; 03-03-2014 at 06:41 AM.
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  #75  
Old 01-20-2011, 07:05 AM
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It's a problem and part of why the 500mm hot zone elements aren't being used anymore.

I had to account for the heater length when I built my hood. I think rolling a furnace on a smooth floor will probably ( probably mind you) work but it depends on the construction and seating of the passage brick. Simply put, they're fragile. I see lots of stuff. I had one man methodically break every one of his elements, one after another trying to pry them out of the bricks from inside the furnace instead of pulling the whole unit. In my shop I once had the elements all out and on the marver and Mary Beth came and sat on them...
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