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Old 03-10-2021, 02:04 AM
Peter Bowles Peter Bowles is offline
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Crown and passage bricks

Hi all, hope this finds you all well.

I'm in the process of building a new (and first for me) moly furnace. There'll be a number of questions down the way around wiring it all up, but first a couple of queries about crowns and passage bricks.

My plan is to run the elements down through the length of K26 bricks that I've put a slight taper on about 3 inches up the length. Now that I'm laying the bricks out (six of them) they look pretty substantial in relation to the overall size of the crown and I'm concerned about the structural integrity of the crown.

My thought was that the bricks would run all the way through the crown so the bottom face of the brick would be flush with the internal surface of the crown. Is this how it works?

I could create the casting with smaller openings for the elements to hang through and make the passage brick sit in a recess, but then I'd loose some of the heating length of the element unless I hang the thicker section partially into the furnace.

Any thoughts?

Thanks in anticipation.
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Old 03-10-2021, 07:04 AM
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ON all the moly's I've worked on, the passage brick rested on the crown in a flattened out point but never went through the crown to the hotface. I would expect a lot of trouble if it did. If I was even considering that, I'd use a 28 brick. The 26's as they stand spall quite a bit in service on the hotface to crown end.
The crown needs to have six ( or whatever) football ( american) shaped holes in it. The hot/cold weld should not get up inside the crown but should sit right at the interface between the crown and the open hearth.

The drill through of the passage brick needs to be perfect- absolutely no tension or compression of the element at all. It should hang plumb.

For 9/18 elements we used a 3"x 7 inch brick. Weld up a maquette from rod to use for checking your fit. Don't try the brick first.
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Old 03-10-2021, 08:02 AM
Josh Bernbaum Josh Bernbaum is offline
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If you can cast the crown upside down, you can leave an indent in the mould for exactly where the bottom of each passage brick will nestle so they will ultimately sit properly and in the exact right spot. And in the center of those recesses, the rounded-slot for the pass through like Pete said. I'm not sure it'll be the best idea to taper the bricks. It may make them easier to slide in later, but that seems to me like you'd just be encouraging space for the escaping gasses and heat to find a path of least resistance. If they are just square bricks, then there is a better chance of nestling the frax around them as flush as possible.
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Old 03-10-2021, 08:10 AM
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Make sure you have a flue mechanism for charging. Otherwise, as Josh implies, you'll have charging gas trying to get out by the passage bricks.

Steve's crowns were all flat on both sides which seemed to work, I don't know why. If the crown was arched, it would be a challenge to cast upside down.
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Old 03-10-2021, 03:07 PM
Peter Bowles Peter Bowles is offline
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This is making me feel much happier, thanks for the replies.
Just to clarify, if the crown is 80mm thick and the length of the taper on the element from 18 - 9 is 15mm, can I have 65mm of the thick end sitting in the oval slot in the crown casting? Or does the top of the taper have to sit at the interface of the passage brick and slot? - I'm concerned about losing some of the heating length of the element inside the crown slot.
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Old 03-10-2021, 03:20 PM
Peter Bowles Peter Bowles is offline
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I once worked from a furnace with a flat crown and every day for a few days after melting, there would be these tough little tadpoles that would drip from the ceiling right into the pot.

I can certainly see the attraction of casting it flat, had also considered casting it flat and sloping it to the back wall, but arched just seems to be the right thing to do.
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Old 03-10-2021, 04:36 PM
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Wink

Arched is the right thing to do but it never stopped anyone. When I make an arched crown, I make a frame that is exactly what the interior dimension of the furnace is. I then stapled the crap out of that shape with muslin stretched as best i can do it.

Then, I pour potters plaster into it and it sags very nicely into a compound arch. Let that dry, flip it and grease it at the least and use it as the inner form for the arch. You still need to scoop out the footballs when you cast. 80 mm seems a touch thin to me. on the outside, when wet, flatten the spots over the footballs for the brick. Use 3" brick if you are using a 9/18 element. You need the extra body. The cold part can be in the brick, don't let it get in the combustion chamber, such as it is. Hang it plumb. There are a lot of ways to hook this up.

My experience with the cable is to use a 350MCM Stranded cable going into a 600MCM clamp. That's just me. Check your connections when you finally hook up after about a month. Then tighten them. I did this with very little input fifteen years ago. Way back in the archives is a thread about the portland vase. Try to find it.
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Old 03-10-2021, 05:41 PM
Peter Bowles Peter Bowles is offline
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I've got a couple of sketches below..one with the element junction just at the point of the bottom of the passage brick, and the other with the junction at the opening into the chamber.

I had sized the elements to have all the hot zone in the chamber and now I'm concerned that if they are partially in the crown I'm going to reduce the heat transfer to the chamber.

What are your thoughts please?

Option 1 on the left with hot zone in the slot in the crown?

or

Option 2 on the right with some of the thick section of the element in the crown?

Thanks
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Old 03-10-2021, 06:37 PM
Josh Bernbaum Josh Bernbaum is offline
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I don't think you need to be concerned with the upper part of the hot zone of the heaters being set within the thickness of the crown a bit, but I think my setup is a bit more like your drawing on the right. I think, I'll have to look up there again next time I'm taking a gather. If you haven't cast it yet, I agree with Pete that 80mm sounds a bit light-duty for crown thickness. I agree that arched crowns are ideal for pot furnaces, and so are crowns that don't have holes in them, but with these Moly's we need to have some acceptable compromises. It'd be great to have the arched crown, but then the complication arises with how the (flat-bottomed) passage bricks sit and nestle into the top of the compound curve of that crown. Flat should make things more straightforward for ensuring that the passage bricks stay plumb. Also, if it's a flat casting, you can cast it upside down and place foam pieces exactly where the passage bricks will later sit. Maybe you'll be able to dig those areas out when the castable is still a bit wet, but again, the passage bricks want to sit plumb and be as flush as possible around the sides and bottoms of the bricks where they nestle in there.
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Old 03-10-2021, 07:05 PM
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That's just well said.
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Old 03-10-2021, 09:20 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is online now
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Mmm
Im not experienced in this type electrical furnaces, but to me , just based on other things, is it a good idea to have a hot zone of the element inside the crown?
Ive built 4 furnaces and have never had less than 150 mm crown, none have failed, except for one when I was talked into something called castolite 30 or some crap, cost me a fortune.
Use Greencast 94- absolutely great, ran hotter than you can imagine for 15 years, looked new when I tore it down.
80 mm sounds risky.
Don't know your elements though
A crown should be a vault, yes, and as Pete says is a challenge to upp side down cast, - Ive done it though, making my floor a bowl shape to contain a pot failure, and avoiding the common USA donut ring leak design failure on US furnaces
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Old 03-11-2021, 07:13 AM
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Placed properly, the weld at the hot cold zone interface should be exactly at the opening into the hearth. If it gets up in the crown, the hot zone is operating at 2800F and will spall the crown eventually.

I consistently use MORCO95 in crown work which comes from the Missouri Refractory Company sold mainly by Hi Temp in Fenton Missouri. No relation to High Temp in Portland. I've never seen it distributed nationally.

I like to cast over a form in a big box. I think I've likely done my last casting. My furnaces have been built following a design from Durk Valkema from back in the '70's. Like an Ice cream cooler with a slant front plate that is removable to access crucible change. Burner comes in on the left and flues out below the burner.
I remember the furnace you had in Denmark with those huge tilted pots we sent you from E.C. I have never seen that done elsewhere to such a degree. I did have tilt in one of my moly's so I could get a bit of extra pot into the furnace. It was really hard to clean.
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Old 03-11-2021, 07:53 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is online now
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Well Pete it was here in Sweden, and it was a one time malfunction- one of the three cast supports underneath the pot gave out during running hot, luckily it was the one at the front so the pot tilted towards the gathering port, and I was able to run the pot its expected life cycle..
All my furnaces have had the gathering port lean in at an angle to facilitate gathering at a steep angle to avoid gathering bubbles in the front of the piece. And also to reach deep into the pot.
You say the castable might spall from the higher temp of the enclosed element... wont the higher temp of the element within the castable shorten its lifetime?
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Old 03-12-2021, 12:19 AM
Peter Bowles Peter Bowles is offline
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Thanks everyone for your input on this. Being new to moly, I've got it in my mind that the element placement is one of the critical things to get right.

I'm taking on board all the comments about the crown thickness, the internal span is only 22 inches, its a pretty small furnace, if I go up to 4 inches thickness the crown will end up just over 100kg using a castable of 2900kg/m3. (How's that for mixing my units). Are the castables you are using as dense?
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Old 03-12-2021, 01:37 AM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is online now
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Almost, GC 94 is 2500 kg/M3. Dense is good.
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Old 03-12-2021, 07:26 AM
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Pete VanderLaan Pete VanderLaan is offline
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Quote:
Originally Posted by Michael Ahlefeldt-Laurvig View Post
Well Pete it was here in Sweden, and it was a one time malfunction- one of the three cast supports underneath the pot gave out during running hot, luckily it was the one at the front so the pot tilted towards the gathering port, and I was able to run the pot its expected life cycle..
All my furnaces have had the gathering port lean in at an angle to facilitate gathering at a steep angle to avoid gathering bubbles in the front of the piece. And also to reach deep into the pot.
You say the castable might spall from the higher temp of the enclosed element... wont the higher temp of the element within the castable shorten its lifetime?
*****
Go Back to my opening sentence :

"Placed properly, the weld at the hot cold zone interface should be exactly at the opening into the hearth. If it gets up in the crown, the hot zone is operating at 2800F and will spall the crown eventually."

***
As to crown thickness, it seems to me, the thinner they are, the sooner they'll fail.
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