more Moly D furnace questions

Hi, this post is a bit long, but it explores my evolving real world Moly D design, so I hope its worth the effort it takes to get through it

I've been designing my first non-resistance element furnace for about a year now, with the help primarily of Jeremy Willis, an Electrical Engineer and the principal of Euclid Kilns, and have followed the Moly D threads on this board with interest. I've got some observations and questions directed primarily to Steve Stadelman and Jon Myers, but anyone feel free to jump right in.

I rejected Silicone Carbide for a couple of reasons, primarily the huge power draws initially when you run then through an SSR to compensate for the lower draws as the elements age, and the ( probably ) shorter element life. While Moly D costs more to set up the control system for, in the long run its more economical in power costs and maintenance : plus I own a bunch of watlow 65 amp 240V Din a Mite phase angle fired SCRs that I bought used from Brad S, and had refurbished by watlow with current limiting added for 100.00 a unit, as well as watlow 8CLS ramp soak controllers that I use one of for oven control, and another for overtemp monitoring, so I have a good foundation for an excellent control system

The design we are currently working on uses a cast cylindrical inner chamber in three parts ; a floor with sill lip large enough to contain an entire 150# pots spill with Mono T9 liner and drain port, a main section ( with some fussing still going on about how to incorporate a front or rear access port big enough to allow a pot to be changed without taking the entire furnace apart... any thoughts anyone ), and an upper chamber in two parts : the side and back walls and crown itself, and a seperate front inclined at about 25 degrees to allow easier and hopefully cooler gathering. This has evolved after a few discussions with Henry H, and my current plans call for casting with Kast o lite 30.

Our initial plan was to use a 100# EC pot, with the chamber of sufficient diameter to take a 150# EC pot. The Moly D elements ( 4 of them )were run from the crown down to just above the spill line, with my thought process ( no scientific proof here )being that full length elements would help equalize heat distribution through the chamber and maybe cut down on the seemingly excessive heat that seems to come out the open doors of furnaces I've seen that use above the pot-line elements, but these also were invested pots, so.... comments welcomed

from the element lengths used by Steve and Jon 12" and 18 ", they must be exclusively above the glass line, which does have some advantages, less likely to get glass on them, and the diameter of the chamber can be smaller ( less refractory to heat, and you don't have to allow for element clearance on the sides ), but I'm left wondering about heat distribution throughout the chamber...what do people think? If it is an issue, then maybe the system I use in my large fuser where I pipe in some compressed air to create air circulation might work...?

In any event, Steve commented that he is going to upgrade to a large EC pot ( currently 100# ) and that his optimum SCR would be a 65 amp Din a Mite similar to what I have, and that piqued my interest...my impression has been that some of the electrics discussed here and else where may be somewhat over the capacity really required in a properly sized and well insulated design. So, my question of him and generally is do you think 65amps at 240V run through a 240 to 120V 25 KVA transformer ( quite a lot less $ than the larger units previously discussed, about $400 new ) would be adequate to get the job in a 150#er ( or larger ) done, and if I'm going to go with the shorter elements, what would be the recommendation for appropriately sized Moly D elements available through Jon to do the job...my impression after some rough calculations is that 6 approximately 16' hot zone 6/12 elements would do it, but I'll leave that to expert comment from Steve and Jon. If 65 amps of 240 isn't adequate to do the job, then what about using two legs, each using a 65 amp SCR feeding into its own transformer and bank of elements?

Well, that about exhausts me, so now I'll let you tear my plan to bits...I'm looking forward to you comments

Bryan

[Pete VanderLaan]

Having seen both furnaces 10 days ago, I think that what you want to do will work. Neither furnace had hot spotting problems and neither furnace was being run close to max when charging.They were idling at about 20% power and hitting 2100F. . I am planning on mine being a six element ( 18 inch) with a 200 Amp SCR and a 50KVA Transformer. I plan to melt a 150 lb pot and two small color pots in the same furnace. They will all be freestanding.

I do know the crucibles and the shorter elements do not pose any thermal shock issues at all for the pots.


Do I recall correctly that I met you at GAS and sold you some pots and you told me about the wool mill on P.E.I.? They are such nice people. I did buy their yarn and love it! I am happily weaving rugs for the house in all my spare time.

yes well temporary power outages are a fact of life here. What would your recommendation be for the refractory in a 150# furnace at the power levels I have available...the literature I have available would indicate that 2800 degree firebrick and Kast o lite 30 are fairly closely matched in thermal conductivity ( perhaps slightly more for the Kast o Lite ) with the kast o lite being stronger and having the advantage of castability, ...or am I missing something here....the way I looked at it the MonoT9 floor lining and the Mizzou pedistal would increase the thermal mass considerably

flowerpot boy

[Steve Stadelman]

Hi Bryan, you are very well set indeed. However you need to check if your transformers will go down to 60 volts. I am assuming that these are the type (like mine) that take 480 down to 240 and 120 depending whether the two secondaries are wired in series or paralell. If you only lower your 240 to 120 you will only have an amperage doubling. Since you are talking about using 65 amp scrs you will only achieve 130 amps. Six millimeter hot section molys can use about 170-180 amps. So in this configuration you will not be using the elements fully and the furnace will be sluggish. If both transformers can be configured to 240 down to sixty volts each scr can be wired to a 60 amp breaker and will draw from 40-45 amps depending on where you set the limeters. If you use elements with 28" hot zones and 16" terminals they have a hot resistance of about .179 ohms and at 166 amps at 2375f take about 30 volts to push the current. Two of these in series will produce just shy of 10kw. Two circuits like this is equal to what Kyle Gribskiv is using to melt using one of Petes 325 pounders. Pete and I will not use two small scrs but one large one and a larger transformer then the power comes from the secondaries in two parallel circuits. Kyle recently lost an element in one circuit and the furnace just limped along on half power. So your redundant power idea is really attractive. Steve.

Steve

one of the things that I've wondered is the size of the chamber in your furnace you have the 100# pot in with elements above the pot....what is its diameter ( and thickness and it is 2800 IFB isn't it ? ), and is it the one you will be upsizing to the big pot

What I'm getting at is how much power requirement is based on the size of the chamber and thus the thermal mass in addition to the pot and glass that needs to be heated...your dimensions would give me a ballpark idea, because if you've sized it to handle the bigger pot then thermal mass isn't quite as big a deal as I've thought...Pete says both your and Jon's furnaces are hefty

If I go to the long elements ( 28 " hot zone ) then you indicate that 2 banks of 2 each with an SCR and transformer will more than do the job up to 325#

If I want to keep the elements totally above the pot line at say 18 " hot zone, then am I correct that 2 banks of 3 elements each with their own SCR and transformer would do the job? ( I'm increasingly concerned about pelletized batch popping and spewing all over the greater surface area presented below the pot line with the longer elements )

also, if that is correct, then could I more immediately power the unit for a 150# pot using one bank of 65 amp SCR to 240 to 60 Volt transformer to 3 elements with 18 inch hot zones...I could design the crown with 6 chimneys for elements and use just 3 for now, plugging the others with dummy bricks

this would allow me to get up and running with the lower power overhead, but able to expand by adding in the second control loop and elements later

again though, I want to be sure that the thermal mass of a chamber sized big enough to eventaully handle a bigger pot isn't going to totally lug down the system on one control loop now

what do you think

I also wonder if there is some way to post on this site somewhere a table of resistance values per unit of length of hot and cold zones of the elements that Jon can supply ( 6/12) and a summary of the calculation process required to come up with sizing Moly elements...I've done this before with Silicone Carbide, its not hard for SiC if you have the tables, but I do not have them for Moly D, and am having to rely on others for that help...what do you think Steve or Jon or Pete, posting this info could expedite discussions about these issues for everyone

Bryan

[Steve Stadelman]

Bryan, my furnace castable comes in at about 1000lb including pot investment. the free area over the pot is a cylender 18"high by 18" dia. In my case my 10 kw input is marginal. If your brickwork has less mass you might get away with it. I have to say I do not know, maybe Pete or Henry Halem or one of our more experienced furnace folks can offer a better opinion but I will not build more than an 100lb furnace again with only 10kw. On element length 3 each 18" hot zone by 18" cold terminal or 4 each 12" hot zone by 18" cold terminal or 2 28" hot zone by 18"cold terminal all add up to the same thing- 170 amps at 60 volts at 2375f. these element lengths all have voltage requirements that divide into 60 so you can balance the load out. That is if the scr shorts out at working temp it will not fry the elements or cause any damage. The furnace will just go up untill your overtemp safety catches it and shuts it off.

Steve, so OK, it looks like the way to go is just to use the two SCR to transformer to element legs from the beginning, and this is made even more palatable by the fact that when I checked on the transformers to make sure they had a 60V secondary tap they didn't, but a model that does is actually a standard stocked item and $100 less ( $300 each ) so that is reasonable to get the redundancy. I'll just have to ponder the issue of 18" above the pot vs 28 " full length elements for a bit : do others out there have ideas of which way they would tend to go?

The other thing I was interesed in relates to the photos Pete took on his visit to Portland, in particular those of the cold zones passing through a brick up to the electrical connections. For hot swapping elements, you obviously would set up one of these ahead of time, pull the bad element and ease the new one down through the frax and into the hole in the crown....what did you do to key the end of the brick to the hole in the crown to get a proper fit?

Others comments on refractories etc are welcomed

Thanks Bryan

[Pete VanderLaan]

They just stuffed fiber in it. Steve has a fan blowing air on to the connector at the strap which is a good idea. If you run with the long elements, make sure you can remove them easily since popping a pot is sometimes brutal.
When I pop mine in the gas furnaces, I pry under them while the furnace is really hot. Then they come out easily when it is cold. I would feel uncomfortable doing the with elements back there even though they would be turned off.
I plan to build mine with the equivalent of a refrigerator door on the front with a big wheel attachment below so I can swing the whole front open.

[Dave Bross]

Ok, so who's going to do the redneck/shade tree/ non UL approved economy version of this furnace. The one that you jump start with an arc welder until up to speed?

What would it take?

I think Pete mentioned something about the folks in Portland starting one off that way?

[Pete VanderLaan]

you can't jump start it. It needs a constant source of power. We did attach a crackerbox to an element in portland and it was starting to get hot. Because the clamp connection was poor, we broke the element end, which was old. The connection is pretty important to have done well.
The elements can be run directly from the power panel but you need a lot of amperage to do it. In most instances, it will take a dedicated panel if you want to avoid the transformer. There is no real savings though because the larger SCR more than makes up for the cost of the transformer not being needed.- No free lunch strikes again. Further, the elements are 24 inch to run right from the wall and that costs more to boot. Even though I have the spare power, I am going with a 50KVA Transformer.. I think that between the SCR, the contactor, the elements and the tranformer that the power set up should run about $3600 dollars. Not cheap but the only thing that gets replaced are the elements.
Again, I am doing it not to save money- I won't. I will however have a silent unit, no open flame, and no combustion gasses in a small building.

so Pete, given you intend to swing open one entire side of the furnace to facilitate pot changes, I take it you are pedistal mounting the pot and not investing it as Steve did? are you intending to use castable or IFB, and how are you handling the construction of what would amount to a pretty massive door?

I like the concept of the furnace having one large and two smaller color pots...how were you going to configure this within the chamber, and how were you going to handle access for gathering from the various pots

Bryan

Pete I had a look at your door pics and I think that probably answers my question about multiple pot access...nice doors, but I'm still having trouble conceptualizing one whole side of the furnace coming open and somehow retaining all that door and sidewall frax well in place

Bryan

[Pete VanderLaan]

If you lifted those doors off of their frames, and removed all of the rigidized fiber, it would expose a plate that runs the length of the furnace. The plate leans against the furnace itself and was cast as a beveled piece. The plate has two openings in it of good size and the doors cover the openings.
When the furnace is cold, the plate is simply picked up and removed. That exposes the guts of the furnace and the pot comes out thru that opening. For smaller pots, they can simply come out thru the door ports.
On long furnaces, this causes a problem with sagging of the crown, and the plate has to snug in under the crown itself. That means that the crown must be jacked up- literally and have the plate removed. Then the jack comes out, new pots go in, then jack it up again and re-install the plate. The entire process takes under an hour. I have used this design on gas furnaces for twenty five years. My only concern is mass at this point although I haven't quite decided on how to install a "kill switch".
I envison having the elements on the remaining three sides. of the furnace.

[Eben Horton]

Pete, about your kill switch-
a really simple way to do it would be to wire in a rubber pressure mat right infront of your furnace. this way all you have to do is wire it in series with your scr. Also you don't have to do any funny stuff with your door-

just a thought-
and now I'm off for philly! If you don't hear from me in a week I've hung myself in the back of my booth-

Eben

[Pete VanderLaan]

Originally posted by Eben Horton
Pete, about your kill switch-
a really simple way to do it would be to wire in a rubber pressure mat right infront of your furnace. this way all you have to do is wire it in series with your scr. Also you don't have to do any funny stuff with your door-

just a thought-
and now I'm off for philly! If you don't hear from me in a week I've hung myself in the back of my booth-

Eben

Well, I suppose that that would keep the flies off of the work... ( good luck)

[Steve Stadelman]

Just a couple of thoughts, The element info I use is from the Kanthal Super Handbook. They will send you one for free. If you use a pressure switch under your "doormat" to kill the furnace be sure to positivly disconnect both power feeds, the scr only switches one leg of power off, leaving one live leg to hurt you with. Disconnecting the signal from the controller will not do it.

Pete, I had considered casting the entire removable front as you suggest and obviously have had great success with. My concern, based on no experience with that design, was whether the joint between the vertical portion and the sloped front ( at I'd guess from the pics 25 to 30 degrees off vertical ) would be subject to cracking, despite it resting against the side walls of the chamber : what thickness have you cast this part in the past , of what castable, and do you butress or reinforce it in any way?

Bryan

[Pete VanderLaan]

It's six inches thick in Kast-o-lite 30. It does eventually crack and I just stuff it back together. There is no reinforcement. A front plate usually lasts about four to five years.

rubber pressure mats as a way to shut off power to the door : I didn't know they existed. Can someone point me to a source of supply?

Bryan

[Mark Wilson]

in the past, i posted a link to motionet.com which is an extremely useful site. here is a link to a page on motionet dealing with safety mats.

http://www.motionnet.com/cgi-bin/sea...e?a=cat&no=923

[Pete VanderLaan]

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