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Old 07-14-2002, 04:21 PM
Bryan Harnois
Posts: n/a
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 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 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

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