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Tom Clifton
02-21-2012, 05:16 PM
From Parker Glass Furnace (http://www.parkerglassfurnace.com/pages/howtobuild.html)

This is important because the next step is to heat the elements in a kiln to 300 F in a kiln Prepare a bath of milk of magnesia and dip the heated elements one at a time and hang them to dry. The manufacturer of the wire sells a product that costs a lot more than milk of magnesia and the manufacturer says will make the elements last longer. Your decision here. The idea is to setup a barrier on the element to lessen the degradation of the element.


Any basis to this?

Pete VanderLaan
02-21-2012, 05:38 PM
I am always skeptical of this sort of stuff. For a while we had people swearing that if you coated the elements in ITC they would last forever. I note that those guys have seemingly gone away. Just don't handle elements without gloves and clean them with acetone if you do.

I really hate top loaders like that.

Tom Clifton
02-21-2012, 06:07 PM
It ran counter to what intuitively seemed right, but there is so much I don't know... I do believe that if anything kanthal needs to be heated to over 1700f (or so) to develope the white aluminum oxide layer as thatis what helps to passivate it.

One more observation/queston. While poking around with 'element calculators' the only one I have found that appears to function is at Spiral Arts (http://www.spiralarts.com/tech/annealer-calc.asp). However, closer invesitgation shows the wire type to be locked in to nichrome-c (based on the resistance per foot). It is interesting that at the very bottom of the page is a note "Wire temperatures not calculated in this version - avoid watt densities on coil < 9 watts/sq.in."

Seems odd to avoid low power densities to me. Perhaps this has something to do with the low resistance of cold nichrome?

Cecil McKenzie
02-21-2012, 09:30 PM
Tom... Thanks for posting that site. I am contemplating building a wire melter myself. I glanced at the directions and would disagree on what he says about mercury relays. He says they fail in the off positions. Maybe I am doing something wrong but I have never had one fail in the off position. Mine have always failed in the closed position meaning they are allowing current to flow. Since I have always used 220 sometimes one side stays closed but the other side opens up so it doesn't overheat whatever it is running but it can still shock you if you touch an element.When both sides fail closed you can have a runaway heat situation.

I have a small Denver with a 220 mercury relay and I have seen it fail closed on the side that runs the 110 safety light and the elapsed time meter so when the door is open even though the safety switch on the door has cut off power to the mercury relay the light stays on and the little meter keeps running. When this happens I know that there is potential of 110 shock. Usually if I tap the mercury relay with a piece of wood it goes back to normal operation. Of course the other side of the relay is not hooked up to a light so I don't know when it is stuck.When this happens the elements are starting to cool even though it is still energized to 110.

I have not really addressed this situation other than to buy a new switch when I notice this happening. I suppose one could go to the trouble of putting a safety light on each side to let one know if the switch has failed or maybe put in two relays or some other type of switching device .

Knock on wood I have never been shocked from the Denver furnace. I do have a rubber mat in front of that furnace.

Jordan Kube
02-22-2012, 01:24 AM
I made one for Kanthal A-1, Tom:

www.wirefurnace.com

Pete VanderLaan
02-22-2012, 05:55 AM
Every merc relay I have ever had fail, and there have been precious few but always too close to the rated ampacity, have the solenoid fail and when it fails it falls in to the open position, meaning no power flows. With Mechanical recays, the opposite is true for me. The points fuse and the power flows.

The coating people are cultists.

Cecil McKenzie
02-22-2012, 08:45 AM
Not trying to belabor the point but I investigated on line a little and found different views.

This paragraph is from an art magazine and it agrees with the fail open or off camp.
"Another obvious replacement for the electro-mechanical relay is the wet, or

mercury, relay. This device consists of a plunger with a small amount

of mercury on top. As the device is activated the plunger moves up and

compresses the mercury to the second contact. The advantage of this

mechanical device is that there is no arc formed when the contacts are

closed. This arc is the main reason dry electro-mechanical contacts are

welded shut. The mercury relays have been tested over 1,000,000 cycles

before failure and have proven themselves to be more reliable than the dry

relays. However, they are still a mechanical device and they will fail.

Fortunately, when they do fail, it is normally in the open or “Off” condition

where gravity tends to pull the plunger away from the contacts. There are

several disadvantages with mercury relays: cost, environmental restrictions

to mercury, and disposal. In addition, the mercury relay must be operated in

an upright position or the mercury will short the contacts in a horizontal

position. Another disadvantage is the power required to activate the coil. The

low voltage controllers are not capable of activating a mercury relay directly.

Usually the controller will activate a dry electro-mechanical relay which in

turn will activate the mercury relay. Now, how does this help if a dry electro-

mechanical relay is required to activate a mercury relay? By far, the majority

of the current is passed through the contacts of the mercury relay and only a

very small amount of current passes through the dry relay. The amount of

current through the dry contacts is not enough to produce the destructive arc.

This in effect increases the amount of cycles the dry relay can perform

before failure, actually to the quality level of the mercury relay."

This paragraph concerns nuclear power plants and seems to support the fail closed or on position camp.


"This bulletin addresses the failures of mercury-wetted relays used in the

logic matrix of the reactor protective system (RPS) of nuclear power plants

designed by Combustion Engineering (C-E). Except for Arkansas Nuclear One

Unit 2 and Palisades, both of which use dry-contact matrix relays, the NRC

understands that all other operating C-E plants use C.P. Clare Model

HG2X-1011 mercury-wetted matrix relays in the RPS.

Mercury-wetted matrix relays manufactured by the Adams and Westlake

Company were initially used in the Palisades plant; however, because of

repeated failures of these relays, they were subsequently replaced with

relays having dry-contacts. GTE, the manufacturer of these dry-contact

relays, however, has since discontinued their production. Thus, although the

dry-contact relays used at Palisades have performed without a failure since

they were installed, they are not available for the other operating nuclear

power plants designed by C-E.

OPERATING EXPERIENCES AND EVALUATION:

To date, operating nuclear power plants designed by C-E have reported


thirty-one (31) failures of mercury-wetted relays used in the logic matrix


of the RPS.

Most of the reported failures were "failed-closed" type (i.e., the type that

could inhibit a reactor trip), and four of the reported events involved

multiple failures (i.e., three relay failures were detected during two

tests; two other failures were detected during two different tests). Because

of the redundancy within the RPS, no reported event would have prevented a

reactor trip; however, the build-up of coincident "failed-closed" failures

of certain sets of relays could result in trip failures for off-normal

events."

Not trying to prove I'm right but I have seen stated many times that mercury

relays fail in the open position which is counter to what I have experienced

and can be dangerous to an operator and to the work in an annealer or

possibly a furnace. I did notice that in the nuclear reactor article they

mention redunduncy which I take to mean switches in series. Also the first

article mentions using a mechanical and mercury relay with the mechanical

relay supplying power to the mercury relay. This would make the

mechanical relay last longer and provide some redundancy and an extra

layer of safety.

I think the idea that mercury relays don't fail comes in part from the

literature on mercury relays that state they can go through hundreds of

thousands of cycles without failure . But that doesn't mean they can't fail.

Tom Clifton
02-22-2012, 11:39 AM
Over the weekend I received Mark Lauckners 40lb Furnace (http://www.mayneislandglass.com/instructionalvideos.html) video. Best money I have spent in quite a while. Should keep me from making a number of dumb mistakes...

Any way - I intend to shrink it down back into the 15lb range which better meets my (perceived) needs. One thing I'm not sure about is the desirable aspect ratio of the crucible. My inclination was to use the small one from guadalupe glass, but I have been told that due to the width/depth combination that I may wind up not being able to get most of the glass out of it easily. As the pool of glass gets shallower the gathers get smaller... Using a narrower crucible with a greater depth reduces this concern. I just don't know the limits of practicality.

The other thing is that with the crucible being only 5 3/4" tall the furnace will be a bit short and getting enough 11ga wire in there for 1700 - 2000 watts is going to be tight. I could just overbuild the furnace for a 40lb crucible, but that puts me into a whole different ballpark...

I'm not inclined to skinny down the wire too much so just looking for input.

Mark Wilson
02-22-2012, 11:52 AM
kanthal a1 or apm has a small amount of aluminum in it. as the wire fires for the 1st time, the aluminum diffuses out and forms an aluminum oxide coating on the outer surface of the wire, which provides protection until microcracks form to let the oxygen in. so this type of wire is already coated and needs no additional coating.

Dave Bross
02-22-2012, 12:22 PM
The shallow pot has big advantages in fining time. Less distance for the bubs to travel.

I put a couple 2x4s under the front legs of my shallow pot, top loading furnace when it's getting near the bottom. The glass on the bottom runs to the back and you can get it easily. Not that you want it for much if it's been there very long but it's good to get it out of there.

Getting the wire stuffed in there is always the trick. Remember you'll want at least a 2 to 1 stretch on the coils too, although you'll have to have whoever makes the coil do that for you in Marks' design. Mark's 40 pounder runs the "garage door spring" elements like the Denvers.

Think about using thinner gauge elements....less "stuffing" room needed...and the variac gives you a whole other world of possibilities on using less wire. Just be sure you have enough watts and watch the watt loading. Size of chamber sure does play in here too when you're talking 2K watts per sq. ft.

I would highly reccomend at least a couple inches between the pot and the walls.

You can cut the chamber size down, minimize tight bends in the elements and even up the heat on the pot by going to a sort-of-round-agon.
You cut enough firebricks at around a 22 degree angle to make a pentagon, octagon or whatever sort of "gon" it will take to get big enough. I figured it out by plotting it out on paper at reduced scale. You can get the exact angles for the cuts that way too.

Dennis Hetland
02-22-2012, 12:46 PM
kanthal a1 or apm has a small amount of aluminum in it. as the wire fires for the 1st time, the aluminum diffuses out and forms an aluminum oxide coating on the outer surface of the wire, which provides protection until microcracks form to let the oxygen in. so this type of wire is already coated and needs no additional coating.

That would point to a good reason to wear gloves and wash with acetone. Could it be possible that the oil from one's hands might inhibit the forming of a complete and even coating of aluminum oxide?
I'm just wondering.

Pete VanderLaan
02-22-2012, 01:01 PM
Just don't handle elements without gloves and clean them with acetone if you do.

*******
From the top

Richard Huntrods
02-22-2012, 06:16 PM
Over the weekend I received Mark Lauckners 40lb Furnace (http://www.mayneislandglass.com/instructionalvideos.html) video. Best money I have spent in quite a while. Should keep me from making a number of dumb mistakes...

Any way - I intend to shrink it down back into the 15lb range which better meets my (perceived) needs. One thing I'm not sure about is the desirable aspect ratio of the crucible. My inclination was to use the small one from guadalupe glass, but I have been told that due to the width/depth combination that I may wind up not being able to get most of the glass out of it easily. As the pool of glass gets shallower the gathers get smaller... Using a narrower crucible with a greater depth reduces this concern. I just don't know the limits of practicality.

The other thing is that with the crucible being only 5 3/4" tall the furnace will be a bit short and getting enough 11ga wire in there for 1700 - 2000 watts is going to be tight. I could just overbuild the furnace for a 40lb crucible, but that puts me into a whole different ballpark...

I'm not inclined to skinny down the wire too much so just looking for input.

I would recommend just building the 40, pretty much exactly as outlined in the video. It's a terrific furnace. There are some mods you may want to consider though...

- forget the garage door spring elements. I used 11ga A1 and it has been working out fine. You do want about 8 times around the furnace though, and the stretch should be 3-1 for maximum element life. I used a 1/2" dovetail bit to groove the bricks, 2 grooves per brick.

- go with inexpensive controller, like the Fuji, and SSR's work just fine as electronic relays. With the Fuji you can do RS485 control with a laptop or other computer to program them.

- I was raised on side gather gas furnaces, so I can't top gather like Mark does in his video. So I beveled the gathering port opening at about 45degs. That was great for gathering but caused endless 'glass drip damage' problems to the maintenance lid. Last maintenance cycle I enlarged the gathering port and used rammable to recreate the sloping gathering port. So far the few glass drips have not not touched it.

Check out my web site if you want to see my version being built, operated, maintained and modified.

http://tinshack.huntrods.com/ for the current (Vancouver Island) studio and

http://tinshack.huntrods.com/tinshack_calgary/index.html for the original build and operation.

Cheers,

-Richard

Tom Clifton
02-22-2012, 08:01 PM
- forget the garage door spring elements. I used 11ga A1 and it has been working out fine.


I called Denver Glass and they wanted $100 each for the 8ga elements for their 60lb furnace. That pretty much convinced me to roll my own... For your 40lb build what was your element resistance? (or calculated power at 220v or 240v)

Also, you said you cut 2 grooves per brick. Did you do a straight wire riser like the Denver elements or did you pigtail them through the insulation between rounds?

One thing I saw in Marks video is that his elements slumped sideways. While to some degree this may be unavoidable, I can't help but believe that it is caused (at least in part) by the large diameter and low stretch. If nothing else, slumped over elements causes hot spots where they touch (or nearly so) and that can cause early failure.

I am also considering using a curtain style element. Pretty much winding the 11ga wire on a 5" wide x 1/2" thick form then suspending it on a mullite rod (well stretched - better than 4:1) mounted about 1/2" off the IFB wall. The intention is to not have the heat concentrated around the element as you do in a groove. What I don't know is if this would be too fragile - moving or bumping the furnace could be an issue.

Cecil McKenzie
02-22-2012, 08:53 PM
Tom ... I noticed where you were concerned about elements slumping over. I have a 40 lb Denver that is really the same as a 60 lb with smaller pot. The elements do slump over and it seems that this could contribute to early failure. The last time I put in new elements I used more substantial refractory clay retaining pads and notice that where those pads were the elements slumped less. I have wondered if having some interior support might keep that( the slumping) from happening. Since the interior is curved they can not be suspended on a rod. I have thought that possibly cutting mullite tubing in short lengths or possibly using ceramic balls like ones used in a ball mill and inserting them into the elements before installation might prevent the slumping and might help keep the elements in their troughs. Just an idea . I have not tried this.

Richard Huntrods
02-22-2012, 10:08 PM
I called Denver Glass and they wanted $100 each for the 8ga elements for their 60lb furnace. That pretty much convinced me to roll my own... For your 40lb build what was your element resistance? (or calculated power at 220v or 240v)

Also, you said you cut 2 grooves per brick. Did you do a straight wire riser like the Denver elements or did you pigtail them through the insulation between rounds?

One thing I saw in Marks video is that his elements slumped sideways. While to some degree this may be unavoidable, I can't help but believe that it is caused (at least in part) by the large diameter and low stretch. If nothing else, slumped over elements causes hot spots where they touch (or nearly so) and that can cause early failure.

I am also considering using a curtain style element. Pretty much winding the 11ga wire on a 5" wide x 1/2" thick form then suspending it on a mullite rod (well stretched - better than 4:1) mounted about 1/2" off the IFB wall. The intention is to not have the heat concentrated around the element as you do in a groove. What I don't know is if this would be too fragile - moving or bumping the furnace could be an issue.

Yes, the elements slump. That's why 3-1 stretch works better than 2-1 in my opinion. Of course, all I know is that my melter has been in operation since somewhere in 2003 on the same set of elements, including a 600km move in a moving truck.

A couple of things help.

- using a good controller and ssr. I see the power being applied in 1 sec pulses no matter what the temp. Not blasting power into the elements (a.k.a. 'full on until temp') has to help.

- The 3-1 stretch. This from the fellow that wound the elements. When I used 2-1 they failed quickly (second set - first set got chewed by glass volcano due to me being inexperienced on first run-up).

- I washed the elements in acetone and then used gloves as recommended here by people who know.

- they fit tightly in the groove,so they can't slump far -i.e. cannot fall out of the groove as it's a dovetail. The element slots in then drops just a bit.

- I have never used greater than a 100F/hr ramp when heating the furnace from any temp to any temp except in the quartz inversion zone, where I use 25F/hr. Again, thanks to Pete for that. I have a bigger 'inversion zone' in my program; from 900F to 1300F.

- sealing the elements off from the glass vapors. This is straight out of Mark's plans and probably does as much as anything to preserve the elements. Glass (and the vapors) are very corrosive. If they can eat holes in brick, they sure can eat Kanthal.

As for power, it's 240V. I really cannot recall the wiring diagram as that was 2003, but it's basically Mark's plan (including the internal vertical rises of elements inside the chamber) except he has 4 turns around the furnace with his big elements, and I ended up doubling that to 8. The only difference is that my element winder didn't want to make 2 great long elements - Mark uses 2 big elements and wraps them around the furnace twice each - so he split them. I therefore have 4 elements. So I have 2 and 2 (or two pairs), and I'm pretty sure the pairs are wired in series. I just can't recall the overall wiring diagram.

The heat chamber itself is almost exactly 1 cubic foot, and the furnace recovers quickly and heats to 2250F no problem, so it's probably the recommended wattage. That says "10 amps" but I know it idles at aound 4.5 amps at 1850.

Cheers,

-Richard