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Cecil McKenzie
03-12-2012, 09:33 AM
I was reading about moly elements this morning and the ad said that since the resistance in the elements doesn't change over time they don't need to be replaced in pairs. This made me wonder if wire elements always need to be changed as a set and if a new one is paired with an older one what are the consequences. I have often changed a single element in my annealler and have not noticed a problem.

I am thinking about a small 40 lb Denver furnace. I have always changed both elements if one went out and am wondering if I really need to do this. I have been turning this one on and off more frequently than I used to so I was contemplating just changing the one that is currently burned out. Is this a bad idea?

I realize the ad may have been referring to silicon carbide elements rather than the kanthal wire elements which I gather from talk here do change resistance over time. Any insights theoretical or from shop practice would be appreciated.

Pete VanderLaan
03-12-2012, 10:10 AM
Given the high risk factor involved in not changing out elements that are known to have short life spans, I would plan element swaps on a denver on a regulalr basis. In your circumstance, losing an element can mean losing the pot, hardly worth it unless it's all invested in which case all is lost philosophically.

Mike Hanson
03-12-2012, 05:45 PM
When I install a new kanthal element to replace a burned out one I make note of the ohm reading on each element that is still working to see if there has been a change. If there is no significant change in the elements ohm reading that are still working they do not get changed. As far as changing all of them out just because one has burned out is a waste of money as far as I am concerned.

Tom Clifton
03-12-2012, 07:09 PM
The elements for a Denver 60lb wire furnace are $100 each , I am quite sure that the same elements are used in the 40# and the 60#. IMHO, As they are only $100 each(It takes a pair) it isn't worth your time to try to save a few bucks by not replacing both at once. Once you heat the kanthal elements they get brittle and you stand a 50/50 chance of breaking the "good" element while taking the bad one out. Also - if the "good" element is slumped over it is on its last legs anyway.

So, I'm with Pete on this one...

Pete VanderLaan
03-12-2012, 07:10 PM
I would check in at duralite and see if they have a better price on those elements.

Tom Clifton
03-12-2012, 07:15 PM
I would check in at duralite and see if they have a better price on those elements.

The Denver elements are the 8ga "garage door springs" - When I was getting prices for my "lauchner 40#" I didn't see anything that heavy at Duralite. Probably should have called to ask, but Mark had crossed them off his suppliers list so I didn't call.

Cecil McKenzie
03-12-2012, 08:12 PM
Thanks for any input it is appreciated. The elements I have were made by Duralite. It is the first ones I have used from them so we will see how they hold up.

Tom.. It seems the elements fall over almost immediately no matter what. The last pair went up and down at least 4 times with campaigns of 4 to 6 weeks between going up and down. Melting sorted or should I sordid catch barrel glass(SPB) that is preheated to 900 before charging. Pretty much leave the glass at 2050 and set the controller to take an hour and a half to get back to 2050 after charging. This last step I think has made it easier on the elements. Glass is suitable for ornaments, small vases, and tumblers, not paperweights.

Tomorrow I will check the resistance. Thanks Mike. I have some old knob and tube tubes and I am going to see if I can insert them inside the element that I am installing to see if it helps them from falling over and extends the life.

Have thought about switching out Mercury Relay with an SSR . Has anyone tried this on a Denver? The other idea was to use both an MDR and an SSR for redunduncy in preventing runaway heat or accidental shock. Don't know if that is very easy or even possible.

How about converting to moly? I saw some right angle elements that looked like they could be retrofitted to the space in a Denver furnace. Anyone using right angle elements in their furnaces?

Pete VanderLaan
03-13-2012, 04:29 AM
I do think it would run far better with an SCR and I can get you one that costs way less than a watlow that runs very well. Converting a Denver to a moly is a lot of expense for a dubious value. Right angle elements are profoundly difficult to ship. You would need the SCR, the transformer, a good relay, cables and 4-20 ma input controller plus the elements. Probably a $5-6K investment in the equivalent of a '63 Corvair. See Robert Gary Parks observations about fuming agents.

Duralite used to make all of Denver's elements but tired of being blamed for what were essentially design flaws in the furnace itself. He does sell the elements to the best of my knowledge.

Bob Meyer
03-16-2012, 02:57 PM
Cecil,

I have a Denver (the larger one), and I had similar experiences. I've since found that the elements fall over like that from being over-heated. Your controller can dictate how long it takes to get to a certain temp, but it's purely either on or off, so the elements go through significant temperature changes no matter how long a time period you set. It was explained to me that it's these temp fluctuations that shorten the life of these elements.

I tried just changing out the MDR for an SSR first, and had virtually no change in layover or element life. At least with the controller that came with my furnace (a Red Lion), there was no % power output adjustment, so the power was still either fully on or fully off. It was only when I also changed to a controller that would let me adjust % power output that I got the desired effect. I usually have it set to 50-60% power - it's slowed the time to get back to working temp after charging, but doesn't effect the temp much during blowing.

I also ended up adding MDR's to the circuit to be able to completely shut off the current. As you've probably read, the SSR's will have some leakage - of which I can personally attest to. So I just used the MDR's that came with it as shut-offs for when the door's open.

Over a 2-year period now, I'm getting absolutely no laying-down of elements, and they're lasting a full year (up to 14 months). That works out great for me, as I can change them during the slow time of year on a predictable basis.

Also, I get my elements from Duralite whenever possible (I say "whenever possible" because they seem to make them as they're ordered, rather than stocking them as Denver does). They're about 2/3 the price of those from Denver, and seem more substantial somehow (slightly heavier guage).

So as far as element life goes, I'm not sure it's Denver's physical design that's the main contributor - it seems more like the electronics are. I was assured for 2 years that their electronics did the same thing as the SSR's, and was having to replace elements every 3-4 months during that time. And they'd throw out various red herrings like that I needed to pre-heat my cullet, and nothing helped (I've done it and not done it, and found element life to be entirely independent of it).

I ended up spent no more replacing the SSR & controller than I did on a single element change - it was well worth it.

Pete VanderLaan
03-16-2012, 03:33 PM
Have you considered putting alumina rods into the elements. I use them on lower fire kanthal and they really keep the elements in line. You can get them from Coorstek, which is the ceramic part of the beer company.

I think part of the reason that Duralite parted ways with Denver was over blame assignment and Mark felt kind of dumped on for stuff he didn't deserve. The electrical aspect of the Denver has always been a low budget affair. An SCR or current control of any type helps enormously as you have pointed out. The stuff about cullet heating etc is the red herring as you have also observed. Always watch the connections between the inbound wiring and the element. It gets loose and then it arcs. Then it fails.

Duralite doesn't stock the element because it has no relationship with Denver and there is no basis to do it. Prudent planning will make sure you have elements on the shelf for the next inevitable failure. I am continually amazed at how people do not keep spares of anything in their shops including moly elements. I frequently find myself shipping them overnight which is such a needless expense. Sign of the economic times I suppose.

Jordan Kube
03-16-2012, 04:26 PM
Lean manufacturing Pete!

Cecil McKenzie
03-17-2012, 12:28 PM
Bob... Thanks for the pointed useful response. I am not that knowledgeable about electronics and even thought the SSRs could be used to limit the flow of power to the elements.

My furnace has a very simple Love ramping and holding controller so it is of the on-off kind. What kind of controller did you change to? I did not know that SSR s " leaked " and it is good to know. I had thought of using the SSR because I thought it was adjustable for power control and also thought that by using both MDR and SSR I would increase the safety factor. It is my experience that MDRs fail in the on position contrary to popular opinion.

I am not familiar at all with SSRs but assume they have two hot wires going in and two coming out and if you incorporated an MDR did you put the MDR ahead of the SSR or the other way around? Were you able to use the controlling wires from the same outputs on the controller or do you need a controller with multiple outlets? Do the MDR and SSR use the same controlling voltage?

Nice to have someone willing to discuss Denver stuff. I am 62 and slowing down a lot and my propane furnace is 26 years old . This little Denver is little more than a toy but you can actually make work with it so why complain. Thanks for your ideas I think there is a good chance I might change the controller so I can make this work as well as possible. I usually limp along at 2050 F trying to take it easy on the elements. What are you keeping yours at? Thanks.

Tom...I know it probably makes sense to change both to save time and effort(that is what I have always done) but it seems I have more time than money these days.

Pete... I use the Coors tube in my annealer and fusing kiln but since the Denver has a curved interior have not used them in that furnace. I asked Mark at Duralite about putting alumina balls inside the elements once and he seemed to think it was a bad idea . I think he thought it might create hot spots. Someone gave me a coffee can full of old knob and tube electrical insulators. I am thinking of cutting them in short pieces if they will fit and inserting them inside one element and seeing if it helps stop the collapsed element problem.

Richard Huntrods
03-17-2012, 01:20 PM
I have difficulty believing that element fall-over is the problem. As soon as the element gets really hot (glass furnace temps) the coils fall over. However, as pointed out in other posts, there is an oxide coating that forms on the elements that should provide sufficient electrical insulation to prevent short circuiting of the fallen coils and hot spots.

My elements fell over within days, but the same elements worked in my furnace since 2003. I firmly believe the following to be true with wire elements:

1. 3-1 stretch is best
2. don't touch the elements with bare fingers when installing, and wipe the elements down with acetone (or other similar solvent but NOT turpentine or paint thinner) before installing.
3. PROTECT THE ELEMENTS from glass fumes. There should be some physical barrier like alumina board & frax creating a solid boundary between the top of the pot and the heating chamber.

Yes, these are my opinions, but borne of building my own wire furnace (Mark Lauckner 40lb plans with some modifications) and running it since 2003. I'm on my 3rd set of elements, and those lasted in 24/7 use since early 2004 until 2006 when the furnace was shut down and moved to the west coast. They continue to run where I am now.

As for controllers, I'm using a Fuji PXR-3 (simple, cheap, 2 programs, RSS485 computer control) and two SSRs. (one on each power leg). The two SSRs are not really needed, but they were cheap and I wasn't sure when I was starting out.

An SSR is just a 'silicon relay'. It's a plug-in replacement for a mercury relay, which I could not get. It is simple on/off.

HOWEVER, a good controller does not just blast full power at the system it controls. My Fuji applies power in '1 second on/1 second off' pulses, at least when you are near the set point. And perhaps that's the other side of the equation - I never just blast full power to the elements, I ALWAYS run a maximum 100F/hr ramp, except if I'm heating thru the quartz inversion zone when I run 25F/hr.

So the slow ramp means the controller always does the 1sec on/1 sec off pulse.

Anyway, this is what I know. My furnace works really, really well.

I now idle at 1300F, blow and charge at 2100F, cook at 2250F (3 hr cook) and squeeze at 1850F. I have only ever had Spruce Pine in my furnace. First charge ever was batch from a friend, and only SP batch ever since.

-R

Pete VanderLaan
03-17-2012, 05:21 PM
SCR's leak power as well as SSR's While Watlow reassuringly runs both hot leads into the SCR, one simple passes through and the other is regulated so you always have a live leg in your system unless you shut off the power. SSR's have one leg in one leg out. The SCR's I m currently using has only one leg going in to the unit so there is no illusion about the other leg which heads straight for the transformer H1. H3 passes through the SCR before being stepped down on voltage.

Bob Meyer
03-17-2012, 09:02 PM
Richard -

It sounds like your experience is with something other than a Denver, though some of your statements will certainly apply.

The elements laying over is not necessarily the problem - but, at least in Denvers, a symptom of the problem. With the original electronics, the elements get far hotter than they need to at times, causing both the laying over and premature failure. Now that I have that addressed, I get neither.

As far as the rest of your comparison, I'm not sure that your experience with the Lauckner furnace is going to transfer to the Denver. I think that elements lasting for years is a phenomenon totally unseen in a Denver, no matter what the approach. And I think it's a physical impossibility to shield the elements from the glass fumes with a Denver - I think it's a much different setup than what you have.

Bob Meyer
03-17-2012, 09:26 PM
Bob... Thanks for the pointed useful response. I am not that knowledgeable about electronics and even thought the SSRs could be used to limit the flow of power to the elements.

My furnace has a very simple Love ramping and holding controller so it is of the on-off kind. What kind of controller did you change to? I did not know that SSR s " leaked " and it is good to know. I had thought of using the SSR because I thought it was adjustable for power control and also thought that by using both MDR and SSR I would increase the safety factor. It is my experience that MDRs fail in the on position contrary to popular opinion.

I am not familiar at all with SSRs but assume they have two hot wires going in and two coming out and if you incorporated an MDR did you put the MDR ahead of the SSR or the other way around? Were you able to use the controlling wires from the same outputs on the controller or do you need a controller with multiple outlets? Do the MDR and SSR use the same controlling voltage?

Nice to have someone willing to discuss Denver stuff. I am 62 and slowing down a lot and my propane furnace is 26 years old . This little Denver is little more than a toy but you can actually make work with it so why complain. Thanks for your ideas I think there is a good chance I might change the controller so I can make this work as well as possible. I usually limp along at 2050 F trying to take it easy on the elements. What are you keeping yours at? Thanks.

Tom...I know it probably makes sense to change both to save time and effort(that is what I have always done) but it seems I have more time than money these days.

Pete... I use the Coors tube in my annealer and fusing kiln but since the Denver has a curved interior have not used them in that furnace. I asked Mark at Duralite about putting alumina balls inside the elements once and he seemed to think it was a bad idea . I think he thought it might create hot spots. Someone gave me a coffee can full of old knob and tube electrical insulators. I am thinking of cutting them in short pieces if they will fit and inserting them inside one element and seeing if it helps stop the collapsed element problem.


Cecil,

Don't think that the SSR's can't be used to limit the flow of electricity to the elements. From what I understand (and I'm sure I'll be corrected if I don't!), the SCR's will literally limit the flow, and it will be a steady flow. The SSR is on/off as Richard states, but on/off fast enough that for these purposes it apparently has the same effect as an SCR with regard to element life (per a Watlow engineer).

However, the SSR can't do that without the appropriate controller. So you need both.

(At least this is what I've come to understand after talking to or reading from a number of people, though everyone seems to have a slightly different spin on it.)

And I'm guessing you're right in that it would be a real trick to get much of anything inside these elements AND be able to get them into place. But I'm not sure of the configuration of your furnace - is it an invested pot with the elements above, like the larger Denvers?

Let me know if you'd like to trade stories on our Denvers, though (via PM?). I've accumulated a bunch of different info on them that might be helpful.

Richard Huntrods
03-17-2012, 11:33 PM
Richard -

It sounds like your experience is with something other than a Denver, though some of your statements will certainly apply.

The elements laying over is not necessarily the problem - but, at least in Denvers, a symptom of the problem. With the original electronics, the elements get far hotter than they need to at times, causing both the laying over and premature failure. Now that I have that addressed, I get neither.

As far as the rest of your comparison, I'm not sure that your experience with the Lauckner furnace is going to transfer to the Denver. I think that elements lasting for years is a phenomenon totally unseen in a Denver, no matter what the approach. And I think it's a physical impossibility to shield the elements from the glass fumes with a Denver - I think it's a much different setup than what you have.

Really? I said nothing that isn't 'wire 101' in my opinion. Proper stretch. Clean the wire and don't contaminate it. Keep it away from glass fumes. Get a good controller package. What part doesn't translate?

-R

Pete VanderLaan
03-18-2012, 07:32 AM
I haven't sen a Denver in years but there was nothing sophisticated in the setup. Brutal doors, big split bolts, heavy elements. The furnace is on!

You get what you pay for. Richard has spent a lot of years on wire equipment and I tend to listen to what he says. Anyone who can nurse elements as he does is worth checking out. I do think that soft start is a key to long element life and that can be achieved in a lot of ways. Cutting amps to a Denver just means a colder furnace. They're one path to getting started with glass but I wouldn't want to live there. The cost of replacement parts suggests that the user is relied on to be pretty clueless.

There are better ways.

Bob Meyer
03-18-2012, 12:07 PM
Really? I said nothing that isn't 'wire 101' in my opinion. Proper stretch. Clean the wire and don't contaminate it. Keep it away from glass fumes. Get a good controller package. What part doesn't translate?

-R

1. Proper stretch: you apparently have not tried stretching the elements used in a Denver. They simply do not stretch like the other wire elements I (and I assume you) have used. I guess I could have Duralite stretch them differently, but I'm guessing they already know more about proper stretch than I do.

2. Contamination of the wire? I've tried it both with meticulous cleaning and without, and have observed absolutely no difference. And believe me, I've changed elements often enough to be able to note such a difference!

3. Keep elements away from glass fumes? You really don't know how a Denver is set up, do you?

4. Get a good controller package. Well, we certainly agree on that. That's the single factor among those you mention that's both within my ability to influence and has worked.

Bob Meyer
03-18-2012, 12:38 PM
I haven't sen a Denver in years but there was nothing sophisticated in the setup. Brutal doors, big split bolts, heavy elements. The furnace is on!

You get what you pay for. Richard has spent a lot of years on wire equipment and I tend to listen to what he says. Anyone who can nurse elements as he does is worth checking out. I do think that soft start is a key to long element life and that can be achieved in a lot of ways. Cutting amps to a Denver just means a colder furnace. They're one path to getting started with glass but I wouldn't want to live there. The cost of replacement parts suggests that the user is relied on to be pretty clueless.

There are better ways.

Pete, the only part of your statement I'd really take issue with is the bolded statement - unless you mean something other than what I'm thinking by "colder". I ran my furnace at 2050 before and after the change in electronics. The single factor that's changed with the electronics has been the recovery time after a temperature drop.

If I'm blowing things where I open the door frequently, I get a slow, moderate (50 degrees max) decrease in temp through the day - usually there's none. The biggest difference comes after "charging" - previously, my furnace would be back up to temp within an hour or two, and now it takes much longer. But it's still ready to go (up to temp) long before I start blowing the next day.

One other thing, though. I do take some offense at the implication of being "clueless" because of maintenance costs. It now costs me $350 a year for new elements, and whatever a pot costs every 3-5 years (being invested) - along with whatever refractory is required. And that's on top of an initial cost of only $4-5,000 for the furnace. It might not get me world-class glass, but that alone is not necessarily one of my goals. What it does do is let me do what I want to be doing.

And then I hear you guys with the moly furnaces talk about the maintenance issues - fragility, availability and cost of the elements, consequences and costs and difficulty in changing out broken pots, etc.. Maybe it just sounds expensive, but I'd really be interested in seeing some kind of breakdown of the long-term costs - including initial expense and electrical consumption (one guy in our area had his electrical expense increase dramatically with his moly) - of the moly furnaces over time. Of all the things about molys I've seen discussed here - and there have been many - somehow it's the expense that's never seemed to come to light. I've gotten a feel for your views on capitalism and pretty much anything mentioned money, Pete, which may explain that. But we - me, at least - just don't live in a world where that aspect can be disregarded. You've mentioned before that using a Denver rather than a moly is like driving something like a Chevy instead of a Porsche - well, I can't say I ever hope to be able to afford driving a Porsche, either!

Cecil McKenzie
03-18-2012, 05:28 PM
I bought my Denver from a neighbor who started blowing glass at 80. A glory hole and the furnace cost $1300 . He was never able to get it running properly and after a month of working with it I found that it had a loose neutral wire to the controller so it would keep starting the program over.

I am not an apologist for Denver but it will melt glass. I have also known very successful glassblowers who have or still use the larger Denver furnaces.

I do want to run my little furnace in the best possible manner I can, which is why I asked about the elements in the first place.

If I could find a modest sized used moly furnace for sale I might consider using one but at my age I hate to spend that much money on equipment. I also might try to build a furnace like Richard is talking about. Everyones comments have been useful to me and I will contact Bob with a PM on particulars about Denver stuff.

Richard.. On a side note about elements. Once one of my annealler elements came loose and touched itself about 6 inches from the end of a loop of the element. When it did this it started contact and the annealler kept working without that 6 inch loop. It worked normally for the rest of the day and the next day I replaced the element but this seems to indicate that even with the oxidized surface that contact can take place when elements touch. I have also wondered if I knew where there was a break in an element could I turn the furnace off and use a stainless steel alligator clip of some kind to reinitiate contact and keep the furnace running for a while. Thanks again for your comments.

Richard Huntrods
03-18-2012, 06:32 PM
1. Proper stretch: you apparently have not tried stretching the elements used in a Denver. They simply do not stretch like the other wire elements I (and I assume you) have used. I guess I could have Duralite stretch them differently, but I'm guessing they already know more about proper stretch than I do.

No, I have not tried stretching the 8ga 'garage door springs' of a denver. The original design of the Lauckner furnace calls for those very elements, but I could not get them easily, so I redesigned for what I could get.

However, anyone who has the Lauckner 40lb furnace video can WATCH Mark Lauckner stretch those very 8ga elements that come from Denver. So they can be stretched, but it does not look like fun.


2. Contamination of the wire? I've tried it both with meticulous cleaning and without, and have observed absolutely no difference. And believe me, I've changed elements often enough to be able to note such a difference!

Maybe cleaning doesn't do anything, but it'seasy and does not appear to hurt the elements. I did the cleaning on the advice of the fellow who made my elements. Cleaning also seemed to be the general recommendation of other wire furnace builders over the years, so I'm not the only one recommending it.


3. Keep elements away from glass fumes? You really don't know how a Denver is set up, do you?

No, but it sounds like the design of a denver is optimized to sell elements to owners.


4. Get a good controller package. Well, we certainly agree on that. That's the single factor among those you mention that's both within my ability to influence and has worked.
Yep.

-R

Pete VanderLaan
03-18-2012, 09:08 PM
well, I just ran my moly for two straight years without changing any elements or pots. I changed without breaking an element. It does require maintenance clearly but if you do it, it doesn't run up costs. It's extremely well insulated which is what I was referring to on the Denver when I said cutting the amps is a colder furnace. Gas furnaces break burner heads and such. They cause fires. I would never leave one unattended which I will with the moly. I did not suggest that anyone was clueless.

Tom Fuhrman
03-19-2012, 08:52 AM
when DENVER started making these furnaces about 25 years ago, they were state of the art and they were the only thing on the market like this. The problem is that they haven't kept with the times and redesigned them to any great extent to take advantage of all the things that have been developed in the way of new designs and technology since their inception. If I were still trying to use the computer I had in 1990, I'd have a hard time, and furnaces have not advanced as rapidly as electronics but things are different now and it's time for them to develop some new designs that are more functional and more atuned to today's technology.
I'm with Pete, I wouldn't want to go back to my gas breathing/sucking monster furnace that I had 25 years ago. Just like I don't want to drive the gas guzzler I owned back then either. We keep learning better ways to do things, granted they may be more complicated, but hopefully better.

Scott Novota
03-19-2012, 09:41 AM
I think I could write a book on all the little things that I have done wrong over the years with running these little wire melters. None of it is rocket science when you look back on it. It sure seemed at the time not to matter due to lack of experience or anyone telling me different.

Everything from simple general knowledge of ramp up schedules to the esoteric knowledge of programming a controller that understands the binary language of moisture vaporators.

Either way this board has been a god send and I hope that everyone will continue to provide thier large and diverse knowledgebase to those of us that have this bug under our skin that we just can't seem to get rid of. We addicted few that drive down the same insanity pocked path you have already run down need the warnings. Thanks for putting up the "LARGE DITCH!" ahead signs.

Pete VanderLaan
03-19-2012, 01:20 PM
I don't quite have the same rose colored glasses that Tom does. DENVER was actually the first commercial furnace available but it certainly was not state of the art in my mind. It was certainly low budget and casting Ipsen crucibles in a giant block of Mizzou seemed odd to me ( Still does) . At that time gas was really still very cheap and electricity was not but it did get a lot of people in to the studio who otherwise might not have been there.

As a portent of things to come, I was hired by Dale to build furnaces for RISD in 1976. I think I got paid about $200.00.

Tom Fuhrman
03-20-2012, 09:21 AM
Dudley still advocated the cast in crucible for many years. He was one of the first to address energy issues in his little book that he wrote.
I know, he was able to get by with things no one else could.
Pete: who was making small electric furnaces for sale in the early 80's, besides Denver?
First sc furnace I saw was on Whidbey Island in the mid 80's and there were a few others at that time. I didn't know anyone who was willing to share their expertise at that time. As I recall Electroglass came on the scene just a year or 2 after this with prices starting at about $20K. That was a lot of $ in the 80's for a small shop.
I can attest to the fact that if you hadn't come up through a educational institution and studied glass, it was a rather tight group to break into. I went to the GAS conferences and got a couple of the small newsletters but aside from that, communication between most glass people was slim. I made trips to personally see whatever I could wherever I could go. None of the summer camps were running electrics and most of their instructors were clueless about it. Spent a couple weeks with Dudley once which helped a lot.

Pete VanderLaan
03-20-2012, 10:07 AM
I am asking Dudley to come over to the house and to give a lecture on pot furnaces when I teach the class. He's about an hour and a half from here.

I don't think anyone else was offering furnaces in that time period but most people didn't want to buy them, they built their own. Electric was a rare bird and was unattractive on operational costs except in the Northwest. Harvey did have his super kanthal furnace in North Carolina in the '80's. Free standing pots were already becoming the norm by 1980 though but the tank was common and still being built out of Crystalites, the brick of choice for decades which interestingly enough had a primary usage in Crematoriums.

It wasn't really until the explosion of glassblowers that occurred around 1986-90 that commercial equipment became a viable way to make a living. The glassworkers had been sufficiently dumbed down by Dale ridiculing building your own stuff, which he started to do back in the '70's that those skills were quickly being lost. Soon , the people teaching at the schools didn't know how to build them either. They seem pretty much gone now and Eben really nailed it with his video suggesting you weren't really any good if you didn't have your equipment built for you.

Cecil McKenzie
04-10-2012, 09:42 PM
Decided to leave the one unbroken element and replace the one bad element. Cut enough knob and tube porcelain insulators (old time electrical items that have a tube with a knob on one end ) to loosely fill the element . This is to keep the elements from falling over on themselves. Got the unit hot and made things for a week or so but on Saturday the furnace was down to 1100 F or so .

I immediately thought that the advice to change both elements should have been followed. I turned the furnace off and checked the continuity on the elements and found there was continuity on both elements so I tapped the mercury relay and restarted the furnace after reattaching the elements. The furnace started to heat up and by Saturday night it was a 2020 F. I figured the relay had stuck in the open position and after tapping it it was working okay. Sunday the furnace was down around 1100 F.

Now I thought that I had just been lucky and had somehow pushed the bad element together enough to work for a day and the element really was bad.
Checked continuity on the elements and both still seemed intact so I took out the MDR and set it up to test it. I found that one side had failed in the closed position and tried to get the other side to to fail in the open position but it seemed to work correctly through maybe 20 cycles. The test seemed to indicate that the relay should have been powering up the elements but that was not the case. Took the relay off my spare annealler and put it on the furnace and it has worked for two days now. Still not positive what happened. I am thinking the relay was bad and just infrequently failed in the open or not powering position.

If it is true that it failed in the open position that is a first for me. With a score of maybe 7 failing in closed position to 1 failing in open position. It seems that the odds are more for it failing in the open position ( for a two pole relay) than in the closed position. To fail open only one side needs to fail but to fail closed both sides have to fail.

So I am still not sure exactly what happened. Maybe the controller is malfunctioning. Just hope I have glass for the next couple weeks.

Will take a picture of my knob and tube experiment sometime. So far it seems to be keeping the elements from falling over. Duralite said that it didn't matter if the element coils touched anyhow so maybe it doesn't matter if they fall over. The elements don't look very comfortable when they are all collapsed.

Andrew Boatman
04-11-2012, 09:21 AM
When I ran the Denver I thought the elements slumping and touching itself seemed bad. When the element would fail it was always in a place it touched onto itself in the next coil. It would seem to super heat and melt. I had a lot of meltage of coils that way. The metal would melt into the castable. Air hammer had to be used to break out that metal and some Sariset patch to rebuild the shelf. It I did not get all the metal out it would fail there quickly.
I wonder too if they fail in the same location each time. More times then not failure occurred to the right side of the middle about six to eight inches over. Once or twice it was on the left.

Sam has two Denvers. He runs one. When it breaks he brings the other online and fixes the other. Rotates around this way.

I know Glenda in Guthrie has had fits with her new Denver. They have never been able to get it running properly.

I think the distance from the furnace wall to the control panel is not enough. There just seems to be too much heat there for me. I would like to see a separate panel for the controller.

We built a good counter balance for our door making for easier opening too.

On an unrelated note: I do not understand why an invested pot is bad. My assumptions would be; they are hard to change out. They break and leak and there is no way to know this. They last for years and years. Energy consumption increases greatly as one has to heat the entire mass of refractory holding the crucible. Expansion is bad as there is no place for the crucible to go.

Pete VanderLaan
04-11-2012, 11:53 AM
Mercury relays can fail and do exactly what you describe Cecil. I had a whole batch I bought from another glassworker who hung around the Corning Surplus and they all did the same thing. The relay warmed up and broke it's connection. Sometimes it reconnected as it cooled and would fire again, sometimes not. I went to a 60AMP MDR and the problem went away. I am still using that same MDR 15 years later.

Tom Clifton
04-16-2012, 11:25 AM
Duralite said that it didn't matter if the element coils touched anyhow so maybe it doesn't matter if they fall over.

As Andrew noted "When the element would fail it was always in a place it touched onto itself in the next coil. It would seem to super heat and melt."

Duralite is technically correct in their statement. The turn to turn voltage drop in the element is so low that the oxide layer doesn't break down so here is no electrical short. However, there is increased local heating where they touch which shortens the life of the element. This is fine for the purveyor of new elements who may have to make boat payments on a regular basis.

The only concerns I have with using mullite or porcelain tubing to support elements set into a groove is that the tubing its self traps heat that is supposed to be radiated out into the furnace which effectively raises the watts/square-inch value for the elements. (It doesn't really raise the value, just means the elements run hotter, and your elements oxidise faster and could fail sooner)

There are a couple of solutions. The first is not to worry about it and just be sure to change elements on a maintenence schedule before they fail and melt into your castable. The second would be to convert to an SCR type controller that does "'current limiting" which can run the element at a more constant temperature (compared to an on-off controller with its full on or full off operation). I believe there have been other threads on this subject recently. Even with an SCR type controller the elements will eventually fail - just not nearly as often, so scheduled maintenence is still a good idea.

Cecil McKenzie
04-21-2012, 02:14 PM
/Users/lisamckenzie/Desktop/interior of furnace/elementsinelectricfurnace.JPG


Here is a picture from of the elements in a Denver Furnace. The top elements have been installed for a longer period of time and they don't have any support .The bottom elements have been hot for almost 4 weeks and have had porcelain tubes inserted inside the element before installing. The tubes are short pieces to allow the element to circle around the inside of the furnace wall.

From the photo it seems the top element has some hot spots where it has collapsed on itself whereas the lower element seems to glow more evenly. Since the elements have seen different amounts of service it is a little like comparing apples to oranges but it seems like putting supports inside might be a good idea.

One other consideration in this idea is if there is any thermal shocking leading to small bits of the tubes getting in the glass. So far there has not been any sign of this.

If you look carefully the tubes can be seen inside the lower elements. They are not as large as the opening in the elements.

Bob Meyer
04-22-2012, 11:26 AM
Cecil, from what I've been told (as well as had the misfortune to experience), using new elements with older ones will cause more problems than I think you're anticipating. Primarily that the older element will heat up more than the new one because of differences in resistance - exacerbating the problems that the Denver electronics already present. The older one is likely to fail faster than it would've if running with similarly aged elements. So, unfortunately, I'm not sure you'll be able to draw any conclusions from this experiment.

It would be interesting, though, to have a head-to-head comparison - with and without the internal support - with all new elements. My past impression has been that the "falling over" of the elements is a symptom of being severely overheated, rather than the cause of it. But maybe there's something to the idea of providing physical support.

Still, it's hard to get around the fact that I've gotten direct input from the head engineer at Duralite, an engineer from Watlow, and now apparently one from Kanthal, that all say that element life is related more to how you apply the current than anything else. And it's really a pretty cheap fix - not much more (or even less than) a set of new elements.

Cecil McKenzie
04-22-2012, 02:13 PM
Bob, I think your suggestion to use a variable controllable power supply is probably a good idea. ( I will probably pick your brain someday when I have time.) I have been short on funds and time so I am trying to get by on a shoe string.

I tested the resistance on the old element and it was the same as the new one so as someone else suggested I decided to try it and decided to try the interior support idea at the same time with the results I have shown. I haven't really drawn any conclusions yet and a head to head comparison might be interesting even though a head to head experiment is probably not really possible since it is hard to examine the grooves these elements lie in and if little bits of old elements are left in the grooves that would contribute to early failure.

If you look at the upper element pictured , on the bottom part a little left of center is a very bright spot. It was at a spot that looked like that where the last element failed.

I realize this post is probably irrelevant to many readers but since I have seen many comments about elements collapsing I thought it might be relevant to a few people. And might be another tool to help prevent early failure or allow for longer element life in general.

I have been using this furnace as a temporary source of glass when my propane furnace is off and have been trying to design work that looks nice even if it is made from catch barrel glass. I only have about 20 50 gallon barrels to go through.

Pete VanderLaan
04-23-2012, 04:59 AM
you guys keep using the term variable power supply. What does that mean to you?

Cecil McKenzie
04-23-2012, 11:14 AM
It is my understanding that either an SCR or an SSR can be used ( Off the top of my head I don't know which is which) to limit how much current is being released into the element or the current is turned on and off a lot so the element isn't just on or off as it is with a simple relay. I think Bob mentioned what he used to fix his Denver to get longer element life. I hope to do this on mine in the future.

Tom Clifton
04-23-2012, 01:17 PM
you guys keep using the term variable power supply. What does that mean to you?


What it means to me in the strictest sense of the word is a power suply that provides an outut voltage (either AC or DC) that is infinately adjustable from zero volts to (name your number). In the case of a "linear" supply it may be as simple as a transformer with "infinite" taps such as a variac, or a magnetic amplifier (used in stage lighting so, so many years ago) The same thing can be done with switched digital supplies that synthesize a sine wave (single phase or multi-phase)

A more liberal definition may include an SCR/Triac type controller that by controlling the firing phase angle can effectively lower the RMS voltage output as done in "current limiting" configurations. Electronically this is easy to do, and reasonably power efficient, though it does make your transformers sing a bit...

A digital switching supply can be very efficient as long as the load is closely matched to the design. If you design a switcher to load at 100 amps and run it there you may get better than 95% efficiency. If hyou design for 100 amps, but only load to 25 amps, your efficiency may be 50% or less...

Variacs are usually a manual controlled affair (twist the giant knob) and aren't often rated for more than 20-30 amps unless stacked, and that uses a lot of iron and copper and gets expensive. They can be bought surplus very cheaply, and do work quite well in a set and forget situation.

Pete VanderLaan
04-23-2012, 05:31 PM
It is my understanding that either an SCR or an SSR can be used ( Off the top of my head I don't know which is which) to limit how much current is being released into the element or the current is turned on and off a lot so the element isn't just on or off as it is with a simple relay. I think Bob mentioned what he used to fix his Denver to get longer element life. I hope to do this on mine in the future.
*********
That would be an SCR if you are chopping the Sine wave into bite sized edible chunks. It makes the "power on" a softer experience for the heater.

Bob Meyer
04-24-2012, 06:31 PM
Cecil - I see where you're coming from now. I can't blame you for wanting to rig up something short-term. Let us know how that new element holds up with the supports!

Bob