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Contoller questions for glass furnace

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VirgilJ

VirgilJ

Electrical
May 10, 2011
9
I’m in the process of building a studio glass furnace and have concerns about the set-up for the control system.
I have 240V single phase
400amps into the building off a 350 aluminum underground line.
Furnace will be on a on a separate 200 amp circuit. Breaker in the box and then fused disconnect 4 feet from the control panel.
Rest of building will draw about a 100amps on their own panel.

2 series of 4each silicon carbide heating elements.
RR 55 x 29 x 1.5 60V each.
Total amps 169 +0/-20 olms: .71 +20/-0

Recommendations by Watlow are:

Controller: EZ-Zone Digi9tal I-O model #PM-9R1 FA-ALAJ-AAA
Multiple Ramp and Guaranteed soak availability.
Temperature and Current limiting

Will have 2 S-type 24gauge, Thermocouples in the furnace
One for regular use and one for over-temp control.
temperatures will be regularly 2130F and (when filling the freestanding pot with glass materials) will be 2350F for up to 20 hours.
The furnace runs 24/7 only shutting down for repairs and a 6 week period in the winter.

SCR : Power series PC-11-F30B-0000
Has a built in fan and fuse.

Some of the questions I can’t seem to get a consistent answer on are:
Element company says no transformer needed.
One Panel builder and Watlow says YES to Tap-transformer if I run the SCR on phase angle. (and should do that to extend the life of the elements.
Other opinions from various engineers:
Can run the furnace on zero –cross (even ssr’s).
Can Run the furnace on SCR zero cross.
Can run the furnace on SCR phase angle WITHOUT a transformer and adjust the ??? as the elements age AND they won’t age prematurely.
Need a safety contactor controlled by the controller between the 240V line coming in and the SCR.

There are more questions, just can’t think of them right now.

ANY THOUGHTS?!?!
 
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40kW single phase... Blech.
Around here (CA) that would be about $800/month.

Why'd you choose gas over electric?

Can't get 3phase?

You want the SCR phase angle control. I don't see a reason to use a transformer, though someone else may have an opinion on a interposing transformer.



Keith Cress
kcress -
 
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Propane in this application (studio glass furnace) is at least twice as expensive.
Three-phase in not available.
With further discussions today with Larry at Watlow it looks like I'll be using the SCR with current limiting and be able to lower the amps. This will reduce costs and take care of the 1.25 x 169amp problem. Also, IsquaredR (element manufacturers) says they go by 20% instead of 25%. This would still put it at 202.8amps, but then dialed down.
Larry at Watlow now also recommended going with zero-cross and 50% setting. He did say, and I still don't get, that even at the 50% the SCR set at 100% amps would be trying to draw 399 (volts or amps????) and could cause light flicker in the building or even overheat the Energy companies pole transformer. Talking to Audrey again (IsquaredR engineer) She was unclear as to what Larry was talking about. I'll be contacting Larry again at Watlow for clarification of this.
 
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Yes you should continue questioning them.

I would not go with zero crossing since it will cause flicker issues.

I don't believe that a system that "can" draw more can just be turned down a little to "fit you" into a smaller service. As that means you or someone one else could just "turnitup" in the future causing a hazard.

I think you're going to regret not having 3phase brought in.

If you don't you should definitely go for the larger current setup. Go with 225A not 200A. This will help with any harmonics heating issues you come up - with and you likely will. It will also prevent you from having to pull it all out and up-sizing it when you find things running too hot.

Do you not have natural gas? Or does that not get hot enough? (Always learning.)

Keith Cress
kcress -
 
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>Need a safety contactor controlled by the controller between the 240V line coming in and the SCR.

Well, if you don't have a safety contactor, what's the high limit controller output going to do? Open the control signal to the SCR? SCR's fail in the ON state. A failed SCR keeps passing current, it does not fail open. It can make for hot time when that happens, I've seen it.

I was a mere lad, calling on a ceramics components distributor (clays and glazes) who had a shop in the back where they manufactured kilns. I was there to talk about temperature controllers on a Friday.

They'd just finished painting a kiln that was going to be fired up to bake out the brick lining over the weekend. In looking at the control panel, I took note of the fact that it was 3ph SCR fired, but that there was no shunt trip breaker upstream of the SCR, their overtemp controller merely opened the 4-20mA to the SCR which was supposed to drive it to the OFF state.

I mentioned to the owner that such a control scheme would not cover the situation where an SCR failed in the ON state, which is what they do. He asked how often that happened. I didn't have an answer (about 2 weeks after starting the job), so he shrugged it off.

The following Monday morning I got a crisis call from the owner - could I come over immediately, they'd nearly had a fire over the weekend.

The control panel had not yet been fastened to its rack on the kiln's shell, it was sitting on the concrete floor with extension wiring which had kept it cool enough to keep running. At least one of the SCR's had failed over the weekend, pouring all those unregulated KW's into the heating elements. The kiln's steel shell had all its paint burned off and the radiant heat from the shell had managed to char the exposed ends of the wooden workbench close to it. It was just far enough from the wall to not have the wall catch on fire. The only fire flames had been the papers in the work order packet taped to the shell, but the ashes had fallen to the concrete floor so the fire hadn't spread.

The owner wanted to know all about that 'shun thing' that I'd mentioned earlier on Friday.

A word to wise . . . .
 
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Given the variety of answers you've received on SCR's, these Googled silicon carbide/SCR app note might be of interest:


Holland apparently writes articles in the heater element field:

It appears that the only recommended type of SCR is phase angle fired (not zero fired), with tapped transformer as an option.
 
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So, after a couple of weeks of educating myself on the SIC element system and molybdenum disilicide (MoSi2) element system I will be going with the Moly elements. Initial costs are much more, but should be cheaper in the long run...longer element life to go with 10 year or more of life of the glass furnace. I will be using a dual controller, 2 s-type thermal-couples - one regular control - one over temperature control, safety contactor before the SCR, phase-angle SCR with limit control, transformer, 8 elements (4 per series so on bank stays on if an element breaks in the other bank).
Thanks for all the feed back on SIC.
ANY THOUGHTS on the Moly option??
 
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not a reduction atmosphere and with the moly AND a transformer I can more power to the elements.
 
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