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Old 03-01-2018, 11:58 AM
Jack Abner Jack Abner is offline
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Preheating Furnace To Reduce Thermal Shock

In our new gas fired free standing pot furnace we have incorporated a removable electric element for overnight preheating. Reducing thermal shock to the refractories and crucible by preheating the furnace to 600 degrees Fahrenheit and also reducing start-up times. The furnace has been operating four months and cycled to temperature 16 times (weekend warriors) with no apparent damage to refractories or crucible. We typically charge 50 pounds of cullet in the 100 pound pot reaching temperature (2060) in 3.5 hours. We slowly bring the temperature from 600 to 1200 over 1.5 hours then from 1200 to 2060 over 2 hours.
Any thoughts or experience with preheating would be appreciated...
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Old 03-01-2018, 12:28 PM
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going from 600 to 1200 in 1.5 hours is really really fast. I'm inclined to congratulate you on being quite lucky
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Old 03-01-2018, 12:47 PM
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Quote:
Originally Posted by Jack Abner View Post
We slowly bring the temperature from 600 to 1200 over 1.5 hours then from 1200 to 2060 over 2 hours.
Any thoughts or experience with preheating would be appreciated...
Best quote ever!
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Old 03-01-2018, 01:13 PM
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Search "quartz inversion" on this site.... you will have an eye-opener....
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Old 03-01-2018, 01:54 PM
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I have used outdoor halogen lightbulbs in the furnace for preheating. Up to 400-500 degrees F over several days. 140 lb. Natural gas fired furnace, venturi burner, freestanding crucible. After that preheat I take another three days to heat up to 2250 before I start a melt.
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Old 03-01-2018, 02:02 PM
Jack Abner Jack Abner is offline
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Thanks Pete 😉 I love this 16 pot it has the perfect dimensions for my needs... I decided on the preheating based on some reading on the effects of moisture on thermal shock... Im amazed at how much moisture is driven out of the furnace during the preheating process.
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Old 03-01-2018, 02:18 PM
James Ennis James Ennis is offline
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we have used space heaters, bbq lighters(electric) and light bulbs.....but your
ramp up to temp is way too fast. you will have an EPIC fail, a free standing
pot wont take this.
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Old 03-01-2018, 03:11 PM
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moisture is totally separate from thermal shock but does need attention. When water is chemically bound which is the case in most castables, it needs to be driven off under 212F so it doesn't turn to steam since steam is water expanded by about 200 fold. That expansion will crack any casting, so, a new furnace needs to be candled at under 212/ Kenny Pieper used to suggest holding a mirror up in the door port and if it fogs, there's still moisture.

The pot doesn't have any moisture, it has been fired to 2380F. It is still totally susceptible to cracking from expanding too fast. Do what Mark suggested about looking in the archives for quartz inversion. It can be deadly between 980 and 1100F when the silicon atom flips. I bring pots up at 50F per hour with a major slowdown at 980-1100F. It's all in the archives and it's just as important for your furnace as it is for your pot. While I recognize your eagerness to be up and running, you are asking for disaster. The fact that the pot has not failed at this point is surprising to me. It could at any time. Pots don't respond well to being shut off even once. They put up with it, no more. but it does shorten its life.
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Old 03-06-2018, 10:43 PM
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Quote:
Originally Posted by Pete VanderLaan View Post
moisture is totally separate from thermal shock but does need attention. When water is chemically bound which is the case in most castables, it needs to be driven off under 212F so it doesn't turn to steam since steam is water expanded by about 200 fold. That expansion will crack any casting, so, a new furnace needs to be candled at under 212/ Kenny Pieper used to suggest holding a mirror up in the door port and if it fogs, there's still moisture.

The pot doesn't have any moisture, it has been fired to 2380F. It is still totally susceptible to cracking from expanding too fast. Do what Mark suggested about looking in the archives for quartz inversion. It can be deadly between 980 and 1100F when the silicon atom flips. I bring pots up at 50F per hour with a major slowdown at 980-1100F. It's all in the archives and it's just as important for your furnace as it is for your pot. While I recognize your eagerness to be up and running, you are asking for disaster. The fact that the pot has not failed at this point is surprising to me. It could at any time. Pots don't respond well to being shut off even once. They put up with it, no more. but it does shorten its life.
"since steam is water expanded by about 200 fold"

No, no: it is much worse (or better ) than that. expansion upon boiling is 1600-1700!

https://van.physics.illinois.edu/qa/listing.php?id=1734
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Old 03-07-2018, 01:05 PM
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1600-1700? Not sure what you are saying here? Anyway Ive talked to others in the business of manufacturing crucibles for melting glass. Ive been told these types of pots have approximately 17% porosity and can absorb moisture from the air especially from condensation... Also been told that reheating multiple times is not and issue for these types of pots as long as the heating and cooling rate is kept to 100 degrees Celsius per hour... Anyway Ill keep doing what is working and keep you posted.
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Old 03-07-2018, 02:04 PM
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"Also been told that reheating multiple times is not and issue for these types of pots as long as the heating and cooling rate is kept to 100 degrees Celsius per hour..."
********
The results from my experience which is not insubstantial would tell me that simply isn't true. I have never seen a pot do "well" being turned on and off. I sell pots and I use them. I would not do what you do under any circumstances. I think you've been advised well by a number of people. Fortunately, I sell pots.
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Old 03-07-2018, 03:48 PM
Josh Bernbaum Josh Bernbaum is offline
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Quote:
Originally Posted by Jack Abner View Post
In our new gas fired free standing pot furnace we have incorporated a removable electric element for overnight preheating. Reducing thermal shock to the refractories and crucible by preheating the furnace to 600 degrees Fahrenheit and also reducing start-up times. The furnace has been operating four months and cycled to temperature 16 times (weekend warriors) with no apparent damage to refractories or crucible. We typically charge 50 pounds of cullet in the 100 pound pot reaching temperature (2060) in 3.5 hours. We slowly bring the temperature from 600 to 1200 over 1.5 hours then from 1200 to 2060 over 2 hours.
Any thoughts or experience with preheating would be appreciated...
I'd be curious about a comparison between your current fuel consumption with this gas furnace going up from room temp once a week it sounds like, versus what your fuel consumption would be if you just let the thing idle midweek at a lower temp (something above quartz inversion). Perhaps your furnace is made with just lightweight materials like lots of fiber frax but typically it takes more energy to heat up harder materials (which have more longevity) from room temp than it takes to have the furnace material's thermal mass do some of the work in maintaining a set-point at higher temperature. I'm really surprised your 16"er hasn't cracked yet, that's got to be luck more than physics from what little I know about these pots that we use.
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Old 03-07-2018, 03:51 PM
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MR T says... Say no to crack. Take it slow sucka
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Old 03-07-2018, 04:28 PM
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You have asked a question, and you have been answered by people who have decades of practical experience in studio situations. There are pages of discussion on heating up crucibles from room temperature, for the first time, and then subsequent times. You can take the suggestions offered here as you may, but in my opinion, this group is the best source of knowledge that you will ever find. Keep us updated on your journey. Good luck....
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Old 03-07-2018, 09:17 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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What pot are you using? And who told you you could do this? And what is your furnace materials?
Yes Ive used fans and elements to preheat a furnace and its a good idea, but I preheat longer than your entire heat up ramp!
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Old 03-08-2018, 07:10 AM
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He's using a high temp, I sold it to him. I may sell him another one sooner than I anticipated. While it is coarse grained tabular alumina, it still has real limits and I hand out ramp schedules that I use which are admittedly quite conservative but I haven't cracked a pot in 10 years, maybe more.

And... this is not the worst schedule I've run into. I had a guy go up in two hours from room temp with it filled with cullet and then it split in half. All my fault. . He was totally pissed at what a crappy product we make.

This sort of stuff is continual. A number of years back, I had a guy making and selling 104 color and he was making hagy seals to test but making them totally wrong, fusing the cane from the top all the way to the bottom. I told him that taught nothing about expansion strain. He stopped buying pots from me.

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Old 03-08-2018, 09:47 AM
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To date I have run the furnace no more than two consecutive days so I dont have a fuel consumption comparison but hope to begin running the furnace 24/7... currently set-up with 100 pound propane bottles. Fuel consumption during ramp and operation (blow glass and using furnace as glory hole) is approximately 1 gallon per hour, 1/2 gallon per hour holding temp at 2060 with door closed. Construction is 18 x 18 x 35 modular mortared IFB double coated with emissivity ceramic refractory coating, lower section sealed with an additional mortar and a through-the-floor drain bung-hole.
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Old 03-08-2018, 01:46 PM
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100 lb bottles in my mind would be getting on the icy/ risky side if you are really using a gallon an hour. A bottle can only generate so much vapor before you start pushing liquified material through your lines.
I use 10 gallons a day per 24 hour day and view that as too much but we have not recuperated the furnace.

Pay close attention to the temperature of the fuel line.
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Old 03-09-2018, 06:15 PM
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I'm running about 1 gallon per hour on a 100lb cylinder that freezes, so I have to switch tanks every 6-8 hours. The "solution" to that is running two cylinders at a time. That's around 24 hours worth... more if it's just cruising and not on high.

I'd say my invested pot lasted 15 or so heating cycles without significant glass deterioration, and that is with heating up in 5-6 hours or so.

Is the glass great? No. But does it work just fine for most things? Yeah. That being said, I can't wait to keep the f***** on permanently. And eventually relegate my current setup for mobile demos only.
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Old 03-09-2018, 06:16 PM
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Quote:
Originally Posted by Jack Abner View Post
To date I have run the furnace no more than two consecutive days so I dont have a fuel consumption comparison but hope to begin running the furnace 24/7... currently set-up with 100 pound propane bottles. Fuel consumption during ramp and operation (blow glass and using furnace as glory hole) is approximately 1 gallon per hour, 1/2 gallon per hour holding temp at 2060 with door closed. Construction is 18 x 18 x 35 modular mortared IFB double coated with emissivity ceramic refractory coating, lower section sealed with an additional mortar and a through-the-floor drain bung-hole.
Is there any reason you are running on propane and not natural gas?
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Old 03-09-2018, 06:26 PM
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many people don't have nat gas. I don't. My point is simply that operator in-experience when you start talking freezing lines is serious stuff. As in exploding buildings. Jack is not overwhelming me with experience at this juncture.
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Old 03-09-2018, 06:47 PM
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Quote:
Originally Posted by Pete VanderLaan View Post
many people don't have nat gas. I don't. My point is simply that operator in-experience when you start talking freezing lines is serious stuff. As in exploding buildings. Jack is not overwhelming me with experience at this juncture.
Totally understand. I wish I had had that advise before I started my whole mobile/propane experience.

I also figured that maybe Jack might have nat gas available but not running on it for one reason or another... like me... or why run on 100lb cylinders instead of getting a real tank and piping everything in. I just went though that entire learning experience. Maybe he doesn't *know*.

Wish I had found this board before I started and gotten some feedback.
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Old 03-09-2018, 08:03 PM
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I want to thank everyone for the advise! Especially advise on freezing propane tank dangers... I was not aware there was a concern with liquid fuel escape due to freezing! The bottles begin to freeze after 9-10 hours and when pressure and or furnace temperature fall I simply switch tanks. I plan on switching to a larger propane tank or running natural gas ASAP!!
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Old 03-09-2018, 08:31 PM
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Propane safety PDF attached...
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Old 03-10-2018, 08:52 AM
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If you actually are experiencing freezing lines periodically, it can be far more dangerous than you realize. While I don't know if you work indoors or outdoors, having two 100 lb bottles inside a building would not make your fire department happy at all and if they have authority to red tag you, they would. The downside of compliance on where you keep your tanks is that if its outside, that's a long run to the appliance with a system prone to icing. If it is outside, it's just that much more susceptible to cold weather affecting it. At the least, you should have an inline BASO valve.

Icing can be a killer. It can strike very fast. It can strike in the dead of night if it's cold enough. The solution is having a large enough tank that the surface area in the tank can generate vapor sufficient to the load with a safety factor built in. Your set up sounds to me like an adventure in risk management.
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