Help please on how to build and operate a furnace to temper glass

[vizion]

Hi

I want to build a furnace to temper glass sheets up to 48"x 36".

Can anyone help me with design and operational guidance

Thanks

David

[Mike_Aurelius]

To temper, not anneal?

What type of glass? Standard window glass? How thick? What will the edge treatment be?

The main problem that you will have is that to temper glass, you have to heat the glass up to just short of the softening point, then rapidly cool it down, forming a "skin" that forms the case (case hardening). With this size sheet, you have to heat the sheet equally across the surface, and then cool it rapidly across the surface. It is going to be very thickness oriented.

You will also have to support the glass in such a way that it does not deform during the heating process or fracture during the cooling process.

This will not be an inexpensive furnace. You are probably looking at somewhere around $25K (just an off-hand guess) or more.

[vizion]

To temper, not anneal?

What type of glass? Standard window glass?
Tempering for safety glass
Using Standard Float Glass

How thick?
2mm to 4mm

What will the edge treatment be?
Emery/sandpaper rounding


The main problem that you will have is that to temper glass, you have to heat the glass up to just short of the softening point, then rapidly cool it down, forming a "skin" that forms the case (case hardening).

Understood

With this size sheet, you have to heat the sheet equally across the surface,

How does one design for even heating?


and then cool it rapidly across the surface.

Will air cooling be adequate or some other method essential?


It is going to be very thickness oriented.

Agreed

You will also have to support the glass in such a way that it does not deform during the heating process or fracture during the cooling process.

1. How is that best done in the furnace?
and
2. How is the glass to be supported whilst transitioning from furnace to cooling zone?

This will not be an inexpensive furnace. You are probably looking at somewhere around $25K (just an off-hand guess) or more.

What are the main components and how is the $25K allocated?

[Mike_Aurelius]

Designing for even heating is accomplished by using coil-type or element-type heaters spaced about 4-6" apart over the entire surface area that needs to be covered. I would suggest heating on both sides of the glass.

Cooling is best accomplished by using pressurized air in an even pattern on both sides with about the same measurements as the heaters.

The support method I've seen is with the glass on-edge, sitting in a stainless steel holder both top and bottom. Glass is slid into the holders, into a cold furnace.

The furnace should be both heating and cooling, unless you have the space to be able to transition directly from the furnace into the chiller.

Your major expense items will be fire brick, heating elements and temperature controllers. Plus some extruded stainless steel channel for the plate, and some sort of iron framework.

I'm not sure a DIY project is reasonable, given the number of companies around the US that provide safety glass in one form or another.

[vizion]

Designing for even heating is accomplished by using coil-type or element-type heaters spaced about 4-6" apart over the entire surface area that needs to be covered. I would suggest heating on both sides of the glass.

That seems reasonable.

Available power supplies are 240V Single phase or 480V Dual Phase.

Can anyone identify suitable element heaters or coils and say how much would be needed for an array to cover 36" x 48"?

How might they be best wired?


Cooling is best accomplished by using pressurized air in an even pattern on both sides with about the same measurements as the heaters.

How much pressure? What nozzle size? At what angle should the nozzles be set in relation to the surface of the glass?



The support method I've seen is with the glass on-edge, sitting in a stainless steel holder both top and bottom.

I like the vertical approach as I understand horizontal rollers tend to generate waves across the surface of the glass.


Glass is slid into the holders, into a cold furnace.

So I understand you suggest the process to be 1. Load 2. Heat 3. Cool 4 Unload all within a single chamber. If the chamber is constructed of firebrick does it cool quickly enough for a single chamber to be viable?


The furnace should be both heating and cooling, unless you have the space to be able to transition directly from the furnace into the chiller.

I would need would probably require up to three 48" x 36" frames to hold the number of sheets of glass required for a batch. Ideally I would have a stock of differing frames to hold multiple sheets in a vertical array.( e.g one frame might hold 12 11"x11" sheets). With three such frames could I get away with two heating arrays or would I have to plan for 4 arrays (one back and front and one in between each frame? )


Your major expense items will be fire brick,

28 Grade Firebricks (rated to 1500 degrees) in UK cost no more £4.00 each and I estimate I will need no more than 450


heating elements

How much are these likely to cost?

temperature controllers.

How many am I likely to need. Could I use cones and a timing device. Would that give me sufficient accuracy?

PS Just found I can get controllers rated up to 1600 degrees for less than $100


Plus some extruded stainless steel channel for the plate,

Is this for the glass to rest on? Do I have to use stainless?

and some sort of iron framework.
Is this to provide an external structure against which the firebrick is erected?

I would also need to construct an opening door to load/unload the furnace.

I'm not sure a DIY project is reasonable, given the number of companies around the US that provide safety glass in one form or another.

I am in UK not US ;-)

Thanks for your help

David

[somewhere]

Down this road lies the path of a mad man. Or however that saying goes.


You will spend huge amounts of money designing building troubleshooting and running this tempering furnace. I have work tempered for public installations and can tell you this is no small feat. Then you have testing after the tempering process to even know if you hit the mark. This equipment either bought or built will be another large sum of money. I would say Mike's 25k estimate to be modest at best.

The questions you are asking are easy ones to answer the fact you are asking them here makes me skeptical of your understanding of the magnitude of what you are trying to accomplish. I would seriously look at farming the tempering process out.

[Greymatter Glass]

Vizion, welcome to The Melting Pot.

Your question is likely beyond the scope of our user-base's ability (and/or willingness) to provide a comprehensive answer. We're primarily artistic glass blowers and while many of us have a rather expansive knowledge of glass and things in general it sounds like you need specific practical advice... that said, I totally wouldn't be surprised if someone here can offer it, so by all means, the question is good and I hope you find some leads if nothing else. But consider this a starting point, you have a LONG ways to go before this happens.

It looks like Mike's given you a pretty decent summary of what needs to be done, but frankly it's nothing you couldn't learn from a quick google search and/or trip to wikipedia. Your best bet is going to be to consult with an engineering firm local to you, which may not be cheap, but if the initial price tag didn't scare you (and I feel it's low too) then maybe it's something you can consider.


Anyways welcome... it's been a little while since we had a random new guy with odd questions, I always find it fascinating what pops in here from time to time... telescopes, auto glass, cooking wares, now tempering.. :) ...

[Mike_Aurelius]

A cheap controller on eBay is not going to handle your needs.

You are going to need multiple controllers: the most efficient way to run a set up like this is to use a zone heating plan, let's say a 2 square foot area. You will need controllers that can work together, this will require either an industrial computer controller, or controllers that can be ganged together. These controllers will have to work in conjunction with a sequence timer that will time out the heat cycle, then switch the controllers to a cooling cycle.

You may also want to consider using a two stage system, with a heating unit and a slide mechanism to move the glass into the cooling chamber, this would increase your brick costs, but would cut the complexity of the overall design by removing the cooling from the heating chamber. You will also gain by increasing brick life -- if there is one thing that firebricks hate it is rapid cooling. The cooling chamber is called a lehr (pronounced leer), and should be the same overall dimensions as the furnace.

I can't take you much further than that, maybe someone can chime in about heating == I would think that the same heaters as used in high temperature glass furnace units, low voltage high amperage moly units would work. You will need a professional electrical engineer to design the system.

And yeah, on further thought, you are probably going to be closer to $50K, and maybe higher.

Is the Chance-Pilkington facility still running? You might want to check with them to help you out further than I can.

I also know that in the EU, there are plenty of plate glass manufacturing companies that also do tempered glass, a lot of them will manufacture and temper to specification. Pilkington, Schott, and several companies in the Czech Republic should be able to help you out, I believe there is even one or two companies in Italy.

[roughnecked]

I would also expect a steep learning curve.

While tempering glass strengthens the glass surface it leaves a huge stress riser on the edge. Flaws from the cutting and grinding may leave imperfections that let go at the slightest bump. All of this makes the process much more challenging.

Tempering is also done in autoclaves. It would probably be easier to just get one of them.

[roughnecked]

Oops autoclaves are for laminating glass not tempering.

[brads]

The discouraging comments above are on the money. This is NOT a project for a beginner. Properly tempering large sheets of glass requires a very high degree of technical knowledge and expensive equipment. Not something you can find on eBay for $100.

I don't want to be rude, but with your obviously limited knowledge of both the process and equipment required, this post borders on trolling. (Which it may, in fact, be.) It is somewhat akin to saying, "I need to fly to Chicago. Please tell me how to build an airplane so I can do it."

[menty666]

I don't know, it may not be trolling. Sometimes when get onto a project I want to scratch build as many of the pieces as I can just because.

That being said, buying the three sheets of tempered glass is going to be a far sight cheaper than 25-50k once you factor in the learning curve, building the furnace and cooling system and the inevitable loss that'll be incurred while trying to create a usable end product.

Out of curiosity, what's your project, vizion?