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John Stevenson
03-25-2010, 03:26 AM
Done a fresh post as i don't want to hijack his furnace build thread.

Last year at a show there was a guy doing casting demonstrations and he also gave talks, I went in to one.

The main point of interest was his furnace which was propane fired and built out of an old spin dryer.

The spin dryer was an old British one very popular here 20 odd years ago. 18" to 20" across and 36" tall or so. He'd lined this with those lightweight fire bricks that look like polystyrene, that light they look useless.

He remarked that his first furnace was a Kasenite one from a school that was fire brick lined, they often come up second-hand. Apparently this furnace took 45 minutes to get to temperature and used so many cubic feet of gas [ can't remember the number ]

His new furnace could melt a pot of brass in 20 minutes using a third of the gas, all down to the insulation.

The bit that stuck in my mind was this furnace was about 3 years old, been dragged all over the country and had done a lot of work but all the white paint on the outside was still intact, none burnt off.

The lid was a 6" thick piece of this insulation roughly cut into a circle and held down with a piece of metal, when the melt was ready he was able to lift these off with gloved hands, no tongs

malbenbut
03-25-2010, 04:21 AM
A good supply of light insulation board can be sourced from defunct combi boilers. My local scrapyard always seems to have a dozen or so lying around.
MBB

philbur
03-25-2010, 05:54 AM
I have one of these.

http://www.carbolite.com/products.asp?id=2&doc=2

It uses the same lightweight insulation material. The insulation is so good it can hold 1,000 deg C with only about 1 Kw. The down side is that the insulation is quite delicate. However for the HSMer and a bit of care it will last OK. The key is to use a more durable ceramic "plate" to stand items on. I have used it for heat treatment and for aluminium casting projects.

Phil:)

kjbllc
03-25-2010, 07:30 AM
someting that might work is fiber insulation. good to 2600 and very easy to work with, although fiber board is stiffer and may have other benefits that I don't know about. I use it in gas furnace for blacksmithing.

Duffy
03-25-2010, 08:35 AM
There seems to be very little that is "new,' Sir John. About 30 years ago, I read a book on pottery. In it were plans for a Raku kiln, built in a galvanized garbage can. There was a photo of a young woman, holding the completed furnace in one hand, and the 20-lb propane bottle to fire it in the other. For those unfamiliar with pottery, Raku is a pretty high firing process, close to porcelain.
Evan, I agree with your choice of hard firebrick, (even if it was all you COULD get.) Pottery kilns always use soft brick, both because it is a better insulator, AND because the kilns are ALWAYS loaded and unloaded cold and carefully. The lining is treated VERY gently. As a heat treating unit, you may not have that luxury, and the interior could get beaten up pretty quick, if soft. Just a thought, Duffy

bob_s
03-25-2010, 08:43 AM
I'm just waiting for the last quarter of this year when the bits and pieces of the Space Shuttle become available. Should be able to make some really nice furnaces from the heat shield tiles.

Evan
03-25-2010, 09:02 AM
Insulating firebrick has the strength of a dry egg meringue, You can poke a hole in it with your finger. It isn't a good candidate for the inner lining of a heat treat oven.

Glenn Wegman
03-25-2010, 10:23 AM
I'm just waiting for the last quarter of this year when the bits and pieces of the Space Shuttle become available. Should be able to make some really nice furnaces from the heat shield tiles.

I believe that is what is in mine.

It stays dead cold on the outside and can be 2050 F inside, and weighs very little. Goes right to temp and maintains it within 1 C.

I've played with a tile from the shuttle and it's pretty interresting stuff. The piece I had was about 6" X 8" X 5/8" thick and weighed nearly nothing. I could lay it on the palm of my hand and heat the other side with a torch util it glowed.

Sorry for the poor image, my photographer was laughing.

http://img.photobucket.com/albums/v647/Fighter1/Furnace-1.jpg (http://smg.photobucket.com/albums/v647/Fighter1/?action=view&current=Furnace-1.jpg)

CountZero
03-25-2010, 11:12 AM
As I understand it raku is a low firing process, raku ware is porous and will not survive outside int the winter. At least not around here.

aboard_epsilon
03-25-2010, 11:20 AM
Three things come to mind

i remember a couple of episodes of the BBC's tomorrow's world program

One demonstrated a product called STARLITE...a coating that you applied to things ..........an egg was shown being heated with a blow torch.

and another demonstrated thermal bricks /blocks /cubes that could be handled with your bear hands when glowing red hot.

and I'm sure there is a North Wales company that makes those blocks ...and even supplies them to NASA ..cant be found on google .but Ive heard it exists ..and its less than 15 miles from me .

all the best.markj

Alistair Hosie
03-25-2010, 01:40 PM
I can't quite understand that Mark doesn't make sense.Surely if it does not create enough heat to burn your hands it will be useless as a heating material.All things that glow red are not neccessarily hot.But as said if you can hold it it can't be very warm! Sorry maybe it's me , but I can't see that one at all.Alistair

aboard_epsilon
03-25-2010, 02:18 PM
It's an insulating material ..that is so good ..the blocks of it be handled with bear hands when glowing red hot


so by using it .. that the furnace or oven will heat up quicker and retain heat.

all the best.markj

rockrat
03-25-2010, 02:37 PM
I'm just waiting for the last quarter of this year when the bits and pieces of the Space Shuttle become available. Should be able to make some really nice furnaces from the heat shield tiles.

My buddy had some a year or two ago. His friend gave him some. It is on the commercial market now. His friend lined his fireplace with the stuff. I have no clue on the structure of the fireplace, just the story.

I thought it was really interesting. I held a 6x6 piece in my hand and put a propane torch to it. Skeptical, I was a bit easy at first with the flame but after a few seconds, I had the torch about 1" away. My hand was warm at best.

Now for the downside. A few years back when this happened, I was told the stuff was very expensive. Now the rumor is that it is quite cheap.

rock~

J Tiers
03-25-2010, 08:31 PM
It's an insulating material ..that is so good ..the blocks of it be handled with bear hands when glowing red hot


so by using it .. that the furnace or oven will heat up quicker and retain heat.

all the best.markj


More to the point, it's own heat CAPACITY is so low that when you pick a glowing piece up, the energy in the hot area you touch is too low to hurt your relatively massive and water-filled hand.

It's teh difference between sitting down on a metal park bench that has been in the sun, vs a wood one.... or leather car seats vs cloth.........Both are essentially equally "hot", but the wood or cloth "holds less heat" (energy) than the metal or leather.

darryl
03-25-2010, 08:52 PM
As JT said. The weight of that material, or the mass, is so low that there's hardly any material to conduct the heat- plus the fact that it reflects heat before any can be absorbed anyway. When you play the torch on it, what heat is not reflected is stopped so quickly that no significant amount of the material is heated, therefore it will come up to temperature quickly (good for the oven) and cool off almost as quickly to the atmosphere, or your hands. Probably by the time the heat registered in your brain, the surface has cooled pretty much to the temperature of your hands, so you might register a heat spike, but no real amount of energy and no burning.

I don't know from practical experience, but at some point as the temperature in the oven rises, the losses due to air leakage will become significant. This will be more pronounced the better the insulation is. Because there's nothing about the oven that you can rely on to be a thermal mass (when your insulation is that good) except the workpiece, it will be all the more important to eliminate air leakage in order to keep a set temperature. Or so it would seem to me- I think the temperature control might be best done by sensing the workpiece temperature- just my thoughts.

crec
03-25-2010, 09:54 PM
Ok guys, I've never posted here so bear with me. I worked for an OEM designing industrial heat treating equpment for 15 years so I might be able to fill in some of the blanks for you.

Hard brick - Strong but not very good insulation. It will soak up heat meaning it will take longer for the furnace to get to temperature, but it will also retain that heat and give it back to the work when the heat source is turned off. Basically it will take longer to heat and use more energy to maintain the temperature, and take longer to cool down.

Insulating Fire Brick (IFB) - Soft compared to hard brick. Can be worked with simple hand tools. Cuts well with 18 tpi hack saw and can be "shaped" by rubbing bricks together. They will wear away quite rapidely if items are "rubbed" over their surfaces. Relatively good in compression. Insulate much better than hard brick resulting in faster heat ups and lower energy consumption. They also retain heat but much less due to their lower mass. Heat ups and cool downs are faster than hard brick.

Ceramic felts - Generally come in 2300 and 2600F ranges. Made from ceramic fibers felts do not retain appreciable amounts of heat rather they reflect heat very effectively. Heat ups and cool downs are very quick. The fibers are generally restrained by organic binders (usually some form of sugar) that will burn off during the first heat up. If you use them run them in a well ventilated area the first time you run them. Also after burn out the fibers can separate and become airborne if care is not taken. Some vendors make coatings that can be applied to the surface to prevent this from happening.

Vacuum Formed Insulation Board - Basically is ceramic felt that has been chopped, mixed with organic and inorganic binders to make a liquid slurry. The slurry is then drawn by a vacuum platen to the required thickness. Like the felt this type of board reflects heat while retaining very little. This type of board is very soft. You can poke a finger through this if you press hard enough. Like the felt it will also have burn off issues - usually worse. Both the felt and the board are excellent refractory insulations but they are delicate.

Insulating Lightwieght Castable- Pours like cement. Provides some insulation value. Obvious advantage is that it can be cast to provide the desired shape.

Dense castable- Stronger/Sturdier that the light wieght but provides less insulation. Some people will take dense castable and add either foam beads or sawdust to the mix and then pour it to the required shape. They will then add a "hot face" coat of unaltered castable to provide a hard internal shell. The beads or sawdust burn out on the first firing to make air pockets to provide some insulating characteristics.

Ok now when selecting any refractory material select the material that is closest to the temperature that you want to achieve. In other words if your max temp is 2200F then use material rated for 2300F not 2600 or 3000F. The reason is that the higher the temperature rating of the material the worse an insulator it is. So in the end you will pay more for the rating AND pay more for the energy to run the equipment.

All of the materials will work and obviously you have to use what you can get, just some of them will cost you more to run and will make the outside of your furnace a bit more uncomfortable to be around.

Hope this helps

Carlos

Mcgyver
03-25-2010, 10:00 PM
Insulating firebrick has the strength of a dry egg meringue, You can poke a hole in it with your finger. It isn't a good candidate for the inner lining of a heat treat oven.

its the only material to use imo, you coat it inside. forget with what, but it makes a durable surface...some refractory product, stove cement, maybe what they use for poring melting furnaces...there is a product for it i just forget

Prove me wrong, but i went through this with an engineer in the refractory business, a single layer of IFB's on their sides are quite enough for 2000 degrees for any length of time. The intent being to keep the outside a safe temp. Read up on those heavy firebricks; half the virtues extolled on them are what great heat sinks they are (ovens and woodstoves).

Evan
03-25-2010, 10:03 PM
Well, it's built and seems to work just fine.

crec
03-25-2010, 10:14 PM
Actually IFB is relatively strong in compression. It actually is a very good candidate for the interior of a heat treating oven as that is what it was made for. As to poking your finger through it, well you would have to poke pretty hard. It is soft, but not that soft. Now vacuum formed insulation board (basically ridgidized felt) you can poke a finger through pretty easily. If wear is a concern then placing a row of hard brick over the bottom row of IFB can be done to improve the wear characteritics of the furnace. Or you could put in a kiln shelf to provide a "hard" surface. As for coatings IFB generally does not require it but you can "hot face" with castable refractory or use one of the coatings like ITC

Carlos

Evan
03-25-2010, 10:39 PM
It actually is a very good candidate for the interior of a heat treating oven as that is what it was made for

That isn't what it was made for. Furnace lining bricks for heat treat furnaces are three time more dense and abrasion resistant.

John Stevenson
03-26-2010, 03:41 AM
That isn't what it was made for. Furnace lining bricks for heat treat furnaces are three time more dense and abrasion resistant.

That was old technology and we have moved on.
In your design you are not using the firebrick as a lining, your lining is the whiteboard.

However I didn't start this thread to detract from your design but to point out the materials available that require less power to run.

I was amazed that the casting guy, Noel, was using propane, which is more fierce than an electric element and just had a lining of this IFB and it hadn't burnt the paint off.



The thread from Carlos above is very good

ptjw7uk
03-26-2010, 11:00 AM
I remember from when I worked for a cable company they had a continuous casting furnace in which the refractory had to be kept above 1000c or else it would crack. The furnace was shut off twice a year christmas and summer break, it then had to be rebuilt usually by some poor souls in the breaks. I was told that the refractory material underwent a phase change at about 950c and above that it was stable. It must have been pretty strong stuff as it was charged with copper from the top!
The stuff Evan is using must be able to under go many temperature changes without being effected but I doubt it will be that impact restant.

peter

BobWarfield
03-26-2010, 11:20 AM
IFB's are definitely made for the purpose:

http://www.sheffield-pottery.com/Bricks_and_Refractories_s/24.htm

They recommend IFB's for indoor use and hard bricks for outdoor use, presumably on a durability basis.

Cheers,

BW

kjbllc
03-26-2010, 11:36 AM
one thing to keep in mind here that I found out through researching my forge. The fibers and dust from the light bricks and fiber blanket break down into very small particles *think asbestos* so you want to be wearing really good face mask and work with it outside. Also the tend to break down somewhat while you are using them and the tiny particles are lifted up with the heat during use. So you should have a very good way to get those things outside of your breathing space. I have a large hood on all the time.
This may be a reason hard firebrick would be better for a heat treat furnace. kevin

Evan
03-26-2010, 11:55 AM
I am holding a piece of IFB in my hand. The stuff is so soft that it wouldn't last at all if heavy metal objects were being constantly placed on it. It's light enough to float and I can scrape the surface off with a fingernail. It is intended as insulation, not as a working surface. Many of the commercial ovens I have looked at online are metal lined inside.

It dooes also present a silica dust hazard which the white board and or the hard bricks do not. Cement board would work just fine as a replacement for the white board I used but a little more space/thickness for rock wool would then be a good idea.

BTW, John. I think this is a brilliant way to hijack a thread.... :D