um...

[Eben Horton]

this might sound like a ridiculous question, but has anyone used household fiberglass insulation on their equipment?? obviously not close to hot faces, but just inside the steel skin of a furnace or glory hole?

I just built a new hole that is a square design with a 3 inch thick castolite 30 crown, followed by 3 inches of fiber, then did 6 inches of fiberglass insulation. It wasnt the insulation that is glued to a paper or foil backing, just loose glass blanket.

I cant forsee any problems, but maybe i am missing something.

[Fredi Vilina]

my understanding, and correct me if i am wrong, is that heat migrates to equalize, so as it works it way towards the air film that's outside the can, the temperature varies throughout the castable,frax etc. so at 2" into the castable it might be 2000 at 2" into the frax it might be 1600 (your supplier of refactory should be able to do a few different scenarios for you if you provide your particulars) anyways the air film is where the temp drops so if you keep moving that outward the temp changes throughout all the materials and actually becomes hotter where once it was cooler. 6" of fiberglass is cheap but fiberglass melts at about 1000d, (again correct me if I am wrong) so there may be a point inside the fiberglass insul. where the temp has moved outwards over the fiberglass working temp.

[David Paterson]

I have a very handy program that calculates hot face, cold face, and intermediate temperatures for insulation materials. The program seems fairly accurate in actual use on both a furnace and glory hole that I built in the last few years.

A quick look at your design using this program reveals a possible problem:

Assuming that fiberglass transmits heat at about the same rate as 8 lb fiber, and assumung an operating temperature of 2200 F, then the transition between the fiber and the fiberglass will be about 1700 F.

The fiberglass will melt.

In actual fact, a quick Google search turned up some thermal conductivity data for fiberglass, and it appears to be a lot better in insulating value compared to 8 lb fiber, although it would certainly depend on how tight it was packed.

If you increased the thickness of ceramic fiber and decreased the thickness of fiberglass, you would eventially get to the point where the transition temperature would be below the melting point of glass.

In any event, you might consider proceeding with caution.

[Hugh Jenkins]

Eben, I have used fiberglass for years. It is great final insulation. I like the 3" thick matts with a waterproof layer on one side. I put that against the can of the furnace and cast the area between it and the frax blanket with Vermiculite castable. BTW I also wrap the blanket with the saran wrap like stuff for wrapping frieght pallets. Now you can cast between water risistent layers. I think putting the fiberglass right against the blanket is still too hot. Get a good opaque layer in between.

[Glenn Randle]

Hugh,
You have also used vermiculite as an inexpensive insulation too, right? As long as there's a shell to contain it, that seems like a good material to use. Does anyone know it's R-value? I know it has a much higher maximum temp rating than fiberglass. "V-block mix is rated at 1600. I'd think the loose material would be at least that high, and have the advantage of being totally recyclable/reusable. Could it be used instead of fiber, or is that beyond it's capabilities?

[Pete VanderLaan]

we used to make our own block mix with vermiculite, tennessee ball clay and a shovel full of cement and just mix it on the studio floor, and then mush it into place. It was easy to sculpt. It worked great, was cheap and lasted forever. I will do it again with the new furnaces. I also think there is no problem with household fiberglass insulation and agree with Hugh.

[Eben Horton]

thanks for the advice guys- I popped the lid and added some fiber blanket- i think fiberglass insulation is made from borosilicate glass and has a higher melting point no???

[Edward Dluzen]

The basis of textile grade glass fibers is silica, SiO2. In its pure form it exists as a polymer, (SiO2)n. It has no true melting point but softens up to 2000°C, where it starts to degrade. At 1713°C, most of the molecules can move about freely. If the glass is then cooled quickly, they will be unable to form an ordered structure. [2] In the polymer it forms SiO4 groups which are configured as a tetrahedron with the silicon atom at the center, and four oxygen atoms at the corners. These atoms then form a network bonded at the corners by sharing the oxygen atoms.

Fiberglass cloth has excellent heat resistance at relatively low cost. Fiberglass cloth retains approximately 50% of room temperature tensile strength at 700°F (371°C); approximately 25% at 900°F (482°C); with a softening point of 1555°F (846°C) and a melting point of 2075° F (1121 °C).


MSDS (http://knaufusa.com/PDF/CuredMSDS012304EN.pdf)

above MSDS States Melting Point: >1300°F and gives various trade names.

ed

[David Paterson]

A Wikipedia article on fiberglass says there are many different types. One, called E-glass, is a borosilicate composition. However, some fiberglass is made from bottle glass cullet.

A google search turned up a softening point of 1560 F for E-glass.

[Eben Horton]

test fired it yesterday-

wow.

I think i am going to send my propane company a sympathy card :D

[David Paterson]

Can you measure the outer skin temperature of the glory hole when it is at operatinmg temperature?

And how many inches of fiberglass did you end up using?

Some of the websites I found when I searched for info on fiberglass said that the thermal conductivity was very low, a lot better than ceramic fiber, and in the same range as micropourous insulation. You may have discovered a cheap and substantially better replacement for ceramic fiber.

[Hugh Jenkins]

There isn't a great "replacement" for ceramic fiber up in the 2300 to 2600 range. But when you get the temp down below 1900 there are lots of less expensive alternatives that are right for that range. The "standard" of 8 inches of blanket is a waste of this material. Layered changes in insulation works the best. Outside of the liner, try Castolite or Kaolite for the hottest face, 3 to 4 inches of ceramic blanket, then Scamol or V block mix, and then 2-3 inches of fiberglass. This gives about a 12" total wall thickness. This system has just been applied to a 110# furnace that is now up to heat for 10 days. It has finally calmed down to 4 gallons of propane per day. My first guess gas use settings were way too high. This is down 20% from my own 80# furnace.

Definitely time to rebuild my own furnace! It has been a great test unit for recuperators but is underinsulated.

[Eben Horton]

Hugh, why am i not surprised you use fiberglass?? great minds think alike I guess... well my mind may not be great, but I do try to think smart :D