looking for mechanical glass properties
looking for mechanical glass properties
(OP)
Hi
I am looking for properties of high temperature glass (borosilicate, maybe quartz).
Specifically, I am designing a pressure housing for a high temperature light. I will be using a glass hemisphere, outside pressures will reach up to about 2000 psi. The temperature differential is what concerns me. From some initial calcs, it looks like the inside surface could see up to about 300F, while the outside surface is at about 90F. at depth (2000 psi) the outside temps will be much cooler, with a corresponding reduction on the inside surface; but the differential should be similar.
I have never worked with glass before, and would appreciate any input someone may have.
I am looking for properties of high temperature glass (borosilicate, maybe quartz).
Specifically, I am designing a pressure housing for a high temperature light. I will be using a glass hemisphere, outside pressures will reach up to about 2000 psi. The temperature differential is what concerns me. From some initial calcs, it looks like the inside surface could see up to about 300F, while the outside surface is at about 90F. at depth (2000 psi) the outside temps will be much cooler, with a corresponding reduction on the inside surface; but the differential should be similar.
I have never worked with glass before, and would appreciate any input someone may have.





RE: looking for mechanical glass properties
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RE: looking for mechanical glass properties
However, because of the thermal gradient, you may need something like Pyrex:
http://www
Their product index:
http://www
TTFN
RE: looking for mechanical glass properties
(After I wrote this I re-read your post. It seems you already have a high temperature envelope, because the filament is much hotter than 300F)
RE: looking for mechanical glass properties
Fused silica is stronger, has greater thermal shock resistance, better optical properties, and better chemical resistance than borosilicate glass (Pyrex, Kimax).
Check the numbers:
CTE of 7.5 x 10-7/oC is only 1/4 that of Pyrex: 32 x 10-7/oC.
The only proplem with fused silica is cost and high fabrication temperature.
Pyrex as a thermally stable reflector mirror substrate became obsolete by the 1960's, when it became feasible to work large fused silica (aka fused quartz). http://grus.berkeley.edu/~jrg/MATERIALS/node7.html
The 96% silica glasses such as Vycor are a lower cost version of pure fused silica, with nearly-as-good properties.
http://en.wikipedia.org/wiki/Fused_quartz
A very basic summary of the various glasses & their properties:
http:
Also of interest:
"Corning 7971 ultralow-expansion( ULE ) glass is fused silica doped with Ti...Results in an CTE at room temperature that is 20 times lower than that of pure fused silica." See the 8.2-meter diameter mirror substrate at
http://grus.berkeley.edu/~jrg/MATERIALS/node8.html
But, you probably don't need such an ultra-low CTE; might have problem with higher CTE mounting materials. Probably, Pyrex, Vycor, fused silica will all work if the design properly withstands the pressure, and Pyrex is easier to form. What diameter is your glass shield?
P.S In an earlier life, I quenched fused silica capsules containing samples of up to 115 atm internal pressure from 900oC into ice-water. Capsule walls 1 mm (5 mm ID, 7 mm OD). From 800oC, used 6x8 mm capsules. None ever broke.
RE: looking for mechanical glass properties
I think the hemisphere will be about 5 inches OD. One of the factors determining the diameter is the temperature gradient that is acceptable in borosilicate at these high temperatures. The bulb can burn at up to 1500 F, and I'm not sure how close I can get the dome to that temp.
The above numbers (300 F internal, 90F water outside) come from calcs based on a 5 inch dome. A smaller dome will be closer to the bulb, and provides less area for heat transfer, giving me a higher thermal gradient.
I am unsure of how to predict the thermal stresses due to this gradient, then incorporate them into my pressure induced stresses. Any info or ideas?
Kenvlach, your quench experience is reasuring, though!
RE: looking for mechanical glass properties
Flanges, with hard rubber gaskets?
Is the housing filled with dry air or inert gas?
What wall thicknesses have you calculated for the glass?
Probably best to stick to existing designs as much as possible.
RE: looking for mechanical glass properties
the housing will be filled with dry air and dessicant. I will probobly use a high temp seal such as a fat silicone o-ring, but I have actually seen standard low temp buna seals used in a similar application.
As far as glass thickness, 1/4" wall is more than enough (something like 70,000 psi) without the thermal stress. After the thermal issue is introduced, I can only guess at this point (and reverse engineer other products ;) To be safe, my guess is a 3/8" wall. the problem is that i'd like to engineer this, and not overbuild out of fear!