Oven Window Touch Temperature

[PTam30]

Hi,

I don't have much background with heat transfer and am hoping someone can help, please. I have an oven that I'm looking to add a window to, so it will go oven interior to ceramic glass pane to air gap to ceramic glass pane to outside (with the option to add more glass panes as necessary). The oven will go up to maximum 1000 deg F. I have the thermal conductivity of the glass on the attached picture.

How do I determine the final touch temperature of the outside glass surface? Thanks for any help!

 

[IRstuff]

How did you determine the touche temperature of the existing surfaces?

Note that designers often forget to verify that the mounting flanges for the windows are also touch-safe.

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[EdStainless]

It is down to radiant heat transfer between the two panes.
Typically, the outer window will have a partially reflective coating on the inside surface.

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[IRstuff]

It is down to radiant heat transfer between the two panes.

A static air gap also contributes gas conduction, and if the gap is too large, some convection can also occur.

The basic path is to design the window structure to the same R-value as the walls of the oven, which I'm assuming to be touch safe.

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[PTam30]

How did you determine the touche temperature of the existing surfaces?

So far I haven't determined temperatures of any surfaces, although I assume the one exposed to the oven interior would be at 1000 deg F. I guess I'm kind of at a loss for what my first steps would be in getting to the final outside glass surface temperature.

 

[EdStainless]

Look up the optical properties of that inner pane, at that temperature.
You need to know how much of which wavelengths are passing as well as what it will be re-radiating.
Then rinse and repeat for the second pane.

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[LittleInch]

What's happening in that 10" gap? Closed air gap or what?

Insulation around the oven?

Looks like infrared is a key element. Is what ever is in the oven glowing?

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[PTam30]

@LittleInch, yes, it's a closed air gap between the two glass panes.

The oven walls are just insulated by the original manufacturer, but I don't have much information beyond that right now.

Right now I'm just considering what happens in an empty oven, so nothing in there to glow. It looks like the heating elements are in a separate compartment and the hot air just gets blown into the interior chamber.

I'm just looking for a ballpark figure, so I want to assume the oven interior chamber is maintained at a constant 1000 F and all of the heat loss is through the window (i.e. glass pane, air gap, glass pane). I was looking at Q*=k*A*(T1-T2)/d (Q* = heat flux, k = thermal conductivity, A = area, T1-T2 = surface temperatures 1 and 2, and d = distance between surfaces), but I wasn't sure how to determine heat flux to begin with if I'm missing one of the temperatures.

For the ceramic glass, I only have the thermal conductivity. Haven't found any optical properties yet. Thanks!

 

[IRstuff]

You already have information to solve the problem. The surface temperature needs to be no more than around 50C to 55C

https://antiscald.com/index.php?route=information/information&information_id=15

I would design for 45C to 55C; as structures age, their thermal performace degrades.

Assuming natural convection, a 45C surface will only convect a certain amount of heat flow into the surrounding ambient. You then compare this heat flow with the heat flow given the temperature difference between the interior and 55C surface temperature. If the heat flow is too much, then you need to thicken the window structure, such as going with a triple pane or much thicker glass.

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[PTam30]

@IRstuff, thanks! I think this makes sense. One last thing, can I add the thermal conductivities together as 1/keq = 1/k1, 1/k2,..., etc.?

 

[LittleInch]

You really need to figure out somehow what the air temperature in the middle can climb to.

So you need insulation thickness, area and what the rest of the outer surface area does in terms of temperature and heat loss to the ambient air or other surface. If the outer edge is insulated other than the window then the temperature could be quite high.



That's more than 500C in the oven.

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[MJCronin]

Homework problem ? ....

kinda smells like it... (a precise 1000F ? 10" x 10" window) ... very round numbers

MJCronin
Sr. Process Engineer

 

[IRstuff]

One last thing, can I add the thermal conductivities together as 1/keq = 1/k1, 1/k2,..., etc.?

You can add thermal reistances, since the resistances encompasses the thicknesses of each material layer; note that this is different than thermal resistivity

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[PTam30]

Thank you all for your guidance and patience. Will give this a shot and see how it goes.

 

[georgeverghese]

The main mode of heat transfer here would most likely be radiation between the 2 glass panes with reradiating-nonconducting side walls. Fig 5-15 on page 5-29 in the Radiant Heat Transfer section of the 7th edn of Perry Chem Engg Handbook tells me the view factor F12 for this is 0.55 (x axis = 10/10 = 1.0). Read up this section here to work it all out. Some trial and error iterative calcs will be required to determine what the outside surface temp of the inner glass pane will be and also the corresponding inside surface temp of the outer glass pane.