Converting Thermal Conductivity to Specific Heat

Specific heat is the ability of the material to store heat and thermal conductivity is the ability of the material to pass it through from a hot source to a cold sink.

What about a different tack: limiting the temperature rise of the component itself either by disconnecting it on overcurrent during the fault or disconnecting it on achieving a certain temperature during the fault event? In these cases the presence or otherwise of the potting compound will make little difference.

If the fault condition is sustained, the specific heat of the compound is irrelevant, as sooner or later (without heat loss to the outside world) it will reach the temperature of the faulty device (which will then get hotter, as there'll be no heat flow when they're in equilibrium, and energy will still be going in).

IMHO you'd be better to consider the thermal conductivity and the heat lost from the outside surface.

Thanks for your replies. Sorry perhaps I should have given more information, the component in a fault condition is a battery and the fault condition is shorting its terminals. This means it put out about 3watts of heat for about 45 minutes. What I am looking to do is calculate the mass of the potting compound that would be able to contain the heat and stop the external surface reaching an unsafe temperature outside the potting compound. Thanks

First,

Until you know the thermal conductivity, and thermal mass, of the specific potting compound, there's not much to learn. That depends on the fillers used to impart thermal conductivity. Not sure containing the heat is the right path.

Beryllium oxide used to be a favorite filler, eons ago when I designed thermal control systems for satellites. (No air in space to help) Maybe you could use this to conduct the heat to a safe place?

Ask your supplier for help, and put the onus on them for your success.

Good luck.

Unfortunately as far as certification of this product is concerned there is no safe place! I need to increase the mass of the potting compound to a level that means the it's outer surface doesn't reach a certain temp within the time this battery is outputting this heat.

Consider something filled with glass microspheres.

If the manufacturer can't supply the information, you'll have to have it tested.

Has this got anything to do with the safety of electrical equipment in potentially explosive atmospheres?

If it has, and it is new equipment that has not been previously type-tested for <...certification...>, then testing by an approved testing operation will be required in order to obtain it. Open a dialogue now. Telephone?

Vacuum is an even better insulator than air, or am I wrong ?

You are correct. In #4 I had mistakenly assumed that OP wanted to remove heat from the package.

After I found that he wanted an insulator, I suggested glass micro sphere filled compound.

If feasible, vacuum would be best, I think.

You are correct; however, it is much easier to package air than a vacuum.

How about using a fuse instead, Boss?

I have to assume that the fault is internal to the battery and any external safety devices have failed I afraid! The only way I can do it is to prove the exterior surface will not reach the maximum permissible temperature.

" ... about 3watts of heat for about 45 minutes ..." - why not 30W for 4.5 minutes, or perchance 300W for 27 seconds (you see where I'm going)?

Hi Chris, looks like you'll have to either guesstimate the specific heat or do a test.

Since it sounds physically small I'd suggest heating a (dead) battery in a oven to a known temperature. allow a long time (hours) for the entire unit to come to the same temperature, drop it into a known mass of water, at a known temperature, in an vacuum insulated flask and then wait a long time for the temperatures to equalise.

It's straight forward to then calculate the battery's specific heat from the various temperatures..

Messy, yes, but fun.

To answer your first question, there is no way to convert thermal conductivity to specific heat. They are independent properties.

However, you may have some degrees of freedom that you haven't discussed. If you insulate the battery from ambient conditions, the heat dissipated during the electrical fault condition won't be able to escape to ambient as rapidly and your external surfaces can be kept cooler. Although your battery may get hotter, your external surfaces will remain cooler.

Wow - using potting compound as a heatsink. I've never tried that - good luck. Let me know how it works out. Personally, with my experience, potting compound does not distribute heat very well. Maybe I'm using the wrong potting compound. I'm open to change.

Thermal conductivity and specific heat are unrelated. Ask the manufacturer for more data, make your best guess based on whatever data you can get, induce the fault, and measure the temperature. If it is too high add more potting compound.

Yes-You have to know what is the power you dissipating PD from the heat source , and the temperature of the source TS and what is the temperature of the ambient TA

The connection is : TS=PD X RSA+TA when RSA is the Thermal Resistance .

The temperature rise shall be TS-TA

Good Thinking=Top Result

Your problem is a transient heat transfer. The way heat travels is related to the thermal diffusivity which is related to both thermal conductivity AND specific heat. It is proportional to heat conductivity and in verse to the product specific heat * specific mass. It is measured in m^2/s. This is valid for the 1st part of the heat generation. When batterie is empty it will cool down since the heat will travel through your protective layer to the environment. This is also a time depending phenomenon. You are interested to define the MAXIMAL temperature at the outside of the protective layer. It can be reached even after the batterie is empty according to the diffusivity of protective layer. Almost all equations for heat transfer you find in books are for a steady state heat transfer not for a transient. So that you should try to find the equations for transients which are more complex.

Glass and stone have sp. ht ~ 0.2 cal/gm/°C, most plastics ~ 0.3 - 0.35. I'd be surprised if your potting compound is outside that range. Is that near enough.

Water has an unusually high sp.ht. at 1 cal/gm/°C (by definition of cal).

No one can help with that quest, either in or outside this forum. See #1, #4 and #14 above.