What causes fires to start in ovens?

[T@P]

an ordinary paper fire is 451 degrees Fahrenheit ( ), and the standard oven temp is 350. how do fires start in ovens? also where does the energy come for the first spark which starts the flame?

 

[vincentchan]

where did spark come from?? battery... for some oven, others? your power outlet...
how do fires start in oven?? didn't you just answer your question... SPARK...

 

[vincentchan]

oh, sorry, it seems like i misunderstand the first part of your question, you are asking why would paper get on fire in oven, right?
oven does not only generate heat, it also generate RADIATION, the air in the oven is 300 degree doesn't mean EVERYTHING in the oven is @ 300degree , oven doesn't heat up the air directly, is the radiation heat stuff up... the radiation heat the turkey, also the plate, the inside of the oven, and these stuff transfer there energy to air... so... the stuff in oven is hotter than the air in oven... understand??

 

[Chronos]

Where you get that oven? Mine goes over 500 degrees. The key is ignition temperature. Fat will ignite at about 475 degrees F. Of course you won't find may recipes that call for cooking at that high a temperature. Oven fires do, however, occur at much lower temperatures. One mechanism is steam bubbles. A steam bubble gets trapped, pressurizes and explosively releases 'spitting' fat [which rises to the surface] onto the heating element. Overflow is another cause of oven fires - dish is too small to contain the heated liquid when it expands.

 

[janzizka]

how would overflow start a fire? i mean if everything in the oven is at 350 then where does the temp build? and not to sound like i didnt hear about the radiation... but isn't that more microwavish? standard (old) ovens just produce heat in some conventioinal way...

 

[NoTime]

how would overflow start a fire? i mean if everything in the oven is at 350 then where does the temp build? and not to sound like i didnt hear about the radiation... but isn't that more microwavish? standard (old) ovens just produce heat in some conventioinal way...

The heating coils turn red in an oven.
This translates to a temperature.
Don't feel like looking it up, but probably somewhere between 1000 and 2000 degrees.
More than hot enough to set something, like splashing grease, on fire.

 

[russ_watters]

standard (old) ovens just produce heat in some conventioinal way...

Yeah - with a red-hot heating element or an open flame.

 

[Chronos]

In fairness to janzizka, the set temperature of a conventional [gas or electrically heated] oven is merely an approximation of the average temperature in the cooking chamber. The heating element is, of course, much hotter than the non heating elements of the oven. It becomes more readily apparent when you take into consideration the inverse square law. Note: Convection ovens do a much nicer job of cooking most foods [just not T-bone steaks, I like mine charred on the outside and red in the middle]. The temperature gradient is far lower.

 

[DaveC426913]

"...somewhere between 1000 and 2000 degrees..."

I would be quite surprised if the temp of the coil in an oven reached that kind of temp. Don't forget, the coils on top of the stove and in your hair dryer glow red too. 1000-2000 degrees??

 

[T@P]

but dave for something to "glow red", the substance that glows red must achieve some temperature, and I am sure that a thick metal would have a harder gatting that glow then an incredibly thin wire as in a blow dryer. but thanks i get the idea about ovens

 

[derekmohammed]

The diffrence between the hair dryer and the oven is that it takes much more energy to get the oven up to the red hot area then the hair dryer. In short the Oven draws more current to obtain higher temps. That is all.

 

[russ_watters]

"...somewhere between 1000 and 2000 degrees..."

I would be quite surprised if the temp of the coil in an oven reached that kind of temp. Don't forget, the coils on top of the stove and in your hair dryer glow red too. 1000-2000 degrees??

THIS is the best I could do with a little google. A candle glows orange at 1900K - I'd think a red heating element is a little, but not much below that. A gas burner is significantly hotter.

The diffrence between the hair dryer and the oven is that it takes much more energy to get the oven up to the red hot area then the hair dryer. In short the Oven draws more current to obtain higher temps. That is all.

More energy because its has more mass and surface area: if the color is the same, the temperature is the same.

 

[Integral]

http://www.sp.se/metrology/temperature/eng/its-90.htm

This sort of thing is pretty crude, but clearly the orange glow of a hot oven element corresponds to about 1000C.

An Optical pyrometer is a very precise "thermometer" which measures temperature by comparison to a heated element. This of course only works on objects hot enough to glow in the visible range.

To get an exact temperature one needs to know the emissivity of the glowing object. Since emissivity is always less then 1, your actual temperature will be HIGHER then that which corresponds to the color.

 

[DaveC426913]

"but dave for something to "glow red", the substance that glows red must achieve some temperature, and I am sure that a thick metal would have a harder gatting that glow then an incredibly thin wire as in a blow dryer. but thanks i get the idea about ovens"


Nono. All I'm saying is, I find it hard to believe that these devices would be built that could hit temperatures of 1000 - 2000 degrees. I mention the blow dryer because it comes within inches of your face.

Surely that hot a temp would be too dangerous in the hands of amateurs.

 

[Integral]

Regardless of what you think ought to be, the color does not lie.

Consider the amount of energy which must be transferred to the rapidly moving air in a blow drier? Consider the temperature $\Delta T$ and the time $\Delta t$ clearly the element temperature must be quite high to get the air as hot as it does in such a short time.

 

[Janus]

"but dave for something to "glow red", the substance that glows red must achieve some temperature, and I am sure that a thick metal would have a harder gatting that glow then an incredibly thin wire as in a blow dryer. but thanks i get the idea about ovens"


Nono. All I'm saying is, I find it hard to believe that these devices would be built that could hit temperatures of 1000 - 2000 degrees. I mention the blow dryer because it comes within inches of your face.

Surely that hot a temp would be too dangerous in the hands of amateurs.

Don't confuse temperature with heat content. The amount of heat contained in a hair dryer element is a lot less than that in an oven element. These elements are also encased such that you can't get close enough to them for enough heat to be transferred to burn you.

 

[Chronos]

For the record [temperatures in fahrenheit]

1000 Dark Red
1200 Blood Red
1375 Low Cherry Red
1425 Medium Cherry Red
1500 Full Cherry Red
1550 Bright Red
1650 Salmon
1725 Orange
1825 Lemon

 

[brewnog]

1200 Blood Red
1375 Low Cherry Red
1425 Medium Cherry Red
1500 Full Cherry Red
1650 Salmon
1725 Orange
1825 Lemon

These aren't colours, they're food!

 

[T@P]

wow that went right over my head. but your probably right so :)

 

[DaveC426913]

So that puts an oven at about 1550?

 

[T@P]

lol my oven still says 350 Fahrenheit when i turn it to 350. I am not sure what I am doing wrong ;)

 

[enigma]

T@P,

To get the inside of the oven to 350 degrees, the part of the oven which is doing the heating must be higher than that.

To ensure you don't have to wait for hours for the oven to get up to temp, it would need to be significantly higher than 350 degrees.

 

[Janus]

lol my oven still says 350 Fahrenheit when i turn it to 350.

That's because your oven has a device called a "thermostat" which controls the temp.
It works like this:

When the oven reaches the correct temperture, it shuts off the element. When the oven then cools down to a preset amount less than the set temp, it turns it back on again. It then repeats the above process. In that way, the oven temp "hovers" around the set temp. As Enigma said, the heating elements must be much hotter then any oven temp you want to reach. Otherwise they would take far to long to bring the oven back up to temp.

 

[T@P]

it all makes sense. i was just kidding with my post hence the ;)

 

[Schneibster]

What is heat? What is temperature?

OK, this is after all the physics forum. Let's talk about heat, specific heat, and temperature. We'll touch briefly on latent heat as well. And we'll make sure that we understand the difference between heat and internal kinetic energy- most people mean the second when they say the first.

Let's start with temperature. Temperature is the average kinetic energy of all the molecules in a substance, region, or object. Gasses circulate pretty freely, so by and large the air is going to be pretty much all the same temperature inside your oven. It might be a little hotter at the top, a little cooler at the bottom. Not much, though- probably only five or ten degrees.

Now, what's heat? Well, actually, what most people call "heat" is really internal kinetic energy. It's the total kinetic energy of all the molecules in a substance, region or object. So why is that not heat? Simple: because heat is the difference in total internal kinetic energy between two objects, regions, or substances- not the total internal kinetic energy itself.

So what's the difference between the two? Simple: if you have, say, a liter of boiling water, it's at 100C. If you add more boiling water, say another liter, it's still at 100C. But the first liter contains about 1513 kJ of energy- and so does the second, so even though the water is still at 100C, because there's two liters now instead of one, you have about 3026 kJ of energy; twice as much.

So what's specific heat? Well, that's the amount of energy it takes to raise a certain amount of water by a certain temperature. In the SI system, it's measured in joules per kilogram-kelvin, or more conveniently kilojoules per kilogram-kelvin, abbreviated kJ/kg*K. Since I know that the specific heat of water at 100C is 4.216 kJ/kg*K, and I know that the temperature is 100C which is 373.15K, and water weighs 0.962kg/l at 100C, I just figure it all out and there you have it.

Now, it takes more than just specific heat to make water turn into steam. We know that, because even though the water is boiling, it doesn't all just suddenly turn into steam and go floating away- it takes a long time for boiling water on the stove to all boil away, which means even after we have our 1513 kJ/l in it, we still have to keep pumping heat into turn it to steam. Why is that? Well, even though the water is at 100C, to turn it to steam takes some additional heat so that the water molecules can "break away" from the liquid water. That heat is called "latent heat of vaporization." (Going from ice to water takes latent heat too- but a different amount. It's called "latent heat of liquifaction.") It's about 2270 kJ/kg. So the amount of latent heat for water is greater than its specific heat at the point of vaporization! Wow, no wonder it takes so long to boil it away!

So, we know just by thinking about it that the specific heat of air is pretty low- you wouldn't be able to open the oven if it weren't, the air would cook you alive. But of course the specific heat of the food and metal inside the oven is much higher- so you might be able to wave your hand around in there for a little bit, but don't touch anything! Now, what do you suppose the oven thermostat is measuring? The temperature, of course!

Cooking happens three ways: convection, which is the air moving around as it gets heated, and heating up whatever it touches; radiation, which (as someone else noted) is the burners putting out infrared energy that gets absorbed by whatever it hits (mostly the food, the cooking pan, and the sides of the oven; but of course it heats the air too); and conduction, where the air conducts heat into the food, and the heat spreads out inside the food.

Now, the burners inside the oven get really hot. Electric ones are made out of a special ceramic (ceramics can take incredibly high temperatures, the most temperature resistant things we know how to make are ceramics) with a high-resistance wire down the middle. The wire is very thin, because that way it makes more heat, so the ceramic protects it from breaking if it gets bumped, and also increases the surface area so that the heat can radiate more efficiently, thus keeping the wire from melting too.

The ceramic is essentially pretty close to a black body (it doesn't reflect much light, you'll notice- that helps it radiate the heat so the wire doesn't burn out) so we can guesstimate the temperature by the fairly red-orange color it is when the thermostat turns it on. Just by guess and by gosh, looking at the chart on the Wikipedia page for black body radiation, I'd have to say it's probably somewhere around 1,500K. That's 1,200C, or 2,250F. I'm going to back off and say that the 1,900F that's been floating around here is probably as good an estimate as any. It might be a bit cooler or hotter than that; but not a lot. Now, that's well above the ignition temperature for just about anything you'd care to call "flammable." And of course, if it's a gas oven, you've got a flame right there; needs no more help than that. Last but not least, if grease collects, it can evaporate at high temperature, and catch fire. So there you have it.