For those of us who hate wading through video, the gist of this is that overheated wheatbags can catch fire and set your house alight
:eek:
Now there’s two mechanisms given for this.
Overheat in the microwave, and it catches fire in the microwave
I get this.
Overheat in the microwave, take it to bed with you, it continues to heat up (?) due to overinsulation (??) and your bed can catch fire up to TWO OR THREE HOURS LATER (???)
I don’t get this. How is this even thermodynamically possible? Unless the wheat is actually smouldering - in which case, wouldn’t you smell it?
There are a few things that do this, charcoal, greasy rags and apparently wheat bags. Unca Cecil has a column IIRC the gist of which is some materials emit some heat and get enough of it it self ignites. Coal bunker fires may have sunk the Maine and the Titanic had one onboard as well. Sorry I can’t link I am on a plane and the interwebs are sketchy
Apparently it’s exactly what the name implies, a cloth bag filled with wheat (or rice). I was curious so I googled it. Seems to be common in the UK, and they claim that wheat bags retain heat well and are more flexible than hot water bottles.
At least that’s what my quick google search taught me.
The usual “spontaneous combustion” that I’v heard of, is a well-insulated pile of flamable material hat may be slowly oxidizing a the time. The classic wet haystack or pile of oily rags come to mind. In both cases there is a slow oxidation, which produces heat. With sufficient insulation, that heat does not escape but builds up. The heat buildup in oily rags just exacerbates the situation until it starts burning. The flames in wet haystacks come from bacterial decay in the hot sun that produces so much heat (due to the large volume) that it does in fact burst into flames.
So is the wheat bag moist enough to create decay, or articficially heated to the point of causing smouldering? I doubt it’s decay, since that would create a noticeable beer smell. Not sure if oxidation of starch dust would cause this? I heard that explosions of grain towers is a risk; when the starch dust-air mixture comes in contact with fire…
Or if there is some even tiny metal contamination; heating a piece of metal in a microwave can create fire due to overheating from indiced current. (Those brown kraft-paper towels risk bursting into flames in a microwave, since they are made from recycled material and the recycle process may include tiny chunks of foil.) Or do they think “hot spots” may develop in the wheat itself during the microwave heating cycle?
If you heated up the center of the bag hot enough to burn, but it didn’t have sufficient oxygen, it could retain that heat until movement allowed some oxygen to get down there. At that point it could theoretically flare up, or at least continue increasing in temperature. You might not smell it precisely because there’s limited exchange with the air.
But is this a real problem? I’m skeptical. And it would certainly be avoided by minimizing the time in the microwave.
They’re certainly common out here on the West coast of the US. I have one that I sometimes use in place of a heating pad.
Round here (Australia) wheat tends to be the medium of choice - I guess people copy off what they’ve seen.
Capt Kirk’s comment about charcoal allowed me to google this, so whadda you know, maybe it is possible. Except that charcoal strikes me as a more readily combustible material in general than wheat.
dracoi’s suggestion sounds like a plausible way for a fire to start, except that it doesn’t seem to have been the mechanism in the linked video, where the woman claims it was tucked up in her bed and caught fire while still there - not much opportunity for extra oxygen getting in under those circumstances.
I use a wheat bag quite frequently - stick it on my neck for tight muscles (a water bottle would fall straight off). But IME they cool way down within about half an hour.
Hmmm…maybe UK wheat bags are generally bigger too? Mine’s not much bigger than two fists together - wouldn’t be much use to warm up a whole bed.
I have not seen massively large wheat-bags in the UK. It seems to be a good choice for a heat pack heated in the microwave - the grains hold a bit of moisture to make a good candidate for microwave heating, and form a flexible mass when constrained in a bag.
If you overmicrowave, they can dry out enough to catch fire, especially if you have a hot-spot in the microwave with no rotation. I agree that the smouldering/ignition path seems unlikely, but it could happen. Something more likely is leaving a wheatbag in a bed then using an electric blanket - that could easily trap enough heat to start a fire.
I can’t speak to your issues specifically, but my doctor’s opinion is that the cool-down time is a feature, not a bug. Most of the injuries people put heat on only need 20-30 minutes, not the continuous heat you get out of a heating pad. In fact, alternating between hot and cold is frequently recommended.
As an engineer who has worked with coal, this is a common problem especially with lower rank coals.
This combustion is a complex process - and a lot of things need to be just right for this to propogate such as:
a> Many organic materials have oxygenated compounds in them - which provide the oxygen necessary for sustainting the combustion.
b> The pile needs to have good heat conductivity hence the counter intuitive phenomenon - if you get it wet a bit - it increases conductivity without significantly harming its ability to heat up and burn (specific heat). Hence a little bit moisture encourages this combustion.
c> The right particle size (effects air ingression) and the initiation with a local high temperature zone (friction, biological,…)