I’ve always been a little embarrassed to ask, but when I have, say, a propane torch, how come the flame doesn’t go back through the line and blow up the can?
In: Engineering
Fire requires a certain fuel/oxygen mixture to combust. A propane tank has a lot of fuel but little to no oxygen.
Reading these explanations now make me wonder how bullets work, because there isn’t any oxygen inside.
The contents of the propane can are under pressure. Gas wants to move from high pressure to low pressure. When you open the valve, gas comes out, because it is pushing against the back of the valve.
The gas can’t burn inside of the hose because it has no oxygen (the hose is full of gas and there’s no room for oxygen,) and because the gas is pressurized, gas is pushing to get out of the hose where it can mix with air and undergo the combustion reaction.
People have answered this for fuel-only torches, but you also have oxyacetylene torches where you supply both oxygen and acetylene. In this case the flame can indeed travel back, known as a flashback. Care and equipment is required to prevent it or at least limit damage.
Another fun one – if you light a candle and blast oxygen at it what will happen?
Answer – it will blow the candle out. Not enough fuel in the right form vs the strong wind of oxygen blowing it out.
Fire needs oxygen. No oxygen, no fire. Some(or atmospheric levels)oxygen-> fire. Lots of oxygen–> more fire.
For oxygen to travel upstream and against the outwards pressure to reach inside the fuel container, it would be like trying to shove toothpaste back in the tube while someone else was squeezing the tube.
Because the tank is pressurized and the oxygen inside of the nozzle has been completely pushed out. As you go into your torch, you hit what’s called the Higher Explosive Limit of the propane.
Basically, all fuel needs air to burn, and how much air it needs to do so is specified as a range. If it is above, or below this range, it won’t light. If the entire space is just filled with propane, you don’t have adequate air to burn, and thus, no flame.
Well, for one thing it’s pressurized; which means the flow of gasses always wants to leave the line, not travel backwards to the can. Secondly, you need a certain mixture of air and propane fumes to enable combustion, and that does not exist inside the can.
have you ever drank milk through a straw? then you must have blown bubbles in the milk too. but why couldnt you drink any milk when you were blowing bubbles? its kind of the same way.
Also the propane is pressurized, so when you open the valve, it really wants to spurt out that little nozzle like air blown it into a balloon, and that energy pushes the flame outward, hence the direction of the flame you see. If it weren’t pressurized, it would just dribble out as a liquid like gasoline from a gas pump which does happen as the propane exhausts itself and so the pressure drops a lot — it dribbles out. Without pressure of some kind, the ignition could burn back down the hose into the tank. Not a pretty sight.
It’s the same with plastic cigarette lighters which are filled with butane and then a shot of pressured gas (?) added. When you flick your Bic, the spark ignites a gas coming out of the lighter under pressure. And you hear a rushing of gas. This is unlike on an older style zippo lighter which operates by an older method much like a kerosene lantern. A gas-soaked cotton pad with a cotton wick attached releases gas fumes when you open the top of the lighter. Flick the friction device to create a spark, and it ignites the end of a wick surrounded by those fumes. That’s why a zippo lighter flame is lazy and wavering like a candle while a Bic lighter flame operates more like a blowtorch.
Puncture a kid’s blow-up swimming pool and you watch the water in the pool, under gravity pressure, pushing the water out very fast. The likelihood of the burning flame going into the container is pretty close to zero due to that pressure. It’s like a punctured tire . . . [Oh,shut up!] . . . .
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