Windscreen overheating canister

[Jerry Adams]

I noticed the top of my canister warm to the touch but not hot.

The side of the canister where the fuel is was cold.

What's unsafe? When the top of the canister is hot to touch or the side where the fuel is?

[And E]

Rule of thumb would be that if it's too hot to touch, it's too hot for safe use. I don't think any part of the canister should get too hot, top or sides.
It shouldn't be more than 50*C, which, when you touch it, should be almost uncomfortably hot (your sensitivity may vary).

It's fine if it's just warm, that would actually be beneficial if you're operating it in cold/snow.

[Jerry Adams]

Yeah Roger's article said that too – hot to touch is unsafe, if not hot to touch then its okay.

But I noticed the top of canister is warm next to where stove screws on, cold on side where the fuel is.

So if the top of canister is hot then it's unsafe?

[And E]

If its just the threading area, then it should be fine, as long as the main canister body area doesn't get too hot. You could try PMing/emailing Hiking Jim of Adventures in Stoving, he'd probably know better than anybody else.

The warmth is probably from the thermal feedback from the stove itself. The stove screws onto the canister, and heat travels down the metal to where it connects to the canister. Just keep tabs on how hot the top of the canister gets, and if it's too hot, turn off the stove and let it cool down (maybe pour some luke warm/room temp water on it) and then fire it up again when its cool.

Boiling 2 cups at a time is more efficient and quicker than boiling a liter at once, and you can let the stove cool between boils that way.

[David Thomas]

Top hot and bottom cool?

Not a problem.

Here's why: The top is only in contact with the fuel vapor inside. All gases are low-density and low-heat-capacity. A little heat into thin metal and a bit of gas will heat it up.

The bottom, however, holds all the liquid fuel. That much larger mass will take much longer to heat up. Because (1) it is a larger mass, (2) because it is more removed from the heat source (feedback from the burner head), (3) the metal of the top of canister functions somewhat like a heat shield for the bottom of the canister (4) the hot vapors in the top of the canister stratify and don't exchange much heat with the liquid below and – most significantly – (5) you keep cooling the liquid by operating the stove (see below).

It is the temperature of the LIQUID fuel that determines the pressure in the canister. If the LIQUID is hot, the pressure will be high, maybe dangerously high. While the liquid is in equilibrium with vapor, it is only in equilibrium with the vapor immediately adjacent to the liquid. So the liquid could be 50F, the vapor immediately above the liquid also 50F, but 2 mm higher the vapor could be 60F, 70F above that, 80F above that, etc and the top of the canister could be at 130-150F but the pressure of the canister would be based on liquid at 50F (whatever your butane-propane mix is, it has a particular vapor pressure at 50F).

So it is not a problem of being too HOT.

You could have a problem of the canister being too COLD even when the top is hot, for all the same reasons. Plus boiling vapor off the liquid will cool the liquid. So if you have thermal feedback to the top of the canister but not to the sides or bottom; you use it long and hard; in cold weather; and you have a high-butane mix, you could lose pressure in your canister even though the top is hot to the touch.

Sometimes in my day job, I've burned hundreds of pounds of propane per day. The top of the tank would be at ambient temperature, but the bottom of the line would be iced up and tank pressure would drop below the pressures I needed for the equipment. It is (on a stove canister or a home, propane-fired BBQ) a great way to check fuel level: run your finger down the side of the canister after it has been running a while. The liquid/gas demarcation will be quite distinct.

The fix for that cold liquid fuel in a BPing stove is a copper wire tied to the LOWER side of the canister and extending into the flame or a heat reflector that redirects more radiant heat lower on the canister, or warming the canister directly with hot water or a bic lighter.

Pro-tip: 140F is limit of what metal object you are able/willing to hold with your bare hand. As such, you can make reasonable estimates of 120-130-140-150F temps within 5F or so with a little practice.

Edited to add yet another parenthetical thought.

[David Thomas]

With a heat source above (radiant heat through the air and conductive heat transfer down the stove stem), the gases will stratify and NOT exchange heat to the liquid below (to any significant extent).

With gases boiling off the top of the liquid causing the upper liquid to cool, the liquid won't stratify – that colder, denser liquid will sink, replaced by slightly warmer liquid from below.

Upshot: the vapor in the headspace is hottest at the top. The liquid at nearly the same temperature throughout the liquid (and colder than any of the gas above it).

[Stuart R]

Top hot and bottom cool

Some guys prefer that way around and some the other way :-))

[Bob Gross]

"Top hot and bottom cool"

Eric Chan prefers the top to be hawt.

–B.G.–

[Roger Caffin]

Hi Jerry

What really matters is the pressure inside the canister. If that gets too high things can go orbital. DoT regulations require that the canister be able to safely handle 50 C. My experience is that the canisters can handle rather more than that – an engineering safety margin.

But what do they mean by 50 C? Well, the pressure inside the canister is created by the liquid fuel inside. The temperature of the fuel is what matters. When you say the side of the canister was cool, you are 'measuring' the temperature of the fuel. So that is fine.

What about the warm top? Yes, that warmth is because it is getting heat down from the stove. The heat is warming the steel body at the Lindal valve, and that warmth or energy is then spreading down the sides into the fuel, helping to vaporise it. That is normal, even useful.

By the way, what does 'hot to touch' mean? If it means you could touch the metal without going 'ouch', that is OK. The important test is 'TOO hot to touch', which happens between 40 C and 50 C. When the sides of the canister where the fuel is touching start to approach 'too hot to touch', then you have a problem. Below that you are fine, but always monitor what is happening.

Cheers

[Jerry Adams]

Ah ha – chemical engineer and stove expert agree : )

I think in addition to direction down stove through Lindal valve, particular windscreens will also cause radiation down to the top of the canister. First, I boiled 2 cups of water with windscreen. Then, I boilded 2 cups of water without windscreen and it seemed like the top of the canister wasn't as warm.

It seems like a windscreen with minimal clearance between windscreen and pot (like 1/4 or 1/2 inch) will cause canister to warm more. The hot exhaust will pool up around the bottom of the pot.

Someone said they made a windscreen based on my article and complained canister heated up. I bet he was just talking about the top of the canister and not the temperature of fuel as measured by feeling the side of the canister towards the bottom.

[Joe L]

On the safety margin: Those canisters do not explode when left in the trunks of cars in Phoenix in June when the temps are 115+ degrees. That might be no ignition of the vapor, but the cars do get much warmer than the outside air temp.

In the winter, I use a regular windscreen on a canister stove but I do monitor the warmth of the canister. That extra heat may help to vaporize the second gas in the canister's gas mixture, the liquid gas with the lower boiling point.

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