Pot boiling times

[mat_swan]

Hi Folks, long time lurker, I had some thoughts on the pot boiling post on the front page so I thought I'd log in and see if anyone else thought I was talking sense or nonsense.

I was interested by the fact that the boil times were so different between the finned and normal pots so I thought I'd crack out some long forgotten science (on a Sunday night!) and see if I could find a reason. As a disclaimer, this is very crude, with rough figures!

First theory is the relative masses of the pots and the energy needed to heat them up.

So, for the first run lets assume, very crudely, that our system consists of a pot, 350ml of water and a heat source delivering a constant supply. For this first crude attempt let's also assume efficient heat transfer between the metal and the water and no heat loss to the outside.

Some numbers then:
Water has specific heat capacity of 4200 J/kg.K, alumnium 913.
Temperature difference from average room temp (23C = 296K) to boiling point (100C = 373K) is 77K
An AK brupot has a mass of 0.222kg
An AK 1l al pot has a mass of 0.143kg
Scaling that for 750ml gives us 0.107kg (if there's a figure going for the actualy pots used, please shout!)

To raise the brupot and the water to boiling point should take ~129kJ
The same for the estimated 750ml pot and water should take ~ 120kJ

Now, this is only a 7% increase on the heat required compared to the 57% increase in boil time observed so there are clearly some other mechanisms dominating! I suspect heat loss to the surroundings may be a key thing, but I'm going to have to brush up on some other subjects before I look into that! I'd be interested to see if you get more benefit from your weight putting an identical heat exchanger inside the pot compared to outside!

Something that could be interesting to test- the effect of having more metal to heat up on the finned pot should mean the the efficiency gap closes the more water you try to boil, so if tested again with, say 550ml and 750ml the difference should reduce. If it doesn't there's something else I really wouldn't expect going on!

Maths is here, please let me know if you spot any mistakes: https://drive.google.com/file/d/0B68LyR ... sp=sharing

Hope that sparks some thoughts and hopefully I'll have something else when I get the time- work lunchtimes perhaps.

Mat

[Bearbonesnorm]

I'll weigh the pots and put the weights up, I'll also try again using different amounts of water.

I'm still thinking that the way the fins are shaped / positioned in relation to the pot, very little of their surface are comes into contact with the pot, so a lot of heat is radiated to the outside air rather than conducted to the pot and ultimately the water inside.

[mat_swan]

I think you're right- that ring at the bottom of the fins is a large surface area, probably half that of the base of the pot, and a lot of it will be dumping heat into the cold air.

It would be interesting to see the boundary of high temperature air around the pot- you'd want to maximise the amount of fin metal area in that and maximise its contact with the pot.

Some fins that started deep in the centre and got shallower towards the edges would probably help.

[VeganGraham]

Some fins that started deep in the centre and got shallower towards the edges would probably help

Thicker where they attach to the pot tapering to thinner at the edges would be better too, a bit like the fins on an air cooled motorbike engine, although they would be heavier and harder to make.

[Zippy]

Interesting.

Whilst my knowledge on thermodynamics is rather limited, I have a fair few comments (which I'll try and get a chance to actually lay out).

The end point to all my ramblings would have been that data collection on experimentation within a controlled environment would yield more accurate and useful results, from which extrapolation (and hopefully interpolation too) would be more useful for the overall outcome. The variations and factors which all have an affect would be greatly based on many assumptions etc. (which to be fair, they have to be and I have to make loads of assumptions at work otherwise I wouldn't calculate anything), but to the point where I think these calcs are going to have less relevance than a bit of measuring.

Now, from a forum point of view, if we can get some data together for different stoves/water temp/external temp/cooking vessels together, that could work out some pretty crafty and useful data.

Off the top of my head, some additional variables to be accounted for that could potentially make a large difference include the surface area to volume ratio of both the water and the heating vessel and external convection i.e. wind (as this will quickly change the heat transfer to the ambient air efficiency) and where the flame is doing the work (I guess percentage efficiency of heat energy used actually for heating, and on the chain reaction from pot to stove too). Also, I'm aware that these calculations appear to be very static (my speciality as a civil eng.!), but I'm pretty sure how heat spreads through is very much a dynamic situation as particles bounce around getting more energy at different rates - especially as we get to boiling point - it must be at least a squared term as going from sub boil to boil involves a phase change, and we're on the border of that occurring.
Also, I guess if something is gonna heat up quick, it's gonna cool down quick too - would be handy to work that out too I guess as it's all part of the same thing kinda.

Like I said, not really within my normal realm of knowledge, but that's the outcomes of a few thoughts I had. I'll take part when I grab a chance and do some reading up.
Theory and practice, in theory the same, in practice it isn't, innit.

[Bearbonesnorm]

I knew you wouldn't be able to resist Zippy

[mat_swan]

Interesting point about a finned pot cooling down faster too- I thought a removable heat exchanger with a flat ring that the pot sits on would work quite well. Much like if you took the whole thing off the bottom of the fire maple pot and turned it upside down.

Another thought, I was under the impression that one of the big pluses for a heat exchanger was that it took enough heat from the flame that you could put a cosy on the side that you otherwise couldn't becuase it would catch fire/melt, or have I been barking up the wrong tree? That would significantly reduce heat loss to the outside air.

[Bearbonesnorm]

Another thought, I was under the impression that one of the big pluses for a heat exchanger was that it took enough heat from the flame that you could put a cosy on the side that you otherwise couldn't becuase it would catch fire/melt, or have I been barking up the wrong tree? That would significantly reduce heat loss to the outside air.

I think that's just a little bonus. A piece of aluminium placed around the bottom of the pot (acting as a skirt) should produce the same effect. If it's the heat exchanger itself radiating heat then a cosy won't alter that ... the heat's lost before the cosy comes into play.

[mat_swan]

Agreed, the heat lost from any fins is pre-cosy (which is a phrase I find oddly pleasing, like being just about to get into a nice warm pub). When you say an aluminium skirt I'm not sure I understand you right- I wouldn't have thought that would divert the hot air from flowing up the sides?

[Joshvegas]

its been a long to since I did physics but

Surely fins only have a limited effect in that while they potentially increase heat transfer you aren't actually increasing surface are thats exposed to water possibly reducing it as there is less heat up the sides?
I have an idea... how expensive is one off titanium printing?

[Chew]

When you say an aluminium skirt I'm not sure I understand you right- I wouldn't have thought that would divert the hot air from flowing up the sides?

I'd imagine you wouldnt want any hot air flow up the side as it would be inefficient as its just passing by the pot rather than being collected and concentrated at the base of the pot?

[Bearbonesnorm]

When you say an aluminium skirt I'm not sure I understand you right- I wouldn't have thought that would divert the hot air from flowing up the sides?

Imagine you got a length of 50mm aluminium strip and wrapped it around the base of your pot but 25mm would stick out below the pot ... like a skirt.

Yes, it would prevent any heat flowing up the sides, which as Chew says is really the point of it. I imagine you mean that it would act as a dam and prevent the burnt gases from escaping up past the side of the pot ... which it likely would but I think some experimentation could produce a 'sweet spot' for the skirt length.

[mat_swan]

Ah, OK, it was the 'not touching the ground' part of the skirt that hadn't clicked. Makes sense now. You could potentially cut, say, a set of 5mm wide tabs, maybe 10mm tall, around the bottom circumference of your skirt and bend them inwards 45 or 90 degrees. I would have thought that should improve heat extraction from the airflow by breaking up the flow.


Titanium printing is expensive. Stainless steel powder is about 50-60 quid a kilo, titanium is somewhere up in the hundreds iirc.