The thinking behind heat exchangers is pretty simple … by attaching something to the bottom of a pot, you will increase its surface area and to a degree help contain the flame at the base of the pot. In theory those two things should speed up boil times which in turn has the knock-on effect that you require less fuel to boil a given amount of water.
Obviously there’s a downside (because there always is and that’s life) and that downside comes in the form of extra weight and a slightly bigger pot. However, if you can speed up the cooking process and use less fuel to do it, then the pros probably outweigh the cons … particularly on longer trips.
The two pots I selected were both aluminium, the one with the heat exchanger had a capacity of 1L and the other 750ml. The heat exchanger pot has a 10mm larger diameter which should actually help reduce boiling times compared to the narrower pot.
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| Heat exchanger or radiator? |
I made a mesh pot stand so I could see what was going on and got to work. The stand was made so that it would sit within the heat exchanger rather than have the heat exchanger sit on top of it. This meant that the base of each pot would be exactly the same height from the top of the stove.
First up was the ‘standard’ pot … 350ml of water and 10ml of meths in the stove. There’s no ‘bloom time’ on the stove, I just lit it, waited 10 seconds to make sure it wasn’t going out, placed the pot on top and started the timer. There was a little flame overspill up the sides which should in theory be a plus point when it came to the heat exchanger. 2 minutes 56 seconds later we had a rolling boil.
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| Standard pot, nothing fancy just aluminium. |
I allowed everything to cool down for an hour and repeated the test, the only thing changed was the pot. It was very obvious that the fins of the heat exchanger held the flame and prevented any of it from disappearing up the sides. The flame colour was good, which indicated that the exchanger wasn’t restricting the stove in anyway … so I was a little surprised when the clock stopped at 4 minutes 36 seconds and we’d only just achieved a proper boil. Although I don’t tend to believe stuff until I try it or see it, I have to say I wasn’t really expecting this … so I let everything cool down and tried a second time. The results were pretty much the same give or take a few seconds.
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| Heat exchanger prevents any flame overspill. |
I made a brew and had a think … The theory of increasing the surface area of the base of the pot is sound but in this case it seemed as though the exchanger was actually ‘robbing’ heat from the pot. I wondered whether there might not be enough of the exchanger element in contact with the pot base … the concertinaed fins of the exchanger certainly have a large surface area but if that area is acting independently of the pot, it will just behave as a heat sink taking heat away from the pot and radiating it into fresh air, which is the exact opposite of what we want.
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| MYOG internal heat exchanger, not posh but it seems to work. |
After a second cup of tea (and a biscuit), I struck upon the idea of increasing the surface area of the pot on the inside. I theorised that any heat that ‘got into’ the exchanger would have no choice but to pass over to the water inside. I cut two strips of aluminium and roughly rolled / folded them so they fitted inside the ‘standard’ pot and held themselves in place. I hoped that being in contact with both the base and side of the pot would mean any flame overspill might also be captured (a little bit). 2 minutes 35 seconds later the water was boiling, now, obviously none of this is particularly scientific but I don’t think a 20 second reduction is a fluke. More experimentation is required, so I’m in the process of making something with a little more ‘design’ to see whether an internal exchanger can produce any significant benefits … I’ll let you know.
Exchanger pot: Fire Maple
Standard pot: Something old, cheap and cheerful
Stove: Proto’ Bear Bones
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