HEAT EXCHANGER POTS FOR HIGH ALTITUDE
THERE ARE TWO HEAT EXCHANGING PARTS TO A DEDICATED HIGH ALTITUDE COOKING SET UP, THE VAPORIZING TUBE IN THE STOVE AND THE EXCHANGER OR ‘FLUX RING’ ON THE POT. THIS ARTICLE IS (MOSTLY) ABOUT THE LATTER.
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FOR HIGH ALTITUDE COOKING & MELTING A LOT OF NOISE IS MADE ABOUT THE STOVE, WHICH THOUGH IMPERATIVE IS ONLY HALF THE SYSTEM. THE BEST STOVE UNDER WRONG POT WILL STILL SEE UNBURNED GAS WASTING AWAY, AND A SYSTEM BURNING INEFFICENTLY WITH YOUR PRECIOUS FUEL.
BY HIGH WE MEAN ABOVE 5000m, WHERE PRESSURE IS A THIRD LESS THAN AT SEA LEVEL AND WATER BOILS AT 80c OR LESS, WHERE EVERY ADVANTAGE IS NEEDED. THIS IS A MAJOR ELEMENT IN THE CYCLE OF GETTING STOVES TO WORK OPTIMALLY AND IS SOMETHING OTHER INDUSTRIES PUT A LOT OF ENERGY INTO (HA HA), BUT MUCH OF THE PROCESS IN MOUNTAINEERING IS EITHER OVERLOOKED, MISUNDERSTOOD OR NOT RELEVANT. HEAT EXCHANGER SYSTEMS MAKE A BIGGER DIFFERENCE THE HIGHER AND COLDER THINGS GET, WHERE MAKING WATER AND FOOD IS THE PRIMARY THING THAT KEEPS YOU ON THE MOUNTAIN. STOVES NEED TO BE EFFICIENT TO MAKE THE HOURS-LONG JOB OF MELTING AS PRODUCTIVE AS POSSIBLE, THAT IS A BATTLE AGAINST LOW AIR PRESSURE.
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ALMOST EVERYTHING WRITTEN ABOUT HEAT EXCHANGERS IN COOKING SETS COMES FROM THE HIKING WORLD WHERE THEY ARE CONCERNED WITH OTHER THINGS. SURE, FUEL EFFICIENCY MATTERS, AND IS RELATIVE TO THE WEIGHT OF THE POT BLAH BLAH BLAH, BUT AT 6000m IN A TIBETAN WINTER THERE’S OTHER THINGS THAT MATTER. THE FLATLAND VERSION WILL TELL YOU HEAT EXCHANGERS ARE ABOUT TRAPPING GAS TO GET MORE OUT OF THE FUEL YOU CARRY, WHICH IS NOT REALLY WHAT HEAT EXCHANGE IS, BUT THEY WILL BE GETTING THE EFFECTS OF HEAT EXCHANGE REGARDLESS OF WHETHER THEY RECOGNIZE IT OR NOT.
REAL HEAT EXCHANGE IS THE PROCESS OF USING AN INTERMEDIARY THERMAL ACTION TO GET BETTER EFFICIENCY FOR MOVING ENERGY FROM ONE PLACE TO ANOTHER. IN A WAY ALL POTS ARE DOING THIS WITH THE CONDUCTIVE METAL THEY ARE MADE OF, BUT HERE IT MEANS AMPLIFYING THAT ACTION TO INCREASE ITS EFFECT. THIS THEN IS HAVING MORE HEATED MATERIAL TO TRANSFER MORE HEAT, WHICH HAS THE RELATED EFFECT OF ALLOWING MORE GAS TO IGNITE BY RAISING THE LOCALIZED PRESSURE, WORKING ON TWO ASPECTS OF PHYSICS SIMULTANEOUSLY. LIKE A HEAT EXCHANGER MASK, IT IS THIS CREATION OF A SPECIFIC THERMAL MICRO-ENVIRONMENT THAT IS THE HEAT EXCHANGE, SOMETHING THAT SIMPLY SHIELDING AND CONTAINING THE GAS DOESN’T SIGNIFICANTLY DO*.
ANYONE USING A STOVE AT ALTITUDE WILL KNOW THE ISSUES WITH REDUCED AIR PRESSURE ON THE CANISTER, AND HEAT EXCHANGERS DON’T DIRECTLY CHANGE THIS, INSTEAD THEY HELP IGNITION OF THE GAS RELEASED AT SUB-OPTIMAL PRESSURE. THIS IN TURN WARMS THE AMBIENT TEMPERATURE THAT DOES HELP THE CANISTER PRESSURE, CREATING A LOOP THAT GETS A SYSTEM AT IT’S BEST.
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AT ALTITUDE AND IN THE COLD – BOTH ASPECTS OF LOW PRESSURE – WHAT AN EXCHANGER DOES PRIMARILY IS PRODUCE HOT METAL THAT LETS CANISTER FUEL VAPORIZE MORE TO BURN. LIKE THE VAPORIZING TUBE IN A PROPER COLD CONDITIONS STOVE, GAS THAT EXPANDED TO FLASH POINT IS WARMED SO IT CAN BURN MORE COMPLETELY. IN THE EXPEDITION STOVE SET, THE VAPORIZING TUBE OF THE STOVE ALONE WON’T BE ENOUGH FOR OPTIMAL COMBUSTION, AND GOOD HEAT EXCHANGERS MATTER A LOT WHEN USING INVERTABLE CANISTERS, WHERE THE LIQUID FEEDING GAS HAS THE GREATEST PRESSURE GRADIENT TO COVER TO COMBUSTION.
THE AIM OF THE EXCHANGER IS TO HEAT MORE GAS TO RELEASE MORE ENERGY TO BE TRANSFERRED TO THE POTS CONTENTS (ALMOST ALWAYS WATER IN THIS CASE), THUS SURFACE AREA OF METAL IS WHAT COUNTS WHICH MEANS THE CONTOURED AND CRINKLED SHAPES THAT EXCHANGERS HAVE. SIMPLY TRAPPING GAS IS ONE THING, AS SOME SYSTEMS DO, BUT THAT GAS NEEDS TO BE EXPANDED AND IGNITED AS EFFICIENCTLY AS POSSIBLE WHICH THE LARGER SURFACE AREA OF THE CONVOLUTED EXCHANGER DOES, THE MORE CRENULATIONS THE BETTER.
THE MECHANISM IS TO GET THE EXCHANGER HEATED AS HIGH AND FAST AS POSSIBLE SO IT IS WORKING AT OPTIMUM, IGNITING AS MUCH GAS AND TRANSFERRING AS MUCH HEAT AS IT WILL SAFELY ALLOW. HEAT EXCHANGER POTS NEED THE INFLOW OF OXYGEN TO GO WITH THE INCREASED COMBUSTION, AND THIS NEEDS TO BE AS DIRECT AS POSSIBLE SO MINIMAL GAS ESCAPES UNBURNED, BUT WITHOUT BEING COMPROMISED BY WIND
A RESULT OF ONLY TRAPPING GAS BUT NOT EFFICIENTLY BURNING IT IS CARBON MONOXIDE, WHICH IS WHERE THE STORIES ABOUT EXCHANGERS BEING DANGEROUS ON THIS ISSUE ARISE. A POT WITH A REGULAR BASE AND JUST A WINDSHIELD DOES NOT INCREASE THE SURFACE AREA THAT DOES THE ACTUAL EXCHANGING, AND THOUGH WILL BE BETTER UNTRAPPED GAS IS FAR FROM OPTIMUM EFFICIENCY.
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NOT ALL EXCHANGER POTS WORK OPTIMALLY, EITHER AS HEATING ELEMENTS, FOR OXYGEN DELIVERY, FOR HEAT TRANSFER, OR SIMPLY AT THE TEMPERATURES INVOLVED, WITH JETBOILS BEING THE BEST EXAMPLE AS THEY ARE MADE FOR THE RELATIVELY LOW OUTPUT OF THEIR BURNERS (1600-2400kcals/hr COMPARED TO THE +3000kcals OF SOME OTHERS) AS WELL AS BE AS LIGHT AS POSSIBLE (TOO LIGHT IN SOME RESPECTS).
POTS WITH JUST A FLUX RING AROUND THE EDGE TRAP GAS WELL AND ALLOW OXYGEN IN, BUT ARE NOT OPTIMAL FOR CREATING THE THERMAL EXCHANGE LAYER NOR TRANSFERRING THIS INTO THE WATER. THIS SLOWS THE STABILIZATION OF AN OPTIMAL EXCHANGE ZONE, MEANING GAS IS NOT BEING FULLY COMBUSTED.
BUILT IN EXCHANGERS ARE FAR MORE EASY TO USE AND EFFICIENT THAN ADD ON ONES AS THE METAL IS THE METAL OF THE POT ITSELF SO IS COMPLETELY WITHIN THE THERMAL SYSTEM, WHEREAS AN ADD ON MIGHT WORK BUT TRANSFERS LESS INTO THE POTS CONTENTS, IN EFFECT ONLY DOING HALF OF THE EQUATION.
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IN USE EXCHANGER POTS ARE BEST WHEN USED KNOWING THE PRINCIPLES AT WORK, AND GETTING THE METAL ELEMENTS HEATED, SIMILAR TO THE WAY A VAPORIZING TUBE MUST BE WARMED AND THE AMBIENT TEMPERATURE RAISED (WHEN IT CAN BE ie INSIDE A TENT). THIS MEANS GETTING THE CANISTER GOING, BUT ALSO GETTING SOMETHING IN THE POT TO HOT ASAP SO THE EXCHANGER CAN STABILIZE, SO GETTING 1/4 OF A POT NEAR-BOILING THEN ADDING MORE (AS IS DONE WITH MELTING SNOW) KEEPS THE GAS BURNING OPTIMALLY AS IT FLOWS INTO THE HOTTEST THERMAL EXCHANGE ZONE POSSIBLE.
IN OUR EXPERIENCE WE HAVE FOUND THAT THE BEST HEAT EXCHANGING SYSTEM HAS SEVERAL THINGS;
- LOTS OF METAL SURFACE AREA SPREAD EVENLY – TO HEAT MORE GAS FASTER AND TRANSFER THE ENERGY DIRECTLY ACROSS AN EVEN AREA
- ADJUSTABLE WIND PROTECTION/GAS CONTAINMENT – TO TRAP AND STABILIZE LOW PRESSURE GAS FOR RAPID COMBUSTION
- GOOD OXYGEN INFLOW – TO FEED THE COMBUSTION
- GOOD HEAT BLEED WHEN NEEDED – WAYS TO ADJUST EXTERNAL PRESSURE ON THE CANISTER
- HIGH KCAL OUTPUT – TO GET THE METAL ELEMENT UP TO OPTIMAL TEMPERATURE FAST AND KEEP IT CONSTANT. THIS INCLUDES CANISTER INVERSION TO GET THE EXCHANGER GOING.
- ROBUST – ALL INTERFACING PARTS COPE WITH THE RIGORS OF THE JOB, NO FRAGILE OR FIDDLY PARTS.
THE POTS THAT NAIL THIS BEST ARE THE MSR REACTOR ONES AND ONES WITH THE SAME DESIGN OF ENCAPSULATED HEAT VANES THAT COVER THE BASE, HOLLOW WALLS THAT CAPTURE GAS AND OXYGEN VENTS THAT SUCK IN AIR BUT PROTECT THE FLAME FROM WIND. THEY ARE ROBUST AS HELL, WITH ALUMINIUM HEAVY ENOUGH TO RETAIN A DECENT AMOUNT OF HEAT, AND THE SHAPE OF THE BASE SITS WELL ON MOST BURNERS. OF THE VERSIONS AVAILABLE THE CLASSIC 1.7L POT IS THE BEST HAVING THE LARGEST BASE AREA THAT TRAPS MORE HEAT AND SITS MORE STABLE.
WHERE MOST POTS FAIL IS IN LACKING THE VANES TO CREATE EXTRA HEATED MASS ACROSS ALL OF THE POTS BASE, WHICH OVERLOADS THE OTHER ELEMENTS OF THE EXCHANGER AND REDUCES PERFORMANCE IN THE HIGH COLD. MANY POTS TOO ARE FLIMSY, UNABLE TO SAFELY COPE WITH THE EXTRA HEAT TRAPPED BY THE EXCHANGER, THIS IS USUALLY A RESULT OF STRIVING FOR LIGHTNESS AND NO SERIOUS INTENTION OF HIGH ALTITUDE USE.
MSR-STYLE POTS ARE HEAVIER THAN SOME OTHERS BECAUSE THEY ARE BUILT FOR HIGHER TEMPERATURES, BUT THIS ADDED ROBUSTNESS MEANS THEY SURVIVE BEING USED TO SHOVEL SNOW AND WE HAVE ONES WELL OVER 15 YEARS OLD WITH HUNDREDS OF DAYS OF USE ON THEM. THIS EXTRA WEIGHT IS MADE UP IN GAS SAVED OVER ABOUT 3 DAYS, AND MEANS THEY CAN BE PAIRED WITH THE MOST POWERFUL STOVES WITH NO RISK OF COMING APART.
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*IN THE CASE OF ‘PSEUDO-HEAT EXCHANGER’ SYSTEMS WHICH ARE REALLY JUST WINDSHIELDS, THE INCREASED EFFECTS COME FROM HEAT-BLEED RAISING AMBIENT TEMPERATURE, SOMETHING THAT THE BEST EXCHANGER SYSTEMS WILL ALSO DO, THOUGH MANY DON’T DUE TO WHAT THEY CALL GREATER EFFICIENCY, WHICH IS ONLY TRUE IN LOWER TEMPERATURES AND ALTITUDES.
