Ultralight Tent Stove- Part 2 A glimmer of success
This post is the second in a four-part series on my efforts to design a truly ultralight tent stove. My aim was to efficiently and cleanly provide cooking and warmth from a small number of bush sticks to enhance the pleasure of skiing and camping in a winter wonderland. Small steps showed glimmer os success.
Ultralight Tent Stove For Alpine Camping- Part 2 Harnessing the abundant heat energy in a hand full of sticks.
Introduction. In part 1 of my stove story, I defined the key feature that I required in my dream ultralight tent stove. I also described the theory of efficient wood combustion and my journey towards my dream stove design. Despite not having full control of the combustion in my micro stoves, I was spurred on by the amazing protracted and intense heat energy that could be released by the efficient combustion of a hand full of bush sticks. All that remained was to design and build a stove that would harness this abundant energy to provide warmth, cooking, and delight to those of us that love to venture into cold, remote, and wild places with a light load on our backs. Here is my first ‘dream’ ultralight tent stove.
Now, just in case you think such a little ultralight tent stove is a toy, look at the next night time photo!
That was good fun but just too hot for stove longevity, but the heating and cooking power for a small tent should be obvious even to a blind person. The next photo is of a more moderate burn.
I should also mention at this stage that a small stove size is obviously important for easy backpacking. Size is also a critical determinant of stove temperature and the quantity of radiant heat that it will provide to the tent occupants. To explain this, my stove has a relatively fixed rate of combustion of a small portion of the fuel load at any given time, burning only ~400g of sticks/hour. This means that my stove has a small fixed rate of energy output and so the stove cooktop/radiator needs to be commensurately small if it is to provide effective comforting radiant heat. A faint hint of red glow from the stove is the point where the tent occupants will say; “I can feel that….arhhh that is nice.” (In contrast, batch loading stoves are generally much larger and can have much higher energy output that can simply be raised just by adding more fuel and air to achieve the desired radiant heat level.)
Temperature measurement. Regarding my temperature estimates, there is a caveat. They are measured with an infrared thermometer and I don’t exactly believe the readings. I am fairly sure that it underestimates low temperatures off shiny surfaces (according to my digital/finger tests). However, when I record a 250 C temperatures measurement on the rough dark surface of the stove a small chip of wood or organic dust will instantly combust on the surface of the cooktop (no finger tests please). This is consistent with wood combustion temperature of 190-260 C (Forensic and Scientific Services). Lastly, within the burner, I am destroying high-temperature paint that is rated to 1093 C. This is not a surprise as a good quality flame can have a temperature of 1473 C, even a candle can reach 1400 C (Forensic and Scientific Services).
The color of the hot outside surface of the stove also indicates the temperature as indicated in the table below [temperatures in degree C] (Forensic and Scientific Services). You be the judge as I am not so good with reds. I think you can imagine the temperatures that are within the burner.
| Dull red | 500°-600° |
| Dark red | 600°-800° |
| Bright red | 800°-1000° |
| Yellow red | 1000°-1200° |
| Bright yellow | 1200°-1400° |
| White | 1400°-1600 |
How does the ultralight tent stove work? I think it is useful to think of the ultralight tent stove as a device that makes hot smoke or wood gas from the upside-down pyrolysis of the bottom ends of fuel sticks in a small confined burner and most of the remaining gas, soot and tar being burned after passing over a hot charcoal bed and into a relatively large heat exchanger labyrinth which doubles as a spark arrestor and cooktop. The preheating and drying of the fuel, on the drying rack and also in the fuel tube, provided by this design is important for good performance, particularly for wet fuel sticks that winter backpackers usually encounter and more particularly when they are on the move from day to day and cannot easily accumulate a supply of dry(ish) wood.
The sticks are fed by hand into the fuel tube and they feed themselves downwards by gravity as the lower ends burn away and collapse into a very hot glowing coal bed. Above the flames and coals, the intense radiant heat impacts the incoming sticks and starts their pyrolysis. The smoke and gas from the pyrolysis is drawn downwards by the flue pipe draft and it ignites and burns as it passes through the glowing coals and a tongue of flame and hot gas moves out into the labyrinth of the stove body. (See the hot stove photos above and you can almost see the flame path.) The labyrinth slows the speed of the flame front, provides turbulence, for good mixing of fuel with oxygen for nearly complete combustion and provides enough surface for heat exchange of the hot gases to the stove body so that a suitable temperature of 300-500 C can be maintained on the cooktop, 255-300 C on the stove walls, and about 200-300 C on the bottom surface above the wood drying rack. The labyrinth path also acts as an effective spark arrestor so that hot embers do not escape and rain down on the lightweight tent fabric or in the bush environment.
Burner self-regulation. Because this ultralight tent stove uses an inverted burner it normally burns only the bottom ends of the fuel sticks. This means that only a small portion of the fuel load can burn at any time and it has self-regulating properties. The fuel sticks feed down by gravity as they convert to charcoal and collapse. This maintains a steady state with dry and hot fuel above the pyrolysis point, roasted fuel at the pyrolysis point and an important bed of coals below.
The air flow in through the stove is greater as the gases in the stove heat up and there is more ‘draft’ in the flue pipe. With an appropriate air flow restriction in the primary air port, the stronger draft tends to ‘pull’ secondary air in faster. Consequently, the flame and the pyrolysis zone moves down lower on the fuel sticks and decreases the drying, roasting, pyrolysis, combustion and burner temperature. “…More draft more pulling and less draft less pulling of the flame…. yes you’ve got it…” This means that the stove naturally achieves a steady burn rate in the burner assembly. The surging of the strength of the draft can be heard as a pulsing or chuffing sound coming from the burner primary air port.
Balancing air port settings. Over the top of this self-regulation, the operator can and should adjust the balance of air entering the primary, secondary and tertiary air inlets to control; stove temperature, air/smoke ratio and also the location of the flame and pyrolysis zone in the burner assembly and the amount of coals in the charcoal bed. It is a bit tricky to describe with words, but with experience, the flame can be moved up into the zone of the fused quartz window to optimize light output. The charcoal load can be deliberately reduced or increased by juggling the three air ports to allow more or less air into the primary air port. Also, it is worth noting that filling the fuel tube aperture with fuel sticks will act to strongly restrict air entry through the secondary air port. “…does this all sound amazing…maybe there is such a thing as intelligent design after all…” Under normal operating conditions there is not much smoke to see. For a bit of fun, the air port controls can also be used to send up a good show of smoke on demand for the fellow campers who just want a good photo of a tent with a stove, chimney and a smoke plume. However, I don’t do it for too long or I will have a dirty flue pipe to roll up and a smelly one to put in my backpack when I move on. The primary air port valve assembly may at first appear to be unnecessarily long. However, it is designed this way to arrest sparks from exploding embers. CAUTION Consequently, it should always be left in place when a strong burn is happening in the burner assembly.
Special care with dry wood. The stove user depends on the above described natural balance of the inverted burner and occasionally the user’s intervention to regulate the burn rate and prevent smoke from coming out the fuel tube into the tent. “…I call this the second unwanted chimney….”. WARNING: Very dry and decayed wood can be great fun because it burns at a temperature as low as 150 C (Forensic and Scientific Services). This means that the ‘pyrolysis front’ can just keep climbing up the burner glass, up the fuel tube and up out of the top of the fuel tube, if there is any wood sticking out there to burn. However, this is not fun. Luckily or unluckily, depending upon the way you look at it, such dry wood is seldom available when there is good skiing, snow and bad weather about and the stove is most needed. If dry wood is available I use it with care by all means, but I try not to load up the fuel tube too much and I keep the snuffer can on, if I do. A small amount of dry wood can be put to good use for starting the burner and can be fun to use at night, if used sparingly, mixed with other wood, to help pull the flame high into the burner glass to generate more light. Other accelerants can be used this way (but that is a story for another day).
WARNING: CARBON MONOXIDE RISK. Similar, but more stealthy, anomalous reverse burning can occur when fuel sticks have dwindled to charcoal (the fate of all fuel sticks) and there is no more follow up fuel for production of; pyrolysis gas, flame and draft production. “… yes, it is hard to accept that a stove full of glowing coals will have very little draft up the intended flue pipe and instead it uses the fuel tube as an alternative chimney that is directly above the static heat source. “….I can hear some of you muttering…yerrr that’s alright in practice,… but, but, but..what about the theory…”. However, remember that this stove uses an ‘up-side-down burner’. To compound this already unfortunate situation, charcoal burning can make a lot of carbon monoxide and if that is not enough, it is odourless, invisible and very toxic to humans.
To eliminate reverse charcoal burning risk I open the primary air port, take off the fuel tube and burner glass and it’s mounting and I cover the burner opening with a thin foil cover (micro ash shovel). This importantly leaves the flue pipe as the only path for the safe exit of the exhaust gases from the slow burn of the last charcoal. I also increase the ventilation of the tent until the stove is extinguished.
CARDINAL RULES: I never, go to sleep before ALL the charcoal is used up or I may wake up dead! Also, I never leave the stove unattended particularly with dry wood in the fuel tube without covering it with the snuffer can. I no longer leave any sticks poking out of the fuel tube while unattended without the snuffer can covering them, as they can catch fire, unbalance and topple out! “…. a scorched cooking device is a reminder to me…and I did not think the sticks were particularly dry!”
Problems with incomplete burning. Incomplete combustion is inefficient, dirty and makes stove pack up unpleasant. My early stoves were always so. They created smelly tar and thick soot in the flue pipe that would not come off until I got home and used a metal scraper to clean it. My ultralight flue pipe brushes never worked under field conditions. Removal of these deposits is a nasty tedious and unpleasant task and can easily result in microscopic cuts that will only be revealed when I wash my hands. It is a task that is best done at home and even better if it can be avoided. “…I have no suitable flame technique for doing this…not yet!…”
Problems with lean burning. While persistent lean burning would be a logical way to prevent soot and tar problems, is also inefficient and in my opinion undesirable. Stove efficiency is reduced with lean burning as the excess air is diluting the hot gas in the stove and wasting it up the flue pipe. This theory can easily be tested by running the stove cooktop at 400 C with open air port controls. If the air input is restricted to the ‘hint of smoke point’ the stove surface will quickly climb to 450 C or even 500 C.
Carbon as an antioxidant? Concerning the excess oxygen present during lean burning, I am fairly certain that this accelerates the oxidation of stove metal. Unprotected titanium and stainless steel shows signs of oxidation in my stoves that are designed to run very hot and, cleanly. I now coat the hottest metal surfaces with sodium silicate refractory render (looking for a better word than paint) to provide protection from rapid oxidation. I still like to restrict the air supply to get a ‘hint of smoke’ from the flue pipe as an additional precaution. The previously mentioned thin foil batch loaded conventional stove designs appear not to have an oxidation problem, even without protective coating. I speculate that this could be because the batch loading combustion process ensures that for most of the time there is no excess of oxygen and the reducing conditions provide protection of the titanium or stainless steel foil from rapid oxidation.
Just in case you are thinking “…. how can carbon protect a hot metal surface in a stove …..” Consider the fused quartz burner glass in my stove photos above or below. You will always see carbon deposits on the inside. This is a very hot part of the stove, particularly when it is filled with glowing wood and burning gas yet even under these conditions the carbon stubbornly sticks to the smooth glass and is very slow to burn away. “…. when you think of the different properties that the element carbon can exhibit…..it can be dirty lamp black, super lubricant in graphite and even diamonds, so this behaviour is not a surprise to me…”. The bottom line is that a hint of smoke from the flue pipe is probably a good bet for maximizing stove longevity, but you need to get the balance right. I am even thinking of putting a little observation peep-hole up in my tent so I will no longer need to stick my head out of the slit door of my tent to check for a hint of smoke. It is also important not to confuse condensing water with smoke that is slightly ‘blueish’.
Heating power of ultralight tent stove. I estimate the heating power of the micro stove to be about 890 w. “….yes similar to a 1000 w radiator, for as long as you or your companions are prepared to keep poking little sticks down its burner tube….”. This estimate is based on the following assumptions and calculations:
Net heat of combustion of logwood with 20% moisture is 4.1 kwh/kg
Ultralight tent stove Mk15 burns 0.325 kg of moist wood per hour (measured by author)
Assume 95% complete combustion (only a hint of smoke from flue pipe with no tar, soot or sparks)
Assume 70% of heat of combustion is transferred into the tent.
Stove power (w) =( heat of combustion/kg of wood*wood burnt/h)*(% combustion/100)*(% heat transfer/100)
= 4.1*0.325*0.95*0.7 =0.887 kw or 887 w
Summing up. So, how does the stove’s performance measure up to the dream? Overall it is pretty good. It provides nice air heating in a small tent. More importantly, if placed in the middle of a 3,000 mm diameter tent it provides very nice comforting radiant heat that can be felt throughout the tent. It can boil 500g of water in about 15 min, a little slow, but there is a lot of time for cooking on long winter nights. Not being able to load it up for hours of burning while you sleep just sucks, but real-world test of other ‘little ‘ stoves don’t do much better than 0.5 hours. But that is a new challenge for us to work on. Ash build up in the stove body below the burner slowly reduces the stove performance. It is a pest but it needs to be cleared after multiple hours of burning.
The light from the burner glass is; variable, generally not strong when you need it most and obscured by carbon buildup, but is still very welcome. It creates a lovely gentle light on the ceiling of the tent, but not much on the floor, but still enough to find your torch! I supplement the stove light with a light from my modified alpine tea light candle with a high-tech ceramic wick (Alpine camping candle), a small LED light. The burner glass needs special care in handling but is tougher than it looks and the stove body provides good protection for it while backpacking. The burner glass certainly helps me manage the burn and the fuel feed and it gets a big tick for the wow factor.
The fact that the stove performs well without the need for a flue damper or spark arrestor is a testament to an improved design. The ability to mount the stove well above a deep snow surface (on a simple bush pole as an option) and accommodating a large wood drying rack below is a great convenience. Regarding safety/risk I consider that there is more risk of using an alcohol or gas stove in a tent vestibule than using a properly installed wood stove inside a tent. With this wood stove, there is no naked flame to catch the tent alight and combustion products are exhausted thoroughly if managed properly and there is no flammable fuel to explode or spill and fireball your tent. “…the biggest surprise can be a gentle ‘pop’ noise and occasionally this is accompanied by a most perfect smoke ring that goes sailing across the tent…have not caught one on camera yet”
The fuel efficiency as low as 324g/h is remarkable. “In disbelief, I have repeated this test several times.”
Future improvements. A more powerful stove that weighs not much more and with a lot more cooking/snow melting surface would be ideal (and possibly another leg or two!), especially if the extra weight, effort and pleasure can be shared by a group of people in one tent or in a larger ultralight community tent with a massive snow pit to sit around in comfort and drink and eat and talk about what you have experienced. Let’s call it a snow lodge tent. How about a thermoelectric generator (TEG) (~80g) that can power LED lighting, charge camera batteries etc and run a good air circulation fan? There is an ideal temperature differential between ice and stove (0-300 C) to drive a powerful high-temperature TEGs. “..Dream on….”
What’s next? Part 3 of this series will be about getting safe, clean, practical, efficient heat from the stove. You will just have to wait for the report on the bigger stove. “…. but it’s looking good…”
Tim
Addendum
This post dates back some time now and could be considered redundant. However, it contains some fresh ideas and lessons that may be helpful to others. In my opinion, it has been largely supplanted by my very light, compact, and hot KISS tent stove.
