A sacrificial guard tube for a KISS tent stove
This post describes a guard tube for the stove pipe on a tent stove. It sacrificially protects the roll up foil stove pipe from damage.
I have had an enquiry about the KISS stoves ‘flat-out’ burning action being a little bit too hot for its own good. The question came from Nick who has asked so many good questions about my stoves. So thanks again to Nick.
The area of concern was the lower end of the rollup flue pipe where it connects to the stove. “Over the years I have dealt with this issue by simply snipping off the ~50mm of damage and accepting that the flue pipe will be just a bit shorter. I have one pipe that has so many hours of powerful burning that it has been ‘trimmed’ twice. “It is still-going-strong.”
Having an extra-long pipe to allow for trimming over the years is a solution. “It could still be sold to ‘the little people’ years later when it got too short for you.” However, this may not be an acceptable solution for others and is certainly not ‘PC’.
The Kiss Stove is my hottest stove design amongst many other hot tent stoves. “The Stella temperature performance when seen will be believed.” This will increase the metal damage in the bottom of the flue system.
I have commented sporadically on this and related issues in many posts. However, was time to consolidate this in a two-part post.
Part 1 described experiments where stove operation was modified to reduce stove temperature to protect the flue pipe (Experiments to cool the hot KISS Stove).
The Part 2 post is about enjoying the flat-out hot stove by having a single cheap sacrificial part that can ‘cop’ the damage and spare the flue pipe (A sacrificial guard tube). Of course, it could be used in conjunction with the moderated stove operation.
This second solution is particularly good for those (Like me and my tent companions.) without enough self-control to resist the lovely warmth that comes from running the stove flat-out in a tent in a freezing outside world. “Did I hear ‘stuff the flue pipe’, we are nice and warm.”
More stove parts with a similar problem. As well as the base of the flue pipe, it’s connector cone and the compression ring within the bottom of the flue pipe suffer similar or greater damage. Consequently, a solution that dealt with all these would be great.
The cause of the damage?
Years ago, but not now, I was perplexed as to why such a tiny portion of the bottom of the flue pipe was so damaged, while the rest was OK. “It was like a fire breathing dragon had a go at it.” The following considerations may help to explain why this is so.
Temperature and insulation. The metal damage near the bottom of the flue pipe and associated fittings is superficially caused by heat being transferred to the metal by very hot flue gas and the heat being unable to be dissipated quickly enough. The flue pipe components are joined with overlapping joints. The outer layer insulates the inner layer of metal and further reduces heat rejection at these joints.
Turbulence. The hot flue gas is very turbulent as it changes direction and ‘flow-shape’ when it transitions from the exhaust channel of the fire dome and then into the flue pipe that is at right angles to the channel.
This turbulence greatly increases the transfer of heat from the gas to the flue system metal at the start of the flue system. Luckily, the gas settles rapidly into a laminar flow for the rest of the passage up the flue pipe. The exponentially rapid decrease in flue pipe surface temperature reflects the rapidly diminished rate of heat transfer from this laminar flow condition.
Surplus oxygen. The KISS Stove (and other related stoves with inverted J-burners) burn very hot and clean without flue gas smoke (Because it is a wood gas (smoke) generator and a very effective gas burner).
This means that at most times there must be a stoichiometric excess of oxygen for the combustion of wood gas and charcoal that is burnt at any instant. I think this surplus unburnt oxygen in the flue gas is a major contributor to the hot metal decay.
It gets worse, the turbulence that increases the transfer of heat from the hot gas, at the same location also increases the contact of the excess oxygen with the hot metal surface in the short turbulent zone.
‘The triple whammy’. These unfortunate metal components at the start of the flue cop ‘the triple whammy’ or is it the ‘quadruple or quintuple whammy’. High temperature plus turbulence plus oxygen plus double insulation.
“Surprise surprise, they are the fastest decaying component that I have ever experienced.”
The one-piece sacrificial guard tube
Once upon a time when this ‘cylindrical tube’ was very short (40mm), I called it a ‘ring’. It grew and grew and grew to a length of 200mm or more! So here is a little ode to Tolkienesque magic rings before I rename it;
Could one stretched hot ring spare them all?
Coping curry atop that red glowing fireball.
One cheap device that we will sacrifice,
Could it save the rest from their untimely fall?
I designed the sacrificial guard tube to be simple, cheap, light, backpack friendly and preferably have second use (or take the place of another existing stove component.
The design is such that a replacement guard tube, if needed, could be made with simple tools and from curbside scrap kitchen stainless steel. If desperate it could be of tin plate metal from a coffee can or similar.
Conclusion
The sacrificial flue pipe guard provides a way of limiting the flue pipe damage so that it can have a long useful life. The guard also eliminates two other small flue parts that were also subject to similar decay problems.
A guard made of 0.4mm thick stainless steel should have a very long life even if it will be four times heavier.
If the guard tube is long enough, it could also have the damage at the bottom cut off to give it a second-or-third-life.
Addendum
The guard tube has multiple benefits and uses. It can be used as a substitute device for safely/conveniently rolling and storing the flue pipe foil backpacking.
The simple and flexible method of construction allows the guard tube to be made conical to use with flue pipes with smaller diameters. This, in turn, means that the rolled up flue pipe can be compactly packed within the fire dome. These issues and more are discussed in other related posts.
Tim
