Part 6.- Narrow flue pipe seam width for backpacking tent stoves.

This post reports on the advantages of roll up flue pipe seam width optimization for compact ultralight backpacking.

Note: This is just one little part of a 12 part series on improved stove pipe rolling. You may wish to start at Part 1 as hors d’oeuvres and then taste from the banquet.

Introduction

The flue pipe seam width needs to have a significant seam overlap to form a mechanically stable flue pipe with an adequate seal along the long overlapping seam when deployed.

My flue pipes, for small tent stoves, have much smaller diameters (30, 40 and 50mm) than most used by others. As is my usual won’t, I made my rollup flue pipes from titanium and stainless steel foils, without reference to the designs used by others. Consequently, I used only a 15mm wide seam overlap. “Whether by good luck or divine providence my design seems to work well.”

For details of my past traditional, but radical, flue pipes please see: roll up flue pipes for tent stoves and hot tenting

I subsequently found that others (Titaniumgoat (Where has it gone?), Seekoutside and Liteoutdoors) use 47-65mm overlaps (calculated by me from their published sheet widths and stated pipe diameters.

The large flue pipe seam width may be more stable and seal better. However, this extra seam width has disadvantages:

• It adds weight,
• Increases the critical roll width for backpacking and
• Makes a ‘less efficient cut’ of multiple flue pipes from a roll of foil of a given width.

Consequently, having a small (~15mm) flue pipe seam width could help to optimize the design for packing the pipe within tiny tent stoves while winter backpacking.

Flue pipe seam width- Optimised for compact backpacking

Experiment 1. Evaluation of a 40mm flue pipe with a slender 9mm seam overlap

Background and rationale. Initially, I contemplated the experimental comparison of flue pipes with 15 and 50 mm overlaps. Because the smaller overlap works so well, I thought that the comparison would be somewhat pointless. Consequently, I decided instead to compare a very radical slender overlap with my regular 15mm one. “According to a repeating theme in my post of finding the breaking-point.”

“I did not expect an improvement, but would it fail? As you will see, if you continue, being a ‘tightarse’, I also had another very noble experimental purpose for the pipe if it should fail.”

Tim

A radical roll up flue pipe. The experimental pipe was 40mm dia*2,200mm long with only a 9mm seam overlap. The overlap was made so small so that, when rolled up, it could just fit inside the tiny fire dome of the KISS Stove and related stoves.

The pipe requires a little more care during first forming and may need more retaining rings to hold a good shape. For this purpose, I have made more compact and stiffer retaining rings so that more of them can be ‘stacked’ on the rolled-up pipe when backpacking.

The 9mm flue pipe seam width did not look as though it would seal quite as well as pipes with wider overlaps. Nevertheless, it did not fail. It indeed works very well.

This complements the KISS Stove’s stellar heating performance with its record-breaking lightweight. Now, the boasting includes a compact flue pipe (stove pipe) that can fit within the stove and pot while ultralight backpacking.

Note: I did a night time heat treatment of the flue pipe with my alcohol flame lance and to my dismay, I could see small strips of light coming from the seam between the holding rings. This meant that there must be a small amount of air leaking into the pipe with only a 9mm overlap.

Experiment 2. Evaluation of a 37mm flue pipe with a 15mm overlap

Background and rationale. Given the light leakage observed above, I thought a wider overlap (15mm) would be prudent to reduce inward air leakage and improved flue pipe draft.

I had already experimentally tested a 30mm dia pipe on the KISS Stove and it worked surprisingly well. However, the stove performance was a little below the ‘stellar standard’ that I have set with the KISS Stove with the 40mm pipe.

I also knew that a flue pipe foil width of 128mm (Allowing for small cutting errors) would fit comfortably across the fire dome of the KISS Stove when rolled up.

Lastly, I had already developed a flue pipe guard tube that connects between the stove and the flue pipe. This connector could be easily tapered to make it fit any smaller flue pipe diameter if required.

Reverse engineering the flue pipe diameter. I determined that a foil roll that was 132mm wide could just fit inside the fire dome of my dome stoves. Then I made the flue pipe 37mm in diameter so that it had a substantial 16mm seam overlap. The calculation was:

Pipe diameter=(width-overlap)*1/pie=(132-16)*7/22=37

This flue pipe feels really stable and appeared to have a very tight seam overlap. “It feels just so stiff and smooth so it is the best that I have made so far.”

Importantly, the flue pipe could drive a very hot stove, as shown below, with this slightly reduced diameter. For more details of the performance of the stove with this pipe please see (KISS Stove).

Flue pipe seam width- Optimised for convenience when ultra-compact backpacking is not required

The above analysis has focused on optimising the seam overlap size of compact backpacking efficiency. Such optimization comes with the requirement of needing more holding rings to stop excessive seam bulging.

Consequently, this next section looks at the advantages of wide seam overlaps where ultra-compact packing is not required or valued. “We are crossing over to the ‘Darkside’.”

Seriously, there even could be a backpacking situation where the flue pipe could be backpacked separately to the stove (eg in a buddies backpack or in a pulk) and a little more length to the roll (~35mm) would be of no consequence if it could make the flue pipe much simpler and possibly cheaper if it requires fewer rings.

Experiment 3. Evaluation of a 62mm flue pipe with a 53mm overlap

For this experiment, I was making a long roll up flue pipe extension for a Gstove. I started with a 3,000mm long *248mm wide strip of 0.1mm thick stainless steel foil. After forming the flue pipe by a tight helical screwing method it formed a 62mm diameter pipe with a seam overlap of approximately 53mm. This was very similar to the overlaps in the popular roll up stove pipes mentioned above. It is very generous by my ‘skimpy’ standards.

The flue pipe, as anticipated, was easy to shape for the first time. Much easier than for my thin pipes. I used a dusting of talcum powder as a refractory lubricant to help the foil layers slide easily over each other. The generous overlap made a good seam seal but made a ‘flatish’ zone in the round flue pipe shape where the two overlapping seam layers resisted bending.

Subsequently, the pipe was easy to deploy and pack up by the fast and crinkle-free methods that I describe in improved flue pipe forming.

“I was pleased that my first foray into such long and large diameter pipes was successful. Also, that my hunch that they would be somewhat easier to form and pack up by my spiral screwing method was ‘born out. I am not sure if the wide seam overlap made the screwing method easier, but it may have. A comparative test of an equivalent pipe with a narrow overlap would be required for a definitive answer.”

The mystery of the bending pipe. While burning in the above blemish-free pipe with a charcoal-fired lance I thought I saw it bend during the heating process. “I could also hear some strange creaking noises.” I had not noticed this bending before with my small pipes with skimpy overlaps.

I examined the pipe further and could see some tiny creases that were adjacent to the seam overlap line. The pipe was perfectly crease-free before burning in.

Origin of bending and creasing. I think the creases formed during the bending action while heat treating the pipe. The pipe that was in contact with the charcoal lance and for some distance above got very hot. I speculate that while the heated round section got very hot and elongated. On the other hand, the metal of the wide seam overlap did not get very hot and did not elongate nearly as much.

The high thermal expansion of stainless steel coupled with the large temperature differential would account for the odd bending of the pipe, the noises and also the tiny creases that were probably thermal stress creases that were left in the pipe surface.

The creases were very small compared to those that are made by the conventional flue pipe rolling method. However, to me, it indicates, that a somewhat narrower overlap may be better for an optimal design.

Conclusion

Narrow seams on thin pipes. A narrower seam overlap of 15mm appears to be quite adequate for stability and sealing of 37mm pipes. The flue pipe will be 20% lighter and when rolled up will be shorter by about 35mm than other commercially available pipes. It will also make it more convenient for backpacking. The narrower overlap also allows more efficient/flexible use of the foil rolls from which they are cut.

The narrower seam overlap, when coupled with a slightly smaller flue pipe diameter, will allow it to be conveniently packed within the KISS Stove fire dome.

Wide seam. The wide seem obviously has a valid place, because it is so popular. It will have advantages over the equivalent narrow seam in that it requires less holding rings to form a good initial flue pipe shape without excessive seam bulges between rings. It will have extra length when rolled up and may not fit inside small stove cavities.

Narrow seams on thick pipes. A test of a large 62mm flue pipe with a 15mm seam would make an interesting test and provide the definitive answer for such pipes with regard to preventing thermal stress creasing (I will add the results of such testing when available).

The magical flue pipe rings. Some readers will be ahead of me by now. However, a simple change of ring size can allow ‘the one flue pipe ‘foil strip’ to be any diameter within a reasonable range. For example, the above 37mm pipe could be 30mm or 40mm just by changing the tiny rings.

An ode to the poor, neglected, forgotten and unseen flue pipe seam:

Should a flue pipe seam be wide or thin?
The definitive answer may lay hidden within?
If both worked well with no leak, nor smell,
For backpacking, one is cheaper, lighter, shorter, (not like my words), to fit in.

Here is another contradictory ode to the flue pipe for a situation where packing space is not ‘tight’. It was written before I discovered the micro creases with the big overlap:

Should a flue pipe seam be wide or thin?
The definitive answer may not be within?
If both worked well with no leak, nor smell,
If not for creases, wide would probably win.

Tim