Steam generator- stick fired for backpacking uses

Background

A steam generator for backpacking uses formed from my regular blower stove and cooking pot with only 80g of extra components.

Note: This post is just about the development of an ultralight stick-fired steam generator for backpacking. Its use for heating a miniature steam bath sauna and other uses will be covered in separate posts. However, I do indulge a little speculation about its possible uses just to attempt to justify this crackpot idea.

Free heat source. For some time I have tinkered with many small stick-burning stoves that can cleanly transform small fallen sticks into intense heat for cooking and comfort while backpack camping. It makes an infinite supply of heat that is not constrained by pack weight considerations. If the heat can be easily managed it can provide considerable comfort when living in a cold winter world with only the light equipment that we carry in our backpacks.

A separate post will explore a dedicated stick-fired water heater/steam generator that may be more suitable for base camp or similar situations where a little extra baggage weight and volume would be an acceptable compromise for the luxury it could provide.

Other backpacking/camping hot water luxuries. On winter trips, heating a pot of water on these stoves can provide abundant warm or hot water for what I call a Soft bottle shower. More hot water and the same cheap soft bottle can be used as a giant Alpine hot water soft bottle for a little extra bedtime warmth (4-8h) when sleeping in a freezing tent. A bonus is that it becomes a source of liquid water for breakfast when all other water is frozen solid. It even can be used for its boring intended use of storing/carrying water.

A 2-litre hot shower is welcome in winter camp and is a big step up from a Spartan cold shower or a rub down with snow (even if it may be so good for health). The 2-3 minute duration and piddling flow rate don’t make it even one half-star luxury experience. So a lightweight sauna bath might lift the luxury level without adding much to backpack weight or becoming too indulgent.

Infinite USB backpacking energy supply. For many years I have sparingly used small USB power supplies to run my blower stoves for essential cooking (2-3h/each 18650 rechargeable battery). However, more recently I have discovered the joy of using larger but very light power banks that can run the blower fan for 27h or more. So indulgent use of such power now seems to be quite fitting. “It is amazing to think that the 1.5 watts of fan power can cause the tiny stove to output 3000 watts of heat. This is a 2000 times multiplier.”

Hot rock steam. In keeping with Scandinavian custom, I experimented with heating rocks on my small stick-burning blower stove to see if the stored heat could make enough steam to run a tiny sauna tent. “Yes, I know that suitable rocks are not a commonly available resource when snow camping, but let’s continue.” Despite the estimated heat output of about 3Kw, it was difficult to transfer enough heat into enough rocks so that they could make lots of sustained steam for a sauna when splashed with water. “After an hour or so of heating a single big rock, it could barely spit steam back at me and we both rapidly ‘ran-out-of-steam for the project”.

I think the rock got to an equilibrium temperature (not much over 100C) while heating. It was losing as much heat to the surroundings as it was gaining from the stove. Even providing the rock with a crafty doona made from two spaced layers of aluminium cooking foil did not improve things much.

As a retired chemist/physicist, I felt embarrassed about this thermodynamic failure. I watched a Youtube video demonstration of a commercial mobile sauna (used with an RV camper, too big for a backpack). They did a lot of sweet raising work; assembling the fold-up sauna tent, setting up seats, preparing a steel rock bucket, filling it with selected rocks, and heating it for ages with a big gas ring burner that was connected to an equally large and heavy gas bottle.

After all this,………. their rocks hardly made any steam (as little as mine did) and I think the pair enjoyed a warm humid air bath that was directly heated by the gas burner (and thankfully it was all well-ventilated). That made me feel a bit better about my hot rock failure.

The wonders of the specific heat and heat of vaporization of water. While tinkering with the above-mentioned hot water soft bottle, I investigated the heat storage capacity (specific heat) of rocks and water and other stuff. I was surprised that water was almost 6 times better than rock (see my Alpine hot water soft bottle post for more details). This triggered the idea of boiling a pot of water on my powerful blower stove and directly using the heat of steam to continuously heat a tiny sauna bath.

Knowing that a kilogram of water can store 6 times more heat than a kilogram of rocks given the same temperature difference may be a surprise to you as it was for me. “It is the Queen of heat storage.” Now, the heat energy that is used to convert the same amount of water into steam (or is released when it condenses back into liquid water in a sauna) is 2260 Kjouls/kg. This is about 7 times greater than the energy used to heat the same amount of water from 20C to 100C for the hot-water bottle. “This makes steam the king of heat storage, but it is not a nice bed fellow!”

Consequently, steam is a marvellous source of heat energy when it can be condensed safely for a useful purpose such as a steam sauna or water heating. It also has the virtue of being a continuous source of energy so long as the stove is fueled with little sticks and the water supply is maintained. “Sounds just perfect for extended bathing!”

1. Could the steam from that water be easily collected and delivered to a tiny sauna tent with a bathtub floor? “That is without blowing myself up.”
2. Could the same device make large volumes of warm water for a generous bush bath formed in the sauna tent structure?
3. Could it be used to desalinate seawater for remote coastal camping?
4. Maybe making frothy coffee is a step too far?

Sauna power requirements. According to Saunaking a general rule of thumb is that a sauna will need about 1kw of power to heat 50 cubic ft of sauna volume. I estimate that this equates to ~1kw/1.4 cubic meter. My proposed minimal backpacking bath/sauna tent would have a volume of only ~0.2 cubic meters (0.6*0.4*0.8M), so it would theoretically only require 0.14kw to run. Consequently, the estimated 3kw power of my blower stove should be up to the task.

First things first- first-generation stick-fired steam generator

Keeping low weight and size in mind, I started by making an experimental backpacking steam generator essentially using my favourite stick-fired blower stove and favourite customized dinner pot that travels with me on most winter adventures. It has a special rebated bottom rim that neatly fits in the top of the blower stove fire bowl and secures the pot and prevents flame leakage. “Like the bucket and the bum that were made for each other.”

The only additions to become a steam generator were very light. These were a silnylon pot cover with an attachment for a silicone rubber steam tube and an aluminium flame shield or awning that could keep the sometimes boisterous exhaust flame away from the delicate silnylon. “Ohh…… if such first-generation dream designs could be so simple in reality!”

Three possible steam generator failures or are there more? Before my first test of the steam generator, my main concern was that it would not produce enough steam to be of any use. Would it be a steampunk success or a bit of a failed fizzer?

At the other end of the spectrum of self-doubt, what would happen if it produced a lot of steam and pressure? Would the tight silicone rubber band hold in place, particularly when it became wet and very slippery against the polished stainless steel wall of the pot?

Lastly, would the protective awning made from pie dish aluminium foil survive in the exhaust flame or melt or oxidize into grey ash?

Steam generator testing. I started with ~900ml load of cold water. As expected from this little stove, after about ~10 minutes the characteristic early sounds of water changing into incipient steam bubbles could be heard. The sound was coming from steam bubbles rising from the bottom of the pot and then collapsing as they rose through the slightly less hot water column. “This sound, while camping, is a signal to get ready for boiling water and a well-earned hot drink.”

Soon a jet of steam shot out of the silicone rubber pipe. My concern about it being a fizzer was gone. However, at the same time out of the corner of my eye, I could also see the pot cover steadily gliding upwards. Then off it popped in a cloud of steam. I think this was a glass-half-full situation and my new challenge was better retention of the pot cover. “At least the first test showed that the design had a natural inbuilt safety pressure relief.” And the aluminium awning deflected the exhaust flames and survived completely unscathed.

Glueing the steam cover. Glueing in place would have been a very simple solution, but the pressure build-up may then express itself as another problem somewhere else with a steam explosion potential. Above all else, it would make the pot incompatible with its main purpose of making snow melting, hot drinks and cooking my dinner.

“Such is the principle of multiuse backpacking items. That ‘extra use’ can’t be at the expense of the other primary functionalities of the item. Such tight rules may not apply to camping in mountain huts, base camps, sledding/pulk and vehicle camps. So glueing is not ruled out.”

Increased friction. During my tinkering with rubber and particularly RTV silicone rubber, I have found that after making and curing my DIY gismos, they have this strange property of sticking to themselves (or other silicone rubberized surfaces) if left in contact for some time, with gentle compressive forces applied. This sticking appears to be enhanced by elevated temperatures. “I think the process is one of delayed vulcanization where the RTV silicone rubber polymerization from liquid to solid is never quite complete and there are always short polymers available to make new and weak polymerization bonds to other short ones.”

Consequently, I thought that coating the rim of the cooking pot with thinned silicone rubber may greatly increase the friction between the pot wall and the steam cover. This trick will be kept up my sleeve as a last resort because it may stick too well and defeat the removability, so reducing the pressure would make a good starting point to solve the problem.

Reducing the steam pressure. The pressure build-up was likely due to pipe friction within the long silicone rubber steam tube with a small diameter (~4-5mm). Consequently, I thought that a much wider steam tube might be part of a simple solution and the redundant outlet might become a pressure relief valve.

An enlarged steam tube using steampunk technology

Finding a long intact tiger snake skin that was shed in my orchard gave me the inspiration to make a glued tube from silnylon. I had recent success with making hydraulic head pressure testing tubes with glued silnylon. They worked well, in that the glued seams could easily withstand a 600mm head of water pressure. However, the base fabric still leaked a little through many micropores in the silnylon.

I thought that a similar, but longer tube might make an excellent ultralight and compact lay-flat steam tube, particularly if the silnylon fabric was first made more water/steam proof by impregnating with extra silicone rubber. ” It could be just like a rollup firefighting hose and occupy negligible space in a backpack and could deliver steam or hot water at a safe distance from the heating device.”

The first silnylon steam tube. I made a 1,400mm long steam tube with a circumference of ~70mm (22mm ID) that had a sectional area that was about 25 times bigger than the hose that caused the above over-pressurisation failure. Before forming the tube, I doped the entire length of the strip of fabric with DIY silicone rubber seam sealer to hopefully block the tiny pores that leaked water in the previous HH testing and would most likely also leak steam. Unfortunately, this doping still left tiny leakage holes that were revealed during HH testing and could be observed with strong back lighting.

The second silnylon steam tube. Next, I made a similar tube. This time I again doped the tube and then added a thin screeded layer of undiluted RTV silicone rubber to both sides before forming the tube. Before fitting the steam tube to the pot cover, I did a hydraulic pressure test with the tube hanging and filled with water. The sealing success is demonstrated in this little video of the tube filled with water alongside the snakeskin that inspired the idea.

Testing the enlarged steam tube

With the large silnylon steam tube attached, I put 800g of water in the pot and set it on the blowers stove and waited nervously for about 8 min for the steam to shoot out of the small (blue vent) in the middle of the silnylon pot cover. It was time to blank off the vent and see if the device would deliver steam through the enlarged steam tube. The boiler held together this time and a strong plume of steam came out of the steam tube. “This at last was looking like a success!”

In order to set the steam tube exit nozzle in the frame for the photo and video, I needed to curl it around through about 270 degrees. The resulting gentle kinking of the soft tube did not noticeably disrupt the steam flow. “It looked like what intestinal gas would do during its passage through our bodies. It even made a corresponding noise of such exiting gas that could be heard over the gentle roar of the blower stove.” Also, water that condensed within the tube seemed to clear out quite easily without blocking the steam flow.

Results and discussion of the steam generator

This little video may better describe the steady plume of steam as it condensed in the cool evening breeze.

Steam for a sauna. Over a ~15 min run time, the steam generator boiler lost ~330g of water. The steam output felt as though it would be more than enough to sustain the warmth and humidity in a tiny backpacking sauna tent, but that will need to be tested.

The energy to boil water is equivalent to 2660kj/kg or 0.739kwh/kg. So I estimate the energy in the 330 g of steam to be 0.244kwh and this equates to a power of 0.98kw. This greatly exceeds the 0.14kw required to run a tiny one-person sauna tent (as estimated above). Some of this surplus power may be needed to compensate for the greater loss of heat from such a small sauna tent with a high surface area to volume ratio. Also, operation in a cold alpine environment as intended may require more heat.

The steaming rate was 22g/min, so an 800g fill of the boiler would provide 36 min steam bath, but it would need to be somewhat less (~25min) to prevent the boiler from running dry. Even this lesser time would make a welcome warm bath in the bush.

The testing showed that the heat transfer from the ~3kw blower stove to the water was only 33% (100*0.98/3.00). This is not unexpected when there was such a big wasteful flame plume burning beyond the exhaust port (as shown in the video, this is normal with dryish fuel sticks).

Damper sticks usually would be used under most winter field conditions and the combustion will be more concentrated below the boiler pot with little or no exhaust flame. Consequently, the overall combustion rate will be considerably lower and the efficiency much higher.

If the amount of steam from the steam tube was excessive for the sauna, then the vent hole in the centre of the pot cover could be used to vent some of the steam to reduce the flow through the steam tube to the sauna.

Steam for desalination. There was a small amount of splashing of boiling water into the steam tube when the boiling first started. This would be unacceptable if the device was being used for desalination. Consequently, A higher steam outlet height would be required as a splash head to prevent the distillate from being contaminated with salt.

The water distillation rate of 1.3L/hour is a good start, but a higher rate would be desirable to make a practical desalinator for seaside camping. A larger heat exchanger surface that was thinner and/or made of more conductive metal may help in this regard.

More testing to come.

Tim

Addendum 1

While the application of the generated steam to various camping purposes will be the subject of more posts, I thought that I would tease you with this little video of my first attempts to heat water with the steady and almost invisible jet of steam. It is a powerful heat source, no doubt, but it is about as difficult as that Tiger snake to manage. However, I did find out that the steam pressure could easily clear water from the submerged nozzle on the steam pipe. So that is another hurdle crossed.