Effect of direct pot heating on stick stove performance

For a miniature stove pipe drafted wood stove, this experiment showed that the indirect heating of a pot made the stove burn hotter and more cleanly. Consequently the cooking was quicker than with direct heating of the same pot.

Introduction

A discontinued design. I designed my early dome tent stoves with both fuel and exhaust ports in the dome wall. This was essentially done, with the mistaken belief, that I needed to leave the whole circle of the fire dome available for effective cooking.

This side ported stove worked well enough, but it has been supplanted by a much better stove that uses the same fire dome but has vastly improved combustion and stove temperatures (KISS stove).

Here is a link to a little Instagram video of the stove.

Using the fire dome as a blower stove. I demonstrated that the fire dome from the above side ported tent stove, can also be inverted to become the fire bowl for a powerful USB blower stove for rapid outdoor cooking.

Brilliantly fast cooking is possible where the bottom of the cooking pot is directly heated by the flames and radiant heat from the glowing charcoal that is whipped into a glowing frenzy that is directly below the pot. “The stove works its best with the pot of water in place.”

Alcohol as fuel. The same fire bowl configuration can also be used to house a tiny alcohol burner as a backup fuel option. The combustion of the alcohol is largely completed in the flame that spreads out beneath the pot. “The direct heating of the stove bottom with the pathetic alcohol flame is essential if your hot coffee hit is to arrive in a reasonable time using only ~7g of ethanol.”

Using the tent stove outside (without the tent). If the alcohol or USB power run out, it would be simple to run the tent stove outside for cooking. The next question was whether the cooking be better with the pot on the normal stove top? Or would it be better with the flames directly under the pot?

Combustion issues with direct heating options. Direct heating with traditional cast iron wood stoves has for a long time been a feature of sophisticated wood-fired range stoves. I have the privilege of staying in the mountains, for a special week each winter in a large remote ski-in chalet. There, we have a massive old cast iron range stove. It can cook for 30+ people. It has one firebox and two large ovens and an enormous cooktop to satisfy the enormous skiers appetites. The stove has an equally enormous appetite for small split pieces of firewood that we must also prepare.

It has all sorts of gizmos to change the heat flows to various parts of the range. “It is a fulltime task just to keep the fire stoked with suitable small pieces of wood to maintain adequate cooking temperatures.” In addition, to enable fast cooking, there are two sets of removable circular/concentric plates/rings that can be removed and covered with large or medium cooking pots or kettles.

[Add a photo of the chalet stove top showing some cooking rings removed]

This wonderful feature allows direct contact of the pots with the hottest combustion gases and some flame. However, the combustion is largely completed in the firebox so the heat absorption by the cooking posts can not adversely affect the stoves wood combustion.

Balancing heat for combustion with heat for cooking in tiny backpacking wood stoves. By contrast, my tiny backpacking stick burning tent stoves are quasi gasifier. This mean’s that there is a negligible separation between; wood gas pyrolysis, its combustion and the heat exchanger/stovetop.

The stovetop is relatively poor at heat exchanging to the pot so the cooking is, let’s say, leisurely. It would be tempting to put the pot directly on the burn chamber to speed things up. The test below shows that direct heating actually spoilt the hot and smoke free combustion and this actually made the cooking slower.

Experiments with flue pipe drafted outdoor stick stoves

Experiment 1- A dome stove with a directly heated pot

For this experiment, I took a redundant fire dome with a 50, 40, and 20mm diameter fuel, exhaust and USB fire blower ports respectively in the fire dome sidewall.

I inverted the dome to make it a fire bowl and fitted a flame guide within the bowl to make the flames and hot gas cross the fire bowl before they could flow back to the flue pipe port.

I fitted a 1850*40mm dia flue pipe via an elbow to the 40mm dia port. The 50mm port was fitted with a short cone to make it a fuel stick feed in and air supply port.

The custom cooking pot was used to cover and seal the top of the fire bowl to form the closed burn chamber. The bottom of the pot was resting on the top of the flame guide. Water was added to the pot before the fire was started. It meant that heat from the flames and radiating heat from the charcoal would directly impact on the bottom of the cooking pot. ‘Sounds great……?”

[Add a photo of the flame guide inside the fire bowl with the pot removed]

Results and discussion. Examination of the water in the bottom of the pot showed that tiny water vapour bubbles rapidly formed over the bottom of the pot as soon as the fire was started. To me, this indicated that there was excellent heat exchanging to the water.

The stove did not burn well and often discharged a considerable amount of thick smoke from the flue pipe. This smoke is normally absent in all my high-performance tent stoves.

There were brief periods with little smoke as the wood component of the fuel ran low and combustion was dominated by charcoal in the fuel mix.

This indicated to me that the heat extracted by the water in the pot was depriving the wood gas of a satisfactory combustion temperature.

However, heavy smoking started once more when fresher portions of the fuel sticks were pushed in or added. By contrast, this phenomenon is absent in the operation of the KISS STOVE.

Smoke from any combustion represents inefficiency in the combustion process. If is not induced by lack of oxygen it usually is as a result of an inadequate combustion temperature.

When the burn test was finished, the bottom of the pot was uniformly coated with soft and shiny black creosote. It was a very unpleasant contamination that stuck to any surface that it contacted. “So nasty, that I forgot to photograph it in my haste to get rid of it.” The remnants of some burnt creosote can be seen a photo in Expt 2. where it stuck to the hot plate that I added to the fire bowl.

The pot cleaning was relatively easy, but similar deposits in the flue pipe will be much harder to clean out under field conditions.

Such deposits can easily double the pack weight of the pipe and I expect that the stickiness would make rolling up of the flue pipe by my new quick flue pipe screwing method impossible. It would also be a grubby process when using the conventional roll up method.

I speculated that the cooking pot was taking too much heat from the combustion chamber and was causing it to have an inadequate temperature for good combustion of the wood gas (or smoke). The next experiment aimed to improve this.

Experiment 2- A stove with an indirectly heated pot

The next experimental stove configuration was the same as the previous one but a thin stainless steel cover plate (0.1mm thick) was attached to the fire dome rim with a linked series of little flat clips. “That is a pretty cute holding system, even if it is me saying so!”

The intention was to somewhat insulate the combustion zone from the cooking pot so that the fire bowl would be much hotter and create better combustion conditions. This would, on the downside, restrict the proportion of the combustion heat that was transferred into the water.

Results and discussion. The indirect heating of the pot greatly improved the combustion. There were sustained periods of combustion with only very slight smoking. However, when significant new fuel was added, it took a substantial time for the smoky phase to clear.

Here is a little Instagram video of the stove in action and boiling water.

General discussion/conclusion

Even with these significant combustion improvements in the second experiment, the stove performance was poor when compared with my best stove/s.

This benchmark stove/s is/are the KISS tent stove which can also convert to a blower stove. They share the same fire dome/fire bowl and custom cooking pot that it/they backpack in.

Lastly, (silly me!) it dawned on me that my KISS tent stove could easily become an ‘unpowered-outside-cooking-stove’ by simply removing the tent if my alcohol and batteries ran out!

This is my Segway to my obligatory ode:

On another outside stove, my mind was bent,
One using natural draft, if my alcohol and batteries were spent,
Should flames heat the pot directly or possibly not? 
Silly, keep KISS stove cooking and remove the tent!

It would just need an alternative way of stabilizing the top of the flue pipe. “Why was I so slow to think of that, as this is what I do at home on my stove testing bench!”

“I think I can claim that this stove has got everything except size and weight…… Stick fired tent heating/cooking, stick fired outdoor blower-stove using USB power or outdoor natural draft stick stove and indoor or outdoor alcohol-fueled cooking.”

The stove below is running directly on the earth without the need to connect the stove bottom that would normally be used when in a tent.

Here is an Instagram video of a liberated tent stove chuffing away happily in the open air while boiling water in a small pot or a large mug.

[Add a photo of large pot boiling on KISS STOVE at night]

Tim