Dean Still explains time and temperature in a Rocket Stove in a YouTube video

Dean Still and Sam Bentson have started collaborating on a series of videos that explain the basics of how Rocket Stoves work, so that stove designers and stove users can get the best performance out of this popular stove design. In this first installment, “Time and Temperature,” Dean explains the importance of high combustion temperature in a Rocket stove where there is limited time to burn up smoke particles. He demonstrates how the Jet-Flame (www.jet-flame.com) helps to increase combustion temperature by blowing air under the fire.

Be sure and subscribe to Sam’s YouTube channel so you never miss an episode! New videos will be added every other week.

Sam Bentson trains Bernard Kabera and colleagues to use the new stove lab equipment

Aprovecho’s General Manager Sam Benston recently returned from a trip to Rwanda, where he helped to set up a new ISO compliant cookstove lab. Here are some photos and information from Sam about his work there:

I was installing the LEMS (Laboratory Emissions Monitoring System) and PEMS (Portable Emissions Monitoring System) and the rest of the new ISO 19867 cookstove laboratory at the Rwanda Standards Board in Kigali. The lab started as an empty room full of equipment in boxes. I trained the laboratory staff on the set-up and use of the equipment for cookstove evaluations according to ISO 19867. Shortly after I left there was a Grand Opening to celebrate on the ISO’s World Standards Day. Here is a twitter link with photos:  https://twitter.com/REMA_Rwanda/.  Our new PEMS with the battery powered gravimetric system is visible.

The PEMS is visible here at the launch of the Cook Stove testing lab in Kigali
The PEMS is visible here at the launch of the Cook Stove testing lab in Kigali.
Photo via @REMA_Rwanda

Aprovecho provides a turnkey cookstove testing laboratory which is useful for cookstove performance certification, design, and basic research. The lab is centered around the ARC manufactured LEMS. It consists of a gas and particle analyzer with a pump and filter PM2.5 sampler, an emissions collection hood, and a dilution tunnel. The LEMS is the result of 20 years of development that started due to the lack of affordable and easy to use equipment suitable for cookstove emissions monitoring.

Testing a stove under the new LEMS hood.
Testing a stove under the newly installed LEMS hood.

Aprovecho develops its equipment as the need arises during research and development activities that occur in its laboratory. Aprovecho’s ability to commission the other instruments that makeup a cookstove testing laboratory is the result of a similar depth of experience.

Bernard Kibera and colleagues training to use the new stove lab equipment
Mr. Bernard Kabera and colleagues training to use the new stove lab equipment.
Sam Bentson trains Bernard Kibera and colleagues to use the new stove lab equipment
Sam Bentson trains Mr. Bernard Kabera and colleagues to use the new stove lab equipment.

It was remarkable to observe how the Rwandan people have protected themselves against COVID. It was a great honor to be part of their community at this time.

–Sam Bentson

Sad cooking pot on a stove
Two cooking pots
Mind the Gap!

Here are the TLUD (Top-Lit Up Draft Stove) derived heat transfer principles that ARC designers use when designing and improving stoves. They are just as important for Rocket stoves as TLUDs:

T: The temperature of the hot gas contacting the pot or griddle should be as hot as possible.

A: Expose as much of the surface area of the pot or griddle to the hot gases as practical.

R: Increasing heat transfer by radiation is important. Move the zone of combustion as close to the surface to be heated without increasing harmful emissions.

P: Optimize the proximity of the hot gases to the pot or griddle by reducing the channel gap without reducing the velocity of the gases. Reduce the thermal resistance with appropriately sized channel gaps under and at the sides of the pot. Match the firepower to the channel gap size and to the size of the pot or griddle.

V: In convective heat transfer, the primary resistance is in the surface boundary layer of very slowly moving gas immediately adjacent to a wall. Increase the velocity of the hot gas as it flows past the pot without reducing the temperature of the gases. As a rule of thumb, heat transfer efficiency can double when the velocity of the hot gases also doubles (N. MacCarty, et al, 2015).

Man at chalkboard
When analyzing a system, try to improve the least efficient part first. 

There are three types of heat exchangers generally used to capture the heat produced in a combustion chamber.

The hot flue gases can:

  1. Heat mass, like heavy stone or masonry
  2. Heat water which then warms the house or…
  3. The easiest and least expensive route – make the hot stove gases efficiently heat the air inside the room

In modern houses with limited air exchange rates heating the air has become the popular option. High mass heat exchangers were created in the days of drafty houses when heating air was a losing proposition. Old houses had air exchange rates of more than 10 exchanges per hour. All the air in the house was replaced ten times or more every hour! It didn’t make sense to heat air that would quickly be outdoors.

Heat exchangers increase heat transfer to the room by making sure that the hot flue gases leaving through the chimney are as cool as possible. Even a smoldering fire turns about 90% of the wood into heat. But, heat transfer efficiency (heat delivered to the room) can be less than 20% in poorly designed systems. As the cartoon shows, a little improvement in heat transfer equals impressive increases in fuel efficiency.

Retaining Heat is Part of the Equation

We cook beans (and other long simmering foods) at Aprovecho using a “haybox.” The pot of food is boiled for ten minutes on a stove and then placed in a well-insulated, airtight box. The beans inside the pot get soft and palatable because the retained heat is sufficient to finish cooking them. We end up using a great deal less fuel because the haybox has improved the heat transfer into the pot. (It’s also a much easier cooking method!)

How a haybox works
A Haybox cooks beans by keeping the heat in the pot. When cooking on a stove, the heat needs to be constantly replaced, using more fuel.

The reason that beans are usually simmered over a fire for a couple of hours is because the pot constantly loses heat to room air. The reduced flame underneath the pot replaces the lost heat.

In the same way, a furnace or a wood replaces the heat in our houses because the house allows the heat to constantly leak away. The house loses heat and the burning wood replaces it. If the house loses a lot of heat, we use a lot of wood per season. If the house loses less heat, we can save trees and are better stewards of this precious resource. If the house loses very little heat, the stove is frequently not even lit because energy in sunlight and interior sources of heat are now equal to the heating demand.

Jet-Flame cross section

Dr. Larry Winiarski would remind me to imagine the languid rising of smoke from a cigarette when thinking about the velocity of natural draft gases in the Rocket stove.  I remember Larry saying that rising smoke is sexy, contemplative, and slow.

Sam Bentson, General Manager of ARC, and Chenkai Wang, Division Business Manager of SSM, spent months designing an inexpensive 2 Watt fan that developed a pressure of 0.75 inches of water column to blow high enough velocity air jets into a Rocket stove fire to increase mixing and combustion efficiency.

When Sam measures the dynamic pressure in the chimneys of household natural draft rocket cooking stoves he finds less than 0.01 inches of water column. Sam estimated, using the Archimedes principle, that a 10” inch in diameter chimney pipe at 700°C for its entire length would have to be 15 meters tall to generate 0.50 inches of water column. It’s amazing how powerful a little electric fan can be!

Jet-Flame cross section drawing
The 2020 Jet-Flame

Kirk Harris writes that he has wondered how an exceedingly small pressure variance could drive the tall flames that we see in some stoves. He envisions fire gas as having exceptionally low density, very light weight, with very low inertia.  Kirk thinks of fire as like a “hole in the atmosphere”, easy to push around. His stoves use static mixers and small velocity induced natural draft pressure differences to mix flame that has been divided into thin sheets. Using these approaches, the Harris TLUD stove achieves less than 1mg/min for PM2.5 when burning pellets.

The Jet-Flame, on the other hand, uses very high-pressure jets of air that blast up into charcoal and then mix wood gas and air as the jets pass through sticks on fire. The bottom air technique requires the equivalent pressure of a 15-meter-high, extremely hot chimney to lower emissions to about the same degree as the Harris stove. It is interesting to think of these two stoves side by side, representing quite different approaches to clean burning. The Harris stove is gently manipulating flame as easy to move around as a hole in the atmosphere while the Jet-Flame is dynamic, a bit loud, creating hot jets of air that drill holes in burning wood.