New Video: Test Kitchen 2021 Natural Draft Hood

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The old saying, “if you can’t handle the heat, get out of the kitchen,” is not lost on us when we are working in the test kitchen. It takes us a while to get the vertical and horizontal grid of Climate Solution Consulting’s HAPEx particulate matter monitors started and positioned. Then ARC’s PEMS-PC partial-capture based emissions sampler has to collect a zero point of the gasses in the atmosphere. Don’t forget the temperature sensors (where is that data?) and the wood. This is after reviewing yesterday’s work, discussing a plan for the day, and watering the garden. So, by the time the young scientist rolls into the test kitchen in the Oregon summer, currently home of America’s largest wildfire, it’s about 100°F and rising. But science must go on.

ARC’s four year old test kitchen is currently being used to test a natural draft hood of our design. Our experiment allows us to operate the fire without being exposed to the emissions from the fire. We used to use a vacuum cleaner as a positive pressure ventilator, but now we sit outside of the room and feed the fire through a glove box.  After seeing that the hood was effective enough to reduce the concentration of PM2.5 in the test kitchen to below 35 ug (averaged over 24 hr), we turned the hood around and made a video for you of us doing the water boiling test while enjoying a smoke free kitchen.

Enjoy the video (and know that those loud pumps and fans go with the bit about it being hot in the kitchen).

Please send your photos and stories of natural draft hoods! We don’t want to lose this beautiful technology.

-Sam Bentsen, Aprovecho Research Center General Manager

Product Development

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General Manager Sam Bentsen is happy about some LEMS test results.

Why test a stove?

Most of the time, our lab uses testing for product development. If we did not test a stove prototype we would be guessing whether it met expectations. Testing in the lab gets us ready for the field testing of prototypes. Then, customers take the prototype and make it work. The factory and distributors frequently ask for design changes as the product gets closer to shelves. From initial design to market usually takes about a year of testing/iteration/development.

Recently, a factory in Africa asked us to design a $10 wholesale, pellet burning forced draft TLUD prototype that achieves Tier 4 for thermal efficiency, CO, and PM2.5. The stove has to last ten years with scheduled maintenance and require as low wattage as possible.

We had tested the Oorja several times during surveys of commercially available stoves. ARC published the results in books and papers trying to inform the public how stoves compare on various measures of performance. We were trying to make available a Consumer’s Report on stoves (see list below). We knew that the Oorja stove met the Tier rankings and that it used a high mass, low cost, durable combustion chamber. We tried a castable refractory in the lab and we also found several manufacturers that make inexpensive ceramic combustion chambers.

The factory wants a high-powered stove to meet the needs of cooks in their region. Protecting health is also a major concern. Delivering a design that can be made for $10 is also very important. All the interconnected partners in the business plan have to make a healthy profit to bring a “Tier 4” technology to the public. The designer is only the first step in a web of stakeholders.

After all of the necessary parts were combined in the lab, testing with the LEMS (Laboratory Emissions Monitoring System) started. Many iterations were needed to get close to optimal performance. Adjusting the primary and secondary air at high power took experimentation. In several weeks of daily testing, the prototype was repeatedly achieving best scores. A CAD drawing was made and the design was sent to the factory. The factory is making their version of the stove, we will test it here and make adjustments if needed, and then field testing of the prototypes will begin, including home trails and test sales in stores.

Does it sound like a lot of work? The payback to know, rather than guess, that the product can be successfully sold. It’s great to make data based decisions, and a careful approach attracts investment. Failing miserably with products we loved (and lost money on) has made ARC consider external input carefully, especially from field testing.

Cook Stove Performance Reports:

The Oorja Forced Draft TLUD: A Different Approach

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Testing the Oorja Stove under the LEMS hood.

There are many forced draft TLUDs that are quite similar to Dr. Tom Reed’s 2001 version, the WoodGas stove. The Oorja stove can be about as clean burning but has several obvious differences: a high mass refractory ceramic combustion chamber, much bigger secondary air holes, and high firepower. Like other forced draft TLUDs the turn down ratio, created by limiting the combustion air, is narrow. The Mimi-Moto had to turn to a smaller combustion chamber for simmering to achieve Tier 4 for low power metrics. It’s a problem for Forced Draft TLUDS.

I have been a fan of the Oorja stove since 1999. In 2003, when I was living in India, hundreds of thousands of British Petroleum Oorja stoves were in use, burning pellets made from field residue. It’s been fascinating recently to read Dr. H. S. Mukunda’s 2010 paper describing the development of the Oorja.* When his team tested the lifespan of a metal combustion chamber it was only about 12 months and cast iron was expected to last about twice as long. The team developed a ceramic combustion chamber to create a better, longer lasting stove. I’m testing an Oorja stove with ceramic combustion chamber that is 20 years old!

Mr. Prasad Kokil from the San Jay Group writes: “We had developed this Oorja stove for BP in our company. We developed the ceramic refractory for the Oorja at that time. Our Elegant Model (now for sale) has a ceramic refractory combustion and is a forced draft TLUD”.

Large secondary air holes near the top of the combustion chamber.

Dr. Mukunda and team decided that at a burn rate of 12 grams per minute the primary air should be 18 g/min, and the secondary air was set at 54 g/min. The 18 secondary air holes, just below the top of the combustion chamber, are larger than in other FD-TLUDs at 6.5 mm in diameter creating a velocity of 1.8 meters per second. Using larger holes means that a low wattage computer fan supplies air jets with sufficient volume and velocity. Emission measurements made by the development team, carried out at fuel consumption rates of 12 and 9 g/min, showed that the CO emissions were 1 and 1.3 g/MJ whereas particulate emissions were 10 and 6 mg/MJ for the high and low power levels. When burning the made charcoal, CO rose but did not exceed the Indian standards.  

The Oorja stove has been tested at various times in our lab with impressive results. Learning from Dr. Mukunda and team how to make stoves that are super clean burning and last a really long time is an important development. Thanks for such a great stove!

* Gasifier stoves: Science, technology and field outreach H. S. Mukunda, et al., CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010 

Fred & Lise Colgan, founders of InStove

Rocket Stove Sterilization for Hospitals

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Fred & Lise Colgan, from InStove
An All American Autoclave

Fred and Lise Colgan created InStove, manufacturing and distributing institutional stoves initially designed by Dr. Larry Winiarski. They developed and sold large Rocket stoves that cooked food, sterilized medical equipment, and pasteurized water.

The autoclave, sold by Wisconsin Aluminum Foundry, works like a big pressure cooker and sterilizes quickly using steam and pressure. The unit fits into a Rocket stove that delivers the heat using less wood compared to traditional stoves. A chimney removes smoke from the room. The system can sterilize about 7 gallons of surgical instruments, dressings, and other medical supplies at a time, making them safe for either reuse or hygienic disposal.

These larger Rocket stoves combine the same strategies that are used in smaller versions. A pot skirt cylinder surrounds the sterilizer creating a narrow channel gap that is especially effective in transferring heat, in part because the pot is larger. Big pots have more surface area so increased percentages of heat pass into the water. When a chimney is attached to the stove, the hot gases are forced to flow down another channel on the outside of the pot skirt. In this way, adding a chimney to the stove does not diminish the fuel efficiency. A lot of the heat has already scraped against the pot and been absorbed before it exits out of the chimney. The light weight bricks used in a larger Rocket stove combustion chamber can be thicker and larger than bricks used in a smaller stove. The Institutional Stove described in the Institutional Rocket Stove pdf on the Publications page can handle pots from 50 to 300 liters. The downloadable Excel worksheet Institutional Stove Gap Calculator can help you determine the measurements of an institutional stove designed to fit the large pot you have available for use.