No More Simmering!

A quick internet search for “retained heat cooker” brings up many modern choices.

In 2018, the World Health Organization concluded, “Every day around the world, billions of children are exposed to unsafe levels of air pollution. The result is a global public health emergency.” 

The WHO recommends that a combination of actions may be most effective. Aprovecho has used Retained Heat Cookers (Hayboxes) for decades and we recommend them, especially where beans make up a part of the diet.

Retained Heat Cookers simmer food to completion reducing CO emissions by 56%, PM emissions by 37% and saving 50% of the fuel and time spent cooking. (Test Results of Cook Stove Performance, 2011). When food is simmering, the fire is constantly replacing the heat lost from the pot. If the heat is captured instead, the retained heat in the boiling pot finishes cooking the food. 

In the same way, a drafty and uninsulated house has to have a big fire going all the time to keep the house warm.  The super-insulated, almost airtight house can stay warm for a long time after the fire is extinguished. In Oregon, Haybox homes are called “Super Good Cents” homes.

Once the boiling pot is in the box, food cooks without further attention. Even pinto beans will finish softening if the box, with waterproof R-7 insulation, is almost airtight.  Makes cooking a lot easier, too!

6mm Gaps Recommended for Pot Skirts

graph helps calculate proper skirt gap for best heat transfer efficiency
From SAMUEL BALDWIN’S “BIOMASS STOVES: ENGINEERING DESIGN, DEVELOPMENT, AND DISSEMINATION,” VITA, 1987

Smaller fires are often cleaner burning compared to larger fires, and higher heat transfer efficiencies allow the use of smaller fires while the time to boil stays acceptable. A pot skirt which creates a narrow channel gap reduces fuel use and time to boil.

Dr. Sam Baldwin recommended 10mm to 11mm channel gaps for household cook stoves at normal high firepowers (4kW to 5kW).

ARC uses tighter channel gaps at 5kW and we do not seem to experience problems. ARC recommends 6mm channel gaps in pot skirts for 20cm in diameter (or larger) pots. 

It is interesting to note that Dr. Baldwin decreased the channel gap to 8mm with bigger institutional sized pots. We just tested a 6mm gap in a skirt surrounding a 40cm in diameter (60 liter) pot. The result, with a small open fire, was around 48% thermal efficiency.

We recommend a 6mm channel gap to achieve close to 50% thermal efficiency with a large range of pots..

The tight channel gap seems to be OK, so far. It would be great to model velocity, temperature drop, etc. as the channel gap changes! We will do it one of these days…