GRATES IMPROVE EFFICIENCY
Dean Still, April 2003

Dear Friends,

Damon Ogle and Mike Hatfield observed in their recent trip to Central America that the shelf placed in the fuel magazine of the Rocket stoves
which lifts the sticks of wood up off the floor of the combustion chamber) was frequently put aside by users who first placed sticks of wood directly
on the floor of the combustion chamber and then after a while, on top of the glowing coals.

Damon and Mike wondered whether including a shelf was important. Luckily, researchers become opinionated only if they get lazy...Damon set up a test
stove in the lab and he and I ran nine one hour water boiling tests. The same stove was tested three times with

  1. a horizontal sheet metal shelf
  2. then without it
  3. then with a solid brick shelf in the feed magazine that blocked air from passing under the shelf and allowed primary air to feed
    the fire in the combustion chamber from underneath a metal grate in the
    floorof the combustion chamber. Air was drawn directly up through the fire.

Checkout a drawing of the shelf concept in the Rocket combustion chamber on the Aprovecho stove page at www.efn.org/~apro

After averaging the results from the three tightly grouped series of tests, the simple shelf improved efficiency by 14%. The solid shelf with under fire
air improved efficiency by 35.7 %.

The amazing Sam Baldwin, in the best book yet written on cookstoves,
"BIOMASS STOVES: ENGINEERING DESIGN, DEVELOPMENT, AND DISSEMINATION"
reported improving efficiency from 18% to 25% by using a grate on a "malgache" metal stove. This represents an improvement of 38.8%, which
parallels our findings.

Grates that allow air to pass under and up through the burning sticks can have a significant positive effect on efficiency.

Both Damon and I also observed that fewer coals are produced by using a shelf or grate. But further testing is needed to determine the extent of the
difference. More hours staring into the fire!

All Best,

Dean Still
Damon Ogle