Components of an IAP Stove

It is helpful to provide names for the parts of a typical IAP stove (which tends to be a subset of rocket stoves - see post on these below for an overview):

• Outer walls ("the box", or similar) - the enclosure that confines the smoke and sends it up the chimney, and it provides a low temperature exterior for safety. Easiest to make from very local materials.
  
• Combustion chamber (or firebox) - constrains the fire so that it is as hot as possible (to burn the smoke), should be inexpensive/high temperature material/low thermal mass; very often it is the "rocket elbow" and is formed by the intersection of the horizontal wood entrance arm and the vertical Rocket/combustion chamber chimney.
• Rocket chimney - the vertical passage above the combustion chamber, for the fire to heat the plancha after the smoke is burned
• Fuel entrance - the part of the rocket elbow that the cook feeds the wood in, after it is pre-heated while being introduced to the fire slowly; the air entering the fire is pre-heated as well
• Grate - it supports the wood inside the wood entrance, so that air can flow in below the fire, and it keeps the wood and charcoal embers up and out of the ashed
• Insulation material - a low thermal mass (porous) loose material that prevents the heat from the fire from traveling far - wasting heat that could be used for cooking food. Typically wood ash or a volcanic material like pumice.
  
• Plancha - the metal (usually) plate upon which the food is cooked, either by intimate contact (dead animal parts, breads, etc.) or inside of pots. Very often there are adjustable pot holes so that the fire can directly contact the bottom of the pots (note that the pot can either be on the surface or allowed to drop below the level of the plancha - hopefully improving heat transfer sufficiently to warrant the extra cost. Inkawasi stoves may not have a plancha, replacing it with a ceramic surface that has pot holes which the pots fit into (dropping their bottoms below the surface so that they are in the flames).
• Flame space - the area underneath the plancha where the flames coming out of the combustion chamber chimney flow under the plancha
• Filler space and material - some of the area under the back of the plancha, where scrap materials can be placed to avoid using too much insulative material (since it should not absorb too much heat).
• Chimney - which carries the smoke out of the kitchen/house, connecting to the flame space. Should be witha long lasting (can't corrode easily) and earthquake resistant material and design.
  

Somewhat in order of importance for engineering design purposes:

• Combustion chamber (plus fuel entrance and combustion chamber chimney) - the whole "rocket elbow" was postulated/developed by Dr. Larry Winiarski at Aprovecho, and is usually made as a single assembly out of the same material necessarily a high temperature one since it comes in direct contact with flame. It is possible to make this whole elbow in one piece (usually crafted by a potter, out of clay) but it tends to be made in 2 parts (fuel entrance and chimney, with the combustion chamber being the intersection of these), and sometimes more. The material is the most important aspect, since at very high temperatures (perhaps up to 1100 oC) most materials are susceptible to degradation and breakage. Previous materials for this part have included mild steel, stainless steel, cast iron, and all types of ceramics (mud, bricks of any kind). Metal, besides cast iron, degrades relatively rapidly and are also expensive, cast iron when too thick requires too much time to heat up, and soft bricks have the potential to be damaged too easily. Hardness and other mechanical properties (including resistance to cracking), thermal characteristics, and cost are of primary concern but also to be considered is the ease of construction and stability over time. More recently, low thermal mass ceramics have been preferred by some so that the majority of the heat flow is limited (and doesn't spread to unwanted places) and thus is more concentrated in the firebox - sometimes proposed is the use of these in a combustion chamber made from several custom "bricks". The six brick architecture is usually used for a complete stove, but placing it inside an IAP stove should not seem unusual - it can be made very locally and inexpensively. This would replace the typical, often fragile/brittle, "baldosa" tile material (as is used in stoves such as the ONIL, in Guatemala). In any cases the materials must either be cut from flat tiles or cast/formed into the appropriate shape before firing (see here for an Aprovecho insulative variety, but the proper plain clay bricks should be sufficient since there is insulation around the combustion chambers in IAP stoves) - it may be that a four brick approach is almost as stable and takes slightly less expertise to make reliably.
• Plancha - the plancha is typically made from 1/8 " - 1/4" thick mild steel - cheap and readily available in developing countries (cast iron ones are best for corrosion resistance but are more typically used only for restaurant-sized versions). The two primary types are the plain flat plate and ones containing adjustable pot holes (plasma torch cut?) - while the pot hole version may be preferred, the flat plate one is much cheaper (since it takes no specialized equipment, and is more commonly available). In either case the durability and structural integrity is important - too thin and it can warp to to thermal stresses, so often it is often reinforced on the underside, adding cost. Also, an additional metal plate may be welded to the underside, immediately above the combustion chamber chimney, since this has the most exposure to very high temperatures and so is more susceptible to early failure. If too thick the plancha can cost too much (though it will be less prone to warping) - metal is sold essentially by the kilogram) so 3/16" is more common than 1/4" since the later is 30% more expensive. Planchas can be made either from new metal (from a ferreteria or local metal supplier) or recycled metal (from a blacksmith), with the latter preferred for flat plate planchas. All planchas must be maintained regularly by the owner - the top scrubbed with a stone to remove rust, and the bottom cleaned to remove carbon build up. I am just checking now on the price of potential plancha materials in the U.S. - new metal with dimensions of 14"x28"x3/16" (ONIL dimensions) from McMaster Carr, as an example of cost vs. thickness and area.
  

• Outer walls -I have typically made the walls from hollow bricks, assuming that the most important characteristic is cost per unit volume, but now I see that the insulating material around the combustion chamber (usually +5 cm all around it) is sufficient for keeping the outsides of the stove cool. Filling hollow bricks with insulating material is noble, but if it is not necessary it consumes too much valuable material (e.g wood ash is hard to get in quantity, and the interior volume of these bricks can be as much as the inside of the stove) - as long as there is sufficient insulation inside the walls they may not need to be filled except at the top adjacent to the flame space, so the wall thickness can be reduced and the overall width of the stove reduced. The ONIL stove uses concrete for the walls, and the TWP stove (and others) use a sheet steel skin to contain the insulative filler - as long as there is sufficient insulation around the combustion chamber, and the material in contact with the flame is resistant to degradation by the flame, almost any material can be considered. In the past, rammed earth was used, but this had a high thermal mass when it was not combined with a porous insulative fill material on the inside. Now, ordinary bricks, and even stone, can be used, as long as the combustion chamber is properly insulated.

• Insulation - This material, usually "loose" (free flowing - not a super fine powder, but granular) is preferably low thermal mass (e.g. filled with air spaces) - wood ash, pumice, vermiculite, perlite, and crushed porous brick are usual materials. Many people think that sand or stones are appropriate, but these are high thermal mass materials which tend to absorb lots of heat from the combustion chamber, preventing a hot fire from developing quickly (which is necessary to burn the smoke). Loose materials tend to be used because they fill small spaces easily, but high temperature fiberglass may be an alternative (or aerogels and simililar high tech materials with lots of air space inside). The local availability of this material is an importyant consideration - contries with volcanoes tend to have easy access to pumice, while wood ash can be more common (but hard to get in quantities need for mass production of stoves. In any case, minimizing the amount of these insulative materials is helpful - it should only be used very close to the combustion chamber and places in contact with flame (high temperatures) and other places where other materials are inappropriate.

• Chimney - this part removes the smoke from the kitchen, and like all components it should be low cost, made from local materials, have good durability and mechanical strength (including earthquake resistance), and be easily maintained (cleanability). Metal tubes are the most common chimneys, but they tend to degrade via corrosion at the bottom and perhaps only last one year; chimneys made from hollow bricks are very cheap and durable (and can be made earthquake proof with steel reinforcing) but cannot easily be cleaned. Mud and stone chimneys are the least expensive to build, using only very local materials, but are not commonly used.

• Filler material - this component is perhaps the least important, since it is far from the combustion chamber so has little possibility of stealing heat which could be used to cook food. Located toward the back of the stove, the materials can be things such as broken bricks and other rubble, stones etc. - anything to reduce the amount of insulating material used.

Materials and Design Recommendations - and Problem Areas
After all this discussion about the various components, how should each component be made - what guidelines should be followed? The general principles of local, inexpensive, simple, and maintainable should be our guiding principles - what is available locally is the first thing to consider, so exploring the neighborhood is the first order of business. Blacksmith shops, construction material suppliers, home repair stores, hardware stores, brick yards, etc. all have to be checked to see what the full range of materials is available for each and every component( i.e. don't arrive at the site with pre-conceived notions of an exact design, set of materials, or cost). Metal plate for the plancha can be tricky, as well as materials for chimneys - these may be the most local-dependent. If planchas without pot holes are not commercially available (or are too expensive (often $USD 30), then don't plan on these, and the same for metal chimneys!

Tricky parts of a non-cast cement mono-wall (like the ONIL - at this point this design seems too expensive) are these few stove areas:

• the part of the front of the stove wall (facing the user and above the fuel entrance) where you must sometimes have an insulating brick material, if it touches the combustion chamber
• the combustion chamber material and design - even if we have good flat materials (like baldosa tile), how can be make the chamber and fuel tunnel so that it can be made, fabricated, and perhaps shipped (from the brick kiln) easily? Of course it must be high temperature resistant, when nothing else in the region may need to be. If this part fails, then we have lost the battle.
• the connection between the fuel tunnel and the combustion chamber - should be tight so that it is mechanically strong, and so that air does not leak in (particularly in the case of a cylindrical combustion chamber).
  
• the thin joint between the plancha and the chimney, to keep air from entering the stove (the same is true everywhere around the plancha - smoke will not blow out, but unwanted cold air can enter).
• the joint between the chimney and the stove body can be a weak point if not designed properly (supporting the chimney, since it has a hole in it to remove the smoke from the flame space it, can be weak point).
  
• the chimney - popular sheet metal chimneys can corrode in the bottom segment as soon as in one year (users may not replace them, defeting the purpose of the stove) so a better design is needed. There is little experience with rock/mud/adobe/block chimneys it seems, or a combination of these materials. And how can the chimney be supported, if there is no house wall or strong roof?