Monday, January 14, 2008

IAP Stove Building in Humanazana, Peru

The engineers from EWB-Princeton have a great step-by-step guide to how they built the Justa-type (an enclosed masonry stove, with a chimney, used particularly for reducing indoor air pollution in Central and South American countries; often using a steel plate/griddle/plancha on the top for multiple pots) stove in Humanzana, Peru (Summer 2007).

Some small details about building any new stove (even now, every stove project is new) are particularly appropriate and important for realizing all the performance for this general design. For this type of stove you want:

  • a very intense fire, to burn the smoke
  • high temperature materials for the firebox
  • a low outside surface temperature
  • ways to make it so that it can't go wrong during operations
  • a design that will burn anything, and always completely
  • minimum ash and emissions, and maximum heat to the pot
  • can cook several dishes at one time
  • can cook some dishes directly on the metal top - breads, etc.
  • almost completely local materials when possible
  • as little heat going out the chimney as possible
  • construction just as simple as possible - foolproof
First you have to strengthen and generally improve the stove building site - I expect that you might regularly have to plan on spending time resurfacing the stove base platform, and often with the whole thing still hot from the last meal cooked. We tried to scrape this one out to a stable position, then packed it closely with stones, filled in the spaces with cement, then smoothed over the top until it was level and smooth - we might as well do a good job. Its easy to lay out the stove blocks where the nrw stove will go - it makes everything much more obvious. Everything is as hollow as possible, because solid structural things have a high thermal conductivity so we want hollow space that we can fill with better things.

After I left they made a great change by building the chimney first, while nothing else was in the way - it is made out of big 4X2 bricks, where some of the vertical shafts are hollow (200 cm2) and 2 or filled with stones, cement, and steel reinforcing bar to provide good stiffness. With the chimney as a locating point, the walls can be laid and another brick used as a spacer on the other end - clumsiness in locating the blocks can lead to lots of times wasted when you fix them together with mortar, so arrange them first.

The chimney just starts off as a leveled space where the special first brick (with holes punched it it for the gases) is sunk into wet cement and leveled, the reinforcing bar is inserted and small rocks are used as used as fill, cement is poured in regularly to fill the porosity between the stones, another brick is lowered down the steel bars and it goes up one brick at a time (30 cm each is this case). The brick is very porous so we soaked it with water first, mortared the "web" sections, then filled mortar into the gaps on the outside.

The bricks of the "wall" (you have to give names so that you can communicate fast in 2 languages) have to be attached to the base using "filet" joints that I know of from woodworking. You are attaching a flying buttress joint on both the inside and outside bottom joints on the wall - you just use your finger to make a nice big concave bead at the base; even though the stove is held together in many ways, this is the only place where it is secured to the platform. The wall bricks are also attached to each other with a layer of cement, and again it is very important to apply water to all brick surfaces before adding cement.

With the walls and chimney in place you start to insulate the hollow bricks - you want your stove to be as hot as it can be on the inside (for complete combustion - very little smoke or ash remaining), but as cool as it can be on the outside surfaces (so that it does not burn family members who happen to touch it). Sifted wood ash is a good insulator - non-combustible, lightweight (lots of air spaces), and non-compacting - but it is not as easy to find a large supply as one might think, so we use it sparingly, only where the temperatures are the highest. Around the combustion chamber and at the very top of the stove are the only places that may justify ash, so fill the tubes toward the front of the stove only with ash, and then the back ones are filled first with lower grade insulators (like crushed brick) and then topped off with ash for the last 10 cm (the hot gases in the stove will only be at the very top, right underneath the plancha. Save room at the very top of each tube for a few centimeters of coarse gravel - the cement that seals these and makes the top of the stove will not stick to ash, but it will anchor itself to the small rocks.

Now you create the top surface by applying a layer of cement across all the tube tops, checking regularly to make sure that it is level. You have to balance aestetics with productivity to create the best look without an unreasonable amount of effort - 1 cm thick is enough, then use a wet glove to create a smooth surface and rounded edges. The metal plancha is now carefully pressed into place - your initial careful measurements will result in it overlapping the wall bricks by ~1 cm on all sides, and by pressing it into the cement you will make a "lip" that both positions the plancha and seals it so that hot gases won't escape. Leave the plancha in place long enough for the cement to set well, but remove it before the plancha becomes permanently attached.

Now how do you show your cooks how to use this new type of stove properly, make sure they understand your instructions, are prepared to use these with their own local dishes, and can troubleshoot any possible problem that may arise? Consider starting by demonstrating cooking on a demo stove, in public, so that you can answer questions and demonstrate some of the benefits - no smoke in the kitchen, little or no smoke out the chimney (the smoke is burned), a cool outer surface that is safe for children, retained heat that keeps food hot after the fire is out, space for several pots with different temperatures depending on how they are placed, and so on. One thing that you can´t show them quickly is how much wood is saved by using this stove, compared to their old one - even stove engineers cannot tell you what the savings are yet, since they are hard to measure (cooking the same meals both ways is required, and tests should be conducted several times to get decent statistics). Remember that fuel savings may not actually be the strong point of this type of stove - it is designed primarily to reduce IAP, and everything else may turn out to be secondary. This type of stove is not meant for single pot cooking if it is to be efficient, since if the plancha surface is not full of pots, the empty spaces are just radiating heat out into the room instead of being used to cook food - this comes from burning wood which is not used efficiently.

No comments: