| Boiling Point No. 23 - December 1990 |
Rocket, Down Draught, Wood Burning Stove by Dr L Winiarski, USA - Editorial Summary
24395 Starr Creek Road Channels Oregon 97333 USA Telephone 503 753-4921
Two articles in Boiling Point No.21 clean Combustion of Wood" and the Silkalon Stove both address principles which I have found very useful in stove design. Figure 1 shows my vertical feed, down draught, Rocket stove which is inexpensive, fuel efficient and very clean burning. It has a variety of uses including bread baking, boiling, frying, and grilling, water and space heating.
An early version ("Estufa Rocky) has been disseminated through an Aprovecho/Ministry of Agriculture/Peace Corps programme in more than 1,000 urban households in Guatemala and subsequent versions have been distributed on a smaller scale in Belize through a Peace Corps volunteer and in Mexico through a local branch of the national Mexican agency, Desarollo de la Familial Stove dissemination work on down draught stoves in Guatemala was described in Aprovecho Institute's Cookstove News.
There are several guidelines I have found useful to follow in developing clean burning down draught stoves. If these guidelines are followed, the stoves can be constructed in a wide variety of configurations and made out of various materials. I have measured efficiencies of over 40% (standard PHU) when very light weight insulation is used and the area of the channel around the pot is about the same as the area of the chimney.
Guidelines for down draught stoves.
• The wood is arranged so that only the ends of the wood are in the intense heat of the insulated combustion zone, thus excessive hydrocarbon vapours are not driven out of the wood at one time.
An insulated chimney provides a strong draught so that the air passes through the burning wood at a high speed much like the blowing of a forge with a bellows. Cooking can be done on top, this chimney. A simple damper like a sliding brick is used to regulate the air and fuel opening so that excess air doesn't cool the fire.
• The wood acts as its own grate. The air passing between the flowing sticks is preheated before it enters the flame so it promotes the combustion of the hydrocarbons rather than quenching the name. Vertical feeding of the fuel with the sticks forming their own grate is similar to that of the Silkalon stove. However, the cleanest burning will occur if the sticks of wood are arranged to stand like a fence across the side of the feed hole closest to the chimney. A square or rectangular hole makes this a little easier. Pushing the sticks against the chimney side of the feed hole effectively blocks off excessive air from passing behind the sticks and cooling the fire. The standing or steeply inclined sticks feed themselves as the ends burn off, so the stove also tends to be self regulating. If too much wood surface is heated, there will not be enough air to burn all the gases that are produced and dense black smoke will occur. The secret is to adjust the number and size of the pieces of wood used to give the proper amount of burning surface for the desired power output. An audible, intense flame with no visible smoke leaving the chimney verifies the good fuel arrangement.Many things can be used as fuel, ranging from metre long wood sticks to twigs, dung, rice husks and coal. The stove should prove useful in many parts of the world where a chimney is too costly or traditionally not used and people are cooking over open flames or charcoal. The stove can be made in separate pieces which can be carried to the cooking area and stacked together. The pieces are moulded or cast from insulating, heat resistant material such as adobe with a high content of straw and ash or made with a lightweight concrete composed of perlite, vermiculite or pumice aggregates and reinforced with chicken wire or fibre. The inner channels that are exposed to the flames have a sandy clay liner that can be moulded in place. A hard firing will tend to turn this into brick. Another possibility is for a pottery shop to sell prefired liners of the correct shape and the user add his own adobe or lightweight concrete insulation around these liners.
The purpose of each of the main components is described below:
The Insulated Combustion Chamber helps the fire burn very hot. This high temperature increases the speed and efficiency of combustion. An ideal insulating material would be lightweight, heat resistant, rigid, durable, inexpensive and easily available.
The Vertical Insulated Chimney that can be enlarged to surround the sides and even the top of the cooking pot, is one of the secrets of the efficiency of these stoves. Since it can be almost a metre tall and very hot inside, it provides a very strong draught. Air rushes through the burning fuel as in a blacksmith's forge. With good insulation little heat is lost to the surroundings and more is available for cooking form of open stair steps. If fuel flow is adjusted so there is not too thick a layer on the step grate, the primary combustion air will produce burning through the fuel on the step grate insert, allowing more fuel to fall down. Some air space is also needed above the fuel on the step grate in order to admit a small amount of secondary air.
The Cross Sectional Area of the fuel loading tube, the combustion tunnel, the chimney, the gap around the pot or under a griddle, and the hole through the oven-lid should all be approximately the same in order
The Fuel Chamber holds the bulk of the fuel away from the heat of the combustion zone. A nearly vertical feed helps the fuel slide into the combustion zone as it burns. Since only a relatively small amount of fuel is being heated (only the ends of sticks of wood are burning), excessive smoke is not produced (most smoke is really oil-like vapours that need a very high temperature to ignite and burn completely).
Particle Fuels like sawdust, rice husks, seeds, coal dust. etc, can be burned if arranged to slide down a grate that extends into the to ensure a good draught through the stove. If the fuel loading tube is too large, the air may circulate within it and allow the fire to burn back up the fuel sticks. If the gap around the pot is too large, part of the hot gas will escape without heating the pot. In a stove with a 10 cm diameter chimney tube, the gap around a 25 cm diameter pot need only be about 1 cm.
Pumice cement or perlite cement stoves are still in use at the Aprovecho Institute after several years. The stoves have developed a number of small cracks but are still quite functional. The fired-in-place insert clay lining is cracked but remains intact. A small amount of chicken wire reinforcement in some of the stoves has made them quite durable to both thermal and physical shock.
High efficiency depends on a number of factors that may not seem important to the untrained stove user. The hot gases must be confined to flow in intimate contact around the pot. For a large household pot the optimum gap at the sides may seem surprisingly small, in the order of 8mm. Excess air must be minimised by careful damper/fuel arrangement.
Cooking on top of the stove chimney may not seem as different as sometimes feared In effect, many stoves already do this. It is just that the chimneys are very short. Simply increasing the height of the chimney does not necessarily make the stove culturally unacceptable. The dissemination in Guatemala has already shown this.
Note: This stove should not tee confused with the Rocket stove developed by 5 Joseph to burn hollow cylindrical briquettes. Ed.