5. Heat, Fire and Stoves

Cooking requires the transformation of the potential energy in fuel into heat energy. To improve the efficiency of cooking, we need to understand how heat is transmitted, how it is released in combustion and how heat loss can be minimized. Finally, we need to know how heat is made useful in a stove.

How heat generated in a fire is transmitted

Radiation

Radiant energy is emitted from hot objects and does not become perceptible heat until absorbed on the surface of another object. It is a form of electromagnetic energy as is light (Fig. 5-1).

Fig. 5-1

- Radiation is emitted equally in all directions. An object moved twice as close to a radiant heat source (e.g. a fire) receives four times as much radiation. (The intensity of radiant heat is inversely proportional to the square of the distance, Fig. 5-2).

Fig. 5-2; Fig. 5-3

- Emission of radiant energy increases dramatically with temperature difference. If the difference in absolute temperature between an object and its environment is doubled, radiant heat transfer will be increased by sixteen times. (Radiant heat transfer increases as the fourth power of the temperature difference.)

Conduction

Conduction is the movement of heat through solid materials (Fig. 5-3). Heat flows rapidly through good conductors like steel or aluminum. Materials which conduct heat slowly, like wood or cement, are called insulators. Substances with many tiny trapped air spaces are really good insulators (charcoal, sawdust, straw, Fig. 5-4).

Fig 5-4: Thermal Conductivity in (cal x cm)/(s x ºC x cm²) x 10³

Convection

Convection involves the transfer! of heat by the movement of a gas or liquid. As air is heated, it tends to rise and is replaced at the heat source by cooler air, which is heated and then rises, and so on. This type of heat transfer, which occurs because of the buoyancy of the heated air, is called natural convection. When heat is carried away from hot objects by air currents, this involve forced convection, or advection.

- Air heated by the flames of a fire will rise in still air (Fig. 5-5).
- In a breeze, the hot air will move with the prevailing current (Fig. 5-6).

Fig. 5-5; Fig. 5-6

How heat is released in combustion

Fuel, oxygen and high temperatures are required for combustion to take place. For a chemically simple fuel such as natural gas, or methane, combustion involves a relatively straightforward conversion of chemical energy into heat. The carbon and hydrogen in methane combine with oxygen from the air to produce carbon dioxide, water, and heat (Fig. 5-7).

Fig.5-7

Once ignited by heat from another source (such as a match), the heat from the burning fuel is adequate to sustain combustion. When pre-mixed with air, as in gas stoves and furnaces, natural gas will burn completely.

Fig.5-8

The combustion of wood also involves the combination of carbon and hydrogen with oxygen to produce heat. However, wood is a much more chemically complex substance than methane. It is composed primarily of carbon, hydrogen, and also oxygen occurring together in the form of cellulose, lignin, gums and resins.

The combustion of wood has these characteristics (fig. 5-8):

1. Wood burns in two stages. First, volatile gases are produced and burn, leaving solid charcoal, which then burns.
2. Oxygen must come from the air surrounding the zone of combustion.
3. The size' shape, and arrangement of fuel pieces affects the rate and completeness of combustion.

1. Wood burns in two stages

When a piece of wood is added to a fire, chemical changes occur in the presence of heat. At first, non-flammable carbon dioxide and water are given off. As the temperature increases, combustible gases and tars are also evolved. This process of chemical degradation in wood is called pyrolysis. When the temperature exceeds about 280 °C (Fig. 5-9), the proportion of flammable gases emitted is high enough to burn. Combustion will then occur only in the presence of oxygen, and at temperatures exceeding the ignition temperature of the fuel. (The average ignition temperature for the evolved gases in a wood fire is about 600 °C.) The gas is ignited by radiant heat from the already burning pieces of wood.

Once ignited, the pyrolyzed gases will burn at a temperature of 1100 °C; the flames then provide radiant heat which maintains and accelerates pyrolysis. The flames of a wood fire are these burning gases.

The flames probably do not even touch the surface of the wood. The flow of gases, which greatly increases with the heat of the flames, prevents oxygen from reaching the surface of the wood. It is only after this flow of gases subsides that the charcoal starts to burn. It burns with only a faint blue flame, and the by-products of combustion are mostly carbon dioxide and carbon.

Fig.5-9

All of these processes are normally occurring simultaneously- in a wood fire; charcoal may be burning on the surface of a small piece of wood within a minute after it is added to the fire, while the center of a much larger piece may not even become warm for an hour or more.

2. Oxygen must come from the air surrounding the zone of combustion

For optimum combustion, the supply of air to the fire is critical.
- Insufficient oxygen, due to restricted air flow or poor air distribution, may allow some combustible gases to escape without burning. A fire which produces a lot of smoke usually indicates a problem of this sort.- Up to a certain point, increased air flow increases both the rate and efficiency of combustion.- Air flow that greatly exceeds that required for combustion may carry off enough heat to lower the temperature of the fuel below its ignition temperature.- Excess air may also lower the concentration of flammable gases so that not enough chemical reactions occur to maintain the high temperatures necessary to sustain combustion.

3. Size, shape and arrangement of fuel pieces
- The rate of combustion depends in part on the size of wood pieces (Fig. 5-10). A larger piece of firewood has a greater volume in proportion to its surface area than does a smaller piece. Smaller pieces therefore have proportionately greater exposure to air flow and will burn faster. Small wood pieces heat quickly and will produce vigorous flames and little charcoal. This is because rapid pyrolysis of wood gives a high yield of flammable gases in proportion to remaining charcoal.

Fig.5-10: Wood cut smaller burns faster.
- The large surge of volatile gases from small wood pieces may not burn completely if the supply of oxygen is limited, such as in an enclosed stove.- Large pieces of wood burn slowly and steadily.- Large pieces of wood may not burn at all, however, if there is not an external source of heat (such as other burning wood). This is because too much heat is conducted into the interior of the piece for the ignition temperature to be sustained on the surface.- Straight pieces of wood arranged in a parallel fashion lie tightly together and impede air flow. As a result, gases will move away from areas of sufficiently high temperature before mixing with enough air to burn.

Moisture in the wood

- Wet wood puts out less heat because a large fraction of the heat generated goes into evaporating water. Up to 12% of the heat energy in green wood may be consumed in this way.
- The evaporation of water from wood will dilute the flammable gases, which slows the combustion rate and decreases combustion efficiency. This results in a smokier fire and increases the condensation of tars in the stove and chimney.

Combustion of other fuels

Charcoal

As charcoal is formed in a wood fire, it combines with oxygen and burns. It is made commercially by slowly heating wood in the absence of air, and the flammable tars and gases produced escape unburned.
- Charcoal is composed mainly of carbon, with some hydrogen.
- It has about 45% energy per unit weight than wood.
- Because charcoal has few remaining volatile components, there are no appreciable flames. Heat is generated on the burning surface.
- Charcoal pieces fit tightly together. For efficient combustion in charcoal stoves, air is usually provided over a large surface area through a grate from underneath.

Agricultural wastes

These are similar in chemical composition to wood. The usual problem with burning rice hulls or sawdust is that air flow through the fuel is very restricted. To provide sufficient air for combustion either the air velocity across the surface of the fuel bed must be increased, or air must be supplied through a grate from below (see Rice Hull Stoves, Chapter 7). On the other hand, if fuels such as straw, corn husks, etc. are stacked loosely, the fuel density is too low to provide a useful amount of heat in a stove.

How heat loss can be minimized

Heat loss to the surrounding area is minimized by enclosing the fire. Two things are accomplished by this:
- The enclosing walls block the wind so that convective heat is retained (Fig, 5-l l).
- The interior walls will absorb the radiant heat and will then re-radiate some of this heat to the cookpot (Fig. 5-12).

Fig. 5-11; Fig. 5-12

Some heat will still be escaping. Heat conducted through the stove walls is lost by radiation' and by convection to the outside air. Metal stoves are very hot to the touch because of this conducted heat loss. Conductive loss may be reduced by insulating the stove walls so that heat is retained. The Thai Buc-Bucket is a metal and terra cotta charcoal stove that uses a layer of ash for insulation. The insulating properties of sand/clay or adobe stoves could be improved upon if finely chopped organic matter, such as rice hulls or sawdust, were added to the mix. These materials form small cavities, in which trapped air functions as an excellent insulator (Fig. 5-13).

Fig. 5-13; Fig. 5-14

Heat is also lost from the cookpot (Fig. 5-14). For heating efficiency, the best cookpot would have a large surface area exposed to the fire, and a small area exposed to the air: Both convective and radiant heat loss from the exposed surface of the pot can be lessened by sinking the pot into the stove. The pot will also gain conducted heat from the hot stove walls. Covering the cookpot (Fig. 5-15) prevents convective and evaporative heat losses from the contents inside. This may reduce total heat loss by up to one half.

The heat lost in the exhaust gases from the fire can be utilized by:
- Directing the rising smoke around the sides of a single cookpot (Fig. 5-16).

Fig. 5-15; Fig. 5-16

- Using the exhaust gases from the fire under the first pot in a multi-pot stove to heat subsequent pots (Fig. 5-17). Hot gases can be directed to the additional pots by a flue, or internal passageway. Hot, light gases rising in the chimney draw hot air through the stove.

Fig. 5-17

How heat is made useful in a stove

The preceding discussion outlined the theoretical principles operative in any cookstove. In this section, the structural design principles of a generalized stove are explained (Fig. 5-18).

Fig. 5-18: Parts of a stove

Firebox

The firebox is where the fire is contained, and combustion occurs.
- It must be large enough to accommodate the size and type of fuel used, but narrow enough to confine the fire beneath the pot (Fig. 5-19).

Fig. 5-19

- The cookpot should be as close to the heat source as possible, to take full advantage of radiant heat, yet not so close that it smothers the fire.

Firebox entrance

The firebox entrance is where fuel is fed into the stove, and where air enters. The size and placement of the entrance will affect the structural arrangement of the fire.
- The firebox entrance should be wide enough to permit easy access to the fire, and allow a cries-cross placement of fuel pieces. With some designs, two or three smaller entrances might work well (e.g. the Louga stove; see Chapter 7).

Dampers

Dampers are doors which control the flow of air. A front damper, placed before the fire, reduces airflow into the firebox. A back damper, placed in the flue downstream from all the pot holes or in the chimney, controls draft through the stove. Dampers can be made of sheet metal, clay blocks, or concrete blocks.
- They should close as tightly as possible.
- The front damper should be designed to focus air on the base of the fire. This improves overall combustion (Fig. 5-20).
- Back damper doors further control air flow by reducing draft from the chimney.
- After the fire dies both dampers can be closed to conserve heat (Fig. 5-21).

Fig. 5-20; Fig. 5-21

- Dampers are important. They should be permanently secured to the stove so they cannot be lost.

Baffles

Baffles are obstructions to the flow of hot air and gases. They increase turbulence and direct airflow around the bottoms of the pots.
- There should be baffles under all pots not over the firebox.
- The space between the bottom of the cookpot and the baffle should be the minimum required to maintain adequate draft.

Chimney

The chimney carries smoke out of the kitchen. The force that pulls smoke through the chimney is called draft; it occurs because the hot gases from the fire are lighter than the surrounding air, and therefore rise, drawing hot air through the stove. The pull of the chimney also draws cold air through the firebox entrance, and any open spaces or cracks in the stove.
- The draft should be strong enough to aid combustion and draw the smoke up the chimney.
- Too strong a draft may draw in excess air which dilutes the heat of exhaust gases.
- Draft increases with the height and diameter of the chimney.
- The chimney should extend 75 cm above the highest point of the roof, for safety and to prevent downdrafts. (Downdrafts may occur with the air turbulence that results when wind flows over a house.)
- The chimney should have a cap to keep rainwater out (Fig. 5-22).
- It the chimney is near any flammable material (e.g. a thatched roof) it should have a screen covering to prevent sparks from flying out.
- Both rain cap and spark screen should be removable to permit regular cleaning of the chimney.
- Where the chimney penetrates a combustible roof or wall, the chimney should have a nonflammable spacer around it. If the chimney becomes very hot (as in a flue fire) heat conducted through a metal spacer might cause a fret It may be wise to avoid metal and use a less conductive chimney material such as clay pipe, or to leave an air space around the chimney where it penetrates the roof.
- The chimney must be cleaned regularly because a black, sticky substance called creosote condenses inside the chimney. Creosote is flammable and can catch fire from a spark. In addition to being a fire hazard, it can clog up the chimney. The chimney should be cleaned at least every six months.

Fig. 5-22