Intermittent Kilns 5r192r

Intermittent Kilns Intermittent kilns work by firing cool wares using a heat source, where the temperature is slowly increased throughout the firing process . Traditionally, intermittent kilns were nothing more than a trench drug in the ground filled with a fuel source and unfired pots. Intermittent kiln may be either rectangular circular or oval shaped. Types of intermittent kilns: There are two types of intermittent kilns: 1- Up draught intermittent kiln 2- Downdraught intermittent kiln 3- Clamp 4- Scove

1)

Up draught intermittent kiln:

Updraft kilns are those in which the flame is introduced into the bottom of the kiln, at or below floor level, and exhausted out the top. Updraft kilns consist of three basic components: the firebox, the damper, and the stack area. The firebox is where the flame enters. The damper is at the top of the kiln and controls the exhaust (and by association, the kiln's atmosphere). The stack area is where the pots are set and is between the firebox and damper. Although an updraft kiln tends to be less fuel efficient than a downdraft kiln, most commercially built fuelburning kilns are updrafts. This is mainly due to their internal structure of up draught Simplicity to build, packs, and ship. kiln Disadvantages of up draught kiln: Up draught kiln have following disadvantages:1- Burning of bricks is not uniform. 2- The supply is not continuous. 3- There is a loss of heat from top most layers of bricks. 4- There is waste of fuel heat as the kiln is to be cooled down. 5- The cost of manufacture is more.

2)

Downdraught intermittent kiln:

Downdraft kilns are designed to force the flame and heated air to circulate through the kiln. Flame is introduced at the bottom of the kiln and naturally flows upward. The construction forces the flame back downward, to exhaust at the bottom of the kiln. Downdraft kilns consist of four main components: the firebox, the stack area, the damper, and the chimney. The addition of the chimney helps create draw, or air flow. Sprung arch downdraft kilns are probably the commonly potter-built kiln type in the United States.

Internal structure of down draught kiln

Advantages of down draught intermittent kiln: The following are the advantages of down draught kiln over the up draught intermittent kiln: 1- The burning of bricks is less or more uniform. 2- The thermal performance is much better and heat loss is much less. 3- Heat can be controlled in a better way and hence such kilns are most useful for burning of structural clay tiles, terra - cotta etc.

3)

Clamp:

Clamp kiln

One of the oldest methods of firing bricks is by clamp. It is 4000 BC old. A clamp is a temporary construction of unfired or green bricks which is dismantled after firing and could be erected near the clay source. Clamps varied from yard to yard but there were general rules which most followed. The floor had to be level and was made of burnt brick. Channels were often made in the floor and filled with fuel, usually breeze but any fuel would suffice and wood, furze, charcoal was also used.

Next came three or four layers of green bricks which were placed on edge and then another layer of fuel was added. After this, green bricks were packed closely together to a height of 14 or 15 feet. The bricks were 'dished' or tilted inward to prevent injury to workmen during firing. Sometimes the outside was sealed with wet pug. Most clamp bricks had a small percentage of breeze added to the clay during manufacture. This helped to 'self fire' them and ensured that a good temperature was reached. Clamps contained 30,000 to 150,000 bricks. An average size would take two or three weeks to burn out, although larger ones could take as much as ten or twelve weeks.

4) The scove kiln: A widely used adaptation of the clamp is the scove kiln, also mistakenly called a clamp. If the fuel available is of a type which cannot be spread as a thin bed at the base of the kiln and/or is not in sufficient quantity to burn all the bricks without the need for replenishment, tunnels can be built through the base of the pile in order to feed additional fuel. This is a suitable method of burning wood, the latter being one of the most frequently used fuels for small-scale brick making in developing countries. Usually, the outer surface of the piled-up bricks is scoved, that is to say plastered all over its sides, with mud. Thus, the name of the scove kiln. The construction of a scove requires a level, dry area of land. Previously fired bricks, if available, lay bed face down to form a good, flat surface. Three or four layers of bricks are used to form the bottom of the tunnels. The width of each tunnel is approximately equal to that of two brick lengths. Three lengths of bricks separate the tunnels. Alternate courses are laid at right angles to each other (i.e. a course of headers, followed by a course . Two short tunnels A coal fired scove clamp (e.g. approxima tely 2 m long) may be sufficient for a small number of bricks. For large numbers of bricks, tunnels cannot be longer than approximately 6 m.

otherwise, fuel inserted from both ends will not reach the centre of the tunnel. Large numbers of bricks are dealt with by extending the number of tunnels to cope with the requirement. The fourth and successive courses of bricks are laid in such a way that rows of brickwork finally meet, and tunnels are thus completed. The progression of the early stages of construction of a scove is shown in figure VII.6. In the foreground, a few courses of fired bricks are set, marking out the tunnel positions. In the middle of the picture, the first corbelled-out course of green bricks is partly set, while further back several courses are laid. Green bricks are set above tunnel level, in alternate courses of headers and stretchers up to a height of at least 3 m above the ground. At the edge of the scove, each course is stepped in a centimeter or so, to give a sloping side. Small spaces are left between the bricks to allow the hot gases from the fires to rise. The required maximum spacing between bricks is a ‘finger width’. This is easy to achieve although a narrower spacing may be satisfactory. As the scove is built up, an outer layer of previously burnt bricks is laid, to provide insulation. This will also allow the proper firing of the outer layers of green bricks. On the top of the green bricks, two or three courses of previously fired bricks should be laid, bed face down and closely packed. The whole structure should then be scoved with wet mud to seal air gaps. Turves are sometimes laid on top to reduce heat losses. The wet mud should not contain a high fraction of clay if cracks are to be avoided during firing. Some of the top bricks half-way between the tunnels must not be scoved so that they may be lifted out to increase air flow through the kiln as required. The provision of this adjustable ventilation can be most useful in controlling the rate of burning. Firewood is set into the tunnels for firing. It should preferably be at least 10 cm across, in pieces about 1 m in length. Kindling should be set in the mouth and bottom of the tunnel. Since the heat of the fire is to rise up into the bricks, it is essential that strong winds do not blow through the tunnels, cooling bricks down, and wasting heat. Such winds may increase fuel consumption by 25 per cent. A number of measures may be taken to avoid this waste of heat, including the blocking of the centre of the tunnel during construction, or the temporary blocking of tunnel mouths with bricks. In the latter case, one end may be bricked up and fire set at the other end. Once the fire is well alight, that end may be bricked up while the previous one is opened and lighted.