III. Kiln designs
In general, large kilns are more economical on the use of fuel than small kilns as less heat is lost through the proportionally smaller outside area of the kiln. Thus, separate teams of brickmakers may use the same large kiln on a co-operative basis, and thus benefit from lower firing costs.
There is a wide variety of kiln types and sizes. These may be split into two major groups: the intermittent kilns and the continuous kilns.
Intermittent kilns are filled with green bricks which are first heated up to the maximum temperature and then cooled before they are drawn out from the kiln. Thus, the kiln structure is also heated during the process.
Consequently, all the heat within the bricks and kiln is lost into the atmosphere during cooling. Intermittent kilns are very adaptable to changing market demands, but are not the most fuel efficient. They include the clamp, scove, scotch and downdraught kilns.
The continuous kilns have fires alight in some part of them all the time. Fired bricks are continuously removed and replaced by green bricks in another part of the kiln which is then heated. Consequently, the rate of output is approximately constant. The continuous group of kilns includes various versions of the Hoffmann kiln, including the Bull’s trench, zig-zag and high draft kilns, and the tunnel kiln. The latter is a capital-intensive, large-scale continuous kiln, which is outside the scope of this memorandum.1 Continuous kilns utilise heat from the cooling bricks to pre-heat green bricks and combustion air, or to dry bricks before they are put into the kiln. Consequently, continuous kilns are economical in the use of fuel.
III.1 The clamp
The clamp is the most basic type of kiln since no permanent kiln structure is built. It consists essentially of a pile of green bricks interspersed with combustible material. Normally, the clay from which the bricks are moulded also includes fuel material. The clamp kilns were commonly used in the United Kingdom. Some of these, containing one and a quarter million bricks, are still used for the production of bricks of various colours and textures.
It is possible to use a variety of burnable waste materials in brick clays (e.g. sifted rubbish, small particles of coke, coal dust with ashes, breeze). In countries where timber is produced, large quantities of sawdust may be mixed with clay before firing. This will reduce the expenditure on the main fuel for burning the bricks. Waste materials should be of relatively small size and should not exceed in weight 5 to 10 per cent of the total mixture. Otherwise, the clay will become difficult to mould or the finished product may become too weak or too porous. Furthermore, the added fuel material should be thoroughly mixed with the clay.
A flat, dry area of land is first chosen, and a checkerwork pattern of spaced out, already burnt bricks laid down over an area of approximately 15 m by 12 m. Fuel in the form of coke, breeze or small coal2 is then spread between the checkerwork bricks, covering the latter with a layer at least 20 cm thick. Dry, green bricks are next closed-laid on edge upon this fuel bed.
A clamp is generally made up of approximately 28 layers of bricks. Its sides are sloped for stability (see figures VII.1 and VII.2). Three or four holes1 at the base of one of the clamp walls are formed in order to allow the initial ignition of the fuel bed. Two courses of already fired bricks are next laid on top of the green bricks for insulation purposes. Fired bricks are also laid against the sloping sides of the clamp as it progresses. Sometimes, a second thin bed of fuel is laid at a higher level in the clamp(51).
Once several metres of the length of the clamp have been built up, the fuel bed may be ignited with wood stuffed into the eyes of the clamp. The latter are bricked up with loosely placed burnt bricks once the fuel bed is alight. As the fire advances, more green bricks are built into the clamp.
During burning, the heat rises through the bricks above, fumes and sometimes smoke leaving the top of the clamp. The rate of burning is not easily controlled and depends upon several factors, including wind strength and direction. Some wind protection with screens can help control the temperature. Ventilation, and hence burning rate, can also be controlled, to some extent by an adjustment of the burnt brick covering the top of the clamp. For example, these bricks may be spaced out or removed in order to speed up the firing of the bricks in a given area. Conversely, they may be tightened up or covered with ashes in order to slow down the burning rate. It is desirable to have the fire advancing with a straight front at a steady rate. Bricks close to the edges of the clamp will tend to be underfired as a result of higher heat losses. This may be partially rectified by placing a little more fuel near the edges of the clamp. Extra fuel may also be spread between the top bricks during firing.
The firing process is indicated by the sinking of the top of the clamp. Under the right circumstances, the latter will settle evenly. Once the fire has passed through a particular point, the bricks start to cool. They may then be withdrawn, sorted into various grades, and sold. Thus, bricks within one clamp are set and drawn simultaneously. After a number of weeks, the fire reaches the end of the clamp. Before then, construction and lighting of a new clamp may be started as previously, if market demand requires it.
If enough air flows through the bricks during firing, the oxidising process will give them a red colour. Where air is scarce, reducing conditions due to the gases from the burnt fuel will yield orange or yellow bricks, especially if a limy clay is used for moulding. Variations in colours will be normal even on a single brick face.
As the fuel is in close contact with the green bricks, the fuel efficiency of a large clamp of 100,000 to 1 million bricks can be fairly high (e.g. about 7,000 MJ per 1,000 bricks). Smaller clamps will be less efficient, as a result of the greater proportion of outer cooling area for a given volume. However, they may be operated successfully with only 10,000 bricks. For low production rates, it is only necessary to fire a clamp occasionally and have the bricks in place until they are sold.
The bricks near the centre of the clamp will be the hardest. Others should be sufficiently good for many uses. They should be sorted for sale as best, “seconds” and soft-burnt bricks. However, 20 per cent of the bricks may still not be saleable. Fortunately, many of these rejects can be put into the next clamp for refiring, or used in the clamp base, sides or top.
III.2 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 (figure VII.3). This is a suitable method of burning wood, the latter being one of the most frequently used fuels for small-scale brickmaking 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 (figure VII.4). 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 of stretchers). Two short tunnels (e.g. approximately 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. Figure VII.5 illustrates the construction of a four-tunnel scove.
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 centimetre 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 (figure VII.7) 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. Thereafter, the fires may be controlled by bricking up tunnel mouths with loose bricks and adjusting the vents on top. As fuel burns away, it must be replenished.
As with all kilns, heat must be gentle at first until all the water in the bricks is driven off. Adequate air flow is therefore essential to remove the steam produced. Thus, the vents should be open, and the fires kept low so long as steam is seen to rise from the top of the scove. This water smoking period may last several days.
Once the water smoking stage is completed, the fires may be built up gradually to increase temperatures up to a maximum over a period of a few days. A maximum temperature is indicated by the charring of dry grass or paper thrown on top of the scove, or the appearance of a red glow by night. The vents should be closed with fired bricks well before the maximum temperature is attained in order to regulate the burning rate and, thus, help to even out the temperature amongst the bricks. The maintaining of this temperature for several hours (i.e. soaking stage) requires a last charge of fuel, the closure of the tunnel mouths and the sealing of closed vents with mud.
The scove should be left to cool naturally for at least three to four days. Then if necessary some bricks may be removed from the outside to speed the later stages of cooling. Subsequently, the bricks may be left in position until sold. Before collection or despatch, under- and over-fired bricks must be discarded and the remainder, if of variable quality, should be sorted out into good quality, ‘seconds’ and soft-fired bricks. Rejects may be incorporated in the next scove.
Although wood is generally the fuel used in scoves, oil-burners are used in some countries. Coal, which is also an alternative fuel for scoves, requires a special grate at each end of the tunnel mouth, and is therefore more appropriate for firing in permanent kilns.
The fuel efficiency of scoves is low, 16000 MJ of heat being required per 1,000 bricks for a typical African scove (10, 52). A square scove has a smaller cooling area than a rectangular scove, for a given number of bricks. However, it will require a relatively longer tunnel which may exceed the allowed length for proper lighting of the scove. Thus, small kilns could be square while larger ones may need to be rectangular.
In order to increase the heat efficiency, the height of a scove should be as great as possible, so long as saleable bricks are obtained from the top. Safety must be borne in mind, however, as high scoves tend to be unstable as a result of shrinkage of bricks during firing. Moreover, a high setting complicates the placing of green bricks on top courses, and increases the risk of accidents. Figure VII.8 shows a high scove of approximately 60,000 bricks, after firing and stripping of the outer bricks.
A scove may be built for firing a few thousand bricks only, but will be less fuel efficient than larger scoves.
III.3 The Scotch kiln
The Scotch kiln is similar to the scove, except that the base, the fire tunnels and the outer walls are permanently built with bricks set in mortar (figure VII.9). The kiln itself has no permanent top, green bricks being set inside the kiln, as shown on the extreme left of figure VII.9. Much basic construction work is thus saved. A plane and section of a seven-tunnel Scotch kiln is shown in figure VII.10. Walls on either side are buttressed, and corners are massively constructed. Access into the kiln is through a doorway in the end walls. This doorway is filled temporarily with closely laid bricks (without mortar) during kiln operations. In some kilns the whole end wall is temporarily erected (40).
Wood is often used for firing the kiln, although oil burners or coal grates may also be installed. The sink of the bricks - after shrinkage - is more easily measured than in the clamp or scove kiln, since the fixed position of the permanent side walls may be used as a reference point. The sink gives an indication of the firing process within the kiln.
The advantages of the Scotch kiln over other permanent kiln structures are its simple design and easy erection. Setting and drawing of bricks are also simple.
The Scotch kilns, like the clamp and scove, are updraught kilns. They have been widely used in developing countries. Their chief failing is the irregular heating and consequent large proportion of under- and over-burnt bricks. This is especially true for clays with a short vitrification range as they cannot be fired without a good temperature control.
Fuel consumption of the order of 16,000 MJ per 1,000 bricks is generally the norm for Scotch kilns (8, 42).
III.4 The down-draught kiln
In the down-draught kiln, hot gases from burning fuel are deflected to the top of the kiln which must have a permanent roof. They then flow down between the green bricks to warm and fire them. The green bricks rest either upon an open-work support of previously fired bricks (figure VII.12) or upon a perforated floor through which the warm gases flow. These gases are then exhausted through a chimney outside the area of the kiln after passage through a flue linking the kiln floor to the chimney. The warm gases rising through the height of the chimney provide sufficient draught to pull the hot gases down continually through the stack of green bricks.
The down-draught kiln is more heat efficient than the up-draught kiln described earlier. It can be used for various ceramic products (e.g. drainage pipes and tiles of various types) in addition to the firing of bricks. The kiln can be operated at high temperatures and may then be used for the production of refractory ware.
Circular down-draught kilns may be built in place of rectangular kilns. They are stronger than the latter, but require reinforcement with steel bands to keep the brickwork from deteriorating through periodic cooling and heating. Rectangular down-draught kilns are more simple to build, although they require also steel tie-bars as a reinforcement. They however have the advantage of being easier to set with green bricks than circular kilns. Figure VII.11 shows the ground level plan of a rectangular down-draught kiln of massive construction, with 14 grates for burning fuel. A number of grates stocked with lighted wood are shown in figure VII.13. The grates may be prefabricated from iron bars, as indicated in figure VII.11. A “flash” wall is built behind the grates to keep the flames off the nearby green bricks. The wall in the figure is continuous. Alternatively, separate “bag” walls can be built around the back of each fire (see figure VII.12 right-hand side). The continuous wall tends to even out the heating effect.
Hot gases rise to the arched crown of the kiln and are drawn down between open set bricks (figure VII.12) by the chimney “suction”, through the perforated floor (shown in the figure) along its centre line. There should be a few small holes at the base of the flash wall, in the underground flue in order to ensure the burning of bricks near the bottom of the wall. A metal sheet damper is available near the bottom of the chimney in order to vary the flow of gases and exercise control over the operation of the kiln. The control of air flow is achieved by the use of metal doors. These should be thick enough to avoid distortions (figure VII.14).
Entrance to the kiln is through small arched doorways (figure VII. 15) referred to as “wickets”. These are bricked up temporarily during firing.
The height of downdraught kilns should not be too great, as it is difficult and time consuming to set bricks at heights that may not be easily reached by workers.
Down-draught kilns may hold from 10,000 to 100,000 bricks. The one shown in figure VII.15 takes 40,000 bricks.
Fuel consumption depends greatly upon the condition of the kiln, the manner of setting the bricks and the control of the firing process. For example, damp foundations absorb heat, and a badly fitting damper may waste fuel. The loss of heat varies from one type of kiln to another. Large kilns consume less fuel than small kilns, for a given number of bricks. An under-filled kiln loses as much heat as one properly filled. An over-filled kiln prevents the passage of hot gases, and this requires a longer burning cycle. Too much draught allows more heat to be wasted up the chimney. Given the above varying circumstances, the heat required by downdraught kilns varies from 12,000 to 19,000 MJ per 1,000 bricks. An exact estimate of heat consumption requires an in-depth study of the characteristics of the kiln (5, 24, 33).
III.5 Original circular Hoffmann kiln
The Hoffmann kiln is a multi-chamber kiln where the air warmed by cooling bricks in some chambers pre-heats the combustion air for the fire, and exhaust gases from combustion pre-heat the green bricks. The main advantage of this kiln is its particularly low fuel consumption rate.
The original Hoffmann kiln was circular (see figure VII.16) and built around a central chimney. An arched-top tunnel surrounds the chimney at a distance of a few metres, and is connected to it by 12 flues passing through the brickwork between the tunnel and the chimney. Each flue can be closed off by dropping a damper. Entrance into the tunnel is through any one of 12 wickets. During operation most of the kiln’s tunnel is full of bricks either warming, being fired or cooling.
A typical condition of the kiln is shown in figure VII.16. All but two neighbouring wickets are closed. Cold fired bricks are drawn from one part of the tunnel adjacent to one of the open wickets and dry green bricks are set by the other wicket. Cold air flows to the warm chimney through both wickets. This air cannot pass through the recently set bricks as they are sealed off with a paper damper across the whole width of the annular tunnel. The air flows through the bricks which are drawn, into warm bricks further down the kiln (counter-clockwise in the figure) close to the fire. As the air flows counter-clockwise, its temperature rises through contact with increasingly hot bricks. The air is thus pre-heated and ready for efficient combustion in the firing zone of the kiln where fuel is fed in through closeable holes in the tunnel roof. Thus, little fuel is consumed for heating the combustion air. The latter also performs the useful task of cooling bricks for drawing, thus making kiln space available on a relatively short time. The hot products of combustion cannot be vented straight to the chimney through the nearest flue, as the latter is closed (this is indicated by a dot in the circle centre in the figure). Instead, the hot gases pre-heat unburnt bricks. Thus, less fuel is required at the firing stage in order to get the bricks to the maximum temperature. Next, the cooled gases flow through recently set green bricks, bringing the latter to the water smoking stage. These bricks are sufficiently warm to exclude the forming of condensation. Figure VII.16 shows the gases leaving from the open damper (no dot in the circle). Subsequently, this damper is closed, the next one (counter-clockwise) is opened and the bricks marked “set” start the water smoking stage. The fuel feed, and the drawing and setting operations, are also moved counter-clockwise at this stage1. Once the part of the tunnel marked “setting” has been filled with green bricks up to the next flue, a paper damper is pasted over the bricks and the wicket (counter-clockwise) is then broken down and cooled fired bricks are withdrawn. The paper dampers can be torn open by reaching through the fueling points with a metal rod.
Figure VII.16 also shows a sectional drawing of the Hoffmann kiln. Bricks in the firing zone are on the left-hand side of the figure, and the empty part of the kiln - between drawing and re-setting - including the closed flue damper, is on the right-hand side. A roof covering protects the kiln from adverse weather. Wickets are shown as two thin walls, separated by an air gap. Thus, heat is kept in the kiln without the need for expensive building work at the wickets.
In the original Hoffmann kiln, fuel fed through the roof falls into hollow pillars formed by bricks set for firing. Ash from the fuel causes some discoloration of the bricks.
The tunnel is subdivided into 12 notional chambers which are identified by the flue positions. Each chamber is approximately 3.5 m long and 5 m wide. The height of each chamber is restricted to about 2.5 m for easy working conditions.
Daily rate of production from such a continuous kiln is at least 10,000 bricks.
The advantages of the original Hoffmann design include the identical chambers, the fairly short flues and low fuel consumption (2,000 MJ per 1,000 bricks(8)).
III.6 Modern Hoffmann kilns
Increased demand for bricks in industrialised countries require the erection of substantially larger kilns than those originally designed by Hoffmann. Consequently, the original circular kiln has been modified for the following reasons:
Under these circumstances, the original design was modified into the so-called elliptical Hoffmann kiln, which has long straight walls and a few curved chambers at the end (see figure VII.17). The operating principle is exactly the same as that of the original design. The main difference relates to the larger number of chambers available in the elliptical design.
The operation of the modern elliptical Hoffmann kiln may be summarised with reference to figure VII.17. It includes the following sequence of events1:
In this type of kiln, gases are too cool for water smoking. As they carry much water vapour, there is a risk of spoiling green bricks by condensation (e.g. softening of bricks, surface cracking, and scumming by salts deposited from the products of combustion). Overcoming these problems - which may arise with exhaust gases at less than 120 °C and which are present to some extent in the original circular Hoffmann kiln - requires a second flue which connects all the chambers. Any of the latter may be connected or disconnected to this so-called hot air flue by opening or closing dampers in the same manner as for the main flue connection. In figure VII.17, the hot air flue is regarded as being in the central island of the kiln. Some of the warmed fresh air is taken off by the chimney suction applied to the flue. It is provided by chambers 3 and 4 where bricks are still hot, passed down the hot air flue, then into chambers 14 and 15 where drying, or water smoking takes place. Hence the drying is done with clean warm air, containing no moisture or products of combustion. This air then flows from chambers 14 to 15 into flues (where damper are open) and is exhausted by the chimney. Meanwhile, fresh green bricks are set in chamber 16.
The production rate of most Hoffmann kilns is approximately 25,000 or more per day, a too large output for the type of brickworks considered in this memorandum. However, small kilns can be built to produce only 2,000 bricks per day(8). Figure VII.18 shows hollow clay blocks set within an elliptical Hoffmann kiln with a capacity of about 10,000 ordinary size bricks per day. This is an interesting kiln since wood is used for firing in place of coal (figure VII. 19). A wide variety of agricultural wastes may also be used in place of wood in the top-fed kiln shown in figure VII. 19. For example, sawdust has been used in Honduras (10).
Fuel consumption of elliptical Hoffmann kilns vary according to the kiln condition and method of operation, as mentioned in the previous section. It is estimated at approximately 5,000 MJ per 1,000 bricks.
VII.7 Bull’s Trench kiln
A large fraction of the cost of construction of the Hoffmann kiln is in the building of the arch of the long tunnel, and in the provision of a chimney, with connecting flues and dampers. Thus, the idea behind the design of an archless kiln by a British engineer (W. Bull) in 1876.
As with the two types of Hoffmann kiln, the Bull’s trench kiln may be circular or elliptical. Both forms have been widely used throughout the Indian subcontinent. Construction of this type of kiln is briefly described below.
A trench is dug in a dry soil area which is not subject to flooding. It is approximately 6 m wide and 2 to 2.5 m deep.1 Alternatively, especially if the soil is not sufficiently dry, the trench may be dug to only half of this depth, while excavated material is piled up on the trench side, and held out off the trench by a brick wall starting at the bottom of the trench (figure VII.20). The total length of the trench is approximately 120 m. It is so constructed as to constitute a continuous trench.
When in operation, the Bull’s Trench is full of bricks warming, being fired or cooling. Cooled bricks are drawn and new green bricks are set, while the fire is moved progressively around the kiln. The exhaust gases are drawn off through 16 m high moveable metal chimneys with wide bases, which fit over the openable vent holes set in the brick and ash top of the kiln. These chimneys are guyed with ropes to protect them from strong winds. The type of chimneys shown in figure VII.21 require six men to move them. This figure also shows the method of fueling whereby small shovelfuls of less than 1.5 cm size coals are transferred from storage bins on top of the kiln, and sprinkled in amongst the hot bricks through the removable cast-iron feed holes. Metal sheet dampers are used within the set bricks to control draught.
Figure VII.22 shows the sequence of events diagramatically. The setting of the bricks within the kiln must be such as to allow sufficient air flow between the bricks and wide enough spaces for the insertion and burning of fuel and the accumulation of ashes. However, the whole setting must be sufficiently strong and stable to ensure safe operation of the kiln. The setting in figure VII.20 shows occasional cross link bricks, between the separate bungs or pillars of bricks, tying bungs together. Information on the firing of these kilns is available (54) and kiln designs are standardised(55).
Modifications to this type of kiln have involved the provision of flues from the trench so that chimneys can be moved on rails located on the centre island rather than over the setting bricks in the kiln. A major problem is the corrosion of the mild steel chimneys normally used. They may rust through within only a few months. Accordingly, some kilns have been redesigned with dampers opening to flues connected to a permanent brick chimney.
The whole Bull’s Trench kiln is very large, a normal output being 28,000 bricks per day. With a narrow trench output could be reduced to 14,000 bricks per day. It is not possible to shorten the trench as this will affect the heat transfer efficiency. The depth of the trench cannot be reduced either without impairing the firing behaviour. The kiln would be very big to roof over, and is most suited to dry weather conditions. The chief advantage of this type of kiln is its low initial construction costs.
Fuel consumption is much better than in intermittent kilns, 4,500 MJ being required for firing 1,000 bricks (10, 22, 56). About 70 per cent first-class bricks can be obtained, the remaining bricks being of poorer quality.
III.8 Habla kiln
The effective tunnel length of the Hoffmann type kiln may be increased by the building of zigzagged chambers. The resulting kilns, known as the zigzag kilns, have a faster firing schedule than the Hoffmann kiln. However, they require a fan - and therefore electrical power - as air must travel a longer path and a simple chimney does not provide sufficient draught for air circulation. Fans provide a more steady draught than chimneys and can be better controlled. They allow a larger transfer of heat to the water-smoking stage, thus saving fuel. However, it is best to avoid condensation on fan blades and subsequent corrosion of the latter by having gases extracted at 120 °C. This is especially important if fuel or clay contain sulphur compounds such as pyrites which are transformed into sulphuric acid in the kiln gases.
The zigzag kiln developed by A. Habla is an archless kiln. One additional simplifying feature of this kiln is that the zigzagging walls are temporary structures of green bricks which may be sold after firing. Habla kilns are of various designs: in some kilns the flues are returned from all chambers to the central island while in others, some of the flues are returned to the outer walls. Figure VII.23 illustrates the former type of kiln. For simplicity, it omits the hot air flue which can be carried above the main flue for providing clean drying air.
The Habla kiln is rectangular, but close to a square. The one illustrated in figure VII.23 has chambers numbered 1 to 20, every second chambers being accessible through a wicket. Partition walls of dried green bricks, with a thickness of only one brick length, are alternatively built out from the central island and the outer wall. These partition walls deflect the gases from the island to the outer wall, through the wide-set bricks between the partitions. As the temperature of any particular chamber rises, the wide-set bricks are first heated. Then, as bricks in the partition shrink a little, draught through the partitions increases, and the bricks in that partition as well as the wide-set ones become fired.
In figure VII.23, chambers 1 and 2 are empty. Bricks are drawn from chamber 3. Air passing through the open wickets is warmed as it cools bricks in chambers 4 and the following ones. After the firing zone, the exhaust gases preheat bricks, flow out of chamber 15 through the chamber flue and open damper (shown in the section drawing), and enter the main flue from where they are expelled through a short chimney by the fan. Bricks in chambers 16 to 19 are water smoked by clean warm air from chambers 4 to 6. Paper dampers are used as in the case of the Hoffmann kiln.
An interesting modification to the layout shown in figure VII.23 is to build a pair of short partitions in line with each other, approaching from the island and outer wall, but leaving a gap in the middle. Secondly, a wall may be built in the middle of the kiln, separated by two gaps from the island and outer walls. The fire can then travel along two paths simultaneously, around both ends of the second partition, through the central gap of the third, then around both sides of the fourth and so on. This modification should help speed the rate of firing.
Large Habla kilns, producing 25,000 bricks per day (57), have been built in a number of countries. Recently, in India, a 24-chamber high-draught kiln of this type has been developed (58) for an output of 30,000 bricks per day. It is fired with coal and wood. Figure VII.24 is based on published information on this Indian kiln. The latter may be reduced in size for a production of 15,000 bricks per day. Roofed zigzag kilns may also be built for as few as 3,000 bricks per day (8).
The Habla kiln is economical to construct and operate. It has a larger capacity relative to its area than other continuous kilns. This feature reduces the costs of land and construction. Furthermore, the kiln has a long firing zone, allowing difficult clays to be fired more easily. The long-firing path assists heat exchange between gases and bricks, thus improving fuel efficiency. Because partitions are of green bricks, less permanent brickwork has to be heated and cooled, thus adding to fuel efficiency. The shrinkage of bricks in the partitions and consequent leakage of hot gases shortens the distance travelled by the latter. Thus, less power is needed to drive the fan. As a result of partition leakage, the kiln has relatively few “dead spaces” where heat is insufficient to fire bricks properly. The building of partitions of green bricks at the start of each operation does not increase labour costs since the bricks are removed for sale and may thus be regarded as part of the whole setting. Another advantage of the Habla kiln is the easy access to the structure.
Fuel consumption of a zigzag kiln is estimated at 3,000 MJ per 1,000 bricks(57). Consumption in the high-draught Indian archless kiln is also approximately 3,000 MJ per 1,000 bricks.
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