Continuous kilns for the firing of ceramic products are not a modern invention. The early Chinese potters often built their kilns as a series of chambers in the side of a hill. The lowest chamber was lit first and the heat of combustion ascended through the chambers higher up the hillside. Similarly, when a kiln chamber had been fired, it was cooled down by air drawn through it, which was subsequently used in the drying of the ware and combustion of the fuel in the chamber higher up. With the move away from firing with stationary fireboxes to that of fuelling kilns through holes in the roof, the fire could be made to move through the kiln. The Chinese climbimg kilns, and much later the big cross-draught intermittent kilns, like the Kassel and Newcastle, were the forerunner of the continuous kilns.
The development of continuous kilns, of which Hoffmann kilns are probably the best known and most widly used, began around 1849. Mason Arnold zu Frstenburg first constructed an annular kiln, in which the chambers were arranged in a circle around a central chimney. At the same time, Maille at Illeneuve le Roi also constructed an annular kiln working on the same psincipal. In England, Joseph Gibbs patented a top firing continuous annular kiln on the 29th of April 1841. Unfortunately, none of these first kiln designs proved to work very well, but the idea of a kiln with a continually moving firing zone had been introduced, and it was only a matter of time before this method of firing was made a practical reality.
In 1856, there was the publication of a paper about another annular continuous kiln by a Berlin Master Builder, Friedrich Hoffman and a Viennese City Councillor, A. Licht, that heralded a new and exiting phase in the history of brick firing technology.
The first annular kiln built according to the Hoffman principal went into operation at Scholwin near Stettin on the 22nd November 1859. In 1870, there were already 331 such kilns in Prussia and a total of 639 Hoffman kilns throughout the world. The main reason for such a fast dissemination of Hoffmanns technology was th two thirds fuel saving over the intermittent and semi-continuous kiln that were used at the time.
The first Hoffmann kilns were in the form of a great circular ring chamber, with massive walls and a large chimney at the centre, to which underground radial flues converged from the inside walls of each of the twelve chambers. The chambers were barrel arched (like a railway tunnel), and in the roof arches there were several small feed holes through which fine coal could be fed into spaces made among the bricks to be fired. Around the outer wall or the kiln were the twelve openings for loading and unloading the individual firing chambers. These chambers were seperated from each other by very large metal dampers, that could be raised and lowered as the fire moved around the kiln from chamber to chamber.
Once the kiln is lit it is allowed to go out, and the sequence of operation is continuous. With the kiln in full operation two chambers will be open, and the other ten sealed up at the door and by the interconecting steel dampers. If the chambers are numbered from 1 to 12, then bricks are being unloaded from 2 and loade in 1. The damper is closed between 1 and 12, but open between all other chambers. Air is drawn through the open door of 2 and through the bricks in 3, 4, 5, and 6, cooling them down and at the same time being heated itself.
The temperatures of the chambers increase from 3 to 6, with the temperature at 6 being close to the firing temperature.
Chamber 7 is being fired, with fuel being fed at intervals through the roof, which is immediately ignited by the hot air from 6. The products of combustion pass on to 8, 9, 10, and 11 drying and pre-firing the bricks in these chambers. In 8, the bricks are at the pre-firing stage, and in 11 they are going through the water smoking stage. From 11 the combustion gases pass through the flue and up the central chimney. All the other 11 flues from the other chambers are closed off with dampers. At regular intervals, the firing zone is moved forward and the corresponding changes made to the dampers between the chambers and into the chimney. The chambers being loaded and unloaded move forward in sequence, and this way heat is extracted from the cooling bricks and also from the hot combustion gases.
The original circular Hoffmann kiln
The original round Hoffman kiln is no longer in use, having been replaced by the more modern version, which takes the form of two parallel tunnels built side by side, connected by curved tunnels at either end. With such an arrangment, the chimney is built outside the kiln structure and may be connected to more than one kiln. Sixteen chambers are about the minimum for effective working; twenty-two chambers are preferable. The original Hoffmann was superseded, because it had such a large heat absorbing mass, and the tapering firing chambers were small and unnecessary complicated to load, while the very large damper between chambers was cumbersome and awkward to operate. This damper was replaced by the pasting of a paper or fabric screen between the sections of the firing tunnel as it is loaded. The screen seals the tunnel at this point and prevents cold air being drawn the wrong way round the kiln to the firing zone. The screen is destroyed by the approaching firing zone, at just the right time, as it moves around the kiln.
There are now many different designs of continuous kiln, based on the moving firing zone perfected in the Hoffmann kiln, but the basic principal of the moving firing zone, coupled with continuous loading and unloading, remains the same.
The path of fire travel in the original Hoffmann was circular, later distorted into an ellipse, the arch of the tunnel being supported on the side walls. In order to increase output, continuous kilns with longer circuits through which the firing zone travels more rapidly are built. Two firing zones running simultaneously are possible on the larger kilns. To save space, the firing circuit is bent into a Zig Zag form. Other designs, in which the circuits are built in the form of a T, Y or X, with the chimney in the centre, are also used. In principal, they do not differ from the Hoffmann Kiln, but with a high rate of fire travel, assisted by a strong fan draught system, they are still popular in some countries.
A modern 16 chamber Hoffmann kiln
This is a modified Hoffmann, in which the fuel does not come into contact with the bricks, and it is capable of higher temperatures. This kiln can be used for firing firebricks and high quality bricks, where firing discolouration is not desirable. Transverse grates are fitted across the tunnel, seperating the kiln into several chambers. Belgium kilns are usually fed with fuel from the front through a stoke hole built into the chamber door. The grates consist of refractory firebars laid across an ash pit. Primary air for the combustion of the coal enters under the grate from outside the kiln through a vent fitted with a steel door. All other similar openings fitted to each section are kept closed.
The far end of the ash pit is connected by a flue to the space between the inner walls of the kiln, which in turn is cnnected to the chimney. These flues are opened and closed by dampers operated from the top of the kiln. The Belgium kiln has most of the advantages of the heat recovery of the Hoffmann kiln, but is less efficient, because the primary air for combustion is cold. The main advantage over the Hoffmann is that, as the fuel is not burnt in the settings, fuel ash does not adhere to the bricks, even at the high temperatures, so there is less brick discoloration and wastage.
It is important to determine where and how the heat is lost during firing, and to see what measures can be taken to keep them as low as possible. A high proportion of the heat lost is carried away in the combustion waste gases, and, more impotantly, a lot of the waste gas heat is carried out by the excess air passing through the kiln. The heat lost through excess air can be as high as 50% of the total heat input. This loss can be measured by monitoring the temperature of the waste gases going up the chimney and comparing it to the fuel: fired brick figures.
The fuel consumed in kilogram per 1000 bricks in a continuous kiln increases from 35 at a waste gas temperature of 100C to 75 at 300C with 500% excess air. This figure shows the importance of controlling the excess air: for example, at 200C, if the excess air is increased from 100% to 800%, for the same rise in temperature, the fuel consumption has to be more than doubled, as most of the available heat from the fuel is going straight up the chimney. If the waste gas temperature falls too low, it is an indication that the draught through the system is impaired. This introduces another set of problems, like inefficient combustion, poor heat distribution and the condensation of flue gases on the green bricks, causing discoloration. A flue gas temperature of 100C is a reasonable temperature to work to, but if the kiln is small, with less then 16 chambers, a temperature this low may not be obtainable. The minimum of excess air is, of course, desirable for maximum efficiency, but some leakage of air is inevitable with such an estensive structure as a continuous kiln.
The other main source of heat loss is by radiation and convection through the kiln structure. One way this can be reduced is to increase the kiln output, because the radiation and convection of heat remain almost constant, whether the output is high or low.Packing more bricks into the kiln by placing the bricks tightly together does not work, as it tneds to decrease the rate of fire travel and leads to a higher percentage of bricks being unevenly fired. There is a best rate of fire travel corresponding to the proportion of solid bricks to gas spaces in the kiln chamber. By reducing the brick stacking density and using a higher draught, the rate of fire travel may be increased, which will result in an increased output. To establish the best and most productive conditions require a lot of experimentation.
With barrel arch and chamber kilns, radiation and convection losses from the kiln structure are usually kept down by building the wall a metre or so thick, through which the heat penetrates slowly. It is true that the thicker the walls the more heat is required to raise the kiln to top temperature, but in continuous kilns this heat is largely recovered. A large heat capacity in a kiln structure, however, reduces the rate of natural cooling, so a chamber of fired bricks will be too hot to unload for a long time. Extra insulation on the top of a kiln will reduce the heat and make the working conditions for the kiln stokers more pleasant. It will also reduce the rate of cooling. If lower density insulation material is used to replace high density firebrick, then heat losses will be reduced at the same time as through draught cooling time reduced. This is because of the lower heat absorbtion and retention of the insulation material.
There is alo a case for insulating the base of continuouse kilns; less heat will pass into the foundations during high temperatures, so the reservoir of heat that returns into the kiln during cooling would be reduced.
On all types of kilns, careful maintance is the first requisite for efficient working. The brickwork of all kilns must be kept in good repair. Cracks let unwanted cold air in and greatly facilitae heat loss during firing. When the doore of the kilns are sealed up, this should be done with double thickness brick, preferabl with an insulation space between them and a good layerofclay slurry plastered across the outside for sealant. Level top firing floors indicate structural soundness and lower heat losses. With intermittent kilns, it is important to see that flues are kept clean, that dampers fit and seal well and that the chamber floors are level and clean. The positioning of the green bricks during loading needs to be done accurately, or increased brick wastage will result.
Loading the chamber of a Hoffmann kiln in Bangladesh (Photo: H. Norske)
|Text and drawings by
Appropriate Development Consultants
Cumbria LA23 3LD
German Appropriate Technology Exchange