The Hoffmann kiln is a series of batch process kilns. Hoffmann kilns are the most common kiln used in production of bricks and some other ceramic products. Patented by German Friedrich Hoffmann for brick making in 1858, it was later used for lime-burning, and was known as the Hoffmann continuous kiln. 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 barrel-arched chambers (like a railway tunnel) have several small feed holes in the roof arches through which fine coal could be fed into spaces made among the bricks to be fired. The Hoffmann kiln is a series of batch process kilns. Hoffmann kilns are the most common kiln used in production of bricks and some other ceramic products. Patented by German Friedrich Hoffmann for brick making in 1858, it was later used for lime-burning, and was known as the Hoffmann continuous kiln.

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 barrel-arched chambers (like a railway tunnel) have several small feed holes in the roof arches 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 separated 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. 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. As the fire passes round the kiln, the chambers in front of the actual firing zone are gradually warmed, and the chambers behind cool off slowly.

The Hybrid 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The original round Hoffman kiln is no longer in use, having been replaced by the more modern version, which takes the form of two parallel tunnels built side by side, connected by curved tunnels at either end.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 arrangement, 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. Although the burning time is only about three days, the bricks are in the kiln for about ten days to allow for raising the temperature and, after burning, subsequent lowering of the temperature before unloading the chambers.

The original Hoffmann was superseded, because it had such a large heat absorbing mass, and the tapering firing chambers were small and unnecessarily 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. 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.

Hybrid Hoffman Kiln

Hybrid Hoffman Kiln

Advantages of Hoffmann kilns

• They are fuel efficient because of the direct stoking into the bricks and their use of waste heat to dry and preheat the green
   bricks before firing. Efficiency figures for Hoffmann kilns using coal as a fuel, in England and China, range from 1,800
   at best to 2,350 kJ per kg fired brick at worst.

• They have the capacity to fire very large quantities of bricks evenly and with minimal wastage.

• The use of the down or cross draught system, with the fuel being burnt at regular intervals amongst the bricks, provides an
   even firing throughout the kiln.

• The firing is controllable by the use of the dampers and careful stoking. A variety of biomass fuels can be used successfully,
   for example, sawdust and wood processing waste, rice and coffee husk. These fuels can be added through the roof of the kiln
   with the use of mechanised fuel charging hoppers that run on compressed air or electricity. This is necessary when burning
   the biomas fuels that have a higher mass and lower calorific value than other fuels, like coal or coke.

Disadvantages of Hoffmann kilns

• They have a large mass that absorbs a lot of heat as the firing zone moves forward through the cold kiln. This is compensated
   by the fact that some of the residual heat in the kiln and fired bricks is used to pre-heat the air for combustion.

• They are expensive to build and require regular maintenance. Some models are large and complicated structures with
   very intricate systems of flues and dampers.

• They occupy a lot of space; because of this they are often built in the clay pit after all the usable clay has been extracted,
   so as not to occupy large areas of clay reserves.

• Direct stoking affects the quality of the bricks directly beneath the stoke holes. These bricks can be over-fired and are
   usually discoloured. This can be avoided by incorporating firing trenches beneath the stoking holes, along the sides
   and in the centre of the firing chamber. These trenches contain burning fuel and prevent localised brick spoilage in the kiln.
   The fitting of firing trenches, however, means that the capacity of the kiln is reduced to produce a higher quantity of evenly
   fired bricks.