In the preceding kinds, the combustion gases and the flames from the furnace reach the chimney by the shortest path. The products to be fired must then be on the direct route. This condition is not always easy to fulfil, and hence it happens that bricks which are not reached remain insufficiently baked, and there are inequalities in the firing.
To obviate this inconvenience and to get a uniform baking of the products, kilns with reversed flame, as it is called, have been constructed. These may be built square or round; the latter form is the better, and should always be preferred. Many examples of this type are known, but their principle is always the same. The furnaces are arranged round them, and are sufficient in number; the arch of the kiln has no orifice; the flooring is pierced with holes which communicate with a circular channel leading to a chimney.
The flame penetrates into the oven and ascends to the arched roof; finding no outlet there, it is forced to descend again towards the bottom, whither the draught attracts it.
In this movement currents are produced which effect the mixture of various warm gases in such a way that a uniform heat reigns throughout the whole interior of the kiln. Moreover, the vault produces an intense radiation which allows of a very high temperature being attained. Thus these reverberatory kilns are used in the firing of stoneware and similar products requiring great heat.
We have introduced some modifications into these kilns with the object of rendering them more easy to fill and more economical in fuel. The furnaces are placed in the thickness of the wall and arranged as gas-generators, the air necessary for combustion being warmed while passing through the "chicanes" which are on each side of the furnace. Figs. 206 to 211 are sections and plan of this kiln.
The dimensions of reverberatory kilns cannot exceed a certain limit, on account of the too great height which they would reach. We shall take as maximum a cubic content of 100 cubic metres (40,000 to 50,000 bricks), which corresponds to a diameter of 6 or 7 metres and a height of 4 or 5 metres. This height has less disadvantages than in the case of continuous kilns, for the best baked bricks are at the top, while in the Hoffmann kilns it is generally those at the base which are most fired, and the considerable contraction which they undergo causes movements in the mass which often deform the bricks. Therefore it is recommended that the height of continuous kilns should not exceed a certain limit. It is evident that small kilns are, proportionately to their size, dearer and less economical than large ones.
In the foregoing kilns which burn coal on fire-bars, the latter are of small or large section. Gratings of large section are particularly suitable for rectangular kilns (Figs. 204, 205); it is calculated that 0.350 kilos (about 12 ounces) of coal per hour are consumed for each square decimetre (about 15 sq. in.) of the fire-bars. For gratings of small section this quantity may be estimated as high as a kilogram (2.2 lbs.); it is always better to have too large gratings, for it is easy to diminish their section by covering them with bricks.
The section of the chimney ought to be larger than for an ordinary grating, for, besides the products of combustion, the water-vapour given out by the green bricks has to be carried away. Reverberatory kilns require especially a strong draught to bring down the flames, which have a tendency to remain under the roof. Some manufacturers give a chimney to each kiln, others build one only for a combination of four or six kilns. In this latter case the kilns are connected by air channels provided with trap-doors or registers to give the firing a kind of continuity. The burning gases of one kiln at full heat help to warm the next one, which is already red when its furnaces are lighted, the following one being "en enfumage." If this continuity should happen at any given moment to hinder the progress of the fire, it would be easy to stop it by altering the registers so as to connect the kilns directly with the chimney-It is difficult to estimate the consumption in covered kilns, with direct or reversed flame; it depends, in fact, on many circumstances : nature of the products, degree of baking, mode of stacking, management of the fire, and draught, all of which have a more or less important influence on the final result.
Fig. 210. Horizontal Section.
Fig. 211. Plan.