This is composed of two parallel galleries in the form of tunnels with slightly elliptical arch, joined at the ends by a channel (Fig. 217). The arrangement of the doors for filling and of the heating holes in the roof is continuous rectangular kiln. (Scale of 7 1/2 millimetres to the metre.) Built by MM. Toisoul and Fradet, the same as in the circular kiln, but the escape of the gases of combustion and consequently the draught are effected in a different way. A central channel (3, Fig. 215) situated between the two galleries stretches the whole length of the kiln, ft is put in communication with the compartments by vertical channels, c, closed by cast-iron traps or doors, V, which are worked from the outside by means of the flywheels v, which turn a threaded rod in a fixed nut. The central channel, C, leads to the chimney, either directly, when it is at the end of the kiln, or by a subterranean channel when it is at some distance from it, as shown in the plan (Figs. 217, 215).
Fig. 2l3. Transverse Section EFGHIJ. The Gallery A' is full of Bricks, the Gallery A is empty.
The most convenient shape for the galleries is the rectangular covered by an arched, or slightly elliptical (Fig. 213) roof. The dimensions of these galleries are very variable; they depend upon the daily production, and the extreme limits of section are from 3 to 10 square metres, and of cubic space from 8 to 60 cubic metres; these are exceptional cases, which it is not always advantageous to adopt. We may take as a good average 20 to 40 cubic metres, that is to say, a space containing from 10,000 to 20,000 ordinary bricks.
The cubic contents of the compartments in the kiln shown in our plans are 28.7 cubic metres, a space capable of accommodating 13,000 bricks. Its dimensions are: 3 metres broad, 4 metres long, and 2.7 metres high under the arch. This height may be reduced without any inconvenience by increasing the breadth.
As to the number of compartments, the minimum for satisfactory progress is sixteen. It is better to have twenty or even more, for the work is then easier; the heat is less strong during charging and discharging, on account of the distance of the fire and the longer period of cooling that the products undergo after being fired. Kilns have even been constructed with a sufficient number of compartments to have one "en grand feu" in each gallery, and this means thirty-two compartments. On account of the length of such a kiln, it may be advantageous to make two of it, side by side.
Kilns are generally built of brick with or without iron plating. Armoured kilns, like the one of which we give the plans, are very solid, very elegant, and do not take up much space; but they are costly. Therefore it is more economical to support the inner walls with pillars and a thick layer of rammed clay covered with a masonry facing. Thus constructed, the kiln resists fire well and costs less than with plating. To bake ordinary products it is not necessary to make the interior of refractory clay; it will be enough to choose bricks which resist a strong firing well.
But if a high temperature has to be attained, it is absolutely necessary to build an inner covering of refractory bricks.
Fig. 214. Continuous rectangular klin (Scale of 5 millimetres to the metre.) Built by MM. Tuisoul and Fradet.
Fig. 214. Section All.
Fig. 215. Section CD.
Fig. 216. Plan of Top.
Fig. 217. Horizontal Section.
The volume of masonry depends upon the size of the kiln, but it is not proportional to this size, because a small kiln requires, for its size, a greater volume than a large one, and it is more advantageous to build a medium-sized kiln than two small ones.
As these are a cause of weakness in the arch, and as it is always they which cause deterioration in it, we must avoid multiplying them unduly, and content ourselves with the number necessary for good firing. We may say that, for a horizontal surface of 12 square metres per compartment, 12 to 14 holes are amply sufficient for firing even difficult products. The arrangement of them varies, and depends upon the method of firing and of filling the kiln.
Without being positive, - for in these questions special conditions are of every importance, - we may say that the symmetrical arrangement is as good as any other recommended by builders. Nevertheless it is right to place the holes near the central channel a little farther from the side than those along the outside walls, because the fire always advances more quickly against this channel than against the outside wall, in consequence of the cooling which this latter undergoes, and also on account of the draught which acts on the slope.
This in some kilns is in the very centre of the building, in others it is at one end; the collecting channel opens into it, and this causes some loss in draught owing to the height of this channel above the ground.
As the chimney represents a certain expense which is not much increased by making it large enough to serve two kilns, it is advisable under these circumstances to place it in the axis of separation of the two kilns. It is an advantage to have a strong draught which can be moderated, while on the other hand it is difficult to remedy a deficiency in draught. Some manufacturers prefer a chimney to each kiln, fearing lest the progress of one should hinder that of the other, if one chimney only is used.
As for dimensions, it must not be forgotten that it must remove not only the gaseous products of combustion but also the water-vapour contained in the products to be fired, which is considerable in quantity.
Fig. 218. Chimney (Scale of 5.4 millimetres to the metre)..
Fig. 219. Chimney with Pedestal.
Continuous Kilns - Chimney.
We give (Fig. 21 8) the plans of a chimney which we have constructed for the kiln just described. The orifice at the top, 1.4 metres across, is sufficient for the draught of two kilns. The Figs. 219 represent another chimney which may also serve two kilns.