A convenient size for a kiln is about 60 feet by 11 feet internal dimensions, and 12 feet high. This will contain about 80,000 bricks. The fire-holes are 3 feet apart. These kilns are often made 12 feet wide, but 11 feet is enough to burn through properly.
A kiln takes on an average a week to burn, and, including the time required for crowding and emptying, it may be burnt about once every three weeks, or ten times in an average season. This will produce about 800,000 bricks, that is about as many as would be turned out by two moulders in full work.
The bricks in the centre of the kiln are generally well burnt. Those at the bottom are likely to be very hard, some clinkered. Those at the top are often badly burnt, soft, and unfit for exterior work.
The following advantages are claimed for kiln-burning over clamp-burning: -
Fig. 11. Plan.
Fig. 12. Cross Section on A B.
1. In kilns the bricks are nearly all turned out of the same quality, being equally burnt, and are more uniform in colour; whereas the bricks produced from different parts of the same clamp vary greatly in quality, and many of them are almost useless.
2. For kiln-burning the bricks need not stand so long on the hacks to dry, because the fires in the kiln can be regulated so as to drive off the moisture gradually.
This prevents warping, which often occurs with bricks clamped in too moist a condition.
3. Though the kiln-burning requires more fuel, yet the speed with which the crowding, burning, and discharging take place, the absence of waste, and the superior quality of the bricks produced, render it preferable even from an economical point of view.
Hoffmann's Kiln is used chiefly in brick-manufactories on a large scale, where a great number of bricks is required annually, and a continuous supply has to be kept up; but it is also employed in making bricks for very extensive works where several millions of bricks are required.
This kiln is circular in plan.
At certain equidistant points there are grooves formed in the sides of this annular chamber, so that it can be screened across by an iron shutter temporarily inserted (see S, Fig. n) at any of these points.
The portions of the kiln between these points are called "chambers" or "compartments."
They are marked 1 to 12 in Fig. II; each of them is connected by means of a flue, /, with a high central chimney, C.
The number of compartments varies in different kilns from 8 to 24, but a 12-chambered kiln is found in practice to be the most convenient for the purpose of the engineer in providing a temporary brickfield to supply special works.
Each flue can be cut off from the chimney by lowering upon it a cast-iron damper, d.
Fire-holes with covers are provided at fh, fh, by which fuel in the shape of powdered coal may be supplied to the bricks.
The object of these arrangements is to utilise all the heat produced by the fuel, and thus to save expense in firing.
Thus in a 12-chambered kiln on a certain day the chambers might be in use as shown in Fig. II.
The annular chamber is closed by an iron shutter at S, between compartments 12 and 1.
The flue from No. 12 is placed in communication with the chimney by raising the damper, dl2; all the other dampers are closed.
The state of things is then as follows: -
Chamber No. 1 is being filled with raw bricks.
„ No. 2 is being emptied of cold burnt bricks.
„ Nos. 3, 4, 5, 6 contain bricks which have been burnt and are cooling. „ Nos. 7, 8 contain bricks which are being burnt, fuel being supplied to them through the fire-holes, fh, fh. „ Nos. 9, 10, 11, 12 are drying and becoming very hot under the influence of the heat from Nos. 7, 8.
The cold air is entering through the open doors of 1 and 2, and proceeds in the direction shown by the arrows. Becoming partly heated by passing over the cooling bricks in 3, 4, 5, 6, it enters 7 and 8, whence it goes on in a highly heated state to dry and heat the raw bricks in Nos. 9, 10, 11, 12. Meeting the screen between 12 and 1, it passes through the flue f12, goes up at d12 into the chimney.
The next day:
No. 2 would be filled with raw bricks.
No. 3 would be emptied of cold burnt bricks.
Nos. 4, 5, 6, 7 would contain burnt bricks cooling.
Nos. 8, 9 would contain bricks burning.
Nos. 10, 11, 12, 1 would be drying.
The screen would be between 1 and 2, and the smoke, etc., would escape up the chimney through the flue f1, the damper d1 being raised, and all the other dampers down. The doors D2 D3 would be open; all the other doors shut.
A similar change is made each day, so that the kiln burns continuously, never being allowed to go out except for repairs.