An aperture or passage for the escape of the smoke and heated air from a furnace or fire-place, and for producing a more perfect combustion by determining a rapid current of air through the fuel. The principle upon which the action of a chimney depends, is, that the air in the chimney becoming rarefied, its specific gravity is diminished, and the weight of the column of air within the chimney becoming less than the weight of a column of the external air of the same altitude, the heated air in consequence escapes at the top of the chimney, and is replaced by the colder and denser air, which enters at the bottom. The greater, therefore, the height of the chimney, the greater will be the effect; for the greater will be the difference in the weight of the two columns of air. In Mr. Tredgold's work on warming and ventilating apartments, the following rale is given for the orifices of chimneys according to the height and magnitude of the fire-places. Multiply by 17 the length of the fireplace in inches, and divide by the square root of the height of the chimney (above the grate) in feet, and the quotient is the area in inches for the aperture of the chimney.
Mr. Hiort, of the Office of Works, Whitehall, has taken out a patent for a new method of building chimney flues and tunnels, which has been adopted in the new palace, Pimlico, and in several public buildings, and has given great satisfaction. The plan consists in building within the usual walls, and incorporated with the common brick-work, circular smoke flues or tunnels, as seen in the annexed plan or horizontal section at b. Each flue or tunnel is surrounded in every direction, from top to bottom, by cavities or hot-air chambers c c, commencing at the back of each fire-place, and connected with each other. The air confined within these chambers is said to be rendered sufficiently warm by the heat of any one fire, to prevent condensation in all the flues contained in the same stack of chimneys. The figure of each brick composing these circular flues is wedge-like, or inclined, as respects its upper and lower surfaces: the external face is composed of two planes, forming a very obtuse angle with each other; and the internal, of the arc of a circle, to the centre of which the two ends of the brick tend or radiate; and the whole circle is completed by placing four bricks together, end to end, as shown in the foregoing plan.
Their inclined figure is best shown in the engraving above, from which it will likewise be seen that the flues or tunnels may be carried in any direction without producing any internal angles, the bricks being readily adapted to any required curvature. To make the flue straight, it will be observed that the thick ends of one course of bricks are placed alternately upon the thin ends of the next course; and in order to make curves, the thick ends are placed together on one side, and the thin ends on the opposite side. The circular flue commences at the throat of the chimney below the usual line of the chimney bar, and immediately over the fire. From below the chimney-bar the flue is continued downward to the hearth in a half-circle, forming the centre of the back of the fire-place. From the construction of these chimneys, and the nature of the materials of which they consist, no danger need be apprehended should the soot ignite (an accident not likely to happen), for such an accumulation of soot as common chimneys are liable to cannot take place within these flues, there being no angles within which it can lodge, the draught of air being much stronger through them, and the necessity for cleansing them may be rendered less frequent by vitrifying the inside of the bricks to prevent adhesion; nevertheless, the operation of cleansing may with facility be performed without the aid of climbing boys, all sharp angular turns, and other impediments which have hitherto opposed the use of machinery for this purpose being totally avoided.
Whilst upon this subject, we have great pleasure in calling the attention of our readers to a very effective machine for sweeping chimneys, invented by Mr Glass, which has been approved by the Society for Superseding the
Necessity of Climbing Boys, and which seems applicable to almost every construction of flues. The brush, Fig. 1, is made of a round stock a, commonly alder, and pierced with small holes, into which bunches, formed of strips of the best whalebone, are inserted and fastened by glue. These strips b are from 8 to 81/2 inches in length, which renders the brush, including the stock, about 20 inches in diameter; it therefore completely fills, and effectually cleanses, the largest flues, which are never more than 14 inches square, and are seldom more than 14 by 9 inches. At the end of the stock c is a very strong brass ferrule, with a wormed socket, which receives the screw of the first joint. Fig. 2 is a representation of the ferrules of the real size; the three first portions d d d, 2 1/2 feet in length, are made of good cane, the rest e e e of ground ash, and of the same length, the number used of course depending upon the height of the chimney; these are made gradually stronger towards the bottom, and are affixed to each other by means of the brass screws and sockets in Fig. 2, before described.
The superiority of this machine consists in extreme pliability, lightness, and strength, which render it peculiarly applicable in high chimneys having a diagonal portion at b, as shown in the annexed Fig. 3. If Glass's machine be introduced through B, it will proceed to the top of A with ease, whilst most other machines generally stick at b. A common defect in the construction of chimneys, although not so great in houses of recent construction, is, that the aperture is generally much larger than necessary for the passage of the smoke, the consequence of which is, that the fuel in the grate not being sufficient to rarefy the whole portion of the air in the flue, the rising current of heated air is met by a descending current of cold air, and the smoke is borne back into the room. By Mr. Tredgold's rule, before given, it appears, for a grate 18 inches wide, a chimney 36 feet in height would require an aperture of only 51 inches area, little more than 7 inches square, whereas no chimneys are less than 14 inches by 9.
It is true that they could not be swept by climbing boys if made of less dimensions; but if carried up in nearly a straight direction, without abrupt bends, they might be easily cleaned by machinery, by which means a barbarous and inhuman practice would be abolished, and the proper dimensions being assigned to the chimney, the annoyance of smoke in the apartments would be got rid of. The most effectual remedy for smoky chimneys is to contract the aperture, and lower the breast of the chimney; and when this hat been carried as far as is practicable, some further benefit may be derived by placing on the summit a revolving cap, turned by a vane, so that the aperture for the smoke shall always be to leeward. A machine of this description, invented by Capt. Halliday, is represented in the engraving on the preceding page, a and b are two square plates of iron, or other metal, the upper one being supported by four vertical pillars; c is the aperture for the passage of the smoke from the brick flue immediately beneath; across this aperture a bar is fixed horizontally, supporting the upright spindle d, on the upper end of which is fixed a double-tailed vane, shown in plan at Fig. 2.
Below the plate b a square plate, forming a screen or guard, is attached by braces to the spindle, and the spindle being turned by the action of the wind upon the vane, the screen is constantly opposed directly to the wind.
The sketch below exhibits a smoke cowl commonly used at Glasgow:- Fig. 1 is an elevation, and Fig. 2 a plan or horizontal section of the contrivance, which consists of a quadrangular box of sheet iron, surmounted by a pyramidical cap, and placed, as exhibited, over the top of the brick flue. There are four doors to it, a b c d; a is connected by an intermediate rod f to the opposite door c, and b, by the rod e to the door d, so that when the wind closes the door opposed to it, the opposite one is opened for the smoke to escape uninfluenced by the wind.
The annexed engraving represents Mr. Fen-ner's apparatus for curing smoky chimneys. It consists of a spiral tube or flue to the upper part of an ordinary chimney. These tubes are made of thin copper, and furnished with a flange at the lowest end upon which it rests on the top of the brick-work of the ordinary flue. The chimney is then continued upwards with a reduced thickness of brick-work, by which means the capacity is sufficiently enlarged for the reception of the spiral tube. The expanded part of the chimney is closed in at top so as to form a hot-air chamber round the tube, which being of thin metal, the heat is readily transmitted to the chamber.