chimneys, draught defined in 1909 yearchimneys, draught - Chimneys, Draught;
chimneys, draught - Before any intelligible account can be given as to the action of chimneys (efficient or non-efficient), it becomes necessary to explain as clearly as possible the phenomenon of "draught," without which chimneys would be of little service for any of the uses to which they are put. It is understood, of course, that by the word "draught" is meant the unceasing up-flow of air that will be found passing through chimneys, entering at the bottom aperture and escaping at the chimney-top. About the only occasion upon which this steady up-flow of air becomes irregular is when a chimney suffers with " down-draught," a trouble that will be referred to presently. At almost all other times the up-draught is constant, only varying as regards its strength, for even new brick chimneys, or those that may have been disused for a long period, are commonly found to have a constant though somewhat feeble draught in them.
The actual cause of draught in chimneys, the regular ascending current, is due to the fact that cold air is heavier than that which is hot.
The action is as follows: Supposing a stove exists at the bottom of a chimney, as Fig. 1. When the fire is lighted, the air within the stove becomes rarefied by the heat, that is, it is made or rendered lighter than it was, owing to its particles having become expanded by the warmth they have absorbed. Immediately they thus become lightened, they are displaced by the superior weight of the cold particles around them, and this displacement causes them to take an upward direction.
The warm air rises up into the chimney, and the cold air which has entered to take its place, gets warmed, and rises also, and so it goes on. The action is continuous, and exceedingly rapid, as can be judged by holding a lighted taper where the air enters the stove. The direction of the flame of the taper will show what a keen inrush or stream of air is entering the aperture.
In addition to air that may be warmed, and so caused to make an up-draught in the chimney, there are the hot products of combustion. The air which has actually passed through the burning mass of fuel, undergoes a chemical change, and becomes a gas with a different name; but it still remains lighter than the cold outer air, and consequently acts in the way described. With stoves very little air passes through without undergoing this change, or becoming very highly heated, and the result is a much stronger and more powerful draught in the chimney than we get with an open grate. With this latter article, the open mouth of the chimney, whether the grate be an old or new pattern, permits of a deal of air passing in which is barely warmed, and this tends to retard, or at any rate interfere with the draught.
It is peculiar to note what a length of time a brick chimney will retain sufficient warmth to induce a draught in it. Chimneys that have not been in use for months are sometimes found to have a steady up-current of air in them, due to the brickwork still being able to impart sufficient warmth to the air in contact, to render it lighter. In perfectly new chimneys a feeble up-draught is sometimes observable, caused by the mere fact of the air inside the building being a degree or two warmer than that which is without.
To illustrate the rapid action induced amongst the particles of a fluid by a very little heat, take a glass flask or jar of small size, holding say a pint, and nearly fill this with cold water. Into this water stir a small quantity of hard-wood dust (fine sawdust of mahogany will do), and then suspend the jar so that a lamp can be placed under, and the results noted. A small paraffin or benzoline lamp is better than a spirit-lamp for this purpose, as the flame can be turned down low, and we can see what a very little heat will do. If we place a lamp under, we shall find that, although the heat which comes from the lamp chimney is far from great, the particles of dust will almost instantly set up a motion, and the motion of some of the ascending particles will be exceedingly rapid. By watching the dust particles we are able to tell what is happening with the particles of the liquid, for it is the movement of these latter that causes the solid particles to take the direction they do.
If we take the average open range (and there are very many still existing in large residences in London as well as in the country), we find that there is a 24-inch to 28-inch space between the top fire-bar and the chimney breast above it. If this space is left open, the smoke from the fire will show great reluctance to pass up the chimney, and in quite 80 per cent, of such cases some of the smoke will ooze out into the kitchen, and the range is said to smoke. The remedy for this state of things is the introduction of a blower, a sheet of metal placed so as to reduce the space just referred to, and the inflowing air is caused to come more within the influence of the fire. This is effectual for the simple reason that it prevents, or at least reduces, the free passage into the chimney of air that is absolutely cold.
An instance once came to the writer's notice in which one of these old-pattern ranges had to be retained in the kitchen, although it was noted for its rarely ceasing its (till then) incurable habit of smoking. An ordinary blower was of no avail - that is to say, it did not wholly remedy matters, although it effected a little improvement. An effectual cure was made in this case with a specially made blower, which reached down to the level of the top fire-bar as Fig. 2. This had its lower part in sections which could be raised to admit of the introduction or withdrawal of the cooking vessels. It will be seen that by this arrangement the open range was almost converted into a close-fire kitchener, so far as its shutting out cold air from the chimney was concerned.
With dog-grates the smoke trouble, when it occurs, is caused by precisely the same conditions as with the open range. These grates are usually fixed in a roomy opening which is tiled round, and the result, so far as appearance is concerned, could not be better. The spaciousness of the opening is, however, what ruins the efficient working of the chimney and the grate, by reason of the abundant presence and inflow of cold air as just lately described.
Blowers are sometimes applied to these openings, to obviate or cure the sluggish action of the draught in carrying the smoke away. Either a piece of bevelled plate, or ordinary sheet in a brass frame, or coloured leaded glass as in Fig. 3, would look and act excellently. This would overcome the smoking, assuming it proceeded from the cause under discussion.
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