THE MODERN HOME.

Waipa Post, Volume 47, Issue 3386, 28 October 1933, Page 3

THE MODERN HOME.

WHY SOME CHIMNEYS MISBEHAVE. CAUSES OF SMOKE NUISANCE. ACTION OF WIND. CARE NEEDED WHEN APPLYING REMEDY.

(By A. C. Brookes.)

Windy spring days, when cold spells of weather necessitate good fires, have a halbit of disclosing those faults of chimney v design which make for smoky rooms. It is advisable to direct attention to this problem now, so that it may be studied and dealt with in the suniimer months when the chimneys are temporarily out of commission. The smoking back, in rough weather, of a domestic fire is usually attributed to the wind blowing down the chimney. Although the statement is sometimes correct, in a majority of cases it is not. The direction of a wind is mainly horizontal, with a slight upward component, and it is only by deflection from obstacles that a downward component can be produced.

Although the flow of gases obeys laws more complicated than those pertaining to the flow of water, both are essentially the same. As the flow of water in a channel is visible, while that of air in. the form of wind is invisible, much information may be obtained 'by watching the flow of the nearest stream:.

In an open channel the surface .of the water is a straight line, inclined to the horizontal at a slight angle, and of course the flow is from the higher toward the lower level. The velocity of the flow varies with the angle of inclination. As an example let us take the water flowing smoothly through a channel. If an obstruction be placed in the channel extending into the stream, but not across it, the following effects will be produced: The surface of the water on the upstream side of the obstruction will be raised, approaching more nearly the horizontal, while that on the donwstream side will be lowered, also approaching the horizontal. The vertical distance between the two levels is a measure of the pressure exerted on the obstacle, each foot being equal to a pressure of approximately J lb per square inch. The total force causing the water to flow is lessened by this pressure, so that the total velocity also is reduced, although where the stream is constricted the velocity is increased. This accumulation of the water varies with the distance to which the obstacle projects into the stream and the angle it makes with the line of flow, the effect being greatest when at right angles, and least when parallel to the "line of flow. The water is also deflected parallel to the surface of the obstacle until it reaches the opening, where, combining with the rest of the unimpeded flow, it produces a current flowing obliquely to the main line of flow. This is in turn deflected, as the side of the channel produces an eddy on the lower side of the obstruction, thereby restoring the uniform flow parallel to the length of the channel.

A similar, although invisible, effect is produced when a wind impinges on a building. A pressure is set up, varying with the angle at which the wind strikes the surface. With perpendi-

cular impact a wind of 15 miles per hour will give a pressure of 18 ounces on each square foot. On a sloping roof of 30 degrees pitch the pressure will be aboult 9 ounces per square foot. Air being compressible, it will act like a spring under this pressure, and will be forced through any opening communicating with an area of lower pressure. The wind will not blow down the chimney, but may set up such a high pressure that air will flow in any direction toward a lower pressure area. In addition, low-pressure areas are produced on the lee side of the buildings, and also in narrow passages between them, which are more or less parallel to the direction of the wind, as the wind blowing through such passages sucks, air out of any doors or windows opening into them. A chimney-stack in a high-pressure area, especially in conjunction with fresh-air inlets into the room in a low-pressure area, is certain to give trouble. . A fire lessens the pressure in the chimney, and its effect is sufficient to produce a stable current up the shaft, if the pressure of the air at the.'.chimney top is the same as that at the inlets into the room- If the former is high or the latter low;, because of the Ibehaviour of the wind, thp fire is unable, to overcome the reverse pressure, and a smoky chimney results, the quantity of smoke expelled varying with the force of the wind.

There are no exceptions to the law that air always flows toward the lower pressure-; therefore, if a chinmqy is subject to down draught, the pressure in the room is less than at the chimney top, and can be cured only by increasing the pressure in the room or by decreasing the pressure, at the chimney fop.

Let- us now return to our stream. When two streams flowing with different velocities meet, if they are almost parallel, the velocity of the combined stream is. less than that of the faster of the two separate watercourses and greater than that of the slower. The final velocity will depend on the relative size of the streams and the angle on which they meet. In untechnical language any stream of air or water flowing obliquely across an opening exerts a suction on that opening, varying in amount with increased parallelism of the two currents. If the streams are exactly at right angles to one another no suction occurs. In the same way, the wind blowing across a chimney-pot usually exerts some slight suction, which may become important under certain conditions.

As previously shown, the wind striking a roof is deflected in a direction more or less parallel to it, so that the wind blowing across the pot (if the stack be on the ridge) will not be horizontal and at right angles to the pot, but somewhat upward, providing the conditions for some suction on the top of the chimney. With a steeper roof or a wider stack the effect will be increased.

iChimney-stacks on gable-ends or at the eaves are subject to this effect in the greatest degree., as the wind deflected from the wall and the stack is flowing vertically parallel to the flue and consequently exerts a powerful effect. This increase in upward current is apparently an advantage, but often gives rise to a particularly awkward, smoky chimney, which is not improved by the that most of the stacks in this position have straight flues. The increased chimney current,

which may be as much as. 20 feet per second during the gusts, draws air out ' of the room much more quickly than it can enter by the ordinary .inlets, thus producing a low pressure in the room), but even so a pressure greater than that at the top of the chimney while the wind is blowing. As s;Qon as the wind drops the pressure at t&e chimney-top increases, and becomes greater than that in the room, and air flows down the chimney until.equilibi'ium is restored. ;... •!. •.. ,:;-' Generally speaking, wind action is a nuisance. It is a variable factor, and the builder must be wary in-in- . stalling devices to overcome a smoky... chimney, of which the action is de- '• pendent on the wind. When wind action is absent on calm days 90 per cent of smoky chimneys are cured. The action of the wind is so wide that although it may increase the ward current it may at the same time so alter the distribution of pressures around the building as to do more than counteract any increased draught. ; In 90 per cent of cases the draught due ; to heated flue gases is sufficient for '■ all purposes. The wind factor is five ' or six times more powerful than the temperature of the gases, and even,.; if it does not produce a smoky chimney it produces a tremendous waste..-. of fuel and unpleasantly draughty,rooms. • ;