56. When the wind blows horizontally, as in Fig. 15, the air which is compressed at A flows up over the edge of the chimney and follows the path of the arrows a and b. This current deflects the wind somewhat, as shown by the arrows c and d, and lifts it above the leeward edge of the chimney. An opportunity is thus given for the chimney gases (which are shown by feathered arrows) to pass over the edge of the chimney into the area of low pressure at B. As the velocity of the wind increases, the pressure at B becomes less, and the chimney draft is augmented correspondingly.

If the wind blows upwards, the area of low pressure is formed close to the top of the chimney, and the escape of the chimney gases is greatly facilitated. But, if the wind blows downwards, as in Fig. 16, the escape of the chimney gases is cut off, and, unless there is sufficient pressure behind them to deflect or lift the wind at the mouth of the chimney, a back draft, or blow-down, will be produced. All that part of the wind which is included between the dotted lines a and b tends to blow downwards in the chimney; but its pressure is reduced as soon as it enters the chimney, because it is then compelled to fill a larger area. Thus, the downward pressure which the wind will exert in the chimney may be found by multiplying the wind pressure by the perpendicular distance between a and b, and dividing that product by the width c d of the chimney. If this quotient exceeds the upward pressure of the hot gases, then a blow-down will occur.

Fig. 15.

Fig. 16.

57. The beneficial effect of the ordinary horizontal winds upon the draft of a chimney may be increased by means of the circular deflector C, shown in Fig. 17. That part of the wind which is intercepted by the curved surface of C is deflected strongly upwards and operates to lift the main current of wind well above the top of the chimney. Thus, the chimney gases are given a good opportunity to escape over the leeward edge, as indicated by the arrows.

Fig. 17.