Draught

Draught, in Engineering, means the supply of air necessary for the combustion of coal, coke, or other fuel in a furnace of any kind. This supply may be induced in various ways, which may be classified as natural draughts or forced draughts. With a column of warm gases passing up an ordinary chimney, a draught is produced simply by the external pressure of the colder air outside overcoming the smaller pressure of the warm gases within. Such a natural draught is intensified by increasing the difference of temperature between "the internal gases and the external air, or by lengthening the chimney. On locomotives the natural draught cannot conveniently be increased by lengthening the chimney, and it is therefore assisted by letting the exhaust steam at the end of each stroke discharge itself upwards in the chimney. This has the effect of drawing the air more vigorously into the furnace. Natural draught is also assisted by placing a cowl or small fan at the top of the chimney. This is rotated by the wind, and acts as a pump to draw the gases up the flue. Constrictions in the flue may considerably modify the draught; dampers are valves placed for the special purpose of veu-ying the draught by introducing greater or less constriction in the flue. True forced draughts are those where the whole supply of air or oxygen is obtained by mechanical means. Thus, it is a general plan to use large pumping engines to force air through the furnace. The supply may be varied at will, and admits of much greater range than is possible with natural draughts. One very convenient means adopted for the production of forced draught in small forges is to work a light fan at quick speed by a long lever. The efficiency of such an arrangement, the supply of forced air passing from the fan to the centre of the furnace, is much greater than that of the ordinary bellows.