CHIMNEY v. MECHANICAL DRAFT.

Progress, Volume II, Issue 2, 1 December 1906, Page 66

CHIMNEY v. MECHANICAL DRAFT.

A chimney with natural draft will have a draft dependent upon its height, the power of which will not vary, except upon the rise or fall of the internal temperature. It has, therefore, no sucking power , in fact, the term suction in this connection is a fallacy. The chimney acts because the external air is heavier than the internal, and thus -presses into the chimney by the only available opening, viz., that at the bottom, the furnace front. The pressure or intensity of the draft fixes the amount of fuel it is possible to burn on a given area of grate. It therefore becomes necessary, when it is desired to increase the steaming capacity of a boiler by increasing its coal consumption, to increase the intensity of the draft, and the only way in chimney draft is to increase the temperature of the gases passing up it, or increase the height of the chimney. The first method, of course, means a large amount of waste, and is a very uneconomical arrangement ; the second is expensive and unusual. A chimney stack 150 ft. high will burn from 15 ft>. to 20 Ib. of coal per sq. ft. of grate area per hour under normal conditions, but in wet or foggy weather it will be very much less than this, as the wet air is lighter than the dry, and thus produces less pressure at the furnace (the weight of water vapour is about half that of air). A fair average of temperature in the furnace is 2,400° F., and that of the escaping gases at the chimney, without economisers, 6oo° F. This means that one-quarter of the total heat generated is sent up the chimney to waste. Thus, on a 2,000 h p. plant, almost 500 h.p. is going up the chimney per hour, and the coal bill necessary to sustain this will come to a big figure in the year. It is not the author's contention, but it has become a well ascertained fact, that it is cheaper and better in every way to provide the necessary supply of air for burning fuel in steam boilers by mechanical means, and to take as much heat out of .the hot gases after they have ceased to be in contact with the boiler itself, before they are turned out into the atmosphere, than to do it 111 the older way by utilising a portion of the heat generated to create the necessary supply of air. This is the primary reason for using a mechanical means of moving the air. The heat previously necessary to create the draft by means of a chimney may now be employed usefully 111 other directions — The Engineering Review {London)