Modern Gasworks Practice

88 MODERN GASWORKS PRACTICE door is also useful for the insertion of a coke bücket when starting up, in order that a natural draught may be stimulated. On the Continent it is occasionally the practice to erect a distinct chimney for each beuch of retorts. It is claimed that in this way each furnace may be worked independently, whilst with the common TION TO UNTAPBBED Chimney. chimney one furnace may influence another. The idea, however, is confmed almost wholly to stop-ended retort settings. As regards the bricks for the construction of retort-house chimneys, where stock-work is used a firebrick lining should be provided for the lower half. Nowadays, however, it is far more common for the whole chimney to be built of firebricks, in which case no lining is necessary. Natural draught depends for its action on the difference of density between the cold air surrounding the shaft and the warm products of combustion. Moreover, the quantity of gas drawn up by a chimney increases in proportion to the square root of the height of the chim-ney; consequently any increase in heiglit prochices only a comparatively small change in the quantity of gases drawn off. As a'matter of faet, it is stated that a shaft creates the best draught when the temperature of the issuing gases dillers from the atmospheric temperature by 273° C. Accordingly, when the difference between the two temperatures is greater than this, the draught is not so good, and a larger and useless loss of heat takes place. On the other hånd, the heat carried. off by the escaping gases must not all be considered as loss, for a eertain temperature is necessary for the creation of a natural draught. Waste Heat Boilers There can be no question that the modern gasworks provides one of the most notable instances of the useless dissipation of waste heat; for, even with the modern producer operated on the regenerative principle, the quantity of heat lost in the chimney gases of a retort setting represents from 18 to 25 per cent, of the total heat supplied. A parallel instance is provided by the coke-oven, with. which, although. the principle of regeneration is made use of, a loss is shown amounting to 40 per cent, of the calorific value of the fuel employed. Loss of Heat in Draught It must be recognized that the whole of the heat issuing from a retort-bench chimney cannot be regarded as altogether a direct loss, owing to the faet that a certain arnount of energy (in the form of heat) is, in ordinary circumstances, necessary to •create a natural draught. If a natural draught is desired, the loss of a portion of the sensible heat in the waste gases is inevitable ; but this loss may be reduced to a negligible figure by providing the necessary energy required for the draught by mechanical means.