Modern Gasworks Practice

THE HORIZONTAL RETORT BENCH 89 So far as retort settings are concerned, ,it will be appreciated that the sensible heat of the waste gases is not the only direction. in which thermal loss occurs, for even with the most careful regulation of air dampers it is almost impossible to prevent mtermittent appearance of unburnt furnace gases in the waste products, while the proportion of combustibles passing to the chimney is increased by the leakage of coal-gas through defective retorts. The quantity of waste heat which may be recovered depends not only upon the temperature of the products but upon their average specific heat. Moreover, if cooling below 212° F.1 can be effected, there will be an additional gain from the latent heat of vaporization of the moisture present. In applying waste-heat recovery in practice the problem offers two alternatives—the recovery of the greater part of the sensible and latent heat by cooling the effluent gases as far as possible, or the reduction of the temperature of the gases to about 400° F., so as to leave a sufficient surplus of energy to provide for a natural draught. In the first instance, it is, of course, necessary to instal apparatus giving an induced draught. Raising Steam It is interesting to record that one of the earliest attempts at recovery of waste heat from retort settings consisted of inserting in the base of the waste-gas flues a series of 2-inch or 2|-inch cold drawn steel tubes, served with a water supply from a tank controlled by a ball-cock. A set of the tubes was placed in the waste-gas flues ■on each side of the producer, and the two sets were then connected across with a rider, from the centre of which. an outlet pointed down into the ash-pan. The approximate quantity of steam which it would be possible to raise from any particular retort beuch may be readily computed, once certain factors are known. While many engineers prefer to make their calculations on the basis of volume of the waste products available in unit time (in which. case the total volume at normal temperature and pressure, inclusive of steam per Ib. of coke, may be taken at 150 .cubic feet), the writer is of opinion that, at any rate so far as the preliminary calcula-tions required in connection with gasworks are concerned, it is preferable to work .on a basis of weight. When 1 Ib. of pure carbon is completely burnt with. the exact ■tlieoretical quantity of air it gives rise to 12-6 Ib. of waste products. In practice -the average analysis by volume of dry waste gases obtained from the modern regenerator retort setting is :—Carbon dioxide, 19 per cent. ; carbon monoxide, nil; oxygen, C-4 per cent. ; and nitrogen, 80-6 per cent. From this it may be seen that the gaseous principle employed for heating retorts is capable of comparatively fine adjustment, the excess air being very much .smaller in quantity than that found with the ordinary boiler installation. The first step in the calculation is to convert the above Volumetrie figures into analysis by weight. This is readily done by multiplying each of the volume figures by the .corresponding molecular weight, taking the sum of the products so obtained, and -.then dividing each separate product by the sum of all. In this way it will be found 1 The Fahrenheit scale has been used here as being more convenient for deducing the formulæ.