Modern Gasworks Practice

210 MODERN GASWORKS PRACTICE character of the superficial skin, and on the porosity of the brick; and (3) 011 the time and temperature the brick is bathed in the reducing atmosphere. Horizontal Retorts It is not within. the power of the retort manufacturers to alter materially the Chemical composition of the clay occurring in their districts ; hence it is by modifi-cation of the texture of the finished article and attention to the details of manu-facture tliat greatly increased refractoriness lias been obtained. Not only do retorts now have to withstand high, heats, but in addition, as discharging and cliarging take place, they are exposed to considerable fluctuations in temperature. Moreover, they must be of a certain mechanical or physical strength in Order to carry the loads to which they are subjected and to resist the strains put upon them by modern stoking machinery. In the case of the full coal charge and “ pusher,” the retort has to withstand some considerable bursting pressure in the event of a charge jamming-up and. refusing to move. For high temperature, a highly refractory material is required ; and to obtain. this, high porosity and a fairly open grain are desirable. This is chiefly due to the faet that an open grain means larger individual particles, and the heat and flame have greater difficulty in penetrating to the interior of these than in the case of smaller dust-like particles. One function of porosity, however, is that of imparting to the material the ability to withstand fluctuation in temperature. For mechanical strength, on the other hand, a close-grained material of high density gives the best results, whilst diffusion of coal-gas outwards or furnace-gas inwards through the walls of the retorts is more efEectively restricted, and better thermal conductivity is obtained. In the manufacture of the. retort, therefore, these contrary requirements have to be reconciled as far as possible, although. greater attention is usually paid to refractoriness. It is in this way that porosity plays so important a part; and no doubt the success of the German-made retort (which even now has a higher porosity than the average English type) can, in all probability, be greatly attributed to this faet. Porosity, however, as will be realized, is a some-what dangerous property to interfere with, unless considerable care is exercised to keep it within the prescribed limits. It cannot be denied that any increase in porosity may be accompanied by a corresponding decrease in the capacity of the material to withstand working stresses ; hence too much zeal in the open-grain doctrine is likely to teil somewhat seriously when the working life of a retort comes to be reckoned up. Evidence of this is seen in a certain number of retorts having ,a porosity up to as much as 30 per cent, which. at one time foimd their way into gasworks with decideclly unhappy results. In this Connection it is particularly interesting to note that, whilst the Refractory Materials Committee specify a minimum porosity of 18 per cent., the German-made retort has an average in the neigh-bourhood of 25 per cent. It has been shown, too, that in this country the German horizontal retorts, with. due care, are capable of surviving about 2,100 days. A comparison between working life and porosity in this way is striking, in that it shows that even with a comparatively open texture a high degree of mechanical strength is obtainable. With regard to conductivity, it has been shown that if an ordinary