Modern Gasworks Practice

224 MODERN GASWORKS PRACTICE Scotch—Contracts SHB Dinas — Expands P~i—H Stourbridge — Contracts Fig. 124.—Compensation Arrangement for Expansion and Con- liable to upset the calculations of those responsible for tlie design of settings, one of the chief consequences being that the retorts, after a short time, fall slightly out of level. In order to avoid this, an arrangement has been introduced by some engineers whereby the varying properties of the two types of material are made to neutralize one another. For instan.ce, the furnace arch. and combustion chamber may be built up of silica wäre and the division walls of material having a contracting nature. On heating up, the expansion of the former is counteracted by the con-traction of the latter—hence everything is maintained on an even. keel. A spt.fing built up in this way is diagrammatically shown in Fig. 124. Attention must be drawn to the practice of running up the cross-walls between the retorts and above the combustion arch. of siliceous material, while 011 the outer side of the setting, where the heat is less, the same walls are continued in aluminous work. In such cases it is frequently found that the retorts tend to overturn in an outward direction (particularly if the joints in the wall have been made fine), this being due to the pushing upwards and outwards of the silica mato-rial, wliereas the aluminous material on the out-side will have undergone a certain amount of shrinkage, thus allowing the movement to take place. In the constniction of all types of division walls, it will be found an excellent policy to leave a big joint liere and there to take up the crowd-ing out, if necessary. With regard to these cross-walls, it must be remembered that, so far as the greater portion of them is concerned, mechanical strength. is desirable rather than extreme re-fractoriness. for the material merely comes in. contact with the circulating gases, and is not likely to be affected by any severe cutting heat. Consequently, a brick of greater density and less porosity may be used, and ordinary aluminous firebricks of the best quality usually give every satisfaction. Speaking of “cutting heats,” it is as well to point out that small combustion chambers, although they have their advocates, are decidedly conducive to these, and also prevent the effective mixing of the gases. It is essential to employ bricks for th.e cross-walls which. are of sufficient re-fractoriness to withstand. the working temperature, as drippings resulting from the fusion, or partial fusion, of these often cause trouble. The disastrous eft’ect which a small proportion of alkaline impurities may have on the remainder of the setting has already been emphasized. It is well known, of course, that in the case of the ordinary stock building brick the original clay