Modern Gasworks Practice

REFRACTORIES FOR GASWORKS PURPOSES 211 fireclay retort is burnt at 1,040° C., and the final substance is taken as giving a standard coefficient of conductivity (t.e., 1), the figure will be increased to 1-6 if burning takes place at 1,300° C. It is for this reason that a retort will give a greater conductivity after having been. in use for some time. The practice of coating the inner surfaces of the retort with. a glaze, which is prevalent in Germany, has never gained mucli favour liere, and is usually carried out only when specified by the purchaser, the additional charge being about Is. 6<Z. per foot length. The glaze is primarily intended to act as an impervious coating to prevent diffusion of gas through the retort walls. The friction of the stoking machinery disposes of the greater part of it; but by this time “ scurf ” will have formed, and. will take up the duty of the glaze. Experiments have recently been conducted in cormection. with the problem of the outward diffusion of gas through the retort walls during the early stages of the charge, when the retorts may be sub-jected to some pressure, and it is a question wh.eth.er the increased. porosity of the material is not an ofEender in this direction. When perfectly new retorts are in use, there is little doubt that some gas must be lost in this way, although the formation of “ scurf ” will soon act as a check (see Fig. 105). Retorts in this country are made by two distinct methods, i.e., hand-moulding and machine-pressed. The former method is still employed in many yards to-day, and consists in plastering the clay to a certain. thickness roimd a “ plug ” of the required shape. The whole length of the retort is not plastered on in one operation, but is formed in stages of about 18 inches to 2 feet in length at a time. The mandril is placed in a vertical position, and after the first stage has been moulded it is per-mitted to stand aside for a day or two until sufficiently dry to bear the weight of the next stage. In some instances, chiefly on the Continent, a different method is used. An outer casing is fitted with. a core so as to form an annular space corres-ponding to the shape of the retort. The clay substance is then. rammed into the space by means of long-liandled punners provided with spiked ends. The object of the spikes is to release air bubbles which. would affect the porosity of the finished article. Machine presses for retort making (Fig. 111) are chiefly operated by steam power, and consist of a large cast-iron cylinder in which. the piston or “ presshead ” works. The cylinder is filled up with a certain quantity of clay, depending upon the capacity of the machine, and as the piston goes down the clay is compressed and forced through, a die at the base of the cylinder. The die is fashioned in accord-an.ce with. the shape of the retort required, and as the moulded clay passes through., it is supported by a movable table, which descends at the same rate as the com-pressing cylinder. The moulded retort issuing from the die and resting on the movable table is shown in Fig. 112. Machines of this description are capable of making 400 to 500 feet of retort per day. Cast Retorts The process of casting gas retorts has not been generally favoured in this country, but recent experiments have shown that, so far as physical properbies are concerned,