The tipping-floor was

existing on the site, Fig. 1, Plate 8. formed by paving the top of the furnaces so that they were subjected to vibration from heavy carts; and common local firebricks were used in the linings. The arches of the furnaces were very flat, and the main flue was formed by a flat arch traversing over the top of the furnaces lengthwise. The temperatures reached were far in excess of the anticipations of the inventor, the furnaces and flues being constantly at a glowing red heat, and sometimes exceeding 2,000° F.1 In spite of these severe conditions the plant worked well, with practically no repairs, for 17 months; but the heat and vibration had their effect, and a short length of the flat arch forming the main flue over the furnaces eventually failed, thus causing a stoppage of the plant. The six furnaces were then reconstructed with the main flue below the feeding hearths of the furnaces instead of over them; ordinary local fire-bricks being again used for the linings. Advantage was taken of the stoppage to make a new inclined road, Fig. 1, Plate 8, with a more moderate gradient than the old one, and also to lower the feeding platform to its present position and to substitute sliding feeding-doors for the old method of feeding through a kind of funnel, which was closed by choking it with refuse. The feeding platform is 11 feet wide by 6 feet below the level of the tipping floor. The bin so formed has a capacity of about 260 cubic feet behind each furnace, when the muck is well heaped up, and as the Oldham refuse measures 40 cubic feet to the ton, this is equal to a storage of about 6 tons behind each furnace, or, say, two-thirds of the maximum amount required to be burned in 24 hours.

After 2 years and 10 months' working repairs became necessary to the six-cell group; and they were then further improved by the addition of side air-boxes, and the upper portion of them was reconstructed to their present form. In the meantime the group of four cells had been added at the end of the existing tipping platform, and these differed in some respects from the six-cell group as originally constructed. Improved rocking grate-bars of the Settle type with finer spaces (about inch) were put in; and for the linings a more refractory brick, containing about 70 per cent. of silica, was used. The furnace arches were built of large special blocks throughout; those in the front part of the arches having round vents forming the exhaust for the products of combustion. The steam blast was of the round

1 See also Engineering for 16 and 30 September, 1892.

as before.

nozzle type; but an improved form of trumpet was used, and two were applied to each furnace instead of one Hardened tool-steel nozzles were used instead of brass, which had been found to wear larger very rapidly. While these alterations gave an improvement in efficiency, the noise caused by the blast was not obviated. It has been found in testing these four cells that if the two blowers in the ash-pit are not working quite equally, owing to a slight difference in the size of the orifices, or to a readier access for the steam to one or the other, or to the action of the wind, the stronger blower overbalances the weak one, causing it to deliver little or no air, or even to blow back. This can be to some extent avoided by so designing the trumpets that the velocity of the air delivered is considerably above that necessary to produce the required pressure in the ash-pits; but the difficulty has been overcome in later constructions by the adoption of adjustable nozzles, which can be set to exactly balance one another.

At the time that the six-cell group again needed repairs, after nearly 3 years' continuous working, it had become evident that the old chimney, and the main flue leading thereto, were inadequate to deal with the gases from the ten cells, in addition to the electric-light station boilers, and it was therefore decided to build a new chimney for the destructor. It had been demonstrated by repeated trials that a large amount of power was available from the destructor gases, and the adoption of a highpressure Lancashire boiler, 8 feet in diameter by 30 feet long, in place of the old multitubular boiler was resolved upon by the Corporation; and also the addition of a Green's fuel economiser. The work was accordingly put in hand, together with the repair and improvement of the six-cell group, which was damped down, and the four-cell group was coupled direct to the existing multitubular boiler independently by means of a flue of rectangular section, 2 feet 6 inches wide, by 4 feet 3 inches high, and 54 feet long from end of furnaces to boiler front. This flue was built with hollow walls, forming an unventilated air-space, for the purpose of preventing loss of heat by radiation. The inner wall of fire-brick, and the outer wall of common brick, were each 41⁄2 inches thick, and the space between was about 2 inches. The flue was covered with 6-inch fire-brick quarries. The object was to keep the old boiler going as long as possible, and to maintain steam for the blast, one mortar-mill, and for the machinery in the horse and provender department, even while the rest of the plant was being reconstructed. It was found that the drop in

temperature from one end of the flue to the other averaged 505° F.; being 2,090° F. at the furnace end, and 1,585° F. at the boiler front, and the steam was easily maintained at 80 lbs. pressure for all purposes. A supply of steam equal to about 50 I.HP. was given right up to the time when the old boiler had to be removed to complete the seating of the new one, and even after that the four-cell group was kept going under natural draught by means of a temporary flue connection direct to the old chimney. Also the six-cell group was dried in a similar manner after reconstruction by means of natural draught from the old chimney, while the new boiler-seating and flues were being constructed. The construction of the new boiler and the chimney and flues was meanwhile proceeding. The chimney is round, 120 feet high, with an inner shell of fire-brick and an air-space, both extending to the top, the air-space being provided with grates at bottom and top for ventilation; it is thus kept cool, and no cracks have appeared. The cap is formed entirely of brickwork, and its upper surface is curved so as to direct the impinging wind in an upward direction.

The economizer consists of ninety-six tubes without scrapers, which have proved unnecessary owing to the absence of soot from the furnace gases. The by-pass from the economizer had to be carried underneath it, Fig. 2, Plate 8, and at the point where the flues part two dampers are hung on the same vertical spindle, one below the other, at right angles to one another, so that when one is open the other is closed. The dampers throughout the works are of cast iron, with carefully proportioned rims, and ribs tapering from the centre, so as to avoid distortion in the heat. They are hung on ball-bearings, and provided with a toothed rack and clip to fix them in any position. The dampers in the flues behind the boiler have stood well, and can be turned with a very slight pressure of the hand; but those in the greatest heat have not been successful, as they are found to warp and crack after 6 months' use. Even if a damper is made which will draw out of the heat when not in use, it is found to warp and jam the first time it is let down; and also as it is generally found that the damper is required to be only partially open for long periods, the iron framework is soon burned away. Rods are provided for regulating the steam blast, either from the back or the front of the furnaces. The blast apparatus adopted is of the type devised by Mr. C. W. James, in conjunction with the Author, in which the steam-nozzle has a flat adjustable orifice from which a ribbon instead of a plug of steam issues. If the air is carried

through the trumpet, mainly by surface friction between the issuing jet of steam and the surrounding air, then a flat jet should have a greater efficiency than a round one of comparatively large diameter. This is found to be the case at Oldham and elsewhere.

Dry steam is secured by wire-drawing it through a reducing valve from 130 lbs., which is the normal boiler-pressure, down to 70 lbs. per square inch, which is found to be a more economical working pressure for the jets. There are two blast nozzles and trumpets to each furnace, Fig. 3, Plate 8, one at each side, delivering into the side boxes, and they draw their air from the tunnel beneath the feeding-bin, and thus the noise of the jets is so muffled as to cease to be any inconvenience. The side boxes have separate face-plates, which can be renewed from time to time, as they burn out. It is found, however, that the steam and cold air passing behind them protects them so efficiently from being overheated that only fifteen new plates have been required in the 2 years and 2 months, during which these furnaces have been at work. As each furnace contains eight plates, their renewal is only a trifling expense compared with the cost of repairing brickwork sides. These side boxes have the great advantage that they prevent the hot clinker from sticking to the sides of the furnace, as it always does to brickwork, bringing away pieces of the brick with it when it is removed, and thus undermining the furnace crown, and letting it fall long before it is really worn out. The side boxes, besides preventing this action, render the operation of removing the clinker easier, and also heat the air in its passage. The vents for the air are all beneath the bars, and are so arranged as to deliver the air-supply very evenly into the ash-pit.

The ash-pit temperature, with the old system of steam blast delivered direct into it, was 130° F., while with the cast-iron side boxes it is very much higher. At Edinburgh and Bradford, where the air is first heated to some extent by its passage along the hot-air duct, before reaching the side boxes (in which it is further heated) the ash-pit temperatures are found to be about 400° F. The clinker doors of all the ten furnaces are of the fourfold hinged type, with a division in the middle. This arrangement makes it somewhat more difficult to withdraw the clinker than when doors the full width of the furnace are employed, and it is also open to the same objection as the old-fashioned flap door, that the open door exposes a red-hot baffle plate which radiates its heat on the stoker's face. These considerations led to the subsequent

adoption at Bradford of a full-width sliding door with vertical lift and balance weight, Fig. 7. Plate 8. This door is lined with 4 inches of firebrick and consequently is always cool, and up to the present it is found quite satisfactory. The staying of the furnaces was carefully arranged with tie-bolts only at the top and bottom and vertical backstays of H iron, pressing the longitudinal stays to the furnace fronts. In order to make these backstays bind against the middle of the furnace, they were given an inward camber, and, when nipped up, they were pulled straight by the tie-bolts. The ten furnaces had eventually to be stopped for a fortnight, while the temporary flues were removed, and the new mattress chamber and permanent flue-work constructed. The mattress chamber is for cremating bulky articles such as baskets, carcases and worn out bedding, and is simply an enlargement of the main flue placed at the end of the six furnaces, and provided with a large opening covered by iron-plate doors on the deck level, and also with a bottom cleaning-door for removing the residuum. The passage of the gases through it is found quite effective in destroying the articles put in.

During the 2 years since the completion of these works, the whole plant has continued to work satisfactorily. The destructor began to supply steam regularly to the electric-light station in the middle of March, 1896, at the required pressure of 130 lbs. per square inch, and to the extent of about 115 I.HP. (reckoning 20 lbs. steam per I.HP. per hour), this being the first instance of the supply of high-pressure steam raised by the burning of refuse to a central electric lighting station for public supply. The installation of a second high-pressure Lancashire boiler, of the same dimensions as the fist, is the only alteration of the plant which has since been made, and the construction and setting of it, and the necessary flues, was carried on without interfering with the working of the existing plant until February, 1898, when the four-cell group was stopped for a few days in order to couple it to the new boiler. As the plant now stands, either boiler can be coupled so as to perform any part of the work; and they join in contributing the whole of their steam to the electricity station whenever required. The boiler of the four-cell group is provided with a separate Kennedy hot-water meter, and its output is registered daily. Either boiler can be fed by a feed-pump, or injector, or both, either with hot water through the economizer or with cold water from the town main or from the reservoir. The hot gases from both groups join in front of the economizer, through which the whole or any proportion of them can be passed.