The Modern Destructor.

Progress, 1 October 1907, Page 450

The Modern Destructor.

[By T C. Ormiston Chant, A.M I.C.E.]

In the earlier days of the refuse destructor it was of first importance to place it as far as possible away from habitations, but in a position as central as was consistent with the reduced cost of carting refuse to the site. With the advent of the modern destructor, and its absolute freedom from nuisance caused by dust and smell, the former consideration no longer weighs in choice of site, but many other points have tOjbe taken into account. In order to be able to select a type of plant suitable for the existing conditions, the following points must be considered in the order given below :—: — [] ] 1. Whether to erect at different parts of the community two or more destructors of small size, or to place one large one m a central position to deal with the whole of the town's refuse. 2. Whether the site (or sites) chosen admits of the economical disposal of superfluous clinker. 3. What type of plant is the most econo mical. Taking the first and second points, it is safe to say that, except in very densely populated towns where the area of collection is small and the sites available are few, no destructor of a greater capacity than 160 tons per day of 24 hours should be installed, unless there are exceptional facilities for disposal of any superfluous clinker. With proper manipulation a good destructor reduces refuse to about a quarter of the quantity of hard vitreous clinker, a most valuable by-product wherever new roads and buildings are being made. In some cities, however, no use can be found for the clinker, and then the cost of carting it away becomes an item of importance. It is easy to imagine that sometimes there may be no available central site large enough for one big destructor in a town. The question then arises as to the advisability of placing one large destructor without, or two smaller ones within, the area of collection of refuse, and in suitable positions. The greater cost of working and maintaining two small plants would be quite likely to exceed the extra cost of carting to the one large plant outside the town. Taking the third point, the most economical destructor is that which costs least to operate and maintain, in proportion to the amount of useful heat and by-products available. The plant best able to satisfy all these conditions is the one which, firstly, maintains a temperature averaging about 2000 R, never falling below 1250° F. and never exceeding 2900° F. Secondly, suitable boilers should be installed capable of raising all the steam that the heat from the furnace can generate. Thirdly, heated forced draught should be used in the furnaces, and the steam used to obtain the blast should be a small proportion of the total steam generated in the boiler. Fourthly, the operations of feeding the refuse into the furnace and withdrawing the clinker should be conducted from different sides of the furnace. Fifthly, the blast should be heated by the waste gases. Sixthly, the boiler feed water should be heated by any exhaust steam available, or where suitable, an economiser. Lastly, the atmosphere in which the men operating the plant have to work should be as pure as possible. In designing the " Heenan " destructor all these points have been given proper thought, and it is here my intention to describe the plant now being

erected in Wellington by the makers, Messrs. Heenan and Froude, Ltd. The Corporation yard at^Clyde quay fj is an_excellent position for the destructor, because it is fairly central for carting, and any excess of clinker can be tipped near the site for reclamation purposes. Moreover, part of the steam generated will be used for pumping up sewage to enable it to gravitate to the outfall at Lyell Bay. The rest of the steam can be used to drive mills for crushing the clinker and making mortar and paving flags, and even generating electricity to light the destructor buildings, pumping station, stables, and part of the neighbourhood. The " Heenan " destructor furnace is composed essentially of two or more intercommunicating cells with independent ashpits, connected to a combustion, or dust-settling, chamber, a boiler, an air heater (or regenerator), and a centrifugal fan for supplying the blast. There will be three such furnaces (or three-cell units as they are termed) in the Wellington destructor, and each unit is entirely independent. The cells m that particular plant are three in number for each furnace, and there are three furnaces. These cells form a firebrick-hned chamber, 15 feet by 5 feet, rooted by an undulating firebrick arch, and provided with three ashpits, and three grates, only separated from each other by an air-cooled, rounded dead grate. The cells discharge their burning gases into a large and deep chamber, called the combustion, or settling chamber, from which the gases, having been completely burnt at a temperature of about 2500° F., pass to the boiler. The type of boiler best suited for use with destructors is the Babcock and Wilcox water-tube W.I.F. type. Three double-drum boilers of this type will be installed at Clyde quay works ; the working pressure will be 160flbs. per square inch, and the heating surface 1966 feet. Immediately after passing through the boiler the gases, still at a temperature of between 700° F. and 1000° F. enter the regenerator, which is composed of about 350 vertical wrought iron tubes 4 feet high. The gases from the bo^er pass through the tubes, while the air to be supplied to the furnaces passes cutside the tubes in a horizontal direction. The temperature of air blast is thus raised to about 350° F., while that of the waste gases is lowered to between 370°F and 670°F , according to their initial temperature From the regenerator the gases pass along a flue comm-jn to each three-cell unit to aCreen's fuel econnmiser (which I perceive was fully described in a recent issue of Progress) also common to each unit. At the back of each cell is a gas-tight lifting counterbalanced door through which the refuse is fed from a hopper. The heterogeneous nature of the refuse makes it absolutely necessary for uniform working that the fires should be charged by hand, and at regular, and not too infrequent, intervals. The fireman must use a certain amount of discretion in shovelling the refuse, so as to get a fire of uniform composition. Any existing system of mechanical charging is unsatisfactory, and in some cases quite useless, because of the high cost of upkeep. At the front of each cell, and at a slightly lower level than that behind, is a larger gas-tight counterbalanced door through which the clinker is withdrawn from the grate. The advantages of the two openings for separate operations are that, (a) there is no chance of unburnt refuse being taken away with the clinker ; (£>) men charging furnaces are completely out of the way of those withdrawing clinker ; (c) the chargmg floor can be nearer the level of the chargmg door, thus facilitating chargmg, and the clmkering door can be at such a height above the floor that the men at work need not stoop. The advantage of the communicating cells is obvious, for if a cell be freshly charged, the fumes from it have to mix with those of the adjoining cells in the furnace itself, thereby ensurmg complete combustion as early as possible. The combustion chamber is carried below floor level, and is sufficiently large to only necessitate cleaning of dust once a week. By thus deepening the chamber, less surface is exposed to the outer air, and loss of heat by air leakage is reduced. The chamber is provided with a gas-tight door of sufficient size to enable a whole carcase of an ox or horse to pass in ; while in the by-flue close to the chamber, offal can be rendered harmless in a few minutes. The offal is dropped down a gas-tight shoot from the tipping floor. |i One of the features of the Wellington plant is that beneath each boiler is a grate for coal fire, should there not be refuse available. Forced draught and hot blast will also be supplied to these coal grates Each unit is piovided with a by-flue through which all gases can pass to the chimney without using either boiler, air heater, or economiser, should it be necessary at any time to burn a large quantity of refuse without generating the corresponding amount of steam.

A complete system of dampers controls the direction the gases take after leaving the combustion chambers. Another feature of the destructor is the method of ventilating the building. In the roof a wood trunk, provided with properly-proportioned openings to the interior of the building, runs the whole length of the building, and has no openings except in the lower windows and the doors, and this trunk is connected up to the inlet of each of the three forced-draught fans. Consequently, when one of the fans is working it sucks air from the trunk in the roof, which collects all dust-laden air from the interior of the building, and thus causes a constant inrush of fresh air. The dust-laden air is then projected by the ian through the air-heater into the furnaces, and the dust eventually settles in the combustion chamber The fans used for the blast are direct coupled to high-speed engines. By this means the blast can be maintained at a pressure of over 3-inch water gauge in the ashpits with a consumption of only about 5 /0/ 0 of the total steam generated. * The very best results ever published from experiments with the steam-jet air blower show 10% of the total steam generated with an ashpit pressure of only l^-inch water gauge , 15% is a more usual figure, while 20% and even 30% are not uncommon after the steam nozzle has been scoured away by three or four months' wear. In order that the clinker withdrawn from the furnaces may be handled as easily as possible, a clinker railway for suspended tipping tubs will be provided to carry the clinker straight to the crushing mill where it will be crushed and screened into' three or four sizes, and stored in hoppers ready for use A mortar mill and paving flag press will also be installed as soon as possible. In conclusion, it may be stated that a complete apparatus has been provided for testing the Wellington plant, and proper provision has also been made for inspection holes, pyrometer and pressure recorder openings, and for cleaning doors.