ing the high pressure exhaust passage with the low pressure slide valve chests. The success of the compound system in point of economy is thoroughly established, and may be explained by the fact that owing to the moderate degree of expansion employed in each cylinder, the metal is exposed to a smaller range of temperature, and there is less liquefaction; consequently the loss of heat by the cooling and re-heating of the cylinders is less than in simple engines. The loss by clearance, also, is comparatively small. In this system economy is obtained without the disadvantage of producing the severe straining actions on the machinery above referred to. SURFACE AND JET CONDENSERS. Figs. 29 and 30 show sections of a jet condenser. The steam, after doing its work in the cylinder, is led to a large chamber C called the condensing chamber, where it meets with a jet of cold sea water. The water is introduced to the chamber by a pipe S with a number of slits or holes in it called a spray pipe. The effect of this is to divide the water into a number of small streams thus condensing the steam and forming a vacuum. The condensed steam and water fall to the bottom of the air-pump chamber K and are pumped by means of the air pump P into the hotwell H. Some of this water is pumped back again to the boiler by the feed pump, but the larger part is discharged overboard by the discharge valve D. Sea water contains one-twentieth of its volume of air at the ordinary pressure of the atmosphere, and this would collect in the condenser and spoil the vacuum. The air pump P pumps out this air, any uncondensed vapour and water from the chamber C, and maintains the vacuum. The foot and delivery valves vv vv of the air pump are generally made of vulcanized indiarubber, circular in shape. The valves are fitted over a gun-metal grating, and on the upper surface of the valve is fixed a guard to prevent the valve from lifting too high. Bilge Injection Pipe is a pipe leading from the condensing chamber to the engine-room bilge and fitted with a grating to prevent chips, etc., getting to the condenser. In the case of a serious leak and the engine-room bilge becoming filled with water, the necessary water for condensing the steam can be obtained by this means from the bilge by opening the bilge injection valve and closing the sea injection valve. Salts in Sea Water and Boiler Scale.-Sea water contains in solution chiefly chloride of sodium (common salt), sulphate and carbonate of lime, and sulphate of magnesia. Common salt is soluble in water at all temperatures, but at a temperature of about 300° Fahr., the sulphate of lime is deposited on the boiler plates, thereby forming a hard scale which is a bad conductor of heat. To reduce the amount of scale formed on the plates, some of the water in the boiler is blown out and its place supplied by the less dense sea water. To prevent scale forming on the plates of the boiler and the loss resulting from blowing out (sometimes 15 per cent. of the whole value of the fuel burnt) the surface condenser was introduced. Figs. 31 and 32 show a vertical surface condenser. The steam passes through a number of small brass tubes and the cold sea water is circulated round the tubes by means of a pump called a centrifugal pump. The condensed steam is pumped by the air pump P, in this case vertical, to the hotwell, and the whole of it pumped by the feed pump to the boiler. The tubes (which have been omitted for the sake of clearness) are secured to the tube plates AA, BB and are parallel to the stays S S S S. The tubes are made water-tight at the ends, and free to expand and contract under changes of temperature by small screwed glands and tape packing. A circulating plate CC (Fig. 32) is fitted in the condenser for circulating the water more thoroughly around the tubes. The course taken by the circulating water is shown by the arrows. Sea Injection.-A spray pipe S is shown in the upper part of the condenser (Fig. 32) into which sea water is admitted. In the event of the circulating pump breaking down, the condenser can be turned into a common jet condenser. Auxiliary or surplus feed pipe is a small pipe for admitting sea water to the condenser or hotwell to make up any waste of fresh water which may take place. Boiler Deterioration.-With the introduction of the surface condenser a very serious drawback was experienced. It was found that the boilers of ships using surface condensation became deteriorated in a very rapid manner. This was found to be due principally to the presence of moist air in the boilers, and most of the precautions that have been adopted are on the basis of excluding moist air from the boilers, or of neutralizing its effect when present. Zinc plates, in good metallic contact with the plates or stays of the boiler and suspended below the water level, are found to afford complete protection against corrosion, whether arising from the water itself, galvanic action, or air admitted with the feed. Pumping Power. The pumps used in a large armoured ship for pumping water from the bilge are: 1. Centrifugal circulating pumps for surface condenser which are worked by separate small engines, and discharge the bilge water through the condensers overboard, or by a separate discharge valve overboard. 2. Engine bilge pumps, worked by the main engines. 3. Fire engine worked by a separate small engine. 4. Auxiliary bilge engines, two in number, worked by separate small engines. 5. Friedmann's ejectors. A jet of steam is admitted to a large pipe discharging overboard. A branch from this pipe is led down into the bilge or other part required to be readily cleared of water. On admitting steam to the discharge pipe a partial vacuum is formed in the suction pipe, and thus the bilge water is discharged overboard. COMBUSTION. COMBUSTION may be defined as the rapid chemical union of the oxygen of the air with the hydrogen and carbon present in the fuel whereby heat and light are evolved. The chief combustible constituents of coal are carbon and hydrogen, and their chemical combination with the oxygen of the air is the source of heat used in steam boilers. The following table gives approximately the composition of North Country and Welsh coals: |