Marine Steam Generators

MARINE STEAM GENERATORS Water-tank Boilers.—In this type of marine steam genera tor the fire passes through the tubes and the water circulates out side the tubes. The principal boiler of the mercantile marine, and, till late years, of the various navies of the world, is the cylin drical Scotch marine boiler (fig. 7), the various parts being clearly indicated. The experience gained after many years with this boiler proved it to be undoubtedly a good, safe and easily under stood generator, and also comparatively easy to clean. These boilers are made single-ended and double-ended, and are cylindri cal in shape with flat ends. The illustration shows a single-ended type. The furnaces or flues are corrugated in order to allow for expansion and compression. One to four furnaces are fitted, either terminating in a common combustion chamber, or, a better plan, being fitted with an independent combustion chamber to each furnace. In the boiler shown the furnaces are fitted each with sep arate combustion chambers, and are stayed as shown. The tubes are of commercial size, being obtainable in practically every port in the world, and are fitted with cap ferrules at the combustion cham ber end to prevent burning away of the tube ends. The boiler is, however, excessively heavy, unequal strains are set up (unless a feed water circulator is used), and the space per horse-power is much greater than with the water-tube types. Another point against this boiler is the length of time taken to raise steam, ten to hours usually being allowed. Against these disadvantages we have the greater quantity of water carried, with consequent ease of keep ing steam and water levels, and the same skill is not required in firing up, cleaning fires, etc., as is necessary with the water-tube types. The boiler is fairly economical as compared with the water tube types. Of course, in the case of explosion, much greater dam age would be done owing to the greater steam and water space.

Water-tube Boilers.

As its name implies, in this type of gen erator the water passes through the tubes, and the heat from the furnaces outside the tubes. It is only within the last few years that the water-tube boiler has been seriously considered as a rival to the cylinder water-tank generator. For naval purposes, of course, there is no question as to which type is suitable; and for both naval and mercantile marine purposes the two outstanding features of the water-tube boiler are common. Briefly, these two points are as follows : (a) the water-tube boiler is very much lighter ; (b) the boiler requires much smaller space for the same horse-power, or gives much greater horse-power for the same space. Both these fac tors are of paramount importance in the merchant service. By the reduction of boiler weight and space the cargo-carrying capacity, or passenger accommodation, is greatly increased, with consequent greater earning power for the vessel.

It must be borne in mind that much higher pressures can be car ried, which is essential for warships, mail steamers, and cross Channel packets, especially when turbine-engined ; and this pres sure can be carried with perfect safety. Forcing, that is, using a higher pressure than that for which the boiler was designed, can be carried out, and steam can be raised very quickly without undue strain on the working parts.

One of the best known marine water-tube boilers is the Pirrow (fig. 8). The construction is simple and thus the process of clean ing, an all-important matter with water-tube boilers, is an easy matter. The Yarrow boiler consists of a large water drum and two lower water drums, connected by a series of inclined generating tubes (fig. 8). The boiler feed water is pumped into the steam drum, descends through the tubes most remote from the fire and rises through the tubes nearest the fire, where it is converted into steam. After leaving the steam drum the steam enters a super heater, where it is superheated, and thence through the steam valve to the engine.

General.

The modern trend is towards much higher boiler pressures, higher temperatures, and the more efficient burning of the fuel and generation of steam in the boilers. There is no reason why pressures of 700 lb. or Boo lb., should not be carried, as the trials of the high-pressure turbine steamer "King George V." were most successful, and showed a marked improvement in the con sumption of fuel, in this case coal, compared with such a vessel using steam at the normal working pressure. Superheating the steam, air preheating, increase in boiler feed water temperature and analysis of the funnel gases are all receiving due attention, as are also the best means to adopt to ensure protection and cleanli ness of the boiler surfaces against corrosion.

Prevention of Boiler Corrosion.

Zinc slabs or bars are fit ted to the interior of marine boilers so as to be electronegative to all other parts of the boiler to attract the galvanic attention of acids. The zinc slabs are suspended in the steam and water spaces of the boilers ; in some water-tube boilers zinc angle bars are used.

It is essential that slabs are fitted so as to protect the whole of the surfaces, and worn slabs should be renewed. The latest method of preventing boiler corrosion is the electrolytic. By this system elec trolytic conductivity is produced in the water by supplying a cur rent of electricity from a source outside the boiler to metallic anodes, insulated from the boiler shell. The current can be sup plied continuously and its strength controlled; i.e., increased or de creased regardless of the wear or deterioration produced on the anodes, thereby ensuring permanent efficiency of protection.