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CHAPTER X.

VENTILATION.

THE problem of the effective ventilation of a ship is a more difficult one than the ventilation of a building. In the case of a building, ventilation is assisted because the porous nature of the walls allows air to diffuse through, this diffusion being all the more effective as the difference of temperature within and without is greater. In the case of a ship, however, the conditions are quite different, and the impervious nature of the skin renders diffusion through impossible; ventilation must be obtained by the actual introduction of fresh air and the withdrawal of the foul air.

Ventilation can be either natural or artificial. By natural ventilation is meant supply and exhaust without the aid of fans. By artificial ventilation is meant that fans are used to draw pure air down to, or draw foul air away from, the space to be ventilated, or both. Natural ventilation is used for the ventilation of the 'tween deck spaces, and for the special cases of the coal-bunkers and spirit-room. Artificial ventilation is used for the other spaces below, as magazines, shell-rooms, store-rooms, etc.

For the 'tween decks cowls are used which can be turned to face the wind, the exhaust taking place through the hatchways, and in some cases through other cowls which can be turned away from the wind. The side-lights are also useful for ventilation purposes. For the spirit-room it is necessary to have an independent supply and exhaust, with cowls on the upper deck, as it is undesirable to have any connection between the spirit-room and any other system of ventilation which might communicate with a magazine.

Coal-bunker Ventilation.-The efficient ventilation of coalbunkers is of extreme importance, because of the gas that comes from the coal. This gas when mixed with air forms an explosive mixture, and if it is allowed to accumulate may cause serious

explosions. The gases are especially liberated from the coal, if a sudden fall of the barometer occurs, or if the temperature rises.

It is worth noting in this connection that it is undesirable to take wet coal on board, because the moisture causes a rapid and dangerous generation of heat and gas. The coal should also be kept as dry as possible (thus when cleaning the main deck the coaling scuttles should be kept closed.) Temperature tubes are provided in all coal-bunkers, so that the temperature in the body of the coal may be ascertained at frequent intervals. No light except a safety lamp must be used inside a coal-bunker until it is ascertained that the bunker does not contain explosive gas. Special precautions in this respect are necessary for a few days after coaling.

In the ventilation of a coal-bunker two pipes are employed, one for the introduction of fresh air and one for the withdrawal of the foul air. The latter is led where possible up a funnel casing. These casings under ordinary circumstances will be hot, and this causes the air in the pipe to rise. This induces a current of air over the top of the bunker, and fresh air is then drawn down the supply pipe leading from the upper deck.

There are slight differences in the fittings of different ships, but the general principles will be understood by Fig. 104, which shows the ventilation of the coal-bunkers of a battle-ship. In this type of ship there are three separate series of bunkers, which are each independently ventilated.

1. The upper bunkers, behind the armour, which extend over the length of the double bottom.

2. The lower bunkers, abreast the engine- and boiler-rooms.

3. The wing bunkers, abreast the engine- and boiler-rooms (can be used for coal if desired, and in the deep load condition these are assumed to be filled).

1. Upper bunkers.-These bunkers are formed into groups, Fig. 102 shows the arrangements for the forward group of four. A trunk is taken into the funnel casing into which all the four bunkers exhaust. Each bunker has a separate supply by means of a pipe leading from the upper deck. The coaling scuttles in the main deck can also be removed and the gratings put in if desired.

2. Lower bunkers.-The supply pipes for these bunkers are taken down the ventilators in order to avoid piercing the thick decks, and the exhausts are taken up the funnel casings. A separate supply and exhaust is taken from each bunker, the supply pipe being taken to the far corner so as to get the current of air well over the top of the bunker.

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3. Wing bunkers.-These bunkers are ventilated similarly to the lower bunkers.

The upper end of each supply pipe is fitted with a louvre with fittings for locking, and the louvre has engraved upon it, "Not to be closed without special order." The top ends of the exhausts are led above the upper deck, each with a mushroom top and a throttle

POAT DECK.

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- PLAN.

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FIG. 104.-Ventilation of coal-bunkers.

valve for closing. Where these pipes have a bend where water is likely to accumulate, a plug is fitted so that the water can be drained away. The flanges of the deck beams have holes as necessary to allow air to circulate freely over the top of the coal when the bunker is full. The slide valves and throttle valves to

the supply and exhaust pipes have been omitted in recent ships. Also the exhausts end in a louvre on the side of the casing.

Double Bottoms, etc.-It is important to note that care must be taken in dealing with confined spaces which are closed for long periods. In such spaces poisonous gases are likely to accumulate, and nobody should be allowed to enter until it has been ascertained that the air is pure enough to breathe. This can be done by putting a lighted candle in for a few minutes. The manholes to such spaces are in pairs (see Fig. 49), fitted as far apart as possible, and hand fans can be used to ventilate the spaces before entering.

Ordinary Ship Ventilation.-1. With large steam-driven fans. -For the artificial ventilation of spaces below, the system adopted, up to and including Canopus class, was by means of a number of large steam-driven fans. Trunks were led from these fans, and these trunks were pierced by louvres to allow air to pass into the compartments through which the trunks pass. Fig. 105 shows some specimen leads in a ship of the Majestic class. In these ships there were ten fans, eight being 6 ft. in diameter, two being 4 ft. 6 in. in diameter. Some of the most important of these are connected together, so that if a fan breaks down the other can do its work. With this system the watertight bulkheads are constantly pierced by the non-watertight trunks, and it is necessary at such places to fit valves which will automatically close the opening. These valves are designed so that if water rises on either side of the bulkhead or floods the flat, a float is lifted which releases the valve closing the opening. Two forms of these automatic valves have been largely adopted, viz. Beck's and Broadfoot's (Figs. 63 and 64). In Beck's valve an ordinary slide valve is used, which is connected to a balance weight. When the float rises it releases this balance weight, and the valve then shuts. In Broadfoot's valve, the valve turns round and closes the opening. In either case a small hole is made in the bulkhead with a pipe leading to the float, so that if water rises on the opposite side to the float, the valve will be shut. This hole is closed by the movement of the lever carrying the balance weight. Small pipes are led from wells in the main or upper deck, so that in case of emergency the valves can be closed by pouring water into these wells.

There are objections to this system, viz.—

(i.) The large fans require steam-engines, which heat the compartments, and which require long lengths of steam and exhaust pipes. (ii.) The use of the automatic valves to keep the flats and

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