The most common types of triple-engines have the cylinders arranged in the sequence— high. intermediate, low; the condenser forms part of the engine-framing, and the pumps are placed at the back of the condenser and worked by levers. In the smaller engines the eylin ders are rigidly bolted together; but in the larger they are free, and connected only by a pair of bar-stars fixed to their centers. This is customary. in order to prevent the extension of the distance between the centers when the engines are heated : but it is a point which appears more important in theory than in practice. and it is doubtful whether the greater rigidity of the bolted cylinders in the smaller engines is not a much more important feature in ordinary work. In naval vessels vertical engines are now- almost uniformly adopted. and the necessary protection for the cylinders is obtained IT an armored hatch. ]n the later designs the larger engines are made open-fronted, with standards of cast steel at the back and wrought-steel pillars in front. Feed, bilge, and circulating pumps are worked by separate engines. For the air-pumps also separate engines have sometimes been adopted, and they possess great merits for purposes, as the vacuum can be maintained and the condenser kept clear of water while the main engines are standing, and the latter are thus ready to answer more in stantly any order which may be given. With the three-crank engine, however, this is of less importance than with the two-crauk type. In modern cruisers, which are designed with the view of steaming upon emergency at a very high speed, and ordinarily at about half that rate, the engines become much too large for the power developed at slow speeds. and in consequence are not economical under the ordinary condition of working. In larger vessels this difficulty is met by separating each set of propelling engines into two sets of half the capacity, the one forward of the other, and so arranged that the forward set may be disconnected, with the after set left to do the work. The propelling engines of the Italian cruisers Lepanto, Italia., IIc rmberto, and Sardegna, and of the British cruisers Blake and Blenheim. have been ar ranged on this plan. The general details of the engine have not undergone many modifica tions, but still have not remained without change.
Pishm - Val high steam pressures have become common, piston-valves have be come the rule for the high-pressure cylimler, and are not unusual for the intermediate. When well designed they have the great advantage of being almost free Erin friction. so far 71.7 the valve itself is concerned. It is usual to fit springless adjustable sleeves. whieli have all the advantages of the old solid ring so for as their freedom from friction is coneellmed, and in ease of leakage they can with ease be adjusted by lining up at their joints. In smaller engines the same springless ring has been used for the pistons of the high-pressure and intermediate cyl inders. It may not give such absolute steam tightness as the spring ring, but tiny little leakage can be picked up in the low-pressure cylinder, and such very slight loss.of efficieney as may be due to this cause should be fairly well compensated by the diminislied friction of the valves. For low-Imressure cylinders piston-valves arc not in favor; if fitted with spring rings their friction is about as great as, and occasionally greater than, that of a well-hahmeecl slide-valve; while if fitted with springless rings there is always sonic leakage, which is irre coverable. But the large port clearances inseparable from the use of piston-valves are most
objectionable; and with triple-engines this is especially so, because with the customary late cut-off it becomes difficult to compress allfliciently fur insuring economy and smoothness, and working Nvhen in "full gear," without some special device.
Feed - Water ileat system is founded on the fail that. if the feed-water as it is drawn from the hot-well he raised in temperature by the heat of a portion of steam introduced into it from one of the steam-receivers, the decrease of the coal necessary to generate steam from the water of the higher temperature bears a greater ratio to the coal required without feed-heating than the power which would be developed in the cylinder by that portion of steam would bear to the whole power developed when passing till the steam through all the cylinders. The temperature of the feed is of course limited by the temperature of the steam in the receiver from which the supply for heating is drawn. Supposing, for example, a triple expansilm engine were working under the following conditions without feed-heating: Boiler pressure, 150 lbs.; indicated horse-power in high-pressure. cylinder. 398; in intermediate and low-pressure cylinders, together, 790; total, 1,118; and temperature of hot-well, It)0° V. Then with feed-heating the same engine might work as follows: The feed might be heated to 220° F., and the percentage of steam from the first receiver required to heat it would be 10.88 per cent. the indicated horse-power in the high-pressure cylinder would be as before. 398, and in the intermediate and low-pressure cylinders it would be per cent less than before, or 705, and the total would be 1,103, or 93 per cent of the power developed without fced-heating. Neanwhile the heat to he added to each pound of the feed-water aL 220° F. for converting, it into steam would be 1,005 units, against 1,125 units with feed at 100' R, equivalent to an ex penditure of only per cent of the heat required without feed-heating. Bence, the expend iture of heat in relation to power would be 804 93 = per cent, equivalent to a heat economy of per cent. If the steam for heating can be taken from the low-pressure re ceiver, the economy is about doubled.
Feed - Water order to make Imp the losses of water clue to leakage of steam from safety-valves, joints, etc., in engines supplied with surface-condensers. it was formerly customary to pump water from the sea, into the boilers. This involved deposit on the internal surfaces, and consequent loss of efficiency and danger of accident through overheating, the plates. With the higher pressures now adopted the danger arising from overheating is much more serious, and the necessity is absolute of maintaining the heating surfaces free from de posit. This Call be done only by filling the boilers with fresh water in the first instance, and maintaining it in that coalition. To do this two methods are adopted, either separately or in conjunction; either a reserve supply of fresh water is carried in tanks, or the supplementary feed is distilled sea-water by special apparatus provided for the purpose.