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VOL. 38. CLEVELAND, AUGUST 6, 1908. NEW YORK No. 6 a ---- The Combination System of Reciprocating Engines and Steam Turbines BY THE. HON. ©. A. PARSONS AND. R: J. WALKER, E50., MEMBER. In the early years of steam turbine design and development it became appar- ent that the turbine engine was capab!e of economically dealing with ratios of expansion far beyond the reach of any reciprocating engine whose limitations in this respect had been experimentally de- termined by many investigations. In 1889 the first condensing turbine of about 100 H. P. was designed for an ex. pansion ratio of 100 by volume, the ex- pansion being effected in two turbines of the double parallel flow type, the low- pressure turbine (Fig. 1, Plate 1,) taking steam from the exhaust of the high-pres- stire at atmospheric pressure and expand- ing it down to 1 lb. absolute. 'The striking feature presented by this design was the very high estimated eft- ciency of this low-pressure portion. A separate low-pressure turbine was not, however, actually constructed till some years later. In 1894 a patent was taken out for the "combination" of a reciprocating engine with a steam turbine whose object was "to increase the power obtainable by the expansion of the steam beyond the limits possible with reciprocating engines." The previous treatment of the steam is, of course, immaterial, provided that its con- dition of pressure and wetness on reach- ing the engine are known. The first instance of a separate turbine worked from the exhaust of other tur- bines was in the Turbinia's machinery in 1897--the pressure at entry of her low- pressure turbine was about 9 lbs. abso- lute, and the exhaust 1 Ib. absolute. The slip ratio of her three shafts show- ed that the low-pressure turbine developed about one-third of the total horsepower obtained from the steam at 160 Ibs. pres- sure, agreeing closely with calculations. In the year 1902 the combination of re- ciprocating engines exhausting into tur- bines was first put to a practical test in H. M. Destroyer Velox. In this vessel two small reciprocating engines were fit- ted for cruising, purposes, of such power that, in combination with the main tur- bines, they would give an economical con- sumption at the speeds of 11 to 13 knots, the usual cruising speeds at the time the Velox was built. The arrangement of michinery con- sisted of one main high-pressure and ore low-pressure turbine on each side of the vessel, each driving a separate shaft, or four shafts in all. The two small recip- rocating engines were coupled at the for- ward end of each of the low-pressure turbines. For speeds up to about 13 knots, steam was admitted to the two re- ciprocating engines, and expanded down to about atmospheric pressure; it then passed through the high-pressure, and thence through the low-pressure turbines to the condenser. This combination gave excellent results at these cruising speeds. For speeds above 13 knots, however, the reciprocating engines had to be cut out and steam admitted to the turbines alone. With the advance of naval efficiency, the cruising speeds of war vessels have been increased, and in vessels subsequent to the Velox additional high-pressure tur- bines have been fitted, an arrangement which permits of good economy over a wide range of cruising speeds. It may be said that perhaps the most important field for the combined system of machinery as applied to marine propul- sion is for those installations where the designed full speed of the vessel falls be- low the range suitable for an all-turbine arrangement, the reciprocating engine working in the region of pressure drop where the conditions are best suited for it, and the turbine utilizing that portion of the expansion diagram which the re- ciprocating engine is not able to utilize efficiently. It is generally well known that an all- turbine arrangement has not been advo- cated by us for ships where the designed speed falls below 15 or 16 knots, excepting in some special cases stich as yachts; and for vessels of moderate or slow speed, the combination system of machinery appears to be eminently suitable. In a good quardruple reciprocating en- gine, the steam is expanded down to the pressure of release, about 10 lbs. absolute, and gains in economy as the vacuum is increased up to about 25 in. or 26 in,, whereas, in a turbine, it is possible to deal economically with very low-pressure steam, and to expand this low-pressure steam to a low absolute pressure corres: ponding to the highest vacuum obtainable in turbine practice. Fig. 2 and 3 show the effect of vacuum upon steam consumption as the result of tests carried out on a reciprocating engine and steam turbine respectively, from which it will be noted that, whilst the curve for the reciprocating engine gives the mini- mum consumption at between 25 in. and 26 in. vacuum, the curve for the turbine continues to fall as the vacuum increases. Fig. 4 (Plate I.) shows in diagrammatic form, the advantage of the combined sys- *Read at the spring meetings of the forty- ninth session of the Institute of Naval Archi- tects,