DIFFERENCE BETWEEN STEAM TURBINES AND RECIPROCATING STEAM ENGINES


The reciprocating steam engine utilizes economically a greater amount of the available energy of the steam at high pressures than at low pressures. The turbine, on the other hand, has the advantage at low pressures and in cases where large expansion of the steam are uses.

To obtain the same steam expansion in a reciprocating engine, the cylinders would be very large with greater cylinder losses. The greater expansion of steam in turbines permits the recovery of a large amount of power from what otherwise (in the reciprocating steam engine) would be waste steam.

The most important energy losses that occur in a turbine include: steam leakage, wheel and blade rotation, left-over velocity of the steam leaving the blades, heat content of the exhaust steam, radiation, and friction.

The steam consumption of the reciprocating engine and the turbine are approximately the same, but the steam turbine operates on the complete expansion cycle, the reciprocating engine operates on a cycle with incomplete expansion.

The turbine, therefore, converts into work a greater amount of heat per pound of steam used.

Although steam pressures and temperatures, and turbine designs continue to improve the overall efficiencies of power plants, nevertheless the thermal efficiency of steam turbines hovers around 35 percent.

The steam turbine, however, is well adapted to the driving of electric generators of the alternating current type as the desirable speeds of the two machines are the same, and is almost universally used in steam plants larger than about 1000 kW capacity.

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