Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGCOMBUSTIONCHAMBERSTARTERPROPULSIONPOWERCOUPLINGCOMPRESSOR SHAFT TURBINEATMOSPHERICAIR INTAKEEXHAUST TOATMOSPHERE147.135Figure 23-1.— Schematic diagram showing relationship of parts in single-shaft gas turbine engine.place in the boiler, while the process by whichthe thermal energy of the steam is converted intomechanical work takes place in the turbine. Inthe reciprocating internal combustion engine,three processes— the compression of atmosphericair, the combustion of a fuel-air mixture,and the conversion of heat to work— all takeplace in one unit, the cylinder. The gas turbineengine is similar to the reciprocating internalcombustion engine in that the same three processes—compression,combustion, and conversionof heat to work— occur; but it is unlike thereciprocating internal combustion engine in thatthese three processes take place in three separateunits rather than in one unit. In the gasturbine engine, the compression of atmosphericair is accomplished in the compressor; the combustionof fuel is accomplished in the combustionchamber; and the conversion of heat to workis accomplished in the turbine.Many different types and models of gasturbine engines are in use. The gas turbineengine shown in figure 23-1 is called a single -shaft type because one shaft from the turbinerotor drives the compressor and an extensionof this same shaft drives the load.The gas turbine engine shown in figure 23-2is called a split- shaft type. This engine is consideredto be split into two sections: the gasproducingsection, or gas generator, and thepower turbine section. The gas-generator section,in which a stream of expanding gases iscreated as a result of continuous combustion,includes the compressor, the combustion chamber(or chambers), and the gas-generator turbine.The power turbine section consists of apower turbine and the power output shaft. In thistype of gas turbine engine, there is no mechanicalconnection between the gas-generator turbineand the power turbine. When the engine isoperating, the two turbines produce basically thesame effect as that produced by a hydraulictorque converter. The split shaft gas turbineengine is well suited for use as a propulsionunit where loads vary, since the gas-generatorsection can be operated at a steady and continuousspeed while the power turbine sectionis free to vary with the load. Starting effortrequired for a split-shaft gas turbine engineis far less than that required for a single-shaftgas turbine engine connected to the reductiongear, propulsion shaft, and propeller.In the twin- spool gas turbine engine (fig.23-3) the air compressor is split into twosections or stages and each stage is drivenby a separate turbine element. The lowpressureturbine element drives the low pressure compressorelement and the high pressure turbineelement drives the high pressure compressor596