Principles of naval engineering - Historic Naval Ships Association

CHAPTER 23GAS TURBINESThe gas turbine engine, long regarded asa promising but experimental prime mover,has in recent years been developed to the pointwhere it is entirely practicable for ship propulsionand for a number of auxiliary applications.Gas turbine engines are currently installedas prime movers on minesweepers,landing craft, PT boats, air-sea rescue boats,hydrofoils, hydroskimmers, and other craft. Inaddition, the gas turbine engine is finding increasingapplication as the driving unit forship's service generators, pumps, and otherauxiliary units.Although the gas turbine engine as a typeneed no longer be regarded as experimental,many specific models of gas turbine engines arestill at least partially experimental and subjectto further change and development. The discussionin this chapter therefore deals primarilywith the general principles of gas turbineengines rather than with specific models. Detailedinformation on any specific model maybeobtained from the manufacturer's technical manualfurnished with the equipment.BASIC PRINCIPLESThermodynamic cycles are discussed in chapter 8 ofthis text.The gas turbine engine bears some resemblanceto an internal combustion engine of thereciprocating type and some resemblance to asteam turbine. However, a brief considerationof the basic principles of a gas turbine enginereveals several ways in which the gas turbineengine is quite unlike either the reciprocatinginternal combustion engine or the steam turbine.Let us look first at the thermodynamiccycles of the three engine types. The reciprocatinginternal combustion engine has an open,heated-engine cycle and the steam turbine*^ hasa closed, unheated- engine cycle. In contrast, thegas turbine has an open, unheated- engine cycle—a combination we have not previously encounteredin our study of naval machinery. The gasturbine cycle is open because it includes theatmosphere; it is an unheated-engine cycle becausethe working substance is heated in a devicewhich is separate from the engine.Another way in which the three types ofengines differ is in the working substance. Theworking fluid in a steam turbine installation issteam. In both the reciprocating internal combustionengine and the gas turbine engine, theworking fluid may be considered as being thehot gases of combustion that result from theburning of fuel in air. However, there are veryimportant differences in the way the workingfluid is used in the reciprocating internal combustionengine and in the gas turbine engine.Still other differences in the three types ofengines become apparent when we consider thearrangement and relationship of component partsand the processes that occur during the cycle.From our study of previous chapters of thistext, we are already familiar with the functionalarrangement of parts in steam turbine installationsand in reciprocating internal combustionengines. Now let us look at the relationship ofthe major components in a basic gas turbineengine, as illustrated schematically in figure23-1.In the steam turbine installation, the processesof combustion and steam generation takeInternal combustion engines are discussed In chapter22.3 Steam turbines are discussed In chapter 12 and Inchapter 16.595