Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGThe MHD conversion is similar to the thermoelectricconversion and the thermionic conversionin some respects but quite unlike themin others. All three conversions involve the directconversion of thermal energy into electricalenergy, without the intervening step of conversionto mechanical energy; in this sense, all threemay be regarded as "direct energy conversions."But the MHD conversion, unlike the thermoelectricand thermionic conversions, requiresa working fluid— namely, a hot ionized gas.In this respect, then, the MHD conversion issomewhat less a "direct energy conversion"than the other two processes.The major problems in connection with magnetohydrodynamicconversion arise from the factthat extremely high temperatures (in excess of4000 ° F) must be developed in order to produceionization of the gas. Obviously, such high temperaturespose materials problems. Also, it isdifficult to achieve such temperatures on thelarge scale desired for MHD generators. Nuclearreactors capable of operating at these ultra- hightemperatures are under development but are notfully operational. When chemical fuels such asoil or powdered coal are used, the desired temperaturescan be obtained only if combustiontakes place with almost pure oxygen or if thecombustion air is preheated to approximately2000° F.In spite of the temperature problem, the magnetohydrodynamicconversion process continuesto arouse great interest among scientists andengineers. It should be noted, in fact, that thetemperature problem is only one side of the coin.On the other side, the use of such high temperaturesleads to thepossibilityof thermal efficienciesfar greater than any that are even theoreticallypossible with conventional heat engines. Ithas been estimated that overall efficiencies ashigh as 50 to 60 percent may be achieved throughMHD conversion, if provision is made for utilizingthe "waste" heat of the MHD process. Theadvantage of utilizing the waste heat is enormous,since the ionized gas is at a temperature of 2500°to 3000° F when it is discharged from the generator.COMBINED POWER PLANTSIn recent years there has been a great dealof interest in the use of combined power plantsfor ship propulsion. In a combined power plant,two basically different kinds of prime movers areused to furnish propulsive power. Each type ofprime mover has its own inherent limitations, aswell as its own unique advantages; the purpose ofcombining two prime movers is to make full useof the specialadvantagesof eachand, atthe sametime, to minimize or bypass its limitations.Combined power plants are of particular interestfor naval ships because of the constantneed to reconcile conflicting operational requirements.On the one hand, a naval ship mustbe able to operate at high speeds when necessary.On the other hand, the ship must be able to cruiseeconomically at lower speeds for extended distancesand extended periods of time. "^ If the primemover is selected specifically for high speedoperation, there is normally some sacrifice ofcruising radius. If the prime mover is selectedspecifically for economical operation at cruisingspeeds, there is normally some sacrifice ofspeed capability. In most cases, then, the selectionof a prime mover represents a compromisebetween high speed capability and large cruisingradius. 8For many naval applications, it appears thatconflicting operational requirements can be reconciledby combining a base-load plant of moderateweight and high efficiency with a boosterplant of very light weight and lesser efficiency.The base- load plant is selected to meet cruisingrequirements, and should be able to go manyhours between overhauls. The booster plant willinevitably require overhauls at much shorter intervalsbut is capable of providing additionalpower for high speed operation when necessary.A combined power plant for ship propulsionusually consists of two prime movers which aremechanically connected by gearing, clutching, orboth. In some combination plants, the two primemovers have interrelated thermodynamic cyclesin which one prime mover utilizes waste heatfrom the other. In other combination plants, thethermodynamic cycles of the two prime moversare entirely separate and independent.A great many combinations of prime moversare possible, though not all combinations areequally feasible or desirable. Also, for any given^More than 80 percent of the total operating time ofnaval ships is at speeds requiring less than a third ofthe power available from the Installed plant.SThis does not necessarily apply to nuclear ships. Obviously,however, the use of nuclear power bringsabout the necessity for another set of compromises.638