PRINCIPLES OF NAVAL ENGINEERING 



The MHD conversion is similar to the ther- 

 moelectric conversion and the thermionic con- 

 version in some respects but quite unlike them 

 in others. All three conversions involve the di- 

 rect conversion of thermal energy into electrical 

 energy, without the intervening step of conver- 

 sion to mechanical energy; in this sense, all three 

 may be regarded as "direct energy conver- 

 sions." But the MHD conversion, unlike the ther- 

 moelectric and thermionic conversions, re- 

 quires a working fluid— namely, a hot ionized gas. 

 In this respect, then, the MHD conversion is 

 somewhat less a "direct energy conversion" 

 than the other two processes. 



The major problems in connection with mag- 

 netohydrodynamic conversion arise from the fact 

 that extremely high temperatures (in excess of 

 4000 ° F) must be developed in order to produce 

 ionization of the gas. Obviously, such high tem- 

 peratures pose materials problems. Also, it is 

 difficult to achieve such temperatures on the 

 large scale desired for MHD generators. Nuclear 

 reactors capable of operating at these ultra- high 

 temperatures are under development but are not 

 fully operational. When chemical fuels such as 

 oil or powdered coal are used, the desired tem- 

 peratures can be obtained only if combustion 

 takes place with almost pure oxygen or if the 

 combustion air is preheated to approximately 

 2000° F. 



In spite of the temperature problem, the mag- 

 netohydrodynamic conversion process continues 

 to arouse great interest among scientists and 

 engineers. It should be noted, in fact, that the 

 temperature problem is only one side of the coin. 

 On the other side, the use of such high tempera- 

 tures leads to thepossibilityof thermal efficien- 

 cies far greater than any that are even theoreti- 

 cally possible with conventional heat engines. It 

 has been estimated that overall efficiencies as 

 high as 50 to 60 percent may be achieved through 

 MHD conversion, if provision is made for utiliz- 

 ing the "waste" heat of the MHD process. The 

 advantage 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 gen- 

 erator. 



used to furnish propulsive power. Each type of 

 prime mover has its own inherent limitations, as 

 well as its own unique advantages; the purpose of 

 combining two prime movers is to make full use 

 of the specialadvantagesof eachand, atthe same 

 time, to minimize or bypass its limitations. 



Combined power plants are of particular in- 

 terest for naval ships because of the constant 

 need to reconcile conflicting operational re- 

 quirements. On the one hand, a naval ship must 

 be able to operate at high speeds when necessary. 

 On the other hand, the ship must be able to cruise 

 economically at lower speeds for extended dis- 

 tances and extended periods of time. "^ If the prime 

 mover is selected specifically for high speed 

 operation, there is normally some sacrifice of 

 cruising radius. If the prime mover is selected 

 specifically for economical operation at cruising 

 speeds, there is normally some sacrifice of 

 speed capability. In most cases, then, the selec- 

 tion of a prime mover represents a compromise 

 between high speed capability and large cruising 

 radius. 8 



For many naval applications, it appears that 

 conflicting operational requirements can be re- 

 conciled by combining a base-load plant of mod- 

 erate weight and high efficiency with a booster 

 plant of very light weight and lesser efficiency. 

 The base- load plant is selected to meet cruising 

 requirements, and should be able to go many 

 hours between overhauls. The booster plant will 

 inevitably require overhauls at much shorter in- 

 tervals but is capable of providing additional 

 power for high speed operation when necessary. 



A combined power plant for ship propulsion 

 usually consists of two prime movers which are 

 mechanically connected by gearing, clutching, or 

 both. In some combination plants, the two prime 

 movers have interrelated thermodynamic cycles 

 in which one prime mover utilizes waste heat 

 from the other. In other combination plants, the 

 thermodynamic cycles of the two prime movers 

 are entirely separate and independent. 



A great many combinations of prime movers 

 are possible, though not all combinations are 

 equally feasible or desirable. Also, for any given 



COMBINED POWER PLANTS 



In recent years there has been a great deal 

 of interest in the use of combined power plants 

 for ship propulsion. In a combined power plant, 

 two basically different kinds of prime movers are 



^More than 80 percent of the total operating time of 

 naval ships is at speeds requiring less than a third of 

 the power available from the Installed plant. 

 SThis does not necessarily apply to nuclear ships. Ob- 

 viously, however, the use of nuclear power brings 

 about the necessity for another set of compromises. 



638 



