air  ejector  condenser  removes  the  air  and noncondensable gases from the condensate before they  enter  the  deaerating  feed  tank  (DFT). Feed The  feed  phase  starts  in  the  DFT.  The  DFT preheats  the  feedwater  and  removes  dissolved gases.  The  dissolved  gases,  if  not  removed,  will cause   erosion   and   deterioration   of   the   boiler tubes. The  main  feed  booster  pump  and  main  feed pump increase the feedwater pressure to a pressure greater than the operating pressure of the boiler. The increased pressure ensures a flow of feedwater through  the  boiler.  That  brings  us  back  to  the point  where  we  started.  Thus,  the  system  is  a closed  system. DIESEL ENGINES Diesel  engines  are  the  favored  means  of  power for medium and light vessels. They are relatively low-cost  power  plants  to  produce,  are  reliable,  and have a high fuel-efficiency rate. They can also be started  from  a  cold-plant  condition  and  rapidly brought  on  line. The  cycle  of  operation  for  diesel  engines  starts with the intake of air. Next the air is compressed. Following  compression,  combustion  occurs.  The combustion produces a rapid expansion of gases in the cylinder. This downward expansion is the power stroke of the cylinder. As the waste gases exhaust, new air intake occurs to start the cycle over  again. Each  cycle  causes  the  pistons  within  the cylinders to reciprocate. The rotary motion of the pistons,  connected  to  the  crankshaft,  drives  the propellers. Among  the  disadvantages  are  the  frequent overhaul and periodic maintenance requirements and the power limitations of the engines. Diesels cannot  develop  enough  power  to  meet  the  high- speed  requirement  of  combatant  ships. GAS TURBINES In  gas  turbines,  as  in  diesel  engines,  the working substance is air. They are open systems; that means the air passes through the engine once and  is  discharged  back  to  the  atmosphere. Air  is  drawn  into  the  compressor  from  the atmosphere. The compressor raises the pressure of  the  air  and  discharges  it  to  the  combustion chamber,  where  fuel  is  admitted.  Here,  as  the fuel-air  mixture  ignites,  combustion  occurs.  The hot combustion gases then expand and enter the turbine.  This  turbine  is  similar  in  design  and theory to that of the conventional steam turbine. Approximately 75 percent of the power developed by the turbine is used to drive the compressor and accessory systems. The remaining power is used as  engine  output. The shaft of a gas turbine ship rotates in one direction  only.  An  external  method  of  reversing the  direction  of  travel  of  the  ship  is  required  to propel   the   ship   forward   or   backward.   This problem  is  overcome  by  the  reversible  pitch propeller. As the shaft turns in one direction, the ship  is  propelled  forward  or  backward  by  a  change in  the  propeller  pitch. Because of the high rotational speed and high temperatures   of   the   gas   turbine,   operational parameters   must   be   closely   monitored.   Auto- mated  central  operating  systems  have  been developed   to   monitor   those   parameters,   thus keeping  the  manning  level  low. Two disadvantages of gas turbines are that the engine must be removed for overhaul and that it needs  a  high  volume  of  air  for  operation. However,  these  two  disadvantages  complement each  other  because  the  engine  can  be  removed through the large ducts needed to accommodate the  high  volume  of  air. Gas  turbines  are  becoming  the  preferred propulsion plant for several ship types. They are very  light  and  compact  and  offer  a  high-power plant that is relatively inexpensive to build. They are as fuel efficient as a conventional steam plant. NUCLEAR  POWER  PLANTS Nuclear   power   plants   are   very   similar   to conventional  steam  turbine  plants.  The  major difference  is  that  a  nuclear  reactor  replaces  the boiler as the device that generates steam. Submarines  are  ideally  suited  for  a  nuclear power plant because their reactor does not need a supply of air from the atmosphere. Before the advent   of   nuclear   power,   submarines   ran   on motors  charged  by  d.c.  batteries  when  submerged. When surfaced, diesel engines supplied power for the submarine and recharged the batteries. The charge of the batteries limited the endurance of the submerged submarine. Nuclear power plants enable  submarines  to  remain  submerged  for extended  periods. Nuclear reactors transfer the energy emitted by  the  fission  of  radioactive  material  into  thermal energy.  A  primary  and  a  secondary  system  (or 17-8