Most  auxiliary  machinery  operates  on  saturated steam.  Reciprocating  machinery,  in  particular, requires  saturated  steam  to  lubricate  internal moving  parts  of  the  steam  end.  Naval  boilers, therefore,   produce   both   saturated   steam   and superheated steam. EXPANSION The  EXPANSION  area  of  the  main  steam system  is  that  part  of  the  basic  steam  cycle  in which steam from the boilers to the main turbines is  expanded.  This  removes  the  heat  energy  stored in  the  steam  and  transforms  that  energy  into mechanical  energy  of  rotation. The main turbines usually have a high-pressure (HP)  turbine  and  a  low-pressure  (LP)  turbine.  The steam flows into the HP turbine and on into the LP  turbine.  Area  B  of  figure  3-1  shows  the expansion  area  of  the  main  steam  system.  This portion  of  the  main  steam  system  contains  HP and  LP  turbines. CONDENSATION Each ship must produce enough feedwater for the  boilers  and  still  maintain  an  efficient engineering  plant.  Therefore,  feedwater  is  used over  and  over  again. As the steam leaves or exhausts from the LP turbine,  it  enters  the  CONDENSATE  system.  The condensate system is that part of the steam cycle in which the steam is condensed back to water. Then  it  flows  from  the  main  condenser  toward the boilers while it is being prepared for use as feedwater. The components of the condensate system are (1) the main condenser, (2) the main condensate pump,  (3)  the  main  air  ejector  condenser,  and (4) the top half of the deaerating feed tank (DFT). These components are shown in area C of figure 3-1. The main condenser receives steam from the LP turbine. It condenses the steam into water. We will  explain  this  process  in  the  next  chapter  on boilers. The main condensate pump takes suction from the main condenser hot well. It delivers the condensate  into  the  condensate  piping  system  and through  the  main  air  ejector  condenser.  As  its name  implies,  the  air  ejector  removes  air  and noncondensable  gases  from  the  main  condenser that leak or are discharged into it during normal operation.  The  condensate  is  used  as  a  cooling medium for condensing the steam in the inter and after  condensers  of  the  main  air  ejector. FEED The  DFT  (fig.  3-2)  is  the  dividing  line  between condensate  and  feedwater.  The  condensate  enters the  DFT  through  the  spray  nozzles  and  turns into  feedwater  in  the  reservoir  section  of  the  DFT. The  DFT  has  three  basic  functions: l  To  remove  dissolved  oxygen  and  non- condensable  gases  from  the  condensate .  To preheat the water .   To act as a reservoir to store feedwater to take   care   of   fluctuations   in   feedwater demand  or  condensate  supply The  condensate  enters  the  DFT  through  the condensate inlet. There it is sprayed into the dome of the tank by nozzles. It is discharged in a fine spray  throughout  the  steam-filled  top.  The  fine spray  and  heating  of  the  condensate  releases trapped air and oxygen. The gas-free condensate falls to the bottom of the tank through the water collecting  cones,  while  the  air  and  oxygen  are exhausted  from  the  tank  vent. The   collected   condensate   in   the   storage section  of  the  DFT  is  now  called  feedwater  and becomes  a  source  of  supply  for  the  main  feed booster pump. The main feed booster pump takes suction from the DFT and maintains a constant discharge  pressure  to  the  main  feed  pump. The  main  feed  pump  receives  the  water (delivered  from  the  booster  pump)  and  discharges it  into  the  main  feed  piping  system.  Area  D  of figure 3-1 shows the path of the water from the DFT  to  the  economizer.  The  discharge  pressure of  the  main  feed  pump  is  maintained  at  100  to 150   psig   above   boiler   operating   pressure   on 600-psi plants. On 1200-psi plants, it is maintained at 200 to 300 psig above boiler operating pressure. The  discharge  pressure  is  maintained  throughout the  main  feed  piping  system.  However,  the quantity  of  water  discharged  to  the  economizer is  controlled  by  a  feed  stop  and  check  valve  or automatic  feedwater  regulator  valve. The economizer is positioned on the boiler to perform  one  basic  function.  It  acts  as  a  preheater. The   gases   of   combustion   flow   around   the economizer   tubes   and   metal   projections   that extend  from  the  outer  tube  surfaces.  The  tubes and  projections  absorb  some  of  the  heat  of combustion  and  heat  the  water  that  is  flowing through  the  economizer  tubes.  As  a  result,  the water is about 100 °F hotter as it flows out of the economizer  to  the  boiler. 3-4