Boiler overload capacity— Boiler overload capacity  is  specified  in  the  design  of  the boiler.  It  is  usually  120  percent  of  boiler full-power  capacity,  for  either  steaming rate  or  firing  rate,  as  specified  for  the individual installation. Superheater outlet pressure— Superheater outlet pressure is the actual pressure at the superheater outlet at any given time. Steam  drum  pressure—  Steam   drum pressure is the actual pressure carried in the boiler steam drum at any given time. Operating  pressure—  Operating  pressure  is the constant pressure at which the boiler is  being  operated.  This  pressure  may  be carried  at  either  the  steam  drum  or  the superheater   outlet,   depending   on   the design  feature  of  the  boiler.  Operating pressure  is  specified  in  the  manufacturer’s technical  manual. Design pressure— Design pressure is the maximum pressure specified by the boiler manufacturer   as   a   criterion   for   boiler design.  Design  pressure  is  not  the  same  as operating pressure. It is somewhat higher than operating pressure. Design pressure is  given  in  the  manufacturer’s  technical manual  for  the  particular  boiler. Design temperature— Design  temperature is the maximum operating temperature at the  superheater  outlet  at  some  specified rate of operation. For combatant ships the specified  rate  of  operation  is  normally  full- power  capacity. Operating  temperature—  Operating  tem- perature is the actual temperature at the superheater  out  let.  Operating  temperature is the same as design temperature ONLY when   the   boiler   is   operating   at   rate specified  in  the  definition  of  design temperature. Boiler  efficiency—   The  efficiency  of  a boiler  is  the  Btu’s  per  pound  of  fuel absorbed by the water and steam divided by  the  Btu’s  per  pound  of  fuel  fired.  In other  words,  boiler  efficiency  is  output divided  by  input,  or  heat  used  divided by  heat  available.  Boiler  efficiency  is expressed as a percent. Superheater surface—  The   superheater surface is that portion of the total heating surface  where  the  steam  is  heated  after leaving  the  boiler  steam  drum. Economizer  surface—  The  economizer surface is that portion of the total heating surface  where  the  feed  water  is  heated before  it  enters  the  boiler  steam  drum. Total  heating  surface—  The total heating surface area is the area of the generating, economizer,  and  superheater  tube  banks exposed in the boiler furnace. These tubes are  that  part  of  the  heat  transfer  that exposes one side to combustion gases and the other side to the water or steam being heated. BOILER  CLASSIFICATION Boilers  vary  considerably  in  detail  and  design. Most  boilers  may  be  classified  and  described  in terms  of  a  few  basic  features  or  characteristics. Some knowledge of the methods of classification provides  a  useful  basis  for  understanding  the design  and  construction  of  the  various  types  of naval  boilers. In   the   following   paragraphs,   we   have considered   the   classification   of   naval   boilers according to intended service, location of fire and water spaces, type of circulation, arrangement of steam  and  water  spaces,  number  of  furnaces, burner location, furnace pressure, type of super- heaters,  control  of  superheat,  and  operating pressure. INTENDED  SERVICE A  good  place  to  begin  in  classifying  boilers is  to  consider  their  intended  service.  By  this method  of  classification,  naval  boilers  are divided  into  two  classes,  PROPULSION  BOILERS and  AUXILIARY  BOILERS.  Propulsion  boilers are  used  to  provide  steam  for  ships’  propulsion and  for  vital  auxiliaries’  services.  Auxiliary  boilers are installed in diesel-driven ships and in many steam-driven  combatant  ships.  They  supply  the steam and hot water for galley, heating, and other hotel services and for other auxiliary requirements in  port. 4-2