pressure. A   few   saturation   pressures   and temperatures  for  water  are  as  follows: Pounds Per Square Inch Degrees Absolute  (psia) Fahrenheit  (°F) 11 . . . . . . . . . . . . . . . . . . . . . . .198 14.7 . . . . . . . . . . . . . . . . . . . . . . .212 110 . . . . . . . . . . . . . . . . . . . . . . . .335 340 . . . . . . . . . . . . . . . . . . . . . . . .429 630 . . . . . . . . . . . . . . . . . . . . . . . .567 1200 . . . . . . . . . . . . . . . . . . . . . ..  .596 2000 . . . . . . . . . . . . . . . . . . . . . . . .636 3000 . . . . . . . . . . . . . . . . . . . . . . . .695 3206.2 . . . . . . . . . . . . . . . . . . . . . . . .705.40 We  know  that  atmospheric  pressure  is  14.7 psia  at  sea  level  and  lesser  at  higher  altitudes. Boiling  water  on  top  of  a  mountain  takes  a  lot longer  than  at  sea  level.  Why  is  this?  As  noted before, temperature and pressure are indications of  internal  energy.  Since  we  cannot  raise  the temperature  of  boiling  water  above  the  saturation temperature  for  that  pressure,  the  internal  energy available  for  boiling  water  is  less  at  higher altitudes than at sea level. By the same lines of reasoning, you should be able to figure out why water boils faster in a pressure cooker than in an open  kettle. A peculiar thing happens to water and steam at  an  absolute  pressure  of  3206.2  psia  and  the corresponding   saturation   temperature   at 705.40°F.  At  this  point,  the  CRITICAL  POINT, the  vapor  and  liquid  are  indistinguishable.  No change  of  state  occurs  when  pressure  increases above  this  point  or  when  heat  is  added.  At  the critical  point,  we  no  longer  refer  to  water  or steam.  At  this  point  we  cannot  tell  the  waterer steam  apart.  Instead,  we  call  the  substance  a fluid or a working substance. Boilers designed to operate at pressures and temperatures above the critical   point   are   SUPERCRITICAL   boilers. Supercritical boilers are not used, at present, in propulsion plants of naval ships; however, some boilers of this type are used in stationary steam power  plants. If we generate steam by boiling water in an open pan at atmospheric pressure, the water and steam that is in immediate contact with the water will remain at 212°F until all the water evaporates. If we fit an absolutely tight cover to the pan so no  steam  can  escape  while  we  continue  to  add heat, both the pressure and temperature inside the vessel  will  rise.  The  steam  and  water  will  both increase in temperature and pressure, and each fluid will be at the same temperature and pressure as the other. In  operation,  a  boiler  is  neither  an  open  vessel nor a closed vessel. It is a vessel designed with restricted openings allowing steam to escape at a uniform rate while feedwater is brought in at a uniform  rate.  Steam  generation  takes  place  in  the boiler  at  constant  pressure  and  constant  tem- perature,   less   fluctuations.   Fluctuations   in constant pressure and constant temperature are caused  by  changes  in  steam  demands. We cannot raise the temperature of the steam in the steam drum above the temperature of the water from which it is being generated until the steam  is  removed  from  contact  with  the  water inside  the  steam  drum  and  then  heated.  Steam  that has  been  heated  above  its  saturation  temperature at a given pressure is SUPERHEATED STEAM. The  vessel  in  which  the  saturated  steam  is superheated  is  a  SUPERHEATER. The   amount   by   which   the   temperature of  superheated  steam  exceeds  the  temperature of  saturated  steam  at  the  same  pressure  is the  DEGREE  OF  SUPERHEAT.  For  example, if  saturated  steam  at  620  psia  with  a  corre- sponding   saturation   temperature   of   490°F   is superheated to 790°F, the degree of superheat is 300°F  (790  –  490  =  300). Most  naval  propulsion  boilers  have  super- heaters.  The  primary  advantage  is  that  super- heating  steam  provides  a  greater  temperature differential  between  the  boiler  and  the  condenser. This  allows  more  heat  to  be  converted  to  work at  the  turbines.  We  will  discuss  propulsion  boilers and  component  parts  more  extensively  in  the  next chapter. Another advantage is that superheated steam is dry and therefore causes relatively little corrosion  or  erosion  of  machinery  and  piping. Also, superheated steam does not conduct or lose heat  as  rapidly  as  saturated  steam.  The  increased efficiency  which  results  from  the  use  of  super- heated   steam   reduces   the   fuel   oil   required to   generate   each   pound   of   steam.   It   also reduces the space and weight requirements for the boilers. 3-3