the steam drum. In the process, the steam loses all   but   20°F   to   30°F   of   its   superheat.   The advantage  of  desuperheated  steam  is  that  it  is certain  to  be  dry,  yet  not  so  hot  as  to  require special  alloy  steels  for  the  construction  of  the piping that carries the desuperheated steam about the ship. Steam use will be discussed later in chapters 3  and  4  of  this  textbook.  We  will  describe  the steam  cycle  and  typical  boilers  used  on  naval ships. Combustion Combustion  refers  to  the  rapid  chemical  union of  oxygen  with  fuel.  Perfect  combustion  of  fuel would  result  in  carbon  dioxide,  nitrogen,  water vapor, and sulphur dioxide. The oxygen required to burn the fuel is obtained from the air. Air is a   mechanical   mixture   containing   by   weight 21   percent   oxygen,   78   percent   nitrogen,   and 1  percent  other  gases.  Only  oxygen  is  used  in combustion. Nitrogen is an inert gas that has no chemical  effect  upon  combustion. The  chemical  combination  obtained  during combustion  results  in  the  liberation  of  heat energy.    A  portion  of  this  energy  is  used  to propel  the  ship.  Actually,  what  happens  is  a rearrangement   of   the   atoms   of   the   chemical elements into new combinations of molecules. In other  words,  when  the  fuel  oil  temperature  (in  the presence  of  oxygen)  is  increased  to  the  ignition point, a chemical reaction occurs. The fuel begins to  separate  and  unite  with  specific  amounts  of oxygen to form an entirely new substance. Heat energy  is  given  off  in  the  process.  A  good  fuel burns  quickly  and  produces  a  large  amount  of heat. Perfect combustion is the objective. However, this has been impossible to achieve as yet in either a  boiler  or  the  cylinders  of  an  internal-combustion engine.  Theoretically,  it  is  simple.  It  consists  of bringing  each  particle  of  the  fuel  (heated  to  its ignition   temperature)   into   contact   with   the correct  amount  of  oxygen.  The  following  factors are  involved: l  Sufficient  oxygen  must  be  supplied. l  The  oxygen  and  fuel  particles  must  be thoroughly  mixed. .   Temperatures   must   be   high   enough   to maintain  combustion. .  Enough  time  must  be  allowed  to  permit completion  of  the  process. Complete  combustion  can  be  achieved.  This . is  accomplished  by  more  oxygen  being  supplied to the process than would be required if perfect combustion  were  possible.  The  result  is  that  some of the excess oxygen appears in the combustion gases. Units of Heat Measurement Both  internal  energy  and  heat  is  measured using  the  British  thermal  unit  (Btu).  For  most practical   engineering   purposes,   1   Btu   is   the thermal  energy  required  to  raise  the  temperature of  1  pound  of  pure  water  to  1°F.  Burning  a wooden  kitchen  match  completely  will  produce about  1  Btu. When  large  amounts  of  thermal  energy  are involved,  it  is  usually  more  convenient  to  use multiples of the Btu. For example, 1 kBtu is equal to 1000 Btu, and 1 MBtu is equal to 1 million Btu. Another unit in which thermal energy maybe measured  is  the  calorie.  The  calorie  is  the  amount of heat required to raise the temperature of 1 gram of pure water 1°C. One Btu equals 252 calories. Sensible Heat and Latent Heat Sensible heat and latent heat are terms often used to indicate the effect that the flow of heat has  on  a  substance.  The  flow  of  heat  from  one substance  to  another  is  normally  reflected  in  a temperature  change  in  each  substance—the  hotter substance  becomes  cooler,  the  cooler  substance becomes hotter. However, the flow of heat is not reflected in a temperature change in a substance that   is   in   the   process   of   changing   from   one physical  state  (solid,  liquid,  or  gas)  to  another. When the flow of heat is reflected in a temperature change,  we  say  that  sensible  heat  has  been  added to or removed from the substance (heat that can be sensed or felt). When the flow of heat is not reflected  in  a  temperature  change,  but  is  reflected in the changing physical state of a substance, we say that latent heat has been added or removed. Does   anything   bother   you   in   this   last paragraph? It should. Here we are talking about sensible heat and latent heat as though we had two  different  types  of  heat  to  consider.  This  is common (if inaccurate) engineering language. So keep  the  following  points  clearly  in  mind:  (1)  heat is   the   movement   (flow)   of   thermal   energy; (2) when we talk about adding and removing heat, we really mean that we are providing temperature differentials so thermal energy can flow from one substance to another; and (3) when we talk about 2-12