Quantcast Chapter 3 Basic Steam Cycle

CHAPTER 3 BASIC STEAM CYCLE To  understand  steam  generation,  you  must know what happens to the steam after it leaves the boiler. A good way to learn the steam plant on  your  ship  is  to  trace  the  path  of  steam  and water throughout its entire cycle of operation. In each cycle, the water and the steam flow through the entire system without ever being exposed to the  atmosphere.  The  four  areas  of  operation  in a main steam system are generation, expansion, condensation,   and   feed.   After   studying   this chapter, you will have the knowledge and ablity to describe the main steam cycle and the functions of the auxiliary steam systems. MAIN  STEAM  SYSTEM The movement of a ship through the water is the  result  of  a  number  of  energy  transformations. Although these transformations were mentioned in  the  last  chapter,  we  will  now  discuss  these transformations as they occur. Figure 3-1 shows the  four  major  areas  of  operation  in  the  basic steam  cycle  and  the  major  energy  transformations that take place. These areas are A—generation, B—expansion,   C—condensation,   and   D—feed. GENERATION—The   first   energy   transfor- mation  occurs  in  the  boiler  furnace  when  fuel oil  burns.  By  the  process  of  combustion, the  chemical  energy  stored  in  the  fuel  oil is   transformed   into   thermal   energy.   Thermal energy   flows   from   the   burning   fuel   to   the water and generates steam. The thermal energy is  now  stored  as  internal  energy  in  steam, as  we  can  tell  from  the  increased  pressure and  temperature  of  the  steam. EXPANSION—When steam enters the turbines and  expands,  the  thermal  energy  of  the  steam converts  to  mechanical  energy,  which  turns  the shaft and drives the ship. For  the  remainder  of  the  cycle,  energy  is returned  to  the  water  (CONDENSATION  and FEED)  and  back  to  the  boiler  where  it  is  again heated and changed into steam. The energy used for  this  purpose  is  the  thermal  energy  of  the auxiliary steam. The  following  paragraphs  will  explain  the  four major areas of operation in the basic steam cycle shown  in  figure  3-1. GENERATION When  a  liquid  boils,  it  generates  a  vapor. Some or all of the liquid changes its physical state from liquid to gas (or vapor). As long as the vapor is  in  contact  with  the  liquid  from  which  it  is being    generated, it  remains  at  the  same temperature  as  the  boiling  liquid.  In  this condition,   the   liquid   and   its   vapors   are   in equilibrium    contact with each other. Area A of figure 3-1 shows the GENERATION area of the basic  steam  cycle. The  temperature  at  which  a  boiling  liquid and  its  vapors  may  exist  in  equilibrium  contact depends  on  the  pressure  under  which  the process  takes  place.  As  the  pressure  increases, the   boiling   temperature   increases.   As   the pressure   decreases, the   boiling   temperature decreases. Determining the boiling point depends on the pressure. When  a  liquid  is  boiling  and  generating vapor, the liquid is a SATURATED LIQUID and the  vapor  is  a  SATURATED  VAPOR.  The temperature  at  which  a  liquid  boils  under  a given   pressure   is   the   SATURATION   TEM- PERATURE,  and  the  corresponding  pressure  is the  SATURATION  PRESSURE.  Each  pressure has a corresponding saturation temperature, and each temperature has a corresponding saturation 3-1


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