Tuesday, 11 May 2010

Thermodynamic Process

Whenever one or more of the properties of a system change, a change in the state of the system occurs.    The  path  of  the  succession  of  states  through  which  the  system  passes  is  called  the thermodynamic process.  One example of a thermodynamic process is increasing the temperature of a fluid while maintaining a constant pressure.   Another example is increasing the pressure of a  confined  gas  while  maintaining  a  constant  temperature.    Thermodynamic  processes  will  be discussed in more detail in later chapters. Cyclic Process When a system in a given initial state goes through a number of different changes in state (going through various processes) and finally returns to its initial values, the system has undergone a cyclic process or cycle.  Therefore, at the conclusion of a cycle, all the properties have the same value  they  had  at  the  beginning.   Steam  (water)  that  circulates  through  a  closed  cooling  loop undergoes a cycle. Reversible Process reversible process  for a system is defined as a process that, once having taken place, can be reversed, and in so doing leaves no change in either the system or surroundings.  In other words the system and surroundings are returned to their original condition before the process took place. In  reality,  there  are  no  truly  reversible  processes;  however,  for  analysis  purposes,  one  uses reversible  to  make  the  analysis  simpler,  and  to  determine  maximum  theoretical  efficiencies. Therefore, the reversible process is an appropriate starting point on which to base engineering study and calculation. Although the reversible process can be approximated, it can never be matched by real processes. One way to make real processes approximate reversible process is to carry out the process in a series of small or infinitesimal steps.   For example, heat transfer may be considered reversible if it occurs due to a small temperature difference between the system and its surroundings.   For example, transferring heat across a temperature difference of 0.00001 °F "appears" to be more reversible  than  for  transferring  heat  across  a  temperature  difference  of  100  °F.   Therefore,  by cooling or heating the system in a number of infinitesamally small steps, we can approximate a reversible  process.    Although  not  practical  for  real  processes,  this  method  is  beneficial  for thermodynamic studies since the rate at which processes occur is not important. Irreversible Process An irreversible process is a process that cannot return both the system and the surroundings to their  original  conditions.    That  is,  the  system  and  the  surroundings  would  not  return  to  their



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