Whenever one or more of the properties of a system change, a change in the state of the systemoccurs. The path of the succession of states through which the system passes is called thethermodynamic process. One example of a thermodynamic process is increasing the temperatureof a fluid while maintaining a constant pressure. Another example is increasing the pressure ofa confined gas while maintaining a constant temperature. Thermodynamic processes will bediscussed in more detail in later chapters.
When a system in a given initial state goes through a number of different changes in state (goingthrough various processes) and finally returns to its initial values, the system has undergone acyclic process or cycle. Therefore, at the conclusion of a cycle, all the properties have the samevalue they had at the beginning. Steam (water) that circulates through a closed cooling loopundergoes a cycle.
A reversible process for a system is defined as a process that, once having taken place, can bereversed, and in so doing leaves no change in either the system or surroundings. In other wordsthe 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 usesreversible to make the analysis simpler, and to determine maximum theoretical efficiencies.Therefore, the reversible process is an appropriate starting point on which to base engineeringstudy 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 aseries of small or infinitesimal steps. For example, heat transfer may be considered reversibleif it occurs due to a small temperature difference between the system and its surroundings. Forexample, 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, bycooling or heating the system in a number of infinitesamally small steps, we can approximate areversible process. Although not practical for real processes, this method is beneficial forthermodynamic studies since the rate at which processes occur is not important.
An irreversible process is a process that cannot return both the system and the surroundings totheir original conditions. That is, the system and the surroundings would not return to their