Thermodynamics

378 | ThermodynamicsEntropy Change of a System, S systemDespite the reputation of entropy as being vague and abstract and the intimidationassociated with it, entropy balance is actually easier to deal withthan energy balance since, unlike energy, entropy does not exist in variousforms. Therefore, the determination of entropy change of a system during aprocess involves evaluating entropy of the system at the beginning and atthe end of the process and taking their difference. That is,orEntropy change Entropy at final state Entropy at initial state¢S system S final S initial S 2 S 1(7–69)Note that entropy is a property, and the value of a property does not changeunless the state of the system changes. Therefore, the entropy change of asystem is zero if the state of the system does not change during the process.For example, the entropy change of steady-flow devices such as nozzles,compressors, turbines, pumps, and heat exchangers is zero during steadyoperation.When the properties of the system are not uniform, the entropy of the systemcan be determined by integration fromS system s dm VsrdV(7–70)where V is the volume of the system and r is density.SurroundingsSYSTEMT b = 400 KQ = 500 kJQS heat =T b= 1.25 kJ/KMechanisms of Entropy Transfer, S in and S outEntropy can be transferred to or from a system by two mechanisms: heattransfer and mass flow (in contrast, energy is transferred by work also).Entropy transfer is recognized at the system boundary as it crosses theboundary, and it represents the entropy gained or lost by a system during aprocess. The only form of entropy interaction associated with a fixed massor closed system is heat transfer, and thus the entropy transfer for an adiabaticclosed system is zero.1 Heat TransferHeat is, in essence, a form of disorganized energy, and some disorganization(entropy) will flow with heat. Heat transfer to a system increases theentropy of that system and thus the level of molecular disorder or randomness,and heat transfer from a system decreases it. In fact, heat rejection isthe only way the entropy of a fixed mass can be decreased. The ratio of theheat transfer Q at a location to the absolute temperature T at that location iscalled the entropy flow or entropy transfer and is expressed as (Fig. 7–58)FIGURE 7–58Heat transfer is always accompaniedby entropy transfer in the amount ofQ/T, where T is the boundarytemperature.Entropy transfer by heat transfer: S heat Q 1T constant2 (7–71)TThe quantity Q/T represents the entropy transfer accompanied by heat transfer,and the direction of entropy transfer is the same as the direction of heattransfer since thermodynamic temperature T is always a positive quantity.