SECOND LAW OF THERMODYNAMICS ThermodynamicsEquation 1-23 shows that the maximum possible efficiency exists when TH is at its largestpossible value or when TC is at its smallest value. Since all practical systems and processes arereally irreversible, the above efficiency represents an upper limit of efficiency for any givensystem operating between the same two temperatures. The system’s maximum possibleefficiency would be that of a Carnot efficiency, but because Carnot efficiencies representreversible processes, the actual system will not reach this efficiency value. Thus, the Carnotefficiency serves as an unattainable upper limit for any real system’s efficiency. The followingexample demonstrates the above principles.Example 1: Carnot EfficiencyAn inventor claims to have an engine that receives 100 Btu of heat and produces 25 Btuof useful work when operating between a source at 140°F and a receiver at 0°F. Is theclaim a valid claim?Solution:TH= 140oF + 460 = 600°RTC= 0oF + 460 = 460°Rh= (600-460)/600 x 100 = 23.3%Claimed efficiency = 25/100 = 25%Therefore, the claim is invalid.The most important aspect of the second law for our practical purposes is the determination ofmaximum possible efficiencies obtained from a power system. Actual efficiencies will alwaysbe less than this maximum. The losses (friction, for example) in the system and the fact thatsystems are not truly reversible preclude us from obtaining the maximum possible efficiency.An illustration of the difference that may exist between the ideal and actual efficiency ispresented in Figure 22 and the following example.Example 2: Actual vs. Ideal EfficiencyThe actual efficiency of a steam cycle is 18.0%. The facility operates from a steamsource at 340°F and rejects heat to atmosphere at 60°F. Compare the Carnot efficiencyto the actual efficiency.HT-01 Page 74 Rev. 0
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