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2300_les15 - V The Second Law of Thermodynamics E The...

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1 Lesson 15, Geof Silcox, Chemical Engineering, University of Utah V. The Second Law of Thermodynamics E. The Carnot Cycle 1. The reversible heat engine (a piston cylinder device) that operates on a cycle between heat reservoirs at T H and T L , as shown below on a P-v diagram, is the Carnot cycle . 2. Because the cycle is reversible, the Carnot cycle, operating between T H and T L , is the most efficient cycle possible. We want to know how much heat, Q L , must be rejected by the engine . 3. If the Carnot heat engine is operated in reverse, we call it a Carnot refrigeration cycle . •1-2 rev., isothermal expansion •2-3 rev., adiabatic expansion •3-4 rev., isothermal compression •4-1 rev., adiabatic compression T H = constant T L = constant Q H Q L 1 2 3 4 W net,out = shaded area Q 41 =0 Q 23 =0 P v H L in out th Q Q 1 Q Q 1 = = η Lesson 15, Geof Silcox, Chemical Engineering, University of Utah V. The Second Law of Thermodynamics F. The Carnot Principles 1. “The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs.” (p. 302) 2. “The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.” (p. 302)
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