Practical steam engines utilize 425°C steam, which is later exhausted at 280°C.

Question

Practical steam engines utilize 425°C steam, which is later exhausted at 280°C. Explicitly show how you follow the steps in the Problem-Solving Strategy for thermodynamics found on pages 361-2 when answering the questions below. (Do this on paper. Your instructor may ask you to turn in this work.)

(a) What is the maximum efficiency that such a heat engine can have?
%
(b) Since 280°C steam is still quite hot, a second steam engine is sometimes operated using the exhaust of the first. What is the maximum efficiency of the second engine if its exhaust has a temperature of 150°C?
%
(c) What is the overall efficiency of the two engines?
%
(d) Show that this is the same efficiency as a single Carnot engine operating between 425°C and 150°C.

Explanation / Answer

A) maximum efficiency E = 1-(Tc/Th) = 1-((280+273)/(425+273)) = 0.2077 = 20.77 %

B) maximum efficiency E = 1-((273+150)/(273+280)) = 0.464 = 0.235 = 23.5 %

D) efficiency os single carnot engone is E =1-((273+150)/(273+450)) = 0.414 = 41.4 %

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