Compare Rankine cycle with Carnot cycle:

Sol.: Rankine cycle without superheat: 1 - A - 2 - 3 - 4 - 1.

Rankine cycle with superheat : 1 - A - 2 - 2′ -3′ - 4 - 1.

       Carnor cyle without superheat: A - 2 - 3 -4″ - A.

Carnot cycle with superheat: A - 2″ - 3′ - 4² - A.

(1) The thermal efficiency of a Rankine cycle is lower than the equivalent Carnot cycle. Temperature of heat supply to Carnot cycle = T_A; Mean temperature of heat supply to Rankine

 

(2) Carnot cycle requires a compressor to handle wet steam mixture whereas in Rankine cycle, small pump is used.

(3) The steam can be superheated at constant pressure along 2 - 2' in a Rankine cycle. Superheating of steam in Carnot cycle at constant temperature along A - ?" is accompanied by a fall of pressure which is difficult to attain in practice because heat transfer and expansion process should go side by side. Therefore Rankine cycle is taken as ideal cycle for steam power plants.