Rankine cycle - Thermodynamics:
The major problems of Carnot cycle is compressing very wet steam mixture from the condenser pressure up to boiler pressure. The trouble can be avoided by condensing steam in the condenser and then compressing the water in a comparatively small feed pump. The effect of modification is to make the cycle practical one. Further, far less work is required to pump liquid than to compress vapour and therefore this modification has the result that the feed pump's work is one or two per cent only of the work developed by turbine. We can thus neglect this term in our cycle analysis.
The idealized cycle for simple steam power plant taking into account above modification is called as Rankine cycle shown in the figure given above. It is made from four practical processes:
(a) 1 - 2 :Heat is added to increase temperature of the high-pressure water up to its saturation value (process 1 to A). The water is evaporated at constant temperature and pressure (process A to 2). Both processes occur within boiler, but not all the heat supplied is at the maximum cycle temperature. Therefore, the mean temperature at which heat is supplied is lower than the equivalent Carnot cycle. Therefore, the basic steam cycle thermal efficiency is lower inherently. Applying first law of thermodynamics to this process.