The Rankine Cycle

All steam power plants make use of the Rankine cycle for their operation. This is because the Carnot cycle is practically impossible to achieve in normal circumstances. The Rankine cycle is a cycle wherin heat addition and rejection take place at isobaric conditions(constant pressure) whereas Carnot cycle occurs isothermally (constant temperature) which makes it hard to achieve practically.

First lets check out the basic Rankine cycle:

Every Basic Rankine cycle consists of the following components:

(1)Feed pump: feeds water to the boiler
(2)Boiler: heats the water and converts it into steam
(3)Turbine: The turbine is the component connected to a generator which generates the power at a steam power plant. its a rotary device fitted with blades which cause the mechanism to rotate when impacted with high velocity steam.
(4)Condenser: the condenser condenses the vapours into fluid or water.
The Rankine cycle consits of the following four processe:

PROCESS 1 TO 2: In this process the feed pump supplies water at a higher pressure to the boiler. since the water is in a liquid state the amount of work to be supplied to it will be less.

PROCESS 2 TO 3: In this process the boiler heats the water to high temperatures at constant pressure hence making it an isobaric process. the steam rate or the amount of steam produced is maintained or kept constant. if the combustion system is efficient, it ensures optimum steam production. the steam obtained from the boiler is in a dry and saturated state.

PROCESS 3 TO 4: This process results in power generation. the steam produced in the boiler is directed towards the turbine. now, this is done using a pipe fitted with a nozzle. the basic purpose of the nozzle is to cause increae in velocity of the steam and decrease in pressure. hence we say that "steam undergoes expansion in the turbine." the high RPM's of the turbine (which is connected to a generator) cause the generation of power.

PROCESS 4 TO 1: After the steam gets expanded, it somewhat gets condensed, although not completely, thus we call this the vapour state. this happens because the pressure of the steam reduces at the turbine (using gay lusacs law, P is directly proportional to T) thus reducing its temperature, condensing it. the wet and cooled steam is sent to a condenser in this stage where all of the steam is converted to water and sent to the feed pump and the same process continues all over again.
There are two other important types of Rankine cycles which ae used widely since it improves the overall efficiency of the system. Their working is the same as the basic Rankine cycle, with a few slight changes.

(1)Regenerative Rankine cycle: In this cycle, the feed water for the boiler is heated with some steam which is directed to the turbine and or the steam released from the turbine (which still contains heat). what this does is reduces the amount of work spent to heat the feed water in the boiler since it already will be at a slightly higher temperature. this helps increase the efficiency of the combustion system and the power plant as a whole.

(2)Reheat Rankine cylce: in this cycle, there are two operational turbines. first the steam from the boiler is sent to the first turbine. after the steam is released from this boiler, it is sent back to to the boiler (the steam is at low pressure low temperature) where it is reheated and sent to the second turbine. hence the name reheat rankine cycle. this increases the work output of the power plant.

There are two other ways in which efficiency can be increased -

(1)superheating the steam: steam is heated further to a higher temperature.

(2)increasing boiler pressure: raising the pressure at which the steam will be heated at increases the efficiency. this is becuase the heat rejected by the system reduces delivering the fluid to the boiler at a higer temperature, which is ideally expected.