There are four processes in the Rankine cycle, each changing the state of the working fluid.
First, the working fluid (water) is enter the pump at state 3 at saturated liquid and it is pumped (ideally isentropically) from low pressure to high (operating) pressure of boiler by a pump to the state 4. During this isentropic compression water temperature is slightly increased. Pumping requires a power input (for example, mechanical or electrical). The conservation of energy relation for pump is given as Wpump = m (h4 - h3)
The high pressure compressed liquid enters a boiler at state 4 where it is heated at constant pressure by an external heat source to become a saturated vapour at statel’which in turn superheated to state 1 through super heater. Common heat source for power plant systems are coal (or other chemical energy), natural gas, or nuclear power. The conservation of energy relation for boiler is given as Qin =m (h1 - h4)
Process 1 –2:
The superheated vapour enter the turbine at state 1 and expands through a turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapour at state 2. The conservation of energy relation for turbine is given as Wturbine = m (h1 –h2)
Process 2 –3:
The vapour then enters a condenser at state 2. At this state, steam is a saturated liquid-vapour mixture where it is cooled to become a saturated liquid at state 3. This liquid then re-enters the pump and the cycle is repeated. The conservation of energy relation for condenser is given as Qout = m (h2 –h3)
The exposed Rankine cycle can also prevent vapour overheating, which reduces the amount of liquid condensed after the expansion in the turbine.