What is Rankine Cycle?
The Rankine cycle is used for analyzing the changes in working substances in steam turbine power plants. Heat is supplied to the working substance so that it can produce some amount of work. Qs amount of heat is supplied to the boiler then water converted into steam, this high thermal energy steam passes through nozzle then this high-velocity stream impinging the turbine blade so that turbine produces WT amount of work then it passes through the condenser, QR amount of heat rejected from the steam so that steam is converted into water then this water pumped into the boiler by using a hydraulic pump. WP amount of work input is required to run the pump.
Ideal Rankine Cycle
A steam turbine power plant consists of a boiler, turbine, condenser and pump. Reversible isobaric heat addition (1-2) takes place in the boiler then the steam passes through the nozzle and high-velocity steam is impinging the turbine blades. Reversible adiabatic expansion or isentropic expansion (2-3) takes place in the turbine. Steam is allowed to pass into the condenser and reversible isobaric heat rejection (3-4) takes place in the condenser. Steam is converted into the water in a condenser and water is pumped into the boiler by using a hydraulic pump. Reversible adiabatic pumping (4-1) takes place in the pump.
h-s and T-s Diagram for Ideal Rankine Cycle
As we discussed above, reversible isobaric heat addition (1-2) takes place in a boiler, reversible adiabatic expansion or isentropic expansion (2-3) in a turbine, reversible isobaric heat rejection (3-4) in the condenser and reversible isentropic pumping (4-1) in the heat pump. The h-s and T-s diagrams for the ideal Rankine cycle are shown in Fig. below.
Efficiency of Ideal Rankine Cycle
The efficiency of the ideal Rankine cycle is defined as the ratio of net work done by the steam turbine power plant Wnet and heat supplied to the boiler Qs. The difference between turbine workout WT and pump work input Wp is defined as net work done Wnet.
The procedure to determine the expression for efficiency of the ideal Rankine cycle is given below.
The process (1-2): reversible isobaric heat addition (Boiler)
Amount of heat supplied in the boiler:
Apply S.F.E.E
h1+Qs=h2
Qs=h2-h1
The process (2-3): reversible adiabatic expansion (isentropic expansion) (turbine)
Amount of work done by the turbine:
Apply S.F.E.E
h2=WT+h3
WT=h2-h3
The process (3-4): reversible isobaric heat rejection (Condenser)
Amount of heat rejected in condenser:
Apply S.F.E.E
h3=QR+h4
QR=h3-h4= h3-hf4
The process (4-1): reversible adiabatic pumping or isentropic pumping (Pump)
Amount of work input in pump:
Apply S.F.E.E
h4+WP=h1
Wp=h1-h4=h1-hf4=vf4(Pb-Pc)
Net work done in steam turbine power plant:
Wnet = WT-WP=(h2-h3)-(h1-hf4)
The efficiency of Rankine cycle
ηRankine=Net Work Done/Heat Supplied
ηRankine=Wnet/Qs=[(h2-h3)-(h1-hf4)]/h2-h1
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