L3-Vapor Power Systems

VAPOR POWER SYSTEM

Objective

Study vapour power plants

• in which working fluid is alternately vaporised and condensed

• water is considered as working fluid

Modelling of VPS

Focus on Subsystem : Converting heat to work

ME 306 Applied Thermodynamics

Focus on Subsystem : Converting heat to work

Analysis of VPS -- Rankine Cycle

• Evaluating principal work and heat transfers

• Ideal Rankine cycle

• Principal irreversibilities and losses

• Improving Performance using

• Superheating, reheating, regeneration

1

SIMPLE VAPOR POWER PLANT

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CYCLE ANALYSIS

TURBINE

CONDENSER

PUMP

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PUMP

BOILER

Thermal Efficiency

Back work ratio (bwr)

IDEAL RANKINE CYCLE

PUMP WORK

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Process 1–2: Isentropic expansion of the working fluid through the turbine from saturated

vapor at state 1 to the condenser pressure.

Process 2–3: Heat transfer from the working fluid as it flows at constant pressure through the

condenser with saturated liquid at state 3.

Process 3–4: Isentropic compression in the pump to state 4 in the compressed liquid region.

Process 4–1: Heat transfer to the working fluid as it flows at constant pressure through the

boiler to complete the cycle.

Moran and Shapiro (2006)

Numerical ProblemSteam is the working fluid in an ideal Rankine cycle. Saturated vapor enters the turbine at 8.0 MPa and

saturated liquid exits the condenser at a pressure of 0.008 MPa. The net power output of the cycle is 100

MW. Determine for the cycle (a) the thermal efficiency, (b) the back work ratio, (c) the mass flow rate of the

steam, in kg/h, (d) the rate of heat transfer, Qin , into the working fluid as it passes through the boiler, in MW,

(e) the rate of heat transfer, Qout, from the condensing steam as it passes through the condenser, in MW, (f)

the mass flow rate of the condenser cooling water, in kg/ h, if cooling water enters the condenser at 15°C

and exits at 35°C.

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Effects of Boiler and Condenser Pressures

Thermal efficiency increases with

• Increase in average temperature at which energy is added through heat transfer

• Decrease in average temperature at which energy is rejected through heat transfer

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Condenser enables

• Higher expansion ratio in the turbine; more work

• Same working fluid may be re-used; no impurities

COMPARISON WITH CARNOT CYCLE

• Heat addition process

• Pumping process

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Moran and Shapiro 2006

• Pumping process

Principal Irreversibilities and Losses

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Numerical Problem

Reconsider the vapor power cycle of the previous example, but include in the analysisthat the turbine and the pump each have an isentropic efficiency of 85%. Determinefor the modified cycle (a) the thermal efficiency, (b) the mass flow rate of steam, inkg/h, for a net power output of 100 MW, (c) the rate of heat transfer into the workingfluid as it passes through the boiler, in MW, (d) the rate of heat transfer from thecondensing steam as it passes through the condenser, in MW, (e) the mass flow rateof the condenser cooling water, in kg/h, if cooling water enters the condenser at 15°Cand exits as 35°C. Discuss the effects on the vapor cycle of irreversibilities within theturbine and pump.

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turbine and pump.

Improving Performance Superheat and Reheat

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SUPER CRITICAL CYCLE

Numerical ProblemSteam is the working fluid in an ideal Rankine cycle with superheat and reheat. Steam

enters the first-stage turbine at 8.0 Mpa, 480C, and expands to 0.7 MPa. It is then reheated

to 440C before entering the second-stage turbine, where it expands to the condenser

pressure of 0.008 MPa. The net power output is 100 MW. Determine (a) the thermal

efficiency of the cycle, (b) the mass flow rate of steam, in kg/h, (c) the rate of heat transfer

from the condensing steam as it passes through the condenser, in MW. Discuss the effects

of reheat on the vapor power cycle.

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Numerical ProblemReconsider the reheat cycle of previous example, but include in the analysis that each turbine

stage has the same isentropic efficiency. (a) If ηt 85%, determine the thermal efficiency. (b)

Plot the thermal efficiency versus turbine stage efficiency ranging from 85 to 100%.

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REGENERATIVE VAPOR POWER CYCLE

open feed water heater

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Cycle analysis

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CLOSED FEED WATER HEATERS

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MULTIPLE FEED WATER HEATERS

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Moran and Shapiro (2006)

BINARY VAPOR CYCLE

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Moran and Shapiro (2006)

Numerical ProblemConsider a regenerative vapor power cycle with one open feedwater heater. Steam enters the

turbine at 8.0 MPa, 480°C and expands to 0.7 MPa, where some of the steam is extracted and

diverted to the open feed water heater operating at 0.7 Mpa. The remaining steam expands

through the second-stage turbine to the condenser pressure of 0.008 MPa. Saturated liquid

exits the open feed water heater at 0.7 MPa. The isentropic efficiency of each turbine stage is

85% and each pump operates isentropically. If the net power output of the cycle is 100 MW,

determine (a) the thermal efficiency and (b) the mass flow rate of steam entering the first turbine

stage, in kg/h.

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