ChemE 260 The Rankine Cycle

ChemE 260 The Rankine Cycle

May 17, 2005

Dr. William BaratuciSenior Lecturer

Chemical Engineering Department

University of Washington

TCD 9: A & BCB 9: 1 & 2

Vapor and Gas Power SystemsVapor and Gas Power Systems• Vapor Cycles

– Use evaporator and condenser for isothermal heat transfer steps

• Gas Cycles– No phase changes– Use isobaric heat exchangers for heat transfer steps

• Closed Cycles– Not piston-and-cylinder devices– Working fluid remains inside the system– Use two HEX’s to absorb and reject heat– Power generation turbines

• Open Cycles– Fresh working fluid drawn into the cycle– Spent working fluid discarded after 1 pass through the cycle– Usually air is the working fluid– Only 1 HEX because the fluid doesn’t actually complete the cycle– Automobile and jet engines

BaratuciChemE 260May 16, 2005

PV DiagramPV Diagram

• In Vapor or Gas Power Cycles, the area enclosed by the cycle path is equal to the specific shaft work produced by the cycle.

P

V

QH

QC

1

2

34

TH

TC

2 3 4 1Sh,cycle

1 2 3 4

W ˆ ˆ ˆ ˆV dP V dP V dP V dP Enclosed Aream

• This is a PV Diagram of a Gas Power Cycle

• A Vapor Power Cycle looks the same, but the 2-phase envelope is also shown on the plot.


TS DiagramTS Diagram• This is a TS Diagram of a Vapor

Power Cycle

• A Gas Power Cycle looks the same, but the 2-phase envelope is not shown on the plot.

Sh,cyclecycleQ W

int rev

QˆmdST

2 3 4 1Sh,cycle

1 2 3 4

W ˆ ˆ ˆ ˆTdS TdS TdS TdS Enclosed Aream

• In Vapor or Gas Power Cycles, the area enclosed by the cycle path is, once again, equal to the specific shaft work produced by the cycle.Baratuci

ChemE 260May 16, 2005

Carnot Is Not PracticalCarnot Is Not Practical

• Why is the Carnot Vapor Power Cycle impractical ?

– Most pumps do not handle vapor-liquid mixtures well

• Pumps that do are more expensive and less efficient.

– Turbines do not work well with low quality

• Quality must be greater than 90% at the outlet

• This is difficult to achieve without superheating in the reboiler

– Using strictly isothermal heating prevents…

• Subcooled liquid feed to the boiler which would make the pump

more efficient

• Superheating in the boiler effluent which would increase turbine

effluent qualityBaratuciChemE 260May 16, 2005

The Rankine CycleThe Rankine Cycle• A practical vapor power cycle

– Minimizes cavitation problems in the pump

– Allows for the possibility of superheating in the boiler

• The Rankine Cycle– Step 1-2: Boiler Heat added at constant

pressure

– Step 2-3 Turbine Isentropic expansion

– Step 3-4: Condenser Heat rejected at constant pressure

– Step 4-1: Pump Isentropic compression

• Internally Reversible– External irreversibility due to heat transfer through a finite temperature

difference in the boilerBaratuciChemE 260May 16, 2005

TS DiagramTS Diagram


Cavitation

BaratuciChemE 260November 23, 2004

Efficiency & Boiler PressureEfficiency & Boiler Pressure

• WSh

• QH

Shth

H

W

Q

th


Efficiency & Condenser PressureEfficiency & Condenser Pressure

• WSh

• QH: same

Shth

H

W

Q

th


Next Class …• CIDR Focus Group Interview• After that … Problem Session• And then …

– Improvements to the Rankine Cycle• Superheat the turbine feed• Pump to supercritical pressure• Use a two-stage turbine with a reheater between stages• Use regeneration with a two-stage turbine and a two-stage pump• Use two vapor power cycles to cover a wider T range

– Non-ideal behavior in vapor power cycles• Boilers and condensers cannot operate at constant P because of

friction• Turbines and pumps are neither adiabatic nor internally reversible.• Pump feed must be subcooled to avoid cavitation.


Documents

ChemE 260 The Rankine Cycle