Lecture 8 - Vapour power cycles PDF

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Lecture Notes: ME2011

Lecture 8 - Vapour power cycles.doc DIS

1.1

Thermodynamic cycles are generally associated with power generation ( refrigeration (

) or

1.2

The devices or systems used to produce a net power output are called thermodynamic cycles they operate on are called

1.3

Thermodynamic cycles can also be categorised by the phase of the working fluid: a) Vapour Power Cycles (where the working fluid is alternatively vaporised and condensed) b) Gas Power Cycles (where the working fluid remains completely in the gas phase)

and the

P high = constant T P low = constant

g

f

s

Is the Carnot Cycle a suitable ideal cycle for a vapour power cycle? Two main possibilities exist for using a steady flow Carnot cycle.

T

T 1

4

1

2

4

3

2

3

s

s 1

Lecture Notes: ME2011

Lecture 8 - Vapour power cycles.doc DIS

Using a Carnot cycle for vapour would cause a lot of practical difficulties. Read Çengel and Boles section 9-1

3.1

The Rankine cycle (with superheat) is the ideal vapour power cycle since many of the impracticalities of the Carnot cycle are eliminated by superheating the steam in the boiler and condensing the steam completely in the condenser. Heat in

1

Evaporator

A Superheater

2

Boiler Feed pump

Turbine Work out

Work in 4

Condenser 3 Heat out

T

s

3.2

The ideal Rankine Cycle consists of four reversible steady flow processes: a) Constant pressure heat addition in a boiler and superheater (1-A-2) b) Isentropic expansion in a turbine (2-3) c) Constant pressure heat rejection in a condenser (3-4) d) Isentropic compression in a pump (4-1) 2

Lecture Notes: ME2011

Lecture 8 - Vapour power cycles.doc DIS

3.3

Deviation of Actual Vapour Power Cycle from the Ideal T

s

3.4

The actual vapour cycle differs from the ideal Rankine Cycle as a result of the irreversibilities in various components (eg. fluid friction and unwanted heat loss)

4.1

The Boiler

4.2

The Turbine

4.3

The Condenser

3

Lecture Notes: ME2011

Lecture 8 - Vapour power cycles.doc DIS

4.4

The Feed Pump

4.5

Net Work Out

4.6

Cycle Efficiency

4.7

Work Ratio

4

Lecture Notes: ME2011

Lecture 8 - Vapour power cycles.doc DIS

T

s

1.1

Advantages a) Reducing the condenser pressure increases the net work output since the temperature at which heat is rejected is lowered.

1.2

Disadvantages b) The boiler heat input required is also increased but the net effect is an increase in thermal efficiency c) The turbine exit steam quality is reduced which may cause problems if it goes into the wet region

T

s

5

Lecture Notes: ME2011

Lecture 8 - Vapour power cycles.doc DIS

2.1

Advantages a) Increase the average temperature at which heat is added and thus improves thermal efficiency. b) Improves the turbine exit steam quality.

2.2

Disadvantages a) The boiler heat input required is also increased but the net effect is an increase in thermal efficiency b) The temperature to which the steam can be raised is limited by the materials (typically...