THERMODYNAMICS Lecture 7: Thermodynamic cycles steam cycles

THERMODYNAMICS Lecture 7: Thermodynamic cycles – steam cycles Pierwsza strona Schematic of steam power plant – Clausius-Rankine cycle Thermodynami...
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THERMODYNAMICS Lecture 7: Thermodynamic cycles – steam cycles

Pierwsza strona

Schematic of steam power plant – Clausius-Rankine cycle

Thermodynamics

A cycle of steam machine is following:

p

2

p1

Stages of a closed cycle:

3

1

1 4 p0

5 V0

4 5

V1 V2 V

2 3

The piston is at rest, vapour goes from the boiler to a cylinder, pressure increases. The piston moves, vapour is delivered, constant pressure, volume increases. Vapour access is closed, adiabatic expansion

Opening of the cylinder to the cooler, rapid decrease of pressure, constant volume. Removal of vapour remainder, constant pressure, volume decreases.

The work is performed during processes 2,3 and 5. It assumes the following values; Thermodynamics

Schematic of steam power plant – Clausius-Rankine cycle

W = m& (h − h ) t

1

2

Q& = m& (h − h ) cond

W = m& (h − h ) p

4

Q&

3

boiler

η =

W −W t

t

Q Thermodynamics

in

p

=

(h

1

− h ) − (h − h (h − h ) 2

4

1

4

3

)

Q& − Q& η = Q& in

t

in

out

2

3

= m& (h − h ) 1

= 1−

(h (h

4

2

1

−h −h

3

2

) )

Schematic of steam plant – wet Clausius-Rankine cycle Steam is a working fluid in ideal C-R cycle. Saturated vapour enters the turbine at 8MPa and saturated liquid exits the condenser at a pressure of 0.008MPa. The net power output of the cycle is 100MW. Determine for the cycle a)thermal efficiency, b) back work ratio, c) mass flow of steam in kg/h, d) rate of heat transfer into the working fluid as it passes through the boiler, e) rate of heattransfer from the condensing steam as it passes through the condenser, f) mass flow rate of condensing cooling water, if cooling water enters the condenser at 15C and exits at 35C.

Thermodynamics

Thermodynamics

Clausius-Rankine cycle – effect of boiler and condenser pressure variation

η

ideal

T = 1− T

C

H

Increasing boiler pressure increases efficiency. Decreasing the condenser pressure increases efficiency. Temperature of surroundings is the lowest temperature to which heat can be discharged (pcond