lecture 7: thermodynamic cycles – steam cycles · pdf filethermodynamics lecture 7:...
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Pierwsza strona
THERMODYNAMICS
Lecture 7: Thermodynamic cycles– steam cycles
Thermodynamics
Schematic of steam power plant – Clausius-Rankine cycle
Thermodynamics
A cycle of steam machine is following:
V0 V1 V2
p0
p1
1
2
3
4
5
p
V
Stages of a closed cycle:
1 The piston is at rest, vapourgoes from the boiler to a cylinder,pressure increases.
2 The piston moves, vapour isdelivered, constant pressure, volume increases.
3 Vapour access is closed, adiabaticexpansion
4 Opening of the cylinder to the cooler, rapid decrease of pressure, constant volume.
5 Removal of vapour remainder, constant pressure, volume decreases.
The work is performed during processes 2,3 and 5. It assumes thefollowing values;
Thermodynamics
Schematic of steam power plant – Clausius-Rankine cycle
)(21
hhmWt
−= &)(
32hhmQ
cond−= &&
)(34
hhmWp
−= &)(
41hhmQ
boiler−= &&
( ) ( )( )
41
3421
hhhhhh
QWW
in
pt
t −−−−
=−
=η ( )( )
21
321hhhh
QQQ
in
outin
t −−
−=−
=&
&&η
Thermodynamics
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 andsaturated liquid exits the condenser at a pressure of 0.008MPa. The net power output of thecycle is 100MW. Determine for the cycle a)thermal efficiency, b) back work ratio, c) mass flow ofsteam 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 flowrate 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 condenserpressure variation
H
C
ideal TT
−= 1η
Increasing boiler pressureincreases efficiency.
Decreasing the condenserpressure increases efficiency.
Temperature of surroundingsis the lowest temperature to which heat can be discharged(pcond<patm)!
Thermodynamics
Ideal Clausius-Rankine cycle vz. Carnot cycle
Thermodynamics
Clausius-Rankine cycle – irreversibilities in turbine and pump
( )( )
sts
t
t hhhh
WW
21
21
−−
==η
( )( )
34
34
hhhh
WW
s
p
ps
p −−
==η
Thermodynamics
Clausius-Rankine cycle – irreversibilities in turbine and pump
Reconsider the vapor cycle of previousexample, but include in the analyssi thatthe turbine and the pump each have theisentropic efficiency of 85%. Determinefor the modified cycle a) thermalefficiency, b) mass flow rate of steam for as net power output of 100MW, d) rateof heat transfer into the working fluid as it passes through the boiler, e) rate ofheattransfer from the condensing steamas it passes through the condenser, f) mass flow rate of condensing coolingwater, if cooling water enters thecondenser at 15C and exits at 35C. Discuss the effects of irreversibilitieswithin the turbine and pump.
Thermodynamics
Clausius-Rankine cycle – superheating and reheating
Thermodynamics
Clausius-Rankine cycle – supercritical cycles
Critical conditionsfor steam:
Pcr=22.09 MPa
Tcr=647,3K
Thermodynamics
Schematic of steam power plant – Clausius-Rankine cycleSteam enters the first-stage turbine at 8MPa and 480C and expands to 0.7MPa. It is thenreheated to 440C before entering the second-stage turbine, where it expands to thecondenser pressure of 0.008MPa. The net power output is 100MW. Determine a) thermalefficiency of the cycle, b) mass flow rate of steam, c) rate of heat transfer from condensingsteam as it passes through the condenser. Discuss the effects of reheat on the vapourpower cycle.
Thermodynamics
Schematic of steam power plant – Clausius-Rankine cycleReconsider the reheat cycle of last example but include in the analysis that each turbinestage has the same isentropic efficiency. If ηt=0.85 determine the thermal efficiency.
Thermodynamics
Clausius-Rankine cycle – regenerative cycles
y=m2/m1
)1)(()(3221
yhhmhhmWt
−−+−= &&
)1)(()(4567
yhhmhhmWp
−−+−= &&
Thermodynamics
Clausius-Rankine cycle – regenerative cycleConsider a regenerative vapour cycle with one open feedwater heater. Steam enters theturbine at 8MPa. 480C and expands to 0.7MPa, where some of the steam is extracted anddiverted to the open feedwater heater operating at 0.7MPa. The remaining steam expandsthrough the second stage turbine to the condenser pressere of 0.008MPa. Saturated liquidexits the open feedwater heater at 0.7MPa. The isentropic efficiency of each turbine stage is85% and each pump operates isentropically. If the net power output of the cycle is 100 MW, determine a) the thermal effciiency, b) mass flow rate of steam entering the first turbinestage.
Thermodynamics
Clausius-Rankine cycle – closed feedwater heaters
Thermodynamics
Regenerative vapour cycle with one closed feedwater heater
( )( )
72
56
hhhhy
−−
=( ) ( )6572
0 hhhhy −+−=
Thermodynamics
Clausius-Rankine cycle – multiple feedwater heaters
Thermodynamics
Clausius-Rankine cycle – binary cycles
Thermodynamics
Clausius-Rankine cycle – cogeneration