prev

next

out of 23

View

327Download

17

Embed Size (px)

Transcript

A. STEAM CYCLEDEFINITIONS: 1. Rankine cycle is the ideal cycle for vapor power plants. 2. In a rankine cycle, water enters the pump as saturated liquid and is compressed isentropically to the operating pressure of the boiler. 3. In the pump, the water pressure and temperature increases somewhat during this isentropic compression process due to a slight decrease in specific volume of water. 4. The superheated vapor enters the turbine and expands isentropically and produces work by the rotating shaft. The temperature and pressure may drop during the process. 5. steam is condensed at constant pressure in the condenser. 6. the boiler and condenser do not involve any work, and the pump and turbine are assumed to be isentropic. 7. Rankine cycle power plant converts 26% of the heat it receives in the boiler to net work. Work input 8. Back work ratio = Work output 9. The lesser the back work ratio, the better is the cycle. 10. Only 0.4 % of the turbine work output is required to operate the pump. 11. In actual condensers, the liquid is usually subcooled to prevent cavitation. That damaged the impeller of the pump. 12. Fluid friction causes pressure drops in the boiler, the condenser, and piping between various components. 13. The pressure in the condenser is usually very small. 14. To compensate pressure drops in rankine cycle, the water must be pumped to a sufficient higher pressure than ideal cycle. 15. The major source of irreversibility is the heat loss from the steam to the surroundings. 16. To increase the thermal efficiency of rankine cycle, increase the average temperature at which heat is transferred to the working fluid in the boiler. 17. To increase the thermal efficiency of rankine cycle, decrease the average temperature at which heat is rejected from the working fluid in the condenser. 18. Lowering the operating pressure of the condenser automatically lower the temperature of the steam, and thus the temperature at which heat is rejected. 19. The overall effect of lowering the condenser pressure is an increase in the efficiency of rankine cycle. 20. To take advantage of the increase efficiencies at low pressures, the condenser of steam power plants usually operate well below the atmospheric pressure. 21. The average temperature at which heat is added to the steam can be increased without increasing the boiler pressure by superheating the steam to high temperature.

22. Superheating the steam to higher temperature decreases the moisture content of the steam at the turbine exit. 23. Presently the highest steam temperature allowed at the turbine inlet is about 620 degree C. ceramics are very promising in this regard. 24. Raises the average temperature at which heat is added to the steam raises the thermal efficiency of the cycle. 25. The average temperature during the reheat process can be increased by increasing the number of expansion and reheat stages. 26. As the number of stages is increased, the expansion and reheat process approached an isothermal process at the maximum temperature. 27. In a reheat cycle, the optimum reheat pressure is about of the maximum cycle pressure. 28. The main purpose of reheating is to reduce the moisture content of the steam at the final stage of expansion. 29. Regeneration also provides a convenient means of dearating the feedwater to prevent corrosion in the boiler. 30. The cycle efficiency increases further as the number of feedwater heater is increased. 31. A trap allows the liquid to be throttled to a lower pressure region but traps the vapor. 32. A closed feedwater heater is more expensive than open feedwater heater. 33. Cogeneration is the production of more than one useful of energy (such as process heat and electric power)from the same energy source. 34. The overall thermal efficiency of a power plant can be increased by binary cycles or combined cycles. 35. A binary cycle is composed of two separate cycles, one at high temperature (topping cycle) and the other at relative low temperature. 36. Combined cycles have a higher thermal efficiency than the steam or gas turbine cycle operating alone. METHODS OF IMPROVING THE EFFICIENCY OF RANKINE CYCLE 1. By lowering the condenser pressure in rankine cycle(1) (4) (4) T (3) (2) Increase in W (3) (2)

S

In a rankine cycle with fixed turbine inlet condition. What is the effect of lowering the condenser pressure? A. The pump work input will increase. B. The turbine work output will increase C. The heat added will increase. D. The heat rejected will decrease. E. The cycle efficiency will increase. F. The moisture content at turbine exit will increase. 2. By increasing the boiler pressure in rankine cycle(1) (1) increase in W decrease in W (4) T (4) (3) (2) (2) T1 = T2

S In an ideal Rankine cycle with fixed turbine inlet temperature and condenser pressure, what is the effect of increasing boiler pressure. A. The pump work input will increase . B. The turbine work output will increase . C. The heat added will decrease. D. The heat rejected will decrease. E. The cycle efficiency will increase. F. The moisture content at turbine exit will increase 3. By superheating the steam to higher temperature in rankine cycle(1) (1) (4) T (3) (2) (2) increase in W

S

In an ideal rankine cycle with fixed boiler and condenser pressures, what is the effect of superheating the steam to a higher temperature. A. The pump work input will remains constant. B. The turbine work output will increase C. The heat added will increase D. The heat rejected will increase E. The cycle efficiency will increase F. The moisture content at turbine exit will decrease 4. By reheating the steam in rankine cycle.(1) (2) (6) T (5) (4) REHEATING (3)

S Assume the mass flow rate is maintained the same, when a simple ideal rankine cycle is modified with reheating, A. The pumpwork input will remains constant. B. The turbine work output will increase C. The heat added will increase. D. The heat rejected will increase. E. The cycle efficiency will increase. F. The moisture content at turbine exit will decrease. 5. Regeneration of the steam in rankine cycle How do the following quantities change when the simple ideal rankine cycle is modified with Regeneration? A. The pump work input will decrease. (1)

1 (2)(1 m) (1 m) (3)

B. The turbine work output will decrease C. The heat added will decrease D. The heat rejected will decrease E. The cycle efficiency will increase F. The moisture content at turbine exit remains constant 1. RANKINE CYCLE Is the standard and most common steam cycle.the working fluid is waterSCHEMATIC DIAGRAM:

QA (1) (4) WP PUMP (2) CONDENSER QR BOILER WT TURBINE

(3)(1) P4 = P1 (4) T S3= S4 (3) P3 = P2 (2) S1 = S2

S FORMULAS QA 1.1 Heat addition in boiler, QA m BOILER (1)

The Boiler conditions: 1. Changes in kinetic and potential energy are negligible 2. There is no work crossing the control surface 3. The process is constant pressure heat addition QA = ( h1-h4 ), kJ/kg QA= m( h1-h4 ), kw Where: m = mass of steam flow 1.2 Turbine work, Wt m (1) WT Turbine (2) The turbine conditions: 1. The turbine is adiabatic Q = 0 and s1 = s2 2. The change in kinetic energy across the turbine is not negligible but in the ideal analysis the kinetic energy change is being ignored since we have no specific means of determining inlet and discharges velocities. W = h1-h2, kJ/kg W = m ( h1-h2 ), kW

1.3 Quality after turbine expansion, X

s = s = sf + x sfg2 1 2

S1 - Sf X2 = Sfg

h2 = hf2 + x2hfg2 1.4 Heat rejected in the condenser, QR The condenser conditions is The same as the boiler conditions QR = h2 - h3, kJ/kg QR = m (h2 - h3), kW m (2) Cooling requirement in condenser QR = Q mw, mass flow of cooling waterWATER

T2

CONDENSER m (h2 - h3) mw = CP (t2 - t1) (3)

QR mw t1COOLING WATER

1.4 Pump work, WP WP

(4)

PUMP (3) Pump conditions: 1. The pump is adiabatic (Q= 0 ) And reversible (s1 = s2) 2. The change in kinetic and potential energy are negligible 3. The fluid is not in compressible (v1 = v2) WP = h4-h3, kJ/kg Wp = v3 (p -p3),kJ/kg WP = m (h4-h3),kW4

Enthalpy calculations at point 4 h4 = v3(P4-P )+h33

1.6 Cycle Efficiency, e turbine work pump work e = heat added WT - WP e = = (h1-h4) (h1 - h2) - ( h4 - h3 )

QA 1.7 Cogeneration efficiency ec ( QR + WT ) ec = QA 1.8 Backwork ratio (BW) W h4 - h3 BW = = W h1 - h2P T

Problems 1. In a rankine cycle steam enters the turbine at 2.5Mpa (enthalpies and entropies given) and condenser of 50kpa (properties given), what is the thermal efficiency of the cycle? Ans. 25.55% 2. In an ideal rankine cycle the steam throttle condition is 4.10Mpa and 440 C. if turbine exhausts is 0.105Mpa ,determine the thermal efficiency of the cycle Ans. 27.55%

3. In a rankine cycle saturated liquid water at 1 bar is compressed isentropically to 150 bar.First by heating in a boiler,and then by super heating at constant pressure of 150 bar. The water substance is brought to 750 K. After adiabatic reversible expansion in a turbine To 1 bar,it is then cooled in a condenser to a saturated liquid. What is the thermal efficiency of the cycle? Ans. 34.24% 2. REHEAT CYCLE Schematic diagram: Turbine(1) HP LP (3) (4)

WT

QA

BOILER

(2)

condenser QRPump (6) (5)

WPREHEATER

(1) (3)P3 = P2 P3 = P2 S1 = S2

T

(6) (5) P5 = P4

(2)

S3 = S4

(4)

S

c (6)

2.2 TURBINE WORK WT = (h1 h2) + (h3 h4), Kj/Kg WT = ms[(h1 h2) + (h3 h4)],kw

(1)

HP(2) (3)

LP

WT

(4)

2.3 PUMP WORK WP = h6 h5, kj/kg WP = ms(h6 h5), kw WP = V5(P6 P5), kj/kg

(6)

PUMP WP

(5)

2.4 ENTHALPY h6 = V5(P6 P5 )+ h5 2.5 HEAT REJECTED IN THE CONDENSER QR = ( h4 h5 ), kj/kg QR = ms ( h4 h5 ), kj/kg COOLING

Recommended

View more >