Vapor and Combined Power Cycles

TOPICSVapor Power CyclesRankine Power Cycle modifications to increase efficiencyReheat and Regenerative cyclesCombined cyclesBinary cycles

STEAM ENGINE

The Carnot Vapor Cycle

Impracticability: ??!!!Impracticability: ??!!

Max temp Tcr2-phase devices

(Is it possible ???!!!)

4

5

6

The General James M Gavin Steam Power Plant ,Ohiofull capacity: 2,600,000kW

9

BOILERTURBINE

PUMP

CONDENSER

q in

w out

qout

w in

1

3

42

We’ll simplify the power plant

Rankine - the ideal cycle for Vapor Power Cycles

Example 10-1

Deviations of actual from ideal

Example 10-2 Actual cycle

Increasing the Efficiency: lowering cond. pr

Increasing the Efficiency: superheating the steam

Increasing the Efficiency: increasing boiler pr.

A supercritical Rankine cycle

Example 10-1

Example 10-3 Effect of Boiler pr, temp on efficiency

Ideal Reheat Rankine Cycle

Example 10-4

Regenerative Rankine Cycle: with Open FWH

Examples of closed feedwater heaters

Regenerative Rankine Cycle: with Closed FWH with a pump

Closed feedwater heater with TRAP

The General James M Gavin Steam Power Plant ,Ohiofull capacity: 2,600,000kW

Gavin power plant schematic

six turbines on two separate parallel shafts,

feedwater pump is driven by a separate 65,000HP turbine (FPT)

The system has four low pressure closed feedwater heaters, one open feedwater heater / de-aerator, and three high pressure closed feedwater heaters.

• Consider a steam power plant operating on the ideal regenerative Rankine cycle with one open feedwater heater. Steam enters the turbine at 15 Mpa and 600°C and is condensed in the condenser at a pressure of 10 kPa. Some steam leaves the turbine at a pressure of 1.2 MPa and enters the open feedwater heater. Determine the fraction of steam extracted from the turbine and the thermal efficiency of the cycle.

Example 10-5

Example 10-6

Problem 10-42

Problem 10-53

Problem 10-61E

Problem 10-62

Problem 10-78

Problem 10-81

Problem 10-91

Problem 10-28

Problem 10-29

Problem 10-30

1.A high critical temperature and a safe maximum pressure. A critical temperature above the metallurgically allowed maximum temperature (about 620°C) makes it possible to transfer a considerable portion of the heat isothermally at the maximum temperature as the fluid changes phase. This makes the cycle approach the Carnot cycle.

2.Very high pressures at the maximum temperature are undesirable because they create material-strength problems

3.A condenser pressure that is not too low. Condensers usually operate below atmospheric pressure. Pressures well below the atmospheric pressure create air-leakage problems. Therefore, a substance whose saturation pressure at the ambient temperature is too low is not a good candidate.

Desirable properties of working fluid

5. A high enthalpy of vaporization (hfg) so that heat transfer to the working fluid is nearly isothermal and large mass flow rates are not needed.

6. Low triple-point temperature. A triple-point temperature below the temperature of the cooling medium prevents any solidification problems.

7. A saturation dome that resembles an inverted U. This eliminates the formation of excessive moisture in the turbine and the need for reheating.

8. Good heat transfer characteristics (high thermal conductivity).

9. Other properties such as being inert, inexpensive, readily available, and nontoxic.

Well, we cannot change the way water behaves during the high-temperature part of the cycle, but we certainly can replace it with a more suitable fluid.

The result is a power cycle that is actually a combination of two cycles, one in the high-temperature region and the other in the low-temperature region.

Such a cycle is called a binary vapor cycle. In binary vapor cycles, the condenser of the high-temperature cycle (also called the topping cycle) serves as the boiler of the low-temperature cycle (also called the bottoming cycle).

That is, the heat output of the high-temperature cycle is used as the heat input to the low-temperature one.

Mercury–water binary vapour cycle.

cascadeCycle.swf

New ideas

• Cogeneration• Gas-vapor Combined power cycles

MODIFIED RANKINE CYCLE

MODIFIED RANKINE