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Steam cycle and steam power

plant

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Elements in steam cycle

Saturation temperature

Subcooled liquid

Compressed liquid

Saturated liquid Vapor

Saturated vapor

Superheated vapor

Degree superheat

Degree subcooled

Wet vapor

Quality

Moisture

Latent heat ofvaporization

Critical point

Sensible heat

Latent heat

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steam cycle

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Specify whether the steam is wet, dry or

superheated for the following conditions:

t = 200C, p = 1.44 MPa

t = 220C, p = 2.318 MPa

p = 1.0 MPa, s = 6.672 kJ/kgK

p = 3.0 MPa, t = 234C

t = 250C, v = 54.2x103 m3/kg

P = 11.0 MPa, h = 2805 kJ/kg

P = 4.0 MPa, s = 5.897 kJ/kgK

p = 15.0 MPa, t = 310C

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Processes of vapors

Constant pressure

Constant volume

Constant temperature Constant entropy

Constant enthalpy

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Rankine cycle

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Ideal Rankine cycle processes (SPSP)

Adiabatic pumping(1 to 2)

Constant pressure heat

addition of heat in thesteam generator (2 to 3)

Isentropic expansion inthe engine (3 to 4)

Constant pressure heatrejection in thecondenser (4 to 1)

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Ideal versus actual rankine cycle

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Improving rankine cycle efficiecy

Lowering the condenser pressure in rankine

cycle

Increasing the boiler pressure in rankine cycle

Superheating the steam to a higher

temperature in rankine cycle

Reheat cycle Regenerative cycle

Reheat regenerative cycle

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Rankine cycle statements

Rankine cycle is the ideal cycle for vapor power plant

In a rankine cycle, water enters the pump as saturatedliquid and is compressed isentropically to the operatingpressure of the boiler

In the pump, the water pressure and temperature increasessomewhat during the isentropic compression process dueto a slight decrease in specific volume of water

The superheated vapor enters the turbine and expandsisentropically and produces work by the rotating shaft. The

temperature and pressure may drop during the process. Steam is condensed at constant pressure in the condenser.

The boiler and condenser do not involve any work, and thepump and turbine are assumed to be isentropic.

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Rankine cycle statements

Rankine cycle power plant converts 26% of the heat itrecieves in the boiler to net work.

The lesser the back work ratio, the better is the cycle.

Only .4% of the turbine work output is required to operate

the pump. In actual condensers, the liquid is usually subcooled to

prevent cavitation.

Fluid friction causes pressure drops in the boiler, thecondenser, and piping between various compunents.

The pressure in the condenser is usually very small. The major source of irreversibility is the heat loss from the

steam to the surroundings.

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Rankine cycle statements

To compensate pressure drops in Rankine cycle, thewater must be pumped to a sufficient higher pressurethan the ideal cycle.

To increase the thermal efficiency of rankine cycle,

increase the average temperature at which heat istransferred to the working fluid in the boiler.

To increase the thermal efficiency of rankine cycle,decrease the average temperature at which heat isrejected from the working fluid in the condenser.

Lowering the operating pressure of the condenserautomatically lowers the temperature of the steam,and thus the temperature at which heat is rejected.

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Rankine cycle statements

The overall effect of lowering the condenser pressure isan increase in the efficiency of rankine cycle.

To take advantage of the increased efficiencies at lowpressure, the condenser of steam power plants usually

operate well below the atmospheric pressure. The average temperature at which heat is added to the

steam can be increased without increasing the boilerpressure by superheating the steam to hightemperature.

Superheating the steam to higher temperaturesdecrease the moisture content of the steam at theturbine exit.

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Rankine cycle statements

Presently the highest steam temperature allowed atthe turbine inlet is about 620C.

Raises the average temperature at which heat is addedto the steam raises the thermal efficiency of the cycle.

The average temperature during the reheat processcan be increased by increasing the number ofexpansion and reheat stages.

As the number of stages is increased, the expansionand reheat process approached an isothermal processat the maximum temperature.

In a reheat cycle, the optimum reheat pressure is about of the maximum cycle.

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Rankine cycle statements

The main purpose of reheating is to reduce themoisture content of the steam at the final stageexpansion.

Regeneration also provides a convenient means of

dearating the feedwater to prevent corrosion in theboiler.

The cycle efficiency increases further as the number offeedwater heaters is increased.

A trap allows the liquid to be throttled to a lowerpressure region but traps the vapor.

A closed feedwater heater is more expansive than theopen feedwater heater.

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Rankine cycle statements

Cogeneration is the production of more thanone useful form of energy from the sameenergy source.

The overall thermal efficiency of a power plantcan be increased by binary cycles or combinedcycles.

A binary cycles is composed of two separatecycle, one at high temperatures and the otherat relative low temperatures.

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Problem solving

Steam is generated at 4.1 MPa and 440C and

condensation occurs at .105 MPa.

(a) For a Rankine cycle operating between these

limits, compute the thermal efficiency and theheat rate.

(b) Considering that a Rankine cycle occurs between

the same limits, determine QA, QB, Wnet, and ec.(c) What mass flow rate is required for a net

output of 30000 kW?

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Problem solving

In a reheat engine cycle steam at 8.0 MPa and485 deg C enters the turbine and expands to1.4 MPa. At this point, the steam is withdrawn

and passed through a reheater. It re enters theturbine at 1.3 Mpa and 720 deg C. Expansionnow occurs to the condenser pressure of .006MPa. For the cycle and 1 kg of steam

determine (a) QA (b) Wnet (c) ec . For theengine determine (d) W (e) ec and (f) thesteam flow for an engine output of 40000 kW.

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Problem solving

Steam is delivered to an engine at 5.4 MPa

and 600 deg C. Before condensation at 31 deg

C, steam is extracted for feedwater heating at

.06 MPa. For and ideal cycle, find (a) theamount of steam extracted (b) W and (c) e.

For the ideal engine and the same states,

compute (d) W and e, and (e) steam rate

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Problem solving

Steam at 5 MPa and 365 deg C enters a

turbine and expands until it becomes

saturated. The steam is withdrwan and

reheated to 330 deg C. After expansion in theturbine to 150 deg C, m1 kg is extracted for

feedwater heating. The remaining steam

expands to the condenser pressure of .016MPa. For 1 kg of steam, find Wnet , ec ,ee and

the ideal steam rate.