Steam Power Plant cycle Lectures - AAST

Steam Power Plant cycle LecturesDr: Aly Hassan Elbatran

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Steam Power Plant Cycle

Basic cycle descriptionThere are four processes in the Rankine cycle, each changing the state of the

working fluid.

• Process 1-2s: First, the working fluid is pumped (ideally isentropically) from low

to high pressure by a pump. Pumping requires a power input (for example

mechanical or electrical).

• Process 2s-3: The high pressure liquid enters a boiler where it is heated at

constant pressure by an external heat source to become a saturated vapor.

Common heat sources for power plant systems are coal, natural gas, or nuclear

power.

• Process 3-4s: The saturated vapor expands through a turbine to generate

power output. Ideally, this expansion is isentropic. This decreases the temperature

and pressure of the vapor.

• Process 4s-1: The vapor then enters a condenser where it is cooled to become

a saturated liquid. This liquid then re-enters the pump and the cycle repeats.

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Steam Power Plant CycleEfficiency improvements in steam power plants

1- Superheating the steam to high temperatures

The average temperature at which heat is supplied in the boiler can be increased by superheating the steam.

Dry saturated steam from the boiler is passed through a second bank of smaller bore tubes within the boiler until

the steam reaches the required temperature. The value of TH, the mean temperature at which heat is added,

increases, while TL remains constant. Therefore, the efficiency increases.

It is well known that the cycle efficiency is generally proportional to

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The question that emerges is how to improve the cycle efficiency; naturally through lowering the heat sink

temperature TL and/or raising the heat source temperature TH.

Efficiency improvements in steam power plants

2- Lowering the condenser pressure

- Setting the steam condenser pressure is generally

restricted by the temperature of the available condenser

water (lake, river, etc.) typically around 25°C or condenser

saturation pressure of around Psat =~3.2 kPa.

- The pressure at the exit of the turbine can be less than

atmospheric pressure with a condenser and the closed loop

of the condenser permits the use of high water quality on

the steam cycle side. However, lowering condenser

pressure is not unlimited as it depends on the design

condenser temperature and the limits of lower steam quality

at turbine exit.

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Steam Power Plant CycleEfficiency improvements in steam power plants

3- Increasing the boiler pressure

- Increasing the operating pressure of the boiler

automatically raises the temperature at which boiling

takes place. This consequently raises the average

temperature at which heat is added to the steam and

thus raises the thermal efficiency of the cycle.

- An increase in boiler pressure results in a higher TH

for the same TL, therefore higher cycle thermal

efficiency η. But state 4′ indicates a lower steam

quality than state 4, then more wet steam at the

turbine exhaust is expected; this may result in

cavitation of the turbine blades at the last low

pressure stages. Consequently, the efficiency is

decreased and the cost of maintenance increases. It

is recommended to keep the steam quality higher

than 90% at the turbine exhaust section.

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Actual Rankine cycle

In actual situations, both the water pumps and the steam

Turbines do not operate isentropically and losses result in

more power demand for pumping and less power actually

generated by steam to blades. Such losses are clearly shown in the following comparisons. That is, h3 – h4 < h3 – h4s and h2 – h1 > h2s – h1

The performance of an actual turbine or pump is usually

expressed in terms of isentropic efficiency. The isentropic

efficiency of a turbine ηT is defined as the ratio of “work

delivered by the actual turbine” to “work delivered by an

isentropic turbine.”

The isentropic efficiency of a pump ηP is defined as the

ratio of “work required by an isentropic pump” to “work

required by the actual pump.”

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Steam Power Plant CycleRankine cycle with reheat

1. The moisture content at the exhaust of the turbine should be no greater than 10% – this can result in physical erosion of the turbine

blades.

2. As higher boiler pressures are required for high efficiency, this leads to a higher moisture content ratio in the low pressure turbine

expansion.

3. To improve the turbine exhaust steam conditions, the steam can be reheated between two turbine expansion stages or steps. The

following points emerge:

• The temperature of the steam entering the turbine is limited by metallurgical constraints.

• Modern boilers can handle up to 30 MPa and a maximum temperature of tmax ≈ 650°C.

• Materials, such as ceramic blades, can handle temperatures up to 750°C.

4- This arrangement provides high steam quality or even slightly superheated vapor at turbine exit. Therefore, for a given TH the Rankine

cycle, efficiency increases without reducing the steam quality at turbine exit.

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Steam Power Plant CycleRankine cycle with reheat

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Rankine cycle with regeneration

In this configuration, the Rankine cycle is provided with feed water heaters (FWHs) to heat the

high-pressure sub-cooled water at the exit of the pump to the saturation temperature. Most of

the heat addition (QH) is performed at high temperature.

Advantages:-

1. It improves the cycle efficiency.

2. It provides a convenient means of deaerating the feed water (removing the air that leaks in at

the condenser) to prevent corrosion in the boiler.

3. It also helps to control the large volume flow rate of the steam at the final stages of the turbine.

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Feed water heatersThere are two different types of FWHs commonly used in power plants:

- Open FWH, where the two streams of high temperature steam and low temperature water mix in an open

heater at constant pressure.

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Feed water heaters

- Closed FWH, where a heat exchanger is used to transfer heat between the two streams but the two streams do

not mix. The two streams can be naturally maintained at different pressures.

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Feed water heaters

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Rankine cycle with regeneration

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Steam Power Plant cycle Lectures - AAST