An Introduction To Steam Power Generating Plant And Components

An Introduction To Steam Power Generating Plant  And Components

Introduction
By far the greater part of our electricity is produced by power stations in which the generators are powered by steam turbines.

steam generating plants have the same major components. A typical arrangement is shown in Figure 1

figure 1 Steam plant circuit

Boilers and superheaters
In coal, gas and oil fired systems, the fuel and air enter the boiler where the hot gases from combustion heat the feed water to produce wet steam. There are two basic kinds of boiler, the fire tube type and the water tube type. In fire tube boilers the hot gases from combustion pass through a system of tubes around which water is circulating. These are usually to be found in small installations where low pressure steam is required for industrial processes and space heating.

fire tube condenser

In water tube boilers, the hot gases from combustion circulate around a system of tubes containing water. This is the type used in power stations for producing large quantities of high pressure steam.

The wet steam passes through a system of tubes in the superheater where additional heat energy is supplied from the combustion gases to produce superheated steam. Every possible unit of heat energy is extracted from the exhaust gases and before escaping to the atmosphere, they are used to heat up the boiler feed water in the economiser. They are also used to pre-heat the incoming air

water type condenser

In nuclear installations, the heat source is enriched uranium. This is bombarded with neutrons in the reactor vessel causing some of the uranium atoms to split and release heat energy. The process is called nuclear fission. There are many different designs of reactor in operation throughout the world.

Turbines
The superheated steam passes to the high pressure turbine where it expands and does work on the rotor blades. It then passes to the intermediate pressure turbine where the blades have a larger diameter. Here it continues to expand and do work. You will note that some of the exhaust steam from the intermediate pressure turbine is fed to the boiler feed water heater where it is injected into the feed water from the hotwell. The remainder of the steam from the intermediate pressure turbine passes to the low pressure turbine.

Here the blades are of a still larger diameter and arranged so that the steam enters centrally, and expands outwards through the two sets of low pressure blades. All three turbines are connected by a common shaft which drives the electrical generator.

Condensers and feed heaters

The exhaust steam from the low pressure turbine passes to the condenser as low pressure wet steam. There are two basic types of condenser. In the spray type, cooling water is injected into the steam causing it to condense.

spray type condenser

In the surface type, the steam condenses on the surface of a system of pipes through which there is a flow of cooling water.The surface type is used in all large power stations. Sea water and river water are used for cooling wherever practical and a great many of our nuclear and gas fired power stations are sited on the coast. Coal fired power stations tend to be sited inland near the remaining coal fields, to reduce transportation costs. Here the cooling water for the condensers is generally re-circulated through cooling towers .

surface type condenser

When the steam condenses it occupies a much smaller volume and as a result, the pressure in the condenser is well below atmospheric pressure. This is beneficial because it creates as large a pressure drop as is possible across the low pressure turbine, allowing steam to expand freely and do the maximum possible amount of work. The condensed steam must however be extracted from the condenser by the condensate extraction pump.

 condensate extraction pump

The condensate passes to a reservoir called the hotwell where make-up water is added for evaporation losses. The feed water from the hotwell is heated first in the feed water heater by exhaust steam, and then in the economiser by the exhaust gases from the boiler. 

FEED WATER PUMP

The boiler feed pump delivers the feed water through the economiser to the boiler. The objective of the feed water heaters is to raise the temperature of the water to as close to its saturation pressure as possible before it enters the boiler.

FEED WATER HEATER

position of economiser  in boiler

Power rating
The output power of a boiler is the heat energy received per second by the feed water as it is changed into steam.

If the initial and final enthalpy values of the feed water and steam are h1 and h2, this can be written as

boiler power rating=(h2- h1)ms (W)

The output power of a turbine is its shaft work output per second. This is the product of its output torque, T (N m) and angular velocity, 𝜔 (rad s⁻¹).

shaft output power=T𝜔 (W)

Thermal and mechanical efficiency (𝜂)

The thermal efficiency of a boiler gives a comparison of the heat energy received per second by the water and steam and the heat energy available per second in the fuel.

If the fuel consumption rate is mf  kilograms per second and its calorific value is CV then

The mechanical efficiency of a turbine gives a comparison of the work done per second by the expanding steam and the actual shaft output power after losses in the blades and friction losses.

If the initial and final enthalpy values of the steam as it passes through the turbine are h1 and h2, this can be written as

The overall thermal efficiency of a steam plant gives a comparison of the heat energy available per second in the fuel and the actual shaft output power.

When using these formulae you should be careful to use the correct units. Steam flow rates and fuel consumption are often given in tonnes per hour which need to be converted to kg s⁻¹.
Calorific values are often given in MJkg⁻¹ and the enthalpy values from steam property tables are given in kJ kg⁻¹. Both need to be converted to J kg⁻¹.
An oil-fired steam generating plant raises 15 tonnes of steam per hour from feed water at a temperature of 75 8C. The fuel consumption rate is 1.5 tonnes per hour with a calorific value of
42MJkg⁻¹. 
The steam enters the high pressure turbine stage at 70 bar and temperature 500 8C and exhausts to the condenser at a pressure of 0.08 bar and 0.89 dry. The mechanical efficiency of the turbine is 72%. Determine 
(a) the power output from the boiler, 
(b) the boiler efficiency, 
(c) the power output from the turbine, 
(d) the overall thermal efficiency of the plant.