51
CHAPTER FIVE
5 ENERGY FLOW IN THE STEAM POWER STATION
5.1 Introduction
Energy flow in a steam power plant can take the form of either energy transformation or
energy transfer as listed below;.
Transformation of Energy
1. Latent chemical energy into heat energy by combustion.
2. Heat energy to mechanical work by expansions of working medium in the turbine.
3. Mechanical work into electrical through the electromagnetic action of the generator.
4. Electrical energy back to mechanical work in electric motors.
5. Electricity to heat for conventional or accurate spot welding..
Transfer of energy can be from fluid to fluid in steam generators, condenser, coolers and
heaters or be carried by these fluids from place to place in the plant’s pipe and dust systems.
The components of steam power plant can also be classified as heat - transferring devices
such as boiler, superheater, economiser and condenser, heat - transforming devices such as
the turbine (in which the mechanical torque is transferred directly to the rotor of the
electrical generator used to overcome the drag of electromagnetic attraction involved in the
generator action and Accessories - mainly feed water pumps, fans, stoker, burners etc.
Functional Relations
All action of modern electric production centers around three equipment namely the boiler,
the turbine and the condenser - which constitute the main power producing group, and all
the remaining equipment such as feed heaters, pumps, fans, stokers etc service these
equipment. The auxiliaries can be classified in two groups. Those concerned with the flow
MPE 571E: POWER PLANT ENGINEERING
, 52
of the covering medium - water loop and those concerned with combustion of fuel and the
flow of the remitting gases - gas loops. The source of thermal energy in a steam power plant
can be the combustion of fuel (oil, gas, coal), nuclear fusion or geothermal
5.2 Thermal level
The selection of the economic operating conditions for the boiler-turbine-condenser group is
the first and most important step in power plant design. The temperature range is of the
order of 1425 °C down to 21 °C and it is the designer’s problem to choose what portion of
this range will give him the most economical installation. The working medium in a power
plant therefore operates between two common levels: high temperature - source and low
temperature - sink. The low - temperature sink depends on the required exhaust pressure,
temperature and quantity of natural water available for condensing. The turbine exhaust
pressure is carried as low as the condensing water permits on the basis of 10-15 °C
temperature difference of the warm end of the condenser tubes. The limitations for the high
temperature are the available of high temperature resisting metals, necessary temperature
difference and cost of equipment. The trend of steam power plant practice is towards higher
pressures and temperatures because of the advantage of heat efficiency of the vapour cycle
and of decreased size of the equipment. However, the use of high temperature, for example,
poses many problems of mechanical design arising out of thermal expansion, change of
structural properties of metals etc. Boiler pressures are standardised as follows 11.25, 12.66,
14.06 and in increments of 1.76 bar above this. Generally the steam pressure at the turbine
throttle valve is taken to be the official plant pressure and thus the boiler pressure must be
above this to take account of frictional losses. Boiler pressure = 1.04 (official pressure +
frictional losses). The 4 % taken is due to the fact that the safety valve will not open until
pressure has risen some 4% above operating pressure.
5.3 Heat flow
All the three forms of heat transfer namely conduction, convection and radiation are found in
power plant engineering. The following are examples:
1. Radiation from fuel bed and luminous gases to boiler tubes and water walls.
MPE 571E: POWER PLANT ENGINEERING
CHAPTER FIVE
5 ENERGY FLOW IN THE STEAM POWER STATION
5.1 Introduction
Energy flow in a steam power plant can take the form of either energy transformation or
energy transfer as listed below;.
Transformation of Energy
1. Latent chemical energy into heat energy by combustion.
2. Heat energy to mechanical work by expansions of working medium in the turbine.
3. Mechanical work into electrical through the electromagnetic action of the generator.
4. Electrical energy back to mechanical work in electric motors.
5. Electricity to heat for conventional or accurate spot welding..
Transfer of energy can be from fluid to fluid in steam generators, condenser, coolers and
heaters or be carried by these fluids from place to place in the plant’s pipe and dust systems.
The components of steam power plant can also be classified as heat - transferring devices
such as boiler, superheater, economiser and condenser, heat - transforming devices such as
the turbine (in which the mechanical torque is transferred directly to the rotor of the
electrical generator used to overcome the drag of electromagnetic attraction involved in the
generator action and Accessories - mainly feed water pumps, fans, stoker, burners etc.
Functional Relations
All action of modern electric production centers around three equipment namely the boiler,
the turbine and the condenser - which constitute the main power producing group, and all
the remaining equipment such as feed heaters, pumps, fans, stokers etc service these
equipment. The auxiliaries can be classified in two groups. Those concerned with the flow
MPE 571E: POWER PLANT ENGINEERING
, 52
of the covering medium - water loop and those concerned with combustion of fuel and the
flow of the remitting gases - gas loops. The source of thermal energy in a steam power plant
can be the combustion of fuel (oil, gas, coal), nuclear fusion or geothermal
5.2 Thermal level
The selection of the economic operating conditions for the boiler-turbine-condenser group is
the first and most important step in power plant design. The temperature range is of the
order of 1425 °C down to 21 °C and it is the designer’s problem to choose what portion of
this range will give him the most economical installation. The working medium in a power
plant therefore operates between two common levels: high temperature - source and low
temperature - sink. The low - temperature sink depends on the required exhaust pressure,
temperature and quantity of natural water available for condensing. The turbine exhaust
pressure is carried as low as the condensing water permits on the basis of 10-15 °C
temperature difference of the warm end of the condenser tubes. The limitations for the high
temperature are the available of high temperature resisting metals, necessary temperature
difference and cost of equipment. The trend of steam power plant practice is towards higher
pressures and temperatures because of the advantage of heat efficiency of the vapour cycle
and of decreased size of the equipment. However, the use of high temperature, for example,
poses many problems of mechanical design arising out of thermal expansion, change of
structural properties of metals etc. Boiler pressures are standardised as follows 11.25, 12.66,
14.06 and in increments of 1.76 bar above this. Generally the steam pressure at the turbine
throttle valve is taken to be the official plant pressure and thus the boiler pressure must be
above this to take account of frictional losses. Boiler pressure = 1.04 (official pressure +
frictional losses). The 4 % taken is due to the fact that the safety valve will not open until
pressure has risen some 4% above operating pressure.
5.3 Heat flow
All the three forms of heat transfer namely conduction, convection and radiation are found in
power plant engineering. The following are examples:
1. Radiation from fuel bed and luminous gases to boiler tubes and water walls.
MPE 571E: POWER PLANT ENGINEERING
