POWER GENERATING PLANTS

Hi, steemians i will like to discuss on this broad topic called STEAM PLANT.Please kindly read on

1.0 STEAM PLANT

1.1 INTRODUCTION
In the steam plant, steam is generated in a boiler from which it passes into the steam
main. The steam main feeds the steam into a turbine or engine or it may pass into some other
plant such as heaters or process machinery. After expanding through the turbine or engine or passing through some other plant, if the plant is working on a “dead-loss” system, then the exhaust steam passes away to atmosphere. But if steam recovery plant is installed, the
exhaust steam passes into condenser where it is condensed to water, called condensate. The
condensate is extracted from the condenser by the condensate extraction pump from which it
passes as feed water by means of feed water pump back to the boiler, the losses in the system
are made up in the condenser by means of a make-up water supply.

1.2 ADVANTAGES OF STEAM RECOVERY PLANT (CONDENSER)
(a) The pressure in the condenser can be operated well below atmospheric pressure. This
means that a greater expansion of the steam can be obtained, which results in more
work.
(b) The water in the circuit can be chemically treated to reduce scale formation in the
boiler.

Effects of Scale Formation in the Boiler
(i) It impedes the transfer of heat from the furnace to the water
(ii) It reduces the boiler efficiency
(iii) It may cause local over-heating with resulting damage
(iv) If overheating is serious, it may cause a burst in the vicinity.

1.3 VARIOUS CIRCUITS IN STEAM PLANT

1.3.1 The furnace Gas Circuit.
Air is taken into the furnace from the atmosphere to supply the necessary oxygen for
combustion. The combustion products pass through the boiler, transferring heat, then pass out
to the atmosphere through the flue. Most furnaces are fired by coal, gas or oil.

1.3.2 The Steam Circuit
Water is passed into the boiler where it is converted into steam. It passes into plant
where it is expanded, giving up some of its energy. It is then condensed in a condenser and
passes as condensate to be pumped back into the boiler.

1.3.3 Condenser Cooling Water Circuit
Cooling water passes into the condenser, has heat transferred into it by the condensing
steam then at a higher temperature, passes out to be cooled in a cooling tower. Cooled water
then circulates back to the condenser.

1.3.4 Cooling Air Circuit
In the case of a cooling tower, cool air passes into the bottom of the tower, from the
atmosphere and heat is transferred into it from the falling hot water spray. The warm air then
passes back to the atmosphere through the top of the tower.

1.3.5 Hot Well
In some steam plant the condenser is passed into a tank, called the “hot well” which
acts as a reservoir for feed water. From the hot well, feed water is pumped through the feed
pump back into the boiler. In this case, make-up water could be fed into the hot well.

1.4 BOILERS
A boiler is the device in which steam is generated. Generally, it must consist of a
water container and some heating devices. In the boiler, steam leave the water at its surface
and pump into what is called the steam space. This is the space in the water container directly
above the water. Steam formed above the surface of water is always wet, therefore, the wet
steam is removed from the steam space and piped into a “ super heater” This consist of a
long tube or series of tubes which are suspended across the path of the hot gases from the
furnace. As the wet steam progresses through the tube or tubes it is gradually dried out and
enebturally superheated. From the superheater it passes to the steam main.

1.4.1 Improvement of Thermal Efficiency of the Boiler
The flue gases will still be hot, having passed through the main boiler then the super
heater. The energy in these flue gases can be used to improve the thermal efficiency of the
boiler in the ways.
(a) Thermal efficiency of the boiler can be improved by passing the flue gases through an
‘economizer’. The economizer is really on heat exchanger in which the feed water
being pumped into the boiler is heated, therefore, less energy is required to raise the
steam.
(b) Having passed the flue gases through the economizer, they are still moderately hot.
Further thermal efficiency improvement can be obtained by passing the flue gases
through an air heater. Air heater is also a heat exchanger in which the air being ducted
to the boiler furnace is heated.

1.4.2 Types of Boiler
There are many designs of boiler, but they can be divided into two types; fir-tube
boilers and water-tube boilers.
(a) Fire-Tube Boiler
A fire-tube boiler is sometimes called on economic boiler it has a cylinderic outer
shell and contains two larger-bore flues into which are set the furnaces. The hot flue gases
pass out of the furnace flues are made to pass through a number of small-bore tubes arranged
above the large-bore furnace flues. These small-bore tubes breake up the water bulk in the
boiler and present a large heating surface to he water. In this type of boiler, the flue gasses
are made to pass inside the tube while the water to be heated is outside the tube.
(b) Water-Tube Boiler
With the increasing demand for higher power output from steam plant it became
necessary to develop boilers with higher pressures and steam outputs that could be handled
by the shell-tube boilers. This led to the development of the water-tube boiler. The furnaces
of around the furnace wall. Most heat energy in this type of boiler is transferred by radiation
to the vertical water tubes and from the tubes to the water inside the tubes.

1.5 BOILER CALCULATION
This is given by the ratio of the energy received by the steam to the energy supplied by
the fuel to produce the steam.
Therefore, Boiler thermal efficiency =
Energy to Steam ÷ Energy from Fuel
If ms = Mass of steam raised in a given time
mf = Mass of fuel used in the same time
Then Boiler Efficiency = ms(h2-h1)÷mf CV x 100%

Equipvalent Evaporation of a Boiler
The size of the boiler, or its capacity, is quoted as the rate in kg/hour at which the
steam is generated. A comparison is sometimes made by an equivalent evaporation, which is
defined as the quantity of steam produced per unit quantity of fuel burned when the
evaporation process-takes place from and at 100°C.
At 1000C it will be the enthalpy of evaporation which is supplied specific enthalpy of
evaporation at 100°C; hfg = 2256.9 kJ/kg.
Therefore, the equivalent evaporation of a boiler, from and at 100°C is:
ms(h2 – h1) ÷2256.9 kg in the given time or per kilogram of fuel.

QUESTION 1.1
A boiler with super heater generates 6000 kg/h of steam at a pressure of 15 bar, 0.98
dry at exit from boiler and at a temperature of 300°C on leaving the super heater. If the feed
water temperature is 80°C and the overall efficiency of the combine boiler and super heater is
85%, determine:
(a) the amount of coal of calorific value 30,000 kJ/kg used per hour.
(b) the equivalent evaporation from and at 100°C for the combined unit.
(c) the heating surface required in the superheater if the rate of heat transmission may be
taken as 450000 kJ/m2
of heating surface per hour.

Solution
(a) Specific enthalpy of feed water (h1) at 80°C is 334.9 kJ/kg (from steam table).
Specific enthalpy of steam generated (h2) at pressure of 15 bar and temperature of
3000C is 303 kJ/kg
Therefore, energy transferred to the steam = m(h2-h1)
= 6000 (3039 – 334.9) = 6000 x 2704.1
= 16224600 kJ/h
This is 85% of the energy from the fuel
Therefore, energy from fuel = (16225 x 10^3)/0.85 = 19088 x 10^3
kJ/h
(b) The equivalent evaporation = 7189 kg/h
(c) Heating surface required in the superheater specific enthalpy of steam at exit from
boiler at 0.98 dryness and 15 bar
Energy transferred in the super heater = m(h2-h)
= 6000 (3039 – 2753.06) = 1716 x 10^3kJ/h
Area= Heat transferred ÷ Heat transfered per unit area
=1716×10^6÷45000=3.18m^2

2.0 THE GAS TURBINE CYCLES

2.1 THE IDEAL GAS TURBINE CYCLE
The ideal cycle for gas-turbine engines was first proposed by George Brayton for use
in the reciprocating oil-burning engine that he developed in 1870. Today it is used for gas
turbines only where the compression and expansion processes take place in rotating
machinery. Gas turbines usually operate on an open cycle as shown in Fig. 2.1.

Fresh air at ambient conditions is drawn into the compressor, where its temperature
and pressure are raised. High-pressure air proceeds into combustion chamber, where the fuel
is burned at constant pressure. The resulting high-temperature gases then enter the turbine,
where they expand to the atmospheric pressure, thus producing power. The exhaust gases
leaving the turbine are thrown out (not recirculated), causing the cycle to be classified as open cycle.
The open gas-turbine cycle can be modelled as a closed cycle, as shown in Fig. 2.2, by
utilizing the air-standard assumption. Here the compression and expansion processes remain the same, but the combustion process is replaced by a constant pressure heat-addition process from an external sources, and the exhaust process is replaced by a constant pressure heat￾rejection process to the ambient air. The ideal cycle that the working fluid undergoes in this
closed loop is the Brayton cycle, which is the same as the Joule cycle or constant pressure
flow process cycle.

The cycle is made up of four internally reversible processes:
Process 1-2: Isentropic compression (in a compressor)
Process 2-3: Constant pressure heat addition (in a heater)
Process 3-4: Isentropic expansion (in a turbine)
Process 4-1 Constant pressure heat rejection (in a cooler)
The steady flow energy equation is used to analyse each process, changes in kinetic and
potential energy being neglected. Therefore, heat transfers to and from the working fluid are:
Qin = h3 – h2 = Cp(T3 – T2 )------------ (3.1)
Qout = h4 – h1 = Cp(T4 – T1 ) ----------(3.2)
The thermal efficiency of the cycle is the same with that of constant pressure (Joule) cycle.
Therefore nth=Wnet÷Qin ---------------(3.3)
The network output Wnet=Cp(T3-T4)-(T2-T1).

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