POWER
PLANT ENGINEERING
Calorific value—of fuel is defined as the amount of heat
produced when unit quantity of fuel is completely burnt under standard
conditions. The calorific value is expressed as kcal/kg the calorific value of
a fuel can be classified in two ways: (a) Higher calorific value (HCV), (b)
Lower calorific value (LCV)
Higher calorific value—is the total amount of
heat produced when unit quantity of fuel is burnt completely and the products
of combustion have been cooled to room temperature generally 150C.
Lower calorific value—is the net amount of
heat produced when unit quantity of fuel is completely burnt and the products
of combustion are not cooled to room temperature but are allowed to escape.
Power plant—is an assembly of equipment that produces and delivers
mechanical and electrical energy. Electrical equipment of a power station
includes generators, transformers, switch gears and control gears.
Requirement of plant design—the factors to be kept
in view while designing a power station are follows: (a) economy of
expenditure, (b) safety of plant and personnel, (c) reliability, (d)
efficiency, (e) ease of maintenance, (f) good working conditions, (g) minm
transmission loss.
Connected load—it is the sum of ratings in kW of equipment
installed in the consumers premises.
Maximum demand—it is the maximum load which a consumer uses
at any time.
Demand factor—it is defined as the ratio of maximum demand
to connected load.
Load curve—it is graphical representation between load in
kW and time in hours.
Load factor—it is defined as the ratio of average load to
maximum demand.
Plant capacity factor—it is defined as the ratio of actual
energy produced in kWh to the maximum possible energy that could have been
produced during the same period.
Plant use factor—it is defined as the ratio of energy produced
in given time to the maximum possible energy that could have been produced the
actual number of hours the plant was in operation.
Diversity factor—it is defined as the ratio of sum of
individual maximum demand to the simultaneous maximum demand of a system.
Load duration curve—represents re-arrangement of all the load
elements of chronological load curve in the order of descending magnitude.
Power plant capacity—depends upon the following factors: (a) maxm
demand of consumers at present, (b) type of load, (c) future load conditions,
(d) availability of fuel, (e) total cost of power plant, (f) possibility of
inter connecting the power plant to other power plants.
Principles of power plant design—while designing a
power plant the following factors should be considered: (a) low capital cost,
(b) reliability of supplying power, (c) low maintenance cost, (d) low operating
cost, (e) high efficiency, (f) low cost of energy generated, (g) reserve
capacity to meet future power demand, (h) simplicity of design.
Types of loads—the various type of loads are as follows:
Residential load—it includes domestic
lights, power needed for domestic appliances such as radio, television, water
heaters, refrigerators and small motor for pumping water.
Commercial load—it includes lighting
for shop, advertisements and electrical appliances used in shops and
restaurants etc.
Industrial load—it consists of load
demand of various industries.
Municipal load—it consists of street
lighting, power required for water supply and drainage purposes.
Irrigation load—electrical power need
for pumps driven by electric motors to supply water to fields is included in
this type of load.
Traction load—it includes tram cars,
trolley, buses and rail-ways.
Depreciation cost—is the amount to be set
aside per year form income to meet the depreciation caused by the age of
service, wear and tear of machinery. The most commonly used methods are as
follows: (a) straight line method, (b) sinking fund method.
Energy rates (tariff)—it is desirable to
charge the consumer according to his maximum demand in kW and the energy
consumed in kWh. The tariff chosen should recover the fixed cost, operating
cost and profit etc.
Types of tariff—the various types of tariffs are as follows:
(a) flat demand rate, (b) straight line meter rate, (c) step meter rate, (d)
block rate tariff, (e) two part tariff, (f) three part tariff.
Flat demand rate—it is based on the number of lamps installed
and a fixed number of hours of use per month or per hour. It is expressed by
the expression: Y=DX
Straight line meter rate—according to this energy rate the
amount to be charged from the consumer depends upon the energy consumed in kWh
which is recorded by a means of kWh meter. It is expressed in the form: Y=EZ
Block rate tariff—according to this tariff a certain price per
units is charged for all or any part of block of each unit and for succeeding
blocks of energy the corresponding unit charges decrease.
Two part tariff—in this tariff the
total charges are based on the maximum demand and energy consumed. It is
expressed as Y=DX+EZ
Three part tariff—according to this
tariff the customer pays some fixed amount in addition to the charges for
maximum demand and energy consumed. The fixed amount to be charged depends upon
the occasional increase in fuel price, rise in wages of labors etc. it is
expressed by the expression: Y=DX+EZ+C
Where, Y = Total amount of bill for the period
considered.
D = Rate per kW of maximum demand.
X = Maximum demand in kW.
E = Energy rate per kW.
Z = Energy consumed in kWh during the given period.
C=
Constant amount to be charged form the consumer during each billing period.
Plant performance and operation characteristics
Boiler, turbines,
generators etc, of a power station should work efficiently. Some curves are
plotted to observe their performance. The various curves used are as follows:
Input output curve—is a graphical
representation betn the net energy output (L) and input (I).
Efficiency curve—the ratio of output of
power station to input is called efficiency. The efficiency curve is obtained
by plotting efficiency against output.
Heat rate curve—the ratio of input to
output is known as heat rate (HR).
Heat rate curve is obtained by plotting values of heat rate against
corresponding value of output.
Incremental rate curve—incremental rate is
obtained by plotting values of IR
against corresponding values of output. Incremental rate is defined as IR=dI/dL
Types of dust collectors—the various types of dust collectors
are as follows:
(a)
Mechanical dust collectors. (b) Electrical dust collectors.
Mechanical dust
collectors—are
sub-divided into wet and dry types. In wet type collectors also known as
scrubbers water sprays are used to wash dust from the air. The basic principles
of mechanical dust collector’s are—(1) By increasing the cross-sectional area
of duct through which dust laden gases are passing, the velocity of gases is
reduced and causes heavier dust particles to fall down. (2) Changing the
direction of flow of flue gases causes the heavier particle of settle out. (3)
Sometimes baffles are provided to separate the heavier particles.
Electrical dust
collectors—it
has two sets of electrodes, insulated from each other, that maintain an
electrostatic field between them at high voltage. The flue gases are made to
pass between these two sets of electrodes. The electric field ionizes the dust
particles that pass through it attracting them to the electrode of opposite
charge. The other electrode is maintained at a negative potential of 30,000 to
60,000 volts. The dust particles are removed from the collecting electrode by
rapping the electrode periodically.
Draught—is defined as the difference absolute gas pressure at any
point in a gas flow passes and the ambient atmospheric pressure. The various
types of draught systems are as follows:
(a) Natural draught, (b)
Mechanical draught, (c) Steam jet draught.
The purpose of draught
is as follows:
§ To supply required
amount of air to the furnace for the combustion of fuel.
§ To remove the gases
products of combustion.
Boiler—is a closed vessel in which water is converted into steam
by the application of heat. Thermal energy released by combustion of fuel is
transferred to water which vaporizes and gets converted into steam at the
desired pressure and temperature. A boiler should fulfill the following
requirements: (a) safety, (b) accessibility, (c) capacity, (d) efficiency, (e)
it should be simple in construction and its maintenance cost should be low, (f)
its initial cost should be low, (g) the boiler should have no joints exposed to
flames, (h) the boiler should be capable of quick starting and loading.
Boiler mountings and accessories
Mountings are the
component used for the safety of boiler. Example: feed
cheek valve, steam stop valve, safety valve, blow off valve, water level
indicator, pressure gauge, fusible plug.
Accessories are used to operate it efficiently.
Example: feed pump, air pre-heater, super-heater, draft equipment to supply
air to furnace.
Boiler performance—the performance of a steam boiler may be
expressed in terms of the followings: (a) heat release per cubic meter of
furnace volume, (b) efficiency, (c) heat transferred per square meter of the
heating surface area per hour, (d) rate of combustion is kcal/m2 of
the grate area per hour for solid fuel, (e) amount of steam produced per hour.
Diesel power plant—is suitable for small and medium outputs. It
is used as central power station for smaller power supplies and as a standby
plant to hydro-electric plants and steam power plant. The diesel power plants
are commonly used where fuel prices or reliability of supply favor oil over
coal, where water supply is limited, where loads are relatively small, and
where electric line services is unavailable or is available at too high rates.
Diesel power plants in common use have capacities up to about 5 MW.
Engine performance
IMEP—in order to determine
the power developed by the engine. From the area of indicator diagram it is
possible to find an average gas pressure which while acting on piston
throughout one stroke would account for the network done. This pressure is
called indicated mean effective pressure
(IMEP)
IHP—the indicated HP of
the engine can be calculated as: IHP= (PmLANn)/
(4500*k)
BHP—is defined as the net
power available at the crankshaft. It is found by measuring the output torque
with a dynamometer.
FHP—the difference of IHP and BHP is called FHP. It is
utilized in overcoming frictional resistance of rotating and sliding parts of
the engine.
Indicated thermal
efficiency—it
is defined as the ratio of indicated work to thermal input.
Brake thermal
efficiency—it
is defined as the ratio of brake output to thermal input.
Mechanical efficiency—it is defined as the
ratio of BHP to IHP.
Nuclear power plant—as large amount of coal and petroleum are
being used to produce energy, time may come when their reserve may not be able
to meet the energy requirements. Thus there is tendency to seek alternative
sources of energy. The discovery that energy can be liberated by the nuclear
fission of materials like uranium, plutonium has opened up new sources of power
of great importance. The heat produced due to fission of U and Pu is used to
heat water to generate steam which is used for running turbo-generator.
Nuclear fuel—fuel
of a nuclear reactor should be fissionable material which can be defined as an
element or isotope whose nuclei can be caused to undergo nuclear fission by
nuclear bombardment and to produce a fission chain reaction. It can be one or
all of the following U233, U235 and Pu239
Hydro-electric power plant—water is the cheapest
source of power. It served as the source of power to our civilization in its
earlier days in the form of water wheels.
Gas turbine power plant—has relatively low cost and can be
quickly put into commission. It requires less space. This plant is of smaller
capacity and is mainly used for peak load service.
TERMS AND DEFINITIONS
Work ratio—it is defined as the ratio of network output
to the total work produced in the turbine.
Thermal efficiency—it is defined as the ratio of network output
the total fuel energy input.
Air ratio—it is defined as the amount of air entering
the compressor inlet per unit of network output of the turbine.
Pressure ratio—it is defined as the ratio of absolute
pressure at the compressor outlet to the absolute atmospheric pressure at
compressor inlet.
Compressor efficiency—is defined as the work
required for ideal compression to the actually required by the compressor for a
given pressure ratio.
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