UNIT-1
1.An overview of power generation
Energy is an important factor for the development of any country. In modern civilization the
standard of living of an given country can be directly related to per capita energy
consumption presently majority of the countries are facing the problem of energy crisis due
to rapid growth in population and due to high living standard. The per capita energy
consumption in USA is 8000 kwh/whereas the per capita energy consumption in India is
150kwh.
India the largest democracy in the world having an area of 1.27 million square miles
with a population around 1000 millions. There are 5.76 lakhs villages in India which covers a
70% of the total population.
The first power plant was commissioned in 18.9% at
Darjeeling.
Energy : It is defined as ability to do work.
Energy exists in the various forms. One form of energy can be easily converted into the
another forms by suitable arrangement by the use of suitable arrangement. Out of various
forms of energy electrical energy is preferred due to following advantages.
(i)Can be easily transported from one place to another.
(ii)Hosses in transport are minimum
(iii)Can be easily subdivided.
(iv)Economical in use.
(v)Easily converted into another forms of energy.
(vi)Easily controlled and regulated to suit requirements.
Power: It is defined as rate of change energy. Any physical unit of energy when divided by
a time automatically becomes a unit of power.
Hydroelectric power plants:
In hydro electric plants energy of water is utilized to move the turbines which in turn run the
electric generators. The energy of water utilized for power generation may be kinetic or
potential. The kinetic energy of water is its energy in motion and is a function of mass and
velocity, while the potential energy is a function of the difference in the level/haad of water
between two points.
Rainfall is the primary source of water and depends upon such factors as temperature,
humidity, cloudiness, wind etc. However it has been observed that only a small part of
rainfall can actually utilized in the power generation.
The first major hydroelectric was started in Mysore with a capacity of 4.5MW.
The need of Hydroelectric power:
1)Being a source of lower cost of generation it contributed to better financial position for
the spodem and this is well established by actual data for system and our country.
2)It also offers advantages in total investment costs of generation through differential in
regard to reserve requirements, auxiliary power needs, longer life of plant etc.
3)Hydroelectric power give the well known advantage in respect of meeting flucation in the
load more economically and more efficiently.
4)It helps in improving the stability and reliability of the system and teads to the better
performance and economy of the fuel burning plant.
5)Hydroelectric generation eliminates the need for the production and transportation to a
corresponding extent.
Advantages of hydroelectric power plants:
(i)The plant is highly reliable and its operation and maintenance charges are very low.
(ii)It is quick starting and can be brought on load within few minutes, and the load can be
increased rapidly.
(iii)Hydro stations are able to respond to rapidly changing loads without loss of efficiency.
(iv)The plant has no stand by loss.
(v)The efficiency of the plant does not change with age.
(vi)The man power requirement for the hydroelectric plant is less and much of the power
plant is under automatic control.
(vii)No fuel transportation charges.
(viii)As water is used as fuel no ash problem and atmosphere is not polluted since no smoke
is produced in the power plant.
(ix)The number of operation required as considerably small compared with thermal plant.
(x)In addition to power generation these plants are also used for flood control and irrigation
purpose.
Disadvantages of power plants:
(i)Initial cost of plant including dam is very high.
(ii)Hydro station has special requirement of site which usually in an isolated area with
difficult access.
(iii)Power generation by the hydro plant is only dependent on the quality of water available,
which in turn depends on rain.
discontinued.
During the dry year the power production may be
(iv)The site of hydroelectric station is selected on the basis of water quality at economical
head. Such sites are usually away from load centers. The transmission of power from
power station to the load centre requires long transmission lines. This substantially increase
the capital expenditure and also there is loss of power in such transmission.
Classification of hydroelectric power plants:
The hydroelectric plants are classified according to
a)availability of head
b)according to nature of load
c)according to quantity of water available
(A)Classification on the availability of head:
(1)Low head plants:- When the head of water available is below 30 meters is known as low
head power plants. Francis, propeller or Kaplan type turbines are generally used in such
power plant.
(2)Medium head plants:- When the operating head of water lies between 30 to 100 meters,
the power plant is known as medium head plant.
(3)High head plants:- When the available head for power generation exceeds 100 meters
the plant is known as high head plant the Francis and pehon wheel are common prime
movers used in high head plants.
Classification according to nature of load:
The electric load on any power plant does not remain constant and varies according to
seasons as well as every hour in a day of twenty four hours.
(1)Base load plant:- This type of plant takes the load on the base portion of the load curve.
The load on the plant remains more or less constant through out the operation period. Base
load plants are generally large in capacity.
(2)Peak load plants:- This type of plant takes the load on the peak portion of the load curve.
They store the water during off peak period and supply during peak periods on the top of
the load curve.
Diagram--------------------------------------------------------------------------Classification according to quantity of water available:
(i)Run- off river plant without pondage.
(ii)Run-off river plant with pondage.
(iii)Storage type plants.
(iv)Pump storage plants.
(v)Mini or Micro hydel plants.
General arrangement and operation of hydro-electric plant:The storage type hydro-projects have large contents of water and the water collected during
heavy rain period is supplied during by period of the year. The collection of water is done
on seasonal basis, therefore the capacity of the reservoir required is extremely large
compared with the other types of hydraullic power plants.
Diagram-------------------------------------------------------------------------The basic requirement of a hydroelectric power plant station is a reservoir where a large
quantity of water is stored during the flood season and used during flood season and used
during dry season. The reservoir is generally built by constructing a dam across a river. The
water from the reservoir is drawn by the fore bay through an open canal or tunnel. The
water from the forebay is supplied to the water prime mover through the penstock which is
located at much lower level than the height of water in the reservoir. The water entering
into the turbine rotates the turbine shaft and generator shaft is coupled with the turbine
shaft.
The functions of the different components used in the hydroelectric power plant.
(i)Reservoir:-The main purpose of the reservoir is to store the water during the rainy season
and supply during dry season.
(ii)Dam:-The function of the dam is to increase the height of water level behind it which
ultimately increase the reservoir capacity.
(iii)Trash rack:-The water intakes from the dam or from the forebay are provided with trash
rack to prevent the entry of debris which might damage the wicket gates and turbine
runners or choke-up the nozzles of the impulse turbine. In winter in order to prevent ice
from clining to trash racks. They are often heated electrically.
(iv)Forebay:-The forebay serves as regulating reservoir temporarily storing when the load
on the plant is reduced for initial increment of an increasing load while water in the canal is
being accelerated.
(v)Surgetank:-A surge tank is introduced in the system between the dam and power house,
to provide better regulation of water pressure in the system during variable load conditions.
(vi)Penstock:-A pipe between the surge tank & the prime mover is known as penstock. The
structural design of the penstock is same as for any other pipe except it has to bear very
high pressure.
(vii)Spillways:-Spillway is considered a safety value for a dam. It must have the capacity to
discharge major floods without damage to the dam and at the same time keeps the
reservoir level below some predetermined maximum level.
(viii)Primemovers:-The main purpose of the primemover is to convert the kinetic energy of
water into mechanical energy to produce the electrical energy.
(ix)Draft tube:-The draft tube is essential part of reaction turbine installation it supplements
the action of othe runner by utilising most of the remaining kinetic energy of water at the
discharge end of water.
(x)Operation:-In hydroelectric power plants the potential energy of the water is converted
into kinetic energy first passing through the tunnel to the power house. The kinetic energy
is converted into mechanical energy in the water turbines. The mechanical energy of the
turbine is further utilized to run electric generator.
Diagram on full page--------------------------------------------------------------------------------------Basic components of Thermal Power plan diagram--------------------------------Wind power plants:- Non-conventional source of energy :The electrical energy can be generated by wind energy. The wind energy which is an
indirect source of oenergy can be used to run wind will in turn drives generator to produce
the electricity.
In India the interest in the wind mills was shown in the last fifties and early sixties. Only in
last 12 to 15 years the development is going on in various institution. An important reason
for this lack of interest in wind energy must be that wind, in India is relatively low and very
appericiable with seasons.
Advantages:(i)It does not pollute the atmosphere.
(ii)Fuel provision & transport are not required in wind power system.
(iii)Wind energy is renewable source of energy.
(iv)Wind energy when produced on the small scale is cheaper, but competitive with
conventional power generating systems when produced on large scale.
Disadvantages of wind energy:(i)It is flucating in nature.
(ii)Due to its irregularity it need storage devices.
(iii)It also produces noise.
Methods of wind recording:(i)Human observation & long book:- It involves using the Beaufort scale of wind strengths
which defines visible “symptoms” attributable to different wind speeds. This method is
cheap but often unreliable.
(ii)Mechanical cup counter:- The majority of the metrological stations are mechanical cupcounter anemometers. By taking the readings twice or thrice a day it is estimated the mean
wind speed. An instruments for measuring (or) indicating the force of the wind.
Types of wind mills:(i)Multiple blade type:-It is the most widely used wind mill. It has 15 to 20 blades made
from metal sheets. The soil type has three blades made by stitching out triangular pieces of
convass cloth. Both these run at low speed of 60 to 80 RPM.
(ii)Savonius type:-This type of wind mill has hollow circular cylinder sliced in half and the
halves are mounted on vertical shaft with a gap in between . Torque is produced by the
pressure difference between the two sides of othe half facing the wind. This is quiet
efficient but needs a larger surface area.
(iii)Darrieus type:-This wind mill needs much less surface area. It is shaped like an egg
beater & has two or three blades shaped like aerofoils.
Diagram----------------------------------------------------------Constructional Details & Principle of operations:
It consists of two half cylinder facing opposite direction in such a way as to have almost
an s-shapped cross-section. These two semicircular drums are mounted on a vertical axis to
the wind direction with a gap at the axis b/w the two drums. Irrespective of the wind
direction the ------------- such as to make the convex sides of the buckets head into the wind
from the rotor shaft we can tape power for our use like water pumping, battery changing,
gram winnowing etc. However, instead of having two edges together to make s-shape, they
overlap to leave the wind space b/w the two inner edges, so that each of these edges is near
the ineteal axis of the opposite half cylinder, as showing fig. The main action of the wind is
very simple, the force of the wind is greater on the cupped face than on the sounded face.
The wind curving around the back side of the cupped face exists a reduced pressure much as
the wind does over the top as an air foil and this helps to drive the rotation. The wide slof
b/w the two inner edges of the half cylinders, lets the air whip around inside the forwardmoving cupped face and then around the -----------------.
Wind – electric generating power plant:
Diagram---------------------------------------------------------The above fig. shows various parts of wind-electric generating power plant.
(i)Wind turbine or rotor.
(ii)Wind mill head – it houses speed increaser, drive shaft clutch coupling etc.
(iii)Electric generator.
(iV)Supporting structure.
The wind mill head performs following functions:(i)It supports the rotor housing and the rotor bearings.
(ii)It also houses any control mechanism incorporated like changing the pitch of the blades
for the safety devices & tail vane to orient the rotor to face the wind, the latter is facilitated
by mounting on the top of the supporting structure with suitable bearings.
The supporting structure is designed to with stand the wind load during gusts. Its type of
height is related to cost & transmission system incorporated.
Wind Energy:-
Conversion of K.E of the wind into mechanical energy that can be utilized
to perform useful work or to generate elect. Wind energy is the form of solar energy
because winds arise primarily from temp differences of the earth’s surface resulting from
unequal explosure to solar radiation.
Most of the Machines for converting wind energy into mechanical energy consists
basically of a no of sails, vares, or blades radiating from hub or central axis. Wind energy
conversion devices are commonly known as wind turbines because they convert the energy
of the wind stream into energy of rotation. The combination of wind turbine and generator
is referred as aerogenerator.
Basic components of wind energy conversion system:
Diagram-------------------------------------------------------------------Aero turbine:convert energy is moving air to rotary mechanical energy. In general, the
require pitch & yaw control for proper operation.
Mechanical Interface:- Consists of setup gear & suitable coupling, transmits, the rotary
mechanical energy to an electric generator. The o/p of generator is connected to the load
or power grid as the application wareants.
Controller:- Is to sense wind speed, wind direction, shafts speeds and torques at one or
more points, o/p pwr and generator temperature as necessary and appropriate control
signals for matching the electrical o/p to the wind energy i/p & protect the system from
extreme conditions brought upon by strong winds, electrical faults and the like.
Solar Pond Electric Power Plant:
A low temperature thermal electri power production scheme using solar pond is shown
below. The energy obtained from a solar pond is used to drive a Rankine cycle head engine.
The hot water from the bottom level of the pond is pumped to the evaporator where the
organic Fluid is vapourized. The vapour then flows under high pressure to the turbine
where it expands and work thus obtained runs an electric generator producing electricity.
The exhaust vapour is then conderse in a condenser and the liquid is pumped back to the
evaporator and the cycle I repeated.
Diagram---------------------------------------------------------------Low temperature solar plant:
Fig. shows a schematic diagram of a low temperature solar power plant. In this system an
array of flat plate collector is used to heat water about 70 0C and this heat is used to boil
butane in a heat exchanger. The high pressure butane vapour thus obtained runs a butane
turbine which in terms operates a hydraulic pump. The pump pumps the water from well
which is pumped by the pump and the condensate is returned to heat exchanger.
(2)Medium temperature system using focusing collectors:
A circular or rectangular parabolic mirror can collect the radiation and focus it an to a small
area a mechanism for moving the collector to follow the sun being necessary.
The fig. shown below shows a concave solar energy collector focusing sun’s rays on the
boiler at a focal point. Generation of steam at 2500C could give turbine efficiencies up to
20-25% .
Diagram------------------------------------------------(3)High temp system: Solar form & solar Tower.
(i)The solar farm:-It consist of a whole field covered with parabolic trough concentrators.
(ii)The solar Tower:-It consist of a central receiver on a tower & a whole field of tracking. In
case of ‘solar farm’ temperature at the point of focus can reach several hundred degrees
celcius.
In case of central receiver “solar tower” concentrators, temperature can reach thousands
of degrees Celsius, since a field reflectors are arranged separately on sun tracking frames to
reflect the sun on to a boiler mounted on a central tower.
Diagram--------------------------------------------Importance of instrumentation for power generation:-
The instruments are used in the power plants for a number of reasons as to operate the
power plant most efficiently. Instruments furnish accurate information for guidance to safe
continuous and proper plant operation. The information given by the recording units from
control room helps to direct its operation so as to achieve the best performance.
The functions which are carried out by the various instruments in the power plant are
listed below.
(i)Operating guidance:-The instruments provide the guidance to operate the power plant
efficiently & economically.
(ii)Performance calculation:-They render help in making performance calculations in respect
of plant working.
(iii)Maintenance & repair guidance:-As the instruments enable us to check the internal
conditions of the equipment thus they provide us maintenance and repair guidance.
(iv)Economical supervision:-They enable us to supervise the plant economically.
(v)Cost allocation:-The instruments extended a helping hand in dealing with the problems
concerning cost accounting & cost allocation.
Diagram----------------------------------------Diagram----------------------------------------------------NUCLEAR POWER PLANT
Introduction:
The number of protons in a given atom is known as the atomic number. It is usually
represented by symbol the number of protons is equals to number of electrons the total
number of protons by neutrons is called as “mass number”. It is denoted by symbol A. The
total number of neutrons in a atom given by (A-Z).
Isotopes:-The atoms which are having some atomic number and different mass numbers
are called as isotopes. The isotopes may be stable or oradio active nearly 280 stable
isotopes and 50 unstables are occurring naturally.
The example of the isotope is --------------------. The naturally occurring elements of
highest atomic weight, such as thorium, radium and uranium consist of unstable isotopes.
These elements undergo a spontaneous change referred as radio active change of αpracticle, β-practicle and γ-practicle.
Nuclear Fission:-When unstable heavy nucleus is bombarded with high energy neutrons, it
splits into two framgments more or less of equal mass. This process is known as “Nuclear
Fission”. The fission fragment formed due to fission are the isotopes which are located in
the middle of periodic table. The nuclear fission is always associated with the release of the
huge amount of energy.
To sustain the fission processes, the following requirements must be fulfilled.
(i)The bombarded neutrons must have sufficient energy to cause fission of another nucleus.
(ii)The number of neutrons produced must be able to increase the rate of fission as certain
loss of neutrons by absorption and leakage is unavoidable.
(iii)The fission process must generate energy.
(iv)The fission process must be controlled.
Nuclear Reactors:
The nuclear reactor may be regarded as substitute for the boiler fire box of steam power
plant. The heat is produced in the nuclear power plant is by fission. The other cycles of
operation and components required exactly same either a steam plant. The steam or the
gas may be used as working fluid in nuclear power plant. The nuclear power plant may be
of steam driven turbine or gas drive turbine as per choice of the fluid.
Diagram-----------------------------------
Fuel: The fuels which are generally used in the reactors are ------------------------- . The fuel is
shaped and located in the reactor in such a manner that the heat production with in the
reactor is uniform. The fuel elements are designed taking into account the heat transfer
corrosion and structural strength. The fuel rods are clad with aluminium, stainless steel to
prevent the oxidation of uranium.
Moderator:-The Moderator is a material which reduces kinetic energy of fast neutrons i.e
13200km/sec to2200m/sec, and this is done in a fraction of second. The function of the
moderator is to increase the probability of reaction. The slowing down of neutrons is
effectively done by the light elements such as H2,D2,N2,O2,C etc.
The Moderator should have some properties which are listed below.
(i)It must be as light as possible to slow down the neutrons.
(ii)It must have resistance to corrosion as it has to work under high pressure & high
temperature.
(iii)It must have high M.P,if it is a solid.
(iv)It must be cheap and easily available.
Reflector:-It is always necessary to conserve the neutrons as much as possible in order to
reduce the consumption of fissile material and to keep the size of reactor small. The
neutrons which are released in fission process can be absorbed by the fuel itself, moderator,
coolant or structural materials.
Some neutrons may escape from the core without
absorption and will be lost forever. To reduce this loss of escape, the reactor inner surface
is surrounded by a material called as reflector.
Coolant:-The main purpose of the coolant in the reactor is to transfer the heat produced
inside the reactor. The same heat carried by the coolant is used in the heat exchanger for
further utilization in power generation.
Control rods:-The control rod performs following functions.
(i)To start the nuclear chain reaction when the reactor is started from cold.
(ii)To maintain the fission reaction in steady state.
(iii)To shut down the reactor automatically under emergency conditions.
Shielding:-The reactor is a source of α,β, and γ radiation which are harmful to human body.
To prevent the effects of these radiation, shielding is used. The shielding absorbs all the
radiations.
Reactor vessel:-The reactor vessel encloses the reactor core, reflector and shield. It can with
stand for the pressure as high as 200 bar.
Thermal Power Plants
The general layout of the thermal power plant consist of mainly 4 circuits as shown in figure.
The four circuits are as follows.
a)Coal and ash circuit:-In this circuit, the coal from the storage is fed to the boiler through
coal handling equipment for the generation of steam. The ash produced due to combustion
of coal is removed to ash storage through ash handling system.
b)Air and gas circuit:Air is supplied to the combustion chamber through F.D or I.D fan or by
using both. The dust from the air is removed before supplying to the combustion chamber.
The exhaust gases carrying sufficient quantity of heat and ash are passed through the air
header where the exhaust heat of the gases is given to the air and then it is passed through
the dust collectors where most of the dust is removed before exhausting the gases to the
atmosphere through chimney.
c)Feed water and steam circuit:-The steam generated in the boiler is fed to the steam prime
mover to develop the power. The steam coming out of prime mover is condensed in the
condenser and then fed to the boiler with the help of pump. The condensate is heated in
the feed heaters using the steam tapped from the different points of the turbine. The feed
heaters may be of mixed type or indirect heating type.
Some of the steam and water is lost passing through different components of the system
therefore fed water is supplied from the external source to compensate this loss. The feed
water supplied from external source is passed through purifying plant to reduce the
dissolved salts to an acceptable levels.
Cooling water circuit:-The quantity of cooling water required to condense the steam is
considerably large and it is taken through the condenser and heated water is discharged to
the lower side of the river.
Working of the thermal power plant:Steam is generated in the boiler of the thermal plant using the heat of the fuel burned in the
combustion chamber. The steam generated is passed through steam turbine where part of
its thermal energy is converted into mechanical energy which is further used for generating
electric power. The steam coming out of the steam turbine is condensed in the condenser
and the condensate is supplied back to the boiler with the help of feed pump and the cycle
is repeated.
The function of the boiler is to generate the steam. The function of the condenser is to
condense the steam coming out of steam turbine at low pressure. The function of steam
turbine is convert the heat energy into mechanical energy.
Diagram--------------------------------------Major Accessories in the thermal power plant:(*)The drought is one of the most essential system of the thermal power plants. The
purpose of the draught is to supply required quantity of air for combustion and remove the
burnt products from the system. To move the air through the fuel bed and to produce a
flow of hot gases through boiler, economizer, preheater and chimney require a difference of
pressure equal to that necessary to accelerate the burnt gases to their final velocity and to
overcome the pressure losses equivalent to pressure head.
The difference of pressure required to maintain flow of air and to discharge the gases
through the chimney to atmosphere is know as draught.
In most of the modern power plants, the draught used must be independent of
atmospheric condition, and it must have generator flexibility to take the flucating loads on
the plants. The artificial draught is produced by a fan and it is known as mechanical (fan)
draught.
(i)Forced draught:Diagram------------------------In this system, a blower is installed near the base of the boiler and air is forced to pass
through the furnace, flues, economizer, air preheater and to the stack. This drought system
is known as positive draught system or forced draught system because the pressure of air
throughout the system is above atmospheric pressure and air is forced to flow through the
system.
(ii)Induced draught:Diagram------------------------------------------------In this system the blower is located near the base of chimney instead of near the grate. The
air is sucked in the system by reducing the pressure through the system below atmosphere.
The induced draught fan sucks the burned gases from the furnace and pressure inside the
furnace is reduced below atmosphere and induces the atmospheric air to flow through
furnace.
(iii)Balanced draught:It is always combine to prefer the both the draught if the forced drought is used alone, then
the furnace an not be opened either for firing or for the inspection because high pressure
air inside the furnace will try to blow out suddenly and there is every chance of blowing out
the fire completely and furnace stops.
If the induced drought is used alone then also furnace cannot be opened either for firing
or inspection as the cold air will try to rush the furnace as the furnace below atmospheric
pressure. The reduces the effective draught and dilutes the combustion.
To over come both the difficulties mentioned above either using forced or induced
draught alone, a balanced draught is always preferred.
The balanced draught is
combination of both forced and induced draught.
Diagram-----------------------------------------------The pressure of air below the grate is above atmosphere and helps to remove the
exhaust gases as quick as possible from the combustion zone. The pressure of air below
grate is above atmosphere and it helps for proper and uniform combustion.
Condenser:
The use of condenser in the power plant improvers the efficiency of the power plant by
decreasing the exhaust pressure of the steam below atmosphere. Another advantage of the
condenser is that the steam condensed may be recovered to provide a source of good pure
feed water to boiler and reduces the water softening plant capacity to considerable extent.
The use of condenser in steam power plant reduces overall cost of generation by
increasing the thermal efficiency of the power plant.
The desirable feature of the good condensing plant are.
(i)Minimum quantity of circulating water
(ii)Minimum cooling surface area per kw capacity.
(iii)Minimum auxiliary power.
(iv)Maximum steam condensed per m2 of surface area.
Different type of Reactors:-(1)on the basis of Nuclear energy.
(a)Fast Reactors:-In this, the fission is effected by fast neutrons without any use of
moderators.
(b)Thermal Reactors:-In these reactors, the fast neutrons are slowed with the use of
moderators. The slow neutrons are obsorbed by the fissionable fuel and chain reaction is
maintained. The moderater is most essential component is these reactors.
(2)On the basis of Fuel used.
(a)Natural fuel:-In this natural uranium is used as fuel and generally heavy water or graphite
is used as moderator.
(b)Enriched Uranium:-In this uranium used contains 5 to 10% U235 and ordinary water can
be used as moderator.
(3)On the basis of Moderator used:(a)Water moderated.
(b)Heavy water moderated.
(c)Graphite moderated.
(d)Beryllium moderated.
(4)On the basis of coolent used:(a)Water cooled reactors.
(b)Gas cooled reactor.
(c)Liquid metal.
(d)Organic liquid cooled reactors.
Pressured water Reactor:-(PWR) In this water reactor is a light water cooled and moderated
reactor. It uses enriched uranium as fuel.
Diagram----------------------------
The pressurized tank included in the kt maintains the constant pressure in the kt
throughout the load range. Electric heating coil in the pressuriser boils the water to form
the steam which is collected in the dome and pressurizes the entire coolant kt before
starting reactor. To reduce the ------------------Boiler Accessories:(i)Economiser:-An huge of quantity of fuel is used in the thermal power plant and very large
quantity of heat is carried by the exhaust gases. The 25% of the heat remains in exhaust
gases. This loss can be holved by installing an economizer in the fuel gases. The economizer
is feed water heater derving heat from the fuels gases discharged from the boiler. The cost
of the economizer depends on the total gain.
Advantages of Economiser:1.The feeding of water with a H2O at a temperature near the boiling point oreduces the
temperature differences in the boiler, prevents the formation of stagnation pockets of the
cold water and thus reduces greatly the thermal stress created in pressure ports of the
boiler.
2.When the feed water is nor as pure as it should be, temporary hardness is deposited on
the side of the economizer tubes and while this necessitates internal cleaning of the
economizer, the evil is not as great as internal cleaning of the boiler.
3.Due to the reduction in the combustion rate of the furnace the boiler will be more
efficient and the actual fuel saving will be greater than the theoretically calculated.
Diagram-------------------------------------------------(ii)Air Pre heater:The heat carried with the flue gas coming out of economizer is further utilized for pre
heating the air before supplying to the combustion chamber.
It has been found that an increase of 20oC in the air temperature increases the boiler
efficiency by 1%.
Air heaters are usually installed an steam generators that burn solid fuels.
Air heaters benefits are
1.Improved combustion
2.Successful use of low grade fuel.
3.Increased thermal efficiency.
4.Saving fuel consumption.
5.Increased steam generation capacity.
They are of two types.
1.Plate type air heater.
Diagram---------------------------------&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
Write about boiler:-In thermal plant.
1.Production control specification of equipment and preparation of equipment request. The
diagram constructed on process flow does not show the scale size of equipment or technical
specification of equipment.
The diagram indicates variables been measured
(temp,flow,level).
2.Whether indication recording or other service is required (DSC/PLC).
3.Whether control or alarm functions like (adjustments of trlling are required).
4.The auxillary features of instrument or controllers.
5.The type of connecting lines (process, preumatic, hydraulic,capillary lines). The point of
measurement and control.
6.Which instrumentation is to be indicated is control and which instrument is to remain is
process unit.
7.The process control has symbols to represent elements of a process control system.
8.Process control diagram is also called piping or instrumentation of P&I.D.
Process furace with cascade control.
Diagram----------------------------------------
Cogeneration power plants or cogeneration plants:
Cogeneration means sequential conversion of energy contained is the fuel into two or more
usable forms. Ex:The energy of coal is converted into heat is boiler to produce steam. The
steam is used to generate electrical energy and is addition provides heat is manufacturing
process. A convention system uses energy of fuels to produce electrical or thermal energy
whereas cogeneration system provides from same fuel.
1.Cogeneration system can either be power generation system or waste utilization system.
2.A boiler uses fuel like coal, oil and produces steam is cogeneration plant steam is used to
produce electrical power is the convention turbo generator and part of steam is used for
other industrial purpose.
This is shown below:
Diagram---------------------------------
Is power plants some steam is exhausted from turbine and supplied to adjacent industries
for manufacturing process.
Advantages:
Fuel economy: Generation results in substantial economy is consumption of primary fuels
like coal, oil,gas.
1.Low capital cost.
2.Saves industry from power cuts and power supply.
Uses:Used is cement industry, steal and casting, petrochemical industries.
Different types of cogeneration technologies:1.Steam turbine system.
2.Gas turbine cogenerator.
3.Combined cycle cogenerator.
Gas turbine cogenerator:-It consists of compressor, combustion chamber, gas turbine and
generator. Air is compressed by compressor and supplied to combustion chamber. Fuel is
burnt is combustion chamber and heats the compressed air. The hot pressure gas expands
is turbine which breaks generator. The exhaust from combustion chamber is used as
process steam. The transfer of electrical energy is generated by this cycle is high gas
turbined need more maintenance than steam turbine.
Diagram-----------------------------------------------Co generation is a combination of gas turbines and steam turbine systems. The gas turbine
exhaust is used is waste heat boiler to rise steam. If necessary additional fuel is added to
waste heat boiler. The system from boiler is used is a back pressure turbine which drives
another alternative to produce electrically. The low pressure exhaust from steam turbine is
used as process steam. The system has greater flexibility.
Explain brief survey of power generations:
1.Thermal power generation.
2.Hydro power.
3.Nuclear
4.Wind
5.Solar
6.Geothermal
7.Biomass
Various types of power generations:-Power can be generated by using different methods:
1.Hydel power generation.
2.Thermal power generation.
3.Nuclear power generation.
4.Wind power generation.
5.Biomass power generation.
6.From ocean energy.
7.Solar power generation.
In energy power plant we have three par:
Boiler
Turbine
Generator
Instrumentation plays an important role is power generation. The branch of science
which deals with all measurements of power plant is called power plant instrumentator.
There are three types of power corporations:
NTPC-National thermal power corporation.
PGC-Power Grid corporation.
Nuclear power corporation, Ltd., in A.P we have
1.APGENCO:Andhra Pradesh Power Generation Corporation.
2.APTRANSCO:Andhra Pradesh Power Transmission corporation.
Various power distributions in A.P are,
1.SPDC:Southern Power Distribution Corp.,
2.EPDC:Eastern Power Distribution Corp.,
3.CPDC:Central Power Distribution Corp.,
4.NPDC:Northern Power Distribution Corp.,
The following measurements plays an important role is power plants:
TemperatureVoltage
Pressure----Current
Flow------PH nsnt(measurement)
Level
In every plant we have field and control room.
In field we can directly see the
measurements by using following devices:
Measurement Device
Temperature temperature guages
Pressure
Flow
pressure guages
flow guages
Level level gauges
In control room we can control all the measurements. In a power plant PLC (Programme
Logic Controller) & DCS (Distributed control systems) plays an important role is control
room.
Far this purpose we can use the following devices:
Measurement Device
Temperature temperature transmit
Pressure
Flow
pressure transmit
flow transmit
Level level transmit
Various parameters used in power plant:
Electrical parameters Non electrical
Voltage
fuel nsnt
Current
air & steam controlling
Power level radiation detector
Frequency
smoke density measurement
Power factor meter
dust monitor
Trivector method
pH measurement
Flow controlling
Pressure controlling
Temp controlling
Feed water flow measurement
Analyser:-Thermal conductive type, Paranagrenetic type, Oxygen type, In fra red type
,spectrum analyser, Hydrogen purity meter, PH meter, chronotography, conductivity cell,
fuel analyser.
Hydro Thermal Power generator:
Diagram--------------------------------------------Thermal and bio-mass power plant have following parts:
Various resources like water or coal or bio-mass.
Steam generator or water boiler.
Turbine
Generator
Control room
Demineralization plant
Air cool generator
Steam generator or boilers can be used to convert various resources like
water/coal/biomass connect into steam energy at high temperature by using furnaces.
Turbine:It converts steam energy into mechanical energy.
Generator:Converts mechanical energy to electrical energy.
Control room:There are two processes used in control rooms of power plants.
PLC – Programme Logic Controller.
DCS – Distributed control systems.
From fields (from boiler & turbine generator, DM plants various connections can be given
to control room). We can control all the nsnts of power plant in control room.
DM plant:- For purification of water we can use DM plant.
Air cool generator:- These are used for cooling feed water or heat water from boilers.
Generator converts mechanical energy into electrical energy. This power can be given to
substations from that the power can be distributed to various parts.
Details of thermal power plant:Coal handling plantEconomizer
Pulverising plantSuper heater & Preheater
Draft fan (induced)Air pre heater
Boiler
Air handling plantAlternator with excitor
Turbine
CondenserProtection & control equipment
Cooling Towers & Ponds
Feed water heaterInstrumentation
1.Coal handling plant:- The function of coal handling plant is automatic feeding of coal to
boiler furnace. A grate (an iron frame) at bottom of surface holds fuel bed. Coal is weighted
& read to a hopper through a conveyer mechanism from hopper through a conveyer
mechanism from hopper it is fed to grate to some form of stoker mechanism. The stoker
may be an overfeed stoker or under feed stoker depending on rather coal entry is
above/below entry of air. A thermal power plant burns erornous account of coal. A 200
MW plant may require 2000 tons of coal daily. In every plant there is enough daily. In every
plant there is enough storage of coal for atleast 15 days.
2.Pulverising plant: (Split coal into small particles):-In modern thermal power plant coal is
pulverized i.e ground to dust like size & carried to furnace in a steam of hot furnace.
Pulverisation is a means of exposing a large surface area to the action of O 2 & consequently
helps the combustion process.
Advantages:
The rate of combustion can be controlled & changed quick to meet the varying load.
Automatic combustion control also can be used.
Wide variety of low grade coal also can be used.
Pulverising mills are classified as contract mill, bill mills & import mills.
3.Draft fan:- The combustion in boiler require or supply of sufficient quantity of air &
removal of exhaust gases. The circulation of air is caused by adifference in pressure known
as draft. The draft is differentiation pressure b/w atmosphere & inside boiler.
Draft is of two types:
Natural Draft
Mechanical Draft
Natural draft is used in small boilers. Modern plants used huge size boilers of capacity 100
tons/hour of coal, such boilers need more air 2 lakh-M3/min.
Mechanical draft is essential. This draft consists of a forced draft (F.D) & Induced draft
(I.D) or both. A F.D can forces air through furnace economizer of air. The pressure of air
through out the system. In above atmospheric air is allowed to flow through system in I.D
system a fan is installed near the base of chimney. The burnt gases aresucked out of boilers,
thus reducing pressure inside boiler to less the stmospheric. This induces fresh air to enter
furnace. A balance drop uses both draft & induced draft parts. The fuel gas is shown below:
Diagram------------------------------------------4.Boiler:-A boiler/steam generator is a close vessel in which water under pressure is
converted into steam. A boiler is designed to absorb maximum amount of heat released in
process of combustion. The heat is transferred to boiler by conduction, convention &
radiations.
Boilers are two types : They are:
Fire Tube Boiler
Water Tube Boiler
A fire tube boiler is a simple, compact rugged & suitable for low pressure here it is used
where steam capacity of 15 tons/h.
In water tube boiler, water flow inside tube & hot gases flow outside the tube. The tubes
are interconnected to common heater & to steam outlet. Water tube boiler are classified as
vertical, horizontal & inlined tube. The circulation of water is boiler may be natural (or)
forced through pumps.
5.Super heater & pre heater:- Super heated steam is that steam which contains more than
saturated steam. This additional heat provides more energy to turbines & thus electrical
o/p power is more. The function of super heater is to remove moisture from saturated
steam & to raise the temperature of steam. The heat of combustion gases from furnace is
used for super heating. The function of pre heater is to super heat fastly expanded steam
from turbine.
This ensure that steam remains throughout the last stage of turbine.
A steam turbine converts heat energy of steam into mechanical energy & drives the
generator. Steam when comes from a small opening, attains a max velocity. There are two
types inpulse & reaction. Both have a no of stages in which pressure drop takes place. A
standard speed after, clear reduction of 1500/3000 rpm for 50Hz generators.
Generators are used to maintain speed constant when load changes.
In inpulse turbine steam is expanded is turbine nozzle & attains high velocity. A steam jet
openings on belt fixed on the rotor periphery. Expansion of steam takes place is nozzle
steam pressure during flow of steam over turbine plates remains const.
In reaction turbine only partial expansion takes place is nozzle & further expansion takes
place as steam flows over rotor plates. The velocity of steam increases as it expands while
passing over pulse. In a reaction turbine, partial expansion takes place is nozzle which is
inpulse reaction.
6.Ash Handling:- Coal contains considerable a of ash. A 200MW plant produces 500 tons of
ash daily. Conveyer belts are used to carry ash to dump size. Trucks & wagons are also used
in transportatic.
7.Condenser:- It is used to condense steam exhausted from turbine, thus it helps
maintaining low pressure at exhaust thereby permitting expansion of steam is turbine to a
very low pressure. This improves plant efficiency. The exhaust steam is condensed & used
as feed water for boiler.
8.Cooling Towers & Ponds: A 200MW plant need 1500*106 gallons of cooling water per day.
A large source of water supply is usually not available in a thermal plant. Hence closed
cooling water system, where the warm water coming out condenser is cooled & reused. A
small plant use spray ponds, medium & large plants use cooling towers. In this ponds &
towers the warm water looses heat to atmosphere & it is reused.
9.Feed water heater:- The steam coming out of the turbine is condensed & the condensar is
feed back to boiler & and condenser is to make up losses in their closed ckt make up water
is used is necessary to heat water before i.e is feed to boiler.
10.Economizer:- Fuel gases coming out of boiler carry lot of heat. An economizer extracts
part of heat from fuel gases and used for heating the feed water. The use of an economizer
results in saving coal consumption and improve efficiency of boiler.
Diagram--------------------------------11.Air pre heater:-After flue gases leave the economizer heat can be extracted from them
and is used to head the incoming gas from combustion.
12.Auxilary system used for protection & controlling equipment:- The auxileries in a steam
plant are divides as:
Urgent auxiliary
Service auxiliary
Urgent auxillaries which are associated with running of fluid and whose loss would cause an
immediate reduction of unit o/p.
The service auxillaries are those whose loss will not effect the o/p until after a
considerable time safety alarm and safety controls are used for protection control.
13.Instrumentation:- Various controls and instruments are used to maintain yielding produ
plant safety and operator information. The instruments are used in field and control in field
we can use pressure gauges, level, flow, level. For controlling various parameters like flow,
level temp, pressure we can use transmitters like flow, level, temperature, pressure
transmitters in DCS/PLC systems.
Details of Boiler process:- Operating control and safety controls used in Boiler.
Diagram-----------------------------------------The various parts in boiler are:
Coal handling
Fuel burning system
Furanace
Pulverising plant
Air pre heater
Super heater
Economizer
Draft fan
Safety & interlocking system
Water treatment plant (D.M plant).
Coal handling & next three & Air pre heater & next As explained before.
Demineralization plant:- Here H2O is purified & demineralized & supplied to various parts is
power plant ie; turbine & boiler. For this purpose we can use various acids.
Safety interlocking and operating control systems:- Operating controls regulate boiler so
it operates under certain set conditions involving load, feed water and combustion. The aim
is to obtain most efficient o/p from boiler for each pound of fuel burned. This controls can
be governing controls that make adjustments of feed water, fuel air, flue gas and ‘steam’
flow as demanded by load.
Safety controls provides upper or lower limits set for safe operation of boiler. If variable
conditions involving the boiler go beyond a certain safe limit an alarm warns that corrective
actions must be taken.
Boilers:- A boiler may be defined as a closed vessal in which steam is produced from water
by combustion of fuel.
Classification of Boilers:The boilers may be classified as follows:
1.Horizantal, vertical (or) Inchined: If the one of the boiler is horizontal, the boiler is called
as horizontal, if the any is vertical, inchined . The penty of a horizontal boiler can be
inspected and repained easily but it occupies more space the vertical occupies less spells.
2.Fire tube and water tube:-in the fire tube boilers the not gases are inside the tubes and
the water surrenmets the tubes.
Examples:Coch ram, Locomitive boiler in the water tube boiler the water is inside the tubes
and hot gases surrend them.
Examples:Babcock and Wilcox.
3.Externally fixed and Internally fined:- The boiler is known as externally fined if the fire is
outside the shell.
Examples:-Babcock, and Wilcox boiler, stirliy boiler.
Incase of internally fined boilers the furned is located inside the boiler shell.
Example :-Cochram boiler.
In forced circulation type of boiler the circulation of water is done by a forced pump.
Example:-Lamant, Benson boiler.
In returnal circulation type of boiles takes place due to natural certain currents produced by
the application of heat.
Example:-Bobcock and wilcox boiler.
5.High pressure and low pressure boiler:*The boiler which produce steam at pressure of 80 ben and above are called high pressure
boilers.
Example: Bobcock and Wilcox, lament boilers.
*The boilers which produce steam at pressure below 80 ben called low pressure boilers.
Example:-Cochram, Locomotive boilers.
6.Single tube and multitube boilers:The fire tube boiler are classified as single tube and multitube boilers dependent upper
where the fire tube is are or more than and single courish, simple vertical boiler. Rest of
boilers are multi tube boilers.
(escending 12%) in the last stages of the turbine is avoided.
Example for water tube boiler Bobcock and Wilcox boiler. Stirling boiler Locomauot
boiler, Benson boiler, Yellow boiler, and Lo-effler boiler.
Example for fire tube boiler simple vertical boiler. Cochran boiler, Lancashire boiler,
Cornish boiler, Scoter marine boiler, Locomotive boiler, and vetcon boiler.
According to position of the furnace.
*internally fixed boilers-The furnace is located inside the boiler most of the fire tube steam
boilers and internally fired.
*Externally fixed boilers-The furnace is arranged underhealth in a brick-work setting, water
tube steam hoilers are always externally fired.
According to the axis of the shell.
*Vertical boilers – The axis of the shell is vertical
Simple vertical boiler and conchran boiler.
*Horizontal boilers-The axis of the shell is horizontal. Lancartive boiler, locomotive boiler,
and Babcock and will cox boiler.
According to the number of tubes.
*single tube steam boiler-there is only on fire tube or water tubes.
*Multitubular boiler-There are two or more fire tubes of water tubes. Lancashive boiler
Locomotive boiler, Babcock and willcox boiler are mult.
Diagram--------------------------------------This is a modern high pressure water tube steam bo9iler working on a forced circulation.
The circulation is maintained by a certifugal pump driven by steam turbine, using steam
from the boiler. The forced circulation causes the feed water to circulate through the water
walls and drums equal to tertimes the mass of steam evaporated. This prevents the tubes
from being over heated. The feed water passes through the economizer to an emporating
drum. It is then drawn to the circulating pump through the tube. The pump deliver the
feed to the headers, at a pressure above the drum pressure. The header distributes water
through nozzlesinto the generating tubes acting in parallel.
The water equal steam fun their tubes pases…………………………………….
Diagram-----------------------------------This is a water tube boiler using a forced circulation. In main principle of working is to
evaporate the feed water by means of super heated steam from the super heater. The hot
gases from the furance are used for super heating.
The feed water from the economizer tubes is forced to mix with the supper heated steam
in the evaporating drum. The saturated steam, thus formed, is drawn from the evaporating
drum by a steam circulating pump. This steam passes through the tubes of combustion
chamber walls and then enters the super heater. From the super heater about one-third of
the super heated steam passes to the turbine and the remaining two-third is used to
evaporate the feed water in the evaporating drum.
Diagram----------------------------------------A simple vertical boiler produces steam at a low pressure and in small enatities. It is used
for low power generation or at place where the space is limited. It consists of a cylindrical
shell surrounding a nearly. Cylindrical fire box. The fire box is grate. The fire box is fitted
with two or more inclined consist tubes F.F is inclination is provided to increane the heating
surface as well as to improve the circulation of water. An uptake tube passes from the top
of the fire box to the chimney. The hand holes are provided opposite to the end of each
water tube for cleaning deposits. A man hole is provided at the top for a man to enter and
clean the boiler. The space between the boiler shell and fire box is filled with water to
heated.
3diagrams-----------------------------------*The water tube boilers are used exclusively when pressure about 10 bar and capacity in
excess of 1000 kg of steam res.hour is required. Babcock and wilcox water tube boiler is an
exempted of horizontal straight tube boiler and may be designed for stationary of marine
purposes.
The fig shows a Bobcock and Wilcox boiler with longitudinal drum. It consists of adrum
connected serving of front and near end header by short …
Unit-2
PARAMETERS AND MEASUREMENTS:
Electrical Measurements:Current and Voltage:Electrical measuring instruments may be classified according to their functions.
1.Indicating instruments – Ex:Ammeter, Volt meter, Water meter.
2.Integrating instruments – Ex: Energy meter (Walt-hour meter).
2.Recording instruments – Which gives continous locord of the variations of the electrical
quantity.
Ex: X-Y Recorder.
An electrical instrument essentially consists of a movable element and a scale to indicates
or register the electrical quantity being measured. The movable element is supported on
jeweled bearings and carries a pointer or sets of dials. The moment of movable element is
caused by utilizing one or more of the following effects of current or voltage.
1.Magnetic effect – M.I. (moving iron) instruments.
2.Electrodynamic effect – i) PMMC
ii) Dynamo meter type.
3.Electromagnetic induction – incluction type instruments.
4.Thermal effect – Hot wire instruments.
5.Chemical effect – Electrolytic instruments.
6.Electro static effect – Electro static instruments.
Essentials of operation of Electrical Instruments:In order to ensure proper operation of indicating instruments, the following three use
required.
1.Deflecting torque (operating)
2.Controlling torque.
3.Damping torque.
Deflecting torque:- One important requirement in indicating instruments is the
arrangement for producing deflecting operating torque (td), when the instrument is
connected in the circuit to measure the given electrical quantity.
The deflecting torque comes the moving system to move from zero position to indicate
an a graduated scale the value of electrical quantity being measured.
Controlling torque:- If deflecting torque were acting alone, the pointer would continue to
move indefinitely and could swing over to the maximum deflected position irrespective of
the magnitude of current to be measured. This necessities to provide some form of
controlling or opposing torque (Tc). This controlling torque should oppose the deflecting
torque should increase with the deflection of the moving system.
The pointer will be brought to rest at a position where the two opposing torques are
equal ie, Td=Tc.
The controlling torque indicating instrument may be provided by :
1.Spring control.
2.Gravity control.
i)Spring control: It is the most common method of providing controlling torque in electrical
instruments. A spiral hair spring made of some non magnetic material like phosphor
bronze, is attached to the moving system of the instrument as shown in figure. With the
deflection of the pointer, the spring is twisted in the opposite direction. This twist in the
spring provides the controlling torque.
ii)In Gravity control instead of spring the weight is used.
Damping torque:-If the moxing system is acted upon by deflecting and controlling torques
then the pointer, due dto inertia, will oscillate about its final deflected position for quite
some time before coming to the rest. This is often undesirable because it makes difficult to
obtain quick and accurate readings. In order to avoid these osciallations of the pointer and
bring it quickly to its final deflected position, a damping torque is provided in the indicating
instruments.
The damping is provided by damping methods :
1.Air friction damping.
2.Fluid friction damping.
3.Eddy current damping.
PMMC:Diagram-------------------------------The basic principle of the ammeter and of the voltmeter is the same. Both are current
operated devices ie, deflecting torque is produced when current flows through their
operating coils. In the ammeter, deflection is produced by the current we wish to measure.
Since the torision torque of a spiral spring is proportional to the angle of twist. The
controlling torque is directly proportional to the deflection of the pointer ie, Tcαθ.
Diagram-----------------------The pointer will come to the rest at a position when controlling Tc is equal to the
deflecting torque.
Td=Tc
In PMMC the deflecting torque is directly proportional to the current following in the
operating coil: id,Td α I. With spring control Tc α θ. In the final deflected position since the
deflection is directly proportional to the I, scale of such on instrument will uniform ora
certain fraction of the current.
In the voltmeters, the deflecting torque is produced by a current which is proportional to
the potential difference we wish to measure.
Construction:- It consists of a light rectangular coil of many turns of the fine wire wound on
an aluminium forever inside which is an iron core as shown in figure.
The coil is delicately pivoted upon jewel bearings and is maintained between the poles of
a permanent horse shoe magnet attached to the poles are two soft – iron pole pieces which
concentrate the magnetic field. The current is led into and out of the coil by means of two
control hair springs. One above and other below the coil., in fig(2). These springs also
provide the controlling torque. The damping torque is provided by eddy currents induced in
the aluminium former on the coil moves from on position to another.
Working:- When the instrument is connected in the circuit to measure current or voltage,
the operating current flows through the coil, since the coil is carrying the current and is
placed in the magnetic field of the permanent magnet a mechanical force acting on it. As a
result, the pointer attached to the moving system moves in a clock wise direction over the
graduated scale to indicate the value of current or voltage being measured. If the current in
the coil is reversed, the deflecting torque will also be reversed. Since the direction of the
field of the permanent magnet is the same consequently the pointer will try to deflect
below zero. Deflection in this direction is prevented by a spring “stop”. Since the deflecting
torque reverses with the reversal of current in the coil.
Deflecting torque:
Diagram---------------------------------------
The magnetic field in the air gap is radial due to the presence of soft iron care, this means
that conductors of the coil will always move at light angles to the filed. When current is
passed through the coil, forces act on its both sides which produces the deflecting torque.
B = flex density in wb/m2.
l = length or depth of coil in M.
b = breadth of coil in M.
N = No. of turns in the coil.
If a current of I amps flows in the coil then force acting on each coil side is given by :
F=BIln Newtons.
Deflecting torque (td) = Force *l2 distance.
=BIlN * b
Td=BINA Newton – meter.
Where (A=B*l) is the area of the coil in M2.
Since B,N and A are fixed Td α I.
The instrument is spring controlled so that Tc α θ.
The pointer will come to rest at a position where Td =Tc.
Θ α I.
Thus, the diffection is directly proportional to the operating current. Hence such instrument
have uniform scale.
Moving iron Ammeters and Voltmeters:-
This instrument used for the measurement of alternating voltage and current. It can also
be used for d.c. measurements.
There are two types of moving iron instruments
1.Attraction type:Diagram--------------------------In which single soft-iron vane (moving iron) is mounted on the spindle and is attracted
towards the coil where operating current flows through it.
2.Repulsion type:In which two soft-iron vanes are used : one fixed and attached to the stationary coil while
the other is movable (M.I) and mounted on the spindle of the instrument. When operating
current flows through the coil, the two vanes are magnetized, developing similar polarity at
the same ends. Re pulsion take place between the vanes and the movable vane causes the
pointer to move over the scale.
Operating of repulsion type:- It consists of two – soft iron pieces or vanes. Sorrounded by a
fixed cylindrical hollow coil which carries the operating current one of these vanes is fixed
and the other is free to move as shown in figure. The movable vane is of cylindrical shape
and is mounted axially on a spindle to which a pointer is attached. The fixed vane which is
wedge-shaped and has a large radius, is attached to stationary coil. The controlling torque
is provided by one spiral spring at the top of the instrument. It may be noted, that in this
instrument, springs do not provide the electrical connection. Damping is provided by air
friction due to the motion of a piston in an air chamber.
When current to be measured or current proportional to the voltage to be measured
flows through the coil, a magnetic fixed is set up by the coil. This magnetic field magnetises
the two vanes in the same direction ie, similar polarites are developed at the same ends of
the vanes are shown in figure. Since the adjacent edges of the vanes are of the same
polarity. The two vanes repel each other.
As the fixed vane cannot move, the movable vane deflects and causes the pointer to
move from zero position. The pointer will come to the controlling torque provided by the
spring.
The deflecting torque results due to the repulsion between the similarly changed softiron pieces or vanes, if the two pieces develop pole strength of m1 ,m2 α H2.
If the perminiately of iron is assured constant then HαP.
Instantaneous deflecting torce α l2.
Average deflecting torque Td α mean of i2 over a cycle.
Since the instrument is spring controlled Tc α θ.
In steady position of deflection Td= Tc.
Θ α mean of i2 over a cycle.
α i2 for dc
α i2 rms for A.C.
Attraction type:
In consists of a cylindrical coil or solenold which is kept fixed. An oval shaped soft iron is
attached to the spindle in such a way that it can move in and out of the coil. A pointer is
attached to the spindle so that it is deflected with the motion of the soft iron piece. The
controlling torque is provided by one spiral spring arranged at the top of the moving
element. The damping device is an alluminium vane attached to the spindle as shown in
figure, which moves in a closed chamber in some instruments damping curved chamber the
piston being attached to the spindle.
Working:- When the instrument is connected in the circuit to measure current or voltage,
the operating current flowing through the coil sets up a magnetic field. In other words, the
coil behaves like a magnet and therefore it attach the soft iron piece towards it. The result
is that the pointer attached to the moving system moves from zero position. The pointer
will come to rest at a position where deflecting torque is equal to the controlling torque. If
current in the coil is reversed the direction of the magnetic field also reversed and so does
the magnetic produced in the soft iron piece, Hence the direction of the deflecting torque
remains unchanged for this reason such instruments can be used for both d.c. and A.c.
measurements.
Deflecting torque:
The force F pulling the soft iron piece towards the coil is directly proportional to:
i)field strength ‘H’ produced by the coil.
ii)pole strength ‘M’ developed in the iron pieces.
F α MH
F α H2 -------------------- but MαH.
Instantaneous deflecting torque α H2.
If the permeatrility of iron is assured constant, than H α i.
Instantaneous deflecting torque α i2.
Average deflecting torque Td α mean of i2 over a cycle.
Since the instrument is spring controlled Tc α θ.
In the steady position of deflection Td = Tc.
Θ α mean of i2 over a cycle.
α i2 – for dc.
α i2 rms – for ac.
Instrument Transformers:Transformers are used in a .c systems for the measurement of current, voltage, power
and energy. Instrument Transformers find a wide application in protection circuits of power
systems for the operation of over current, under voltage and earth fault and various other
types of relays.
“Transformers used in conjunction with measuring instruments for
measuring purposes called instrument transformers”. Transformers used for measurement
of current is called ‘current transformer’ and transformer used for voltage measurement are
called voltage transformer (potential transformers).
i)Current transformers:diagram-----------------------------------A current transformer has a primary winding of a few turns of thick wire became current I
following through the primary winding, which is connected increases with the line is to be
measured as shown in figure.
The secondary winding consist of a large number of turns of thin wire, across which, a
low range A.C. Ammeter is connected say (0-5A). If the turns ratio N2/N1=n1 then the
current flowing through the secondary winding is I/n. Hence the range of the ammeter
required in the secondary winding is I/n., to measure a current I-ampers in the primary
winding.
Ex: A 1000/5A current transformer with a single turn primary will have 200 secondary turns.
It steps down current in the ratio 200:1. An Ammeter of range 5A connected across the
secondary winding, can measure current up to 1000A, flows through the primary winding.
The resistance of the ammeter connected across the secondary winding is very small and
hence the current transformer operates with secondary almost under short circuit
conditions. Hence, wherever an ammeter is to be connected to the secondary winding it
must be first short circuited and then the ammeter must be connected.
ii)Potential transformer:diagram-------------------------------
A potential transformer is an – accurate – ratio step down transformer. The secondary
winding consists of a few no. of turns N2, and the primary winding consists of a large no of
turns N1. The low range voltmeter is connected across the secondary winding as shown in
figure.
The high voltage to be measured on the primary side is given by the product of the ratio
N1/N2 times the voltage measured on the secondary side.
V1=N1/N2*V2
V2/V1=N2/N1
For example, if the ration N1/N2=100 and if the voltage measured on the secondary side
by the voltmeter is 100v, then the voltage measured on the high voltage side is
100*100=10,000 Volts.
Measurement of power:A Walt meter measures electric power given to or developed by an circuit. In d.c. circuits
we can measure power easily by taking the voltmeter and ammeter readings.
However in A.C. circuits P=VI cos θ. It depends not only voltage and current but also the
phase shift between them therefore a Walt meter is necessary for a.c. power
measurements.
There are two types of Walt meters:
1.Dynamo meter Walt meter – used for d.c & A.C.
2.Induction Walt meter – used for a.c. power only.
i) Dynamo meter Walt meter:It is used to measure both d.c. as well as A.C. it works on the dynamo meter principle ie,
mechanical force exists between two current carrying conductors or coils.
Diagram --------------------------Construction:
In this meter the fixed coils are connected in series with, the load and carry the load
current I1 while moving coil is connected across the load through a series multiplier and
carries a current I2 proportional to the load voltage asshown in figure. The fixed coils are
called current coils and the movable coil is called potential coil. Air friction damping is
provided in such instruments. A pointer is attached to the movable coil.
Working:
Diagram-----------------When the Walt meter is connected in the circuit to measure power, the current coil
carries the load current, and potential coil carries current proportional to the load voltage.
Due to currents in the coils, mechanical force exists between them. The result is that
movable coil moves the pointer over thescale. The pointer comes to rest at a position
where deflecting torque is equal to the controlling torque. Reversal of current reverses
current in both the fixed coils and the movable coil so that the direction of deflecting torque
remains unchanged. Hence d.c. as well as A.C. power.
Deflecting torque:
Case i :-in d.c. circuits P=VI1
Td α I1,I2. Since I2 is directly proportional to V.
Deflecting torque Td α VI1 α load power.
Case ii:- In A.C. measurements of power current through the load is I, and voltage across the
load is V1 Let the load power factor be cosø logging.
Then V=Vm sin θ.
I=im sin (θ-ø).
But, instantaneous deflecting torque TdαVi.
Hence the instrument indicates the average power.
Average deflecting torque, Td α Average of Vi over a cycle.
Formula and eauations;------------------------------Since the instrument is used to measure d.c. or a.c. power deflecting torque is
proportional to load power.
Since the instrument is spring controlled.
Tc α θ.
In steady position of deflection Td = Tc.
Θ α load power.
Induction Walt meters:This Walt meter can be used to measure a.c. power only in contrast to dynamo meter
Walt meter , which can be used to measure d.c. as well as a.c. power.
It works on the induction principle:
Diagram-----------------------Construction:- It consists of two laminated electromagnets. One electromagnet called shunt
magnet is connected across the supply and carries current proportional to the supply
voltage. The coil of this magnet is made highly inductive so that current in it lags behind the
supply voltage by 900.
The other electromagnet, called series magnet is connected in series with the supply and
carries the load current. The coil of this magnet is made highly noninductive so that angle of
lag or lead is wholly determined by the load.
A thin Aluminium disc mounted on the spindle is placed between the two magnets so
that it cuts the flux of both the magnets. The controlling torque is provided by spiral
springs. The damping is electromagnetic and is usually provided by a permanent magnet
embracing the aluminium disc. Two or more copper rings (shading rings) are provided on
the central limb of the shunt magnet by adjusting the position of these rings, the shunt
magnet flux can be made to lag behind the supply voltage by exactly 900.
100 r, King:- When the Walt meter is connected in the circuit is measure a.c. power, the
shunt magnet carries current proportional to the supply voltage and the series magnet
carries the load current. The two fluxes produced by the magnets induce eddy currents in
the aluminium disc. The interaction between the fluxes and eddy currents produces the
deflecting torque on the disc, causing the pointer connected to the moving system to move
over the scale. The pointer comes to rest at a position where deflecting torque is equal to
the controlling torque.
Let V supply voltage.
Ivcurrent carried by shunt magnet.
Ic current carried by series magnet CosѲ  lagging power factor of the load.
Diagram--------------Current Iv in the shunt magnet lags the supply voltage by 90 o and so cloes the Фv
producedbyit.
The current Ic in the series magnet is the load current and hence lags behind the supply
voltage V by Ф. The flux Фc produced by this current (ie,Ic) is in phase with it, it is clear that
the phase angle Ѳ between the two fluxes is 90-Ф ie, Ѳ=90-Ф.,
Mean deflecting torque
Td
Ф V Ф c sinѲ.
Equation here---------------------------
Measurement of smoke and dust:
The importance of measuring smoke density with addition of sutable alarms and
recorders is now increasing because of the smoke nuisance. Modern combustion unitsdhas
grown tremendously in size producing several million cubic meter of combustion products
per minit. The emission control regulations for also imposed by various governmental
agencies and there fore it has become essential to control the emission of particulates in
stack gases.
Methods used for measuring smoke and dust:
i)photocell type smoke meters:
diagram------------------This type of meter the obscuration principle of smoke to the light is used to measure the
smoke density. A focused light beam through the chimney is passed on a photocell and
thevariation in the signal of the photocell circuit are measured. The variation of the
obscuration of the light source give to the smoke and dust in the gases is measured. The
indicator is scaled in percentage obscruration.
The recorders and alarms are fitted that a countinuous record of smoke emission is
provided and the boiler operators are warned if this exceeds a predetermined level .
The major problem with this type of meter is the fouling of the projector and receiver
lenses by smoke and dust. It has been expermentally determined that the dust will not be
deposited at the end of the tube at right angle to the dust if the ratio of length to the
diameter is minimum 4:1 for coarser particles and 16:1 for fumes and smoke. There fore in
order to provide the lens windows clean for long period of operation, it is necessary to
mount the lamp and photocell units on extension tubes with the possibility of their length
being varied from 4:1 to 16:1 ratio. To install the equipment , two holes of 5 to 10 cms in
diameter are required on oposite sides of the stack. If possible, a position should be chosen
at a point in the dust where the suction exists so that sorrounding air will be drawn to the
duct, pass the glasses protecting the lenses and so keep them clean.
ii)Reflected light dust monitor:
In this meter, the reflected light from the dust particles is measured instead of recording
the light absorbed by dust the arrangement is as shown in figure.
A simple lamp and a photocell are mounted side by side. A light from the lamp is beamed
into the dust through a suitable opening. Some of this light is reflected back on the
photocell. The indication of the reflected light varies with variation of the dust quantity in
the gas. If the dust size remains constant, then the instrument can be calibrated on a weght
basis.
Diagram--------------------------Power factor meter:
Diagram---------------------------------Power factor is defined has cosine of the phase angle between voltage and current.
Naturally determination of the power factor involves measurement of phase angle.
A single phase power factor is shown in figure, it is essentially electrodynamo meter
movement in which the moving element consists of two coils mounted on the same shaft
but at right angles to each other as shown in figure they are called crossed coils. They
rotate in the magnetic field produced by field coils through which line current flows.
The field coils are connected in series with the power line. As such they carry the line
current, one of the movable coils is connected in series with resister R and the combination
is connected across the line. It receives current from the applied potential difference. The
second coil of the movable element is connected in series with inductor (L) across the line.
The movable system of coils is not provided with central springs. As such, the balance
position of the movable coil system depends on the resulting torque developed by the
crossed coils. When the moving coil system is stationary, the torque produced by the two
coils are equal and opposite. The torque developed in each coil depends on the current
through the coil A and as such it depends on the impedance of that coil circuit. The torque
is also proportional to the mutual inductance between each part of the crossed coil
elements with respect to the position of the field coil. Then themovable system is at
balance, its angler displacement depends on the phase angle between the line current in
(filed coil) and the line voltage (crossed coils). The indication of the pointer which is
attached to the movable system is calibrated in terms of phase angle or power factor.
Frequency meter:
Diagram-----------------------The fixed coil is devided into two parts 1 and 2. The two parts of the fixed coil form two
separate resonant circuits. Fixed coil 1 is in series of with in an inductance L 1 and a
capacitance C1 forming a resonant circuit of frequency F2 slight below the lower end of the
instrument scale. Fixed coil 2 is in series with inductance L2 and capacitance C2 forming a
resonant circuit of frequency f2 slightly higher than the upper end of the instrument scale.
In the case of instrument for power frequency measurements the circuits may be tuned to
frequencies of 40 Hz and 60 Hz respectively with 50 Hz in the middle of the scale.
The two parts of fixed coil are arranged as shown in figure, their return circuits being
through the movable coil. The torque on the movable element is proportional to the
current through the moving coil. The current is the sum of the currents in the two parts of
the fixed coil. For applied frequency, with in the limits of the frequency range of the
instrument, the circuit of fixed coi. 1, operaters above resonant frequency (as XL1 > XC1) with
in current through it, lagging the applied voltage. The circuit of the coil 2 operates below
the resonant frequency (as XC2 > XL2) with current i2 leading the applied voltage. One fixed
coil circuit is inductive and the other is capcitive and therefore the torques produced by the
two currents i1 and i2 act in opposite on the moving coil. The resultant torque is a function
of frequency of the applied voltage and therefore the meter scale can be calibrated in
terms of frequency. The instrument scale spreads over anangle of about 900 controlling
torquen is provided by iron vane.
Electro magnetic flow meter:
Diagram----------------------------The electromagnetic flow meters are particularly suitable for the flow measurements of
any electrically conductivity liquid, slurries.
It consists of basically a pair of insulated electrodes lacer in the opposite sides of a non
conducting non magnetic pipe carrying the liquid whose flow is to be measured. The pipe is
sorrounded by an electromagnet which produces a magnetic field. The arrangement is
analagous, toa conductor moving across a magnetic field. Therefore voltage is induced
across the electrodues. This voltage is given by E=B l V volts.
Where B  flux density sb/m2.
l length of conductor.
V velocity of conductor m/sec.
Assuming a constant magnetic field, the magnitude of the voltage appearing across the
lectrodes will be directly proportional to the velocity.
Non conducting pipe has to be used as the output voltage gets short circuted if metallic
pipes are used. But when liquies of high conductivity are measured the short circuit has no
effect. stainless pipes can then be used. The voltage produced is small specially at low flow
rates. Therefore the meter greatly on a high gain amplifier to convert the induced voltage
into a usable form.
Turbine meters:
Turbine flow meters are volumetiric flow meters and are available in wide ranges. The
output is usually in the form of a digital electrical signal whose frequency is directly
proportional to flow rate and whose total count is proportional to the total quantity as each
pulse represents a discrete volume.
Figure shows a magnetic pick up type of turbine flow meter. A feature of this turbine
meter is a hydraulically supported turbine rotar. A permanent magnet seated inside the
rotor body is polarized at 900 to the axis of the rotation. As the rotar rates does the magnet
and therefore rotating magnetic field is produced. This produces an A.C. voltage pulse in
the pick up coil located external to the meter housing. The frequency of this voltage is
directly proportional to the rate of flow. The pulse can be totalized by a counter to give the
value of total flow over a particular interval of time.
Temparature Measurement:
The temparature measurement is also necessary for indication to the operator to atleast
him to unacceptable conditions. As part of schemes to protect plant against excessive
temparature and an automatic temparature control system. Temparature can be measured
only by indirect methods. Generally by transferring heat to an instrument which can
respond to that energy.
i)Thermo couple circuits:
In practical arrangement one of the conductors is broken and an EMF measuring device is
introduced to indicate the temparature between the two junctions. Hot junction and cold
junction (Ref. junction).
Usually the actual temparature of hot junction is required, so reference junction is kept at
a known and constant temparature.
Diagram---------------------A practical thermocouple circuit is shown in figure. The system consists of the thermo
couple (together with its protection housing or protection circuit). The extension wires, a
reference junction temparature control or compensation unit switch unit connecting one
thermo couple at a time to a measuring system and indication (or control alrm device) a
transmitter and thevarious types of measuring equipment.
Choice of thermo couples:
1.Resistance to mechanical and chemical deterioration in the operating environment.
2.The need for a reletively high change in emf output per degree change in temparature.
3.Constancy of calibration. This is very dependent on freedom from contamination and
mechanical strain.
4.Re producatrility – Each thermo couple should have similar characteristies, so that the
system will not have to be re calibrated when a new thermo couple is fitted.
The metals used for the manufacture of thermo couples be divided into base metals and
rare metals.
The base metal group has a temparature limit of upto 11000C with possible extension to
12000C for limited periods. The group has the advantage of producing a high emf output
compased with the rare metals.
The rare metal group has a advantage of a higher temparature range maximum
temparature 15000C. although its emf output issubstantially lower than the base metal
group, it’s accuracy ishigher.
The lower temparature limits of both groups is usually decided by the limiting emf which
can be employed with the measured device. Because base metals thermo couples produce
a relatively large emf. The have the advantage over the rare metal thermo couples in this
respect. In order to ensure that any one thermo couple type maintain its temparature/EMF
output, it is necessary that thematerial conductors have an accurately controlled
composition. Any impurities included in the alloy must not materially affect the values of
the output at any temparature through out the operating range.
Types:
T-type-----upto 4000C
J-type-----
6000C
E-type-----
7000Cα
K-type-------
11000C --- used in power industry.
Types of Radiations:
i)Alpha particles:- The α-particle is a helium nucleus consisting of two portions and one
neuton at has a mass of 62
10-27 Kg and carries a + ve charge of 3.2
10-19 coloumb. Α –
particles is the heaviest and slowest travelling at speeds of firm 1/10 to 1/100 of the velocity
of light. It has greatest mass and slowest speed. It is least ppenentration about 0.2 mm of
aluminium foil.
ii)β – particle:- It is an electron with man of 9.03
10-31 Kg and a charge of 1.6
10-19
coloumbs. Since it is lighter tan α-particle and has a speed approximately equal to that of
light, it can perentrate much further than α-particle about 100 times the perentation of αparticle.
iii) -particles:- It has shorter wave length 0.03 to 3 angstrom speed of this particle issame
as that of light having very high penctrating power.
iv)Neutrons:- The man of neutron between α and β particles. Having the same mass as a
proton-penentrating power is small and it has no charge it is very difficult to detect.
Radiation detectors
1.Geiger – Muller Tube:
Diagram------------------It is used to detect and measurement of α,β and
-rays. This tube is a diode consisting
of a cathode which is a fire wire running through the centre of cylinder. Both of them are
mounted in a thin walled, air tight, glass envelope, scaled by an extremely thin window
through which radiation copy pass at one end in figure.
The air is evacuated from the envelope and a small amount of an inert gas such as argon
is added. The voltage of the battery is kept below the ionization potential of the gas. Now if
radiation percent rates through the window and enters the envelope it would ionize same
of the gas alorms. The resulting –ve ions go towards the anode and +ve ions towards the
cathode. In their passage the ions collide with some of the gas atoms causing them to be
ionized inturn. This process continues till whole of the gas atoms are ionized. In fact
complete ionization takes place in no time. A pulse of current flows through the tube. This
current flows through the resistance R, which is connected in the arode circuit the resulting
voltage deop R is the output voltage.
Once ionization is established it is expected to continue indefinitely. However, since
resistance R which has ahigh resistance is connected in series with the anode and the
battery when the gas ionizes a heavy current flows through the resistance causing a large
voltage drop. This voltage drop is sufficient to reduce the emode voltage below the
ionization potential and therefore ionization ceases.
As long as the gas is not ionized no current is flows through the resistance R and hence
there is no outputs, thus when gas deionizes, the current ceares to flow through resistance
R and sothere is no output.
Now if radiation again enters the tube, the ionization starts and a current again flows
through resistance R giving an output voltage. This way a series of α or β particles or
burstsof
-rays cause a output pulse from resistance R may be amplified and registered by
a counting device.
By countidng the number of pulses, we can know the number of
particles entering the Geiges muller tube in a particular interval of time. The number
counted gives the intensity of radiation.
The pulses may be stored in some cases and the total count may be calibrated directly in
terms of radio activity.
2.Ionisation chamber:
This is quite similar to Geiger muller tube. The ionisation chamber is a metal cylinder (the
outer cylinder) scaled at one end by a window. The other end of the cylinder is closed, and
a metal rod (the center electrode) prottrucles outside the cylinder on this end.
The centrl electrode is grounded through resistance R Resistor R has a very high
resistance. A large tre potential is opplied to the outer electrocle. The chamber contains a
gas which may be air , co2, Nitrogen, Oxygen or methane.
Let us consider that the chamber is filled with air. When nuclear radiation enters the
chamber through the window, some of the atoms of the air ale ionized, the resulting ions
travel towards their respective electrocles. Ie., +ve ions towards the central electrode and
the –ve ions towards the outer electrode. In the process the rushing electrodes, these ions
collicle with other atoms of air causing ionization of these atoms.
The action is
instantaneous.
As the +ve ions collect on the center electrocle (which is at ground potential),the d.c.
potential of this electrode is increased. This increased potential, which is the output signal
of the chamber, may then be amplified and measured. Since the increase in this potential is
a function of the no. of radiation particles that have entered the chamber, the intensity of
the nuclear radiation may be determined. The main use of this chamber is to detect and
measure α –particles.
Diagram--------------------iii) Scintillation counters:
Certain crystals such as zinc sulphide, sodium iodide, anthracene and naphthalene etcl.,
produce a brief flash of light each time they are struck by an α,β, or
particles the
production of a flash light by striking the crystals mentioned above with α,β, or
rays is
called scintillation.
The construction of scintillation counter is shown on back side. Now every time a
radiation strikes the scintillation crystals a tiny flash of light is produced. The flash of light is
ampliphide by a photomultiplier incorporated in the scintillation counter.
Thus, each
particle produces a pulse of anode current at the out put of the photomultiplier the out put
of the photomultipler applied to a high speed electronic counter which counts each tiny
flash generated by scintillation crystals. Thus by counting the number of pulses in a given
period of time, the intensity of radiation may be detected.
Diagram------------------Advantages:
1.This is more sensitive than Gieger muller tube and hence can detect low levels of
radiation.
2.The crystals based in this devices produce flash of light when struct by x-rays. Hence this
counter are transducer may be used to detect x-rays.
3.Counting rate is very fast.
Frequency Meters:- The different types of frequency meters.
1.Mechanical resonance type.
2.Electrical resonance type.
3.Electro dynamo meter type.
4.Weston type.
5.Ratio meter type.
6.Saturabel care type.
*The frequency can also be measured and compared by other arrangements like electric
counter, frequency bridges, stroboscopic methods and cathod ray oscillosoure.
 Electrical Resonance Type frequency meter:- Two types of electrical resonance frequency
meters being desirable here.
1.Ferrodynamic type frequency meter.
2.Electro dynamometer type frequency meter.
1.Ferrodynamic Type freq meter:The following figure shows the construction of an electrical resonance freq meter.
Diagram-----------------------------*It consists of a fixed coil which is connected across the supply whose frequency is to be
measured. This coil is called magneting coil.
*The magnetizing coil is mounted an a laminated iron core. This iron core has a cross
section which varies gradually over the length, being maximum near the end where the
magnetizing coil is mounted and minimum at the other end.
*A moving coil is pivoted over this iron core. A poute is attached to the moving coil. The
terminals of the moving coil are connected to a suitable capitor C. There is no provision for
a controlling force.
*The operation of the instrument can be understood from the three phason aligance shown
below.
Diagram------------------------
*The magneting coil carries a current I and this current produces a flux. Ѳ, if we neglect the
resystence of the coil and the iron losses in the core flux. Ѳ is in phase with current I flux Ѳ
being alternating in nature, induces and every E in the moving coil. They every lags behind
the flux by 900.
*The eng induced circulates a current iron in the moving coil.
*The phase of this current iron depends upon the inducture L of the moving coil and
captiture ‘C’.
*In fig (a) the circuit of the moving coil is assumed to be inductive and therefore current iron
lags behind the eny E by an angle ‘α’ the torque acting on the moving coil is they.
Td α Im cos(900+α)
*In fig (b) the moving coil elct is assumed to be largely capeuitive and therefore current I m
leads the eny E by an angle β and therefore the deflecting torque is
Td α Im cos (900-β)
This torque is abviously opposite to what use in case of fig (a).
*In fig(c) the inductive return is supposed to be equal to the capatitive reetoue and
therefore the elct is under is purely resytine and so current Im is in phase with eny E. This is
because inductive regulate Xc = 2πfl equals the capaiture reacture Xc=1/2πfc and the net
realay of the let is zero.
The deflecting torque Td α Im cos 900=0.
*Hence the deflecting torque on the moving coil is zero. When the inductive reactive early
to the capature reacture.
*Coming to actual operation of the instrument for a fixed freq the capiture reacture is
constant. But the inductive reactance of moving coil is not constant. This is because the
inductive of moving coil occupies on the iron core. This inductance and hence inductive
reactance is maximum. When the moving coil occupies a position close to the magnetizing
coil and minimum when it is at the other end.
*The value of capceitance C is so chosen that the moving coil occupies a convenient mean
position on the iron core. When the freq. is at its normal value the fig shows that the
position of the moving coil for normal freq this means that at this position the value of
inductance is such that for normal freq the inductive reactor is equal to the capiture
reactance.
*Now suppose that the freq increases above the nominal value under these conditionary
the inductive reactance becomes larger than the capaciture reacture this is because X lαf and
Xc α 1/f thus the elct because largely inductive (XL > Xc) and therefore these is torque.
Produced thus torque tries to pull in moving coil to an equillibium position i.e a position
where the inductive reacture should exeed the capaiture reacture. Since inductive reacture
is large than the capeiture reacture this torque must move the coil to a position where its
inductive reacture must care down to a value such that it equals the capaiture reactance.
*A decreasing inductive reactance is obtained it the moving coil moves away from the
megnetinary coil.
*The coil will come to rest when XL=Xc (02) when equation---------------- i.e under conditive of
electrical reasonance.
*If the freq decreases below the nominal value of capaiture reactance because more than
the inductive reactance and hence there is a torque produced fig(b) this torque moves the
coil to a position where its inductive reactance tends to because large and they equals
capacitive reactance therefore the moving coil deflectly towards the magneting coil.
*Advantage of this instrument is that greater sensitivity is achieved with its use as the
inductive of moving coil changes gradually with variation of its position on the iron core.
Weston frequency meter:
This frequency meter consists of two coils mounted perpendicular to each other . Each
coil is divided into two sections the connection are shown below.
Diagram----------------------------
*The branch elct of coil A has resytor RA connected in series with it while coil B has a
reaetance coil LB in series.
*Elct of coil A is in parallel with a reactance coil LA while that of coil B is in parallel with
resystance RD. The moving element is a soft iron needle.
*This needle is pivoted as a spindle which also carries a pointer and damping values.
*There is no controlling force.
The series reactance coil ‘L’ acts to suppress higher
harmonics in the current of instrument and therefore, tends to minimize the wave farm
erroly in its indication.
*The meter is connected across the supply and the two coils carry currents these currents
set up two magnetic fields which are at right analyses to each other.
*The magnitude of the field depends upon the value of current flowing in the coil. Both
these fields act upon the soft iron needle and the needle takes up a position which depends
upon the relative may of the two fields and hence of the currents.
*The meter is so designed that the values of various resistances and inductances are such
that for normal frequ of supply the value of voltage drops across reacture LA and resystance
RB send equal currents through coils A and B.
*Therefore the needle takes up a position which is at 45 0 to both the coils and the pointer is
at the center of the scale.
*Now if the freq increases above its nominal value, reactures of LA and LB increase while
resystences RA and RB remain the same.
*This means that with an increases in frequency the voltage impressed upon coil A increases
as compared with that across coil B. Hence the current in coil A increases while it decrease
in coil B. The tendency of the needle is to deflect towards the stranger field and therefore it
tends to set itself in line with axis of coil A thus the pointer deflects to the left.
*When the freq. decrease an opposite action takes place and the pointer deflects to the
result.
Ratiometer Type frequency meter:
1.A ratio meter type frequency meter consists of a ratio meter which gives a linear
relationship between the current ratio and the deflection.
2.The two coils of this ratio meter one feel with the rectified o/p currently of two separate
bridge rectifies as shown below.
3.The input sides of the two bridge rectifies one connected to alternating current supply
whose freq to be measured.
Diagram-----------------------------4.Input side of one of the bridge rectifier has a series captauiance C and the other has a
series resystance R.
5.Let V be the supply voltage and f be its freq output current of bridge rectifier is :
I1 α Ic α 2pie fVC.
Out put current of bridge rectifier 2 is
I2α IR α V/R.
The above -------------- hold good only if the wave fere is simusoidal.
Reflection θ = K I1/I2.
=equation here-----------------Now K1 C and R are constants.
Θ =K2f where K2 =constant = equation here.
Power factor meters:
*On measuring the current, voltage and power in an a.c clicls its power factor can be
calculated from the relationship
Cosθ=P/VI
*This method of determining the power factor of an electric clct is however of low accuracy
has number of disauls and is rarely used in practice.
*Power factor meters indicates directly by a single reading the power factor of the clct to
which they are connected.
*Power factor meters like wattmeters – name a current clct and a pressut clct the current
clct carries the current in the clct whose power factor is to be measured.
*The pressure elct is connected across the clct whose power factor power factor to be
measured and is usually split up into two parallel pathy – one is inductive and other is noninductive.
*The deflections of the instrument depends upon the phase assurance b/w the main current
and currents in the two party of the pressure clet i.e upon the phase angle (or) power factor
of the elct the deflection is indicated by pointer.
*They are two types of power factor meters
1.Electrodynamometer type
2.Moving Iron type.
*Single phase electrodynamometer power factor meter:
*The construction of a single phase electrodynamometer type power factor meter is shown
below
Diagram--------------------------------
*It consists of a fixed coil which acts as the current coil. This coils is split up into two parts
and carries the current of the clct under test. Therefore the magnetic field produced by this
coil is proportional to the main current.
*Two indicator pressure coils A and B pivoted on a spindle constitute the moving system
pressure coil A has a non-inductive resystance R connected in being with it and coil B has a ----------- inductive choice coil L connected in series with it the two coils are connected across
the voltage of elct.
*The values of R and L are so adjusted that the two coils carry the same value of current at
normal freq i.e R=WL.
*The current through coil A is in phase with the elct voltage while that through coil B lays
the voltage by an angle A which is nearly equal to 900.
*The angle b/w the phases of coils is also made equal to -------triangle symble---- there si no
controlling device.
*In order to simplifies the problem, we assume that the current through coil B lays the
voltage by exactly 900. Also that the angle b/w planes as coils is exactly 900 (i.e tri =900).
*Now there will be two deflecting torque are acting an coil A and the other on coil B. The
coil winely are so arranged that the torque due to the two coils are opposite in direction.
There fore the pointer will take up a position where these two torques are equal.
*Let us consider the case of a lagging power factor of cosθ.
Deflecting torque acting on coil A is
TA=KVIM max cosθsine.
Θ= angular deflection from the phase of reference.
Mmax= maximum value of natural inductance b/w the two coils
They torque acts in clock wise direction.
Deflecting torque acting on coil B is
IB = KVIMmax cos(900-θ) sin (900+θ)
KVI Mmax sinθ cosθ.
This torque acts in anticlockwise direction the value of Mmax is the same in the two exp
owing to similar construction of the coils.
*The coils will take up such a position that the two torques are equal.
Hence at equilism TA = TB.
KVI Mmax cos θ sin =KVI Mmax sinθ cosθ
Θ=ø.
There fore the deflection of the instrument is a means of phase angle of the clet. The scale
of instrument can be calibrated in directly in they of power factor.
Three phase Electrodynamometer power factor meter:
*The following fig shows the construction and connection of a three phase
electrodynamometer type power factor meter.
Diagram-----------------*This meter is only useful for balanced loads.
*The two moving coils are so placed that the angle b/w three plaines is 120 0 they are
connected across two different phases of the supply elct. Each coil has a series resystance
there is no necessity for phase splitting by artificial means since the required phase
displacement b/w current IA and IB in the two moving coils can be obtained from the supply
it self as shown in fig.
*Voltage applied across coil A is V12 and as its clet is rejustive current IA is in phase with V12.
*Voltage applied across coil B is V13 as the clet as coil is resystence
Let ø = phase angle of the clet.
Θ = angular deflection from the phase of
Now V1=V2=V3=V
Torque acting an coil A is !
TA = KV12 I Mmax cos(300+θ) sin(600+θ)
Equation here ----------------------Torques TA and TB act in the opposite directions and the moving suggestion takes up a
position where
Where TA=TB.
Cos(300+ø) sin(600+θ) = cos (300-ø) sin (1200+ø) solving the above exp we have θ=ø.
They the angular deflection of the pointer from the plane of reference is equal to the phase
angle of the clet to which the meter is connected.
UNIT----------------------------------------IIIIIIIIIIIIIIIIIIIIIIIIII
Measurement of Temp:
RTU:
*The resystance of a conductor changes when its temp is changed. This property is utilized
for measurement of Temperature.
*The variation of resystance R with Temp T (0K) can be represented by the following
relationship for most of the meltals as:
R=R0(1+α1T+α2T2+-------+αnTn+--------)
R0=resistance at temp T=0 and α1,α2---------αn one constants.
*The resistance thermometer uses the change is electrical resistance of conductor to
determine the temp and the platinum as an element in the resistance thermometer and it is
used as the Primary element in all high accuracy resistance thermometer.
*The platinum resistance temperature detector (PRTD) is used as an interpolations standard
from oxygen point (-182.960C) to antimony – point (630.740C).
*Platinum is especially suited for this purpose as it can withstand high temp while
maintanery excellent stability and it is limited suscapability to contamination.
*All metal producer a positive change in resistance with temp.
*This implies a metal with a high value of resistivity should be used for RTDS.
*The requirements of a conductor material to be used in RTDs are:
1.The change in resistance of material per unit change in temp should be as large as
possible.
2.The material should have a high value of resistance so that minimum volume of materials
is used for the construction of RTD.
3.The resistance of materials should have a counting and stable relationship with temp.
Diagram-----------------*Gold and Silver are rarely used for construction of RTDs an account of then low resistivities
Tungsten has relatively a high resistivity but is resumed for high temp application as it is
extremely battle and difficult to work.
*Copper is used occasionally as an RTD element its low resistivity forces the element but its
low linerling and low cost variable it an economical alternative its upper limit of temp is
about 1200C.
*The most common RTDs are made of either platinum nichel (02) alloys. The economical
nichel wires are used over a limited temp rase for measurement of integrity platinum is the
abvious choice.
*The common values of resistance for a platinum RTD range from 1hour to several
thoroughly or for the film RTD.
*The single most common value is 100 r at 00C with resistance temp col of 0.00385%C. The
more chemically pure platinum wire has a resistance coe of 0.003920C.
*The characteristics of various materials used for resistance thermometer are.
Diagram-----------------*Linear Approximation:A linear approximation means that we may develop an equation for a st.line which
approximate the resistance VS temp curve over a specified span.
*The following fig shows a curve of variation of resistance ‘R’ with temp θ0C.
Diagram----------------------------*A straight line has been drawn b/w the parts of the curve which represents θ0C with θ0C
represents the main part temp. The eq of this St.leic is the lenear approximation of the
curve from θ0 1C to θ02 C.
The eq of the St.line is written as:
Equations here-------------------*The reason for using α
θ
as fractional slope of resistance-temp curve is that this sene
caysent be used for conductors have the same material with dysent dimension. The value
of α θ can be formel from the value of resistance and temp.
Equation------------------Quadratic Approximation:
A Quadratic approximation of the resistance temp curve is more accurate representation
of the curve over a limited range of temp. The Quadratic approximation relationship include
both lenear term as in eq and an addition has a term which values as the sequence of the
difference in term.
The approximation is
Equation here-----------------------------------Thermistors:
*Thermistors is a contraction of a term “Thermal resytory”. Thermestors are generally
composed of semi conductor materials.
*Most Thermistors have a negative coe of temp resistance i.e their resistance decreases
with increases of temp.
*The negative temp coe of resistance can be large as several percent per degree celcius.
This allows the thermistor elcts to detect very small changes in temp which could not be
observed with an TRD and thermo couple.
*In same cases the resistance of thermistor at room temp may decrease as much as 5
percent for each 10C rise in temp.
*This high sensitivity to temp changes makes thermistor extremenly useful for precision
temp measurements control and compensation.
*Thermistors are widely used in applicants which involves measurements in the range of 600C to 150C the resistance of thermisters ranges from 0.5mr to 0.75mr. Thermestor is a
highly sey device.
*The thermestor exhibits a highly non-linear characterstics of resistance versus temp.
*Construction of Thermystors:
Thermistors are composed of sintered maximum of matalic oxides such as managanese,
nickel, cobalt,copper,iron and uranium. They are available in variety of sizes and shapes.
The thermystors may be inform as beads, rods and discs. Same of the common forms are
shown below:
Diagram----------------Resytance – Temp characteristic of Thermstors:
The mathematical exp for the relationship b/w resistance of a thermistor and temp of
thermistor is
Equation &&&&&&&&&&&&&&&& diagram--------------------------Thermocouples :- The thermocouple is one of the simplest and most commonly used
methods of measuring process temp.
*The operation of a thermocouple is based upon “seeback effect”.
*When heat is applied to junction (not junction) of two dissimilar metals an emf is
generated which can be measured at the other junction (cold junction). The two dissimilar
metals form an electric clet and a current flows as a result of the generated emf as shown
below.
Diagram----------------------------*This current will continue to flow as long as T1 > T2 metal B is described as –ve with respect
to a. Metal A if current flows into it at the cold junction.
*The emf produced is function of the difference in temp of not and cold junction and is
given by
Equation here-----------*Thermo electric Laws:1.The application of heat to a single homogeneous metal is in itself not capable of producing
(or) sytaining an electric current.
2.A thermo –electric emf is produced when the junction of two dissimilar homogenious
metals are kept at different temp. This emf is not affected by temp.
Gradiants along the conductor as shown below.
Diagram-------------------3.In a clct consisting of two dissimilar homogenious metals having the junctions at different
temp, the emf developed will not be affected when thermal homogenious metal is made a
pen of the clet, provide the temp of two junctions are the same as shown in below. This is
called law of “intermediate metals”.
Diagram----------------------4.The thermal emf produced when a clet of two homogenious metals exists b/w a first temp
and a second and thermal emf produced when the same clet exists b/w the second temp
and a thermal are algebraically equal to the thermal emf produced when the elct exists b/w
first and third temp. This is called low of Intermediate temp.
Diagram-----------5.The algebraic sum of the eajs produced in a clet containing two (or) more thermocouples
all at the same tap is zero.
6.The net emf of a clet containing two thermocouples is unaffected by the addition of
ancore thermic at the same temp as any of the other as shown:
Diagram---------------*Thermocouples are used for measurement of temp up to 14000C.
*Clet of thermocouple:
Digram----------------------*When two metals having different work functions are placed together, a voltage is
generated at the junction which is nearly proportional to the temp. This junction is called a
thermocouple. This principle is used to coment heat energy to electrical energy at the
junction of two conductors as shown above.
*The heat at the junction is produced by the electrical current flowing in the heater
elemned while the thermocouple produces an emf at its o/p thermals which can be
measured with help of PMMC.
*The emf produced is proportional to the temp and hence to the rms value of the current.
*The thermocouple type of inst can be used for both d.c and a.c applications.
*The most attractive feature of thermocouple inst is that they can be used for measurement
of current and voltages at very high freq.
*The thermal emf developed in a clet composed of two desimilar metals with junctions kept
at obsolute temp T1 and T2 (T1>T2) may be approximate written as:
Diagram------------------------Quartz crystal thermometers:- A Quartz crystal posses a properly that its resistant freq
changes with the change in temp.
This property is made use of in Quartz crystal
thermometer for measurement of temp.
*The advantage of this thermometer is that sensitivity of the order of 0.0010C are obtained.
Biometallic Thermometers:Biometalic thermometers are extensively used in process industries for local temp
measurement.
These thermomenters use two fundamental principles:(i)All metals expand or contract with change in temp.
(ii)The temp coe of expansion is not the same for all metals and therefore their rates of
expansion (or) contraction are different. The difference in thermal expansion rates is used
to produce defelctions proportional to temp charges.
*A bimetallic thermometers consists of a bimetallic strip which is constructed by bonelis
together two this strips of two different metals such that they cannot make relative to each
other.
*Since all metals try to change their physical diversities at different rates when subjected to
some change in temp these two metallic strips change their leyses at different rates.
*The differential charges of expansion of two metals result in bealing of the bimetallic strip
with change in temp.
*The following fig shows a bimetallic strip in the form of straight cantilener bean with are
end fixed the temp changes cause the trend to deflect.
Diagram------------------*The range of over which a linear relationship exists b/w deflection and temp depends upon
the combination of metals used for the bimetallic strip.
*The deflection of the free end is directly proportioned to the temp change and resume of
the length of strip and inversibly proportional to the thick range through out the linear
portion of deflection temp characterstics.
*The following fig shows a bimetallic strip made up of two metals A and B having different
thermal expansion col banded together at a temp T1 a change in temp (T2-T1) causes a
differential expansion of the strip and if the motion is constrained the strip deflects into a
major circular arc.
The radius of arc is given by
Equation here---------Diagram------------------*It is clear that the bimetallic strip ends found the side whose metal has a lower thermal exp
coe when there is increase in temp and resume happy when there is decrease in temp.
*In most practical applications the metals and their dencious are so chosen that their
moduler of elasticity and thickness are equal. i.e
Equation--------------*Let us consider the case of a bimetallic strip in the form of a centimeter of length L as show
below. The strip is assumed to bend through a circular arc when subjected to a change in
term the thickness of each metal is t/2.
Diagram------------------------Measurement of Pressure:
Measurement of pressuring Electrical Transfer as secondary transfer:
The measurement of force or pressure can be alone by transferring the applied force or
pressure into a disp by elastic elements (bourdon tube or diapharm) which act as primary
transferencess which ect as secondary transferness the o/p of S.T is a fn of which in term is
a fn of pressure. Mechanical methods have to be used to connect the applied force or
pressure to disp. There devices are called force resuming devices.
The choice and design of the shape of rising elements used spends on the mag of force or
pressure to be sensered commonly used summing devices are
Diagrams, Bellows, Burdon tube, Centilener Suspension, Pirot.
Secondary trans:
The disp created by the action of force surming measured is converted into change of some
electrical transfer. The force surnoming member actuates a transolner which converts the
disp into en o/p of electrical format. The various drandness used are
1.Resisfine
2.Inductive
3.Capaciture
4.Prezo-electric
5.Photo-electric
1.Resistance transfer: The electrical strain engaged attached to a diapharm as shown, may
be used for the measurement of pressure the o/p of the stain ganges is a fn of the bal stain
which in turn is a fn of diapharma deflection and the differential pressure. The deflection
generally follows a linear rasiation with differential pressure P=P2-P1.
Change is resistance of strain ganges or account of application of pressure is calibrated in
terms of differential pressure.
Diagram-----------------------2.Indefine Transfers:
Diagram-----------------------Inductive transfer have been successfully used as secondary transferness, along with a
disphargm for measurement of pressure, fig shown, which uses two coils an upper and
lower coil which form the two arms of an a.c bridge the coils have egnal number of turns.
The other two arms of the bridge are formed by two equal resistance each of value R.
The diapharagon is synometrically placed with respect to the coil and so P when P 1=P2,
the relnafarames of the paths of mag flux for both coils are equal and hence inductance of
the coil are equal now initial EQATION HERE---------Suppose, P2 is greater than P1 is there fore the pressure P=P2-P1 deflects the diaphragm
upwards through a distance ‘d’. For small disp of diharagm, the reference of the flux path of
the upper coil is ------------equations-----------Since K, Ro, D are constant, she o/p voltage is α to differential pressure P=P 2-P1. Hence the
o/p voltage number elibrated in terms.
LVDT: The LVDT is used as secondary transfer for instrument pressure with bosdon fure or
belows or diaphragm acting as primary fsansolenss ie as and force susroming device below
Digram---------*With application pressure, the belos contract or expand, which moves the core of the
LVDT, which produces the o/p voltage.
5.Capacitive Transferness:
Dig: plate1 and plate2 are fixed plates and a diapharagm is placed industry b/w the two
plates.
Suppose, capacitance between plate and diaphragm is C1 and plate2 and diagram is
The capacitance between these plates and the electrically grounded diapharagm varies as
fn of the deflection the relation between the deflection by at any point is the pressure is
given by
Equation-----------------Diagram---------------------
Advantages:
1.small size
2.high frequency response
3.adaptability for high temperature operation
4.good lineaty and resolution.
Low pressure measurement (vacume measurement)
Iomition guage
Diagram----------------------Electrons are emitted from the heated cathode the process of iomision means, resnoming of
from an action to get free and true charged ion.
Ometion may be produced by collision of high speed and from the atom.
Electrons are emitted from heated cathode using a filment is are accesferated forwards
the grid which is truly changed. Some of the e are capfused by the grid producing girid
current .
Electrons having high K.E pass through and hit the gas modules present in the tube. I causes
iomization of gas atoms the true ions so produced are attracted to plate and producing a
plate current (IP). Which is proportional to the density of the gas in term proportional to
the pressure. The grid current is assentially instat and negible compare to plate current it is
found that pressure of the gas is directly to =equation.-------------Flow measurements;
Ultrasonic flow transducess.
Use two district measurement principles.
1Transmit time
2Doppler frequently shift.
Basically an ultrasonic transindence for flow rate consists of two piezoelectric crystals in the
liquid or gas separated by a distance one of the crystal acts as frames mitter and other as a
receiver.
When ultrasonic waves pulsed for a very short direction are transmitted across the fluid,
the velocity of the ultrasonic waves is increased or decreased by the fluid velocity
depending upon the direction of the fluid flow.
The transmitter T emits an ultrasonic pulse which is received at the receiver, R Q time at
later the transmit time in the direction of flow is
Diagram------------------------Proper frequency shift:
Velocity of propogation of ultrasonic signal changes if the media moves, here the medium is
flowing fluid whose velocity to be measured.
Transmites A and B are piezoelectric crystals which produces us waves mounted at an
angle to the pipe. The us waves are pulsed for a short period and transmitted across the
fluid, the velocity of us waves increases or decreases by fluid velocity depending upon the
direction of the fluid flow.
For transmitter A, the fluid velocity auole the transmission and velocity of us singnal
increases to c+ v cosθ.
Equation ----------------------------For transmitter B the velocity of us signal reduces due to opposing flow direction. Therefore
the pulse repetation frequency received by received ‘B’ will be
Equation---------------------------Therefore measuring the difference in the repeatation freq θ,l,the velocity can be computed
from
Equation-----------------------Hot – wire Anemometers:The hotwire anemometers is a device that is most often used in research applications to
study varying flow conditions.
When a fluid flows once a heated insface, heat is transferred from the surface and
therefore its temperature reduces, the rate of reduction of temp is related to flow rate.
In a hot wire anemometer, heat is supplied electrically to a fire wire placed in the flow
stereem.
(The temperature of wire determined by measuring its resistance with
wholestone bridge), heat is dissipated by the wire through convention thus causing a drop in
transparence and consequently a diminishing charge in resistance. The temp of the wise
determined by measuring its resistance with heatstone bridge.
The wise attains an equilibrium temp when I2R heat generated in it is just balanced by
the heat loss from its inforce.
The loss of heat from the hot wise is:
Equation---------------Thus if resistance and the temp of the wise are kept constant the rate of fluid flow an be
measured by measuring current ,I through the wise
The hot wire anemometer may be used to measure the velocity in two modes.
Diagram--------------------------The first mode known as constant assent method used an electric ckt adjusted to feed a
constant current to the hot wire knowledge of this current is the resistance of the wise
defines power being fed to the wise which is a fn of the flow velocity. In this operation, a
large resistance is pnt in series with the hot wire and a thermal compensating ckt is also
applied to the o/p a.c voltage beoz, the temp of flow sensing element changes depending
upon the flow rate.
In the second mode known as constant resistance or constant temp, the assient in the
hot wire sensor is continuously adjusted to maintain the wire resistance is hence the wire
temp at a const value throughout the range of hot wire operation. The current or no guage
across the wire is then measure of heat transfer rates and consequently of the flow
velocities.
The constant assunt sys is used at higher frequencies relatively smaller signals for a
electronic stability and amplification reasons but it issued only of limited range of velocities.
Radiation Pyrometers:
*When temp being measured one high and physical contact with the process to be
measured is impracticable (02) impossible use is made of thermal radiation methods (or)
optical pyrometers are used.
*These pyrometers are used under conditionary where corrosine vapours or liquids would
destroy thermocouples. These pyrometers final applications for temp which are above the
range of thermocouples and also for rapidly moving objects.
*Radiation pyrometer measures the radiant heat emitted (or) reflected by a hot object.
Thermal relation is electromagnetic radiation emitted as a result of temp. Thermal radiation
lies in the wave length neighbours from about 0.1 to 100 um.
*The operation of thermal radiation pyrometer is based upon blackbody concepts the total
thermal radiation emitted by a black body is
Equation----------------------------*In practical applications for thermal pyrometers, the transfer of thermal energy takes place
at temp above absolute zero for a two black bodies in right each will radiate energy to the
other and have the net energy transfer is equation------------------*If T1 is much higher than T2 it is safe to pressure that the radiation is proportional to T1 as
the tem T2 become insignificant.
*In general a rough blank surface radiates more heat than a smooth bright surface. This
effect is called emissivity and is emp as
Equation---------------The value of emissitivity from 0 to 1.
*The energy is radiated over a wide range of freq of the electromagnetic spectrum. The
distribution for any particular wavelength ‘ ‘ is given by plan radiation law.
Eqiuation----------------Principle used for Relation Temp Measuring Devices:
There are two principle used for construction of radiation temp measuring devices.
1.Total Radiation Pyrometer:- In this case the total radiant energy from a neated barely is
measured this energy is rep by the area under the curves of fig shown as explained
earlier and is given by steam Boltzman Law. The radiation pyrometer then is inleneel
to receive max amount of radiant energy at widest range of wavelengthy.
2.Selective (or partial) Radiation pyrometery:
*In this case we may measure the spectral radiant industry of the radiated energy from the
heated body at a given wavelength for example it a vertical line is drawn the
variation of invensity with temp for given wave length can be found. The optical
pyrometer uses this principle.
*Actually it is difficult to build a practicle pyrometer that is responsive to radiation of all
wavelength (or) to spectral wavelengths.
Black body conditions:- When a hot body is totally enclosed by walls at the same temp
then both walls and body radiate and absolves heat at the same rate. If a small role
is made in the contains this area will behave as a perfect blackbody. Since rays
leaving the enclose would have been reflected many a times.
*This principle applies very closing to furnace with a small hole through which a radiation
pyrometer may be sighted either an the body as shown in below or on the ceiling of
the furnace.
This can be explained as under:
The walls and the neated body are at the same steady temp the body obsolbs a pent of
radiation from the walls and reflects the rest that is
Equation-------------------Diagram--------------------------*When the neated body is viewed from out side the reflected radiant energy is seen. Since
EQUAION----- for black body radiation, the heated body must radiate the same
amount of enrgy that it absorbs otherwise temp equilibrium will not established.
*The total relation energy emitted from the body and the calculates the temp.
Radiation Receiving Elements:
The purpose of a radiations temp measuring device is to connect the radiant energy into a
suitable forever for indication of temp.
1.Vaccum Thermocouple:
2.Thermopile: To assemble several thermocouples in series to form a thermopile.
3.Bolometer: A bolometer is a thermal device that changes its electrical resistance with
temp.
4.Photo – electric Transferences:
Diagram-------------------
CONTROL LOOPS AND INTERLOCKS IN BOILERS:
Combustion control:
The efficient combustion of fuel in the combustion chamber and efficient transfer of beat
energy to the water for steam generation are essential for the economical working of power
plant.
“The purpose of the combustion control is to regulate the rate of flow of the coal/air
mixture to the burner and at the same time to maintain the correct composition of the
mixture”.
ON – OFF Combustion control circuit:ON-OFF controls are united to ft and ------ boilers. As the name inplies, a drop in pressure
actuates a pressure start or meseury switch to start the sroker or burners and opens the air
damper. Since control is limited in vapuring the length of an and off periods, combustion
efficiency is low.
A typical ON-OFF combustion control circuit for a low pressure steam heating boiler.
Operation:On demand for heat in steam system, the thermostat will actuate (by means of low
voltage) the clapper in the velay. As the clapper is pulled in, it transfer current from the
No.1 to the No.3 terminals
Diagram------------------------------------
Pressure control
Low
voltage
thermostat
Low water out off
Primary
control
Hot ground
Junction
box
Line switch
fuse
Burner motor
Ignition
T/F
On the primary control. The burners motor and iquition will then come on operating the
unit. Either one of two controls determines the sequence of normal operation of the
system. These are the thermostat and pressure control. It the thermostat is satisfied by a
rise in room temperature, it will break its contract cleenerquze the relay, thus interrupting
the flow of current from No.1 terminal to the No.3, and there by stopping the burner. But if
a longer period of time is required to bring up enough heat to satisfy the thermostat, the
pressure (limit) control setting may b e reached. This control will then shut –off the current
to the No.1 terminal on the primary control. This will deenerqize the primary of the stopalown transformers. Then the thermostat will no longer be able to hold the clapper in even
though it is making contact the primary control is deprived of all current the burner will
stop.
When the steam pressure drops, the circuit will be restored, and if the thermostat is still
calling for heat (making contact), the unit will resume operation, In case of abnormal
operation (no fire or insufficient fire appearing), the safety function of the primary control
takes over and the unit shuts down, going into safely.
Automatic Combustion Control:-
In longer power plant the automatic combustion control is used.
It regulates
automatically charges in demand of steam and also effects quick and stable charges in other
variables so, as to maintain constant steam pressure and proper combustion conditions. It
maintains constant steam pressure at all loads.
For controlling rate of combustion the conditions to be regulated are draught, air supply,
and fuel supply. The draught and air supply may be regulated either by dampers, vanes, or
by the speed of the faas or both. The fuel supply is regulated according to the method of
firing used. It saves normal labour and increases the operation efficiencies.
Feed water and Drum level (Three Element Control)
Diagram------------------------------The objective of the steam generator (boiler) control is to provide the steam flow
required by the turbine at desired pressure and temperature. The variables that are
controlled are fuel firing rate, air flow, gas flow distribution, feed water flow, and turbine
valve setting.
Most modern control instrumentation employees closed loop control, in this mole, the
actual output of the system is measured and compared to some demand signal (set point).
The difference between the measurement and demand, called the error signal is then used
to reduce the difference between measurement and set point to zero. Proportional control
is the simplest type of closed loop control. The control signal will be either directly or
inversely proportional to error signal.
“Feed water and therefore steam flow is controlled to meet load demand by the turbine
and at the same time maintain the level of water in the steam drum with in relatively
narrow limits. Normally the water level in the drum is maintained half-full up to the
diametrical plane.
A high steam consumption by the turbine, combine with low feed water supply would
lower the water level in the drum.
“A three element automatic control system of which the drum level is one element as
shown in fig. above. The drum level senser responds to the error between actual drum level
and its set point, such as in the case of high steam consumption and low feed water supply,
and acts on the controller to increase the feed water valve opening to meet the steam flow
demand. This action may be too slow and is supplemented by sensors for feed water and
steam flow.
The difference between the signals from these two sensors anticipates champes in drum
level and sends a signal to the controller to actuate the value in the desired direction.
Control of Main header pressure:
Diagram---------------------------The steam pressure control system maintained steam pressure by adjusting fuel and
combustion air flows to meet the desired pressure.
From the above fig. when the pressure drops the flows are increased. A steam pressure
sensor acts directly on the pulverized coal power drives and forced drought fans to offer
desired changes.
A trining signal from fuel flow and air flow sensor maintain the proper fuel/air ratio.
Since it is often difficult to obtain accurate fuel flows, a steam flow sensor is some times
substituted for flow sensor. Usually above s-sec delay is allowed when changing coal flow
and air flow ratio ensure the prevention of a monetary rich mixture with high fuel-air ratio
and thus assure smoke-free combustion.
Excess Air Control:
*The provision of the light amount of excess air is a premodinant factor in the setting of
automatic controllers for optimum combustion efficiency.
*Carefully conducted tests on boiler plant at -----------loads are necessary to determine the
amount of excess air required and this can be represented by the percentage carbon dioxide
in the fuel gases.
*The Co2 percentage will change (for the same amount of excess air) if the percentage of
carbon and hydrogen in the fuel changes clearing the tests where as the oxygen content is
solely dependant on the air supply.
*Modern catrol component therefore includes the measured of oxygen in the fuel gases
which can be dare with better sensitivity than with Co2 instruments.
*The out put from the air flow controllers is applied t a metering controller which sends a
signal to the fan damper positioning control or a fan motor speed regulator.
*The air flow in combustion chamber is metered and a feed back signal applied to the
metering controller to ensure that it gives the right adjusted air supply.
*Associated with this operation is the separate control action taken by the fuel supply which
is also directly affected by the steam pressure signal.
Relationship between Heat Release and air Supply:-
Heat and reheat steam Temperature control:
An accurate control of superheat temperature is important for sufficient power plant.
The principle affection sulpher heat temperature are
1.Furance temperature.
2.Cleanliness of radiant and pendant super heaters.
3.Temperature of gases entering the convective superheaters.
4.Cleanliness of convective super heaters.
5.Mass flow rate of gases through the convective super heaters
6.variation of load on the unit.
A reduction in steam temperature results loss in plant efficiency a drop of about 20 0C in
steam temperature results in about 1% increase in heat rate. On the other hand rise in
temperature above design value may result in over heating and failure of super heater and
reheater tubes and turbine blades.
“Today steam temperature of 10000 F is common and units are being installed for 1050
and 11000F.
Because these high temperatures are limited only by inetallurgy steam
temperatures must be held to close units for safety as well as for economy.
There are 6 basic methods are used for controlling the temperatures of steam leaving the
boiler.
1.By pass / damper control.
2.Spray type desuper heater.
3.Gas re circulation control.
4.Burner fitting up control.
5.Condenser control.
6.Attemperator control.
By pass / damper control:If at any time the flow of gases through a convective super heaters is reduced without
changing the steam flow, then the final steam temperature will be reduced. Gas by-passing
of the super heater or damper control utilizes this principle. If it is desired to give the
proper steam temperature at 3/4th steam flow. At greater steam flows, same of the flue gas
are by passed around the super heater to maintain the desired steam temperature by
partial closing of the dampers under the super heater.
Diagram-----------------------------
Gas recirculation:
Diagram-----------------------In this system, gas from some point down stream the super heater reheaters, mostly
from the economises out let, but sometimes from the air pre heaters outlet is recirculated
to the furnace by means of a gas recirculation fan. The function of the recirculated gas is to
reduce furnace heat obsorption by dialuting the furence gas and lowering the furnace zone
temp. (radiant zone )
As load fall, greater gas mass is recirculated to maintain full
superheat.
The gas recirculation amount influences the heat absorption in the economises primary
and secondary superheaters, reheaters and the furnace as shown in the figure.
Diagram-------------------------------------The range of control is possible with recirculation is, however, limited laugely by the
power consumption of the recirculation fan. In some gas recirculation systems, recirculated
gas is admitted hear the furnace exit, this is called as gas tempering. The furnace exit the
temperature is here reduced without affecting the furnace heat absorption.
Gas recirculation is sometimes used in series with de superheating or atemperation for
effective steam temperature control as shown in below fig.
Diagram---------------------------------Burner Tilting up:Tilting or Vertically adjustable burners change the gas temp entering the
super heater section by changing the elevation of the fireball within the furnace. A+ low
loads (steam flow) the burners are tilted upward (the usual range # 20o) so that the lower
portion of the furnace became less effective in absorbing energy. Thus, the gases enter the
super heater at high temp than if the burners were fixed, and the steam temp. leaving the
convection super heater can be maintained constant from less than one hall (50%) load to
full load (100%). This is a satisfactory and economical method of temp control.
De superheating and attemperation:Control by attemperation means that the steam temp, is reduced by
removing energy from the steam. In a tabular type, a portion of steam (w1) taken out
through tirbes from a point b/w the primary and secondary superheaters by an automatic
valve diverted to a shell and tube heat exchanger where boiled water from the drum may be
circulated. The steam gives up some of its energy to that water and re-mines with the
primary stream (w2) before entering the secondary superheater as shown in the figure.
Fig 1.
Fig:- 1 control of steam temp by attemperation.
A spray type attemperator, also called desuperheater reduces the steam
temp. by spraying low temp. water from the boiler dram or economizer exit into the line
b/w the primary & Secondary superheaters or the radiant and pendant superheaters. The
spray nozzle injects water into the throat of a mixing venture, where the water mixes with
high velocity steam in the throat vaporizes and cools the steam. The venture and a thermal
sleeve protect the main steam pipe from thermal shook caused by any un vaporized water
droplets that otherwise might impact on the pipe. The water used for spray must be of high
purity so that no deposits are added on the super heater tubes pipes, and turbine blades.
This provides a rapid and sensitive means for temp. control By regulating the amount of
spray water, steam temp. is controlled to produce a flat temp. Curve beyond point ‘a’ steam
is heated from ‘m’ to ‘n’ in the primary superheater, cooled from ‘n’ to ‘o’ in the
desuperheater by spray water, and again heated from ‘o’ to ‘p’ in the secondary
superheater.
Fig2
Fig3
By Snelgy balance at steady state (fig (3))
Wshs1+Wwhw=(Ws+Ww)hs2
Where
Ws + Ww = mass flow rates of steam + water;
respectively, Kg/s.
hs + hw = Specific enthalpies of steam + water,
Kj/Kg
If the desuperheater is located after the last stage superheated, the steam thump.
Execs the
max desired temp before attemperation which is harmful. On the other hand , the
desuperheater is located before last stage super heater, then at rated load with no
attemperation the steam temp. at the location should be, say 510oC, which increases to
560oC by absorption of heat in the last stage superheater. If at a reduced load the steam
temp at this location shouts up to 520oC the water will be sprayed to bring the temp down
to 510oC
Air fuel Ratio Control :- [ standard boilers operator by Clonka, Kohan]
Valve control of the fuel/Air ratio is achieved by use of constant pressure variable
areas. A simple mechanism can be used to cause the opening area of two values in
proportion to one another. If the valve characteristics are not the same, the towel and air
floss will match at only two points throughout the range. If the movement is not directly
proportional, the mixture will be lean at some firing rates and rich at others.
Fig4.
Fig5
Fig(2) Parallel –arm-linkage- connected fuel and air valves.
Fig1 shows two rotary-type valves on a common shaft. Fig2 show two rotary- type
valves driver by a parallel – arm-linkage. One or preferably both of the valves should
embody manual adjustment of the valve opening (in addition to the handle adjustment) so
as to perorate the adjustment of the fuel/air ratio. The valve control system requires an air
blower with a constant pressure characteristic and oil or gas pressure regulator ahead of the
control valve. Thus the upstream pressure for both air and fuel must be constant at the
valve because valiations in the oil viscosity would affect the flow rate.
Pulverised control:The Pulverosed fuel firing system was used for generating the steam in the early
1920’s The conventional fuel firing methods (sloke firing) were found to be unable to take
the fluctuating loads on the plant due to limited capacity of combustion.
The pulverized fuel systems are nowadays universally used for large capacity plants
and using low cost (low grade) fuel as it gives high thermal efficiency and better control as
per load demand.
In a pulverized fuel firing system, the coal is reduced to a fire powder with the heop
of grinding mill and their projected in to the combustion chamber with the help of hot air
current. The amount of air required (known as secondary air) to complete the combustion is
supplied separately to the combustion chamber. The amount of aire used to carry the coal
and to dry it before entering into the combustion chamber is known as “Primary Air” and
the amount of air which is supplied separately for completing combustion is known as
“Secondary Air”.
The efficiency of pulverised fuel firing system mostly depends up-on the size of the
powder. Many modern power plants (thermal) use pulverised fuel systems.
Advantages & Disadvantages:Adv:- 1. Greater Surface area of coal per unit mass of coal allows faster combustion.
2. Wide variety of low grade coal can be burnt more easily.
3. It gives fast response to load changes as rate of combustion can be controlled easily
and immediately.
4. The system is perfectly free from clinker and slagging troubles.
5. This system works properly in combination with gas & Oil
6. The pulverizing system can be repaired without cooling the unit.
7. Large amount of heat release is possible.
8. The banking losses are low compared with stroke firing system.
9. Practically no ash handling problems.
Dis Adv:1. The capital cost of the pulverized system is considerably high.
2. This system produces fly-ash (fine-dust) which requires special and costly fly-ash
removal equipments as electrostatic precipitations.
3. The flame temperatures are high.
4. The possibilities of explosion are more as coal burns like a gas.
5. The fine grinding of fuel at all loads is not possible particularly in this system
6. The building space required is large particularly for central system.
7. The skilled opened are required.
8. Special starting-up equipments are required.
Boiler feed pump (BFP) recirculation control:Or
Feed water control system:A feed water control system has to perform three functions.
i.
Control the level of water in the boiler drum and prevent damage from burning
out boiler tubes because of low water in the boiler drum.
ii.
Prevents an abnormally high water level in the boiler drum.
iii.
Regulate the water in such a manner that the entire system is dept
approximately in balance The types of water control systems are
1. Single element self operated.
2. Single element relay operated.
3. Two clement self operated.
4. Three element relay operated.
For large capacity boilers the last method is most commonly used. The Three
element relay operated control system as shown in fig.
Fig6
Fig- Three element relay operated feed water control system.
In this system the feed water flow to the boiler is measured and compared with the
steam flow out of the boiler. Ratio relay and proportionate control affects the change in
position of the control valve.
Boiler water in the drum is tranenitted. And any change in it will affect the
proportionate and react relay control. The net variation is passed on through a manual (or)
Automatic selector station to water regulating valve (or) pump speed control drive some
tines valuable speed motor and installed between a constant speed motor and a feed pump
(or) a throttle valve is used at the discharge of the pump to obtain constant pressure.
Hot well control system:The principal parts of surface condensers in addition to the shell and water boxes
(or) channels are a large exhaust steam inlet port shell side air-removal outlets and a holwell
equipped with condensate and air outlets.
Fig7
The arrangement of water circulation and different components as shown in fig.
The feed water from hotwell is supplied to a storage and separating drum through
the economiser most of the sensible heat is supplied feed water passing through the
economiser.
When more heat is available that can be used in increasing the sensible heat of the
feed water or pass it through an air heater. However in most economizers the feed water is
not heated higher than to with in 25oC of the temperature corresponding to the saturation
temperature of the steam in the boiler thus preventing steam formations in the economizer.
A water temperature of 85oC in the hot well is the maximum at which the feed
pump works satisfactorily as there is a slight negative pressure on the suction side of the
pump. At temperatures over 85oC steam bubbler begin to form and the boiler feed pump
will not be able to pump steam and water and flow stops. Therefore the feed water is
pumped through and heated in the economizer. Since it is on the pressure side of the pump,
the water can be heated to a much higher temp than the hot well temperature. The
maximum temperature to which the water can be heated in the economizer is 25oC below
steam forming temperature in the boiler.
Fig
The use of condenser in steam power plant reduces the overall cost of generation by
increasing the thermal efficiency of the power plant.
Fig
Fig:- Effect of cooling water temp on condenser vacuum.
Decorator Level Control:One of the feed water heaters is a contact type open heater known as deaerador
others being closed heaters. It is used for the purpose of deaerating the feed water.
The presence of dissolved gases like oxygen and carbon dioxide (Co2) in water makes
the water conserve, as they react with the metal to form iron oxide.
The solubility of there gases in water decreases with increase in temperature and becomes
340 at the boiler or saturation temperature. These gases are removed in the desecrator,
where feed water is heated to the saturation temperature. By the steam extracted from the
turbine.
Fig
feed water alter passing through a heat exchanges, called rest condenser is sprayed from
the top so as to expose large surface area and the bled steam from the turbine is fed from
the bottom. By contact the steam condensers and the feed water is heated to the saturation
temp. Dissolved O2 and Co2 gases get release from the water and leave along with some
vapor, which is condoned back in the vent condenser and the gases are vented out.
Furnace draft control:-
The draft is one of the most essential systems of thermal power plant the purpose of
draught is to supply required quantity of air for combustion and remove the burnt products
from the system.
To move the air through the fuel bed and to produce a flow of hot gases through the
boiler, econoniser pre heater equal to that necessary to accelerate the burnt gases to their
final velocity. This difference of pressure required to maintain the constant flow of air and
to discharge the gases through the chimney to atmosphere is known as draught.
There are two types of draughts are there
1. Chimney type draught
2. Artificial type draught
Chimney draught:Draught can be obtained by use of chimney only is called chimney draught.
Advantages and limitations of chimney draught.
Advantages:1. It does not require any external power for producing the draught.
2. Capital value (investment) is less.
3. It has long life.
4. Chimney keeps the flue gases at high place in the atmosphere.
Limitations:1. The chimney has no flexibility to create more draught.
2. As there is no thorough mixing of air and fuel in the combustion chamber due to low
velocity of air, therefore combustion is very poor. This increases the specific fuel
consumption.
Artificial draughts:When the draught is produced by any other means except chimney it is known as
artificial draught.
There are three types of Artificial draughts one there
1. Forced draught.
2. Induced draught.
3. Balanced draught.
Forced draught:Fig
The forced draught is installed near the base of the boiler and air is forced to pass
through the furnace, flues, economizer, air-prep heater and to the stack.
Induced draught:Fig
Balanced draught:To overcome the difficulties in induced and forced draught we are using balanced
draught system. It contains both forced draught and induced draught.
Fig
Pressure distribution through the system when the balanced draught in used.
Furnace draught control:Balanced draught boilers are not normally designed for +ve furnace pressure. There
for the furnace pressure must be –ve to prevent low gas leakage.
Pressure taps for measuring furnace pressure may be located some distance below
the top. Because of the chimney effect of the not furnace gases
Intuition b/w Airflow and furnace pressure:Additionally stability problems and intuitions may occur in the overall system
because of measurement lags. It is recommended that the pressure transmitter connections
to the boiler furnace be made with the pipe at least 25mn diameter because of the very low
pressure involved.
Either the forced or the induced draft fan can be used to control the furnace draft
with the other fan performing the basic airflow control function. Interaction can not be
completely eliminate b/w these two loops boil it can be minimized by system designs such
as those shown in the figures.
Signal to induced draft fan :If air flow is detected by
(a) air draft venture
(b) air duct orifice
(c) air side up of air heater
(d) piezoneter ring in forced draft fan inlet
(e) area averaging picot on air side and if air flow is controlled by throttle the
forced draft fan.
The common rule is that air flow should be measured and controlled on the same
side of the furnace to minimize interaction b/w the flow and pressure loops.
Choicer of furnace draft control :Signal to forced Draft fan:
If air flow is detected by
(a) boiler differential
(b) gas side /\P of air heater
(c) venture in stack.
and if the air flow is controlled by throttling the induced draft fan.