Basic Mech- Qn Bank - Part B

7/28/2019 Basic Mech- Qn Bank - Part B

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Page 1 of 49 Compiled byDr.R.KUMARAVELAN

VELALAR COLLEGE OF ENGINEERING AND TECHNOLOGYTHINDAL, ERODE -638 012

DEPARTMENT OF MECHANICAL ENGINEERING

BASIC MECHANICAL ENGINEERING - QUESTION BANKPART B

UNITIII - POWER PLANT ENGINEERING

1. Explain in details the layout of steam power plant with a neat sketch. Also mention its

merits and demerits.

Working Principle

A thermal power plant is also known as steam power plant, which uses steam as the

working fluid. Steam power plant works based on the Rankine cycle. Steam is produced in a

boiler using coal as fuel and is used to drive the steam turbine; heat energy is converted into

mechanical energy by the steam turbine and that mechanical energy is used for generating power

with the help of generator. The steam coming out of the turbine is condensed in the condenser

and the condensate is supplied back to the boiler with the help of feed pump.

The layout of the steam power plant consists of four main circuits. These are

1. Coal and ash circuit2. Air and flue gas circuit3. Water and steam circuit4. Cooling water circuit

1. Coal and ash circuit

This circuit consists of coal delivery, preparation, coal handling, boiler furnace, ash

handling and ash storage. Coal from the storage yard is transferred to the boiler furnace by

means of coal handling equipment like belt conveyor, bucket elevator, etc. This coal is sized by

crushers, breakers, etc. The coal is burnt in the boiler furnace and ash is formed by burning of

coal. Ash coming out of the furnace to hot, dusty and accompanied by some poisonous gases.

The ash is transferred to ash storage. The Indian coal contains 30 to 40% of ash. A power plant

of 100 MW capacities produces 20 to 25 tones of hot ash per hour. Hence the space needed for

the storage of ash is also very high.

2. Air and Flue Gas Circuit

It consist of force draught fan, air preheated, boiler furnace, super heater, economizer,

dust collector, induced draught fan, chimney, etc. Air is taken from the atmosphere by the action

of a forced draught fan. It is passed through an air preheated. The air is preheated by the flue gas


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in the preheated. This preheated air is supplied to the furnace to aid the combustion of fuel. Due

to combustion of fuel, hot gases (flue gases) are formed.

The flue gases from the furnace pass over boiler tubes and super heater tubes. In boiler,

wet steam is generated is converted in to super heated steam by means of flue gases. Then the

flue gases pass through the economizer to heat the feed water. After that, it passes through the

air pre-heater to pre heat the incoming air. Finally, the hot flue gases leave to the atmosphere

through chimney.

5. Water and steam circuit:

It consists of feed pump, economizer, boiler drum, super heater, turbine, condenser etc.

Feed water is pumped to the economizer from the hot well. This water is preheated by the flue

gases in the economizer. This pre heated water is then supplied to the boiler drum. Heat is

transferred to the water by the burning of coal. Due to this, water is converted in to steam. The

steam raised in boiler is passed through a super heater. It is super heated by the flue gases. The

super heated steam is then expanded in a turbine to do work. The turbine drives a generated to

produce electric power. The exhaust steam is then passed through the condenser. In condenser,

the steam is condensed in to water and re circulated.

6. Cooling water circuit

This circuit consists of a pump, condenser, cooling tower etc. cooling water is required to

condense the steam in the condenser. Large quantity of water is required for the purpose. Such

large quantity of water is taken either from river or lake, provided adequate water supply is

available from the river or lake through out the year.


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If adequate quantity of water is not available at the plant site, the hot water from the

condenser is cooled in the cooling tower or cooling ponds and circulated again.

Advantages

1. The Coal used as fuel is cheap.

2 .The power plants can be located near load centre, so the transmission cost and

transmission losses are considerably reduced.

3. The power production does not depend on nature mercy.

4. The initial cost of construction of the plant is low compared to hydro electric plant.

5. Suitable for varying load conditions.

6. There are no transmission losses since they are located near load centre.

7. It takes less time for its construction as compared to hydel power plant.

8. Life of the plant is more (25-30 years) as compared to diesel plant (2-5 years).

Disadvantages

1. Thermal power plant discharges large quantities of sulphur dioxide (SO2) which maycause acid rain.

2. Ash disposal is a serious problem in thermal plant and large areas are needed for ashstorage.

3. Cooling water required is more.4. Fuel transportation, handling and storage charges are more.5. Power generation is considerably high when compared to hydro plant.

6. Cannot be used during peak load.

7. High cost of transmission and distribution if the planet is located far away from load

centres.

8. Coal reserves are depleting continuously.

2. Explain in details the layout of hydro-electric power plant. What are the pros and cons

of hydroelectric power plant?

Working principle

The source of energy used in hydroelectric power plant is water. Water at higher altitude

possesses potential energy. Moving water, such as a river or a waterfall, has mechanical energy.

Mechanical energy is the energy that is possessed by an object due to its motion or stored

energy of position. This means that an object has mechanical energy if its in motion or has the

potential to do work (the movement of matter from one location to another,) based on its

position. The energy of motion is called kinetic energy and the stored energy of position is


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called potential energy. Water has both the ability and the potential to do work. Therefore, water

contains mechanical energy (the ability to do work), kinetic energy (in moving water, the energy

based on movement), and potential energy (the potential to do work.)

When the water falls from a higher level to a lower level, its potential energy is

converted in to kinetic energy and this kinetic energy is converted into mechanical energy by

allowing the water to flow through hydraulic turbine. This mechanical energy is utilized to run

an electric generator which is coupled to the turbine shaft.

Components of hydroelectric power plant:

1. Water reservoir:

The area behind the dam where water is stored is called the reservoir. The water there is

called gravitational potential energy. The main purpose of reservoir is to store water during rainy

season and supply it during dry season. Continuous availability of water is a basic necessity for a

hydroelectric plant. The level of water surface in the reservoir is called head water level.

2. Dam:

The dam is used to increase the height of water level and thereby it increases the capacity

of reservoir. The dam is usually built on a large river that has a drop in elevation, so as to use

the forces of gravity to aid in the process of creating electricity. A dam is built to trap water,

usually in a valley where there is an existing lake. An artificial storage reservoir is formed by

constructing a dam across a river. Notice that the dam is much thicker at the bottom than at the

top, because the pressure of the water increases with depth.

3.Spillway:


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Spillway is constructed to act as a safety valve. It discharges the overflow water to the

down stream side when the reservoir is full. These are generally constructed of concrete and

provided with water discharge opening, shut off by metal control gates. By changing the degree

to which the gates are opened, the discharge of the head water to the tail race can be regulated in

order to maintain water level in reservoir.

4. Gate:

A gate is used to regulate or control the flow of water from the dam.

5. Pressure tunnel:

It carries water from the reservoir to surge tank.

6. Penstock:

Pipes which carry water from the reservoir to the hydraulic turbine are known as

penstock. Penstock is made up of steel or reinforced concrete pipes.

7. Surge tank:

When the load on the generator decreases, the gates admitting water must be closed. Due

to sudden decrease in the rate of flow of water to the turbine, pressure in the penstock increases

suddenly. This sudden increase in pressure causes hammering action on the penstock. This

hammering action is known as water hammer. A surge tank is provided to avoid water hammerin the penstock.

8. Water turbine:

Water through the penstock enters into the turbine through an inlet valve. The Pelton

wheel, Francis turbine, Kaplan turbine are the commonly used hydraulic turbines. The potential

energy of water entering the turbine is converted to mechanical energy. The mechanical energy

available at the turbine shaft is used to run the electric generator. The water is then discharged

through the draft tube.

9. Draft Tube:

It is connected to the outlet of the turbine. It allows the turbine to be placed over tailrace

level. Draft tube is a metallic pipe or concrete tunnel. Draft tube has increasing cross sectional

area towards outlet.

10. Tail race:

Tail race is a passage for discharging water leaving the turbine into the river. The water

held in the tail race is called tail race water level.


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11. Electric generator, step-up transformer:

As the water rushes through the turbine, it spins the turbine shaft, which is coupled to the

electric generator. The generator has a rotating electromagnet called a rotor and a stationary part

called a stator. The rotor creates a magnetic field that produces an electric charge in the stator.

The charge is transmitted as electricity. The step-up transformer increases the voltage of the

current from the stator. The electricity is distributed through power lines to consumers.

Classification of hydro-electric power plants

Hydro-electric power plants are usually classified according to the available head of water.

1. High head power plants:In these power plants the head of water is more than 300 meters. The turbine used

in such plants is Pelton wheel.

2. Medium head power plants:In these power plants the head of water ranges from 30 to 300 meters. The

turbine used in such plants is Francis turbine.

3. Low head power plants:

In these power plants the head of water ranges from 3 to 30 meters. The turbine

used in such plants is Kaplan turbine and Francis turbine.

Merits of the hydro-electric power plant:

1. Water is a renewable source of energy. Water which is the operating fluid hencepollution free.

2. Water is the cheapest source of energy because it exists as a free gift of nature hencethere is no fuel cost.

3. Hydro-electric power plant is highly reliable.4. Running cost of the plant is low.5. Variable load do not affect the efficiency in the case of hydro-electric power plant.6. There is no ash disposal problem as in the case of thermal power plants.7. Water stored in the hydro-electric power plant can also be used for domestic water

supply.

8. It requires less supervising staff.9. Maintenance cost is low.10.Number of operations required to run a hydro-electric power plant is less, compared

to thermal power plant.

Demerits of the hydro-electric power plant:


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1. The hydro-electric power plant are often far away from the load center and requirelong transmission lines to deliver power.

2. Initial cost of the plant is high.3. It takes long time for the erection of such plants and the construction of a dam,

compared to thermal power plant.

4. Power generation is dependent on the quantity of water available, which may varyseason to season and year to year.

5. Water in the reservoir is lost by evaporation.

3. Briefly explain the working principle of gas turbine power plant with a neat sketch. Also

mention its advantages and disadvantages.

Working principle of gas turbine power plant

Gas turbines are described thermodynamically by the Brayton cycle, in which air is

compressed isentropically, combustion occurs at constant pressure, and expansion over the

turbine occurs isentropically back to the starting pressure. The atmospheric air enters the

compressor where it is compressed to a high pressure. This high pressure air then enters the

combustion chamber where it mixes with the fuel (natural gas or petrol) and combustion takes

place. The high pressure and high temperature combustion gases expand in the gas turbine and

thus mechanical energy is produced. Part of this mechanical energy is used in driving the

compressor which is usually mounted on the same shaft as that of the turbine shaft and the

remaining part of this mechanical energy is utilized for producing electric power.

Uses of gas power plant

1. Gas turbine plants are used in jet, aircraft and ships.

2. They are used as standby plants for hydro-electric power plants.

Elements of a gas turbine power plant
http://upload.wikimedia.org/wikipedia/commons/3/3c/Brayton_cycle.svg


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The gas turbine plant consists of


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Page 9 of 49 Compiled by Dr.R.KUMARAVELAN

1. Starting motor2. Low pressure compressor (LPC)3. Intercooler4. High pressure compressor (HPC)

5. Regenerator6. Combustion chamber7. Gas turbine8. Reheating unit

1. Starting motor

Gas turbines are not self starting. They require a starting motor to first bring the turbine to

the minimum speed called coming in speed, for this purpose a starting motor is required

2. Low pressure Compressor (LPC)

In gas turbine plant, the axial and centrifugal flow compressors are used. The major function

of the compressor is to compress the air. Atmospheric air is drawn in and passed through the air

filter. It then flows into the low pressure compressor. Major percentage of power developed (66 %)

by the turbine is used to run the compressor. The power required to run the compressor can be

reduced by compressing the air in two stages, i.e., in low pressure and high pressure compressors

and also by incorporating an intercooler between the two.

3. Intercooler

Intercooler is used to reduce the work of the compressor and increase the efficiency. The air

after compression in the LPC is hot. It is cooled by the intercooler. The intercooler is circulated with

cooling water. An intercooler is provided in between the two stages of compression in order to

minimize the power required for compression at the higher stages.

4. High pressure compressor (HPC)

The cooled air coming out of the intercooler is further compressed in the high pressure

compressor. The pressure and temperature of air further increases in the high pressure compressor.

5. Regenerator

The air from the H.P compressor enters a regenerator which is a type of heat exchanger.

Here the air is preheated by the exhaust gases from the low pressure turbine before it enters the

combustion chamber. The main objective of preheating the compressed air is to reduce the fuel

consumption and to increase the efficiency.

.


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6. Combustion chamber

As the name suggests, it is a chamber where combustion takes place. For any combustion,

fuel and air is required. The fuel used in gas power plants may be natural gas, coal gas, kerosene or

gasoline. Hot air from regenerator flows to the combustion chamber; fuel is injected into the

combustion chamber. After the fuel injection, the combustion takes place. These high pressures,

high temperature products of combustion are passed through the turbine.

7. Gas turbine

Two types of gas turbines are used in the gas turbine plant. 1. High pressure turbine. 2. Low

pressure turbine. The products of combustion from the combustion chamber are first expanded in

high pressure turbine and then in low pressure turbine. The part of the work developed by the gases

passing through the turbine is used to run the compressor and the remaining about (34 %) is used to

generate electric power.

8. Reheating unit

The output of the plant can be further improved by providing a reheating combustion

chamber between high pressure and low pressure turbines. In this, fuel added to reheat the exhaust

gases of high pressure turbine. The addition of the regenerator, intercooler and reheating

combustion chamber are to increase the overall efficiency of the plant.

Advantages of Gas Turbine power plant

1. The initial cost is lower than an equivalent thermal plant.2. Very high power-to weight ratio, compared to reciprocating engines.3. Natural gas is a very suitable fuel and where this is available cheap, it is an ideal source

of power in gas turbine.

4. Gas turbine plant is small and compact in size as compared to steam power plants.5. It can be started quickly and can be put on load in a very short time.6. It requires less water as compared to a steam power plant.7. Moves in one direction only, with far less vibration than a reciprocating engine.8. Fewer moving parts than reciprocating engines.9. High operation speeds.(40,000 to 100,000 rpm)10.The exhaust of the gas turbine is free from smoke.

Disadvantages of Gas Turbine power plant

1. 66% of the power developed is used to drive the compressor; the gas turbine unit has alow thermal efficiency.

2. It requires special metals and alloys for deferent components because the operatingtemperature (2000

0C) and speed (100,000) are very high.

3. Life of the gas turbine power plant is less.4. Part load efficiency is poor.5. Gas turbine power plant creates more noise in it operation


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4. with a schematic diagram, explain the working principle of diesel power plant. What are its

merits and demerits?

Working principle of diesel power plant

Diesel generating plants have an important role in power plants as well as in industries andcommercial installations to meet continuous and emergency standby power requirements for day to-

day use. A diesel power plant may use two stroke or four stoke engine. Diesel engine works on

diesel cycle this cycle is also known as constant pressure cycle. Diesel engine is mostly employed in

Stationary Power plants, Ships, Heavy Motor Vehicles. In diesel engine, diesel oil and light and

heavy oil used as fuel. This fuel is ignited by being injected into the engine cylinder containing air

compressed to a very high pressure; the temperature of this air is sufficiently high to ignite the fuel.

That is why there is no spark plug used in diesel engine. This high temperature compressed air used

in the form of very fine spray is injected at a controlled rate so that the combustion of fuel proceeds

at constant pressure.

Main Components of Diesel Power Plant

The essential components of a diesel power plant are

1. Diesel engine

2. Starting systems

3. Fuel supply systems

4. Air intake system

5. Exhaust system

6. Cooling system

7. Lubricating system

Layout of diesel power plant


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1. Diesel Engine

A diesel engine is a compression ignition (C.I) engine. The main component of a diesel

power plant is diesel engine which is used to produce power. A diesel engine may be four stroke or

two stroke cycle engine. The two stroke engine is more favored for diesel power plant. Air is

admitted into the cylinder of the engine and is compressed; fuel (diesel) is injected into the cylinder

through fuel injectors. Due to the high temperature and pressure obtained during compression, it

ignites. The fuel burns and the burnt gases expand to do work on the moving part inside the cylinder

called piston. This movement of the piston rotates a flywheel. The engine is directly coupled to the

electric generator. The gases after expansion inside the cylinder are exhausted into the atmosphere

and passes through a silencer in order to reduce the noise.

The fuel for the diesel engine is drawn through a filter from the all day tank. The air required

for the diesel engine is drawn through the air filter from the atmosphere.

2. Starting System:

This includes air compressor and starting air tank. Diesel engine used in diesel power plants

is not self starting. The engine is started from cold condition with the help of an air compressor.

3. Fuel supply system:

It includes the storage tank, fuel pump, fuel transfer pump strainers and heaters. Pump draws

diesel from storage tank and suppliers it to the small day tank through the filter. Day tank supplies

the daily fuel need of the engine. The day tank is usually placed high so that diesel flows to engine

under gravity. Diesel is again filtered before being injected into the engine by the fuel injection

pump. The fuel is supplied to the engine according to the load on the plant.

4. Air intake system:

The air required for the combustion of fuel inside the diesel engine cylinder is drawn

through the air filter. The purpose of the filter is to remove dust from the incoming air. The dry


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filter used may be made of felt, wood or cloth. In wet filter, oil bath is used, in this the air passes

over a bath of oil where the dust particles get coated on the oil.

5. Exhaust system

An exhaust manifold is provided to the diesel engine for conveying the exhaust gases to the

atmosphere. The exhaust system should silence the exhaust noise to requisite levels. The muffling

of the exhaust noise is met by using silencers. The gases must be discharged sufficiently high above

the ground level to avoid low level pollution of air.

6. Cooling system

The primary purpose of cooling system is to carry the heat from engine cylinder and to keep

the temperature of the cylinder within safe limits. The temperature of the burning fuel inside the

engine cylinder is in the order of 1500o

Cto 2000o

C. If the excess heat is not removed, it would

overheat the engine and may damage the piston, piston rings and cylinder walls of the engine. Small

engine are cooled by air. Large stationary engines are cooled by water.

Cooling of engines is done by circulating water through jackets. In this, raw water is made

to flow through the heat exchanger when it takes up the excess of heat of jacket water and then is

returned back to the cooling tower.

7. Lubricating system:

This circuit includes lubricating oil tank, oil pump and oil cooler. The purpose of the

lubrication system is to reduce the wear of the engine moving parts. Part of the cylinder such as

piston, shafts, and valves must be lubricated. Lubrication also helps to cool the engine.

In the lubrication system the oil is pumped from the lubricating oil tank through the oil

cooler where the oil is cooled by the cold water entering the engine. The hot oil after cooling the

moving parts and return to the lubrication oil tank.

Advantages of diesel power plant

1. Plant layout is simple

2. Diesel engine power plant can be located near the load centre.

3. Quick starting and easy pick-up load in a short time.

4. Fuel handling is easier and no problem of ash disposal exists.

5. It occupies less space.

6. It requires less quantity of water for cooling purposes.

7. Diesel power plants operate at high overall efficiency than steam power plant.8. It can respond to varying load without any difficulty

9. The operating of the plant is easy and requires less number of personnel

Disadvantages of diesel power plant

1. Plant capacity is limited to about 50 MW of power.2. Diesel fuel is much more expansive than coal.3. High maintenance and lubrication cost.4. In a plant noise is a serious problem.


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5. Life of the plant is low when compared to thermal plants.6. Diesel engines are not guaranteed for operation under continuous overloads, while steam

turbine can work under 25 % of overload continuously

5. Draw the neat sketch of nuclear power plant and explain its working principle. Also

mention its advantages and disadvantages.

1. Fuel:

The fuel which is used in the nuclear reactors are U235

, Pu239

and U233

2. Nuclear reactor:

It consists of reactor core, reflector, shield etc. It may be regarded as a substitute for the

boiler fire box of a steam power plant. During the fission the large amount of heat is liberated by

U235

, This large amount of heat is absorbed by the coolant and it is circulated through the core. The

various types of reactors used in nuclear power plant is

a. Boiling water reactor (BWR)b. Pressurized water reactor (PWR)c. Heavy Water-cooled reactor (HWR)

3. Steam Generator:

The steam generator is fed with feed water which is converted into steam by the heat of the

hot coolant. The purpose of the coolant is to transfer the heat generated in the reactor core and use it

for steam generation. Ordinary water of heavy water is a common coolant.

4. Steam turbine:

Steam turbine is used to convert the heat energy into mechanical (rotational) energy. Thesteam produced in the steam generator is allowed to expand inside the steam turbine by which the

turbine blades (fixed and movable) are made to rotate. Since the turbine blades are fitted around the

turbine shaft, the turbine shaft is rotated. The turbine shaft in turn is coupled to a generator at the

other end that generates electricity.

5. Moderator

It is a material which reduces the speed of fast moving neutrons. During the chain reaction,

neutrons moving with high velocity are produced. Fast moving neutrons try to escape from the


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reactor without causing fission of U235

. Hence, their speed is reduced. This is done by colliding

them with the nuclei of other light material (moderator) which does not capture the neutrons but

scatters them. During collision the kinetic energy is lost and the speed of fast moving neutrons is

reduced. The slow neutrons thus produced are easily absorbed by the nuclear fuel and chain reaction

takes places smoothly. Beryllium, graphite or heavy water is normally used as a moderator.

6. Reflector:

The neutrons produced by fission process are absorbed by fuel rods, moderator, coolant or

the surrounding construction. Some neutrons may escape from the reactor core without absorption.

This is a loss and should be eliminated. This loss can be minimized by surrounding the reactor core

with a material called reflector. The reflector material reflects the escaping neutrons back into the

core. The reflected neutrons cause more fission and improve the neutrons economy of the reactor.

Water, carbon, graphite, beryllium are generally used as reflectors.

7. Control rods:

The control of chain reaction is carried out by removing the fuel rod or the neutrons. It is

easy to absorb neutrons. Absorbing of neutrons is done by inserting neutron absorbing material rods

known as control rods in the reactor core. The control rods must be able to absorb excess neutrons.

The position of these rods are regulated by electronic or electro mechanical devices. The material

used for control rods are boron, cadmium, hafnium etc.

8. Coolant pump and Feed Pump:

The steam from the turbine flows to the condenser where cooling water is circulated.

Coolant pump and feed pump are provided to maintain the flow of coolant and feed water

respectively.

9. Biological shield (Radiation shield):

The reactor produces intense radio activity and these radiations are very harmful to the

human life. The common radiations from the reactors are alpha rays, beta rays, gamma rays and fast

moving neutrons. These radiations must be absorbed before emitted to atmosphere. Thick layers of

lead concrete or steel are provided all around the reactor. These layers absorb the gamma rays,

neutrons etc. Concrete and steel are used as shielding material.

Advantages of nuclear power plant:1. The nuclear power plant requires very small quantity of fuel. Hence fuel

transportation cost is less.

2. Space requirement is less when compared with other power plants of equalcapacity.

3. It is not affected by adverse weather conditions.4. Suitable for large power generation.5. Very large amount of heat is liberated by a very small quantity of fuel.6. Fuel storage facilities are not needed as in the case of the thermal power plant.


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7. Conserve the fossil fuel (coal, oil, gas etc.) for other energy requirements.8. Reliability of operation.9. Does not require large quantities of water for cooling.

Disadvantages of nuclear power plant:

1. Nuclear radiation can be extremely dangerous.2. Radioactive wastes should be disposed carefully, Otherwise it may have bad effect on

the health of workers and the environment as a hole.

3. It requires high initial cost.4. Availability of nuclear fuel is scarce and cost is high.5. Maintenance cost is higher.6. It is not suited for varying load conditions.7. It requires well-trained personnel.

6. Explain the working principle of single acting reciprocating pump.

If a reciprocating pump uses one side of the piston for pumping liquid, then it is known as a

Single Acting Reciprocating Pump.

The main parts of a single acting reciprocating pump are:

1. Cylinder, Piston, Piston Rod, Connecting Rod and Crank.2. Suction Pipe3. Suction Valve4. Delivery Pipe5. Delivery Valve

1. Cylinder, Piston, Piston Rod, Connecting Rod and Crank

A single acting reciprocating pump consists of a piston, which moves forwards and

backwards inside a close fitting cylinder. The movement of the piston is obtained by connecting the

piston rod to the crank by means of a connecting rod. The crank is rotated by an electric motor.


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2. Suction Pipe

A suction pipe connects the lower sump and the cylinder. At the bottom end of the suction

pipe, strainer and foot valve are provided. Foot valve (non-return valve) allows water to flow in the

upward direction only.

3. Suction Valve:

Suction valve is one way valve or a non-return valve. It allows the liquid to flow in one

direction only. That is, it allows the liquid from the suction pipe to the cylinder.

4. Delivery Pipe:

A delivery pipe connects the cylinder and the upper sump and water is discharged from the

cylinder into the delivery pipe.

5. Delivery Valve:

Delivery valve is also one way valve or non-return valve. It allows the liquid to flow in one

direction only. That is, it allows the liquid from the cylinder to the delivery pipe.

Working Principle:

It consists of a piston which moves forwards and backwards in a close fitting

cylinder. The movement of the piston is obtained by connection the piston rod to crank by means of

a connecting rod. The crank is rotated by means of an electric motor. Suction and delivery pipes

with suction valve and delivery valve are connected to the cylinder. The suction and delivery valves

are one way valves or non-return valves, which allow the water to flow in one direction only.

Suction valve allows water from suction pipe to the cylinder which delivery valve allows water

form cylinder to delivery pipe only.

When crank starts rotating, the piston moves to and fro in the cylinder. When crank is at A.,

the piston is at the extreme left position in the cylinder. As the crank is rotating from A to C, (i.e.,

from = 0 to = 1800), the piston is moving towards right in the cylinder. The movement of the

piston towards right creates a partial vacuum in the cylinder. But on the surface of the liquid in the

sump atmospheric pressure is acting, which is more than the pressure inside the cylinder. Thus the

liquid is forced in the suction pipe from the sump. This liquid opens the suction valve and enters the

cylinder. When crank is rotating from C to A (i.e., from = 1800to = 360

0), the piston from its

extreme right position starts moving towards left in the cylinder. The movement of the piston

towards left increases the pressure of the liquid inside the cylinder more than atmospheric pressure.Hence suction valve closes and delivery valve opens. The liquid is forced into the delivery pipe and

is raised to a required height.

6. Briefly explain the types of reciprocating pumps.

If the mechanical energy is converted into hydraulic energy (pressure energy) by sucking the

liquid into a cylinder, in which a piston is reciprocating, the pump is known as Reciprocating Pump.


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Reciprocating pump uses piston or plunger to positively displace a given volume of liquid during

each stroke of the piston. The piston is moving backwards and forwards, exerting a thrust on the

liquid, thereby increasing its pressure energy.

Classification of Reciprocating Pump:

According to the liquid contact on sides of the piston, reciprocating pumps are classified as

1. Single acting reciprocating pump2. Double acting reciprocating pump.

According to the number of cylinder

1. Single Cylinder Reciprocating Pump2. Double Cylinder Reciprocating Pump3. Triple Cylinder Reciprocating Pump4. Duplex Double Acting Pump5. Quintuplex Pump

Single Acting Reciprocating Pump

If a reciprocating pump uses one side of the piston for pumping liquid, then it is known as a

Single Acting Reciprocating Pump.

The main parts of a single acting reciprocating pump are:

1. Cylinder, Piston, Piston Rod, Connecting Rod and Crank.2. Suction Pipe3. Suction Valve4. Delivery Pipe5. Delivery Valve

1. Cylinder, Piston, Piston Rod, Connecting Rod and Crank

A single acting reciprocating pump consists of a piston, which moves forwards and

backwards inside a close fitting cylinder. The movement of the piston is obtained by connecting the

piston rod to the crank by means of a connecting rod. The crank is rotated by an electric motor.

2. Suction Pipe

A suction pipe connects the lower sump and the cylinder. At the bottom end of the suction

pipe, strainer and foot valve are provided. Foot valve (non-return valve) allows water to flow in theupward direction only.


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3. Suction Valve:

Suction valve is one way valve or a non-return valve. It allows the liquid to flow in one

direction only. That is, it allows the liquid from the suction pipe to the cylinder.

4. Delivery Pipe:

A delivery pipe connects the cylinder and the upper sump and water is discharged from the

cylinder into the delivery pipe.

5. Delivery Valve:

Delivery valve is also one way valve or non-return valve. It allows the liquid to flow in one

direction only. That is, it allows the liquid from the cylinder to the delivery pipe.

Working Principle:

It consists of a piston which moves forwards and backwards in a close fitting

cylinder. The movement of the piston is obtained by connection the piston rod to crank by means of

a connecting rod. The crank is rotated by means of an electric motor. Suction and delivery pipes

with suction valve and delivery valve are connected to the cylinder. The suction and delivery valves

are one way valves or non-return valves, which allow the water to flow in one direction only.

Suction valve allows water from suction pipe to the cylinder which delivery valve allows water

form cylinder to delivery pipe only.

When crank starts rotating, the piston moves to and fro in the cylinder. When crank is at A.,

the piston is at the extreme left position in the cylinder. As the crank is rotating from A to C, (i.e.,

from = 0 to = 1800), the piston is moving towards right in the cylinder. The movement of the

piston towards right creates a partial vacuum in the cylinder. But on the surface of the liquid in the

sump atmospheric pressure is acting, which is more than the pressure inside the cylinder. Thus the

liquid is forced in the suction pipe from the sump. This liquid opens the suction valve and enters the

cylinder.

When crank is rotating from C to A (i.e., from = 1800 to = 360

0), the piston from its

extreme right position starts moving towards left in the cylinder. The movement of the piston

towards left increases the pressure of the liquid inside the cylinder more than atmospheric pressure.

Hence suction valve closes and delivery valve opens. The liquid is forced into the delivery pipe and

is raised to a required height.

Double Acting Reciprocating Pump:If the liquid is in contact with both the sides of the piston, it is known as Double Acting

Reciprocating Pump. A double acting reciprocating pump consists of the following main parts.

1. Cylinder:

The cylinder consists of a piston which moves forwards and backwards. The cylinder is a

closely fitted one and the suction and delivery pipes with one-way valves are connected with the

cylinder. The piston inside the cylinder is connected with a piston rod.


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2. Connecting Rod and Crank:

The movement of the piston is obtained by connecting the piston rod to a crank by means of

a connecting rod. The crank is rotated clockwise by means of an electric motor. The crank, by its

rotary movement provides the to and fro movement of the piston and connecting rod.

3. Suction and Delivery Pipes:

The suction and delivery pipes are connected with the cylinder. Two suction pipes and two

delivery pipes are used with suction and delivery valves respectively.

Working Principle

A double acting reciprocating pump has two suction ant two delivery pipes with suction and

delivery valves as shown.

During each stroke, when suction takes place on one side of the piston, the other side

delivers the liquid. In this way, in the case of a double acting pump, in one complete revolution of

the crank, there are two suction strokes and two delivery strokes. Therefore, the liquid is delivered

to the pipes by the pump during these two delivery strokes.

In the case of a double acting reciprocating pump, when there is a suction stroke on one side

of the piston, its other side has a delivery stroke. Thus, for one complete revolution of the crank,

there are two delivery strokes. The liquid is delivered by the pump during these two delivery

strokes.

If the speed of the crank is N rpm, then the number of delivery strokes will be 2N per minute

or (N / 30) per second. However, due to the presence of the piston rod on one side, the volumes of

liquid delivered from both the sides of the piston will not be equal.

8. Explain briefly the working of centrifugal pump.

Centrifugal Pump is a Rotodynamic pump which uses the mechanical energy of the rotating

impeller to increase the velocity of a fluid by the application of centrifugal force. The fluid enters

the pump impeller along the rotating axis and gets accelerated, flows radically outwards into acasing and exits through the piping system. It is generally used for large discharge through smaller

heads.

Main Parts of a Centrifugal Pump:

The following are the main parts of a centrifugal pump:

1. Impeller.

2. Casing

3. Suction pipe with a foot valve and a strainer.


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4. Delivery pipe.5. Shaft6. Stuffing Box

1. Impeller:

The rotating part of the centrifugal pump is called Impeller. It consists of backward curved

vanes of blades. The impeller is mounted on a shaft. The shaft is coupled to the shaft of an electric

motor. It is enclosed in the casing. The fluid enters in the central portion known as the eye of the

impeller and flows out radially outward and it is then discharged around the entire circumference

into the casing.

Impellers may be classified as:

a. Closed or Shrouded Impeller

b. Semi-Open Impeller

c. Open Impeller

2. Casing:

The centrifugal pump has a stationary outer casing. It is an air-tight passage surrounding the

impeller. Its shape is designed in such a way that the kinetic energy of liquid discharged at the outlet

of the impeller is converted into pressure energy before the liquid leaves the casing and enters the

delivery pipe. Two openings are provided in the casing for suction and delivery of water. The

casing contains bearings for supporting the pump shaft. The following three types of casings are

commonly adopted:

a. Volute casing

b. Vortex casing

c. Casing with guide blades (or) Diffuser casing.

3. Suction pipe with a foot valve and a strainer.

Suction Pipe is a pipe connected at its upper end tot eh inlet of the pump at the center of the

impeller. The center of the impeller is known as Eye of the Impeller. The lower end of the suction

pipe dips into liquid in a suction tank or a sump from which the liquid is to be pumped or lifted up.

A Strainer is fitted at the lower end of the suction pipe. The liquid from the sump first enters

the strainer. The strainer is provided in order to keep the debris (impurities) such as leaves, wooden

pieces and other rubbish away from the pump. It then passes through the foot valve to enter thesuction pipe.


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A foot valve is a not-return valve or one-way type of valve. It is fitted at the lower end of the

suction pipe. The foot valve opens only in the upward direction. Therefore, the liquid will pass

through the foot valve only upwards. It will not allow the liquid to flow downwards back to the

sump.

4. Delivery Pipe and Delivery Valve:

A pipe whose one end is connected to the outlet of the pump and the other end delivers the

liquid at the required height is known as Delivery pipe.

Delivery Valve: Just near the outlet of the pump on the delivery pipe, a delivery valve is provided.

A deliver valve is a regulating valve. It is required to be provided to control the flow of the liquid

from the pump into the delivery pipe.

5. Shaft:

A shaft of a pump performs the duties of transferring the torque from the motor to the

impeller, transferring the hydraulically induced radial loading to the journal bearing without undue

deflection, transferring the axial thrust from the impeller to the bearing and providing and

acceptable environment for the shaft seals.

6. Stuffing Box:

It is used to stop leakage of air into the casing when the pressure in the casing is below

atmospheric. It also stops the leakage of liquid under pressure from the casing of the pump. The

stuffing box packing consists of a soft semi-plastic material. This plastic material is cut in rings. It

fits around the shaft or shaft sleeve.

Working Principle:

The first step in the operation of a centrifugal pump is priming. After the pump is primed,

the delivery valve is still kept closed. The electric motor is now started to rotate the impeller. The

delivery valve is kept closed in order to reduce the starting torque for the motor.

The rotation of the impeller in the casing full of liquid produces centrifugal force. This

centrifugal force imparts a centrifugal head to the liquid. This result is an increase of pressure

throughout the liquid mass. Thus, if the speed of the impeller is sufficiently high, the pressure in the

liquid surrounding the impeller is increased. As long as the delivery valve is closed and the impeller

is rotating, it jut churns the liquid in the casing.When the delivery valve is opened, the liquid is made to flow in an outward radial direction.

And the liquid is leaving the vanes of the impeller at the outer circumference with a high pressure.

In the mean while, a partial vacuum is created at the eye of the impeller due to the centrifugal

action. This causes sucking of the liquid form the sump (which is at atmospheric pressure) to rush

through the suction pipe to the eye of the impeller.

Thus, this rushing liquid is replacing the liquid which is being discharged from the entire

circumference of the impeller. The high pressure of the liquid leaving the impeller is utilized in

lifting the liquid to the required height through the delivery pipe.


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Uses of Centrifugal Pumps:

Centrifugal pumps are most commonly employed for pumping large volumes of liquids at

low pressures. However, by multi-staging, it is possible to increase the outlet pressures. The

capacity of the pump depends upon the impeller diameter, its width and speed. The head of the

pump depends upon the impeller diameter and speed.

Centrifugal pumps are sued widely in power plants. They are also used for plumbing,

drainage and marine works. They find applications in residences, agriculture and industries such as

petroleum, sugar paper, pharmaceutical, chemicals, etc.

9. Write a brief note on steam turbine.

In steam turbine, the heat energy of steam is first converted into kinetic energy in nozzles.

This high velocity steam impinges on the curved blades which changes the direction of flow of

steam. The change in the flow direction causes the force to be extorted on the blades which are

attached to the shaft. As a result, the shaft rotates and the turbine produces work.

The main parts of the steam turbine are fixed nozzles, rotor, fixed and moving blades, outer

casing, etc. The rotor is a circular disc fixed to a horizontal shaft. On the periphery of the rotor, a

number of blades are fixed at uniform intervals.

Classification of Steam Turbines:

Steam turbines are classified as follows:

1. According to the method of steam expansion in the turbine.

a. Impulse turbine

b. Reaction turbine

c. Combination of impulse and Reaction turbine

2. According to the steam flow direction:

a. Axial turbine

b. Radial turbine

c. Tangential turbine

d. Mixed flow turbine

3. According to the number of stages

a. Single stage turbine b. Multi state turbine.

4. According to the steam pressure

a. High pressure turbine

b. Low pressure turbine

c. Medium pressure turbine

Simple Impulse Turbine

Delaval turbine is known as simple impulse turbine. Simple impulse turbine consists of only

one set of nozzles and moving blades.

As the steam expands in the nozzle, velocity of steam increases at the expense of its

pressure. The high velocity steam jet impinges on the moving blades which move in the direction of

the jet. The movement of blades makes the shaft to rotate.


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All the kinetic energy of steam is absorbed by one set of moving blades, the velocity of

turbine shaft is too high (25,000 to 30,000 rpm).The speed of the impulse turbine is very high and it

has many practical problems. With this high speed small amount of kinetic energy of steam can be

utilized. These difficulties associated with the use of single stage turbines for large pressure drops

can be solved by compounding, i.e., the use of more than one stage.

Compounding may be of three types:

1. Velocity compounding2. Pressure compounding3. PressureVelocity compounding

Reaction Turbine

In reaction turbines, there is no sudden pressure drop. Reaction turbines, steam expands as it

flows over the fixed blades which act as nozzles. In this turbine there is a gradual pressure drop

takes place continuously over the fixed and moving blades. In fact this pressure drop gives a

reaction and hence the rotor starts moving. A number of wheels are fixed to the rotating shaft. Fixed

guide ways are provided in between each pair of rotating wheels.

The fixed blades act as guides and also as nozzles. The guide blades serve the following

functions.

1. It changes the direction of steam.2. It guides the steam to enter the next wheel3. It allows the steam to expand to a higher velocity as steam flows through them.

Differences between Impulse and Reaction Turbines:

Sl.

NoImpulse Turbines Reaction Turbines

1.Impulse turbine consist of moving blades

and nozzles

Reaction turbine consists of fixed blades and

moving blades (moving blades act as nozzles)

2.Steam strikes the blades with kinetic

energy.

Steam passes over the moving blades with

pressure and kinetic energy

3.Pressure drops in nozzles and not in movingblades

Pressure drops in fixed bladed (nozzles) aswell as moving blades


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4.

Due to sudden and steep reassure drop at the

end (tip) of the nozzle, blade speed and

steam speed increase.

Due to low level pressure drop, blade speed

and steam speed are comparatively less.

5.Due to more pressure drop, number of

stages required is less.

Number of stages required is more due to

more pressure drop.

6. Overall efficiency is low. Overall efficiency is high.

7.The amount of space occupied by the

impulse turbine is less.

The amount of space occupied by the reaction

turbine is more.

8.Very much suitable for small powergeneration.

Very much suitable for high powergeneration.

10. Compare the differences between centrifugal pump and reciprocating pump.

Sl.

NoCentrifugal Pumps Reciprocating Pumps

1. The discharge is continuous and smooth The discharge is fluctuating and pulsating

2. It can handle large quantity of liquid. It handles small quantity of liquid only.

3.It can be used for lifting highly viscousliquids

It is used only for lifting pure water of lessviscous liquids.

4.It is used for large discharge through

smaller heads.It is meant for small discharge and high heads.

5.Cost of centrifugal pump is less as

compared to reciprocating pump.

Cost of reciprocating pump is approximately

four times the cost of centrifugal pump.

6.Centrifugal pump runs at high speed.They can be coupled to electric motor.

Reciprocating pump runs at low speed. Speed is

limited due to consideration of separation and

cavitation.

7.The operation of centrifugal pump issmooth and without much noise. Themaintenance cost is low.

The operation of reciprocating pump iscomplicated and with much noise. Themaintenance cost is high.

8.Centrifugal pump needs smaller floor areaand installation cost is low.

Reciprocating pump requires large floor areaand installation cost is high.

9. Efficiency is high. Efficiency is low.

UNIT IV INTERNAL COMBUSTION ENGINES

1. Explain the working principle of two stroke petrol engine and neat sketches.

Two stroke cycle is very widely employed where small power required for motor cycle, auto

rickshaw, scooters. These types of engines are compact in size, easy for manufacturing and simple

in operations. One drawback is there, Specific Fuel Consumption (S.F.C) is more (means fuel per

Break Horse Power (B.H.P.) per hour is more).

There are no inlet and exhaust valves as in four stroke engine but we have inlet and exhaustportsonly, due to which suction and exhaust stroke are eliminated in two stroke cycle engine. Here the

burnt exhaust gases are forced out through the exhaust port by a fresh charge of fuel which enters

the cylinder nearly at the end of working stroke through inlet port. This process is called as

Scavenging.

First stroke (suction and compression)

From bottom dead centre, when the piston moves up, it first closes the transfer port and then

exhaust port. The charge of fuel which previously entered in the cylinder is now compressed. When
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the piston is nearing the upward movement the inlet port opens and afresh charge of air-fuel mixture

from the carburetor enters the crankcase.

Second stroke (ignition, expansion and exhaust stroke)

The ignition starts due to the spark given by spark plug when the piston is nearing the completion ofits compression stroke. As a result, piston is pushed down performing the working stroke. So the

piston moves from TDC to BDC. During the downward movement it closes the inlet port and

pushes the fresh charges in the crank case towards up by means of transfer port. So the fresh up

going charges reach the combustion chamber and push the burned gases through the exhaust port.

Likewise the cycle is repeated.

2. Explain the working principle of two stroke diesel engine and neat sketches.

A two-stroke diesel engine shares the same operating principles as other internal combustion

engines. It has all of the advantages that other diesel engines have over gasoline engines. A two-

stroke diesel engine does not produce as much power as a four-stroke diesel engine; however, it

runs smoother than the four-stroke diesel. This is because it generates a power stroke each time the

piston moves downward; that is, once for each crankshaft revolution. The two-stroke diesel engine

has a less complicated valve train because it does not use intake valves. Instead, it requires a

supercharger to force air into the cylinder and force exhaust gases out, because the piston cannot do

this naturally as in four-stroke engines. The two-stroke diesel takes in air and discharges

exhaust through a system called scavenging. Scavenging begins with the piston at bottom dead

center. At this point, the intake ports are uncovered in the cylinder wall and the exhaust valve is

open. The supercharger forces air into the cylinder, and, as the air is forced in, the burned gases

from the previous operating cycle are forced out


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First stroke (suction, compression):

As the piston moves towards top dead center, it covers the intake ports. The exhaust valves closed.

Due to the piston continues upward movement, the air in the cylinder is tightly compressed As in

the four-stroke cycle diesel, a tremendous amount of heat is generated by the compression. At

the same time the fresh air get entered into the crank case through the inlet port.

Second stroke (ignition, expansion and exhaust):

At the end of the compression stroke, fuel is injected. Due to the compression the air which is

compressed there is in high temperature the burning of fuel taking place. So the force developed due

to the burned gases will push down the piston. This downward movement is taken by the crankshaft for vehicle movement. During the downward movement the piston close the inlet port and

open the exhaust port. At the same time the piston push the fresh air in the crank case towards up

through the transfer port. So the fresh air goes into the combustion chamber and pushes the burned

gases out through the exhaust port. (I.e. scavenging taking place)

Now the engine is ready for next cycle of operation.

Compression ratio

The amount of compression in an engine cylinder is a ratio of the original volume and the final

volume. A compression ratio of 2:1 means the air has been compressed to half its original volume.

A ratio of 3:1 indicates compression to one-third of the original volume. The compression ratio of a

gasoline engine varies from about 6:1 to 10:1. The compression ratio for a diesel engine varies from

about 12:1 to 24:1. The diesels higher compression ratios are necessary to create air temperatures

hot enough to ignite fuel on contact.

3. Explain the working principle of Four Stroke petrol engine and neat sketches

Generally the vehicles using petrol / gasoline engine have four strokes as they are more

efficient than two stroke engine and give complete combustion of fuel to optimum use.

The four-stroke cycle engine has four strokes namely intake (suction), compression, power, and

exhaust strokes.


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A) Suction or intake stroke:

Initially when engine is started piston moves downwards towards bottom of the cylinder

which creates low pressure at top. Due to this intake valve opens and the fuel mixture containing

petrol vapors and air are sucked in by the cylinder from the Carburetor.

B) Compression stroke:

After this the inlet valve gets closed. The piston now moves towards the top of cylinder and

compresses the fuel mixture to one tenth of its initial volume. The temperature and pressure inside

the cylinder increases due to compression caused.

C) Power stroke:

During this stroke the inlet and exhaust valve remains closed. As the piston reaches near top

position spark plug produces an electric spark. Due to this the combustion taking place inside the

cylinder of the engine. The spark produced causes explosion of fuel. The hot gases expand and

force the piston to move downwards. The piston is linked to the piston rod and the piston rod to the

crank shaft. They all move each other due to the link between them.

D) Exhaust stroke:


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In this stroke the exhaust valve remains open at the start. The piston is forced to move

upwards because of the momentum gained. This forces gases to move through the exhaust valve

into the atmosphere. Now the exhaust valve closes and the intake valve opens. Then the cycle of

operation repeated.

Carburetor

It is the heart of gasoline engine. They meter the fuel and mix it with the air in precise

proportions. Old carburetors do spark advance by measuring the difference in pressure between the

outside and inside of the carburetor. The amount of throttle advance is also measured. The engine's

remains which may be carbon monoxide or unburned hydrocarbons shows how well the carburetor

is working. In new engines, a small computer is used to calculate these parameters and control one

or more electric injectors. Most of the new cars use electronic fuel injection as it allows the engine

computer to precisely control the fuel air mixture which increases energy efficiency and reduces

pollution.

Applications:

These engines are widely used in vehicles, portable power plants to supply the power to run

pumps and other machineries on farms. Many small boats, aero planes, trucks and buses also use it.

4. Explain the working principle of Four Stroke diesel engine and neat sketches.

Suction stroke:-

In this stroke, the piston moves down from the top dead centre. As a result, inlet valve opens

and air is drawn into the cylinder. After sufficient quantity of air with pressure is drawn, suction

valve closes at the end of the stroke. The exhaust valve remains closed during this stroke.

Compression stroke:-

In this stroke, piston moves up from the bottom dead centre. During this stroke both inlet

and exhaust valve are closed. The air drawn into the cylinder during suction stroke is entrapped

inside the cylinder and compressed due to upward movement of the piston. In diesel engine, the

compression ratio used is very high as a result; the air is finally compressed to a very high pressure

and temperature. Finally the fuel is injected and the burning taking place inside the cylinder.

Working or power stroke:-

In this stroke, both inlet and exhaust valve remain closed. The hot gases (which are

produced due to ignition of fuel during compression stroke) and compressed air now expand in the

cylinder pushing the piston down and hence work is done. At the end of stroke, the piston finally

reaches the bottom dead centre.


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Exhaust stroke:-

In this stroke, the piston again moves upward. The exhaust valve opens, while inlet and fuel

valve are closed. The upward movement of the piston pushes the gases out through the exhaust

valve. And the engine ready for next cycle.

5. Differentiate 4 stroke engine with 2 stroke engine

S.No Four stroke engine Two stroke engine

1Two revolution of flywheel having

one power stroke.

One revolution of flywheel having one power

stroke.

2 Valves are provided. Ports are provided.

3 Water cooling arrangement. Air cooling arrangement.

4 More space is required. Less space is required.

5 Heavier flywheel is required. Lighter flywheel is required.

6 Thermal efficiency is more. Thermal efficiency is less.

7 Volumetric efficiency is more. Volumetric efficiency is less.

8 Less fuel consumption. More fuel consumption.


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9 Requires more lubrication. Not essential.

10 Torque is not uniform. Torque is uniform.

6. Differentiate petrol engine with diesel engine

7. Discuss the working of a Benson boiler.

The presence of steam bubbles in contact with the surface of tubes seriously affects the heat

transfer from flue gases to water. By rising the boiler pressure to the critical level (220 kgf /sq.cm),the problem is overcome. This was suggested by Mark Benson in 1922.

Here they eliminate the boiler drum and send the water in a single stage. And they use two types of

evaporators (radiant evaporator, convective evaporator).

Economizer: It is used to pre heat the water to the boiler.

Radiant evaporator: They are parallel tubes in construction. This is exactly placed over the grate

area. So the water in the pipe line of radiant evaporator receives the heat energy from the

combustion of fuel in the grate area. Majority of the water converted in to steam in it.

S.NO PETROL ENGINE (S.I ENGINE) DIESEL ENGINE (C.I.ENGINE)

1 Operates based on Otto cycle. Operates based on diesel cycle.

2 Spark plug is used for fuel ignition. Fuel is ignited by hot compressed air.

3 Low compression ratio (6 to 8). High compression ratio (15 to 20).

4 Operates at high speed. Operates at low speed.

5 Petrol is the costlier fuel. Diesel is the less cost fuel.

6 Lighter in weight. Heavier in weight.

7 Carburetor is used. Fuel injection pump is used.

8 Fuel consumption is more. Fuel consumption is less.

9 Less vibration. More vibration.

10Petrol is the high volatile liquid, so

handling is more risk.

Diesel is less volatile liquid. So handling is

less risky.


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Convection evaporator: The remaining water in the pipe line is fully converted into wet steam in

the convection evaporator by receiving the heat energy from hot flue gases.

Convection super heater: when wet steam used to rotate the turbine blade, it will create corrosion

problems. So for removing the water vapor, they pass the steam through the super heater. Here all

the water particles are removed and the steam becomes super heated steam. This is the steam used

to rotate the blades of turbine for power production.

Super heater, Air pre heater, economizer and evaporator receive heat from flue gases.

Working

The water from the well is treated for boiler usage in the separate plant. Then its passed in

to the boiler through a pipe line. The feed pump is used to pump the water to economizer unit.

There the water slightly heated by the hot flue gases from the burned area below. Then the hot water

passes through the radiant evaporator, which is very near to the grate area. Radiant evaporator is

nothing but a series of parallel pipe lines. So the water moving time and exposure to the heating

area will become high. By this most of the water become steam in this area. Here the water in the

pipe line directly in contact with firing of coal. Next the water entered in to convective evaporator;

here the remaining water is converted into saturated steam. For avoiding corrosion problem from

the usage of wet steam, they send the water through super heater. Here the water out put becomes

super heated steam. This is the steam required to rotate the blades of the turbine.

Advantages

1.

Smaller in size.2. Less weight.3. Simple in design compared to la mount boiler.

8. Explain the working principle of La-Mont High Pressure boilers

La mount boiler is a modern high pressure boiler working on forced circulation system. A

forced circulation boiler was introduced by la mount in the year of 1925 which is used in Europe

and America.

Feed water pump : Thepump supplies the water to the economizer.


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Economizer : This device used to preheat the water before to drum.

Drum : This is for storing water and also steam.

Circulating pump : Centrifugal pump is used to pump the water to evaporator unit.

Evaporator : It is used to convert the hot feed water to wet steam.

Super heater : The moisture in the wet steam will corrode the turbine blades,

So removing the water vapor in steam we pass the wet steam through

the super heater.

Air pre heater :Its used to preheat the air by means of flue gases from burned area.

Working

Feed water is pumped to the boiler drum by the feed pump through the economizer.

Economizer preheats the incoming water to the boiler drum. Then using the centrifugal pump the

hot water is pumped to evaporator unit. Evaporator used to convert the water from water to wet

steam level by means of burning the coal in the grate area. Then the wet steam is passed through the

super heater to avoid water vapors in the steam content. This is steam required to rotate the blades

of the turbine unit. The economizer and super heater get the heat energy from flue gases.

Advantages

1. Output of the boiler steam is at high pressure.

2. Usage of air pre heater and economizer increase the efficiency of the boiler.

3. Small in size.

4. Forced circulation of water increase the boiler efficiency and heat transfer rate.

Disadvantages

1. Ash removal problem is high it is because of its compact size.2. Salt sedimentation and cleaning problem.3. Ash and salt deposition decrease the boiler efficiency.

9. Explain the working principle of Babcock and Wilcox boilers (high pressure boiler) and

neat sketches.


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This kind of boiler producing steam at a pressure of more than 80 bar. Mostly water tube

boilers are preferred for high pressure steam production.

Its a horizontal, externally fired, high pressure, water tube boiler.

PartsShell

Its a horizontal water filled drum. Halfportion is filled by water and the remaining is for steam

collection.

Water tubes

Number of water tubes is placed between the drum and furnace with the inclination of 10 to 15

degree. This arrangement is for increasing the water circulation at the time of operation.

Uptake header and down comer

The water tubes are connected to the drum by small pipes at one end is called uptake header

by long pipes at the other end is called down comer.

Furnace

This is the place of burning for the coal. The coal is transferred from the storage area to

furnace by means of conveyers. Fire door is the entrance for the burning area.

Baffles

Baffles are the part used to redirect the flue gases around the water tubes more time. This is

done for increasing better heat transfer.

Mud box

This is placed at the bottom of the entire arrangement. The clay like impurities are collected

in the mud box. And send out through the blow off cock arrangement.


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Inspection doors

This is the passage for repair and inspection work for the labors.

Working

Coal from the storage area is transferred to the furnace area by means of conveyor. The

burned gases from the combustion are goes around the water tubes; here the baffle plates are used to

deflect the hot gases more time around the water tubes. This will increase the heat transfer

effectively from flue gas to water. The steam produced due to heating of water is stored in the boiler

drum. The uptake header and uptake header are used to increase the steam and water circulation in

the tubes to the drum. The collected steam passes through the super heater for further processing.

10. Draw and explain any 4 boiler mountings?

For having smooth and safety operation some of the equipments are very essential in

running of boiler unit.

They are,

1. Water level indicator,2. Pressure gauge,3. Fusible plug,4. Safety valve.

Water level indicator

Its used to show the level of water in the

boiler drum. The indicator made up of glass

material to show the water level.

And it has inlet and outlet as shown in

figure. There are some readings in the body.

This equipment helps to avoid the over

heating of boiler when the level of water

become less in the drum.

Pressure gauge

Its mainly used to indicate the steam pressure inside the boiler. This is made up of bronze

alloy. The pressure gauge consists of a curved metallic tube (bourdon tube) of elliptical cross

section. One of its ends is connected with toothed sector by a link and the other end is open and is

connected to the steam space of the boiler. There is a pointer which is connected to the sectorarrangement.

When the steam enters into the tube, the elliptical structure tries to expand into a circle. This

causes the free end of the tube to move. This effect is transfer to through the links and the pointer

shows some reading in the gauge.


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Fusible plug

Fusible plug used to put off the fire in the boiler furnace, when the water level below on

unsafe limit. It prevents the overheating of tubes and shell thereby avoiding explosion of the boiler.

It consists of gun metal body with a hexagonal flange screwed with the firebox crown plate. A

hollow gun metal plug is screwed into the gun metal body. Another copper plug is locked with the

hollow plug by a low melting point fusible metal (tin or lead).

The fusible plug is submerged in water in water during the normal operation. When the

water level falls below the safe limit, the fusible plug is uncovered from water. The plug gets over

heated and the fusible metal melts by the heat of the furnace. The copper plug drops down and the

water from the put off the fire. Gun metal body prevents the plug from falling into the furnace.

Lever safety valve

Safe valves are used to maintain a constant

safe pressure inside the boiler. When the pressure

inside the boiler exceeds the safe limit, the valve

automatically relieves the excess pressure.

In its body, it has lever, valve seat, valve,

fulcrum and hinges.

The required operating pressure level of

weight is added on the lever end. When the pressure


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of steam inside the boiler exceeds the normal level, the valve lifted up and the excess pressure of

steam released out.

11. Draw and explain any 4 boiler accessories?

These are the devices used to increase the efficiency of the boiler and also for proper working of

the boiler.

1. Economizer2. Air preheater3. Super heater4. Steam separator

Economizer

This is the device used to pre heat the

water before it goes into the boiler. Water

enters in the lower portion and leaves from

the upper portion to boiler. The hot flue gases

from the boiler used to pre heat the water. By

doing these kind of operations we increase the

boiler efficiency.

Air preheater

The air taken from the atmosphere for

burning of coal in the furnace is sent through a

pre heating arrangement for increasing the boiler

efficiency. Hot air passed in the lower portion of

the unit and leaves out from the top outlet as

shown in figure. The hot flue gases from the

burning area heats the pipe line of the atmosphere

air. Baffle plates are used to deflect the hot air

more time around the tubes. So better heat

transfer takes place in the arrangement. This will increase the efficiency of the boiler.

Super heater

This is the level of steam require to

rotate the blades of the turbine. The structure

of the unit has many tubes. The hot saturated

steam from the evaporator is heated once again

up to super heated level (removing of all water

vapors in the steam content) by means of flue

gases from the burning of coal in the grate

area.


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Steam seperator

This is the part used to separate water particles from steam before it is supplied to a steam

engine or turbine. It prevents the damaging of turbine blades

due to moisture present in steam. It is located in the supplynear the turbine or engine.

It has cylindrical container with baffle plates as shown in

figure. A water gauge is fitted to indicate the water collected

in the separator. A drain cock is provided at the bottom of

the separator to drain away the separated water.

During the operation the steam strikes the baffle plates

of the separator and the direction changed. As a result,

heavier water particles in the steam falls down to the bottom of the separator. The separated steam

is free from water particles. It is passed to the turbine through the outlet pipe.


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1. Explain with neat sketch the working of a vapour compression refrigeration system.

Vapor-compression refrigeration is one of the many refrigeration cycles available for use. It

has been and is the most widely used method forair-conditioning of large public buildings, private

residences, hotels, hospitals, theaters, restaurants and automobiles. It is also used in domestic and

commercial refrigerators, large-scale warehouses for storage of foods and meats, refrigerated trucks

and railroad cars, and a host of other commercial and industrial services. Oil refineries,

petrochemical and chemical processing plants, and natural gas processing plants are among the

many types of industrial plants that often utilize large vapor-compression refrigeration systems.

It consists of the following five essential parts

1. Compressor.

2. Condenser.

3. Receiver.

4. Expansion valve or Throttle valve.

5. Evaporator.

1. Compressor

The main function of compressor is to increase the pressure and temperature of the vapour

refrigeration above atmosphere. The compressor used here is reciprocating type. However for very

big plant, centrifugal compressor directly coupled with high speed rotating engines (gas turbine) are

used.

2. Condenser

The condenser consists of coils of tubes made of copper in which the high pressure and

temperature refrigerant is condensed into liquid refrigerant. The refrigerant, while passing through

the condenser gives up its latent heat to the surrounding condensing medium which is normally air

or water.
http://en.wikipedia.org/wiki/Refrigeration_cyclehttp://en.wikipedia.org/wiki/Air_conditioninghttp://en.wikipedia.org/wiki/Oil_refineryhttp://en.wikipedia.org/wiki/Petrochemicalhttp://en.wikipedia.org/wiki/Chemical_planthttp://en.wikipedia.org/wiki/Natural_gas_processinghttp://en.wikipedia.org/wiki/Natural_gas_processinghttp://en.wikipedia.org/wiki/Chemical_planthttp://en.wikipedia.org/wiki/Petrochemicalhttp://en.wikipedia.org/wiki/Oil_refineryhttp://en.wikipedia.org/wiki/Air_conditioninghttp://en.wikipedia.org/wiki/Refrigeration_cycle


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3. Receiver

The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver

from where it is supplied to the evaporator through the expansion valve.

4. Expansion valve or Throttle valve

It is also called throttle valve or refrigerant control valve. Its function is to allow the liquid

refrigerant under high pressure and temperature to pass at a controlled rate after reducing its

pressure and temperature (i.e. expansion in the throttle valve) into the evaporator.

5. Evaporator

The evaporator is a coil of tubes made of copper and it is kept in the space to be cooled.

When the low pressure and temperature liquid refrigerant flows through the evaporator picks up

heat from the space which is to be cooled and become fully vapourised .

Working principle

Figure shows the basic components of a vapour compression refrigeration system. As

shown in the figure the basic system consists of an evaporator, compressor, condenser and an

expansion valve. The refrigeration effect is obtained in the cold region as heat is extracted by the

vaporization of refrigerant in the evaporator. The refrigerant vapour from the evaporator is

compressed in the compressor to a high pressure at which its saturation temperature is greater than

the ambient or any other heat sink. Hence when the high pressure, high temperature refrigerant

flows through the condenser, condensation of the vapour into liquid takes place by heat rejection to

the heat sink. To complete the cycle, the high pressure liquid is made to flow through an expansion

valve. In the expansion valve the pressure and temperature of the refrigerant decrease. This low

pressure and low temperature refrigerant vapour evaporates in the evaporator taking heat from the

cold region. It should be observed that the system operates on a closed cycle. The system requires

input in the form of mechanical work. It extracts heat from a cold space and rejects heat to a high

temperature heat sink. A refrigeration system can also be used as a heat pump, in which the useful

output is the high temperature heat rejected at the condenser. Alternatively, a refrigeration system

can be used for providing cooling in summer and heating in winter. Such systems have been built

and are available now.

2. Draw neat sketch of vapour absorption refrigeration system and explain its working

principle.As the name implies, absorption refrigeration systems involve the absorption of a refrigerant

by a transport medium. The most widely used absorption refrigeration system is the ammoniawater

system, where ammonia (NH3) serves as the refrigerant and water (H2O) as the transport medium.

Other absorption refrigeration systems include waterlithium bromide and waterlithium chloride

systems, where water serves as the refrigerant.

It consists of the following components.

1. Absorber 2. H eat exchanger

3. Generator 4. Pump


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5. Condenser 6. Receiver

7. Expansion valve 8. Evaporator

1. Absorber

The absorber contains weak ammonia solution. Its function is to absorb the low pressure

vapour refrigerant from the evaporator into this solution. The water in the absorber has the ability to

absorb very large quantity of ammonia vapor and the solution thus formed is known as aqua-

ammonia. Some form of cooling arrangement (usually water cooling) is employed in the absorber

to remove the heat evolved during absorption.

2. Heat exchanger:

The function of the heat exchanger is to transfer heat to the strong ammonia solution from

the hot weak solution of ammonia returning from generator.

3. Pump:

The strong ammonia solution formed in the absorber is pumped to the generator by the

liquid pump. The pump increases the pressure of the solution up to 10 kg/m2.

4. Generator:

The heat is added to the generator from an external source to heat the ammonia solution. It

may be a gas burner, electric heater or steam passing through the coil.

5. Condenser:

The condenser consists of coils of tubes made of copper in which the high pressure andtemperature vapour refrigerant is condensed into liquid refrigerant. The refrigerant, while passing

through the condenser gives up its latent heat to the surrounding condensing medium which is

normally air or water.

6. Receiver:

The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver

from where it is supplied to the evaporator through the expansion valve.


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7. Expansion valve:

It is also called as throttle value or refrigerant control valve. Its function is to allow the

liquid refrigerant under high pressure and temperature to pass at a controlled rate after reducing its

pressure and temperature (i.e. expansion in the throttle valve) into the evaporator.

8. Evaporator:

The evaporator is a coil of tubes made of copper and it is kept in the space to be cooled.

When the low pressure and temperature liquid refrigerant flows through the evaporator picks up

heat from the space which is to be cooled and become fully vaporized.

Working principle:

Dry ammonia vapour at low pressure passes in to the absorber from the evaporator. In the

absorber, the dry ammonia vapor is dissolved in cold water and strong solution ammonia is formed.

Heat evolved during absorption of ammonia is removed by circulating cold water through coils kept

in the absorber.

The highly concentrated ammonia solution (known as strong solution or aqua-ammonia) is

then pumped by a pump to the generator through a heat exchanger. In the heat exchanger, the strong

ammonia solution is heated by the hot weak solution returning from the generator to the absorber. In

the generator, the warm solution is further heated by steam coils, gas or electricity and the ammonia

vapour is driven out of solution. The boiling point of ammonia is less than that of water. Hence, the

vapours leaving the generator are mainly of ammonia. The weak ammonia solution is returned to

the absorber through the heat exchanger.

Ammonia vapours leaving the generator may contain some water vapour. If this water

vapour is allowed directly to the condenser and expansion valve, it may feeze resulting in chocked

flow. Analyzer and rectifiers are incorporated in the system before the condenser. The ammonia

vapour from the generator passes through a series of trays in the analyzer and ammonia is separated

from wat

Basic Mech- Qn Bank - Part B

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