Basic Mechanical Engineering

Unit 1) Introduction to Mechanical Engineering

Important Sub branches of Mechanical Engineering

a) Thermal Engineering
b) Manufacturing
c) Design Engineering
d) Automobile Engineering
e) Material Science Engineering
f) Computer Aided Design and Manufacturing (CAD/CAM)
g) Mechatronics

I. What is Mechanical Engineering?

Before entering into the Mechanical Engineer role, let us first understand the concept of
Mechanical Engineering. Mechanical Engineering is defined as the branch of engineering that
deals with the design, development, construction, and operation of mechanical systems and tools.
It include machines, tools, and equipment used in various industries, such as transportation,
manufacturing, power generation, and medical devices etc.
II. What role does a Mechanical Engineer play in our society and in Industries? Mechanical
engineers are involved in almost every aspect of human existence and welfare, including
machines, cars and other vehicles, aircraft, power plants, automobile parts, and manufacturing
plants etc. A Mechanical Engineer plays a significant role in designing, developing, and testing
machines as well as thermal devices. It also includes systems that are essential to many aspects of
modern society and Industries. They use their knowledge of mechanics, thermodynamics,
materials science, and energy to create solutions that improve the quality of life of people.
Besides, the role of a mechanical engineer in our society is contributed as: 1.Power Generation:
Mechanical engineers design and develop power-generating machines such as internal combustion
engines, gas turbines, and steam and wind turbines etc.
2. Heating and Cooling Systems: They design and develop heating, ventilation, refrigeration and
air conditioning systems for buildings and other structures. 3.Transportation: Mechanical
engineers are involved in designing and developing transportation systems, including cars, trains,
airplanes, steamers and boats. 4.Industrial Equipment: They design, develop and maintain
industrial equipment such as machine tools, robots, and conveyor systems & belts.
5.Infrastructure: Mechanical engineers play a key role in the design and maintenance of
infrastructure, including buildings, bridges, roads, and transportation systems
. Overall, Mechanical Engineers are involved in designing, building, and maintaining the engines,
machines, and structures that make modern life possible and comfortable. They contribute to
society by using their skills to improve the safety, security, efficiency, and comfort of the systems
and devices that we rely on every day.

Introduction to Lows of Thermodynamics

Thermodynamics and Energy

Thermodynamics can be defined as the study of energy, energy transformations and its relation to matter. The a
nalysis of thermal systems is achieved through the application of the governing conservation equations, namely
Conservation of Mass, Conservation of Energy (1st law of thermodynamics), the 2nd law of thermodynamics and
the property relations. Energy can be viewed as the ability to cause changes.
 First law of thermodynamics:
one of the most fundamental laws of nature is the conservation of energy principle. It simply states that during an
interaction, energy can change from one form to another but the total amount of energy remains constant.

Second law of thermodynamics:

energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.
Whenever there is an interaction between energy and matter, thermodynamics is involved. Some examples inclu
de heating and air‐conditioning systems, refrigerators, water heaters, etc.

System and Its Types

A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the
system is called the surroundings.

Boundary: the real or imaginary surface that separates the system from its surroundings. The boundaries of a sys
tem can be fixed or movable. Mathematically, the boundary has zero thickness, no mass, and no volume.

Closed system or control mass: consists of a fixed amount of mass, and no mass can cross its boundary. But, en
ergy in the form of heat or work, can cross the boundary, and the volume of a closed system does not have to be
fixed.

Open system or control volume: is a properly selected region in space. It usually encloses a device that involves
mass flow such as a compressor. Both mass and energy can cross the boundary of a control volume. Important n
ote: some thermodynamics relations that are applicable to closed and open systems are different. Thus, it is extre
mely important to recognize the type of system we have before start analyzing it.

Isolated system: A closed system that does not communicate with the surroundings by any means.

Adiabatic system : A closed or open system that does not exchange energy with the surroundings by heat
Energy

In thermodynamics, we deal with change of the total energy only. Thus, the total energy of a system can be assig
ned a value of zero at some reference point. Total energy of a system has two groups: macroscopic and microsco
pic.

Macroscopic forms of energy

: forms of energy that a system posses as a whole with respect to some outside reference frame, such as kinetic an
d potential energy. The macroscopic energy of a system is related to motion and the influence of some external ef
fects such as gravity, magnetism, electricity, and surface tension.

Kinetic energy: energy that a system posses as a result of its relative motion relative to some reference frame,

where V is the velocity of the system in (m/s). kJ

Potential energy: is the energy that a system posses as a result of its elevation in a gravitational field,

where g is the gravitational acceleration and z is the elevation of the center of gravity of the system relative to so
me arbitrary reference plane.

Microscopic forms of Energy are those related to molecular structure of a system. They are independent of outs
ide reference frames. The sum of microscopic energy is called the internal energy, U.

The total energy of a system consists of the kinetic, potential, and internal energies:

where the contributions of magnetic, electric, nuclear energy are neglected.

Internal energy is related to the molecular structure and the degree of molecular activity and it may be viewed as t
he sum of the kinetic and potential energies of molecules.

Properties of a System

Any characteristic of a system is called a property. In classical thermodynamics, the substance is assumed to be a
continuum, homogenous matter with no microscopic holes. This assumption holds as long as the volumes, and le
ngth scales are large with respect to the intermolecular spacing.

Intensive properties are those that are independent of the size (mass) of a system, such as temperature, pressure
, and density. They are not additive.

Extensive properties values that are dependant on size of the system such as mass, volume, and total energy U.
They are additive.

Generally, uppercase letters are used to denote extensive properties (except mass m), and lower case letters are us
ed for intensive properties (except pressure P, temperature T).

Extensive properties per unit mass are called specific properties, e.g. specific volume (v=V/m).
State and Equilibrium

At a given state, all the properties of a system have fixed values. Thus, if the value of even one property changes,
the state will change to different one.

In an equilibrium state, there are no unbalanced potentials (or driving forces) within the system. A system in equi
librium experiences no changes when it is isolated from its surroundings.

Thermal equilibrium : when the temperature is the same throughout the entire system.

Mechanical equilibrium: when there is no change in pressure at any point of the system. However, the pressure
may vary within the system due to gravitational effects.

Phase equilibrium:an equilibrium level. in a two phase system, when the mass of each phase reaches

Chemical equilibrium: when the chemical composition of a system does not change with time, i.e., no chemical
reactions occur.

Processes and Cycles

Any change a system undergoes from one equilibrium state to another is called a process, and the series of states t
hrough which a system passes during a process is called a path.

Heat engine

Any engine that converts thermal energy to mechanical work output. Examples: steam engine, diesel engine, and
gasoline (petrol) engine.

Heat engine can be classified On the basis of how thermal energy is being delivered to working fluid.

1. Internal Combustion engine ; an engine combustion of working fluid takes place inside the engine
cylinder. i.e Petrol engine, Diesel engine, Rocket engine etc
2. External Combustion engine ; an engine combustion of working fluid takes place outside the engine
cylinder. i.e Steam engine etc
Classification of Internal Combustion engine
a) On the basis of ignition ; S.I engine, C.I engine
b) On the basis of strokes ; 2-Stroke engine, 4-Stroke engine
c) On the basis of engine design ;Reciprocating engine, Rotary engine(single rotor and multi rotor)
d) On the basis of cylinders ; Single Cylinder engine, Multi cylinder engine
e) On the basis of working cycles ; otto cycle engine, diesel cycle engine, dual cycle engine f) On the
basis of ignition ; Spark ignition(S.I) engine, Compression ignition(C.I) engine
g) On the basis of Valve/ Port design ; Poppet Valve, Rotary Valve, Reed Valve, Piston Controlled
Porting
h) On the basis of Valve Location ; The T-head, The L-head, The F-head, The I-head(Over head Valve
(OHV), Over head Cam (OHC))
i) On the basis of Fuel ; Petrol engine, Diesel engine, others(Gasoline, CNG, LPG etc) engine j) On the
basis of Cooling ; Direct Air-cooling, Indirect Air-cooling (Liquid Cooling)

IC engine Terminology
 Bore (D); The nominal inside diameter of the engine cylinder.
Stroke(L); The maximum distance travelled by the piston from TDC to BDC. It is equal to twice the
radius of the crank. L = 2r
Dead centre; The extreme positions of piston.it is of two types.
i. Top Dead Centre (TDC); The extreme position of the piston at the top of the cylinder of the
vertical engine is called top dead centre (TDC), Incase of horizontal engines. It is known as inner
dead centre (IDC).
ii. Bottom Dead Centre (BDC); The extreme position of the piston at the bottom of the cylinder of
the vertical engine called bottom dead centre (BDC). In case of horizontal engines, it is known as
outer dead center (ODC).
Crank throw(r)/ Crank radius; it is central distance between crank pin to center of crank shaft.

Compression ratio; Compression ratio is a ratio of the Total volume when the piston is at bottom dead
centre to the Clearance volume when the piston is at top dead centre.
The compression ratio varies from 5 : 1 to 10 : 1 for petrol engines and from 12:1 to 22 : 1 for diesel
engines.

Piston Displacement(Swept Volume) Vs; The volume swept by the piston during one stroke i.e while
moving from TDC to BDC.
Petrol engine is also known as Spark Ignition (SI) engine.
Invented by Nicolaus A. Otto in 1876 that is why petrol engine is also known as Otto engine. Since
ignition occurs due to a spark petrol engines are called spark ignition (SI) engines.
A four stroke engine gives a power stroke in every set of four strokes of the piston or two revolution of the
crankshaft.
The petrol engine operates on theoretical Otto cycle.
It is also called as constant volume combustion cycle as the combustion takes place at constant volume with
increase of pressure.
The cycle of operation of a four stroke petrol engine consists of the following strokes:
i. Suction or intake stroke
ii. Compression stroke,
iii. Expansion or power stroke, and
iv. Exhaust stroke.
i. Suction Stroke of 4-Stroke Petrol engine: During this stroke, the piston moves from Top Dead
Centre (TDC) to Bottom Dead Centre (BDC) creating a vaccum inside the cylinder. During this
stroke, the inlet valve is kept opened and the exhaust valve is kept closed The vacuum created
inside the cylinder draws the air petrol mixture (which is also known as charge) into the cylinder
through the inlet valve. It is performed till the piston reaches BDC. The above process is known as
suction and this stroke is called the suction stroke.
ii. Compression Stroke of 4-Stroke Petrol engine : During this stroke, both the inlet and exhaust
valves are closed. The air petrol mixture is compressed as the piston moves upwards from BDC to
TDC.As a result of this compression, pressure and temperature of the air fuel mixture or charge is
increased. Just before the piston reaches the TDC, the air petrol mixture (charge) is ignited by a
spark plug; suddenly burning of the air fuel mixture takes place almost instantaneously. It
increases the pressure and temperature inside the cylinder. Volume remains constant during
combustion. These two strokes (i.e., suction and compression stroke) complete one revolution of
the crankshaft.
iii. Expansion or Power Stroke or Working Stroke: During this stroke, both the inlet and exhaust
valves remain closed. The high pressure of the products of combustion (due to expansion of
charge) pushes tile piston from TDC to BDC. It is also called as working stroke as work is done by
the expansion of hot gases. The force above the piston is transmitted to the crankshaft through the
connecting rod and crank mechanism. Excess energy due to the combustion is stored in the
flywheel which helps for the operation of three idle strokes.
iv. Exhaust Stroke: At the end of the expansion stroke, the exhaust valve opens and the pressure
inside falls suddenly. Thus during this stroke, the inlet valve is closed and the exhaust valve is kept
opened. The upward movement of the piston from BDC to TDC, pushes out the products of
combustion from the engine cylinder through the exhaust valve into the atmosphere. The cycle of
 operation is then repeated. These two strokes (i.e., expansion and exhaust strokes) complete one
revolution of the crankshaft.

Four Stroke Diesel engine

 Diesel engine is also known as compression ignition (CI) Engine.

 It is invented by Rudolf Diesel (1892)
 The four stroke diesel engine is similar to four stroke petrol engine except that it operates at a higher
compression ratio (14 to 22).
 In a diesel engine, only air is sucked from the atmosphere instead of air fuel mixture during the suction
stroke.
 In diesel engines, spark plug is not required for igniting the air fuel mixture. Because the fuel is injected
and forms an explosive mixture, which ignites spontaneously under pressure.
 Diesel engine works on the principle of diesel cycle.
 It is also called as constant pressure combustion cycle as the combustion of fuel takes place at constant
pressure with increase of temperature.
 Since ignition results due to high temperature of compressed air, these are called compression ignition
(CI) engines.
The cycle of operation of a four stroke diesel engine consists of the following strokes:
 Suction or intake stroke,
 Compression stroke,
 Expansion or power stroke, and
 Exhaust stroke.

1. Suction Stroke: During suction stroke, the inlet valve opens and the exhaust valve closes. The piston
moves from TDC to BDC. This piston movement reduces the pressure inside the cylinder below the
atmospheric pressure. Due to the pressure difference, the fresh air is sucked into the cylinder through the
inlet valve.

2. Compression Stroke: During this stroke, both the inlet and exhaust valves are closed. The air in the
cylinder is compressed as the piston moves upwards from BDC to TDC.As a result of this compression,
pressure and temperature of the air is increased. Just before the piston reaches the TDC, the diesel is
injected into the cylinder in the form of a fine spray. The fuel gets vaporized and self ignited due to the
heat of compressed air. The fuel burns instantaneously at constant pressure.

3. Expansion or Power Stroke: During this stroke, both inlet and exhaust valves are closed. The
combustion of fresh fuel injected into the cylinder is due to the high pressure and temperature developed
during compression stroke. The fuel is continuously injected for 20% of the expansion stroke. The high
pressure of the combustion products due to expansion of charge pushes piston from TDC to BDC. It is
also called as working stroke as work is done by the expansion of hot gases.

4. Exhaust Stroke: During this stroke, inlet valve is closed and the exhaust valve is opened The piston
moves from BDC to TDC. The burnt waste gases are sent out through exhaust valve and the cycle is
repeated.
First Stroke (Upward Stroke of the Piston) of 2-Stroke Petrol engine
(a) Compression and Inductance: During the upward movement of the piston from BDC to TDC, both
the transfer and exhaust ports are covered by the piston.The petrol air mixture which is already transferred
into the engine cylinder is compressed by the moving piston. Thus, the pressure and temperature of the
charge increases at the end of compression.The compression process is continued until the piston reaches
TDC.At the same time, the inlet port is uncovered by the moving piston and the fresh petrol air mixture
enters the crankcase through the inlet port.
(b) Ignition and Inductance: After the piston almost reaches the TDC, the compressed petrol air mixture
is ignited by means of an electric spark produced by a spark plug. The admission of fresh charge into the
crankcase continues till the piston reaches the TDC.

Second Stroke (Downward Stroke of the Piston) of 2-Stroke Petrol engine

(c) Expansion and Crankcase Compression: The ignited gases expand and forces the piston to move
down, thus useful work is obtained. When the piston moves down, the petrol air mixture is partially
compressed in the crankcase. Thus compression is known as crankcase compression.
(d) Exhaust and Transfer: Almost at the end of expansion, the exhaust port is uncovered and the
combustion products escape to the atmosphere. Immediately, the transfer port is also uncovered and the
partially compressed air fuel mixture from the crankcase enters the cylinder through transfer port. The
crown of the piston is made of a deflected shape, so the fresh air – petrol mixture entering the cylinder is
deflected upward in the cylinder. Thus the escape of fresh charge along with the exhaust gases is reduced.
The cycle of operations are then repeated.
First Stroke (Upward Stroke of the Piston) of 2-Stroke Diesel(C.I) engine
(a) Compression and Inductance: During the upward movement of the piston from BDC to TDC, both
the transfer and exhaust ports are covered by the piston.The air which is already transferred into the
engine cylinder is compressed by the moving piston. This increases the pressure and temperature of the
air.The compression process is continued until the piston reaches TDC. At the same time, the inlet port is
uncovered by the moving piston and the fresh air enters the crankcase through the inlet port.
(b) Injection and Inductance: After the piston almost reaches the TDC, the fuel (diesel) is injected
through the fuel injector in the cylinder.The combustion of fresh fuel injected into the cylinder takes place
due to the high temperature already developed in the cylinder during compression of the air.The
admission of fresh air into the crankcase continues till the piston reaches the TDC.

Second Stroke (Downward Stroke of the Piston) of 2-Stroke Diesel(C.I) engine

(c) Expansion and Crankcase Compression: The burnt gases expand and forces the piston to move
down, thus useful work is obtained. When the piston moves down, the air is partially compressed in the
crankcase. This compression is known as crankcase compression.
(d) Exhaust and Transfer: Nearly at the end of expansion, the exhaust port is uncovered and the
combustion products escape to the atmosphere. Immediately the transfer port is also uncovered and the
partially compressed air from the crankcase enters the cylinder through the transfer port. 12 The cycle of
the operations are then repeated.
 Power Plants:

Thermal/Steam Power plant

COMPONENTS • High pressure boiler • Prime mover • Condensers and cooling towers • Coal handling
system • Ash and dust handling system • Draught system • Feed water purification plant • Pumping system • Air
Pre-heater, Economizer, Super Heater, Feed Heaters.

High pressure Boiler: Steam generator is a device or equipment which burns the fuel and facilitates the
exchange of heat produced to the water to generate required quantity and quality of steam. Thus, it is a heat
exchanger which has the place for burning of fuel and flow of hot flue gases produced and also has space for
storing of water and steam. As steam is produced & stored at high pressure than the atmospheric pressure, steam
generator is also a pressure vessel. To handle the hot flue gases and to keep high pressure steam, certain other
mountings and accessories are also required for its safe and efficient operation. In this way steam generator is
not simply a vessel to boil water but it is a complete unit performing the complete task of producing & handling
the high-pressure steam by burning of the fuel and exhausting the flue gases efficiently and safely. Most of the
boilers are actually a type of shell & tube type heat exchangers
COAL DELIVERY: The coal from supply points is delivered by ships or boats to power stations situated near
to sea or river whereas coal is supplied by rail or trucks to the power stations which are situated away from sea
or river. The transportation of coal by trucks is used if the railway facilities are not available.
Super heater: WORKING Steam stop valve is opened. The steam (wet or dry) from the evaporator drum is
passed through the super heater tubes. First the steam is passed through the radiant super heater and then to the
convective super heater. The steam is heated when it passes through these super heaters and converted into
superheated steam.This superheated steam is supplied to the turbine through a valve
Condenser: Working of condensers In a jet condenser, the steam to be condensed and the cooling water come
in direct contact and the temperature of the condensate is the same as that of the cooling water leaving the
condenser? For jet condensers the recovery of the condensate for reuse as boiler feed water is not possible.
Depending upon the arrangement of the removal of condensate, the jet condensers are subdivided in to the
following categories.
COOLING TOWERS : A cooling tower is a semi-enclosed device for evaporative cooling of water by contact
with air. It is a wooden, steel, concrete structure and corrugated surfaces or troughs or baffles or perforated trays
are provided inside the tower for uniform distribution and better atomization of water in the tower. The hot
water coming out from the condenser are fed to the tower on the top and allowed to tickle in the form of thin
sheets or drops. The air flows from the bottom of the atmosphere after effective cooling. An evaporative cooling
tower is a machine of relatively simple conception and operation. The water to be cooled for a chiller, industrial
process or refrigeration installation is pumped and distributed through spray nozzles over a fill pack or heat
exchange surface through which passes an air current commonly generated buy a fan. A small fraction of this
water evaporates and the remainder is cooled thanks to the absorption of latent heat of evaporation by the
passing air, and fall under gravity into a basin from there it is pumped back to the heat load source

A generating station in which diesel engine is used as the prime mover for the generation of electrical energy is
known as diesel power station. In a diesel power station, diesel engine is used as the prime mover. The diesel
burns inside the engine and the products of this combustion act as the working fluid to produce mechanical
energy. The diesel engine drives alternator which converts mechanical energy into electrical energy. As the
generation cost is considerable due to high price of diesel, therefore, such power stations are only used to
produce small power. Although steam power stations and hydro-electric plants are invariably used to generate
bulk power at cheaper costs, yet diesel power stations are finding favour at places where demand of power is
less, sufficient quantity of coal and water is not available and the transportation facilities are inadequate. This
plants are also standby sets for continuity of supply to important points such as hospitals, radio stations, cinema
houses and telephone exchanges.
Advantages
(a) The design and layout of the plant are quite simple.
(b) It occupies less space as the number and size of the auxiliaries is small.
(c) It can be located at any place.
(d) It can be started quickly and it can pickup load in a short time.
(e) There are no standby losses.
(f) It requires less quantity of water for cooling.
(g) The overall cost is much less than that of steam power station of same capacity.
(h) The thermal efficiency of the plant is higher than that of a steam power station.
(i) It requires less operating staff
Disadvantages
(a) The plant has high running charges as the fuel (diesel) used is costly.
(b) The plant doesn’t work satisfactorily under overload conditions for a longer period.
(c) The plant can only generate small power.
(d) The cost of lubrication is generally high.
(e) The maintenances charges are generally high

HYDRO ELECTRIC POWER PLANT: Water reservoir Continuous availability of water is the basic
necessity for a hydro- electric plant. Water collected from catchment area during rainy season is stored in the
reservoir. Water surfaces in the storage reservoir us known as head race. Dam The function of a dam is to
increase the height of water level behind it which ultimately increases the reservoir capacity. The dam also helps
to increase the working heat of the power plant.

Spillway :Water after a certain level in the reservoir overflows through spillway without allowing the increase
in water level in the reservoir during rainy season.
Pressure tunnel :It carries water from the reservoir to surge tank. Penstock: Water from surge tank is taken to
the turbine by means of penstocks, made up of reinforced concrete pipes or steel.
Surge tank :There is sudden increase of pressure in the penstock due to sudden backflow of water, as load on
the turbine is reduced. The sudden rise of pressure in the penstock is known as water hammer. The surge tank is
introduced between the dam and the power house to keep in reducing the sudden rise of pressure in the
penstock. Otherwise, penstock will be damaged by the water hammer.
Water turbine :Water through the penstock enters into the turbine through and inlet valve. Prime movers
which are in common use are Pelton turbine, Francis turbine and Kaplan turbine. The potential energy of water
entering the turbine is converted into 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. Draft tube It is connected
to the outlet of the turbine. It allows the turbine to be placed over tail race level.
Tail race: Tail race is a water way to lead the water discharged from the turbine to the river. The water held in
the tail race is called tail race water level.
Power house: The power house accommodates the turbine, generator, and transformer and control room

NUCLEAR POWER PLANT: Basics Atoms consist of nucleus and electrons. The nucleus is composed of
protons and neutrons. Protons are positively charged whereas neutrons are electrically neutral. Atoms with
nuclei having same number of protons but difference in their masses are called isotopes. They are identical in
terms of their chemical properties but differ with respect to nuclear properties.

Energy from Nuclear Reactions: The sum of masses of protons and neutrons exceeds the mass of the atomic
nucleus and this difference is called mass defect ∆m. In a nuclear reaction the mass defect is converted into
energy known as binding energy according to Einstein’s equation (E=∆m c2). Fissioning one amu of mass
results in release of 931 MeV of energy. It has been found that element having higher and lower mass numbers
are unstable. Thus the lower mass numbers can be fused or the higher mass numbers can be fissioned to produce
more stable elements. This results in two types of nuclear reactions known as fusion and fission. The total
energy per fission reaction of U235 is about 200 MeV. Fuel burn-up rate is the amount of energy in MW/days
produced by each metric ton of fuel.
Nuclear Fission When unstable heavy nuclei are bombarded with high energy neutrons, it splits into several
smaller fragments. These fragments, or fission products, are about equal to half the original mass. This process
is called Nuclear Fission. Two or three neutrons are also emitted. The sum of the masses of these fragments is
less than the original mass. This missing mass (about 0.1 percent of the original mass) has been converted into
energy. Fission can occur when a nucleus of a heavy atom captures a neutron, or it can happen spontaneously.
Nuclear Fusion Fusion :is the opposite of fission, it is the joining together of two light nuclei to form a heavier
one (plus a small fragment). For example if two 2H nuclei (two deuterons) can be made to come together they
can form He and a neutron NUCLEAR POWER REACTORS A nuclear reactor produces and controls the
release of energy from splitting the atoms of elements such as uranium and plutonium. In a nuclear power
reactor, the energy released from continuous fission of the atoms in the fuel as heat is used to make steam. The
steam is used to drive the turbines which produce electricity (as in most fossil fuel plants).
There are several components common to most types of reactors: Fuel Usually pellets of uranium oxide (UO2)
arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core. Moderator
This is material which slows down the neutrons released from fission so that they cause more fission. It is
usually water, but may be heavy water or graphite.
Control Rods These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are
inserted or withdrawn from the core to control the rate of reaction, or to halt it. (Secondary shutdown systems
involve adding other neutron absorbers, usually in the primary cooling system.)
Coolant : A liquid or gas circulating through the core so as to transfer the heat from it. In light water reactors the
moderator functions also as coolant.
Pressure Vessel or Pressure Tubes Usually a robust steel vessel containing the reactor core and
moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the
moderator.
Steam Generator Part of the cooling system where the heat from the reactor is used to make steam for the
turbine.
Reflectors Some of the neutrons produced during fission will be partly absorbed by the fuel elements,
moderator, coolant and other materials. The remaining neutrons will try to escape from the reactor and will be
lost. Such losses are minimized by surrounding (lining) the reactor core with a material called a reflector which
will reflect the neutrons back to the core. They improve the neutron economy. Economy: Graphite, Beryllium.
Shielding During Nuclear fission σ α γ particles and neutrons are also produced. They are harmful to human
life. Therefore it is necessary to shield the reactor with thick layers of lead, or concrete to protect both the
operating personnel as well as environment from radiation hazards.