B.Tech
First
Semester Examination
Basics
of Mechanical Engineering (ME-101F)
Q. 1. (a) Differentiate between water tube boiler and
fire tube boiler.
Ans. Comparison between 'Fire-tube and Water tube' Boiler:
Things
|
Water-tube boiler
|
Fire-tube boiler
|
1) Position of water and hot gases
|
Hot gases are inside the tubes and water outside the tubes.
|
Water is inside the tubes and hot gases outside the tubes.
|
2) Mode of firing
|
Generally internally fired.
|
Externally.
|
3) Rate of steam production
|
Lower.
|
Higher.
|
4) Operating pressure
|
Operating pressure limited to 16 bar.
|
Can work under as high pressure as 100 bar.
|
5) Suitability
|
Not suitable for large power plants.
|
Suitable for large power plants.
|
6) Risk of bursting
|
Lesser risk of bursting due to lower working pressure.
|
Higher risk of bursting due to high pressure.
|
Q. 1. (b) What
do you mean by boiler mountings and accessories? Discuss any five Accessories
in detail with neat sketch. 10
Ans. Mountings : The items such as stop valve.
Safety valves, water level gauges, fusible plug,
blow-off cock, pressure gauges. Water level
indicator etc. are termed as mountings and a boiler cannot
work safely without them.
Accessories : The items such
as super heaters, economiser, feed pumps etc. are termed as
accessories and they form integral
part of the boiler. They increase the efficiency of the boiler. The following
accessories are discussed :
Economiser : An economiser
is a device in which the waste heat of the flue gases is utilised for
heating the feed water.
Injector : The function of an injector is
to feed water into the boiler. It is commonly employed for
vertical and locomotive boilers and does not find
its application in large capacity high pressure boilers. It
is also used where the space is not available for
the installation of a feed pump. In an injector the water is delivered to the
boiler by steam pressure; the kinetic energy of steam is used to increase the
pressure and velocity of the feed water.
Superheater: The function of a superheater is
to increase the temperature of the steam above its
saturation point. The superheater is very important
accessory of a boiler and can be used both on fire-tube
and water-tube boilers. The small boilers are not
commonly provided with a superheater.
Steam Separator: Thesteam available from a boiler
may be either wet, dry; or superheated; but in
many cases there will be loss of heat from it during
its passage through the steam pipe from the boiler to the engine tending to
produce wetness. The use of wet steam in an engine or turbine is uneconomical
besides involving some risk ; hence it is usual to endeavour to separate any
water that may be present from the steam before the latter enters the engine.
This is accomplished by the use of a steam separator. Thus, the tunction of a
steam separator is to remove the entrained water particles from the steam
conveyed to the steam engine or turbine. It is installed as close to the steam
engine as possible on the main steam pipe from the boiler.
Air Preheater : The function of the air
pre-heater is to increase the temperature of air before it
enters the furnace. It is generally placed after the
economiser; so the flue gases pass through the
economiser
and then to the air preheater.
Q. 2 (a) Diffrentiate between jet condensers and surface condensers.
Ans.
Jet
Condensers
|
Surface Condensers
|
1.) In jet condensers, the exhaust steam and water
come in direct contact with each other
|
1.) In surface condensers, the exhaust steam and water
do not come into direct contact.
|
2.) The cooling water is usually sprayed
into the exhaust steam to cause rapid condensation.
|
2.) The steam passes over the outer
surface of tubes through which a supply of cooling water is maintained.
|
3.) A jet condenser is simpler and
cheaper.
|
3.) A surface condenser is complicated
and costlier than the jet condenser.
|
4.) It should be installed when the
cooling water
is cheaply and easily made suitable for boiler feed or when a cheap source of
boiler.
|
4.) A surface condenser is most
commonly used because the condensate obtained is not
thrown as a
waste but returned to the boiler.
|
Q. 2(b) What do
you mean by cooling towers and cooling ponds ? Classify various types of
cooling towers with neat sketch.
Ans. The cooling ponds are the
simplest of the device for recooling of the cooling water. A cooling
pond consists of a large, shallow pool into which
the hot water is allowed to come in contact with the
atmospheric air. In power plants the hot water from
condenser is cooled in cooling tower, so that it can be reused in condenser for
condensation of steam. In a cooling tower water is made to trickle down drop by
drop so that it comes in contact with the air moving in the opposite direction.
The cooling towers may also be classified as
follows:
(i) Natural draught cooling towers
(ii) Mechanical draught cooling towers
(iii) Forced draught cooling towers
(iv) Induced draught cooling towers.
(i) Natural Draught Cooling Tower : In this type of
tower, the hot water from the condenser is
pumped to the troughs and nozzles
situated near the bottom.
(ii) Mechanical Draught Cooling Towers : In these towers the draught of
air for cooling the tower is
produced mechanically by means of propeller fans.
These towers are usually built in cells or units, the
capacity
depending upon the number of cells used.
Q. 3. (a)
Comparison of Otto, Diesel and Dual cycles.
10
Ans. Figure shows comparison of otto, diesel and
dual combustion cycles at various compression
ratio
and with given cut off ratio for diesel and dual cycles. For the given
compression ratio.
Q. 3. (b)
Differentiate between two stroke and four stroke engines.
Ans.
Comparison of
four stroke and two stroke cycle engines :
S. No.
|
Four Stroke Cycle
Engines
|
Two Stroke Cycle
Engines
|
1.
|
The cycle is completed in four strokes
Of the piston or in two revolutions of the
crankshaft. Thus one power stroke is obtained in every two revolutions of the
crankshaft
|
The cycle is completed in two strokes of the piston
or in one revolution of the revolution of the crankshaft. Thus, one power stroke is obtained in each
revolution of the crankshaft.
|
2.
|
Turning moment is not so uniform and hence heavier
flywheel is needed.
|
More uniform turning movement and hence lighter
fly-wheel is needed.
|
3.
|
Power produced for same size of engine is small or
for the same power the engine is heavy and bulky.
|
Power produced for same size of engine in more or
for the same power the engine is light and compact
|
4.
|
Lesser cooling and lubrication requirements. Lesser
rate of wear and tear.
|
Greater cooling and lubrication requirement. Greater rate of wear & tear.
|
5.
|
The four stroke engine contains valve & valve
mechanism.
|
Two stroke engines have no valves but only ports.
|
Q. 4. (a) It is
desired to generator 1000 kW of power and survey reveals that 450 m of static
head and a minimum flow of 03 m /sec is available. Comment whether the task can
be accomplished by installing a Pelton wheel that turns 1000 revolutions per
minute and has an efficiency of 80%. Further design the Pelton wheel by
assuming suitable data for coefficient of velocity, speed ratio and velocity coefficient
of the jet. 12
Q. 4. (b)
Differentiate between Impulse and Reaction hydraulic turbines.
Ans.
S. No.
|
Impulse turbine
|
Reaction
turbines
|
1.
|
All the available energy of
fluid is converted into kinetic energy by nozzle.
|
Only portion of energy is transformed
into kinetic energy before fluid enters the runner.
|
2.
|
The jet is not continued &
at atmospheric pressure throughout the action of water on the runner.
|
Water enter the runner with
excess pressure and then both velocity and pressure change as water passes
runner.
|
3.
|
The unit is installed above the
tail race.
|
Unit is kept entirely submerged
in water in water below tail race.
|
4.
|
Flow regulation is possible
without loss.
|
Flow regulation is always with
loss.
|
5.
|
The relative velocity of water
on runner may remain constant or reduce slightly due to friction.
|
Relative velocity always
increases.
|
6.
|
Casing has no hydraulic
function to perform.
|
Casing is essential since unit
is sealed from atmosphere.
|
Q. 5. (a) Derive
V.R. for single start, double start and triple start worm and worm wheel and compare
their results.
Ans.
Assume that worm rotates
through one revolution about its axis. Then angle turned by it will be
2K. The lead of worm is the distance moved by
pitch circle at the gear wheel. Thus, angle turned by it
during the same time will be I/R2 or 2I/d2.
Q.6 (a) Prove that
for crossed belt drive system the length of belt is given by
,
r1,r2 are
radii of drive and driven pulleys x is the distance between centers of two
pulleys.
Ans.
Q. 6. (b) What do
you mean by clutch ? Discuss single plate clutch system with neat sketch.
Ans. A
clutch is a device used to transmit the rotary motion of one shaft to another
when desired.
Disc Clutch (Single-Plate Clutch): A disc clutch consists of a
clutch plate attached to a splined hub
which is free to slide axially on splines cut on the
driven shaft. The clutch plate is made of steel and has a
ring of friction lining on each side. The engine
shaft supports a rigidly fixed flywheel. A spring-loaded pressure plate presses
the clutch plate firmly against the flywheel when the clutch is engaged. When
disengaged, the springs press against a cover attached to the flywheel. Thus,
both the flywheel and the pressure plate rotate with the input shaft. The
movement of the clutch pedal is transferred to the pressure plate through a
thrust bearing.
Figure shows the pressure plate pulled back by the
release levers and the friction linings on the
clutch plate are no longer in contact with the
pressure plate or the flywheel. The flywheel rotates without
driving the clutch plate and thus, the driven shaft.
When the foot is taken off the clutch pedal, the pressure on the thrust bearing
is released. As a result, the springs become free to move the pressure plate to
bring it in contact with the clutch plate. The clutch plate slides on the
splined hub and is tightly gripped between the pressure plate and the flywheel.
The friction between the linings on the clutch plate, and the flywheel on one
side and the pressure plate on the other, cause the clutch plate and hence, the
driven shaft to rotate.
Q. 7. (a) Derive
an expression for extension of a tapered bar.
Ans. Let diameter of bar 'd’ at
distance x from AB
Q. 7. (b) A thin
walled tube, when subjected to internal pressure and Torque; has the following
set
of stresses
produced at a point on its wall surface.
(i) 50 N/mm
& 30 N/mm at right angles to each other, both tensile.
(ii)
Complimentary shear stress of 40 N/mm determine the normal, tangential stresses
on the
planes which are
equally inclined to the direct stresses. How the results would be affected if
due to end
thrust, 30 N/mm
stress becomes compressive and other stresses remain unchanged.
Ans. The normal and tangential stresses on any plane
Ans. The normal and tangential stresses on any plane
Q.
8. Determine the reactions and construct the shear force and bending moment
diagram for the beam loaded as shown in the figure. Also find the points of
contraflexure.
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