EXPERIMENT NO. 5
Aim: To study the Vapour Compression Refrigeration System.
Introduction
The art of producing & maintaining the temperature in an enclosed space below surrounding
temperature is known as refrigeration. In order to maintain low temperature in refrigerator
space, it is necessary to remove heat continuously equal to the amount of heat leaking into the
system & reject the same to the surrounding atmosphere at higher temperature. A machine
which produces cold is called refrigerator & the process is called refrigeration. Refrigeration
has wide applications in chemical industries, food industries, air conditioning plants & many
other industrial processes.
The capacity of refrigeration system is given in tons of refrigeration. One ton of refrigeration
is the amount of heat to be removed in order to form one ton of ice at 0°C in 24 hours when
the temperature of water supplied is 0°C.
Working
In an air refrigeration system, air is used as working fluid to remove heat from refrigerated
space & discharge the same to surrounding atmosphere. In vapour refrigeration system
instead of air, vapour like ammonia, carbon dioxide, sulphur dioxide are used as working
fluids.
In vapour compression refrigeration system, the refrigerant used alternately, undergoes a
change of phase from vapour to liquid & liquid to vapour during the cycle. The latent heat of
vaporization is used for absorbing the heat at low temperature from refrigerated space & the
same is rejected during condensation at high pressure.
The arrangement of the components of this refrigeration system is shown in fig 1. The liquid
refrigerant coming out from a condenser at high pressure is passed through a throttle valve &
the pressure of refrigerant is reduced. The temperature of the refrigerant falls as its pressure is
reduced. A mixture of vapour & liquid coming out of the throttle valve at low temperature
enters the evaporator (refrigerator) & absorbs heat from the evaporator in the form of latent
heat & is converted into vapour. Then it is further passed to compressor where the
compression takes place & so its temperature as well as pressure increases. The high
pressure, high temperature vapour coming out of compressor enters the condenser where the
latent heat of refrigerant is removed by using water or air & it is converted into liquid. The
high pressure liquid refrigerant is again passed through the throttle valve & the cycle is
repeated.
The major advantages of this system over air refrigeration system are smaller size of machine
& high COP (4 to 5 ).
Coefficient Of Performane (C.O.P.):
It is the ratio of heat extracted from the system to the work done on the refrigerant in the
compressor. Its value is always greater than one.
Where, Q = Refrigerating effect or the amount of refrigeration produced.
W= Amount of work done in the compressor.
Here the refrigeration effect & energy input should be expressed in the same units.
Fig. 5.1 Vapour Compression Refrigeration Cycle
EXPERIMENT NO. 6
Aim: To study the functioning of Window Room Air Conditioner.
Introduction
Man feels comfortable at a particular temperature, humidity and air motion. Therefore winter
and summer are equally uncomfortable for human beings. The simultaneous control of
temperature, humidity, purity and air motion within enclosed space is known as air
conditioning. Air conditioning provides comfortable and healthy conditions for the occupants
in residences, theaters, office buildings, hospitals and railway coaches. Air conditioning
increases the working efficiency of the employees in factories and offices. The conditioned
air has comfort effect on health and psychological effect on the human minds.
Description
The Window Type Air Conditioner is the simplest type of packaged Air
Conditioner. In this unit, filtering, cooling and air distribution are combined in a compact
package. A Window Air Conditioner consists of a case, which is divided into two parts such
as Out Door part and In Door part by a partition as shown in figure 6.1. The out door part
consists of (hermetically sealed) motor driven compressor, condenser, motor driven fan and a
tray as shown in figure. The outdoor portion is also divided into two parts by a partition.
The compressor, motor and the tray are housed in the bottom part and condenser
and fan with small motor to run the fan are housed in the top part. The indoor part is divided
into bottom and top parts. The evaporator, the fan with motor is housed at the top portion. A
tray is also provided below evaporator to collect the water removed from the air by
dehumidifying. Both the trays are connected by a pipeline and fed to outside for draining.
The air filter and power connection are housed in the bottom part as shown in the figure.
The air conditioner is fitted in an opening in the wall such that the outdoor portion
remains outside the wall side. The indoor portion is fitted with bottom and top shutters which
can be set at different inclinations.
Working
When the conditioner is working, the low-pressure vapour refrigerant is drawn
from the evaporator to the compressor and it is compressed to high pressure and temperature.
This high pressure and high temperature vapour is condensed passing through the condenser.
The condensed liquid at high pressure is passed through the capillary and then flows through
the evaporator. As refrigerant comes out of the capillary, its pressure and temperature fall and
it starts boiling absorbing the heat in the evaporator compartment from the air and is
vaporized. The compressor again draws this low-pressure vapour and the cycle is repeated.
For condensing the high pressure refrigerant vapour , the air is drawn by fan F1
from the bottom of the outside compartment & passed over the condenser & discharged to the
atmosphere from top of the outside compartment.
The air from the room to be cooled is drawn with the help of fan F2 through the air
filter from the bottom of the compartment and passed over the evaporator. It loses its heat &
moisture giving out its heat to the refrigerant passing through the evaporator. The cooled &
dehumidified air is thrown into the room through the dampers from top of the inside
compartment as shown in figure. The moisture removed from the air by the evaporator drops
into the tray located at the bottom of the evaporator and led to the atmosphere through the
pipe.
The water collected in the tray evaporates to some extent & helps in cooling
the compressor & condenser.
Fig 6.1 Window Room Air Conditioner
EXPERIMENT NO. 7
Aim : To calculate the Mechanical Advantage, Velocity Ratio, and efficiency of Single start,
Double start and Triple start Worm & Worm Wheel.
Apparatus Required: Worm & worm wheel mounted on the wall, weights, weight hanger,
pan, ropes, thread, weight box, metre scale & vernier caliper.
Description:
Figure shows the arrangement of Worm and Worm Wheel. It consists of a square threaded
screw called Worm and a toothed wheel called Worm Wheel. The screw is in mesh with the
worm wheel and their axis are at right angles to each other. On the axis of worm is effort
wheel of large diameter, over which a rope passes. The effort (p) is applied at the end of this
rope. On the axis of the worm wheel, a small pulley or a load drum is provided over which a
rope passes at the end of which load to be lifted is attached. The worm and worm wheel
arrangements may be of the following types:
1. SINGLE START: If the worm is single threaded, then for each round of effort wheel,
the worm will push the worm wheel through one tooth.
2. DOUBLE START: For double threaded worm, the worm will push the worm wheel
through two teeth in each revolution of the effort wheel.
3. TRIPLE START: If the worm is triple threaded, then for each revolution of effort
wheel, the worm will push the worm wheel through three teeth.
Case 1:
SINGLE START WORM & WORM WHEEL
For each revolution of effort wheel by applying P effort,
(i) Distance moved by effort P = 2πl = πD
(ii) Load drums turns through = 1/T revolution
(iii) Distance moved by load W = πd / T
Velocity Ratio (V.R.) = Distance moved by effort P/ Distance moved by load W
= πD / πd /T = DT/d
Mechanical Advantage (M.A.) = Load lifted / effort applied = W/P
Efficiency (η) = M.A./ V.R.
Case 2:
DOUBLE START WORM & WORM WHEEL
Distance moved by load W= 2 πd/T
Velocity ratio (V.R.) = πD/2πd/T = DT/2d
Case 3:
TRIPLE START WORM & WORM WHEEL
Distance moved by load W= 3πd/T
Velocity ratio (VR) = πD/3πd/T = DT/3d
Procedure:
Tie one end of the rope with the hole provided in the grooved drum. Wind this rope on the
groove of the drum in clockwise direction & hang the load to be lifted (W) from the other end
of the rope. Note down the value of total weight to be lifted, W = weight placed on the hanger
+ weight of the hanger = W1 + W2. Note down the weight of empty hanger pertaining to
effort pulley. Tie one end of the other rope with the hole provided in the groove of effort
pulley & wind this rope on the groove of the drum in anticlockwise direction keeping the pan
freely tied on the free end of the rope. Increase the weights gradually on effort pan & get the
position to descend it steadily & the weight starts moving upwards with the same speed.
Allow effort pulley to rotate steadily for one complete revolution. In this condition, total
weight of the effort pan (weight of empty pan + weight placed on the pan = P1 + P2) will be
effort applied P and the total weight of the hanger (weight of the empty hanger + weight
placed on the hanger = W1 + W2 ) will be load lifted W. Repeat the experiment to get
different values of efforts P for different values of loads W. Calculate M.A. and V.R. and get
the values of efficiencies.
Observations:
(i)
Thickness of the rope of effort pulley, t1 =
(ii)
Thickness of the rope of drum, t2 =
(iii)
Effective circumference of the effort pulley, π(D+ t1) =
(iv)
Effective circumference of the drum, π(D+ t2) =
(v)
No. of teeth on the worm wheel, N =
(vi)
Weight of the empty hanger, W1 =
(vii)
Weight of the empty, effort pan P1 =
Observation Table:
S.NO. Velocity ratio
Load lifted
V.R.= π(D+t1)N/ W= W1 + W2
π(D+t2)
Applied effort
P = P1 + P2
MECHANICAL % Efficiency
η = M.A./ V.R. ×
ADVANTAGE
M.A. = W/P
100
1
2
3
Result:
Average values of , i) MA =
ii) VR =
iii) % η =
Precautions:
1) Lubricate the apparatus to reduce friction.
2) Weight W should be lifted gradually upwards at uniform speed.
3) To get effective values of circumference the thickness of the ropes should be
measured.
4) Weights of empty hanger and effort pan should always be determined.
5) For the load W and effort P the weights should be placed gently.
6) Load W and effort P should hang freely without touching wall etc.
7) The wound of the ropes on the circumferences of the load drum and effort pulley
should be single i.e. rope should not overlap.
Fig. 7.1 Worm and Worm Wheel
EXPERIMENT NO. 8
Aim: To calculate the Mechanical Advantage, Velocity Ratio and efficiency of
Single and Double Purchase Winch Crabs
Appratus Required:
Single and Double purchase winch crab apparatus, weights, hangers, rope etc.
Theory
Winch crabs are the lifting machines in which velocity ratio is increased by a gear system. If
only one set of gears is used, the winch crab is called a Single Purchase Winch Crab, and if
two sets are used, it is called Double Purchase Winch Crab.
(i) Single Purchase Winch Crab
It consists of a load drum of radius r connected to an axle by gears. The toothed wheel on
load drum is called Spur wheel and the small toothed wheel on axle is called Pinion. The axle
is provided with an effort pulley of diameter D.
Let, number of teeth on spur wheel and pinion be T1 and T2 respectively.
The effort P be applied at the effort pulley.
When one revolution is made by the pulley, the distance moved by the effort = 2πR
= πD
When the axle makes one revolution, due to gear arrangement the load drum also moves T2
number of teeth, which means it makes T2 / T1 revolutions.
The distance over which the load moves = 2π r ( T2 / T1)
Velocity Ratio = distance moved by effort / distance moved by the load
= 2πR / 2π r ( T2 / T1)
= D / d ( T1 / T2)
Mechanical Advantage (M.A.) = W / P
Efficiency ,
η = M.A. / V.R.
(ii) Double Purchase Winch Crab
Velocity Ratio of a Winch Crab can be increased by providing another axle with a pair of
pinion and gear. Since two pairs of pinion and gear are used it is called Double purchase
winch crab. It is used for lifting heavier loads.
Let, the number of teeth on the two spur wheels be T1 and T3
and number of teeth on the two pinions be T2 and T4 respectively.
The effort P be applied at the effort pulley.
When one revolution is made by the pulley, the distance moved by the effort = 2πR
= πD
When axle A makes one revolution, axle B is moved by T2 teeth i.e., it makes T2 / T1
revolutions and,
The load axle moves by ( T2 / T1) / ( T4 / T3) revolutions.
Therefore, the distance moved by the load = 2π r ( T2 / T1) / ( T4 / T3)
Velocity Ratio = distance moved by effort / distance moved by the load
= 2πR / 2π r ( T2 / T1) / ( T4 / T3)
= D / d [( T1 / T2) ( T3 / T4)]
Mechanical Advantage (M.A.) = W / P
Efficiency ,
η = M.A. / V.R.
Procedure
Measure the circumference of the effort wheel and load axle with the help of thread and scale.
Count the number of teeth on pinion and gear wheel. Hang the load W to the thread passing
round the load axle. Now place sufficient weights in the effort pan till the load W just starts
moving upwards. Note down the weights added in the pan with weight of the pan. Repeat the
procedure for different weights.
Observations
1) No of teeth on pinion-P1 = T2
2) No of teeth on pinion-P2 = T4
3) No of teeth on spur gear S1 =T1
4) No of teeth on spur gearS2 = T3
5) Diameter of the effort pulley =2R
6) Diameter of the load axle = 2r
S.NO.
LOAD PLACED EFFORT
(W)
APPLIED
(P)
Result
M.A. =
V.R. =
M.A.
=W/P
V.R.
η
= Average η
M.A./V.R.
Remarks
Efficiency of the machine, η =
Precautions
1) Bearing of axles, the pinion and gear wheel should be properly lubricated so as to
reduce friction.
2) The weights should be put gently in the pan.
3) Weight W should be lifted gradually upwards at uniform speed.
4) To get effective values of circumference the thickness of the ropes should be
measured.
5) Weights of empty hanger and effort pan should always be determined.
6) Load W and effort P should hang freely without touching wall etc.
7) The wound of the rope on the circumferences of the effort pulley should be single i.e.
rope should not overlap.
Fig 8.1 Single Purchase Winch Crab
Fig 8.2 Double Purchase Winch Crab
EXPERIMENT NO. 9
Aim: To find the Mechanical Advantage, Velocity Ratio and Efficiency of simple and
compound screw-jack.
Apparatus Required: Simple and compound screw jack, Load to be lifted (W), Weights or
Effort to be applied (P), Vernier caliper, Pan, Weight box.
Theory:
Screw Jack
It is a device used for lifting heavy loads which are usually centrally loaded by applying
smaller effort. It works on the principle of inclined plane. The device consists of a nut and
screw. The load is carried by screw head. The body consisting of a nut is fixed and screw is
rotated by means of a lever.
The axial distance moved by the screw when it makes one complete revolution is
known as the Lead of the screw. The distance between two consecutive threads is called Pitch
of the screw. For single threaded screw Lead = Pitch, and for double threaded screw L = 2p
Mechanical Advantage
It is the ratio of weight lifted to effort applied.
M.A. = W/ P
Velocity Ratio
It is the ratio of distance moved by the effort (y) to the distance moved by the load (x).
V.R. = y/ x
In one complete revolution of the lever by effort P:
Distance traveled by effort = 2 π R
And, distance traveled by the load = p
Therefore, Velocity Ratio = 2 π R / p
Compound Screw Jack
In one complete revolution of the effort wheel, the distance moved by the
effort, y = π ( D + d )
But since it is a compound screw jack,
Therefore for the load to be lifted through a distance p, the no. of revolutions required by the
effort wheel = No. of teeth on the gear, N.
Therefore for p distance moved by the load, the distance moved by the effort =
π ( D + d )N
Therefore, Velocity ratio = π ( D + d )N / p
Mechanical Efficiency = M.A./ V.R.
=W/P
π ( D + d )N / p
= W. p
P π ( D + d )N
Procedure :
Put load on the jack and start applying efforts gradually and record the observations as
the load just moves.
Observations :
Let, Load Lifted = W
Effort Applied = P
Effort Wheel Diameter ( D) = 130 mm
Diameter of the rope (d ) = 5 mm
Pitch of the screw ( p ) = 2.5 mm
No. of teeth on the gear ( N ) = 40
Weight of the Pan ( w ) = 150 gm
S. No. W (kg )
1.
2.
3.
4.
5.
P( kg )
MA = W/P
VR = π(D+d)W/p
15
17
19
21
23
Precautions:
1. Lubricate the jack well before use.
2. Apply effort gently.
3. Note the effort readings as the load just moves.
Fig. 9.1 Simple Screw Jack
MA/VR =
Wp/Pπ(D+d)N
EXPERIMENT NO. 10
Aim: To determine the Mechanical Advantage, Velocity Ratio, & Efficiency of a
Differential Wheel and axle.
Appratus Required:
Differential Wheel and Axle apparatus mounted on a wall, weights hanger, weights, weights
box, 2 rope pieces, string, scale, vernier calipers etc
Theory
1. Simple Wheel and Axle
In fig10.1 is shown a Simple wheel & axle, in which the wheel A & axle B are keyed
to the same shaft. The shaft is mounted on ball bearings, in order to reduce the frictional
resistance to minimum. A string is wound round the axle B, which carries the load to be
lifted. A second string is wound round the wheel A in the opposite direction to that of
string on B.
Let, W = Load lifted
P = Effort applied to lift the load
D = Diameter of effort wheel, and
D = Diameter of the load drum
One end of the string is fixed to the wheel, while the other is free and the effort is
applied to this end. Since the two strings are wound in the opposite directions, so the
downward motion of the effort (P) will raise the load (W).
Since the wheel A and axle B are keyed to the same shaft, so when the wheel rotates
through one revolution, axle will also rotate through one revolution.
We know that the distance moved by the effort in one revolution the effort wheel
= πD
& distance moved by the load in one revolution = πd
V.R. = Distance moved by effort / Distance moved by load = πD/ πd = D/ d
Now ,
M.A. = Load lifted / Effort applied = W/P
and Efficiency
η = M.A. / V.R.
2. Differential Wheel and Axle
It is an improved form of Simple wheel & axle, in which the Velocity Ratio is
intensified with the help of a load axle. In figure 10.2 is shown a Differential wheel &
axle. In this case, load axle BC is made up of two parts of different diameters. Like
Simple wheel & axle, the wheel A, & axle B and C are keyed to the same shaft, which is
mounted on ball bearings, in order to reduce the frictional resistance to minimum.
The effort string is wound round the wheel A. Another string is wound round the axle
B, which after passing round the pulley (to which the weight W is attached) is wound
round the axle C in the opposite directions to that of the axle B; care being taken to wind
the string on the wheel A & axle C in the same direction. As a result of this, when the
string unwinds from the wheel A, the other string also unwinds from the axle C. But it
winds on the axle B in figure 10.2.
Let
D = diameter of effort wheel A,
d1 = diameter of the axle B
d2 = diameter of the axle C
W = Weight lifted by the machine, &
P = Effort applied to lift the weight
We know distance moved by the effort in one revolution of the effort wheel A= πD
Therefore, Length of string, which will wound on the axle B in one revolution = πd1
& Length of string, which will unwound from the axle C in one revolution = πd2
Therefore, Length of string, which will wound in one revolution = πd1 - πd2
= π(d1 - d2)
& distance moved by the weight = 1/2× π (d1 - d2)
= π/2(d1 - d2)
therefore,
and,
V.R. = Distance moved by effort P/ Distance moved by load
= πD/ π/2(d1 - d2)
= 2D/(d1 - d2)
M.A.= W/ P
and, Efficiency,
η = M.A./ V.R.
Observations:
1.
2.
3.
4.
Weight of the empty effort pan P1 =
Weight of the empty hanger, W1 =
Thickness of effort wheel’s string, t1 =
Thickness of load axle’s rope , t2 =
Observation Table:
S.N
O.
Effective circumference of
Wheel
π (D + t1)
1
2
3
Axle
π (d + t2)
Velocity
Ratio
VR =
π (D + t1) /
π (D + t2)
Load
Effort
lifted
Applied
W = W1 + P = P1+P2
W2
Mechanical
Advantage
MA = W/P
Efficiency
= MA/VR
Calculations:
Result:
Average value of: i) M.A. =
ii) V.R. =
iii) % η =
Precautions:
1. Weight of empty hanger and effort pan should always be taken.
2. Weight W should be lifted gradually upwards at uniform speed.
3. To get effective values of circumference, the thickness of the ropes should be
measured.
4. Lubricate the bearings apparatus to reduce friction.
Fig 10.1 Simple Wheel and Axle
Fig 10.2 Wheel and Differential Axle
EXPERIMENT NO. 11
Aim : To draw stress-strain diagram for a mild steel specimen under tensile test.
Theory :
Stress- strain curve can be drawn experimentally by subjecting a metallic bar of
uniform cross-section to a gradually increasing tensile load till failure of the bar occurs. The
test is conducted on a tensile testing machine on a test specimen having the shape as shown
in the figure 11.1. The specimen has collars provided at both the ends for gripping it firmly
in the fixtures of the machine. The central portion of the test specimen is somewhat smaller
than the end regions and this central section constitutes the gauge length over which
elongations are measured. An extensometer is used to measure very small changes in the
length. After that vernier scale on the machine is used to measure the extension. Load and
extension are simultaneously measured till the specimen breaks. Stress is calculated by
dividing the load by the original cross-sectional area of the test specimen. Strain is
calculated by dividing the extension of given length by original unstrained length.
change in length
Strain =
original length
Load
Area
The typical behavior of stress-strain curve for mild steel specimen and its salient features are:
Stress =
1. Proportional Limit:
Stress is a linear function of strain and the material obeys Hooke’s law.
This proportionality extends up to point A and this point is called Proportional limit.
O-A is a straight line and its slope represents the value of Modulus of Elasticity.
2. Elastic Limit :
Beyond proportional limit, stress and strain depart from straight line
relationship. The material however remains elastic upto state point B. The word
elastic implies that the stress developed in the material is such that there is no
residual or permanent deformation when the load is removed. Upto this point, the
deformation is reversible or recoverable. Stress at B is called the elastic limit stress,
this represents the maximum unit stress to which a material can be subjected to and
is still able to return to its original form upon removal of load.
3. Yield Point :
Beyond Elastic limit, the material shows considerable strain even
though there is no increase in load or stress. This strain is not fully recoverable i.e.
there is no tendency of the atoms to return to their original positions. The behavior of
the material is inelastic and the onset of plastic deformation is called yielding of the
material. Yielding pertains to the region C-D and there is drop in load at the point D.
The point C is called the upper yield point & point D is lower yield point. The
difference between the upper and lower yield point is small and the quoted yield
stress is usually the lower value.
4. Ultimate Strength or Tensile Strength :
After yielding has taken place, the material becomes strain hardened and an
increase in load is required to take the material to its maximum stress at point E.
Strain in this portion is about 100 times than that of the portion from 0 to D. Point E
represents the maximum ordinate of the curve or tensile stress of the material.
5. Breaking Strength :
In the portion EF, there is falling off the load from the maximum until
fracture takes place at F. The point F is referred to as the fracture or breaking point
and the corresponding stress is called breaking stress.
The apparent fall in stress from E to F may be attributed to the fact that
stress calculations are made on the basis of original cross-sectional area. In fact
elongation of the specimen is accompanied by reduction in cross-sectional area and
this reduction becomes significant near the ultimate stress. In case stress calculations
are based on actual area, the curve would be seen to rise until fracture occurs. For
mild steel, the test piece breaks making a cup and cone type fracture; the two pieces
can be joined together to find out the diameter at the neck under the specimen breaks.
Fig. 11.1 Tensile Test Specimen
Fig. 11.2 Stress –Strain Diagram